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arXiv · 2604.12494

Multiwavelength Study of Blue Straggler Stars in Tombaugh 2: Evidence for Binary Mass Transfer and Constraints on Cluster Dynamical State

D. Bisht·Ing-Guey Jiang·K. Belwal·D. C. Cınar·Arvind K. Dattatrey·Geeta Rangwal·A. Raj·Shraddha Biswas·Mohit Singh Bisht·Alok Durgapal
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Abstract

We present a focused multiwavelength study of blue straggler stars (BSSs) in the intermediate-age open cluster Tombaugh 2, located in the outer Galactic disk, to constrain the dominant formation pathways of BSSs in a low-density environment. Cluster members are identified using Gaia DR3 astrometry through a Gaussian Mixture Model, yielding a clean sample of high-probability members. Color-magnitude diagram analysis indicates an age of 1.74 Gyr. The radial surface density profile is well described by a King model, indicating a centrally concentrated overall structure, while the cluster exhibits only weak or no clear evidence of mass segregation among its stellar populations. We identify 26 BSS candidates and 2 YSS candidates. Spectral energy distributions constructed from ultraviolet, optical, and infrared photometry reveal that 9 BSSs (32%) exhibit significant ultraviolet excess, indicating an additional hot component. Binary SED decomposition identifies stripped companions with effective temperatures Teff $\sim$ (1.5-8) $\times$ 10$^4$ K and radii R $\sim$ 0.04-0.28 R_$\odot$, consistent with proto-white dwarfs, extremely low-mass pre-helium white dwarfs, and young hot remnants formed through recent mass transfer. A slight central concentration of BSSs, together with stripped companions, suggests that binary mass transfer is an important formation channel, with no evidence for merger-driven formation. Multi-epoch VLT/FLAMES spectroscopy reveals radial-velocity variability in several systems, providing independent evidence for binarity. Our results highlight that optical-infrared photometric analyses alone may fail to detect hot compact companions, while spectroscopy and ultraviolet observations provide complementary constraints, with ultraviolet data offering a direct probe of such companions in intermediate-age open clusters.

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D. Bisht, Ing-Guey Jiang, K. Belwal, D. C. Cınar, Arvind K. Dattatrey, Geeta Rangwal, A. Raj, Shraddha Biswas, Mohit Singh Bisht, Alok Durgapal. 2026-04-14. Multiwavelength Study of Blue Straggler Stars in Tombaugh 2: Evidence for Binary Mass Transfer and Constraints on Cluster Dynamical State. https://arxiv.org/abs/2604.12494

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