[Paper Review] Monitoring evolved stars for binarity with the HERMES spectrograph

N. Gorlova, H. Van Winckel+5 more|arXiv (Cornell University)|Mar 10, 2014

Stellar, planetary, and galactic studies3 references3 citations

TL;DR

This study uses high-precision radial velocity monitoring with the HERMES spectrograph on the Mercator telescope to detect long-period, eccentric binaries among evolved stars, including hot subdwarfs, barium/S stars, and post-AGB stars with disks. Key findings include the discovery of systems with orbital periods up to 1,210 days and eccentricities exceeding standard binary evolution predictions, along with evidence of ongoing mass transfer and scattered light in disk systems.

ABSTRACT

Binarity is often invoked to explain peculiarities that can not be explained by the standard theory of stellar evolution. Detecting orbital motion via the Doppler effect is the best method to test binarity when direct imaging is not possible. However, when the orbital period exceeds the duration of a typical observing run, monitoring often becomes problematic. Placing a high-throughput spectrograph on a small semi- robotic telescope allowed us to carry out a radial-velocity survey of various types of peculiar evolved stars. In this review we highlight some findings after the first four years of observations. Thus, we detect eccentric binaries among hot subdwarfs, barium, S stars, and post- AGB stars with disks, which are not predicted by the standard binary interaction theory. In disk objects, in addition, we find signs of the on- going mass transfer to the companion, and an intriguing line splitting, which we attribute to the scattered light of the primary.

Motivation & Objective

• To test binarity in evolved stars using radial velocity monitoring, especially where direct imaging is not feasible.
• To investigate the role of binary interactions in explaining peculiarities in post-MS stars such as barium stars, S stars, and post-AGB objects with disks.
• To characterize orbital parameters, stellar properties, and circumstellar dynamics to constrain binary evolution theories.
• To identify systems with ongoing mass transfer and detect scattered light signatures in circumstellar environments.
• To resolve the lack of empirical evidence linking different evolutionary stages of post-MS stars through binary evolution.

Proposed method

• Utilizes the high-throughput, high-resolution (R = 85,000) HERMES fiber echelle spectrograph on the 1.2-m Mercator telescope at La Palma.
• Employs a temperature-controlled environment to achieve radial velocity precision better than 200 m s⁻¹.
• Applies a Python-based pipeline for spectral reduction and radial velocity determination via cross-correlation with tailored line masks for various spectral types and metallicities.
• Conducts long-term, queue-mode observations with cadences from weekly to several times per semester to build dense time series of spectra.
• Uses independent orbital fitting on radial velocity curves of both components to measure systemic velocity differences and orbital elements.
• Analyzes Hα profiles and cross-correlation functions (CCFs) to detect spectral features indicative of mass transfer and scattered light.

Experimental results

Research questions

• RQ1 What is the orbital distribution of long-period, eccentric binaries among evolved stars such as hot subdwarfs and post-AGB stars?
• RQ2 Can radial velocity monitoring detect ongoing mass transfer in systems with circumstellar disks, and what spectral signatures does it produce?
• RQ3 Why do observed orbital periods and eccentricities in evolved binaries exceed predictions from standard binary interaction theories?
• RQ4 What causes the asymmetric or double-peak Hα profiles observed in some disk systems, and how do they correlate with orbital phase?
• RQ5 Can the observed satellite features in CCFs be attributed to scattered light rather than a physical companion in post-AGB stars with disks?

Key findings

• Eight long-period hot subdwarf binaries were discovered with orbital periods between 491 and 1,363 days, all with eccentricities higher than predicted by stable Roche-lobe overflow theory.
• The systemic velocity difference between the subdwarf and its cool companion was measured at ~1 km s⁻¹, consistent with gravitational redshift effects.
• In post-AGB stars with disks, Hα profiles showed double-peak emission that became asymmetric or replaced by blue-shifted absorption during primary's superior conjunction, indicating ongoing mass transfer.
• A P Cyg-like profile was observed in some systems, intensifying at specific orbital phases, suggesting pole-on viewing geometry and active accretion.
• A satellite feature in the cross-correlation function of IRAS 19135+3937 and BD+46 442 was detected, interpreted as reflected light from the circumbinary disk rather than a physical companion.
• Photometric monitoring of IRAS 19135+3937 showed a bluer color during minimum light, supporting the scattered light interpretation, consistent with interferometric findings in similar systems.