V4334 Sgr (Sakurai's Object): still churning out the dust

TL;DR

This study presents a near-infrared spectrum of Sakurai's Object from 2020, revealing recent hot dust formation, extended emission lines, and changes in electron density, providing new insights into its late thermal pulse evolution.

Contribution

First near-infrared spectrum of Sakurai's Object in 2020 showing recent dust formation and spectral changes, offering updated understanding of its late thermal pulse phase.

Findings

Brightening continuum with blackbody temperature of 624 K

Detection of hot dust formation in the environment

Increase in electron density between 2013 and 2020

Abstract

We present a $0.8-2.5\,\mu$m spectrum of the Very Late Thermal Pulse object V4334 Sgr (Sakurai's Object), obtained in 2020 September. The spectrum displays a continuum that rises strongly to longer wavelengths, and is considerably brighter than the most recent published spectrum obtained seven years earlier. At the longer wavelengths the continuum is well fitted by a blackbody with a temperature of $624\pm8$ K. However, there is excess continuum at the shortest wavelengths that we interpret as being due to hot dust that has very recently formed in an environment with C/O $\simeq2.5$. Other possible sources for this excess continuum are discussed - such as the stellar photosphere dimly seen through the dust shell, and light scattered off the inner wall of the dust torus - but these interpretations seem unlikely. Numerous emission lines are present, including those of HeI, CI, [CI], and OI. Our observations confirm that emission in the HeI 1.083$\,\mu$m and [CI] 0.9827/0.9852$\,\mu$m lines is spatially extended. The [CI] line fluxes suggest that the electron density increased by an order of magnitude between 2013 and 2020, and that these two lines may soon disappear from the spectrum. The flux ratio of the 1.083$\,\mu$m and 2.058$\,\mu$m HeI lines is consistent with the previously-assumed interstellar extinction. The stellar photosphere remains elusive, and the central star may not be as hot as suggested by current evolutionary models.