Constraining Mass-transfer Histories of Blue Straggler Stars with COS Spectroscopy of White Dwarf Companions

Abstract

Recent studies show that the majority of blue straggler stars (BSSs) in old open clusters are formed through mass transfer from an evolved star onto a main-sequence companion, resulting in a BSS and white dwarf (WD) in a binary system. We present constraints on the mass transfer histories of two BSS-WD binaries in the open cluster NGC 188, using WD temperatures and surface gravities measured with HST COS far-ultraviolet spectroscopy. Adopting a Gaia-based cluster distance of 1847 ± 107 pc, we determine that one system, WOCS 4540, formed through Case C mass transfer resulting in a COcore white dwarf with T eff =17000 +140 −200 K and a log g=7.80 +0.06 −0.06 , corresponding to a mass of 0.53 +0.03 −0.03 M and a cooling age of 105 +6 −5 Myr. The other system, WOCS 5379, formed through Case B mass transfer resulting in a He-core white dwarf with T eff =15500 +170 −150 K and a log g=7.50 +0.06 −0.05 , corresponding to a mass of 0.42 +0.02 −0.02 M and an age of 250 +20 −20 Myr. The WD parameters are consistent across four different cluster distance assumptions. We determine possible progenitor binary systems with a grid of accretion models using MESA, and investigate whether these systems would lead to stable or unstable mass transfer. WOCS 4540 likely resulted from stable mass transfer during periastron passage in an eccentric binary system, while WOCS 5379 challenges our current understanding of the expected outcomes for mass transfer from red giant branch stars. Both systems are examples of the value in using detailed analyses to fine-tune our physical understanding of binary evolutionary processes.

WIYN Open Cluster Study. LXXVII. Radial-Velocity Measurements and Spectroscopic Binary Orbits in the Open Cluster NGC 7789

“…WOCS 5011 and WOCS 20009 are both long-period SB1s with periods of 2710 d and 4190 d, respectively. Such periods are consistent with mass transfer from asymptotic branch giants (e.g., Mathieu 2011 andGosnell et al 2019, albeit will less massive turnoffs). WOCS 36011, on the other hand, has a shorter period of 217 d and a much higher eccentricity with e ∼ 0.74.…”

“…WOCS 36011, on the other hand, has a shorter period of 217 d and a much higher eccentricity with e ∼ 0.74. Blue stragglers with periods on the order of 10 2 days are likely the result of mass transfer from an RGB star, as in the case of WOCS 5379 in the open cluster NGC 188 (Gosnell et al 2014;Gosnell et al 2019). Interestingly, the CMD location of WOCS 36011 in NGC 7789 relative to the ZAMS turnoff is very similar to that of WOCS 5379 in NGC 188 (although the primary star of WOCS 36011 is of higher mass).…”

Formation of millisecond pulsars with long orbital periods by accretion-induced collapse of white-dwarfs

“…This relation was confirmed in different kinds of observed peculiar binaries, such as binary MSPs, blue straggler binaries, sdB binaries, etc (see, e.g. Rappaport et al 1995;Carney, Latham & Laird 2005;Chen et al 2013a;Gosnell et al 2019). On the basis of this relation, one can get the mass of the WD companion once the orbital period is obtained.…”

Unexpected Short-Period Variability in Dwarf Carbon Stars from the Zwicky Transient Facility

“…Blue straggler stars are another class similar to dCs in that they may have experienced mass transfer from a previous AGB companion. As discussed by Gosnell et al (2019), blue straggler stars in a cluster color-magnitude diagram are more luminous and bluer than the main sequence turnoff. While some are likely formed in mergers or collisions, most blue straggler stars are thought, like dC stars, to be the result of mass transfer from a giant to a main sequence dwarf.…”