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CAS01 - Maser emission from OH at the 6035-MHz transition

Short title: Caswell (2001)
Authors: Caswell, J. L.
Journal: Monthly Notices of the Royal Astronomical Society, Volume 326, Issue 2, pp. 805-820.
Bibcode:
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Target type: OH
Observations category: SF.general
Facility type: interferometer
Transitions: OH_6030,OH_6035

Abstract
The Australia Telescope Compact Array (ATCA) has been used to measure positions with subarcsec accuracy for more than 50 masers at the 6035-MHz transition of excited OH. Eleven of the masers are new discoveries; the remainder complete the detailed study of masers from an earlier single-dish survey of the southern Galactic plane using the Parkes telescope. All previously known masers in the region were strong enough to be studied in the present observations, despite the common occurrence of variability. Typically, the intensity varies by less than a factor of two over several years, but one of the new masers is, remarkably, 100 times stronger than the upper limit measured six years earlier. The precise positions confirm that most of the masers coincide with a 1665-MHz ground-state OH maser counterpart that is usually stronger. However, there are a few instances where any 1665-MHz counterpart is either much weaker or not detected. When surveyed with comparable sensitivity, we show that the 6035-MHz masers are approximately one-third as common as 1665-MHz masers. We also add to earlier evidence that 1720-MHz masers preferentially occur at the sites hosting 6035-MHz masers. These facts are discussed in the context of current maser pumping models. Towards most of the 6035-MHz maser sites we find a coincident 6668-MHz methanol maser, and towards some of the maser sites we report new detections of ultracompact Hii regions. Clusters of two or three sites, each showing one, or several, of the principal molecular masing transitions, are found to be common; future comparative studies of these sites within small clusters will elucidate the precise conditions favouring different maser species. Several instances of absorption occur at 6035MHz, one of them especially striking since it extends over a broad velocity range adjacent to maser emission related to the ultracompact Hii region 5.885-0.392. The absorption provides new clues to understanding the geometry and excitation of 5.885-0.392. Three sources were observed at 6030MHz, an OH transition from the same excited state (²Î�_3/2, J=5/2) as the 6035-MHz line; in one case, the matching velocity patterns at 6030 and 6035MHz, their similar circular polarization characteristics, and their spatial coincidence, reveal Zeeman splitting in a magnetic field of -9mG, a field strength close to the upper limit found for any such masers.

Abstract

The Australia Telescope Compact Array (ATCA) has been used to measure positions with subarcsec accuracy for more than 50 masers at the 6035-MHz transition of excited OH. Eleven of the masers are new discoveries; the remainder complete the detailed study of masers from an earlier single-dish survey of the southern Galactic plane using the Parkes telescope. All previously known masers in the region were strong enough to be studied in the present observations, despite the common occurrence of variability. Typically, the intensity varies by less than a factor of two over several years, but one of the new masers is, remarkably, 100 times stronger than the upper limit measured six years earlier. The precise positions confirm that most of the masers coincide with a 1665-MHz ground-state OH maser counterpart that is usually stronger. However, there are a few instances where any 1665-MHz counterpart is either much weaker or not detected. When surveyed with comparable sensitivity, we show that the 6035-MHz masers are approximately one-third as common as 1665-MHz masers. We also add to earlier evidence that 1720-MHz masers preferentially occur at the sites hosting 6035-MHz masers. These facts are discussed in the context of current maser pumping models. Towards most of the 6035-MHz maser sites we find a coincident 6668-MHz methanol maser, and towards some of the maser sites we report new detections of ultracompact Hii regions. Clusters of two or three sites, each showing one, or several, of the principal molecular masing transitions, are found to be common; future comparative studies of these sites within small clusters will elucidate the precise conditions favouring different maser species. Several instances of absorption occur at 6035MHz, one of them especially striking since it extends over a broad velocity range adjacent to maser emission related to the ultracompact Hii region 5.885-0.392. The absorption provides new clues to understanding the geometry and excitation of 5.885-0.392. Three sources were observed at 6030MHz, an OH transition from the same excited state (²Î�_3/2, J=5/2) as the 6035-MHz line; in one case, the matching velocity patterns at 6030 and 6035MHz, their similar circular polarization characteristics, and their spatial coincidence, reveal Zeeman splitting in a magnetic field of -9mG, a field strength close to the upper limit found for any such masers.

