Cillian O'Donnell | Searcharxiv

arXiv2026

An analysis of the Type Ia SN 2024gy and a comparison of different host extinction estimation techniques

Type Ia supernovae (SNe Ia) are well-known standardisable candles, and are one of the main ways to measure the distance to their host galaxies. However, extinction due to interstellar dust causes objects to appear fainter and redder. Correcting for this requires estimating the amount of intervening material and how the extinction changes as a function of wavelength. We present and analyse optical and near-infrared data of the well-observed SN 2024gy and use these to compare different extinction estimation techniques, making use of photometric, spectroscopic, and polarimetric data. SN 2024gy is a normal SN Ia with high velocity (HV) components in Si II $\lambda6355$ (phase $<-10$ days) and a particularly strong HV feature in the Ca II near-infrared triplet (up to peak). Modelling SN 2024gy with TARDIS shows better matches with a double-detonation scenario compared to a delayed-detonation scenario due to a better match to the Ca II HV component. A measurement of the stable Ni/Fe ratio however favours a delayed-detonation scenario. Host extinction estimates range from $E(B-V)_{host}=0.12\pm0.02$ mag (narrow interstellar absorption lines) to $E(B-V)_{host}=0.24\pm0.06$ mag (Lira law) with a mean of $E(B-V)_{host}=0.22\pm0.04$ mag, assuming $R_V=3.1$. The spread between different methods highlights the challenge of accurately estimating the amount of extinction light suffers before being observed.

astro-ph.GA↗

arXiv2026

Violent mergers revisited: The origin of the fastest stars in the Galaxy

Binary systems of two carbon-oxygen white dwarfs are one of the most promising candidates for the progenitor systems of Type Ia supernovae. Violent mergers, where the primary white dwarf ignites when the secondary white dwarf smashes onto it while being disrupted on its last orbit, were the first proposed double degenerate merger scenario that ignites dynamically. However, violent mergers likely contribute only a few per cent to the total Type Ia supernova rate and do not yield normal Type Ia supernova light curves. Here we revisit the scenario, simulating a violent merger with better methods, and in particular a more accurate treatment of the detonation. We find good agreement with previous simulations, with one critical difference. The secondary white dwarf, being disrupted and accelerated towards the primary white dwarf, and impacted by its explosion, does not fully burn. Its core survives as a bound object. The explosion leaves behind a $0.16\,\mathrm{M_\odot}$ carbon-oxygen white dwarf travelling $2800\,\mathrm{km/s}$, making it an excellent (and so far the only) candidate to explain the origin of the fastest observed hyper-velocity white dwarfs. We also show that before the explosion, $5\times10^{-3}\,\mathrm{M_\odot}$ of material consisting predominantly of helium, carbon, and oxygen has already been ejected at velocities above $1000\,\mathrm{km/s}$. Finally, we argue that if a violent merger made D6-1 and D6-3, and violent mergers require the most massive primary white dwarfs in binaries of two carbon-oxygen white dwarfs, there has to be a much larger population of white dwarf mergers with slightly lower-mass primary white dwarfs. Because of its size, this population can essentially only give rise to normal Type Ia supernovae, likely exploding via the quadruple detonation channel and leaving no bound object behind.

astro-ph.HE↗