分类： 天文学 >> 天文学 提交时间： 2023-02-19
Yi Yang
Hauyu Baobab Liu
Takayuki Muto
Jun Hashimoto
Ruobing Dong
Kazuhiro Kanagawa
Munetake Momose
Eiji Akiyama
Yasuhiro Hasegawa
Takashi Tsukagoshi
Mihoko Konishi
Motohide Tamura
摘要： Crescent-like asymmetric dust structures discovered in protoplanetary disks indicate dust aggregations. Thus, the research on them helps us understand the planet formation process. Here we analyze the ALMA data of the protoplanetary disk around the T-Tauri star SR 21, which has asymmetric structures detected in previous sub-millimeter observations. Imaged at ALMA Band 6 (1.3 mm) with a spatial resolution of about 0.$\arcsec$04, the disk is found to consist of two rings and three asymmetric structures, with two of the asymmetric structures being in the same ring. Compared to the Band 6 image, the Band 3 (2.7 mm) image also shows the three asymmetric structures but with some clumps. The elongated asymmetric structures in the outer ring could be due to the interactions of a growing planet. By fitting the Band 3 and Band 6 dust continuum data, two branches of solutions of maximum dust size in the disk are suggested: one is larger than 1 mm, and the other is smaller than 300 $\mu m$. High-resolution continuum observations at longer wavelengths as well as polarization observations can help break the degeneracy. We also suggest that the prominent spiral previously identified in VLT/SPHERE observations to the south of the star at 0.$\arcsec$25 may be the scattered light counterpart of the Inner Arc, and the structure is a dust-trapping vortex in nature. The discovered features in SR 21 make it a good target for studying the evolution of asymmetric structures and planet formation.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Ryuta Orihara
Munetake Momose
Takayuki Muto
Jun Hashimoto
Hauyu Baobab Liu
Takashi Tsukagoshi
Tomoyuki Kudo
Sanemichi Takahashi
Yi Yang
Yasuhiro Hasegawa
Ruobing Dong
Mihoko Konishi
Eiji Akiyama
摘要： In this study, we reported the results of high-resolution (0.14 arcsec) Atacama Large Millimeter/submillimeter Array (ALMA) observations of the 225 GHz dust continuum and CO molecular emission lines from the transitional disk around SY Cha. Our high-resolution observations clearly revealed the inner cavity and the central point source for the first time. The radial profile of the ring can be approximated by a bright narrow ring superimposed on a fainter wide ring. Furthermore, we found that there is a weak azimuthal asymmetry in dust continuum emission. For gas emissions, we detected $\rm{}^{12}CO$(2$-$1), $\rm{}^{13}CO$(2$-$1) and $\rm{}C^{18}O$(2$-$1), from which we estimated the total gas mass of the disk to be $2.2\times10^{-4}M_\odot$, assuming a CO/H$_2$ ratio of $10^{-4}$. The observations showed that the gas is present inside the dust cavity. The analysis of the velocity structure of the $\rm{}^{12}CO$(2$-$1) emission line revealed that the velocity is distorted at the location of the dust inner disk, which may be owing to warping of the disk or radial gas flow within the cavity of the dust disk. High-resolution observations of SY Cha showed that this system is composed of a ring and a distorted inner disk, which may be common, as indicated by the survey of transitional disk systems at a resolution of $\sim$0.1~arcsec.
