Log Teff Research Articles

The evolution of accreting population III stars at 10−6–103 M⊙ yr−1

Context. The first stars formed over five orders of magnitude in mass by accretion in primordial dark matter halos. Aims. We study the evolution of massive, very massive and supermassive primordial (Pop III) stars over nine orders of magnitude in accretion rate. Methods. We use the stellar evolution code GENEC to evolve accreting Pop III stars from 10−6–103 M⊙ yr−1 and study how these rates determine final masses. The stars are evolved until either the end central Si burning or they encounter the general relativistic instability (GRI). We also examine how metallicity affects the evolution of the star at one accretion rate. Results. At rates below ∼2.5 × 10−5 M⊙ yr−1 the final mass of the star falls below that required for pair-instability supernovae. The minimum rate required to produce black holes with masses above 250 M⊙ is ∼5 × 10−5 M⊙ yr−1, well within the range of infall rates found in numerical simulations of halos that cool via H2, ≲10−3 M⊙ yr−1. At rates of 5 × 10−5 M⊙ yr−1 to 4 × 10−2 M⊙ yr−1, like those expected for halos cooling by both H2 and Lyα, the star collapses after Si burning. At higher accretion rates the GRI triggers the collapse of the star during central H burning. Stars that grow at above these rates are cool red hypergiants with effective temperatures log(Teff) = 3.8 and luminosities that can reach 1010.5 L⊙. At accretion rates of 100–1000 M⊙ yr−1 the gas encounters the general relativistic instability prior to the onset of central hydrogen burning and collapses to a black hole with a mass of ∼106 M⊙ without ever having become a star. Conclusions. Our models corroborate previous studies of Pop III stellar evolution with and without hydrodynamics over separate, smaller ranges in accretion rate. They also reveal for the first time the critical transition rate in accretion above which catastrophic baryon collapse, like that which can occur during galaxy collisions in the high-redshift Universe, produces supermassive black holes via dark collapse.

SOLAR-TYPE ECLIPSING BINARY KIC 4832197: PHYSICAL PROPERTIES AND INTRINSIC VARIABILITY OF THE COMPONENTS

Comprehensive analysis of optical spectroscopy and space photometry of the solar type eclipsing binary system KIC 4832197 is presented. The system is composed of F7V + F9V components with masses of M1 = 1.16 ± 0.12 M☉, M2 = 1.07 ± 0.10 M☉ and radii of R1 = 1.26 ± 0.04 R☉, R2 = 1.03 ± 0.03 R☉. The position of the components on the Log Teff – Log L/L☉ plane suggests an age of 2.8 ± 0.8 Gyr for the system. Inspection of out-of-eclipse brightness in time reveals a wave-like variability pattern, whose amplitude and shape quickly change on order of days. Frequency analysis of this variability results in two significant peaks in the amplitude spectrum, which are interpreted as rotational modulation of spots on the components. Assuming both spots are on the same component, a lower limit for the differential rotation coeffcient is computed as k = 0.12, which is weaker compared to the solar value of k☉ = 0.189.

The evolutionary stage of Betelgeuse inferred from its pulsation periods

Abstract Betelgeuse is a well known bright red supergiant that shows semi-regular variations with four approximate periods of 2200, 420, 230, and 185 days. While the longest period was customarily regarded as LSP (long secondary period) of unknown origin, we identify the ∼2200-d period as the radial fundamental mode, and the three shorter periods as the radial first, second, and third overtones. From a linear nonadiabatic pulsation analysis including the pulsation/convection coupling, we have found that these radial pulsation modes are all excited in the envelope of a model in a late stage of the core-carbon burning. Models with similar pulsation property have masses of 11 ∼ 12 M⊙ (19 M⊙ at ZAMS) with luminosities (log L/L⊙ = 5.27 ∼ 5.28) and effective temperatures (log Teff ≈ 3.53) that are consistent with the range of the observational determinations. We also find that a synthetic light curve obtained by adding the fundamental and the first-overtone mode is comparable with the light curve of Betelgeuse up to the Great Dimming. We conclude that Betelgeuse is likely in the late stage of core carbon burning, and a good candidate for the next Galactic Type II supernova.

