Molecular masers: a unique tool for Milky Way dynamics and star formation

The process of formation of high-mass (> 7 M_⊙) stars presents still many open questions. The massive (proto)stars start to burn hydrogen while still accreting mass from the natal cocoon, and emit such a powerful ionizing radiation to halt the mass infall, with the contradictory result that high-mass stars could never form. Numerical simulations of the formation of massive stars (see, for instance, Kuiper et al. 2010) indicate that disk-mediated accretion could partly solve this paradox, by boosting the mass infall rate through the equatorial plane and beaming the ionizing photons along the perpendicular direction. To test these models, the physical conditions and kinematics of the gas close to the massive (proto)stars have to be determined.

Molecular masers

Several intense maser (microwave amplified stimulated emission radiation) lines of relatively abundant molecules as hydroxyl (OH), water (H2O) and methanol (CH3OH) are observed in high-mass star forming regions. The maser emission originates from multiple compact spots with size of a few milliarcsec (mas), each of them emitting over a narrow bandwidth < 1 km/s.

The motion of the persistent maser spots can be tracked on the plane of the sky, which, combined with the line of sight (LOS) velocities measured through the Doppler effect, provides the three-dimensional (3D) gas velocities. Therefore the maser emission is an unique tool to sample the full kinematics of the gas surrounding the high-mass (proto)stars with superb spatial and velocity resolution.

Trigonometric parallax and the BeSSeL project

The sky-projected trajectory of a given maser spot relative to a very distant (extragalactic) source is the combination of a linear (superposition of the differential Galactic rotation and the peculiar motions of the sun and the target) and an elliptical pattern corresponding to the projection along the LOS of the earth orbit around the sun. The measurement of the major-axis of this ellipse provides the trigonometric parallax of the maser source and a very accurate determination of the distance of the region where the masers reside. Left panel of Figure 1 shows the maser parallax towards the source IRAS 20126+4104, whose derived distance is1.64 +- 0.05 kpc (Moscadelli et al. 2011).

The Arcetri team participates in the international Bar and Spiral Structure Legacy (BeSSeL) program, whose main goal is to derive the structure and kinematics of the Milky Way (MW) by measuring accurate positions, distances (via trigonometric parallaxes) and proper motions (PM) of 6.7~GHz methanol and 22~GHz water masers in hundreds of high-mass star forming regions distributed over the Galactic Disk (Reid et al. 2014, 2019). The BeSSeL observations has already provided absolute positions and velocities of individual masers with accuracies of a few mas and a few km/s, respectively, for hundreds of sources. Right panel of Figure 2 shows that the maser positions delineate the spiral arms of the MW disk.

Fig. 1: Left: Parallax measurement of the source IRAS 20126+4104 (Moscadelli et al. 2011). Residual motions of two different maser spots (red and green) in right ascension and declination, obtained after subtraction of the linear motion. The curves are the fits consisting only of the annual parallax. The solid and dashed curves correspond, respectively, to the RA and Dec offsets. Right: Plan view of the Milky Way from the north Galactic pole showing locations of high-mass star-forming regions with measured maser trigonometric parallaxes (Reid et al. 2019). The Galactic center (red asterisk) is at (0,0) and the Sun (red Sun symbol) is at (0, 8.15) kpc. Colors indicate the assignment of the maser sources to different spiral arms: 3-kpc arm, yellow; Norma-Outer arm,r ed; Scutum-Centaurus-OSC arm, blue; Sagittarius-Carina arm, purple; Local arm, cyan; Perseus arm, black.

Disks and jets in high-mass (proto)stars

Accurate distances for hundreds of star-forming regions are a fundamental contribution provided by the maser observations to star-formation studies. Besides, a very detailed picture of high-mass star formation is obtained by combining the 3D gas velocities at scales of 10-100 au measured with the masers, with the information on the physical and kinematical conditions of the surrounding ambient at scales of 100-1000 au through sensitive (cm and mm) interferometric (JVLA and ALMA) observations. Using this strategy, we have identified the kinematic structures traced by the different types of molecular masers and characterized their properties. We employ the 22 GHz water masers, emerging from shocked molecular gas at the interface between the fast flow and the ambient material, to study the structure of (proto)stellar outflows close to (within 10–100 AU from) the high-mass (proto)stars (Moscadelli et al. 2007, 2011, 2013). The 6.7 GHz methanol masers trace more quiescent gas than the water masers, possibly located in the area of interaction between the fast protostellar jet and the surrounding disk/envelope (Sanna et al. 2010). A good example of multiple maser observations is that of the high-mass (proto)star IRAS20126+4104, shown in Fig. 2, where the water masers emerge at the surface of a fast collimated, conical jet, and the methanol masers are separated in two kinematically-distinct groups: group~1 include spots with small PMs mainly tracing the edge-on disk rotation; group~2 have larger PMs directed almost parallel to the jet axis, marking material entrained in the flow.

Fig. 2: Water (triangles) and methanol (dots) masers in IRAS 20126+4104 (Moscadelli et al. 2011). The cones indicate the 3D velocities of the maser features with respect to the star, for the H2O masers, or relative to feature 3 (marked with a cross), for the CH3OH masers The starred polygon marks the location of the star, obtained from a model fit to the H2O maser positions and 3D velocities. Labels 1 and 2 denote two elongated groups of CH3OH maser features.

The last phases of a high-mass (proto)star: hypercompact HII regions

The formation process and early evolution of OB stars must heavily depend on the interaction between the stellar radiation pressure and ionizing ux, on the one hand, and the accreting material, on the other. Models (see, for instance, Keto et al. 2003) predict that infall may confine the HII region inside a small radius and that accretion can continue through the ionized gas. Figure 3 shows that, towards the hypercompact HII region G24,78+0.08 A1, the water masers draw an arc at the border of the ionization front and their PMs indicate a motion of recession from the centre of the ionized gas. We are investigating wether the masers are tracing the final expansion of the HII region or just a wide bow-shock ahead of a fast outflow ejected from the (proto)star at the centre of the HII region.

Fig. 3: 3-D motions of the water masers in G24.78+ 0.08 A1 (Moscadelli et al. 2007, 2018). Colored dots and arrows show the positions and proper motions of the water masers. The grayscale image and black contours (10 to 90%, at step of 10% of the image peak of 11 mJy beam-1) reproduce the VLA A-Array 7mm continuum.

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