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ID	Obs. RA	Obs. Dec	l	b	Group id	grpv	Image	Descr.	Line	Frequency	V_peak	F_peak	Telescope	Configuration	V_step	RMS	Beam	Detection	Class	Dist. (parallax)	Date
	(J2000)	(J2000)	(deg.)	(deg.)					OH	(MHz)	(km/s)	(Jy)			(km/s)	(mJy)	(arcsec)	Yes/No		(kpc)
1	13 21 23	-63 00 29.3	306.3221	-0.3337	G306.322-0.334	G306.3221-0.3337	Image		6035 MHz;	6035	-23.5	0.14	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH II +OH	SFR_MIXED		99feb
	13 21 23	-63 00 29.3	306.3221	-0.3337	G306.322-0.334	G306.3221-0.3337	Image		6035 MHz;	6035	-23.5	0.14	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH II +OH	SFR_MIXED		99feb
3	13 50 41.8	-61 35 10.1	309.9205	0.4789	G309.921+0.479	G309.9205+0.4789	Image		6035 MHz;	6035	-59.7	11.8	ATCA	6-km	0.2	3.000	2	-H₂O +CH₃OH II +OH	SFR		99feb
	13 50 41.8	-61 35 10.1	309.9205	0.4789	G309.921+0.479	G309.9205+0.4789	Image		6035 MHz;	6035	-59.7	11.8	ATCA	6-km	0.2	3.000	2	-H₂O +CH₃OH II +OH	SFR		99feb
5	13 50 41.8	-61 35 10.1	309.9206	0.4789	G309.921+0.479	G309.9205+0.4789	Image		6030;	6031	-58.1	2.6	ATCA	6-km	0.2	3.000	2	-H₂O +CH₃OH II +OH	SFR		99feb
6	14 44 56.2	-59 48 0.7	316.7624	-0.0115	G316.763-0.012	G316.7627-0.0117	Image	Descr	6035 MHz;	6035	-37.5	0.19	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +OH	SFR		98nov
	14 44 56.2	-59 48 0.7	316.7624	-0.0115	G316.763-0.012	G316.7627-0.0117	Image	Descr	6035 MHz;	6035	-37.5	0.19	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +OH	SFR		98nov
8	15 03 17.4	-58 36 13.3	319.3982	-0.0123	G319.398-0.012	G319.3983-0.0120	Image	Descr	6035 MHz;	6035	-12.7	0.15	ATCA	6-km	0.2	3.000	2	+H₂O -CH₃OH II +OH	SFR		98jun/nov
	15 03 17.4	-58 36 13.3	319.3982	-0.0123	G319.398-0.012	G319.3983-0.0120	Image	Descr	6035 MHz;	6035	-12.7	0.15	ATCA	6-km	0.2	3.000	2	+H₂O -CH₃OH II +OH	SFR		98jun/nov
10	15 57 58.3	-53 59 23.2	328.2365	-0.5474	G328.237-0.547	G328.2366-0.5475	Image	Descr	6035 MHz;	6035	-41.0	0.24	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		98nov
	15 57 58.3	-53 59 23.2	328.2365	-0.5474	G328.237-0.547	G328.2366-0.5475	Image	Descr	6035 MHz;	6035	-41.0	0.24	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		98nov
12	16 01 10	-53 16 2.3	329.0664	-0.3076	G329.066-0.308	G329.0663-0.3076	Image	Descr	6035 MHz;	6035	-42.7	0.3	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		98jun/99oct
	16 01 10	-53 16 2.3	329.0664	-0.3076	G329.066-0.308	G329.0663-0.3076	Image	Descr	6035 MHz;	6035	-42.7	0.3	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		98jun/99oct
14	16 00 33.2	-52 44 39.9	329.3389	0.1481	G329.339+0.148	G329.3389+0.1481	Image		6035 MHz;	6035	-104.2	0.4	ATCA	6-km	0.2	3.000	2	-H₂O -CH₃OH I +CH₃OH II +OH	SFR		00jun
	16 00 33.2	-52 44 39.9	329.3389	0.1481	G329.339+0.148	G329.3389+0.1481	Image		6035 MHz;	6035	-104.2	0.4	ATCA	6-km	0.2	3.000	2	-H₂O -CH₃OH I +CH₃OH II +OH	SFR		00jun
