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CAS97 - Coincidence of maser emission from OH at 6.035 GHz and methanol at 6.668 GHz

Short title: Caswell (1997)
Authors: Caswell, J. L.
Journal: Monthly Notices of the Royal Astronomical Society, Volume 289, Issue 1, pp. 203-224.
Bibcode:

Non-detection table: Show non-detection table
Detection table: Show detection table (40 detection)
Target type: Known_Class_II
Transitions: OH_6035,CH3OH_6(II)

Abstract
Massive newly formed stars are obscured by dust, and their clearest signature is often at radio frequencies from strong molecular maser emission. In some cases, both hydroxyl and methanol masers are present. Maser position measurements of both species are then especially useful for exploring the immediate surroundings of the new-born star. The present study used the Australia Telescope Compact Array to observe 6.035-GHz OH maser emission in 35 fields 30 of these were also observed at the 6.668-GHz methanol transition. In many of the fields the masers were found to be grouped into several distinct maser sites, allowing a total of 62 sites to be studied. Relative positions at 6.035 and 6.668GHz could be registered to an accuracy of 0.1arcsec. 29 sites harbour both 6.035-GHz OH and 6.668-GHz methanol, and most of these have sufficient maser features to reveal that the distributions of OH and methanol are largely co-extensive, with total extents between 10 and 30mpc (milliparsec). Elongated morphologies are common in both the OH and methanol distributions. The new-born star associated with each group of masers is not visible, but is expected to generate an enveloping ultracompact H II region. Such H II regions have been detected (as compact radio sources, mostly by searching continuum images made from the present observations) at 23 of the present maser sites. For these, the projected distribution of the masers was mapped relative to the H II region. In most cases, the masers are confined to a zone that covers only a portion of the H II region, and is commonly offset from the centre of the H II region, but rarely lies at its projected boundary. The masing zone can be plausibly identified as centred on the (unseen) principal exciting star of the H II region, with the maser distribution corresponding to a torus or ring, in some cases small enough to suggest that it may be embedded within the H II region. The fact that the typical extent is confined to less than 30mpc can be used in some cases to establish distances once the angular extent has been measured. The sample of maser sites studied here is OH-selected and thus biased towards sites with strong OH emission. The high rate of strong H II region detections among them indicates a marked trend (but not a close correlation) for H II regions to be more prominent towards OH-favoured maser sites than methanol-favoured sites.

Abstract

Massive newly formed stars are obscured by dust, and their clearest signature is often at radio frequencies from strong molecular maser emission. In some cases, both hydroxyl and methanol masers are present. Maser position measurements of both species are then especially useful for exploring the immediate surroundings of the new-born star. The present study used the Australia Telescope Compact Array to observe 6.035-GHz OH maser emission in 35 fields 30 of these were also observed at the 6.668-GHz methanol transition. In many of the fields the masers were found to be grouped into several distinct maser sites, allowing a total of 62 sites to be studied. Relative positions at 6.035 and 6.668GHz could be registered to an accuracy of 0.1arcsec. 29 sites harbour both 6.035-GHz OH and 6.668-GHz methanol, and most of these have sufficient maser features to reveal that the distributions of OH and methanol are largely co-extensive, with total extents between 10 and 30mpc (milliparsec). Elongated morphologies are common in both the OH and methanol distributions. The new-born star associated with each group of masers is not visible, but is expected to generate an enveloping ultracompact H II region. Such H II regions have been detected (as compact radio sources, mostly by searching continuum images made from the present observations) at 23 of the present maser sites. For these, the projected distribution of the masers was mapped relative to the H II region. In most cases, the masers are confined to a zone that covers only a portion of the H II region, and is commonly offset from the centre of the H II region, but rarely lies at its projected boundary. The masing zone can be plausibly identified as centred on the (unseen) principal exciting star of the H II region, with the maser distribution corresponding to a torus or ring, in some cases small enough to suggest that it may be embedded within the H II region. The fact that the typical extent is confined to less than 30mpc can be used in some cases to establish distances once the angular extent has been measured. The sample of maser sites studied here is OH-selected and thus biased towards sites with strong OH emission. The high rate of strong H II region detections among them indicates a marked trend (but not a close correlation) for H II regions to be more prominent towards OH-favoured maser sites than methanol-favoured sites.

