48 research outputs found
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&
Signs of a faint disc population at polluted white dwarfs
Observations of atmospheric metals and dust discs around white dwarfs provide
important clues to the fate of terrestrial planetary systems around
intermediate mass stars. We present Spitzer IRAC observations of 15 metal
polluted white dwarfs to investigate the occurrence and physical properties of
circumstellar dust created by the disruption of planetary bodies. We find
subtle infrared excess emission consistent with warm dust around KUV 15519+1730
and HS 2132+0941, and weaker excess around the DZ white dwarf G245-58, which,
if real, makes it the coolest white dwarf known to exhibit a 3.6 micron excess
and the first DZ star with a bright disc. All together our data corroborate a
picture where 1) discs at metal-enriched white dwarfs are commonplace and most
escape detection in the infrared (possibly as narrow rings), 2) the discs are
long lived, having lifetimes on the order of 10^6 yr or longer, and 3) the
frequency of bright, infrared detectable discs decreases with age, on a
timescale of roughly 500 Myr, suggesting large planetesimal disruptions decline
on this same timescale.Comment: 11 pages, 6 figures, 5 tables, MNRAS accepted. Minor changes to match
published versio
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
Does GD 356 have a Terrestrial Planetary Companion?
GD 356 is unique among magnetic white dwarfs because it shows Zeeman-split
Balmer lines in pure emission. The lines originate from a region of nearly
uniform field strength (delta B/B is approximately 0.1) that covers 10 per cent
of the stellar surface in which there is a temperature inversion. The energy
source that heats the photosphere remains a mystery but it is likely to be
associated with the presence of a companion. Based on current models we use
archival Spitzer IRAC observations to place a new and stringent upper limit of
12 Jupiter masses for the mass of such a companion. In the light of this result
and the recent discovery of a 115 min photometric period for GD 356, we exclude
previous models that invoke accretion and revisit the unipolar inductor model
that has been proposed for this system. In this model a highly conducting
planet with a metallic core orbits the magnetic white dwarf and, as it cuts
through field lines, a current is set flowing between the two bodies. This
current dissipates in the photosphere of the white dwarf and causes a
temperature inversion. Such a planet is unlikely to have survived the RGB/AGB
phases of evolution so we argue that it may have formed from the circumstellar
disc of a disrupted He or CO core during a rare merger of two white dwarfs. GD
356 would then be a white dwarf counterpart of the millisecond binary pulsar
PSR 1257+12 which is known to host a planetary system.Comment: 9 pages, 4 figures, accepted by MNRA
A Word to the WISE: Confusion is Unavoidable for WISE-selected Infrared Excesses
Stars with excess infrared radiation from circumstellar dust are invaluable
for studies of exoplanetary systems, informing our understanding on processes
of planet formation and destruction alike. All-sky photometric surveys have
made the identification of dusty infrared excess candidates trivial, however,
samples that rely on data from WISE are plagued with source confusion, leading
to high false positive rates. Techniques to limit its contribution to
WISE-selected samples have been developed, and their effectiveness is even more
important as we near the end-of-life of Spitzer, the only facility capable of
confirming the excess. Here, we present a Spitzer follow-up of a sample of 22
WISE-selected infrared excess candidates near the faint-end of the WISE
detection limits. Eight of the 22 excesses are deemed the result of source
confusion, with the remaining candidates all confirmed by the Spitzer data. We
consider the efficacy of ground-based near-infrared imaging and astrometric
filtering of samples to limit confusion among the sample. We find that both
techniques are worthwhile for vetting candidates, but fail to identify all of
the confused excesses, indicating that they cannot be used to confirm
WISE-selected infrared excess candidates, but only to rule them out. This
result confirms the expectation that WISE-selected infrared excess samples will
always suffer from appreciable levels of contamination, and that care should be
