EDP Sciences
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Volume 388, Number 1, June II 2002
Page(s) 298 - 308
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20020504

A&A 388, 298-308 (2002)
DOI: 10.1051/0004-6361:20020504

H $\alpha$ spectroscopy and $\vec{BV}$ photometry of RT Lacertae

A. Frasca1, Ö. Çakirli2, S. Catalano1, C. Ibanoglu2, E. Marilli1, S. Evren2 and G. Tas2

1  Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
2  Ege University Observatory, Bornova, Izmir, Turkey
    e-mail: afr, scat, ema@ct.astro.it; cakirli,ibanoglu@astronomy.sci.ege.edu.tr

(Received 8 January 2002 / Accepted 19 March 2002 )

Contemporaneous spectroscopic and photometric B V observations of the RS CVn type eclipsing binary RT Lacertae were performed in summer 2000. The photometric observations were obtained at the Ege University Observatory, while the spectroscopic ones were carried out at Catania Astrophysical Observatory in the spectral range 5860-6700 Å. We obtained a high quality radial velocity curve of the system that allowed us to give more accurate values of the orbital parameters. A steady decrease of the barycentric velocity from 1920 to 2000 has been pointed out and has been discussed in the context of a third body hypothesis. Through the subtraction of a "synthetic" spectrum, built up with spectra of inactive standard stars, we detected H $\alpha$ excess emission which fills in the photospheric absorption profiles of both components. With the exception of a few spectra, taken close to the eclipses, in which some extra absorption or a faint double-peaked broad emission appears, there is no further evidence of circumstellar matter in this system, as suggested in previous works. The hotter and more massive star appears also as the more active at a chromospheric level, since it has a H $\alpha$ flux about ten times greater than the companion, on average. Rotational modulation of the H $\alpha$ emission has been detected in both stars. The hemisphere of the more massive star facing the observer at phase $0\fp75$ appears brighter (in H $\alpha$) than that seen at phase $0\fp25$, while for the less massive G9 IV star the maximum H $\alpha$ emission is seen around phase 0 $\fp0$-0 $\fp1$. From the analysis of the contemporaneous light curve (Lanza et al. 2002), the more massive G5 IV star results to be more active than the companion at a photospheric level, in agreement with the chromospheric behaviour observed in H $\alpha$. In addition, the starspots of the G5 IV star are mainly located in the H $\alpha$ brighter hemisphere, suggesting a close spatial association of spots and plages in this star. The G9 IV star displays instead the maximum H $\alpha$ emission at the phase of maximum visibility of the smaller spotted area found from the light-curve analysis. The minimum H $\alpha$ emission occurs when the more heavily spotted region is visible.

Key words: stars: activity -- stars: binaries: spectroscopic -- stars: late-type -- stars: starspots -- stars: individual: RT Lac

Offprint request: A. Frasca, afr@ct.astro.it

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