Infrared Absorption-Line Spectroscopy

Submillimeter-wave Spectroscopy of Molecular Clouds and Star-Forming Regions

Summary: Submillimeter-wave spectroscopy opens up a new window of exploration on molecular clouds and star-forming regions. It is now possible to probe warmer, denser regions of molecular clouds where stars are being formed. A comprehensive spectroscopic study of the same sources for which we have high quality infrared observations is being undertaken at the 10-meter Heinrich Hertz Telescope (HHT) of the Submillimeter Telescope Observatory. Multi-line observations of CO and its isotopes, H₂CO, and CS provide interesting constraints on the physical and structural properties of the surrounding molecular cloud as well as the material more intimately associated with the star forming process. Observations of CH₃OH, CH₂CH₃OH, and vibrationally excited HCN and CS make sensitive probes of the hot, processed material in these regions. Detections of reactive ions such as HCO⁺, DCO⁺, HCS⁺, N₂H⁺ and H₃O⁺ are discussed in the light of measured H₃⁺ abundances, detections of abundant warm H₂O, and estimated cosmic ray ionization rates in dense clouds. This study highlights the value of multi-wavelength studies in deciphering and disentangling the complicated structure and evolution of molecular clouds and star-forming regions. Exciting new advances in infrared and submillimeter instrumentation offer tantalizing prospects for advances in our understanding of the star-forming process. In particular, new submillimeter-wave heterodyne array receivers being developed for the HHT (Arizona) and AST/RO (South Pole) will be discussed.

Description: With the recent direct detections of cold H₂ in dense molecular clouds and star forming regions, it is only natural to try to exploit the complementary nature of (sub)millimeter-wave emission line techniques. Although H₂ itself cannot be detected at radio wavelengths, almost 120 individual molecular species are readily seen in molecular clouds. Commonly, CO is used as a "tracer" of H₂. Submillimeter observations carry two fundamental advantages over the infrared techniques described elsewhere:

The environments of star formation, even entire molecular clouds can be readily mapped out in radio emission lines, not just towards special "pencil beam" lines of sight.
Radio measurements are more sensitive. Astrochemists can probe complicated molecular species that are not in abundance in interstellar clouds, and weakly molecular regions can be detected where infrared techniques lack the necessary sensitivity.

Below is a depiction of the NGC 2024 star-forming region. The visible image shows a dark, dusty molecular cloud. As one moves to longer wavelengths, one can probe through the obscuring material and see the newly-born stars buried within (infrared image). At the SMT, we have mapped out CO and its isotopes towards NGC 2024. The ¹²CO J=3-2 rotational transition is mapped out at the bottom-left of the figure, and you can see that it peaks in intensity at the same location where the stars are being formed and where the obscuration is the highest. At each pixel in this map, we have spectra of CO in several rotational transitions, each of which has different requirements for excitation. Thusly, each transition probes different material in the cloud, and therefore tells a different story. In addition, we have observed the isotopically rarer species like ¹³CO, C¹⁸O, and C¹⁷O. A sampling of these lines is portrayed for a single point in the map; the same point for which we have detected H₂, H₃⁺ and CO directly in the infrared.

Notice the difference in the line profiles. This gives us direct information on the distribution of molecular material in the cloud, and in principle can be used to construct a three-dimensional picture of the cloud. In practice, this exercise is frustrated by the simultaneous effects of line saturation and radiative transfer, changes in the line excitation and elemental abundance variations across the source. These difficulties are not nightmares best forgotten, but rather are important diagnostic tools; by careful modeling, there is still an incredible amount of information that can be gleaned from these spectral line maps.

Looking at the distribution of spectral lines in molecular clouds can be visually illuminating. We have performed a comprehensive spectral line survey of the rho Ophiuchi molecular cloud. Below is an image of the submillimeter continuum emission from interstellar dust, courtesy of Doug Johnstone, and made with SCUBA at the JCMT; atop which we overplot the scope of our spectral line maps.

We use the AST/RO telescope to map out the entire cloud in submillimeter CO J=4-3 line emission at 461 GHz, and atomic carbon in the ³P₁-³P₀ transition at 492 GHz:

The small box marked "EW4" in the rho Oph overview is mapped with the 10-meter HHT in numerous molecular lines with 3000 AU resolution:

It is stunning to see how different molecular species probe different components of the cloud. The different lines of CO probe different depths into the cloud, owing to line opacity. HCO⁺ and CS both probe dense gas, yet look entirely different due to chemistry that enhances their abundances in different situations. Atomic carbon's abundance is a function of the incident ultraviolet radiation field and the subsequent shielding efficiency of dust grains and hydrogen molecules. All of these issues allow us to disentangle some of the complicated structure of molecular clouds and star forming regions.

More complex molecules provide insight into the processing of the elements in the vicinities of star formation. Molecules like methyl alcohol, CH₃OH, are formed predominantly in heated regions where complex interstellar ices can be evaporated from dust grains, for example, near a forming star. For example, in the high-mass young stellar object GL 2591, a single narrow spectrum from 337.5 GHz to 338.5 GHz shows a wealth of weak methanol lines.

Analysis of these lines allows us to use methanol as a kind of thermometer, probing the temperature of the material close to the star. In this case, observations of methanol bands at 241 and 338 GHz yield an estimate of the gas temperature where the methanol is being processed, of about 320 Kelvin (about 50C or 120F). This is very warm compared to the ambient cloud gas, which is at a temperature of about 40K (-233C, or -390F).

These high-temperature probes can be taken to an extreme. Adjacent to the bright, purely rotational 4-3 transition of HCN (hydrogen cyanide) at 354 GHz is a weak feature (in the center of the bandpass) due to the 4-3 rotational transition from a vibrationally-excited state. Comparison of this feature in the 3-2 and 4-3 transitions yields the gas temperature of collisionally-excited HCN, which is of the order of 1000K (700C, or 1300F). Most of the HCN vibrational emission is radiatively "pumped" from the bright mid-infrared continuum emission of the central source (GL 2591), yielding the location of the HCN emitting gas.

To be continued... watch this space...

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Craig Kulesa

Last modified: Tue Apr 16 15:29:13 MST 2002