We present a new analytical three-parameter formula to fit observed column density profiles of prestellar cores. It represents a line-of-sight integral through a spherically symmetric or disc-like isothermal cloud. The underlying model resembles the Bonnor-Ebert model in that it features a flat central region leading into a power-law decline \propto r^{-2} in density, and a well-defined outer radius. However, we do not assume that the cloud is in equilibrium, and can instead make qualitative statements about its dynamical state (expansion, equilibrium, collapse) using the size of the flat region as a proxy. Instead of having temperature as a fitting parameter, our model includes it as input, and thus avoids possible inconsistencies. It is significantly easier to fit to observational data than the Bonnor-Ebert sphere. We apply this model to L1689B and B68. We show that L1689B cannot be in equilibrium but instead appears to be collapsing, while our model verifies that B68 is not far from being a hydrostatic object.
We investigate differences in the molecular abundances between magnetically super- and sub-critical prestellar cores, performing three-dimensional non-ideal magnetohydrodynamical (MHD) simulations with varying densities and magnetic field strengths, and post-processing the results with a time-dependent gas-grain chemical code. Most molecular species show significantly more central depletion in subcritical models, due to the longer duration of collapse. However, the directly observable quantities - the molecule to hydrogen column density ratios - are generally too similar for observational data…
We present a detailed study of the Orion B clouds (d~400 pc), imaged with the PACS/SPIRE cameras at 70-500 $μ$m by the Herschel Gould Belt survey (HGBS). We release new high-res. maps of column density and dust temperature. In the filamentary sub-regions NGC2023/2024, NGC2068/2071, and L1622, 1768 starless dense cores were identified, ~28-45% of which are self-gravitating prestellar cores. A total of 76 protostellar dense cores were also found. The typical lifetime of the prestellar cores was found to be $t_{\rm pre}=1.7_{-0.6}^{+0.8}$ Myr. The prestellar core mass function (CMF) peaks at ~0.5…
Young massive stars are usually found embedded in dense massive molecular clumps and are known for being highly obscured and distant. During their formation process, deuteration is regarded as a potentially good indicator of the very early formation stages. In this work, we test the observability of the ground-state transition of ortho-H$_2$D$^+$ $J_{\rm {K_a, K_c}} = 1_{10}$-$1_{11} $ by performing interferometric and single-dish synthetic observations using magneto-hydrodynamic simulations of high-mass collapsing molecular cores, including deuteration chemistry. We studied different evolutio…
We deploy and demonstrate the capabilities of the magnetic field model developed by Ewertowski & Basu (2013) by fitting observed polarimetry data of the prestellar core FeSt 1-457. The analytic hourglass magnetic field function derived directly from Maxwell's equations yields a central-to-surface magnetic field strength ratio in the equatorial plane, as well as magnetic field directions with relative magnitudes throughout the core. This fit emerges from a comparison of a single plane of the model with the polarization map that results from the integrated properties of the magnetic field and du…
We present Herschel SPIRE and PACS maps of the Cepheus Flare clouds L1157, L1172, L1228, L1241, and L1251, observed by the Herschel Gould Belt Survey (HGBS) of nearby star-forming molecular clouds. Through modified blackbody fits to the SPIRE and PACS data, we determine typical cloud column densities of 0.5-1.0 $\times$ 10$^{21}$ cm$^{-2}$ and typical cloud temperatures of 14-15 K. Using the getsources identification algorithm, we extract 832 dense cores from the SPIRE and PACS data at 160-500 $μ$m. From placement in a mass vs. size diagram, we consider 303 to be candidate prestellar cores, an…
