A λ 3 mm and 1 mm line survey toward the yellow hypergiant IRC +10420
1 Instituto de Ciencia de Materiales de Madrid, Sor Juana Inés de la Cruz, 3, Cantoblanco, 28049 Madrid, Spain
e-mail: email@example.com; firstname.lastname@example.org
2 Observatorio Astronómico Nacional (IGN), Ap 112, 28803 Alcalá de Henares, Spain
3 Department of Astrophysics, Astrobiology Center (CSIC-INTA), Postal address: ESAC campus, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
4 Institut de RadioAstronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin d’ Hères, France
5 Observatorio Astronómico Nacional (IGN), Alfonso XII N°3, 28014 Madrid, Spain
Received: 3 November 2015
Accepted: 7 May 2016
Aims. Our knowledge of the chemical properties of the circumstellar ejecta of the most massive evolved stars is particularly poor. We aim to study the chemical characteristics of the prototypical yellow hypergiant star, IRC +10420. For this purpose, we obtained full line surveys at 1 and 3 mm atmospheric windows.
Methods. We have identified 106 molecular emission lines from 22 molecular species. Approximately half of the molecules detected are N-bearing species, in particular HCN, HNC, CN, NO, NS, PN, and N2H+. We used rotational diagrams to derive the density and rotational temperature of the different molecular species detected. We introduced an iterative method that allows us to take moderate line opacities into account.
Results. We have found that IRC +10420 presents high abundances of the N-bearing molecules compared with O-rich evolved stars. This result supports the presence of a N-rich chemistry, expected for massive stars. Our analysis also suggests a decrease of the 12C/13C ratio from ≳ 7 to ~3.7 in the last 3800 yr, which can be directly related to the nitrogen enrichment observed. In addition, we found that SiO emission presents a significant intensity decrease for high-J lines when compared with older observations. Radiative transfer modeling shows that this variation can be explained by a decrease in the infrared (IR) flux of the dust. The origin of this decrease might be an expansion of the dust shell or a lower stellar temperature due to the pulsation of the star.
Key words: molecular processes / circumstellar matter / radio lines: stars / stars: individual: IRC +10420
Based on observations carried out with the IRAM 30 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).
The reduced spectra (FITS files) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/592/A51
© ESO, 2016