Probing the mass loss history of the yellow hypergiant IRC+10420

TL;DR

This study images and analyzes the molecular envelope of the yellow hypergiant IRC+10420, revealing asymmetries, multiple shells from past mass-loss episodes, and unusually high gas temperatures driven by the star's luminosity.

Contribution

First detailed imaging and modeling of IRC+10420's molecular envelope, identifying multiple shells, asymmetries, and high-temperature conditions with implications for stellar evolution.

Findings

Detected a large, clumpy, asymmetric envelope with bipolar outflow.

Identified two shells indicating past mass-loss episodes.

Estimated high mass-loss rates and elevated gas temperatures.

Abstract

We have used the sub-millimeter array to image the molecular envelope around IRC+10420. Our observations reveal a large and clumpy expanding envelope around the star. The molecular envelope shows a clear asymmetry in $^{12}$CO J=2--1 emission in the South-West direction. The elongation of the envelope is found even more pronounced in the emission of $^{13}$CO J=2--1 and SO J$_{\rm K}$=6$_5$--5$_4$. A small positional velocity gradient across velocity channels is seen in these lines, suggesting the presence of a weak bipolar outflow in the envelope of IRC+10420. In the higher resolution $^{12}$CO J=2--1 map, we find that the envelope has two components: (1) an inner shell (shell I) located between radius of about 1"-2"; (2) an outer shell (shell II) located between 3" to 6" in radius. These shells represent two previous mass-loss episodes from IRC+10420. We attempt to derive in self-consistent manner the physical conditions inside the envelope by modelling the dust properties, and the heating and cooling of molecular gas. We estimate a mass loss rate of $\sim$9 10$^{-4}$ M$_\odot$ yr$^{-1}$ for shell I and 7 10$^{-4}$ M$_\odot$ yr$^{-1}$ for shell II. The gas temperature is found to be unusually high in IRC+10420 in comparison with other oxygen-rich envelopes. The elevated gas temperature is mainly due to higher heating rate, which results from the large luminosity of the central s tar. We also derive an isotopic ratio $^{12}$C/$^{13}$C = 6.