16	16 03 32.1	-53 09 29.9	329.4052	-0.4592	G329.405-0.459	G329.4051-0.4593	Image		6035 MHz;	6035	-68.5	0.15	ATCA	6-km	0.2	3.000	2	+H₂O -CH₃OH I +CH₃OH II +OH	SFR		98nov
	16 03 32.1	-53 09 29.9	329.4052	-0.4592	G329.405-0.459	G329.4051-0.4593	Image		6035 MHz;	6035	-68.5	0.15	ATCA	6-km	0.2	3.000	2	+H₂O -CH₃OH I +CH₃OH II +OH	SFR		98nov
18	16 09 52.4	-51 54 57.6	330.9528	-0.1823	G330.953-0.182	G330.9530-0.1822	Image	Descr	6035 MHz;	6035	-87.9	0.46	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		98nov/99may
	16 09 52.4	-51 54 57.6	330.9528	-0.1823	G330.953-0.182	G330.9530-0.1822	Image	Descr	6035 MHz;	6035	-87.9	0.46	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		98nov/99may
20	16 20 11	-50 53 12.7	332.8263	-0.5486	G332.826-0.549	G332.8258-0.5486	Image	Descr	6035 MHz;	6035	-54.9	1.1	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH II +OH	SFR		99may
	16 20 11	-50 53 12.7	332.8263	-0.5486	G332.826-0.549	G332.8258-0.5486	Image	Descr	6035 MHz;	6035	-54.9	1.1	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH II +OH	SFR		99may
22	16 33 29.2	-48 03 43.7	336.3583	-0.1374	G336.358-0.137	G336.3583-0.1374	Image	Descr	6035 MHz;	6035	-75.1	0.3	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH II +OH	SFR_MIXED		99feb
	16 33 29.2	-48 03 43.7	336.3583	-0.1374	G336.358-0.137	G336.3583-0.1374	Image	Descr	6035 MHz;	6035	-75.1	0.3	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH II +OH	SFR_MIXED		99feb
24	16 34 38.3	-47 36 32.9	336.8223	0.0278	G336.822+0.028	G336.8223+0.0279	Image	Descr	6035 MHz;	6035	-77.4	0.55	ATCA	6-km	0.2	3.000	2	-H₂O +CH₃OH II +OH	SFR		98nov
	16 34 38.3	-47 36 32.9	336.8223	0.0278	G336.822+0.028	G336.8223+0.0279	Image	Descr	6035 MHz;	6035	-77.4	0.55	ATCA	6-km	0.2	3.000	2	-H₂O +CH₃OH II +OH	SFR		98nov
26	16 35 55.2	-47 38 45.4	336.9410	-0.1563	G336.941-0.156	G336.9410-0.1563	Image	Descr	6035 MHz;	6035	-65.7	1.27	ATCA	6-km	0.2	3.000	2	-H₂O +CH₃OH II +OH	UNK		99may/oct
	16 35 55.2	-47 38 45.4	336.9410	-0.1563	G336.941-0.156	G336.9410-0.1563	Image	Descr	6035 MHz;	6035	-65.7	1.27	ATCA	6-km	0.2	3.000	2	-H₂O +CH₃OH II +OH	UNK		99may/oct
28	16 36 12.4	-47 37 57.8	336.9833	-0.1831	G336.983-0.183	G336.9833-0.1830	Image	Descr	6035 MHz;	6035	-76.2	0.23	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH II +OH	SFR		99may/oct
	16 36 12.4	-47 37 57.8	336.9833	-0.1831	G336.983-0.183	G336.9833-0.1830	Image	Descr	6035 MHz;	6035	-76.2	0.23	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH II +OH	SFR		99may/oct
30	16 38 50.5	-47 28 3.2	337.4037	-0.4024	G337.404-0.402	G337.4039-0.4022	Image		6035 MHz;	6035	-35.6	0.67	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		99feb
	16 38 50.5	-47 28 3.2	337.4037	-0.4024	G337.404-0.402	G337.4039-0.4022	Image		6035 MHz;	6035	-35.6	0.67	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		99feb
