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.
The old open cluster M67, populated with blue straggler stars (BSSs), is a well known test bed to study the BSS formation pathways. Here, we report the first direct detection of a white dwarf (WD) companion to a BSS in M67, using far-UV images from the Ultra Violet Imaging telescope (UVIT) on ASTROSAT. Near-simultaneous observations in three far-UV bands combined with GALEX, IUE, ground and space based photometric data covering 0.14 -11.5 µm range for WOCS1007 were found to require a binary fit to its spectral energy distribution (SED), consisting of a BSS and a hot companion. On the other hand, a single spectral fit was found to be satisfactory for the SEDs of two other BSSs, WOCS1006 and WOCS2011, with the latter showing a deficient far-UV flux. The hot companion of WOCS1007 is found to have a T ef f ∼ 13250-13750K and a radius of 0.09±0.01 R . A comparison with WD models suggests it to be a low mass WD (∼ 0.18M ), in agreement with the kinematic mass from the literature. As a low mass WD (< 0.4M ) necessitates formation through mass transfer (MT) in close binaries, WOCS1007 with a known period of 4.2 days along with its fast rotation, is likely to be formed by a case A or case B binary evolution.
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.
Blue straggler stars (BSSs) are the most massive stars in a cluster formed via binary or higher-order stellar interactions. Though the exact nature of such formation scenarios is difficult to pin down, we provide observational constraints on the different possible mechanism. In this quest, we first produce a catalogue of BSSs using Gaia DR2 data. Among the 670 clusters older than 300 Myr, we identified 868 BSSs in 228 clusters and 500 BSS candidates in 208 clusters. In general, all clusters older than 1 Gyr and massive than 1000 M⊙ have BSSs. The average number of BSSs increases with cluster age and mass, and there is a power-law relation between the cluster mass and the maximum number of BSSs in the cluster. We introduce the term fractional mass excess ( $\mathcal {M}_{e}$) for BSSs. We find that at least 54 per cent of BSSs have $\mathcal {M}_{e}$ < 0.5 (likely to have gained mass through a binary mass transfer (MT)), 30 per cent in the 1.0 < $\mathcal {M}_{e}$ <0.5 range (likely to have gained mass through a merger) and up to 16 per cent with $\mathcal {M}_{e}$ > 1.0 (likely from multiple mergers/MT). We also find that the percentage of low $\mathcal {M}_{e}$ BSSs increases with age, beyond 1–2 Gyr, suggesting an increase in formation through MT in older clusters. The BSSs are radially segregated, and the extent of segregation depends on the dynamical relaxation of the cluster. The statistics and trends presented here are expected to constrain the BSS formation models in open clusters.
We present a study of six open clusters (Berkeley 67, King 2, NGC 2420, NGC 2477, NGC 2682 and NGC 6940) using the Ultra Violet Imaging Telescope (UVIT) aboard ASTROSAT and Gaia EDR3. We used combinations of astrometric, photometric and systematic parameters to train and supervise a machine learning algorithm along with a Gaussian mixture model for the determination of cluster membership. This technique is robust, reproducible and versatile in various cluster environments. In this study, the Gaia EDR3 membership catalogues are provided along with classification of the stars as members, candidates and field in the six clusters. We could detect 200–2500 additional members using our method with respect to previous studies, which helped estimate mean space velocities, distances, number of members and core radii. UVIT photometric catalogues, which include blue stragglers, main-sequence and red giants are also provided. From UV–Optical colour-magnitude diagrams, we found that majority of the sources in NGC 2682 and a few in NGC 2420, NGC 2477 and NGC 6940 showed excess UV flux. NGC 2682 images have ten white dwarf detection in far-UV. The far-UV and near-UV images of the massive cluster NGC 2477 have 92 and 576 members respectively, which will be useful to study the UV properties of stars in the extended turn-off and in various evolutionary stages from main-sequence to red clump. Future studies will carry out panchromatic and spectroscopic analysis of noteworthy members detected in this study.
The blue straggler stars (BSSs) are main-sequence (MS) stars, which have evaded stellar evolution by acquiring mass while on the MS. The detection of extremely low mass (ELM) white dwarf (WD) companions to two BSSs and one yellow straggler star (YSS) from our earlier study using UVIT/AstroSat, as well as WD companions to main-sequence stars (known as blue lurkers) suggest a good fraction of post-mass transfer binaries in M67. Using deeper UVIT observations, here we report the detection of another blue lurker in M67, with an ELM WD companion. The post-mass transfer systems with the presence of ELM WDs, including BSSs, are formed from Case A/B mass transfer and are unlikely to show any difference in surface abundances. We find a correlation between the temperature of the WD and the v sin i of the BSSs. We also find that the progenitors of the massive WDs are likely to belong to the hot and luminous group of BSSs in M67. The only detected BSS?WD system by UVIT in the globular cluster NGC 5466 has a normal WD and suggests that open cluster like environment might be present in the outskirts of low density globular clusters.
Binary stars play a vital role in astrophysical research, as a good fraction of stars are in binaries. Binary fraction (BF) is known to change with stellar mass in the Galactic field, but such studies in clusters require binary identification and membership information. Here, we estimate the total and spectral-type high-mass-ratio (HMR) BF (f 0.6) in 23 open clusters using unresolved binaries in color–magnitude diagrams using Gaia DR2 data. We introduce the segregation index ( ) parameter to trace mass segregation of HMR (total and mass) binaries and the reference population. This study finds that in open clusters, (1) HMR BF for the mass range 0.4–3.6 M ⊙ (early M to late B-type stars) has a range of 0.12–0.38 with a peak at 0.12–0.20; (2) older clusters have a relatively higher HMR BF; (3) the mass-ratio distribution is unlikely to be a flat distribution and BF (total) ∼(1.5–2.5) × f 0.6; (4) a decreasing BF (total) from late B to K-type stars, in agreement with the Galactic field stars; (5) older clusters show radial segregation of HMR binaries; (6) B-type and A–F type HMR binaries show radial segregation in some young clusters suggesting a primordial origin. This study will constrain the initial conditions and identify the major mechanisms that regulate binary formation in clusters. Primordial segregation of HMR binaries could result from massive clumps spatially segregated in the collapse phase of the molecular cloud.
King 2, one of the oldest clusters in the Milky Way, with an age of $ 6 Gyr and distance of $ 5700 pc, has been observed with UVIT payload on the AstroSat. With membership information derived from Gaia EDR3, the cluster is found to have 39 blue straggler stars (BSSs). We created multi-wavelength spectra-energy distributions (SED) of all the BSSs. Out of 10 UV detected BSSs, 6 bright ones were fitted with double component SEDs and were found to have hotter companions with properties similar to extreme horizontal branch (EHB)/subdwarf B (sdB) stars, with a range in luminosity and temperature, suggesting a diversity among the hot companions. We suggest that at least 15% of BSSs in this cluster are formed via mass-transfer pathway. When we compared their properties to EHBs and hotter companions to BSS in open and globular clusters, we suggest that EHB/sdBs like companions can form in binaries of open clusters as young as 6 Gyr. Keywords. Open star clusters (1160)-blue straggler stars (168)-extreme horizontal branch stars (513)-B subdwarf stars (129)-ultraviolet astronomy (1736)-spectral energy distribution (2129)-binary stars (154).This article is part of the Special Issue on ''AstroSat: Five Years in Orbit''.
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