Single-star Evolutionary Models Research Articles

Anomalously low-mass core-He-burning star in NGC 6819 as a post-common-envelope phase product

Precise masses of red giant stars enable a robust inference of their ages, but there are cases where these age estimates are very precise but also very inaccurate. Examples are core-helium-burning (CHeB) stars that have lost more mass than predicted by standard single-star evolutionary models. Members of star clusters in the database represent a unique opportunity to identify such stars because they combine exquisite asteroseismic constraints with independent age information (members of a star cluster share a similar age and chemical composition). We focus on the single metal-rich ($Z Li-rich low-mass CHeB star KIC4937011, which is a member of the open cluster NGC 6819 (turn-off mass of $ i.e. an age of $ 2.4$ Gyr). This star has a lower mass by $ than expected for its age and metallicity, which might be explained by binary interactions or mass loss along the red giant branch (RGB). To infer formation scenarios for this object, we performed a Bayesian analysis by combining the binary stellar evolutionary framework binary\_c v2.2.3 with the dynamic nested-sampling approach contained in the dynesty v2.1.1 package. We find that this star probably is the result of a common-envelope evolution (CEE) phase during the RGB stage of the primary star in which the low-mass ($<0.71$ main-sequence companion does not survive. The mass of the primary star at the zero-age main sequence is in the range with a log-orbital period in the range days )$. During the CEE phase, $ of material is ejected from the system, and the final star reaches the CHeB stage after helium flashes as if it were a single star with a mass of $ which is what we observe today. Although the proposed scenario is consistent with photometric and spectroscopic observations, a quantitative comparison with detailed stellar evolution calculations is needed to quantify the systematic skewness of the radius, luminosity, and effective temperature distributions towards higher values than observations.

The SN 2023ixf Progenitor in M101. II. Properties

We follow our first paper with an analysis of the ensemble of the extensive preexplosion ground- and space-based infrared observations of the red supergiant (RSG) progenitor candidate for the nearby core-collapse supernova SN 2023ixf in Messier 101, together with optical data prior to the explosion obtained with the Hubble Space Telescope (HST). We have confirmed the association of the progenitor candidate with the supernova (SN), as well as constrained the metallicity at the SN site, based on SN observations with instruments at Gemini-North. The internal host extinction to the SN has also been confirmed from a high-resolution Keck spectrum. We fit the observed spectral energy distribution (SED) for the star, accounting for its intrinsic variability, with dust radiative-transfer modeling, which assumes a silicate-rich dust shell ahead of the underlying stellar photosphere. The star is heavily dust obscured, likely the dustiest progenitor candidate yet encountered. We found median estimates of the star’s effective temperature and luminosity of 2770 K and 9.0 × 104 L ⊙, with 68% credible intervals of 2340–3150 K and (7.5–10.9) × 104 L ⊙, respectively. The candidate may have a Galactic RSG analog, IRC −10414, with a strikingly similar SED and luminosity. Via comparison with single-star evolutionary models we have constrained the initial mass of the progenitor candidate from 12 M ⊙ to as high as 14 M ⊙. We have had available to us an extraordinary view of the SN 2023ixf progenitor candidate, which should be further followed up in future years with HST and the James Webb Space Telescope.

The first measurements of carbon isotopic ratios in post-RGB stars: SZ Mon and DF Cyg

