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1. Discovery of 22 GHz Water Masers in the Serpens South Region

   https://doi.org/10.3847/1538-3881/ac052c  

   Ortiz-León, Gisela N.; Plunkett, Adele L.; Loinard, Laurent; Dzib, Sergio A.; Rodríguez-Garza, Carolina B.; Pillai, Thushara; Gong, Yan; Brunthaler, Andreas (July 2021, The Astronomical Journal)

   Abstract Using the Karl G. Jansky Very Large Array (VLA), we have conducted a survey for 22 GHz, 6 1,6 –5 2,3 H 2 O masers toward the Serpens South region. The masers were also observed with the Very Long Baseline Array following the VLA detections. We detect for the first time H 2 O masers in the Serpens South region that are found to be associated to three Class 0–Class I objects, including the two brightest protostars in the Serpens South cluster, known as CARMA-6 and CARMA-7. We also detect H 2 O masers associated to a source with no outflow or jet features. We suggest that this source is most probably a background asymptotic giant branch star projected in the direction of Serpens South. The spatial distribution of the emission spots suggest that the masers in the three Class 0–Class I objects emerge very close to the protostars and are likely excited in shocks driven by the interaction between a protostellar jet and the circumstellar material. Based on the comparison of the distributions of bolometric luminosity of sources hosting 22 GHz H 2 O masers and 162 young stellar objects covered by our observations, we identify a limit of L Bol ≈ 10 L ⊙ for a source to host water masers. However, the maser emission shows strong variability in both intensity and velocity spread, and therefore masers associated to lower-luminosity sources may have been missed by our observations. We also report 11 new sources with radio continuum emission at 22 GHz. 

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2. Protostellar and Protoplanetary Disk Masses in the Serpens Region

   https://doi.org/10.3847/1538-4357/ac8ff0  

   Anderson, Alexa R.; Williams, Jonathan P.; Marel, Nienke van der; Law, Charles J.; Ricci, Luca; Tobin, John J.; Tong, Simin (October 2022, The Astrophysical Journal)

   Abstract We present the results from an Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm continuum and¹²CO (J= 2 − 1) line survey spread over 10 deg²in the Serpens star-forming region of 320 young stellar objects, 302 of which are likely members of Serpens (16 Class I, 35 flat-spectrum, 235 Class II, and 16 Class III). From the continuum data, we derive disk dust masses and show that they systematically decline from Class I to flat-spectrum to Class II sources. Grouped by stellar evolutionary state, the disk mass distributions are similar to other young (<3 Myr) regions, indicating that the large-scale environment of a star-forming region does not strongly affect its overall disk dust mass properties. These comparisons between populations reinforce previous conclusions that disks in the Ophiuchus star-forming region have anomalously low masses at all evolutionary stages. Additionally, we find a single deeply embedded protostar that has not been documented elsewhere in the literature and, from the CO line data, 15 protostellar outflows, which we catalog here. 

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3. Evidence for an Interaction between the Galactic Center Clouds M0.10–0.08 and M0.11–0.11

   https://doi.org/10.3847/1538-4357/ac887c  

   Butterfield, Natalie O.; Lang, Cornelia C.; Ginsburg, Adam; Morris, Mark R.; Ott, Jürgen; Ludovici, Dominic A. (September 2022, The Astrophysical Journal)

   Abstract We present high-resolution (∼2–3″; ∼0.1 pc) radio observations of the Galactic center cloud M0.10−0.08 using the Very Large Array at K and Ka band (∼25 and 36 GHz). The M0.10−0.08 cloud is located in a complex environment near the Galactic center Radio Arc and the adjacent M0.11−0.11 molecular cloud. From our data, M0.10−0.08 appears to be a compact molecular cloud (∼3 pc) that contains multiple compact molecular cores (5+; <0.4 pc). In this study, we detect a total of 15 molecular transitions in M0.10−0.08 from the following molecules: NH 3 , HC 3 N, CH 3 OH, HC 5 N, CH 3 CN, and OCS. We have identified more than sixty 36 GHz CH 3 OH masers in M0.10−0.08 with brightness temperatures above 400 K and 31 maser candidates with temperatures between 100 and 400 K. We conduct a kinematic analysis of the gas using NH 3 and detect multiple velocity components toward this region of the Galactic center. The bulk of the gas in this region has a velocity of 51.5 km s −1 (M0.10−0.08) with a lower-velocity wing at 37.6 km s −1 . We also detect a relatively faint velocity component at 10.6 km s −1 that we attribute to being an extension of the M0.11−0.11 cloud. Analysis of the gas kinematics, combined with past X-ray fluorescence observations, suggests M0.10−0.08 and M0.11−0.11 are located in the same vicinity of the Galactic center and could be physically interacting. 

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4. An Updated Reference Frame for the Galactic Inner Parsec

   https://doi.org/10.3847/1538-4357/acec71  

   Darling, Jeremy; Paine, Jennie; Reid, Mark J.; Menten, Karl M.; Sakai, Shoko; Ghez, Andrea (September 2023, The Astrophysical Journal)

   Abstract Infrared observations of stellar orbits about Sgr A* probe the mass distribution in the inner parsec of the Galaxy and provide definitive evidence for the existence of a massive black hole. However, the infrared astrometry is relative and is tied to the radio emission from Sgr A* using stellar SiO masers that coincide with infrared-bright stars. To support and improve this two-step astrometry, we present new astrometric observations of 15 stellar SiO masers within 2 pc of Sgr A*. Combined with legacy observations spanning 25.8 yr, we reanalyze the relative offsets of these masers from Sgr A* and measure positions and proper motions that are significantly improved compared to the previously published reference frame. Maser positions are corrected for epoch-specific differential aberration, precession, nutation, and solar gravitational deflection. Omitting the supergiant IRS 7, the mean position uncertainties are 0.46 mas and 0.84 mas in R.A. and decl., and the mean proper motion uncertainties are 0.07 mas yr⁻¹and 0.12 mas yr⁻¹, respectively. At a distance of 8.2 kpc, these correspond to position uncertainties of 3.7 and 6.9 au and proper motion uncertainties of 2.7 and 4.6 km s⁻¹. The reference frame stability, the uncertainty in the variance-weighted mean proper motion of the maser ensemble, is 8μas yr⁻¹(0.30 km s⁻¹) in R.A. and 11μas yr⁻¹(0.44 km s⁻¹) in decl., which represents a 2.3-fold improvement over previous work and a new benchmark for the maser-based reference frame. 

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5. Kinematics in the Galactic Center with SiO masers

   https://doi.org/10.1017/S1743921323002272  

   Paine, Jennie; Darling, Jeremy (February 2024, Proceedings of the International Astronomical Union)

   Hirota, T; Imai, H; Menten, K; Pihlström, Y (Ed.)

   Abstract Stellar SiO masers are found in the atmospheres of asymptotic giant branch (AGB) stars with several maser transitions observed around 43 and 86 GHz. At least 28 SiO maser stars have been detected within ∼2 pc projected distance from Sgr A* by the Very Large Array (VLA) and Atacama Millimeter/submillimeter Array (ALMA). A subset of these masers have been studied for several decades and form the basis of the radio reference frame that anchors the reference frame for infrared stars in the Galactic Center (GC). We present new observations of the GC masers from VLA and ALMA. These new data combined with extant maser astrometry provide 3D positions, velocities, and acceleration limits. The proper motions and Doppler velocities are measured with unprecedented precision for these masers. We further demonstrate how these measurements may be used to trace the stellar and dark matter mass distributions within a few pc of Sgr A*. 

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