THE OVERCONTACT BINARY V563 LYRAE

Radial velocity (RV) and light curve (LC) data for the overcontact binary V563 Lyr have been obtained and analysed with the 2003 version of the WilsonDevinney (WD) code with results K1 = 147.4 ± 2.1 km/s, K2 = 247.2 ± 1.1 km/s, RVγ = 22.5 ± 1.5 km/s, qsp = 0.596 ± 0.008, M1 = 2.49(4)M☉, M2 = 1.45(4)M☉, R1 = 2.23(2)R☉, R2 = 1.81(2)R☉. A third component was identified, with radial velocity RV3 = 18.8 ± 6.7 km/s. Inserting the derived parameters of the eclipsing pair into a Log (L)-Log (Teff) plot for each star using data from Yakut and Eggleton (2005) suggested that both stars are over-luminous and evolved to, and perhaps past, the terminal age main sequence (TAMS). The companion (star 3) has a spectral type of A0 ± 1 spectral subclass but cannot be gravitationally bound to the eclipsing pair, as its flux would dominate that of the pair, which was not observed. The companion must lie at some other distance.

The overcontact binary SS arietis

New radial velocity and photometric data for SS Ari have been obtained and analysed using the 2003 versions of the Wilson-Devinney code. Values for fundamental quantities have been obtained: M1 = 0.510 (15) M ʘ, M 2 = 1.521 (26) M ʘ, R1 = 0.87 (2) Rʘ, R2 = 1.42 (2) Rʘ, L1 = 0.88 (8) L ʘ, and L 2 = 2.13 (21) L ʘ. In addition to the values provided by the Wilson-Devinney output, luminosities have been computed directly using photometry from APASS, the Gaia distance, and estimates of the interstellar absorption. Although the two sets of luminosities compare well, the accuracy of the latter set is limited by the uncertainty in the APASS magnitudes. Although the TYC magnitudes for the comparison star are more precise, they produce higher luminosities (but still within error limits). In any case, the good agreement between the luminosities determined by the two methods suggests that they, and all other results in this paper, might be more reliable than previous values in the literature. Inserting the parameters into a Log(L)-Log(Teff) plot for each star using data from Yakut and Eggleton (2005) suggests that the more massive star is somewhat over-luminous and therefore evolved, whereas its companion is under-luminous. Analysis of the period behaviour with the aid of an eclipse timing difference (OC) plot reveals that the double sine relationship Kim et al. (2003) is no longer valid. A new LiTE relationship adding two companion stars has been obtained, but, in view of the fact the putative period of one of them is in the range of ∼600 years, the relationship must be regarded as very tentative indeed.

High-amplitude γ Doradus variables

ABSTRACT According to most literature sources, the amplitude of the pulsational variability observed in γ Doradus stars does not exceed 0.1 mag in Johnson V. We have analysed fifteen high-amplitude γ Doradus stars with photometric peak-to-peak amplitudes well beyond this limit, with the aim of unraveling the mechanisms behind the observed high amplitudes and investigating whether these objects are in any way physically distinct from their low-amplitude counterparts. We have calculated astrophysical parameters and investigated the location of the high-amplitude γ Doradus stars and a control sample of fifteen low-amplitude objects in the log Teff versus log L/L⊙ diagram. Employing survey data and our own observations, we analysed the photometric variability of our target stars using discrete Fourier transform. Correlations between the observed primary frequencies, amplitudes and other parameters like effective temperature and luminosity were investigated. The unusually high amplitudes of the high-amplitude γ Doradus stars can be explained by the superposition of several base frequencies in interaction with their combination and overtone frequencies. Although the maximum amplitude of the primary frequencies does not exceed an amplitude of 0.1 mag, total light variability amplitudes of over 0.3 mag (V) can be attained in this way. Low- and high-amplitude γ Doradus stars do not appear to be physically distinct in any other respect than their total variability amplitudes but merely represent two ends of the same, uniform group of variables.