ABSTRACT Dusty post-red giant branch (post-RGB) stars are low- and intermediate-mass stars where the RGB evolution was prematurely terminated by a poorly understood binary interaction. These binary stars are considered to be low-luminosity analogues of post-asymptotic giant branch (post-AGB) binary stars. In this study, we investigated the chemical composition of two dusty post-RGB binary stars, SZ Mon and DF Cyg, using multiwavelength spectroscopic data from HERMES/Mercator (optical) and the APOGEE survey (near-infrared). Owing to challenges posed by existing spectral analysis tools for the study of evolved stars with complex atmospheres, we developed E-iSpec: a dedicated spectral analysis tool for evolved stars, to consistently determine atmospheric parameters, elemental abundances, and carbon isotopic ratios. Our abundance analysis revealed that observed depletion patterns and estimated depletion efficiencies resemble those found in post-AGB binary stars. However, the onset of chemical depletion in post-RGB targets occurs at higher condensation temperatures ($T_{\rm turn-off,~post-RGB}\approx 1\, 400$ K), than in most post-AGB stars ($T_{\rm turn-off,~post-AGB}\approx 1\, 100$ K). Additionally, our study resulted in the first estimates of carbon isotopic ratios for post-RGB stars (12C/13CSZ Mon = 8 ± 4, 12C/13CDF Cyg = 12 ± 3). We found that the observationally derived CNO abundances and the carbon isotopic ratios of our post-RGB binary targets are in good agreement with theoretical predictions from the ATON single star evolutionary models involving first dredge-up and moderately deep extra mixing. This agreement emphasizes that in post-RGB binary targets, the observed CNO abundances reflect the chemical composition expected from single star nucleosynthesis (i.e. convective and non-convective mixing processes) occurring during the RGB phase before it is terminated.

The Origin of the Consistent Planetary Nebula Luminosity Function Bright-end Cutoff

The [O iii] 5007 Å line is typically the brightest line in planetary nebula (PN) spectra. Observations show that the brightest [O iii] 5007 Å PN in a galaxy—the planetary nebula luminosity function (PNLF) bright-end cutoff—is surprisingly independent of galaxy type. To understand the origin of this puzzling uniformity, we simulate PNe with a range of cloud and star parameters using the photoionization code CLOUDY. We find that the peak [O iii] 5007 Å luminosity depends weakly on both the central stellar effective temperature at high temperature and on the total PN ejecta mass; however, the peak [O iii] 5007 Å luminosity depends strongly on the central stellar luminosity and the PN dust-to-gas mass ratio. We explain these scalings physically. They imply that a higher dust-to-gas mass ratio at higher central stellar luminosity can help explain a constant bright-end cutoff in the PNLF across galaxy types. This prediction is testable with a survey of galactic PNe. The surviving remnants of double white dwarf (WD) mergers should also produce photoionized nebulae analogous to PNe. These may be preferentially present at the high luminosity end of the [O iii] PLNF and could explain the existence of PNe in early-type galaxies that are more luminous in [O iii] than expected from single-star evolutionary models. The presence of WD mergers in both young and old stellar populations could contribute to the uniformity of the [O iii] PNLF across galaxy types; such nebulae would lack the hydrogen lines otherwise characteristic of PNe.

Spectroscopic and evolutionary analyses of the binary system AzV 14 outline paths toward the WR stage at low metallicity

Context. The origin of the observed population of Wolf-Rayet (WR) stars in low-metallicity galaxies, such as the Small Magellanic Cloud (SMC), is not yet understood. Standard, single-star evolutionary models predict that WR stars should stem from very massive O-type star progenitors, but these are very rare. On the other hand, binary evolutionary models predict that WR stars could originate from primary stars in close binaries. Aims. We conduct an analysis of the massive O star, AzV 14, to spectroscopically determine its fundamental and stellar wind parameters, which are then used to investigate evolutionary paths from the O-type to the WR stage with stellar evolutionary models. Methods. Multi-epoch UV and optical spectra of AzV 14 are analyzed using the non-local thermodynamic equilibrium (LTE) stellar atmosphere code PoWR. An optical TESS light curve was extracted and analyzed using the PHOEBE code. The obtained parameters are put into an evolutionary context, using the MESA code. Results. AzV 14 is a close binary system with a period of P = 3.7058 ± 0.0013 d. The binary consists of two similar main sequence stars with masses of M1, 2 ≈ 32 M⊙. Both stars have weak stellar winds with mass-loss rates of log Ṁ/(M⊙ yr−1) = −7.7 ± 0.2. Binary evolutionary models can explain the empirically derived stellar and orbital parameters, including the position of the AzV 14 components on the Hertzsprung-Russell diagram, revealing its current age of 3.3 Myr. The model predicts that the primary will evolve into a WR star with Teff ≈ 100 kK, while the secondary, which will accrete significant amounts of mass during the first mass transfer phase, will become a cooler WR star with Teff ≈ 50 kK. Furthermore, WR stars that descend from binary components that have accreted significant amount of mass are predicted to have increased oxygen abundances compared to other WR stars. This model prediction is supported by a spectroscopic analysis of a WR star in the SMC. Conclusions. Inspired by the binary evolutionary models, we hypothesize that the populations of WR stars in low-metallicity galaxies may have bimodal temperature distributions. Hotter WR stars might originate from primary stars, while cooler WR stars are the evolutionary descendants of the secondary stars if they accreted a significant amount of mass. These results may have wide-ranging implications for our understanding of massive star feedback and binary evolution channels at low metallicity.

