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BAR09 - The diversity of methanol maser morphologies from VLBI observations

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Short title: Bartkiewicz et al. (2009)
Authors: Bartkiewicz, A. Szymczak, M. van Langevelde, H. J. Richards, A. M. S. Pihlström, Y. M.
Journal: Astronomy and Astrophysics, Volume 502, Issue 1, 2009, pp.155-173
Bibcode:

Target type: Known_Class_II,YSO
Transitions: CH3OH_6(II)

Abstract
Context: The 6.7 GHz methanol maser marks an early stage of high-mass star formation, but the origin of this maser is currently a matter of debate. In particular it is unclear whether the maser emission arises in discs, outflows or behind shocks running into rotating molecular clouds. Aims: We investigated which structures the methanol masers trace in the environment of high-mass protostar candidates by observing a homogenous sample of methanol masers selected from Torun surveys. We also probed their origins by looking for associated H II regions and IR emission. Methods: We selected 30 methanol sources with improved position accuracies achieved using MERLIN and another 3 from the literature. We imaged 31 of these using the European VLBI Network's expanded array of telescopes with 5-cm (6-GHz) receivers. We used the VLA to search for 8.4 GHz radio continuum counterparts and inspected Spitzer GLIMPSE data at 3.6-8 mum from the archive. Results: High angular resolution images allowed us to analyze the morphology and kinematics of the methanol masers in great detail and verify their association with radio continuum and mid-infrared emission. A new class of ``ring-like'' methanol masers in star-forming regions appeared to be suprisingly common, 29% of the sample. Conclusions: The new morphology strongly suggests that methanol masers originate in the disc or torus around a proto- or a young massive star. However, the maser kinematics indicate the strong influence of outflow or infall. This suggests that they form at the interface between the disc/torus and a flow. This is also strongly supported by Spitzer results because the majority of the masers coincide with 4.5 mum emission to within less than 1´´. Only four masers are associated with the central parts of UC H II regions. This implies that 6.7 GHz methanol maser emission occurs before H II region observable at cm wavelengths is formed. Tables 1-3, 6 and Figs. 3, 6 are only available in electronic form at http://www.aanda.org eprintid: arXiv:0905.3469

Abstract

Context: The 6.7 GHz methanol maser marks an early stage of high-mass star formation, but the origin of this maser is currently a matter of debate. In particular it is unclear whether the maser emission arises in discs, outflows or behind shocks running into rotating molecular clouds.
Aims: We investigated which structures the methanol masers trace in the environment of high-mass protostar candidates by observing a homogenous sample of methanol masers selected from Torun surveys. We also probed their origins by looking for associated H II regions and IR emission.
Methods: We selected 30 methanol sources with improved position accuracies achieved using MERLIN and another 3 from the literature. We imaged 31 of these using the European VLBI Network's expanded array of telescopes with 5-cm (6-GHz) receivers. We used the VLA to search for 8.4 GHz radio continuum counterparts and inspected Spitzer GLIMPSE data at 3.6-8 mum from the archive.
Results: High angular resolution images allowed us to analyze the morphology and kinematics of the methanol masers in great detail and verify their association with radio continuum and mid-infrared emission. A new class of ``ring-like'' methanol masers in star-forming regions appeared to be suprisingly common, 29% of the sample.
Conclusions: The new morphology strongly suggests that methanol masers originate in the disc or torus around a proto- or a young massive star. However, the maser kinematics indicate the strong influence of outflow or infall. This suggests that they form at the interface between the disc/torus and a flow. This is also strongly supported by Spitzer results because the majority of the masers coincide with 4.5 mum emission to within less than 1´´. Only four masers are associated with the central parts of UC H II regions. This implies that 6.7 GHz methanol maser emission occurs before H II region observable at cm wavelengths is formed. Tables 1-3, 6 and Figs. 3, 6 are only available in electronic form at eprintid:

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Detection table from Bartkiewicz et al. (2009)

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Obs. RA	Obs. Dec	RA offset	Dec offset	#	Source name	V_peak	F_peak
(J2000)	(J2000)	(mas)	(mas)			(km/s)	(Jy)

Non-detection table from Bartkiewicz et al. (2009)