我们研究星前核中CCH的丰度，既因为它在化学中的作用，也因为它是磁场的潜在探针。我们还考虑了CCH的N=1-0和N=2-1跃迁的非LTE行为，并改进了目前CCH光谱常数的估计。我们使用IRAM 30 m射电望远镜绘制了CCH向星前核L1498和CB 246的N=1-0和N=2-1跃迁。对CB 246，我们还绘制了1.3mm尘埃发射、N_2H ~+的J=1-0跃迁和C_（18）O的J=2-1跃迁。我们用蒙特卡罗辐射转移程序分析了L1498的CCH观测。我们推导了CCH柱密度的分布，并与由沙尘排放推断的H2柱密度进行了比较。我们发现，虽然存在N=1-0和N=2-1线的不同分量的非LTE强度比，但它们的重要性较小，并且不妨碍基于LTE分析的CCH列密度确定。此外，我们的Monte-Carlo计算与观测的比较表明，非LTE偏差可以定性地理解。对于L1498，我们的观测结果与蒙特卡罗程序相结合，暗示了一个半径为9 x 10^{16} cm的CCH耗尽空穴，与其他含C物质的发现相似。我们简要讨论了所观察到的CCH丰度分布的意义。最后，我们使用我们的观测结果来提供CCH（1-0）线的所有六个分量和CCH（2-1）的七个分量的剩余频率的改进估计。基于这些结果，我们计算改进的光谱常数CCH。我们还简要讨论了利用CCH测量磁场强度的前景。
英文摘要：
We study the abundance of CCH in prestellar cores both because of its role in the chemistry and because it is a potential probe of the magnetic field. We also consider the non-LTE behaviour of the N=1-0 and N=2-1 transitions of CCH and improve current estimates of the spectroscopic constants of CCH. We used the IRAM 30m radiotelescope to map the N=1-0 and N=2-1 transitions of CCH towards the prestellar cores L1498 and CB246. Towards CB246, we also mapped the 1.3 mm dust emission, the J=1-0 transition of N2H+ and the J=2-1 transition of C18O. We used a Monte Carlo radiative transfer program to analyse the CCH observations of L1498. We derived the distribution of CCH column densities and compared with the H2 column densities inferred from dust emission. We find that while non-LTE intensity ratios of different components of the N=1-0 and N=2-1 lines are present, they are of minor importance and do not impede CCH column density determinations based upon LTE analysis. Moreover, the comparison of our Monte-Carlo calculations with observations suggest that the non-LTE deviations can be qualitatively understood. For L1498, our observations in conjunction with the Monte Carlo code imply a CCH depletion hole of radius 9 x 10^{16} cm similar to that found for other C-containing species. We briefly discuss the significance of the observed CCH abundance distribution. Finally, we used our observations to provide improved estimates for the rest frequencies of all six components of the CCH(1-0) line and seven components of CCH(2-1). Based on these results, we compute improved spectroscopic constants for CCH. We also give a brief discussion of the prospects for measuring magnetic field strengths using CCH.
We have carried out observations of CCH and its two $^{13}$C isotopologues, $^{13}$CCH and C$^{13}$CH, in the 84 - 88 GHz band toward two starless cores, L1521B and L134N (L183), using the Nobeyama 45 m radio telescope. We have detected C$^{13}$CH with a signal-to-noise (S/N) ratio of 4, whereas no line of $^{13}$CCH was detected in either the dark clouds. The column densities of the normal species were derived to be ($1.66 \pm 0.18$)$\times 10^{14}$ cm$^{-2}$ and ($7.3 \pm 0.9$)$\times 10^{13}$ cm$^{-2}$ ($1 σ$) in L1521B and L134N, respectively. The column density ratios of $N$(C$^{13}$CH)/$…
Recent observations indicate that organic molecules are prevalent towards starless and prestellar cores. Deuteration of these molecules has not been well-studied during the starless phase. Published observations of singly-deuterated methanol, CH$_2$DOH, have only been observed in a couple of well-studied, dense and evolved prestellar cores (e.g. L1544, L183). Since the formation of gas-phase methanol during this cold phase is believed to occur via desorption from the icy grain surfaces, observations of CH$_2$DOH may be useful as a probe of the deuterium fraction in the ice mantles of dust grai…
Probing the ion-neutral drift velocity towards the L1544 prestellar core: Detection of ambipolar diffusion using N$_2$D$^+$ and para-NH$_2$D
Institute for Advanced Study, Kyushu University（九州大学高等研究院）；Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University（九州大学理学部地球行星科学系）；National Astronomical Observatory of Japan（日本国立天文台）；Max-Planck-Institut für Extraterrestrische Physik（马克斯·普朗克地外物理研究所）；Kaghoshima University（鹿儿岛大学）；Faculty of Science and Engineering, Kyushu Sangyo University（九州产业大学理工学部）；Institut de Radioastronomie Millimétrique (IRAM)（毫米波射电天文学研究所）
