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The Astrophysical Journal, 757:177 (7pp), 2012 October 1 doi:10.1088/0004-637X/757/2/177
C
2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
SEISMOLOGY OF A MASSIVE PULSATING HYDROGEN ATMOSPHERE WHITE DWARF
S. O. Kepler1, Ingrid Pelisoli1, Viviane Pe¸canha1,J.E.S.Costa
1, Luciano Fraga2,J.J.Hermes
3,D.E.Winget
3,
Barbara Castanheira3,4,A.H.C
´
orsico5, A. D. Romero5, Leandro Althaus5,S.J.Kleinman
6, A. Nitta6,
D. Koester7, Baybars K ¨
ulebi8, Stefan Jordan9, and Antonio Kanaan10
1Instituto de F´
ısica, Univ
ersidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre, RS, Brazil;
[email protected]gs.br2Southern Observatory for Astrophysical Research, Casilla 603, La Serena, Chile
3Department of Astronomy and McDonald Observatory, University of Texas, Austin, TX 78712-1083, USA
4Institut f¨
ur Astronomie, T¨
urkenschanzstr. 17, A-1180 Wien, Austria
5Facultad de Ciencias Astron´
omicas y Geof´
ısicas, Universidad Nacional de La Plata, Argentina
6Gemini Observatory, Northern Operations Center, 670 North A’ohoku Place, Hilo, HI 96720, USA
7Institut f¨
ur Theoretische Physik und Astrophysik, Universit¨
at Kiel, D-24098 Kiel, Germany
8Institut de Ci`
encies de L’Espai, Universitat Aut`
onoma de Barcelon and Institute for Space Studies of Catalonia, c/Gran Capit`
a2-4,
Edif. Nexus 104, E-08034 Barcelona, Spain
9Astronomisches Rechen-Institut, ZAH, M¨
onchhofstr. 12-14, D-69120 Heidelberg, Germany
10 Departamento de F´
ısica, Universidade Federal de Santa Catarina, Florian´
opolis, SC, Brazil
Received 2012 June 23; accepted 2012 August 8; published 2012 September 17
ABSTRACT
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=0.88 ±0.02 M
and Teff =12,100 ±140 K. These values are consistent with those derived from the optical spectra and photometric
colors.
Key words: stars: fundamental parameters – stars: oscillations – white dwarfs
Online-only material: color figures
1. INTRODUCTION
Roughly 97% of all stars born have main-sequence masses
less than 8–10 M; these stars will evolve to become white
dwarf stars (e.g., Weidemann 2000). This is in agreement
with Podsiadlowski et al. (2004), who estimate 11 ±1Mas
the lower limit for stars that evolve to neutron stars. White
dwarf stars are therefore records of stellar evolution history
(Fontaine & Brassard 2008; Winget & Kepler 2008; Althaus
et al. 2010b). Because of their compact nature and high internal
temperatures, they are also useful as probes of high energy
density physics (e.g., Kepler et al. 2005; Isern et al. 2010).
Spectroscopically, more than 80% of all white dwarf stars
show only hydrogen lines and are classified as spectral type
DA (e.g., Kepler et al. 2007; Bergeron et al. 2011). As DAs
cool below an effective temperature ∼13,000 K, they develop a
partial ionization zone near the surface that results in surface
convection. As the convection zone becomes deep enough,
near 12,200 K, depending on the mass of the star, most if
not all DA white dwarf stars start to pulsate in non-radial
g-mode pulsations (e.g., Castanheira et al. 2010). We report
the discovery of pulsations in the higher than average mass DA
SDSS J132350.28+010304.22.
We fit the two existing Sloan Digital Sky Survey (SDSS)
spectra of this star with synthetic spectra, based on atmosphere
models, using ML2/α =0.6 (e.g., Bergeron et al. 1995)
mixing length convection parameterization, and Tremblay et al.
(2010) non-ideal Stark broadening (Koester et al. 2009; Koester
2010). Each SDSS spectra can be uniquely identified by a
Plate-Modified Julian Date-Fiber number. The two spectra, with
Plate-Modified Julian Date-Fiber =0297-51959-332 and S/N=
14, yield a fit of Teff =11,780±160 K and log g=8.56±0.06,
and M=0.94 ±0.03 M(Figure 1), while 0297-51663-324,
with S/N=11, yields a fit of Teff =11,900 ±230 K and
log g=8.45 ±0.08, and M=0.88 ±0.05 M; the masses
were obtained using the evolutionary models of Althaus et al.
(2010a). Tremblay et al. (2011) independently fit the same SDSS
spectra with their α=0.8 models and find similar values:
Teff =11,680 ±200 K and log g=8.59 ±0.07, and a mass
M=0.98 ±0.05 M.
Castanheira et al. (2010) discuss the 148 DAVs known to
date and none have reported magnetic field determinations.
Wickramasinghe & Ferrario (2005) quote a mean mass of
0.93 Mfor magnetic white dwarfs, compared with 0.6 Mfor
normal white dwarfs, based on Liebert et al. (2003) determina-
tions. As magnetic field broadening of the lines can be mistaken
as higher gravity, we need to determine if the mass estimated
from the spectra is affected by a magnetic field.
We report higher signal-to-noise (S/N) spectra and polar-
ization measurements showing no clear evidence that SDSS
J132350.28+010304.22 is magnetic to our detection limit
around 1 MG.
2. OBSERVATIONS: PHOTOMETRIC
AND SPECTROSCOPIC
We obtained time series of 30 s images with the B filter on the
night of 2011 April 28, using the SOAR Optical Imager attached
to the SOAR 4.1 m telescope, detecting ZZ Ceti-type pulsations
1