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Contribution à l'étude du milieu interstellaire par l'observation de
la raie en absorption à 21 cm de longueur d'onde de l'hydrogène

Abstract : One of the essential components of the interstellar medium is neutral
atomic hydrogen. A classical, but efficient method to study this
component is the observation of the hyperfine transition at 21 cm
wavelength of neutral hydrogen. The observation of this emission line
can be supplemented usefully by the measurement of the line in
absorption on the continuum radiation of discrete radiosources. The
absorption profile gives the optical depth of neutral hydrogen on
the line of sight, which, compared with the emission profile, makes
it possible in theory to determine the column density and the
excitation temperature of hydrogen in the studied direction.

We present here observations of absorption profiles 21 cm in the
direction of 819 extragalactic radiosources, carried out with the
Nançay radio telescope (Chapter II). We discuss the efficiency of
this instrument for this kind of measurement (Appendix to Chapter II).

Then we use the sample of detected absorption components (300 at |b| >
10°) for a statistical analysis of nearby hydrogen clouds. The study
of radial velocities makes it possible to obtain the motion of the Sun
relative to local interstellar gas; in retrieving the average effect
of differential galactic rotation, we détermine the average distance
of neutral hydrogen clouds from the galactic plane, as well as the
radial velocity dispersion radial of these clouds (Chapter III).

We study some observational bias which affect our sample: the blending
of the spectral components coming from clouds with close radial
velocities, the presence of spurious spectral components due to the
contamination of the absorption profiles by the 21 cm line in emission
(Chapter IV, 1 and 2). Then we present the histogram of the internal
velocity dispersion of the clouds (chapter IV, 5). We then try to
determine the distribution of optical depths of the clouds, as well as
the probability of meeting a cloud of given optical depth on a line of
sight, taking into account observational bias and sensitivity limits
(Chapter IV, 4). The comparison between our results and the
predictions of recent descriptive models of the interstellar
environment indicates that these models must be revised (Chapter IV,

To specify the relationship between dense molecular clouds and diffuse
neutral hydrogen clouds, we carried out radio observations of the OH
radical hydroxyl and the CO molecule in some of the directions where
absorption to 21 cm is known. The presence of molecules in some
diffuse clouds suggests a continuity between diffuse clouds and
molecular clouds (Chapter 7).
Complete list of metadatas
Contributor : Jacques Crovisier <>
Submitted on : Tuesday, January 16, 2007 - 4:39:55 PM
Last modification on : Thursday, December 10, 2020 - 11:09:06 AM
Long-term archiving on: : Tuesday, April 6, 2010 - 10:09:25 PM


  • HAL Id : tel-00124895, version 1


Jacques Crovisier. Contribution à l'étude du milieu interstellaire par l'observation de
la raie en absorption à 21 cm de longueur d'onde de l'hydrogène
neutre.. Astrophysique [astro-ph]. Université Paris-Diderot - Paris VII, 1978. Français. ⟨tel-00124895⟩



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