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Luminous Blue Variables (LBVs) are massive stars that are likely to be a transitionary phase between O stars and hydrogen-free classical Wolf–Rayet stars. The variability of these stars has been an area of study for both professional and amateur astronomers for more than a century. In this paper, we present 5 yr of precision photometry of the classical LBV P Cygni taken with the BRITE-Constellation nanosatellites. We have analyzed these data with Fourier analysis to search for periodicities that could elucidate the drivers of variability for these stars. These data show some long-time-scale variability over the course of all six calendar years of observations, but the frequencies needed to reproduce the individual light curves are not consistent from 1 yr to the next. These results likely show that there is no periodic phenomenon present for P Cygni, meaning that the variability is largely stochastic. We interpret the data as being caused by internal gravity waves similar to those seen in other massive stars, with P Cygni exhibiting a larger amplitude and lower characteristic frequency than the main-sequence or blue supergiant stars previously studied. These results show evidence that LBVs may be an extrapolation of the blue supergiants, which have previously been shown to be an extension of main-sequence stars in the context of the stochastic low-frequency photometric variability.

A research on stellar magnetism in Crimea was initiated by pioneer works of A.B. Severny, V.E. Stepanov, and D.N. Rachkovsky. Today, the study of stellar magnetic fields is a key field of research at the Crimean Astrophysical Observatory (CrAO). The 2.6 m Shajn telescope equipped with the echelle spectrograph ESPL, CCD, and Stokesmeter (a circular polarization analyzer) allows us to study the magnetic field of bright stars up to 5m–6m. The Single Line (SL) technique is developed for measuring magnetic fields at CrAO. This technique is based on the calculation of the Zeeman effect in individual spectral lines. A key advantage of the SL technique is its ability to detect local magnetic fields on the surface of stars. Many results in the field of direct measurements of stellar magnetic fields were obtained at CrAO for the first time. In particular, the magnetic field on supergiants (ǫ Gem), as well as on a number of subgiants, giants, and bright giants was first detected. This, and investigations of other authors, confirmed the hypothesis that a magnetic field is generated at all the stages of evolution of late-type stars, including the stage of star formation. The emergence of large magnetic flux tubes at the surface of stars of V-IV-III luminosity classes (61 Cyg A, β Gem, β Aql) was first registered. In subgiants, the magnetic field behavior with the activity cycle was first established for β Aql. Using the long-term Crimean spectroscopic and spectropolarimetric observations of α Lyr, the 22-year variability cycle of the star, supposedly associated with meridional flows, is confirmed. Magnetic field variability with the pulsation period was first detected for different types of pulsating variables: the classical Cepheid β Aql, the low-amplitude β Cep-type variable γ Peg, and others. In this review we cover more than a half-century history of the formation of the Crimean scientific school for high-precision direct measurements of stellar magnetic fields.

Evidence of wind variability and velocity stratification in the extended atmosphere has been found in the spectra of the supergiant V340 Ser ( $${=}$$ IRAS 17279 $$-$$ 1119) taken at the 6-m BTA telescope with a resolution $$R\geq 60\,000$$ . The H $$\alpha$$ line has a P Cyg profile whose absorption component ( $$V_{\odot}={+}34$$ km s $${}^{-1}$$ ) is formed in the upper layers of the expanding atmosphere close the circumstellar medium. For four dates the mean velocity has been derived from the positions of 300–550 symmetric metal absorptions with an accuracy better than $${\pm}0.1$$ km s $${}^{-1}$$ : $$V_{\odot}=59.30$$ , $$60.09$$ , $$58.46$$ , and $$55.78$$ km s $${}^{-1}$$ . A lot of low-excitation metal lines have an inverse P Cyg profile. The mean positions of their emission components, $$V_{\odot}=46.3\pm 0.4$$ km s $${}^{-1}$$ , differ systematically from the velocity inferred from symmetric absorptions, suggesting the presence of a velocity gradient in the supergiant’s extended atmosphere. The multicomponent profile of the Na I D lines contains the interstellar, $$V_{\odot}={-}11.2$$ km s $${}^{-1}$$ , and circumstellar, $$V_{\odot}={+}10$$ km s $${}^{-1}$$ , components and the component forming in the upper atmospheric layers, $$V_{\odot}={+}34.0$$ km s $${}^{-1}$$ . The mean velocity from 20–30 diffuse interstellar bands (DIBs) identified in the spectra, $$V_{\odot}\textrm{(DIBs)}={-}11.6\pm 0.2$$ km s $${}^{-1}$$ , agrees with the velocity from interstellar Na I and K I components. The equivalent width of the oxygen triplet $$\textrm{W(O\,I 7774)}=1.25$$ A corresponds to an absolute magnitude of the star $$Mv\approx{-}4.6^{m}$$ , which, given the total (interstellar $$+$$ circumstellar) extinction, leads to a distance to the star $$d\approx 2.3$$ kpc.

The research on stellar magnetism in the Crimea was initiated by pioneer works of A.B. Severny, V.E. Stepanov, and D.N. Rachkovsky. Today, the study of stellar magnetic fields is a key field of research at the Crimean Astrophysical Observatory (CrAO). The 2.6 m Shajn telescope equipped with the echelle spectrograph ESPL, CCD, and Stokesmeter (a circular polarization analyzer) allows us to study the magnetic field of bright stars up to 5m–6m. The Single Line (SL) technique is developed for measuring magnetic fields at СrAO. This technique is based on the calculation of Zeeman effect in individual spectral lines. A key advantage of the SL technique is its ability to detect local magnetic fields on the surface of stars. Many results in the field of direct measurements of stellar magnetic fields were obtained at CrAO for the first time. In particular, the magnetic field on supergiants (e Gem), as well as a number of subgiants, giants, and bright giants was first detected. This, and investigations of other authors, confirmed the hypothesis that a magnetic field is generated at all stages of the evolution of late-type stars, including the stage of star formation. The emergence of large magnetic flux tubes at the surface of stars of V-IV-III luminosity classes (61 Cyg A, β Gem, β Aql) was first registered. For β Aql, the behavior of a magnetic field with an activity cycle in subgiants was first established. Using long-term Crimean spectroscopic and spectropolarimetric observations of α Lyr, the 22-year variability cycle of the star, supposedly associated with meridional flows, is confirmed. Magnetic field variability with the pulsation period was first detected for different types of pulsating variables: the classical Cepheid β Aql, the low-amplitude β Cep-type variable γ Peg, and others. In this review we cover more than a half-century history of the formation of the Crimean scientific school for high-precision direct measurements of stellar magnetic fields.

—The article presents the results of multicolor photometry, medium and low resolution spectroscopy of the red nova V838 Mon remnant for 16 years after the 2002 outburst. We also used the archival photometry with the photographic plates of the Sonneberg and Moscow collections from 1928 to 1994. Analysis of these observational data confirmed that the progenitor of the V838 Mon explosion was a wide pair of B3V type stars of reduced luminosity. A brighter component exploded; it was 36 per cent brighter than its companion, and located on the zero-age main sequence of the Spectrum–Luminosity Diagram. Immediately after the outburst, in the fall of 2002, the remnant was a brown L-type supergiant (sgL), but in the fall of 2003 its spectrum changed to M type with a blue radiation excess appeared in the spectral energy distribution, which we interpreted as the reflection effect of the B type companion on the dust formed on the M star. In 2008, the companion was engulfed by the expanding explosion remnant, a type M supergiant (sgM). When the companion was immersing in the expanding M-star, a void was discovered under the M-star upper layer, in which the companion moved for about 200 days. Over the past 10 years, the luminosity of the M star has increased in the V filter by a factor of 10, and the spectral type has changed from M7.5 to M5.5. Based on radial velocities in the BaII 6497 A and CaI 6572 A lines, a deceleration of the expanding envelope of the M supergiant was detected, and in 2018, the envelope velocity approached to the heliocentric velocity of the star +71 km s−1. Quasi-periodic changes with a period of 320 days appeared then in the light curves, especially clearly expressed in I filter. We assume that the remnant has an elongated structure, and its rotation period is about 640 days. This is probably a gigantic contact system that will become a detached binary system in future development. The observations do not confirm the assumption that the explosion of one of the V838 Mon components was due to the merger of a compact binary system components located in a hierarchical triple one. Two hypotheses were proposed on the nature of the explosion of one of the V838 Mon components, directly based on the early age of this system: (1) the ignition of thermonuclear burning of hydrogen in the core after the gravitational compression of a protostar; (2) the fragmentation of the core inside a rapidly rotating star in the stage of gravitational compression of a protostar, and later, the subsequent defragmentation (merger of the core components) due to the loss of torque.

