Determination of physical parameters of five large amplitude δ Scuti stars

Astronomy and Astrophysics Supplement Series, Jul 1999

photometry of the large amplitude δ Scuti stars BP Peg, DY Peg, DY Her, CY Aqr and YZ Boo is presented. Since the data were obtained almost simultaneously in all filters, 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.

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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, Send o print requests to: J.H. Pen~a 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)


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J. H. Peña, D. González, R. Peniche. Determination of physical parameters of five large amplitude δ Scuti stars, Astronomy and Astrophysics Supplement Series, 1999, pp. 11-18, Volume 138, Issue 1, DOI: 10.1051/aas:1999264