32	16 39 39	-46 41 28.3	338.0747	0.0118	G338.075+0.012	G338.0748+0.0118	Image	Descr	6035 MHz;	6035	-48.7	0.35	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH II +OH	SFR_MIXED		99feb
	16 39 39	-46 41 28.3	338.0747	0.0118	G338.075+0.012	G338.0748+0.0118	Image	Descr	6035 MHz;	6035	-48.7	0.35	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH II +OH	SFR_MIXED		99feb
34	16 40 33.5	-45 41 37.2	338.9250	0.5565	G338.925+0.556	G338.9251+0.5565	Image	Descr	6035 MHz;	6035	-63.0	0.32	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR_MIXED		98nov
	16 40 33.5	-45 41 37.2	338.9250	0.5565	G338.925+0.556	G338.9251+0.5565	Image	Descr	6035 MHz;	6035	-63.0	0.32	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR_MIXED		98nov
36	16 44 49	-46 10 13.1	339.0530	-0.3146	G339.053-0.315	G339.0530-0.3146	Image		6035 MHz;	6035	-111.3	0.42	ATCA	6-km	0.2	3.000	2	-H₂O +CH₃OH II +OH	SFR		98jun/99oct
	16 44 49	-46 10 13.1	339.0530	-0.3146	G339.053-0.315	G339.0530-0.3146	Image		6035 MHz;	6035	-111.3	0.42	ATCA	6-km	0.2	3.000	2	-H₂O +CH₃OH II +OH	SFR		98jun/99oct
38	16 43 43.1	-45 42 08	339.2822	0.1359	G339.282+0.136	G339.2822+0.1359	Image	Descr	6035 MHz;	6035	-70.8	0.52	ATCA	6-km	0.2	3.000	2	-H₂O +CH₃OH II +OH	SFR		98jun/99oct
	16 43 43.1	-45 42 08	339.2822	0.1359	G339.282+0.136	G339.2822+0.1359	Image	Descr	6035 MHz;	6035	-70.8	0.52	ATCA	6-km	0.2	3.000	2	-H₂O +CH₃OH II +OH	SFR		98jun/99oct
40	16 46 06	-45 36 44.1	339.6219	-0.1206	G339.622-0.121	G339.6220-0.1206	Image	Descr	6035 MHz;	6035	-30.6	0.64	ATCA	6-km	0.2	3.000	2	+H₂O -CH₃OH I +CH₃OH II +OH	SFR		98nov
	16 46 06	-45 36 44.1	339.6219	-0.1206	G339.622-0.121	G339.6220-0.1206	Image	Descr	6035 MHz;	6035	-30.6	0.64	ATCA	6-km	0.2	3.000	2	+H₂O -CH₃OH I +CH₃OH II +OH	SFR		98nov
42	16 52 4.6	-46 08 34	339.8839	-1.2586	G339.884-1.259	G339.8840-1.2589	Image		6035 MHz;	6035	-37.4	25.5	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		00jun
	16 52 4.6	-46 08 34	339.8839	-1.2586	G339.884-1.259	G339.8840-1.2589	Image		6035 MHz;	6035	-37.4	25.5	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		00jun
44	16 52 4.6	-46 08 34	339.8839	-1.2586	G339.884-1.259	G339.8840-1.2589	Image	Descr	6030;	6031	-37.4	3.3	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		00jun
45	17 02 8.6	-41 47 10.2	344.4185	0.0436	G344.419+0.045	G344.4200+0.0444	Image		6035 MHz;	6035	-72.2	0.2	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH II +OH	SFR_MIXED		98nov/99oct
	17 02 8.6	-41 47 10.2	344.4185	0.0436	G344.419+0.045	G344.4200+0.0444	Image		6035 MHz;	6035	-72.2	0.2	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH II +OH	SFR_MIXED		98nov/99oct
47	17 09 35.4	-41 35 57.1	345.4068	-0.9519	G345.407-0.952	G345.4070-0.9519	Image	Descr	6035 MHz;	6035	-16.1	0.29	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH			98nov
	17 09 35.4	-41 35 57.1	345.4068	-0.9519	G345.407-0.952	G345.4070-0.9519	Image	Descr	6035 MHz;	6035	-16.1	0.29	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH			98nov