We evolve stellar models with zero-age main sequence (ZAMS) mass of $M_{\rm ZAMS} \gtrsim 18 M_\odot$ under the assumption that they experience an enhanced mass-loss rate when crossing the instability strip at high luminosities and conclude that most of them end as type Ibc supernovae (SNe Ibc) or dust-obscured SNe II. We explore what level of enhanced mass-loss rate during the instability strip would be necessary to explain the `red supergiant (RSG) problem'. This problem refers to the dearth of observed core-collapse supernovae progenitors with $M_{\rm ZAMS} \gtrsim 18 M_\odot$. Namely, we examine what enhanced mass loss rate could make it possible for all these stars actually to explode as CCSNe. We find that the mass-loss rate should increase by a factor of at least about ten. We reach this conclusion by analyzing the hydrogen mass in the stellar envelope and the optical depth of the dusty wind at the explosion, and crudely estimate that under our assumptions only about a fifth of these stars explode as unobscured SNe II and SNe IIb. About 10-15 per cent end as obscured SNe II that are infrared-bright but visibly very faint, and the rest, about 65-70 per cent, end as SNe Ibc. However, the statistical uncertainties are still too significant to decide whether many stars with $M_{\rm ZAMS} \gtrsim 18M_\odot$ do not explode as expected in the neutrino driven explosion mechanism, or whether all of them explode as CCSNe, as expected by the jittering jets explosion mechanism., Accepted for publication in MNRAS

We investigate the impact of optically thick clumping on stellar wind diagnostics in O supergiants and constrain wind parameters associated with porosity in velocity space. This is the first time the effects of optically thick clumping have been investigated for a sample of massive hot stars, using models including a full optically thick clumping description. We re-analyse spectroscopic observations of a sample of eight O supergiants. Using a genetic algorithm wrapper around the NLTE atmosphere code FASTWIND we obtain simultaneous fits to optical and UV spectra and determine photospheric and wind properties and surface abundances. We provide empirical constraints on a number of wind parameters including the clumping factors, mass-loss rates and terminal wind velocities. Additionally, we establish the first systematic empirical constraints on velocity filling factors and interclump densities. These parameters describe clump distribution in velocity-space and density of the interclump medium in physical-space, respectively. We observe a mass-loss rate reduction of a factor of 3.6 compared to theoretical predictions from Vink et al. (2000), and mass-loss rates within a factor 1.4 of predictions from Bj\"orklund et al. (2021). We confirm that including optically thick clumping allows simultaneous fitting of recombination lines and resonance lines, including the unsaturated UV phosphorus lines (Pv 1118-1128), without reducing the phosphorus abundance. We find that, on average, half of the wind velocity field is covered by dense clumps. We also find that these clumps are 25 times denser than the average wind, and that the interclump medium is 3-10 times less dense than the mean wind. The former result agrees well with theoretical predictions, the latter suggests that lateral filling-in of radially compressed gas might be critical for setting the scale of the rarefied interclump matter., Comment: Accepted for publication in A&A; 35 pages, 20 figures

Since its discovery about 50 years ago, HD144941 has generally been classified as a peculiar member of the extreme helium (EHe) supergiant stars, a very rare class of low-mass hydrogen-deficient stars. We report the detection of a strong longitudinal magnetic field based on spectropolarimetry with FORS2 on the ESO VLT with surface-averaged longitudinal field strengths as large as -9kG. This is further constrained by the detection of Zeeman splitting of spectral lines to a field strength of at least 15kG, explaining the recent finding of surface spots for this star. The quantitative analysis of the stellar atmosphere based on a hybrid non-local thermodynamic equilibrium approach and new optical spectra yields an effective temperature of 22000$\pm$500K, a logarithmic surface gravity of 4.20$\pm$0.10, and a surface helium fraction of 0.950$\pm$0.002 by number. While the metal abundances are about a factor of 10 sub-solar in absolute number, the metal-to-hydrogen ratios are typical of massive early-type stars, indicating that helium fallback in a weak, fractionated stellar wind in the presence of a magnetic field took place -- the canonical mechanism for the formation of the helium-strong phenomenon. Both the spectroscopic and the Gaia EDR3 parallax imply HD144941 to be a luminous massive star. Kinematically, we argue that HD144941 has reached its high Galactic latitude as a runaway star. We conclude that instead of being a comparatively high-gravity low-mass EHe star, HD144941 is by far the most extreme member of the magnetic massive helium-strong stars, with almost all atmospheric hydrogen substituted by helium., 9 pages, 8 figures, see A&A for the published version

At the surface of ∼7% of single hot stars stable mainly dipolar strong magnetic fields have been detected. The main hypothesis today is that these magnetic fields are of fossil origin. In other words, these fields formed from the seed field in the molecular clouds from which the stars were formed. The recent observational and theoretical results confi- rm this theory: the properties of the observed fields correspond to those expected from fossil fields. Massi- ve stars are stars whose initial mass exceeds about 8 solar masses. Massive stars play a significant role in the chemical and dynamical evolution of galaxies. However, much of their variability, particularly during their evolved supergiant stage, is poorly understood. To date magnetic field was registered only at three hot stars of I-II luminosity types: ρ Leo (B1 Ib), ζ Ori Aa (O9.2 Ib), and ? CMa (B1.5 II). We performed high-accuracy spectropolarimetric observation of the hot supergiant ζ Per (B1 Ib) over 26 nights from 1997 to 2012 with long-slit spectrograph mounted in the coude focus of 2.6-m reflector ZTSh at the Crimean Astrophysical Observatory. We also used circularly polarized spectra obtained during 2 nights in 2008 wi- th echelle spectrograph ESPADONS mounted at 3.6- m CFHT. Effective magnetic field B e (longitudinal component of the field integrated over visible hemi- sphere) of ζ Per was calculated in the line He I 6678.149 A. Statistically significant longitudinal magnetic field (B e /σ B > 3) was registered in 14 from 199 single measurements. These significant magnetic field values are all in the range from −145 to +148 G with the mean error 27 G. We suppose the supergiant ζ Per can be magnetic, but its magnetic field properties is diffi- cult to detect likely due to the insufficient precision of the used spectropolarimetric measurements compared to the expected field strength.

We have revisited and updated the $uvby$ Str\"omgren colour and colour-index distributions of A, F and early G-type main sequence stars. For this aim, we selected 7054 dwarf stars along with 65 MK standard stars within the same spectral range but covering all luminosity classes. The standard stars were selected following the MK mandate strictly, using spectra taken at classification resolution recorded on photographic plates. We used the colours of these stars to determine the effective temperature and surface gravity. After correcting for systematic offsets using fundamental parameters and considering a few exceptions, we find an one-to-one correspondence, among MK spectral types, Str\"omgren photometry, and their associated physical properties. We have applied a principal component analysis to the mean Str\"omgren indices for dwarf stars complemented by MK standards for higher luminosity classes. We have used the projections to introduce three new photometric metaindices, namely SM1, SM2, and SM3. We have defined a 3D-box, which allowed us to segregate dwarf stars from bright giants and supergiant stars, with the aid of the metaindices. Two of the planes show that the projections of dwarfs and supergiants are ordered by temperature; however, the temperature dependence for the supergiants is not as well defined as for the dwarfs. Following the MK Process, we were able to form an automatic classifier. We present some applications and assigned synthetical spectral types. We suggest that our metaindices formalism allows the extension of Str\"omgren photometric outside its original mandate (i.e., later types), without requiring the introduction of additional photometric filters., Comment: 20 pages, 22 figures, 9 tables, accepted for publication in MNRAS

The results of ultrahigh-time-resolution ( $$\Delta T\sim 12$$ s) observations of the B1 Iab supergiant $$\rho$$ Leo with the 1.25-m telescope at the Crimean Astronomical Station of the SAI MSU are presented. Regular line profile variations with periods of 15–30 min have been revealed. Significant changes in the average line profiles on time intervals of 1–3 days have been detected. Evidence for the existence of short-period line profile variations on time scales of 15–25 min is provided. The nature of the short-period line profile variations is discussed. The possibility of studying the fast variations in the profiles of broad lines (mostly the Balmer hydrogen and He I lines) using low-resolution spectrographs with $$R=1000{-}2000$$ at 1–2-m telescopes is pointed out.

