Abstract
Using the PIONIER visitor instrument that combines the light of the four Auxiliary Telescopes of ESO's Very Large Telescope Interferometer, we measure precisely the diameters of several symbiotic and related stars: HD 352, HD 190658, V1261 Ori, ER Del, FG Ser, and AG Peg. These diameters - in the range of 0.6 to 2.3 milli-arcseconds - are used to assess the filling factor of the Roche lobe of the mass-losing giants and provide indications on the nature of the ongoing mass transfer. We also provide the first spectroscopic orbit of ER Del, based on CORAVEL and HERMES/Mercator observations. The system is found to have an eccentric orbit with a period of 5.7 years. In the case of the symbiotic star FG Ser, we find that the diameter is changing by 13% over the course of 41 days, while the observations of HD 352 are indicative of an elongation. Both these stars are found to have a Roche filling factor close to 1, as is most likely the case for HD 190658 as well, while the three other stars have factors below 0.5-0.6. Our observations reveal the power of interferometry for the study of interacting binary stars - the main limitation in our conclusions being the poorly known distances of the objects.
Highlights
Symbiotic stars show in their spectra the blended characteristics of a cool star, a hot star, as well as emission lines coming from a high-excitation nebula
There is a well-known apparent contradiction between the radius derived from the rotational velocities, which in most cases indicates that the giant fills about only half its Roche lobe, and the fact that many of the symbiotic stars show the clear signature of ellipsoidal variations in their light curve, indicative
We indicate the mass of the giant, M2; of its hot companion, M1; the semi-major axis, a; the bolometric magnitude; the Roche lobe radius, RL; the radius of the giant, R2; the filling factor, f = R2/RL; the rotational velocity, v sin i, assuming rotation is coplanar to the orbital motion; and the resulting parallax, in mas
Summary
Symbiotic stars show in their spectra the blended characteristics of a cool star (generally a K or M giant), a hot star (a white dwarf in most cases), as well as emission lines coming from a high-excitation nebula. With orbital periods in the range of a few hundred to a thousand days, they are thought to be among the interacting binary stars with the longest periods, in which the mass-losing red giant is transferring mass to its hot companion (Mikołajewska 2007). A critical question related to symbiotic stars is whether mass transfer is taking place through a stellar wind or Roche lobe overflow (Mikołajewska 2012), i.e. what is the Roche-lobefilling factor of the giant in those systems. This paper aims at addressing this question in the most direct way
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