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PAN08 - Resolving distance ambiguities towards 6.7 GHz methanol masers

Short title: Pandian et al. (2008)
Authors: Pandian, J. D.Momjian, E.Goldsmith, P. F.
Journal: Astronomy and Astrophysics, Volume 486, Issue 1, 2008, pp.191-208
Bibcode:

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

Abstract
Context: Distances to most star forming regions are determined using kinematics, through the assumption that the observed radial velocity arises from the motion of the source with respect to the Sun resulting from the differential rotation of Galaxy. The primary challenge associated with the application of this technique in the inner Galaxy is the kinematic distance ambiguity. Aims: In this work, we aim to resolve the kinematic distance ambiguity towards a sample of 6.7 GHz methanol masers, which are signposts of the early stages of massive star formation. Methods: We measured 21 cm H I absorption spectra using the Very Large Array in C and CnB configurations. A comparison of the maximum velocity of H I absorption with the source velocity and tangent point velocity was used to resolve the kinematic distance ambiguity. Results: We resolved the distance ambiguity towards 41 sources. Distance determinations that are in conflict with previous measurements are discussed. The NE2001 spiral arm model is broadly consistent with the locations of the star forming complexes. We find that the use of vertical scale height arguments to resolve the distance ambiguity can lead to erroneous classifications for a significant fraction of sources.

Abstract

Context: Distances to most star forming regions are determined using kinematics, through the assumption that the observed radial velocity arises from the motion of the source with respect to the Sun resulting from the differential rotation of Galaxy. The primary challenge associated with the application of this technique in the inner Galaxy is the kinematic distance ambiguity.
Aims: In this work, we aim to resolve the kinematic distance ambiguity towards a sample of 6.7 GHz methanol masers, which are signposts of the early stages of massive star formation.
Methods: We measured 21 cm H I absorption spectra using the Very Large Array in C and CnB configurations. A comparison of the maximum velocity of H I absorption with the source velocity and tangent point velocity was used to resolve the kinematic distance ambiguity.
Results: We resolved the distance ambiguity towards 41 sources. Distance determinations that are in conflict with previous measurements are discussed. The NE2001 spiral arm model is broadly consistent with the locations of the star forming complexes. We find that the use of vertical scale height arguments to resolve the distance ambiguity can lead to erroneous classifications for a significant fraction of sources.

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Detection table from Pandian et al. (2008)

