Main-sequence B-type Stars Research Articles

Ultra-high-energy gamma-ray bubble around microquasar V4641 Sgr.

Microquasars are laboratories for the study of jets of relativistic particles produced by accretion onto a spinning black hole. Microquasars are near enough to allow detailed imaging of spatial features across the multiwavelength spectrum. The recent extension measurement of the spatial morphology of a microquasar, SS 433, to TeV gamma rays1 localizes the acceleration of electrons at shocks in the jet far from the black hole2. V4641 Sagittarii (V4641 Sgr) is a similar binary system with a black hole and B-type main-sequence companion star and has an orbit period of 2.8 days (refs. 3,4). It stands out for its super-Eddington accretion5 and for its radio jet, which is one of the fastest superluminal jets in the Milky Way. Previous observations of V4641 Sgr did not report gamma-ray emission6. Here we report TeV gamma-ray emission from V4641 Sgr that reveals particle acceleration at similar distances from the black hole as SS 433. Furthermore, the gamma-ray spectrum of V4641 Sgr is among the hardest TeV spectra observed from any known gamma-ray source and is detected above 200 TeV. Gamma rays are produced by particles, either electrons or protons, of higher energies. Because energetic electrons lose energy more quickly the higher their energy, such a spectrum either very strongly constrains the electron-production mechanism or points to the acceleration of high-energy protons. This suggests that large-scale jets from microquasars could be more common than previously expected and that they could be a notable source of galactic cosmic rays7-9.

Boron depletion in Galactic early B-type stars reveals two different main sequence star populations

Context. The evolution and fate of massive stars are thought to be affected by rotationally induced internal mixing. The surface boron abundance is a sensitive tracer of this in early B-type main sequence stars. Aims. We test current stellar evolution models of massive main sequence stars which include rotational mixing through a systematic study of their predicted surface boron depletion. Methods. We construct a dense grid of rotating single star models using MESA, for which we employ a new nuclear network which follows all the stable isotopes up to silicon, including lithium, beryllium, boron, as well as the radioactive isotope aluminium-26. We also compile the measured physical parameters of the 90 Galactic early B-type stars with boron abundance information. We then compare each observed stars with our models through a Bayesian analysis, which yields the mixing efficiency parameter with which the star is reproduced the best, and the probability that it is represented by the stellar models. Results. We find that about two-thirds of the sample stars are well represented by the stellar models, with the best agreement achieved for a rotational mixing efficiency of ∼50% compared to the widely adopted value. The remaining one third of the stars, of which many are strongly boron depleted slow rotators, are largely incompatible with our models, for any rotational mixing efficiency. We investigate the observational incidence of binary companions and surface magnetic fields, and discuss their evolutionary implications. Conclusions. Our results confirm the concept of rotational mixing in radiative stellar envelopes. On the other hand, we find that a different boron depletion mechanism, and likely a different formation path, is required to explain about one-third of the sample stars. The large spread in the surface boron abundances of these stars may hold a clue to understanding their origin.

Detection of Two Totally Eclipsing B-type Binaries with Extremely Low Mass Ratios

The detection of O- and B-type stars with extremely low-mass companions is very important for understanding the formation and evolution of binary stars. However, finding them remains a challenge because the low-mass components in such systems contribute such small flux to the total. During our search for pulsations among O- and B-type stars using the Transiting Exoplanet Survey Satellite (TESS) data, we found two short-period and B-type (B9) eclipsing binaries with orbital periods of 1.61613 and 2.37857 days. Photometric solutions of the two close binaries were derived by analyzing the TESS light curves with the Wilson–Devinney method. It is discovered that both of them are detached binaries with extremely low mass ratios of 0.067(2) for TIC 260342097 and 0.140(3) for TIC 209148631. The determined mass ratio indicates that TIC 260342097 is one of the lowest mass ratios among known B-type binary systems. We showed that the two systems have total eclipses with a broad and flat secondary minimum, suggesting that the photometric parameters could be derived reliably. The absolute parameters of the two binaries are estimated and it is found that the secondary components in the two systems are overluminous and oversize when compared with the normal low-mass and cool main-sequence (MS) stars. These findings may imply that the two systems are composed of a B-type MS primary and a cool pre-MS secondary with orbital periods shorter than 2.5 days. They are valuable targets to test theories of binary star formation and evolution.

