In order to carry out the previously described techniques, the photometric values in the standard system presented in the section of Observations were
used first to determine reddening. This was done using specific techniques which have been previously described (see, for example, Peña et al. 1993)
and which are based on a calibration by Nissen (1988) based, in turn, on the fundamental calibrations of Crawford and his co-workers (1975, 1979) for
A and F stars. This is applicable since, as is well-known, the Scuti stars are within these spectral limits in the HR diagram. For both
types, Crawford derived empirical calibrations for Mv and (b-y)0 and in the latter case, also the [Fe/H] metallicity. With these calibrations,
slightly modified by Nissen (1988), a computing code was written to determine reddening and unreddened parameters for each set of photometric data. A
slight interpolation would be necessary since the H
values differ slightly in time from uvby photometry. However, since the difference is so
small, in practice they were treated as if they were taken at the same time.
A brief description for each star is given which emphasizes the main characteristics relevant to the findings of the present paper:
BP Peg. Broglia (1959)
reported a linear ephemeris with a period of 0.10954347 d,
detecting
also a possible variation of the period between the 1953 and 1958 and
1959 observations. Furthermore, they found a modulation period on the
order of 0.37 d with an amplitude in the maxima of 0.45 mag in
the
visual. Rodríguez (1989) presented uvby observations of this
star and
identified two main frequencies, 9.1260 and 11.8303 c/d
corresponding to the fundamental mode and to the first overtone. Kim
et al. (1989) secured new and spectroscopic
observations of this star. They determined a photometric reddening
value of E(b-y) equal to 0.067 mag. A superficial gravity value
of 3.85 and a ratio [Fe/H] of -0.08 were obtained. An average
of 7470 K was determined from their photometry and theoretical grids.
From this and theoretical models, a bolometric magnitude of
mag, a mass of
and an age of
yr were finally derived. Rodríguez et al. (1992)
also reported uvby-
of three large amplitude
Scuti
stars, BP Peg, among them. They were able to determine the same
parameters previously established with their photometry.
CY Aqr. Detre & Chang (1960) and Hardie & Tolbert (1961)
reported an ephemeris with a negative quadratic term. Percy (1975) reported that the period remained constant between 1934 and 1951, when it suffered a drastic change and then remained constant. Elst (1972) determined a modulation period of 0.1222 d, but with a very small amplitude. Fitch (1973) suggested the modulation period might be 0.17766 d, which could be explained by the pulsational theory, whereas
Elst's (1972) period, could not. Percy (1975) found that the modulation period proposed by Elst (1972) was not real but merely a result of its method of analysis.
Geyer & Hoffmann
(1974, 1975) did not find any modulation period. Mahdy & Szeidl (1980) derived an abrupt change of the period of
d around 1952, the period being constant before and after this date. Rolland et al. (1986) and Peña et al. (1987), proposed a period decay consistent with the theoretical model of a pre-white dwarf of
in opposition to the abrupt period changes suggested. Rodríguez (1989) determined a linear ephemeris for his data and, in a later study,
(Rodríguez et al. 1990) determined some observational parameters such as effective temperature of Te 7590 K, superficial gravity of
and metallicity
[Fe/H] = - 0.7. Besides these values, they determined, considering pulsation, the following:
,
,
and an age of
yr. Coates et al. (1994) determined a period change and concluded that the
star was pulsating with only one period. This conclusion is not supported by the study of Powell et al. (1995) which stated that the new data did not present significant variations from cycle to cycle, but that an abrupt period change occurred in 1989 and that the star is now pulsating with a period of 0.061038612 d.
DY Her. Since its discovery as a variable star in the thirties
it has been a subject of considerable interest. In works by McNamara
(1978) with spectra and by Breger et al. (1978) from , a
value of
was determined. Szeidl (1979) stated that the
radius should be larger than that previously reported. Breger et al.
(1978) determined the effective temperature and superficial gravity
with Strömgren photometry with a one channel photometer.
Rodríguez (1989) determined the metallicity of the star with
simultaneous Strömgren photometry. López de Coca et al.
(1990) included it in a comprehensive review of the observational
characteristics of all the
Scuti stars known. Antonello
(1990) carried out a comparison with theoretical models concerning
the Strömgren m1 index. For DY Her he derived
of 7360 K,
of 3.87 and [Fe/H]
of 0.16. The physical parameters of mass, bolometric magnitude and
age were later determined from
photometry by
Rodríguez et al. (1990). Dawei et al.
(1993) confirmed the period change of DY Her based on studies by
Madhy & Szeidl (1980) with a compilation of 60 maxima from 1938 up
to 1979. Yang et al. (1993) made a compilation of 30 years of
observations determining a variation in the period of P0 =
0.14863130 d,
yr. Milone et al.
(1994) derived the mean radius and bolometric
luminosity of
and
mag,
respectively, from visual and infrared photometry and the effective
temperature derived by Breger et al. (1978).
DY Peg.
An excellent review of the previous observations can be found in
Rodriguez (1989). Between 1966 and 1975 several observers obtained
new photometric data with the purpose of confirming light curve
variations. Quigley & Africano (1979, 1981) derived a quadratic
ephemeris which predicted a decrease in the period. Peña & Peniche
(1986) derived a decrease in the period, consistent with the
theoretical models of Dziembowski & Kozlowski (1974) for a
pre-white-dwarf in opposition to the abrupt period changes proposed
by Percy (1975) and Mahdy & Szeidl (1980). Rodriguez (1989) reported
observations of DY Peg in the Strömgren system determining a period
of 0.0729 d. Recently, Kilambi & Rahman (1993) presented
UBV observations and determined mass, radius, gravity and variations of
the temperature over one cycle of pulsation. Rodriguez et al. (1993)
reported light curves in the system.
YZ Boo. Studies by Szeidl & Mahdy (1981) led them to conclude
that YZ Boo had suffered a period change. Joner & McNamara (1983)
derived its physical parameters ( and
,
[Fe/H]= -0.025,
) through UBV and
Strömgren photometry and concluded that it is a dwarf Cepheid of
Pop I, which, according to Breger (1979) could be considered to be
Scuti star. Since the period changes can provide information
on the evolutive stage of the stars, several studies have been done
with this goal in mind: this star was first studied by Szeidl &
Mahdy (1981), later by Peniche et al. (1985); Jiang (1986),
determined new ephemeris with a quadratic term. Peniche et al. (1985)
concluded that it is impossible to decide if the period is constant
or not with the currently available data bases. Kim & Joner (1994a)
reported photometric observations in Coussins VRc system and in a
later paper Kim & Joner (1994b) determined Te = 7410 K,
and a metallicity [M/H] = -0.025.
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