分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The inheritance of material across the star and planet formation process is traced by deuterium fractionation. We report here the first detection of doubly deuterated methanol towards pre-stellar cores. We study the deuterium fractionation of methanol, CH$_{3}$OH, towards two starless and two pre-stellar cores. We derive a D/H ratio of 0.8-1.9$\%$ with CH$_{2}$DOH in pre-stellar cores H-MM1 and L694-2, consistent with measurements in more evolved Class 0/I objects and comet 67P/Churyumov-Gerasimenko, suggesting a direct chemical link arising in the pre-stellar stage. Furthermore, the column density ratios of CHD$_{2}$OH/CH$_{2}$DOH are $\sim$50-80$\%$, consistently high as that towards Class 0/I objects, indicating an efficient formation mechanism of CHD$_{2}$OH, possibly through H atom additions to D$_{2}$CO. The CH$_{2}$DOH/CH$_{3}$OH and CHD$_{2}$OH/CH$_{3}$OH column density ratios in the two pre-stellar cores are larger than that in the two starless cores B68 and L1521E, representing an evolutionary trend of methanol deuteration in early-stage cores.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Turbulence is the dominant source of collisional velocities for grains with a wide range of sizes in protoplanetary disks. So far, only Kolmogorov turbulence has been considered for calculating grain collisional velocities, despite the evidence that turbulence in protoplanetary disks may be non-Kolmogorov. In this work, we present calculations of grain collisional velocities for arbitrary turbulence models characterized by power-law spectra and determined by three dimensionless parameters: the slope of the kinetic energy spectrum, the slope of the auto-correlation time, and the Reynolds number. The implications of our results are illustrated by numerical simulations of the grain size evolution for different turbulence models. We find that for the modeled cases of the Iroshnikov-Kraichnan turbulence and the turbulence induced by the magneto-rotational instabilities, collisional velocities of small grains are much larger than those for the standard Kolmogorov turbulence. This leads to faster grain coagulation in the outer regions of protoplanetary disks, resulting in rapid increase of dust opacity in mm-wavelength and possibly promoting planet formation in very young disks.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Pre-stellar cores are the basic unit for the formation of stars and stellar systems. The anatomy of the physical and chemical structures of pre-stellar cores is critical for understanding the star formation process. L1544 is a prototypical pre-stellar core, which shows significant chemical differentiation surrounding the dust peak. We aim to constrain the physical conditions at the different molecular emission peaks. This study allows us to compare the abundance profiles predicted from chemical models together with the classical density structure of Bonnor-Ebert (BE) sphere. We conducted multi-transition pointed observations of CH$_{3}$OH, c-C$_{3}$H$_{2}$ and HNCO with the IRAM 30m telescope, towards the dust peak and the respective molecular peaks of L1544. With non-LTE radiative transfer calculations and a 1-dimensional model, we revisit the physical structure of L1544, and benchmark with the abundance profiles from current chemical models. We find that the HNCO, c-C$_{3}$H$_{2}$ and CH$_{3}$OH lines in L1544 are tracing progressively higher density gas, from $\sim$10$^{4}$ to several times 10$^{5}$ cm$^{-3}$. Particularly, we find that to produce the observed intensities and ratios of the CH$_{3}$OH lines, a local gas density enhancement upon the BE sphere is required. This suggests that the physical structure of an early-stage core may not necessarily follow a smooth decrease of gas density profile locally, but can be intercepted by clumpy substructures surrounding the gravitational center. Multiple transitions of molecular lines from different molecular species can provide a tomographic view of the density structure of pre-stellar cores. The local gas density enhancement deviating from the BE sphere may reflect the impact of accretion flows that appear asymmetric and are enhanced at the meeting point of large-scale cloud structures.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： A big question in the field of star and planet formation is the time at which substantial dust grain growth occurs. The observed properties of dust emission across different wavelength ranges have been used as an indication that millimeter-sized grains are already present in the envelopes of young protostars. However, this interpretation is in tension with results from coagulation simulations, which are not able to produce such large grains in these conditions. In this work, we show analytically that the production of millimeter-sized grains in protostellar envelopes is impossible under the standard assumptions about the coagulation process. We discuss several possibilities that may serve to explain the observed dust emission in the absence of in-situ grain growth to millimeter sizes.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： To study the early phases of massive star formation, we present ALMA observations of SiO(5-4) emission and VLA observations of 6 cm continuum emission towards 32 Infrared Dark Cloud (IRDC) clumps, spatially resolved down to $\lesssim 0.05$ pc. Out of the 32 clumps, we detect SiO emission in 20 clumps, and in 11 of them the SiO emission is relatively strong and likely tracing protostellar outflows. Some SiO outflows are collimated, while others are less ordered. For the six strongest SiO outflows, we estimate basic outflow properties. In our entire sample, where there is SiO emission, we find 1.3 mm continuum and infrared emission nearby, but not vice versa. We build the spectral energy distributions (SEDs) of cores with 1.3 mm continuum emission and fit them with radiative transfer (RT) models. The low luminosities and stellar masses returned by SED fitting suggest these are early stage protostars. We see a slight trend of increasing SiO line luminosity with bolometric luminosity, which suggests more powerful shocks in the vicinity of more massive YSOs. We do not see a clear relation between the SiO luminosity and the evolutionary stage indicated by $L/M$. We conclude that as a protostar approaches a bolometric luminosity of $\sim 10^2 \: L_{\odot}$, the shocks in the outflow are generally strong enough to form SiO emission. The VLA 6 cm observations toward the 15 clumps with the strongest SiO emission detect emission in four clumps, which is likely shock ionized jets associated with the more massive ones of these protostellar cores.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Shinyoung Kim
