A Dynamical Study of the Non-Star Forming Translucent Molecular Cloud MBM16: Evidence for Shear Driven Turbulence in the Interstellar Medium

We present the results of a velocity correlation study of the high latitude cloud MBM16 using a fully sampled $^{12}$CO map, supplemented by new $^{13}$CO data. We find a correlation length of 0.4 pc. This is similar in size to the formaldehyde clumps described in our previous study. We associate this correlated motion with coherent structures within the turbulent flow. Such structures are generated by free shear flows. Their presence in this non-star forming cloud indicates that kinetic energy is being supplied to the internal turbulence by an external shear flow. Such large scale driving over long times is a possible solution to the dissipation problem for molecular cloud turbulence.Comment: Uses AAS aasms4.sty macros. Accepted for publication in Ap

Mechanisms for the Origin of Turbulence in Non-Star-forming Clouds: The Translucent Cloud MBM 40

We present a multiline, high spatial and velocity resolution CO, H I, and IRAS 100 μm study of the high-latitude, low-mass, non-star-forming, translucent molecular cloud MBM 40. The cloud mass is distributed into two ridges, or filaments, that form a hairpin structure. Velocity channel maps indicate a highly ordered flow in the molecular gas, with the northeastern part of the filament moving away from and the southwestern filament moving toward the observer relative to the mean cloud radial velocity. Significant changes in emissivity occur over 0.03 pc, indicating large transverse density gradients along the ridges. However, the velocity field appears to be continuous, showing no evidence for shock compression. The neutral hydrogen at the same velocity envelops the molecular gas but shows a decrease along the hairpin, indicating that the atomic hydrogen has converted to H2; the strongest 100 μm emission coincides with the CO, not the H I, emission peak. These results indicate that MBM 40 is condensing out of a larger scale flow and is structured by thermal instability and shear flow turbulence. This externally driven turbulence does not produce large compression and may explain why gravitational collapse and star formation do not occur in MBM 40

CH 3 GHz Observations of Molecular Clouds Along the Galactic Plane

Spectra in the CH $^2\Pi_{1/2}$, J=1/2, F=1-1 transition at 3335 MHz were obtained in three 5-point crosses centered on the Galactic plane at $\ell =$ 50\arcdeg, 100\arcdeg, and 110\arcdeg. The lines of sight traverse both Giant Molecular Clouds (GMCs) and local, smaller entities. This transition is a good tracer of low-density molecular gas and the line profiles are very similar to CO(1-0) data at nearly the same resolution. In addition, the CH 3335 MHz line can be used to calibrate the CO-H$_2$ conversion factor (X$_{\rm CO}$) in low-density molecular gas. Although this technique underestimates X$_{\rm CO}$ in GMCs, our results are within a factor of two of X$_{\rm CO}$ values calibrated for GMCs by other techniques. The similarity of CH and CO line profiles, and that of X$_{\rm CO}$ values derived from CH and more traditional techniques, implies that most of the molecular gas along the observed lines of sight is at relatively low densities ($n \le$ 10$^3$ cm$^{-3}$).Comment: 26 pages, 12 figures, submitted to the AJ, revised after referee repor

Hydroxyl as a Tracer of H2 in the Envelope of MBM40

We observed 51 positions in the OH 1667 MHz main line transitions in the translucent, high latitude cloud MBM40. We detected OH emission in 8 out of 8 positions in the molecular core of the cloud and 24 out of 43 in the surrounding, lower extinction envelope and periphery of the cloud. Using a linear relationship between the integrated OH line intensity and E(B-V), we estimate the mass in the core, the envelope, and the periphery of the cloud to be 4, 8, and 5 solar masses. As much as a third of the total cloud mass may be found in the in the periphery (E(B-V) $<$ 0.12 mag) and about a half in the envelope (0.12 $\le$ E(B-V) $\le$ 0.17 mag). If these results are applicable to other translucent clouds the OH 1667 MHz line is an excellent tracer of gas in very low extinction regions and high-sensitivity mapping of the envelopes of molecular clouds may reveal the presence of significant quantities of molecular mass.Comment: 26 pages, 3 figures, and 5 table

CH 3 GHz Observations of the Galactic Center

A 3 $\times$ 3 map of the Galactic Center was made at 9\arcmin resolution and 10\arcmin spacing in the CH $^2\Pi_{1/2}$, J=1/2, F=1-1 transition at 3335 MHz. The CH emission shows a velocity extent that is nearly that of the CO(1-0) line, but the CH line profiles differ markedly from the CO. The 3335 MHz CH transition primarily traces low-density molecular gas and our observations indicate that the mass of this component within $\sim$ 30 pc of the Galactic Center is $\sim$ 9 $\times$ 10$^6$ M$_\odot$. The CO-H$_2$ conversion factor obtained for the low-density gas in the mapped region is greater than that thought to apply to the dense molecular gas at the Galactic Center. In addition to tracing the low-density molecular gas at the Galactic Center, the CH spectra show evidence of emission from molecular clouds along the line of sight both in the foreground and background. The scale height of these clouds ranges from 27 - 109 pc, consistent with previous work based on observations of molecular clouds in the inner Galaxy.Comment: 29 pages, 12 figure

A dirty window: diffuse and translucent molecular gas in the interstellar medium

This book provides an introduction to the physics of interstellar gas in the Galaxy. It deals with the diffuse interstellar medium which supplies a complex environment for exploring the neutral gas content of a galaxy like the Milky Way and the techniques necessary for studying this non-stellar component. After an initial exposition of the phases of the interstellar medium and the role of gas in a spiral galaxy, the authors discuss the transition from atomic to molecular gas. They then consider basic radiative transfer and molecular spectroscopy with particular emphasis on the molecules useful for studying low-density molecular gas. Observational techniques for investigating the gas and the dust component of the diffuse interstellar medium throughout the electromagnetic spectrum are explored emphasizing results from the recent Herschel and Planck missions. A brief exposition on dust in the diffuse interstellar medium is followed by a discussion of molecular clouds in general and high-latitude molecular clouds in particular. Ways of calibrating CO observations with the molecular hydrogen content of a cloud are examined along with the dark molecular gas controversy. High-latitude molecular clouds are considered in detail as vehicles for applying the techniques developed in the book. Given the transient nature of diffuse and translucent molecular clouds, the role of turbulence in the origin and dynamics of these objects is examined in some detail. The book is targeted at graduate students or postdocs who are entering the field of interstellar medium studies

The mixing of dust and gas in the high latitude translucent cloud MBM 40

Context. High latitude molecular clouds (hereafter HLMCs) permit the study of interstellar gas dynamics and astrochemistry with good accuracy due to their proximity, generally clear lines of sight, and lack of internal star-forming activity which can heavily modify the physical context. MBM 40, one of the nearest HLMCs, has been extensively studied, making it a superb target to infer and study the dust-to-gas mixing ratio (DGMR). Aims. The mixing of dust and gas in the interstellar medium remains a fundamental issue to keep track of astrochemistry evolution and molecular abundances. Accounting for both molecular and atomic gas is difficult because H2 is not directly observable and H