The Formaldehyde Masers in Sgr B2: Very Long Baseline Array and Very Large Array Observations

Abstract

Observations of two of the formaldehyde (H2CO) masers (A and D) in Sgr B2 using the VLBA+Y27 (resolution ~0.01") and the VLA (resolution ~9") are presented. The VLBA observations show compact sources (<10 milliarcseconds, <80 AU) with brightness temperatures >10^8 K. The maser sources are partially resolved in the VLBA observations. The flux densities in the VLBA observations are about 1/2 those of the VLA; and, the linewidths are about 2/3 of the VLA values. The applicability of a core-halo model for the emission distribution is demonstrated. Comparison with earlier H2CO absorption observations and with ammonia (NH3) observations suggests that H2CO masers form in shocked gas. Comparison of the integrated flux densities in current VLA observations with those in previous observations indicates that (1) most of the masers have varied in the past 20 years, and (2) intensity variations are typically less than a factor of two compared to the 20-year mean. No significant linear or circular polarization is detected with either instrument.

20 pages, 3 figures, 5 tables, accepted to ApJ

References in corpus (10)

• Unified Schemes for Radio-Loud Active Galactic Nuclei
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• Water Masers in W49 North and Sagittarius B2
• VLBA Observations of Astrometric Reference Sources in the Galactic Center
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Cited by in corpus (13)

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• An H2CO 6cm Maser Pinpointing a Possible Circumstellar Torus in IRAS18566+0408
• The Emission Structure of Formaldehyde MegaMasers
• New Maser Emission from Nonmetastable Ammonia in NGC 7538. IV. Coincident Masers in Adjacent States of Para-ammonia
• New Maser Emission from Nonmetastable Ammonia in NGC 7538
• Masers and star formation
• The physical and chemical structure of Sagittarius B2, VIII. Full molecular line survey of hot cores
• Formaldehyde and H110a observations towards 6.7 GHz methanol maser sources
• A Review of H2CO 6cm Masers in the Galaxy
• Long-term Variability of H${}_2$CO Masers in Star-forming Regions
• Discovery of non-metastable ammonia masers in Sagittarius B2