分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Accretion disks are an essential component in the paradigm of the formation of low-mass stars. Recent observations further identify disks surrounding low-mass pre-main-sequence stars perturbed by flybys. Whether disks around more massive stars evolve in a similar manner becomes an urgent question. We report the discovery of a Keplerian disk of a few solar masses surrounding a 32 solar-mass protostar in the Sagittarius C cloud around the Galactic Center. The disk is gravitationally stable with two embedded spirals. A combined analysis of analytical solutions and numerical simulations demonstrates that the most likely scenario to form the spirals is through external perturbations induced by a close flyby, and one such perturber with the expected parameters is identified. The massive, early O-type star embedded in this disk forms in a similar manner with respect to low-mass stars, in the sense of not only disk-mediated accretion, but also flyby-impacted disk evolution.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Feng-Wei Xu
Ke Wang
Tie Liu
Paul F. Goldsmith
Qizhou Zhang
Mika Juvela
Hong-Li Liu
Sheng-Li Qin
Guang-Xing Li
Anandmayee Tej
Guido Garay
Leonardo Bronfman
Shanghuo Li
Yue-Fang Wu
Gilberto C. Gómez
Enrique Vázquez-Semadeni
Ken'ichi Tatematsu
Zhiyuan Ren
Yong Zhang
L. Viktor Toth
摘要： We present ALMA Band-3/7 observations towards "the Heart" of a massive hub-filament system (HFS) SDC335, to investigate its fragmentation and accretion. At a resolution of $\sim0.03$ pc, 3 mm continuum emission resolves two massive dense cores MM1 and MM2, with $383(^{+234}_{-120})$ $M_\odot$ (10-24% mass of "the Heart") and $74(^{+47}_{-24})$ $M_\odot$, respectively. With a resolution down to 0.01 pc, 0.87 mm continuum emission shows MM1 further fragments into six condensations and multi-transition lines of H$_2$CS provide temperature estimation. The relation between separation and mass of condensations at a scale of 0.01 pc favors turbulent Jeans fragmentation where the turbulence seems to be scale-free rather than scale-dependent. We use the H$^{13}$CO$^+$ (1-0) emission line to resolve the complex gas motion inside "the Heart" in position-position-velocity space. We identify four major gas streams connected to large-scale filaments, inheriting the anti-clockwise spiral pattern. Along these streams, gas feeds the central massive core MM1. Assuming an inclination angle of $45(\pm15)^{\circ}$ and a H$^{13}$CO$^+$ abundance of $5(\pm3)\times10^{-11}$, the total mass infall rate is estimated to be $2.40(\pm0.78)\times10^{-3}$ $M_\odot$ yr$^{-1}$, numerically consistent with the accretion rates derived from the clump-scale spherical infall model and the core-scale outflows. The consistency suggests a continuous, near steady-state, and efficient accretion from global collapse, therefore ensuring core feeding. Our comprehensive study of SDC335 showcases the detailed gas kinematics in a prototypical massive infalling clump and calls for further systematic and statistical analyses in a large sample.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Feng-Wei Xu
Ke Wang
Tie Liu
Paul F. Goldsmith
Qizhou Zhang
Mika Juvela
Hong-Li Liu
Sheng-Li Qin
Guang-Xing Li
Anandmayee Tej
Guido Garay
Leonardo Bronfman
Shanghuo Li
Yue-Fang Wu
Gilberto C. Gómez
Enrique Vázquez-Semadeni
Ken'ichi Tatematsu
Zhiyuan Ren
Yong Zhang
L. Viktor Toth
摘要： We present ALMA Band-3/7 observations towards "the Heart" of a massive hub-filament system (HFS) SDC335, to investigate its fragmentation and accretion. At a resolution of $\sim0.03$ pc, 3 mm continuum emission resolves two massive dense cores MM1 and MM2, with $383(^{+234}_{-120})$ $M_\odot$ (10-24% mass of "the Heart") and $74(^{+47}_{-24})$ $M_\odot$, respectively. With a resolution down to 0.01 pc, 0.87 mm continuum emission shows MM1 further fragments into six condensations and multi-transition lines of H$_2$CS provide temperature estimation. The relation between separation and mass of condensations at a scale of 0.01 pc favors turbulent Jeans fragmentation where the turbulence seems to be scale-free rather than scale-dependent. We use the H$^{13}$CO$^+$ (1-0) emission line to resolve the complex gas motion inside "the Heart" in position-position-velocity space. We identify four major gas streams connected to large-scale filaments, inheriting the anti-clockwise spiral pattern. Along these streams, gas feeds the central massive core MM1. Assuming an inclination angle of $45(\pm15)^{\circ}$ and a H$^{13}$CO$^+$ abundance of $5(\pm3)\times10^{-11}$, the total mass infall rate is estimated to be $2.40(\pm0.78)\times10^{-3}$ $M_\odot$ yr$^{-1}$, numerically consistent with the accretion rates derived from the clump-scale spherical infall model and the core-scale outflows. The consistency suggests a continuous, near steady-state, and efficient accretion from global collapse, therefore ensuring core feeding. Our comprehensive study of SDC335 showcases the detailed gas kinematics in a prototypical massive infalling clump and calls for further systematic and statistical analyses in a large sample.