Contact tracing of binary stars: Pathways to stellar mergers

Stellar mergers lead to diverse phenomena: rejuvenated blue stragglers, magnetised and peculiar stars, transients and nebulae. Using a grid of about 6000 detailed 1D binary evolution models (initial component masses of 0.5-20$\,\text{M}_{\odot}$ at solar metallicity), we investigate which initial binary-star configurations lead to contact and classical common-envelope (CE) phases and assess the likelihood of a subsequent merger. Considering rotation and tides, we identify five mechanisms leading to contact and mergers: runaway mass transfer, $\text{L}_{2}$-overflow, accretor expansion, tidally-driven orbital decay, and non-conservative mass transfer. At least 40% of mass-transferring binaries with initial primary masses of 5-20$\,\text{M}_{\odot}$ enter contact, with >12% and >19% likely merging and evolving into a classical CE phase, respectively. Classical CE evolution occurs in late Case-B and Case-C binaries for initial mass ratios $q_{\text{i}}$ < 0.15-0.35, stable mass transfer for larger $q_{\text{i}}$. Early Case-B binaries enter contact for $q_{\text{i}}$ < 0.15-0.35 and in initially wider Case-A binaries, this occurs for $q_{\text{i}}$ < 0.35. All initially closest Case-A systems form contact binaries. We predict that binaries entering contact with $q$ < 0.5 merge or detach on a thermal timescale, while those formed with $q$ > 0.5 lead to long-lived contact phases. The fact that contact binaries are almost exclusively observed with $q$ > 0.5 confirms our expectations. Our contact, merger and classical CE incidences are lower limits because the mass transfer in our models is non-conservative. In most binaries, the non-accreted mass cannot be ejected and may settle in disks or lead to contact phases and mergers. Overall, contact binaries are a frequent and fascinating result of binary mass transfer of which the exact outcomes still remain to be understood and explored further.