Evidence of Rocky Planetesimals Orbiting Two Hyades Stars

The Hyades is the nearest open cluster, relatively young and containing numerous A-type stars; its known age, distance, and metallicity make it an ideal site to study planetary systems around 2-3 Msun stars at an epoch similar to the late heavy bombardment. Hubble Space Telescope far-ultraviolet spectroscopy strongly suggests ongoing, external metal pollution in two remnant Hyads. For ongoing accretion in both stars, the polluting material has log[n(Si)/n(C)] > 0.2, is more carbon deficient than chondritic meteorites, and is thus rocky. These data are consistent with a picture where rocky planetesimals and small planets have formed in the Hyades around two main-sequence A-type stars, whose white dwarf descendants bear the scars. These detections via metal pollution are shown to be equivalent to infrared excesses of Lir/L* ~ 1e-6 in the terrestrial zone of the stars.Comment: 7 pages, 3 figures, 2 tables, accepted to MNRA

The frequency of planetary debris around young white dwarfs

(Abridged) We present the results of the first unbiased survey for metal pollution among H-atmosphere (DA) white dwarfs with cooling ages of 20-200 Myr and 17000K < Teff < 27000K, using HST COS in the far UV between 1130 and 1435 A. The atmospheric parameters and element abundances are determined using theoretical models, which include the effects of element stratification due to gravitational settling and radiative levitation. We find 48 of the 85 DA white dwarfs studied, or 56% show traces of metals. In 25 stars, the elements can be explained by radiative levitation alone, although we argue that accretion has very likely occurred recently. The remaining 23 white dwarfs (27%) must be currently accreting. Together with previous studies, we find no accretion rate trend in cooling age from ~40 Myr to ~2 Gyr. The median, main sequence progenitor of our sample corresponds to a star of ~2 Msun, and we find 13 of 23 white dwarfs descending from 2-3 Msun late B- and A-type stars to be currently accreting. Only one of 14 targets with Mwd > 0.8 Msun is found to be currently accreting, which suggests a large fraction are double-degenerate mergers, and the merger discs do not commonly reform large planetesimals or otherwise pollute the remnant. We reconfirm our previous finding that two white dwarf Hyads are currently accreting rocky debris. At least 27%, and possibly up to ~50%, of all white dwarfs with cooling ages 20-200 Myr are accreting planetary debris. At Teff > 23000K, the luminosity of white dwarfs is likely sufficient to vaporize circumstellar dust, and hence no stars with strong metal-pollution are found. However, planetesimal disruption events should occur in this cooling age and Teff range as well, and likely result in short phases of high mass transfer rates. It appears that the formation of rocky planetary material is common around 2-3 Msun late B- and A-type stars.Comment: Accepted by A&

Atmospheric parameters and carbon abundance for hot DB white dwarfs

Atmospheric parameters for hot DB (helium atmosphere) white dwarfs near effective temperatures of 25000K are extremely difficult to determine from optical spectroscopy. This is particularly unfortunate, because this is the range of variable DBV or V777 Her stars. Accurate atmospheric parameters are needed to help or confirm the asteroseismic analysis of these objects. Another important aspect is the new class of white dwarfs - the hot DQ - detected by Dufour et al. (2007), with spectra dominated by carbon lines. The analysis shows that their atmospheres are pure carbon. The origin of these stars is not yet understood, but they may have an evolutionary link with the hotter DBs as studied here. Our aim is to determine accurate atmospheric parameters and element abundances and study the implications for the evolution white dwarfs of spectral classes DB and hot DQ. High resolution UV spectra of five DBs are studied with model atmospheres. We determine stellar parameters and abundances or upper limits of C and Si. These objects are compared with cooler DBs below 20000K. We find photospheric C and no other heavy elements - with extremely high limits on the C/Si ratio - in two of the five hot DBs. We compare various explanations for this unusual composition, which have been proposed in the literature: accretion of interstellar or circumstellar matter, radiative levitation, carbon dredge-up from deeper interior below the helium layer, and a residual stellar wind. None of these explanations is completely satisfactory, and the problem of the origin of the hot DQ remains an open question