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Supersonic motions are common in molecular clouds. (Sub)sonic turbulence is usually detected toward dense cores and filaments. However, it remains unknown whether (sub)sonic motions at larger scales ($\gtrsim$1~pc) can be present in different environments or not. Located at a distance of about 110 pc, Ophiuchus North 1 (Oph N1) is one of the nearest molecular clouds that allows in-depth investigation of its turbulence properties by large-scale mapping observations of single-dish telescopes. We carried out the $^{12}$CO ($J=1-0$) and C$^{18}$O ($J=1-0$) imaging observations toward Oph N1 with the Purple Mountain Observatory 13.7 m telescope. The observations have an angular resolution of $\sim$55\arcsec (i.e., 0.03~pc). Most of the whole C$^{18}$O emitting regions have Mach numbers of $\lesssim$1, demonstrating the large-scale (sub)sonic turbulence across Oph N1. Based on the polarization measurements, we estimate the magnetic field strength of the plane-of-sky component to be $\gtrsim$9~$\mu$G. We infer that Oph N1 is globally sub-Alfv{\'e}nic, and is supported against gravity mainly by the magnetic field. The steep velocity structure function can be caused by the expansion of the Sh~2-27 H{\scriptsize II} region or the dissipative range of incompressible turbulence. Our observations reveal a surprising case of clouds characterised by widespread subsonic turbulence and steep size-linewidth relationship. This cloud is magnetized where ion-neutral friction should play an important role.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Recently studies discovered that part of the Gould Belt belongs to a 2.7 kpc-long {coherent, thin} wave consisting of a chain of clouds, where a damped undulation pattern has been identified from the spatial arrangement of the clouds. We use the proper motions of Young Stellar Objects (YSOs) anchored inside the clouds to study the kinematic structure of the Radcliffe Wave in terms of $v_z$, and identify a damped, wave-like pattern from the $v_z$ space, which we call "velocity undulation". We propose a new formalism based on the Ensemble Empirical Mode Decomposition (EEMD) to determine the amplitude, period, and phase of the undulation pattern, and find that the spatial and the velocity undulation share an almost identical spatial frequency of about 1.5 kpc, and both are damped when measured from one side to the other. Measured for the first cycle, they exhibit a phase difference of around $2\pi/3$. The structure is oscillating around the midplane of the Milky Way disk with an amplitude of $\sim\,130\,\pm\,20\,\rm pc$. The vertical extent of the Radcliffe Wave exceeds the thickness of the molecular disk, suggesting that the undulation of the undulation signature might originate from a perturbation, e.g. the passage of a dwarf galaxy.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Molecular clouds have complex density structures produced by processes including turbulence and gravity. We propose a triangulation-based method to dissect the density structure of a molecular cloud and study the interactions between dense cores and their environments. In our {approach}, a Delaunay triangulation is constructed, which consists of edges connecting these cores. Starting from this construction, we study the physical connections between neighboring dense cores and the ambient environment in a systematic fashion. We apply our method to the Cygnus-X massive GMC complex and find that the core separation is related to the mean surface density by $\Sigma_{\rm edge} \propto l_{\rm core }^{-0.28 }$, which can be explained by {fragmentation controlled by a scale-dependent turbulent pressure (where the pressure is a function of scale, e.g. $p\sim l^{2/3}$)}. We also find that the masses of low-mass cores ($M_{\rm core} 10\, M_{\odot}$) grow mostly through accretion. The transition from fragmentation to accretion coincides with the transition from a log-normal core mass function (CMF) to a power-law CMF. By constructing surface density profiles measured along edges that connect neighboring cores, we find evidence that the massive cores have accreted a significant fraction of gas from their surroundings and thus depleted the gas reservoir. Our analysis reveals a picture where cores form through fragmentation controlled by scale-dependent turbulent pressure support, followed by accretion onto the massive cores, {and the method can be applied to different regions to achieve deeper understandings in the future.