Phenomenology

The proto-stellar environment where young stars form has physical conditions suitable to induce population inversions between specific pairs of energy levels in several abundant molecules, including water (H2O), methanol (CH3OH), hydroxyl (OH), ammonia (NH3), silicon monoxide (SiO), and formaldehyde (H2CO). The resulting non-thermal maser emission (acronym of, Microwave Amplification by Stimulated Emission of Radiation) in the corresponding spectral transitions provides a beacon whose brightness temperature (typically brighter than 100 million K) far exceeds that of the more commonly-excited thermal emission lines. 

Maser emission arises from compact parcels (blobs) of gas with size from a few to several astronomical units, called cloudlets (middle panel below), where gas particles coherently move with quasi-constant velocities along the line-of-sight. Under this condition, the stimulated radiation of individual particles (either atoms or molecules) can be amplified and "beamed" towards the observer, giving rise to measured flux densities much larger than a Jansky (Jy) typically. Since the intrinsic line-width of the maser radiation is also very narrow (left panel below), and smaller than 1 km/s in most cases, these properties allow for a clear cut with respect to (molecular) lines that are excited by thermal processes, whose line broadening, for instance, implies widths an order of magnitude larger at average inter-stellar conditions. 

Maser maps typically show a large number of bright and compact spots at different velocities (right panel below) that can be grouped into individual cluodlets. Their enhanced brightness temperature has provided the sole target for spectroscopic Very Long Baseline Interferometry (VLBI) observations at an instrumental sensitivity of ca. 1 mJy/beam. The exceptional angular resolution of VLBI observations, of the order of the milli-arcsecond (mas) at centimeter wavelengths, is the only one able to resolve single maser spots at typical heliocentric distances of 1 kpc and more. Since early ‘80 (e.g., Genzel et al. 1981; Gwinn et al. 1992), iterated VLBI observations of maser spots have been used to image how individual cloudlets move in time on the plane-of-the-sky. This latter velocity component  (called, the maser proper motion) can be combined with the maser line-of-sight velocity (VLSR), inferred from the Doppler-shift of the maser line, providing all-together the full-space motion of local gas. In this context, maser cloudlets can be thought as gas bullets that provide an unique probe of star formation kinematics.

        for a reference (historical) summary about masers

                                                                                                                                                                                                                                                                                          for a reference (recent) study about maser kinematics

What are we tracing?

When talking about gas dynamics near young forming stars, and typically within radii of a few 1000s au only, one commonly is reminded of two major circum-stellar structures, namely, what we generally call disk-like structures and the jets and winds launched from these regions. In the literature, two molecular masers, water and methanol, have traditionally been used to study these complementary environments, whose association with either one or the other maser species directly follows from the different excitation mechanisms. Firstly, water masers in the 22.2 GHz transition, whose pumping mechanism is predominantly collisional, were identified as tracer of shock phenomena in the vicinity of stars (e.g., Elitzur et al. 1989, 1992; Kaufman & Neufeld 1996; Hollenbach et al. 2013), either very young or evolved, and their observations has been key to reveal the dynamics of fast shocked layers of gas moving away from the star. Subsequently, methanol masers in the 6.7 GHz transition, whose pumping mechanism is predominantly radiative, were identified as tracer of warm and quiescent gas in the inner circum-stellar regions of massive stars, where the requirements of an IR field and high methanol column densities are satisfied (e.g., Cragg et al. 1992, 2005; Sobolev et al. 1997). 

The spectrum below shows an example of a rich Galactic site of methanol maser emission revealing the presence of a young massive star.  By observing this emission at VLBI resolution, one can parcel out the spectrum in single spectral features and thus cloudlets, measuring their motions (see the corresponding map in the slideshow). 

                                                                                                                                                                                                                                                                                     for a recent summary about maser science  perspectives