100 research outputs found
Anomalous flux event in the TESS Sector 43 light curve of the white dwarf photometric standard HZ 4 was caused by a passing asteroid
Frymire & Ardila (2023) reported an anomalous flux variation in the
Transiting Exoplanet Survey Satellite (TESS) Sector 43 light curve of the white
dwarf HZ 4. We show that this flux variation was caused by the main-belt
asteroid 4382 Stravinsky traversing the nearby TESS pixels, and it is therefore
not a cause for concern regarding the continued use of HZ 4 as a photometric
standard star.Comment: Accepted for publication in Research Notes of the American
Astronomical Societ
The sdA problem - III. New extremely low-mass white dwarfs and their precursors from Gaia astrometry
The physical nature of the sdA stars---cool hydrogen-rich objects with
spectroscopic surface gravities intermediate between main sequence and
canonical mass white dwarfs---has been elusive since they were found in Sloan
Digital Sky Survey Data Release 12 spectra. The population is likely dominated
by metal-poor A/F stars in the halo with overestimated surface gravities, with
a small contribution of extremely low-mass white dwarfs and their precursors,
i.e., ELMs and pre-ELMs. In this work, we seek to identify (pre-)ELMs with
radii smaller than is possible for main sequence stars, allowing even for very
low metallicity. We analyse 3891 sdAs previously identified in the Sloan
Digital Sky Survey using Gaia DR2 data. Our Monte Carlo analysis supports that
90 of these are inconsistent with the main sequence. 37 lie close to or within
the canonical white dwarf cooling sequence, while the remaining 53 lie between
the canonical white dwarfs and main sequence, which we interpret as likely
(pre-)ELMs given their spectral class. Of these, 30 pass more conservative
criteria that allow for higher systematic uncertainties on the parallax, as
well as an approximate treatment of extinction. Our identifications increase
the number of known (pre-)ELMs by up to 50 per cent, demonstrating how Gaia
astrometry can reveal members of the compact (pre-)ELM subpopulation of the sdA
spectral class.Comment: 13 pages, 19 figures, 1 table. Accepted for publication in MNRA
Destroying Aliases from the Ground and Space: Super-Nyquist ZZ Cetis in K2 Long Cadence Data
With typical periods of order 10 minutes, the pulsation signatures of ZZ Ceti
variables (pulsating hydrogen-atmosphere white dwarf stars) are severely
undersampled by long-cadence (29.42 minutes per exposure) K2 observations.
Nyquist aliasing renders the intrinsic frequencies ambiguous, stifling
precision asteroseismology. We report the discovery of two new ZZ Cetis in
long-cadence K2 data: EPIC 210377280 and EPIC 220274129. Guided by 3-4 nights
of follow-up, high-speed (<=30 s) photometry from McDonald Observatory, we
recover accurate pulsation frequencies for K2 signals that reflected 4-5 times
off the Nyquist with the full precision of over 70 days of monitoring (~0.01
muHz). In turn, the K2 observations enable us to select the correct peaks from
the alias structure of the ground-based signals caused by gaps in the
observations. We identify at least seven independent pulsation modes in the
light curves of each of these stars. For EPIC 220274129, we detect three
complete sets of rotationally split ell=1 (dipole mode) triplets, which we use
to asteroseismically infer the stellar rotation period of 12.7+/-1.3 hr. We
also detect two sub-Nyquist K2 signals that are likely combination (difference)
frequencies. We attribute our inability to match some of the K2 signals to the
ground-based data to changes in pulsation amplitudes between epochs of
observation. Model fits to SOAR spectroscopy place both EPIC 210377280 and EPIC
220274129 near the middle of the ZZ Ceti instability strip, with Teff =
11590+/-200 K and 11810+/-210 K, and masses 0.57+/-0.03 Msun and 0.62+/-0.03
Msun, respectively.Comment: 13 pages, 9 figures, 7 tables; accepted for publication in Ap
Discovery of pulsations, including possible pressure modes, in two new extremely low mass, He-core white dwarfs
We report the discovery of the second and third pulsating extremely low mass
white dwarfs (WDs), SDSS J111215.82+111745.0 (hereafter J1112) and SDSS
J151826.68+065813.2 (hereafter J1518). Both have masses < 0.25 Msun and
effective temperatures below 10,000 K, establishing these putatively He-core
WDs as a cooler class of pulsating hydrogen-atmosphere WDs (DAVs, or ZZ Ceti
stars). The short-period pulsations evidenced in the light curve of J1112 may
also represent the first observation of acoustic (p-mode) pulsations in any WD,
which provide an exciting opportunity to probe this WD in a complimentary way
compared to the long-period g-modes also present. J1112 is a Teff = 9590 +/-
140 K and log(g) = 6.36 +/- 0.06 WD. The star displays sinusoidal variability
at five distinct periodicities between 1792-2855 s. In this star we also see
short-period variability, strongest at 134.3 s, well short of expected g-modes
for such a low-mass WD. The other new pulsating WD, J1518, is a Teff = 9900 +/-
140 K and log(g) = 6.80 +/- 0.05 WD. The light curve of J1518 is highly
non-sinusoidal, with at least seven significant periods between 1335-3848 s.
Consistent with the expectation that ELM WDs must be formed in binaries, these
two new pulsating He-core WDs, in addition to the prototype SDSS
J184037.78+642312.3, have close companions. However, the observed variability
is inconsistent with tidally induced pulsations and is so far best explained by
the same hydrogen partial-ionization driving mechanism at work in classic
C/O-core ZZ Ceti stars.Comment: 9 pages, 5 figures, accepted to The Astrophysical Journa
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