2,674 research outputs found
Discovery of an Ultracool White Dwarf Companion
The discovery of a low luminosity common proper motion companion to the white
dwarf GD392 at a wide separation of is reported. photometry
suggests a low temperature ( K) while data strongly
indicate suppressed flux at all near infrared wavelengths. Thus, GD392B is one
of the few white dwarfs to show significant collision induced absorption due to
the presence of photospheric and the first ultracool white dwarf
detected as a companion to another star. Models fail to explain GD392B as a
normal mass white dwarf. If correct, the cool companion may be explained as a
low mass white dwarf or unresolved double degenerate. The similarities of
GD392B to known ultracool degenerates are discussed, including some possible
implications for the faint end of the white dwarf luminosity function.Comment: 27 pages, 9 figures, 4 tables, re-accepted to ApJ after some revisio
Calibration of White Dwarf cooling sequences: theoretical uncertainty
White Dwarf luminosities are powerful age indicators, whose calibration
should be based on reliable models. We discuss the uncertainty of some chemical
and physical parameters and their influence on the age estimated by means of
white dwarf cooling sequences. Models at the beginning of the white dwarf
sequence have been obtained on the base of progenitor evolutionary tracks
computed starting from the zero age horizontal branch and for a typical halo
chemical composition (Z=0.0001, Y=0.23). The uncertainties due to nuclear
reaction rates, convection, mass loss and initial chemical composition are
discussed. Then, various cooling sequences for a typical white dwarf mass
(M=0.6 Mo) have been calculated under different assumptions on some input
physics, namely: conductive opacity, contribution of the ion-electron
interaction to the free energy and microscopic diffusion. Finally we present
the evolution of white dwarfs having mass ranging between 0.5 and 0.9 Mo. Much
effort has been spent to extend the equation of state down to the low
temperature and high density regime. An analysis of the latest improvement in
the physics of white dwarf interiors is presented. We conclude that at the
faint end of the cooling sequence (log L/Lo=-5.5) the present overall
uncertainty on the age is of the order of 20%, which correspond to about 3 Gyr.
We suggest that this uncertainty could be substantially reduced by improving
our knowledge of the conductive opacity (especially in the partially degenerate
regime) and by fixing the internal stratification of C and O.Comment: 14 figures, accepted by Ap
Low Luminosity Companions to White Dwarfs
This paper presents results of a near-infrared imaging survey for low mass
stellar and substellar companions to white dwarfs. A wide field proper motion
survey of 261 white dwarfs was capable of directly detecting companions at
orbital separations between and 5000 AU with masses as low as 0.05
, while a deep near field search of 86 white dwarfs was capable of
directly detecting companions at separations between and 1100 AU with
masses as low as 0.02 . Additionally, all white dwarf targets were
examined for near-infrared excess emission, a technique capable of detecting
companions at arbitrarily close separations down to masses of 0.05 .
No brown dwarf candidates were detected, which implies a brown dwarf
companion fraction of % for white dwarfs. In contrast, the stellar
companion fraction of white dwarfs as measured by this survey is 22%,
uncorrected for bias. Moreover, most of the known and suspected stellar
companions to white dwarfs are low mass stars whose masses are only slightly
greater than the masses of brown dwarfs. Twenty previously unknown stellar
companions were detected, five of which are confirmed or likely white dwarfs
themselves, while fifteen are confirmed or likely low mass stars.
Similar to the distribution of cool field dwarfs as a function of spectral
type, the number of cool unevolved dwarf companions peaks at mid-M type. Based
on the present work, relative to this peak, field L dwarfs appear to be roughly
2-3 times more abundant than companion L dwarfs. Additionally, there is no
evidence that the initial companion masses have been altered by post main
sequence binary interactions.Comment: 149 pages, 59 figures, 11 tables, accepted to ApJ Supplement
The Formation Rate, Mass and Luminosity Functions of DA White Dwarfs from the Palomar Green Survey
Spectrophotometric observations at high signal-to-noise ratio were obtained
of a complete sample of 347 DA white dwarfs from the Palomar Green (PG) Survey.
