The Origin of B-type Runaway Stars: Non-LTE Abundances as a Diagnostic

Abstract

There are two accepted mechanisms to explain the origin of runaway OB-type stars: the Binary Supernova Scenario (BSS), and the Cluster Ejection Scenario (CES). In the former, a supernova explosion within a close binary ejects the secondary star, while in the latter close multi-body interactions in a dense cluster cause one or more of the stars to be ejected from the region at high velocity. Both mechanisms have the potential to affect the surface composition of the runaway star. TLUSTY non-LTE model atmosphere calculations have been used to determine atmospheric parameters and carbon, nitrogen, magnesium and silicon abundances for a sample of B-type runaways. These same analytical tools were used by Hunter et al. (2009) for their analysis of 50 B-type open cluster Galactic stars (i.e. non-runaways). Effective temperatures were deduced using the silicon-ionization balance technique, surface gravities from Balmer line profiles and microturbulent velocities derived using the Si spectrum. The runaways show no obvious abundance anomalies when compared with stars in the open clusters. The runaways do show a spread in composition which almost certainly reflects the Galactic abundance gradient and a range in the birthplaces of the runaways in the Galactic disk. Since the observed Galactic abundance gradients of C, N, Mg and Si are of a similar magnitude, the abundance ratios (e.g., N/Mg) are, as obtained, essentially uniform across the sample.

Massive runaway and walkaway stars

“…These features should in principle leave observable signatures in the spectra of these stars (e.g., Maíz Apellániz et al 2018). However, the effects of the metallicity gradient in the radial direction of the Galaxy complicates the spectral identification of binary products, as pointed out by McEvoy et al (2017).…”

Search for Galactic runaway stars using Gaia Data Release 1 and HIPPARCOS proper motions

“…Morgan et al (1955) classified this star as B1 Ib but we have not observed it with GOSSS. It is not clear whether this object had been previously identified as a runaway: McEvoy et al (2017) indicates that it is one but use Martin (2004) as a reference for that claim. What Martin (2004) actually says is that HD 119 608 is a Post Asymptotic Giant Branch (PAGB) star instead of a runaway B supergiant, while the latter Szczerba et al (2007) says it is not a PAGB, something that unpublished data from the IACOB project (Simón-Díaz et al 2015b) also indicates.…”

The Early-type Stars from LAMOST survey: Atmospheric parameters

“…We also predict the stellar labels for a sample of pre-labeled high-resolution spectra (HRS) from publications to verify the consistency of our training set. We select a sample of 28 early-type stars with pre-estimated stellar labels from works of Trundle et al (2007), Nieva & Przybilla (2012) and McEvoy et al (2017). In order to directly compare the results of these archived HRS spectra to those of the LAMOST-LRS and LAMOST-MRS data, we first degraded the resolution of the HRS spectra down to resolutions of R ∼ 1, 800 (HRS-LRS) and R ∼ 7, 500 (HRS-MRS).…”

“…A comprehensive study of the early B-type stars located within the solar neighborhood is carried out by Nieva & Przybilla (2012) using multiple sets of high-resolution spectra. McEvoy et al (2017) measured atmospheric parameters and metallic abundance for a sample of runaway B-type stars in the Galaxy to trace their formation mechanism. These studies above all utilized high signalto-noise ratio (SNR) spectra that cover the wavelength regions rich in atmospheric absorption lines to proceed the line diagnostic analysis but the investigations were limited to a small sample of observations.…”