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The present work is devoted to the analysis of the internal structure of relativistic jets under the condition that the velocity of the plasma flow at the jet axis vanishes. It is shown that in spite of the seemingly fundamental difference in the formulation of the problem at the axis, the key properties of the internal structure of such relativistic jets remain the same as for nonzero velocity along the axis. In both cases, at a sufficiently low ambient pressure, a dense core appears near the axis, the radius of which is close to the size of the light cylinder., 8 pages, 7 figures, accepted by MNRAS

In this study, we investigate the period changes of eight short-period Type II Cepheids of the BL Her subtype, i.e., with periods in the 1-4 day range. The $O-C$ diagrams for these stars are constructed using all suitable observational data from ground and space surveys. This spans a time interval of over one century and includes digitized photographic plates as well as photometry from the literature. The $O-C$ diagrams show parabolic evolutionary trends, which indicate the presence of both increasing and decreasing periods for these eight short period stars. These period changes are in good agreement with the recent theoretical evolutionary framework and stellar evolution models for BL Her stars. The pulsation stability test proposed by Lombard and Koen also suggests that the changes in the periods are real., Comment: Accepted for publication in MNRAS

We present multiwavelength observations and a model for flat spectrum radio quasar NVSS J141922-083830, originally classified as a blazar candidate of unknown type (BCU II object) in the Third Fermi-LAT AGN Catalog (3LAC). Relatively bright flares (>3 magnitudes) were observed on 21 February 2015 (MJD 57074) and 8 September 2018 (MJD 58369) in the optical band with the MASTER Global Robotic Net (MASTER-Net) telescopes. Optical spectra obtained with the Southern African Large Telescope (SALT) on 1 March 2015 (MJD 57082), during outburst, and on 30 May 2017 (MJD 57903), during quiescence, showed emission lines at 5325\r{A} and at $\approx$3630\r{A} that we identified as the Mg II 2798\r{A} and C III] 1909\r{A} lines, respectively, and hence derived a redshift z = 0.903. Analysis of Fermi-LAT data was performed in the quiescent regime (5 years of data) and during four prominent flaring states in February-April 2014, October-November 2014, February-March 2015 and September 2018. We present spectral and timing analysis with Fermi-LAT. We report a hardening of the gamma-ray spectrum during the last three flaring periods, with a power-law spectral index $\Gamma = 2.0$-$2.1$. The maximum gamma-ray flux level was observed on 24 October 2014 (MJD 56954) at $(7.57 \pm 1.83) \times 10^{-7}$ ph~cm$^{-2}$s$^{-1}$. The multi-wavelength spectral energy distribution during the February-March 2015 flare supports the earlier evidence of this blazar to belong to the FSRQ class. The SED can be well represented with a single-zone leptonic model with parameters typical of FSRQs, but also a hadronic origin of the high-energy emission can not be ruled out., Comment: 15 pages, 11 figures, 8 tables, accepted for publication in MNRAS (29 July 2022)

We report the results of optical \'echelle spectroscopy with the Southern African Large Telescope (SALT) of the mass donor star BSDL\,923 in the neutron star (NS) high-mass X-ray binary XMMU\,J051342.6$-$672412 associated with the LMC supernova remnant (SNR) MCSNR\,J0513$-$6724. We found that BSDL\,923 is a B0.7\,III star with double peaked emission lines originating in a circumbinary disk-like structure. This classification and the presence of double-peaked emission lines imply that BSDL\,923 is a Be star. Modelling with the stellar atmosphere code {\sc fastwind} was used to derive the effective temperature $T_{\rm eff}=27\pm1$\,kK, surface gravity $\log g=3.22\pm0.10$, projected rotational velocity $v\sin i\approx100\pm45 \, \kms$, colour excess $E(B-V)=0.53\pm0.05$\,mag, and luminosity $\log(L_*/\lsun)=5.46\pm0.10$ of BSDL\,923, as well as to show that the surface of this star is polluted with $\alpha$-elements (O, Mg and Si) from the supernova ejecta. We found also that the NS is orbiting BSDL\,923 in an eccentric ($e=0.158\pm0.061$) orbit with the orbital period of $1.280\pm0.006$\,d and the semi-major axis of $17\pm3 \, \rsun$, and the radius of BSDL\,923 is $25\pm5 \,\rsun$. We speculate that the NS is embedded in the atmosphere of BSDL\,923 either because it was kicked at birth towards this star or because of inflation of BSDL\,923 caused by the energy input from the supernova blast wave. Using long-slit spectroscopy with SALT, we searched for possible signs of the SNR shell in the 2D spectrum, but did not find them. This lack of detection is consistent with the young age ($\approx4^{+2} _{-1}$\,kyr) of MCSNR\,J0513$-$6724, implying that it is still in the adiabatic (non-radiative) phase., Comment: 13 pages, 11 figures and 6 tables. Accepted for publication by MNRAS

Stellar counter-rotation in disk galaxies directly relates to the complex phenomenon of the disk mass assembly believed to be driven by external processes, such as accretion and mergers. The detailed study of such systems makes it possible to reveal the source of external accretion and establish the details of this process. In this paper, we investigate the galaxy PGC 66551 (MaNGA ID~1-179561) which hosts two large-scale counter-rotating stellar disks suspected in the SDSS MaNGA data and then confirmed using deep follow-up spectroscopy with the 10-m Southern African Large Telescope. We measured properties of ionized gas and stellar populations of both counter-rotating disks in PGC 66551. We found that the counter-rotating disk is compact, contains young stars with subsolar metallicity, and has a stellar mass $5\times10^{9}$ M$_\odot$ which amounts to $\approx$20\% of the galaxy's total. Surprisingly, the main 8 Gyr old disk has a significantly lower metallicity -0.8 dex than other counter-rotating galaxies. We developed a simple analytic model for the metal enrichment history, which we applied to PGC 66551 and constrained the parameters of the galactic outflow wind and estimated the metallicity of the infalling gas that formed the counter-rotating disk to be $-0.9 ... -0.5$ dex. Our interpretation prefers a merger with gas-rich satellite over cold accretion from a cosmic filament as a source of gas, which then formed the counter-rotating disk in PGC 66551., 18 pages, 10 figure, submitted to ApJ

Voids occupy about 3/4 of the volume of the Universe and contain about 15% of its mass. Due to various observational selection effects, these structure elements and galaxies populating voids, are highly under-explored. This especially relates to the lowest mass galaxies which comprise the main void population. Studying the nearby voids allows us to improve our understanding of the most elusive void objects. We present the brief overview of the current status and the prospects of the study of the nearest voids and their galaxies. First, we summarize the pioneer study of a hundred galaxies residing in the nearby Lynx-Cancer void which clearly evidence for the slower evolution of void galaxies and finds also the unusual very metal-poor and gas-rich dwarfs. Then we describe the recently defined sample of the nearby voids within the sphere with R = 25 Mpc and a sample of 1350 galaxies residing in these voids (~20% of all galaxies within this volume). We discuss the current results obtained for several directions of the study of this sample. They include: the search for Very Young Galaxies, the study of HI properties, the clustering of void galaxies and its relation to the void substructures, and the unbiased study of 260 void galaxies within the Local Volume (R < 11 Mpc). Altogether, this opens a perspective way to address the suggested peculiarities of the void galaxy formation and evolution. Finally, we briefly overview the expected advancements in the void galaxy studies related to the upcoming new facilities., 8 pages, 2 figures. Paper, reflecting a talk at the conference "Multifaceted Universe: Theory and Observations" - 2022, held at SAO RAS, Nizhnij Arkhyz, Russia, 23-27 May, 2022

