Search

Microlensing of stars in strongly lensed galaxies can lead to temporary extreme magnification factors (μ> 1000), enabling their detection at high redshifts. Following the discovery of Icarus, several stars at cosmological distances (z > 1) have been observed using the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST). This emerging field of gravitational lensing holds promise to study individual high redshift stars. It also offers the opportunity to study the substructure in the lens plane with implications for dark matter models, as more lensed stars are detected and analysed statistically. Due to the computational demands of simulating microlensing at large magnification factors, it is important to develop fast and accurate analytical approximations for the probability of magnification in such extreme scenarios. In this study, we consider different macro-model magnification and microlensing surface mass density scenarios and study how the probability of extreme magnification factors depends on these factors. To achieve this, we created state-of-the-art large simulations of the microlensing effect in these scenarios. Through the analysis of these simulations, we derived analytical scaling relationships that can bypass the need for expensive numerical simulations. Our results are useful to interpret current observations of stars at cosmic distances which are extremely magnified and under the influence of microlenses. [ABSTRACT FROM AUTHOR]

We searched for OB-runaway stars inside supernova remnants (SNRs) as a pre-supernova binary companion. As the majority of massive stars are found in close binary systems, a runaway star ejected by the orbital energy after the supernova (SN) is expected to be found. Considering a binary mass fraction, q = 0.25−1.0, the runaway star is likely to have an OB spectral type. We selected 12 SNRs at Galactic longitudes 109°−189°. Using Gaia astrometry, we selected stars having consistent distances with those of the SNRs and showing a peculiar proper motion directed away from the central region of the SNRs. We also determined the radial distribution of the extinction towards the SNRs and estimated the spectral types of the OB-runaway candidates through Gaia and Two Micron All Sky Survey photometry. We found two candidates among 12 SNRs. By spectroscopic observations, Gaia DR3 195632152560621440 inside SNR G166.0 + 4.3 was found to be an evolved A3 type star that cannot be the pre-SN binary companion to the progenitor. Gaia DR3 513927750767375872 inside SNR HB 3 is the only OB-runaway candidate (2D space velocity of 33.3 ± 3.3 km s−1). Based on photometric study, the star can be an early B-type main-sequence star with a slightly higher extinction relative to the SNR; however, it might instead be an evolved A-type star at the same distance. The other 11 SNRs do not host an OB-runaway star within their central regions. Although the runaway search was performed in a large interval of distance and extinction, we also estimated distances to the SNRs. [ABSTRACT FROM AUTHOR]

Stellar mergers and accretion events have been crucial in shaping the evolution of the Milky Way (MW). These events have been dynamically identified and chemically characterized using red giants and main-sequence stars. RR Lyrae (RRL) variables can play a crucial role in tracing the early formation of the MW since they are ubiquitous, old (t ≥ 10 Gyr) low-mass stars and accurate distance indicators. We exploited Data Release 3 of the GALAH survey to identify 78 field RRLs suitable for chemical analysis. Using synthetic spectra calculations, we determined atmospheric parameters and abundances of Fe, Mg, Ca, Y, and Ba. Most of our stars exhibit halo-like chemical compositions, with an iron peak around [Fe/H] ≈ −1.40, and enhanced Ca and Mg content. Notably, we discovered a metal-rich tail, with [Fe/H] values ranging from −1 to approximately solar metallicity. This sub-group includes almost 1/4 of the sample, it is characterized by thin disc kinematics and displays sub-solar α-element abundances, marginally consistent with the majority of the MW stars. Surprisingly, they differ distinctly from typical MW disc stars in terms of the s-process elements Y and Ba. We took advantage of similar data available in the literature and built a total sample of 535 field RRLs for which we estimated kinematical and dynamical properties. We found that metal-rich RRLs (1/3 of the sample) likely represent an old component of the MW thin disc. We also detected RRLs with retrograde orbits and provided preliminary associations with the Gaia–Sausage–Enceladus, Helmi, Sequoia, Sagittarius, and Thamnos stellar streams. [ABSTRACT FROM AUTHOR]

