Ewine F. van Dishoeck

Molecular Cloud Structure in the Magellanic Clouds: Effect of Metallicity

The chemical structure of neutral clouds in low-metallicity environments is examined, with particular emphasis on the H to H2 and C+ to CO transitions. We observed near-IR H2 (1, 0) S(1), (2, 1) S(1), and (5, 3) O(3) lines and the 12CO J = 1 → 0 line from 30 Doradus and N159/N160 in the Large Magellanic Cloud and from DEM S 16, DEM S 37, and LI-SMC 36 in the Small Magellanic Cloud. We find that the H2 emission is UV-excited and that (weak) CO emission always exists (in our surveyed regions) toward positions where H2 and [C II] emission have been detected. Using a PDR code and a radiative transfer code, we simulate the emission of line radiation from spherical clouds and from large planar clouds. Because [C II] emission and H2 emission arise on the surface of the cloud and because the lines are optically thin, these lines are not affected by changes in the relative sizes of the neutral cloud and the CO-bearing core, while the optically thick CO emission can be strongly affected. The sizes of clouds are estimated by measuring the deviation of CO emission strength from that predicted by a planar cloud model of a given size. The average cloud column density, and therefore its size, increases as the metallicity decreases. Our result agrees with the photoionization-regulated star formation theory of McKee.

Photon-dominated Regions in Low-Ultraviolet Fields: A Study of the Peripheral Region of L1204/S140

We have carried out an in-depth study of the peripheral region of the molecular cloud L1204/S140, where the far-ultraviolet radiation and the density are relatively low. Our observations test theories of photon-dominated regions (PDRs) in a regime that has been little explored. Knowledge of such regions will also help to test theories of photoionization-regulated star formation. [C II] 158 μm and [O I] 63 μm lines are detected by the Infrared Space Observatory at all 16 positions along a one-dimensional cut in right ascension. Emission from H2 rotational transitions J = 2 → 0 and J = 3 → 1 at 28 and 17 μm, respectively, was also detected at several positions. The [C II], [O I], and H2 intensities along the cut show much less spatial variation than do the rotational lines of 12CO and other CO isotopes. The average [C II] and [O I] intensities and their ratio are consistent with models of PDRs with low values of far-ultraviolet radiation (G0) and density. The best-fitting model has G0 ~ 15 and density n ~ 103 cm-3. Standard PDR models underpredict the intensity in the H2 rotational lines by up to an order of magnitude. This problem has also been seen in bright PDRs and attributed to factors, such as geometry and gas-grain drift, that should be much less important in the regime studied here. The fact that we see the same problem in our data suggests that more fundamental solutions, such as higher H2 formation rates, are needed. Also, in this regime of low density and small line width, the [O I] line is sensitive to the radiative transfer and geometry. Using the ionization structure of the models, a quantitative analysis of timescales for ambipolar diffusion in the peripheral regions of the S140 cloud is consistent with a theory of photoionization-regulated star formation. Observations of [C II] in other galaxies differ from both those of high-G0 PDRs in our galaxy and the low-G0 regions we have studied. The extragalactic results are not easily reproduced with mixtures of high- and low-G0 regions.

4. Molecular Photodissociation

Photodissociation is the dominant removal process of molecules in any region exposed to intense ultraviolet (UV) radiation. This includes diffuse and translucent interstellar clouds, dense photon-dominated regions, high velocity shocks, the surface layers of protoplanetary disks, and cometary and exoplanetary atmospheres. The rate of photodissociation depends on the cross sections for absorption into a range of excited electronic states, as well as on the intensity and shape of the radiation field at each position into the region of interest. Thus, an acccurate determination of the photodissociation rate of even a simple molecule like water or carbon monoxide involves many detailed considerations ranging from its electronic structure to its dissociation dynamics and the specifics of the radiation field that the molecule is exposed to. In this review chapter, each of these steps in determining photodissociation rates is discussed systematically and examples are provided.

High-excitation molecular lines from circumstellar disks

Observations of submillimeter lines of CO, HCO + , HCN and their isotopes from circumstellar disks around low-mass pre-main sequence stars can be used to set constraints on the temperature and density distributions in these disks. The lines considered here originate from levels with higher excitation temperatures and critical densities than studied before (CO 6–5, HCO + and HCN 4–3), and are combined with interferometer data on lower excitation lines. We discuss the results for two disks, i.e., those around LkCa 15 and TW Hya. We find that the TW Hya disk has a warm surface layer and agrees well with a flaring disk geometry, while the LkCa 15 disk is cooler and can be described by either dust-settling in a flared disk or a flatter disk overall. The densities are well described by disk models in the literature.