344 research outputs found
Discovery of Non-radial pulsations in PQ Andromedae
We have detected pulsations in time-series photometry of the WZ Sge dwarf
nova PQ And. The strongest peak in the power spectrum occurs at a period of
10.5 minutes. Similar periods have been observed in other WZ Sge systems and
are attributed to ZZ Ceti type non-radial pulsations. There is no indication in
the photometry of an approximately 1.7 hour orbital period as reported in
previous spectroscopic observations.Comment: 7 pages, 5 figure
New Elemental Abundances for V1974 Cygni
We present a new analysis of existing optical and ultraviolet spectra of the
ONeMg nova V1974 Cygni 1992. Using these data and the photoionization code
Cloudy, we have determined the physical parameters and elemental abundances for
this nova. Many of the previous studies of this nova have made use of incorrect
analyses and hence a new study was required. Our results show that the ejecta
are enhanced, relative to solar, in helium, nitrogen, oxygen, neon, magnesium
and iron. Carbon was found to be subsolar. We find an ejected mass of ~2x10e-4
solar masses. Our model results fit well with observations taken at IR, radio,
sub-millimeter and X-ray wavelengths.Comment: 18 pages, 4 figures, accepted for publication in Ap
Synthesis of intermediate-mass elements in classical novae: from Si to Ca
Thermonuclear runaways driven by accretion into degenerate white dwarf cores
are the source that power classical nova outbursts.
In this paper, we identify the dominant nuclear paths involved in the
synthesis of intermediate-mass elements, from Si to Ca, during such violent
events.
New evolutionary sequences of 1.35 Mo ONe novae have been computed, using
updated nuclear reaction rates.
The main nuclear activity in this region is powered by the leakage from the
NeNa-MgAl region, where the activity is confined during the early stages of the
explosion. We discuss the critical role played by 30P(p,gamma) in the synthesis
of nuclear species beyond sulfur and point out the large uncertainty that
affects its rate, which has dramatic consequences for studies of nova
nucleosynthesis in the Si-Ca mass region.Comment: Accepted for publication in The Astrophysical Journal. 23 pages
(aastex), 6 Postscript figure
Abundance Analysis of the Extremely Fast ONeMg Novae V838 Herculis and V4160 Sagittarii
V838 Her and V4160 Sgr were two of the fastest classical novae ever observed, exhibiting light curve declines of 2 mag in less than 2 days. Both novae also showed strong neon emission lines, indicative of an outburst occurring on an oxygen-neon-magnesium white dwarf. Being the brighter of the two, V838 Her has an extensive set of X-ray to radio observations obtained during its first year after outburst. V4160 Sgr has a more modest set of ultraviolet and optical spectra, which show it to be similar to V838 Her, not just in its light curve evolution but also in its spectral development. The observed attributes imply that these novae occurred on extremely massive white dwarfs. This paper uses the Cloudy photoionization code to fit multiple epochs of emission line spectra to determine the elemental abundances of the ejecta of V838 Her and V4160 Sgr
Nucleosynthesis in ONeMg Novae: Models versus Observations to Constrain the Masses of ONeMg White Dwarfs and Their Envelopes
Nucleosynthesis in ONeMg novae has been investigated with the wide ranges of
three parameters, i.e., the white dwarf mass, the envelope mass at ignition,
and the initial composition. A quasi-analytic one-zone approach is used with an
up-to-date nuclear reaction network. The nucleosynthesis results show
correlation with the peak temperatures or the cooling timescales during
outbursts. Among the combinations of white dwarf and envelope masses which give
the same peak temperature, the explosion is more violent for a lower white
dwarf mass owing to its smaller gravitational potential. Comparison of the
nucleosynthesis results with observations implies that at least two-third of
the white dwarf masses for the observed ONeMg novae are ,
which are significantly lower than estimated by previous hydrodynamic studies
but consistent with the observations of V1974 Cyg. Moreover, the envelope
masses derived from the comparison are , which are in
good agreement with the ejecta masses estimated from observations but
significantly higher than in previous hydrodynamic studies. With such a low
mass white dwarf and a high mass envelope, the nova can produce interesting
amounts of -ray emitters Be, Na, and Al. We suggest
that V1974 Cyg has produced Na as high as the upper limit derived from
the COMPTEL survey. In addition, a non-negligible part of the Galactic
Al may originate from ONeMg novae, if not the major contributors. Both
the future INTEGRAL survey for these -ray emitters and abundance
estimates derived from ultraviolet, optical, and near infrared spectroscopies
will impose a severe constraint on the current nova models.Comment: 21 pages, 23 figures, to appear in the Astrophysical Journal, Vol.