Mapping the Galactic spiral structure is a difficult task since the Sun is located in the Galactic plane and because of dust extinction. For these reasons, molecular masers in radio wavelengths have been used with great success to trace the Milky Way spiral arms. Recently, Gaia parallaxes have helped in investigating the spiral structure in the Solar extended neighborhood. In this paper, we propose to determine the location of the spiral arms using Cepheids since they are bright, young supergiants with accurate distances (they are the first ladder of the extragalactic distance scale). They can be observed at very large distances; therefore, we need to take the Galactic warp into account. Thanks to updated mid-infrared photometry and to the most complete catalog of Galactic Cepheids, we derived the parameters of the warp using a robust regression method. Using a clustering algorithm, we identified groups of Cepheids after having corrected their Galactocentric distances from the (small) effects of the warp. We derived new parameters for the Galactic warp, and we show that the warp cannot be responsible for the increased dispersion of abundance gradients in the outer disk reported in previous studies. We show that Cepheids can be used to trace spiral arms, even at large distances from the Sun. The groups we identify are consistent with previous studies explicitly deriving the position of spiral arms using young tracers (masers, OB(A) stars) or mapping overdensities of upper main-sequence stars in the Solar neighborhood thanks to Gaia data., Accepted in A&A

KK242 is a LV dwarf of transition type residing in the void environment. Koda et al. present clear indications on its connection with Scd galaxy NGC6503. This implies the distance to KK242 of ~6.3 Mpc and its M_B = -10.5 mag. Its radial velocity, known from the Effelsberg radio telescope \HI\ observations, reveals, however, the difference with that of NGC6503, dV ~ 400 km/s. If real, this fact implies the substantial constraints on its origin. To clear-up the issue of KK242 radial velocity, we obtained with the SAO 6-m telescope spectra of its faint star-forming (SF) complex. H-alpha and H-beta emission is detected in two adjacent compact regions, the southern and northern, separated by ~2" (~60 pc). Their mean radial velocity is V_hel = -66 km/s, ~100 km/s lower than that of NGC6503. We use the HST Legacy Archive images and photometry of individual stars from the Extragalactic Distance Database, available for KK242, to identify in the SF complex the exciting hot stars, the probable BHeB and RHeB stars and a supernova remnant. We address, based on the possible range of its gas metallicity, the probable evolutionary paths of KK242. Using package Cloudy and parameters of the exciting B0V stars, we conclude that the observed flux ratio of [Sii] doublet to H-alpha is consistent with the value of 12+log(O/H) ~7.35+/-0.18 dex, expected for a stripped void dIrr galaxy., Comment: 15 pages, 8 figures, 4 tables, published at MNRAS

We have estimated masses of components of visual binaries from their spectral classification. We have selected pairs, where the less massive component looks more evolved. Spectral observations of some of such pairs were made, and at least one pair, HD~156331, was confirmed to have components of different age. Since mass exchange is excluded in wide binaries, it means that HD~156331 can be formed by the capture., 6 pages, 2 figures. accepted to Open Astronomy

We search for LBV stars in galaxies outside the Local Group. Here we present a study of two bright $H\alpha$ sources in the NGC 247 galaxy. Object j004703.27-204708.4 ($M_V=-9.08 \pm 0.15^m$) shows the spectral lines typical for well-studied LBV stars: broad and bright emission lines of hydrogen and helium He I with P Cyg profiles, emission lines of iron Fe II, silicon Si II, nitrogen N II and carbon C II, forbidden iron [Fe II] and nitrogen [N II] lines. The variability of the object is $\Delta B = 0.74\pm0.09^m$ and $\Delta V = 0.88\pm0.09^m$, which makes it reliable LBV candidate. The star j004702.18-204739.93 ($M_V=-9.66 \pm 0.23^m$) shows many emission lines of iron Fe II, forbidden iron lines [Fe II], bright hydrogen lines with broad wings, and also forbidden lines of oxygen [O I] and calcium [Ca II] formed in the circumstellar matter. The study of the light curve of this star also did not reveal significant variations in brightness ($\Delta V = 0.29\pm0.09^m$). We obtained estimates of interstellar absorption, the photosphere temperature, as well as bolometric magnitudes $M_\text{bol}=-10.5^{+0.5}_{-0.4}$ and $M_\text{bol}=-10.8^{+0.5}_{-0.6}$, which corresponds to bolometric luminosities $\log(L_\text{bol}/L_{\odot})=6.11^{+0.20}_{-0.16}$ and $6.24^{+0.20}_{-0.25}$ for j004703.27-204708.4 and j004702.18-204739.93 respectively. Thus, the object j004703.27-204708.4 remains a reliable LBV candidate, while the object j004702.18-204739.93 can be classified as B[e]-supergiant., Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journal 2020 July 15

The NGC 346 young stellar system and associated N66 giant HII region in the Small Magellanic Cloud are the nearest example of a massive star forming event in a low metallicity ($Z\approx0.2Z_{\odot}$) galaxy. With an age of $\lesssim$3Myr this system provides a unique opportunity to study relationships between massive stars and their associated HII region. Using archival data, we derive a total H$\alpha$ luminosity of L(H$\alpha$)=4.1$\times$10$^{38}$ergs$^{-1}$ corresponding to an H-photoionization rate of 3$\times$10$^{50}$s$^{-1}$. A comparison with a predicted stellar ionization rate derived from the more than 50 known O-stars in NGC 346, including massive stars recently classified from HST FUV spectra, indicates an approximate ionization balance. Spectra obtained with SALT suggest the ionization structure of N66 could be consistent with some leakage of ionizing photons. Due to the low metallicity, the far ultraviolet luminosity from NGC 346 is not confined to the interstellar cloud associated with N66. Ionization extends through much of the spatial extent of the N66 cloud complex, and most of the cloud mass is not ionized. The stellar mass estimated from nebular L(H$\alpha$) appears to be lower than masses derived from the census of resolved stars which may indicate a disconnect between the formation of high and low mass stars in this region. We briefly discuss implications of the properties of N66 for studies of star formation and stellar feedback in low metallicity environments., Comment: 15 pages, 6 figures, Accepted to be published in PASP

We have completed our observational program to search for wide binary systems with non-coeval components in the southern sky and report our results here. The final set of four systems was spectroscopically investigated in this paper. No binary systems with components of different ages were found among them. Taking into account our previous studies, we estimate the fraction of such binaries (i.e., binaries formed, presumably, by capture) not higher than 0.06~\%. The study will be continued on the northern sky., Comment: 9 pages, 3 figures, 2 tables. Published in Research in Astronomy and Astrophysics

