IMPLICATIONS FOR THE FORMATION OF BLUE STRAGGLER STARS FROMHSTULTRAVIOLET OBSERVATIONS OF NGC 188

Abstract

We present results of a Hubble Space Telescope far-ultraviolet (FUV) survey searching for white dwarf (WD) companions to blue straggler stars (BSSs) in open cluster NGC 188. The majority of NGC 188 BSSs (15 of 21) are single-lined binaries with properties suggestive of mass-transfer formation via Roche lobe overflow, specifically through an asymptotic giant branch star transferring mass to a main sequence secondary, yielding a BSS binary with a WD companion. In NGC 188, a BSS formed by this mechanism within the past 400 Myr will have a WD companion hot and luminous enough to be directly detected as a FUV photometric excess with HST. Comparing expected BSS FUV emission to observed photometry reveals four BSSs with WD companions above 12,000 K (younger than 250 Myr) and three WD companions with temperatures between 11,000-12,000 K. These BSS+WD binaries all formed through recent mass transfer. The location of the young BSSs in an optical color-magnitude diagram (CMD) indicates that distance from the zero-age main sequence does not necessarily correlate with BSS age. There is no clear CMD separation between mass transfer-formed BSSs and those likely formed through other mechanisms, such as collisions. The seven detected WD companions place a lower limit on the mass-transfer formation frequency of 33%. We consider other possible formation mechanisms by comparing properties of the BSS population to theoretical predictions. We conclude that 14 BSS binaries likely formed from mass transfer, resulting in an inferred mass-transfer formation frequency of approximately 67%.

A Hot Companion to a Blue Straggler in NGC 188 as Revealed by the Ultra-Violet Imaging Telescope (Uvit) on Astrosat

“…The data also confirm that the temperature of the hot component is not high enough to be classified as a sub-dwarf, as demonstrated by the slope of the FUV part of the SED. The estimated temperature of the cooler component confirms that it is indeed a BSS, as it is similar to the temperature of other BSSs in NGC188 (Table 1; Gosnell et al 2015) and higher than the turn-off temperature of NGC 188 (∼ 5500 K, based on the turn-off color). The opti-cal region of the SED is fitted well by the combined flux from the two components.…”

“…The hot star is about 3 times more luminous than hot B sub-dwarfs (T ef f > 27000 K L ∼ 10 L ) and blue horizontal branch stars (L ∼ 10 L ) (Heber 2016). Also, the FUV luminosity is found to be much higher than those of the BSS+WD of similar temperature (e.g., WOCS-4540, WOCS-5379) as estimated by Gosnell et al (2015). These two observations suggest that the hot component cannot be a WD or a sub-dwarf, therefore we speculate that it is a post-AGB star or a post-HB star (depending on the initial mass of the star).…”

“…WOCS-5885 is probably the progenitor of systems composed of a BSS and hot-WD companion, like those identified by Gosnell et al (2015). The temperature of the companion rules out the possibility this has a red-giant along with a pre-WD, as suggested by Siegel et al (2014).…”

“…Recently, Gosnell et al (2014Gosnell et al ( , 2015 used HST to study BSSs in NGC188 in the UV region, detecting white dwarf (WD) companions to seven BSS and thereby confirming that at least some BSSs can form by mass transfer. Three of their BSSs were found to have cool WD companions; four were found to have hotter WDs.…”

Far-ultraviolet observation of the globular cluster NGC 6397

“…Nine of our FUV selected BSs are optically bright, the remaining FUV bright BS is optically faint and located in the MS turn-off region. Its FUV excess might be explained with a WD companion which contributes flux to the FUV (see Gosnell et al 2015, who found BS-WD binaries in the old open cluster NGC 188)…”