Non-local Thermodynamic Equilibrium Model Atmospheres Research Articles

Discovery of an extended G giant chromosphere in the 2019 eclipse of γ Per

The November 2019 eclipse of γ Per was a rare opportunity to seek evidence for a chromosphere of the G8 giant, hitherto suspected but not detected. Twenty-nine years after the only other observed eclipse, we aim to find chromospheric absorption in the strong Ca II H&K lines, and to determine its column densities and scale height. Using the Telescopio Internacional de Guanajuato Robótico-Espectroscópico (TIGRE) in Guanajuato (central Mexico) before, during and after the 8 days of total eclipse, we obtained good S/N spectra of the G8 giant alone and composite spectra of the partial phases, near eclipse and far from eclipse. In the near UV of the Ca II H&K and Hϵ lines, the G giant spectrum that was adequately scaled was subtracted from the composite spectra in partial phases, near and far from eclipse, to obtain the A3 companion spectra with and without traces of chromospheric absorption. In addition, we used PHOENIX full non-local thermodynamic equilibrium model atmospheres on the blue A star spectrum, iSpec spectral analysis of the red G giant spectrum, and evolution tracks to study both components of γ Per. For the first time, we present evidence for this rare type of a not very extended G giant chromosphere, reaching out about half of an A-star radius (~1.5 Gm) with a scale height of only 0.17 Gm. By its location in the Hertzsprung-Russell diagram, the γ Per G8 giant is very close to the onset of more extended chromospheres. Furthermore, we show that this giant has a rather inactive chromosphere, and a recent 5 ksec XMM pointing reveals only a very faint, low-energy corona. While the γ Per primary has a mass of ~3.6 M⊙, and its A3 companion has one of ~2.4 M⊙, the latter is too cool (8400 ± 300 K), which is too evolved on the main sequence to be the same age as the primary. The high eccentricity of the 5329.08 days long-period orbit may therefore be reminiscent of a rare capture event. Using the eclipse method, we resolve a pivotal case of a G giant chromosphere, which seems to represent a low-gravity analogue of the inactive Sun. A systematic change of giant chromospheric extent by Hertzsprung-Russell diagram position is confirmed. Compared to the solar chromosphere, the density scale height increases with gravity by ∝ ɡ−1.5.

New insight into the massive eccentric binary HD 165052: self-consistent orbital solution, apsidal motion, and fundamental parameters

ABSTRACT HD 165052 is a short-period massive eccentric binary system that undergoes apsidal motion. As the rate of apsidal motion is directly related to the internal structure constants of the binary components, its study allows getting insight into the internal structure of the stars. We use medium- and high-resolution spectroscopic observations of HD 165052 to provide constraints on the fundamental properties of the binary system and the evolutionary state of its components. We apply a spectral disentangling code to reconstruct artefact-free spectra of the individual stars and derive the radial velocities (RVs) at the times of the observations. We perform the first analysis of the disentangled spectra with the non-local thermodynamic equilibrium model atmosphere code CMFGEN to determine the stellar properties. We derive the first self-consistent orbital solution of all existing RV data, including those reported in the literature, accounting for apsidal motion. We build, for the very first time, dedicated stellar evolution tracks with the Clés code requesting the theoretical effective temperatures and luminosities to match those obtained from our spectroscopic analysis. The binary system HD 165052, consisting of an O6.5 V((f)) primary ($T_\text{eff,P}=37\, 500 \pm 1000$ K) and an O7 V((f)) secondary ($T_\text{eff,S}=36\, 000 \pm 1000$ K), displays apsidal motion at a rate of $(11.30^{+0.64}_{-0.49})^\circ$ yr−1. Evolutionary masses are compared to minimum dynamical masses to constrain the orbital inclination. Evolutionary masses $M_\text{ev,P}=24.8\pm 1.0\, \mathrm{M}_\odot$ and $M_\text{ev,S}=20.9\pm 1.0\, \mathrm{M}_\odot$ and radii $R_\text{ev,P}=7.0^{+0.5}_{-0.4}\, \mathrm{R}_\odot$ and $R_\text{ev,S}=6.2^{+0.4}_{-0.3}\, \mathrm{R}_\odot$ are derived, and the inclination is constrained to 22.1° ≤ i ≤ 23.3°. Theoretical apsidal motion rates, derived assuming an age of 2.0 ± 0.5 Myr for the binary, are in agreement with the observational determination. The agreement with theoretical apsidal motion rates enforces the inferred values of the evolutionary stellar masses and radii.