We present intensity interferometry of the luminous blue variable P Cyg in the light of its H$\alpha$ emission performed with 1\,m-class telescopes. We compare the measured visibility points to synthesized interferometric data based on the CMFGEN physical modeling of a high-resolution spectrum of P Cyg recorded almost simultaneously with our interferometry data. Tuning the stellar parameters of P Cyg and its H$\alpha$ linear diameter we estimate the distance of P Cyg as $1.56\pm0.25$~kpc, which is compatible within $1\sigma$ with $1.36\pm0.24$~kpc reported by the Gaia DR2 catalogue of parallaxes recently published. Both values are significantly smaller than the canonic value of $1.80\pm0.10$~kpc usually adopted in literature. Our method used to calibrate the distance of P Cyg can apply to very massive and luminous stars both in our galaxy and neighbour galaxies and can improve the so-called Wind-Momentum Luminosity relation that potentially applies to calibrate cosmological candles in the local Universe., Comment: To be published in MNRAS

The Galactic emission-line object MWC 349A is one of the brightest radio stars in the sky. The central object is embedded in an almost edge-on oriented Keplerian rotating thick disc that seems to drive a rotating bipolar wind. The dense disc is also the site of hot molecular emission such as the CO bands with its prominent band heads in the near-infrared spectral range. Despite numerous studies, the nature of MWC 349A is still controversial with classifications ranging from a pre-main sequence object to an evolved supergiant. We collected new high-resolution near-infrared spectra in the K- and L-bands using the GNIRS spectrograph at Gemini-North to study the molecular disc of MWC 349A, and in particular to search for other molecular species such as SiO and the isotope 13CO. The amount of 13CO, obtained from the 12CO/13CO ratio, is recognized as an excellent tool to discriminate between pre-main sequence and evolved massive stars. We find no signatures of SiO band emission, but detect CO band emission with considerably lower intensity and CO gas temperature compared to previous observations. Moreover, from detailed modelling of the emission spectrum, we derive an isotope ratio of 12CO/13CO = 4 +- 1. Based on this significant enrichment of the circumstellar environment in 13CO we conclude that MWC 349A belongs to the group of B[e] supergiants, and we discuss possible reasons for the drop in CO intensity., 8 pages 5 figures, accepted for publication in MNRAS

Non-LTE analysis (where LTE is local thermodynamic equilibrium) of the oxygen abundances for 51 Galactic A-, F- and G-type supergiants and bright giants is performed. In contrast with carbon and nitrogen, oxygen does not show any significant systematic anomalies in its abundances log ε(O). There is no marked difference from the initial oxygen abundance, within the errors, for the log ε(O) determination across the Teff interval from 4500–8500 K and the $\log \, g$ interval from 1.2–2.9 dex. This result agrees well with theoretical predictions for stellar models with rotation. With our new data for oxygen and our earlier non-LTE determinations of the N and C abundances for stars from the same sample, we constructed the [N/C] versus [N/O] relation for 17 stars. This relation is known to be a sensitive indicator of stellar evolution. A pronounced correlation between [N/C] versus [N/O] is found; the observed [N/C] increase from 0 to 1.6 dex is accompanied by a [N/O] increase from 0 to 0.9 dex. When comparing the observed [N/C] versus [N/O] relation with the theoretical one, we show that this relation reflects a strong dependence of the evolutionary changes in CNO abundances on the initial rotation velocities of stars. Given that the initial rotational velocities of these stars are expected to satisfy V0 < 150 km s−1, it is found that they are mostly post-first-dredge-up (post-FDU) objects. It is important that such initial velocities V0 are typical for about 80 per cent of the stars in question (i.e. stars with masses 4–19 M$\odot$). A constancy of the total C+N+O abundance during stellar evolution is confirmed. The mean value of log ε(C+N + O) = 8.97 ± 0.08 found for AFG supergiants and bright giants seems to be very close to the initial values of 8.92 (the Sun) or 8.94 (unevolved B-type main-sequence stars).

Galactic, young massive star clusters are approximately coeval aggregates of stars, close enough to resolve the individual stars, massive enough to have produced large numbers of massive stars, and young enough for these stars to be in a pre-supernova state. As such these objects represent powerful natural laboratories in which to study the evolution of massive stars. To be used in this way, it is crucial that accurate and precise distances are known, since this affects both the inferred luminosities of the cluster members and the age estimate for the cluster itself. Here we present distance estimates for three star clusters rich in Red Supergiants ($\chi$ Per, NGC 7419 and Westerlund 1) based on their average astrometric parallaxes $\bar{\pi}$ in Gaia Data Release 2, where the measurement of $\bar{\pi}$ is obtained from a proper-motion screened sample of spectroscopically-confirmed cluster members. We determine distances of $d=2.25^{+0.16}_{-0.14}$kpc, $d=3.00^{+0.35}_{-0.29}$kpc, and $d=3.87^{+0.95}_{-0.64}$kpc for the three clusters respectively. We find that the dominant source of error is that in Gaia's zero-point parallax offset $\pi_{\rm ZP}$, and we argue that more precise distances cannot be determined without an improved characterization of this quantity., Comment: 5 pages, 9 figures, to appear in MNRAS

The results of our second stage (1998-2018) of the detailed spectroscopy of peculiar supergiants identified with galactic infrared sources, performed mainly at the 6-meter BTA telescope are summarized. The main aspect of the program is a search for the evolutionary variations in the chemical composition of stars, past the AGB stage and the TDU, as well as an analysis of spectral manifestations of kinematic processes in their extended, often unstable, atmospheres and in the envelopes. The most significant result is detection of the s-process element excesses in seven single post-AGB stars, which confirms the theory of evolution of this type of stars. In three of these stars we for the first time discovered the ejection of the s-process heavy metals to the circumstellar envelopes. A lithium excess was found in the atmospheres of two peculiar supergiants V2324 Cyg and V4334 Sgr. The results of investigation of the kinematical state of atmospheres and envelopes will clarify the equilibrium of matter produced by stars in the AGB and post-AGB stages and delivered to the interstellar medium., Comment: 35 pages, 10 figures

We have studied the interstellar lines and diffuse bands in the spectrum of the supergiant κ Cas using the CCD spectra obtained with the Echelle spectrograph of the 2 meter telescope at the peak Terskol observatory. A synthetic stellar spectrum was used to correctly reveal the interstellar features, computed for a model atmosphere with Teff = 21 500 K, log g = 2.60, v sin i = 83 kms-1, [M/H] = 0.0, ξ = 15 kms-1 and with partial selection of main chemical element abundances. Eleven interstellar bands and 91 diffuse interstellar bands were identified. A further 14 absorption features were revealed, possibly also diffuse bands. The spectrophotometric diffuse band parameters and their accuracies were tabulated. We present maps of observed and synthetic spectral regions containing interstellar diffuse bands.