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
Ruobing Dong
Hauyu Baobab Liu
Nicolas Cuello
Christophe Pinte
Peter Abraham
Eduard Vorobyov
Jun Hashimoto
Agnes Kospal
Eugene Chiang
Michihiro Takami
Lei Chen
Michael Dunham
Misato Fukagawa
Joel Green
Yasuhiro Hasegawa
Thomas Henning
Yaroslav Pavlyuchenkov
Tae-Soo Pyo
Motohide Tamura
摘要： Close encounters between young stellar objects in star forming clusters are expected to dramatically perturb circumstellar disks. Such events are witnessed in numerical simulations of star formation, but few direct observations of ongoing encounters have been made. Here we report sub-0".1 resolution Atacama Large Millimeter Array (ALMA) and Jansky Very Large Array (JVLA) observations towards the million year old binary protostar Z CMa in dust continuum and molecular line emission. A point source ~4700 au from the binary has been discovered at both millimeter and centimeter wavelengths. It is located along the extension of a ~2000 au streamer structure previously found in scattered light imaging, whose counterpart in dust and gas emission is also newly identified. Comparison with simulations shows signposts of a rare flyby event in action. Z CMa is a "double burster", as both binary components undergo accretion outbursts, which may be facilitated by perturbations to the host disk by flybys.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Yuxin Lin
Friedrich Wyrowski
Hauyu Baobab Liu
Andrés Izquierdo
Timea Csengeri
Silvia Leurini
Karl M. Menten
摘要： OB star clusters originate from parsec-scale massive molecular clumps. We aim to understand the evolution of temperature and density structures on the intermediate-scale ($\lesssim$0.1-1 pc) extended gas of massive clumps. We performed $\sim$0.1 pc resolution observations (SMA+APEX) of multiple molecular line tracers (e.g., CH$_{3}$CCH, H$_{2}$CS, CH$_{3}$CN, CH$_{3}$OH) which cover a wide range of excitation conditions, towards a sample of eight massive clumps. Based on various radiative transfer models, we constrain the gas temperature and density structures and establish an evolutionary picture, aided by a spatially-dependent virial analysis and abundance ratios of multiple species. We determine temperature radial profiles varying between 30-200 K over a continuous scale, from the center of the clumps out to 0.3-0.4 pc radii. The clumps' radial gas density profiles, described by radial power-laws with slopes between -0.6 and $\sim$-1.5, are steeper for more evolved sources, as suggested by results based on both dust continuum, representing the bulk of the gas ($\sim$10$^{4}$ cm$^{-3}$), and CH$_{3}$OH lines probing the dense gas ($\gtrsim$10$^{6}$-10$^{8}$ cm$^{-3}$) regime. The density contrast between the dense gas and the bulk gas increases with evolution, and may be indicative of spatially and temporally varying star formation efficiencies. The radial profiles of the virial parameter show a global variation towards a sub-virial state as the clump evolves. The line-widths decline with increasing radius around the central core region and increase in the outer envelope, with a slope shallower than the case of the supersonic turbulence ($\,\propto\,$$r^{0.5}$) and the subsonic Kolmogorov scaling ($\,\propto\,$$r^{0.33}$). In the context of clump evolution, we also find that the abundance ratios of [CCH]/[CH$_{3}$OH] and [CH$_{3}$CN]/[CH$_{3}$OH] show correlations with clump $L/M$.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Roberto Galván-Madrid
Qizhou Zhang
Andrés Izquierdo
Charles J. Law
Thomas Peters
Eric Keto
Hauyu Baobab Liu
Paul T. P. Ho
Adam Ginsburg
Carlos Carrasco-González
摘要： We present ALMA observations with a 800 au resolution and radiative-transfer modelling of the inner part ($r\approx6000$ au) of the ionized accretion flow around a compact star cluster in formation at the center of the luminous ultra-compact (UC) HII region G10.6-0.4. We modeled the flow with an ionized Keplerian disk with and without radial motions in its outer part, or with an external Ulrich envelope. The MCMC fits to the data give total stellar masses $M_\star$ from 120 to $200~M_\odot$, with much smaller ionized-gas masses $M_\mathrm{ion-gas} = 0.2$ to $0.25~M_\odot$. The stellar mass is distributed within the gravitational radius $R_g\approx 1000$ to 1500 au, where the ionized gas is bound. The viewing inclination angle from the face-on orientation is $i = 49$ to $56~\deg$. Radial motions at radii $r > R_g$ converge to $v_{r,0} \approx 8.7$ km/s, or about the speed of sound of ionized gas, indicating that this gas is marginally unbound at most. From additional constraints on the ionizing-photon rate and far-IR luminosity of the region, we conclude that the stellar cluster consists of a few massive stars with $M_\mathrm{star} = 32$ to $60~M_\odot$, or one star in this range of masses accompanied by a population of lower-mass stars. Any active accretion of ionized gas onto the massive (proto)stars is residual. The inferred cluster density is very large, comparable to that reported at similar scales in the Galactic Center. Stellar interactions are likely to occur within the next Myr.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Junhao Liu
Qizhou Zhang
Keping Qiu
Hauyu Baobab Liu
