19 research outputs found
The search for ZZ Ceti stars in the original Kepler mission
We report the discovery of 42 white dwarfs in the original Kepler mission
field, including nine new confirmed pulsating hydrogen-atmosphere white dwarfs
(ZZ Ceti stars). Guided by the Kepler-INT Survey (KIS), we selected white dwarf
candidates on the basis of their U-g, g-r, and r-H_alpha photometric colours.
We followed up these candidates with high-signal-to-noise optical spectroscopy
from the 4.2-m William Herschel Telescope. Using ground-based, time-series
photometry, we put our sample of new spectroscopically characterized white
dwarfs in the context of the empirical ZZ Ceti instability strip. Prior to our
search, only two pulsating white dwarfs had been observed by Kepler.
Ultimately, four of our new ZZ Cetis were observed from space. These rich
datasets are helping initiate a rapid advancement in the asteroseismic
investigation of pulsating white dwarfs, which continues with the extended
Kepler mission, K2.Comment: 9 pages, 6 figures, accepted for publication in MNRA
The fastest stars in the Galaxy
We report a spectroscopic search for hypervelocity white dwarfs (WDs) that
are runaways from Type Ia supernovae (SNe Ia) and related thermonuclear
explosions. Candidates are selected from Gaia data with high tangential
velocities and blue colors. We find six new runaways, including four stars with
radial velocities (RVs) and total space velocities
. These are most likely the surviving donors from
double-degenerate binaries in which the other WD exploded. The other two
objects have lower minimum velocities, , and may
have formed through a different mechanism, such as pure deflagration of a WD in
a Type Iax supernova. The four fastest stars are hotter and smaller than the
previously known "D stars," with effective temperatures ranging from
20,000 to 130,000 K and radii of . Three
of these have carbon-dominated atmospheres, and one has a helium-dominated
atmosphere. Two stars have RVs of and -- the
fastest systemic stellar RVs ever measured. Their inferred birth velocities,
, imply that both WDs in the progenitor binary
had masses . The high observed velocities suggest that a
dominant fraction of the observed hypervelocity WD population comes from
double-degenerate binaries whose total mass significantly exceeds the
Chandrasekhar limit. However, the two nearest and faintest D stars have the
lowest velocities and masses, suggesting that observational selection effects
favor rarer, higher-mass stars. A significant population of fainter low-mass
runaways may still await discovery. We infer a birth rate of D stars that
is consistent with the SN Ia rate. The birth rate is poorly constrained,
however, because the luminosities and lifetimes of stars are
uncertain.Comment: 26 pages, 17 figures. Accepted to OJ
Orbital decay in an accreting and eclipsing 13.7 minute orbital period binary with a luminous donor
We report the discovery of ZTF J0127+5258, a compact mass-transferring binary
with an orbital period of 13.7 minutes. The system contains a white dwarf
accretor, which likely originated as a post-common envelope carbon-oxygen (CO)
white dwarf, and a warm donor ().
The donor probably formed during a common envelope phase between the CO white
dwarf and an evolving giant which left behind a helium star or helium white
dwarf in a close orbit with the CO white dwarf. We measure gravitational
wave-driven orbital inspiral with significance, which yields a
joint constraint on the component masses and mass transfer rate. While the
accretion disk in the system is dominated by ionized helium emission, the donor
exhibits a mixture of hydrogen and helium absorption lines. Phase-resolved
spectroscopy yields a donor radial-velocity semi-amplitude of , and high-speed photometry reveals that the system is eclipsing.
We detect a {\it Chandra} X-ray counterpart with . Depending on the mass-transfer rate, the system will
likely evolve into either a stably mass-transferring helium CV, merge to become
an R Crb star, or explode as a Type Ia supernova in the next million years. We
predict that the Laser Space Interferometer Antenna (LISA) will detect the
source with a signal-to-noise ratio of after 4 years of observations.
