Universität zu Köln

Yusef-Zadeh, F., Wardle, M., Lis, D., Viti, S., Brogan, C., Charnbers, E., Pound, M. and Rickert, M. (2013). 74 MHz Nonthermal Emission from Molecular Clouds: Evidence for a Cosmic Ray Dominated Region at the Galactic Center. J. Phys. Chem. A, 117 (39). S. 9404 - 9420. WASHINGTON: AMER CHEMICAL SOC. ISSN 1089-5639

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Abstract

We present: 74 MHz radio continuum observations of the Galactic center region. These measurements show nonthermal radio emission arising from molecular clouds that is unaffected by free-free absorption along the line of sight We focus on one cloud, G0.13-0.13, representative of the population of molecular clouds that are spatially correlated with steep spectrum (alpha(74MHz)(327MHz) = 1.3 +/- 0.3) nonthermal emission from the Galactic center region. This cloud lies adjacent to the nonthermal radio filaments of the Arc near l similar to 0.2 degrees and is a strong source of 74 MHz continuum, SiO (2-1), and Fe I K alpha 6.4 keV line emission. This three-way correlation provides the most compelling evidence yet that relativistic electrons, here traced by 74 MHz emission, are physically associated with the G0.13-0.13 molecular cloud and that low energy cosmic ray electrons are responsible for the Fe I K alpha line emission. The high cosmic ray ionization rate similar to 10(-13) s(-1) H-1 is responsible for heating the molecular gas to high temperatures and allows the disturbed gas to maintain a high velocity dispersion. Large velocity gradient (LVG) modeling of multitransition SiO observations of this cloud implies H-2 densities similar to 10(4-5) cm(-3) and high temperatures. The lower limit to the temperature of G0.13-0.13 is similar to 100 K, whereas the upper limit is as high as 1000 K. Lastly, we used a time-dependent chemical model in which cosmic rays drive the chemistry of the gas to investigate for molecular line diagnostics of cosmic ray heating. When the cloud reaches chemical equilibrium, the abundance ratios of HCN/HNC and N2H+/HCO+ are consistent with measured values. In addition, significant abundance of SiO is predicted in the cosmic ray dominated region of the Galactic center. We discuss different possibilities to account for the origin of widespread SiO emission detected from Galactic center molecular clouds.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Yusef-Zadeh, F. UNSPECIFIED UNSPECIFIED UNSPECIFIED Wardle, M. UNSPECIFIED UNSPECIFIED UNSPECIFIED Lis, D. UNSPECIFIED UNSPECIFIED UNSPECIFIED Viti, S. UNSPECIFIED UNSPECIFIED UNSPECIFIED Brogan, C. UNSPECIFIED UNSPECIFIED UNSPECIFIED Charnbers, E. UNSPECIFIED UNSPECIFIED UNSPECIFIED Pound, M. UNSPECIFIED UNSPECIFIED UNSPECIFIED Rickert, M. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-474281
DOI:	10.1021/jp311240h
Journal or Publication Title:	J. Phys. Chem. A
Volume:	117
Number:	39
Page Range:	S. 9404 - 9420
Date:	2013
Publisher:	AMER CHEMICAL SOC
Place of Publication:	WASHINGTON
ISSN:	1089-5639
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language ABSORPTION-LINE SURVEY; X-RAY; SIO EMISSION; LARGE-SCALE; THERMAL SIO; GAS; SAGITTARIUS; (HCO+)-C-13; WARM; ABUNDANCES Multiple languages Chemistry, Physical; Physics, Atomic, Molecular & Chemical Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/47428