A Multiwavelength Classification and Study of Red Supergiant Candidates in NGC 6946

We have combined resolved stellar photometry from Hubble Space Telescope (HST), Spitzer, and Gaia to identify red supergiant (RSG) candidates in NGC 6946, based on their colors, proper motions, visual morphologies, and spectral energy distributions. We start with a large sample of 17,865 RSG candidates based solely on HST near-infrared photometry. We then chose a small sample of 385 of these candidates with Spitzer matches for a more detailed study. Using evolutionary models and isochrones, we isolate a space where RSGs would be found in our photometry catalogs. We then visually inspect each candidate and compare them to Gaia catalogs to identify and remove foreground stars. As a result, we classify 95 potential RSGs, with 40 of these being in our highest-quality sample. We fit the photometry of the populations of stars in the regions surrounding the RSGs to infer their ages. Placing our best candidate RSG stars into three age bins between 1 and 30 Myr, we find 27.5% of the candidates falling between 1–10 Myr, 37.5% between 10–20 Myr, and 35% between 20–30 Myr. A comparison of our results to the models of massive star evolution shows some agreement between model luminosities and the luminosities of our candidates for each age. Three of our candidates appear significantly more consistent with binary models than single-star evolution models.

The Gaia-ESO survey: A spectroscopic study of the young open cluster NGC 3293

We present a spectroscopic analysis of the GIRAFFE and UVES data collected by the Gaia-ESO survey for the young open cluster NGC 3293. Archive spectra from the same instruments obtained in the framework of the ‘VLT-FLAMES survey of massive stars’ are also analysed. Atmospheric parameters, non-local thermodynamic equilibrium (LTE) chemical abundances for six elements (He, C, N, Ne, Mg, and Si), or variability information are reported for a total of about 160 B stars spanning a wide range in terms of spectral types (B1 to B9.5) and rotation rate (up to 350 km s−1). Our analysis leads to about a five-fold increase in the number of cluster members with an abundance determination and it characterises the late B-star population in detail for the first time. We take advantage of the multi-epoch observations on various timescales and a temporal baseline, sometimes spanning ∼15 years, to detect several binary systems or intrinsically line-profile variables. A deconvolution algorithm is used to infer the current, true (deprojected) rotational velocity distribution. We find a broad, Gaussian-like distribution peaking around 200–250 km s−1. Although some stars populate the high-velocity tail, most stars in the cluster appear to rotate far from critical. We discuss the chemical properties of the cluster, including the low occurrence of abundance peculiarities in the late B stars and the paucity of objects showing CN-cycle burning products at their surface. We argue that the former result can largely be explained by the inhibition of diffusion effects because of fast rotation, while the latter is generally in accord with the predictions of single-star evolutionary models under the assumption of a wide range of initial spin rates at the onset of main-sequence evolution. However, we find some evidence for a less efficient mixing in two quite rapidly rotating stars that are among the most massive objects in our sample. Finally, we obtain a cluster age of ∼20 Myr through a detailed, star-to-star correction of our results for the effect of stellar rotation (e.g., gravity darkening). This is significantly older than previous estimates from turn-off fitting that fully relied on classical, non-rotating isochrones.

