2.2. Interstellar objects ( ~ 1 to 50 pc)
2.2.1. Excess line width Wexcess relationship with Object size R
There is a relationship between an object size (R) and a parameter of the emitted spectral line from an object, namely the line width (= Wexcess) in excess of (i) the usual thermal width and in excess of (ii) the width due to large scale motion . Statistics for objects with sizes R from 1 pc up to 1000 pc, including globules, molecular clouds, HII regions in our Galaxy and extragalactic HII regions, suggest a law of the form Wexcess ~ Rq.
An observed general law with q = 0.5 ± 0.1 is indicated for objects with R > 1 pc (e.g., Larson 1979; Vallée 1994e).
Several theoretical models which predicted values of q >> 0.5 or q << 0.5 could be eliminated, due to the small error bars (± 0.1). The remaining models with q 0.5 use (i) turbulences, or else (ii) virial balance for gravitationally-bound clouds. (i) Models with small scale turbulence often employ non-magnetic turbulence (e.g., stellar winds, outflows embedded in gaseous clouds, thermal instabilities around cloud edges, clump collisions in clouds (e.g., Miesch & Bally 1994), although later on in the presence of a magnetic field these non-magnetic turbulences can be theoretically converted into weak MHD waves (e.g., Arons & Max 1975). Heithausen (1996) has shown that high-latitude molecular clouds, with low gas density n 10 cm-3 and small sizes 2 pc, are not in virial balance with self-gravity; these clouds are compressed by the external rarefied HI gas, implying that the cloud relation Wexcess ~ rq is due to internal turbulences needed to oppose the external compression. (ii) For larger density clouds, models using the virial balance with self-gravity also involve the internal magnetic field pressure. Questions to be addressed concern the strength of the cloud magnetic field. Future trends: can strong magnetic support be ruled out in most clouds ? Which degree of magnetic support of a cloud is reasonable, i.e. 1/5 of the overall energy due to magnetism, gravitation, kinetic/thermal motions, turbulent motions, and cloud rotation (e.g., Myers and Goodman 1990) ?