Planck Galactic Cold Clumps (PGCCs) possibly represent the early stages of star formation. To understand better the properties of PGCCs, we studied 16 PGCCs in the L1495 cloud with molecular lines ...and continuum data from Herschel, JCMT/SCUBA-2, and the PMO 13.7 m telescope. Thirty dense cores were identified in 16 PGCCs from 2D Gaussian fitting. The dense cores have dust temperatures of Td = 11-14 K, and H2 column densities of N H 2 = (0.36-2.5) × 1022 cm−2. We found that not all PGCCs contain prestellar objects. In general, the dense cores in PGCCs are usually at their earliest evolutionary stages. All the dense cores have non-thermal velocity dispersions larger than the thermal velocity dispersions from molecular line data, suggesting that the dense cores may be turbulence-dominated. We have calculated the virial parameter and found that 14 of the dense cores have <2, while 16 of the dense cores have >2. This suggests that some of the dense cores are not bound in the absence of external pressure and magnetic fields. The column density profiles of dense cores were fitted. The sizes of the flat regions and core radii decrease with the evolution of dense cores. CO depletion was found to occur in all the dense cores, but is more significant in prestellar core candidates than in protostellar or starless cores. The protostellar cores inside the PGCCs are still at a very early evolutionary stage, sharing similar physical and chemical properties with the prestellar core candidates.
Context. The sub-millimetre polarisation of dust emission from star-forming clouds carries information on grain properties and on the effects that magnetic fields have on cloud evolution. Aims. Using ...observations of a dense filamentary cloud G035.39-00.33, we aim to characterise the dust emission properties and the variations of the polarisation fraction. Methods. JCMT SCUBA-2/POL-2 observations at 850 μm were combined with Planck 850 μm(353 GHz) data to map polarisation fraction at small and large scales. With previous total intensity SCUBA-2 observations (450 and 850 μm) and Herschel data, the column densities were determined via modified black-body fits and via radiative transfer modelling. Models were constructed to examine how the observed polarisation angles and fractions depend on potential magnetic field geometries and grain alignment processes. Results. POL-2 data show clear changes in the magnetic field orientation. These are not in contradiction with the uniform orientation and almost constant polarisation fraction seen by Planck, because of the difference in the beam sizes and the POL-2 data being affected by spatial filtering. The filament has a peak column density of N(H2) ~ 7 × 1022 cm−2, a minimum dust temperature of T ~ 12 K, and a mass of ~4300 M⊙ for the area N(H2) > 5 × 1021 cm−2. The estimated average value of the dust opacity spectral index is β ~ 1.9. The ratio of sub-millimetre and J-band optical depths is τ (250 μm)∕τ (J) ~ 2.5 × 10−3, more than four times the typical values for diffuse medium. The polarisation fraction decreases as a function of column density to p ~ 1% in the central filament. Because of noise, the observed decrease of p(N) is significant only at N(H2) > 2 × 1022 cm−2. The observations suggest that the grain alignment is not constant. Although the data can be explained with a complete loss of alignment at densities above ~104 cm−3 or using the predictions of radiative torques alignment, the uncertainty of the field geometry and the spatial filtering of the SCUBA-2 data prevent strong conclusions. Conclusions. The G035.39-00.33 filament shows strong signs of dust evolution and the low polarisation fraction is suggestive of a loss of polarised emission from its densest parts.
Abstract
The internal structure of the prestellar core G208.68-19.02-N2 (G208-N2) in the Orion Molecular Cloud 3 (OMC-3) region has been studied with the Atacama Large Millimeter/submillimeter Array. ...The dust continuum emission revealed a filamentary structure with a length of ∼5000 au and an average H
2
volume density of ∼6 × 10
7
cm
−3
. At the tip of this filamentary structure, there is a compact object, which we call a
nucleus
, with a radius of ∼150–200 au and a mass of ∼0.1
M
⊙
. The nucleus has a central density of ∼2 × 10
9
cm
−3
with a radial density profile of
r
−1.87±0.11
. The density scaling of the nucleus is ∼3.7 times higher than that of the singular isothermal sphere (SIS). This as well as the very low virial parameter of 0.39 suggests that the gravity is dominant over the pressure everywhere in the nucleus. However, there is no sign of CO outflow localized to this nucleus. The filamentary structure is traced by the N
2
D
+
3–2 emission, but not by the C
18
O 2–1 emission, implying the significant CO depletion due to high density and cold temperature. Toward the nucleus, the N
2
D
+
also shows the signature of depletion. This could imply either the depletion of the parent molecule, N
2
, or the presence of the embedded very-low luminosity central source that could sublimate the CO in the very small area. The nucleus in G208-N2 is considered to be a prestellar core on the verge of first hydrostatic core (FHSC) formation or a candidate for the FHSC.