Catalogue of the central stars of planetary nebulae

Planetary nebulae represent a potential late stage of stellar evolution, however, their central stars (CSPNe) are relatively faint and, therefore, pertinent information is available for merely < 20% of the Galactic sample. Consequently, the literature was surveyed to construct a new catalogue of 620 CSPNe featuring important spectral classifications and information. The catalogue supersedes the existing iteration by 25% and includes physical parameters such as luminosity, surface gravity, temperature, magnitude estimates, and references for published spectra. The marked statistical improvement enabled the following pertinent conclusions to be determined: the H-rich/H-poor ratio is 2:1, there is a deficiency of CSPNe with types [WC 5-6], and nearly 80% of binary central stars belong to the H-rich group. The last finding suggests that evolutionary scenarios leading to the formation of binary central stars interfere with the conditions required for the formation of H-poor CSPN. Approximately 50% of the sample with derived values of log L⋆, log Teff, and log g, exhibit masses and ages consistent with single stellar evolutionary models. The implication is that single stars are indeed able to form planetary nebulae. Moreover, it is shown that H-poor CSPNe are formed by higher mass progenitors. The catalogue is available through the Vizier database.

Pulsational properties of ten new slowly pulsating B stars

Context. Slowly pulsating B (SPB) stars are upper main-sequence multi-periodic pulsators that show non-radial g-mode oscillations driven by the κ mechanism acting on the iron bump. These multi-periodic pulsators have great asteroseismic potential and can be employed for the calibration of stellar structure and evolution models of massive stars. Aims. We collected a sample of ten hitherto unidentified SPB stars with the aim of describing their pulsational properties and identifying pulsational modes. Methods. Photometric time series data from various surveys were collected and analyzed using diverse frequency search algorithms. We calculated astrophysical parameters and investigated the location of our sample stars in the log Teff vs. log L/L⊙ diagram. Current pulsational models were calculated and used for the identification of pulsational modes in our sample stars. An extensive grid of stellar models along with their g-mode eigenfrequencies was calculated and subsequently cross-matched with the observed pulsational frequencies. The best-fit models were then used in an attempt to constrain stellar parameters such as mass, age, metallicity, and convective overshoot. Results. We present detected frequencies, corresponding g-mode identifications, and the masses and ages of the stellar models producing the best frequency cross-matches. We partially succeeded in constraining stellar parameters, in particular concerning mass and age. Where applicable, rotation periods have been derived from the spacing of triplet component frequencies. No evolved SPB stars are present in our sample. We identify two candidate high-metallicity objects (HD 86424 and HD 163285), one young SPB star (HD 36999), and two candidate young SPB stars (HD 61712 and HD 61076). Conclusions. We demonstrate the feasibility of using ground-based observations to perform basic asteroseismological analyses of SPB stars. Our results significantly enlarge the sample of known SPB stars with reliable pulsational mode identifications, which provides important input parameters for modeling attempts aiming to investigate the internal processes at work in upper main-sequence stars.

EVR-CB-001: An Evolving, Progenitor, White Dwarf Compact Binary Discovered with the Evryscope

Abstract We present EVR-CB-001, the discovery of a compact binary with an extremely low-mass (0.21 ± 0.05M ⊙) helium core white dwarf progenitor (pre-He WD) and an unseen low-mass (0.32 ± 0.06M ⊙) helium white dwarf (He WD) companion. He WDs are thought to evolve from the remnant helium-rich core of a main-sequence star stripped during the giant phase by a close companion. Low-mass He WDs are exotic objects (only about 0.2% of WDs are thought to be less than 0.3 M ⊙), and are expected to be found in compact binaries. Pre-He WDs are even rarer, and occupy the intermediate phase after the core is stripped, but before the star becomes a fully degenerate WD and with a larger radius (≈0.2R ⊙) than a typical WD. The primary component of EVR-CB-001 (the pre-He WD) was originally thought to be a hot subdwarf (sdB) star from its blue color and under-luminous magnitude, characteristic of sdBs. The mass, temperature (T eff = 18,500 ± 500 K), and surface gravity ( ) solutions from this work are lower than values for typical hot subdwarfs. The primary is likely to be a post-red-giant branch, pre-He WD contracting into a He WD, and at a stage that places it nearest to sdBs on color–magnitude and T eff–log(g) diagrams. EVR-CB-001 is expected to evolve into a fully double degenerate, compact system that should spin down and potentially evolve into a single hot subdwarf star. Single hot subdwarfs are observed, but progenitor systems have been elusive.