Searching for OB-type pre-supernova binary companions inside supernova remnants

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

Bayesian sampling with BeAtlas, a grid of synthetic Be star spectra I. Recovering the fundamental parameters of α Eri and β CMi

ABSTRACT Classical B emission (Be) stars are fast rotating, near-main-sequence B-type stars. The rotation and the presence of circumstellar discs profoundly modify the observables of active Be stars. Our goal is to infer stellar and disc parameters, as well as distance and interstellar extinction, using the currently most favoured physical models for these objects. We present BeAtlas, a grid of $61\, 600$ non-local thermodynamic equilibrium radiative transfer models for Be stars, calculated with the hdust code. The grid was coupled with a Monte Carlo Markov chain (MCMC) code to sample the posterior distribution. We test our method on two well-studied Be stars, α Eri and β CMi, using photometric, polarimetric, and spectroscopic data as input to the code. We recover literature determinations for most of the parameters of the targets, in particular the mass and age of α Eri, the disc parameters of β CMi, and their distances and inclinations. The main discrepancy is that we estimate lower rotational rates than previous works. We confirm previously detected signs of disc truncation in β CMi and note that its inner disc seems to have a flatter density slope than its outer disc. The correlations between the parameters are complex, further indicating that exploring the entire parameter space simultaneously is a more robust approach, statistically. The combination of BeAtlas and Bayesian-MCMC techniques proves successful, and a powerful new tool for the field: The fundamental parameters of any Be star can now be estimated in a matter of hours or days.

Multiwavelength Characterization of the High-mass X-Ray Binary Population of M33

We present multiwavelength characterization of 65 high-mass X-ray binary (HMXB) candidates in M33. We use the Chandra ACIS survey of M33 (ChASeM33) catalog to select hard X-ray point sources that are spatially coincident with UV-bright point-source optical counterparts in the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region catalog, which covers the inner disk of M33 at near-IR, optical, and near-UV wavelengths. We perform spectral energy distribution fitting on multiband photometry for each point-source optical counterpart to measure its physical properties including mass, temperature, luminosity, and radius. We find that the majority of the HMXB companion star candidates are likely B-type main-sequence stars, suggesting that the HMXB population of M33 is dominated by Be X-ray binaries (Be-XRBs), as is seen in other Local Group galaxies. We use spatially resolved recent star formation history maps of M33 to measure the age distribution of the HMXB candidate sample and the HMXB production rate for M33. We find a bimodal distribution for the HMXB production rate over the last 80 Myr, with a peak at ∼10 and ∼40 Myr, which match theoretical formation timescales for the most massive HMXBs and Be-XRBs, respectively. We measure an HMXB production rate of 107–136 HMXBs/(M ⊙ yr−1) over the last 50 Myr and 150–199 HMXBs/(M ⊙ yr−1) over the last 80 Myr. For sources with compact object classifications from overlapping NuSTAR observations, we find a preference for giant/supergiant companion stars in black hole HMXBs and main-sequence companion stars in neutron star HMXBs.

