585 research outputs found
Carbon-rich (DQ) white dwarfs in the Sloan Digital Sky Survey
Among the spectroscopically identified white dwarfs, a fraction smaller than
2% have spectra dominated by carbon lines, mainly molecular C2, but also in a
smaller group by CI and CII lines. These are together called DQ white dwarfs.
We want to derive atmospheric parameters Teff,log g, and carbon abundances for
a large sample of these stars and discuss implications for their spectral
evolution. Sloan Digital Sky Survey spectra and ugriz photometry were used,
together with GAIA Data Release 2 parallaxes and G band photometry. These were
fitted to synthetic spectra and theoretical photometry derived from model
atmospheres. We found that the DQs hotter than Teff ~10000 K have masses ~0.4
Msun larger than the cooler ones, which have masses typical for the majority of
white dwarfs, ~0.6 Msun. A significant fraction of the hotter objects with Teff
> 14500 K have atmospheres dominated by carbon.Comment: Accepted by Astronomy & Astrophysic
The temporal changes of the pulsational periods of the pre-white dwarf PG 1159-035
PG 1159-035, a pre-white dwarf with T=140000 K, is the prototype of the
PG1159 spectroscopic class and the DOV pulsating class. Changes in the star
cause variations in its oscillation periods. The measurement of temporal change
in the oscillation periods, dP/dt, allows us to estimate directly rates of
stellar evolutionary changes, such as the cooling rate and the envelope
contraction rate, providing a way to test and refine evolutionary models for
pre-white dwarf pulsating stars.
We measured 27 pulsation modes period changes. The periods varied at rates of
between 1 and 100 ms/yr, and several can be directly measured with a relative
standard uncertainty below 10%. For the 516.0 s mode (the highest in amplitude)
in particular, not only the value of dP/dt can be measured directly with a
relative standard uncertainty of 2%, but the second order period change,
d(dP/dt)/dt, can also be calculated reliably. By using the (O-C) method we
refined the dP/dt and estimated the d(dP/dt)/dt for six other pulsation
periods. As a first application, we calculated the change in the PG 1559-035
rotation period, dP_rot/dt = -2.13*10^{-6} s/s, the envelope contraction rate
dR/dt = -2.2*10^{-13} solar radius/s, and the cooling rante dT/dt =
-1.42*10^{-3} K/s.Comment: 8 pages; 2 figures; 2 tables; appendix with 2 table
Pulsating White Dwarfs
The Sloan Digital Sky Survey has allowed us to increase the number of known
white dwarfs by a factor of five and consequently the number of known pulsating
white dwarfs also by a factor of five. It has also led to the discovery of new
types of variable white dwarfs, as the variable hot DQs, and the pulsating
Extremely Low Mass white dwarfs. With the Kepler Mission, it has been possible
to discover new phenomena, the outbursts present in a few pulsating white
dwarfs.Comment: 10 pages, 2 figures, Wide-field variability surveys: a 21st-century
perspective, 22nd Los Alamos Stellar Pulsation Conference Series meeting, San
Pedro de Atacama, Chile, Nov. 28 - Dec. 2, 201
Seismology Of White Dwarfs: The ZZ Ceti Stars
We calculate an extensive adiabatic model grid for pulsating white dwarfs with H dominated atmospheres, the ZZ Ceti stars. We developed a new approach for asteroseismology, using the relative observed amplitudes as weights, and compared the computed modes with the observed ones for the class of ZZ Ceti stars. We measure the H layer mass for 83 stars and found an average of M(H)/M(*) = 10(-6.3), which is thinner than the predicted value of M(H)/M(*) = 10(-4). Our results indicate that the stars lose more mass during their evolution than previously expected.Astronom
Panchromatic fits to the Globular Cluster NGC 6366
We present panchromatic isochrone fits to the color magnitude data of the
globular cluster NGC 6366, based on HST ACS/WFC and SOAR photometric data.