Model atmosphere analysis of the extreme DQ white dwarf GSC2U J131147.2+292348

A new model atmosphere analysis for the peculiar DQ white dwarf discovered by Carollo et al. (2002) is presented. The effective temperature and carbon abundance have been estimated by fitting both the photometric data (UBJ,VRF,IN,JHK) and a low resolution spectrum (3500<lambda<7500 A) with a new model grid for helium-rich white dwarfs with traces of carbon (DQ stars). We estimate Teff ~ 5120 +/- 200 K and log[C/He] ~ -5.8 +/- 0.5, which make GSC2U J131147.2+292348 the coolest DQ star ever observed. This result indicates that the hypothetical transition from C2 to C2H molecules around Teff = 6000 K, which was inferred to explain the absence of DQ stars at lower temperatures, needs to be reconsidered.Comment: 4 pages, 2 figures, accepted for publication in Astronomy and Astrophysics Letter

On the Nature of the Peculiar Hot Star in the Young LMC Cluster NGC1818

The blue star reported in the field of the young LMC cluster NGC1818 by Elson et al. (1998) has the wrong luminosity and radius to be a "luminous white dwarf" member of the cluster. In addition, unless the effective temperature quoted by the authors is a drastic underestimate, the luminosity is much too low for it to be a cluster member in the post-AGB phase. Other possibilities, including that of binary evolution, are briefly discussed. However, the implication that the massive main sequence turnoff stars in this cluster can produce white dwarfs (instead of neutron stars) from single-star evolution needs to be reconsidered.Comment: 5 pages, no figures, Ap J Letters in pres

The cool end of the DZ sequence in the SDSS

We report the discovery of cool DZ white dwarfs, which lie in the SDSS (u-g) vs. (g-r) two-color diagram across and below the main sequence. These stars represent the extension of the well-known DZ sequence towards cooler temperatures.Comment: To appear in the proceedings of the "17th European Workshop on White Dwarfs", Tuebingen, Germany, August 16-20, 201

Non-LTE models for the gaseous metal component of circumstellar discs around white dwarfs

Gaseous metal discs around single white dwarfs have been discovered recently. They are thought to develop from disrupted planetary bodies. Spectroscopic analyses will allow us to study the composition of extrasolar planetary material. We investigate in detail the first object for which a gas disc was discovered (SDSS J122859.93+104032.9). Therefor we perform non-LTE modelling of viscous gas discs by computing the detailed vertical structure and line spectra. The models are composed of carbon, oxygen, magnesium, silicon, calcium, and hydrogen with chemical abundances typical for Solar System asteroids. Line asymmetries are modelled by assuming spiral-arm and eccentric disc structures as suggested by hydrodynamical simulations. The observed infrared Ca II emission triplet can be modelled with a hydrogen-deficient metal gas disc located inside of the tidal disruption radius, with an effective temperature of about 6000 K and a surface mass density of 0.3 g/cm^2. The inner radius is well constrained at about 0.64 Solar radii. The line profile asymmetry can be reproduced by either a spiral-arm structure or an eccentric disc, the latter being favoured by its time variability behaviour. Such structures, reaching from 0.64 to 1.5 Solar radii, contain a mass of about 3 to 6*10^21 g, the latter equivalent to the mass of a 135-km diameter Solar System asteroid.Comment: 7 pages, 10 figures, accepted for publication in A&

The pre-cataclysmic variable, LTT 560

Aims. System parameters of the object LTT560 are determined in order to clarify its nature and evolutionary status. Methods. We apply time-series photometry to reveal orbital modulations of the light curve, time-series spectroscopy to measure radial velocities of features from both the primary and the secondary star, and flux-calibrated spectroscopy to derive temperatures of both components. Results. We find that LTT 560 is composed of a low temperature (T ∼ 7500 K) DA white dwarf as the primary and an M5.5±1 mainsequence star as the secondary component. The current orbital period is Porb = 3.54(07) h.We derive a mass ratio Msec/Mwd = 0.36(03) and estimate the distance to d = 25–40 pc. Long-term variation of the orbital light curve and an additional Hα emission component on the white dwarf indicate activity in the system, probably in the form of flaring and/or accretion events