taken in their interpretation regardless of the filters applied.Comment: 13 pages, 4 Figures; Accepted for publication in Ap
The puzzling source IGR J17361-4441 in NGC 6388: a possible planetary tidal disruption event
On 2011 August 11, INTEGRAL discovered the hard X-ray source IGR J17361-4441
near the centre of the globular cluster NGC 6388. Follow up observations with
Chandra showed the position of the transient was inconsistent with the cluster
dynamical centre, and thus not related to its possible intermediate mass black
hole. The source showed a peculiar hard spectrum (Gamma \approx 0.8) and no
evidence of QPOs, pulsations, type-I bursts, or radio emission. Based on its
peak luminosity, IGR J17361-4441 was classified as a very faint X-ray
transient, and most likely a low-mass X-ray binary. We re-analysed 200 days of
Swift/XRT observations, covering the whole outburst of IGR J17361-4441 and find
a t^{-5/3} trend evident in the light curve, and a thermal emission component
that does not evolve significantly with time. We investigate whether this
source could be a tidal disruption event, and for certain assumptions find an
accretion efficiency epsilon \approx 3.5E-04 (M_{Ch}/M) consistent with a
massive white dwarf, and a disrupted minor body mass M_{mb}=1.9E+27(M/M_{Ch}) g
in the terrestrial-icy planet regime. These numbers yield an inner disc
temperature of the order kT_{in} \approx 0.04 keV, consistent with the
blackbody temperature of kT_{in} \approx 0.08 keV estimated by spectral
fitting. Although the density of white dwarfs and the number of free-floating
planets are uncertain, we estimate the rate of planetary tidal disruptions in
NGC 6388 to be in the range 3E-06 to 3E-04 yr^{-1}. Averaged over the Milky Way
globular clusters, the upper limit value corresponds to 0.05 yr^{-1},
consistent with the observation of a single event by INTEGRAL and Swift.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society Main Journal on 2014 July 16; 9 pages, 5 figures. Added references;
corrected typo
The WIRED Survey. III. An Infrared Excess around the Eclipsing Post-common Envelope Binary SDSS J030308.35+005443.7
We present the discovery with WISE of a significant infrared excess associated with the eclipsing post-common envelope binary SDSS J030308.35+005443.7, the first excess discovered around a non-interacting white dwarf+main-sequence M dwarf binary. The spectral energy distribution of the white dwarf+M dwarf companion shows significant excess longward of 3 Îźm. A T_(eff) of 8940 K for the white dwarf is consistent with a cooling age >2 Gyr, implying that the excess may be due to a recently formed circumbinary dust disk of material that extends from the tidal truncation radius of the binary at 1.96 R_â out to <0.8 AU, with a total mass of ~10^(20) g. We also construct WISE and follow-up ground-based near-infrared light curves of the system and find variability in the K band that appears to be in phase with ellipsoidal variations observed in the visible. The presence of dust might be due to (1) material being generated by the destruction of small rocky bodies that are being perturbed by an unseen planetary system or (2) dust condensing from the companion's wind. The high inclination of this system and the presence of dust make it an attractive target for M dwarf transit surveys and long-term photometric monitoring
Planetesimals at DZ stars â I. Chondritic compositions and a massive accretion event
There is a wealth of evidence to suggest that planetary systems can survive beyond the main sequence. Most commonly, white dwarfs are found to be accreting material from tidally disrupted asteroids, whose bulk compositions are reflected by the metals polluting the stellar photospheres. While many examples are known, most lack the deep, high-resolution data required to detect multiple elements, and thus characterize the planetesimals that orbit them. Here, spectra of seven DZ white dwarfs observed with Keck High Resolution Echelle Spectrometer (HIRES) are analysed, where up to nine metals are measured per star. Their compositions are compared against those of Solar system objects, working in a Bayesian framework to infer or marginalize over the accretion history. All of the stars have been accreting primitive material, similar to chondrites, with hints of a Mercury-like composition at one star. The most polluted star is observed several Myr after its last major accretion episode, in which a Moon-sized object met its demise