Radial abundance gradients provide sound constraints for chemo-dynamical models of galaxies. Azimuthal variations of abundance ratios are solid diagnostics to understand their chemical enrichment. In this paper we investigate azimuthal variations of abundances in the Milky Way using Cepheids. We provide the detailed chemical composition (25 elements) of 105 Classical Cepheids from high-resolution SALT spectra observed by the MAGIC project. Negative abundance gradients, with abundances decreasing from the inner to the outer disc, have been reported both in the Milky Way and in external galaxies, and our results are in full agreement with literature results. We find azimuthal variations of the oxygen abundance [O/H]. While a large number of external spirals show negligible azimuthal variations, the Milky Way seems to be one of the few galaxies with noticeable [O/H] azimuthal asymmetries. They reach ~0.2 dex in the inner Galaxy and in the outer disc, where they are the largest, thus supporting similar findings for nearby spiral galaxies as well as recent 2D chemo-dynamical models., Comment: 9 pages, 8 figures, accepted for publication in MNRAS

The dwarf irregular galaxy HIPASS J1131-31 was discovered as a source of HI emission at low redshift in such close proximity of a bright star that we call it Peekaboo. The galaxy resolves into stars in images with Hubble Space Telescope, leading to a distance estimate of 6.8+-0.7 Mpc. Spectral optical observations with the Southern African Large Telescope reveal HIPASS J1131-31 to be one of the most extremely metal-poor galaxies known with the gas-phase oxygen abundance 12+log(O/H) = 6.99+-0.16 dex via the direct [OIII] 4363 line method and 6.87+-0.07 dex from the two strong line empirical methods. The red giant branch of the system is tenuous compared with the prominence of the features of young populations in the color-magnitude diagram, inviting speculation that star formation in the galaxy only began in the last few Gyr., Comment: 11 pages, 7 figures, published in MNRAS

Although S0 galaxies are usually considered "red and dead", they often demonstrate star formation organized into ring structures. We aim to clarify the nature of this phenomenon and how it differs from star formation in spiral galaxies. We investigated the nearby, moderate-luminosity S0 galaxy NGC 254 using long-slit spectroscopy taken with the South African Large Telescope and publicly available imaging data. Applying a full spectral fitting, we analyzed gaseous and stellar kinematics as well as ionized gas excitation and metallicity and stellar population properties resolved by radius. An advanced approach of simultaneously fitting spectra and photometric data allowed us to quantify the fraction of hidden counter-rotating stars in this galaxy. We find that the ionized gas is counter-rotating with respect to the stars throughout NGC 254 disk, indicating an external origin of the gas. We argue the gas-rich galaxy merger from retrograde orbit as a main source of counter-rotating material. The star formation fed by this counter-rotating gas occurs within two rings: an outer ring at R=55 - 70 arcsec and an inner ring at R=18 arcsec. The star formation rate is weak, 0.02 solar mass per year in total, and the gas metallicity is slightly subsolar. We estimated that the accretion of the gas occurred about 1 Gyr ago, and about 1% of all stars have formed in situ from counter-rotating gas., 11 pages, 9 figures, accepted for publication in Astronomy & Astrophysics

In the framework of an ongoing project aimed at searching for and studying eXtremely Metal-Poor (XMP) very gas-rich blue dwarfs in nearby voids, we conducted spectroscopy with the 11-m Southern African Large Telescope (SALT) of 26 candidates, preselected in the first paper of this series (PEPK19). For 23 of them, we detected Oxygen lines, allowing us to estimate the gas O/H ratio. For ten of them, the oxygen abundance is found to be very low, in the range of 12+log(O/H)=6.95-7.30 dex. Of those, four void dwarfs have 12+log(O/H) < 7.19, or Z < Zo/30. For the majority of observed galaxies, the faint line [OIII]4363A used to estimate O/H with the direct T_e method appeared either too noisy or was not detected. We therefore use the semi-empirical method of Izotov, Thuan (2007) for these spectra, or, when applicable, the new 'Strong line' method of Izotov et al. (2019b). We present and discuss the results for all void dwarfs observed in this work. We also compare their O/H values with O/H values of ~140 void galaxies available from our recent papers. We address the properties of the newly found unusual void XMP dwarfs and compare them with those for ten known prototype void XMP objects. The latter small group is outstanding based on their very small mass fraction of stars (only 0.01--0.02 of the baryonic mass), the blue colours of stars in the outer body (indicating a non-cosmological age for the main star-forming episode), and the low gas metallicity (several times lower than expected for their luminosity)., 18 pages, 5 figures, 10 tables, submitted to MNRAS

This paper presents results from photometric and statistical-parallax analysis of a sample of 850 field RR Lyrae (RRL) variables. The photometric and spectroscopic data for sample RRLs are obtained from (1) our new spectroscopic observations (for 448 RRLs) carried out with the Southern African Large Telescope (SALT); (2) our photometric observations using the 1.0-m telescope of the South African Astronomical Observatory (SAAO), and (3) literature. These are combined with accurate proper motion data from the second release of \textit{Gaia} mission (DR2). This study primarily determines the velocity distribution of solar neighborhood RRLs, and it also calibrates the zero points of the RRLs visual V-band luminosity-metallicity (LZ or $M_V-$[\text{Fe/H}]) relation and their period-luminosity-metallicity (PLZ) relations in the \textit{WISE} $W_1-$ and \textit{2MASS} $Ks-$band. The calibrated PLZ and LZ relations are used to estimate the Galactic Center distance and the distance modulus of the Large Magellanic Cloud (LMC), which are found to be 7.99$\pm$0.49\,kpc and 18.46$\pm$0.09 \,mag, respectively. All our results are in excellent agreement with available literature based on statistical parallax analysis, but are considerably more accurate and precise. Moreover, the zero-points of our calibrated PLZ and LZ relations are quite consistent with current results found by other techniques and yield the LMC distance modulus that is within 0.04\,mag of the current most precise estimate., 20 pages,21 figueres

We report the results of optical spectroscopy of the Small Magellanic Cloud supernova remnant (SNR) MCSNR J0127-7332 and the mass donor Be star, 2dFS 3831, in its associated high-mass X-ray binary SXP 1062 carried out with the Southern African Large Telescope (SALT). Using high-resolution long-slit spectra, we measured the expansion velocity of the SNR shell of \approx 140 km/s, indicating that MCSNR J0127-7332 is in the radiative phase. We found that the observed line ratios in the SNR spectrum can be understood if the local interstellar medium is ionized by 2dFS 3831 and/or OB stars around the SNR. We propose that MCSNR J0127-7332 is the result of supernova explosion within a bubble produced by the stellar wind of the supernova progenitor and that the bubble was surrounded by a massive shell at the moment of supernova explosion. We estimated the age of MCSNR J0127-7332 to be \la 10 000 yr. We found that the spectrum of 2dFS 3831 changes with orbital phase. Namely, the equivalent width of the Halpha emission line decreased by \approx 40 per cent in \approx 130 d after periastron passage of the neutron star and then almost returned to its original value in the next \approx 100 d. Also, the spectrum of 2dFS 3831 obtained closest to the periastron epoch (about three weeks after the periastron) shows a noticeable emission line of He II \lambda 4686, which disappeared in the next about two weeks. We interpret these changes as a result of the temporary perturbation and heating of the disk as the neutron star passes through it., Comment: 12 pages, 8 figures, accepted for publication in MNRAS

