Hints of the Existence of C-rich Massive Evolved Stars

Abstract

We aim to study the properties of a particular type of evolved stars, 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 = 2-1 emission 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 and 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 an 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 ∼ 5-9 M _⊙, which could correspond to an AGB or an RSG and AFGL 2233 an RSG with M _init ∼ 20 M _⊙, which would confirm the existence of massive C-rich evolved stars. Two scenarios are proposed to form these types of objects. The first one is capable of producing high-mass AGB stars up to ∼8 M _⊙ and the second one is capable of forming C-rich RSGs like AFGL 2233.

Based on observations carried out with the IRAM NOEMA and 30 m. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).