UOCS – VI. UVIT/AstroSat detection of low-mass white dwarf companions to four more blue stragglers in M67

Abstract

Blue stragglers stars (BSSs) in M67 have attracted attention from observations and theory to unravel their formation mechanisms. In the series of Ultra Violet Imaging Telescope (UVIT on AstroSat) study of Open clusters (UOCS), here we report the detection of hot companions to 4 more BSSs (WOCS 2013, WOCS 3013, WOCS 4006 and WOCS 5005), using Far-UV photometry obtained in two epochs from UVIT. We characterise the hot companions to be low to extremely low mass (LM/ELM) white dwarfs (WDs) with Teff ∼ 13–23 kK, R/R⊙ ∼ 0.035–0.051, M/M⊙ ∼ 0.19–0.3 and age ∼ 25–300 Myr using WD models. Two BSSs (WOCS 1025 and WOCS 3005) showed UV excess, and may have a hot companion, but we are unable to confirm/characterise. 13 BSSs are detected by UVIT in the two epochs of data, of the total 14 present in M67. We have performed detailed analysis on 10 BSSs, including our previous studies. Five BSSs are found to have LM/ELM WD companions, suggesting Case-B mass-transfers (MT) to be prevalent in M67 (with a lower limit of 37.5 per cent, 5/14), along with other mechanisms. Three BS+WD systems have orbital parameters outside the limit for stable MT as per the models. We speculate the following three possibilities - their orbits are altered due to cluster dynamics, some may be in triple systems with LM/ELM WDs in an unknown closer orbit, or a modified MT mechanism may be required to enable their formation.

UOCS –VIII. UV study of the open cluster NGC 2506 using ASTROSAT★

“…The hot temperatures (7750-9750 K) of NGC 2506 BSS are consistent with the young age (2.2 Gyr) of the cluster. This temperature range of BSS is comparable to temperatures of the BSS of other intermediate-age OCs such as NGC 7789 that varies from 7250-10250 K (Vaidya et al 2022) and NGC M67 having a temperature range of 6250-9000 K (Sindhu et al 2019;Pandey et al 2021). We note that from the SEDs, that the temperatures of YSS varies from 6500-6750 K. This shows that YSS are cooler than BSS which is expected since the YSS are presumably evolved BSS.…”

“…For the same cluster, Subramaniam et al (2016a) discovered a post AGB/HB companion of a BSS (WOCS 5885) using AS-TROSAT /Ultraviolet Imaging Telescope (UVIT) data. Similarly, Sindhu et al (2019), Pandey et al (2021), Jadhav et al (2021b), andVaidya et al (2022) used UVIT data to study the OCs M67, King 2, and NGC 7789, respectively, and reported BSS with hot companions. UV observations also detect other interesting objects in star clusters, such as the yellow stragglers stars (YSS) and red clump (RC) stars.…”

A uvbyCaHbeta CCD Analysis of the Open Cluster Standard, M67,and its Relation to NGC 752

“…The literature in support of this scenario is heavily tied to M67 and growing (see, e.g. Leiner et al (2019); Subramaniam et al (2020); Pandey, Subramaniam, & Jadhav (2021); Leiner & Geller (2021); Geller et al (2021) and the many references therein.) The high percentage of binaries among BS in M67 is detailed in Geller, Latham, & Mathieu (2015), supplying an obvious source for most photometric and/or spectroscopic anomalies.…”

A uvbyCaHβ CCD Analysis of the Open Cluster Standard, M67, and Its Relation to NGC 752

“…The commonly accepted scenario for the formation of such systems is a binary mass transfer/merger event that turns the lower-mass companion into a higher-mass star, potentially leaving a visually faint but UV-bright white dwarf companion behind (McCrea 1964;Hills & Day 1976;Leonard 1989;Perets & Fabrycky 2009). The literature in support of this scenario is heavily tied to M67 and growing (see, e.g., Leiner et al 2019;Subramaniam et al 2020;Geller et al 2021;Leiner & Geller 2021;Pandey et al 2021, and the many references therein). The high percentage of binaries among BSs in M67 is detailed in Geller et al (2015), supplying an obvious source for most photometric and/or spectroscopic anomalies.…”

“…As already noted, all radial velocity binary and/or BS systems identified in Geller et al (2015) have been excluded from our analysis, so detection of a BS binary origin for the seven bluer stars of Figure 8 must come from an alternative technique; the majority of M67 BS systems have been detected in the UV via their white dwarf companion (Pandey et al 2021). We have searched the primary sources for matches to our seven anomalous stars, the UVIT Catalog of Open Clusters (Jadhav et al 2021) and the potential blue lurkers identified in M67 (Jadhav et al 2019;Leiner et al 2019;Subramaniam et al 2020).…”