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We study the effects of passages of compact objects such as stars, star clusters, and black holes through molecular clouds, and propose that the gravitational interaction between the compact object and the ambient gas can lead to the formation of thin and collimated features made of dense gas, which we call "molecular contrails". Supercritical contrails can collapse further leading to triggered star formation. The width of a molecular contrail is determined by the mass and velocity of the compact object and the velocity dispersion of the ambient molecular medium. Under typical conditions in the Milky Way, passages of stellar-mass objects lead to the formation of width $d\gtrsim 0.01\;\rm {parsec}$ contrails, and passages of star clusters lead to the formation of $d\gtrsim 1\;\rm {parsec}$ contrails. We present a few molecular contrail candidates from both categories identified from ALMA 1.3mm continuum observations of star-forming regions and the $^{13}$CO(1-0) map from the Galactic Ring Survey respectively. The contrails represent an overlooked channel where stars and gas in the Galactic disk interact to for structures. They also present a potential way of detecting dark compact objects in the Milky Way.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We propose a new, efficient multi-scale method to decompose a map (or signal in general) into components maps that contain structures of different sizes. In the widely-used wave transform, artifacts containing negative values arise around regions with sharp transitions due to the application of band-limited filters. In our approach, the decomposition is achieved by solving a modified, non-linear version of the diffusion equation. This is inspired by the anisotropic diffusion methods, which establish the link between image filtering and partial differential equations. In our case, the artifact issue is addressed where the positivity of the decomposed images is guaranteed. Our new method is particularly suitable for signals which contain localized, non-linear features, as typical of astronomical observations. It can be used to study the multi-scale structures of astronomical maps quantitatively and should be useful in observation-related tasks such as background removal. We thus propose a new measure called the "scale spectrum", which describes how the image values distribute among different components in the scale space, to describe maps. The method allows for input arrays of an arbitrary number of dimensions, and a python3 implementation of the algorithms is included in the Appendix and available at https://gxli.github.io/Constrained-Diffusion-Decomposition/.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The Central Molecular Zone (CMZ) is a ring-like structure sitting at the center of the Milky Way. Using the 870 $\mu$m continuum map from the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL), we study the anisotropy of the density structure of the gas in the Central Molecular Zone (CMZ) using the 2D correlation function. To quantify the spatial anisotropy, we define the critical angle $\theta_{\rm half}$, as well as the anisotropy parameter $A\equiv \frac{\pi}{4\theta_{\rm half}}-1$. We find that the density structure is strongly anisotropic at the large scale (~ 100 pc), and the degree of spatial anisotropy decreases with the decreasing scale. At the scale of ~ 10 pc, the structure is still mildly anisotropic. In our analyses, we provide a quantitative description of the anisotropic density structure of gas in the CMZ, and the formalism can be applied to different regions to study their differences.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： To investigate the effects of massive star evolution on surrounding molecules, we select 9 massive clumps previously observed with the Atacama Pathfinder Experiment (APEX) telescope and the Submillimeter Array (SMA) telescope. Based on the observations of APEX, we obtain luminosity to mass ratio L$_{\rm clump}$/M$_{\rm clump}$ that range from 10 to 154 L$_{\sun}$/M$_{\sun}$, where some of them embedded Ultra Compact (UC) H\,{\footnotesize II} region. Using the SMA, CH$_3$CN (12$_{\rm K}$--11$_{\rm K}$) transitions were observed toward 9 massive star-forming regions. We derive the CH$_3$CN rotational temperature and column density using XCLASS program, and calculate its fractional abundance. We find that CH$_3$CN temperature seems to increase with the increase of L$_{\rm clump}$/M$_{\rm clump}$ when the ratio is between 10 to 40 L$_{\sun}$/M$_{\sun}$, then decrease when L$_{\rm clump}$/M$_{\rm clump}$ $\ge$ 40 L$_{\sun}$/M$_{\sun}$. Assuming the CH$_3$CN gas is heated by radiation from the central star, the effective distance of CH$_3$CN relative to the central star is estimated. The distance range from $\sim$ 0.003 to $\sim$ 0.083 pc, which accounts for $\sim$ 1/100 to $\sim$ 1/1000 of clump size. The effective distance increases slightly as L$_{\rm clump}$/M$_{\rm clump}$ increases (R$_{\rm eff}$ $\sim$ (L$_{\rm clump}$/M$_{\rm clump}$)$^{0.5\pm0.2}$). Overall, the CH$_3$CN abundance is found to decrease as the clumps evolve, e.g., X$_{\rm CH_3CN}$ $\sim$ (L$_{\rm clump}$/M$_{\rm clump}$)$^{-1.0\pm0.7}$. The steady decline of CH$_3$CN abundance as the clumps evolution can be interpreted as a result of photodissociation.