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• Target type: UCHII
  Observations category: SF.general
  Facility type: interferometer
  Transitions: OH_1665,OH_1667
  

• Abstract

  We present measurements of the distribution of the OH masers at 1665 and 1667MHz towards the cometary ultracompact Hii region in the complex G34.3+0.2. The results are based on observations made in both senses of circular polarization with a very long baseline interferometry (VLBI) array having an angular resolution of 5×20mas2. 38 maser features are identified in the region. 33 of these lie on an arc at the edge of the cometary Hii region. Five are located in a cluster offset toward the north-east by 3arcsec, and are probably associated with an independent ultracompact Hii region. There is a velocity gradient of 30kms-1pc-1 across the arc. We identify five Zeeman pairs and determine that the magnetic field varies between 1 and 7mG, but is always directed away from the Earth. The OH masers may arise in clumps in a shell of gas in a bow shock caused by the motion of the exciting star through the molecular cloud. The stand-off distance and the thickness of the shocked shell are roughly consistent with those predicted by such a bow-shock model. Also, the position of the exciting star(s), as estimated from the focus of the parabolic bow shock, closely matches that of the peak emission from the cometary Hii region. However, the north-south velocity gradient in the ionized material remains difficult to explain in the context of the bow-shock model.

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ID	Obs. RA	Obs. Dec	RA offset	Dec offset	l	b	Group id	grpv	#	Source name	Image	Line	Frequency	Vpeak	Fpeak	Telescope	Vstep	RMS	Beam	PA	Detection	Class	Dist. (parallax)	Date
	(J2000)	(J2000)	(mas)	(mas)	(deg.)	(deg.)						OH	(MHz)	(km/s)	(Jy)		(km/s)	(Jy/beam)	(arcsec)	(deg.)	Yes/No		(kpc)
1	+18 53 18.646	+01 14 58.1152			34.257415	0.153098	G34.257+0.153	G34.2575+0.1531		G34.3+0.2	Image	1665 MHz;	1665	55.3 .. 63.1	0.7 .. 27	NRAOVLA	0.2	0.100	0.02 x 0.005	-9	+H2O +CH3OH I +CH3OH II +OH	SFR_MIXED		1987 Feb 28
	+18 53 18.8049	+01 14 57.5465	2381.9 mas	-567.1 mas	34.257577	0.152437			1	G34.3+0.2			1665	55.3	1
	+18 53 18.6472	+01 14 58.2166	16.3 mas	103 mas	34.257443	0.153106			2	G34.3+0.2			1665	58.1	1.4
	+18 53 18.6482	+01 14 58.2752	31.8 mas	161.6 mas	34.257459	0.153110			3	G34.3+0.2			1665	58.3	5.4
	+18 53 20.0712	+01 15 0.5856	21376.9 mas	2472 mas	34.260734	0.148127			4	G34.3+0.2			1665	58.5	2.6
	+18 53 18.827	+01 14 56.5508	2713.3 mas	-1562.8 mas	34.257372	0.152229			5	G34.3+0.2			1665	58.7	1
	+18 53 18.6495	+01 14 58.2394	51 mas	125.8 mas	34.257453	0.153101			6	G34.3+0.2			1665	58.7	10
	+18 53 18.648	+01 14 58.2221	28.5 mas	108.5 mas	34.257446	0.153104			7	G34.3+0.2			1665	59.1	1.3
	+18 53 18.6466	+01 14 58.1314	7.6 mas	17.8 mas	34.257421	0.153098			8	G34.3+0.2			1665	59.3	2.1
	+18 53 18.6837	+01 14 59.7688	564.8 mas	1655.2 mas	34.257896	0.153167			9	G34.3+0.2			1665	59.5	10.6