Context. Stars that are found on the blue horizontal-branch (BHB) evolved from low-mass stars that have completed their core hydrogen-burning main sequence (MS) stage and undergone the helium flash at the end of their red giant phase. Hence, they are very old objects that can be used as markers in studying galactic structure and formation history. The fact that their luminosity is virtually constant at all effective temperatures also makes them good standard candles. Aims. We have compiled a catalogue of BHB stars with stellar parameters calculated from spectral energy distributions (SEDs) constructed using data from multiple large-scale photometric surveys. In addition, we update our previous Gaia Early Data Release 3 (EDR3) catalogue of BHB stars with parallax errors less than 20% by using the SED results to define the selection criteria. The purpose of these catalogues is to create a set of BHB star candidates with reliable stellar parameters. In addition, they provide a more complete full-sky catalogue with candidate objects found along the whole BHB from where RR-Lyrae are found on the instability strip to the extreme horizontal-branch (EHB). Methods. We selected a large dataset of Gaia Data Release 3 (DR3) objects based only on their position on the colour-magnitude diagram (CMD), along with the tangential velocity and parallax errors. The SEDs were then used to evaluate contamination levels in the dataset and derive optimised data quality acceptance constraints. This allowed us to extend the Gaia DR3 colour and absolute magnitude criteria further towards the EHB. The level of contamination found using SED analysis was confirmed by acquiring spectra using the Ondrejov Echelle spectrograph, attached to the Perek 2m telescope at the Astronomical Institute of the Czech Academy of Sciences. Results. We present a catalogue of 9172 Galactic halo BHB candidate stars with atmospheric and stellar parameters calculated from synthetic SEDs. We also present an extended Gaia DR3-based catalogue of 22 335 BHB candidate stars with a wider range of effective temperatures and Gaia DR3 parallax errors of less than 20%. This represents an increase of 33% compared to the our 2021 catalogue, with a contamination level of 10%. [ABSTRACT FROM AUTHOR]

The kinematics of the ϵ Cha young stellar association close to the Sun has been studied based on a list of candidate stars from the Dickson-Vandervelde et al. work. The working sample consists of 26 stars with parallaxes, proper motions from the Gaia DR3 catalog and radial velocities taken from literary sources. The orbits of the stars back to the past were constructed, and the moment when the association had a minimum spatial size was determined, as well as an analysis of the dependencies of the velocities on the coordinates was carried out. It is shown that the initial sample is divided into two parts with different kinematic properties. The first sample included 9 stars. Based on the construction of the orbits of these 9 stars, an age estimate of million years was obtained. An expansion coefficient in the plane with the value km/s/kpc was also found for them, on the basis of which another age estimate Ma was obtained. The second sample included 17 stars. The construction of their orbits gave an estimate of age Ma, and based on the gradient 248 km/s/kpc, a second estimate of their age Ma was obtained. This suggests that the Cha association either consists of two groupings of different ages, or a younger one arose as a result of a recent outbreak of star formation within a common star system. The question of the gravitational connection of the groupings has not been considered in the framework of this work. [ABSTRACT FROM AUTHOR]

The authors have proposed a model geometry for the interior of a regular black hole (BH) mimicker, the frozen star, whose most startling feature is that each spherical shell in its interior is a surface of infinite redshift. The geometry is a solution of the Einstein equations which is sourced by an exotic matter with maximally negative radial pressure. The frozen star geometry is previously presented in singular coordinates for which −gtt$-g_{tt}$ and grr$g^{rr}$ vanish in the bulk and connect smoothly to the Schwarzschild exterior. Additionally, the geometry is mildly singular in the center of the star. Here, the authors present regular coordinates for the entirety of the frozen star. Each zero in the metric is replaced with a small, dimensionless parameter ε; in both −gtt$-g_{tt}$ and grr$g^{rr}$ thus maintaining maximally negative radial pressure. The authors also regularize the geometry, energy density and pressure in the center of the star in a smooth way. The frozen star solution presented here is a completely regular solution of Einstein's equations whose compactness is arbitrarily close to that of a Schwarzschild BH. It obeys the null energy condition, the universally agreed‐upon energy condition, and it is free of any known pathology. As far as it is known, this is the first solution that obeys all of these constraints and, in addition, as will be shown in a future publication, can mimic all of the thermodynamic properties of a standard BH. Our initial analysis uses Schwarzschild‐like coordinates and applies the Killing equations to show that an infalling, point‐like object will move very slowly, effectively sticking to the surface of the star and never coming out. If one nevertheless follows the trajectory of the object into the interior of the star, it moves along an almost‐radial trajectory until it comes within a small distance from the star's center. Once there, if the object has any amount of angular momentum at all, it will be reflected outwards by a potential barrier onto a different almost‐radial trajectory. Finally, using Kruskal‐like coordinates, the causal structure of the regularized frozen star and discuss its ε→0$\varepsilon \rightarrow 0$ limit, for which the geometry degenerates and becomes effectively two dimensional is considered. [ABSTRACT FROM AUTHOR]

