2,599 research outputs found
The Ultramassive White Dwarf EUVE J1746-706
We have obtained new optical and extreme ultraviolet (EUV) spectroscopy of
the ultramassive white dwarf EUVE J1746-706. We revise Vennes et al.'s (1996a,
ApJ, 467, 784) original estimates of the atmospheric parameters and we measure
an effective temperature of 46,500 +/- 700 K and a surface gravity log g = 9.05
+/- 0.15 (~1.2 M_o), in agreement with Balmer line profiles and the EUV
continuum. We derive an upper limit on the atmospheric abundance of helium of
He/H = 1.3 x 10^{-4} and a neutral hydrogen column density in the local
interstellar medium N_HI = 1.8 +/- 0.4 x 10^{19} cm^{-2} from the EUV spectrum.
Our upper limit corresponds to half the helium abundance observed in the
atmosphere of the ultramassive white dwarf GD 50. We discuss the possibility
that EUVE J1746-706 represents an earlier phase of evolution relative to GD 50
and may, therefore, help us understand the origin and evolution of massive
white dwarfs.Comment: 6 pages, 4 postscript figures, uses aastex, to be published in ApJ
Letter
Recognition of micro-scale deformation structures in glacial sediments - pattern perception, observer bias and the influence of experience
It is a scientist's mission to try to remain unbiased. However, certain factors play a role in scientific analyses that are not controlled by conscious thought. These factors are potentially very important in areas of science where interpretations are based on a scientist's ability to identify patterns or structures. One such area is the micromorphology of glacial sediments. In this paper we investigate the role of an analyst's experience in relation to pattern perception with specific reference to turbate microstructures in glacial diamictons. An experiment was conducted on 52 participants, which demonstrated that, as may be expected, more experienced (glacial) micromorphologists tend to exhibit a higher sensitivity-to-signal, but that complete novices, if given clear instructions, can reach levels of sensitivity similar to those of experts. It also showed, perhaps more surprisingly, that response bias does not decrease with experience. We discuss psychological factors, such as the drive for success and consistency, that may have contributed to these results and investigate their possible implications in the micromorphological analysis and interpretation of glacial sediments
Alan D. Weidemann
Dr. Weidemann is an adjunct faculty member at University of Southern Mississippi, Department of Marine Science, as well as a researcher at the Naval Research Laboratory. His areas of specialization are optical oceanography, propagation of ultraviolet, visible and near-IR electromagnetic radiant energy in oceanic environments, and relationship between inherent and apparent optical properties. Present research is measurement of optical properties on unmanned gliders and development of a flow cytometer to function on the unmanned glid
An empirical initial-final mass relation from hot, massive white dwarfs in NGC 2168 (M35)
The relation between the zero-age main sequence mass of a star and its
white-dwarf remnant (the initial-final mass relation) is a powerful tool for
exploration of mass loss processes during stellar evolution. We present an
empirical derivation of the initial-final mass relation based on spectroscopic
analysis of seven massive white dwarfs in NGC 2168 (M35). Using an internally
consistent data set, we show that the resultant white dwarf mass increases
monotonically with progenitor mass for masses greater than 4 solar masses, one
of the first open clusters to show this trend. We also find two massive white
dwarfs foreground to the cluster that are otherwise consistent with cluster
membership. These white dwarfs can be explained as former cluster members
moving steadily away from the cluster at speeds of <~0.5 km/s since their
formation and may provide the first direct evidence of the loss of white dwarfs
from open clusters. Based on these data alone, we constrain the upper mass
limit of WD progenitors to be >=5.8 solar masses at the 90% confidence level
for a cluster age of 150 Myr.Comment: 14 pages, 3 figures. Accepted for publication in the Astrophysical
Journal Letters. Contains some acknowledgements not in accepted version (for
space reasons), otherwise identical to accepted versio
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
A Dusty Disk Around WD1150-153: Explaining the Metals in White Dwarfs by Accretion from the Interstellar Medium versus Debris Disks
We report the discovery of excess K-band radiation from a metal-rich DAV
white dwarf star, WD1150-153. Our near infrared spectroscopic observations show
that the excess radiation cannot be explained by a (sub)stellar companion, and
is likely to be caused by a debris disk similar to the other DAZ white dwarfs
with circumstellar debris disks. We find that the fraction of DAZ white dwarfs
with detectable debris disks is at least 14%. We also revisit the problem of
explaining the metals in white dwarf photospheres by accretion from the
interstellar medium (ISM). We use the observed interstellar column densities
toward stars in close angular proximity and similar distance as DAZ white
dwarfs to constrain the contribution of accretion from the ISM. We find no
correlation between the accretion density required to supply metals observed in
DAZs with the densities observed in their interstellar environment, indicating
that ISM accretion alone cannot explain the presence of metals in nearby DAZ
white dwarfs. Although ISM accretion will certainly contribute, our analysis
indicates that it is not the dominant source of metals for most DAZ white
dwarfs. Instead, the growing number of circumstellar debris disks around DAZs
suggests that circumstellar material may play a more dominant role in polluting
the white dwarf atmospheres.Comment: ApJ, in pres
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 New Class of Dusty DAZ White Dwarfs
Our mid-infrared survey of 124 white dwarfs with the Spitzer Space Telescope
and the IRAC imager has revealed an infrared excess associated with the white
dwarf WD 2115-560 naturally explained by circumstellar dust. This object is the
fourth white dwarf observed to have circumstellar dust. All four are DAZ white
dwarfs, i.e. they have both photospheric Balmer lines and photospheric metal
lines.
We discuss these four objects as a class, which we abbreviate "DAZd", where
the "d" stands for "dust". Using an optically-thick, geometrically-thin disk
model analogous to Saturn's rings, we find that the inner disk edges are at
>~0.1 to 0.2 Ro and that the outer disk edges are ~0.3 to 0.6 Ro. This model
naturally explains the accretion rates and lifetimes of the detected WD disks
and the accretion rates inferred from photospheric metal abundances.Comment: 27 pages, 7 figures, ApJ accepte
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
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