In recent years, our understanding of solar-like oscillations from main sequence to red giant stars has improved dramatically thanks to pristine data collected from space telescopes. One of the remaining open questions focuses around the observational identification of solar-like oscillations in pre-main sequence stars. We aim to develop an improved method to search for solar-like oscillations in pre-main sequence stars and apply it to data collected by the Kepler K2 mission. Our software APOLLO includes a novel way to detect low signal-to-noise ratio solar like oscillations in the presence of a high background level. By calibrating our method using known solar-like oscillators from the main Kepler mission, we apply it to T Tauri stars observed by Kepler K2 and identify several candidate pre-main sequence solar-like oscillators. We find that our method is robust even when applied to time-series of observational lengths as short as those obtained with the TESS satellite in one sector. We identify EPIC 205375290 as a possible candidate for solar-like oscillations in a pre-main sequence star with $\nu_\mathrm{max} \simeq 242\,\mu$Hz. We also derive EPIC 205375290's fundamental parameters to be $T_\mathrm{eff}$ = 3670$\pm$180 K, log $g$ = 3.85$\pm$0.3, $v$sin$i$ = 8 $\pm$ 1 km s$^{-1}$, and about solar metallicity from a high-resolution spectrum obtained from the Keck archive., Comment: 14 pages, 13 figures

We present the results of study of the Galactic candidate luminous blue variable Wray 15-906, revealed via detection of its infrared circumstellar shell (of \approx2 pc in diameter) with the Wide-field Infrared Survey Explorer and the Herschel Space Observatory. Using the stellar atmosphere code CMFGEN and the Gaia parallax, we found that Wray 15-906 is a relatively low-luminosity, log(L/Lsun)\approx5.4, star of temperature of 25\pm2 kK, with a mass-loss rate of \approx3\times10^{-5} Msun/yr, a wind velocity of 280\pm50 km/s, and a surface helium abundance of 65\pm2 per cent (by mass). In the framework of single star evolution, the obtained results suggest that Wray 15-906 is a post-red supergiant star with initial mass of \approx26\pm2 Msun and that before exploding as a supernova it could transform for a short time into a WN11h star. Our spectroscopic monitoring with the Southern African Large Telescope (SALT) does not reveal significant changes in the spectrum of Wray 15-906 during the last 8 yr, while the V-band light curve of this star over years 1999--2019 shows quasi-periodic variability with a period of \approx1700 d and an amplitude of \approx0.1 mag. We estimated the mass of the shell to be 2.9\pm0.5 Msun assuming the gas-to-dust mass ratio of 200. The presence of such a shell indicates that Wray 15-906 has suffered substantial mass loss in the recent past. We found that the open star cluster C1128-631 could be the birth place of Wray 15-906 provided that this star is a rejuvenated product of binary evolution (a blue straggler)., 18 pages, 15 figures, accepted to MNRAS

We have obtained imaging data in two photometric bands, g and r, for a sample of 42 isolated lenticular galaxies with the Las Cumbres Observatory one-meter telescope network. We have analyzed the structure of their large-scale stellar disks. The parameters of surface brightness distributions have been determined including the radial profile shapes and disk thicknesses. After inspecting the radial brightness profiles, all the galaxies have been classified into pure exponential (Type I), truncated (Type II), and antitruncated (Type III) disks. By comparing the derived statistics of the radial profiles shapes with our previous sample of the cluster S0s, we noted a prominent difference between stellar disks of S0s galaxies in quite rarefied environments and in clusters: it is only in sparse environments that Type II disks, with profile truncations, can be found. This finding implies probable different dynamical history of S0 galaxies in different environments., 22 pages, 9 figures. Accepted to The Astronomical Journal. arXiv admin note: text overlap with arXiv:1809.05202

Classical novae are thermonuclear explosions that occur on the surfaces of white dwarf stars in interacting binary systems (Bode & Evans 2008). It has long been thought that the luminosity of classical novae is powered by continued nuclear burning on the surface of the white dwarf after the initial runaway (Gallaher & Starrfield 1978). However, recent observations of GeV $\gamma$-rays from classical novae have hinted that shocks internal to the nova ejecta may dominate the nova emission. Shocks have also been suggested to power the luminosity of events as diverse as stellar mergers (Metzger & Pejcha 2017), supernovae (Moriya et al. 2018), and tidal disruption events (Roth et al. 2016), but observational confirmation has been lacking. Here we report simultaneous space-based optical and $\gamma$-ray observations of the 2018 nova V906 Carinae (ASASSN-18fv), revealing a remarkable series of distinct correlated flares in both bands. The optical and $\gamma$-ray flares occur simultaneously, implying a common origin in shocks. During the flares, the nova luminosity doubles, implying that the bulk of the luminosity is shock-powered. Furthermore, we detect concurrent but weak X-ray emission from deeply embedded shocks, confirming that the shock power does not appear in the X-ray band and supporting its emergence at longer wavelengths. Our data, spanning the spectrum from radio to $\gamma$-ray, provide direct evidence that shocks can power substantial luminosity in classical novae and other optical transients., Comment: 49 pages including supplementary information. Accepted and published in Nature Astronomy

Galactic stellar discs, such as that of the Milky Way, have usually a complex structure consisting of a thin and a thick component. The study of galactic disc substructures and their differences can shed light on the galaxy assembling processes and their evolution. However, due to observational difficulties there is a lack of information about the stellar populations of the thick disc components in external galaxies. Here we investigate three edge-on early-type disc galaxies in the Fornax cluster IC335, NGC1380A, NGC1381 by using publicly available photometrical data and our new deep long-slit spectroscopy along galactic mid-planes obtained with the 10-m SALT telescope. We report that significant changes of the stellar population properties beyond the radius where photometrical profiles demonstrate a knee are caused by an increasing thick disc contribution. Stellar population properties in the outermost thick-disc dominated regions demonstrate remarkably old ages and a low metallicity. We interpret these findings as a consequence of star formation quenching in the outermost regions of the discs due to ram pressure gas stripping from the disc periphery at the beginning of the cluster assembly while subsequent star formation occurring in the inner discs being gradually extinguished by starvation., Comment: 12 pages, 7 figures; Accepted for publication in MNRAS

We report the results of our study of the high-velocity (\approx160 km/s) runaway O star CPD-64 2731 and its associated horseshoe-shaped nebula discovered with the Wide-field Infrared Survey Explorer. Spectroscopic observations with the Southern African Large Telescope and spectral analysis indicate that CPD-64 2731 is a fast-rotating main-sequence O5.5 star with enhanced surface nitrogen abundance. We derive a projected rotational velocity of \approx300 km/s which is extremely high for this spectral type. Its kinematic age of \approx6 Myr, assuming it was born near the Galactic plane, exceeds its age derived from single star models by a factor of two. These properties suggest that CPD-64 2731 is a rejuvenated and spun-up binary product. The geometry of the nebula and the almost central location of the star within it argue against a pure bow shock interpretation for the nebula. Instead, we suggest that the binary interaction happened recently, thereby creating the nebula, with a cavity blown by the current fast stellar wind. This inference is supported by our results of 2D numerical hydrodynamic modelling., Comment: 15 pages, 11 figures, accepted for publication in MNRAS

