Interstellar clumps: physical relations, magnetism, and energy

Introduction: Clouds

In interstellar molecular clouds, the mean physical parameters such as the gas density n, diameter D, magnetic field B, and gas linewidth W are governed by universal scaling relations (for recent magnetic field reviews, see Vallée, 1997 and 1998). The scaling relations follow the form:

<n> ~ <D>c	[Equ.1]
<B> ~ <n>k	[Equ.2]
<W> ~ <D>q	[Equ.3]
<W> ~ <D>q	[Equ.4]

Following the pioneering work of Larson (1981) in molecular clouds, with diameter D >1 pc and D <100 pc, the exponents were found as follows:

Cloud physics deals with media in rough energy equilibrium (turbulence, gravity).

New Results: Clumps

There is a natural separation between clouds and clumps, occurring near 0.5 pc. One does not expect the physical behaviors of clumps (D <0.5 pc) to be the same as for clouds (D >1 pc).

Below, I summarize a statistical study of the exponent values c, k, p, q for clumps, and of the energy components for clumps. Full details will appear in a forthcoming issue of ApJ. (Vallée, 2000). In cold clumps, the gas density is higher than in clouds. Clumps are beginning to reveal their secrets, as attested by published findings at high angular resolution through the Zeeman effect (using some data from Crutcher, 1999) and through cool dust and neutral gas emissions.

1. Clumps turned out to also satisfy universal scaling relations, similar to the one above for clouds, but with different exponent values.
2. In molecular clumps, with diameters D >0.01 pc and <0.5 pc, I find the exponents to be (Vallée, 2000): c = -1.5; k = +1.0; p = -1.5; q = +0.8 for D  <0.1 pc but q = +0.3 for D >0.1 pc.

   Some of these exponents for clumps can be seen here:

   Figure 1 -- gas density vs size	Figure 2 -- magnetic field strength vs gas density	Figure 3 -- field strength vs size

3. For clumps, the relations above [Equ. 1, 2, 3] predict that p = k . c, and this is found observationally [Fig. 1, 2, 3].
4. The energy distribution in clumps reveals that the support against gravitational collapse in clumps with sizes >0.1 pc comes mainly from turbulent energy, while clumps with sizes <0.1 pc are supported by more magnetic and less turbulent energies. The clump size of 0.1 pc is critical in many other aspects.
5. Clump physics, as indicated by the exponents above, may deal with ongoing accretion processes in shocked and compressed media.

Comparison: Supershells and edges

The k=1 exponent for clumps [Fig. 3 here; and Vallée 2000] is the same as found in some other media. A k=1 was found in the expanding supershells, of thickness ~20 pc, encircling OB associations (Fig. 2 in Vallée, 1993).

A k=1 was also found in the compressed carbon-ionized C II edges, of depths ~0.2 pc, around molecular clouds when clouds are heated by nearby stars (Vallée, 1989). Supershells and C II edges deal with physics of shocked and compressed media.

References
Crutcher, R.M. 1999, Astrophys. J., 520, 706-713.
Larson, R.B. 1981, MNRAS, 194, 809-826.
Vallée, J.P. 1989, Astron. & Astrophys., 224, 191-198.
Vallée, J.P. 1993, Astrophys. J., 419, 670-673.
Vallée, J.P. 1997, Fundam. Cosmic Physics, 19, 1-89.
Vallée, J.P. 1998, Fundam. Cosmic Physics, 19, 319-422.
Vallée, J.P. 2000, Astrophys. J., 537, in press.

Jacques P. Vallée    <Jacques.Vallee@hia.nrc.ca> Dr. Jacques Vallée is an Astronomer (more than 125 research articles in refereed journals), a Research Council Officer at NRC's HIA in Victoria, an Information Agent (for the Media and the Public), an Outreach person (for HIA web, NRC Communication Team, TIS Internal Communication Team) and a Technical Secretary (for committees of university members). After his degrees from Montreal (1968-1969) and Toronto (1970-1973), his astronomical career took him to Leiden (1974-1975), Ottawa (1976, 1981-1987, 1992-1995), Kingston (1977-1980), Grenoble (1988), Edinburgh (1989-1991), and Victoria (1996-present). 'Magnetic astronomy' is his research field, which he reviewed in Fundamentals of Cosmic Physics, v.19, p. 1-89 & p. 319-422. Dr. Jacques Vallée est un Astronome (plus de 125 articles de recherches dans des revues arbitrées), un Agent du Conseil de Recherches pour le CNRC à l'IHA à Victoria, un Agent d'information (pour les Médias et le Public), une personne pour campagne publique (toile de l'IHA, Équipe de communication du CNRC, l'Équipe des communications internes du STI), et un Secrétaire technique (pour des comités d'universitaires). Après ses diplômes Montréal (1968-1969) et Toronto (1970-1973), sa carrière astronomique l'a conduit à Leiden (1974-1975), Ottawa (1976, 1981-1987, 1992-1995), Kingston (1977-1980), Grenoble (1988), Edimbourg (1989-1991), et Victoria (1996-présent). Son champ de recherche est 'l'Astronomie Magnétique', qu'il a synthétisé dans "Fundamentals of Cosmic Physics", v.19, p. 1-89 & p. 319-422.

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