Context.
Blue horizontal-branch stars evolve from low-mass stars that have completed their main-sequence lifetimes and undergone a helium flash at the end of their red-giant phase. As such, blue ...horizontal-branch stars are very old objects that can be used as markers in studies of the Galactic structure and formation history. To create a clean sky catalogue of blue horizontal-branch stars, we cross-matched the
Gaia
data release 2 (DR2) dataset with existing reference catalogues to define selection criteria based on
Gaia
DR2 parameters. Following the publication of
Gaia
early data release 3 (EDR3), these methods were verified and subsequently applied to this latest release.
Aims.
Previous catalogues of blue horizontal-branch stars were developed using spectral analyses or were restricted to individual globular clusters. The purpose of this catalogue is to identify a set of blue horizontal-branch star candidates that have been selected using photometric and astrometric observations and exhibits a low contamination rate. This has been deemed important as the success of the
Gaia
mission has changed the way that targets are selected for large-scale spectroscopic surveys, meaning that far fewer spectra will be acquired for blue horizontal-branch stars in the future unless they are specifically targeted.
Methods.
We cross-matched reference blue horizontal-branch datasets with the
Gaia
DR2 database and defined two sets of selection criteria. Firstly, in
Gaia
DR2 – colour and absolute
G
magnitude space, and secondly, in
Gaia
DR2 – colour and reduced proper motion space. The main-sequence contamination in both subsets of the catalogue was reduced, at the expense of completeness, by concentrating on the Milky Way’s Galactic halo, where relatively young main-sequence stars were not expected. The entire catalogue is limited to those stars with no apparent neighbours within 5 arcsec. These methods were verified and subsequently applied to the
Gaia
EDR3.
Results.
We present a catalogue, based on
Gaia
EDR3, of 57 377 blue horizontal-branch stars. The
Gaia
EDR3 parallax was used in selecting 16 794 candidates and the proper motions were used to identify a further 40 583 candidates.
Abstract
We report the discovery of three stars that, along with the prototype LP 40−365, form a distinct class of chemically peculiar runaway stars that are the survivors of thermonuclear ...explosions. Spectroscopy of the four confirmed LP 40−365 stars finds ONe-dominated atmospheres enriched with remarkably similar amounts of nuclear ashes of partial O- and Si-burning. Kinematic evidence is consistent with ejection from a binary supernova progenitor; at least two stars have rest-frame velocities indicating they are unbound to the Galaxy. With masses and radii ranging between 0.20 and 0.28 M$\odot$ and between 0.16 and 0.60 R$\odot$, respectively, we speculate these inflated white dwarfs are the partly burnt remnants of either peculiar Type Iax or electron-capture supernovae. Adopting supernova rates from the literature, we estimate that ∼20 LP 40−365 stars brighter than 19 mag should be detectable within 2 kpc from the Sun at the end of the Gaia mission. We suggest that as they cool, these stars will evolve in their spectroscopic appearance, and eventually become peculiar O-rich white dwarfs. Finally, we stress that the discovery of new LP 40−365 stars will be useful to further constrain their evolution, supplying key boundary conditions to the modelling of explosion mechanisms, supernova rates, and nucleosynthetic yields of peculiar thermonuclear explosions.
In light of substantial new discoveries of hot subdwarfs by ongoing spectroscopic surveys and the availability of new all-sky data from ground-based photometric surveys and the
Gaia
mission Data ...Release 2, we compiled an updated catalogue of the known hot subdwarf stars. The catalogue contains 5874 unique sources including 528 previously unknown hot subdwarfs and provides multi-band photometry, astrometry from
Gaia
, and classifications based on spectroscopy and colours. This new catalogue provides atmospheric parameters of 2187 stars and radial velocities of 2790 stars from the literature. Using colour, absolute magnitude, and reduced proper motion criteria, we identified 268 previously misclassified objects, most of which are less luminous white dwarfs or more luminous blue horizontal branch and main-sequence stars.
