938 research outputs found
Rapid variability of accretion in AM Herculis
We present the last pointed observation of AM Her carried out during the life
of the BeppoSAX satellite. It was bright at the beginning of the observation,
but dropped to the lowest X-ray level ever observed so far. The X-ray emission
during the bright period is consistent with accretion occurring onto the main
pole of the magnetized white dwarf. The rapid change from the active state to
the low deep state indicates a drop by a factor of 17 in the accretion rate and
hence that accretion switched-off. The short timescale (less than one hour) of
this variation still remains a puzzle. Optical photometry acquired
simultaneousy during the low state shows that the white dwarf remains heated,
although a weak emission from the accretion stream could be still present.
Cyclotron radiation, usually dominating the V and R bands, is negligible thus
corroborating the possibility that AM Her was in an off-accretion state. The
X-ray emission during the inactive state is consistent with coronal emission
from the secondary late type star.Comment: 6 pages A&A-Latex, 6 Figures, accepted for publication in A&
Detection of the white dwarf and the secondary star in the new SU UMa dwarf nova HS 2219+1824
We report the discovery of a new, non-eclipsing SU UMa-type dwarf nova, HS 2219+1824. Photometry obtained in
quiescence (V â 17.5) reveals a double-humped light curve from which we derive an orbital period of 86.2 min. Additional photometry obtained during a superoutburst reaching V 12.0 clearly shows superhumps with a period of 89.05 min. The optical spectrum contains double-peaked Balmer and He I emission lines from the accretion disc as well as broad absorption troughs of HÎČ, HÎł, and HÎŽ from the white dwarf primary star. Modelling of the optical spectrum implies a white dwarf temperature of 13 000 K < âŒ
T eïŹ < ⌠17 000 K, a distance of 180 pc < ⌠d <
⌠230 pc, and suggests that the spectral type of the donor star is later than M 5. Phase-resolved spectroscopy obtained during quiescence reveals a narrow Hα emission line component which has a radial velocity amplitude and phase consistent with an origin on the secondary star, possibly on the irradiated hemisphere facing the white dwarf. This constitutes the first detection of line emission from the secondary star in a quiescent SU UMa star
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
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
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&
A new soft X-ray spectral model for polars with an application to AM Herculis
We present a simple heuristic model for the time-averaged soft X-ray
temperature distribution in the accretion spot on the white dwarf in polars.
The model is based on the analysis of the Chandra LETG spectrum of the
prototype polar AM Her and involves an exponential distribution of the emitting
area vs. blackbody temperature a(T) = a0 exp(-T/T0). With one free parameter
besides the normalization, it is mathematically as simple as the single
blackbody, but is physically more plausible and fits the soft X-ray and
far-ultraviolet spectral fluxes much better. The model yields more reliable
values of the wavelength-integrated flux of the soft X-ray component and the
implied accretion rate than reported previously.Comment: 5 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
Runaway accretion of metals from compact debris disks onto white dwarfs
It was recently proposed that metal-rich white dwarfs (WDs) accrete their
metals from compact debris disks found to exist around more than a dozen of
them. At the same time, elemental abundances measured in atmospheres of some
WDs imply vigorous metal accretion at rates up to g/s, far in excess
of what can be supplied solely by Poynting-Robertson drag acting on such debris
disks. To explain this observation we propose a model, in which rapid transport
of metals from the disk onto the WD naturally results from interaction between
this particulate disk and spatially coexisting disk of metallic gas. The latter
is fed by evaporation of debris particles at the sublimation radius located at
several tens of WD radii. Because of pressure support gaseous disk orbits WD
slower than particulate disk. Resultant azimuthal drift between them at speed
~1 m/s causes aerodynamic drag on the disk of solids and drives inward
migration of its constituent particles. Upon reaching the sublimation radius
particles evaporate, enhancing the density of metallic gaseous disk and leading
to positive feedback. Under favorable circumstances (low viscosity in the disk
of metallic gas and efficient aerodynamic coupling between the disks) system
evolves in a runaway fashion, destroying debris disk on time scale of yr, and giving rise to high metal accretion rates up to
g/s, in agreement with observations.Comment: 5 pages, 2 figures, submitted to ApJ
The unbiased frequency of planetary signatures around single and binary white dwarfs using and
This paper presents combined IRAC and COS
results for a double-blind survey of 195 single and 22 wide binary white dwarfs
for infrared excesses and atmospheric metals. The selection criteria include
cooling ages in the range 9 to 300 Myr, and hydrogen-rich atmospheres so that
the presence of atmospheric metals can be confidently linked to ongoing
accretion from a circumstellar disc. The entire sample has infrared photometry,
whereas 168 targets have corresponding ultraviolet spectra. Three stars with
infrared excesses due to debris discs are recovered, yielding a nominal
frequency of per cent, while in stark contrast, the
fraction of stars with atmospheric metals is per cent. Thus, only one
out of 30 polluted white dwarfs exhibits an infrared excess at 3-4 m in
IRAC photometry, which reinforces the fact that atmospheric metal pollution is
the most sensitive tracer of white dwarf planetary systems. The corresponding
fraction of infrared excesses around white dwarfs with wide binary companions
is consistent with zero, using both the infrared survey data and an independent
assessment of potential binarity for well-established dusty and polluted stars.
In contrast, the frequency of atmospheric pollution among the targets in wide
binaries is indistinct from apparently single stars, and moreover the
multiplicity of polluted white dwarfs in a complete and volume-limited sample
is the same as for field stars. Therefore, it appears that the delivery of
planetesimal material onto white dwarfs is ultimately not driven by stellar
companions, but by the dynamics of planetary bodies.Comment: 15 pages, 6 figures, 3 tables; accepted for publication in MNRA
Thermal emission from low-field neutron stars
We present a new grid of LTE model atmospheres for weakly magnetic
(B<=10e10G) neutron stars, using opacity and equation of state data from the
OPAL project and employing a fully frequency- and angle-dependent radiation
transfer. We discuss the differences from earlier models, including a
comparison with a detailed NLTE calculation. We suggest heating of the outer
layers of the neutron star atmosphere as an explanation for the featureless
X-ray spectra of RX J1856.5-3754 and RX J0720.4-3125 recently observed with
Chandra and XMM.Comment: 8 pages A&A(5)-Latex, 6 Figures, A&A in press. The model spectra
presented here are available as XSPEC tables at
http://www.astro.soton.ac.uk/~btg/outgoing/nsspec
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