The initial spin distribution of B-type stars revealed by the split main sequences of young star clusters

Spectroscopic observations of stars in young open clusters have revealed evidence for a dichotomous distribution of stellar rotational velocities, with 10−30% of stars rotating slowly and the remaining 70−90% rotating fairly rapidly. At the same time, high-precision multiband photometry of young star clusters shows a split main sequence band, which is again interpreted as due to a spin dichotomy. Recent papers suggest that extreme rotation is required to retrieve the photometric split. Our new grids of MESA models and the prevalent SYCLIST models show, however, that initial slow (0−35% of the linear Keplerian rotation velocities) and intermediate (50−65% of the Keplerian rotation velocities) rotation are adequate to explain the photometric split. These values are consistent with the recent spectroscopic measurements of cluster and field stars, and are likely to reflect the birth spin distributions of upper main-sequence stars. A fraction of the initially faster-rotating stars may be able to reach near-critical rotation at the end of their main-sequence evolution and produce Be stars in the turn-off region of young star clusters. However, we find that the presence of Be stars up to two magnitudes below the cluster turnoff advocates for a crucial role of binary interaction in creating Be stars. We argue that surface chemical composition measurements may help distinguish these two Be star formation channels. While only the most rapidly rotating, and therefore nitrogen-enriched, single stars can evolve into Be stars, slow pre-mass-transfer rotation and inefficient accretion allows for mild or no enrichment even in critically rotating accretion-induced Be stars. Our results shed new light on the origin of the spin distribution of young and evolved B-type main sequence stars.

The spins of stripped B stars support magnetic internal angular momentum transport

In order to predict the spins of stellar remnants we need to understand the evolution of the internal rotation of stars, and to identify at which stage the rotation of the contracting cores of evolved stars decouples from their expanding envelopes. The donor stars of mass transferring binaries lose almost their entire envelope and may thus offer a direct view on their core rotation. After the mass transfer event they contract and fade rapidly, although they are well observable when caught in the short-lived B-star phase. The B-type primary of the galactic binary system LB-1, which was originally suggested to contain a massive black hole, is nicely explained as a stripped star accompanied by a fainter Be star. The narrow absorption lines in the primary’s spectrum signify extremely slow rotation, atypical of B-type main-sequence stars. Here we investigate the evolution of mass donors in generic grids of detailed binary evolution models, where both stars include differential rotation, internal angular momentum transport, and spin-orbit coupling. Whereas the mass gainers are typically spun-up during the mass transfer, we find that the spins of the stripped donor models depend sensitively on the employed mechanism for internal angular momentum transport. Purely hydrodynamic transport cannot explain the observed slow rotation, while models including magnetic angular momentum transport are able to reproduce the observed rotation of LB-1 and similar stars, independent of the initial rotation rate. In such models the spin of the white dwarfs that emerge at the end of the evolution is independent of the mass stripping. We find evidence that the mass transfer in LB-1 was moderately non-conservative.

HD 133729: A blue large-amplitude pulsator in orbit around a main-sequence B-type star

Blue large-amplitude pulsators (BLAPs) form a small group of hot objects pulsating in a fundamental radial mode with periods of the order of 30 min. Proposed evolutionary scenarios explain them as evolved low-mass stars: ∼0.3 M⊙ shell-hydrogen-burning objects with a degenerated helium core, more massive (0.5–0.8) M⊙ core-helium-burning stars, or ∼0.7 M⊙ surviving companions of type Ia supernovae. Therefore, their origin remains to be established. Using data from Transiting Exoplanet Survey Satellite, we discovered that HD 133729 is a binary consisting of a late B-type main-sequence star and a BLAP. The BLAP pulsates with a period of 32.37 min decreasing at a rate of ( − 7.11 ± 0.33) × 10−11. The light curve is typical for BLAPs, but it shows an unusual 40-s drop at the descending branch. Due to light dilution by a brighter companion, the observed amplitude of pulsation is much smaller than in other BLAPs. From available photometry, we derived times of maximum light, which revealed the binary nature of the star via an O−C diagram. The diagram shows variations with a period of 23.08433 d that we attribute to the light-travel-time effect in the system. The analysis of these variations allowed us to derive the spectroscopic parameters of the BLAP’s orbit around the binary’s centre of mass. The presence of a hot companion in the system was confirmed by the analysis of its spectral energy distribution, which was also used to place the components in the Hertzsprung-Russell diagram. The obtained position of the BLAP fully agrees with the location of the other members of the class. With the estimated V ≈ 11 mag and the Gaia distance of less than 0.5 kpc, the BLAP is the brightest and the nearest of all known BLAPs. It may become a key object in the verification of the evolutionary scenarios for this class of variables. We argue that low-mass progenitors of the BLAP are excluded if the components are coeval and no mass transfer between the components took place.

