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

Abstract

Blue straggler stars (BSSs) are formed through mass transfer 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 case 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.

Transitory Tidal Heating and Its Impact on Cluster Isochrones

“…WOCS 1007 is known to pulsate, but this is due to its location in the δ Scuti instability region rather than tidally induced perturbations. Although no eclipses were found in the light curves of WOCS 5005 and WOCS 1025 (see Figure 9 for the locations of these systems in the M67 CMD), thereby ruling out high-inclination close binary systems, it does not exclude the possibility of lowinclination close binary companions (Vernekar et al 2023). No pulsations were found in these systems.…”

“…Other systems do have shorter orbital periods. Vernekar et al (2023) studied the K2 and TESS data of five BSSs in M67 that are known to be spectroscopic binaries. They estimate an orbital period for WOCS 1007 of P orb = 4.2 days, e = 0.21, m 1 = 1.95 ± 0.26 M e (similar to what is obtained from isochrones), m 2 = 0.22 ± 0.05 M e , and R 2 = 0.07 R e (confirming the secondary to be a low-mass WD, which could not have formed through a single-star evolution scenario).…”

“…They estimate an orbital period for WOCS 1007 of P orb = 4.2 days, e = 0.21, m 1 = 1.95 ± 0.26 M e (similar to what is obtained from isochrones), m 2 = 0.22 ± 0.05 M e , and R 2 = 0.07 R e (confirming the secondary to be a low-mass WD, which could not have formed through a single-star evolution scenario). Vernekar et al (2023) postulate that the progenitor system of WOCS 1007 was an almost equal-mass (∼1.35 M e ) binary that has experienced mass transfer. WOCS 1007 is known to pulsate, but this is due to its location in the δ Scuti instability region rather than tidally induced perturbations.…”

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

“…We also search for variability signatures of BSSs using Transiting Exoplanet Survey Satellite (TESS) data. Recently, a similar analysis was presented by Vernekar et al (2023), where they studied the light curves of five BSSs that are known to be SB1s in the OC M67 using Kepler and TESS data. From the light curves, they provided supporting evidence of the MT as the formation mechanism of BSSs to the earlier multi-wavelength SED analysis.…”

“…The BSSs in this cluster are found to have compact objects like white dwarfs as companions, which have a low probability of exhibiting eclipses. A similar system is shown in Vernekar et al (2023). Furthermore, even if many more systems are eclipsing, their detection is limited by the amount of data available using TESS.…”