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We report observations of oxygen-rich (O-rich) radio molecular line tracers, including H2CO, H2O, and HCO+ toward 28 silicate carbon stars (SCSs). SCSs are special evolved star systems that consist of a carbon star associated with peculiar O-rich circumstellar material. Absorption in the 6 cm transition was detected from seven SCSs using the Shanghai 65 m Tianma radio telescope. In order to further investigate their O-rich material, several 3 mm molecular lines including HCO+ and CO were also observed with the Purple Mountain Observatory 13.7 m telescope. Double-peak profiles in some of these lines suggest the possible presence of a companion in the SCS system. A negative association between H2O masers and H2CO absorption was found in our sample of SCSs. The maser sources may correspond to an earlier evolutionary stage when the circumstellar envelope (CSE) material is O-rich and limited to a compact and denser region. As the star evolves the CSE gradually expands and carbon abundance is enhanced, resulting in the masers disappearing and H2CO absorption arising. [ABSTRACT FROM AUTHOR]

We report the detection of a new class I methanol maser candidate from the 52−41E transition (266.8 GHz). This methanol transition has been detected toward a nearby high-mass star-forming region G352.630-1.067 (distance ∼0.7 kpc), in Submillimeter Array (SMA) observations. The new candidate transition has a similar spatial distribution as the 42−31E (218.4 GHz) and 8−1−70E (229.7 GHz) transitions, which are known class I maser transitions. Thermal methanol emission in this source is confined to a central hot core, while the three class I maser transitions are detected in two additional regions. These two maser-only emission regions are clearly associated with shocked gas traced by 2 μm Ks-band and thermal v = 0, J = 5−4 SiO molecular emission. In contrast to the thermal methanol emission from the hot core, the three class I maser transitions show an positive trend in the rotation diagram for the two maser regions. Large velocity gradient modeling of the 266.8, 218.4, and 229.7 GHz transitions shows that the 266.8 GHz transition can be a maser for a wide range of conditions. The intensity ratios for the three methanol transitions detected in maser regions can be reproduced under conditions that are typical for class I methanol maser sites. These facts all support the hypothesis that the detected emission from the 266.8 GHz methanol (52−41E) transition is masing. [ABSTRACT FROM AUTHOR]

Young or forming high-mass stars that are nearby and not within a cluster environment have the potential to provide fundamental insights into star formation. In this paper we report such a candidate (G352.630-1.067), for which we have measured the distance through very long baseline interferometry parallax observations of the associated 6.7 GHz class II methanol maser. We determine the distance to the source to be kpc, which makes it the second nearest high-mass star formation region (HMSFR) known, with only the Orion Nebula being closer. This may place this source, not within a Galactic spiral arm, but in the region between the Local and Sagittarius arms, indicating that molecular clouds in interarm regions may also generate high-mass stars. Kinematic association between this source and the Sagittarius Arm suggests that it may be located in a spur extending outward from this arm. Comparison with the known, nearby HMSFRs (distances less than 1 kpc), reveal that G352.630-1.067 is in a more isolated environment than others, hence providing an excellent candidate for investigations of the processes that form individual high-mass stars. We find a good spatial correlation between the 6.7 GHz methanol maser and high angular resolution images of the infrared outflow, suggesting that the class II methanol masers are closely associated with a jet/outflow in this source. [ABSTRACT FROM AUTHOR]

We report the results of wide field CO mapping in the region of IRAS 19312+1950. This Infrared Astronomical Satellite (IRAS) object exhibits SiO/H2O/OH maser emission, and is embedded in a chemically rich molecular component, the origin of which is still unknown. In order to reveal the entire structure and gas mass of the surrounding molecular component for the first time, we have mapped a wide region around IRAS 19312+1950 in the 12CO J = 1–0, 13CO J = 1–0 and C18O J = 1–0 lines using the Nobeyama 45 m telescope. In conjunction with archival CO maps, we investigated a region up to 20′ × 20′ in size around this IRAS object. We calculated the CO gas mass assuming local thermal equilibrium, the stellar velocity through the interstellar medium assuming an analytic model of bow shock, and the absolute luminosity, using the latest archival data and trigonometric parallax distance. The derived gas mass (225 M⊙–478 M⊙) of the molecular component and the relatively large luminosity (2.63 × 104L☉) suggest that the central SiO/H2O/OH maser source is a red supergiant rather than an asymptotic giant branch (AGB) star or post-AGB star. [ABSTRACT FROM AUTHOR]