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Castanheira B.G.,Institute For Astronomie | Kepler S.O.,Federal University of Rio Grande do Sul | Kleinman S.J.,Gemini Observatory | Nitta A.,Gemini Observatory | Fraga L.,Southern Observatory for Astrophysical Research
Monthly Notices of the Royal Astronomical Society | Year: 2013

Using the SOuthern Astrophysical Research telescope (SOAR) Optical Imager at the SOAR 4.1 m telescope, we report on the discovery of five new massive pulsating white dwarf stars. Our results represent an increase of about 20 per cent in the number of massive pulsators. We have detected both short and long periods, low and high amplitude pulsation modes, covering the whole range of the ZZ Ceti instability strip. In this paper, we present a first seismological study of the new massive pulsators based on the few frequencies detected. Our analysis indicates that these stars have masses higher than average, in agreement with the spectroscopic determinations. In addition, we study for the first time the ensemble properties of the pulsating white dwarf stars with masses above 0.8 M⊙. We found a bimodal distribution of the main pulsation period with the effective temperature for the massive DAVs, which indicates mode selection mechanisms. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Source


Paunzen E.,Masaryk University | Iliev I.K.,Bulgarian Academy of Science | Pintado O.I.,Instituto Superior Of Correlacion Geologica | Baum H.,University of Vienna | And 7 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

Globular clusters are main astrophysical laboratories to test and modify evolutionary models. Thought to be rather homogeneous in their local elemental distribution of members, results suggest a wide variety of chemical peculiarities. Besides different main sequences, believed to be caused by different helium abundances, peculiarities of blue horizontal-branch stars and on the red giant branch were found. This whole zoo of peculiar objects has to be explained in the context of stellar formation and evolution. The tool of Δ photometry is employed in order to detect peculiar stars in the whole spectral range. This three filter narrowband system measures the flux distribution in the region from 4900 to 5600 Å in order to find any peculiarities around 5200 Å. It is highly efficient to detect classical chemically peculiar stars of the upper main sequence, Be/Ae, shell and metal-weak objects in the Milky Way and Magellanic Clouds. We present Δa photometry of 2266 stars from 109 individual frames for three globular clusters (NGC 104, NGC 6205, and NGC 7099). A comparison with published abundances, for three horizontal-branch stars, only, yields an excellent agreement. According to the 3s detection limit of each globular cluster, about 3 per cent of the stars lie in abnormal regions in the diagnostic diagrams. The first observations of three widely different aggregates give very promising results, which will serve as a solid basis for follow-up observations including photometric as well as spectroscopic studies. © 2014 The Authors.Published by Oxford University Press on behalf of the Royal Astronomical Society. Source


Castanheira B.G.,Federal University of Rio Grande do Sul | Castanheira B.G.,Institute For Astronomie | Kepler S.O.,Federal University of Rio Grande do Sul | Kleinman S.J.,Gemini Observatory | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

Using the The SOuthern Astrophysical Research (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. and Mullally et al., which are inside the ZZ Ceti instability strip. With the two pulsators discovered by Castanheira et al., 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 10 NOVs located near the edges of the ZZ Ceti instability strip. Our results are consistent with a pure mass-dependent ZZ Ceti instability strip. © 2010 The Authors. Journal compilation © 2010 RAS. Source


Dalessio J.,University of Delaware | Dalessio J.,Delaware Asteroseismology Research Center | Sullivan D.J.,Victoria University of Wellington | Provencal J.L.,University of Delaware | And 7 more authors.
Astrophysical Journal | Year: 2013

Variations in the pulsation arrival time of five independent pulsation frequencies of the DB white dwarf EC 20058-5234 individually imitate the effects of reflex motion induced by a planet or companion but are inconsistent when considered in unison. The pulsation frequencies vary periodically in a 12.9 year cycle and undergo secular changes that are inconsistent with simple neutrino plus photon-cooling models. The magnitude of the periodic and secular variations increases with the period of the pulsations, possibly hinting that the corresponding physical mechanism is located near the surface of the star. The phase of the periodic variations appears coupled to the sign of the secular variations. The standards for pulsation-timing-based detection of planetary companions around pulsating white dwarfs, and possibly other variables such as subdwarf B stars, should be re-evaluated. The physical mechanism responsible for this surprising result may involve a redistribution of angular momentum or a magnetic cycle. Additionally, variations in a supposed combination frequency are shown to match the sum of the variations of the parent frequencies to remarkable precision, an expected but unprecedented confirmation of theoretical predictions. © 2013. The American Astronomical Society. All rights reserved.. Source


Kepler S.O.,Federal University of Rio Grande do Sul | Pelisoli I.,Federal University of Rio Grande do Sul | Pecanha V.,Federal University of Rio Grande do Sul | Costa J.E.S.,Federal University of Rio Grande do Sul | And 14 more authors.
Astrophysical Journal | Year: 2012

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 4.1m SOAR Telescope, complemented by contemporary McDonald Observatory 2.1m 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= 0.88 ± 0.02 M and T eff = 12, 100 ± 140 K. These values are consistent with those derived from the optical spectra and photometric colors. © 2012. The American Astronomical Society. All rights reserved. Source

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