HERSCHEL/PACS SPECTROSCOPIC SURVEY OF PROTOSTARS IN ORION: THE ORIGIN OF FAR-INFRARED CO EMISSION
- Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627 (United States)
- Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218 (United States)
- Department of Physics and Astronomy, University of Toledo, 2801 West Bancroft Street, OH 43606 (United States)
- European Space Agency, ESAC/SRE-OAH, P.O. Box 78, E-28691 Villanueva de la Canada, Madrid (Spain)
- Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109 (United States)
- National Radio Astronomy Observatory, Charlottesville, VA 22903 (United States)
- Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, D-69117 Heidelberg (Germany)
- NHSC/IPAC/Caltech, 770 South Wilson Avenue, Pasadena, CA 91125 (United States)
- US Naval Research Laboratory, Code 7210, Washington, DC 20375 (United States)
- National Research Council of Canada, Herzberg Institute of Astrophysics, Department of Physics and Astronomy, University of Victoria, Victoria, BC V9E 2E7 (Canada)
- Instituto de Astrofisica de Andalucia, CSIC, Camino Bajo de Huetor 50, E-18008 Granada (Spain)
We present far-infrared (57-196 {mu}m) spectra of 21 protostars in the Orion molecular clouds. These were obtained with the Photodetector Array Camera and Spectrometer (PACS) on board the Herschel Space observatory as part of the Herschel Orion Protostar Survey program. We analyzed the emission lines from rotational transitions of CO, involving rotational quantum numbers in the range J {sub up} = 14-46, using PACS spectra extracted within a projected distance of {approx}<2000 AU centered on the protostar. The total luminosity of the CO lines observed with PACS (L {sub CO}) is found to increase with increasing protostellar luminosity (L {sub bol}). However, no significant correlation is found between L {sub CO} and evolutionary indicators or envelope properties of the protostars such as bolometric temperature, T {sub bol}, or envelope density. The CO rotational (excitation) temperature implied by the line ratios increases with increasing rotational quantum number J, and at least 3-4 rotational temperature components are required to fit the observed rotational diagram in the PACS wavelength range. The rotational temperature components are remarkably invariant between protostars and show no dependence on L {sub bol}, T {sub bol}, or envelope density, implying that if the emitting gas is in local thermodynamic equilibrium, the CO emission must arise in multiple temperature components that remain independent of L {sub bol} over two orders of magnitudes. The observed CO emission can also be modeled as arising from a single-temperature gas component or from a medium with a power-law temperature distribution; both of these require sub-thermally excited molecular gas at low densities (n(H{sub 2}) {approx}< 10{sup 6} cm{sup -3}) and high temperatures (T {approx}> 2000 K). Our results suggest that the contribution from photodissociation regions, produced along the envelope cavity walls from UV-heating, is unlikely to be the dominant component of the CO emission observed with PACS. Instead, the 'universality' of the rotational temperatures and the observed correlation between L {sub CO} and L {sub bol} can most easily be explained if the observed CO emission originates in shock-heated, hot (T {approx}> 2000 K), sub-thermally excited (n(H{sub 2}) {approx}< 10{sup 6} cm{sup -3}) molecular gas. Post-shock gas at these densities is more likely to be found within the outflow cavities along the molecular outflow or along the cavity walls at radii {approx}> several 100-1000 AU.
- OSTI ID:
- 22167155
- Journal Information:
- Astrophysical Journal, Vol. 763, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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