Delta Scuti Stars

Astron. & Astrophys. 23, 221-240 (1973) 1973A&A....23..221B Delta Scuti Stars A. Baglin+, M. Breger*, C. Chevalierx, B. Hauck0 , J.M. le Contel+, J.P. Sareyanv and J.C. Valtier+ Observatoire de Nice+, University of Texas at Austin*, Observatoire de Meudon, Department d'Astrophysique Fondamentalex, Institut d'Astronomie de l'Universite de Lausanne et Observatoire de Geneve0 , Observatoire de ParisV, Departement de Photometrie Received August 28, 1972 Summary. The extensive observational and theoretical Key words: b Scuti stars - cepheid variables - short data on b Scuti variables (published and previously period pulsators unpublished) are collected and analyzed. Properties of pulsation are discussed, and relationships between b Scuti stars and other types of pulsators and non variable stars are established. 1. Introduction b Scuti variables are short-period pulsators situated in of variable stars. At the time the b Scuti variable group the extension of the cepheid instability strip, which had five members. Only after 1965 were numerous crosses the main-sequence between A2 V and FO V. discoveries of b Scuti variables made. This is due to Pulsation periods are between 34 minutes and 5 hours. the fact that the small amplitudes of most variables The amplitudes in light and radial velocity are generally require modem photoelectljc techniques. Several sys very small: a few hundredths of a magnitude and about tematic searches for more (j Scuti variables have been 10 km s-1 respectively. In addition beats are often ob made by Danziger and Dickens (1967), Millis (1967b), served. The variables discovered so far lie either on J0rgensen et al. (1971) and Breger (1966, 1969a, the main-sequence or in a band up to three magnitudes 1972a, b). above. In Fig. 1, a typical light curve in the V filter is shown. We suggest the following definition of ab Scuti variable: The observations were made with the Lick 24-inch a b Scuti variable is a pulsating star which (i) is situated reflector. The variability and .period were confirmed on in the cepheid instability strip (or its extension) and subsequent nights. At present, more than 70 b Scuti (ii) has a pulsation period less than one day. In variables are known and a thousand more should be practice, this means a Population I short-period within reach of a small telescope. pulsator of spectral type A or F. b Scuti stars are With the numerous recent developments in the study therefore the younger analogs to the RR Lyrae stars of these stars we thought it necessary to review the and have many characteristics similar to the AI Vel subject and to give reliable basic data for future in stars. vestigations. (j Scuti variables are also of relevance to The first mention of the b Scuti variables was made by other fields of Astronomy, and an attempt has been Wright (1900) who announced the radial velocity of made to point out these directions. the star b Set as variable. Better observations required an improvement of photographic plates. It was only around 1935 that Colacevich (1935) and Fath (1935) My+ Const. HR 6290 made their relatively accurate simultaneous radial -0.010 velocity and photometric measurements of (j Set. They determined a period and placed the star in the f3 Canis Majoris variable star group. Further in 0.000 vestigations made in the following years suggested that b Set resembled the cepheids rather than the +0.010 (hotter) /3 Canis Majoris variables. Walker (1953) and (Moy 22,1969) Lindblad and Eggen (1953) discovered the stars DQ Cep U.T. 02.00 03.00 04.00 05.00 06.00 07.00 and CC And to be variable. In an important develop ment, Eggen (1956) found the variability of l! Pup and Fig. 1. Light curve of the variable H.R. 6290 in the V filter obtained (j Del and pointed out the existence of a separate type at the Lick 24 inch reflector by M.B. © European Southern Observatory • Provided by the NASA Astrophysics Data System 222 A. Baglin et al. 2. Variability Measurements differential reduction method should correct for these effects, assuming their linearity in time. Observations of c5 Scuti variables have been under (iv) Filter width and color-dependent extinction co taken with three main aims: to establish which stars efficients. Since the comparison stars are likely to have are variable, to determine the amplitude and period different colors from the variable star, in wideband 1973A&A....23..221B of variations and to study beat phenomena. The filters the effective wavelengths may differ. These amplitudes of variation are generally near the limit of effective wavelengths are extinction-dependent. This detection, and even can be almost zero at certain may be a problem during nights of variable extinction epochs when beats exist. So, long observational se and at large hour angles. So it is more interesting to quences on the same object are needed, if possible, work with narrow-band filters. For bright stars under on several consecutive nights. For example HR 5005 these circumstances, an intermediate-band photometric has been observed as constant by Valtier (1971) during system such as the uvby system may be appropriate. 