The document titled "Exoplanet Division Meeting, 2023 September 30: Variations on an exoplanet theme" provides an update on the Asteroids & Remote Planets Section Exoplanet Division Meeting held on September 30, 2023. The meeting aimed to review the variations in exoplanet transits and demonstrate how these variations can be measured to infer possible causes. Presentations were given on topics such as the effects of stellar activity on transit timings, the causes of transit-timing variations, and the potential habitability of transiting exoplanets. Various software packages for analyzing exoplanet data were also discussed. The document concludes with a list of new members welcomed by the British Astronomical Association. [Extracted from the article]

Astrometry, the measurement of accurate star positions and motions, was first carried out from space by the European Space Agency's Hipparcos mission in the 1990s. Today, it is undergoing a particularly dramatic advance with ESA's ongoing Gaia mission, launched in 2013. I explain why star positions are of such importance in astronomy, and outline the 2000-year history of the field. This illustrates the profound scientific insights that have been gained over the past centuries as measurement accuracies have advanced, as well as the technical and computational challenges involved. I explain the reasons why measurements from space became necessary in order to advance the field, and outline the measurement principles underpinning these two space mission. I conclude with a summary of the contents of the latest Gaia catalogue release, list some of the scientific highlights that have been revealed by Gaia so far and, in the process, demonstrate how these measurements are revolutionising our understanding of the origin, structure, and evolution of our Galaxy. [ABSTRACT FROM AUTHOR]

Up to now and probably for still a long time, the only support of information used to detect exoplanets has been the analysis of light, either visible or infrared. In the vast majority of cases it is the light from a star and not the light from the planet itself which is used, because the huge contrast in brightness between the star and a planet orbiting it as well as the extremely short angular distance between them makes direct imaging a real challenge. It is then a subtle effect detected on the starlight that in general indicates the planet's presence and provides information on some of its characteristics: mass, radius, distance to the star, temperature, etc. As an introduction to the different contributions appearing in this volume, this article proposes a kind of brief review of the various methods imagined by astronomers to exploit one of the properties of the light to succeed in detecting and characterizing exoplanets. We'll show that even direct detection became a reality and contributes to the more than 5000 exoplanets detected today. [ABSTRACT FROM AUTHOR]

The optical spectra of the B-supergiant LS III +52°24 (IRAS 22023+5249) obtained at the 6-m BTA telescope with a resolution in 2010–2021 revealed signs of wind variability and velocity stratification in the extended atmosphere. The neutral hydrogen lines H and H have a P Cyg profile; their wind absorption component changes position in the range from to km/s. The H emission intensity reaches record-high values with respect to the local continuum: . The stationary radial velocity according to the positions of symmetric forbidden emissions and pemitted metal emissions is taken as the systemic velocity km/s. For the first time for this star, a time variability of the radial velocity in the range from to km/s was found based on the positions of absorptions of NII and OII ions, which indicates the possible presence of a companion and/or atmospheric pulsations. The variability of the oxygen triplet profile (OI 7774) due to the appearance of unstable emission was recorded. The set of interstellar absorptions of the NaI D-line profile in the range from to km/s is formed in the Local Arm and subsequent arms of the Galaxy. The distance to the star, kpc, combined with the high velocity indicates that the star is located in the interarm space of the Galaxy beyond the Scutum–Crux arm. [ABSTRACT FROM AUTHOR]

