UOCS – VII. Blue straggler populations of open cluster NGC 7789 with UVIT/AstroSat

Context. Binary stars play a crucial role in our understanding of the formation and evolution of star clusters and their stellar populations. Aims. We use Gaia Data Release 3 to homogeneously analyze 78 Galactic open clusters and the unresolved binary systems they host, each composed of two main sequence (MS) stars. Methods. We first investigated the structural parameters of these clusters, such as the core radius and the central density, and determined the cluster mass function (MF) and total mass by interpolating the density profile of each cluster. We measured the fraction of binaries with a large mass ratio and the fraction of blue straggler stars (BSSs), and finally investigated possible connections between the populations of binary stars and BSSs with the main parameters of the host cluster. Results. Remarkably, we find that the MFs of 78 analyzed open clusters follow a similar trend and are well reproduced by two single power-law functions, with a change in slope around masses of 1 M⊙. The fraction of binary stars ranges from ∼15% to more than ∼60% without significant correlation with the mass and the age of the host cluster. Moreover, we detect hints of a correlation between the total fraction of binary stars and the central density of the host cluster. We compared the fraction of binary stars with that of BSSs, finding that clusters with high and low central density exhibit different trends. The fraction of binaries does not significantly change with the mass of the primary star and the mass ratio. The radial distribution of binary stars depends on cluster age. The binaries of clusters younger than ∼800 Myr typically show a flat radial distribution, with some hints of a double peak. In contrast, the binaries of the remaining clusters are more centrally concentrated than the single stars, which is similar to what is observed in globular clusters.

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“…We confirm the previous conclusion that clusters younger than ∼300 Myr do not host BSSs (Vaidya et al 2022;Rain et al 2021). As shown in Fig.…”

Section: Binaries and Blue Stragglerssupporting

confidence: 93%

Photometric binaries, mass functions, and structural parameters of 78 Galactic open clusters

Cordoni

Milone

Marino

et al. 2023

A&A

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“…However, since the known metallicity of Berkeley 17 ranges from −0.19 dex to −0.01 dex (Friel et al 2005), a change in metallicity can shift the equal mass binary isochrone of ZAMS towards the red BSS sequence. It has been revealed through spectroscopic and UV observations that in many OCs, such as NGC 188 (Geller et al 2008;Gosnell et al 2015), M67 (Geller et al 2015;Sindhu et al 2019;Jadhav et al 2019), NGC 6819 (Milliman et al 2015, and NGC 7789 (Nine et al 2020;Vaidya et al 2022), ∼33% -85% of BSS formed via binary system evolution, primarily through the mass-transfer channel. The triple-mediated interactions are also considered as an important formation channel for some BSS of OCs, such as M67 and NGC 188 (Leigh & Sills 2011;Leiner et al 2016;Bertelli Motta et al 2018), Melotte 66 (Rao et al 2022), and NGC 2506 (Panthi et al 2022).…”

Section: Discussionmentioning

confidence: 99%

Discovery of double BSS sequences in the old Galactic open cluster Berkeley 17

Rao

Bhattacharya

Vaidya

et al. 2022

Monthly Notices of the Royal Astronomical Society: Letters

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Blue straggler stars (BSS) are peculiar objects which normally appear as a single broad sequence along the extension of the main sequence. Only four globular clusters (GCs) have been observed to have two distinct and parallel BSS sequences. For the first time for any open cluster (OC), we report double BSS sequences in Berkeley 17. Using the machine-learning based membership algorithm ML-MOC on Gaia EDR3 data, we identify 627 cluster members, including 21 BSS candidates out to 15′ from the cluster center. Both the BSS sequences are almost equally populated and parallel to one another in Gaia as well as in Pan-STARRS colour-magnitude diagram (CMD). We statistically confirm their presence and report that both BSS sequences are highly segregated compared to the reference population out to ∼5.5′ and not segregated thereafter. The lower densities of OCs make BSS formation impossible via the collisional channel. Therefore, mass transfer seems to be the only viable channel for forming candidates of both sequences. The gap between the red and blue BSS sequences, on the other hand, is significant and presents a great opportunity to understand the connection between BSS formation and internal as well as external dynamics of the parent clusters.

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“…The age of the universe is also a limiting factor at the lower end of the WD mass. ELMs, on the other hand, can be found in binary form in the universe (Jadhav et al 2019;Ratzloff et al 2019Vaidya et al 2022). The lower limit on the mass of the ELM WD can be attributed to mass loss during the early stages of evolution.…”

Section: Discussionmentioning

confidence: 99%

GlobULeS. IV. UVIT/AstroSat Detection of Extremely Low Mass White Dwarf Companions to Blue Straggler Stars in NGC 362

Dattatrey

Yadav

Rani

et al. 2023

ApJ

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We report the discovery of extremely low mass white dwarfs (ELM WDs) as a companion of blue straggler stars (BSSs) in the Galactic globular cluster NGC 362 using images from AstroSat's Ultra Violet Imaging Telescope (UVIT). Spectral energy distributions (SEDs) for 26 far-UV (FUV) bright member BSSs are created using data from the UVIT, the UltraViolet and Optical Telescope (UVOT), Gaia EDR3, and the 2.2 m ESO/MPI telescope. A single SED is fitted to 14 BSSs, whereas double-SED fits revealed ELM WDs as binary companions in 12 of the 26 BSSs studied. The effective temperature, radius, luminosity, and mass of the 12 ELM WDs are found to have a range of T eff = 9750−18,000 K, R = 0.1−0.4 R ⊙, L = 0.4−3.3 L ⊙, and M = 0.16−0.20 M ⊙. These suggest that 12 BSSs are post-mass-transfer systems formed through the case A/B mass transfer pathway. To the best of our knowledge, this is the first finding of ELM WDs as companions to BSSs in globular clusters. This cluster is known to have a binary BSS sequence, and the 12 binary and 14 single BSSs (as classified by the SEDs) follow the mass transfer and collisional sequence of BSSs in the color–magnitude diagram. The cooling ages of nine of the ELM WDs are found to be younger than 500 Myr. Though the binary BSSs may have formed during the core collapse (∼200 Myr) or as part of the dynamical evolution of the cluster, they provide new insights on the dynamics of this cluster.

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UOCS – VII. Blue straggler populations of open cluster NGC 7789 with UVIT/AstroSat

Cited by 16 publications

References 65 publications

Photometric binaries, mass functions, and structural parameters of 78 Galactic open clusters

Photometric binaries, mass functions, and structural parameters of 78 Galactic open clusters

Discovery of double BSS sequences in the old Galactic open cluster Berkeley 17

GlobULeS. IV. UVIT/AstroSat Detection of Extremely Low Mass White Dwarf Companions to Blue Straggler Stars in NGC 362

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