Identification of New Classical Be Stars from the LAMOST Medium Resolution Survey

Be stars are B-type main-sequence stars that display broad Balmer emission lines in their spectra. Identification of the Be population is essential to further examine the formation and evolutionary models. We report the detection of classical Be (CBe) stars from observations with the Large sky Area Multi-Object fiber Spectroscopic Telescope Medium Resolution Survey Data Release 7 (LAMOST MRS DR7). We used a deep convolutional neural network, ResNet, with an 18 layer module to examine the morphology of the Hα profile. We identified 1162 candidate Be stars from the collection of 2,260,387 spectra for 789,918 stars in the database. The ResNet network achieves a Be-star classification accuracy of 99.5%. Among the detections, 151 of these are prior known Be stars crossmatched from the literature. By applying a three-step test, we identified 183 new CBe stars. We find that 41 CBe stars are members of known open clusters. Based on an investigation of the kinematics of the identified CBe stars from the Gaia EDR3 astrometric solutions, we identified 16 new runaways. These new identifications will provide a reference for future follow-ups to further investigate their physical properties.

Astrometric and Photometric Verification of Faint Blue White Dwarfs in the Gaia Catalog of nearby Stars

Abstract The Gaia catalog of nearby stars (GCNS) divided all objects with parallaxes >10 mas into GCNS-selected and GCNS-rejected 100 pc samples. Below the white dwarf (WD) sequence in the complete GCNS color–magnitude diagram (CMD), at Gabs > 14.7 + 4.7(G − RP), there appear 60 GCNS-selected faint blue white dwarfs (FBWDs). However this CMD region is also populated by 411 GCNS-rejected objects, mainly from crowded regions toward the Galactic center and the Magellanic Clouds. The WD catalog of Gentile et al. lists only 47 GCNS-selected but also 8 GCNS-rejected objects. I confirm 59 of the GCNS-selected but none of the GCNS-rejected objects as FBWDs from visual inspection and a proper motion check using additional optical sky surveys. Hence FBWDs form an additional branch in the CMD. Compared to the full GCNS-selected 100 pc sample, FBWDs have relatively high proper motions and tangential velocities. They represent interesting targets for studies of ultracool or infrared-faint, and possibly also ultramassive WDs.

Stellar Inclination Angles from Be-star Hα Emission Line Profiles

We demonstrate that the angle between a star's rotation axis and the observer's line of sight, usually called the inclination angle, can be reliably determined for Be stars via Hα emission line profile fitting. We test our method on a sample of 11 Be stars with available long-baseline interferometric data from the Navy Precision Optical Interferometer. We fit the Hα emission line profile of each star to obtain a spectroscopic inclination angle iHα. We then obtain an independent inclination angle estimate, , by fitting the observed interferometric visibilities with model visibilities based on a purely geometric representation of the light distribution on the sky. The sample differences, , are normally distributed with a mean of zero and a standard deviation of 67, and the linear correlation coefficient between iHα and is r = 0.93. As Be stars comprise upwards of one-fifth of all main-sequence B-type stars, this Hα line profile fitting technique has the potential to provide an efficient method for detecting correlated stellar spin axes in young open clusters. Furthermore, if the orientation of the Be star's circumstellar disk on the plane of the sky can be constrained by polarization measurements, it is possible to determine the full 3D stellar rotation vector of each Be star.

