Abstract
We have carried out polarization calibration for archival Jansky Very Large Array (JVLA) (∼9 mm) full polarization observations toward the Class 0 young stellar object (YSO) OMC-3/MMS 6 ...(also known as HOPS-87), and then compared the results with the archival Atacama Large Millimeter Array (ALMA) 1.2 mm observations. The resolved spectral indices show that the innermost ∼100 au region of OMC-3/MMS 6 is marginally optically thin (e.g.,
τ
≲ 1) at ∼9 mm wavelength, such that the JVLA observations can directly probe the linearly polarized emission from nonspherical 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. The geometry of this system is consistent with a magnetically regulated dense (pseudo)disk, although this “hourglass” appears to be ∼40° inclined with respect to the previously reported outflow axis. In contrast, the inner ∼100 au region of this YSO is likely very optically thick (e.g.,
τ
≫ 1) at ∼1 mm wavelength. The electric field position angles resolved by JVLA and ALMA present ∼90° offsets on this region, which indicate that the dominant polarization mechanism at 1 mm wavelength is dichroic extinction. This is the second case where the (sub)millimeter dichroic extinction is demonstrated by the direct comparison between the JVLA and ALMA polarization observations.
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of the dust continuum emission at 1.3 mm and 12CO line emission of the transitional disk around DM Tau. DM Tau's disk is ...thought to possess a dust-free inner cavity inside a few au, from the absence of near-infrared excess on its spectral energy distribution (SED). Previous submillimeter observations were, however, unable to detect the cavity; instead, a dust ring ∼20 au in radius was seen. The excellent angular resolution achieved in the new ALMA observations, 43 × 31 mas, allows discovery of a 4 au radius inner dust ring, confirming previous SED modeling results. This inner ring is symmetric in continuum emission, but asymmetric in 12CO emission. The known (outer) dust ring at ∼20 au is recovered and shows azimuthal asymmetry with a strong-weak side contrast of ∼1.3. The gap between these two rings is depleted by a factor of ∼40 in dust emission relative to the outer ring. An extended outer dust disk is revealed, separated from the outer ring by another gap. The location of the inner ring is comparable to that of the main asteroid belt in the solar system. As a disk with a "proto-asteroid belt," the DM Tau system offers valuable clues to disk evolution and planet formation in the terrestrial-planet-forming region.
Spatially resolved structures in protoplanetary disks hint at unseen planets. Previous imaging observations of the transitional disk around MWC 758 revealed an inner cavity, a ring-like outer disk, ...emission clumps, and spiral arms, all possibly generated by companions. We present ALMA dust continuum observations of MWC 758 at 0.87 mm wavelength with 43 × 39 mas angular resolution (6.9 × 6.2 au) and 20 Jy beam−1 rms. The central submillimeter emission cavity is revealed to be eccentric; once deprojected, its outer edge can be well fitted by an ellipse with an eccentricity of 0.1 and one focus on the star. The broad ring-like outer disk is resolved into three narrow rings with two gaps in between. The outer two rings tentatively show the same eccentricity and orientation as the innermost ring bounding the inner cavity. The two previously known dust emission clumps are resolved in both the radial and azimuthal directions, with radial widths equal to ∼4× the local scale height. Only one of the two spiral arms previously imaged in near-infrared (NIR) scattered light is revealed in ALMA dust emission, at a slightly larger stellocentric distance owing to projection effects. We also submit evidence of disk truncation at ∼100 au based on comparing NIR imaging observations with models. The spirals, the north clump, and the truncated disk edge are all broadly consistent with the presence of one companion exterior to the spirals at roughly 100 au.
We present 1.3 mm ALMA dust polarization observations at a resolution of ∼0.02 pc for three massive molecular clumps, MM1, MM4, and MM9, in the infrared dark cloud G28.34+0.06. With these sensitive ...and high-resolution continuum data, MM1 is resolved into a cluster of condensations. The magnetic field structure in each clump is revealed by the polarized emission. We found a trend of decreasing polarized emission fraction with increasing Stokes I intensities in MM1 and MM4. Using the angular dispersion function method (a modified Davis-Chandrasekhar-Fermi method), the plane-of-sky magnetic field strengths in two massive dense cores, MM1-Core1 and MM4-Core4, are estimated to be ∼1.6 mG and ∼0.32 mG, respectively. The virial parameters in MM1-Core1 and MM4-Core4 are calculated to be ∼0.76 and ∼0.37, respectively, suggesting that massive star formation does not start in equilibrium. Using the polarization-intensity gradient-local gravity method, we found that the local gravity is closely aligned with intensity gradient in the three clumps, and the magnetic field tends to be aligned with the local gravity in MM1 and MM4 except for regions near the emission peak, which suggests that the gravity plays a dominant role in regulating the gas collapse. Half of the outflows in MM4 and MM9 are found to be aligned within 10° of the condensation-scale (<0.05 pc) magnetic field, indicating that the magnetic field could play an important role from condensation to disk scale in the early stage of massive star formation.
