Subluminous Type Ia supernovae, such as the Type Iax–class prototype SN 2002cx, are described by a variety of models such as the failed detonation and partial deflagration of an accreting ...carbon-oxygen white dwarf star or the explosion of an accreting, hybrid carbon-oxygen-neon core. These models predict that bound remnants survive such events with, according to some simulations, a high kick velocity. We report the discovery of a high proper motion, low-mass white dwarf (LP 40-365) that travels at a velocity greater than the Galactic escape velocity and whose peculiar atmosphere is dominated by intermediate-mass elements. Strong evidence indicates that this partially burnt remnant was ejected following a subluminous Type Ia supernova event. This supports the viability of single-degenerate supernova progenitors.
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We have measured the orbital parameters of seven close binaries, including six new objects, in a radial velocity survey of 38 objects comprising a hot subdwarf star with orbital periods ranging from ...∼0.17 to 3 d. One new system, GALEX J2205−3141, shows reflection on an M dwarf companion. Three other objects show significant short-period variations, but their orbital parameters could not be constrained. Two systems comprising a hot subdwarf paired with a bright main-sequence/giant companion display short-period photometric variations possibly due to irradiation or stellar activity and are also short-period candidates. All except two candidates were drawn from a selection of subluminous stars in the Galaxy Evolution Explorer ultraviolet sky survey. Our new identifications also include a low-mass subdwarf B star and likely progenitor of a low-mass white dwarf (GALEX J0805−1058) paired with an unseen, possibly substellar, companion. The mass functions of the newly identified binaries imply minimum secondary masses ranging from 0.03 to 0.39 M⊙. Photometric time series suggest that, apart from GALEX J0805−1058 and J2205−3141, the companions are most likely white dwarfs. We update the binary population statistics: close to 40 per cent of hot subdwarfs have a companion. Also, we found that the secondary mass distribution shows a low-mass peak attributed to late-type dwarfs, and a higher mass peak and tail distribution attributed to white dwarfs and a few spectroscopic composites. Also, we found that the population kinematics imply an old age and include a few likely halo population members.
We assembled a catalogue of bright, hot subdwarf and white dwarf stars extracted from a joint ultraviolet, optical and infrared source list. The selection is secured using colour criteria that ...correlate well with effective temperatures T
eff≳ 12 000 K. We built a N
UV− V versus V − J diagram for ≳60 000 bright sources using the Galaxy Evolution Explorer (GALEX) N
UV magnitude (N
UV < 14), the associated Guide Star Catalog (GSC2.3.2) photographic quick-V magnitude and the Two-Micron All-Sky Survey J and H magnitudes. This distillation process delivered a catalogue of ≈700 sources with N
UV− V < 0.5 comprising ∼160 known hot subdwarf stars and another ∼60 known white dwarf stars. A reduced proper-motion diagram built using the proper-motion measurements extracted from the Naval Observatory Merged Astrometric Dataset allowed us to identify an additional ∼120 new hot subdwarf candidates and ∼10 hot white dwarf candidates. We present a spectroscopic study of a subset of 52 subdwarfs, 48 of them analysed here for the first time, and with nine objects brighter than V ∼ 12. Our sample of spectroscopically confirmed hot subdwarfs comprises 10 sdO-type stars and 42 sdB-type stars suitable for pulsation and binary studies. We also present a study of 50 known white dwarfs selected in the GALEX survey and six new white dwarfs from our catalogue of subluminous candidates. Ultraviolet, optical and infrared synthetic magnitudes employed in the selection and analysis of white dwarf stars are listed in appendix.
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We have identified a new cool magnetic white dwarf in the New Luyten Two-Tenths (NLTT) catalogue. The high proper-motion star NLTT 10480 (μ = 0.5″ yr-1) shows weak Zeeman-split lines of calcium as ...well as characteristic Hα and β Zeeman triplets. Using VLT X-shooter spectra, we measured a surface-averaged magnetic field BS ~ 0.5 MG. The relative intensity of the π and σ components of the calcium and hydrogen lines imply a high inclination (i ≳ 60°). The optical-to-infrared V − J colour index and the Ca I/Ca II ionization balance indicate a temperature between 4900 and 5200 K, while the Balmer line profiles favour a higher temperature of 5400 K. The discrepancy is potentially resolved by increasing the metallicity to 0.03 × solar, hence increasing the electron pressure. However, the measured calcium abundance and abundance upper limits for other elements (Na, Al, Si, and Fe) imply a low photospheric metallicity ≲ 10-4 solar. Assuming diffusion steady-state, a calcium accretion rate of log Ṁ(g s-1) = 5.6 ± 0.3 is required to sustain a calcium abundance of log n(Ca)/n(H) = −10.30 ± 0.30 in the white dwarf atmosphere. We examine the implications of this discovery for the incidence of planetary debris and weak magnetic fields in cool white dwarf stars.
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We present a model atmosphere analysis of cool hydrogen-rich white dwarfs observed at the Very Large Telescope (VLT) with the X-shooter spectrograph. The intermediate-dispersion and high ...signal-to-noise ratio of the spectra allowed us to conduct a detailed analysis of hydrogen and heavy element line profiles. In particular, we tested various prescriptions for hydrogen Balmer line broadening parameters and determined the effective temperature and surface gravity of each star. The spectra of three objects (NLTT 1675, 6390, and 11393) show heavy elements (Mg, Al, Ca, or Fe). Our abundance analysis revealed a relatively high iron-to-calcium ratio in NLTT 1675 and NLTT 6390. We also present an analysis of spectropolarimetric data obtained at the VLT using the focal reducer and low dispersion spectrograph (FORS). We establish strict upper limits to the magnetic field strengths in three of the DAZ white dwarfs and determine the longitudinal magnetic field strength in the DAZ NLTT 10480. The class of DAZ white dwarfs comprises objects that are possibly accreting material from their immediate circumstellar environment and the present study helps to establish the class properties.