Young massive stars are usually found embedded in dense and massive molecular clumps which are known for being highly obscured and distant. During their formation process, the degree of deuteration can be used as a potential indicator of the very early formation stages. This is particularly effective when employing the abundance of H$_2$D$^+$. However, its low abundances and large distances make detections in massive sources hard to achieve. We present an application of the radiative transfer code POLARIS, with the goal to test the observability of the ortho-H$_2$D$^+$ transition $1_{10}$-$1_{…
Context: The $^{15}$N fractionation has been observed to show large variations among astrophysical sources, depending both on the type of target and on the molecular tracer used. These variations cannot be reproduced by the current chemical models. Aims: Until now, the $^{14}$N/$^{15}$N ratio in N$_2$H$^+$ has been accurately measured in only one prestellar source, L1544, where strong levels of fractionation, with depletion in $^{15}$N, are found ($^{14}$N/$^{15}$N$\: \approx 1000$). In this paper we extend the sample to three more bona fide prestellar cores, in order to understand if the anti…
High-mass analogues of low-mass prestellar cores are searched for to constrain the models of high-mass star formation. Several high-mass cores, at various evolutionary stages, have been recently identified towards the massive star-forming region W43-MM1 and amongst them a high-mass prestellar core candidate. We aim to characterise the chemistry in this high-mass prestellar core candidate, referred to as W43-MM1 core #6, and its environment. Using ALMA high-spatial resolution data of W43-MM1, we have studied the molecular content of core #6 and a neighbouring high-mass protostellar core, referr…
Ambipolar diffusion likely plays a pivotal role in the formation and evolution of dense cores in weakly-ionized molecular clouds. Linear analyses show that the evolutionary times and fragmentation scales are significantly greater than the hydrodynamic (Jeans) values even for clouds with mildly supercritical mass-to-flux ratio. We utilize values of fragmentation scales and growth times that correspond to typical ionization fractions within a molecular cloud, and apply to the context of the observed estimated lifetime of prestellar cores as well as the observed number of such embedded cores form…
We aim to reveal the physical properties and chemical composition of the cores in the California molecular cloud (CMC), so as to better understand the initial conditions of star formation. We made a high-resolution column density map (18.2") with Herschel data, and extracted a complete sample of the cores in the CMC with the \textsl{fellwalker} algorithm. We performed new single-pointing observations of molecular lines near 90 GHz with the IRAM 30m telescope along the main filament of the CMC. In addition, we also performed a numerical modeling of chemical evolution for the cores under the phy…
Context. The deuterium fraction in low-mass prestellar cores is a good diagnostic indicator of the initial phases of star formations, and it is also a fundamental quantity to infer the ionisation degree in these objects. Aims. With the analysis of multiple transitions of $\rm N_2H^+$, $\rm N_2D^+$, $\rm HC^{18}O^+$ and $\rm DCO^+$ we are able to determine the molecular column density maps and the deuterium fraction in $\rm N_2H^+$ and $\rm HCO^+$ toward the prototypical prestellar core L1544. This is the preliminary step to derive the ionisation degree in the source. Methods. We use a non-loca…
Understanding the chemical processes during starless core and prestellar core evolution is an important step in understanding the initial stages of star and disk formation. This project is a study of deuterated ammonia, o-NH$_2$D, in the L1251 star-forming region toward Cepheus. Twenty-two dense cores (twenty of which are starless or prestellar, and two of which have a protostar), previously identified by p-NH$_3$ (1,1) observations, were targeted with the 12m Arizona Radio Observatory telescope on Kitt Peak. o-NH$_2$D J$_{\rm{K_a} \rm{K_c}}^{\pm} =$ $1_{11}^{+} \rightarrow 1_{01}^{-}$ was det…
Inventories of Rich Carbon-Chain Chemistry in Prestellar and Starless Cores in the Perseus Molecular Cloud
University of California, Berkeley（加州大学伯克利分校）；National Radio Astronomy Observatory（美国国家射电天文台）；RIKEN Cluster for Pioneering Research（理化学研究所先导研究集群）；Steward Observatory, University of Arizona（亚利桑那大学斯图尔德天文台）；Centro de Astrobiología (CAB), CSIC-INTA（天体生物学中心，西班牙科学研究中心-国家航空航天技术研究所）