Images recorded with the Gemini Multi-Object Spectrograph (GMOS) on Gemini South are combined with archival images from other facilities to search for variable stars in the southern portion of the nearby disk galaxy NGC 247. Fifteen new periodic and non-periodic variables are identified. These include three Cepheids with periods < 25 days, four semi-regular variables, one of which shows light variations similar to those of R CrB stars, five variables with intrinsic visible/red brightnesses and colors that are similar to those of luminous blue variables (LBVs), and three fainter blue variables, one of which may be a non-eclipsing close binary system. The S Doradus instability strip defines the upper envelope of a distinct sequence of objects on the (i', g'-i') color-magnitude diagram (CMD) of NGC 247. The frequency of variability with an amplitude > 0.1 magnitude in the part of the CMD that contains LBVs over the seven month period when the GMOS images were recorded is ~ 0.2. The light curve of the B[e] supergiant J004702.18--204739.9, which is among the brightest stars in NGC 247, is also examined. Low amplitude variations on day-to-day timescales are found, coupled with a systematic trend in mean brightness over a six month time interval., Accepted for publication in the Astronomical Journal

Canopus, the brightest and closest yellow supergiant to our Solar System, offers a unique laboratory for understanding the physics of evolved massive stars. The accurate and precise PIONIER data allowed us to simultaneously measure the angular diameter and the limb darkening (LD) profile using different analytical laws. We found that the power-law LD, being also in agreement with predictions from stellar atmosphere models, reproduces the interferometric data well. For this model we measured an angular diameter of $7.184 \pm 0.0017 \pm 0.029$ mas and an LD coefficient of $0.1438 \pm 0.0015$, which are respectively $\gtrsim 5$ and $\sim15-25$ more precise than in our previous A\&A paper on Canopus from 2008. From a dedicated analysis of the interferometric data, we also provide new constraints on the putative presence of weak surface inhomogeneities. Additionally, we analyzed the SED in a innovative way by simultaneously fitting the reddening-related parameters and the stellar effective temperature and gravity. We find that a model based on two effective temperatures is much better at reproducing the whole SED, from which we derived several parameters, including a new bolometric flux estimate. The Canopus angular diameter and LD measured in this work with PIONIER are the most precise to date, with a direct impact on several related fundamental parameters. Moreover, thanks to our joint analysis, we were able to determine a set of fundamental parameters that simultaneously reproduces both high-precision interferometric data and a good quality SED and, at the same time, agrees with stellar evolution models., 14 pages, 7 figures

Thorne-Zytkow objects are stars that have a neutron star core with an extended hydrogen-rich envelope. Massive Thorne-Zytkow objects are proposed to explode when the nuclear reactions sustaining their structure are terminated by the exhaustion of the seed elements. In this paper, we investigate the observational properties of the possible Thorne-Zytkow object explosions. We find that Thorne-Zytkow object explosions are observed as long-duration transients lasting for several years. If the accretion disk triggering the explosions does not last for a long time, Thorne-Zytkow object explosions have a luminosity plateau with about 1e39 erg/s lasting for a few years, and then they suddenly become faint. They would be observed as vanished stars after a bright phase lasting for a few years. If the accretion disk is sustained for long time, the Thorne-Zytkow object explosions become as bright as supernovae. They would be observed as supernovae with rise times of several hundred days. We found that their photospheric velocities are 2000 km/s at most, much smaller than those found in supernovae. Supernovae with extremely long rise times such as HSC16aayt and SN 2008iy may be related to the explosions of Thorne-Zytkow objects., 5 pages, 6 figures, accepted by Monthly Notices of the Royal Astronomical Society

We characterize the extreme heartbeat star system MACHO 80.7443.1718 in the LMC using TESS photometry and spectroscopic observations from the Magellan Inamori Kyocera Echelle (MIKE) and SOAR Goodman spectographs. MACHO 80.7443.1718 was first identified as a heartbeat star system in the All-Sky Automated Survey for SuperNovae (ASAS-SN) with $P_{\rm orb}=32.836\pm0.008\,{\rm d}$. MACHO 80.7443.1718 is a young (${\sim}6$~Myr), massive binary, composed of a B0 Iae supergiant with $M_1 \simeq 35 M_\odot$ and an O9.5V secondary with $M_2 \simeq 16 M_\odot$ on an eccentric ($e=0.51\pm0.03$) orbit. In addition to having the largest variability amplitude amongst all known heartbeats stars, MACHO 80.7443.1718 is also one of the most massive heartbeat stars yet discovered. The B[e] supergiant has Balmer emission lines and permitted/forbidden metallic emission lines associated with a circumstellar disk. The disk rapidly dissipates at periastron which could indicate mass transfer to the secondary, but re-emerges immediately following periastron passage. MACHO 80.7443.1718 also shows tidally excited oscillations at the $N=25$ and $N=41$ orbital harmonics and has a rotational period of 4.4 d., 21 pages, 16 figures. Submitted to MNRAS

We probe the radial clumping stratification of OB stars in the intermediate and outer wind regions (r>~2 R*) to derive upper limits for mass-loss rates, and compare to current mass-loss implementation. Together with archival multi-wavelength data, our new far-infrared continuum observations for a sample of 25 OB stars (including 13 B Supergiants) uniquely constrain the clumping properties of the intermediate wind region. We derive the minimum radial stratification of the clumping factor through the stellar wind, fclmin(r), and the corresponding maximum mass-loss rate, Mdotmax, normalising clumping factors to the outermost wind region (clfar=1). The clumping degree for r>~2 R* decreases or stays constant with increasing radius for almost the whole sample. There is a dependence on luminosity class and spectral type at the intermediate region relative to the outer ones: O Supergiants (OSGs) present a factor 2 larger clumping factors than B Supergiants (BSGs). The maximum clumping of roughly 1/3 of the OB Supergiants occurs close to the wind base (r, Comment: 23 pages + 12 appendices pages, 18 figures, accepted in Astronomy & Astrophysics

Wolf-Rayet stars are amongst the rarest but also most intriguing massive stars. Their extreme stellar winds induce famous multi-wavelength circumstellar gas nebulae of various morphologies, spanning from circles and rings to bipolar shapes. This study is devoted to the investigation of the formation of young, asymmetric Wolf-Rayet gas nebulae and we present a 2.5-dimensional magneto-hydrodynamical toy model for the simulation of Wolf-Rayet gas nebulae generated by wind-wind interaction. Our method accounts for stellar wind asymmetries, rotation, magnetisation, evolution and mixing of materials. It is found that the morphology of the Wolf-Rayet nebulae of blue supergiant ancestors is tightly related to the wind geometry and to the stellar phase transition time interval, generating either a broadened peanut-like or a collimated jet-like gas nebula. Radiative transfer calculations of our Wolf-Rayet nebulae for dust infrared emission at 24 $\mu$m show that the projected diffuse emission can appear as oblate, bipolar, ellipsoidal or ring structures. Important projection effects are at work in shaping observed Wolf-Rayet nebulae. This might call a revision of the various classifications of Wolf-Rayet shells, which are mostly based on their observed shape. Particularly, our models question the possibility of producing pre-Wolf-Rayet wind asymmetries, responsible for bipolar nebulae like NGC 6888, within the single red supergiant evolution channel scenario. We propose that bipolar Wolf-Rayet nebulae can only be formed within the red supergiant scenario by multiple/merged massive stellar systems, or by single high-mass stars undergoing additional, e.g. blue supergiant, evolutionary stages prior to the Wolf-Rayet phase., Comment: 18 pages, 10 figures

We present the far ultraviolet (FUV) imaging of the nearest Jellyfish or Fireball galaxy IC3418/VCC 1217, in the Virgo cluster of galaxies, using Ultraviolet Imaging Telescope (UVIT) onboard the ASTROSAT satellite. The young star formation observed here in the 17 kpc long turbulent wake of IC3418, due to ram pressure stripping of cold gas surrounded by hot intra-cluster medium, is a unique laboratory that is unavailable in the Milkyway. We have tried to resolve star forming clumps, seen compact to GALEX UV images, using better resolution available with the UVIT and incorporated UV-optical images from Hubble Space Telescope archive. For the first time, we resolve the compact star forming clumps (fireballs) into sub-clumps and subsequently into a possibly dozen isolated stars. We speculate that many of them could be blue supergiant stars which are cousins of SDSS J122952.66+112227.8, the farthest star (~17 Mpc) we had found earlier surrounding one of these compact clumps. We found evidence of star formation rate (4 - 7.4 x 10^-4 M_sun per yr ) in these fireballs, estimated from UVIT flux densities, to be increasing with the distance from the parent galaxy. We propose a new dynamical model in which the stripped gas may be developing vortex street where the vortices grow to compact star forming clumps due to self-gravity. Gravity winning over turbulent force with time or length along the trail can explain the puzzling trend of higher star formation rate and bluer/younger stars observed in fireballs farther away from the parent galaxy., Comment: 12 pages, 9 figures. Accepted for publication in the Special Issue of the Journal of Astrophysics and Astronomy on ASTROSAT

The results of spectral studies of photospheric lines of the late B-supergiant HD 199478 (B8Iae) in the visual range are presented. Our results for the first time ever, have shown that the scale of temporal variations in photospheric lines is in good coincidence with the quasi-cyclic variability of the emission component of the line Halpha. The instability of the periodicity according to the data of different years complicates the obtaining of accurate information about the pulsation modes and astroseismic information, which is required to study the photospheric variability in BA supergiants. The scale of temporal variations of 40-50 days is typical for the full cycle of variations in hydrogen. Our data revealed that these time scales are in good agree with absorption lines too. The distribution of the velocity gradient along the line shows the existence mainly radial, and to a lesser level, non-radial pulsations. The question of the main reason for the activity in the stellar atmosphere remains open. Given the above, to explain the observed spectral variations, we tend to accept the concept of the appearance of inhomogeneity on the surface of the star. For the explanation of the observed results, apparently, the magnetic field can play a key role. Measurements of the longitudinal magnetic field BA type SGs show that the maximum value of the magnetic field strength for beta Ori is 130 G, and for the remaining BA SGs it is several gauss. However, spots can cause the inhomogeneity on the surface of the star, where the local magnetic field is much higher.