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We present an analysis of X-shooter spectra of the polluted, hydrogen-rich white dwarf NLTT 25792. The spectra show strong lines of calcium (Ca H&K, near-infrared calcium triplet, and Ca ...sub(I)lambda4226) and numerous lines of iron along with magnesium and aluminum lines from which we draw the abundance pattern. Moreover, the photospheric Ca H&K lines are possibly blended with a circumstellar component shifted by -20 km s super(-1) relative to the photosphere. A comparison with a sample of four white dwarfs with similar parameters show considerable variations in their abundance patterns, particularly in the calcium to magnesium abundance ratio that varies by a factor of five within this sample. The observed variations, even after accounting for diffusion effects, imply similar variations in the putative accretion source. Also, we find that silicon and sodium are significantly underabundant in the atmosphere of NLTT 25792, a fact that may offer some clues on the nature of the accretion source.
We present a detailed model atmosphere analysis of high-dispersion and high signal-to-noise ratio spectra of the heavily polluted DAZ white dwarf GALEX J1931+0117. The spectra obtained with the Very ...Large Telescope (VLT)-Kueyen/UV-Visual Echelle Spectrograph show several well-resolved Si ii spectral lines enabling a study of pressure effects on line profiles. We observed large Stark shifts in silicon lines in agreement with theoretical predictions and laboratory measurements. Taking into account Stark shifts in the calculation of synthetic spectra, we reduced the scatter in individual line radial velocity measurements from ∼3 to ≲1 km s−1. We present revised abundances of O, Mg, Si, Ca and Fe based on a critical review of line-broadening parameters and oscillator strengths. The new measurements are generally in agreement with our previous analysis with the exception of magnesium with a revised abundance of a factor of 2 lower than previously estimated. The magnesium, silicon and iron abundances exceed solar abundances, but the oxygen and calcium abundances are below solar. Also, we compared the observed line profiles to synthetic spectra computed with variable accretion rates and vertical abundance distributions assuming diffusive steady state. The inferred accretion rates vary from
for calcium to 2 × 109 g s−1 for oxygen. We find that the accretion flow must be oxygen rich while being deficient in calcium relative to solar abundances. The lack of radial velocity variations between two measurement epochs suggests that GALEX J1931+0117 is probably not in a close binary and that the source of the accreted material resides in a debris disc.
We report on the discovery of the shortest period binary comprising a hot subdwarf star (CD-30 11223, GALEX J1411-3053) and a massive unseen companion. Photometric data from the All Sky Automated ...Survey show ellipsoidal variations of the hot subdwarf primary and spectroscopic series revealed an orbital period of 70.5 minutes. The large velocity amplitude suggests the presence of a massive white dwarf in the system (M sub(2)/M sub(middot in circle) > 0.77) assuming a canonical mass for the hot subdwarf (0.48 M sub(middot in circle)), although a white dwarf mass as low as 0.75 M sub(middot in circle) is allowable by postulating a subdwarf mass as low as 0.44 M sub(middot in circle). The amplitude of ellipsoidal variations and a high rotation velocity imposed a high-inclination to the system (i > ~ 68degrees) and, possibly, observable secondary transits (i > ~ 74degrees). At the lowest permissible inclination and assuming a subdwarf mass of ~0.48 M sub(middot in circle), the total mass of the system reaches the Chandrasekhar mass limit at 1.35 M sub(middot in circle) and would exceed it for a subdwarf mass above 0.48 M sub(middot in circle). The system should be considered, like its sibling KPD 1930+2752, a candidate progenitor for a Type Ia supernova. The system should become semi-detached and initiate mass transfer within asymptotically =30 Myr.
We report on the discovery of the extremely low-mass, hydrogen-rich white dwarf, NLTT 11748. Based on measurements of the effective temperature (8540 ± 50 K) and surface gravity (log g = 6.20 ± 0.15) ...obtained by fitting the observed Balmer line profiles with synthetic spectra, we derive a mass of 0.167 ± 0.005 $M_\odot$. This object is one of only a handful of white dwarfs with masses below 0.2 $M_\odot$ that are believed to be the product of close binary evolution with an episode of Roche lobe overflow onto a degenerate companion (neutron star or white dwarf). Assuming membership in the halo population, as suggested by the kinematics and adopting a cooling age of $4.0{-}6.3$ Gyr for the white dwarf, we infer a progenitor mass of 0.87-0.93 $M_\odot$. The likely companion has yet to be identified, but a search for radial velocity variations may help constrain its nature.
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We report on the discovery of a new heavily polluted white dwarf. The DAZ white dwarf GALEX J193156.8+011745 was identified in a joint Galaxy Evolution Explorer (GALEX)/GSC survey of ...ultraviolet-excess objects. Optical spectra obtained at ESO New Technology Telescope show strong absorption lines of magnesium and silicon, and a detailed abundance analysis based on Very Large Telescope-Kueyen UVES spectra reveal super-solar abundances of silicon and magnesium and near-solar abundances of oxygen, calcium and iron. The overall abundance pattern bears the signature of on-going accretion on to the white dwarf atmosphere. The infrared spectral energy distribution shows an excess in the H and K bands likely associated with the accretion source.
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