HD 61273: a new binary system with a hot component showing Hα emission line

Presented at the meeting of the Working Group on Active B Stars during the 26th IAU General Assembly in Prague, Czech Republic on 2006 August 18

D. Briot & F. Royer

Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon, France; danielle.briot@obspm.fr, frederic.royer@obspm.fr

Received: 2007 March 29; Accepted: 2007 July 13.

1. Introduction

HD 61273 is a 7th magnitude star, classified as a B9-type star in the HD catalog. The only few papers concerning this star always misclassified it. Stetson (1991) identified it as a "field blue-straggler." HIPPARCOS (ESA 1997) detected its light variability, P = 6.458 d, and it was categorized as an ACV (α² CVn type) variable star. Actually HD 61273 is a binary system that we detected from spectroscopic observations.

2. Spectroscopic observations

We obtained 32 spectra of HD 61273 with the ÉLODIE spectrometer (OHP, France) along a 9 years time span, in the spectral range 3900--6800 Å, and with a resolving power R = 42000. These observations reveal periodic variations in radial velocity and permanent emission in Hα.

Periodic variations of radial velocities are attributed to a binary system. The orbital period of the system is P = 12.9190 d and is twice the photometric period detected by HIPPARCOS, suggesting that the system is an ellipsoidal variable with a component filling its Roche lobe. The radial velocity curve of the cold component is determined with a precision better than 1 km s^-1. Unfortunately, radial velocities of the hot component can be hardly determined. The mass ratio can nevertheless be estimated on the assumption of spin-orbit synchronization of the Roche-lobe filling giant component using only the RV curve of this star (Harmanec 1990). We obtain a mass ratio Q = 0.14. The cool component is thus the less massive of the system. This corresponds to the so-called Algol paradox and implies a mass transfer from the now less massive star toward the now more massive component (Crawford 1955). This is moreover strengthened by the presence of emission in the Hα line which is the signature of the presence of circumstellar material.

Although it cannot be strictly classified as an Algol on the basis of its light curves, which do not show eclipses, the physical processes occurring in the binary system and accounting of the observed spectra are the same as physical processes occurring in Algol-type stars.

Hα line profile: signature of circumstellar material

The Hα line of HD 61273 shows always double peaked emission superimposed to absorption components, whatever the phase of the orbital period (see Fig. 1). Huang & Struve (1956) showed that the gas ejected by one component, which touches the inner contact surface, circulates around the other component, thus forming a gaseous ring where emission lines originate. Peters (1980, 1989) finds that there is permanent emission in lines of long-period Algol systems. If P > 6 d, permanent Hα disk is observed. However, as noted in Peters (2001), this cutoff period is only an approximation. In the case of long-period systems, the stream of matter deviated by the Coriolis acceleration misses the now primary star and forms a circumstellar structure spiraling around it.

Summary

HD 61273 is a new binary system, in which all the processes of Algol type stars occur. It corresponds to a long period Algol and then shows a permanent emission in Hα, attributed to the matter coming from the now cold star and forming a disk around the now hot star.

A more detailed analysis is in progress to disentangle the composite spectrum, derive the physical parameters of each component and model the system.

References

Crawford, J. A. 1955, ApJ, 121, 71
ESA 1997, The Hipparcos and Tycho Catalogues, ESA-SP 1200
Harmanec, P. 1990, A&A, 237, 91
Huang, S. S., & Struve, O. 1956, AJ, 61, 300
Peters, G. J. 1980, in "IAU Symp. 88: Close Binary Stars: Observations and Interpretation," 287
Peters, G. J. 1989, Space Science Reviews, 50, 9
Peters, G. J. 2001, in "The Influence of Binaries on Stellar Population Studies," ed. D. Vanbeveren (Dordrecht, Kluwer), 79
Stetson, P. B. 1991, AJ, 102, 589