Updating the theoretical tidal evolution constants: Apsidal motion and the moment of inertia

Context. The theoretical apsidal motion constants are key tools to investigate the stellar interiors in close eccentric binary systems. In addition, these constants and the moment of inertia are also important to investigate the tidal evolution of close binary stars as well as of exo-planetary systems. Aims. The aim of the paper is to present new evolutionary models, based on the MESA package, that include the internal structure constants (k2, k3, and k4), the radius of gyration, and the gravitational potential energy for configurations computed from the pre-main-sequence up to the first ascent giant branch or beyond. The calculations are available for the three metallicities [Fe/H] = 0.00, −0.50, and −1.00, which take the recent investigations in less metallic environments into account. This new set of models replaces the old ones, published about 15 years ago, using the code GRANADA. Methods. Core overshooting was taken into account using the mass-fov relationship, which was derived semi-empirically for models more massive than 1.2 M⊙. The differential equations governing the apsidal motion constants, moment of inertia, and the gravitational potential energy were integrated simultaneously through a fifth-order Runge-Kutta method with a tolerance level of 10−7. Results. The resulting models (from 0.8 up to 35.0 M⊙) are presented in 54 tables for the three metallicities, containing the usual characteristics of an evolutionary model (age, initial masses, log Teff, log g, and log L), the constants of internal structure (k2, k3, and k4), the radius of gyration β, and the factor α that is related with the gravitational potential energy.

Absolute dimensions of the low-mass eclipsing binary system NSVS 10653195

Context. Low-mass stars in eclipsing binary systems show radii larger and effective temperatures lower than theoretical stellar models predict for isolated stars with the same masses. Eclipsing binaries with low-mass components are hard to find due to their low luminosity. As a consequence, the analysis of the known low-mass eclipsing systems is key to understand this behavior. Aims. We aim to investigate the mass–radius relation for low-mass stars and the cause of the deviation of the observed radii in low-mass detached eclipsing binary stars (LMDEB) from theoretical stellar models. Methods. We developed a physical model of the LMDEB system NSVS 10653195 to accurately measure the masses and radii of the components. We obtained several high-resolution spectra in order to fit a spectroscopic orbit. Standardized absolute photometry was obtained to measure reliable color indices and to measure the mean Teff of the system in out-of-eclipse phases. We observed and analyzed optical VRI and infrared JK band differential light-curves which were fitted using PHOEBE. A Markov chain Monte-Carlo (MCMC) simulation near the solution found provides robust uncertainties for the fitted parameters. Results. NSVS 10653195 is a detached eclipsing binary composed of two similar stars with masses of M1 = 0.6402 ± 0.0052 M⊙ and M2 = 0.6511 ± 0.0052 M⊙ and radii of R1 = 0.687+0.017−0.024 R⊙ and R2 = 0.672+0.018−0.022 R⊙. Spectral types were estimated to be K6V and K7V. These stars rotate in a circular orbit with an orbital inclination of i = 86.22 ± 0.61 degrees and a period of P = 0.5607222(2) d. The distance to the system is estimated to be d = 135.2+7.6−7.9 pc, in excellent agreement with the value from Gaia. If solar metallicity were assumed, the age of the system would be older than log (age) ∼ 8 based on the Mbol–log Teff diagram. Conclusions. NSVS 10653195 is composed of two oversized and active K stars. While their radii is above model predictions their Teff are in better agreement with models.