Magnetic field plays a crucial role in shaping molecular clouds and regulating star formation, yet the complete information on the magnetic field is not well constrained owing to the limitations in ...observations. We study the magnetic field in the massive infrared dark cloud G035.39-00.33 from dust continuum polarization observations at 850 m with SCUBA-2/POL-2 at JCMT for the first time. The magnetic field tends to be perpendicular to the densest part of the main filament (FM), whereas it has a less defined relative orientation in the rest of the structure, where it tends to be parallel to some diffuse regions. A mean plane-of-the-sky magnetic field strength of ∼50 G for FM is obtained using the Davis-Chandrasekhar-Fermi method. Based on 13CO (1-0) line observations, we suggest a formation scenario of FM due to large-scale (∼10 pc) cloud-cloud collision. Using additional NH3 line data, we estimate that FM will be gravitationally unstable if it is only supported by thermal pressure and turbulence. The northern part of FM, however, can be stabilized by a modest additional support from the local magnetic field. The middle and southern parts of FM are likely unstable even if the magnetic field support is taken into account. We claim that the clumps in FM may be supported by turbulence and magnetic fields against gravitational collapse. Finally, we identified for the first time a massive (∼200 M ), collapsing starless clump candidate, "c8," in G035.39-00.33. The magnetic field surrounding "c8" is likely pinched, hinting at an accretion flow along the filament.
Abstract
The Submillimetre Common User Bolometer Array 2 (SCUBA-2) is the James Clerk Maxwell Telescope’s continuum imager, operating simultaneously at 450 and 850
μ
m. SCUBA-2 was commissioned in ...2009–2011, and since that time, regular observations of point-like standard sources have been performed whenever the instrument is in use. Expanding the calibrator observation sample by an order of magnitude compared to previous work, in this paper we derive updated opacity relations at each wavelength for a new atmospheric extinction correction, analyze the Flux Conversion Factors used to convert instrumental units to physical flux units as a function of date and observation time, present information on the beam profiles for each wavelength, and update secondary calibrator source fluxes. Between 07:00 and 17:00 UTC, the portion of the night that is most stable to temperature gradients that cause dish deformation, the total flux uncertainty and the peak flux uncertainty measured at 450
μ
m are found to be 14% and 17%, respectively. Measured at 850
μ
m, the total flux and peak flux uncertainties are 6% and 7%, respectively. The analysis presented in this work is applicable to all SCUBA-2 projects observed since 2011.
Abstract We observe the magnetic field morphology toward a nearby star-forming filamentary cloud, G202.3+2.5, using James Clerk Maxwell Telescope/POL-2 850 μ m thermal dust polarization observations ...with an angular resolution of 14.″4 (∼0.053 pc). The average magnetic field orientation is found to be perpendicular to the filaments, while showing different behaviors in the four subregions, suggesting various effects from the filaments’ collision in these subregions. With the kinematics obtained by the N 2 H + observation by IRAM, we estimate the plane-of-sky magnetic field strength by two methods, the classical Davis–Chandrasekhar–Fermi (DCF) method and the angular dispersion function (ADF) method, giving B pos,dcf and B pos,adf of ∼90 and ∼53 μ G. We study the relative importance between the gravity ( G ), magnetic field ( B ), and turbulence ( T ) in the four subregions, and find G > T > B , G ≥ T > B , G ∼ T > B , and T > G > B in the north tail, west trunk, south root, and east wing, respectively. In addition, we investigate the projection effects on the DCF and ADF methods, based on a similar simulation case, and find the 3D magnetic field strength may be underestimated by a factor of ∼3 if applying the widely used statistical B pos -to- B 3D factor when using the DCF or ADF methods, which may further underestimate/overestimate the related parameters.
The low dust temperatures (<14 K) of Planck Galactic cold clumps (PGCCs) make them ideal targets to probe the initial conditions and very early phase of star formation. "TOP-SCOPE" is a joint survey ...program targeting ∼2000 PGCCs in J = 1-0 transitions of CO isotopologues and ∼1000 PGCCs in 850 m continuum emission. The objective of the "TOP-SCOPE" survey and the joint surveys (SMT 10 m, KVN 21 m, and NRO 45 m) is to statistically study the initial conditions occurring during star formation and the evolution of molecular clouds, across a wide range of environments. The observations, data analysis, and example science cases for these surveys are introduced with an exemplar source, PGCC G26.53+0.17 (G26), which is a filamentary infrared dark cloud (IRDC). The total mass, length, and mean line mass (M/L) of the G26 filament are ∼6200 M☉, ∼12 pc, and ∼500 M☉ pc−1, respectively. Ten massive clumps, including eight starless ones, are found along the filament. The most massive clump as a whole may still be in global collapse, while its denser part seems to be undergoing expansion owing to outflow feedback. The fragmentation in the G26 filament from cloud scale to clump scale is in agreement with gravitational fragmentation of an isothermal, nonmagnetized, and turbulent supported cylinder. A bimodal behavior in dust emissivity spectral index (β) distribution is found in G26, suggesting grain growth along the filament. The G26 filament may be formed owing to large-scale compression flows evidenced by the temperature and velocity gradients across its natal cloud.