Discovery of O stars in the tidal Magellanic Bridge

The Magellanic Bridge, stretching between the Small and the Large Magellanic Cloud (SMC and LMC), is the nearest tidally stripped intergalactic environment. The Bridge has a significantly low average metallicity of Z ≲ 0.1 Z⊙. Here we report the first discovery of O-type stars in the Magellanic Bridge. Three massive O stars were identified thanks to the archival spectra obtained by the ESO’s Very Large Telescope FLAMES instrument. We analyze the spectra of each star using the Potsdam Wolf-Rayet (PoWR) non-local thermodynamic equilibrium model atmosphere code, which provides the physical parameters, ionizing photon fluxes, and surface abundances. The ages of the newly discovered O stars suggest that star formation in the Bridge is ongoing. Furthermore, the discovery of O stars in the Bridge implies that tidally stripped galactic tails containing low-density but highly dynamical gas are capable of producing massive O stars. The multi-epoch spectra indicate that all three O stars are binaries. Despite their spatial proximity to one another, these O stars are chemically distinct. One of them is a fast-rotating giant with nearly LMC-like abundances. The other two are main-sequence stars that rotate extremely slowly and are strongly metal depleted. We discover the most nitrogen-poor O star known to date. Taking into account the previous analyses of B stars in the Bridge, we interpret the various metal abundances as the signature of a chemically inhomogeneous interstellar medium (ISM), suggesting that the Bridge gas might have accreted during multiple episodes of tidal interaction between the Clouds. Attributing the lowest derived metal content to the primordial gas, the time of the initial formation of the Bridge may date back several billion years. Using the Gaia and Galex color-magnitude diagrams, we roughly estimate the total number of O stars in the Bridge and their total ionizing radiation. Comparing this with the energetics of the diffuse ISM, we find that the contribution of the hot stars to the ionizing radiation field in the Bridge is less than 10% and conclude that the main sources of ionizing photons are leaks from the LMC and SMC. We estimate a lower limit for the fraction of ionizing radiation that escapes from these two dwarf galaxies.

NLTE spectral analysis of the intermediate helium-rich subdwarf B star CPD−20°1123

ABSTRACT Subdwarf B (sdB) stars are core helium-burning stars with stratified atmospheres. Their atmospheres are dominated by hydrogen (H) while the helium (He) and metal abundances are shaped by an interplay of gravitational settling and radiative levitation. However, a small fraction of these show spectra dominated by He i absorption lines. In between these groups of He-deficient and extreme He-rich sdBs, some are found to have intermediate surface He abundances. These objects are proposed to be young ‘normal’ (He-deficient) sdBs for which the dynamical stratification of the atmosphere is still ongoing. We present an analysis of the optical spectrum of such an intermediate He-rich sdB, namely CPD−20°1123, by means of non-local thermodynamic equilibrium (NLTE) stellar atmosphere models. It has a He-to-H number ratio of He/H = 0.13 ± 0.05 and its effective temperature of $\mbox{$T_\mathrm{eff}$} = 25\, 500 \pm 1000 \, \mathrm{K}$ together with a surface gravity of $\log \, (g$ / cm s−2) = 5.3 ± 0.3 places the star close to the high-temperature edge until which it may be justified to use LTE model atmospheres. This work states a test of the Tübingen NLTE Model Atmosphere Package for this temperature regime. We present the first application of revised, elaborated model atoms of low ionization stages of light metals usable with this atmosphere code.

