16 research outputs found
Periodic optical variability and debris accretion in white dwarfs: a test for a causal connection
Recent Kepler photometry has revealed that about half of white dwarfs (WDs)
have periodic, low-level (~ 1e-4 - 1e-3), optical variations. Hubble Space
Telescope (HST) ultraviolet spectroscopy has shown that up to about one half of
WDs are actively accreting rocky planetary debris, as evidenced by the presence
of photospheric metal absorption lines. We have obtained HST ultraviolet
spectra of seven WDs that have been monitored for periodic variations, to test
the hypothesis that these two phenomena are causally connected, i.e. that the
optical periodic modulation is caused by WD rotation coupled with an
inhomogeneous surface distribution of accreted metals. We detect photospheric
metals in four out of the seven WDs. However, we find no significant
correspondence between the existence of optical periodic variability and the
detection of photospheric ultraviolet absorption lines. Thus the null
hypothesis stands, that the two phenomena are not directly related. Some other
source of WD surface inhomogeneity, perhaps related to magnetic field strength,
combined with the WD rotation, or alternatively effects due to close binary
companions, may be behind the observed optical modulation. We report the
marginal detection of molecular hydrogen in WD J1949+4734, only the fourth
known WD with detected H_2 lines. We also re-classify J1926+4219 as a
carbon-rich He-sdO subdwarf.Comment: MNRAS, in pres
The RR Lyrae Delay-Time Distribution: A Novel Perspective on Models of Old Stellar Populations
The delay-time distribution (DTD) is the occurrence rate of a class of
objects as a function of time after a hypothetical burst of star formation.
DTDs are mainly used as a statistical test of stellar evolution scenarios for
supernova progenitors, but they can be applied to many other classes of
astronomical objects. We calculate the first DTD for RR Lyrae variables using
29,810 RR Lyrae from the OGLE-IV survey and a map of the stellar-age
distribution (SAD) in the Large Magellanic Cloud (LMC). We find that of the OGLE-IV RR Lyrae are associated with delay-times older than 8 Gyr
(main-sequence progenitor masses less than 1 M), and consistent with
existing constraints on their ages, but surprisingly about of RR Lyrae
appear have delay times Gyr (main-sequence masses between
M at LMC metallicity). This intermediate-age signal also persists
outside the Bar-region where crowding is less of a concern, and we verified
that without this signal, the spatial distribution of the OGLE-IV RR Lyrae is
inconsistent with the SAD map of the LMC. Since an intermediate-age RR Lyrae
channel is in tension with the lack of RR Lyrae in intermediate-age clusters
(noting issues with small-number statistics), and the age-metallicity
constraints of LMC stars, our DTD result possibly indicates that systematic
uncertainties may still exist in SAD measurements of old-stellar populations,
perhaps stemming from the construction methodology or the stellar evolution
models used. We described tests to further investigate this issue.Comment: 21 pages, 11 figures. Accepted to Ap
A dearth of small particles in the transiting material around the white dwarfWD 1145+017
White dwarf WD 1145+017 is orbited by several clouds of dust, possibly emanating from actively disintegrating bodies. These dust clouds reveal themselves through deep, broad, and evolving transits in the star's light curve. Here, we report two epochs of multiwavelength photometric observations of WD 1145+017, including several filters in the optical, Ks and 4.5 μmbands in 2016 and 2017. The observed transit depths are different at these wavelengths. However, after correcting for excess dust emission at Ks and 4.5 μm, we find the transit depths for the white dwarf itself are the same at all wavelengths, at least to within the observational uncertainties of ~5-10 per cent. From this surprising result, and under the assumption of low optical depth dust clouds, we conclude that there is a deficit of small particles (with radii s ≲ 1.5 μm) in the transiting material. We propose a model wherein only large particles can survive the high equilibrium temperature environment corresponding to 4.5 h orbital periods around WD 1145+017, while small particles sublimate rapidly. In addition, we evaluate dust models that are permitted by our measurements of infrared emission
Shallow Ultraviolet Transits of WD 1145+017
WD 1145+017 is a unique white dwarf system that has a heavily polluted
atmosphere, an infrared excess from a dust disk, numerous broad absorption
lines from circumstellar gas, and changing transit features, likely from
fragments of an actively disintegrating asteroid. Here, we present results from
a large photometric and spectroscopic campaign with Hubble, Keck , VLT,
Spitzer, and many other smaller telescopes from 2015 to 2018. Somewhat
surprisingly, but consistent with previous observations in the u' band, the UV
transit depths are always shallower than those in the optical. We develop a
model that can quantitatively explain the observed "bluing" and the main
findings are: I. the transiting objects, circumstellar gas, and white dwarf are
all aligned along our line of sight; II. the transiting object is blocking a
larger fraction of the circumstellar gas than of the white dwarf itself.