We present a method to estimate distances to AGB stars, utilizing the rich infrared data sets available for these infrared-bright targets. The method is based on the assumption that stars with intrinsically similar properties (metallicity, initial mass, etc.) produce similar spectral energy distributions (SEDs) and similar luminosities. We here discuss the results for AGB stars belonging to the BAaDE survey sample whose distances were calibrated using the template SEDs of stars with their VLBI parallaxes. As VLBI parallaxes are only known for a handful of sources, the resulting templates only cover a small subset of the BAaDE sample. Additional methods to derive suitable templates will therefore also be required. The work on expanding the template set is promising, although more fine tuning is still needed. [ABSTRACT FROM AUTHOR]

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 . The H line has a P Cyg profile whose absorption component ( km s ) 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 km s : , , , and km s . A lot of low-excitation metal lines have an inverse P Cyg profile. The mean positions of their emission components, km s , 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, km s , and circumstellar, km s , components and the component forming in the upper atmospheric layers, km s . The mean velocity from 20–30 diffuse interstellar bands (DIBs) identified in the spectra, km s , agrees with the velocity from interstellar Na I and K I components. The equivalent width of the oxygen triplet Å corresponds to an absolute magnitude of the star , which, given the total (interstellar circumstellar) extinction, leads to a distance to the star kpc. [ABSTRACT FROM AUTHOR]

Large pristine samples of red clump stars are highly sought after given that they are standard candles and give precise distances even at large distances. However, it is difficult to cleanly select red clumps stars because they can have the same Teff and log g as red giant branch stars. Recently, it was shown that the asteroseismic parameters, $\rm {\Delta }$ P and $\rm {\Delta \nu }$ , which are used to accurately select red clump stars, can be derived from spectra using the change in the surface carbon to nitrogen ratio ([C/N]) caused by mixing during the red giant branch. This change in [C/N] can also impact the spectral energy distribution. In this study, we predict the $\rm {\Delta }$ P, $\rm {\Delta \nu }$ , Teff, and log g using 2MASS, AllWISE, Gaia, and Pan-STARRS data in order to select a clean sample of red clump stars. We achieve a contamination rate of ∼20 per cent, equivalent to what is achieved when selecting from Teff and log g derived from low-resolution spectra. Finally, we present two red clump samples. One sample has a contamination rate of ∼20 per cent and ∼405 000 red clump stars. The other has a contamination of ∼33 per cent and ∼2.6 million red clump stars that includes ∼75 000 stars at distances >10 kpc. For |b| > 30 deg, we find ∼15 000 stars with contamination rate of ∼9 per cent. The scientific potential of this catalogue for studying the structure and formation history of the Galaxy is vast, given that it includes millions of precise distances to stars in the inner bulge and distant halo where astrometric distances are imprecise. [ABSTRACT FROM AUTHOR]

The systematic offset of Gaia parallaxes has been widely reported with Gaia 's second data release, and it is expected to persist in future Gaia data. In order to use Gaia parallaxes to infer distances to high precision, we develop a hierarchical probabilistic model to determine the Gaia parallax zero-point offset along with the calibration of an empirical model for luminosity of red clump stars by combining astrometric and photometric measurements. Using a cross-matched sample of red clump stars from the Apache Point Observatory Galactic Evolution Experiment and Gaia Data Release 2 (DR2), we report the parallax zero-point offset in DR2 to be |$\varpi _0 = -48 \pm 1\,\mu \text{as}$|⁠. We infer the red clump absolute magnitude to be −1.622 ± 0.004 in Ks , 0.435 ± 0.004 in G , −1.019 ± 0.004 in J , and −1.516 ± 0.004 in H. The intrinsic scatter of the red clump is ∼0.09 mag in J, H , and Ks , or ∼ 0.12 mag in G. We tailor our models to accommodate more complex analyses such as investigating the variations of the parallax zero-point with each source's observed magnitude, observed colour, and sky position. In particular, we find fluctuations of the zero-point across the sky to be of order or less than a few 10s of |$\mu \text{as}$|⁠. [ABSTRACT FROM AUTHOR]

