We report the first high spatial resolution measurement of magnetic fields surrounding LkH 101, part of the Auriga-California molecular cloud. The observations were taken with the POL-2 polarimeter ...on the James Clerk Maxwell Telescope within the framework of the B-fields In Star-forming Region Observations (BISTRO) survey. Observed polarization of thermal dust emission at 850 m is found to be mostly associated with the redshifted gas component of the cloud. The magnetic field displays a relatively complex morphology. Two variants of the Davis-Chandrasekhar-Fermi method, unsharp masking and structure function, are used to calculate the strength of magnetic fields in the plane of the sky, yielding a similar result of BPOS ∼ 115 G. The mass-to-magnetic-flux ratio in critical value units, λ ∼ 0.3, is the smallest among the values obtained for other regions surveyed by POL-2. This implies that the LkH 101 region is subcritical, and the magnetic field is strong enough to prevent gravitational collapse. The inferred δB/B0 ∼ 0.3 implies that the large-scale component of the magnetic field dominates the turbulent one. The variation of the polarization fraction with total emission intensity can be fitted by a power law with an index of = 0.82 0.03, which lies in the range previously reported for molecular clouds. We find that the polarization fraction decreases rapidly with proximity to the only early B star (LkH 101) in the region. Magnetic field tangling and the joint effect of grain alignment and rotational disruption by radiative torques can potentially explain such a decreasing trend.
We present the results of simultaneous 450 μm and 850 μm polarization observations toward the massive star-forming region NGC 2071IR, a target of the BISTRO (B-fields in STar-forming Region ...Observations) Survey, using the POL-2 polarimeter and SCUBA-2 camera mounted on the James Clerk Maxwell Telescope. We find a pinched magnetic field morphology in the central dense core region, which could be due to a rotating toroidal disklike structure and a bipolar outflow originating from the central young stellar object IRS 3. Using the modified Davis-Chandrasekhar-Fermi method, we obtain a plane-of-sky magnetic field strength of 563 ± 421 μG in the central ~0.12 pc region from 850 μm polarization data. The corresponding magnetic energy density of 2.04 × 10-8 erg cm-3 is comparable to the turbulent and gravitational energy densities in the region. We find that the magnetic field direction is very well aligned with the whole of the IRS 3 bipolar outflow structure. We find that the median value of polarization fractions is 3.0% at 450 μm in the central 3' region, which is larger than the median value of 1.2% at 850 μm. The trend could be due to the better alignment of warmer dust in the strong radiation environment. We also find that polarization fractions decrease with intensity at both wavelengths, with slopes, determined by fitting a Rician noise model of 0.59 ± 0.03 at 450 μm and 0.36 ± 0.04 at 850 μm, respectively. We think that the shallow slope at 850 μm is due to grain alignment at the center being assisted by strong radiation from the central young stellar objects.
In the early stages of star formation, a protostar is deeply embedded in an optically thick envelope such that it is not directly observable. Variations in the protostellar accretion rate, however, ...will cause luminosity changes that are reprocessed by the surrounding envelope and are observable at submillimeter wavelengths. We searched for submillimeter flux variability toward 12 Planck Galactic Cold Clumps detected by the James Clerk Maxwell Telescope (JCMT)-SCUBA-2 Continuum Observations of Pre-protostellar Evolution (SCOPE) survey. These observations were conducted at 850 using the JCMT/SCUBA-2. Each field was observed three times over about 14 months between 2016 April and 2017 June. We applied a relative flux calibration and achieved a calibration uncertainty of ∼3.6% on average. We identified 136 clumps across 12 fields and detected four sources with flux variations of ∼30%. For three of these sources, the variations appear to be primarily due to large-scale contamination, leaving one plausible candidate. The flux change of the candidate may be associated with low- or intermediate-mass star formation assuming a distance of 1.5 kpc, although we cannot completely rule out the possibility that it is a random deviation. Further studies with dedicated monitoring would provide a better understanding of the detailed relationship between submillimeter flux and accretion rate variabilities while enhancing the search for variability in star-forming clumps farther away than the Gould Belt.