We have performed survey-type observations in 1 mm continuum and molecular lines toward dense cores (32 prestellar + 7 protostellar) with an average density of $\gtrsim$10$^5$ cm$^{-3}$ in the Taurus molecular clouds using the Atacama Large Millimeter/submillimeter Array-Atacama Compact Array (ALMA-ACA) stand-alone mode with an angular resolution of 6.$''$5 ($\sim$900 au). The primary purpose of this study is to investigate the innermost part of dense cores toward understanding the initial condition of star formation. In the protostellar cores, contributions from protostellar disks dominate th…
Using Scattered Near-Infrared Light to Map Water Ice in Prestellar Cores with SPHEREx
University of California Berkeley（加州大学伯克利分校）；Massachusetts Institute of Technology（麻省理工学院）；Max-Planck-Institut für Extraterrestrische Physik（马克斯·普朗克地外物理学研究所）
We investigate the amplification of turbulence through gravitational contraction of the primordial gas in minihalos. We perform numerical simulations to follow the cloud collapse, assuming polytropic equations of state for different initial turbulent Mach numbers and resolutions. We find that the turbulent velocity is amplified solely by gravitational contraction, and eventually becomes comparable to the sound speed, even for small initial turbulent Mach numbers (${\cal M}_0 \gtrsim 0.05$). We derive an analytic formula for the amplification of turbulent velocity in a collapsing cloud, and fin…
We report Atacama Large Millimeter Array observations of 3 mm dust continuum emission and line emission, in HCO$^{+}$, H$^{13}$CO$^{+}$, N$_{2}$H$^{+}$ and CH$_{3}$CN, towards two massive and dense clumps (MDCs) in early but distinct evolutionary phases (prestellar and protostellar), made with the goal of investigating their fragmentation characteristics at angular scales of $\sim$1 $\arcsec$. Towards the prestellar clump we detected ten compact structures (cores), with radius from 1200 to 4500 AU and masses from 1.6 to 20~M$_\odot$. Half of these cores exhibit inverse P Cygni profiles in HCO$…
Models of prestellar cores often assume that the cores are isolated from their environment - material outside the core boundary plays no role in the subsequent evolution. This is unlikely to be the case in reality, where cores are located within hierarchically substructured molecular clouds. We investigate the dynamical and chemical evolution of prestellar cores, modelled as Bonnor-Ebert spheres, and show that the density of the ambient medium has a large impact on the resulting chemical properties of the cores. Models embedded in high-density, low-temperature surroundings have greatly enhance…
The origin of the stellar initial mass function (IMF) is one of the most debated issues in astrophysics. Here, we explore the possible link between the quasi-universal filamentary structure of star-forming molecular clouds and the origin of the IMF. Based on our recent comprehensive study of filament properties from Herschel Gould Belt survey observations (Arzoumanian et al.), we derive, for the first time, a good estimate of the filament mass function (FMF) and filament line mass function (FLMF) in nearby molecular clouds. We use the observed FLMF to propose a simple toy model for the origin …
To deepen our understanding of the chemical properties of the Planck Galactic Cold Clump (PGCC) G168.72-15.48, we performed observations of nine molecular species, namely, \ce{c-C3H}, \ce{H2CO}, \ce{HC5N}, \ce{HC7N}, \ce{SO}, \ce{CCH}, \ce{N2H+}, \ce{CH3OH}, and \ce{CH3CCH}, toward two dense cores in PGCC G168.72-15.48 using the Tianma Radio Telescope and Purple Mountain Observatory Telescope. We detected \ce{c-C3H}, \ce{H2CO}, \ce{HC5N}, \ce{N2H+}, \ce{CCH}, and \ce{CH3OH} in both G168-H1 and G168-H2 cores, whereas \ce{HC7N} and \ce{CH3CCH} were detected only in G168-H1 and SO was detected on…
Non-ideal magnetohydrodynamic (MHD) effects are thought to be gravity's closest ally in overcoming the support of magnetic fields and in forming stars. Here, we modify the publicly available version of the adaptive mesh refinement code FLASH (Fryxell et al. 2000; Dubey et al. 2008) to include a detailed treatment of non-ideal MHD and study such effects in collapsing prestellar cores. We implement two very extended non-equilibrium chemical networks, the largest of which is comprised of $\sim$ 300 species and includes a detailed description of deuterium chemistry. The ambipolar-diffusion, Ohmic …
