1997A&AS..125..303R


Query : 1997A&AS..125..303R

1997A&AS..125..303R - Astron. Astrophys., Suppl. Ser., 125, 303-312 (1997/October-2)

The Cepheus molecular cloud. I. Multi-transition observations in CO and 13CO.

REMY G.P., GRENIER I.A., DUVERT G. and THADDEUS P.

Abstract (from CDS):

The eastern cloud of the nearby Cepheus complex, located at 111°<l<117° and 14°<b<19°, has been mapped in the CO(J=1->0) transition at a resolution of 0.8 pc with the CfA 1.2m telescope. This massive, but fairly diffuse cloud, with visual extinction <2mag, has also been sampled in the J=1->0 and 2->1 rotational transitions of CO and 13CO at the same resolution, using the CfA and POM-2 millimeter telescopes. The radiative transfer of the lines has been treated in the LTE and LVG approximations, the two yielding comparable results. In CO, low excitation temperatures have been found in the range of 5-11 K, with moderate optical depths (τ<3) and CO column-densities up to 4x1016cm–2. In 13CO, the gas is optically thin with slightly lower excitation temperatures of 4-8 K, for 13CO column-densities up to 6x1015cm–2. Under these conditions, the observed ratios of 2->1 over 1->0 velocity-integrated intensities, W_2 ->1/W1 ->0_, have been found to be consistent with a uniform value over the cloud of 0.88 ±0.09 in CO and 0.60 ±0.07 in 13CO, as typical of many clouds. The ratios of CO over 13CO velocity-integrated intensities, W(12CO)/W(13CO), have been found to decrease with 13CO intensity as expected from the progressive saturation of the CO lines. The large scatter about this relation, observed at scales of 0.8 and 0.2pc, cannot be accounted for by instrumental error, beam dilution, or the dispersion of excitation temperatures and line widths measured in the cloud. The W(12CO)/W(13CO) fluctuations are therefore indicative of intrinsic variations in the molecular abundances. In particular, very low W(12 CO)/W(13CO) ratios may result from efficient isotopic fractionation in this cold environment. Similar variations in 13CO intensity have been reported at a scale of 0.2 pc in two other dark clouds, HCL 2 (Cernicharo & Guelin 1987A&A...176..299C) and IC 5146 (Lada et al., 1994ApJ...429..694L), from the behaviour of their W(12CO)/W(13CO) or W(13CO)/AV ratios with visual extinction. Together with the present results, they suggest that the 13CO abundance integrated along the line of sight largely varies inside the diffuse envelopes of molecular clouds.

Abstract Copyright:

Journal keyword(s): ISM: Cepheus cloud (Cep cloud) - ISM: abundances - ISM: clouds - ISM: molecules - radio lines: ISM

Simbad objects: 11

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Number of rows : 11
N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2024
#notes
1 NAME MCLD 126.6+24.5 MoC 04 23 55.8 +85 46 47           ~ 27 0
2 TMC-1 MoC 04 41 45.9 +25 41 27           ~ 1686 0
3 M 42 HII 05 35 17 -05 23.4           ~ 4093 0
4 NAME Ori A MoC 05 38 -07.1           ~ 3021 0
5 NAME Flame Nebula HII 05 41 42.7 -01 54 44           ~ 703 1
6 NAME Ori B MoC 05 41 43.0 -01 54 44           ~ 1381 0
7 NGC 2024 Cl* 05 41 43 -01 50.5           ~ 1156 1
8 LDN 1075 MoC 21 33 56 +54 41.5           ~ 21 0
9 IC 5146 OpC 21 53 29.3 +47 14 46           ~ 468 2
10 NAME Cepheus Cloud SFR 22 00.0 +76 30           ~ 149 0
11 IRAS 22376+7455 TT* 22 38 42.5032929744 +75 11 45.489819660   17.65 16.10 14.90 13.62 ~ 49 0

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