Searching for hot subdwarf stars from the LAMOST Spectra. II. Pure spectroscopic identification method for hot subdwarfs

Abstract Employing a new machine-learning method, named the hierarchical extreme learning machine (HELM) algorithm, we identified 56 hot subdwarf stars in the first data release (DR1) of the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) survey. The atmospheric parameters of the stars are obtained by fitting the profiles of hydrogen (H) Balmer lines and helium (He) lines with synthetic spectra calculated from non-local thermodynamic equilibrium (NLTE) model atmospheres. Five He-rich hot subdwarf stars were found in our sample with their log (nHe/nH) > −1, while 51 stars are He-poor sdB, sdO and sdOB stars. We also confirmed the two He sequences of hot subdwarf stars found by Edelmann et al. (2003, A&A, 400, 939) in a Teff–log(nHe/nH) diagram. The HELM algorithm works directly on the observed spectroscopy and is able to filter out spectral properties without supplementary photometric data. The results presented in this study demonstrate that the HELM algorithm is a reliable method to search for hot subdwarf stars after suitable training is performed, and it can also be used to search for other objects which have obvious features in their spectra or images.

Properties of Kepler Stars with the Most Powerful Flares

We analyze physical characteristics of late-type stars wherein the Kepler mission registered superflares. We use the revised stellar fundamental parameters, i.e. effective temperatures Teff and surface gravitational accelerations log g, from the Kepler archive published in 2017, as compared to previous studies by Balona (2015) based on the release of 2011. Among superflare stars there are both single objects and members of eclipsing binaries. We select the late-type stars (with Teff $< 6500$ K) wherein occured the most powerful flares with the total flare energy $> 10^{35}$ erg and consider their locations in the (Teff - log g) diagram. Both components of binaries and single stars appear to reside mostly in between the main sequence and the subgiant branches and therefore have larger radii compared to that of the Sun. Besides, as a rule these single stars are fast rotators and can be considered as young objects that it is difficult to attribute to "solar-type stars". Extremely high flare energy of these stars requires quite strong magnetic fields that cannot be generated even due to scaling of the solar dynamo. Apparently, for explanation of the strongest non-stationary phenomena on stars considered, it would be worthwhile to attract another regime of the dynamo mechanism that can be realized in these objects.

Pulsational instabilities driven by the ∈ mechanism in hot pre-horizontal branch stars

Context. The ∈ mechanism is a self-excitation mechanism of stellar pulsations that acts in regions inside the star where nuclear burning takes place. It has been shown that the ∈ mechanism can excite pulsations in models of hot pre-horizontal branch stars before they settle into the stable helium core-burning phase. Moreover, it has been shown that this mechanism could explain the shortest periods of LS IV-14°116, a mild He-sdBV star. Aims. We aim to study the ∈ mechanism in stellar models appropriate for hot pre-horizontal branch stars to predict their pulsational properties and the instability domain in the log g − log Teff plane. Methods. We performed detailed computations of non-adiabatic non-radial pulsations on stellar models during the helium subflashes just before the helium-core burning phase. Computations were carried out for different values of initial helium composition, metallicity, and envelope mass at the moment of helium flash. Results. We find an instability domain of long-period gravity modes due to the ∈ mechanism in the log g − log Teff plane at roughly 22 000 K ≲ Teff ≲ 50 000 K and 4.67 ≲ log g ≲ 6.15. Consequently, we find instabilities due to the ∈ mechanism on pre-extreme horizontal branch stellar models (Teff ≳ 22 000 K), but not on pre-blue horizontal branch stellar models (Teff ≲ 21 000 K). The periods of excited modes range between ~200 and ~2000 s. Comparison with the three known pulsating He-rich subdwarfs shows that ∈ mechanism can excite gravity modes in stars with similar surface properties (He abundances, log g, log Teff), but in our models it is only able to excite modes in the range of the shortest observed periods. Conclusions. We predict a new instability strip for hot-subdwarf stars of which LS IV-14°116 could be the first inhabitant. Based on simple estimates we expect 1 to 10 stars in the current samples of hot-subdwarf stars to be pulsating by the ∈ mechanism. Our results could constitute a theoretical basis for future searches of pulsators in the Galactic field.

