Testing the circumstellar disc hypothesis: a search for H₂outflow signatures from massive young stellar objects with linearly distributed methanol masers

Abstract

The results of a survey searching for outflows using near‐infrared imaging are presented. Targets were chosen from a compiled list of massive young stellar objects associated with methanol masers in linear distributions. Presently, it is a widely held belief that these methanol masers are found in (and delineate) circumstellar accretion discs around massive stars. If this scenario is correct, one way to test the disc hypothesis is to search for outflows perpendicular to the methanol maser distributions. The main objective of the survey was to obtain wide‐field near‐infrared images of the sites of linearly distributed methanol masers using a narrow‐band 2.12‐μm filter. This filter is centred on the H2v= 1–0 S(1) line; a shock diagnostic that has been shown to successfully trace CO outflows from young stellar objects. 28 sources in total were imaged of which 18 sources display H2 emission. Of these, only two sources showed emission found to be dominantly perpendicular to the methanol maser distribution. Surprisingly, the H2 emission in these fields is not distributed randomly, but instead the majority of sources are found to have H2 emission dominantly parallel to their distribution of methanol masers. These results seriously question the hypothesis that methanol masers exist in circumstellar discs. The possibility that linearly distributed methanol masers are instead directly associated with outflows is discussed.

SiO outflow signatures toward massive young stellar objects with linearly distributed methanol masers

“…Taken by itself, this appears to support the interpretation of De Buizer (2003) that the H 2 emission near these sources are being excited in the bow shocks of outflows.…”

“…4b, knots of H 2 emission, as well as some diffuse H 2 emission was found at this site (De Buizer 2003). The maser location is coincident with a bright near-IR continuum source (De Buizer 2003), that was also detected in the mid-IR (De Buizer et al 2000) and re-imaged here with T-ReCS at higher angular resolution (Fig. 4a, Sect.…”

“…Also, the H 2 emission is situated exactly parallel to the position angle of the methanol maser distribution. In De Buizer (2003) this source was considered to be the best candidate for further observations in disproving the circumstellar disk hypothesis for linearly distributed methanol masers. Our ATCA observations have revealed SiO emission distributed at the same angle as the H 2 emission and the methanol maser position angle (Fig.…”

“…However, recent modeling by several authors (e.g. McKee & Tan 2002, 2003Krumholz et al 2005) has shown that despite all the alleged problems, the highest mass stars may indeed be formed theoretically in a scaled-up version of low-mass star formation via core accretion onto disks. However, it is not known with certainty if massive stars form in this way because direct imaging of the accretion disks that are hypothesized to be feeding very young B and O type stars is very difficult.…”

“…They concluded that the H 2 emission most likely arises in terminal shocks at the tips of high-speed bipolar outflows and that the orientation of the line of methanol masers for one source (IRAS 16076-5134) was consistent with the masers being excited in an edge-on accretion disk that could be driving the outflow responsible for the H 2 emission. In a larger study of 28 sources specifically chosen to have linear distributions of methanol masers, De Buizer (2003) tried to test the maserdisk hypothesis by searching for outflows perpendicular to the methanol maser distributions. He obtained wide-field images of the sites of linearly distributed methanol masers using the 2.12 μm H 2 (1-0) S(1) line as the outflow diagnostic.…”