Craig Kulesa
Steward Observatory
933 N. Cherry Ave
Tucson, AZ 85721
15 July 2001
Although most interstellar molecules have been detected through mm-wave spectroscopy, pivotal non-polar molecules like H2 and H3+ have no radio spectrum and are essentially invisible in the prevailing cold conditions in molecular clouds and star forming regions. Radio astronomers have long adopted the CO molecule as a tracer for H2, but the quantitative relation between CO and H2 in dense clouds has been indirect at best. It has only been with the advent of sensitive high-resolution infrared spectrometers like Phoenix, that detection of cold H2 and H3+ has been recently achieved in absorption against the IR continuum of obscured sources behind molecular clouds. It is now possible to *directly* probe the bulk of the mass of molecular gas in H2, and measure the molecular ion H3+ that is responsible for initiating the chemistry that forms the majority of the 120+ molecules detected in interstellar clouds. Additionally, all absorption-line measurements probe a milliarcsecond "pencil beam" line of sight through the cloud for unparalleled spatial resolution. The tests of theory offered by these observations are both significant and many.
At the Kitt Peak 2.1-meter and 4-meter telescopes, we have successfully used Phoenix to perform the first simultaneous observations of H2, H3+ and CO toward a small sample of bright young stellar objects embedded in well-known molecular clouds [see NOAO Newsletter, June 2000]. We are now at the threshold of making this technique directly comparable to the mapping techniques of traditional mm-wave emission line spectroscopy. With Gemini, we should be able to "map" the structure and chemistry of a molecular cloud by analyzing multiple lines of sight through a single cloud. Comparison with [sub]millimeter spectroscopy mapping over the same region will provide a direct test of over 30 years of using interstellar CO as a tracer of H2. It will also test recent large-scale maps of near-infrared extinction as probes of the physical structure of molecular clouds.
As a demonstration of this project at Gemini, we propose a comprehensive spectroscopic study of several deeply embedded IR sources in a single well known molecular cloud. One such target for December observations is NGC 2024 (Flame Nebula, part of the Orion B complex), naturally pending team collaboration with other Demonstration Science proposers. For example, the starless clouds used in near-IR extinction mapping would also serve as excellent targets.
Successful completion of these observations will provide the astronomical community with unique observations of an important component of the Galaxy and will demonstrate the unique capabilities of Phoenix at Gemini South. The proposed spectroscopy will provide crucial tests of theoretical models of molecular clouds. Specifically, this observing program (1) establishes directly the abundance of cold H2 in a sample of dense clouds where many other molecules are observed, (2) measures the abundance, excitation, and physical environments of CO and 13CO in molecular clouds with an accuracy that cannot be achieved by radio frequency measurements, (3) constrains the cosmic ray ionization rate and H2O chemistry via direct observations of the pivotal molecular ion H3+, and (4) complements and tests related medium-resolution spectroscopy with ISO/SIRTF, submillimeter-wave spectroscopy at telescopes like the JCMT, CSO and HHT, and near-IR extinction mapping using instruments like FLAMINGOS, Pisces (MMT) and NICMOS on HST.
A stand-alone data reduction pipeline is currently in development.
Additional responsibilities of the proposers:
Sincerely,
Craig A. Kulesa
Steward Observatory/University of Arizona
John H. Black
Onsala Space Observatory
