The \hi gas kinetic temperature of high galactic latitude cirrus clouds
R. Stark, J. M. Dickey, W. B. Burton, A. Wennmacher;
AaA, 1994, 281, 199
ABSTRACT:We present the results of a combined absorption and emission study of the H I 21 cm line toward several cirrus clouds at high galactic
latitude.
The absorption measurements were obtained with the Very Large Array (VLA)
synthesis telescope, the emission data with the Effelsberg telescope.
The
three cirrus complexes studied were selected from the Deul & Burton (1990)
study.
Deul & Burton had shown that these cirrus clouds have associated H I emission
with radial velocities near 0 km/s as well as with more extreme velocities of
v less than -20 km/s.
Absorption against background continuum sources
was detected in 40% of the lines of sight investigated.
The H I line was
always found to be optically thin, tau less than 0.6.
The O km/s component was
found in each direction where absorption was detected; in addition, three
directions showed absorption at intermediate negative velocities.
The derived
apparent spin temperatures vary between 20 K and greater than 350 K.
The
emission profiles reveal much more widely distributed gas than is visible in
absorption.
Only the cool clumps in the cirrus show up in absorption.
The observations
suggest a collision of a high velocity cloud with the galactic disk gas, by which
different parts of the cirrus are at present seen in different phases of shock
completion.
This model may explain the large observed differences in the H I emission and
absorption profiles over small distances on the sky, as well as the variation of the
far-infrared brightness ratio.
KEYWORDS: gas temperature, h i regions, infrared cirrus (astronomy), interstellar matter, kinematics, absorption spectra, astronomical models, emission spectra, radial velocity, spin temperature, very large array (vla)
PERSOKEY:h_i, ,
CODE: stark94