11,323 research outputs found
Pulsation in carbon-atmosphere white dwarfs: A new chapter in white dwarf asteroseismology
We present some of the results of a survey aimed at exploring the
asteroseismological potential of the newly-discovered carbon-atmosphere white
dwarfs. We show that, in certains regions of parameter space, carbon-atmosphere
white dwarfs may drive low-order gravity modes. We demonstrate that our
theoretical results are consistent with the recent exciting discovery of
luminosity variations in SDSS J1426+5752 and some null results obtained by a
team of scientists at McDonald Observatory. We also present follow-up
photometric observations carried out by ourselves at the Mount Bigelow 1.6-m
telescope using the new Mont4K camera. The results of follow-up spectroscopic
observations at the MMT are also briefly reported, including the surprising
discovery that SDSS J1426+5752 is not only a pulsating star but that it is also
a magnetic white dwarf with a surface field near 1.2 MG. The discovery of
-mode pulsations in SDSS J1426+5752 is quite significant in itself as it
opens a fourth asteroseismological "window", after the GW Vir, V777 Her, and ZZ
Ceti families, through which one may study white dwarfs.Comment: 7 pages, 4 figures, to appear in Journal of Physics Conference
Proceedings for the 16th European White Dwarf Worksho
Improved determination of the atmospheric parameters of the pulsating sdB star Feige 48
As part of a multifaceted effort to exploit better the asteroseismological
potential of the pulsating sdB star Feige 48, we present an improved
spectroscopic analysis of that star based on new grids of NLTE, fully
line-blanketed model atmospheres. To that end, we gathered four high S/N
time-averaged optical spectra of varying spectral resolution from 1.0 \AA\ to
8.7 \AA, and we made use of the results of four independent studies to fix the
abundances of the most important metals in the atmosphere of Feige 48. The mean
atmospheric parameters we obtained from our four spectra of Feige 48 are :
Teff= 29,850 60 K, log = 5.46 0.01, and log N(He)/N(H) =
2.88 0.02. We also modeled for the first time the He II line at 1640
\AA\ from the STIS archive spectrum of the star and we found with this line an
effective temperature and a surface gravity that match well the values obtained
with the optical data. With some fine tuning of the abundances of the metals
visible in the optical domain we were able to achieve a very good agreement
between our best available spectrum and our best-fitting synthetic one. Our
derived atmospheric parameters for Feige 48 are in rather good agreement with
previous estimates based on less sophisticated models. This underlines the
relatively small effects of the NLTE approach combined with line blanketing in
the atmosphere of this particular star, implying that the current estimates of
the atmospheric parameters of Feige 48 are reliable and secure.Comment: Accepted for publication in ApJ, April 201
Follow-up Studies of the Pulsating Magnetic White Dwarf SDSS J142625.71+575218.3
We present a follow-up analysis of the unique magnetic luminosity-variable
carbon-atmosphere white dwarf SDSS J142625.71+575218.3. This includes the
results of some 106.4 h of integrated light photometry which have revealed,
among other things, the presence of a new periodicity at 319.720 s which is not
harmonically related to the dominant oscillation (417.707 s) previously known
in that star. Using our photometry and available spectroscopy, we consider the
suggestion made by Montgomery et al. (2008) that the luminosity variations in
SDSS J142625.71+575218.3 may not be caused by pulsational instabilities, but
rather by photometric activity in a carbon-transferring analog of AM CVn. This
includes a detailed search for possible radial velocity variations due to rapid
orbital motion on the basis of MMT spectroscopy. At the end of the exercise, we
unequivocally rule out the interacting binary hypothesis and conclude instead
that, indeed, the luminosity variations are caused by g-mode pulsations as in
other pulsating white dwarfs. This is in line with the preferred possibility
put forward by Montgomery et al. (2008).Comment: 11 pages in emulateApJ, 12 figures, accepted for publication in Ap
The enigmatic He-sdB pulsator LS IV14116: new insights from the VLT
The intermediate Helium subdwarf B star LS IV14116 is a unique
object showing extremely peculiar atmospheric abundances as well as long-period
pulsations that cannot be explained in terms of the usual opacity mechanism.
One hypothesis invoked was that a strong magnetic field may be responsible. We
discredit this possibility on the basis of FORS2 spectro-polarimetry, which
allows us to rule out a mean longitudinal magnetic field down to 300 G.
Using the same data, we derive the atmospheric parameters for LS
IV14116 to be = 35,150111 K, =
5.880.02 and = 0.620.01. The high
surface gravity in particular is at odds with the theory that LS
IV14116 has not yet settled onto the Helium Main Sequence, and that
the pulsations are excited by an mechanism acting on the
Helium-burning shells present after the main Helium flash.
