Discovery of a Barium Blue Straggler Star in M67 and “Sighting” of Its White Dwarf Companion

• Published in: Astrophysical journal. Letters Vol. 970; no. 2; p. L39
• Main Authors: Pal, Harshit, Subramaniam, Annapurni, Reddy, Arumalla B. S., Jadhav, Vikrant V.
• Format: Journal Article
• Language: English
• Published: Austin IOP Publishing 01-08-2024
• Subjects: Asymptotic giant branch stars; Barium; Barium stars; Binary stars; Blue straggler stars; Chemical detection; Companion stars; Kinematics; Mass transfer; Nuclear capture; Open star clusters; S-process; Spectral energy distribution; Spectroscopic analysis; Spectroscopic binary stars; Stars; Thermal neutrons; White dwarf stars
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ad6316

We report the discovery of a barium blue straggler star (BSS) in M67, exhibiting enhancements in slow neutron-capture ( s -)process elements. Spectroscopic analysis of two BSSs (WOCS 9005 & WOCS 1020) and four stars located near the main-sequence turn-off using GALAH spectra, showed that WOCS 9005 has a significantly high abundance of the s -process elements ([Ba/Fe] = 0.75 ± 0.08, [Y/Fe] = 1.09 ± 0.07, and [La/Fe] = 0.65 ± 0.06). The BSS (WOCS 9005) is a spectroscopic binary with a known period, eccentricity, and a suspected white dwarf (WD) companion with a kinematic mass of 0.5 M ⊙ . The first “sighting” of the WD in this barium BSS is achieved through multiwavelength spectral energy distribution (SED) with the crucial far-UV data from the UVIT/AstroSat. The parameters of the hot and cool companions are derived using binary fits of the SED using two combinations of models, yielding a WD with T eff in the range 9750–15,250 K. Considering the kinematic mass limit, the cooling age of the WD is estimated as ∼60 Myr. The observed enhancements are attributed to a mass transfer (MT) from a companion asymptotic giant branch star, now a WD. We estimate the accreted mass to be 0.15 M ⊙ , through wind accretion, which increased the envelope mass from 0.45 M ⊙ . The detection of chemical enhancement, as well as the sighting of WD in this system, have been possible due to the recent MT in this binary, as suggested by the young WD.