We present a study of the high-resolution optical spectrum for the hot post-asymptotic giant branch (post-AGB) star, Hen 3-1013, identified as the optical counterpart of the infrared sourceIRAS 14331-6435. For the first time the detailed identifications of the observed absorption and emission features in the wavelength range 3700-9000 \AA\ is carried out. Absorption lines of HI, HeI, CI, NI, OI, NeI, CII, NII, OII, SiII, SII, ArII, FeII, MnII, CrII, TiII, CoII, NiII, SIII, FeIII and SIV were detected. From the absorption lines, we derived heliocentric radial velocities of $V_r=-29.6\pm0.4$ km/s. We have identified emission permitted lines of OI, NI, FeII, MgII, SiII and AlII. The forbidden lines of [NI], [FeII], [CrII] and [NiII] have been identified also. Analysis of [NiII] lines in the gaseous shell gives an estimate for the electron density $N_e\sim10^7$ cm$^{-3}$ and the expansion velocity of the nebula $V_{exp}=12$ km/s. The mean radial velocity as measured from emission features of the envelope is $V_r=-36.0\pm0.4$ km/s. The Balmer lines H$\alpha$, H$\beta$ and H$\gamma$ show P Cyg behaviour which indicate ongoing post-AGB mass-loss. Based on ASAS and ASAS-SN data, we have detected rapid photometric variability in Hen 3-1013 with an amplitude up to 0.2 mag in the V band. The star's low-resolution spectrum underwent no significant changes from 1994 to 2012. Based on archival data, we have traced the photometric history of the star over more than 100 years. No significant changes in the star brightness have been found., Comment: 19 pages, 6 figures, 8 tables, accepted for publication in MNRAS

We present a sample of 66 galaxies belonging to the equatorial part (Dec.= -7$^o$, +7$^o$) of the large so called Eridanus void (after Fairall 1998). The void galaxies are selected as to be separated from the luminous galaxies ($M_{\rm B} < M_{\rm B}^{*} +1$), delineating the void, by more than 2 Mpc. Our main goal is to study systematically the evolutionary parameters of the void sample (metallicity and gas content) and to compare the void galaxy properties with their counterparts residing in denser environments. Besides the general galaxy parameters, compiled mainly from the literature, we present the results of dedicated observations to measure the oxygen abundance O/H in HII-regions of 23 void galaxies obtained with the 11-m SALT telescope (SAAO) and the 6-m BTA telescope (SAO), as well as the O/H estimates derived from the analysis of the SDSS DR12 spectra for 3 objects. We compiled all available data on O/H in 36 these void galaxies, including those for 11 galaxies available in the literature (for one object both SDSS and SALT spectra were used), and analyze this data in relation to galaxy luminosity ($\log$(O/H) versus $M_{\rm B}$). Comparing them with the control sample of similar type galaxies from the Local Volume, we find clear evidence for a substantially lower average metallicity of the Eridanus void galaxies. This result matches well the conclusions of our recent similar study for galaxies in the Lynx-Cancer void., Published in MNRAS; 39 pages, 13 figures, 19 tables

We present the analysis of medium-resolution spectra obtained at the Southern African Large Telescope (SALT) for nuclear globular clusters (GCs) in two dwarf spheroidal galaxies (dSphs). The galaxies have similar star formation histories, but they are situated in completely different environments. ESO269-66 is a close neighbour of the giant S0 NGC5128. KKs3 is one of the few truly isolated dSphs within 10 Mpc. We estimate the helium abundance $Y=0.3$, $\rm age=12.6\pm1$ Gyr, $[Fe/H]=-1.5,-1.55\pm0.2$ dex, and abundances of C, N, Mg, Ca, Ti, and Cr for the nuclei of ESO269-66 and KKs3. Our surface photometry results using HST images yield the half-light radius of the cluster in KKs3, $\rm r_h=4.8\pm0.2$ pc. We demonstrate the similarities of medium-resolution spectra, ages, chemical compositions, and structure for GCs in ESO269-66 and KKs3 and for several massive Galactic GCs with $[Fe/H]\sim-1.6$ dex. All Galactic GCs posses Extended Blue Horizontal Branches and multiple stellar populations. Five of the selected Galactic objects are iron-complex GCs. Our results indicate that the sample GCs observed now in different environments had similar conditions of their formation $\sim$1 Gyr after the Big Bang., Comment: 14 pages, 8 figures, 10 tables. Table 1 and two references were corrected. Accepted for publication in MNRAS

A large sub-mm survey with Herschel will enable many exciting science opportunities, especially in an era of wide-field optical and radio surveys and high resolution cosmic microwave background experiments. The Herschel-SPIRE Legacy Survey (HSLS), will lead to imaging data over 4000 sq. degrees at 250, 350, and 500 micron. Major Goals of HSLS are: (a) produce a catalog of 2.5 to 3 million galaxies down to 26, 27 and 33 mJy (50% completeness; 5 sigma confusion noise) at 250, 350 and 500 micron, respectively, in the southern hemisphere (3000 sq. degrees) and in an equatorial strip (1000 sq. degrees), areas which have extensive multi-wavelength coverage and are easily accessible from ALMA. Two thirds of the of the sources are expected to be at z > 1, one third at z > 2 and about a 1000 at z > 5. (b) Remove point source confusion in secondary anisotropy studies with Planck and ground-based CMB data. (c) Find at least 1200 strongly lensed bright sub-mm sources leading to a 2% test of general relativity. (d) Identify 200 proto-cluster regions at z of 2 and perform an unbiased study of the environmental dependence of star formation. (e) Perform an unbiased survey for star formation and dust at high Galactic latitude and make a census of debris disks and dust around AGB stars and white dwarfs., White paper supplement to the proposal submitted by the HSLS science team to ESA for Herschel open-time programs

Rings in S0s are enigmatic features which can however betray the evolutionary paths of particular galaxies. We have undertaken long-slit spectroscopy of five lenticular galaxies with UV-bright outer rings. The observations have been made with the Southern African Large Telescope (SALT) to reveal the kinematics, chemistry, and the ages of the stellar populations and the gas characteristics in the rings and surrounding disks. Four of the five rings are also bright in the H-alpha emission line, and the spectra of the gaseous rings extracted around the maxima of the H-alpha equivalent width reveal excitation by young stars betraying current star formation in the rings. The integrated level of this star formation is 0.1-0.2 solar mass per year, with the outstanding value of 1 solar mass per year in NGC 7808. The difference of chemical composition between the ionized gas of the rings which demonstrate nearly solar metallicity and the underlying stellar disks which are metal-poor implies recent accretion of the gas and star formation ignition; the star formation history estimated by using different star formation indicators implies that the star formation rate decreases with e-folding time of less than 1 Gyr. In NGC 809 where the UV-ring is well visible but the H-alpha emission line excited by massive stars is absent, the star formation has already ceased., 22 pages including the Appendix, 11 figures. Accepted to the Astronomical Journal

We report the results of spectroscopic observations and numerical modelling of the H II region IRAS 18153-1651. Our study was motivated by the discovery of an optical arc and two main-sequence stars of spectral type B1 and B3 near the centre of IRAS 18153-1651. We interpret the arc as the edge of the wind bubble (blown by the B1 star), whose brightness is enhanced by the interaction with a photoevaporation flow from a nearby molecular cloud. This interpretation implies that we deal with a unique case of a young massive star (the most massive member of a recently formed low-mass star cluster) caught just tens of thousands of years after its stellar wind has begun to blow a bubble into the surrounding dense medium. Our two-dimensional, radiation-hydrodynamics simulations of the wind bubble and the H II region around the B1 star provide a reasonable match to observations, both in terms of morphology and absolute brightness of the optical and mid-infrared emission, and verify the young age of IRAS 18153-1651. Taken together our results strongly suggest that we have revealed the first example of a wind bubble blown by a main-sequence B star., Comment: 12 pages, 10 figures, accepted for publication in MNRAS