Thushara Pillai
Josep Miquel Girart
Zhi-Yun Li
Ke Wang
摘要： We present 1.3 mm ALMA dust polarization observations at a resolution of $\sim$0.02 pc of three massive molecular clumps, MM1, MM4, and MM9, in the infrared dark cloud G28.34+0.06. With the 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 strength in two massive dense cores, MM1-Core1 and MM4-Core4, are estimated to be $\sim$1.6 mG and $\sim$0.32 mG, respectively. \textbf{The ordered magnetic energy is found to be smaller than the turbulent energy in the two cores, while the total magnetic energy is found to be comparable to the turbulent energy.} The total virial parameters in MM1-Core1 and MM4-Core4 are calculated to be $\sim$0.76 and $\sim$0.37, respectively, suggesting that massive star formation does not start in equilibrium. Using the polarization-intensity gradient-local gravity method, we found that the local gravity is closely aligned with intensity gradient in the three clumps, and the magnetic field tends to be aligned with the local gravity in MM1 and MM4 except for regions near the emission peak, which suggests that the gravity plays a dominant role in regulating the gas collapse. Half of the outflows in MM4 and MM9 are found to be aligned within 10$^{\circ}$ of the condensation-scale ($<$0.05 pc) magnetic field, indicating that the magnetic field could play an important role from condensation to disk scale in the early stage of massive star formation. We also found that the fragmentation in MM1-Core1 cannot be solely explained by thermal Jeans fragmentation or turbulent Jeans fragmentation.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Junhao Liu
Qizhou Zhang
Patrick M. Koch
Hauyu Baobab Liu
Zhi-Yun Li
Shanghuo Li
Josep Miquel Girart
Huei-Ru Vivien Chen
Tao-Chung Ching
Paul T. P. Ho
Shih-Ping Lai
Keping Qiu
Ramprasad Rao
Ya-wen Tang
摘要： We present ALMA dust polarization and molecular line observations toward 4 clumps (I(N), I, IV, and V) in the massive star-forming region NGC 6334. In conjunction with large-scale dust polarization and molecular line data from JCMT, Planck, and NANTEN2, we make a synergistic analysis of relative orientations between magnetic fields ($\theta_{\mathrm{B}}$), column density gradients ($\theta_{\mathrm{NG}}$), local gravity ($\theta_{\mathrm{LG}}$), and velocity gradients ($\theta_{\mathrm{VG}}$) to investigate the multi-scale (from $\sim$30 pc to 0.003 pc) physical properties in NGC 6334. We find that the relative orientation between $\theta_{\mathrm{B}}$ and $\theta_{\mathrm{NG}}$ changes from statistically more perpendicular to parallel as column density ($N_{\mathrm{H_2}}$) increases, which is a signature of trans-to-sub-Alfv\'{e}nic turbulence at complex/cloud scales as revealed by previous numerical studies. Because $\theta_{\mathrm{NG}}$ and $\theta_{\mathrm{LG}}$ are preferentially aligned within the NGC 6334 cloud, we suggest that the more parallel alignment between $\theta_{\mathrm{B}}$ and $\theta_{\mathrm{NG}}$ at higher $N_{\mathrm{H_2}}$ is because the magnetic field line is dragged by gravity. At even higher $N_{\mathrm{H_2}}$, the angle between $\theta_{\mathrm{B}}$ and $\theta_{\mathrm{NG}}$ or $\theta_{\mathrm{LG}}$ transits back to having no preferred orientation or statistically slightly more perpendicular, suggesting that the magnetic field structure is impacted by star formation activities. A statistically more perpendicular alignment is found between $\theta_{\mathrm{B}}$ and $\theta_{\mathrm{VG}}$ throughout our studied $N_{\mathrm{H_2}}$ range, which indicates a trans-to-sub-Alfv\'{e}nic state at small scales as well. The normalised mass-to-flux ratio derived from the polarization-intensity gradient (KTH) method increases with $N_{\mathrm{H_2}}$.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The azimuthal asymmetries of dust rings in protoplanetary disks such as a crescent around young stars are often interpreted as dust traps, and thus as ideal locations for planetesimal and planet formations. Whether such dust traps effectively promote planetesimal formation in disks around very-low-mass stars (VLM; a mass of $\lesssim$0.2~$M_\odot$) is debatable, as the dynamical and grain growth timescales in such systems are long. To investigate grain growth in such systems, we studied the dust ring with crescent around the VLM star ZZ~Tau~IRS using the Karl G. Jansky Very Large Array (JVLA) at centimeter wavelengths. Significant signals were detected around ZZ~Tau~IRS. To estimate the maximum grain size ($a_{\rm max}$) in the crescent, we compared the observed spectral energy distribution (SED) with SEDs for various $a_{\rm max}$ values predicted by radiative transfer calculations. We found $a_{\rm max} \gtrsim$~1~mm and $\lesssim$~60~$\mu$m in the crescent and ring, respectively, though our modeling efforts rely on uncertain dust properties. Our results suggest that grain growth occurred in the ZZ~Tau~IRS disk, relative to sub-micron-sized interstellar medium. Planet formation in crescent with mm-sized pebbles might proceed more efficiently than in other regions with sub-millimeter-sized pebbles via pebble accretion scenarios.
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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