4,355 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
SDSS J142625.71+575218.3: the First Pulsating White Dwarf With A Large Detectable Magnetic Field
We report the discovery of a strong magnetic field in the unique pulsating carbon- atmosphere white dwarf SDSS J142625.71 + 575218.3. From spectra gathered at the MMT and Keck telescopes, we infer a surface field of B(s) similar or equal to 1.2 MG, based on obvious Zeeman components seen in several carbon lines. We also detect the presence of a Zeeman- splitted He I lambda 4471 line, which is an indicator of the presence of a nonnegligible amount of helium in the atmosphere of this "hot DQ" star. This is important for understanding its pulsations, as nonadabatic theory reveals that some helium must be present in the envelope mixture for pulsation modes to be excited in the range of effective temperature where the target star is found. Out of nearly 200 pulsating white dwarfs known today, this is the first example of a star with a large detectable magnetic field. We suggest that SDSS J142625.71 + 575218.3 is the white dwarf equivalent of a rapidly oscillating Ap star.NSERCNSF AST 03-07321Reardon FoundationAstronom
On the formation of hot DQ white dwarfs
We present the first full evolutionary calculations aimed at exploring the
origin of hot DQ white dwarfs. These calculations consistently cover the whole
evolution from the born-again stage to the white dwarf cooling track. Our
calculations provide strong support to the diffusive/convective-mixing picture
for the formation of hot DQs. We find that the hot DQ stage is a short-lived
stage and that the range of effective temperatures where hot DQ stars are found
can be accounted for by different masses of residual helium and/or different
initial stellar masses. In the frame of this scenario, a correlation between
the effective temperature and the surface carbon abundance in DQs should be
expected, with the largest carbon abundances expected in the hottest DQs. From
our calculations, we suggest that most of the hot DQs could be the cooler
descendants of some PG1159 stars characterized by He-rich envelopes markedly
smaller than those predicted by the standard theory of stellar evolution. At
least for one hot DQ, the high-gravity white dwarf SDSS J142625.70+575218.4, an
evolutionary link between this star and the massive PG1159 star H1504+65 is
plausible.Comment: 4 pages, 2 figures. To be published in The Astrophysical Journal
Letter
Hot DQ White Dwarfs: Something Different
We present a detailed analysis of all the known Hot DQ white dwarfs in the
Fourth Data Release of the Sloan Digital Sky Survey (SDSS) recently found to
have carbon dominated atmospheres. Our spectroscopic and photometric analysis
reveals that these objects all have effective temperatures between ~18,000 and
24,000 K. The surface composition is found to be completely dominated by
carbon, as revealed by the absence of Hbeta and HeI 4471 lines (or
determination of trace amount in a few cases). We find that the surface gravity
of all objects but one seems to be ''normal'' and around log g = 8.0 while one
is likely near log g = 9.0. The presence of a weak magnetic field is directly
detected by spectropolarimetry in one object and is suspected in two others. We
propose that these strange stars could be cooled down versions of the weird
PG1159 star H1504+65 and form a new family of hydrogen and helium deficient
objects following the post-AGB phase. Finally, we present the results of full
nonadiabatic calculations dedicated specifically to each of the Hot DQ that
show that only SDSS J142625.70+575218.4 is expected to exhibit luminosity
variations. This result is in excellent agreement with recent observations by
Montgomery et al. who find that J142625.70+575218.4 is the only pulsator among
6 Hot DQ white dwarfs surveyed in February 2008.Comment: 33 pages, 7 figures, accepted for publication in Ap
High Carbon in I Zwicky 18: New Results from Hubble Space Telescope Spectroscopy
We present new measurements of the gas-phase C/O abundance ratio in both the
NW and SE components of the extremely metal-poor dwarf irregular galaxy I Zw
18, based on ultraviolet spectroscopy of the two H II regions using the Faint
Object Spectrograph on the Hubble Space Telescope. We determine values of log
C/O = -0.63 +/- 0.10 for the NW component and log C/O = -0.56 +/- 0.09 for the
SE component. In comparison, log C/O = -0.37 in the sun, while log C/O = -0.85
+/- 0.07 in the three most metal-poor irregular galaxies measured by Garnett et
al. (1995a). Our measurements show that C/O in I Zw 18 is significantly higher
than in other comparably metal-poor irregular galaxies, and above predictions