In this paper, we report the results of spectroscopic and photometric observations of the candidate evolved massive star MN48 disclosed via detection of a mid-infrared circular shell around it with the Spitzer Space Telescope. Follow-up optical spectroscopy of MN48 with the Southern African Large Telescope (SALT) carried out in 2011--2015 revealed significant changes in the spectrum of this star, which are typical of luminous blue variables (LBVs). The LBV status of MN48 was further supported by photometric monitoring which shows that in 2009--2011 this star has brightened by approx 0.9 and 1 mag in the V and I_c bands, respectively, then faded by approx 1.1 and 1.6 mag during the next four years, and apparently started to brighten again recently. The detected changes in the spectrum and brightness of MN48 make this star the 18th known Galactic bona fide LBV and increase the percentage of LBVs associated with circumstellar nebulae to more than 70 per cent. We discuss the possible birth place of MN48 and suggest that this star might have been ejected either from a putative star cluster embedded in the HII region IRAS 16455-4531 or the young massive star cluster Westerlund 1., 11 pages, 10 figures, accepted for publication in MNRAS

The mass-luminosity relation is a fundamental law of astrophysics. We have suggested that the currently used mass-luminosity relation is not correct for the M/M_sun > 2.7 range of mass since it was created using double-lined eclipsing binaries, where the components are synchronized and consequently change each other's evolutionary path. To exclude this effect we have started a project to study long-period massive eclipsing binaries in order to construct radial velocity curves and determine masses for the components. We outline our project and present the selected test sample together with the first HRS/SALT spectral observations and the software package, FBS (Fitting Binary Stars), that we developed for the analysis of our spectral data. As the first result we present the radial velocity curves and best-fit orbital elements for the two components of the FP Car binary system from our test sample., 12 pages, 6 figures, accepted by Research in Astronomy and Astrophysics

Gravitational wave emission can lead to the coalescence of close pairs of compact objects orbiting each other. For the case of neutron stars such mergers may yield masses above the Tolman-Oppenheimer-Volkoff limit, leading to the formation of black holes. For the case of white dwarfs the merger product may exceed the Chandrasekhar limit, leading either to a thermonuclear explosion as Type Ia supernova, or to a collapse forming a neutron star. If a Type Ia supernova explosion is avoided, the merger of two massive white dwarfs is expected to form a hydrogen- and helium-free circumstellar nebula with a hot and luminous, rapidly rotating and highly magnetized central star for several 10,000 yr before its final collapse. Here we report the discovery of a hot star with an emission line dominated spectrum in the centre of a circular mid-infrared nebula. Both the star and the nebula appear to be free of hydrogen and helium. Our tailored stellar atmosphere and wind models indicate a stellar surface temperature of about 200,000 K, and a record outflow velocity of 16,000 km/s. This extreme velocity, together with the derived mass outflow rate, imply rapid stellar rotation and a strong magnetic field aiding the wind acceleration. The Gaia distance of the star leads to a luminosity of 10^{4.5} Lsun, which matches models of the post-merger evolution of super-Chandrasekhar mass white dwarfs. The high stellar temperature and the nebular size argue for a short remaining lifetime of the star, which will produce a bright optical and high-energy transient upon collapse. Our observations indicate that super-Chandrasekhar mass white dwarf mergers can indeed avoid a thermonuclear explosion as Type Ia supernova, and provide empirical evidence for magnetic field generation in stellar mergers., Comment: 14 pages, 3 figures, original version of the manuscript submitted to Nature; accepted version is published at https://www.nature.com/articles/s41586-019-1216-1 and is accessible at https://rdcu.be/bDo6c

We report the discovery of a spiral-like nebula with the Wide-field Infrared Survey Explorer (WISE) and the results of optical spectroscopy of its associated star TYC 8606-2025-1 with the Southern African Large Telescope (SALT). We find that TYC 8606-2025-1 is a G8 III star of $\approx3$ M$_\odot$, showing a carbon depletion by a factor of two and a nitrogen enhancement by a factor of three. We also derived an excess of s-process elements, most strongly for barium, which is a factor of three overabundant, indicating that TYC 8606-2025-1 is a mild barium star. We thereby add a new member to the small group of barium stars with circumstellar nebulae. Our radial velocity measurements indicate that TYC 8606-2025-1 has an unseen binary companion. The advanced evolutionary stage of TYC 8606-2025-1, together with the presence of a circumstellar nebula, implies an initial mass of the companion of also about 3 M$_\odot$. We conclude that the infrared nebula, due to its spiral shape, and because it has no optical counterpart, was ejected by the companion as a consequence of a very late thermal pulse, during about one orbital rotation., Comment: 11 pages, 7 figures, accepted for publication in MNRAS

We report the discovery of a square axisymmetric circumstellar nebula around the emission-line star HD 93795 in archival Spitzer Space Telescope 24 micron data. We classify HD 93795 as an B9 Ia star using optical spectra obtained with the Southern African Large Telescope (SALT). A spectral analysis carried out with the stellar atmosphere code FASTWIND indicates that HD 93795 only recently left the main sequence and is evolving redward for the first time. We discuss possible scenarios for the origin of the nebula and suggest that HD 93795 was originally a binary system and that the nebula was formed because of merger of the binary components. We also discuss a discrepancy between distance estimates for HD 93795 based on the Gaia data and the possible membership of this star of the Car OB1 association, and conclude that HD 93795 could be at the same distance as Car OB1., Comment: 11 pages, 7 figures, accepted for publication in MNRAS

We introduce a project aimed at systematically searching for eXtremely Metal-Poor (XMP) very gas-rich blue dwarfs in voids in the nearby Universe. Several such galaxies were first identified in the course of an unbiased study of the galaxy population in the nearby Lynx-Cancer void. These very rare and unusual galaxies appear to be the best proxies for the so-called Very Young Galaxies (VYGs) defined recently in the model simulations by Tweed et al. (2018). We discuss the main properties of ten prototype objects residing in nearby voids and formulate criteria to search for similar dwarfs in other voids. The recently published sample of 1354 Nearby Void Galaxies (NVG) is used to identify a subsample of 60 void dwarf XMP candidates. We provide a list of these XMP candidates with their main parameters and finding charts. These candidates are the subjects of subsequent spectral, photometric and HI studies in the accompanying papers. Looking ahead, with reference to the submitted accompanying papers, we find that this study results in the discovery of many new XMP dwarfs with 12+log(O/H) ~ 7.0-7.3 dex., Comment: 14 pages, 5 figures, 2 tables. Accepted for publication in MNRAS

LMC P3 is the most luminous gamma-ray binary discovered to date and the first detected outside of the Galaxy, with an orbital period of 10.301 d. We report on optical spectroscopic observations undertaken with the Southern African Large Telescope (SALT) using the High Resolution spectrograph (HRS). We find the binary is slightly eccentric, $e = 0.40\pm0.07$, and place the time of periastron at HJD $2457412.13 \pm 0.29$. Stellar model fitting finds an effective temperature of $T_{\rm eff} = 36351 \pm 53$ K. The mass function, $f = 0.0010 \pm 0.0004$ M$_{\odot}$, favours a neutron star compact object. The phases of superior and inferior conjunctions are $0.98$ and $0.24$, respectively (where phase 0 is at the Fermi-LAT maximum), close to the reported maxima in the GeV and TeV light curves., Comment: Published in MNRAS. This version: a few minor typo corrections

