Search for Collisionally Pumped 1720 MHz OH Masers in Star-forming Regions: A VLA Survey of 18 cm OH Masers toward 80 Class I Methanol Masers
Class I methanol masers (cIMMs) and 1720 MHz OH masers are believed to arise from a common collisional pumping mechanism, and both trace shocked gas regions in supernova remnants (SNRs). However, their coexistence in star formation regions (SFRs) and their association with a front of bipolar outflow (a source of shock-stimulated collisional pumping) remain unclear. To search for collisionally pumped OH(1720) masers, we conducted a Very Large Array survey of the 18 cm OH masers and continuum emission toward a sample of 80 SFRs associated with 44 GHz cIMMs. Main-line OH maser emission was detected in 50% of the sample, and OH(1720) maser emission was detected in 20%. Continuum emission was detected in 28% of sources. A catalog of the detected OH masers is presented. Individual OH masers are found in close proximity, and the regions of masers of different transitions are often overlapping. A typical linear projected offset between the OH(1720) and OH(1665) masers is ~0.04 pc. A remarkable number of ~81% of the OH(1720) masers are associated with the continuum emission. The median separation between the targeted 44 GHz cIMMs and OH(1720) masers is ~0.2 pc, which is similar to reported distances between cIMMs and H II regions. The observed properties of the detected OH(1720) masers are different from those in SNRs and indicate that a nonlocal line overlap mechanism is responsible for their excitation. Thus, while both 44 GHz cIMMs and OH(1720) masers trace shocked gas in SNRs, physical conditions favorable for excitation of cIMMs, but not OH(1720) masers, are present in SFRs.