First multi-band CCD photometric study for two eclipsing binaries: ASAS J212234-4627.6 and ASAS J212319-4622.4

We present the first BVRI bands CCD photometric light curves of ASAS J212234-4627.6 and ASAS J212319-4622.4 observed with the SARA 0.6-m telescope at the Cerro Telolo Inter-American Observatory (CTIO), Chile on August 16, and September 28, 2014. From our multi-band photometry data, we obtained three minimum light times of ASAS J212234-4627.6 and four minimum light times of ASAS J212319-4622.4. We used the least square method to fit the minimum light times and got the linear ephemeris for the two eclipsing binary systems. Using the WD program, we solved the photometric solutions of the two objects: The photometric solution of ASAS J212234-4627.6 indicates that it is a semi-contact eclipsing binary star, the secondary component accurately filling its limiting lobe, and the asymmetry of the light curves may be due to the presence of a dark spot on the primary component; the photometric solution of ASAS J212319-4622.4 shows that it is a contact eclipsing binary system with a contact factor f = 5.899 ( ± 0.003)%. In addition, we also estimated the global parameters and the evolution state of the two objects. The Log Teff - Log (L/Lsun) diagram shows that the primary and secondary components of ASAS J212234-4627.6 and ASAS J212319-4622.4 are all MS stars.

K2 observations of 95 Vir: δ Scuti pulsations in a chromospherically active star

We have searched for photometric variability in 95 Vir, a fast rotating, chromospherically active early F-type star, which was observed in the framework of Campaign 6 of the Kepler K2 mission. Available literature information on 95 Vir was procured, and well-established calibrations were employed to verify the derived astrophysical parameters. We have investigated the location of our target star in the MBol versus log Teff diagram, which provides information on the evolutionary status. We have discussed our results in detail, drawing on literature information and the theoretical predictions of state-of-the-art pulsation models, with the aim of unravelling the underlying variability mechanisms. From an analysis of 3400 long-cadence measurements, we have identified two main frequencies and several harmonics in our target star. We attribute the main frequency, f 1 = 9.537 28 d−1, to δ Scuti pulsations. The origin of the secondary signal, f 2 = 1.071 29 d−1, is less clear. We have investigated three possible interpretations of the low-frequency variation: binarity, pulsation and rotational modulation. Current evidence favours an interpretation of f 2 as a signature of the rotational period caused by the presence of cool star-spots, which goes along well with the observed chromospheric activity. However, phase-resolved spectroscopy is needed to verify this assumption. We briefly consider other chromospherically active δ Scuti stars that have been presented in the literature. A search for star-spot-induced photometric variability in these objects might be of great interest, as well as an investigation of the interplay between chromospheric and pulsational activity.

An investigation of four chemically peculiar stars with photometric periods below 12 h

We present an investigation of three chemically peculiar (CP) stars and one CP star candidate which exhibit photometric periods below 12 hours. New spectroscopic observations have been acquired which confirm the peculiar nature of all objects. HD 77013 and HD 81076 are classical CP1 (Am) stars; HD 67983 is a marginal CP1 (Am: ) star, and HD 98000 is a CP2 (Ap) star. We have procured observations from the ASAS-3 and SuperWASP archives and obtained additional photometry in order to verify the results from the sky survey data. We have derived astrophysical parameters and investigated the positions of our target stars in the MBol versus log Teff diagram, from which information on evolutionary status has been derived. We present period analyses and discuss each object in detail. From the available data, we propose pulsational variability as the underlying mechanism for the variability in HD 67983, HD 77013 and HD 81076, which offer the opportunity to study the interaction of atomic diffusion and pulsation. HD 67983 and HD 77013 exhibit multiperiodic variability in the gamma Doradus frequency realm; HD 81076 is a delta Scuti star. The CP2 star HD 98000 exhibits monoperiodic variability with a frequency of f~2.148 c/d (P~0.466 d), which we interpret as the rotational period. If this assumption is correct, HD 98000 is the alpha2 Canum Venaticorum (ACV) variable with the shortest period hitherto observed and thus a very interesting object that might help to investigate the influence of rotational mixing on chemical peculiarities.