The analysis of stellar spectra depends upon the effective temperature (Teff) and the surface gravity (log g). However, the estimation of log g with high accuracy is challenging. A classical approach is to search for log g that satisfies the ionization balance, i.e., the abundances from neutral and ionized metallic lines being in agreement. We propose a method of using empirical relations between Teff, log g and line-depth ratios, for which we meticulously select pairs of FeI and FeII lines and pairs of CaI and CaII lines. Based on YJ-band (0.97-1.32 micron) high-resolution spectra of 42 FGK stars (dwarfs to supergiants), we selected five FeI-FeII and four CaI-CaII line pairs together with 13 FeI-FeI pairs (for estimating Teff, and derived the empirical relations. Using such relations does not require complex numerical models and tools for estimating chemical abundances. The relations we present allows one to derive Teff and log g with a precision of around 50 K and 0.2 dex, respectively, but the achievable accuracy depends on the accuracy of the calibrators' stellar parameters. It is essential to revise the calibration by observing stars with accurate stellar parameters available, e.g., stars with asteroseismic log g and stars analyzed with complete stellar models taking into account the effects of non-local thermodynamic equilibrium and convection. In addition, the calibrators we used have a limited metallicity range, [Fe/H] between -0.2 and +0.2 dex, and our relations need to be tested and re-calibrated based on a calibrating dataset for a wider range of metallicity., 14 pages plus 6 additional pages (table and plots), 9 figures, 4 tables. Accepted for publication in MNRAS

Simeis 57 (HS 191) is an optically bright nebula in the Cygnus X region with a peculiar appearance that suggests an outflow from a rotating source. Newly obtained observations and archival data reveal Simeis 57 as a low-density (ne ~ 100 cm−3) nebula with an east-to-west excitation gradient. The extinction of the nebula is AV ≤ 2 mag. The nebula is recognizable but not prominent in mid- and far-infrared images. In its direction, half a dozen small CO clouds have been identified at VLSR = +5 km s−1. One of these coincides with both the optical nebula and a second CO cloud at the nebular velocity VLSR ≈−10 km−1. No luminous stars are embedded in these molecular clouds, nor are any obscured by them and no sufficiently luminous stars are found in the immediate vicinity of the nebula. Instead, all available data points to the evolved star HD 193793 = WR 140 (an O4-5 supergiant and WC7 Wolf–Rayet binary) as the source of excitation, notwithstanding its large separation of 50′, about 25 pc at the stellar distance of 1.7 kpc. Simeis 57 appears to be a part of a larger structure surrounding the HI void centered on HD 193793.

We calculate models of stellar evolution for very massive stars and include the effects of modified gravity to investigate the influence on the physical properties of blue supergiant stars and their use as extragalactic distance indicators. With shielding and fifth force parameters in a similar range as in previous studies of Cepheid and tip of the red giant branch (TRGB) stars we find clear effects on stellar luminosity and flux-weighted gravity. The relationship between flux weighted gravity, g_F = g/Teff^4, and bolometric magnitude M_bol (FGLR), which has been used successfully for accurate distance determinations, is systematically affected. While the stellar evolution FGLRs show a systematic offset from the observed relation, we can use the differential shifts between models with Newtonian and modified gravity to estimate the influence on FGLR distance determinations. Modified gravity leads to a distance increase of 0.05 to 0.15 magnitudes in distance modulus. These change are comparable to the ones found for Cepheid stars. We compare observed FGLR and TRGB distances of nine galaxies to constrain the free parameters of modified gravity. Not accounting for systematic differences between TRGB and FGLR distances shielding parameters of 5*10^-7 and 10^-6 and fifth force parameters of 1/3 and 1 can be ruled out with about 90% confidence. Allowing for potential systematic offsets between TRGB and FGLR distances no determination is possible for a shielding parameter of 10^-6. For 5*10$^-7 a fifth force parameter of 1 can be ruled out to 92% but 1/3 is unlikely only to 60%., To be published in the Astrophysical Journal

Massive stars lose a significant fraction of mass during their evolution. However, the corresponding mass-loss rates are rather uncertain. To improve this, we calculated global line-driven wind models for Galactic B supergiants. Our models predict radial wind structure directly from basic stellar parameters. The hydrodynamic structure of the flow is consistently determined from the photosphere in nearly hydrostatic equilibrium to supersonically expanding wind. The radiative force is derived from the solution of the radiative transfer equation in the comoving frame. We provide a simple formula that predicts theoretical mass-loss rates as a function of stellar luminosity and effective temperature. The mass-loss rate of B supergiants slightly decreases with temperature down to about 22.5 kK, where the region of recombination of Fe IV to Fe III starts to appear. In this region, which is about 5 kK wide, the mass-loss rate gradually increases by a factor of about 6. The increase of the mass-loss rate is associated with a gradual decrease of terminal velocities by a factor of about 2. We compared the predicted wind parameters with observations. While the observed wind terminal velocities are reasonably reproduced by the models, the situation with mass-loss rates is less clear. The mass-loss rates derived from observations that are uncorrected for clumping are by a factor of 3 to 9 higher than our predictions on cool and hot sides of the studied sample, respectively. These observations can be reconciled with theory assuming a temperature-dependent clumping factor. On the other hand, the mass-loss rate estimates that are not sensitive to clumping agree with our predictions much better. Our predictions are by a factor of about 10 lower than the values currently used in evolutionary models appealing for reconsideration of the role of winds in the stellar evolution., 13 pages, accepted for publication in Astronomy & Astrophysics, ver. 2 (inclusion of more precise values in Table 3)

We present synthetic spectra and SEDs computed along evolutionary tracks at Z=1/5 Zsun and Z=1/30 Zsun, for masses between 15 and 150 Msun. We predict that the most massive stars all start their evolution as O2 dwarfs at sub-solar metallicities. The fraction of lifetime spent in the O2V phase increases at lower metallicity. The distribution of dwarfs and giants we predict in the SMC accurately reproduces the observations. Supergiants appear at slightly higher effective temperatures than we predict. More massive stars enter the giant and supergiant phases closer to the ZAMS, but not as close as for solar metallicity. This is due to the reduced stellar winds at lower metallicity. Our models with masses higher than ~60 Msun should appear as O and B stars, whereas these objects are not observed, confirming a trend reported in the recent literature. At Z=1/30 Zsun, dwarfs cover a wider fraction of the MS and giants and supergiants appear at lower effective temperatures than at Z=1/5 Zsun. The UV spectra of these low-metallicity stars have only weak P-Cygni profiles. HeII 1640 sometimes shows a net emission in the most massive models, with an equivalent width reaching ~1.2 A. For both sets of metallicities, we provide synthetic spectroscopy in the wavelength range 4500-8000 A. This range will be covered by the instruments HARMONI and MOSAICS on the ELT and will be relevant to identify hot massive stars in Local Group galaxies with low extinction. We suggest the use of the ratio of HeI 7065 to HeII 5412 as a diagnostic for spectral type. We show that this ratio does not depend on metallicity. Finally, we discuss the ionizing fluxes of our models. The relation between the hydrogen ionizing flux per unit area versus effective temperature depends only weakly on metallicity. The ratios of HeI and HeII to H ionizing fluxes both depend on metallicity, although in a slightly different way., 18 pages, 17 figures + appendix. Accepted in Astronomy and Astrophysics