We investigate star formation at very early evolutionary phases in five massive clouds in the inner 500 pc of the Galaxy, the Central Molecular Zone (CMZ). Using interferometer observations of H2O ...masers and ultra-compact H ii regions, we find evidence of ongoing star formation embedded in cores of 0.2 pc scales and 105 cm−3 densities. Among the five clouds, Sgr C possesses a high (9%) fraction of gas mass in gravitationally bound and/or protostellar cores, and follows the dense ( 104 cm−3) gas star formation relation that is extrapolated from nearby clouds. The other four clouds have less than 1% of their cloud masses in gravitationally bound and/or protostellar cores, and star formation rates 10 times lower than predicted by the dense gas star formation relation. At the spatial scale of these cores, the star formation efficiency is comparable to that in Galactic disk sources. We suggest that the overall inactive star formation in these CMZ clouds could be because there is much less gas confined in gravitationally bound cores, which may be a result of the strong turbulence in this region and/or the very early evolutionary stage of the clouds when collapse has only recently started.
Abstract
Embedded class 0/I protostellar disks represent the initial condition for planet formation. This calls for a better understanding of their bulk properties and the dust grains within them. We ...model multiwavelength dust continuum observations of the disk surrounding the class I protostar TMC1A to provide insight on these properties. The observations can be well fit by a gravitationally self-regulated (i.e., marginally gravitationally unstable and internally heated) disk model with surface density Σ ∼ 1720(
R
/10 au)
−1.96
g cm
−2
and midplane temperature
T
mid
∼ 185(
R
/10 au)
−1.27
K. The observed disk contains an
m
= 1 spiral substructure; we use our model to predict the spiral’s pitch angle, and the prediction is consistent with the observations. This agreement serves as both a test of our model and strong evidence of the gravitational nature of the spiral. Our model estimates a maximum grain size
a
max
∼
196
(
R
/
10
au
)
−
2.45
μ
m
, which is consistent with grain growth being capped by a fragmentation barrier with a threshold velocity of ∼1 m s
−1
. We further demonstrate that the observational properties of TMC1A are typical among the observed population of class 0/I disks, which hints that traditional methods of disk data analysis based on Gaussian fitting and the assumption of optically thin dust emission could have systematically underestimated the disk size and mass and overestimated the grain size.
Aims.
Accretion and luminosity bursts can be triggered by three distinct mechanisms: the magnetorotational instability (MRI) in the inner disk regions, clump infall in gravitationally fragmented ...disks, and close encounters with an intruder star. We study all three of these burst mechanisms to determine the disk kinematic characteristics that can help to distinguish between them.
Methods.
Numerical hydrodynamics simulations in the thin-disk limit were employed to model the bursts in disk environments that are expected for each burst mechanism.
Results.
We found that the circumstellar disks featuring accretion bursts can bear kinematic features that are distinct for different burst mechanisms, which can be useful when identifying the origin of a particular burst. The disks in the stellar encounter and clump-infall models are characterized by deviations from the Keplerian rotation of tens of per cent, while the disks in the MRI models are characterized by deviations of only a few per cent, which is mostly caused by the gravitational instability that fuels the MRI bursts. Velocity channel maps also show distinct kinks and wiggles, which are caused by gas disk flows that are particular to each considered burst mechanism. The deviations of velocity channels in the burst-hosting disks from a symmetric pattern typical of Keplerian disks are strongest for the clump-infall and collision models, and carry individual features that may be useful for the identification of the corresponding burst mechanism. The considered burst mechanisms produce a variety of light curves with the burst amplitudes varying in the Δ
m
= 2.5−3.7 limits, except for the clump-infall model where Δ
m
can reach 5.4, although the derived numbers may be affected by a small sample and boundary conditions.
Conclusions.