Stellar laboratories

Context.Accurate atomic data is an essential ingredient for the calculation of reliable non-local thermodynamic equilibrium (NLTE) model atmospheres that are mandatory for the spectral analysis of hot stars.Aims.We aim to search for and identify for the first time spectral lines of copper (atomic numberZ= 29) and indium (Z= 49) in hot white dwarf (WD) stars and to subsequently determine their photospheric abundances.Methods.Oscillator strengths of Cu IV–VIIwere calculated to include radiative and collisional bound-bound transitions of Cu in our NLTE model-atmosphere calculations. Oscillator strengths of In IV - VIwere compiled from the literature.Results.We newly identified 1 Cu IV, 51 Cu V, 2 Cu VI, and 5 In Vlines in the ultraviolet (UV) spectrum of DO-type WD RE 0503−289. We determined the photospheric abundances of 9.3 × 10−5(mass fraction, 132 times solar) and 3.0 × 10−5(56 600 times solar), respectively; we also found Cu overabundances in the DA-type WD G191−B2B (6.3 × 10−6, 9 times solar).Conclusions.All identified Cu IV-VIlines in the UV spectrum of RE 0503−289 were simultaneously well reproduced with our newly calculated oscillator strengths. With the detection of Cu and In in RE 0503−289, the total number of trans-iron elements (Z> 28) in this extraordinary WD reaches an unprecedented number of 18.

New Hot Subdwarf Stars Identified in Gaia DR2 with LAMOST DR5 Spectra. II.

Abstract Three hundred and eighty eight hot subdwarf stars have been identified by using the Hertzsprung–Russell (HR) diagram built from the second data release of the Gaia mission. By analyzing their observed Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) spectra, we characterized 186 sdB, 73 He-sdOB, 65 sdOB, 45 sdO, 12 He-sdO, and 7 He-sdB stars. The atmospheric parameters of these stars (e.g., T eff, logg, log(nHe/nH)) are obtained by fitting the hydrogen (H) and helium (He) line profiles with synthetic spectra calculated from non-local thermodynamic equilibrium model atmospheres. Among these stars, we have 135 new identified hot subdwarfs which have not been cataloged before. Although 253 stars appear in the catalog by Geier et al., only 91 of them have atmospheric parameters. Together with the 294 hot subdwarf stars found by Lei et al., we identified 682 hot subdwarf stars in total by using the Gaia HR-diagram and LAMOST spectra. These results demonstrate the efficiency of our method to combine large surveys to search for hot subdwarf stars. We found a distinct gap in our He-sdOB stars based on their He abundance, which is also presented in extreme horizontal branch (EHB) stars of the globular cluster ω Cen. The number fraction of the sample size for the two subgroups is very different between the two counterparts. However, the distinct gap between the H-sdB stars and He-sdOB stars in ω Cen is not visible in our sample. More interestingly, the He-sdB population with the highest He abundance in our sample is completely missing in ω Cen. The discrepancy between our field hot subdwarf stars and the EHB stars in ω Cen indicate different origins for the two counterparts.

Two’s company, three’s a crowd: SALT reveals the likely triple nature of the nucleus of the extreme abundance discrepancy factor planetary nebula Sp 3

Abstract The substantial number of binary central stars of planetary nebulae (CSPNe) now known ( $\sim 50$ ) has revealed a strong connection between binarity and some morphological features including jets and low-ionisation structures. However, some morphological features and asymmetries might be too complex or subtle to ascribe to binary interactions alone. In these cases, a tertiary component, that is, a triple nucleus, could be the missing ingredient required to produce these features. The only proven triple, NGC 246, is alone insufficient to investigate the shaping role of triple nuclei, but one straightforward way to identify more triples is to search for binaries in nuclei with known visual companions. Here we demonstrate this approach with the SALT HRS (High Resolution Spectrograph on the Southern African Large Telescope) discovery of a 4.81-d orbital period in the CSPN of Sp 3 which has a visual companion 0.31 arcsec away. The spectroscopic distance of the visual companion is in agreement with distance estimates to the nebula, the Gaia DR2 parallax of the central star, and the gravity distance of the central star. This supports a physical association between the visual companion and the inner 4.81 d binary, making the nucleus of Sp 3 a likely triple. We determine $T_\mathrm{eff}=68^{+12}_{-6}\ \text{kK},\ \log g=4.6\pm0.2\ \text{cm s}^{-2}$ , and $v_\mathrm{rot}=80\pm20\ \text{km s}^{-1}$ for the primary from non-local thermodynamic equilibrium model atmosphere analysis. The peculiar nebula presents an apparent bipolar morphology, jets, and an unexpected ‘extreme’ oxygen abundance discrepancy factor (adf) of $24.6^{+4.1}_{-3.4}$ . The adf is inconsistent with the purported trend for longer orbital period post-common-envelope (CE) PNe to exhibit normal adfs, further highlighting the dominant influence of selection effects in post-CE PNe. Lastly, the Type I nebular abundances of Sp 3, whose origin is often attributed to more massive progenitors, are incongruous with the likely Galactic Thick Disk membership of Sp 3, possibly suggesting that rotation and binarity may play an important role in influencing the AGB nucleosynthesis of PNe.

