1,066 research outputs found
Two new pulsating low-mass pre-white dwarfs or SX Phenix stars?*
Context. The discovery of pulsations in low-mass stars opens an opportunity
for probing their interiors and to determine their evolution, by employing the
tools of asteroseismology. Aims. We aim to analyze high-speed photometry of
SDSSJ145847.02070754.46 and SDSSJ173001.94070600.25 and discover
brightness variabilities. In order to locate these stars in the diagram we fit optical spectra (SDSS) with synthetic non-magnetic
spectra derived from model atmospheres. Methods. To carry out this study, we
used the photometric data obtained by us for these stars with the 2.15m
telescope at CASLEO, Argentina. We analyzed their light curves and we apply the
Discrete Fourier Transform to determine the pulsation frequencies. Finally, we
compare both stars in the diagram, with known two
pre-white dwarfs, seven pulsating pre-ELM white dwarf stars, Scuti and
SX Phe stars. Results. We report the discovery of pulsations in
SDSSJ145847.02070754.46 and SDSSJ173001.94070600.25. We determine their
effective temperature and surface gravity to be = 7 972 200
K, = 4.25 0.5 and = 7 925 200 K, =
4.25 0.5, respectively. With these parameters these new pulsating
low-mass stars can be identified with either ELM white dwarfs (with ~ 0.17 Mo)
or more massive SX Phe stars. We identified pulsation periods of 3 278.7 and 1
633.9 s for SDSSJ145847.02070754.46 and a pulsation period of 3 367.1 s for
SDSSJ173001.94070600.25. These two new objects together with those of Maxted
et al. (2013, 2014) indicate the possible existence of a new instability domain
towards the late stages of evolution of low-mass white dwarf stars, although
their identification with SX Phe stars cannot be discarded.Comment: 5 pages, 5 figures, 1 table, accepted for publication in A&A
The environment of the fast rotating star Achernar - Thermal infrared interferometry with VLTI/MIDI and SIMECA modeling
Context: As is the case of several other Be stars, Achernar is surrounded by
an envelope, recently detected by near-IR interferometry.
Aims: We search for the signature of circumstellar emission at distances of a
few stellar radii from Achernar, in the thermal IR domain.
Methods: We obtained interferometric observations on three VLTI baselines in
the N band (8-13 mic), using the MIDI instrument.
Results: From the measured visibilities, we derive the angular extension and
flux contribution of the N band circumstellar emission in the polar direction
of Achernar. The interferometrically resolved polar envelope contributes 13.4
+/- 2.5 % of the photospheric flux in the N band, with a full width at half
maximum of 9.9 +/- 2.3 mas (~ 6 Rstar). This flux contribution is in good
agreement with the photometric IR excess of 10-20% measured by fitting the
spectral energy distribution. Due to our limited azimuth coverage, we can only
establish an upper limit of 5-10% for the equatorial envelope. We compare the
observed properties of the envelope with an existing model of this star
computed with the SIMECA code.
Conclusions: The observed extended emission in the thermal IR along the polar
direction of Achernar is well reproduced by the existing SIMECA model. Already
detected at 2.2mic, this polar envelope is most probably an observational
signature of the fast wind ejected by the hot polar caps of the star.Comment: A&A Letter, in pres
Testing White Dwarf Crystallization Theory with Asteroseismology of the Massive Pulsating DA Star BPM 37093
It was predicted more than 40 years ago that the cores of the coolest white
dwarf stars should eventually crystallize. This effect is one of the largest
sources of uncertainty in white dwarf cooling models, which are now routinely
used to estimate the ages of stellar populations in both the Galactic disk and
the halo. We are attempting to minimize this source of uncertainty by
calibrating the models, using observations of pulsating white dwarfs. In a
typical mass white dwarf model, crystallization does not begin until the
surface temperature reaches 6000-8000 K. In more massive white dwarf models the
effect begins at higher surface temperatures, where pulsations are observed in
the ZZ Ceti (DAV) stars. We use the observed pulsation periods of BPM 37093,
the most massive DAV white dwarf presently known, to probe the interior and
determine the size of the crystallized core empirically. Our initial
exploration of the models strongly suggests the presence of a solid core
containing about 90% of the stellar mass, which is consistent with our
theoretical expectations.Comment: minor changes for length, accepted for ApJ Letter
Evidence For Temperature Change And Oblique Pulsation From Light Curve Fits Of The Pulsating White Dwarf GD 358
Convective driving, the mechanism originally proposed by Brickhill for pulsating white dwarf stars, has gained general acceptance as the generic linear instability mechanism in DAV and dbV white dwarfs. This physical mechanism naturally leads to a nonlinear formulation, reproducing the observed light curves of many pulsating white dwarfs. This numerical model can also provide information on the average depth of a star's convection zone and the inclination angle of its pulsation axis. In this paper, we give two sets of results of nonlinear light curve fits to data on the dbV GD 358. Our first fit is based on data gathered in 2006 by the Whole Earth Telescope; this data set was multiperiodic containing at least 12 individual modes. Our second fit utilizes data obtained in 1996, when GD 358 underwent a dramatic change in excited frequencies accompanied by a rapid increase in fractional amplitude; during this event it was essentially monoperiodic. We argue that GD 358's convection zone was much thinner in 1996 than in 2006, and we interpret this as a result of a short-lived increase in its surface temperature. In addition, we find strong evidence of oblique pulsation using two sets of evenly split triplets in the 2006 data. This marks the first time that oblique pulsation has been identified in a variable white dwarf star.Delaware Asteroseismic Research CenterNational Science Foundation AST-0909107, AST-0607840Norman Hackerman Advanced Research Program 003658-0255-2007Crystal Trust FoundationMt. Cuba ObservatoryUniversity of DelawareAstronom
Seismology Of A Massive Pulsating Hydrogen Atmosphere White Dwarf
We report our observations of the new pulsating hydrogen atmosphere white dwarf SDSS J132350.28+010304.22. We discovered periodic photometric variations in frequency and amplitude that are commensurate with nonradial g-mode pulsations in ZZ Ceti stars. This, along with estimates for the star's temperature and gravity, establishes it as a massive ZZ Ceti star. We used time-series photometric observations with the 4.1 m SOAR Telescope, complemented by contemporary McDonald Observatory 2.1 m data, to discover the photometric variability. The light curve of SDSS J132350.28+010304.22 shows at least nine detectable frequencies. We used these frequencies to make an asteroseismic determination of the total mass and effective temperature of the star: M-star = 0.88 +/- 0.02 M-circle dot and T-eff = 12,100 +/- 140 K. These values are consistent with those derived from the the optical spectra and photometric colors.CNPqFAPERGS/PronexUS National Science Foundation AST-0909107Norman Hackerman Advanced Research Program 003658-0252-2009MICINN grant AYA08-1839/ESPESF EUROCORES Program EuroGENESIS (MICINN grant) EUI2009-04170Generalitat de Catalunya 2009SGR315EU-FEDER fundsAGENCIA through the Programa de Modernizacion Tecnologica BID 1728/OC-ARCONICET PIP 112-200801-00940Astronom
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