Western Illinois University: Macomb Campus
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Department of Physics
Student Seminar on Wednesday, April 20
Apr 14, 2011
PHYS600 Wednesday, April 20, 4:00-4:50 pm
Currens Hall 205
All physics faculty and students are invited.
Speaker: Liang Yuan, (Graduate Student, Physics Department, WIU)
Title: "The Nature of Formaldehyde 2 cm Emission in NGC 7538 IRS1"
Formaldehyde is a tracer of high density gas in massive star forming regions. The K-doublet lines from the three lowest rotational energy levels of ortho-formaldehyde correspond to wavelengths of 6, 2 and 1 cm. Thermal emission of these transitions is rare, and maser emission has only been detected in the 6 cm line. NGC 7538 is an active site of massive star formation in the Galaxy, and one of only a few regions known to harbor 6 cm formaldehyde (H2CO) masers. Using the NRAO 100 m Green Bank Telescope (GBT), we detected 2 cm H2CO emission toward NGC 7538 IRS1. The velocity of the 2 cm H2CO line is very similar to the velocity of one of the 6 cm H2CO masers but the linewidth is greater. To investigate the nature of the 2 cm emission, we conducted observations of the 1 cm H2CO transition, and obtained a cross-scan map of the 2 cm line. We detected 1 cm emission and found that the 2 cm emission is extended (61"), which implies brightness temperatures of <0.2 K. Assuming optically thin mission, LTE, and that the 1 cm and 2 cm lines originate from the same volume of gas, both these detections are consistent with thermal emission of gas at <40 K. We conclude that the 1 cm and 2 cm H2CO lines detected with the GBT are thermal, which implies molecular densities above ~10^5 cm^-3.