Light curves and spectra for stellar collisions between main-sequence stars in galactic nuclei

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching bei München, Germany
² JILA, University of Colorado and National Institute of Standards and Technology, 440 UCB, Boulder, 80308 CO, USA
³ Department of Astrophysical and Planetary Sciences, 391 UCB, Boulder, 80309 CO, USA
⁴ Institut d’Astrophysique de Paris, CNRS-Sorbonne Université, 98 bis Boulevard Arago, F-75014 Paris, France

^⋆ Corresponding author.

Received: 7 February 2025
Accepted: 15 May 2025

Abstract

High-velocity stellar collisions in galactic nuclei produce ejecta that generate potentially observable electromagnetic radiation, making them promising nuclear transients. However, the photometric and spectroscopic properties of these collisions, which would more frequently involve main-sequence stars, remain largely unexplored. Here, using 3D hydrodynamics and 1D radiative-transfer simulations, we investigate the properties and observables of the debris produced in high-velocity collisions between terminal-age main-sequence stars, covering a wide range of collision configurations. The ejecta produce bright ultraviolet (UV) transients with bolometric luminosities typically peaking at ≳10⁴³ erg s⁻¹, declining steeply as t⁻²−t⁻⁴ to reach ≳10⁴¹−10⁴² erg s⁻¹ at 0.5 d, and leveling off on a plateau at 10³⁹−10^41.5 erg s⁻¹ (M_V between −10 to −15 mag) after a few days. The total radiated energy is less than 10⁴⁹ ergs, which corresponds to 10⁻³−10⁻⁵ of the initial collision kinetic energy. Their spectra evolve considerably during the first few days, morphing from UV- to optical-dominated. The UV range shows numerous resonance transitions from metals like C, N, and O, whereas the optical primarily shows H I Balmer lines. These properties are qualitatively similar to those observed, as well as obtained in models of Type II supernovae. Observables from these events exhibit clear correlations with collision configurations, including impact parameter, relative velocity, and stellar masses. We provide fitting formulae to describe these correlations. Detecting these transients requires sub-day cadence surveys such as ULTRASAT, combined with spectroscopic observations to disentangle degeneracies and infer collision characteristics.