Mass-loss rates and terminal wind velocities are key parameters that determine the kinetic wind energy and momenta of massive stars. Furthermore, accurate mass-loss rates determine the mass and rotational velocity evolution of mass stars, and their fates as neutron stars and black holes in function of metallicity (Z). Here we update our Monte Carlo mass-loss Recipe with new dynamically-consistent computations of the terminal wind velocity -- as a function of Z. These predictions are particularly timely as the HST ULLYSES project will observe ultraviolet spectra with blue-shifted P Cygni lines of hundreds of massive stars in the low-Z Large and Small Magellanic Clouds, as well as sub-SMC metallicity hosts. Around 35 000 K, we uncover a weak-wind "dip" and we present diagnostics to investigate its physics with ULLYSES and X-Shooter data. We discuss how the dip may provide important information on wind-driving physics, and how this is of key relevance towards finding a new gold-standard for OB star mass-loss rates. For B supergiants below the Fe IV to III bi-stability jump, the terminal velocity is found to be independent of Z and M, while the mass-loss rate still varies as $\dot{M} \propto Z^{0.85}$. For O-type stars above the bi-stability jump we find a terminal-velocity dependence of $v_{\infty} \propto Z^{0.19}$ and the Z-dependence of the mass-loss rate is found to be as shallow as $\dot{M} \propto Z^{0.42}$, implying that to reproduce the `heavy' black holes from LIGO/VIRGO, the `low Z' requirement becomes even more stringent than was previously anticipated., Comment: MNRAS Accepted. 12 pages, 13 figures. Notes on IDL/Python Mass-loss Recipe Usage added to the Appendix

We present updated results of the Large Binocular Telescope Search for Failed Supernovae. This search monitors luminous stars in 27 nearby galaxies with a current baseline of 11~yr of data. We re-discover the failed supernova (SN) candidate N6946-BH1 as well as a new candidate, M101-OC1. M101-OC1 is a blue supergiant that rapidly disappears in optical wavelengths with no evidence for significant obscuration by warm dust. While we consider other options, a good explanation for the fading of M101-OC1 is a failed SN, but follow-up observations are needed to confirm this. Assuming only one clearly detected failed SN, we find a failed SN fraction $f = 0.16^{+0.23}_{-0.12}$ at 90 per~cent confidence. We also report on a collection of stars that show slow ($\sim$decade), large amplitude ($\Delta L/L > 3$) luminosity changes., Comment: 14 pages, 11 figures, 5 tables, 2 machine-readable tables (included as ancillary txt files)

GCIRS 7, the brightest star in the Galactic central parsec, formed $6\pm2$ Myr ago together with dozens of massive stars in a disk orbiting the central black-hole. It has been argued that GCIRS 7 is a pulsating body, on the basis of photometric variability. We present the first medium-resolution ($R=500$), K-band spectro-interferometric observations of GCIRS 7, using the GRAVITY instrument with the four auxiliary telescopes of the ESO VLTI. We looked for variations using two epochs, namely 2017 and 2019. We find GCIRS 7 to be moderately resolved with a uniform-disk photospheric diameter of $\theta^*_\text{UD}=1.55 \pm 0.03$ mas ($R^*_\text{UD}=1368 \pm 26$ $R_\odot$) in the K-band continuum. The narrow-band uniform-disk diameter increases above 2.3 $\mu$m, with a clear correlation with the CO band heads in the spectrum. This correlation is aptly modeled by a hot ($T_\text{L}=2368\pm37$ K), geometrically thin molecular shell with a diameter of $\theta_\text{L}=1.74\pm0.03$ mas, as measured in 2017. The shell diameter increased ($\theta_\text{L}=1.89\pm0.03$ mas), while its temperature decreased ($T_\text{L}=2140\pm42$ K) in 2019. In contrast, the photospheric diameter $\theta^*_\text{UD}$ and the extinction up to the photosphere of GCIRS 7 ($A_{\mathrm{K}_\mathrm{S}}=3.18 \pm 0.16$) have the same value within uncertainties at the two epochs. In the context of previous interferometric and photo-spectrometric measurements, the GRAVITY data allow for an interpretation in terms of photospheric pulsations. The photospheric diameter measured in 2017 and 2019 is significantly larger than previously reported using the PIONIER instrument ($\theta_*=1.076 \pm 0.093$ mas in 2013 in the H band). The parameters of the photosphere and molecular shell of GCIRS 7 are comparable to those of other red supergiants that have previously been studied using interferometry., Comment: 12 pages, 11 figures, 3 tables. Accepted for publication in Astronomy and Astrophysics (A&A)

The Humphreys-Davidson (HD) limit sets the boundary between evolutionary channels of massive stars that either end their lives as red supergiants (RSGs) or as the hotter blue supergiants (BSGs) and Wolf-Rayet stars. Mixing in the envelopes of massive stars close to their Eddington limit is crucial for investigating the upper luminosity limit of the coolest supergiants. We study the effects of excess mixing in superadiabatic layers that are dominated by radiation pressure, and we critically investigate the effects of mixing and mass loss on the evolution of RSGs with log (Teff/K) < 3.68 - as a function of metallicity. Using MESA, we produce grids of massive star models at three metallicities: Galactic (Zsol), LMC (1/2 Zsol) and SMC (1/5 Zsol), with both high and low amounts of overshooting to study the upper luminosity limit of RSGs. We systematically study the effects of excess mixing in the superadiabatic layers of post-main sequence massive stars, overshooting above the hydrogen core and yellow supergiant (YSG) mass-loss rates on the fraction of core helium burning time spent as a RSG. We find that the excess mixing in the superadiabatic layers is stronger at lower metallicities, as it depends on the opacities in the hydrogen bump at log (Teff/K) ~ 4, which become more pronounced at lower metallicity. This shifts the cutoff luminosities to lower values at lower metallicities, thus balancing the first-order effect of mass loss. The opposing effects of mass loss and excess envelope mixing during post-main sequence evolution of stars with higher overshooting potentially results in a metallicity-independent upper luminosity limit., Accepted for publication in MNRAS. 15 pages, 13 figures

We investigate the dust growth in oxygen-rich stellar outflows for a set of nine well-observed massive supergiants with optically thin dust shells. Models of the infrared emission from their circumstellar dust shells are compared to their observed infrared spectra so as to derive the essential parameters that govern dust formation in the extended envelope of these stars. The results obtained from the comparative study are also compared with the predictions of a model for silicate dust condensation solely based on laboratory data and basic stellar properties. The infrared emission in the wavelength range between 6 and 25 mu can be reproduced rather well by a mixture of non-stoichiometric iron-bearing silicates, alumina, and metallic iron dust particles for all nine objects. The observed spectra obtained from three objects, mu Cep, RW Cyg, and RS Per, can be reproduced by a stationary and (essentially) spherically symmetric outflow which enables a direct comparison with predictions from a theoretical dust growth model. The temperature at the onset of massive silicate dust growth is of the order of 920 K and the corresponding outflow velocity of the order of the sound velocity for these objects. The condensation temperature suggests that the silicate dust grows on the corundum dust grains that are formed well in the interior of the silicate dust shell at a much higher temperature. Our results propose that regarding the two major problems of dust formation in stellar outflows: (i) formation of seed nuclei; (ii) their growth to macroscopic dust grains, we are gradually coming close to a quantitative understanding of the second item., 29 pages, 9 figures, accepted by Astronomy & Astrophysics

Type IIb supernovae (SNe IIb) present a unique opportunity for investigating the evolutionary channels and mechanisms governing the evolution of stripped-envelope SN progenitors due to a variety of observational constraints available. Comparison of these constraints with the full distribution of theoretical properties not only help ascertain the prevalence of observed properties in nature, but can also reveal currently unobserved populations. In this follow-up paper, we use the large grid of models presented in Sravan et al. 2019 to derive distributions of single and binary SNe IIb progenitor properties and compare them to constraints from three independent observational probes: multi-band SN light-curves, direct progenitor detections, and X-ray/radio observations. Consistent with previous work, we find that while current observations exclude single stars as SN IIb progenitors, SN IIb progenitors in binaries can account for them. We also find that the distributions indicate the existence of an unobserved dominant population of binary SNe IIb at low metallicity that arise due to mass transfer initiated on the Hertzsprung Gap. In particular, our models indicate the existence of a group of highly stripped (envelope mass ~0.1-0.2 M_sun) progenitors that are compact (, Comment: Resubmitted to the Astrophysical Journal after incorporating suggestions from the referee

