Infrared OH absorption lines in 1612MHz OH maser sources

ISO mid-infrared absorption data helps to distinguish between radiative and collisional pumping mechanisms in 1612MHz OH masers in various environments. Archive data of OH absorption at 34.6 and 53.3 μm shows different behavior and different pumping rates for different types of maser sources. 1. Mid-Infrared absorption in 1612MHz OH masers Mid-infrared spectroscopy by the Infrared Space Observatory (ISO) first provided a way to directly distinguish between infrared pumping and collisional pumping in OH masers in various astrophysical environments using the presence of IR absorption lines. The ISO archive spectra for all known Galactic OH 1612MHz masers have been searches for OH related absorption. Absorption at 34.6μm is detected by the ISO Short Wavelength Spectrometer (SWS) in only 5 sources (out of 87) while at 53.3μm absorption is detected by the ISO Long wavelength Spectrometer (LWS) in 11 sources (out of 48). The non-detection of the two lines in most of the sources is due to the low spectral resolution and/or sensitivity of the ISO instruments. However, we found respectively 16 and 7 sources with spectra of good enough quality that do not show the 34.6 and 53.3μm absorption lines (He et al. 2005, He & Chen 2004) possibly due to clumpy shell or limb-filling emission. The spectra of the detected lines are reproduced in Fig. 1 (He et al. 2005, He & Chen 2004). Panel A shows the five 34.6μm line profiles while Panels B and C present the 53.3μm line profiles of the 11 sources. The spectra of the 53.3μm spectra in panels B and C are grouped according to the following classification: RaBe – redshifted absorption and blueshifted infilling emission, Asymm – asymmetrical profile, GCMega – symmetrical profile of GC or megamasers, and BaRe – blueshifted absorption and redshifted infilling emission. The weak infilling emission features are found to be ubiquitous: blueshifted RaBe infilling in stellar OH masers, and redshifted Asymm or BaRe infilling in interstellar OH masers. Infilling is not present in the symmetric GCMega profiles of GC and megamaser sources. The shifted infilling emission is possibly due to emission in only one of the 53.3μm doublet components. The alternative of a velocity shift of about 560 km/s (≈ 0.1μm) from the nominal line center is unreasonable in such sources. However, it is not clear how such doublet asymmetry arises in the two types of OH masers. The OH maser pumping routes tracing technique developed by M. Gray (2007, these proceedings) may provide an answer. The exact pump rate may be defined as: the integrated photon flux of the blue+red OH maser peaks divided by the integrated 35+53μm absorption photon flux. The pump rate was calculated for six sources with both infrared OH absorption lines (at least tentatively)