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Detection table from Caswell (1997)

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ID	Obs. RA	Obs. Dec	l	b	Group id	Source name	Descr.	Frequency	V_peak	F_peak	Telescope	Detection	Class	Dist. (parallax)	Date
	(J2000)	(J2000)	(deg.)	(deg.)				(MHz)	(km/s)	(Jy)		Yes/No		(kpc)
1	07 44 52	-24 07 42.3	240.3158	0.0712	G240.316+0.071	240.316+0.071	Descr	6035	63.6	2.2	ATCA	+H₂O -CH₃OH II +OH	SFR		95jul
2	10 24 10.7	-57 52 34	284.3505	-0.4182	G284.351-0.418	284.351-0.418		6035	4.7	2.7	ATCA	+H₂O -CH₃OH I +CH₃OH II +OH	SFR		95jul.94may
3	10 31 29.9	-58 02 18.5	285.2634	-0.0500	G285.263-0.050	285.263-0.050	Descr	6035	9.3	2.1	ATCA	+H₂O +CH₃OH II +OH	SFR_MIXED		94may
4	11 35 32.2	-63 14 43	294.5114	-1.6212	G294.512-1.622	294.511-1.621		6035	-12	7.6	ATCA	+H₂O -CH₃OH I +CH₃OH II +OH	SFR		95jul
5	12 34 53.3	-61 39 39.9	300.9688	1.1476	G300.969+1.148	300.969+1.148		6035	-37.8	4.7	ATCA	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		93nov
6	13 11 16.9	-62 45 54.7	305.2000	0.0191	G305.200+0.019	305.200+0.019		6035	-33.2	7.2	ATCA	+H₂O -CH₃OH I +CH₃OH II +OH	SFR		93nov
7	13 50 41.8	-61 35 10.1	309.9205	0.4789	G309.921+0.479	309.921+0.479	Descr	6035	-59.8	17	ATCA	-H₂O +CH₃OH II +OH	SFR		95jul
8	14 06 18.4	-62 00 15.3	311.5955	-0.3984	G311.596-0.398	311.596-0.398	Descr	6035	31.4	0.7	ATCA	+OH	SFR		95jul
9	14 06 38.7	-61 58 23.1	311.6427	-0.3800	G311.643-0.380	311.643-0.380	Descr	6035	33.8	0.5	ATCA	+H₂O +CH₃OH II +OH	SFR_MIXED		95jul
10	15 29 19.3	-56 31 21.3	323.4591	-0.0785	G323.459-0.079	323.459-0.079	Descr	6035	-70.2	23.6	ATCA	-H₂O +CH₃OH II +OH	SFR		94may
11	15 54 6.4	-53 11 41.1	328.3070	0.4303	G328.307+0.430	328.307+0.430		6035	-90.4	2.4	ATCA	+H₂O -CH₃OH II +OH	SFR_MIXED		94may
12	15 55 48.4	-52 43 6.7	328.8082	0.6331	G328.809+0.633	328.808+0.633		6035	-45.7	8.9	ATCA	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		94may
13	15 55 48.7	-52 43 5.5	328.8091	0.6328	G328.809+0.633	328.809+0.633		6035	-45	1.3	ATCA	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		94may
14	16 12 9.8	-51 28 37.8	331.5114	-0.1023	G331.512-0.103	331.511-0.102	Descr	6035	-90	3	ATCA	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		94may
15	16 12 9.9	-51 28 37.1	331.5118	-0.1025	G331.512-0.103	331.512-0.103	Descr	6035	-86.8	1.5	ATCA	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		94may
16	16 12 09	-51 25 47.7	331.5423	-0.0665	G331.543-0.066	331.542-0.066		6035	-86	5.1	ATCA	-H₂O +CH₃OH I +CH₃OH II +OH	SFR		94may
17	16 21 2.9	-50 35 10	333.1353	-0.4315	G333.127-0.434	333.135-0.431		6035	-51.4	7.3	ATCA	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		93nov
18	16 21 2.8	-50 35 12	333.1347	-0.4317	G333.127-0.434	333.135-0.431s		6035	-50	1.1	ATCA	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		93nov
19	16 22 11	-50 05 56.5	333.6076	-0.2147	G333.607-0.215	333.608-0.215		6035	-51.6	3.1	ATCA	+H₂O +CH₃OH I +OH	SFR		94may
20	16 38 9.5	-47 04 59.9	337.6125	-0.0597	G337.613-0.060	337.613-0.060		6035	-42	2.4	ATCA	+H₂O +CH₃OH II +OH	SFR		94may
21	16 38 29.7	-47 00 35.8	337.7053	-0.0532	G337.705-0.053	337.705-0.053		6035	-50.6	1.1	ATCA	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		95jul
22	16 38 9.1	-46 11 3.1	338.2803	0.5424	G338.280+0.542	338.280+0.542		6035	-56.8	2.3	ATCA	+H₂O +CH₃OH II +OH	SFR		94may
23	16 50 14.8	-44 42 26.7	340.7849	-0.0960	G340.785-0.096	340.785-0.096	Descr	6035	-102	7.5	ATCA	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		94may