4.5 hours but was discovered precedingly by Danziger However, the width and the wavelengths of the filters and Dickens (1967) with a period of 3 hours. depend also on the purpose of the observations The brightness or radial velocity variability of these (detection, or measure of physical parameters such stars appears most promising to measure, but changes as T,,). in surface gravity and shapes of line profiles also may (v) The chromatic variation of the photomultiplier be looked for. The best possible instrumentation, response is a related problem. The photomultiplier weather conditions as well as observing and reduction should be kept at a constant temperature (Young, 1963). techniques are required. If the Johnson V filter and a 1 P 21 photomultiplier tube is used, then the long wavelength cutoff is given by the tube and quite variable. In the differential 2.1. Photometry reduction mode, again these effects should cancel out Light amplitudes under 0.05 magnitudes have to be if the comparison stars have the same color. detected; this rules out classical photographic technics. In the method proposed by Sareyan and Ischi (1972) Photoelectrically, both D.C. and pulse-counting tech the chromatic effect and the fatigue of the P.M. are niques have been employed successfully. Differential reduced. star-star methods are required. Unfortunately, most (vi) Spurious signal. In the pulse-counting mode weaker observers have not described their observing techniques pulses are not counted and extrinsic spurious pulses and in the case of a few controversial variables, critical may be detected by sudden count increases. This evaluation by other astronomers may be impossible. important problem may be overcome by eliminating We would like to list some of the problems to be the sources of such pulses. Shielding of the cold box is guarded against and possible ways to solve them: desirable and it may be necessary to have a pre (i) Photomultiplier memory, fatigue and instability. amplifier inside the shielded system. In the D.C. mode This is always present to some degree. Then, stability similar problems are present, but may be more difficult has to be insured on HT sources (Lallemand, 1962), to detect. filters and P.M. temperatures (Young, 1967) terrestrial (vii) Photon Statistics. This excludes fainter objects. magnetic field' (Rufener, 1966). P.M. memory and (viii) Seeing and background changes. A relatively large fatigue effect are minimized by keeping a constant diaphragm should be used to measure all the light from voltage between the cathod and the first three dynodes, the star and to avoid the small light losses when the and varying the voltage on the following stages seeing disk increases in size. A diaphragm size of 20" (Sareyan and Ischi, 1972). In each case, it is better or larger is recommended. This, however, also increases observing the stars in the same manner as well as using the sky contribution. If this sky contribution is signi two comparison stars. ficant, it has to be monitored very carefully. (ii) One has to be careful of obtaining a proper Fabry (ix) Outputs. (field lens) action and stable position on the photo It is important to have the possibility of knowing the cathod to avoid errors due to movements of the image seeing conditions at any time. An analog output of the stars inside the diaphragm and to different system is necessary. It permits also a rapid estimate of sensitivities of different parts of the photocathod. the quantity of information needed to get a given (This is easier to obtain on P.M. in which the photo precision. Young (1968) has shown that with a counting cathod is perpendicular to the incident flux: e.g. Lalle system the necessary information is obtained more mand, EMI. ...) rapidly. In addition reductions are more rapid and (iii) Atmospheric changes such as thin undetected personal errors are avoided. clouds, variable extinction coefficients and transpar (x) Pulsational amplitudes are usually larger at shorter ency changes. If the atmospheric changes are slower wavelengths where one might expect them to be easily than the time needed to complete one observing cycle detectable. On the other hand, the atmosphere appears of two comparison and one variable stars, then the to be_ more stable in the visual region. The blue or © European Southern Observatory • Provided by the NASA Astrophysics Data System Delta Scuti Stars 223 visible region appears to be a good compromise for 2.3. Spectrophotometric Measurements detection in one filter. It is desirable to determine the variations of effective (xi) Choice of comparison stars.

Load more