Pulsation among TESS A and B stars and the Maia variables

ABSTRACT Classification of over 50 000 TESS stars in sectors 1–18 has resulted in the detection of 766 pulsating main-sequence B stars as well as over 5000 δ Scuti, 2300 γ Doradus, and 114 roAp candidates. Whereas it has been assumed that high-frequency pulsations among B-type main-sequence stars are confined to the early B-type β Cephei stars, the observations indicate that high frequencies are to be found over the whole B-star range, eventually merging with δ Scuti stars. The cool B stars pulsating in high frequencies are called Maia variables. It is shown that Maia variables are not rapidly rotating and thus cannot be β Cephei pulsators that appear to have lower temperatures due to gravity darkening. In the region where β Cephei variables are found, the proportion of pulsating stars is larger and amplitudes are higher and a considerable fraction pulsate in a single mode and low rotation rate. There is no distinct region of slowly pulsating B stars (SPB stars). Stars pulsating solely in low frequencies are found among all B stars. At most, only one-third of B stars appear to pulsate. These results, as well as the fact that a large fraction of A and B stars show rotational modulation, indicate a need for a revision of current ideas regarding stars with radiative envelopes.

A stripped helium star in the potential black hole binary LB-1

Context. The recently claimed discovery of a massive (MBH = 68−13+11 M⊙) black hole in the Galactic solar neighborhood has led to controversial discussions because it severely challenges our current view of stellar evolution. Aims. A crucial aspect for the determination of the mass of the unseen black hole is the precise nature of its visible companion, the B-type star LS V+22 25. Because stars of different mass can exhibit B-type spectra during the course of their evolution, it is essential to obtain a comprehensive picture of the star to unravel its nature and, thus, its mass. Methods. To this end, we study the spectral energy distribution of LS V+22 25 and perform a quantitative spectroscopic analysis that includes the determination of chemical abundances for He, C, N, O, Ne, Mg, Al, Si, S, Ar, and Fe. Results. Our analysis clearly shows that LS V+22 25 is not an ordinary main sequence B-type star. The derived abundance pattern exhibits heavy imprints of the CNO bi-cycle of hydrogen burning, that is, He and N are strongly enriched at the expense of C and O. Moreover, the elements Mg, Al, Si, S, Ar, and Fe are systematically underabundant when compared to normal main-sequence B-type stars. We suggest that LS V+22 25 is a stripped helium star and discuss two possible formation scenarios. Combining our photometric and spectroscopic results with the Gaia parallax, we infer a stellar mass of 1.1 ± 0.5 M⊙. Based on the binary system’s mass function, this yields a minimum mass of 2–3 M⊙ for the compact companion, which implies that it may not necessarily be a black hole but a massive neutron- or main sequence star. Conclusions. The star LS V+22 25 has become famous for possibly having a very massive black hole companion. However, a closer look reveals that the star itself is a very intriguing object. Further investigations are necessary for complete characterization of this object.

Age Determination of Galactic B-type Stars in Double-lined Eclipsing Binaries

Abstract We present the results of age determination for galactic B-type main-sequence stars that are components of double-lined eclipsing binaries. Only detached systems are considered. We analyze 38 binary systems that meet such criteria. The analysis is based on evolutionary computations, and we consider that the age is determined if there is a common value from the radius–age diagrams and the agreement in the position of both components in the Hertzsprung–Russell diagram. In some cases, to meet these two conditions, it was necessary to adjust the value of the metallicity, Z, or/and the parameter of overshooting from the convective core, α ov. We determine a consistent age for 33 of the 38 binaries. Additionally, we completed extensive computations, and for each system, we give the range of α ov and Z for which a consistent solution exists. The age of the studied B-type main-sequence stars ranges, as counted from the Zero Age Main Sequence, from about 2.5 Myr to about 200 Myr.