Hypervelocity stars are rare objects, mostly main-sequence (MS) B stars, traveling so fast that they will eventually escape from the Milky Way. Recently, it has been shown that the popular Hills ...mechanism, in which a binary system is disrupted via a close encounter with the supermassive black hole at the Galactic center, may not be their only ejection mechanism. The analyses of Gaia data ruled out a Galactic center origin for some of them, and instead indicated that they are extreme disk runaway stars ejected at velocities exceeding the predicted limits of classical scenarios (dynamical ejection from star clusters or binary supernova ejection). We present the discovery of a new extreme disk runaway star, PG 1610+062, which is a slowly pulsating B star bright enough to be studied in detail. A quantitative analysis of spectra taken with ESI at the Keck Observatory revealed that PG 1610+062 is a late B-type MS star of 4–5 M⊙ with low projected rotational velocity. Abundances (C, N, O, Ne, Mg, Al, Si, S, Ar, and Fe) were derived differentially with respect to the normal B star HD 137366 and indicate that PG 1610+062 is somewhat metal rich. A kinematic analysis, based on our spectrophotometric distance (17.3 kpc) and on proper motions from Gaia’s second data release, shows that PG 1610+062 was probably ejected from the Carina-Sagittarius spiral arm at a velocity of 550 ± 40 km s−1, which is beyond the classical limits. Accordingly, the star is in the top five of the most extreme MS disk runaway stars and is only the second among the five for which the chemical composition is known.
The quasar Q0918+1636 (z = 3.07) has two intervening high-metallicity Damped Lyman α Absorbers (DLAs) along the line of sight, at redshifts of z = 2.412 and 2.583. The z = 2.583 DLA is located at a ...large impact parameter of 16.2 kpc, and despite this large impact parameter it has a very high metallicity (consistent with solar), a substantial fraction of H2 molecules and it is dusty as inferred from the reddened spectrum of the background QSO. The z = 2.412 DLA has a metallicity of M/H = −0.6 (based on Zn ii and Si ii). In this paper we present new observations of this interesting sightline consisting of deep multiband imaging and further VLT spectroscopy. By fitting stellar population synthesis models to the photometric Spectral Energy Distribution we constrain the physical properties of the z = 2.583 DLA galaxy, and we infer its morphology by fitting a Sérsic model to its surface brightness profile. We find it to be a relatively massive (M
1010 M), strongly star-forming (SFR 30 M yr−1), dusty (E(B − V) = 0.4) galaxy with a disc-like morphology. We detect strong emission lines from the z = 2.583 DLA (O ii λ3727, O iii λλ4960, 5007, Hβ and Hα, albeit at low signal-to-noise ratio except for the O iii λ5007 line). The metallicity derived from the emission lines is consistent with the absorption metallicity (12 + log (O/H) = 8.8 ± 0.2). We also detect O iii λ5007 emission from the galaxy counterpart of the z = 2.412 DLA at a small impact parameter (<2 kpc). Overall our findings are consistent with the emerging picture that high-metallicity DLAs are associated with relatively luminous and massive galaxy counterparts, compared to typical DLAs.
Context.
Thermonuclear supernovae (SNe), a subset of which are the highly important SNe Type Ia, remain one of the more poorly understood phenomena known to modern astrophysics. In recent years, the ...single degenerate helium (He) donor channel, where a white dwarf star accretes He-rich matter from a hydrogen-depleted companion, has emerged as a promising candidate progenitor scenario for these events. An unresolved question in this scenario is the fate of the companion star, which would be evident as a runaway hot subdwarf O/B stars (He sdO/B) in the aftermath of the SN event.
Aims.
Previous studies have shown that the kinematic properties of an ejected companion provide an opportunity to closer examine the properties of an SN progenitor system. However, with the number of observed objects not matching predictions by theory, the viability of this mechanism is called into question. In this study, we first synthesize a population of companion stars ejected by the aforementioned mechanism, taking into account predicted ejection velocities, the inferred population density in the Galactic mass distribution, and subsequent kinematics in the Galactic potential. We then discuss the astrometric properties of this population.
Methods.
We present 10
6
individual ejection trajectories, which were numerically computed with a newly developed, lightweight simulation framework. Initial conditions were randomly generated, but weighted according to the Galactic mass density and ejection velocity data. We then discuss the bulk properties (Galactic distribution and observational parameters) of our sample.
Results.
Our synthetic population reflects the Galactic mass distribution. A peak in the density distribution for close objects is expected in the direction of the Galactic centre. Higher mass runaways should outnumber lower mass ones. If the entire considered mass range is realised, the radial velocity distribution should show a peak at 500 km s
−1
. If only close US 708 analogues are considered, there should be a peak at (∼750 − 850) km s
−1
. In either case, US 708 should be a member of the high-velocity tail of the distribution.
Conclusions.
We show that the puzzling lack of confirmed surviving companion stars of thermonuclear SNe, though possibly an observation-related selection effect, may indicate a selection against high mass donors in the SD He donor channel.
