Molecular clouds, rotation

In the dense interstellar medium characteristic of sites of star fonuation, for example, scattering of visible/UV light by sub-micron-sized dust grains makes molecular clouds optically opaque and lowers their internal temperature to only a few tens of Kelvin. The thenual radiation from such objects therefore peaks in the FIR and only becomes optically thin at even longer wavelengths. Rotational motions of small molecules and rovibrational transitions of larger species and clusters thus provide, in many cases, the only or the most powerfiil probes of the dense, cold gas and dust of the interstellar medium. 

Hydrogen chloride is the first chlorine-bearing interstellar molecule to have been detected. Its lowest rotational transition (J = 1 -> 0) at 625.9 GHz has been observed in the Orion Molecular Cloud (OMC-1) in emission with the Kuiper Airborne Observatory, (Blake, Keene, and Phillips, 1985) since atmospheric opacity at this... 

The collapse of rotating molecular cloud cores leads to the formation of massive accretion disks that evolve to more tenuous protoplanetary disks. Disk evolution is driven by a combination of viscous evolution, grain coagulation, photoevaporation, and accretion to the star. The pace of disk evolution can vary substantially, but massive accretion disks are thought to be typical for stars with ages < 1 Myr and lower-mass protoplanetary disks with reduced or no accretion rates are usually 1-8 Myr old. Disks older than 10 Myr are almost exclusively non-accreting debris disks (see Figs. 1.3 and 1.5). 

The material falling from the parent molecular cloud directly on the forming star is rapidly dissociated and ionized and solids are vaporized, once the matter passes the accretion shock on the stellar surface. The material falling on the inner parts of the accretion disk suffers the same fate, since the matter has to pass through a standing shockwave on the surface of the disk. At this shock the infall of matter is stopped and the flow characteristics change from infall of envelope material to nearly Keplerian rotation of disk material. The location of this shock is shown in Fig. 2.10 as the heavy line marked by crosses. 

Figure 3.3 Left measured accretion rate as a function of age (Sicilia-Aguilar etal. 2006). The line is the prediction of a single Lynden-Bell Pringle model as a function of time. Right measured accretion rates as a function of stellar mass for a cluster of a given age, in this case in Ophiucus. The lines are model predictions for a given initial dimensionless rotation rate of the parent molecular cloud core from which these stars were formed. From Dullemond et at. (2006b).

In Section B we have discussed how the basic quantities of line emission and absorption, the excitation temperature Tex and optical depth r can be determined from observations. Energies required for rotational excitation are generally low enough (< 200 cm-1) so that the rotational levels are expected to be populated even at the very low kinetic temperatures of the interstellar molecular clouds. On the other hand, with a few exceptions such as H20 and NH3, one may assume that only the lowest energy levels of interstellar molecules are populated. Under these conditions the observable fractional column density Nx may not deviate appreciably from the total column density N of a molecule, which can be computed by means of Eq. (17) on the assumption of LTE. 

However, as a result of the relatively large beamwidth of radiotelescopes the apparent turbulent velocities of molecular clouds are partly the result of large-scale systematic motions, such as rotation. 

Linear hydrocarbon radicals have been the subject of intensive laboratory spectroscopic and radio-astronomical research since the early 1980s. In recent years, a considerable number of rotational spectroscopic studies of medium to longer hydrocarbon chains such as C5H, CeH, CgH, and ChH have been carried out using a pulsed molecular beam FTMW spectrometer. The high resolution offered by such a spectrometer allowed the detection of the hyperfine sphtting of rotational transitions. These measurements improved fine and hyperfine coupling constants and provided rest frequencies with accuracies better than 0.30 km s in equivalent radial velocity up to 50 GHz. Indeed, some of the small C H radicals with n < 9 have subsequently been detected in space, in molecular cloud cores, and in certain circumstellar shells. These hydrocarbon chains are among the most abundant reactive space molecules known. 

Boss A. P. and Myhill E. A. (1995) Collapse and fragmentation of molecular cloud cores III. Initial differential rotation. 

The study of such species is important cosmochemically, but is quite difficult at microwave frequencies where the rotational spectra are weak, and nearly impossible at IR or optical wavelengths due to the extinction present in dense molecular clouds and young stellar objects. 

Figure 2 shows apart of the emission spectrum ofOMC-1 (Orion Molecular Cloud), a region of high-mass star formation, in the 208-232 GHz range. Many rotational lines are observed, only a few of them have been assigned by now. The rotational emission... 

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