Oxygen abundance and the N/C versus N/O relation for AFG supergiants and bright giants

ABSTRACT Non-LTE analysis (where LTE is local thermodynamic equilibrium) of the oxygen abundances for 51 Galactic A-, F- and G-type supergiants and bright giants is performed. In contrast with carbon and nitrogen, oxygen does not show any significant systematic anomalies in its abundances log ε(O). There is no marked difference from the initial oxygen abundance, within the errors, for the log ε(O) determination across the Teff interval from 4500–8500 K and the $\log \, g$ interval from 1.2–2.9 dex. This result agrees well with theoretical predictions for stellar models with rotation. With our new data for oxygen and our earlier non-LTE determinations of the N and C abundances for stars from the same sample, we constructed the [N/C] versus [N/O] relation for 17 stars. This relation is known to be a sensitive indicator of stellar evolution. A pronounced correlation between [N/C] versus [N/O] is found; the observed [N/C] increase from 0 to 1.6 dex is accompanied by a [N/O] increase from 0 to 0.9 dex. When comparing the observed [N/C] versus [N/O] relation with the theoretical one, we show that this relation reflects a strong dependence of the evolutionary changes in CNO abundances on the initial rotation velocities of stars. Given that the initial rotational velocities of these stars are expected to satisfy V0 < 150 km s−1, it is found that they are mostly post-first-dredge-up (post-FDU) objects. It is important that such initial velocities V0 are typical for about 80 per cent of the stars in question (i.e. stars with masses 4–19 M$\odot$). A constancy of the total C+N+O abundance during stellar evolution is confirmed. The mean value of log ε(C+N + O) = 8.97 ± 0.08 found for AFG supergiants and bright giants seems to be very close to the initial values of 8.92 (the Sun) or 8.94 (unevolved B-type main-sequence stars).

The spiral potential of the Milky Way

Context. The location of young sources in the Galaxy suggests a four-armed spiral structure, whereas tangential points of spiral arms observed in the integrated light at infrared and radio wavelengths indicate that only two arms are massive. Aims. Variable extinction in the Galactic plane and high light-to-mass ratios of young sources make it difficult to judge the total mass associated with the arms outlined by such tracers. The current objective is to estimate the mass associated with the Sagittarius arm by means of the kinematics of the stars across it. Methods. Spectra of 1726 candidate B- and A-type stars within 3◦ of the Galactic center (GC) were obtained with the FLAMES instrument at the VLT with a resolution of ≈6000 in the spectral range of 396–457 nm. Radial velocities were derived by least-squares fits of the spectra to synthetic ones. The final sample was limited to 1507 stars with either Gaia DR2 parallaxes or main-sequence B-type stars having reliable spectroscopic distances. Results. The solar peculiar motion in the direction of the GC relative to the local standard of rest (LSR) was estimated to U⊙ = 10.7 ± 1.3kms−1. The variation in the median radial velocity relative to the LSR as a function of distance from the sun shows a gradual increase from slightly negative values near the sun to almost 5 km s−1 at a distance of around 4 kpc. A sinusoidal function with an amplitude of 3.4 ± 1.3kms−1 and a maximum at 4.0 ± 0.6 kpc inside the sun is the best fit to the data. A positive median radial velocity relative to the LSR around 1.8 kpc, the expected distance to the Sagittarius arm, can be excluded at a 99% level of confidence. A marginal peak detected at this distance may be associated with stellar streams in the star-forming regions, but it is too narrow to be associated with a major arm feature. Conclusions. A comparison with test-particle simulations in a fixed galactic potential with an imposed spiral pattern shows the best agreement with a two-armed spiral potential having the Scutum–Crux arm as the next major inner arm. A relative radial forcing dFr ≈ 1.5% and a pattern speed in the range of 20–30 km s−1 kpc−1 yield the best fit. The lack of a positive velocity perturbation in the region around the Sagittarius arm excludes it from being a major arm. Thus, the main spiral potential of the Galaxy is two-armed, while the Sagittarius arm is an inter-arm feature with only a small mass perturbation associated with it.