Search for trans-iron elements in hot, helium-rich white dwarfs with the HST Cosmic Origins Spectrograph

The metal abundances in the atmospheres of hot white dwarfs (WDs) entering the cooling sequence are determined by the preceding Asymptotic Giant Branch (AGB) evolutionary phase and, subsequently, by the onset of gravitational settling and radiative levitation. In this paper, we investigate three hot He-rich WDs, which are believed to result from a late He-shell flash. During such a flash, the He-rich intershell matter is dredged up and dominates the surface chemistry. Hence, in contrast to the usual H-rich WDs, their spectra allow direct access to s-process element abundances in the intershell that were synthesized during the AGB stage. In order to look for trans-iron group elements (atomic number Z > 29), we performed a non-local thermodynamic equilibrium model atmosphere analysis of new ultraviolet spectra taken with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. One of our program stars is of PG 1159 spectral type; this star, PG 1707+427, has effective temperature Teff = 85 000 K, and surface gravity logg = 7.5. The two other stars are DO white dwarfs: WD 0111+002 has Teff = 58 000 K and log g = 7.7, and PG 0109+111 has Teff = 70 000 K and log g = 8.0. These stars trace the onset of element diffusion during early WD evolution. While zinc is the only trans-iron element we could detect in the PG 1159 star, both DOs exhibit lines from Zn, Ga, Ge, Se; one additionally exhibits lines from Sr, Sn, Te, and I and the other from As. Generally, the trans-iron elements are very abundant in the DOs, meaning that radiative levitation must be acting. Most extreme is the almost six orders of magnitude oversolar abundance of tellurium in PG 0109+111. In terms of mass fraction, it is the most abundant metal in the atmosphere. The two DOs join the hitherto unique hot DO RE 0503−289, in which 14 trans-iron elements had even been identified.

Spectral analysis of four surprisingly similar hot hydrogen-rich subdwarf O stars

Context. Post-extreme horizontal branch stars (post-EHB) are helium-shell burning objects evolving away from the EHB and contracting directly towards the white dwarf regime. While the stars forming the EHB have been extensively studied in the past, their hotter and more evolved progeny are not so well characterized. Aims. We perform a comprehensive spectroscopic analysis of four such bright sdO stars, namely Feige 34, Feige 67, AGK+81°266, and LS II+18°9, among which the first three are used as standard stars for flux calibration. Our goal is to determine their atmospheric parameters, chemical properties, and evolutionary status to better understand this class of stars that are en route to become white dwarfs. Methods. We used non-local thermodynamic equilibrium model atmospheres in combination with high quality optical and UV spectra. Photometric data were also used to compute the spectroscopic distances of our stars and to characterize the companion responsible for the infrared excess of Feige 34. Results. The four bright sdO stars have very similar atmospheric parameters with Teff between 60 000 and 63 000 K and log g (cm s-2) in the range 5.9 to 6.1. This places these objects right on the theoretical post-EHB evolutionary tracks. The UV spectra are dominated by strong iron and nickel lines and suggest abundances that are enriched with respect to those of the Sun by factors of 25 and 60. On the other hand, the lighter elements, C, N, O, Mg, Si, P, and S are depleted. The stars have very similar abundances, although AGK+81°266 shows differences in its light element abundances. For instance, the helium abundance of this object is 10 times lower than that observed in the other three stars. All our stars show UV spectral lines that require additional line broadening that is consistent with a rotational velocity of about 25 km s-1. The infrared excess of Feige 34 is well reproduced by a M0 main-sequence companion and the surface area ratio of the two stars suggests that the system is a physical binary. However, the lack of radial velocity variations points towards a low inclination and/or long orbital period. Spectroscopic and Hipparcos distances are in good agreement for our three brightest stars. Conclusions. We performed a spectroscopic analysis of four hot sdO stars that are very similar in terms of atmospheric parameters and chemical compositions. The rotation velocities of our stars are significantly higher than what is observed in their immediate progenitors on the EHB, suggesting that angular momentum may be conserved as the stars evolve away from the EHB.

