A&A 443, 251-269 (2005)

DOI: 10.1051/0004-6361:20053294

## Structural parameters of the hot pulsating B subdwarf Feige 48 from asteroseismology

**S. Charpinet**

^{1}, G. Fontaine^{2}, P. Brassard^{2}, M. Billères^{3}, E. M. Green^{4}and P. Chayer^{5, 6}^{1}UMR 5572, Université Paul Sabatier et CNRS, Observatoire Midi-Pyrénées, 14 Av. E. Belin, 31400 Toulouse, France

e-mail: scharpin@ast.obs-mip.fr

^{2}Département de Physique, Université de Montréal, C.P. 6128, Succursale Centre-Ville, Montréal QC, H3C 3J7, Canada

e-mail: [fontaine;brassard]@astro.umontreal.ca

^{3}European Southern Observatory, Santiago Headquarters, Av. A. de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile

e-mail: mbillere@eso.org

^{4}Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA

e-mail: bgreen@as.arizona.edu

^{5}Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218-2686, USA

e-mail: chayer@pha.jhu.edu

^{6}Primary affiliation: Department of Physics and Astronomy, University of Victoria, PO Box 3055, Victoria, BC V8W 3P6, Canada

(Received 25 April 2005 / Accepted 21 July 2005)

** Abstract **

We present observations followed by a thorough analysis of the
rapidly pulsating subdwarf B star (or EC 14026 star)
Feige 48
. This
work is part of a long term multifaceted effort to exploit the strong
asteroseismological potential of sdB pulsators which involves high
sensitivity photometric observations, accurate spectroscopic measurements,
and the development of appropriate modelling tools dedicated to the
interpretation of the seismic data. Our model atmosphere analysis
of the time averaged optical spectrum of
Feige 48
obtained
at the new Multiple Mirror Telescope (MMT) leads to estimates of
K and
(with
), in excellent
agreement with previous spectroscopic measurements of its atmospheric
parameters. This places
Feige 48
close to the red edge of the
EC 14026 instability region in the
plane. A
standard Fourier analysis of our high signal-to-noise ratio
Canada-France-Hawaii Telescope (CFHT) light curves reveals the presence
of nine distinct harmonic oscillations with periods in the range 343-383
s, a significant improvement over previous reported observations that
recovered only five periods (Koen et al. 1998, MNRAS, 300, 1105; Reed et al. 2004, MNRAS, 348, 1164). Out of these nine periods, only four
turn out to be independent modes having different *k* and/or
indices suitable for detailed asteroseismology. The remaining periods
can be interpreted as rotationally split components of modes with the
same (*k*, ) values, but different *m* indices that bear the
signature of the rotation of the star. On the basis of the four
independent periods, we have carried out a detailed asteroseismic
analysis of
Feige 48
using the well-known forward method. Our
analysis leads objectively to the identification of the (*k*, )
indices of the four independent periods identified in the star, and to
the determination of its structural parameters. The periods correspond
to low-order modes with adjacent values of *k* and with , 1, and 2.
They define a band of unstable modes, in agreement with nonadiabatic
pulsation theory. Furthermore, the average dispersion between the
observed periods and the periods of the corresponding theoretical modes
of the optimal model is only ~0.005%, quite close to the actual accuracy
of the observations. We emphasise that radiative levitation is a key
ingredient in the determination of accurate pulsation periods for sdB
stars, and that standard models with uniform metallicity fail to
reproduce the observed periods in Feige 48 because they do not
incorporate this key piece of constitutive physics. On the basis of our
combined spectroscopic and asteroseismic analysis, the inferred global
structural parameters of
Feige 48
are
K,
,
,
(i.e.,
close to the canonical mass of extreme horizontal branch stars),
, and
. Combined
with detailed model atmosphere calculations, we estimate, in addition,
that this star has an absolute visual magnitude
and is located at a distance
pc (using
).
Finally, the analysis of the fine structure indicate a rotation period
of
h, leading to an equatorial velocity of
km s^{-1}. With a
km s^{-1} limit set by Heber et al. (2000, A&A, 363, 198), this result means
that
Feige 48
is a moderate rotator which, with an inclination
angle
, is seen nearly pole-on.

**Key words:**stars: interiors

**--**stars: oscillations

**--**stars: subdwarfs

**--**stars: individual: Feige 48

SIMBAD Objects

**©**

*ESO 2005*