Open AccessVery Large Array observations of broad 6‐cm excited‐state OH lines in W49A
Author(s) -
Palmer Patrick,
Goss W. M.
Publication year - 2005
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2005.09085.x
Subject(s) - physics , maser , excited state , astrophysics , brightness , emission spectrum , ground state , brightness temperature , atomic physics , line (geometry) , spectral line , astronomy , mathematics , geometry
Using the Very Large Array (VLA), we observed all three of the 6‐cm lines of the 2 Π 1/2 , J = 1/2 state of OH with sub‐arcsecond resolution (∼0.4 arcsec) in W49A. While the spatial distribution and the range in velocities of the 6‐cm lines are similar to those of the ground‐state (18‐cm) OH lines, a large fraction of the total emission in all three 6‐cm lines has large linewidths (∼5–10 km s −1 ) and is spatially extended, very unlike typical ground‐state OH masers, which typically are point‐like at VLA resolutions and have linewidths ≤1 km s −1 . We find brightness temperatures of 5900, 4700 and ≥730 K for the 4660, 4750 and 4765‐MHz lines, respectively. We conclude that these are indeed maser lines. However, the gains are ∼0.3, again very unlike the 18‐cm lines, which have gains ≥10 4 . We compare the excited‐state OH emission with that from other molecules observed with comparable angular resolution to estimate physical conditions in the regions emitting the peculiar, low‐gain maser lines. We also comment on the relationship with the 18‐cm masers.