Alpha Virginis: line-profile variations and orbital elements^⋆

¹ Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany
e-mail: dharring96@gmail.com
² Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
³ Applied Research Labs, University of Hawaii, 2800 Woodlawn Drive, Honolulu, HI 96822, USA
⁴ Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Ave. Universidad S/N, Cuernavaca, 62210 Morelos, Mexico
⁵ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁶ Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70-264, D.F. 04510 México, Mexico
⁷ Centro de Investigación en Ciencias, Universidad Autónoma del Estado de Morelos, Cuernavaca, 62210 Mexico, Mexico

Received: 9 May 2015
Accepted: 18 February 2016

Abstract

Context. Alpha Virginis (Spica) is a B-type binary system whose proximity and brightness allow detailed investigations of the internal structure and evolution of stars undergoing time-variable tidal interactions. Previous studies have led to the conclusion that the internal structure of Spica’s primary star may be more centrally condensed than predicted by theoretical models of single stars, raising the possibility that the interactions could lead to effects that are currently neglected in structure and evolution calculations. The key parameters in confirming this result are the values of the orbital eccentricity e, the apsidal period U, and the primary star’s radius, R₁.

Aims. The aim of this paper is to analyze the impact that Spica’s line profile variability has on the derivation of its orbital elements and to explore the use of the variability for constraining R₁.

Methods. We use high signal-to-noise and high spectral resolution observations obtained in 2000, 2008, and 2013 to derive the orbital elements from fits to the radial velocity curves. We produce synthetic line profiles using an ab initio tidal interaction model.

Results. The general variations in the line profiles can be understood in terms of the tidal flows, whose large-scale structure is relatively fixed in the rotating binary system reference frame. Fits to the radial velocity curves yield e = 0.108 ± 0.014. However, the analogous RV curves from theoretical line profiles indicate that the distortion in the lines causes the fitted value of e to depend on the argument of periastron; i.e., on the epoch of observation. As a result, the actual value of e may be as high as 0.125. We find that U = 117.9 ± 1.8, which is in agreement with previous determinations. Using the value R₁ = 6.8 R_⊙ derived by Palate et al. (2013) the value of the observational internal structure constant k_2,obs is consistent with theory. We confirm the presence of variability in the line profiles of the secondary star.