18 research outputs found
The role of magnetic fields in the fragmentation of the Taurus B213 filament into Sun-type star-forming cores
Fragmentation is a key step in the process of transforming clouds (and their
substructures such as filaments, clumps, and cores) into protostars. The
thermal gas pressure and gravitational collapse are believed to be the primary
agents governing this process, referred to as the thermal Jeans fragmentation.
However, the contributions of other factors (such as magnetic fields and
turbulence) to the fragmentation process remain less explored. In this work, we
have tested possible fragmentation mechanisms by estimating the mean core mass
and mean inter-core separation of the B213 filament. We have used the 14"
resolution James Clerk Maxwell Telescope (JCMT) Submillimetre Common-User
Bolometer Array 2 (SCUBA-2)/POL-2 850 m dust continuum map and combined it
with a Planck 850 m map and Herschel data. We find that in addition to the
thermal contribution, the presence of ordered magnetic fields could be
important in the fragmentation of the B213 filament.Comment: 10 pages, 3 figures, and 5 tables ; Accepted for publication in JOA
Spatial and temporal variations of tap water 17O-excess in China
Compared to tap water δ2H and δ18O, tap water 17O-excess preserves additional information about source water dynamics. In this study, we provide the first report of 17O-excess variations of tap water across China (652 samples). Annual 17O-excess of tap waters at the national scale did not show obvious spatial pattern, and was almost unaffected by local environmental factors except in the Qinghai-Tibet Plateau region with a strong latitudinal trend. The mean 17O-excess values in different seasons were not significantly different. The isotopic compositions of most of the tap waters at the annual and seasonal scale were likely influenced by the equilibrium fractionation effect (δ′18O-δ′17O slope ranged from 0.5277 to 0.5301), except for the northwest region in the summer (slope = 0.5264) influenced by kinetic fractionation associated with re-evaporation effect. Based on the information of tap water source distribution, site aridity index and the known precipitation δ18O values, a subset of the tap water can be considered as precipitation proxy. Different from the obvious spatial characteristics of precipitation δ18O, precipitation 17O-excess did not show a clear spatial pattern. But it revealed much detailed precipitation formation mechanisms related to different climate regions and geographical conditions. The lower 17O-excess values of the precipitation-sourced tap waters were caused by kinetic fractionation associated with supersaturation process in snow or glacier formation and re-evaporation effect in some arid regions. The higher 17O-excess values of the precipitation-sourced tap waters in the inland were caused by continental moisture recycling, while likely caused by multiple factors in the southeast coastal region including short transport from ocean source and the humid local environment. Overall, this study provides a unique tap water 17O-excess dataset across China, and probes the precipitation formation mechanisms using tap waters
Initial conditions of star formation at 2000 au: physical structure and NH depletion of three early-stage cores
Pre-stellar cores represent a critical evolutionary phase in low-mass star
formation. We aim to unveil the detailed thermal structure and density
distribution of three early-stage cores, starless core L1517B, and prestellar
core L694-2 and L429, with the high angular resolution observations of the
NH (1,1) and (2,2) inversion transitions obtained with VLA and GBT. In
addition, we explore where/if NH depletes in the central regions.
Applying the mid-infrared extinction method to the 8m
map we obtain a high angular resolution hydrogen column density map, and derive
the gas density profile to assess the variation of NH abundance as a
function of gas volume density. The measured temperature profiles of L429 and
L1517B show a minor decrease towards the core center, dropping from 9\~K
to below 8\~K, and 11 K to 10 K, while L694-2 has a rather uniform
temperature distribution around 9 K. Among the three cores, L429 has the
highest central gas density, close to sonic velocity line-width, and largest
localised velocity gradient, all indicative of an advanced evolutionary stage.
We resolve that the abundance of NH becomes two times lower in the
central region of L429, occurring around a gas density of
4.410. Compared to Ophiuchus/H-MM1 which shows an even
stronger drop of the NH abundance at 210, the
abundance variations of the three cores plus Ophiuchus/H-MM1 suggest a
progressive NH depletion with increasing central density of the core.Comment: 23 pages, 23 figures incl. appendix; A&A accepte
The magnetic field in colliding filaments G202.3+2.5
We observe the magnetic field morphology towards a nearby star-forming
filamentary cloud, G202.3+2.5, by the JCMT/POL-2 850 {\mu}m thermal dust
polarization observation 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 filaments' collision in these subregions. With the
kinematics obtained by N2H+ observation by IRAM, we estimate the plane-of-sky
(POS) magnetic field strength by two methods, the classical
Davis-Chandrasekhar-Fermi (DCF) method and the angular dispersion function
(ADF) method, B_{pos,dcf} and B_{pos,adf} are ~90 {\mu}G and ~53 {\mu}G. We
study the relative importance between the gravity (G), magnetic field (B) and
turbulence (T) in the four subregions, 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 effect 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 DCF or ADF method, which may
further underestimate/overestimate related parameters.Comment: Accepted by ApJ. 20 pages, 9 figure
The HASHTAG Project: The First Submillimeter Images of the Andromeda Galaxy from the Ground
Observing nearby galaxies with submillimeter telescopes on the ground has two major challenges. First, the brightness is significantly reduced at long submillimeter wavelengths compared to the brightness at the peak of the dust emission. Second, it is necessary to use a high-pass spatial filter to remove atmospheric noise on large angular scales, which has the unwelcome side effect of also removing the galaxy\u27s large-scale structure. We have developed a technique for producing high-resolution submillimeter images of galaxies of large angular size by using the telescope on the ground to determine the small-scale structure (the large Fourier components) and a space telescope (Herschel or Planck) to determine the large-scale structure (the small Fourier components). Using this technique, we are carrying out the HARP and SCUBA-2 High Resolution Terahertz Andromeda Galaxy Survey (HASHTAG), an international Large Program on the James Clerk Maxwell Telescope, with one aim being to produce the first high-fidelity high-resolution submillimeter images of Andromeda. In this paper, we describe the survey, the method we have developed for combining the space-based and ground-based data, and we present the first HASHTAG images of Andromeda at 450 and 850 μm. We also have created a method to predict the CO(J = 3-2) line flux across M31, which contaminates the 850 μm band. We find that while normally the contamination is below our sensitivity limit, it can be significant (up to 28%) in a few of the brightest regions of the 10 kpc ring. We therefore also provide images with the predicted line emission removed
Triple isotope variations of monthly tap water in China
Tap water isotopic compositions could potentially record information on local climate and water management practices. A new water isotope tracer 17O-excess becomes available in recent years providing additional information of the mechanisms of various hydrological processes. Detailed data records of tap water 17O-excess have not been reported. In this report, monthly tap water samples (n = 652) were collected from December 2014 to November 2015 from 92 sites across China. The isotopic variations (δ2H, δ18O, and δ17O) of tap water were analyzed by a Triple Water Vapor Isotope Analyzer (T-WVIA) based on Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) technique and two second-order isotopic variables (d-excess and 17O-excess) were calculated. The geographic location information of the 92 collection sites including latitude, longitude, and elevation were also provided in this dataset. This report presents national-scale tap water isotope dataset at monthly time scale. Researchers and water resource managers who focus on the tap water issues could use them to probe the water source and water management strategies at large spatial scales