Because most circumstellar lines are concentrated in the UV, the UV flux
appears to be less blocked compared to the optical during a transit, leading to
a shallower UV transit. This scenario is further supported by the strong
anti-correlation between optical transit depth and circumstellar line strength.
We have yet to detect any wavelength-dependent transits caused by the
transiting material around WD 1145+017.Comment: 16 pages, 11 figures, 6 tables, ApJ, in pres
An irradiated-Jupiter analogue hotter than the Sun
Planets orbiting close to hot stars experience intense extreme-ultraviolet
radiation, potentially leading to atmosphere evaporation and to thermal
dissociation of molecules. However, this extreme regime remains mainly
unexplored due to observational challenges. Only a single known ultra-hot giant
planet, KELT-9b, receives enough ultraviolet radiation for molecular
dissociation, with a day-side temperature of ~4,600K. An alternative approach
uses irradiated brown dwarfs as hot-Jupiter analogues. With atmospheres and
radii similar to those of giant planets, brown dwarfs orbiting close to hot
Earth-sized white-dwarf stars can be directly detected above the glare of the
star. Here we report observations revealing an extremely irradiated low-mass
companion to the hot white dwarf WD0032-317. Our analysis indicates a day-side
temperature of ~8,000K, and a day-to-night temperature difference of ~6,000K.
The amount of extreme-ultraviolet radiation (with wavelengths 100-912\r{A})
received by WD0032-317B is equivalent to that received by planets orbiting
close to stars as hot as a late B-type stars, and about 5,600 times higher than
that of KELT-9b. With a mass of ~75-88 Jupiter masses, this
near-hydrogen-burning-limit object is potentially one of the most massive brown
dwarfs known.Comment: Authors' version of the article published in Nature Astronomy (DOI
https://doi.org/10.1038/s41550-023-02048-z
The Gravity Collective: A Search for the Electromagnetic Counterpart to the Neutron Star-Black Hole Merger GW190814
We present optical follow-up imaging obtained with the Katzman Automatic
Imaging Telescope, Las Cumbres Observatory Global Telescope Network, Nickel
Telescope, Swope Telescope, and Thacher Telescope of the LIGO/Virgo
gravitational wave (GW) signal from the neutron star-black hole (NSBH) merger
GW190814. We searched the GW190814 localization region (19 deg for the
90th percentile best localization), covering a total of 51 deg and 94.6%
of the two-dimensional localization region. Analyzing the properties of 189
transients that we consider as candidate counterparts to the NSBH merger,
including their localizations, discovery times from merger, optical spectra,
likely host-galaxy redshifts, and photometric evolution, we conclude that none
of these objects are likely to be associated with GW190814. Based on this
finding, we consider the likely optical properties of an electromagnetic
counterpart to GW190814, including possible kilonovae and short gamma-ray burst
afterglows. Using the joint limits from our follow-up imaging, we conclude that
a counterpart with an -band decline rate of 0.68 mag day, similar to
the kilonova AT 2017gfo, could peak at an absolute magnitude of at most
mag (50% confidence). Our data are not constraining for ''red'' kilonovae and
rule out ''blue'' kilonovae with (30% confidence). We
strongly rule out all known types of short gamma-ray burst afterglows with
viewing angles 17 assuming an initial jet opening angle of
and explosion energies and circumburst densities similar to
afterglows explored in the literature. Finally, we explore the possibility that
GW190814 merged in the disk of an active galactic nucleus, of which we find
four in the localization region, but we do not find any candidate counterparts
among these sources.Comment: 86 pages, 9 figure