Abstract
We present spectroscopic and photometric observations of the star HD 173977. It appears that the star is part of a double line binary system, with a period of 1.801 d, corresponding to twice the period of the photometric variations. Hence the star is an ellipsoidal variable. The system is probably synchronized. The physical parameters of both components were derived through two independant methods, one based on evolutionary tracks, the other being the result of the behaviour of light curves in a close binary system. After removing the ellipsoidal variations, 3 frequencies are detected in the photometric data: 8.56, 14.51 and 16.42 d -1 , while 2 additional frequencies are also possible: 10.96 and 12.11 d -1 . In accordance with its position in the HR diagram, the primary component of HD 173977 should be considered as a δ Scuti star and no longer as a γ Doradus star. In addition, HD 173844, used as a check star, is discovered variable with a 15.79 d -1 frequency and is classified as a δ Scuti star.
Highlights
Fekel et al (2003) obtained 2 spectra and found large variations both in line profile and in radial velocity, which led them to classify the star as a single line spectroscopic binary
A rapid inspection of the first spectra showed that two components were present, so we decided to study this star in more detail to disentangle binary motions and pulsational variations
Standard reductions were performed with the IRAF package and radial velocity measurements were made using a Gaussian fit to the observed line profiles of the lines cited above
Summary
HD 173977 (HN Dra, HIP 91983, F1, B=8.43, V=8.11) was discovered to be variable by the Hipparcos group (ESA, 1997) with a period of 0.9 d It was classified as “a prime γ Doradus candidate” by Handler (1999) who derived two periods, namely 0.900 and 1.327 d from the same photometric Hipparcos data. Fekel et al (2003) obtained 2 spectra and found large variations both in line profile and in radial velocity, which led them to classify the star as a single line spectroscopic binary. This star was included in an observational photometric and spectroscopic campaign (Mathias et al, 2004) to compare its photometric and spectroscopic periods and to determine its pulsational modes from the line profile and radial velocity variations.
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