The far-ultraviolet spectra of two hot PG 1159 stars

PG1159 stars are hot, hydrogen-deficient (pre-) white dwarfs with atmospheres mainly composed of helium, carbon, and oxygen. The unusual surface chemistry is the result of a late helium-shell flash. Observed element abundances enable us to test stellar evolution models quantitatively with respect to their nucleosynthesis products formed near the helium-burning shell of the progenitor asymptotic giant branch stars. Because of the high effective temperatures (Teff), abundance determinations require ultraviolet spectroscopy and non-local thermodynamic equilibrium model atmosphere analyses. Up to now, we have presented results for the prototype of this spectral class and two cooler members (Teff in the range 85,000-140,000 K). Here we report on the results for two even hotter stars (PG1520+525 and PG1144+005, both with Teff = 150,000 K) which are the only two objects in this temperature-gravity region for which useful far-ultraviolet spectra are available, and revisit the prototype star. Previous results on the abundances of some species are confirmed, while results on others (Si, P, S) are revised. In particular, a solar abundance of sulphur is measured in contrast to earlier claims of a strong S deficiency that contradicted stellar evolution models. For the first time, we assess the abundances of Na, Al, and Cl with newly constructed non-LTE model atoms. Besides the main constituents (He, C, O), we determine the abundances (or upper limits) of N, F, Ne, Na, Al, Si, P, S, Cl, Ar, and Fe. Generally, good agreement with stellar models is found.

Stellar laboratories

For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. New of Kr IV - VII oscillator strengths for a large number of lines allow to construct more detailed model atoms for our NLTE model-atmosphere calculations. This enables us to search for additional Kr lines in observed spectra and to improve Kr abundance determinations. We calculated Kr IV - VII oscillator strengths to consider radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Kr lines exhibited in high-resolution and high-S/N ultraviolet (UV) observations of the hot white dwarf RE 0503-289. We reanalyzed the effective temperature and surface gravity and determined Teff = 70 000 +/- 2000 K and log (g / cm/s**2) = 7.5 +/- 0.1. We newly identified ten Kr V lines and one Kr VI line in the spectrum of RE 0503-289. We measured a Kr abundance of -3.3 +/- 0.3 (logarithmic mass fraction). We discovered that the interstellar absorption toward RE 0503-289 has a multi-velocity structure within a radial-velocity interval of -40 km/s < vrad < +18 km/s. Reliable measurements and calculations of atomic data are a prerequisite for state-of-the-art NLTE stellar-atmosphere modeling. Observed Kr V - VII line profiles in the UV spectrum of the white dwarf RE 0503-289 were simultaneously well reproduced with our newly calculated oscillator strengths.

SERENDIPITOUS DETECTION OF X-RAY EMISSION FROM THE HOT BORN-AGAIN CENTRAL STAR OF THE PLANETARY NEBULA K 1-16

We report the serendipitous detection of point-like X-ray emission from the hot, PG1159-type central star of the planetary nebula (CSPN) K 1-16 by the XMM-Newton and Chandra X-Ray Observatories. The CSPN lies superimposed on a galaxy cluster that includes an X-ray-bright quasar, but we have successfully isolated the CSPN X-ray emission from the strong diffuse background contributed by the quasar and intracluster gas. We have modeled the XMM-Newton and Chandra X-ray data, taking advantage of the contrasting detection efficiencies of the two observatories to better constrain the low-energy spectral response of Chandra's Advanced CCD Imaging Spectrometer. We find that the CSPN X-ray spectrum is well characterized by the combination of a non-local thermodynamic equilibrium model atmosphere with T ~ 135 kK and a carbon-rich, optically thin thermal plasma with T X ~ 1 MK. These results for X-ray emission from the K 1-16 CSPN, combined with those obtained for other PG1159-type objects, lend support to the born-again scenario for Wolf-Rayet and PG1159 CSPNe, wherein a late helium shell flash dredges up carbon-rich intershell material and ejects this material into the circumstellar environment.

