UVIT Open Cluster Study. II. Detection of Extremely Low Mass White Dwarfs and Post–Mass Transfer Binaries in M67

Abstract

A detailed study of the ultraviolet (UV) bright stars in the old open star cluster, M67 is presented based on the far-UV observations using the Ultra Violet Imaging Telescope (UVIT) on ASTROSAT. The UV and UV-optical colour-magnitude diagrams (CMDs) along with overlaid isochrones are presented for the member stars, which include blue straggler stars (BSSs), triple systems, white dwarfs (WDs) and spectroscopic binaries (SB). The CMDs suggest the presence of excess UV flux in many members, which could be extrinsic or intrinsic to them. We construct multi-wavelength spectral energy distribution (SED) using photometric data from the UVIT, Gaia DR2, 2MASS and WISE surveys along with optical photometry. We fitted model SEDs to 7 WDs and find 4 of them have mass > 0.5 M and cooling age of less than 200 Myr, thus demanding BSS progenitors. SED fits to 23 stars detect extremely low mass (ELM) WD companions to WOCS2007, WOCS6006 and WOCS2002, and a low mass WD to WOCS3001, which suggest these to be post mass transfer (MT) systems. 12 sources with possible WD companion need further confirmation. 9 sources have X-ray and excess UV flux, possibly arising out of stellar activity. This study demonstrates that UV observations are key to detect and characterise the ELM WDs in non-degenerate systems, which are ideal test beds to explore the formation pathways of these peculiar WDs. The increasing detection of post-MT systems among BSSs and main-sequence stars suggest a strong MT pathway and stellar interactions in M67.

UVIT/AstroSat studies of blue straggler stars and post-mass transfer systems in star clusters: detection of one more blue lurker in M67

“…We have detected WD companions to BSSs in the OC M67 (Sindhu et al 2019(Sindhu et al . 2020Jadhav et al 2019), and in the outskirts of the GC NGC 5466 (Sahu et al 2019), using the UVIT images. Leiner et al (2019) identified a fast rotating MS star and suggested that it could be a post-MT product.…”

“…As this star is located on the MS, rather than the normal BSSs, which are brighter and bluer than the MS tip, they named it a blue-lurker (BL). Jadhav et al (2019), detected the presence of WD companions to two MS stars in M67, which can be classified as BLs. One has a confirmed post-MT nature, as its companion is a ELM WD.…”

UOCS. IV. Discovery of diverse hot companions to blue stragglers in the old open cluster King 2

“…Ultraviolet imaging of binary systems reveals the presence of hotter companions in the binary system, given that the hotter companion is luminous in UV. Old OCs such as NGC 188 and NGC 2682 are rich with BSSs, binary stars and contain many such optically sub-luminous UV-bright companions (Subramaniam et al 2016;Sindhu et al 2019;Jadhav et al 2019). Similar companions have been identified to BSSs in the outskirts of GCs (Sahu et al 2019;Singh et al 2020).…”

“…Otherwise, the secondary will be fainter than the MSTO and can be classified as a blue lurker. The blue lurkers are identified by their high stellar rotation (Leiner et al 2019) or evidence of extremely low-mass (ELM) white dwarf (WD) companion (Jadhav et al 2019). Depending on the evolutionary status of binary components, the binary system can be observed as main sequence (MS)?MS, contact binaries (Rucinski 1998), common envelop, MS?horizontal branch (HB;Subramaniam et al 2016), MS?extreme HB (EHB; Singh et al 2020), MS?…”

“…Depending on the evolutionary status of binary components, the binary system can be observed as main sequence (MS)?MS, contact binaries (Rucinski 1998), common envelop, MS?horizontal branch (HB;Subramaniam et al 2016), MS?extreme HB (EHB; Singh et al 2020), MS? subdwarf-B (sdB; Han et al 2002), MS?WD (Jadhav et al 2019), WD?WD (Marsh et al 1995) and many more combinations. The binary evolution also depends on external factors such as collisions in a high-density environment which can decouple the binary (Heggie 1975) and tertiary star which can expedite the mass transfer/ merger by reducing the orbital separation (Kozai 1962).…”