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.
The left panel shows the spectral profile of a maser cloudlet, usually referred to as a maser spectral feature. Each point in this spectrum corresponds to a single maser spot, which is defined by its velocity in the Local-Standard-of-Rest (LSR) within a given spectral resolution. The maser feature has a Gaussian profile with line-width of a few tenths of kilometer-per-second only.
The middle panel shows the actual appearance of the maser cloudlet in space, which extends for a few tens of milli-arcseconds. By repeating the same observation after some months (or years, depending on the heliocentric distance), one will measure a similar map where the cloudlet has moved to a different position, tracing the maser proper motion.
The right panel shows the distribution of individual spots (colored circles) that, grouped together, make up the cloudlet imaged in the middle panel. In this kind of plot, each circle marks the peak position of a spot and has a counterpart in the spectral feature to the left (same symbols). This visual representation is useful to show that a cloudlet has a regular sub-structure, which reflects internal gas motions (check here for a direct application of these internal gradients).
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).
This emission was observed towards one of the strongest Galactic methanol maser sources, in the star-forming region G023.01-00.04, at 5 different epochs (as listed to the right).
At each epoch, this spectrum is the composition of tens of individual maser spectral features (and thus cloudlets), whose emission all together ranges over ca. 10 km/s in velocity and reaches flux densities of hundreds of Jansky.