Arcsecond-resolution CO mapping of the yellow hypergiants IRC +10420 and AFGL 2343 ^*

¹ Institut de RadioAstronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin-d'Hères, France e-mail: [ccarrizo;neri]@iram.fr
² Observatorio Astronómico Nacional (IGN), Apdo. 112, 28803 Alcalá de Henares, Spain e-mail: [g.quintana;v.bujarrabal]@oan.es
³ Observatorio Astronómico Nacional (IGN), c/ Alfonso XII n13, 28014 Madrid, Spain e-mail: j.alcolea@oan.es

Received: 3 August 2006
Accepted: 16 December 2006

Abstract

Context.IRC +10420 and AFGL 2343 are the unique, known yellow hypergiants (YHGs) presenting a heavy circumstellar envelope (CSE).

Aims.We aim to study the morphology, exceptional kinematics, and excitation conditions of their CSEs, and the implications for mass-loss processes.

Methods.We have mapped the ¹²CO and 1-0 emission in these YHGs with the IRAM Plateau de Bure interferometer and the 30 m telescope. We developed LVG models in order to analyze their circumstellar characteristics.

Results.The maps show that the overall shape of both CSEs is approximately spherical, although they also reveal several aspherical features. The CSE around IRC +10420 shows a rounded extended halo surrounding a bright inner region, with both components presenting aspherical characteristics. It presents a brightness minimum at the center. The envelope around AFGL 2343 is a detached shell, showing spherical symmetry and clumpiness at a level of ~15% of the maximum brightness. The envelopes expand isotropically at ~35 km s^-1, about two or three times faster than typical CSEs around AGB stars. High temperatures (~200 K) are derived for the innermost regions in IRC +10420, while denser and cooler (~30 K) gas is found in AFGL 2343.

Conclusions.The mass-loss processes in these YHGs have been found to be similar. The deduced mass-loss rates (~10  yr^-1) are much higher than those obtained in AGB stars, and they present significant variations on time scales of ~1000 yr.