Early formation of carbon monoxide in the Centaurus A supernova SN 2016adj

TL;DR

This study reports the earliest detection of carbon monoxide in a supernova, modeling its properties and exploring implications for supernova chemistry and classification.

Contribution

It presents near-infrared spectroscopic observations of SN 2016adj, detecting CO formation within 58 days, and constrains the isotopic ratio of carbon, offering new insights into supernova ejecta composition.

Findings

First early detection of CO in a supernova at 58 days.

Constraints on the $^{12}$C/$^{13}$C ratio from near-infrared data.

Discussion on the supernova's classification and carbon-rich ejecta.

Abstract

We present near-infrared spectroscopy of the NGC 5128 supernova SN 2016adj in the first 2 months following discovery. We report the detection of first overtone carbon monoxide emission at $\sim58.2$ d after discovery, one of the earliest detections of CO in an erupting supernova. We model the CO emission to derive the CO mass, temperature and velocity, assuming both pure $^{12}$CO and a composition that includes $^{13}$CO; the case for the latter is the isotopic analyses of meteoritic grains, which suggest that core collapse supernovae can synthesise significant amounts of $^{13}$C. Our models show that, while the CO data are adequately explained by pure $^{12}$CO, they do not preclude the presence of $^{13}$CO, to a limit of $^{12}$C/$^{13}$C $> 3$, the first constraint on the $^{12}$C/$^{13}$C ratio determined from near-infrared observations. We estimate the reddening to the object, and the effective temperature from the energy distribution at outburst. We discuss whether the ejecta of SN 2016adj may be carbon-rich, what the infrared data tell us about the classification of this supernova, and what implications the early formation of CO in supernovae may have for CO formation in supernovae in general.