Burst-triggering mechanisms are associated with distinct kinematic features in the burst-hosting disks that may be used for their identification. Further studies including a wider model parameter space and the construction of synthetic disk images in thermal dust and molecular line emission are needed to constrain the mechanisms that lead to FU Orionis bursts.
Filamentary structures are ubiquitous in high-mass star-forming molecular clouds. Their relation with high-mass star formation is still to be understood. Here we report interferometric observations ...toward eight filamentary high-mass star-forming clouds. A total of 50 dense cores are identified in these clouds, most of which present signatures of high-mass star formation. Five of them are not associated with any star formation indicators and hence are prestellar core candidates. Evolutionary phases of these cores and their line widths, temperatures, abundances, and virial parameters are found to be correlated. In a subsample of four morphologically well-defined filaments, we find that their fragmentation cannot be solely explained by thermal or turbulence pressure support. We also investigate distributions of gas temperatures and nonthermal motions along the filaments and find a spatial correlation between nonthermal line widths and star formation activities. We find evidence of gas flows along these filaments and derive an accretion rate along filaments of ∼10−4 . These results suggest a strong relationship between massive filaments and high-mass star formation, through (i) filamentary fragmentation in very early evolutionary phases to form dense cores, (ii) accretion flows along filaments that are important for the growth of dense cores and protostars, and (iii) enhancement of nonthermal motion in the filaments by the feedback or accretion during star formation.
We are motivated by the recent measurements of dust opacity indices (β) around young stellar objects (YSOs), which suggest that efficient grain growth may have occurred earlier than the Class I ...stage. The present work makes use of abundant archival interferometric observations at submillimeter, millimeter, and centimeter wavelength bands to examine grain growth signatures in the dense inner regions (<1000 au) of nine Class 0 YSOs. A systematic data analysis is performed to derive dust temperatures, optical depths, and dust opacity indices based on single-component modified blackbody fittings to the spectral energy distributions (SEDs). The fitted dust opacity indices (β) are in a wide range of 0.3-2.0 when single-component SED fitting is adopted. Four out of the nine observed sources show β lower than 1.7, the typical value of the interstellar dust. Low dust opacity index (or spectral index) values may be explained by the effect of dust grain growth, which makes . Alternatively, the very small observed values of β may be interpreted by the presence of deeply embedded and hot inner disks, which only significantly contribute to the observed fluxes at long wavelength bands. This possibility can be tested by the higher angular resolution imaging observations of ALMA or more detailed sampling of SEDs in the millimeter and centimeter bands. The β values of the remaining five sources are close to or consistent with 1.7, indicating that grain growth would start to significantly reduce the values of β no earlier than the late Class 0 stage for these YSOs.
Context. Gravitational collapse of molecular cloud or cloud core/clump may lead to the formation of geometrically flattened, rotating accretion flow surrounding the new born star or star cluster. ...Gravitational instability may occur in such accretion flow when the gas to stellar mass ratio is high (e.g., over ~10%). Aims. This paper takes the OB cluster-forming region G10.6-0.4 as an example. We introduce the enclosed gas mass around its central ultra compact (UC) Hii region, address the gravitational stability of the accreting gas, and outline the observed potential signatures of gravitational instability. Methods. The dense gas accretion flow around the central UC Hii region in G10.6-0.4 is geometrically flattened, and is in an approximately edge-on projection. The position-velocity (PV) diagrams of various molecular gas tracers on G10.6-0.4 consistently show asymmetry in the spatial- and the velocity domain. We deduce the morphology of the dense gas accretion flow by modeling velocity distribution of the azimuthally asymmetric gas structures, and by directly de-projecting the PV diagrams. Results. We find that within the 0.3 pc radius, an infall velocity of 1–2 km s-1 may be required to explain the observed PV diagrams. In addition, the velocity distribution traced in the PV diagrams can be interpreted by spiral arm-like structures, which may be connected with exterior infalling gas filaments. We propose that the morphology of dense gas structures appears very similar to the spatially resolved gas structures around the OB cluster-forming region G33.92+0.11 with similar gas mass and size, which is likely, however, to be in an approximately face-on projection. Conclusions. The dense gas accretion flow around G10.6-0.4 appears to be Toomre-unstable, which is consistent with the existence of large-scale spiral arm-like structures, and the formation of localized gas condensations. The proposed approaches for data analyses may be applied to the observations of Class 0/I low-mass protostars, in diagnosis of disk gravitational instability.
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