[Paper Review] Calcium Emission in Interacting Binary Be Stars

P. Koubský, L. Kotková+8 more|arXiv (Cornell University)|May 10, 2012

Stellar, planetary, and galactic studies3 citations

TL;DR

This study investigates calcium II infrared triplet (Ca II IR) emission in Be stars using spectroscopic data from the Ondřejov 2-m telescope, finding that Ca II emission is primarily linked to peculiar disc environments rather than binary interaction per se. However, Ca II emission without Paschen-line emission is a strong indicator of interacting binaries, leading to the discovery of a new interacting Be binary, HD 81357, with an orbital period of 33.73 days and a semi-amplitude of 79 km s⁻¹ for the secondary.

ABSTRACT

Polidan (1976) suggested that Be stars showing the CaII IR triplet in emission are interacting binaries. With the advent of the Gaia satellite, which will host a spectrometer to observe stars in the range 8470--8750 Å, we carried out a spectroscopic survey of 150 Be stars, including Be binaries. We show that the Ca II triplet in emission, often connected with emission in Paschen lines, is an indicator of a peculiar environment in a Be star disc rather than a signature of an interacting binary Be star. However, Ca II emission without visible emission in Paschen lines is observed in interacting binary stars, as well as in peculiar objects. During the survey, a new interacting Be binary - HD 81357 - was discovered.

Motivation & Objective

• To investigate the origin of Ca II infrared triplet emission in Be stars, particularly in relation to binary interactions.
• To determine whether Ca II emission is a reliable indicator of interacting binary systems in Be stars.
• To characterize the circumstellar environment of Be stars through high-resolution spectroscopy in the Hα and Gaia RVS (8470–8750 Å) regions.
• To identify and analyze new interacting Be binary systems through long-term monitoring of radial velocity variations.
• To assess the utility of Ca II triplet emission as a diagnostic tool for circumstellar disc structure and binary interactions.

Proposed method

• Spectra of ~150 Be stars were obtained using the Ondřejov 2-m telescope equipped with a Coudé spectrograph and a SITe 2000x800 CCD, covering 6250–6750 Å and 8392–8900 Å with R ≈ 12,700.
• Stellar and calibration spectra (Th-Ar and tungsten lamps) were reduced using the IRAF software package for consistent wavelength calibration and flux correction.
• Radial velocity curves were derived from the Ca II triplet lines (especially 8498 Å), with particular attention to emission components and orbital modulations.
• Orbital elements were determined by fitting the radial velocity variations of the secondary star (via Fe I and Ca II absorption lines) and the primary (via Hα and Ca II emission shifts).
• Trailing spectra were constructed over multiple epochs to visualize orbital phase-dependent changes in line profiles and emission components.
• Comparative analysis was performed with known interacting Be binaries (e.g., BR CMi, KX And) to identify similarities in spectral behavior.

Experimental results

Research questions

• RQ1 Is Ca II infrared triplet emission in Be stars a reliable indicator of binary interaction?
• RQ2 How does the behavior of Ca II emission correlate with Paschen line emission in Be star discs?
• RQ3 Can Ca II emission in the absence of Paschen-line emission serve as a diagnostic for interacting binaries?
• RQ4 What are the orbital parameters of newly discovered Be binary systems, such as HD 81357?
• RQ5 To what extent does the circumstellar environment of Be stars influence the formation and morphology of Ca II emission lines?

Key findings

• Ca II infrared triplet emission in Be stars is primarily associated with peculiar disc structures rather than binary interaction per se.
• In systems like AX Mon, KX And, and BR CMi, strong Ca II emission correlates with disc activity, but not all interacting binaries show this emission.
• Ca II emission without visible Paschen-line emission is a distinctive signature of interacting binaries, observed in HD 81357 and other peculiar systems.
• HD 81357 was identified as a new interacting Be binary with an orbital period of 33.73 days and a secondary radial velocity semi-amplitude of 79 km s⁻¹.
• The radial velocity curve of the Ca II 8498 Å line in HD 81357 shows an emission component that moves in anti-phase, suggesting a possible disc around the primary star.
• The orbital solution derived from Hipparcos photometry supports a double-wave light curve, consistent with the 33.73-day period, while a 25.7-day period yields a simpler curve.