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Register a new userExtremely high excitation SiO lines in disk-outflow system in Orion Source I
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We present high-resolution images of the submillimeter SiO line emissions of a massive young stellar object Orion Source I using the Atacama Large Millimeter/ Submillimeter Array (ALMA) at band 8. We detected the 464 GHz SiO v=4 J=11-10 line in Source I, which is the first detection of the SiO v=4 line in star-forming regions, together with the 465 GHz 29SiO v=2 J=11-10 and the 428 GHz SiO v=2 J=10-9 lines with a resolution of 50 AU. The 29SiO v=2 J=11-10 and SiO v=4 J=11-10 lines have compact structures with the diameter of <80 AU. The spatial and velocity distribution suggest that the line emissions are associated with the base of the outflow and the surface of the edge-on disk. In contrast, SiO v=2 J=10-9 emission shows a bipolar structure in the direction of northeast-southwest low-velocity outflow with ~200 AU scale. The emission line exhibits a velocity gradient along the direction of the disk elongation. With the assumption of the ring structure with Keplerian rotation, we estimated the lower limit of the central mass to be 7 solar mass and the radius of 12 AU< r <26 AU.
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One of the outstanding problems in star-formation theory concerns the transfer of angular momentum such that mass can accrete onto a newly born young stellar object (YSO). From a theoretical standpoint, outflows and jets are predicted to play an essential role in angular momentum transfer and their rotation motions have been reported for both low- and high-mass YSOs. However, little quantitative discussion on outflow launching mechanisms have been presented for high-mass YSOs due to a lack of observational data. Here we present a clear signature of rotation in the bipolar outflow driven by Orion Source I, a high-mass YSO candidate, using the Atacama Large Millimeter/Submillimeter Array (ALMA). A rotational transition of silicon monoxide (Si18O) reveals a velocity gradient perpendicular to the outflow axis which is consistent with that of the circumstellar disk traced by a high-excitation water (H2O) line. The launching radii and outward velocity of the outflow are estimated to be >10 au and 10 km s-1, respectively. These parameters rule out a possibility that the observed outflow is produced by entrainment of a high-velocity jet, and that contribution from stellar-wind or X-wind which have smaller launching radii are significant in the case of Source I. Thus, present results provide a convincing evidence of a rotating outflow directly driven by the magneto-centrifugal disk wind launched by a high-mass YSO candidate.
Vibration-rotation lines of H$_{2}$ from highly excited levels approaching the dissociation limit have been detected at a number of locations in the shocked gas of the Orion Molecular Cloud (OMC-1), including in a Herbig-Haro object near the tip of one of the OMC-1 fingers. Population diagrams show that while the excited H$_{2}$ is almost entirely at a kinetic temperature of $sim$1,800 K, (typical for vibrationally shock-excited H$_{2}$), as in the previously reported case of Herbig-Haro object HH 7 up to a few percent of the H$_{2}$ is at a kinetic temperature of $sim$5,000~K. The location with the largest fraction of hot H$_{2}$ is the Herbig-Haro object, where the outflowing material is moving at a higher speed than at the other locations. Although theoretical work is required for a better understanding of the 5,000 K H$_{2}$, (including how it cools), its existence and the apparent dependence of its abundance relative to that of the cooler component on the relative velocities of the outflow and the surrounding ambient gas appear broadly consistent with it having recently reformed. The existence of this high temperature H$_{2}$ appears to be a common characteristic of shock-excited molecular gas.
We present observational results of the submillimeter H2O and SiO lines toward a candidate high-mass young stellar object Orion Source I using ALMA. The spatial structures of the high excitation lines at lower-state energies of >2500 K show compact structures consistent with the circumstellar disk and/or base of the northeast-southwest bipolar outflow with a 100 au scale. The highest excitation transition, the SiO (v=4) line at band 8, has the most compact structure. In contrast, lower-excitation transitions are more extended than 200 au tracing the outflow. Almost all the line show velocity gradients perpendicular to the outflow axis suggesting rotation motions of the circumstellar disk and outflow. While some of the detected lines show broad line profiles and spatially extended emission components indicative of thermal excitation, the strong H2O lines at 321 GHz, 474 GHz, and 658 GHz with brightness temperatures of >1000 K show clear signatures of maser action.
In this {it Letter}, we present sensitive millimeter SiO (J=5-4; $ u$=0) line observations of the outflow arising from the enigmatic object Orion Source I made with the Atacama Large Millimeter/Submillimeter Array (ALMA). The observations reveal that at scales of a few thousand AU, the outflow has a marked butterfly morphology along a northeast-southwest axis. However, contrary to what is found in the SiO and H$_2$O maser observations at scales of tens of AU, the blueshifted radial velocities of the moving gas are found to the northwest, while the redshifted velocities are in the southeast. The ALMA observations are complemented with SiO (J=8-7; $ u$=0) maps (with a similar spatial resolution) obtained with the Submillimeter Array (SMA). These observations also show a similar morphology and velocity structure in this outflow. We discuss some possibilities to explain these differences at small and large scales across the flow.
Using Science Verification data from the Atacama Large Millimeter/Submillimeter Array (ALMA), we have identified and imaged five rotational transitions (J=5-4 and J=6-5) of the three silicon monoxide isotopologues 28SiO v=0, 1, 2 and 29SiO v=0 and 28Si18O v=0 in the frequency range from 214 to 246 GHz towards the Orion BN/KL region. The emission of the ground-state 28SiO, 29SiO and 28Si18O shows an extended bipolar shape in the northeast-southwest direction at the position of Radio Source I, indicating that these isotopologues trace an outflow (~18 km/s, P.A. ~50deg, ~5000 AU in diameter) that is driven by this embedded high-mass young stellar object (YSO). Whereas on small scales (10-1000 AU) the outflow from Source I has a well-ordered spatial and velocity structure, as probed by Very Long Baseline Interferometry (VLBI) imaging of SiO masers, the large scales (500-5000 AU) probed by thermal SiO with ALMA reveal a complex structure and velocity field, most likely related to the effects of the environment of the BN/KL region on the outflow emanating from Source I. The emission of the vibrationally-excited species peaks at the position of Source I. This emission is compact and not resolved at an angular resolution of ~1.5 (~600 AU at a distance of 420 pc). 2-D Gaussian fitting to individual velocity channels locates emission peaks within radii of 100 AU, i.e. they trace the innermost part of the outflow. A narrow spectral profile and spatial distribution of the v=1 J=5-4 line similar to the masing v=1 J=1-0 transition, provide evidence for the most highly rotationally excited (frequency > 200 GHz) SiO maser emission associated with Source I known to date. The maser emission will enable studies of the Source I disk-outflow interface with future ALMA longest baselines.
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