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Register a new userALMA High Angular Resolution Polarization Study; An Extremely Young Class 0 Source, OMC-3/MMS 6
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Satoko Takahashi Dr
Publication date
2018
fields
Physics
and research's language is
English
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Using the $approx$15km ALMA long baselines, we imaged the Stokes $I$ emission and linearly polarized intensity ($PI$) in the 1.1-mm continuum band of a very young intermediate-mass protostellar source, MMS 6, in the Orion Molecular Cloud-3. The achieved angular resolution, $0.02{times}0.03$ ($approx$10 AU), shows for the first time a wealth of data on the dust emission polarization in the central 200 AU of a protostar. The $PI$ peak is offset to the south-west (SW) by $approx$20 AU with respect to the Stokes $I$ peak. Its polarization degree is 11 % with its $E$-vector orientation of P.A.${approx}135^{circ}$. A partial ring-like structure with a radius of $approx$80 AU is detected in $PI$ but not in the Stokes $I$. NW (north-west) and SE (south-east) parts of the ring are bright with a high polarization degree of $gtrsim$10 %, and their $E$-vector orientations are roughly orthogonal to those observed near the center. We also detected arm-like polarized structures, extending to 1000 AU scale to the north, with the $E$-vectors aligned along the minor axis of the structures. We explored possible origins of the polarized emission comparing with magnetohydrodynamical (MHD) simulations of the toroidal wrapping of the magnetic field. The simulations are consistent with the $PI$ emission in the ring-like and the extended arm-like structures observed with ALMA. However, the current simulations do not completely reproduce observed polarization characteristics in the central 50 AU. Although the self-scattering model can explain the polarization pattern and positional offset between the Stokes $I$ and $PI$, this model is not able to reproduce the observed high degree of polarization.
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Using the Atacama Large Millimeter/submillimeter Array (ALMA), we investigated a peculiar millimeter source MMS 3 located in the Orion Molecular Cloud 3 (OMC-3) region in the 1.3 mm continuum, CO ($J$=2-1), SiO ($J$=5-4), C$^{18}$O ($J$=2-1), N$_2$D$^+$ ($J$=3-2), and DCN ($J$=3-2) emissions. With the ALMA high angular resolution ($sim$0.2), we detected a very compact and highly centrally condensed continuum emission with a size of 0.45 $times$ 0.32 (P.A.=0.22$^circ$). The peak position coincides with the locations of previously reported $Spitzer$/IRAC and X-ray sources within their positional uncertainties. We also detected an envelope with a diameter of $sim$6800 au (P.A.=75$^circ$) in the C$^{18}$O ($J$=2-1) emission. Moreover, a bipolar outflow was detected in the CO ($J$=2-1) emission for the first time. The outflow elongates roughly perpendicular to the long axis of the envelope detected in the C$^{18}$O ($J$=2-1) emission. Compact high-velocity CO gas in the (red-shifted) velocity range of 22-30 km s$^{-1}$, presumably tracing a jet, was detected near the 1.3 mm continuum peak. A compact and faint red-shifted SiO emission was marginally detected on the CO outflow lobe. The physical quantities of the outflow in MMS 3 are relatively smaller than those in other sources in the OMC-3 region. The centrally condensed object associated with the near-infrared and X-ray sources, the flattened envelope, and the faint outflow indicate that MMS 3 harbors a low mass protostar with an age of $sim$10$^3$ yr.
Both high- and low-velocity outflows are occasionally observed around a protostar by molecular line emission. The high-velocity component is called `Extremely High-Velocity (EHV) flow, while the low-velocity component is simply referred as `(molecular) outflow. This study reports a newly found EHV flow and outflow around MMS $5$ in the Orion Molecular Cloud 3 observed with ALMA. In the observation, CO $J$=2--1 emission traces both the EHV flow ($|v_{rm{LSR}} - v_{rm{sys}}|$ $simeq$ 50--100 $rm{km s^{-1}}$) and outflow ($|v_{rm{LSR}} - v_{rm{sys}}|$ $simeq$ 10--50 $rm{km s^{-1}}$). On the other hand, SiO $J$=5--4 emission only traces the EHV flow. The EHV flow is collimated and located at the root of the V-shaped outflow. The CO outflow extends up to $sim$ 14,000,AU with a position angle (P.A.) of $sim79^circ$ and the CO redshifted EHV flow extends to $sim$11,000 AU with P.A. $sim96^circ$. The EHV flow is smaller than the outflow, and the dynamical timescale of the EHV flow is shorter than that of the outflow by a factor of $sim 3$. The flow driving mechanism is discussed based on the size, time scale, axis difference between the EHV flow and outflow, and the periodicity of the knots. Our results are consistent with the nested wind scenario, although the jet entrainment scenario could not completely be ruled out.
