J/A+A/486/191 Distances towards 6.7GHz methanol masers (Pandian+, 2008)
Resolving distance ambiguities towards 6.7 GHz methanol masers.
Pandian J.D., Momjian E., Goldsmith P.F.
<Astron. Astrophys., 486, 191-208 (2008)>
=2008A&A...486..191P 2008A&A...486..191P
ADC_Keywords: Masers ; Radial velocities ; Stars, distances
Keywords: Galaxy: kinematics and dynamics - Galaxy: structure - H II regions
Abstract:
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.
In this work, we aim to resolve the kinematic distance ambiguity
towards a sample of 6.7GHz methanol masers, which are signposts of
the early stages of massive star formation.
We measured 21cm HI absorption spectra using the Very Large Array in C
and CnB configurations. A comparison of the maximum velocity of HI
absorption with the source velocity and tangent point velocity was
used to resolve the kinematic distance ambiguity.
Description:
The observations were made in June 2005 using the C-Band receiver of
Green Bank Telescope. The GBT spectrometer was used in 16 intermediate
frequency (IF) mode to cover 8 frequencies in two polarizations.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 76 41 6.7 GHz methanol maser properties
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See also:
VIII/65 : 1.4GHz NRAO VLA Sky Survey (NVSS) (Condon+ 1998)
J/A+AS/134/115 : Medicina 6.7GHz methanol masers survey (Slysh+ 1999)
J/A+AS/143/269 : 6.7GHz methanol maser emission survey (Szymczak+, 2000)
J/A+A/403/1095 : 6.7GHz methanol masers survey of low-mass YSO (Minier+, 2003)
J/other/ChJAA/3.49 : Velocity and distance of methanol maser sources (Xu+ 2003)
J/A+A/432/737 : General Cat. of 6.7GHz Methanol Masers (Pestalozzi+, 2005)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 10 A10 --- Source Methanol maser source name (LL.ll+D.dd)
11 A1 --- m_Source [AB] Multiplicity index for sources which have
two recombination line velocities in the
same line of sight
12 A1 --- n_Source [abcde] Note on Source (1)
14- 26 A13 --- NVSS NVSS source (HHMMSS+DDMMSS) associated with
the methanol maser
27 A1 --- u_NVSS [?] Uncertainty flag on NVSS
29- 31 I3 km/s Vmin ? LSR Velocity extent of the maser emission,
lower limit
33- 35 I3 km/s Vmax ? LSR Velocity extent of the maser emission,
upper limit
37- 41 F5.1 km/s Vsys Systemic velocity, derived from the radio
recombination line
43- 44 A2 --- KDA [NFT? ] KDA resolution (2)
46- 49 F4.1 kpc d ?=- Distance (3)
51- 53 F3.1 kpc E_d ? Error on d, upper limit (5)
55- 57 F3.1 kpc e_d ? Error on d, lower limit (5)
59- 62 F4.1 kpc d' ?=- Modified kinematic distance (4)
64- 66 F3.1 kpc E_d' ? Error on d', upper limit (5)
68- 70 F3.1 kpc e_d' ? Error on d', lower limit (5)
72 A1 --- q_KDA [AB ] Quality grade of the KDA resolution
74- 76 F3.1 mJy sigma rms noise in the 21cm continuum image
(in mJy/beam)
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Note (1): Note as follows:
a = Distance from Blum et al. (2001AJ....121.3149B 2001AJ....121.3149B)
b = Source has ambiguity between two recombination lines
c = The continuum source associated with the A and B sources
may be reversed
d = Distance from Genzel et al. (1981ApJ...247.1039G 1981ApJ...247.1039G)
e = d' for this source is the distance to the tangent point
Note (2): kinematic distance ambiguity (KDA) resolution specifying
whether the source is as follows:
N = at the near distance
F = at the far distance
T = at the tangent point
? = KDA is unresolved
Note (3): calculated from the conventional method using the C85 rotation curve.
Note (4): calculated using the methodology described in Sect. 4.
Note (5): Errors for distance are not listed when vsys+10 exceeds the
tangent point velocity, vt, since such velocities are forbidden, and
cannot be converted to a meaningful distance.
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 30-Sep-2008