1,945 research outputs found
Precise Atmospheric Parameters for the Shortest Period Binary White Dwarfs: Gravitational Waves, Metals, and Pulsations
We present a detailed spectroscopic analysis of 61 low mass white dwarfs and
provide precise atmospheric parameters, masses, and updated binary system
parameters based on our new model atmosphere grids and the most recent
evolutionary model calculations. For the first time, we measure systematic
abundances of He, Ca and Mg for metal-rich extremely low mass white dwarfs and
examine the distribution of these abundances as a function of effective
temperature and mass. Based on our preliminary results, we discuss the
possibility that shell flashes may be responsible for the presence of the
observed He and metals. We compare stellar radii derived from our spectroscopic
analysis to model-independent measurements and find good agreement except for
those white dwarfs with Teff < 10,000 K. We also calculate the expected
gravitational wave strain for each system and discuss their significance to the
eLISA space-borne gravitational wave observatory. Finally, we provide an update
on the instability strip of extremely low mass white dwarf pulsators.Comment: 18 pages, 13 figures, 3 tables, accepted for publication in Ap
A New Gravitational Wave Verification Source
We report the discovery of a detached 20 min orbital period binary white
dwarf. WD0931+444 (SDSS J093506.93+441106.9) was previously classified as a WD
+ M dwarf system based on its optical spectrum. Our time-resolved optical
spectroscopy observations obtained at the 8m Gemini and 6.5m MMT reveal
peak-to-peak radial velocity variations of 400 km/s every 20 min for the WD,
but no velocity variations for the M dwarf. In addition, high-speed photometry
from the McDonald 2.1m telescope shows no evidence of variability nor evidence
of a reflection effect. An M dwarf companion is physically too large to fit
into a 20 min orbit. Thus, the orbital motion of the WD is almost certainly due
to an invisible WD companion. The M dwarf must be either an unrelated
background object or the tertiary component of a hiearchical triple system.
WD0931+444 contains a pair of WDs, a 0.32 Msol primary and a >0.14 Msol
secondary, at a separation of >0.19 Rsol. After J0651+2844, WD0931+444 becomes
the second-shortest period detached binary WD currently known. The two WDs will
lose angular momentum through gravitational wave radiation and merge in <9 Myr.
The log h ~ -22 gravitational wave strain from WD0931+444 is strong enough to
make it a verification source for gravitational wave missions in the
milli-Hertz frequency range, e.g. the evolved Laser Interferometer Space
Antenna (eLISA), bringing the total number of known eLISA verification sources
to nine.Comment: MNRAS Letters, in pres
When flux standards go wild: white dwarfs in the age of Kepler
White dwarf stars have been used as flux standards for decades, thanks to
their staid simplicity. We have empirically tested their photometric stability
by analyzing the light curves of 398 high-probability candidates and
spectroscopically confirmed white dwarfs observed during the original Kepler
mission and later with K2 Campaigns 0-8. We find that the vast majority (>97
per cent) of non-pulsating and apparently isolated white dwarfs are stable to
better than 1 per cent in the Kepler bandpass on 1-hr to 10-d timescales,
confirming that these stellar remnants are useful flux standards. From the
cases that do exhibit significant variability, we caution that binarity,
magnetism, and pulsations are three important attributes to rule out when
establishing white dwarfs as flux standards, especially those hotter than
30,000 K.Comment: Accepted for publication in MNRAS; 7 pages, 4 figures, 2 table
Discovery of an ultramassive pulsating white dwarf
We announce the discovery of the most massive pulsating hydrogen-atmosphere
(DA) white dwarf (WD) ever discovered, GD 518. Model atmosphere fits to the
optical spectrum of this star show it is a 12,030 +/- 210 K WD with a log(g) =
9.08 +/- 0.06, which corresponds to a mass of 1.20 +/- 0.03 Msun. Stellar
evolution models indicate that the progenitor of such a high-mass WD endured a
stable carbon-burning phase, producing an oxygen-neon-core WD. The discovery of
pulsations in GD 518 thus offers the first opportunity to probe the interior of
a WD with a possible oxygen-neon core. Such a massive WD should also be
significantly crystallized at this temperature. The star exhibits
multi-periodic luminosity variations at timescales ranging from roughly 425-595
s and amplitudes up to 0.7%, consistent in period and amplitude with the
observed variability of typical ZZ Ceti stars, which exhibit non-radial g-mode
pulsations driven by a hydrogen partial ionization zone. Successfully
unraveling both the total mass and core composition of GD 518 provides a unique
opportunity to investigate intermediate-mass stellar evolution, and can
possibly place an upper limit to the mass of a carbon-oxygen-core WD, which in
turn constrains SNe Ia progenitor systems.Comment: 5 pages, 3 figures, Astrophysical Journal Letters, 771, L2 (2013
Generalized parton correlation functions for a spin-1/2 hadron
The fully unintegrated, off-diagonal quark-quark correlator for a spin-1/2
hadron is parameterized in terms of so-called generalized parton correlation
functions. Such objects, in particular, can be considered as mother
distributions of generalized parton distributions on the one hand and
transverse momentum dependent parton distributions on the other. Therefore, our
study provides new, model-independent insights into the recently proposed
nontrivial relations between generalized and transverse momentum dependent
parton distributions. We find that none of these relations can be promoted to a
model-independent status. As a by-product we obtain the first complete
classification of generalized parton distributions beyond leading twist. The
present paper is a natural extension of our previous corresponding analysis for
spin-0 hadrons.Comment: 41 pages, 3 figures; v2: added referenc
Controllability on infinite-dimensional manifolds
Following the unified approach of A. Kriegl and P.W. Michor (1997) for a
treatment of global analysis on a class of locally convex spaces known as
convenient, we give a generalization of Rashevsky-Chow's theorem for control
systems in regular connected manifolds modelled on convenient
(infinite-dimensional) locally convex spaces which are not necessarily
normable.Comment: 19 pages, 1 figur
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Rapid Orbital Decay in the 12.75-Minute Binary White Dwarf J0651+2844
We report the detection of orbital decay in the 12.75-minute, detached binary white dwarf (WD) SDSS J065133.338+284423.37 (hereafter J0651). Our photometric observations over a 13 month baseline constrain the orbital period to 765.206543(55) s and indicate that the orbit is decreasing at a rate of (-9.8 +/- 2.8) x 10(-12) s s(-1) (or -0.31 +/- 0.09 ms yr(-1)). We revise the system parameters based on our new photometric and spectroscopic observations: J0651 contains two WDs with M-1 = 0.26 +/- 0.04 M-circle dot and M-2 = 0.50 +/- 0.04 M-circle dot. General relativity predicts orbital decay due to gravitational wave radiation of (-8.2 +/- 1.7) x 10(-12) s s(-1) (or -0.26 +/- 0.05 ms yr(-1)). Our observed rate of orbital decay is consistent with this expectation. J0651 is currently the second-loudest gravitational wave source known in the milli-Hertz range and the loudest non-interacting binary, which makes it an excellent verification source for future missions aimed at directly detecting gravitational waves. Our work establishes the feasibility of monitoring this system's orbital period decay at optical wavelengths.NSF AST-0909107, AST-1008734Norman Hackerman Advanced Research Program 003658-0252-2009Astronom
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