An interferometric study of molecular emission around NGC 7538 at millimeter wavelengths
Pratap, Preethi
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https://hdl.handle.net/2142/23635
Description
Title
An interferometric study of molecular emission around NGC 7538 at millimeter wavelengths
Author(s)
Pratap, Preethi
Issue Date
1990
Doctoral Committee Chair(s)
Snyder, Lewis E.
Department of Study
Physics, Astronomy and Astrophysics
Discipline
Physics, Astronomy and Astrophysics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Astronomy and Astrophysics
Language
eng
Abstract
NGC 7538 is a visible H II region with an associated molecular cloud, situated in the Perseus arm of the Galaxy. A group of infrared sources, IRS 1, IRS 2, and IRS 3, is situated at the interface between the molecular cloud and the visible nebula. Maser emission from OH, H$\sb2$O, H$\sb2$CO, CH$\sb3$OH, and NH$\sb3$ has been detected toward IRS 1. In order to examine the model for the excitation of the formaldehyde (H$\sb2$CO) maser, proposed by Boland and de Jong (1981), interferometric observations of the cloud around IRS 1 have been made in the J = 1-0 transitions of HCN, HCO$\sp{+}$, and $\sp{13}$CO. The HCN and HCO$\sp{+}$ maps show the presence of a cavity around IRS 1 which is not seen in the $\sp{13}$CO maps. Since HCN and HCO$\sp{+}$ trace densities $>$10$\sp5$ cm$\sp{-3}$, the cavity indicates a lack of high density material around the H II region. Continuum observations at 88 GHz and 110 GHz result in a 2-component model for the H II region. There is a cool, extended component toward which all the masers are seen. The parameters calculated for this component, along with the H$\sb2$ densities ($\sim$10$\sp4$ cm$\sp{-3}$) from the molecular observations, were put into the H$\sb2$CO master model. The results show that the H$\sb2$CO gas must be at a distance of 0.011 pc from the H II region for maximum inversion in the H$\sb2$CO transition and the abundance of H$\sb2$CO relative to H$\sb2$ must be $\sim$4 $\times$ 10$\sp{-7}$.
"The molecular observations also show that the molecular cloud is extremely clumpy. These clumps around IRS 1 could provide the varying densities required for the other masers observed in the region. The HCO$\sp{+}$ results also indicate the presence of a high density molecular outflow which originates about 15"" south of IRS 1. The position angle of the outflow agrees with that of a CO outflow detected toward this source. The results imply that there may be another source powering the outflow which is situated south of IRS 1. The new source appears to be situated at the center of an elongated structure, also seen in HCO$\sp{+}$, which is oriented perpendicular to the direction of the outflow."
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