NLTE spectroscopic analysis of the3He anomaly in subluminous B-type stars

Several B-type main-sequence stars show chemical peculiarities. A particularly striking class are the3He stars, which exhibit a remarkable enrichment of3He with respect to4He. This isotopic anomaly has also been found in blue horizontal branch (BHB) and subdwarf B (sdB) stars, which are helium-core burning stars of the extreme horizontal branch. Recent surveys uncovered 113He sdBs. The3He anomaly is not due to thermonuclear processes, but caused by atomic diffusion in the stellar atmosphere. Using a hybrid local/non-local thermodynamic equilibrium (LTE/NLTE) approach for B-type stars, we analyzed high-quality spectra of two known3He BHBs and nine known3He sdBs to determine their isotopic helium abundances and4He/3He abundance ratios. We redetermined their atmospheric parameters and analyzed selected He Ilines, includingλ4922 Å andλ6678 Å, which are very sensitive to4He/3He. Most of the3He sdBs cluster in a narrow temperature strip between 26000 K and 30000 K and are helium deficient in accordance with previous LTE analyses. BD+48° 2721 is reclassified as a BHB star because of its low temperature (Teff= 20700 K). Whereas4He is almost absent (4He/3He < 0.25) in most of the known3He stars, other sample stars show abundance ratios up to4He/3He ∼2.51. A search for3He stars among 26 candidate sdBs from the ESO SPY survey led to the discovery of two new3He sdB stars (HE 0929–0424 and HE 1047–0436). The observed helium line profiles of all BHBs and of three sdBs are not matched by chemically homogeneous atmospheres, but hint at vertical helium stratification. This phenomenon has been seen in other peculiar B-type stars, but is found for the first time for sdBs. We estimate helium to increase from the outer to the inner atmosphere by factors ranging from 1.4 (SB 290) up to 8.0 (BD+48° 2721).

A New Hyper-runaway Star Discovered from LAMOST and Gaia: Ejected Almost in the Galactic Rotation Direction

Abstract In this paper, we report the discovery of a new late-B-type unbound hyper-runaway star (LAMOST-HVS4) from the LAMOST spectroscopic surveys. According to its atmospheric parameters, it is either a B-type main-sequence (MS) star or a blue horizontal branch (BHB) star. Its Galactocentric distance and velocity are 30.3 ± 1.6 kpc and 586 ± 7 km s−1 if it is an MS star, and they are 13.2 ± 3.7 kpc and 590 ± 7 km s−1 if a BHB star. We track its trajectories back and find that the trajectories intersect with the Galactic disk, and the Galactic center lies outside of the intersection region at the 3σ confidence level. We investigate a number of mechanisms that could be responsible for the ejection of the star and find that it is probably ejected from the Galactic disk by supernova explosion or multiple-body interactions in dense young stellar clusters.

Tidal breakup of quadruple stars in the Galactic Centre

The most likely origin of hypervelocity stars (HVSs) is the tidal disruption of a binary star by the supermassive black hole (MBH) in the Galactic Centre (GC). However, HE0437-5439, a $9$ M$_\odot$ B-type main-sequence star moving with a heliocentric radial velocity of about $720$ km s$^{-1}$ at a distance of $\sim 60$ kpc, and the recent discovered hypervelocity binary candidate (HVB), traveling at $\sim 570$ km s$^{-1}$, challenge this standard scenario. Recently, Fragione & Gualandris (2018) have demonstrated that the tidal breakup of a triple star leads to an insufficient rate. Observations show that quadruple stars made up of two binaries orbiting their common center of mass (the so-called 2+2 quadruples) are $\approx 4\%$ of the stars in the solar neighborhood. Although rarer than triples, 2+2 quadruple stars may have a role in ejecting HVBs as due to their larger energy reservoir. We present a numerical study of 2+2 quadruple disruptions by the MBH in the GC and find that the production of HVBs has a probability $\lesssim 2-4\%$, which translates into an ejection rate of $\lesssim 1$ Gyr$^{-1}$, comparable to the triple disruption scenario. Given the low ejection rate, we suggest that alternative mechanisms are responsible for the origin of HVBs, as the ejection from the interaction of a young star cluster with the MBH in the GC and the origin in the Large Magellanic Cloud.