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

Abstract

We have used the submillimeter array to image the molecular envelope around the yellow hypergiant IRC+10420. Our observations reveal a large and clumpy expanding envelope around the star. The molecular envelope shows a clear asymmetry in (CO)-C-12 J = 2-1 emission in the southwest direction. The elongation of the envelope is found even more pronounced in the emission of (CO)-C-13 J = 2-1 and SO J(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 (CO)-C-12 J = 2-1 map, we find that the envelope has two components: (1) an inner shell (shell I) located between radius of about 1 '' and 2 ''; (2) an outer shell (shell II) located between 3 '' and 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 modeling the dust properties, and the heating and cooling of molecular gas. We estimate a mass-loss rate of similar to 9 x 10(-4) M-circle dot yr(-1) for shell I and 7 x 10(-4) M-circle dot 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 star. We also derive an isotopic ratio C-12/C-13 = 6.