Chang Won Lee
Mario Tafalla
Maheswar Gophinathan
Paola Caselli
Philip C. Myers
Eun Jung Chung
Shanghuo Li
摘要： We present mapping results of two prestellar cores, L1544 and L694-2, embedded in filamentary clouds in C$^{18}$O (3-2), $^{13}$CO (3-2), $^{12}$CO (3-2), HCO$^+$ (4-3), and H$^{13}$CO$^+$ (4-3) with the JCMT telescope to examine the role of the filamentary structures in the formation of dense cores in the clouds, with new distance estimates for L1544 ($175_{-3}^{+4}$ pc) and L694-2 ($203_{-7}^{+6}$ pc). From these observations, we found that the non-thermal velocity dispersion of two prestellar cores and their surrounding clouds are smaller than or comparable to the sound speed. This may indicate that the turbulence has already been dissipated for both filaments and cores during their formation time. We also found a $\lambda/4$ shift between the periodic oscillations in the velocity and the column density distributions implying the possible presence of gravitational core-forming flow motion along the axis of the filament. The mass accretion rates due to these flow motions are estimated to be 2-3 M$_\odot$ Myr$^{-1}$, being comparable to that for Serpens cloud but much smaller than those for the Hub filaments, cluster, or high mass forming filaments by 1 or 2 order of magnitudes. From this study, we suggest that the filaments in our targets might be formed from the shock-compression of colliding clouds, and then the cores are formed by gravitational fragmentation of the filaments to evolve to the prestellar stage. We conclude that the filamentary structures in the clouds play an important role in the entire process of formation of dense cores and their evolution.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Yu Cheng
Jonathan C. Tan
Paola Caselli
Laura Fissel
Hector G. Arce
Francesco Fontani
Matthew D. Goodson
Mengyao Liu
Nicholas Galitzki
摘要： We study star formation in the Center Ridge 1 (CR1) clump in the Vela C giant molecular cloud, selected as a high column density region that shows the lowest level of dust continuum polarization angle dispersion, likely indicating that the magnetic field is relatively strong. We observe the source with the ALMA 7m-array at 1.05~mm and 1.3~mm wavelengths, which enable measurements of dust temperature, core mass and astrochemical deuteration. A relatively modest number of eleven dense cores are identified via their dust continuum emission, with masses spanning from 0.17 to 6.7 Msun. Overall CR1 has a relatively low compact dense gas fraction compared with other typical clouds with similar column densities, which may be a result of the strong magnetic field and/or the very early evolutionary stage of this region. The deuteration ratios, Dfrac, of the cores, measured with N2H+(3-2) and N2D+(3-2) lines, span from 0.011 to 0.85, with the latter being one of the highest values yet detected. The level of deuteration appears to decrease with evolution from prestellar to protostellar phase. A linear filament, running approximately parallel with the large scale magnetic field orientation, is seen connecting the two most massive cores, each having CO bipolar outflows aligned orthogonally to the filament. The filament contains the most deuterated core, likely to be prestellar and located midway between the protostars. The observations permit measurement of the full deuteration structure of the filament along its length, which we present. We also discuss the kinematics and dynamics of this structure, as well as of the dense core population.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Simon E. T. Smith
Rachel Friesen
Antoine Marchal
Jaime E. Pineda
Paola Caselli
Michael Chun-Yuan Chen
Spandan Choudhury
James Di Francesco
Adam Ginsburg
Helen Kirk
Chris Matzner
Anna Punanova
Samantha Scibelli
Yancy Shirley
摘要： Filamentary structures have been found nearly ubiquitously in molecular clouds and yet their formation and evolution is still poorly understood. We examine a segment of Taurus Molecular Cloud 1 (TMC-1) that appears as a single, narrow filament in continuum emission from dust. We use the Regularized Optimization for Hyper-Spectral Analysis (ROHSA), a Gaussian decomposition algorithm which enforces spatial coherence when fitting multiple velocity components simultaneously over a data cube. We analyze HC$_5$N (9-8) line emission as part of the Green Bank Ammonia Survey (GAS) and identify three velocity-coherent components with ROHSA. The two brightest components extend the length of the filament, while the third component is fainter and clumpier. The brightest component has a prominent transverse velocity gradient of $2.7 \pm 0.1$ km s$^{-1}$ pc$^{-1}$ that we show to be indicative of gravitationally induced inflow. In the second component, we identify regularly spaced emission peaks along its length. We show that the local minima between pairs of adjacent HC$_5$N peaks line up closely with submillimetre continuum emission peaks, which we argue is evidence for fragmentation along the spine of TMC-1. While coherent velocity components have been described as separate physical structures in other star-forming filaments, we argue that the two bright components identified in HC$_5$N emission in TMC-1 are tracing two layers in one filament: a lower density outer layer whose material is flowing under gravity towards the higher density inner layer of the filament.
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