Context. The interaction between low-mass companions and the debris discs they reside in is still not fully understood. A debris disc can evolve due to self-stirring, a process in which planetesimals can excite their neighbours to the point of destructive collisions. In addition, the presence of a companion could further stir the disc (companion-stirring). Additional information is necessary to understand this fundamental step in the formation and evolution of a planetary system, and at the moment of writing only a handful of systems are known where a companion and a debris disc have both been detected and studied at the same time. Aims. Our primary goal is to augment the sample of these systems and to understand the relative importance between self-stirring and companion-stirring. Methods. In the course of the VLT/NaCo Imaging Survey for Planets around Young stars (ISPY), we observed HD 193571, an A0 debris disc hosting star at a distance of 68 pc with an age between ∼60 and 170 Myr. We obtained two sets of observations in L′ band and a third epoch in H band using the GPI instrument at Gemini-South. Results. A companion was detected in all three epochs at a projected separation of ∼11 au (∼0.17″), and co-motion was confirmed through proper motion analysis. Given the inferred disc size of 120 au, the companion appears to reside within the gap between the host star and the disc. Comparison between the L′ and H band magnitude and evolutionary tracks suggests a mass of ∼0.31 − 0.39 M⊙. Conclusions. We discovered a previously unknown M-dwarf companion around HD 193571, making it the third low-mass stellar object discovered within a debris disc. A comparison to self- and companion-stirring models suggests that the companion is likely responsible for the stirring of the disc. [ABSTRACT FROM AUTHOR]

Asteroseismology provides fundamental properties (mass, radius and effective temperature) of solar-like oscillating stars using so-called scaling relations. These properties allow the computation of the asteroseismic distance of stars. We compare the asteroseismic distances with the recently released Gaia distances for 74 stars studied in Yıldız et al. There is a very good agreement between these two distances; for 64 of these stars, the difference is less than 10 per cent. However, a systematic difference is seen if we use the effective temperature obtained by spectroscopic methods; the Gaia distances are about 5 per cent greater than the asteroseismic distances. [ABSTRACT FROM AUTHOR]

Highlights: [•] We obtained BVgri photometry of 100,000 stars over the Aquarius stream sky area. [•] 15 separate observing epochs were used in looking for RR Lyr variables. [•] 71 RR Lyr variables were discovered providing a census complete to V=15.2 or 8kpc. [•] Pulsation periods, temperatures, lightcurves, magnitudes and distances are given. [ABSTRACT FROM AUTHOR]

We present the discovery of a very cold, very low mass, nearby brown dwarf using data from the NASA Wide-field Infrared Survey Explorer (WISE). The object, WISE J064723.23–623235.5, has a very red WISE color of W1–W2 > 3.77 mag and a very red Spitzer Space Telescope color of ch1–ch2 = 2.82 ± 0.09 mag. In JMKO –ch2 color (7.58 ± 0.27 mag) it is one of the two or three reddest brown dwarfs known. Our grism spectrum from the Hubble Space Telescope (HST) confirms it to be the seventeenth Y dwarf discovered, and its spectral type of Y1 ± 0.5 makes it one of the four latest-type Y dwarfs classified. Astrometric imaging from Spitzer and HST, combined with data from WISE, provides a preliminary parallax of π = 115 ± 12 mas (d = 8.7 ± 0.9 pc) and proper motion of μ = 387 ± 25 mas yr–1 based on 2.5 yr of monitoring. The spectrum implies a blue J–H color, for which model atmosphere calculations suggest a relatively low surface gravity. The best fit to these models indicates an effective temperature of 350-400 K and a mass of ∼5-30 MJup. Kinematic analysis hints that this object may belong to the Columba moving group, which would support an age of ∼30 Myr and thus an even lower mass of <2 MJup, but verification would require a radial velocity measurement not currently possible for a J = 22.7 mag brown dwarf. [ABSTRACT FROM AUTHOR]