The Extreme Scarcity of Dust-enshrouded Red Supergiants: Consequences for Producing Stripped Stars via Winds

Quiescent mass loss during the red supergiant (RSG) phase has been shown to be far lower than prescriptions typically employed in single-star evolutionary models. Importantly, RSG winds are too weak to drive the production of Wolf-Rayet (WR) stars and stripped-envelope supernovae (SE-SNe) at initial masses of roughly 20–40 M ⊙. If single stars are to make WR stars and SE-SNe, this shifts the burden of mass loss to rare dust-enshrouded RSGs (DE-RSGs), objects claimed to represent a short-lived, high-mass-loss phase. Here, we take a fresh look at the purported DE-RSGs. By modeling the mid-IR excesses of the full sample of RSGs in the Large Magellanic Cloud, we find that only one RSG has both a high mass-loss rate ( ≥ 10−4 M ⊙ yr−1) and a high optical circumstellar dust extinction (7.92 mag). This RSG is WOH G64, and it is the only one of the 14 originally proposed DE-RSGs that is actually dust enshrouded. The rest appear to be either normal RSGs without strong IR excess, or lower-mass asymptotic giant branch stars. Only one additional object in the full catalog of RSGs (not previously identified as a DE-RSG) shows strong mid-IR excess. We conclude that if DE-RSGs do represent a pre-SN phase of enhanced in single stars, it is extremely short-lived, only capable of removing ≤2 M ⊙ of material. This rules out the single-star post-RSG pathway for the production of WR stars, luminous blue variables, and SE-SNe. Single-star models should not employ -prescriptions based on these extreme objects for any significant fraction of the RSG phase.

Unveiling the Nature of SN 2011fh: A Young and Massive Star Gives Rise to a Luminous SN 2009ip−like Event

Abstract In recent years, many Type IIn supernovae have been found to share striking similarities with the peculiar SN 2009ip, whose true nature is still under debate. Here, we present 10 yr of observations of SN 2011fh, an interacting transient with spectroscopic and photometric similarities to SN 2009ip. SN 2011fh had an M r ∼ −16 mag brightening event, followed by a brighter M r ∼ −18 mag luminous outburst in 2011 August. The spectra of SN 2011fh are dominated by narrow to intermediate Balmer emission lines throughout its evolution, with P Cygni profiles indicating fast-moving material at ∼6400 km s−1. HST/WFC3 observations from 2016 October revealed a bright source with M F814W ≈ −13.3 mag, indicating that we are seeing the ongoing interaction of the ejecta with the circumstellar material or that the star might be going through an eruptive phase five years after the luminous outburst of 2011. Using HST photometry of the stellar cluster around SN 2011fh, we estimated an age of ∼4.5 Myr for the progenitor, which implies a stellar mass of ∼60 M ⊙, using single-star evolution models, or a mass range of 35–80 M ⊙, considering a binary system. We also show that the progenitor of SN 2011fh exceeded the classical Eddington limit by a large factor in the months preceding the luminous outburst of 2011, suggesting strong super-Eddington winds as a possible mechanism for the observed mass loss. These findings favor an energetic outburst in a young and massive star, possibly a luminous blue variable.