We have carried out polarization calibration for archival JVLA ($sim$9 mm) full polarization observations towards the Class 0 young stellar object (YSO) OMC-3/MMS 6 (also known as HOPS-87), and then compared with the archival ALMA 1.2 mm observations. We found that the innermost $sim$100 au region of OMC-3/MMS 6 is likely very optically thick (e.g., $taugg$1) at $sim$1 mm wavelength such that the dominant polarization mechanism is dichroic extinction. It is marginally optically thin (e.g., $taulesssim$1) at $sim$9 mm wavelength such that the JVLA observations can directly probe the linearly polarized emission from non-spherical dust. Assuming that the projected long axis of dust grains is aligned perpendicular to magnetic field (B-field) lines, we propose that the overall B-field topology resembles an hourglass shape, while this hourglass appears $sim$40$^{circ}$ inclined with respect to the previously reported outflow axis. The geometry of this system is consistent with a magnetically regulated dense (pseudo-)disk. Based on the observed 29.45 GHz flux density and assuming a dust absorption opacity $kappa^{abs}_{29.45,GHz}=$0.0096 cm$^{2} $g$^{-1}$, the derived overall dust mass within a $sim$43 au radius is $sim$14000 $M_{oplus}$. From this case study, it appears to us that some previous 9 mm surveys towards Class 0/I YSOs might have systematically underestimated dust masses by one order of magnitude, owing to that they assumed the too high dust absorption opacity ($sim$0.1 cm$^{2}$ g$^{-1}$) for $sim$9 mm wavelengths but without self-consistently considering the dust scattering opacity.
OMC-2 FIR 4 is one of the closest known young intermediate-mass protoclusters, located at a distance of 420 pc in Orion. This region is one of the few where the complete 500-2000 GHz spectrum has been observed with the heterodyne spectrometer HIFI on board the Herschel satellite, and unbiased spectral surveys at 0.8, 1, 2 and 3 mm have been obtained with the JCMT and IRAM 30-m telescopes. In order to investigate the morphology of this region, we used the IRAM Plateau de Bure Interferometer to image OMC-2 FIR 4 in the 2-mm continuum emission, as well as in DCO+(2-1), DCN(2-1), C34S(3-2), and several CH3OH lines. In addition, we analysed observations of the NH3(1,1) and (2,2) inversion transitions made with the Very Large Array of the NRAO. The resulting maps have an angular resolution which allows us to resolve structures of 5, equivalent to 2000 AU. Our observations reveal three spatially resolved sources within OMC-2 FIR 4, of one or several solar masses each, with hints of further unresolved substructure within them. Two of these sources have elongated shapes and are associated with dust continuum emission peaks, thus likely containing at least one molecular core each. One of them also displays radio continuum emission, which may be attributed to a young B3-B4 star that dominates the overall luminosity output of the region. The third source identified displays a DCO+(2-1) emission peak, and weak dust continuum emission. Its higher abundance of DCO+ relative to the other two regions suggests a lower temperature and therefore its possible association with either a younger low-mass protostar or a starless core. It may alternatively be part of the colder envelope of OMC-2 FIR 4. Our interferometric observations evidence the complexity of this region, where multiple cores, chemical differentiation and an ionised region all coexist within an area of only 10000 AU.
We present high angular resolution dust polarization and molecular line observations carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the Class 0 protostar Serpens SMM1. By complementing these observations with new polarization observations from the Submillimeter Array (SMA) and archival data from the Combined Array for Research in Millimeter-wave Astronomy (CARMA) and the James Clerk Maxwell Telescopes (JCMT), we can compare the magnetic field orientations at different spatial scales. We find major changes in the magnetic field orientation between large (~0.1 pc) scales -- where the magnetic field is oriented E-W, perpendicular to the major axis of the dusty filament where SMM1 is embedded -- and the intermediate and small scales probed by CARMA (~1000 AU resolution), the SMA (~350 AU resolution), and ALMA (~140 AU resolution). The ALMA maps reveal that the redshifted lobe of the bipolar outflow is shaping the magnetic field in SMM1 on the southeast side of the source; however, on the northwestern side and elsewhere in the source, low velocity shocks may be causing the observed chaotic magnetic field pattern. High-spatial-resolution continuum and spectral-line observations also reveal a tight (~130 AU) protobinary system in SMM1-b, the eastern component of which is launching an extremely high-velocity, one-sided jet visible in both CO(2-1) and SiO(5-4); however, that jet does not appear to be shaping the magnetic field. These observations show that with the sensitivity and resolution of ALMA, we can now begin to understand the role that feedback (e.g., from protostellar outflows) plays in shaping the magnetic field in very young, star-forming sources like SMM1.
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