The system is the first \emph{LISA}-loud mass-transferring binary with an
intrinsically luminous donor, a class of sources that provide the opportunity
to leverage the synergy between optical and infrared time domain surveys, X-ray
facilities, and gravitational-wave observatories to probe general relativity,
accretion physics, and binary evolution.Comment: 13 pages, 7 figures, 2 tables, submitted to ApJ
A rotating white dwarf shows different compositions on its opposite faces
White dwarfs, the extremely dense remnants left behind by most stars after
their death, are characterised by a mass comparable to that of the Sun
compressed into the size of an Earth-like planet. In the resulting strong
gravity, heavy elements sink toward the centre and the upper layer of the
atmosphere contains only the lightest element present, usually hydrogen or
helium. Several mechanisms compete with gravitational settling to change a
white dwarf's surface composition as it cools, and the fraction of white dwarfs
with helium atmospheres is known to increase by a factor ~2.5 below a
temperature of about 30,000 K; therefore, some white dwarfs that appear to have
hydrogen-dominated atmospheres above 30,000 K are bound to transition to be
helium-dominated as they cool below it. Here we report observations of ZTF
J203349.8+322901.1, a transitioning white dwarf with two faces: one side of its
atmosphere is dominated by hydrogen and the other one by helium. This peculiar
nature is likely caused by the presence of a small magnetic field, which
creates an inhomogeneity in temperature, pressure or mixing strength over the
surface. ZTF J203349.8+322901.1 might be the most extreme member of a class of
magnetic, transitioning white dwarfs -- together with GD 323, a white dwarf
that shows similar but much more subtle variations. This new class could help
shed light on the physical mechanisms behind white dwarf spectral evolution.Comment: 45 pages, 11 figure
The McDonald Observatory search for pulsating sdA stars: Asteroseismic support for multiple populations
Context. The nature of the recently identified âsdAâ spectroscopic class of stars is not well understood. The thousands of known sdAs have H-dominated spectra, spectroscopic surface gravity values between main sequence stars and isolated white dwarfs, and effective temperatures below the lower limit for He-burning subdwarfs. Most are likely products of binary stellar evolution, whether extremely low-mass white dwarfs and their precursors or blue stragglers in the halo. Aims. Stellar eigenfrequencies revealed through time series photometry of pulsating stars sensitively probe stellar structural properties. The properties of pulsations exhibited by sdA stars would contribute substantially to our developing understanding of this class. Methods. We extend our photometric campaign to discover pulsating extremely low-mass white dwarfs from the McDonald Observatory to target sdA stars classified from SDSS spectra. We also obtain follow-up time series spectroscopy to search for binary signatures from four new pulsators. Results. Out of 23 sdA stars observed, we clearly detect stellar pulsations in 7. Dominant pulsation periods range from 4.6 min to 12.3 h, with most on timescales of approximately one hour. We argue specific classifications for some of the new variables, identifying both compact and likely main sequence dwarf pulsators, along with a candidate low-mass RR Lyrae star. Conclusions. With dominant pulsation periods spanning orders of magnitude, the pulsational evidence supports the emerging narrative that the sdA class consists of multiple stellar populations. Since multiple types of sdA exhibit stellar pulsations, follow-up asteroseismic analysis can be used to probe the precise evolutionary natures and stellar structures of these individual subpopulations
Discovery of Two Polars from a Crossmatch of ZTF and the SRG/eFEDS X-ray Catalog
Magnetic CVs are luminous Galactic X-ray sources but have been difficult to
find in purely optical surveys due to their lack of outburst behavior. The
eROSITA telescope aboard the Spektr-RG (SRG) mission is conducting an all-sky
X-ray survey and recently released the public eROSITA Final Equatorial Depth
Survey (eFEDS) catalog. We crossmatched the eFEDS catalog with photometry from
the Zwicky Transient Facility (ZTF) and discovered two new magnetic cataclysmic
variables (CVs). We obtained high-cadence optical photometry and phase-resolved
spectroscopy for each magnetic CV candidate and found them both to be polars.
Among the newly discovered magnetic CVs is ZTFJ0850+0443, an eclipsing polar
with orbital period hr, white dwarf mass and accretion rate /yr.
We suggest that ZTFJ0850+0443 is a low magnetic field strength polar, with
MG. We also discovered a non-eclipsing polar,
ZTFJ0926+0105, with orbital period hr, magnetic field
strength MG, and accretion rate /yr.Comment: Submitted to Ap
Orbital Decay in an Accreting and Eclipsing 13.7 Minute Orbital Period Binary with a Luminous Donor
We report the discovery of ZTF J0127+5258, a compact mass-transferring binary with an orbital period of 13.7 minutes. The system contains a white dwarf accretor, which likely originated as a postâcommon envelope carbonâoxygen (CO) white dwarf, and a warm donor (Teff,donor = 16,400 ± 1000 K). The donor probably formed during a common envelope phase between the CO white dwarf and an evolving giant that left behind a helium star or white dwarf in a close orbit with the CO white dwarf. We measure gravitational waveâdriven orbital inspiral with âŒ51Ï significance, which yields a joint constraint on the component masses and mass transfer rate. While the accretion disk in the system is dominated by ionized helium emission, the donor exhibits a mixture of hydrogen and helium absorption lines. Phase-resolved spectroscopy yields a donor radial velocity semiamplitude of 771 ± 27 km sâ1, and high-speed photometry reveals that the system is eclipsing. We detect a Chandra X-ray counterpart with LX ⌠3 Ă 1031 erg sâ1. Depending on the mass transfer rate, the system will likely either evolve into a stably mass-transferring helium cataclysmic variable, merge to become an R CrB star, or explode as a Type Ia supernova in the next million years. We predict that the Laser Space Interferometer Antenna (LISA) will detect the source with a signal-to-noise ratio of 24 ± 6 after 4 yr of observations. The system is the first LISA-loud mass-transferring binary with an intrinsically luminous donor, a class of sources that provide the opportunity to leverage the synergy between optical and infrared time domain surveys, X-ray facilities, and gravitational-wave observatories to probe general relativity, accretion physics, and binary evolution