Fits of observed Balmer lines to synthetic spectra calculated from
pure-hydrogen model atmospheres were used to obtain robust values of Teff, log
g, masses, radii, and cooling ages. The luminosity function of the sample,
weighted by 1/Vmax, was obtained and compared with other determinations. The
mass distribution of the white dwarfs is derived, after important corrections
for the radii of the white dwarfs in this magnitude-limited survey and for the
cooling time scales. The formation rate of DA white dwarfs from the PG is
estimated to be 0.6x10^(-12) pc^(-3) yr^(-1). Comparison with predictions from
a theoretical study of the white dwarf formation rate for single stars
indicates that >80% of the high mass component requires a different origin,
presumably mergers of lower mass double degenerate stars. In order to estimate
the recent formation rate of all white dwarfs in the local Galactic disk,
corrections for incompleteness of the PG, addition of the DB-DO white dwarfs,
and allowance for stars hidden by luminous binary companions had to be applied
to enhance the rate. An overall formation rate of white dwarfs recently in the
local Galactic disk of 1.15+/-0.25x10^(-12) pc^(-3) yr^(-1) is obtained. Two
recent studies of samples of nearby Galactic planetary nebulae lead to
estimates around twice as high. Difficulties in reconciling these
determinations are discussed.Comment: 73 pages, 18 figures, accepted for publication in the ApJ Supplemen
The Future is Now: the Formation of Single Low Mass White Dwarfs in the Solar Neighborhood
Low mass helium-core white dwarfs (M < 0.45 Msun) can be produced from
interacting binary systems, and traditionally all of them have been attributed
to this channel. However, a low mass white dwarf could also result from a
single star that experiences severe mass loss on the first ascent giant branch.
A large population of low mass He-core white dwarfs has been discovered in the
old metal-rich cluster NGC 6791. There is therefore a mechanism in clusters to
produce low mass white dwarfs without requiring binary star interactions, and
we search for evidence of a similar population in field white dwarfs. We argue
that there is a significant field population (of order half of the detected
systems) that arises from old metal rich stars which truncate their evolution
prior to the helium flash from severe mass loss. There is a consistent absence
of evidence for nearby companions in a large fraction of low mass white dwarfs.
The number of old metal-rich field dwarfs is also comparable with the
apparently single low mass white dwarf population, and our revised estimate for
the space density of low mass white dwarfs produced from binary interactions is
also compatible with theoretical expectations. This indicates that this channel
of stellar evolution, hitherto thought hypothetical only, has been in operation
in our own Galaxy for many billions of years. One strong implication of our
model is that single low mass white dwarfs should be good targets for planet
searches because they are likely to arise from metal-rich progenitors. We also
discuss other observational tests and implications, including the potential
impact on SN Ia rates and the frequency of planetary nebulae.Comment: ApJ published versio
The Early Palomar Program (1950-1955) for the Discovery of Classical Novae in M81: Analysis of the Spatial Distribution, Magnitude Distribution, and Distance Suggestion
Data obtained in the 1950-1955 Palomar campaign for the discovery of
classical novae in M81 are set out in detail. Positions and apparent B
magnitudes are listed for the 23 novae that were found. There is modest
evidence that the spatial distribution of the novae does not track the B
brightness distribution of either the total light or the light beyond an
isophotal radius that is 70\arcsec from the center of M81. The nova
distribution is more extended than the aforementioned light, with a significant
fraction of the sample appearing in the outer disk/spiral arm region. We
suggest that many (perhaps a majority) of the M81 novae that are observed at
any given epoch (compared with say years ago) are daughters of
Population I interacting binaries. The conclusion that the present day novae
are drawn from two population groups, one from low mass white dwarf secondaries
of close binaries identified with the bulge/thick disk population, and the
other from massive white dwarf secondaries identified with the outer thin
disk/spiral arm population, is discussed. We conclude that the M81 data are
consistent with the two population division as argued previously from (1) the
observational studies on other grounds by Della Valle et al. (1992, 1994),
Della Valle & Livio (1998), and Shafter et al. (1996) of nearby galaxies, (2)
the Hatano et al. (1997a,b) Monte Carlo simulations of novae in M31 and in the
Galaxy, and (3) the Yungelson et al. (1997) population synthesis modeling of
nova binaries. Two different methods of using M81 novae as distance indicators
give a nova distance modulus for M81 as , consistent with the
Cepheid modulus that is the same value.Comment: 24 pages, 7 figures, accepted to PAS
An Intermediate-Mass Black Hole in the Globular Cluster G1: Improved Significance from New Keck and Hubble Space Telescope Observations