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The interstellar medium (ISM) is an inseparable part of the Milky Way ecosystem whose evolutionary history remains a challenging question. We trace the evolution of the molecular ISM using a sample of Young Stellar Objects (YSO) association --molecular cloud complex (YSO-MC complex). We derive their three-dimensional (3D) velocities by combining the Gaia astrometric measurements of the YSO associations and the CO observations of the associated molecular clouds. Based on the 3D velocities, we simulate the motions of the YSO-MC complexes in the Galactic potential and forecast the ISM evolution by tracing the motions of the individual complexes, and reveal the roles of shear and stellar feedback in determining ISM evolution: Galactic shear stretches Galactic-scale molecular cloud complexes, such as the G120 Complex, into Galactic-scale filaments, and it also contributes to the destruction of the filaments; while stellar feedback creates interconnected superbubbles whose expansion injects peculiar velocities into the ISM. The Galactic-scale molecular gas clumps are often precursors of the filaments and the Galactic-scale filaments are transient structures under a constant stretch by shear. This evolutionary sequence sets a foundation to interpret other gas structures. Animations are available at https://gxli.github.io/ISM-6D/movie.html.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： HII regions made of gas ionized by radiations from young massive stars, are widely distributed in the Milky Way. They are tracers for star formation, and their distributions are correlated with the Galactic spiral structure. Radio recombination lines (RRLs) of hydrogen and other atoms allows for the precisest determination of physical parameters such as temperature and density. However, RRLs at around 1.4 GHz from HII regions are weak and their detections are difficult. As a result, only a limited number of detections have been obtained yet. The 19-beam receiver on board of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) can simultaneously cover 23 RRLs for H$n{\alpha}$, He$n{\alpha}$, and C$n{\alpha}$ ($n = 164 - 186$), respectively. This, combined with its unparalleled collecting area, makes FAST the most powerful telescope to detect weak RRLs. In this pilot survey, we use FAST to observe nine HII regions at L band. We allocate 20 minutes pointing time for each source to achieve a sensitivity of around 9 mK in a velocity resolution of 2.0 km/s. In total, 21 RRLs for H$n{\alpha}$ and C$n{\alpha}$ at $1.0 - 1.5$ GHz have been simultaneously detected with strong emission signals. Overall, the detection rates for the H167${\alpha}$ and C167${\alpha}$ RRLs are 100%, while that for the He167${\alpha}$ RRL is 33.3%. Using hydrogen and helium RRLs, we measure the electron density, electron temperature, and pressure for three HII regions. This pilot survey demonstrates the capability of FAST in RRL measurements, and a statistically meaningful sample with RRL detection, through which knowledges about Galactic spiral structure and evolution can be obtained, is expected in the future.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Lijie Liu
Martin Bureau
Guang-Xing Li
Timothy A. Davis
Dieu D. Nguyen
Fu-Heng Liang
Woorak Choi
Mark R. Smith
Satoru Iguchi
摘要： We present a study of molecular structures (clumps and clouds) in the dwarf galaxy NGC 404 using high-resolution (0.86x0.51 pc^2) Atacama Large Millimeter/sub-millimeter Array ^{12}CO(2-1) observations. We find two distinct regions in NGC 404: a gravitationally-stable central region (Toomre parameter Q=3-30) and a gravitationally-unstable molecular ring (Q<=1). The molecular structures in the central region have a steeper size -- linewidth relation and larger virial parameters than those in the molecular ring, suggesting gas is more turbulent in the former. In the molecular ring, clumps exhibit a shallower mass -- size relation and larger virial parameters than clouds, implying density structures and dynamics are regulated by different physical mechanisms at different spatial scales. We construct an analytical model of clump-clump collisions to explain the results in the molecular ring. We propose that clump-clump collisions are driven by gravitational instabilities coupled with galactic shear, that lead to a population of clumps whose accumulation lengths (i.e. average separations) are approximately equal to their tidal radii. Our model-predicted clump masses and sizes (and mass -- size relation) and turbulence energy injection rates (and size -- linewidth relation) match the observations in the molecular ring very well, suggesting clump-clump collisions is the main mechanism regulating clump properties and gas turbulence in that region. As expected, our collision model does not apply to the central region, where turbulence is likely driven by clump migration.