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SUN18 - Discovery of H₂O, CH₃OH, and OH Masers in the Extreme Outer Galaxy

Short title: Sun et al. (2018)
Authors: Sun, YanXu, YeChen, XiFang, MinHenkel, ChristianYang, JiMenten, Karl M.Chen, Xue-PengJiang, Zhi-Bo
Journal: The Astrophysical Journal, Volume 869, Issue 2, article id. 148, 13 pp. (2018).
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Target type: CO_clumps
Observations category: SF.general
Facility type: single-dish
Transitions: OH_4765,CH3OH_6(II)

Abstract
We present the results of a single dish interstellar maser survey toward 200 molecular clouds (MCs) in the extreme outer Galaxy (EOG) possibly lying in the Outer Scutum-Centaurus (OSC) arm. The observations resulted in the detections of two new 22.2 GHz water masers, two new 6.7 GHz class II methanol masers, and one new 4765 MHz excited-state OH maser. These new detections include one high Galactic latitude (b>2) methanol maser, one farthest Galactic methanol maser, and one farthest Galactic OH maser from the Sun. The velocities of the masers and the CO emission are in agreement with each other, which strongly indicates an association between the maser and the EOG cloud. The presence of the masers provides reliable evidence for ongoing massive star-forming activity in the OSC arm. The EOG clouds with detected interstellar masers are all associated with young stellar objects. Masers are primarily detected toward the brighter and more massive MCs. Because of the large distances involved, the faint masers with luminosity below 10^-6 L _sun are not accessible by our current observations. The low number of masers in the EOG regions supports the scenario of decreasing metallicity and decreasing star formation rate/efficiency at large Galactocentric radii. Our new maser detections are valuable targets for future VLBI parallax and proper motion measurements.

Abstract

We present the results of a single dish interstellar maser survey toward 200 molecular clouds (MCs) in the extreme outer Galaxy (EOG) possibly lying in the Outer Scutum-Centaurus (OSC) arm. The observations resulted in the detections of two new 22.2 GHz water masers, two new 6.7 GHz class II methanol masers, and one new 4765 MHz excited-state OH maser. These new detections include one high Galactic latitude (b>2) methanol maser, one farthest Galactic methanol maser, and one farthest Galactic OH maser from the Sun. The velocities of the masers and the CO emission are in agreement with each other, which strongly indicates an association between the maser and the EOG cloud. The presence of the masers provides reliable evidence for ongoing massive star-forming activity in the OSC arm. The EOG clouds with detected interstellar masers are all associated with young stellar objects. Masers are primarily detected toward the brighter and more massive MCs. Because of the large distances involved, the faint masers with luminosity below 10^-6 L _sun are not accessible by our current observations. The low number of masers in the EOG regions supports the scenario of decreasing metallicity and decreasing star formation rate/efficiency at large Galactocentric radii. Our new maser detections are valuable targets for future VLBI parallax and proper motion measurements.

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Detection table from Sun et al. (2018)

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ID	Obs. RA	Obs. Dec	l	b	Group id	grpv	Source name	Image	Line	V_range	V_peak	F_peak	∫F dv	Telescope	RMS	Beam	Detection	Class	Distance	Dist. (parallax)	Date
	(J2000)	(J2000)	(deg.)	(deg.)					CH₃OH (class II)	(km/s)	(km/s)	(Jy)	(Jy*km/s)		(Jy)	(arcsec)	Yes/No		(kpc)	(kpc)
1	18 38 34.9	-03 32 4.8	28.3200	1.2430	G28.319+1.245	G28.3188+1.2446	28.32+1.24	Image	6.6 GHz (II);	-43.3;-41.2	-42.2	0.55	0.47	TMRT	0.058	40	+H₂O +CH₃OH II -OH	SFR	20.800		2017 June-August
2	18 57 14.3	+08 16 26.8	40.9583	2.4834	G40.956+2.476	G40.9583+2.4834	40.96+2.48	Image	6.6 GHz (II);	-63.3;-61.5	-62.4	0.27	0.15	TMRT	0.019	40	+H₂O +CH₃OH II -OH	SFR			2017 June-August

Non-detection table from Sun et al. (2018)