We present the methods and first results of a survey of nearby high proper motion main-sequence stars to probe for cool companions with the Gemini camera at Lick Observatory. This survey uses a sample of old (age > 2 Gyr) stars as targets to probe for companions down to temperatures of 500 K. Multi-epoch observations allow us to discriminate comoving companions from background objects. So far, our survey has successfully rediscovered the wide T8.5 companion to GJ 1263 and has discovered a companion to the nearby M0V star GJ 660.1. The companion to GJ 660.1 (GJ 660.1B) is ~4 mag fainter than its host star in the J-band and is located at a projected separation of ~ 120 AU. Known trigonometric parallax and Two Micron All Sky Survey magnitudes for the GJ 660.1 system indicate a spectral type for the companion of M9 ± 2. [ABSTRACT FROM AUTHOR]

The article explores the authorship of an astronomical manuscript most likely composed by English astronomer Jeremiah Horrocks in the 17th century. The author describes Horrocks' astronomical observations as outlined in the manuscript, with emphasis given to his work studying the distance between Mercury and the stars Lanx borea and Lanx austrina. The work described in the manuscript is then related to Horrocks' calculation of ephemerides. The manuscript was also largely critical of the astronomical calculations of German astronomer Philip van Lansberge related to stellar parallax.

Galileo found the Copernican heliocentric theory of the universe so persuasive owing to its mathematical elegance that he embraced it even when his theory of the tides stood in opposition to it. Further support for Galileo’s deep commitment to the Copernican heliocentric theory is found in his recently discovered unpublished observations of the double star Mizar in 1617, which exhibited no annual stellar parallax and hence indicated that the Earth does not move, in contradiction to the Copernican heliocentric theory. Further, Galileo did not mention this contradiction in his Dialogue Concerning the Two Chief World Systems of 1632. I conclude that he was so deeply committed to the Copernican heliocentric theory that he was unswayed even when observations undermined it, and I suggest that if he had published his observations on the double star Mizar, general acceptance of the Copernican heliocentric theory by astronomers would have been delayed even more than it was. [ABSTRACT FROM AUTHOR]

We report the discovery of 21 hitherto unknown bright southern ultracool dwarfs with spectral types in the range M7 to L5.5, together with new observations of a further three late M dwarfs previously confirmed. Three more objects are already identified in the literature as high proper motion stars; we derive their spectral types for the first time. All objects were selected from the Two Micron All-Sky Survey (2MASS) all-sky and SuperCOSMOS point source data bases on the basis of their optical/near-infrared colours, J-band magnitudes and proper motions. Low-resolution JH spectroscopy with the European Southern Observatory (ESO)/New Technology Telescope (NTT) Son of Isaac (SOFI) spectrograph has confirmed the ultracool nature of 24 targets, out of a total of 25 candidates observed. Spectral types are derived by direct comparison with template objects and compared to results from H2O and FeH indices. We also report the discovery of one binary, as revealed by SOFI acquisition imaging; spectra were taken for both components. The spectral types of the two components are L2 and L4 and the distance ∼19 pc. Spectroscopic distances and transverse velocities are derived for the sample. Two ∼L5 objects lie only at ∼0 pc distant. Such nearby objects are excellent targets for further study to derive their parallaxes and to search for fainter, later companions with adaptive optics and/or methane imaging. [ABSTRACT FROM AUTHOR]