At the end of its life, a very massive star is expected to collapse into a black hole. The recent detection of an 85 Msun black hole from the gravitational wave event GW 190521 appears to present a fundamental problem as to how such heavy black holes exist above the approximately 50 Msun pair-instability limit where stars are expected to be blown to pieces with no remnant left. Using MESA, we show that for stellar models with non-extreme assumptions, 90..100 Msun stars at reduced metallicity (Z/Zsun < 0.1) can produce blue supergiant progenitors with core masses sufficiently small to remain below the fundamental pair-instability limit, yet at the same time lose an amount of mass via stellar winds that is small enough to end up in the range of an "impossible" 85 Msun black hole. The two key points are the proper consideration of core overshooting and stellar wind physics with an improved scaling of mass loss with iron (Fe) contents characteristic for the host galaxy metallicity. Our modelling provides a robust scenario that not only doubles the maximum black hole mass set by pair instability, but also allows us to probe the maximum stellar black hole mass as a function of metallicity and Cosmic time in a physically sound framework., MNRAS Accepted. 9 pages, 3 figures. Prior Submission in September 2020 to Nature (see Research Square)

Several decades of observations of the most massive and most luminous stars have revealed a complex upper HR Diagram, shaped by mass loss, and inhabited by a variety of evolved stars exhibiting the consequences of their mass loss histories. This review presents a brief historical overview of the HR Diagram for massive stars, highlighting some of the primary discoveries and results from their observation in nearby galaxies. The chapters in this volume include reviews of our current understanding of different groups of evolved massive stars, all losing mass and in different stages of their evolution; the Luminous Blue Variables (LBVs), B[e] supergiants, the warm hypergiants, Wolf-Rayet stars, and the population of OB stars and supergiants in the Magellanic Clouds., Introductory review chapter for e-book "Luminous stars in Nearby Galaxies" (https://www.mdpi.com/journal/galaxies/special_issues/Luminous)

Massive stars briefly pass through the yellow supergiant (YSG) phase as they evolve redward across the HR diagram and expand into red supergiants (RSGs). Higher-mass stars pass through the YSG phase again as they evolve blueward after experiencing significant RSG mass loss. These post-RSG objects offer us a tantalizing glimpse into which stars end their lives as RSGs, and why. One telltale sign of a post-RSG object may be an instability to pulsations, depending on the star's interior structure. Here we report the discovery of five YSGs with pulsation periods faster than 1 day, found in a sample of 76 cool supergiants observed by \tess at two-minute cadence. These pulsating YSGs are concentrated in a HR diagram region not previously associated with pulsations; we conclude that this is a genuine new class of pulsating star, Fast Yellow Pulsating Supergiants (FYPS). For each FYPS, we extract frequencies via iterative prewhitening and conduct a time-frequency analysis. One FYPS has an extracted frequency that is split into a triplet, and the amplitude of that peak is modulated on the same timescale as the frequency spacing of the triplet; neither rotation nor binary effects are likely culprits. We discuss the evolutionary status of FYPS and conclude that they are candidate post-RSGs. All stars in our sample also show the same stochastic low-frequency variability (SLFV) found in hot OB stars and attributed to internal gravity waves. Finally, we find four $\alpha$ Cygni variables in our sample, of which three are newly discovered., Comment: 29 pages, 13 figures, 8 tables. Accepted for publication in ApJ. Comments welcome

We have detected a broad interstellar absorption band centred close to 7700 \r{A} and with a FWHM of 176.6$\pm$3.9 \r{A}. This is the first such absorption band detected in the optical range and is significantly wider than the numerous diffuse interstellar bands (DIBs). It remained undiscovered until now because it is partially hidden behind the A telluric band produced by O$_2$. The band was discovered using STIS@HST spectra and later detected in a large sample of stars of diverse type (OB stars, BA supergiants, red giants) using further STIS and ground-based spectroscopy. The EW of the band is measured and compared with our extinction and K I $\lambda\lambda$7667.021,7701.093 measurements for the same sample. The carrier is ubiquitous in the diffuse and translucent Galactic ISM but is depleted in the environment around OB stars. In particular, it appears to be absent or nearly so in sightlines rich in molecular carbon. This behaviour is similar to that of the $\sigma$-type DIBs, which originate in the low/intermediate-density UV-exposed ISM but are depleted in the high-density UV-shielded molecular clouds. We also present an update on our previous work on the relationship between $E(4405-5495)$ and $R_{5495}$ and incorporate our results into a general model of the ISM., Comment: 17 pages, accepted for publication in MNRAS

Detections of massive stars in the direction of the H II region CS 78 in NGC 6334 and of G332.809-0.132 are here presented. The region covered by the G332.809-0.132 complex coincides with the RCW 103 stellar association. In its core (40' in radius), approximately 110 OB candidate stars (Ks < 10 mag and 0.4 < AKs < 1.6 mag) were identified using 2MASS, DENIS, and GLIMPSE data. This number of OB stars accounts for more than 50% of the observed number of Lyman continuum photons from this region. Medium-resolution K-band spectra were obtained for seven early types, including one WN 8 star and one Ofpe/WN 9 star; the latter is located near the RCW 103 remnant and its luminosity is consistent with a distance of about 3 kpc. The area analyzed encloses 9 of the 34 OB stars previously known in RCW 103, as well as IRAS 16115-5044, which we reclassify as a candidate luminous blue variable. The line of sight is particularly interesting, crossing three spiral arms; a molecular cloud at -50 (with RCW 103 in the Scutum-Crux arm) and another at -90 km s-1 (in the Norma arm) are detected, both rich in massive stars and supernova remnants. We also report the detection of a B supergiant as the main ionizing source of CS 78, 2MASS J17213513-3532415. Medium-resolution H and K band spectra display H I and He I lines, as well as Fe II lines. By assuming a distance of 1.35 kpc, we estimate a bolometric magnitude of -6.16, which is typical of supergiants., 44 pages, 15 figures

Common-envelope (CE) evolution in massive binary systems is thought to be one of the most promising channels for the formation of compact binary mergers. In the case of merging binary black holes (BBHs), the essential CE phase takes place at a stage when the first BH is already formed and the companion star expands as a supergiant. We study which BH binaries with supergiant companions will evolve through and potentially survive a CE phase. To this end, we compute envelope binding energies from detailed massive stellar models at different evolutionary stages and metallicities. We make multiple physically extreme choices of assumptions that favor easier CE ejection as well as account for recent advancements in mass transfer stability criteria. We find that even with the most optimistic assumptions, a successful CE ejection in BH (and also NS) binaries is only possible if the donor is a massive convective-envelope giant, a red supergiant (RSG). In other words, pre-CE progenitors of BBH mergers are BH binaries with RSG companions. We find that due to its influence on the radial expansion of massive giants, metallicity has an indirect but a very strong effect on the envelope structure and binding energies of RSGs. Our results suggest that merger rates from population synthesis models could be severely overestimated, especially at low metallicity. Additionally, the lack of observed RSGs with luminosities above log($L/L_{\odot}$) = 5.6-5.8, corresponding to stars with $M > 40 M_{\odot}$, puts into question the viability of the CE channel for the formation of the most massive BBH mergers. Either such RSGs elude detection due to very short lifetimes, or they do not exist and the CE channel can only produce BBH systems with total mass $< 50 M_{\odot}$. We discuss an alternative CE scenario, in which a partial envelope ejection is followed by a phase of possibly long and stable mass transfer., 20 pages + App., accepted for publication in A&A. For $\lambda_{\rm CE}$ fits, see: https://ftp.science.ru.nl/astro/jklencki/

Most massive stars exchange mass with a companion, leading to evolution which is altered drastically from that expected of stars in isolation. Such systems are the result of unusual binary evolution pathways and, as such, may be used to place stringent constraints on the physics of these interactions. We use the R4 system's B[e] supergiant, which has been postulated to be the product of a binary stellar merger, to guide our understanding of such outcomes by comparing observations of R4 to the results of simulations of mergers performed with the 3d hydrodynamics code FLASH. Our approach tailors the simulation initial conditions to the observed properties of R4 and implements realistic stellar profiles generated by the 1d stellar evolution code MESA onto the 3d grid, resolving the merger inspiral to within $0.02\, R_{\odot}$. We then map the merger remnant into MESA to track its evolution on the HR diagram over a period of $10^4$ years. This generates models for a B[e] supergiant with stellar properties, age, and nebula structure in qualitative agreement with that of the R4 system. Our calculations provide concrete evidence to support the idea that R4 was originally a member of a triple system in which the inner binary merged after its most massive member evolved off the main sequence, producing a new object that is of similar mass yet significantly more luminous than the A supergiant companion. The potential applications of the code framework presented in this paper are wide ranging and can be used to generate models of a variety of merger stellar remnants.

