Abstract
In a previous study, we analysed the spectra of 230 cool (Teff < 9000 K) white dwarfs exhibiting strong metal contamination, measuring abundances for Ca, Mg, Fe and in some cases Na, Cr, Ti, ...or Ni. Here, we interpret these abundances in terms of the accretion of debris from extrasolar planetesimals, and infer parent body compositions ranging from crust-like (rich in Ca and Ti) to core-like (rich in Fe and Ni). In particular, two white dwarfs, SDSS J0823+0546 and SDSS J0741+3146, which show log Fe/Ca > 1.9 dex, and Fe to Ni ratios similar to the bulk Earth, have accreted by far the most core-like exoplanetesimals discovered to date. With cooling ages in the range 1–8 Gyr, these white dwarfs are among the oldest stellar remnants in the Milky Way, making it possible to probe the long-term evolution of their ancient planetary systems. From the decrease in maximum abundances as a function of cooling age, we find evidence that the arrival rate of material on to the white dwarfs decreases by three orders of magnitude over a ≃ 6.5 Gyr span in white dwarf cooling ages, indicating that the mass-reservoirs of post-main sequence planetary systems are depleted on a ≃ 1 Gyr e-folding time-scale. Finally, we find that two white dwarfs in our sample are members of wide binaries, and both exhibit atypically high abundances, thus providing strong evidence that distant binary companions can dynamically perturb white dwarf planetary systems.
Little is known about the incidence of magnetic fields among the coolest white dwarfs. Their spectra usually do not exhibit any absorption lines as the bound–bound opacities of hydrogen and helium ...are vanishingly small. Probing these stars for the presence of magnetic fields is therefore extremely challenging. However, external pollution of a cool white dwarf by, e.g. planetary debris, leads to the appearance of metal lines in its spectral energy distribution. These lines provide a unique tool to identify and measure magnetism in the coolest and oldest white dwarfs in the Galaxy. We report the identification of seven strongly metal polluted, cool (T
eff < 8000 K) white dwarfs with magnetic field strengths ranging from 1.9 to 9.6 MG. An analysis of our larger magnitude-limited sample of cool DZ yields a lower limit on the magnetic incidence of 13 ± 4 per cent, noticeably much higher than among hot DA white dwarfs.
The Gaia 20 pc white dwarf sample Hollands, M A; Tremblay, P-E; Gänsicke, B T ...
Monthly notices of the Royal Astronomical Society,
11/2018, Letnik:
480, Številka:
3
Journal Article
ABSTRACT
We present the first volume-limited sample of cataclysmic variables (CVs), selected using the accurate parallaxes provided by the second data release (DR2) of the European Space Agency Gaia ...space mission. The sample is composed of 42 CVs within 150 pc, including two new systems discovered using the Gaia data, and is $(77 \pm 10)$ per cent complete. We use this sample to study the intrinsic properties of the Galactic CV population. In particular, the CV space density we derive, $\rho =(4.8^{+0.6}_{-0.8}) \times 10^{-6}\, \mbox{$\mathrm{pc}^{-3}$}$, is lower than that predicted by most binary population synthesis studies. We also find a low fraction of period bounce CVs, seven per cent, and an average white dwarf mass of $\langle M_\mathrm{WD} \rangle = (0.83 \pm 0.17)\, \mathrm{M}_\odot$. Both findings confirm previous results, ruling out the presence of observational biases affecting these measurements, as has been suggested in the past. The observed fraction of period bounce CVs falls well below theoretical predictions, by at least a factor of five, and remains one of the open problems in the current understanding of CV evolution. Conversely, the average white dwarf mass supports the presence of additional mechanisms of angular momentum loss that have been accounted for in the latest evolutionary models. The fraction of magnetic CVs in the 150 pc sample is remarkably high at 36 per cent. This is in striking contrast with the absence of magnetic white dwarfs in the detached population of CV progenitors, and underlines that the evolution of magnetic systems has to be included in the next generation of population models.
Abstract
White dwarfs with metal lines in their spectra act as signposts for post-main-sequence planetary systems. Searching the Sloan Digital Sky Survey (SDSS) Data Release 12, we have identified ...231 cool (<9000 K) DZ white dwarfs with strong metal absorption, extending the DZ cooling sequence to both higher metal abundances and lower temperatures, and hence longer cooling ages. Of these 231 systems, 104 are previously unknown white dwarfs. Compared with previous work, our spectral fitting uses improved model atmospheres with updated line profiles and line-lists, which we use to derive effective temperatures and abundances for up to eight elements. We also determine spectroscopic distances to our sample, identifying two halo members with tangential space velocities >300 km s−1. The implications of our results on remnant planetary systems are to be discussed in a separate paper.