We report the discovery of a new eclipsing polar, MASTER OT J061451.70-272535.5, detected as an optical transient by MASTER auto-detection software at the recently commissioned MASTER-SAAO telescope. Time resolved (10-20 s) photometry with the SAAO 1.9-m, and 1.0-m telescopes, utilizing the SHOC EM-CCD cameras, revealed that the source eclipses, with a period of 2.08 hours (7482.9$\pm$3.5$\,$s). The eclipse light curve has a peculiar morphology, comprising an initial dip, where the source brightness drops to ${\sim}$50% of the pre-eclipse level before gradually increasing again in brightness. A second rapid ingress follows, where the brightness drops by ${\sim}$60-80%, followed by a more gradual decrease to zero flux. We interpret the eclipse profile as the result of an initial obscuration of the accretion hot-spot on the magnetic white dwarf by the accretion stream, followed by an eclipse of both the hot-spot and the partially illuminated stream by the red dwarf donor star. This is similar to what has been observed in other eclipsing polars such as HU Aqr, but here the stream absorption is more pronounced. The object was subsequently observed with South African Large Telescope (SALT) using the Robert Stobie Spectrograph (RSS). This revealed a spectrum with all of the Balmer lines in emission, a strong HeII 4686\AA{} line with a peak flux greater than that of H$\beta$, as well as weaker HeI lines. The spectral features, along with the structure of the light curve, suggest that MASTER OT J061451.70-272535.5 is a new magnetic cataclysmic variable, most likely of the synchronised Polar subclass., Comment: 15 pages, 11 figures, accepted for publication in MNRAS

We report the discovery of a handful of optical hydrogen-poor knots in the central part of an extended infrared nebula centred on the [WO1] star WR 72, obtained by spectroscopic and imaging observations with the Southern African Large Telescope (SALT). Wide-field Infrared Survey Explorer (WISE) images of the nebula show that it is composed of an extended almost circular halo (of $\approx6$ arcmin or $\approx2.4$ pc in diameter) and an elongated and apparently bipolar inner shell (of a factor of six smaller size), within which the knots are concentrated. Our findings indicate that WR 72 is a new member of the rare group of hydrogen-poor planetary nebulae, which may be explained through a very late thermal pulse of a post-AGB star, or by a merger of two white dwarfs., Comment: 7 pages, 5 figures, accepted for publication in MNRAS

We report the discovery of a circular shell centred on the Be X-ray binary (BeXB) SXP 1323 in the Small Magellanic Cloud (SMC). The shell was detected in an Halpha image obtained with the Very Large Telescope (VLT). Follow-up spectroscopy with the Southern African Large Telescope (SALT) showed that the shell expands with a velocity of $\approx$ 100 km/s and that its emission is due to shock excitation. We suggest that this shell is the remnant of the supernova explosion that led to the formation of the SXP 1323's neutron star $\approx$ 40 000 yr ago. SXP 1323 represents the second known case of a BeXB associated with a supernova remnant (the first one is SXP 1062). Interestingly, both these BeXBs harbour long period pulsars and are located in a low-metallicity galaxy., Comment: 5 pages, 5 figures, accepted for publication in MNRAS Letters

We report the discovery of two mid-infrared nebulae in the northern hemisphere with the Wide-field Infrared Survey Explorer and the results of optical spectroscopy of their central stars, BD+60 2668 (composed of two components, separated from each other by \approx 3 arcsec) and ALS 19653, with the Calar Alto 3.5-m telescope and the Southern African Large Telescope (SALT), respectively. We classify the components of BD+60 2668 as stars of spectral types B0.5 II and B1.5 III. ALS 19653 is indicated in the SIMBAD data base as a planetary nebula, while our observations show that it is a massive B0.5 Ib star, possibly in a binary system. Using the stellar atmosphere code FASTWIND, we derived fundamental parameters of the three stars as well as their surface element abundances, implying that all of them are either on the main sequence or only recently left it. This provides further evidence that massive stars can produce circumstellar nebulae while they are still relatively unevolved. We also report the detection of optical counterparts to the mid-infrared nebulae and a second, more extended optical nebula around ALS 19653, and present the results of SALT spectroscopy of both nebulae associated with this star. The possible origin of the nebulae is discussed., 24 pages, 13 figures, 6 tables, accepted for publication in AJ

We report the results of optical spectroscopy of the candidate evolved massive star MN44 revealed via detection of a circular shell with the Spitzer Space Telescope. First spectra taken in 2009 May--June showed the Balmer lines in emission as well as numerous emission lines of iron, which is typical of luminous blue variables (LBVs) near the visual maximum. New observations carried out in 2015 May--September detected significant changes in the spectrum, indicating that the star became hotter. We found that these changes are accompanied by significant brightness variability of MN44. In particular, the I_c-band brightness decreased by \approx 1.6 mag during the last six years and after reaching its minimum in 2015 June has started to increase. Using archival data, we also found that the I_c-band brightness increased by \approx 3 mag in \approx 30 yr preceding our observations. MN44 therefore represents the seventeenth known example of the Galactic bona fide LBVs. We detected a nitrogen-rich knot to the northwest of the star, which might represent an interstellar cloudlet interacting with the circumstellar shell. We discuss a possible association between MN44 and the INTEGRAL transient source of hard X-ray emission IGR J16327-4940, implying that MN44 might be either a colliding-wind binary or a high-mass X-ray binary., Comment: 13 pages, 7 figures, accepted for publication in MNRAS

By obtaining imaging data in two photometric bands for 60 lenticular galaxies - members of 8 southern clusters - with the Las Cumbres Observatory one-meter telescope network, we have analyzed the structure of their large-scale stellar disks. The parameters of radial surface-brightness profiles have been determined (including also disk thickness), and all the galaxies have been classified into pure exponential (Type I) disk surface-brightness profiles, truncated (Type II) and antitruncated (Type III) piecewise exponential disk surface-brightness profiles. We confirm the previous results of some other authors that the proportion of surface-brightness profile types is very different in environments of different density: in the clusters the Type-II profiles are almost absent while according to the literature data, in the field they constitute about one quarter of all lenticular galaxies. The Type-III profiles are equally presented in the clusters and in the field, while following similar scaling relations; but by undertaking an additional structural analysis including the disk thickness determination we note that some Type-III disks may be a combination of a rather thick exponential pseudobulge and an outer Type-I disk. Marginally we detect a shift of the scaling relation toward higher central surface brightnesses for the outer segments of Type-III disks and smaller thickness of the Type-I disks in the clusters. Both effects may be explained by enhanced radial stellar migration during disk galaxy infall into a cluster that in particular represents an additional channel for Type-I disk shaping in dense environments., Comment: The paper is published in the Astronomical Journal, vol. 156, September issue of 2018; the atlas of 60 S0 galaxies (the results of isophote analysis, color maps, surface-brightness profiles) is added into the ArXiv version

Results of comparison of Gaia DR2 parallaxes with data derived from a combined analysis of 2MASS (Two Micron All-Sky Survey), SDSS (Sloan Digital Sky Survey), GALEX (Galaxy Evolution Explorer), and UKIDSS (UKIRT Infrared Deep Sky Survey) surveys in four selected high-latitude | b | &gt, 48&deg, sky areas are presented. It is shown that multicolor photometric data from large modern surveys can be used for parameterization of stars closer than 4400 pc and brighter than g S D S S = 19 . m 6 , including estimation of parallax and interstellar extinction value. However, the stellar luminosity class should be properly determined.

