Hints of the existence of C-rich massive evolved stars

Astrophys J. 2019 May 10;876(2):10.3847/1538-4357/ab133e. doi: 10.3847/1538-4357/ab133e.

Abstract

We aim at studying the properties of a particular type of evolved stars, the C-rich evolved stars with high expansion velocities. For this purpose we have focused on the two best studied objects within this group, IRC +10401 and AFGL 2233. We focused on determining their luminosity by studying their spectral energy distribution. Also we have obtained single-dish line profiles and interferometric maps of the CO J=1-0 and J=2-1emission lines for both objects. We have modeled this emission using a LVG radiative transfer code to determine the kinetic temperature and density profiles of the gas ejected by these stars. We have found that the luminosities obtained for these objects (log(L/L ) = 4.1 & 5.4) locate them in the domain of the massive Asymptotic Giant Branch stars (AGBs) and the Red Supergiant stars (RSGs). In addition, the mass-loss rates obtained (1.5 × 10-5 - 6 × 10-3 M yr-1) suggest that while IRC+10401 might be an AGB star, AFGL 2233 could be a RSG star. All these results, together with those from previous works suggest that both objects are massive objects, IRC +10401 a massive evolved star with M init ~ 5 - 9M which could correspond to an AGB or a RSG and AFGL 2233 a RSG with M init ~ 20M , which would confirm the existence of massive C-rich evolved stars. Two scenarios are proposed to form these types of objects. The first one capable of producing high mass AGB stars up to ~ 8M and a second one capable of forming C-rich RSGs as AFGL 2233.

Keywords: Astrochemistry; nuclear reactions, nucleosynthesis, abundances; stars: circumstellar matter; stars: evolution; stars: supergiants.