分类： 天文学 >> 天文学 提交时间： 2023-02-19
Shanghuo Li
Xing Lu
Qizhou Zhang
Chang-Won Lee
Patricio Sanhueza
Henrik Beuther
Izaskun
Jiménez-Serra
Keping Qiu
Aina Palau
Siyi Feng
Thushara Pillai
Kee-Tae Kim
Hong-Li Liu
Josep Miquel. Girart
Tie Liu
Junzhi Wang
Ke Wang
Hauyu Baobab Liu
Howard A. Smith
摘要： Pre-stellar cores represent the initial conditions of star formation. Although these initial conditions in nearby low-mass star-forming regions have been investigated in detail, such initial conditions remain vastly unexplored for massive star-forming regions. We report the detection of a cluster of low-mass starless and pre-stellar core candidates in a massive star protocluster forming cloud, NGC6334S. With the ALMA observations at a $\sim$0.02 pc spatial resolution, we identified 17 low-mass starless core candidates that do not show any evidence of protostellar activity. These candidates present small velocity dispersions, high fractional abundances of NH$_{2}$D, high NH$_{3}$ deuterium fractionations, and are completely dark in the infrared wavelengths from 3.6 up to 70~$\mu$m. Turbulence is significantly dissipated and the gas kinematics are dominated by thermal motions toward these candidates. Nine out of the 17 cores are gravitationally bound, and therefore are identified as pre-stellar core candidates. The embedded cores of NGC6334S show a wide diversity in masses and evolutionary stages.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The fragmentation process in massive star-forming regions is one of the contemporary problems in astrophysics, and several physical processes have been proposed to control the fragmentation including turbulence, magnetic field, rotation, stellar feedback, and gravity. However, the fragmentation process has been poorly studied at small spatial scales well below 1000 AU. We aim to use ALMA (Atacama Large Millimeter and Submillimeter Array) high angular resolution data to identify the fragments in W51 IRS2 and to study the fragmentation properties on a spatial scale of 200 AU. We used ALMA data of W51 IRS2 from three projects, which give an angular resolution of 0.028$^{\prime\prime}$ (144 AU) at millimeter wavelengths. We identified compact fragments by using {\it uv}-range constrained 1.3 mm continuum data. A Mean Surface Density of Companions (MSDC) analysis has been performed to study the separations between fragments. A total number of 33 continuum sources are identified and 29 out of them are defined as fragments in the surveyed region.The MSDC analysis reveals two breaks corresponding to spatial sales of 1845 AU and 7346 AU, indicative of a two-level clustering phenomenon, along with a linear regime below 1845 AU, mostly associated with W51 North, whose slope is consistent with the slope for the clustering regime of other cluster-like regions in the Galaxy. The typical masses and separations of the fragments as well as the relation between density and number of fragments can be explained through a thermal Jeans process operating at high temperatures of 200--400 K, consistent with previous measurements of the temperature in the region, and produced by the nearby massive stars. Therefore, although W51 IRS2 seems to be undergoing a thermally inhibited fragmentation phase, this does not seem to prevent the formation of a protocluster associated with W51 North.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Shanghuo Li
Patricio Sanhueza
Chang Won Lee
Qizhou Zhang
Henrik Beuther
Aina Palau
Hong-Li Liu
Howard Smith
Hauyu Baobab Liu
Izaskun
Jiménez-Serra
Kee-Tae Kim
Siyi Feng
Josep Miquel. Girart
Tie Liu
Junzhi Wang
Di Li
Keping Qiu
Xing Lu
Ke Wang
摘要： We present a study of narrow filaments toward a massive infrared dark cloud, NGC 6334S, using the Atacama Large Millimeter/submillimeter Array (ALMA). Thirteen gas filaments are identified using the H$^{13}$CO$^{+}$ line, while a single continuum filament is revealed by the continuum emission. The filaments present a compact radial distribution with a median filament width of $\sim$0.04 pc narrower than the previously proposed `quasi-universal' 0.1~pc filament width. The higher spatial resolution observations and higher-density gas tracer tend to identify even narrower and lower mass filaments. The filament widths are roughly twice the size of embedded cores. The gas filaments are largely supported by thermal motions. The nonthermal motions are predominantly subsonic and transonic in both identified gas filaments and embedded cores, which may imply that stars are likely born in environments of low turbulence. A fraction of embedded objects show a narrower velocity dispersion compared with their corresponding natal filaments, which may indicate that the turbulent dissipation is taking place in these embedded cores. The physical properties (mass, mass per unit length, gas kinematics, and width) of gas filaments are analogous to those of narrow filaments found in low- to high-mass star-forming regions. The more evolved sources are found to be farther away from the filaments, a situation that may have resulted from the relative motions between the YSOs and their natal filaments.
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