Orbit and physical characteristics of the components of the massive Algol V622 Per, a member of the open star cluster χ Per

Analysis of the radial velocities based on spectra of high (near the Hα line) and moderate (4420–4960 A) resolutions supplemented by the published radial velocities has revealed the binarity of a bright member of the young open star cluster χ Per, the star V622 Per. The derived orbital elements of the binary show that the lines of both components are seen in its spectrum, the orbital period is 5.2 days, and the binary is in the phase of active mass exchange. The photometric variability of the star is caused by the ellipsoidal shape of its components. Analysis of the spectroscopic and photometric variabilities has allowed the absolute parameters of the binary’s orbit and its components to be found. V622 Per is shown to be a classical Algol with moderate mass exchange in the binary. Mass transfer occurs from the less massive (\({M_1} = 9.1 \pm 2.7{M_ \odot }\)) but brighter (\(\log {L_1} = 4.52 \pm 0.10{L_ \odot }\)) component onto the more massive (\({M_2} = 13.0 \pm 3.5{M_ \odot }\)) and less bright (\(\log {L_2} = 3.96 \pm 0.10{L_ \odot }\)) component. Analysis of the spectra has confirmed an appreciable overabundance of CNO-cycle products in the atmosphere of the primary component. Comparison of the positions of the binary’s components on the Teff–log g diagram with the age of the cluster χ Per points to a possible delay in the evolution of the primary component due to mass loss by no more than 1–2Myr.

HOT SUBDWARF STARS OBSERVED IN LAMOST DR1—ATMOSPHERIC PARAMETERS FROM SINGLE-LINED SPECTRA

ABSTRACT We present a catalog of 166 spectroscopically identified hot subdwarf stars from LAMOST DR1, 44 of which show the characteristics of cool companions in their optical spectra. Atmospheric parameters of 122 subdwarf stars with non-composite spectra were measured by fitting the profiles of hydrogen (H) and helium (He) lines with synthetic spectra from non-LTE model atmospheres. Most of the sdB stars scatter near the Extreme Horizontal Branch in the T eff– log g ?> diagram and two well defined groups can be outlined. A clustering of He-enriched sdO stars appears near T eff = 45,000 K and log g = 5.8. The sdB population separates into several nearly parallel sequences in the T eff–He abundance diagram with clumps corresponding to those in the T eff– log g ?> diagram. Over 38,000 K (sdO) stars show abundance extremes; they are either He-rich or He-deficient and we observe only a few stars in the abundance range &minus; 1 &lt; log y &lt; 0 ?> . With increasing temperature these extremes become less prominent and the He abundance approaches log y ∼ −0.5. A unique property of our sample is that it covers a large range in apparent magnitude and galactic latitude, therefore it contains a mix of stars from different populations and galactic environments. Our results are consistent with the findings of Hirsch and we conclude that He-rich and He-deficient sdB stars (log y &lt; 1) probably originate from different populations. We also find that most sdO and sdB stars lie in a narrow strip in the plane of luminosity and helium abundance, which suggests that these atmospheric parameters are correlated.

The impact of enhanced iron opacity on massive star pulsations: updated instability strips

Abstract Recently, Bailey et al. made a direct measurement of the iron opacity at the physical conditions of the solar tachocline. They found that the wavelength-integrated iron opacity is roughly 75 per cent higher than what the Opacity Project (OP) and OPAL models predict. Here, we compute new opacity tables with enhanced iron and nickel contributions to the Rosseland mean opacity by 75 per cent each, and compute three dense mesa grids of evolutionary models for Galactic O- and B-type stars covering from 2.5 to 25 M⊙ from zero-age main sequence (ZAMS) until Teff = 10 000 K after the core hydrogen exhaustion. We carry out non-adiabatic mode stability analysis with gyre, and update the extension of the instability strips of heat-driven p- and g-mode pulsators, and the hybrid slowly pulsating B (SPB) - β Cep stars. We compare the position of two confirmed late O-type β Cep and eight confirmed hybrid B-type pulsators with the new instability domains, and justify that ∼75 per cent enhancement, only in iron opacity, is sufficient to consistently reproduce the observed position of these stars on the log Teff versus log g plane. We propose that this improvement in opacities be incorporated in the input physics of new stellar models.