On the most luminous planetary nebulae of M 31

Context.The planetary nebula luminosity function (PNLF) is a standard candle that comprises a key rung on the extragalactic distance ladder. The method is based on the empirical evidence that the luminosity function of planetary nebulae (PNe) in the [O III]λ5007 nebular emission line reaches a maximum value that is approximately invariant with population age, metallicity, or host galaxy type. However, the presence of bright PNe in old stellar populations is not easily explained by single-star evolutionary models.Aims.To gain information about the progenitors of PNe at the tip of the PNLF, we obtained the deepest existing spectra of a sample of PNe in the galaxy M 31 to determine their physico-chemical properties and infer the post-asymptotic giant branch (AGB) masses of their central stars (CSs). Precise chemical abundances allow us to confront the theoretical yields for AGB stellar masses and metallicities expected at the bright end of the PNLF. Central star masses of the sampled PNe provide direct information on the controversial origin of the universal cutoff of the PNLF.Methods.Using the OSIRIS instrument at the 10.4 m Gran Telescopio Canarias (GTC), optical spectra of nine bright M 31 PNe were obtained: four of them at the tip of the PNLF, and the other five some 0.5 mag fainter. A control sample of 21 PNe with previous GTC spectra from the literature is also included. We analyze their physical properties and chemical abundances (He, N, O, Ar, Ne, and S), searching for relevant differences between bright PNe and the control samples. The CS masses are estimated with Cloudy modeling using the most recent evolutionary tracks.Results.The studied PNe show a remarkable uniformity in all their nebular properties, and the brightest PNe show relatively large electron densities. Stellar characteristics also span a narrow range: ⟨L*/L⊙⟩ = 4300 ± 310, ⟨Teff⟩ = 122 000 ± 10 600 K for the CSs of the four brightest PNe, and ⟨L*/L⊙⟩ = 3300 ± 370, ⟨Teff⟩ = 135 000 ± 26 000 K for those in the control set. This groups all the brightest PNe at the location of maximum temperature in the post-AGB tracks for stars with initial massesMi = 1.5M⊙.Conclusions.These figures provide robust observational constraints for the stellar progenitors that produce the PNLF cutoff in a star-forming galaxy such as M 31, where a large range of initial masses is in principle available. Inconsistency is found, however, in the computed N/O abundance ratios of five nebulae, which are 1.5 to 3 times larger than predicted by the existing nucleosynthesis models for stars of these masses.

The Physical Parameters of Four WC-type Wolf–Rayet Stars in the Large Magellanic Cloud: Evidence of Evolution*

Abstract We present a spectral analysis of four Large Magellanic Cloud (LMC) WC-type Wolf–Rayet (WR) stars (BAT99-8, BAT99-9, BAT99-11, and BAT99-52) to shed light on two evolutionary questions surrounding massive stars. The first is: are WO-type WR stars more oxygen enriched than WC-type stars, indicating further chemical evolution, or are the strong high-excitation oxygen lines in WO-type stars an indication of higher temperatures. This study will act as a baseline for answering the question of where WO-type stars fall in WR evolution. Each star’s spectrum, extending from 1100 to 25000 Å, was modeled using cmfgen to determine the star’s physical properties such as luminosity, mass-loss rate, and chemical abundances. The oxygen abundance is a key evolutionary diagnostic, and with higher resolution data and an improved stellar atmosphere code, we found the oxygen abundance to be up to a factor of 5 lower than that of previous studies. The second evolutionary question revolves around the formation of WR stars: do they evolve by themselves or is a close companion star necessary for their formation? Using our derived physical parameters, we compared our results to the Geneva single-star evolutionary models and the Binary Population and Spectral Synthesis (BPASS) binary evolutionary models. We found that both the Geneva solar-metallicity models and BPASS LMC-metallicity models are in agreement with the four WC-type stars, while the Geneva LMC-metallicity models are not. Therefore, these four WC4 stars could have been formed either via binary or single-star evolution.

Revised Stellar Parameters for V471 Tau, A Post-common Envelope Binary in the Hyades

Abstract V471 Tau is a post-common-envelope binary consisting of an eclipsing DA white dwarf and a K-type main-sequence star in the Hyades star cluster. We analyzed publicly available photometry and spectroscopy of V471 Tau to revise the stellar and orbital parameters of the system. We used archival K2 photometry, archival Hubble Space Telescope spectroscopy, and published radial-velocity measurements of the K-type star. Employing Gaussian processes to fit for rotational modulation of the system flux by the main-sequence star, we recovered the transits of the white dwarf in front of the main-sequence star for the first time. The transits are shallower than would be expected from purely geometric occultations owing to gravitational microlensing during transit, which places an additional constraint on the white-dwarf mass. Our revised mass and radius for the main-sequence star is consistent with single-star evolutionary models given the age and metallicity of the Hyades. However, as noted previously in the literature, the white dwarf is too massive and too hot to be the result of single-star evolution given the age of the Hyades, and may be the product of a merger scenario. We independently estimate the conditions of the system at the time of common envelope that would result in the measured orbital parameters today.