24	17 00 10.9	-42 07 19.4	343.9294	0.1249	G343.929+0.125	343.929+0.125		6035	13.6	4.2	ATCA	-H₂O +CH₃OH II +OH	SFR		94may
25	17 05 11.2	-41 29 07	345.0034	-0.2244	G345.003-0.223	345.003-0.224		6035	-26.2	3.2	ATCA	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		94may
26	16 56 47.6	-40 14 25.7	345.0095	1.7924	G345.010+1.792	345.010+1.792	Descr	6035	-21.5	1 1	ATCA	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		94may
27	17 06 50.6	-40 50 59.6	345.6982	-0.0897	G345.698-0.090	345.698-0.090	Descr	6035	-4.8	3.8	ATCA	+H₂O -CH₃OH I +CH₃OH II +OH	SFR		95jul
28	17 11 50.9	-39 09 29	347.6279	0.1486	G347.628+0.148	347.628+0.149		6035	-96.7	5	ATCA	+H₂O +CH₃OH II +OH	SFR		93nov
29	17 19 20.4	-39 03 51.6	348.5498	-0.9788	G348.550-0.979	348.550-0.979	Descr	6035	-13.1	1.4	ATCA	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		95jul
30	17 20 53.4	-35 47 1.2	351.4169	0.6453	G351.417+0.645	351.417+0.845		6035	-10.4	135	ATCA	+H₂O +CH₃OH I +CH₃OH II +OH	SFR_MIXED		95jul
31	17 25 25.1	-36 01 02	351.7428	-0.2432	G351.743-0.243	351.581-0.353	Descr	6035	-93.8	10	ATCA	+CH₃OH II +OH			93nov
32	17 26 42.6	-36 09 16	351.7750	-0.5362	G351.775-0.537	351.775-0.536	Descr	6035	-8.7	2	ATCA	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		94may
33	17 30 26.2	-34 41 45.7	353.4098	-0.3602	G353.410-0.360	353.410-0.360	Descr	6035	-20.7	31.5	ATCA	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		94may
34	17 31 15.5	-33 14 5.6	354.7243	0.3001	G354.724+0.300	354.724+0.300		6035	90.2	0.6	ATCA	+H₂O +CH₃OH II +OH	SFR		95jul
35	17 33 29.1	-32 47 58.8	355.3437	0.1472	G355.344+0.148	355.344+0.147		6035	18	3.4	ATCA	+H₂O -CH₃OH I +CH₃OH II +OH	SFR		95jul
36	17 43 25.6	-29 39 18.3	359.1372	0.0315	G359.137+0.031	359.138+0.031	Descr	6035	-1.8	1.3	ATCA	+H₂O -CH₃OH I +CH₃OH II +OH	SFR_MIXED		95jul
37	17 47 18.6	-28 22 54.6	0.6663	-0.0289	G0.6667-0.0344	0.666-0.029		6035	72.6	9.4	ATCA	+H₂O +CH₃OH I +CH₃OH II -OH	SFR		93nov
38	17 47 20.1	-28 23 6.2	0.6664	-0.0353	G0.6667-0.0344	0.666-0.035		6035	67	2.3	ATCA	+H₂O +CH₃OH I +CH₃OH II -OH	SFR		93nov
39	18 12 11.4	-18 41 29.1	11.9040	-0.1411	G11.904-0.141	11.904-0.141	Descr	6035	42.8	4.4	ATCA	+H₂O -CH₃OH I +CH₃OH II +OH	SFR_MIXED		95jul
40	18 20 24.8	-16 11 34.1	15.0347	-0.6770	G15.033-0.674	15.034-0.677	Descr	6035	21.2	22	ATCA	+H₂O +CH₃OH I +CH₃OH II +OH	GALAXY		95jul

Distribution of sources in all sky (l,b)

Distribution of sources in all sky using Aladin

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Distribution of sources in IRAS color-color diagram

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Original color diagram is taken form van der Veen & Habing (1987)

Type	Description
I	Oxygen-rich non-variable stars without circumstellar shells
II	Variable stars with "young" O-rich circumstellar shells
IIIa	Variable stars with more evolved O-rich circumstellar shells
IIIb	Variable stars with thick O-rich circumstellar shells
IV	Variable stars with very thick O-rich circumstellar shells
V	Planetary nebulae and non- variable stars with very cool circumstellar shells
VIa	Variable stars with relatively cold dust at large distances; a significant part of the stars is carbon-rich
VIb	Variable stars with relatively hot dust close to the star and relatively cold dust at large distances; Some of the objects have proven to be O-rich
VII	Variable stars with more evolved C-rich circumstellar shells
VIII	Different sort of objects

Distribution of the detection velocities versus galactic longitude from CAS97

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