Before performing the isochrone fits, we corrected the photometric data for
differential reddening and calculated the mean ridge line of the color
magnitude diagrams. We compared the isochrones of Dartmouth Stellar Evolution
Database and PAdova and TRieste Stellar Evolution Code (with microscopic
diffusion starting on the main sequence). Based on previous determinations of
the metallicity of this cluster we test it from [Fe/H]=-1.00 to [Fe/H]= -0.50,
and the age from 9 to 13 Gyrs. The uncertainties do not decrease when we fit
simultaneous colors. We also find that the Dartmouth Stellar Evolution Database
isochrones have a better fit in the sub giant branch and low main sequence than
the PAdova and TRieste Stellar Evolution Code. Considering the most recent
spectroscopic determination of the metallicity ([Fe/H]= -0.67), we find
E(B-V)=0.69+/-0.02, (m-M)_V=15.02+/-0.07 and 11+/-2 Gyr for NGC 6366.Comment: 4 pages, 2 figures. Proceedings of the conference "Reading the book
of globular clusters with the lens of stellar evolution", to be published in
Memorie della Societ\'a Astronomica Italian
Low mass variable stars in the globular cluster NGC 6397
We have conducted a photometric survey of the globular cluster NGC 6397 in a
search for variable stars. We obtained ~11h of time-resolved photometric images
with one ne European Southern Observatory-Very Large Telescope using the FOcal
Reducer and low dispersion Spectrograph imager distributed over two consecutive
nights. We analyzed 8391 light curves of stars brighter than magnitude 23 with
the 465 nm-filter, and we identified 412 variable stars, reaching ~ 4.8 +- 0.2
per cent of variability with timescales between 0.004 and 2d, with amplitudes
variation greater than +- 0.2 mag.Comment: 9 figures, complementary dat
Unified line profiles for hydrogen perturbed by collisions with protons: satellites and asymmetries
We present new calculations of unified line profiles for hydrogen perturbed
by collisions with protons. We report on new calculations of the potential
energies and dipole moments which allow the evaluation of profiles for the
lines of the Lyman series up to Lyman and the Balmer series up to
Balmer10. Unified calculations only existed for the lines Lyman to
Lyman and Balmer including the H quasi-molecule. These
data are available as online material accompanying this paper and should be
included in atmosphere models, in place of the Stark effect of protons, since
the quasi-molecular contributions cause not only satellites, but large
asymmetries that are unaccounted for in models that assume Stark broadening of
electrons and protons are equal.Comment: 13 pages, 25 figures. Accepted for publication in MNRA
The age-metallicity dependence for white dwarfs
We present a theoretical study on the metallicity dependence of the
initialtofinal mass relation and its influence on white dwarf age
determinations. We compute a grid of evolutionary sequences from the main
sequence to K on the white dwarf cooling curve, passing through
all intermediate stages. During the thermally-pulsing asymptotic giant branch
no third dredge-up episodes are considered and thus the photospheric C/O ratio
is below unity for sequences with metallicities larger than . We
consider initial metallicities from to , accounting for
stellar populations in the galactic disk and halo, with initial masses below
. We found a clear dependence of the shape of the
initialtofinal mass relation with the progenitor metallicity, where metal
rich progenitors result in less massive white dwarf remnants, due to an
enhancement of the mass loss rates associated to high metallicity values. By
comparing our theoretical computations with semi empirical data from globular
and old open clusters, we found that the observed intrinsic mass spread can be
accounted for by a set of initialtofinal mass relations characterized by
different metallicity values. Also, we confirm that the lifetime spent before
the white dwarf stage increases with metallicity. Finally, we estimate the mean
mass at the top of the white dwarf cooling curve for three globular clusters
NGC 6397, M4 and 47 Tuc, around , characteristic of old stellar
populations. However, we found different values for the progenitor mass, lower
for the metal poor cluster, NGC 6397, and larger for the younger and metal rich
cluster 47 Tuc, as expected from the metallicity dependence of the
initialtofinal mass relation.Comment: Accepted for publication in MNRA
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