ALMA Compact Array observations of the Fried Egg nebula

Evidence for large-scale asymmetric mass-loss from the yellow hypergiant IRAS 17163-3907

¹ Department of Earth and Space SciencesChalmers University of Technology, Onsala Space Observatory, 439-92 Onsala, Sweden
e-mail: swallstrom@asiaa.sinica.edu.tw
² Laboratoire Lagrange, UMR 7293, Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, Boulevard de l’Observatoire, 06304 Nice Cedex 4, France
³ Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse, France
⁴ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse, France
⁵ Instituto de Radioastronomía y Astrofísica, UNAM, A.P. 3-72, Xangari, 58089 Morelia, Mexico
⁶ Astrophysics Research Institute, Liverpool John Moores University, Liverpool L3 5RF, UK
⁷ School of Physics and Astronomy, The University of Leeds, Leeds, LS2 9JT, UK
⁸ Instituto de Ciencia de Materiales de Madrid, CSIC, C/Sor Juana Inés de la Cruz 3, 28049 Cantoblanco, Spain
⁹ Nicolaus Copernicus Astronomical Center, Rabianska 8, 87-100 Torun, Poland
¹⁰ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Heverlee, Belgium
¹¹ Jodrell Bank Centre for Astrophysics, Alan Turing Building, Manchester M13 9PL, UK

Received: 1 March 2016
Accepted: 2 December 2016

Abstract

Yellow hypergiants are rare and represent a fast evolutionary stage of massive evolved stars. That evolutionary phase is characterised by a very intense mass loss, the understanding of which is still very limited. Here we report ALMA Compact Array observations of a 50′′-mosaic toward the Fried Egg nebula, around one of the few Galactic yellow hypergiants IRAS 17163-3907. The emission from the ¹²CO J = 2–1 line, H30α recombination line, and continuum is imaged at a resolution of ~8′′, revealing the morphology of the molecular environment around the star. The continuum emission is unresolved and peaks at the position of the star. The radio recombination line H30α shows unresolved emission at the star, with an approximately Gaussian spectrum centered on a velocity of 21 ± 3km s^-1 with a width of 57 ± 6km s^-1. In contrast, the CO 2–1 emission is complex and decomposes into several components beyond the contamination from interstellar gas in the line of sight. The CO spectrum toward the star is a broad plateau, centered at the systemic velocity of +18 km s^-1 and with an expansion velocity of 100 ± 10km s^-1. Assuming isotropic and constant mass-loss, we estimate a mass-loss rate of 8 ± 1.5 × 10^-5M_⊙ yr^-1. At a radius of 25′′ from the star, we detect CO emission associated with the dust ring previously imaged by Herschel. The kinematics of this ring, however, is not consistent with an expanding shell, but show a velocity gradient of v_sys ± 20km s^-1. In addition, we find a puzzling bright feature radially connecting the star to the CO ring, at a velocity of +40 km s^-1 relative to the star. This spur feature may trace a unidirectional ejection event from the star. Our ACA observations reveal the complex morphology around IRAS 17163 and illustrate the breakthroughs that ALMA will bring to the field of massive stellar evolution.