Near-infrared studies of nova V1674 Herculis: A shocking record-breaker

TL;DR

This study presents near-infrared spectroscopy of the fastest nova on record, V1674 Her, revealing early coronal line emission likely caused by shocks, with detailed analysis of its properties and implications for nova physics.

Contribution

It provides the earliest observed coronal lines in a nova, suggesting shock ionization, and offers new insights into the timing and origin of coronal emission in classical novae.

Findings

Coronal lines appeared at day 11.5, the earliest for any nova.

The nova's ejected hydrogen mass is estimated at 1.4 x 10^{-3} solar masses.

Coronal emission is likely caused by shocks, not photoionization.

Abstract

We present near infrared spectroscopy of Nova Herculis 2021 (V1674 Her), obtained over the first 70 days of its evolution. This fastest nova on record displays a rich emission line spectrum, including strong coronal line emission with complex structures. The hydrogen line fluxes, combined with a distance of 4.7 (+1.3 / -1.0) kpc, give an upper limit to the hydrogen ejected mass of 1.4 (+0.8 / -1.2) 10^{-3} solar masses. The coronal lines appeared at day 11.5, the earliest onset yet observed for any classical nova, before there was an obvious source of ionizing radiation. We argue that the gas cannot be photoionized, at least in the earliest phase, and must be shocked. Its temperature is estimated to be 10^{5.57 +/- 0.05} K on day 11.5. Tentative analysis indicates a solar abundance of aluminum and an underabundance of calcium, relative to silicon, with respect to solar values in the ejecta. Further, we show that the vexing problem of whether collisional or photoionization is responsible for coronal emission in classical novae can be resolved by correlating the temporal sequence in which the X-ray supersoft phase and the near-infrared coronal line emission appear.