Circumstellar masers in the Magellanic Clouds

¹ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
² UMIST, PO Box 88, Manchester M60 1QD, UK
³ Observatorio Astronómico Nacional, Campus Universitario, Apartado 1143, 28800 Alcalá de Henares, Spain
⁴ European Southern Observatory, Casilla 19001, Santiago 19, Chile
⁵ Onsala Space Observatory, 439 92 Onsala, Sweden

Corresponding author: Jacco van Loon, jacco@ast.cam.ac.uk

Received: 9 October 2000
Accepted: 3 January 2001

Abstract

Results are presented of a search for 22 GHz H₂O , 43 GHz SiO, 86 GHz SiO and 129 GHz SiO maser emission from bright IRAS point sources in the Small and Large Magellanic Clouds -mostly circumstellar envelopes around obscured red supergiants and Asymptotic Giant Branch stars (OH/IR stars). The aim of this effort was to test whether the kinematics of the mass loss from these stars depends on metallicity. H₂O maser emission was detected in the red supergiants IRAS 04553-6825 and IRAS 05280-6910, and tentatively in the luminous IR object IRAS 05216-6753 and the AGB star IRAS 05329-6708, all in the LMC. SiO maser emission was detected in IRAS 04553-6825. The double-peaked H₂O maser line profiles of IRAS 04553-6825 and IRAS 05280-6910, in combination with the OH (and SiO) maser line profiles, yield the acceleration of the outflows from these stars. The outflow velocity increases between the H₂O masing zone near the dust-formation region and the more distant OH masing zone from to 26 km s^-1 for IRAS 04553-6825 and from to 17 km s^-1 for IRAS 05280-6910. The total sample of LMC targets is analysed in comparison with circumstellar masers in the Galactic Centre. The photon fluxes of circumstellar masers in the LMC are found to be very similar to those in the Galactic Centre. The expansion velocities in the LMC appear to be ~20% lower than for similarly bright OH masers in the Galactic Centre, but the data are still consistent with no difference in expansion velocity. OH/IR stars in the LMC appear to have slower accelerating envelopes than OH/IR stars in the Galactic Centre. The masers in the LMC have blue-asymmetric emission profiles. This may be due to the amplification of stellar and/or free-free radiation, rather than the amplification of dust emission, and may be more pronounced in low metallicity envelopes. The SiO maser strength increases with the photometric amplitude at 2.2 μm but is independent of the photometric amplitude at 10 μm. This suggests a strong connection between shocks in the dust-free SiO masing zone and the dust formation process. The LMC masers obey the same trend as the Galactic Centre masers. Appendices describe H₂O maser emission from the moderately mass-losing AGB star R Dor in the Milky Way, optical echelle spectroscopy of IRAS 04553-6825, and the properties of circumstellar masers in the Galactic Centre.