2,433 research outputs found
The ELM Survey. I. A Complete Sample of Extremely Low Mass White Dwarfs
We analyze radial velocity observations of the 12 extremely low-mass <0.25
Msol white dwarfs (WDs) in the MMT Hypervelocity Star Survey. Eleven of the 12
WDs are binaries with orbital periods shorter than 14 hours; the one
non-variable WD is possibly a pole-on system among our non-kinematically
selected targets. Our sample is unique: it is complete in a well-defined range
of apparent magnitude and color. The orbital mass functions imply that the
unseen companions are most likely other WDs, although neutron star companions
cannot be excluded. Six of the 11 systems with orbital solutions will merge
within a Hubble time due to the loss of angular momentum through gravitational
wave radiation. The quickest merger is J0923+3028, a g=15.7 ELM WD binary with
a 1.08 hr orbital period and a <130 Myr merger time. The chance of a supernova
Ia event among our ELM WDs is only 1%-7%, however. Three binary systems
(J0755+4906, J1233+1602, and J2119-0018) have extreme mass ratios and will most
likely form stable mass-transfer AM CVn systems. Two of these objects, SDSS
J1233+1602 and J2119-0018, are the lowest surface gravity WDs ever found; both
show Ca II absorption likely from accretion of circumbinary material. We
predict that at least one of our WDs is an eclipsing detached double WD system,
important for constraining helium core WD models.Comment: ApJ, in pres
Direct Distance Measurement to the Dusty White Dwarf GD 362
We present trigonometric parallax observations of GD 362 obtained over seven
epochs using the MDM 2.4m Hiltner Telescope. The existence of a dust disk
around this possibly massive white dwarf makes it an interesting target for
parallax observations. The measured parallax for GD 362 places it at a distance
of 50.6 pc, which implies that its radius and mass are ~ 0.0106 Rsun and 0.71
Msun, respectively. GD 362 is not as massive as initially thought (1.2Msun).
Our results are entirely consistent with the distance and mass estimates (52.2
pc and 0.73 Msun) by Zuckerman et al., who demonstrated that GD 362 has a
helium dominated atmosphere. Dropping GD 362 from the list of massive white
dwarfs, there are no white dwarfs with M > 0.9 Msun that are known to host
circumstellar dust disks.Comment: ApJ Letters, in pres
The ELM Survey. III. A Successful Targeted Survey for Extremely Low Mass White Dwarfs
Extremely low mass (ELM) white dwarfs (WDs) with masses <0.25 Msun are rare
objects that result from compact binary evolution. Here, we present a targeted
spectroscopic survey of ELM WD candidates selected by color. The survey is 71%
complete and has uncovered 18 new ELM WDs. Of the 7 ELM WDs with follow-up
observations, 6 are short-period binaries and 4 have merger times less than 5
Gyr. The most intriguing object, J1741+6526, likely has either a pulsar
companion or a massive WD companion making the system a possible supernova Type
Ia or .Ia progenitor. The overall ELM Survey has now identified 19 double
degenerate binaries with <10 Gyr merger times. The significant absence of short
orbital period ELM WDs at cool temperatures suggests that common envelope
evolution creates ELM WDs directly in short period systems. At least one-third
of the merging systems are halo objects, thus ELM WD binaries continue to form
and merge in both the disk and the halo.Comment: 10 pages, accepted to Ap
Orbital Evolution of Compact White Dwarf Binaries
The new-found prevalence of extremely low mass (ELM, Mhe<0.2 Msun) helium
white dwarfs (WDs) in tight binaries with more massive WDs has raised our
interest in understanding the nature of their mass transfer. Possessing small
(Menv~1e-3 Msun) but thick hydrogen envelopes, these objects have larger radii
than cold WDs and so initiate mass transfer of H-rich material at orbital
periods of 6-10 minutes. Building on the original work of D'Antona et al., we
confirm the 1e6 yr period of continued inspiral with mass transfer of H-rich
matter and highlight that the inspiraling direct-impact double WD binary HM
