Abstract
We present high angular resolution ($\sim$ 0.3$"$) submillimeter continuum (0.85 mm) and line observations of the O-type protostar IRAS 16547$-$4247 carried out with the Atacama Large Millimeter/Submillimeter Array (ALMA). In the 0.85 mm continuum band, the observations revealed two compact sources (with a separation of 2$"$), one of them associated with IRAS 16547$-$4247, and the other one to the west. Both sources are well resolved angularly, revealing a clumpy structure. On the other hand, the line observations revealed a rich variety of molecular species related to both continuum sources. In particular, we found a large number of S-bearing molecules, such as the rare molecule methyl mercaptan (CH$_3$SH). At scales larger than 10,000 AU, molecules (e.g., SO$_2$ or OCS) mostly with low excitation temperatures in the upper states (E$_k$ $\lesssim$ 300 K) are present in both millimeter continuum sources, and show a southeast-northwest velocity gradient of 7 km s$^{-1}$ over 3$"$ (165 km s$^{-1}$ pc$^{-1}$). We suggest that this gradient probably is produced by the thermal (free-free) jet emerging from this object with a similar orientation at the base. At much smaller scales (about 1000 AU), molecules with high excitation temperatures (E$_k$ $\gtrsim$ 500 K) are tracing a rotating structure elongated perpendicular to the orientation of the thermal jet, which we interpret as a candidate disk surrounding IRAS 16547$-$4247. The dynamical mass corresponding to the velocity gradient of the candidate to disk is about 20 M$_\odot$, which is consistent with the bolometric luminosity of IRAS 16547$-$4247.
Highlights
Recent theoretical studies by Krumholz et al (2009), Peters et al (2010), and Kuiper et al (2010) have demonstrated that stars up to 140 M⊙ can be formed in a similar way to the low- and intermediate-mass stars, that is, through flattened accretion disks
In the cases of IRAS 183600537, NGC 7538 IRS1, and W51 North there is some evidence that the innermost parts of the disks are Keplerian, similar to the rotating structures found in low-mass stars
This mass corresponds to an early O-type protostar with a bolometric luminosity of ∼ 106 L⊙ (Panagia 1973), which exceeds by one order of magnitude the bolometric luminosity of IRAS 16547 (∼105 L⊙). We suggest that this gradient might be produced by the thermal jet that entrains the molecular gas, and that has a similar orientation
Summary
Recent theoretical studies by Krumholz et al (2009), Peters et al (2010), and Kuiper et al (2010) have demonstrated that stars up to 140 M⊙ can be formed in a similar way to the low- and intermediate-mass stars, that is, through flattened accretion disks. Observations with high spatial resolution have revealed a rotating structure associated with IRAS 16547, traced at small scales (∼50 AU) by the H2O masers and at moderate scales (∼1000 AU) by the thermal emission of SO2. Both rotating structures have slightly different position angles and most likely are tracing different parts of IRAS 16547. We present submillimeter line and continuum observations, made with the Atacama Large Millimeter/Submillimeter Array (ALMA) of the massive protostar IRAS 16547.
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