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SLY09 - Structure of the class I methanol masers OMC-2 and NGC 2264

Interferometry data

Short title: Slysh & Kalenskii (2009)
Authors: Slysh, V. I. Kalenskii, S. V.
Journal: Astronomy Reports, Volume 53, Issue 6, pp.519-527
Bibcode:

Target type: High_mass_SFR
Transitions: OH_1665,SiO_1-0v0,CH3OH_9(I),CH3OH_23.4(I),CH3OH_25-30_GHz(I),CH3OH_36(I),CH3OH_44(I),CH3OH_95(I),CH3OH_132(I),CH3OH_6(II),CH3OH_23.1(II)

Abstract
Results of interferometric observations of the class I methanol masers OMC-2 and NGC 2264 in the 7₀-6₁ A ⁺ and 8₀-7₁ A ⁺ lines at 44 and 95 GHz, respectively, are presented. The maser spots are distributed along the arcs bent toward infrared sources, which are young stellar objects. The distributions of the maser spots at 44 and 95 GHz are virtually identical, and the fluxes from the brightest spots are similar. The measured sizes of the maser spots at 44 GHz are, on average, about 50 AU. The brightness temperature of the strongest components at 44 GHz is 1.7 � 10⁷ K and 3.9 � 10⁷ K for OMC-2 and NGC 2264, respectively. A simple model for the excitation of Class I methanol masers is proposed it yields an estimate of the limiting brightness temperature of the emission. The model is based solely on the properties of the methanol molecule without invoking the physical parameters of the medium. Using it, we showed that the emission opening angles for NGC 2264 and OMC-2 do not exceed 3� and 4.5�, respectively. The depth of the masing region is about 1000 AU. The emission directivity is naturally realized in the model of of maser consisting of a thermalized core and a thin inverted envelope, probably, with an enhanced methanol abundance. The maser emission has the greatest intensity in the direction tangential to the envelope. The size of the masing envelope estimated from the measured depth and spot extens is &tilde2 � 10⁴ AU, or 0.15 pc. This size is close to the sizes of the dense molecular cores surrounding the young stellar objects IRS 4 in OMC-2 and IRS 1 in NGC 2264.

Abstract

Results of interferometric observations of the class I methanol masers OMC-2 and NGC 2264 in the 7₀-6₁ A ⁺ and 8₀-7₁ A ⁺ lines at 44 and 95 GHz, respectively, are presented. The maser spots are distributed along the arcs bent toward infrared sources, which are young stellar objects. The distributions of the maser spots at 44 and 95 GHz are virtually identical, and the fluxes from the brightest spots are similar. The measured sizes of the maser spots at 44 GHz are, on average, about 50 AU. The brightness temperature of the strongest components at 44 GHz is 1.7 � 10⁷ K and 3.9 � 10⁷ K for OMC-2 and NGC 2264, respectively. A simple model for the excitation of Class I methanol masers is proposed it yields an estimate of the limiting brightness temperature of the emission. The model is based solely on the properties of the methanol molecule without invoking the physical parameters of the medium. Using it, we showed that the emission opening angles for NGC 2264 and OMC-2 do not exceed 3� and 4.5�, respectively. The depth of the masing region is about 1000 AU. The emission directivity is naturally realized in the model of of maser consisting of a thermalized core and a thin inverted envelope, probably, with an enhanced methanol abundance. The maser emission has the greatest intensity in the direction tangential to the envelope. The size of the masing envelope estimated from the measured depth and spot extens is &tilde2 � 10⁴ AU, or 0.15 pc. This size is close to the sizes of the dense molecular cores surrounding the young stellar objects IRS 4 in OMC-2 and IRS 1 in NGC 2264.

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Detection table from Slysh & Kalenskii (2009)

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ID	Obs. RA	Obs. Dec	RA offset	Dec offset	l	b	Group id	#	Maser spot	Source name	Image	Line	V_peak	F_peak	∫F dv	Telescope	Beam	Detection	Class	Dist. (parallax)	Date
	(J2000)	(J2000)	(mas)	(mas)	(deg.)	(deg.)						CH₃OH (class I)	(km/s)	(Jy)	(Jy*km/s)		(arcsec)	Yes/No		(kpc)
1	+05 35 27.813	-05 09 42.77			208.818107	-19.238905	G208.816-19.239			OMC-2	Image	44 GHz (I);	11.94	2	..	NRAOVLA	0.15	+H₂O +CH₃OH I +CH₃OH II	MIXED		October 1998
	+05 35 27.7109	-05 09 46.2016	-1530 mas	-3430 mas	208.818804	-19.239715		1	A	OMC-2			11.94	2

Non-detection table from Slysh & Kalenskii (2009)