OO Dra: An Algol-type Binary Formed through an Extremely Helium-poor Mass Accretion Revealed by Asteroseismology

Abstract Based on 2 minutes of Transiting Exoplanet Survey Satellite data, we analyzed intrinsic oscillations of the primary component and identified seven confident independent δ Scuti frequencies (f 1, f 2, f 3, f 4, f 7, f 11, and f 12). Both single-star evolutionary models and mass-accreting models are computed to reproduce the δ Scuti frequencies and the fitting results match well with each other. The stellar parameters of the primary star yielded by asteroseismology are M = 1.92 − 0.02 + 0.10 M ⊙, Z = 0.011 − 0.001 + 0.006 , R = 2.068 − 0.007 + 0.050 R ⊙, log g = 4.090 − 0.002 + 0.010 , T eff = 8346 − 320 + 244 K, and L = 18.65 − 2.82 + 3.31 L ⊙, which match well with the dynamic ones using the binary model. Furthermore, our asteroseismic results show that OO Dra is another Algol system that has just undergone the rapid mass-transfer stage. The fitting results of single-star evolutionary models indicate that the pulsator is a helium-poor star with an age of 8.22 − 1.33 + 0.12 Myr, and the further mass-accreting models show that the primary star looks like an almost unevolved star formed by an extremely helium-poor mass accretion in the Case A evolutionary scenario.

The luminous red nova AT 2018bwo in NGC 45 and its binary yellow supergiant progenitor

Luminous red novae (LRNe) are astrophysical transients associated with the partial ejection of a binary system’s common envelope shortly before its merger. Here we present the results of our photometric and spectroscopic follow-up campaign of AT 2018bwo (DLT 18x), a LRN discovered in NGC 45, and investigate its progenitor system using binary stellar-evolution models. The transient reached a peak magnitude of Mr = −10.97 ± 0.11 and maintained this brightness during its optical plateau of tp = 41 ± 5 days. During this phase, it showed a rather stable photospheric temperature of ∼3300 K and a luminosity of ∼1040 erg s−1. Although the luminosity and duration of AT 2018bwo is comparable to the LRNe V838 Mon and M31-2015LRN, its photosphere at early times appears larger and cooler, likely due to an extended mass-loss episode before the merger. Toward the end of the plateau, optical spectra showed a reddened continuum with strong molecular absorption bands. The IR spectrum at +103 days after discovery was comparable to that of a M8.5 II type star, analogous to an extended AGB star. The reprocessed emission by the cooling dust was also detected in the mid-infrared bands ∼1.5 years after the outburst. Archival Spitzer and Hubble Space Telescope data taken 10−14 yrs before the transient event suggest a progenitor star with Tprog ∼ 6500 K, Rprog ∼ 100 R⊙, and Lprog = 2 × 104 L⊙, and an upper limit for optically thin warm (1000 K) dust mass of Md < 10−6 M⊙. Using stellar binary-evolution models, we determined the properties of binary systems consistent with the progenitor parameter space. For AT 2018bwo, we infer a primary mass of 12–16 M⊙, which is 9–45% larger than the ∼11 M⊙ obtained using single-star evolution models. The system, consistent with a yellow-supergiant primary, was likely in a stable mass-transfer regime with −2.4 ≤ log(Ṁ/M⊙ yr−1) ≤ −1.2 a decade before the main instability occurred. During the dynamical merger, the system would have ejected 0.15–0.5 M⊙ with a velocity of ∼500 km s−1.

