Observations of the Optical Transient in NGC 300 with AKARI/IRC: Possibilities of Asymmetric Dust Formation

TL;DR

This study presents multi-epoch near-infrared observations of the optical transient in NGC 300, revealing asymmetric dust formation in the ejecta, with implications for understanding dust geometry and evolution in transient events.

Contribution

First multi-epoch NIR spectra of NGC 300-OT showing dust formation with evidence for asymmetric dust distribution, such as tori or bipolar outflows.

Findings

Dust temperature decreased from 810K to 670K over time.

NIR emission is optically thick, indicating significant dust mass.

Dust likely formed in asymmetric structures like tori or clumps.

Abstract

We present the results of near-infrared (NIR) multi-epoch observations of the optical transient in the nearby galaxy NGC300 (NGC300-OT) at 398 and 582 days after the discovery with the Infrared Camera (IRC) onboard AKARI. NIR spectra (2--5 um) of NGC300-OT were obtained for the first time. They show no prominent emission nor absorption features, but are dominated by continuum thermal emission from the dust around NGC300-OT. NIR images were taken in the 2.4, 3.2, and 4.1 um bands. The spectral energy distributions (SED) of NGC300-OT indicate the dust temperature of 810 (+-14) K at 398 days and 670 (+-12) K at 582 days. We attribute the observed NIR emission to the thermal emission from dust grains formed in the ejecta of NGC300-OT. The multi-epoch observations enable us to estimate the dust optical depth as larger than about 12 at 398 days and larger than about 6 at 582 days at 2.4 um, by assuming an isothermal dust cloud. The observed NIR emission must be optically thick, unless the amount of dust grains increases with time. Little extinction at visible wavelengths reported in earlier observations suggests that the dust cloud around NGC300-OT should be distributed inhomogeneously so as to not screen the radiation from the ejecta gas and the central star. The present results suggest the dust grains are not formed in spherically symmetric geometry, but rather in a torus, a bipolar outflow, or clumpy cloudlets.