A new method for determining the CO to H2 conversion factor for translucent clouds
L. Magnani, J. S. Onello;
ApJ, 1995, 443, 169
ABSTRACT:In this paper we describe a new technique for obtaining the conversion factor between the molecular hydrogen column density and the CO(J = 1-0)
integrated antenna temperature.
This factor, typically known as
XCO is often to be of order a few times 1020/sq cm/K km/s) for the
molecular clouds in the Galaxy and is one of the primary means of determining the
molecular cloud mass from CO observations.
However, for the low-extinction
interstellar clouds known as the translucent molecular clouds, estimates of
XCO vary by up to a factor of 60 depending on the object and techniques
employed to calibrate XCO.
Since the cloud mass is directly
proportional to XCO uncertainties in mass estimates of translucent
clouds can be more than an order of magnitude.
We calibrate the H2 content in
translucent clouds by using the linear relationship between the CH and H2 column
densities.
The CH column density is readily determined from observations of the CH
ground-state hyperfine main-line transition at 3335 MHz.
Using CH as a surrogate
tracer for H2 and CO(J = 1-0) observations of a sample of translucent and dark
molecular clouds, we find a wide variation in values for XCO.
For
translucent clouds, XCO ranges from 0.3 to 6.8 x 1020 and
for dark clouds the values range from 0.8 to 8.6.
Although the average
values for both types of cloud are similar to the canonical value determined
for the Galactic molecular cloud ensemble (2-4 x 1020), the
scatter in individual XCO values may indicate that
XCO for a given translucent cloud cannot be determined a priori and must be
obtained for each cloud so that a reliable mass determination may be made.
KEYWORDS: carbon monoxide, density (number/volume), hydrogen, interstellar matter, mass distribution, molecular clouds, data conversion routines, galactic mass, molecular excitation, translucence
CODE: magnani95