We report 850 m dust polarization observations of a low-mass (∼12 M ) starless core in the Ophiuchus cloud, Ophiuchus C, made with the POL-2 instrument on the James Clerk Maxwell Telescope (JCMT) as ...part of the JCMT B-fields In STar-forming Region Observations survey. We detect an ordered magnetic field projected on the plane of the sky in the starless core. The magnetic field across the ∼0.1 pc core shows a predominant northeast-southwest orientation centering between ∼40° and ∼100°, indicating that the field in the core is well aligned with the magnetic field in lower-density regions of the cloud probed by near-infrared observations and also the cloud-scale magnetic field traced by Planck observations. The polarization percentage (P) decreases with increasing total intensity (I), with a power-law index of −1.03 0.05. We estimate the plane-of-sky field strength (Bpos) using modified Davis-Chandrasekhar-Fermi methods based on structure function (SF), autocorrelation function (ACF), and unsharp masking (UM) analyses. We find that the estimates from the SF, ACF, and UM methods yield strengths of 103 46 G, 136 69 G, and 213 115 G, respectively. Our calculations suggest that the Ophiuchus C core is near magnetically critical or slightly magnetically supercritical (i.e., unstable to collapse). The total magnetic energy calculated from the SF method is comparable to the turbulent energy in Ophiuchus C, while the ACF method and the UM method only set upper limits for the total magnetic energy because of large uncertainties.
We present 850 m imaging polarimetry data of the Oph-A core taken with the Submillimeter Common-User Bolometer Array-2 (SCUBA-2) and its polarimeter (POL-2) as part of our ongoing survey project, ...-fields In STar forming RegiOns (BISTRO). The polarization vectors are used to identify the orientation of the magnetic field projected on the plane of the sky at a resolution of 0.01 pc. We identify 10 subregions with distinct polarization fractions and angles in the 0.2 pc Oph-A core; some of them can be part of a coherent magnetic field structure in the Oph region. The results are consistent with previous observations of the brightest regions of Oph-A, where the degrees of polarization are at a level of a few percent, but our data reveal for the first time the magnetic field structures in the fainter regions surrounding the core where the degree of polarization is much higher (>5%). A comparison with previous near-infrared polarimetric data shows that there are several magnetic field components that are consistent at near-infrared and submillimeter wavelengths. Using the Davis-Chandrasekhar-Fermi method, we also derive magnetic field strengths in several subcore regions, which range from approximately 0.2 to 5 mG. We also find a correlation between the magnetic field orientations projected on the sky and the core centroid velocity components.
We present the results of a single dish survey toward 95 VeLLOs in optically thick (HCN 1-0) and thin (\(\rm N_2H^+\) 1-0) lines performed for the purpose of understanding the physical processes of ...inward motions in the envelopes of the VeLLOs and characterizing their true nature. The normalized velocity differences (\(\delta V_{HCN}\)) between the peak velocities of the two lines were derived for 41 VeLLOs detected in both lines. The \(\delta V\) distribution of these VeLLOs is found to be significantly skewed to the blue, indicating the dominance of infalling motions in their envelopes. The infall speeds were derived for 15 infall candidates by using the HILL5 radiative transfer model. The speeds were in the range of 0.03 \(\rm km~s^{-1}\) to 0.3 \(\rm km~s^{-1}\), with a median value of 0.16 \(\rm km~s^{-1}\), being consistent with the gravitational free-fall speeds from pressure-free envelopes. The mass infall rates calculated from the infall speeds are mostly of the order of \(10^{-6} M_{\odot}~yr^{-1}\) with a median value of \(\rm 3.4 \pm 1.5 \times 10^{-6} M_{\odot}~yr^{-1}\). These are found to be also consistent with the values predicted with the inside-out collapse model and show a fairly good correlation with the internal luminosities of the VeLLOs. This again indicates that the infall motions observed toward the VeLLOs are likely to be due to the gravitational infall motions in their envelopes. Our study suggests that most of the VeLLOs are potentially faint protostars while two of the VeLLOs could possibly be proto-brown dwarf candidates.