The recently discovered hypervelocity white dwarf LP 40−365 (aka GD 492) has been suggested as the outcome of the failed disruption of a white dwarf in a subluminous Type Ia supernova (SN Ia). We ...present new observations confirming GD 492 as a single star with unique spectral features. Our spectroscopic analysis suggests that a helium-dominated atmosphere, with 33% neon and 2% oxygen by mass, can reproduce most of the observed properties of this highly unusual star. Although our atmospheric model contrasts with the previous analysis in terms of dominant atmospheric species, we confirm that the atmosphere of GD 492 is strongly hydrogen deficient, , and displays traces of 11 other and iron-group elements (with sulfur, chromium, manganese, and titanium as new detections), indicating nuclear processing of carbon and silicon. We measure a manganese-to-iron ratio seven times larger than solar. While the observed abundances of GD 492 do not fully match any predicted nuclear yields of a partially burned supernova remnant, the manganese excess strongly favors a link with a single-degenerate SN Ia event over alternative scenarios.
A DZ white dwarf with a 30 MG magnetic field Hollands, M A; Stopkowicz, S; Kitsaras, M-P ...
Monthly notices of the Royal Astronomical Society,
02/2023, Letnik:
520, Številka:
3
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
Magnetic white dwarfs with field strengths below 10 MG are easy to recognize since the Zeeman splitting of spectral lines appears proportional to the magnetic field strength. For fields ≳100 ...MG, however, transition wavelengths become chaotic, requiring quantum-chemical predictions of wavelengths and oscillator strengths with a non-perturbative treatment of the magnetic field. While highly accurate calculations have previously been performed for hydrogen and helium, the variational techniques employed become computationally intractable for systems with more than three to four electrons. Modern computational techniques, such as finite-field coupled-cluster theory, allow the calculation of many-electron systems in arbitrarily strong magnetic fields. Because around 25 per cent of white dwarfs have metal lines in their spectra, and some of those are also magnetic, the possibility arises for some metals to be observed in very strong magnetic fields, resulting in unrecognizable spectra. We have identified SDSS J114333.48+661531.83 as a magnetic DZ white dwarf, with a spectrum exhibiting many unusually shaped lines at unknown wavelengths. Using atomic data calculated from computational finite-field coupled-cluster methods, we have identified some of these lines arising from Na, Mg, and Ca. Surprisingly, we find a relatively low field strength of 30 MG, where the large number of overlapping lines from different elements make the spectrum challenging to interpret at a much lower field strength than for DAs and DBs. Finally, we model the field structure of SDSS J1143+6615 finding the data are consistent with an offset dipole.
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.
ABSTRACT
The accretion of planetary debris into the atmospheres of white dwarfs leads to the presence of metal lines in their spectra. Cool metal-rich white dwarfs, which left the main sequence many ...Gyr ago, allow the study of the remnants of the oldest planetary systems. Despite their low effective temperatures (Teff), a non-negligible amount of their flux is emitted in the near-ultraviolet (NUV), where many overlapping metal lines can potentially be detected. We have observed three metal-rich cool white dwarfs with the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope, and compare the results determined from the NUV data with those previously derived from the analysis of optical spectroscopy. For two of the white dwarfs, SDSS J1038−0036 and SDSS J1535+1247, we find reasonable agreement with our previous analysis and the new combined fit of optical and NUV data. For the third object, SDSS J0956+5912, including the STIS data leads to a 10 per cent lower Teff, though we do not identify a convincing explanation for this discrepancy. The unusual abundances found for SDSS J0956+5912 suggest that the accreted parent body was composed largely of water ice and magnesium silicates, and with a mass of up to ≃ 2 × 1025 g. Furthermore, SDSS J0956+5912 shows likely traces of atomic carbon in the NUV. While molecular carbon is not observed in the optical, we demonstrate that the large quantity of metals accreted by SDSS J0956+5912 can suppress the C2 molecular bands, indicating that planetary accretion can convert DQ stars into DZs (and not DQZs/DZQs).
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