Cores and filamentary structures are the prime birthplaces of stars, and play key roles in the process of star formation. Latest advances in the methods of multi-scale source and filament extraction, and in making high-resolution column density map from $Herschel$ multi-wavelength observations enable us to detect the filamentary network structures in highly complex molecular cloud environments. The statistics for physical parameters shows that core mass strongly correlates with core dust temperature, and $M/L$ strongly correlates with $M/T$, which is in line with the prediction of the blackbod…
We report a comprehensive study of the cyanopolyyne chemistry in the prototypical prestellar core L1544. Using the 100m Robert C. Byrd Green Bank Telescope (GBT) we observe 3 emission lines of HC$_3$N, 9 lines of HC$_5$N, 5 lines of HC$_7$N, and 9 lines of HC$_9$N. HC$_9$N is detected for the first time towards the source. The high spectral resolution ($\sim$ 0.05 km s$^{-1}$) reveals double-peak spectral line profiles with the redshifted peak a factor 3-5 brighter. Resolved maps of the core in other molecular tracers indicates that the southern region is redshifted. Therefore, the bulk of the…
Modelling the magnetic field in prestellar cores can serve as a useful tool for studying the initial conditions of star formation. The analytic hourglass model of Ewertowski and Basu (2013) provides a means to fit observed polarimetry measurements and extract useful information. The original model does not specify any radial distribution of the electric current density. Here, we perform a survey of possible centrally-peaked radial distributions of the current density, and numerically derive the full hourglass patterns. Since the vertical distribution is also specified in the original model, we…
Observational Diagnostics of Self-Gravitating MHD Turbulence in Giant Molecular Clouds
我们研究了自引力MHD湍流的可观测的签名，通过应用概率密度函数（PDF）和空间密度功率谱的合成柱密度图。我们发现坍缩云的演化有三个阶段，我们称之为“早期”、“中期”和“晚期”。“在早期，即$t<0.15t_{ff}$，柱密度具有类似于非重力超音速湍流的功率谱斜率和对数正态分布。在中间阶段，即$0.15t_{ff}< t \leq 0.35t_{ff}$，在较浅的PDF和功率谱幂律斜率中存在星前核的特征。在这些时间的列密度PDF幂律尾部具有范围从-2.5到-1.5的视线平均斜率，其中较浅的值属于具有较低磁场强度的模拟。密度功率谱斜率变浅，可用$P(k)=A_1k^{β_2}e^{-k/k_c}$表征，其中$A_1$描述振幅，$k^{β_2}$描述经典幂律行为，尺度$k_c$表征从湍流主导到自引力主导的转变。在坍缩的高级阶段，即$\approx t>0.35t_{ff}$，功率谱斜率为正值，并且观察到PDF矩和Tsallis增量PDF参数急剧增加，这引起PDF-音速马赫数关系之间的偏差。最后，我们证明了在非引力湍流中，引力对密度功率谱的影响可以通过引入一个δ函数来复制，该函数的振幅相当于给定自引力映射中的最大值点。通过对高密度物质的空间滤波，我们发现湍流功率谱得到了恢复。
(Abridged) In this paper, we present analyses of images taken with the Herschel ESA satellite from 70mu to 500mu. We first constructed column density and dust temperature maps. Next, we identified compact cores in the maps, and characterize the cores using modified blackbody fits to their SEDs: we identified 684 starless cores, of which 199 are bound and potential prestellar cores, and 132 protostars. We also matched the Herschel-identified young stars with GAIA sources to model distance variations across the Perseus cloud. We measure a linear gradient function with right ascension and declina…
We revisit the mass-size relation of molecular cloud structures based on the column density map of the Cygnus-X molecular cloud complex. We extract 135 column density peaks in Cygnus-X and analyze the column density distributions around these peaks. The averaged column density profiles, $N(R)$, around all the peaks can be well fitted with broken power-laws, which are described by an inner power-law index $n$, outer power-law index $m$, and the radius $R_{\rm TP}$ and column density $N_{\rm TP}$ at the transition point. We then explore the $M-R$ relation with different samples of cloud structur…
We report the first detection of the molecule cyanomethyl, CH$_2$CN, in a protoplanetary disk. Until now, CH$_2$CN had only been observed at earlier evolutionary stages, in the giant molecular clouds TMC-1 and Sgr 2, and the prestellar core L1544. We detect six transitions of ortho-CH$_2$CN towards the disk around nearby T Tauri star TW Hya. An excitation analysis reveals that the disk-averaged column density, $N$, for ortho-CH$_2$CN is $(6.3\pm 0.5)\times10^{12}$ cm$^{-2}$, which is rescaled to reflect a 3:1 ortho-para ratio, resulting in a total column density, $N_{\rm tot}$, of $(8.4\pm 0.7…