Archival UVES spectroscopy reveals LS IV14116 to have a radial
velocity of 149.12.1 km/s. Running a full kinematic analysis, we find that
it is on a retrograde orbit around the Galactic centre, with a Galactic radial
velocity component =13.238.28 km/s and a Galactic rotational velocity
component =55.5622.13 km/s. This implies that LS IV14116
belongs to the halo population, an intriguing discovery.Comment: accepted for publication in A&
Follow-up Observations of the Second and Third Known Pulsating Hot DQ White Dwarfs
We present follow-up time-series photometric observations that confirm and
extend the results of the significant discovery made by Barlow et al.(2008)
that the Hot DQ white dwarfs SDSS J220029.08-074121.5 and SDSS
J234843.30-094245.3 are luminosity variable. These are the second and third
known members of a new class of pulsating white dwarfs, after the prototype
SDSS J142625.71+575218.3 (Montgomery et al. 2008). We find that the light curve
of SDSS J220029.08-074121.5 is dominated by an oscillation at 654.397+-0.056 s,
and that the light pulse folded on that period is highly nonlinear due to the
presence of the first and second harmonic of the main pulsation. We also
present evidence for the possible detection of two additional pulsation modes
with low amplitudes and periods of 577.576+-0.226 s and 254.732+-0.048 s in
that star. Likewise, we find that the light curve of SDSS J234843.30-094245.3
is dominated by a pulsation with a period of 1044.168+-0.012 s, but with no
sign of harmonic components. A new oscillation, with a low amplitude and a
period of 416.919+-0.004 s, is also probably detected in that second star. We
argue, on the basis of the very different folded pulse shapes, that SDSS
J220029.08-074121.5 is likely magnetic, while SDSS J234843.30-094245.3 is
probably not.Comment: 12 pages, 19 figures, accepted for publication in Ap
Spectral analysis and abundances of the post-HB star HD 76431
HD76431 is a slow rotating post-HB star that shows an underabundance of
helium by 0.5 dex relative to the solar value. These observational facts
suggest that atomic diffusion could be active in its atmosphere. We have used
the MMT and Bok spectra to estimate the atmospheric parameters of the target
star using the model atmospheres and synthetic spectra calculated with TLUSTY
and SYNSPEC. The derived values of the effective temperature, surface gravity,
helium abundance are consistent with those obtained by Ramspeck et al. (2001b).
It appears that NLTE effect are not important for HD76431. We have used Stokes
I spectra from ESPaDOnS at CFHT to perform an abundance analysis and a search
for observational evidence of vertical stratification of the abundance of
certain elements. The results of our abundance analysis are in good agreement
with previously published data with respect to average abundances. Our
numerical simulations show that carbon and nitrogen reveal signatures of
vertical abundance stratification in the atmosphere of HD76431. It appears that
the carbon abundance increases toward the deeper atmospheric layers. Nitrogen
also shows a similar behaviour, but in deeper atmospheric layers we obtain a
significant dispersion for the estimates of its abundance. To our knowledge,
this is the first demonstration of vertical abundance stratification of metals
in a post-HB star and up to now it is the hottest star to show such
stratification features. We also report the detection of two SiIII and one
TiIII emission lines in the spectra of HD76431 that were not detected in
previous studies.Comment: 7 pages, 5 figures, accepted for publication in MNRA
Radiative levitation: a likely explanation for pulsations in the unique hot O subdwarf star SDSS J160043.6+074802.9
Context. SDSS J160043.6+074802.9 (J1600+0748 for short) is the only hot sdO star for which unambiguous multiperiodic luminosity variations have been reported so far. These rapid variations, with periods in the range from ~60 s to ~120 s, are best qualitatively explained in terms of pulsational instabilities, but the exact nature of the driving mechanism has remained a puzzle.
Aims. Our primary goal is to examine quantitatively how pulsation modes can be excited in an object such as J1600+0748. Given the failure of uniform-metallicity models as well documented in the recent Ph.D. thesis of C. Rodríguez-López, we consider the effects of radiative levitation on iron as a means to boost the efficiency of the opacity-driving mechanism in models of J1600+0748.
Methods. We combine high sensitivity time-averaged optical spectroscopy and full nonadiabatic calculations to carry out our study. In the first instance, this is used to estimate the location of J1600+0748 in the log plane. Given this essential input, we pulsate stellar models consistent with these atmospheric parameters. We construct both uniform-metallicity models and structures in which the iron abundance is specified by the condition of diffusive equilibrium between gravitational settling and radiative levitation.
Results. On the basis of NTLE H/He synthetic spectra, we find that the target star has the following atmospheric parameters: log g = 5.93 0.11, = 71 070 2725 K, and log N(He)/N(H) = -0.85 0.08. This takes into account our deconvolution of the spectrum of J1600+0748 as it is polluted by the light of a main sequence companion. We confirm that uniform-metallicity stellar models with Z in the range from 0.02 to 0.10 cannot excite pulsation modes of the kind observed. On the other hand, we find that the inclusion of radiative levitation, as we implemented it, leads to pulsational instabilities in a period range that overlaps with, although it is narrower than, the observed range in J1600+0748. The excited modes correspond to low-order, low-degree p-modes.
Conclusions. We infer that radiative levitation is a likely essential ingredient in the excitation physics at work in J1600+0748
- …