Ten protostellar outflows in the Orion molecular clouds were mapped in the
$^{12}$CO/$^{13}$CO ${J=6\rightarrow5}$ and $^{12}$CO ${J=7\rightarrow6}$
lines. The maps of these mid-$J$ CO lines have an ...angular resolution of about
10$''$ and a typical field size of about 100$''$. Physical parameters of the
molecular outflows were derived, including mass transfer rates, kinetic
luminosities, and outflow forces. The outflow sample was expanded by
re-analyzing archival data of nearby low-luminosity protostars, to cover a wide
range of bolometric luminosities. Outflow parameters derived from other
transitions of CO were compared. The mid-$J$ ($J_{\rm up} \approx 6$) and
low-$J$ ($J_{\rm up} \leq 3$) CO line wings trace essentially the same outflow
component. By contrast, the high-$J$ (up to $J_{\rm up} \approx 50$)
line-emission luminosity of CO shows little correlation with the kinetic
luminosity from the ${J=6\rightarrow5}$ line, which suggests that they trace
distinct components. The low/mid-$J$ CO line wings trace long-term outflow
behaviors while the high-$J$ CO lines are sensitive to short-term activities.
The correlations between the outflow parameters and protostellar properties are
presented, which shows that the strengths of molecular outflows increase with
bolometric luminosity and envelope mass.
CS Depletion in Prestellar Cores Kim, Shinyoung; Chang Won Lee; Gopinathan, Maheswar ...
arXiv.org,
02/2020
Paper, Journal Article
Odprti dostop
The CS molecule is known to be absorbed onto dust in the cold and dense conditions, causing it to get significantly depleted in the central region of cores. This study is aimed to investigate the ...depletion of the CS molecule using the optically thin C\(^{34}\)S molecular line observations. We mapped five prestellar cores, L1544, L1552, L1689B, L694-2, and L1197 using two molecular lines, C\(^{34}\)S \((J=2-1)\) and N\(_2\)H\(^+\) \((J=1-0)\) with the NRO 45-m telescope, doubling the number of cores where the CS depletion was probed using C\(^{34}\)S. In most of our targets, the distribution of C\(^{34}\)S emission shows features that suggest that the CS molecule is generally depleted in the center of the prestellar cores. The radial profile of the CS abundance with respect to H\(_2\) directly measured from the CS emission and the Herschel dust emission indicates that the CS molecule is depleted by a factor of \(\sim\)3 toward the central regions of the cores with respect to their outer regions. The degree of the depletion is found to be even more enhanced by an order of magnitude when the contaminating effect introduced by the presence of CS molecules in the surrounding envelope that lie along the line-of-sight is removed. Except for L1197 which is classified as relatively the least evolved core in our targets based on its observed physical parameters, we found that the remaining four prestellar cores are suffering from significant CS depletion at their central region regardless of the relative difference in their evolutionary status.
Prestellar cores are self-gravitating dense and cold structures within molecular clouds where future stars are born. They are expected, at the stage of transitioning to the protostellar phase, to ...harbor centrally concentrated dense (sub)structures that will seed the formation of a new star or the binary/multiple stellar systems. Characterizing this critical stage of evolution is key to our understanding of star formation. In this work, we report the detection of high density (sub)structures on the thousand-au scale in a sample of dense prestellar cores. Through our recent ALMA observations towards the Orion molecular cloud, we have found five extremely dense prestellar cores, which have centrally concentrated regions \(\sim\) 2000 au in size, and several \(10^7\) \(cm^{-3}\) in average density. Masses of these centrally dense regions are in the range of 0.30 to 6.89 M\(_\odot\). {\it For the first time}, our higher resolution observations (0.8\('' \sim \) 320 au) further reveal that one of the cores shows clear signatures of fragmentation; such individual substructures/fragments have sizes of 800 -1700 au, masses of 0.08 to 0.84 M\(_\odot\), densities of \(2 - 8\times 10^7\) \(cm^{-3}\) and separations of \(\sim 1200\) au. The substructures are massive enough (\(\gtrsim 0.1~M_\odot\)) to form young stellar objects and are likely examples of the earliest stage of stellar embryos which can lead to widely (\(\sim\) 1200 au) separated multiple systems.
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