523, No.1, September 20, 1999; preprint with embedded images can be obtained
from http://th.nao.ac.jp/~wanajo/journal/onenova.p
Magnetic White Dwarfs from the SDSS II. The Second and Third Data Releases
Fifty-two magnetic white dwarfs have been identified in spectroscopic
observations from the Sloan Digital Sky Survey (SDSS) obtained between mid-2002
and the end of 2004, including Data Releases 2 and 3. Though not as numerous
nor as diverse as the discoveries from the first Data Release, the collection
exhibits polar field strengths ranging from 1.5MG to ~1000MG, and includes two
new unusual atomic DQA examples, a molecular DQ, and five stars that show
hydrogen in fields above 500MG. The highest-field example, SDSSJ2346+3853, may
be the most strongly magnetic white dwarf yet discovered. Analysis of the
photometric data indicates that the magnetic sample spans the same temperature
range as for nonmagnetic white dwarfs from the SDSS, and support is found for
previous claims that magnetic white dwarfs tend to have larger masses than
their nonmagnetic counterparts. A glaring exception to this trend is the
apparently low-gravity object SDSSJ0933+1022, which may have a history
involving a close binary companion.Comment: 20 pages, 4 figures Accepted for publication in the Astronomical
Journa
Spectroscopic analysis of DA white dwarfs from the McCook & Sion catalog
For some years now, we have been gathering optical spectra of DA white dwarfs
in an effort to study and define the empirical ZZ Ceti instability strip.
However, we have recently expanded this survey to include all the DA white
dwarfs in the McCook & Sion catalog down to a limiting visual magnitude of
V=17.5. We present here a spectroscopic analysis of over 1000 DA white dwarfs
from this ongoing survey. We have several specific areas of interest most
notably the hot DAO white dwarfs, the ZZ Ceti instability strip, and the DA+dM
binary systems. Furthermore, we present a comparison of the ensemble properties
of our sample with those of other large surveys of DA white dwarfs, paying
particular attention to the distribution of mass as a function of effective
temperature.Comment: 8 pages, 7 figures, to appear in Journal of Physics Conference
Proceedings for the 16th European White Dwarf Worksho
The early X-ray emission from V382 Velorum (=Nova Vel 1999): An internal shock model
We present the results of ASCA and RXTE observations of the early X-ray
emission from the classical nova V382 Velorum. Its ASCA spectrum was hard
(kT~10 keV) with a strong (10**23 cm**-2) intrinsic absorption. In the
subsequent RXTE data, the spectra became softer both due to a declining
temperature and a diminishing column. We argue that this places the X-ray
emission interior to the outermost ejecta produced by V382 Vel in 1999, and
therefore must have been the result of a shock internal to the nova ejecta. The
weakness of the Fe K alpha lines probably indicates that the X-ray emitting
plasmas are not in ionization equilibrium.Comment: 16 pages (including 4 figures), accepted for publication in Ap
Modeling of the Super-Eddington Phase for Classical Novae: Five IUE Novae
We present a light curve model for the super-Eddington luminosity phase of
five classical novae observed with IUE. Optical and UV light curves are
calculated based on the optically thick wind theory with a reduced effective
opacity for a porous atmosphere. Fitting a model light curve with the UV 1455
\AA light curve, we determine the white dwarf mass and distance to be (1.3
M_sun, 4.4 kpc) for V693 CrA, (1.05 M_sun, 1.8 kpc) for V1974 Cyg, (0.95 M_sun,
4.1 kpc) for V1668 Cyg, (1.0 M_sun, 2.1 kpc) for V351 Pup, and (1.0 M_sun, 4.3
kpc) for OS And.Comment: 9 pages including 8 figures, to appear in the Astrophysical Journa
New Low Accretion-Rate Magnetic Binary Systems and their Significance for the Evolution of Cataclysmic Variables
Discoveries of two new white dwarf plus M star binaries with striking optical
cyclotron emission features from the Sloan Digital Sky Survey (SDSS) brings to
six the total number of X-ray faint, magnetic accretion binaries that accrete
at rates < 10^{-13} Msun/yr, or <1% of the values normally encountered in
cataclysmic variables. This fact, coupled with donor stars that underfill their
Roche lobes and very cool white dwarfs, brand the binaries as post
common-envelope systems whose orbits have not yet decayed to the point of
Roche-lobe contact. They are pre-magnetic CVs, or pre-Polars. The systems
exhibit spin/orbit synchronism and apparently accrete by efficient capture of
the stellar wind from the secondary star, a process that has been dubbed a
``magnetic siphon''. Because of this, period evolution of the binaries will
occur solely by gravitational radiation, which is very slow for periods >3 hr.
Optical surveys for the cyclotron harmonics appear to be the only means of
discovery, so the space density of pre-Polars could rival that of Polars, and
the binaries provide an important channel of progenitors (in addition to the
asynchronous Intermediate Polars). Both physical and SDSS observational
selection effects are identified that may help to explain the clumping of all
six systems in a narrow range of magnetic field strength around 60 MG.Comment: 25 pages, 13 figures, Accepted to Ap
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