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تسجيل مستخدم جديدNew developments of the ZZ Ceti instability strip: The discovery of eleven new variables
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Using the SOAR 4.1 m telescope, we report on the discovery of low amplitude pulsations for three stars previously reported as Not-Observed-to-Vary (NOV) by Mukadam et al. (2004) and Mullally et al. (2005), which are inside the ZZ Ceti instability strip. With the two pulsators discovered by Castanheira et al. (2007), we have now found variability in a total of five stars previously reported as NOVs. We also report the variability of eight new pulsating stars, not previously observed, bringing the total number of known ZZ Ceti stars to 148. In addition, we lowered the detection limit for ten NOVs located near the edges of the ZZ Ceti instability strip. Our results are consistent with a pure mass dependent ZZ Ceti instability strip.
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We report the discovery of eleven new ZZ Cetis using telescopes at OPD (Observatorio do Pico dos Dias/LNA) in Brazil, the 4.1 m SOAR (Southern Astrophysical Research) telescope at Cerro Pachon, Chile, and the 2.1 m Otto Struve telescope at McDonald o
bservatory. The candidates were selected from the SDSS (Sloan Digital Sky Survey) and SPY (ESO SN Ia progenitor survey), based on their Teff obtained from optical spectra fitting. This selection criterion yields the highest success rate of detecting new ZZ Cetis, above 90% in the Teff range from 12000 to 11000 K. We also report on a DA not observed to vary, with a Teff placing the star close to the blue edge of the instability strip. Among our new pulsators, one is a little bit cooler than this star for which pulsations were not detected. Our observations are an important constraint on the location of the blue edge of the ZZ Ceti instability strip.
We present our results on the continuation of our survey searching for new ZZ Ceti stars, inspired by the recently launched TESS space mission. The seven targets were bright DA-type white dwarfs located close to the empirical ZZ Ceti instability stri
p. We successfully identified one new pulsator candidate, namely PM J22299+3024, derived detection limits for possible pulsations of four objects for the first time, and determined new detection limits for two targets.
We report on the discovery of six new ZZ Ceti stars. They were selected as candidates based on preparatory photometric observations of objects from the Hamburg Quasar Survey (HQS), and based on the spectra of the Supernova Ia Progenitor Survey (SPY).
Time-series photometry of 19 candidate stars was carried out at the Nordic Optical Telescope (NOT) at Roque de Los Muchachos Observatory, Spain. The new variables are relatively bright, 15.4<B<16.6. Among them is WD1150-153, which is the third ZZ Ceti star that shows photospheric CaII in its spectrum.
We present the results of a comparative period search on different time-scales and modelling of the ZZ Ceti (DAV) star GD 154. We determined six frequencies as normal modes and four rotational doublets around the ones having the largest amplitude. Tw
o normal modes at 807.62 and 861.56 microHz have never been reported before. A rigorous test revealed remarkable intrinsic amplitude variability of frequencies at 839.14 and 861.56 microHz over a 50 d time-scale. In addition, the multimode pulsation changed to monoperiodic pulsation with an 843.15 microHz dominant frequency at the end of the observing run. The 2.76 microHz average rotational split detected led to a determination of a 2.1 d rotational period for GD 154. We searched for model solutions with effective temperatures and log g close to the spectroscopically determined ones. The best-fitting models resulting from the grid search have M_H between 6.3 x 10^-5 and 6.3 x 10^-7 M*, which means thicker hydrogen layer than the previous studies suggested. Our investigations show that mode trapping does not necessarily operate in all of the observed modes and the best candidate for a trapped mode is at 2484 microHz.
The pulsating DA white dwarfs (ZZ Ceti stars) are $g$-mode non-radial pulsators. Asteroseismology provides strong constraints on their global parameters and internal structure. Since all the DA white dwarfs falling in the ZZ Ceti instability strip do
pulsate, the internal structure derived from asteroseismology brings knowledge for the DA white dwarfs as a whole group. HS 0507+0434B is one of the ZZ Ceti stars which lies approximately in the middle of the instability strip for which we have undertaken a detailed asteroseismological study. We carried out multisite observation campaigns in 2007 and from December 2009 to January 2010. In total, 206 hours of photometric time-series have been collected. They have been analysed by means of Fourier analysis and simultaneous multi-frequency sine-wave fitting. In total, 39 frequency values are resolved including 6 triplets and a number of linear combinations. We identify the triplets as $ell$=1 $g$-modes split by rotation. We derived the period spacing, the rotational splitting and the rotation rate. From the comparison of the observed periods with the theoretical periods of a series of models we estimate the fundamental parameters of the star: its total mass M$_{*}$/M$_{odot}$ = 0.675, its luminosity L/L$_{odot}$=3.5$times 10^{-3}$, and its hydrogen mass fraction M$_{H}$/M$_{*}$= 10$^{-8.5}$.
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