	+18 53 18.6742	+01 15 0.0995	421.6 mas	1985.9 mas	34.257960	0.153245			10	G34.3+0.2			1665	59.9	5
	+18 53 18.6871	+01 15 0.1546	615.2 mas	2041 mas	34.257998	0.153204			11	G34.3+0.2			1665	56.1	27
	+18 53 18.6481	+01 14 58.231	29.7 mas	117.4 mas	34.257448	0.153105			12	G34.3+0.2			1665	58.1	0.9
	+18 53 18.6503	+01 14 58.1765	63.2 mas	62.9 mas	34.257439	0.153090			13	G34.3+0.2			1665	58.5	4.4
	+18 53 18.6731	+01 15 0.2884	405.9 mas	2174.8 mas	34.258004	0.153273			14	G34.3+0.2			1665	58.7	2.3
	+18 53 18.658	+01 14 58.0345	178.7 mas	-79.1 mas	34.257418	0.153043			15	G34.3+0.2			1665	59.1	1.4
	+18 53 18.6578	+01 14 58.1523	175.1 mas	38.7 mas	34.257447	0.153059			16	G34.3+0.2			1665	59.3	1.6
	+18 53 18.6858	+01 15 0.0558	595.1 mas	1942.2 mas	34.257971	0.153196			17	G34.3+0.2			1665	59.7	0.7
	+18 53 18.6499	+01 14 58.2812	57.3 mas	167.6 mas	34.257464	0.153104			18	G34.3+0.2			1665	61.5	2.2
	+18 53 20.0807	+01 15 0.422	21518.6 mas	2308.4 mas	34.260711	0.148071			19	G34.3+0.2			1665	62.9	0.8
	+18 53 20.0801	+01 15 0.796	21510.6 mas	2682.4 mas	34.260803	0.148121			20	G34.3+0.2			1665	63.1	1.6
2	+18 53 18.646	+01 14 58.1152			34.257415	0.153098	G34.257+0.153	G34.2575+0.1531		G34.3+0.2	Image	1667 MHz;	1667	57.5 .. 62.7	0.5 .. 60.6	NRAOVLA	0.2	0.100	0.02 x 0.005	-9	+H2O +CH3OH I +CH3OH II +OH	SFR_MIXED		1987 Feb 28
	+18 53 18.8316	+01 14 56.5616	2783.2 mas	-1552 mas	34.257384	0.152213			1	G34.3+0.2			1667	57.5	0.6
	+18 53 18.6625	+01 14 58.036	246.4 mas	-77.6 mas	34.257427	0.153027			2	G34.3+0.2			1667	57.5	0.5
	+18 53 18.6466	+01 14 58.2049	7.4 mas	91.3 mas	34.257439	0.153107			3	G34.3+0.2			1667	58.3	0.9
	+18 53 18.6455	+01 14 58.2272	-8.6 mas	113.6 mas	34.257442	0.153114			4	G34.3+0.2			1667	58.7	14.2
	+18 53 18.647	+01 14 58.1853	13.5 mas	71.7 mas	34.257435	0.153103			5	G34.3+0.2			1667	59.1	4.5
	+18 53 18.6462	+01 14 58.0868	1.4 mas	-26.8 mas	34.257409	0.153094			6	G34.3+0.2			1667	57.5	0.7
	+18 53 18.5924	+01 14 56.4776	-805.2 mas	-1636 mas	34.256909	0.153089			7	G34.3+0.2			1667	57.9	2
	+18 53 18.6465	+01 14 58.3013	5.8 mas	187.7 mas	34.257462	0.153120			8	G34.3+0.2			1667	58.1	1
	+18 53 18.6451	+01 14 57.9344	-14.9 mas	-179.2 mas	34.257369	0.153078			9	G34.3+0.2			1667	58.5	1.6
	+18 53 18.646	+01 14 58.1197	-0.9 mas	6.1 mas	34.257417	0.153098			10	G34.3+0.2			1667	58.91	60.6
	+18 53 18.6464	+01 14 58.1731	4.5 mas	59.5 mas	34.257430	0.153104			11	G34.3+0.2			1667	59.3	10.3
	+18 53 18.6465	+01 14 57.9948	6.5 mas	-118.8 mas	34.257387	0.153081			12	G34.3+0.2			1667	59.5	1.6
	+18 53 18.6464	+01 14 58.2998	5.2 mas	186.2 mas	34.257462	0.153120			13	G34.3+0.2			1667	59.9	1
	+18 53 18.6461	+01 14 58.1188	0.7 mas	5.2 mas	34.257417	0.153098			14	G34.3+0.2			1667	60.3	0.6
	+18 53 18.6466	+01 14 58.0043	8.2 mas	-109.3 mas	34.257389	0.153081			15	G34.3+0.2			1667	60.7	0.6
	+18 53 18.646	+01 14 58.2308	-0.5 mas	117.2 mas	34.257444	0.153112			16	G34.3+0.2			1667	60.9	0.8
	+18 53 18.6461	+01 14 58.1202	0.4 mas	6.6 mas	34.257417	0.153098			17	G34.3+0.2			1667	61.1	0.9
	+18 53 18.5417	+01 15 0.7477	-1565 mas	2634.1 mas	34.257868	0.153818			18	G34.3+0.2			1667	62.7	1.3