We present an analysis of the photometric parallaxes of stars in 21 of the Beijing–Arizona–Taiwan–Connecticut Survey fields carried out with the National Astronomical Observatories (NAOC) 60/90 cm Schmidt Telescope in 15 intermediate-band filters from 3000 to 10 000 Å. In this study, we have adopted a three-component (thin disc, thick disc and halo) model to analyse star count information. By calculating the stellar space density as a function of distance from the Galactic plane, we determine that the range of scaleheight for the thin disc varies from 220 to 320 pc. Although 220 pc seems an extreme value, it is close to the lower limit in the literature. The range of scaleheight for the thick disc is from 600 to 1100 pc, and the corresponding space number density normalization is 7.0–1.0 per cent of the thin disc. We find that the scaleheight of the disc may be variable with the observed direction, which cannot simply be attributed to statistical errors. Possibly the main reasons can be attributed to the disc (mainly the thick disc) being flared, with a scaleheight increasing with radius. The structure is consistent with the merger origin for the thick disc formation. Adopting a de Vaucouleurs law halo, we also find that the axis ratio towards the Galactic Centre is somewhat flatter (∼0.4), while the shape of the halo in the anticentre and antirotation direction is rounder with . Our results show that star counts in different lines of sight can be used directly to obtain a rough estimate of the shape of the stellar halo. Our solutions support the Galactic models with a flattened inner halo, possibly formed by an early merger in the Galaxy's history. [ABSTRACT FROM AUTHOR]

The first charge-coupled device photometric study in the area of the doubtful open cluster NGC 2129 is presented. Photometry of a field offset 15 arcmin northwards is also provided, to probe the Galactic disc population towards the cluster. Using star counts, proper motions from the UCAC2 catalogue, colour–magnitude and colour–colour diagrams, we demonstrate that NGC 2129 is a young open cluster. The cluster radius is 2.5 arcmin, and across this region we find evidence of significant differential reddening, although the reddening law seems to be normal towards its direction. Updated estimates of the cluster fundamental parameters are provided. The mean reddening is found to be and the distance modulus is . Hence, NGC 2129 is located at 2.2 ± 0.2 kpc from the Sun inside the Local spiral arm. The age derived from 37 photometrically selected members is estimated to be approximately 10 Myr. These stars are used to provide new estimates of the cluster absolute proper-motion components. [ABSTRACT FROM AUTHOR]

The central stars of highly evolved planetary nebulae (PNe) are expected to have closely similar absolute visual magnitudesMV. This enables us to determine approximate distances to these sources where one knows their central star visual magnitudes, and levels of extinction. We find that such an analysis implies values ofDwhich are similar to those determined by Phillips; Cahn, Kaler&Stanghellin; Acker, and Daub. However, our distances are very much smaller than those of Zhang; Bensby&Lundstrom, and van de Steene&Zijlstra. The reasons for these differences are discussed, and can be traced to errors in the assumed relation between brightness temperature and radius.Finally, we determine that the binary companions of such stars can be no brighter than, implying a spectral type of K0 or later in the case of main-sequence stars. [ABSTRACT FROM AUTHOR]

The accurate determination of galaxy relative distances is extremely important for the empirical calibration of the uncertain metallicity dependence of some standard candles like Cepheid stars, or for studying the galaxy space distribution and peculiar velocities. Here we have investigated the reliability of the widely usedI-band Tip of the Red Giant Branch (TRGB) relative distances for a sample of Local Group galaxies with complex star formation histories (SFHs) and age–metallicity relationships (AMRs) namely the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC) and LGS3. The use of theKband is also discussed. By employing theoretical stellar population synthesis techniques, we find that using actual determinations of SFH and AMR of the LMC and SMC, their RGB is populated by stars much younger (by∼9 Gyr) than the Galactic globular cluster counterparts, on which theI-band (andK-band) TRGB absolute magnitude is calibrated. This age difference induces a bias in both the photometric metallicity estimates based on the comparison of RGB colours with globular cluster ones, and the TRGB distances. The extent of the distance bias– which is not influenced by the actual value of the TRGB absolute magnitude zero-point– is strongly dependent on the specific TRGB technique applied, and on the assumedI-band bolometric correctionscale adopted; the correction to apply to the SMC–LMC distance modulus ranges from 0 up to+0.10 mag. LGS3 is an example of a galaxy populated mainly by old stars, so that photometric metallicity and distance estimates using globular cluster calibrations are reliable. However, the relative distance moduli between the Magellanic Clouds and LGS3 are affected by the population effects discussed for the LMC and SMC. The correction to apply to the LGS3–LMC distance modulus ranges between−0.05 and+0.14 mag, whereas in the case of the LGS3–SMC distance modulus it goes from−0.07 to+0.04 mag. In the case of all three relative distances discussed before, the correction to apply to theK-band TRGB distances are larger than theI-band case.Our results clearly show that the presence of a well-developed RGB in the colour– magnitude diagram of a stellar system with a complex SFH does not guarantee that it is populated by globular cluster-like red giants, and therefore the TRGB method for distance determination has to be applied with caution. A definitive assessment of the appropriate corrections for population effects on TRGB distances has, however, to wait for a substantial reduction in the uncertainties on thescale for cold stars. [ABSTRACT FROM AUTHOR]

