W UMa-Type Contact Binaries in the Tidal Tails of Young Open Cluster COIN-Gaia 25 and Mamajek 4

TL;DR

This study identifies and models two W UMa-type contact binaries in young open clusters' tidal tails, providing insights into their formation, evolution, and the clusters' ages, with detailed photometric analysis and modeling.

Contribution

First systematic search and detailed modeling of contact binaries in the tidal tails of young open clusters, revealing their properties and formation mechanisms.

Findings

Two confirmed contact binaries in cluster tidal tails.

One binary classified as a deep, low-mass-ratio contact system.

Age constraint for one binary supports rapid contact binary evolution.

Abstract

Star clusters, as dynamically rich environments, are thought to be important sites for the formation of contact binaries. To investigate this, we conducted a systematic search for contact binaries within two young open clusters, COIN-Gaia 25 and Mamajek 4, and their associated tidal tails. From this search, we identified and confirmed two contact binary systems: ASASSN-V J064923.44+013758.4 in the tidal tail of COIN-Gaia 25, and ASASSN-V J173229.06-613712.5 in the tidal tail of Mamajek 4. Using TESS light curve data, we performed detailed modeling with a Markov Chain Monte Carlo (MCMC) method within the 2015 version of the Wilson-Devinney (W-D) code. The resulting photometric parameters are: q_ph = 0.316 (0.013), i= 76.9 (+1.3, -0.9) degrees, f=23.1 (+1.4, -1.9)% for ASASSN-V J064923.44+013758.4, and q_ph = 0.130 (0.004), i= 68.4 (+1.4, -1.3) degrees, f=66.3 (+5.0, -5.3)% for ASASSN-V J173229.06-613712.5. A cool-spot model located on the more massive component was successfully implemented for each system. The derived parameters classify ASASSN-V J173229.06-613712.5 as a deep, low-mass-ratio contact binary. Its location in the tidal tail of Mamajek 4 constrains its age to < 371 Myr, supporting the view that the total lifetime of some contact binaries may be as short as a few thermal timescales. After evaluating standard formation mechanisms, we propose that ASASSN-V J173229.06-613712.5 likely formed via efficient orbital hardening (potentially mediated by gas dynamical friction) during the early, gas-rich phase of its host cluster's evolution. This study demonstrates the value of young clusters and their tidal tails in providing robust age constraints to explore the formation and rapid evolution of contact binaries.