1,726 research outputs found
Keck Speckle Imaging of the White Dwarf G29-38: No Brown Dwarf Companion Detected
The white dwarf Giclas 29-38 has attracted much attention due to its large
infrared excess and the suggestion that excess might be due to a companion
brown dwarf. We observed this object using speckle interferometry at the Keck
telescope, obtaining diffraction-limited resolution (55 milliarcseconds) at K
band, and found it unresolved. Assuming the entire K band excess is due to a
single point-like companion, we place an upper limit on the binary separation
of 30 milliarcseconds, or 0.42 AU at the star's distance of 14.1 pc. This
result, combined with astroseismological data and other images of G29-38,
supports the hypothesis that the source of the near-infrared excess is not a
cool companion but a dust cloud.Comment: 7 pages, 2 figure
Prospects for Measuring Differential Rotation in White Dwarfs Through Asteroseismology
We examine the potential of asteroseismology for exploring the internal
rotation of white dwarf stars. Data from global observing campaigns have
revealed a wealth of frequencies, some of which show the signature of
rotational splitting. Tools developed for helioseismology to use many solar
p-mode frequencies for inversion of the rotation rate with depth are adapted to
the case of more limited numbers of modes of low degree. We find that the small
number of available modes in white dwarfs, coupled with the similarity between
the rotational-splitting kernels of the modes, renders direct inversion
unstable. Accordingly, we adopt what we consider to be plausible functional
forms for the differential rotation profile; this is sufficiently restrictive
to enable us to carry out a useful calibration. We show examples of this
technique for PG 1159 stars and pulsating DB white dwarfs. Published frequency
splittings for white dwarfs are currently not accurate enough for meaningful
inversions; reanalysis of existing data can provide splittings of sufficient
accuracy when the frequencies of individual peaks are extracted via
least-squares fitting or multipeak decompositions. We find that when mode
trapping is evident in the period spacing of g modes, the measured splittings
can constrain dOmega/dr.Comment: 26 pages, 20 postscript figures. Accepted for publication in The
Astrophysical Journa
On the Spectral Evolution of Cool, Helium-Atmosphere White Dwarfs: Detailed Spectroscopic and Photometric Analysis of DZ Stars
We present a detailed analysis of a large spectroscopic and photometric
sample of DZ white dwarfs based on our latest model atmosphere calculations. We
revise the atmospheric parameters of the trigonometric parallax sample of
Bergeron, Leggett, & Ruiz (12 stars) and analyze 147 new DZ white dwarfs
discovered in the Sloan Digital Sky Survey. The inclusion of metals and
hydrogen in our model atmosphere calculations leads to different atmospheric
parameters than those derived from pure helium models. Calcium abundances are
found in the range from log (Ca/He) = -12 to -8. We also find that fits of the
coolest objects show peculiarities, suggesting that our physical models may not
correctly describe the conditions of high atmospheric pressure encountered in
the coolest DZ stars. We find that the mean mass of the 11 DZ stars with
trigonometric parallaxes, = 0.63 Mo, is significantly lower than that
obtained from pure helium models, = 0.78 Mo, and in much better agreement
with the mean mass of other types of white dwarfs. We determine hydrogen
abundances for 27% of the DZ stars in our sample, while only upper limits are
obtained for objects with low signal-to-noise ratio spectroscopic data. We
confirm with a high level of confidence that the accretion rate of hydrogen is
at least two orders of magnitude smaller than that of metals (and up to five in
some cases) to be compatible with the observations. We find a correlation
between the hydrogen abundance and the effective temperature, suggesting for
the first time empirical evidence of a lower temperature boundary for the
hydrogen screening mechanism. Finally, we speculate on the possibility that the
DZA white dwarfs could be the result of the convective mixing of thin
hydrogen-rich atmospheres with the underlying helium convection zone.Comment: 67 pages, 32 figures, accepted for publication in Ap
The mass of the neutron star in the binary millisecond pulsar PSR J1012+5307
We have measured the radial velocity variation of the white dwarf secondary in the binary system containing the millisecond pulsar PSR J1012 + 5307. Combined with the orbital parameters of the radio pulsar, we infer a mass ratio q (=M-1/M-2) = 10.5 +/- 0.5 OUT optical spectroscopy has also allowed us to determine the mass of the white dwarf companion by fitting the spectrum to a grid of DA model atmospheres: we estimate M-2 = 0.16 +/- 0.02 M., and hence the mass of the neutron star is 1.64 +/- 0.22 M., where the error is dominated by that of M-2. The orbital inclination is 52 +/- 4 deg. For an initial neutron star mass of similar to 1.4 M., only a few tenths of a solar mass at most has been successfully accreted over the lifetime of the progenitor low-mass X-ray binary. If the initial mass of the secondary was similar to 1 M., our result suggests that the mass transfer may have been non-conservative
Cool Customers in the Stellar Graveyard I: Limits to Extrasolar Planets Around the White Dwarf G29-38
We present high contrast images of the hydrogen white dwarf G 29-38 taken in
the near infrared with the Hubble Space Telescope and the Gemini North
Telescope as part of a high contrast imaging search for substellar objects in
orbit around nearby white dwarfs.
We review the current limits on planetary companions for G29-38, the only
nearby white dwarf with an infrared excess due to a dust disk. We add our
recent observations to these limits to produce extremely tight constraints on
the types of possible companions that could be present. No objects 6
M are detected in our data at projected separations 12 AU, and no
objects 16 M are detected for separations from 3 to 12 AU, assuming
a total system age of 1 Gyr. Limits for companions at separations 3 AU come
from a combination of 2MASS photometry and previous studies of G29-38's
pulsations. Our imaging with Gemini cannot confirm a tentative claim for the
presence of a low mass brown dwarf. These observations demonstrate that a
careful combination of several techniques can probe nearby white dwarfs for
large planets and low mass brown dwarfs.Comment: 20 pages, 4 figures, Accepted to Ap
High Resolution Spectroscopy of the Pulsating White Dwarf G29-38
We present the analysis of time-resolved, high resolution spectra of the cool
white dwarf pulsator, G29-38. From measuring the Doppler shifts of the H-alpha
core, we detect velocity changes as large as 16.5 km/s and conclude that they
are due to the horizontal motions associated with the g-mode pulsations on the
star. We detect seven pulsation modes from the velocity time-series and
identify the same modes in the flux variations. We discuss the properties of
these modes and use the advantage of having both velocity and flux measurements
of the pulsations to test the convective driving theory proposed for DAV stars.
Our data show limited agreement with the expected relationships between the
amplitude and phases of the velocity and flux modes. Unexpectedly, the velocity
curve shows evidence for harmonic distortion, in the form of a peak in the
Fourier transform whose frequency is the exact sum of the two largest
frequencies. Combination frequencies are a characteristic feature of the
Fourier transforms of light curves of G29-38, but before now have not been
detected in the velocities, nor does published theory predict that they should
exist. We compare our velocity combination frequency to combination frequencies
found in the analysis of light curves of G29-38, and discuss what might account
for the existence of velocity combinations with the properties we observe.
We also use our high-resolution spectra to determine if either rotation or
pulsation can explain the truncated shape observed for the DAV star's line
core. We are able to eliminate both mechanisms: the average spectrum does not
fit the rotationally broadened model and the time-series of spectra provides
proof that the pulsations do not significantly truncate the line.Comment: 24 pages, 9 figures, Accepted for publication in ApJ (June
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