Cancri likely has an ELM WD donor. The ELM WDs have less of a radius expansion
under mass loss, thus enabling a larger range of donor masses that can stably
transfer matter and become a He mass transferring AM CVn binary. Even once in
the long-lived AM CVn mass transferring stage, these He WDs have larger radii
due to their higher entropy from the prolonged H burning stage.Comment: 9 pages, 8 figures. Accepted for publication in the Astrophysical
Journa
The Binary Fraction of Low Mass White Dwarfs
We describe spectroscopic observations of 21 low-mass (<0.45 M_sun) white
dwarfs (WDs) from the Palomar-Green Survey obtained over four years. We use
both radial velocities and infrared photometry to identify binary systems, and
find that the fraction of single, low-mass WDs is <30%. We discuss the
potential formation channels for these single stars including binary mergers of
lower-mass objects. However, binary mergers are not likely to explain the
observed number of single low-mass WDs. Thus additional formation channels,
such as enhanced mass loss due to winds or interactions with substellar
companions, are likely.Comment: 9 pages, accepted to Ap
SDSS J163030.58+423305.8: A 40 minute Orbital Period Detached White Dwarf Binary
We report the discovery of a new detached, double white dwarf system with an
orbital period of 39.8 min. We targeted SDSS J163030.58+423305.8 (hereafter
J1630) as part of our radial velocity program to search for companions around
low-mass white dwarfs using the 6.5m MMT. We detect peak-to-peak radial
velocity variations of 576 km/s. The mass function and optical photometry rule
out main-sequence companions. In addition, no milli-second pulsar companions
are detected in radio observations. Thus the invisible companion is most likely
another white dwarf. Unlike the other 39 min binary SDSS J010657.39-100003.3,
follow-up high speed photometric observations of J1630 obtained at the McDonald
2.1m telescope do not show significant ellipsoidal variations, indicating a
higher primary mass and smaller radius. The absence of eclipses constrain the
inclination angle to <82deg. J1630 contains a pair of white dwarfs, 0.3 Msun
primary + >0.3 Msun invisible secondary, at a separation of >0.32 Rsun. The two
white dwarfs will merge in less than 31 Myr. Depending on the core composition
of the companion, the merger will form either a single core-He burning subdwarf
star or a rapidly rotating massive white dwarf. The gravitational wave strain
from J1630 is detectable by instruments like the Laser Interferometer Space
Antenna (LISA) within the first year of operation.Comment: MNRAS Letters, in pres
Two New Tidally Distorted White Dwarfs
We identify two new tidally distorted white dwarfs (WDs), SDSS
J174140.49+652638.7 and J211921.96-001825.8 (hereafter J1741 and J2119). Both
stars are extremely low mass (ELM, < 0.2 Msun) WDs in short-period, detached
binary systems. High-speed photometric observations obtained at the McDonald
Observatory reveal ellipsoidal variations and Doppler beaming in both systems;
J1741, with a minimum companion mass of 1.1 Msun, has one of the strongest
Doppler beaming signals ever observed in a binary system (0.59 \pm 0.06%
amplitude). We use the observed ellipsoidal variations to constrain the radius
of each WD. For J1741, the star's radius must exceed 0.074 Rsun. For J2119, the
radius exceeds 0.10 Rsun. These indirect radius measurements are comparable to
the radius measurements for the bloated WD companions to A-stars found by the
Kepler spacecraft, and they constitute some of the largest radii inferred for
any WD. Surprisingly, J1741 also appears to show a 0.23 \pm 0.06% reflection
effect, and we discuss possible sources for this excess heating. Both J1741 and
J2119 are strong gravitational wave sources, and the time-of-minimum of the
ellipsoidal variations can be used to detect the orbital period decay. This may
be possible on a timescale of a decade or less.Comment: 6 pages, 4 figures, accepted for publication in the Astrophysical
Journa
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