Spectral modelling of theαVirginis (Spica) binary system

Abstract

Context. The technique of matching synthetic spectra computed with theoretical stellar atmosphere models to the observations is widely used in deriving fundamental parameters of massive stars. When applied to binaries, however, these models generally neglect the interaction effects present in these systems. Aims. The aim of this paper is to explore the uncertainties in binary stellar parameters that are derived from single-star models. Methods. Synthetic spectra that include the tidal perturbations and irradiation effects are computed for the binary system α Virginis (Spica) using our recently-developed CoMBiSpeC model. The synthetic spectra are compared to S /N ∼ 2000 observations and optimum values of T eff and log g are derived.Results. The binary interactions have only a small effect on the strength of the photospheric absorption lines in Spica (<2% for the primary and <4% for the secondary). These differences are comparable to the uncertainties inherent to the process of matching synthetic spectra to the observations and thus the derived values of T eff and log g are unaffected by the binary perturbations. On the other hand, the interactions do produce significant phase-dependent line profile variations in the primary star, leading to systematic distortions in the shape of its radial velocity curve. Migrating sub-features ("bumps") are predicted by our model to be present in the same photospheric lines as observed, and their appearance does not require any a priori assumptions regarding non-radial pulsation modes. Matching the strength of lines in which the most prominent "bumps" occur requires synthetic spectra computed with larger "microturbulence" than that required by other lines.

Stellar modelling of Spica, a high-mass spectroscopic binary with a β Cep variable primary component

“…The authors concluded that tidal flows exerted by the mainsequence secondary on the evolved primary are the main contributor to the short-term variability observed in the line profiles of the Spica system. Palate et al (2013) arrived to a similar conclusion by analysing the data set obtained by Harrington et al (2009) and using improved methodology. The conclusions of these two papers will be discussed in the context of our own results in the last section of the current study.…”

“…To do so, we have selected all individual spectra in a narrow range of orbital phase to make sure that all dynamics we see in the line profiles is due to variability intrinsic to either of the binary components and not due to the orbital motion within the system. Also, if all variability was due to tides and/or tidally locked (see, e.g., Harrington et al 2009;Palate et al 2013), one would expect the signal to be repetitive with orbital cycle, i.e. nearly the same line profile pattern would be observed at a given orbital phase every orbital cycle (see Harrington et al 2009, Figure 6).…”

Short-term spectroscopic variability of Plaskett’s star

“…The analysis of the line profile variations in the slightly eccentric binary system Spica (Palate et al 2013a and references therein) has shown that tidal interactions could explain Spica's line profile variations. Although Plaskett's star is not an eccentric system, the non-synchronicity of the secondary rotation could lead to tidally induced variations such as those reported by Moreno et al (2005).…”

On X-ray pulsations inβCephei-type variables

“…Although no optical eclipses have been observed in this system (Palate et al 2013), an influence of binarity on the X-ray light curve cannot be excluded a priori.…”

“…The primary component (B1.5IV-V) is classified as a β Cep-type variable, but its pulsational behavior is not stable (see discussion in Palate et al 2013).…”