Progenitor mass distribution for 22 historic core-collapse supernovae

ABSTRACT We infer the progenitor mass distribution for 22 historic core-collapse supernovae (CCSNe) using a Bayesian hierarchical model. For this inference, we use the local star formation histories to estimate the age for each supernova (SN). These star formation histories often show multiple bursts of star formation; our model assumes that one burst is associated with the SN progenitor and the others are random bursts of star formation. The primary inference is the progenitor age distribution. Due to the limited number of historic SNe and highly uncertain star formation at young ages, we restrict our inference to the slope of the age distribution and the maximum age for CCSNe. Using single-star evolutionary models, we transform the progenitor age distribution into a progenitor mass distribution. Under these assumptions, the minimum mass for CCSNe is $M_\textrm {min}~=~8.60^{+0.37}_{-0.41}\ \mathrm M_\odot$ and the slope of the progenitor mass distribution is $\alpha = -2.61^{+1.05}_{-1.18}$. The power-law slope for the progenitor mass distribution is consistent with the standard Salpeter initial mass function (α = −2.35). These values are consistent with previous estimates using precursor imaging and the age-dating technique, further confirming that using stellar populations around SN and supernova remnants is a reliable way to infer the progenitor masses.

DY Pegasi: An SX Phoenicis Star in a Binary System with an Evolved Companion

Abstract In this work, the photometric data from the American Association of Variable Star Observers are collected and analyzed on the SX Phoenicis star DY Pegasi (DY Peg). From the frequency analysis, we get three independent frequencies: f 0 = 13.71249 c d−1, f 1 = 17.7000 c d−1, and f 2 = 18.138 c d−1, in which f 0 and f 1 are the radial fundamental and first overtone mode, respectively, while f 2 is detected for the first time and should belong to a nonradial mode. The O − C diagram of the times of maximum light shows that DY Peg has a period change rate (1/P 0)(dP 0/dt) = −(5.87 ± 0.03) × 10−8 yr−1 for its fundamental pulsation mode, and should belong to a binary system that has an orbital period P orb = 15425.0 ± 205.7 days. Based on the spectroscopic information, single star evolutionary models are constructed to fit the observed frequencies. However, some important parameters of the fitted models are not consistent with that from observations. Combing the information from observation and theoretical calculation, we conclude that DY Peg should be an SX Phoenicis star in a binary system and accreting mass from a dust disk, which was the residue of its evolved companion (most probably a hot white dwarf at the present stage) produced in the asymptotic giant branch phase. Further observations are needed to confirm this inference, and it might be potentially a universal formation mechanism and evolutionary history for SX Phoenicis stars.

KIC 10736223: An Algol-type Eclipsing Binary That Has Just Undergone the Rapid Mass-transfer Stage

Abstract This paper reports the discovery of an Algol system KIC 10736223 that just passed the rapid mass transfer stage. From the light-curve and radial-velocity modeling we find KIC 10736223 to be a detached Algol system with the less-massive secondary nearly filling its Roche lobe. Based on the short-cadence Kepler data, we analyzed intrinsic oscillations of the pulsator and identified six secured independent δ Scuti-type pulsation modes (f 1, f 3, f 9, f 19, f 42, and f 48). We compute two grids of theoretical models to reproduce the δ Scuti frequencies, and find that fitting results of mass-accreting models agree well with those of single-star evolutionary models. The fundamental parameters of the primary star yielded with asteroseismology are M ⊙, Z = 0.009 ± 0.001, R ⊙, , K, and L = L ⊙. The asteroseismic parameters match well with the dynamical parameters derived from the binary model. Moreover, our asteroseismic results show that the pulsator is an almost unevolved star with an age between 9.46 and 11.65 Myr for single-star evolutionary models and 2.67–3.14 Myr for mass-accreting models. Therefore, KIC 10736223 may be an Algol system that has just undergone the rapid mass-transfer process.