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ID	Obs. RA	Obs. Dec	l	b	Group id	Source name	Other name	Line	V_range	Detection	Class	Distance	Dist. (parallax)
	(J2000)	(J2000)	(deg.)	(deg.)			NVSS	CH₃OH (class II)	(km/s)	Yes/No		(kpc)	(kpc)
1	18 09 0.8	-20 04 56.7	10.3240	-0.1559	G10.322-0.148	10.32-0.15	180900-200456	6.6 GHz (II);	4;17	+H₂O +CH₃OH I +CH₃OH II	SFR_MIXED	3.400
2	18 08 35.7	-19 52 19	10.4604	0.0321	G10.472+0.027	10.47+0.02	180835-195219	6.6 GHz (II);	58;77	+H₂O +CH₃OH I +CH₃OH II +OH	SFR	10.800
3	18 10 28.4	-19 55 48	10.6232	-0.3824	G10.623-0.383	10.62-0.38	181028-195548	6.6 GHz (II);	-10;7	+H₂O +CH₃OH I +CH₃OH II +OH	SFR_MIXED	3.400
4	18 09 43	-19 26 29.5	10.9649	0.0095	G10.958+0.022	10.95+0.02	180942-192629	6.6 GHz (II);	23;25	+H₂O -CH₃OH I +CH₃OH II	SFR	14.300
5	18 09 39	-19 21 16.4	11.0335	0.0652	G11.034+0.062	11.03+0.06	180939-192116	6.6 GHz (II);	15;21	+H₂O -CH₃OH I +CH₃OH II +OH	SFR	15.000
6	18 14 1.1	-18 53 24.2	11.9371	-0.6161	G11.937-0.616	11.93-0.61	181401-185324	6.6 GHz (II);	30;44	+H₂O +CH₃OH I +CH₃OH II	SFR	4.000
7	18 12 45.5	-18 40 39.6	11.9806	-0.2525	G11.984-0.249	11.99-0.27	181245-184039	6.6 GHz (II);	60;61	-H₂O +CH₃OH II	SFR	11.500
8	18 12 42.9	-18 24 17.8	12.2149	-0.1125	G12.209-0.107	12.20-0.10A	181242-182417	6.6 GHz (II);	2;15	+H₂O +CH₃OH I +CH₃OH II +OH	SFR	13.600
9	18 12 42.9	-18 24 17.8	12.2149	-0.1125	G12.209-0.107	12.20-0.10B	181242-182417	6.6 GHz (II);		+H₂O +CH₃OH I +CH₃OH II +OH	SFR	11.800
10	18 13 55.6	-18 01 24.7	12.6875	-0.1823	G12.681-0.182	12.68-0.18	181355-180124	6.6 GHz (II);	50;62	+H₂O +CH₃OH I +CH₃OH II +OH	SFR	4.900
11	18 14 14.1	-17 55 43.8	12.8058	-0.2014	G12.797-0.193	12.79-0.19	181414-175543	6.6 GHz (II);	31;41	+H₂O +CH₃OH I +CH₃OH II	SFR	3.600
12	18 14 44.7	-17 52 23.6	12.9125	-0.2813	G12.908-0.260	12.90-0.26	181444-175223	6.6 GHz (II);	35;47	+H₂O +CH₃OH I +CH₃OH II +OH	SFR	13.600
13	18 14 5.3	-17 28 32.7	13.1869	0.0460	G13.179+0.060	13.18+0.06	181405-172832	6.6 GHz (II);	48;49	+H₂O +CH₃OH I +CH₃OH II	SFR	4.600
14	18 15 45.1	-16 38 58.7	14.1027	0.0909	G14.101+0.087	14.09+0.10	181545-163858	6.6 GHz (II);	4;17	+H₂O -CH₃OH I +CH₃OH II	SFR	14.800
15	18 23 35.9	-13 09 20.7	18.0775	0.0705	G18.073+0.077	18.06+0.08A	182335-130920	6.6 GHz (II);		-H₂O +CH₃OH II	SFR_MIXED	14.800
16	18 23 35.9	-13 09 20.7	18.0775	0.0705	G18.073+0.077	18.06+0.08B	182335-130920	6.6 GHz (II);	45;57	-H₂O +CH₃OH II	SFR_MIXED	11.900
17	18 24 36.4	-12 51 6.8	18.4610	-0.0042	G18.461-0.004	18.46-0.00	182436-125106	6.6 GHz (II);	47;50	+H₂O -CH₃OH I +CH₃OH II +OH	SFR	12.100
18	18 26 4.3	-11 52 34.7	19.4907	0.1352	G19.485+0.151	19.48+0.15	182604-115234	6.6 GHz (II);	20;27	+H₂O +CH₃OH I +CH₃OH II +OH	SFR	14.500
19	18 27 16.9	-11 53 22.1	19.6171	-0.1331	G19.612-0.134	19.61-0.13	182716-115322	6.6 GHz (II);	52;54	+H₂O -CH₃OH I +CH₃OH II +OH	SFR	12.000
20	18 34 38.8	-08 30 43.5	23.4495	-0.1756	G23.437-0.184	23.43-0.18	183438-083043	6.6 GHz (II);	94;113	+H₂O +CH₃OH I +CH₃OH II +OH	SFR	9.600
