Determination of physical parameters of five large amplitude δ Scuti stars
Astron. Astrophys. Suppl. Ser.
Determination of physical parameters of Scuti stars
J.H. Pen~a 0
D. Gonzalez 0
R. Peniche 0
0 Instituto de Astronom a, UNAM , Mexico Apartado Postal 70-264, D.F. , Mexico
uvby − photometry of the large amplitude Scuti stars BP Peg, DY Peg, DY Her, CY Aqr and YZ Boo is presented. Since the data were obtained almost simultaneously in all lters, meaningful physical parameters have been derived for each star along their pulsation cycles using the calibrations from Nissen (1988) for A and F stars to determine the reddening and derive the unreddened photometric values. The utilization of the theoretical grids of Relyea & Kurucz (1978) has allowed temperature and gravity values to be determined. A comparison with previously reported values is presented.
stars; fundamental parameters |
1. Introduction
The determination of physical parameters such as e ective
temperature and super cial gravity can be done through
the Str¨omgren photometric data reduced to the standard
system for B − F stars, once corrected for interstellar
extinction. This procedure has been done previously by the
authors for scuti variables and for stars in open
cluster which lie within these spectral; its description can be
found in
Peniche et al. (1990)
, for example.
We must emphasize that for these fast pulsators, some
of which are multi-periodic or with unexplained light curve
variations, the data gathered in the present work has the
advantage that it need not be adjusted in phase as it would
have been with a one-channel photometer. This also allows
an extensive time coverage along the cycle, in particular if
the star is faint. On the other hand, the simultaneous
measurements in the di erent lters that de ne the Str¨omgren
system let us avoid problems due to interpolation which
worsen if more than one frequency is present since it is well
known
(Breger 1998)
that many high amplitude Scuti
stars (HADS) are double mode radial pulsators. Hence,
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the analysis of the photometry presented in the current
work shows the real variation of the physical
characteristics of the stars along their pulsation cycles.
If the photometric system is well-de ned and
calibrated, it will provide an e cient way to investigate
physical conditions. A comparison with theoretical models, such
as those of
Relyea & Kurucz (1978)
, allows a direct
comparison with intermediate or wide band photometry
measured from the stars with those obtained theoretically for
early type stars.
Relyea & Kurucz (1978)
calculated grids
for stellar atmospheres for G, F, A, B and O stars for the
solar abundance [Fe=H] = 0:00 in a temperature range
from 5500 K up to 50 000 K. They also considered
abundances of 0:1 and 0:001. A comparison of the photometric
unreddened indexes (b − y)0 and c0 obtained for each star
with such models permits the determination of the e
ective temperature Te and super cial gravity log g along the
cycle of pulsation. Therefore, the research goals for Scuti
stars should be to try to determine not only the frequency
content but also the maximum number of physical
characteristics of as many stars as possible, along each star's
cycle variation. Furthermore, there still exists the
question of the real physical di erences between the Scuti
stars and Dwarf Cepheids; Breger (1980) stated that the
majority of dwarf Cepheids mimic Pop I Scuti stars in
basically all aspects, except for a few stars in the subgroup
of Dwarf Cepheids known as the SX Phe that show low
metallicity, high space velocities and low luminosities and
that do not conform the theoretical period gravity
relation in the direction of low mass. Hence motivation of the
present work.
2. Observations
The photometric system utilized has the advantage, that
the uvby photometry is acquired simultaneously and the
N and W lters that de ne H almost simultaneously.
All the observations were carried out at the Observatorio
Astronomico Nacional at San Pedro Martir, Mexico.
The data was acquired in di erent observing seasons:
July-August, 1986; August-September, 1986;
SeptemberOctober, 1992 and March, 1994. All were carried out
at the 1.5 m telescope with the same
spectrophotometer. A description of this instrument can be found in
Nissen (1984)
. The reduction procedure was done through
the numerical packages NABAPHOT (Arellano-Ferro &
Parrao 1989) and DAMADAP
(Parrao et al. 1998)
which
reduce the data into a standard system. The system that
was chosen was that de ned by the standard values of
Olsen (1983)
and the transformation equations are those
de ned by
Crawford & Barnes (1970)
and by
Crawford
& Mander (1966)
. An estimate of accuracy was done by
comparing the uvby − data obtained with those of
Olsen
(1983)
and Gr¨onbech & Olsen (1977). The uncertainties
were evaluated in the following manner: The average
differences, present data of the standard stars minus Olsen's
data, were evaluated and provide an uncertainty for the
transf (...truncated)