Photometric variability of blue straggler stars in M67 with TESS and K2

Abstract

Blue straggler stars (BSSs) are formed through mass transfers or mergers in binaries. The recent detections of white dwarf (WD) companions to BSSs in M67 suggested a mass transfer pathway of formation. In search of a close companion to five BSSs in M67 that are known to be spectroscopic binaries, we study the light curves from K2 and TESS data. We use PHOEBE to analyse the light curves and estimate the properties of the companions. We detect variability in WOCS 1007, and the light curve is dominated by ellipsoidal variation. Using the light curve and radial velocity measurements, we estimate its orbital period to be 4.212 ± 0.041 d and e = 0.206 ± 002. The mass of the companion is estimated to be 0.22 ± 0.05 M⊙ with a radius of 0.078 ± 0.027 R⊙, confirming it to be an LM WD with Teff = 14300 ± 1100 K. The estimated mass of the BSS, 1.95 ± 0.26 M⊙, is similar to that estimated from isochrones. The BSS in WOCS 1007 shows δ Scuti pulsations, although it is slightly deformed and likely to be formed through an efficient mass transfer. Though we detect a light curve for WOCS 4003 showing grazing eclipse with ellipsoidal variation, the estimated parameters are inconclusive. Apart from the 0.44 d period, we found smaller eclipses with a period of 1.1 d, suggesting a compact triple system. In the cases of WOCS 4003, WOCS 5005, and WOCS 1025, no eclipses or pulsations are detected, confirming the absence of any short-period inner binary with high inclination in these BSSs.