The Relationship Between Masers and Massive Star Formation: What Can Be Learned from the Infrared?

The infrared represents an alternative wavelength regime in which to study the environments of maser emission, while at the same time complementing the information obtained through radio techniques. The near infrared (1-2 μm) yields information on outflows, shocks, and reflected dust emission, while the thermal infrared (3-30 μm) yields information on the thermal dust distribution around stars. Thus, the infrared regime yields important clues in determining whether masers exist in shocks, outflows, circumstellar accretion disks, or in the dense medium close to protostars. 1. How Does Infrared Complement Radio? Observations of centimeter radio continuum sources in the vicinity of galactic masers established the idea that masers are related to young massive stars. However, massive stars of spectral types later than B3 do not generate enough ionizing flux to create observable UCHII regions, given the sensitivities of modern centimeter radio telescopes and the kiloparsec distances to typical massive star forming regions. Furthermore, accurate astrometry from connected element interferometers has shown that masers are often not coincident with UCHII regions. Therefore, in order to figure out exactly how masers are related to young massive stars, and to observe the environments of the masers, one needs to observe at wavelengths other than centimeter. However, there is generally large extinction due to dust and gas in the environments near massive stars that are in the process of forming. This makes observing young massive stars difficult or impossible at wavelengths less the 1 μm. One could use far-infrared and submillimeter instruments, but these technologies presently do not have the spatial resolution to give detailed information on the maser environments close to young stellar sources. This means that the near and thermal infrared are presently the best alternative ground-based spectral regimes in which to study the formation of the individual massive stars and their nearby environments, and how they relate to maser emission. In star forming regions, near infrared (1-2 μm) photons are usually photospheric emission or reflected and scattered photospheric emission off of dust near a star. The near-infrared regime is valuable because observations can peer through the extinction near newly forming stars, if the stars are not too embedded. The thermal infrared (3-30 μm) refers to the spectral regime which one looks at heat wavelength information. When referring to star formation re-