The NGC 346 massive star census

Previous analyses of two large spectroscopic surveys of early-type stars in the Large Magellanic Cloud (LMC) have found an excess of nitrogen enriched B-type targets with avesini ≤ 40 km s−1compared with the predictions of single star evolutionary models that incorporate rotational mixing. By contrast, the number of such targets with 40 < vesini ≤ 80 km s−1was consistent with such models. We have undertaken a similar analysis for 61 B-type targets which lie towards the young cluster, NGC 346 in the Small Magellanic Cloud (SMC). These again have projected rotational velocities,vesini ≤ 80 km s−1, are not classified as supergiants, and are apparently single. Approximately 65% of these SMC targets could have nitrogen enhancements of less than 0.3 dex, which is consistent with them having experienced only small amounts of mixing due to their low rotational velocities. However, as with the previous LMC surveys, an excess of stars with low projected rotational velocities,vesini ≤ 40 km s−1, and significant nitrogen enrichments is found. This is estimated to be approximately 5% of the total population of apparently single B-type stars or 40% of all stars with current rotational velocities of less than 40 km s−1; these percentages are similar to those found previously for the two LMC samples. For all three surveys, the presence of undetected binaries and other uncertainties imply that these percentages might be underestimated and that it is indeed possible for all the single stars with current rotational velocities of less than 40 km s−1to be nitrogen enriched. Two possible explanations incorporate the effects of the magnetic field, via either a stellar merger followed by magnetic braking or the evolution of a single star with a large magnetic field. Both mechanisms would appear to be compatible with the observed frequency of nitrogen-enriched stars in the Magellanic Clouds. Differences in the properties of the nitrogen-enriched stars compared with the remainder of the sample would be consistent with the former mechanism. For the latter, a qualitative comparison with Galactic evolutionary models that incorporate magnetic fields is encouraging in terms of the amount of nitrogen enrichment and its presence in stars near the zero-age main sequence.

A Mass-accreting Gamma Doradus Pulsator with a Synchronized Core in Kepler Eclipsing Binary KIC 7385478

Abstract The short-period (P ≈ 1.7 days), Algol-type eclipsing binary KIC 7385478 consists of an F-type primary star (M 1 ≈ 1.71M ⊙) and an evolved K-type secondary (M 2 ≈ 0.37M ⊙). We study the variability of the Kepler light curve and attribute many frequency peaks in the Fourier spectrum to the spot modulation. These frequencies are in the form of orbital harmonics and are highly variable in amplitude. They are most likely from the mass-accreting primary star. In addition, we identify a series of prograde dipole g modes from the primary star that show a quasi-linear period spacing pattern and are very stable in amplitude. The period spacing pattern reveals an asymptotic period spacing value in agreement with fundamental parameters of the primary star and also implies that the near-convective-core rotation rate is almost the same as the orbital period. Thus, both the surface and the core of this Gamma Dor pulsator have synchronized with the binary orbit. We find that a lower stellar mass ≈1.50M ⊙ and higher effective temperature are needed in order to be compatible with the asteroseismic constraints from single-star evolutionary models.

Clues on the Origin and Evolution of Massive Contact Binaries: Atmosphere Analysis of VFTS 352

Abstract The massive O4.5 V + O5.5 V binary VFTS 352 in the Tarantula Nebula is one of the shortest-period and most massive overcontact binaries known. Recent theoretical studies indicate that some of these systems could ultimately lead to the formation of gravitational waves via black hole binary mergers through the chemically homogeneous evolution pathway. By analyzing ultraviolet–optical phase-resolved spectroscopic data, we aim to constrain atmospheric and wind properties that could be later used to confront theoretical predictions from binary evolution. In particular, surface abundances are powerful diagnostics of the evolutionary status, mass transfer, and internal mixing processes. From a set of 32 Very Large Telescope/FLAMES visual and eight Hubble Space Telescope/Cosmic Origins Spectrograph ultraviolet spectra, we used spectral disentangling to separate the primary and secondary components. Using a genetic algorithm wrapped around the NLTE model atmosphere and the spectral synthesis code fastwind, we perform an 11-parameter optimization to derive the atmospheric and wind parameters of both components, including the surface abundances of He, C, N, O, and Si. We find that both components are hotter than expected compared to single-star evolutionary models, indicating that additional mixing processes may be at play. However, the derived chemical abundances do not show significant indications of mixing when adopting baseline values typical of the system environment.