Explores on how astronomers measure the distance to stars and the roles of observation and interference in the development of scientific knowledge. Usability and significance of heliocentric parallax measurements on the determination of the distance to nearby stars; Units for measuring the distances to stars and galaxies; Description of a parsec and its advantages. INSETS: Two units for astronomical distances.;Helping students understand parallax..

Recent, and older accurate, data on (limb-darkened) angular diameters is compiled for 221 stars, as well asBVRIJK[12][25] magnitudes for those objects, when available. Nine stars (all M-giants or supergiants) showing excess in the[12–25] colour are excluded from the analysis as this may indicate the presence of dust influencing the optical and near-infrared colours as well. Based on this large sample, Baade–Wesselink surface brightness (SB) relations are presented for dwarfs, giants, supergiants and dwarfs in the optical and near-infrared. M-giants are found to follow different SB relations from non-M-giants, in particular inVversus. The preferred relation for non-M-giants is compared to the earlier relation by Fouqué and Gieren (based on 10 stars) and Nordgren et al. (based on 57 stars). Increasing the sample size does not lead to a lower rms value. It is shown that the residuals do not correlate with metallicity at a significant level. The finally adopted observed angular diameters are compared to those predicted by Cohen et al. for 45 stars in common, and there is reasonable overall, and good agreement whenmas. Finally, I comment on the common practice in the literature to average, and then fix, the zero-point of theVversusversusandKversusrelations, and then rederive the slopes. Such a common zero-point at zero colour is not expected from model atmospheres for thecolour and depends on gravity. Relations derived in this way may be biased. [ABSTRACT FROM AUTHOR]

Contact binaries can be used for distance determinations of stellar systems. They are easy to discover and identify and are very abundant among solar-type stars, particularly for MV>+3. The period–luminosity–colour (PLC) relations have similar properties to those for pulsating stars and can currently predict individual values of MV to about ±0.25 mag. [Copyright &y& Elsevier]

We present the results of a spectroscopic programme aiming to investigate the kinematics of the local interstellar medium components towards the Southern Coalsack and Chamaeleon–Musca dark clouds. The analysis is based upon high-resolution spectra of the interstellar Na i D absorption lines towards 63 B-type stars selected to cover these clouds and the connecting area defined by the Galactic coordinates: and . The radial velocities, column densities, velocity dispersions, colour excess and photometric distances to the stars are used to understand the kinematics and distribution of the interstellar cloud components. The analysis indicates that the interstellar gas is distributed in two extended sheet-like structures permeating the whole area, one at and another around 120–150 pc from the Sun. The nearby feature is approaching the local standard of rest with an average radial velocity of −7 km s−1, has low average column density and velocity dispersion . The more distant feature has column densities between , average velocity dispersion and seems associated with the dust sheet observed towards the Coalsack, Musca and Chamaeleon direction. Its velocity is centred around 0 km s−1, but there is a trend for increasing from −3 km s−1 near to 3 km s−1 near . The nearby low column density feature indicates a general outflow from the Sco-Cen association, in agreement with several independent lines of data in the general searched direction. The dust and gas feature around 120–150 pc seem to be part of an extended large-scale feature of similar kinematic properties, supposedly identified with the interaction zone of the Local and Loop I Bubbles. Assuming that the interface and the ring-like volume of dense neutral matter that would have been formed during the collision of the two bubbles have similar properties, our results suggest that the interaction zone between the bubbles is twisted and folded. [ABSTRACT FROM AUTHOR]