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NOAO Newsletter - NOAO Highlights - June 2000 - Number 62


A Nebulous, but Absorbing Probe

Based on a Solicited Contribution from Craig Kulesa

Craig Kulesa (Arizona) and John Black (Onsala Space Observatory) are taking advantage of the unique combination of high resolution and sensitivity afforded by NOAO's Phoenix spectrometer and the 2.1-m telescope at Kitt Peak to probe the intervening interstellar and circumstellar gas towards a sample of luminous young stellar objects embedded in molecular clouds. Their infrared spectroscopic study has produced the first direct, simultaneous observations of cold molecular hydrogen (H2), the pivotal molecular ion H3+, and CO in several isotopes. These measurements, which critically test both theoretical models and microwave observations of molecular clouds, provide unique insight into the physical properties of star-forming regions.


Caption: Both H2, the dominant constituent of molecular clouds, and the pivotal molecular ion H3+, are detected in absorption along a line of sight through the Flame Nebula, NGC 2024 (JHK composite image; spectrum taken taken at the location marked by the box). Vertical dashes indicate the expected locations of the absorption lines. The column densities represented by these observations are 3.5 x 1022 and 9.5 x 1012 cm-2 for H2 and H3+, respectively.

Although infrared H2 line emission is now routinely observed in very energetic molecular environments, the ubiquitous H2 molecule's widely spaced rotational energy levels and lack of permanent dipole moment renders it essentially invisible at the cold temperatures that prevail in dense molecular clouds. With the advent of the high-resolution infrared spectrometer Phoenix, it is now possible to detect the same infrared transitions of cold H2 in absorption towards bright infrared continuum sources embedded deeply within molecular material. Comparison of the abundance of H2 with other common tracers of molecular material, like CO, is of critical importance to understanding the physical structure and mass content of molecular clouds in both the Milky Way and external galaxies. Of the lines of sight observed so far, [CO/H2] ranges from (1-5) x 10-4, a significant departure from an often-assumed value of 8 x 10-5.

The pivotal molecular ion H3+ has long been predicted to be the cornerstone for the ion-molecule chemistry that is partly responsible for forming about 120 known molecules in molecular clouds. However H3+, which, like H2, lacks a permitted radio spectrum, has also eluded detection until quite recently. The measured abundances of H3+ are in good agreement with the abundances of observed species that stem from the existence of H3+ and generally confirm model expectations of gas phase chemistry in dense molecular clouds. Furthermore, these observations allow a direct determination of the cosmic ray ionization rate of H2. This parameter is a critical parameter for physical and chemical models of molecular clouds, since cosmic ray ionization is the dominant heating source in the UV-shielded cores of dense clouds. These observations with Phoenix also constrain the formation processes of warm H2O as measured by the Infrared Space Observatory (ISO) and the Submillimeter Wave Astronomy Satellite (SWAS) for at least two sources.

Observations of CO can be performed using the same infrared absorption techniques, a procedure that has important advantages over observing CO in emissions at [sub]millimeter wavelengths. In the infrared, an entire CO rotational-vibrational band spectrum can be obtained in a single integration, and all observations correspond to the same milli-arcsecond pencil-beam column of molecular gas. Physical conditions and abundances derived with this technique are more accurate and complete than those typically derived from single-dish radio observations.

These observations represent the first simultaneous detections of cold H2, H3+ and CO in a sample of dense molecular clouds where other species are already well measured. Follow-up spectroscopy and imaging at infrared and submillimeter wavelengths is now underway to map the environments surrounding these pencil-beam lines of sight to gain a more comprehensive understanding of the physical structure of molecular clouds and the evolution of the star-forming regions within them.

Caption: CO spectra obtained with Phoenix highlight the power of measuring physical conditions and chemical abundances of star-forming regions in the infrared. The high spectral resolution of Phoenix separates the ambient molecular cloud from molecular outflows in GL 2591, and an excitation analysis using the large number of lines uncovers multiple temperature components in both sources, even when kinematically indistinct.

For more information about this project, see http://loke.as.arizona.edu/~ckulesa/research/.


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