In this Letter, we report the results of spectroscopic and photometric monitoring of the candidate luminous blue variable (LBV) WS1, which was discovered in 2011 through the detection of a mid-infrared circular shell and follow-up optical spectroscopy of its central star. Our monitoring showed that WS1 brightened in the B, V and I bands by more than 1 mag during the last three years, while its spectrum revealed dramatic changes during the same time period, indicating that the star became much cooler. The light curve of WS1 demonstrates that the brightness of this star has reached maximum in 2013 December and then starts to decline. These findings unambiguously proved the LBV nature of WS1 and added one more member to the class of Galactic bona fide LBVs, bringing their number to sixteen (an updated census of these objects is provided)., Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letters

We report the discovery of a circular mid-infrared shell around the emission-line star Wray 16-137 using archival data of the Spitzer Space Telescope. Follow-up optical spectroscopy of Wray 16-137 with the Southern African Large Telescope revealed a rich emission spectrum typical of the classical luminous blue variables (LBVs) like P Cygni. Subsequent spectroscopic and photometric observations showed drastic changes in the spectrum and brightness during the last three years, meaning that Wray 16-137 currently undergoes an S Dor-like outburst. Namely, we found that the star has brightened by \approx 1 mag in the V and I_c bands, while its spectrum became dominated by Fe ii lines. Taken together, our observations unambiguously show that Wray 16-137 is a new member of the family of Galactic bona fide LBVs., Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letters

We present the results of spectroscopic and photometric observations for three hot southern-hemisphere post-AGB objects, Hen 3-1347 = IRAS 17074-1845, Hen 3-1428 = IRAS17311-4924, and LSS 4634 = IRAS 18023-3409. In the spectrograms taken with the 1.9-m telescope of the South African Astronomical Observatory (SAAO) in 2012, we have measured the equivalent widths of the most prominent spectral lines. Comparison of the new data with those published previously points to a change in the spectra of Hen 3-1428 and LSS 4634 in the last 20 years. Based on ASAS data, we have detected rapid photometric variability in all three stars with an amplitude up to 0.3 mag, 0.4 mag in the $V$ band. A similarity between the patterns of variability for the sample stars and other hot protoplanetary nebulae is pointed out. We present the results of $UBV$ observations for Hen 3-1347, according to which the star undergoes rapid irregular brightness variations with maximum amplitudes $\Delta V$=0.25 mag, $\Delta B$=0.25 mag, and $\Delta U$=0.30 mag and shows color-magnitude correlations. Based on archival data, we have traced the photometric history of the stars over more than 100 years. Hen 3-1347 and LSS 4634 have exhibited a significant fading on a long time scale. The revealed brightness and spectrum variations in the stars, along with evidence for their enhanced mass, may be indicative of their rapid post-AGB evolution., Comment: 17 pages, 6 figures

We present the results of a photometric study of 85 objects from the updated sample of galaxies residing in the nearby Lynx--Cancer void. We perform our photometry on u, g, r, and i-band images of the Sloan Digital Sky Survey. We determine model-independent galaxy parameters such as the integrated magnitudes and colors, effective radii and the corresponding surface brightness values, optical radii and Holmberg radii. We analyze the radial surface brightness profiles to determine the central brightness values and scale lengths of the model discs. We analyze the colors of the outer parts of the galaxies and compare them with model evolutionary tracks computed using the PEGASE2 software package. This allowed us to estimate the time T_SF elapsed since the onset of star formation, which turned out to be on the order of the cosmological time T_0 for the overwhelming majority of the galaxies studied. However, for 13 galaxies of the sample the time T_SF does not exceed T_0/2 ~ 7 Gyr, and for 7 of them T_SF < 3.5 Gyr. The latter are mostly unevolved objects dominated by low-luminosity galaxies with M_B > -13.2. We use the integrated magnitudes and colors to estimate the stellar masses of the galaxies. We estimate the parameter M(HI)/L_B and the gas mass fractions for void galaxies with known HI-line fluxes. A small subgroup (about 10%) of the gas-richest void galaxies with M(HI)/L_B > 2.5 has gas mass fractions that reach 94-99%. The outer regions of many of these galaxies show atypically blue colors. To test various statistical differences between void galaxies and galaxies from the samples selected using more general criteria, we compare some of the parameters of void galaxies with similar data for the sample of 195 galaxies from the Equatorial Survey (ES) based on a part of the HIPASS blind HI survey. abridged, 28 pages, 17 figures, 5 tables. To simplify access to photometrical data, we add in this version all related tables. Published in Astrophysical Bulletin, 2014, vol. 69, p.247-265. Pleiads Press. The MW extinction in estimates of stellar mass was missed. Here we correct this error. This affects stellar mass and gas mass fraction in Table 1 and in Fig.8. The stellar mass median correction is 1.08

Recent observations reveal that the central star of the planetary nebula Abell 48 exhibits spectral features similar to massive nitrogen-sequence Wolf-Rayet stars. This raises a pertinent question, whether it is still a planetary nebula or rather a ring nebula of a massive star. In this study, we have constructed a three-dimensional photoionization model of Abell 48, constrained by our new optical integral field spectroscopy. An analysis of the spatially resolved velocity distributions allowed us to constrain the geometry of Abell 48. We used the collisionally excited lines to obtain the nebular physical conditions and ionic abundances of nitrogen, oxygen, neon, sulphur and argon, relative to hydrogen. We also determined helium temperatures and ionic abundances of helium and carbon from the optical recombination lines. We obtained a good fit to the observations for most of the emission-line fluxes in our photoionization model. The ionic abundances deduced from our model are in decent agreement with those derived by the empirical analysis. However, we notice obvious discrepancies between helium temperatures derived from the model and the empirical analysis, as overestimated by our model. This could be due to the presence of a small fraction of cold metal-rich structures, which were not included in our model. It is found that the observed nebular line fluxes were best reproduced by using a hydrogen-deficient expanding model atmosphere as the ionizing source with an effective temperature of 70 kK and a stellar luminosity of 5500 L_sun, which corresponds to a relatively low-mass progenitor star (~ 3 M_sun) rather than a massive Pop I star., 12 pages, 7 figures, 12 tables, accepted by MNRAS

The Advanced LIGO observatory recently reported the first direct detection of gravitational waves predicted by Einstein (1916). We report on the first optical observations of the Gravitational Wave (GW) source GW150914 error region with the Global MASTER Robotic Net. We detected several optical transients, which proved to be unconnected with the GW event. Our result is consistent with the assumption that gravitational waves were produced by a binary black hole merger. The detection of the event confirmed the main prediction of the population synthesis performed with the "Scenario Machine" formulated in Lipunov1997b., 11 pages, 7 figures, 4 tables