We use the resonant dipole-dipole interaction between Rydberg atoms and a periodic external microwave field to engineer XXZ spin Hamiltonians with tunable anisotropies. The atoms are placed in ...one-dimensional (1D) and two-dimensional (2D) arrays of optical tweezers. As illustrations, we apply this engineering to two iconic situations in spin physics: the Heisenberg model in square arrays and spin transport in 1D. We first benchmark the Hamiltonian engineering for two atoms and then demonstrate the freezing of the magnetization on an initially magnetized 2D array. Finally, we explore the dynamics of 1D domain-wall systems with both periodic and open boundary conditions. We systematically compare our data with numerical simulations and assess the residual limitations of the technique as well as routes for improvement. The geometrical versatility of the platform, combined with the flexibility of the simulated Hamiltonians, opens up exciting prospects in the fields of quantum simulation, quantum information processing, and quantum sensing.
We analyzed the near-infrared to UV data of 16 quasars with redshifts ranging from 0.71 <z< 2.13 to investigate dust extinction properties. The sample presented in this work was obtained from the ...High AV Quasar (HAQ) survey. The quasar candidates were selected from the Sloan Digital Sky Survey (SDSS) and the UKIRT Infrared Deep Sky Survey (UKIDSS), and follow-up spectroscopy was carried out at the Nordic Optical Telescope (NOT) and the New Technology Telescope (NTT). To study dust extinction curves intrinsic to the quasars, we selected 16 cases from the HAQ survey for which the Small Magellanic Cloud (SMC) law could not provide a good solution to the spectral energy distributions (SEDs). We derived the extinction curves using the Fitzpatrick & Massa (1986, ApJ, 307, 286, FM) law by comparing the observed SEDs to a combined previously published quasar template. The derived extinction, AV, ranges from 0.2–1.0 mag. All the individual extinction curves of our quasars are steeper (RV = 2.2–2.7) than that of the SMC, with a weighted mean value of RV = 2.4. We derived an average quasar extinction curve for our sample by simultaneously fitting SEDs by using the weighted mean values of the FM law parameters and a varying RV. The entire sample is well fit with a single best-fit value of RV = 2.2 ± 0.2. The average quasar extinction curve deviates from the steepest Milky Way and SMC extinction curves at a confidence level ≳95%. Such steep extinction curves suggest that a significant population of silicates is involved in producing small dust grains. Another possibility might be that the large dust grains may have been destroyed by the activity of the nearby active galactic nuclei (AGN), resulting in steep extinction curves.
Type Ia supernovae (SN Ia) are the most important standard candles for measuring the expansion history of the universe. The thermonuclear explosion of a white dwarf can explain their observed ...properties, but neither the progenitor systems nor any stellar remnants have been conclusively identified. Underluminous SN Ia have been proposed to originate from a so-called double-detonation of a white dwarf. After a critical amount of helium is deposited on the surface through accretion from a close companion, the helium is ignited causing a detonation wave that triggers the explosion of the white dwarf itself. We have discovered both shallow transits and eclipses in the tight binary system CD-30°11223 composed of a carbon/oxygen white dwarf and a hot helium star, allowing us to determine its component masses and fundamental parameters. In the future the system will transfer mass from the helium star to the white dwarf. Modelling this process we find that the detonation in the accreted helium layer is sufficiently strong to trigger the explosion of the core. The helium star will then be ejected at such high velocity that it will escape the Galaxy. The predicted properties of this remnant are an excellent match to the so-called hypervelocity star US 708, a hot, helium-rich star moving at more than 750 km s-1, sufficient for it to leave the Galaxy. The identification of both progenitor and remnant provides a consistent picture of the formation and evolution of underluminous SNIa.
The project Massive Unseen Companions to Hot Faint Under-luminous Stars from SDSS (MUCHFUSS) aims at finding hot sub-dwarf stars with massive compact companions like massive white dwarfs, neutron ...stars, or stellar-mass black holes. We present orbital and atmospheric parameters and put constraints on the nature of the companions of 12 close hot sub-dwarf B star (sdB) binaries found in the course of the MUCHFUSS project. We identified 16 systems where the dM companion will fill its Roche Lobe within a Hubble time and will evolve into a cataclysmic variable; two of them will have a brown dwarf as donor star. Twelve systems with confirmed white dwarf companions will merge within a Hubble time, two of them having a mass ratio to evolve into a stable AM Cvn-type binary and another two which are potential supernova Ia progenitor systems. The remaining eight systems will most likely merge and form RCrB stars or massive C/O white dwarfs depending on the structure of the white dwarf companion.
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