Frances Verter

X-Ray Shadows by High-latitude Molecular Clouds. I. Cartography

We have observed X-ray shadowing by nine nearby high-latitude molecular clouds with the ROSAT PSPC. The

A comparison of CO(J = 1 yields 0) and CO(J = 2 yields 1) emission in the Milky Way molecular ring

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Infrared Properties of Molecular Cirrus. II. Cloud-to-Cloud Variations in Graphite and Polycyclic Aromatic Hydrocarbon Content

--> keV emission from the Local Bubble is used to calculate an average electron density, ne, toward all of our sample clouds. Given the uniformity of emission within the Local Bubble, and previously determined cloud distances, one can determine whether the clouds are within or without the Local Bubble. We confirm that three of the sample clouds, MBM 12, MBM 16, and possibly MBM 20, which have been thought to be within the Local Bubble, are actually within. A combination of

The separation between gas and dust filaments at the edge of the expanding shell in Eridanus

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A Search for Star Formation in the Translucent Cloud MBM 40

--> keV and ne measures suggest that MBM 6 and LDN 1563 may also lie within the Local Bubble or its boundary layer. The shadows cast by the clouds at

Infrared Properties of Molecular Cirrus. I. Photometry of Extended Sources on IRAS Image Products

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Systematic properties of CO emission from galaxies. I. Luminosity function

--> keV and 1.5 keV imply little if any foreground emission in those bands, and produce lower limits for the distance to the hotter emission component responsible for the observed flux. The results are in agreement with previous estimates for the distances to the Loop I Bubble and the Eridion Bubble.

Effect of Malmquist bias on correlation studies with IRAS data base

We have carried out a CO (J = 2 right arrow 1) survey of the Scutum Arm region of the Milky Way molecular ring. Our goals are to compare CO (J = 2 right arrow 1) maps of individual Galactic clouds with the large-scale CO (J = 2 right arrow 1) emission from the Galactic plane, and to predict the CO (J = 2 right arrow 1) appearance of a Galactic cloud ensemble in an external galaxy. The angular resolution and spatial coverage of our survey are compatible with the existing CO (J = 1 right arrow 0) survey of this region by Sanders et al., which we use for comparison. We identify 34 molecular clouds in our map region; their relationships between size and line width and between virial mass and luminosity are consistent with the relationships seen in CO (J = 1 right arrow 0) emission. However, we note that previous studies have shown considerable variation in these relationships; we attribute much of this variation to differences in their cloud definition algorithm and statistical method. We find that the median ratio of integrated emission in the two lowest CO transitions for the clouds in our sample is I(2 right arrow 1)/ I(1 right arrow 0) = 0.69, implying that the typical emitting region in the line of sight contains cold gas that is not actively star-forming. Our conclusion that the molecular ring emission is not dominated by star-forming regions is consistent with other large-scale studies of the Milky Way. Our distributions of cloud size and temperature also imply that relatively massive molecular clouds that lac k star formation, such as Maddalenas cloud, are not rare in the inner Galaxy.

The CO luminosity function of galaxies

We have conducted a photometric survey of translucent molecular clouds in the four IRAS wave bands. We find notable cloud-to-cloud variations in mid-IR emission, with the ratio I12/I25 varying by up to 1 order of magnitude and often above unity. Because the clouds in our sample are nearby and translucent, the cloud heating is well constrained, so that the observed differences in infrared emission must represent differences in grain content. We show from first principles that a dust model containing only two components, Mathis-Rumpl-Nordsieck (MRN) grains in thermal equilibrium and very small grains undergoing stochastic heating, can never produce I12/I25 ≥ 1 under the conditions that prevail in the cloudy interstellar medium. We fit the clouds in this sample with a three-component model which contains continuum emission from both MRN grains and very small graphite grains, plus emission features from a mixture of polycyclic aromatic hydrocarbon (PAH). The cloud-to-cloud variations in mid-IR emission require significant differences in their composition of small grains and PAH molecules. Models which describe the formation and evolution of these particles in the interstellar medium must explain the presence of these variations in a cloud sample which is located in the solar neighborhood.

CO(J = 2 - 1) study of molecular clouds in the southwest arm of M31

Observations obtained from IRAS for a 120 sq deg region at the boundary of a large expanding shell in Eridanus are presented. It is found that the separation between the structures in the H I and IR emission is not due to a transition of the gas into molecular hydrogen at the location of the IR filaments. The molecular hydrogen column density of these filaments is at least two orders of magnitude below what is typically seen in high-latitude molecular clouds. As the H I and dust in this region are associated with a large expanding shell (or superbubble), the action of shocks and radiation fields may be responsible for the separations of gas and dust. Unless the gas and dust structures are resolved and lie in a favorable viewing geometry, the effect tends to be hidden. The results indicate that it is not always evident, from observations with spatial resolutions coarser than a few arcminutes, how interstellar gas and dust are related.