Quantum Stark broadening data for the C iv, N v, O vi, F vii and Ne viii resonance doublets

In this paper, we present new Stark electron impact widths of the 2s–2p resonance doublets of the C IV ,N V ,O VI ,F VII and Ne VIII ions, obtained using our quantum mechanical method. The present results are required to develop future spectral analysis using non-local thermodynamic equilibrium model atmospheres, which can make the determination of photospheric properties more accurate. Results are presented for a large range of electron temperatures required for plasma modelling. These are compared to the available theoretical ones. They are also used to check the previously obtained semiclassical perturbation results, and very good agreement is found. The atomic structure and collision data, used for the calculations of line broadening for the five ions studied here, are also calculated and compared to available theoretical results. The agreement found between the two calculations ensures that our line-broadening procedure uses the adequate structure and collision data. Scalings of linewidths with effective charge Z are also studied.

THE UNUSUAL VARIABLE HOT B SUBDWARF LS IV–14°116

We first present the results of follow-up photometric observations of the He-rich hot B subdwarf LS IV?14?116, which confirm the presence of multiperiodic luminosity variations in the light curve of this star. Rather surprisingly, no other follow-up observations of this kind seem to have been published after the initial suggestion in 2005 that LS IV?14?116 could be a pulsating star of a new kind. We were able to extract from our data at least six significant periodicities ranging from 1954?s to 5084?s, including the two oscillations uncovered previously. We also present the results of an analysis combining a high signal-to-noise optical spectrum of LS IV?14?116 with recently developed non-local thermodynamic equilibrium model atmospheres and synthetic spectra. Our best estimates of the atmospheric parameters of this star are T eff = 34950 ? 250?K, log g = 5.93 ? 0.04, and log N(He)/N(H) = ?0.62 ? 0.03 (formal fitting errors only). These place LS IV?14?116 very near the region of maximum instability in the T eff-log g plane for short-period p-mode pulsators of the hot subdwarf type. If the luminosity variations are indeed due to pulsations, then LS IV?14?116 poses a real challenge to current theory: how can such long observed periods (which would have to be associated with medium- to high-order g-modes) be excited at such a high effective temperature and surface gravity, while the short-period p-modes, more typically excited in this domain, are not observed in this particular star?

A Spectroscopic Analysis of Blue Stragglers, Horizontal Branch Stars, and Turnoff Stars in Four Globular Clusters

We present a spectroscopic analysis of HST STIS and FOS low- and intermediate-resolution spectroscopy of 55 stars in four globular clusters (47 Tucanae, M3, NGC 6752, and NGC 6397). Stars hotter than Teff = 5750 K and with a signal-to-noise ratio larger than 15 were analyzed with non-local thermodynamic equilibrium model atmospheres, and values for their effective temperatures and gravities were obtained. Using photometric fluxes, we also obtained radii, luminosities, and spectroscopic masses. Twenty-four stars in our sample are blue stragglers (BSs). Their photometric colors and magnitudes place these BSs above and redward of the clusters' zero-age main sequence: this is consistent with the gravities we find for these stars, which are lower than zero-age main-sequence gravities. A comparison with stellar evolutionary tracks shows that almost all of our BSs are in the Hertzsprung gap. This is contrary to theory, because of the short timescale expected for stars in this evolutionary phase. The mean BS mass is 1.04 M☉ for 14 nonvariable stars, or 1.07 M☉ counting all 24 BSs in our sample. For the nonvariable stars the mean BS masses for individual clusters are 1.73, 1.01, 0.95, and 0.72 M☉ for NGC 6397, NGC 6752, 47 Tuc, and M3, respectively. Adding the variable stars (which improves the statistics but increases the uncertainty), the mean masses become 1.27, 1.05, 0.99, and 0.99 M☉, respectively. Although there is considerable scatter, the BS spectroscopic masses correlate with both effective temperature and brightness of the stars, as expected. The mean nonvariable turnoff star mass (0.58 M☉) is significantly below the values determined for the BSs and below the main-sequence turnoff mass. The mean nonvariable horizontal-branch (HB) star mass is higher than expected (0.79 M☉). In particular, several HB stars have masses well above the main-sequence turnoff mass. Some of these HB stars are suspected of actually being BSs, since most of them reside at ambiguous locations on the CMD, making them prone to misclassification. Values and limits to the stellar rotation rates (v sin i) are imposed by fitting weak metal lines, the Ca II K line wings, or the helium lines for the hotter stars. Five BSs with reasonably constrained rotations show average and median v sin i values of 109 and 100 km s-1, respectively, suggesting v ~ 160 km s-1. At least some GC BSs are very rapid rotators, but this information cannot yet constrain their origin as stellar collision or binary mergers because of the lack of clear theoretical predictions. Six extreme HB stars have rotation rates v sin i between 50 and 200 km s-1, which are high for these stars and might indicate a binary origin. De Marco et al. found that four BSs and two HB stars in our sample have Balmer jumps that are too large for the effective temperatures implied by the slopes of their Paschen continua. Two additional HB stars are now identified in the current study as having the same feature. For these stars, the presence of a disk of partly ionized material is suspected, although high stellar rotation rates could also partly explain the data.