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The evolution of molecular interstellar clouds is a complex, multi-scale process. The power-law density exponent describes the steepness of density profiles, and it has been used to characterize the density structures of the clouds yet its usage is usually limited to spherically symmetric systems. Importing the Level-Set Method, we develop a new formalism that generates robust maps of a generalized density exponent $k_{\rho}$ at every location for complex density distributions. By applying it to high fidelity, high dynamical range map of the Perseus molecular cloud constructed using data from the Herschel and Planck satellites, we find that the density exponent exhibits a surprisingly wide range of variation ($-3.5 \lesssim k_{\rho} \lesssim -0.5$). Regions at later stages of gravitational collapse are associated with steeper density profiles. Inside a region, gas located in the vicinities of dense structures has very steep density profiles with $k_{\rho} \approx -3$, which forms because of depletion. This density exponent analysis reveals diverse density structures, forming a coherent picture that gravitational collapse leads to a continued steepening of the density profile. We expect our method to be effective in studying other power-law-like density structures, including granular materials and the Large-Scale Structure of the Universe.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： DR21(OH) ridge, the central part of a high-mass star and cluster forming hub-filament system, is resolved spatially and kinematically into three nearly parallel fibers (f1, f2, and f3) with a roughly north-south orientation, using the observations of molecular transitions of H$^{13}$CO$^+$ (1-0), N$_2$H$^+$ (1-0), and NH$_2$D (1$_{1,1}$-1$_{0,1}$) with the Combined Array for Research in Millimeter Astronomy. These fibers are all mildly supersonic ($\sigma_{\rm V}$ about 2 times the sound speed), having lengths around 2 pc and widths about 0.1 pc, and they entangle and conjoin in the south where the most active high-mass star formation takes place. They all have line masses 1 - 2 orders of magnitude higher than their low-mass counterparts and are gravitationally unstable both radially and axially. However, only f1 exhibits high-mass star formation all the way along the fiber, yet f2 and f3 show no signs of significant star formation in their northern parts. A large velocity gradient increasing from north to south is seen in f3, and can be well reproduced with a model of free-fall motion toward the most massive and active dense core in the region, which corroborates the global collapse of the ridge and suggests that the disruptive effects of the tidal forces may explain the inefficiency of star formation in f2 and f3. On larger scales, some of the lower-density, peripheral filaments are likely to be the outer extensions of the fibers, and provide hints on the origin of the ridge.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The evolution of the molecular interstellar medium is controlled by processes such as turbulence, gravity, stellar feedback, and Galactic shear. AL a part of the ISM-6D https://gxli.github.io/ISM-6D/ project, using Gaia astrometric measurements towards a sample of young stellar objects (YSOs), we study morphology and kinematic structure of the associated molecular gas. We identify 150 YSO associations with distance $d \lesssim 3 \;\rm kpc$. The YSO associations are elongated, with a median aspect ratio of 1.97, and are oriented parallel to the disk midplane, with a median angle of 30$^{\circ}$. The turbulence in the molecular clouds as probed by the YSOs is isotropic, and the velocity dispersions are related to the sizes by $\sigma_{v,{\rm 2D}} = 0.74\;(r/{\rm pc})^{0.67} \;({\rm km/s})\;$. The slope is on the steeper side, yet consistent with previous measurements. The energy dissipation rate of turbulence $\dot{\epsilon} = \sigma_{v,{\rm 3D}}^3 /L$ decreases with the Galactocentric distance, with a gradient of 0.2 $\rm dex \; kpc^{-1}$, which can be explained if turbulence is driven by cloud collisions. In this scenario, the clouds located in the inner Galaxy have higher chances to accrete smaller clouds and are more turbulent. Although the density structures of the complexes are anisotropic, the turbulence is consistent with being isotropic. If the alignment between density structures and the Galactic-disk mid-plane is due to shear, we expect $t_{\rm cloud} \gtrsim t_{\rm shear}\approx 30\; \rm Myr$. This cloud lifetime is longer than the turbulence crossing time, and a continuous energy injection is required to maintain the turbulence.
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