ABSTRACT
To study the high-transition dense-gas tracers and their relationships to the star formation of the inner ∼2 kpc circumnuclear region of NGC 253, we present HCN J = 4−3 and HCO+ J = 4−3 maps ...obtained with the James Clerk Maxwell Telescope. Using the spatially resolved data, we compute the concentration indices r90/r50 for the different tracers. HCN and HCO+ 4–3 emission features tend to be centrally concentrated, which is in contrast to the shallower distribution of CO 1–0 and the stellar component. The dense-gas fraction (fdense, traced by the velocity-integrated-intensity ratios of HCN/CO and HCO+/CO) and the ratio R31 (CO 3–2/1–0) decline towards larger galactocentric distances, but increase with higher star formation rate surface density. The radial variation and the large scatter of fdense and R31 imply distinct physical conditions in different regions of the galactic disc. The relationships of fdense versus Σstellar, and SFEdense versus Σstellar are explored. SFEdense increases with higher Σstellar in this galaxy, which is inconsistent with previous work that used HCN 1–0 data. This implies that existing stellar components might have different effects on the high-J HCN and HCO+ than their low-J emission. We also find that SFEdense seems to be decreasing with higher fdense which is consistent with previous works, and it suggests that the ability of the dense gas to form stars diminishes when the average density of the gas increases. This is expected in a scenario where only the regions with high-density contrast collapse and form stars.
Abstract
Hot corinos are of great interest due to their richness in interstellar complex organic molecules (COMs) and the consequent potential prebiotic connection to solar-like planetary systems. ...Recent surveys have reported an increasing number of detected hot corinos in Class 0/I protostars; however, the relationships between their physical properties and the hot-corino signatures remain elusive. In this study, our objective is to establish a general picture of the detectability of hot corinos by identifying the origins of the hot-corino signatures in the sample of young stellar objects (YSOs) obtained from the Atacama Large Millimeter/submillimeter Array Survey of Orion Planck Galactic Cold Clumps project. We apply spectral energy distribution modeling to our sample and identify the physical parameters of the modeled YSOs directly, linking the detection of hot-corino signatures to the envelope properties of the YSOs. Imaging simulations of the methanol emission further support this scenario. We therefore posit that the observed COM emission originates from the warm inner envelopes of the sample YSOs, based on both the warm region size and the envelope density profile. The former is governed by the source luminosity and is additionally affected by the disk and cavity properties, while the latter is related to the evolutionary stages. This scenario provides a framework for detecting hot-corino signatures toward luminous Class 0 YSOs, with fewer detections being observed toward similarly luminous Class I sources.
ABSTRACT
We present ALMA Band 7 polarization observations of the OMC-1 region of the Orion molecular cloud. We find that the polarization pattern observed in the region is likely to have been ...significantly altered by the radiation field of the >104 L⊙ high-mass protostar Orion Source I. In the protostar’s optically thick disc, polarization is likely to arise from dust self-scattering. In material to the south of Source I – previously identified as a region of ‘anomalous’ polarization emission – we observe a polarization geometry concentric around Source I. We demonstrate that Source I’s extreme luminosity may be sufficient to make the radiative precession time-scale shorter than the Larmor time-scale for moderately large grains ($\gt 0.005\!-\!0.1\, \mu$m), causing them to precess around the radiation anisotropy vector (k-RATs) rather than the magnetic field direction (B-RATs). This requires relatively unobscured emission from Source I, supporting the hypothesis that emission in this region arises from the cavity wall of the Source I outflow. This is one of the first times that evidence for k-RAT alignment has been found outside of a protostellar disc or AGB star envelope. Alternatively, the grains may remain aligned by B-RATs and trace gas infall on to the Main Ridge. Elsewhere, we largely find the magnetic field geometry to be radial around the BN/KL explosion centre, consistent with previous observations. However, in the Main Ridge, the magnetic field geometry appears to remain consistent with the larger-scale magnetic field, perhaps indicative of the ability of the dense Ridge to resist disruption by the BN/KL explosion.
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