Astronomy and Astrophysics, volume 368, 950-968 (2001/3-4)
Circumstellar masers in the Magellanic Clouds.
VAN LOON J.T., ZIJLSTRA A.A., BUJARRABAL V. and NYMAN L.-A.
Abstract (from CDS):
Result. are presented of a search for 22 GHz H2O 616->523, 43 GHz SiOv=1(J=1->0), 86 GHz SiOv=1(J=2->1) and 129 GHz SiOv=1(J=3->2) 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. SiOv=1(J=2->1) 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 v∼18 to 26km/s for IRAS 04553-6825 and from v∼6 to 17km/s 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 2.2µm but is independent of the photometric amplitude at 10µm. 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.
masers - circumstellar matter - stars: mass loss - stars: AGB and post-AGB - supergiants - Magellanic Clouds