Multiwavelength study of blue straggler stars in Tombaugh 2: Evidence for binary mass transfer and constraints on cluster dynamical state

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 two yellow straggler star candidates. Spectral energy distributions constructed from ultraviolet, optical, and infrared photometry reveal that nine BSSs (∼32%) exhibit significant ultraviolet excess, indicating an additional hot component. Binary spectral energy distribution decomposition identifies stripped companions with effective temperatures Teff ≈ (1.5–8) × 104 K and radii R ≈ 0.04–0.28 R⊙, 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. Multiepoch Very Large Telescope/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.