On the α‐Element Abundance Gradients in the Disk of the Sculptor Spiral Galaxy NGC 300

We have carried out a detailed spectral analysis of six early B-type supergiants in the Sculptor Group spiral galaxy NGC 300. To this end, we used state-of-the-art unified blanketed non-local thermodynamic equilibrium model atmospheres (computed with the code FASTWIND), aimed at the determination of the stellar parameters and of detailed surface abundance patterns. We discuss the individual evolutionary stage of each star in view of their CNO surface abundance, which we compare to massive star evolutionary models with rotation. Although the quantitative comparison does not yield a good agreement, the qualitative behavior is consistent with the model predictions. This issue supports the idea that, although rather evolved, the stars are directly evolving from the main sequence and therefore are in a pre-red supergiant phase. We derive the stellar abundance gradients in the disk of NGC 300 for the elements O, Mg, and Si. Using the available literature data on H II regions in NGC 300, we carry out a detailed comparison of nebular oxygen abundances resulting from different, widely used empirical calibrations. Finally, we compare the abundance gradients derived from the interstellar medium to those derived from the blue supergiants studied in this paper. We find a O/H abundance gradient of -0.033 ± 0.026 dex arcmin-1 over a distance equal to the isophotal radius of the galaxy. This trend is also followed in good agreement by Mg and Si abundances. The abundance gradients derived from our stellar data are shallower than those obtained from most previous H II region analyses, and we obtain a lower oxygen abundance in the central region of the galaxy (8.57 ± 0.13 dex), which is, however, in agreement with the H II region abundance results derived with recent calibrations of several statistical indicators.

A study of two post-common envelope binary systems

We present intermediate-resolution, phase-resolved spectra of the close-binary systems EC 11575-1845 and V664 Cas, the central star of the planetary nebula HFG 1. Both systems have a very rich emission-line spectrum originating from the face of the cool star, which is irradiated by the hot subdwarf star. The H 1 emission lines are wide, consistent with Stark broadening. These have a complex shape due to strong absorption near the line centre. We report on new spectroscopic orbits for both subdwarf stars, based on radial velocity measurements of their He II lines. The orbits of the heated face of the cool companions were measured from narrow emission lines, and from the wide H I profiles; some differences were found between ions. At phases around 0, absorption lines from the photosphere of the secondary star of V664 Cas are seen. These are used to estimate a spectral type of mid-F to early K, and we report on their radial velocities. Synthetic spectra from the non-local thermodynamic equilibrium model atmospheres were used to fit features in the optical spectrum of the hot subdwarf star of EC 11575-1845, from which we estimate T eff = 105 000 ± 20000 K, logg = 6.7 ± 0.3, log n He /n H = -0.8 ± 0.1. Finally, we consider the most likely range of component masses for these two systems.