for the expected C/O from massive star nucleosynthesis. These results suggest
that carbon in I Zw 18 has been enhanced by an earlier population of lower-mass
carbon producing stars; this idea is supported by stellar photometry of I Zw 18
and its companion, which demonstrate that the current bursts of massive stars
were not the first. Despite its very low metallicity, it is likely that I Zw 18
is not a ``primeval'' galaxy.Comment: 14 pages including 4 figures; uses aaspp4.sty. Accepted for
publication in ApJ. Postscript version also available by e-mail request to
author at [email protected]
First space-borne measurements of methanol inside aged southern tropical to mid-latitude biomass burning plumes using the ACE-FTS instrument
International audienceFirst measurements from space of upper tropospheric and lower stratospheric methanol profiles within aged fire plumes are reported. Elevated levels of methanol at 0–45° S from 30 September to 3 November 2004 have been measured by the high resolution infrared spectrometer ACE-FTS onboard the SCISAT satellite. Methanol volume mixing ratios higher than 4000 pptv are detected and are strongly correlated with other fire products such as CO, C2H6, and HCN. A sensitivity study of the methanol retrieval, accounting for random and systematic contributions, shows that the retrieved methanol profile for a single occultation exceeds 100% error above 16.5 km, with an accuracy of about 20% for measurements inside polluted air masses. The upper tropospheric enhancement ratio of methanol with respect to CO is estimated from the correlation plot between methanol and CO for aged tropical biomass burning plumes. This ratio is in good agreement with the ratio measured in the free troposphere (up to 12 km) by recent aircraft studies and does not suggest any secondary production of methanol by oxidation in aged biomass burning plumes
A spectroscopy approach to measure the gravitational mass of antihydrogen
We study a method to induce resonant transitions between antihydrogen
() quantum states above a material surface in the gravitational field
of the Earth. The method consists of applying a gradient of magnetic field,
which is temporally oscillating with the frequency equal to a frequency of
transition between gravitational states of antihydrogen. A corresponding
resonant change in the spatial density of antihydrogen atoms could be measured
as a function of the frequency of applied field. We estimate an accuracy of
measuring antihydrogen gravitational states spacing and show how a value of the
gravitational mass of the atom could be deduced from such a
measurement. We also demonstrate that a method of induced transitions could be
combined with a free-fall-time measurement in order to further improve the
precision
First space-borne measurements of methanol inside aged tropical biomass burning plumes using the ACE-FTS instrument
International audienceFirst measurements from space of upper tropospheric and lower stratospheric methanol profiles within aged fire plumes are reported. Elevated levels of methanol at 0–45° S from 30 September to 3 November 2004 have been measured by the high resolution infrared spectrometer ACE-FTS onboard the SCISAT satellite. Methanol volume mixing ratios higher than 4000 pptv are detected and are strongly correlated with other fire products such as CO, C2H6, and HCN. A sensitivity study of the methanol retrieval, accounting for random and systematic contributions, shows that the retrieved methanol profile is reliable from 8.5 to 16.5 km, with an accuracy of about 20% for measurements inside polluted air masses. The upper tropospheric enhancement ratio of methanol with respect to CO is estimated from the correlation plot between methanol and CO for aged tropical biomass burning plumes. This ratio is in good agreement with the ratio measured in the free troposphere (up to 12 km) by recent aircraft studies and does not suggest any secondary production of methanol by oxidation in aged biomass burning plumes
Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs
We present an analysis of spectroscopic and photometric data for cool DQ
white dwarfs based on improved model atmosphere calculations. In particular, we
revise the atmospheric parameters of the trigonometric parallax sample of
Bergeron et al.(2001), and discuss the astrophysical implications on the
temperature scale and mean mass, as well as the chemical evolution of these
stars. We also analyze 40 new DQ stars discovered in the first data release of
the Sloan Digital Sky Survey.Comment: 6 pages,3 figures, 14th European Workshop on White Dwarfs, ASP
Conference Series, in pres
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