We present dynamical models for the massive globular cluster G1. The goal is
to measure or place a significant upper limit on the mass of any central black
hole. Whether or not globular clusters contain central massive black holes has
important consequences for a variety of studies. We use new kinematic data
obtained with Keck and new photometry from the Hubble Space Telescope. The Keck
spectra allow us to obtain kinematics out to large radii that are required to
pin down the mass-to-light ratio of the dynamical model and the orbital
structure. The Hubble Space Telescope observations give us a factor of two
better spatial resolution for the surface brightness profile. By fitting
non-parametric, spherical, isotropic models we find a best-fit black hole mass
of 1.7(+-0.3)e4 Msun. Fully general axisymmetric orbit-based models give
similar results, with a black hole mass of 1.8(+-0.5)e4 Msun. The no-black hole
model has Delta_chi^2=5 (marginalized over mass-to-light ratio), implying less
than 3% significance. We have taken into account any change in the
mass-to-light ratio in the center due to stellar remnants. These results are
consistent with our previous estimate in Gebhardt, Rich & Ho (2002), and
inconsistent with the analysis of Baumgardt et al. (2003) who claim that G1
does not show evidence for a black hole. These new results make G1 the best
example of a cluster that contains an intermediate-mass black hole.Comment: accepted for publication in the Astrophysical Journa
The White Dwarf Population in NGC 1039 (M34) and the White Dwarf Initial-Final Mass Relation
We present the first detailed photometric and spectroscopic study of the
white dwarfs (WDs) in the field of the ~225 Myr old (log tau_cl = 8.35) open
cluster NGC 1039 (M34) as part of the ongoing Lick-Arizona White Dwarf Survey.
Using wide-field UBV imaging, we photometrically select 44 WD candidates in
this field. We spectroscopically identify 19 of these objects as WDs; 17 are
hydrogen-atmosphere DA WDs, one is a helium-atmosphere DB WD, and one is a cool
DC WD that exhibits no detectable absorption lines. We find an effective
temperature (T_eff) and surface gravity (log g) for each DA WD by fitting
Balmer-line profiles from model atmospheres to the observed spectra. WD
evolutionary models are then invoked to derive masses and cooling times for
each DA WD. Of the 17 DAs, five are at the approximate distance modulus of the
cluster. Another WD with a distance modulus 0.45 mag brighter than that of the
cluster could be a double-degenerate binary cluster member, but is more likely
to be a field WD. We place the five single cluster member WDs in the empirical
initial-final mass relation and find that three of them lie very close to the
previously derived linear relation; two have WD masses significantly below the
relation. These outliers may have experienced some sort of enhanced mass loss
or binary evolution; however, it is quite possible that these WDs are simply
interlopers from the field WD population. Eight of the 17 DA WDs show
significant CaII K absorption; comparison of the absorption strength with the
WD distances suggests that the absorption is interstellar, though this cannot
be confirmed with the current data.Comment: 24 pages, 13 figures. Accepted for publication in the Astronomical
Journal. Figures 1, 2 and 3 reduced in resolutio
Can groundwater be successfully implemented as a bulk water resource within rural Limpopo Province? Analysis based on GRIP datasets
Groundwater is a strategic water resource in rural Limpopo Province and it accounts for almost 70% of rural domestic water supply. The resource is available throughout the Province in varying quantities and qualities depending on the hydrogeological properties of the underlying aquifer. These properties are mostly secondary in nature and are constrained by the processes of weathering, faulting, fracturing and the influence of intrusive rocks. These properties define typical fractured aquifers in which the selection of drilling sites requires a thorough scientific approach to locate a successful productive borehole. While most water service authorities in the Province have been randomly developing new boreholes with limited success rates, the analysis of datasets in the newly established groundwater data repository, the Limpopo Groundwater Resource Information Project (GRIP) demonstrates that large quantities of groundwater can be obtained and used for bulk supply if the drilling sites are scientifically selected. The GRIP dataset contains 24 922 entries of which 4 000 were tested. The pumping test results indicate that an estimated 576 000 m3/d (210 Mm3/a) can be sustainably abstracted from boreholes in approximately 2 500 villages that are dispersed throughout the Province. However, more than 50% of these boreholes areunequipped of which 3 000 can provide a combined yield of 109 Mm3/a based on a 24 h abstraction rate. These results show that groundwater can be developed as a potential viable bulk-water supply source. This paper attempts to demonstrate, using the existing GRIP dataset, that groundwater is an underutilised resource that can be viably and cost-effectively developed as an alternative bulk water source or conjunctively with surface water
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