21	18 36 6.2	-07 31 6.5	24.4977	-0.0386	G24.494-0.040	24.50-0.03	183606-073106	6.6 GHz (II);	108;116	+H₂O +CH₃OH I +CH₃OH II +OH	SFR	9.300
22	18 36 52	-07 24 53.4	24.6766	-0.1589	G24.676-0.151	24.67-0.14	183652-072453	6.6 GHz (II);	111;114	+H₂O +CH₃OH I +CH₃OH II	SFR	6.400
23	18 36 10.5	-07 11 24.1	24.7973	0.0970	G24.790+0.083	24.78+0.08	183610-071124	6.6 GHz (II);	106;115	+H₂O +CH₃OH I +CH₃OH II +OH	SFR	6.100
24	18 38 4.7	-06 25 23.9	25.6955	0.0294	G25.709+0.044	25.70+0.04A	183804-062523	6.6 GHz (II);		+H₂O +CH₃OH I +CH₃OH II +OH	SFR	12.000
25	18 36 10.5	-07 11 24.1	24.7973	0.0970	G24.790+0.083	27.28+0.15	183610-071124	6.6 GHz (II);	34;37	+H₂O +CH₃OH I +CH₃OH II +OH	SFR	12.900
26	18 44 15.1	-04 17 57.2	28.2873	-0.3643	G28.295-0.377	28.30-0.38A	184415-041757	6.6 GHz (II);		+H₂O +CH₃OH I +CH₃OH II +OH	SFR	12.300
27	18 44 22.3	-04 17 46	28.3038	-0.3898	G28.295-0.377	28.30-0.38B	184422-041746	6.6 GHz (II);	79;93	+H₂O +CH₃OH I +CH₃OH II +OH	SFR	10.600
28	18 47 35.9	-02 01 43	30.6889	-0.0712	G30.702-0.068	30.70-0.07	184735-020143	6.6 GHz (II);	85;90	+H₂O +CH₃OH I +CH₃OH II +OH	SFR	9.000
29	18 47 38.5	-01 57 31.1	30.7562	-0.0490	G30.758-0.050	30.76-0.05	184738-015731	6.6 GHz (II);	88;95	+H₂O +CH₃OH I +CH₃OH II +OH	SFR	9.000
30	18 47 46	-01 54 51.2	30.8099	-0.0563	G30.817-0.057	30.82-0.05	184746-015451	6.6 GHz (II);	91;110	+H₂O +CH₃OH I +CH₃OH II +OH	SFR	9.000
31	18 51 24.3	+00 04 12.9	32.9907	0.0394	G32.993+0.038	32.97+0.04	185124+000412	6.6 GHz (II);	89;93	+H₂O +CH₃OH I +CH₃OH II +OH	SFR	8.700
32	18 52 08	+00 08 19.5	33.1346	-0.0913	G33.133-0.092	33.13-0.09	185207+000819	6.6 GHz (II);	71;81	+H₂O +CH₃OH I +CH₃OH II +OH	SFR	8.700
33	18 52 20.4	+00 25 52.7	33.4187	-0.0041	G33.418-0.004	33.40+0.01A	185220+002552	6.6 GHz (II);		+H₂O -CH₃OH I +CH₃OH II	SFR	10.000
34	18 52 20.4	+00 25 52.7	33.4187	-0.0041	G33.418-0.004	33.40+0.01B	185216+002531?	6.6 GHz (II);	96;107	+H₂O -CH₃OH I +CH₃OH II	SFR	7.100
35	19 07 11.4	+07 09 56.7	41.1007	-0.2158	G41.123-0.220	41.10-0.22	190711+070956	6.6 GHz (II);	63;64	-H₂O +CH₃OH I +CH₃OH II	SFR	9.400
36	19 14 26.1	+09 22 36.9	43.8899	-0.7833	G43.890-0.784	43.87-0.77	191426+092236	6.6 GHz (II);	47;53	+H₂O +CH₃OH I +CH₃OH II +OH	SFR	9.100
37	19 13 20.3	+10 50 54.6	45.0681	0.1389	G45.071+0.132	45.07+0.13	191320+105054	6.6 GHz (II);	57;60	+H₂O +CH₃OH I +CH₃OH II +OH -SiO	SFR	8.000
38	19 22 14.8	+14 03 19.4	48.9194	-0.2846	G48.902-0.273	48.90-0.27	192214+140319	6.6 GHz (II);	63;73	+H₂O +CH₃OH I +CH₃OH II	SFR	7.000
39	19 21 27.5	+15 44 6.2	50.3116	0.6742	G50.315+0.676	50.29+0.69	192127+154406	6.6 GHz (II);	26;33	+H₂O -CH₃OH I +CH₃OH II -OH	SFR	9.300
40	20 38 37.8	+42 37 59.1	81.8791	0.7815	G81.876+0.782	81.87+0.78	203837+423759	6.6 GHz (II);	0;13	+H₂O +CH₃OH I +CH₃OH II +OH	SFR_MIXED	1.200

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 PAN08

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