Direct observational measurements of the magnetic field strength in prestellar cores typically find supercritical mass-to-flux ratios, suggesting that the magnetic field is insufficient to prevent gravitational collapse. These measurements suffer from significant uncertainties; an alternative approach is to utilise the sensitivity of prestellar chemistry to the evolutionary history, and indirectly constrain the degree of magnetic support. We combine non-ideal magnetohydrodynamic simulations of prestellar cores with time-dependent chemistry and radiative transfer modelling, producing synthetic …
Context. The different theoretical models concerning the formation of high-mass stars make distinct predictions regarding their progenitors, i.e. the high-mass prestellar cores. However, so far no conclusive observation of such objects has been made. Aims. We aim to study the very early stages of high-mass star formation in two infrared-dark, massive clumps, to identify the core population that they harbour. Methods. We obtained ALMA observations of continuum emission at 0.8mm and of the ortho-$\rm H_2D^+$ transition at 372GHz towards the two clumps. We use the SCIMES algorithm to identify cor…
Prestellar cores, the birthplace of Sun-like stars, form from the fragmentation of the filamentary structure that composes molecular clouds, from which they must inherit at least partially the kinematics. Furthermore, when they are on the verge of gravitational collapse, they show signs of subsonic infall motions. How extended these motions are, which depends on how the collapse occurs, remains largely unknown. We want to investigate the kinematics of the envelope that surrounds the prototypical prestellar core L1544, studying the cloud-core connection. To our aims, we observed the $\rm HCO^+$…
Stability of filaments in star-forming clouds and the formation of prestellar cores in them
现在人们普遍认为，分子气体的致密细丝是恒星诞生过程中不可或缺的一部分。虽然数值模拟已经成功地再现了湍流气体中的对流结构，并且分析计算已经预测了通过径向坍缩形成致密气体细丝，但是产生这种细丝然后在其中形成星前核的确切过程尚不清楚。因此，在这项工作中，我们研究了形成一个密集的细丝使用一个相对简单的设置，一个均匀密度的圆柱体的压力平衡与其封闭介质。特别是，我们研究，如果它的倾向，形成一个致密的灯丝，并进一步，星前核心的形成在此灯丝承担的初始体积的气体的重力状态。我们报告了一个径向塌陷，导致形成一个致密的对流云是可能的，当气体的初始体积至少是临界稳定的（其特征是该体积的质量线密度与其最大值之间的近似相等）。虽然自引力，但这一体积的气体并没有自由落体。然后，由于类似牛仔裤的不稳定性的增长，这种塌陷后的细丝沿着其长度碎裂，形成像\emph{beads on a string}这样的星前核心。我们认为，在典型的恒星形成云中，在以下假设下被归类为引力超临界的致密细丝不太可能经历引力自由落体：（i）等温性，而事实上，它们不是，和（ii）相对于压力截断的扩展径向轮廓，从而导致其质量线密度的显著高估。这个坍缩后灯丝的径向密度和温度分布与最近对附近恒星形成区的调查中绘制的典型对流云的径向密度和温度分布是一致的。
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…
We study the gravitationally-dominated, accretion-driven evolution of a prestellar core. In our model, as the core's density increases, it remains immersed in a constant-density environment and so it accretes from this environment, increasing its mass and reducing its Jeans length. Assuming a power-law density profile $ρ\propto r^{-p}$, we compute the rate of change of the slope $p$, and show that the value $p=2$ is stationary, and furthermore, an attractor. The radial profile of the Jeans length scales as $r^{p/2}$, implying that, for $p<2$, there is a radius below which the region is smaller…
Prestellar cores are self-gravitating dense and cold structures within molecular clouds where future stars are born. They are expected, at the stage of transitioning to the protostellar phase, to harbor centrally concentrated dense (sub)structures that will seed the formation of a new star or the binary/multiple stellar systems. Characterizing this critical stage of evolution is key to our understanding of star formation. In this work, we report the detection of high density (sub)structures on the thousand-au scale in a sample of dense prestellar cores. Through our recent ALMA observations tow…
Sun-like stars form from the contraction of cold and dense interstellar clouds. How the collapse proceeds and what are the main physical processes driving it, however, is still under debate and a final consensus on the timescale of the process has not been reached. Does this contraction proceed slowly, sustained by strong magnetic fields and ambipolar diffusion, or is it driven by fast collapse with gravity dominating the entire process? One way to answer this question is to measure the age of prestellar cores through statistical methods based on observations or via reliable chemical chronomet…
Unveiling Central ortho-H2D+ Depletion at Sub-kau Scales in Prestellar Core G205.46-14.56M3: The First Interferometric Evidence and Implications for Deuterium Chemistry