The search for extrasolar planets has driven rapid advances in instrumentation, resulting in cameras such as SPHERE at the VLT, GPI at Gemini South and SCExAO at Subaru, capable of achieving very high contrast ($\sim10^{6}$) around bright stars with small inner working angles ($\sim 0\farcs{1}$). The optimal exploitation of data from these instruments depends on the availability of easy-to-use software to process and analyse their data products. We present a pure-python pipeline, {\sc precision}, which provides fast, memory-efficient reduction of data from the SPHERE/IRDIS near-infrared imager, and can be readily extended to other instruments. We apply {\sc precision} to observations of the extreme red supergiant VX~Sgr, the inner outflow of which is revealed to host complex, asymmetric structure in the near-IR. In addition, optical polarimetric imaging reveals clear extended polarised emission on $\sim0.5^{\prime\prime}$ scales which varies significantly with azimuth, confirming the asymmetry. While not conclusive, this could suggest that the ejecta are confined to a disc or torus, which we are viewing nearly face on, although other non-spherical or clumpy configurations remain possible. VX~Sgr has no known companions, making such a geometry difficult to explain, as there is no obvious source of angular momentum in the system., 13 pages, 5 figures. Accepted by MNRAS

We present differential Stromgren uvby Four College Automated Photometric Telescope (FCAPT) observations of five supergiants with spectral types near A0. For three stars we supplemented our new data with our earlier published data, and for two more stars we present new data. We used two computer programs Period04 and VStar in attempts to determine if significant periodicities are present. Some periods we found may not be real as their yearly coverages are only less about four months. Five years of 9 Per photometry and 11 years of HR 8020 photometry both suggest a period of 248 days which is 2/3 of a year. 13 Mon and HR 1035 may be variable, but the data we have taken does not permit us to determine their periods. Our most complete datasets for Deneb and for HR 8020 indicate very complex variability. This is not unexpected for stars with atmospheres where convection is the dominant mode of energy transportation. Among our five supergiants, 13 Mon has the smallest amplitudes for u, v, b, and y. Better sets of photometry for these stars require observing seasons to be as long as possible and should cover many observations per night rather than one. This should remove any effects of data being undersampled.

The supergiant High Mass X-ray Binary IGR J16318-4848 was the first source detected by the INTEGRAL satellite in 2003 and distinguishes itself by its high intrinsic absorption and B[e] phenomenon. It is the perfect candidate to study both binary interaction and the environment of supergiant B[e] stars. This study targets the local properties of IGR J16318-4848. We aim to clarify the geometry of this system, and distinguish different key emitting regions in the binary. We provide optical to near-infrared spectra from VLT/X-Shooter and analyse both fine structures of the lines and the broadband spectral energy distribution by adding archival mid-infrared Spitzer and Herschel data. We also performed a stellar atmosphere and wind modeling of the optical to near-infrared spectrum using the PoWR code. We determine the contribution of the irradiated inner edge of the dusty circumbinary disk, derive the velocity of an equatorial stellar wind, and suggest the compact object orbits within the cavity between the star and the disk. We report on flat-topped lines originating from a spherically symetric disk wind, along with the first detection of what is likely the polar component of the stellar wind. Stellar atmosphere and wind modeling shows that the central star may have a helium-enhanced atmosphere, likely because of its intense wind sheding part of its hydrogen envelope. Finally we compare the properties of IGR J16318-4848 with a similar source, CI Cameleopardis., ApJ in press, 20 pages, 16 figures

We present the results of a spectroscopic and photometric investigation of 15 objects from the open cluster NGC 5662, which contains the Cepheid V Cen. Besides the Cepheid, we studied one G-supergiant, two K-giants, four B-giants, and seven main sequence stars. Radial velocities (RV), v sin i, Teff, log g, and [Fe/H] were determined using model fitting. We have derived the color-excesses, reddenings, and intrinsic colors for these stars to determine their true Teff and log g from comparison to the atmophere models, especially for hot stars, and to determine their absolute magnitudes. RV and GAIA DR2 2018 parallax/distance values for these stars allowed us to determine their membership in the cluster and to construct it 3D kinematic models. These parallaxes and reddenings led to the distances in a range of 700– 875 pc, while photometric MV gave the distances of 550–660 pc. Two objects from our list were found to be non-members of the cluster, and two other objects to be background stars. All members have [Fe/H] = −0.1±0.04 dex. The main sequence stars and B-giants have abundances of C, N, and Na close to the solar one, while the cool supergiants and giants show a deficit of carbon, an overabundance of nitrogen, and a small overabundance or close to the solar sodium content. Two Li-rich K-giants, HD 127733 and HD 127753, were discovered in the cluster.

Supergiant X-ray Binaries host a compact object, generally a neutron star, orbiting an evolved O/B star. Mass transfer proceeds through the intense radiatively-driven wind of the stellar donor, a fraction of which is captured by the gravitational field of the neutron star. The subsequent accretion process onto the neutron star is responsible for the abundant X-ray emission from those systems. They also display variations in time of the X-ray flux by a factor of a few 10, along with changes in the hardness ratios believed to be due to varying absorption along the line-of-sight. We used the most recent results on the inhomogeneities (aka clumps) in the non-stationary wind of massive hot stars to evaluate their impact on the time-variable accretion process. We ran three-dimensional simulations of the wind in the vicinity of the accretor to witness the formation of the bow shock and follow the inhomogeneous flow over several spatial orders of magnitude, down to the neutron star magnetosphere. In particular, we show that the impact of the clumps on the time-variability of the intrinsic mass accretion rate is severely damped by the crossing of the shock, compared to the purely ballistic Bondi-Hoyle-Lyttleton estimation. We also account for the variable absorption due to clumps passing by the line-of-sight and estimate the final effective variability of the mass accretion rate for different orbital separations. These results are confronted to recent analysis of Vela X-1 observations with Chandra by Grinberg et al. (2017). It shows that clumps account well for time-variability at low luminosity but can not generate, per se, the high luminosity activity observed.

NGC 300 ULX1 is the fourth to be discovered in the class of the ultra-luminous X-ray pulsars. Pulsations from NGC 300 ULX1 were discovered during simultaneous XMM-Newton / NuSTAR observations in Dec. 2016. The period decreased from 31.71 s to 31.54 s within a few days, with a spin-up rate of -5.56 x 10^{-7} s s^{-1}, likely one of the largest ever observed from an accreting neutron star. Archival Swift and NICER observations revealed that the period decreased exponentially from ~45 s to ~17.5 s over 2.3 years. The pulses are highly modulated with a pulsed fraction strongly increasing with energy and reaching nearly 80% at energies above 10keV. The X-ray spectrum is described by a power-law and a disk black-body model, leading to a 0.3-30 keV unabsorbed luminosity of 4.7 x 10^{39} erg s^{-1}. The spectrum from an archival XMM-Newton observation of 2010 can be explained by the same model, however, with much higher absorption. This suggests, that the intrinsic luminosity did not change much since that epoch. NGC 300 ULX1 shares many properties with supergiant high mass X-ray binaries, however, at an extreme accretion rate., Comment: 5 pages, 5 figures, to appear in Proc. IAUS 346. arXiv admin note: text overlap with arXiv:1802.10341

We present the results of multicolor (UBVJHKLM) photometry (2009-2017) and low-resolution spectroscopy (2016-2017) of the semi-regular variable V1427 Aql=HD 179821, a yellow supergiant with gas-dust envelope. The star displays low-amplitude ({\Delta}V, Comment: 30 pages, 12 figures, Accepted for publication in Astronomy Letters