49	17 09 35.4	-41 35 55	345.4072	-0.9515	G345.407-0.952	G345.4070-0.9519	Image	Descr	6035 MHz;	6035	-26.4	0.26	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH			98nov
	17 09 35.4	-41 35 55	345.4072	-0.9515	G345.407-0.952	G345.4070-0.9519	Image	Descr	6035 MHz;	6035	-26.4	0.26	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH			98nov
51	17 04 28.1	-40 46 25.3	345.4874	0.3144	G345.487+0.314	G345.4874+0.3144	Image		6035 MHz;	6035	-22.1	3.9	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH II +OH	SFR		98nov
	17 04 28.1	-40 46 25.3	345.4874	0.3144	G345.487+0.314	G345.4874+0.3144	Image		6035 MHz;	6035	-22.1	3.9	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH II +OH	SFR		98nov
53	17 19 59	-38 58 13.5	348.6983	-1.0274	G348.704-1.038	G348.6980-1.0273	Image	Descr	6035 MHz;	6035	-16.3	0.55	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		98nov
	17 19 59	-38 58 13.5	348.6983	-1.0274	G348.704-1.038	G348.6980-1.0273	Image	Descr	6035 MHz;	6035	-16.3	0.55	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		98nov
55	17 19 25.6	-37 10 4.4	350.1131	0.0949	G350.110+0.085	G350.1131+0.0949	Image	Descr	6035 MHz;	6035	-72.1	0.26	ATCA	6-km	0.2	3.000	2	+H₂O -CH₃OH I +CH₃OH II +OH			99feb
	17 19 25.6	-37 10 4.4	350.1131	0.0949	G350.110+0.085	G350.1131+0.0949	Image	Descr	6035 MHz;	6035	-72.1	0.26	ATCA	6-km	0.2	3.000	2	+H₂O -CH₃OH I +CH₃OH II +OH			99feb
57	17 23 28.6	-37 01 48.5	350.6861	-0.4908	G350.686-0.491	G350.6861-0.4909	Image	Descr	6035 MHz;	6035	-15.1	1.09	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		98jun/99oct
	17 23 28.6	-37 01 48.5	350.6861	-0.4908	G350.686-0.491	G350.6861-0.4909	Image	Descr	6035 MHz;	6035	-15.1	1.09	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		98jun/99oct
59	17 30 26.2	-34 41 46	353.4097	-0.3602	G353.410-0.360	G353.4099-0.3602	Image		6035 MHz;	6035	-20.7	29	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		99feb
	17 30 26.2	-34 41 46	353.4097	-0.3602	G353.410-0.360	G353.4099-0.3602	Image		6035 MHz;	6035	-20.7	29	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		99feb
61	17 30 26.2	-34 41 45.5	353.4098	-0.3601	G353.410-0.360	G353.4099-0.3602	Image	Descr	6030;	6031	-22.5	15.5	ATCA	6-km	0.2	3.000	2	+H₂O +CH₃OH I +CH₃OH II +OH	SFR		99feb
62	17 54 38.7	-25 34 45	3.9103	0.0013	G3.910+0.001	G3.9104+0.0012	Image	Descr	6035 MHz;	6035	17.8	0.32	ATCA	6-km	0.2	3.000	2	+H₂O -CH₃OH I +CH₃OH II +OH	SFR		99feb
	17 54 38.7	-25 34 45	3.9103	0.0013	G3.910+0.001	G3.9104+0.0012	Image	Descr	6035 MHz;	6035	17.8	0.32	ATCA	6-km	0.2	3.000	2	+H₂O -CH₃OH I +CH₃OH II +OH	SFR		99feb
64	18 00 30.4	-24 04 3.1	5.8847	-0.3924	G5.885-0.392	G5.8852-0.3924	Image

Display data from paper

CAS01 - Maser emission from OH at the 6035-MHz transition

View as list of objects

Detection table from Caswell (2001)