The influence of turbulent mixing on the chemistry of the interstellar medium has so far received little attention. Previous studies of this effect have suggested that it might play an important role in mixing the various phases of the interstellar medium. In this paper we examine the potential effects of turbulent diffusion on the deuterium chemistry within molecular clouds. We find that such mixing acts to reduce the efficiency of deuteration in these clouds by increasing the ionization fraction and reducing freeze-out of heavy molecules. This leads to lower abundances for many deuterated species. We also examine the influence of turbulent mixing on the transition from atomic hydrogen to H2 and from atomic deuterium to HD near the cloud edge. We find that including turbulent diffusion in our models serves to push these transitions deeper into the cloud and helps maintain a higher atomic fraction throughout the cloud envelope. Based on these findings, we propose a new process to account for the significant scatter in the observed atomic D/H ratio for galactic sightlines extending beyond the Local Bubble. Although several mechanisms have been put forward to explain this scatter, they are unable to fully account for the range in D/H values. We suggest a scenario in which turbulent mixing of atomic and molecular gas at the edges of molecular clouds causes the observed atomic D/H ratio to vary by a factor of ~2.
Deuterium fractionation is considered as an important process to infer the chemical ages of prestellar cores in filaments. We present here the first magneto-hydrodynamical simulations including a chemical network to study deuterium fractionation in magnetized and turbulent filaments and their substructures. The filaments typically show widespread deuterium fractionation with average values $\gtrsim0.01$. For individual cores of similar age, we observe the deuteration fraction to increase with time, but also to be independent of their average properties such as density, virial or mass-to-magnet…
We investigate water and deuterated water chemistry in turbulent protoplanetary disks. Chemical rate equations are solved with the diffusion term, mimicking turbulent mixing in vertical direction. Water near the midplane is transported to the disk atmosphere by turbulence and destroyed by photoreactions to produce atomic oxygen, while the atomic oxygen is transported to the midplane and reforms water and/or other molecules. We find that this cycle significantly decreases column densities of water ice at r < 30 AU, where dust temperatures are too high to reform water ice effectively. The radial…
Context. Millimetric observations have measured high degrees of molecular deuteration in several species seen around low-mass protostars. The Herschel Space Telescope, launched in 2009, is now providing new measures of the deuterium fractionation of water, the main constituent of interstellar ices. Aims. We aim at theoretically studying the formation and the deuteration of water, which is believed to be formed on interstellar grain surfaces in molecular clouds. Methods. We used our gas-grain astrochemical model GRAINOBLE, which considers the multilayer formation of interstellar ices. We varied…
High-mass stars are expected to form from dense prestellar cores. Their precise formation conditions are widely discussed, including their virial condition, which results in slow collapse for super-virial cores with strong support by turbulence or magnetic fields, or fast collapse for sub-virial sources. To disentangle their formation processes, measurements of the deuterium fractions are frequently employed to approximately estimate the ages of these cores and to obtain constraints on their dynamical evolution. We here present 3D magneto-hydrodynamical simulations including for the first time…
Much of the dense gas in molecular clouds has a filamentary structure but the detailed structure and evolution of this gas is poorly known. We have observed 54 cores in infrared dark clouds (IRDCs) using N$_2$H$^+$ (1-0) and (3-2) to determine the kinematics of the densest material, where stars will form. We also observed N$_2$D$^+$ (3-2) towards 29 of the brightest peaks to analyse the level of deuteration which is an excellent probe of the quiescent of the early stages of star formation. There were 13 detections of N$_2$D$^+$ (3-2). This is one of the largest samples of IRDCs yet observed in…
c-C3H2 deuteration towards prestellar and starless cores in the Perseus Molecular Cloud
RIKEN Cluster for Pioneering Research（理化学研究所前沿研究集群）；National Radio Astronomy Observatory（国家射电天文台）；University of Virginia（弗吉尼亚大学）；University of California, Berkeley（加州大学伯克利分校）；University of Arizona（亚利桑那大学）；Centro de Astrobiología (CAB), CSIC-INTA（天体生物学中心（CAB），西班牙高等科学研究委员会-国家航空航天技术研究所）
Deuterium fractionation, i.e. the enhancement of deuterated species with respect to the non-deuterated ones, is considered to be a reliable chemical clock of star-forming regions. This process is strongly affected by the ortho-to-para (o-p) H$_2$ ratio. In this letter we explore the effect of the o-p H$_2$ conversion on grains on the deuteration timescale in fully depleted dense cores, including the most relevant uncertainties that affect this complex process. We show that (i) the o-p H$_2$ conversion on grains is not strongly influenced by the uncertainties on the conversion time and the stic…
The deuterium fractionation of water can serve as a tracer for the chemical and physical evolution of water during star formation and can constrain the origin of water in Solar System bodies. We determine the HDO/H$_2$O ratio in the inner warm gas toward three low-mass Class 0 protostars selected to be in isolated cores, i.e., not associated with any cloud complexes. Previous sources for which the HDO/H$_2$O ratio have been established were all part of larger star-forming complexes. Targeting these isolated protostars allows comparison of the water chemistry in isolated and clustered regions t…
Turbulence influences the behavior of many astrophysical systems, frequently by providing non-thermal pressure support through random bulk motions. Although turbulence is commonly studied in systems with constant volume and mean density, turbulent astrophysical gases often expand or contract under the influence of pressure or gravity. Here, we examine the behavior of turbulence in contracting volumes using idealized models of compressed gases. Employing numerical simulations and an analytical model, we identify a simple mechanism by which the turbulent motions of contracting gases "adiabatical…
The collapse of weakly turbulent prestellar cores is a critical stage in the process of star formation. Being highly non-linear and stochastic, the outcome of collapse can only be explored theoretically by performing large ensembles of numerical simulations. Standard practice is to quantify the initial turbulent velocity field in a core in terms of the amount of turbulent energy (or some equivalent) and the exponent in the power spectrum (n \equiv -d log Pk /d log k). In this paper, we present a numerical study of the influence of the details of the turbulent velocity field on the collapse of …
The ordered magnetic field observed via polarized synchrotron emission in nearby disc galaxies can be explained by a mean-field dynamo operating in the diffuse interstellar medium (ISM). Additionally, vertical-flux initial conditions are potentially able to influence this dynamo via the occurrence of the magneto-rotational instability (MRI). We aim to study the influence of various initial field configurations on the saturated state of the mean-field dynamo. This is motivated by the observation that different saturation behavior was previously obtained for different supernova rates. We perform…
Large-scale turbulent driving regulates star formation in high-redshift gas-rich galaxies II: Influence of the magnetic field and the turbulent compressive fraction
Context. Molecular clouds are known to be turbulent and strongly affected by stellar feedback. Moreover, stellar feedback is believed to drive turbulence at large scales in galaxies. Aims. We study the role played by supernovae in molecular clouds and the influence of the magnetic field on this process. Methods. We performed three-dimensional numerical simulations of supernova explosions, in and near turbulent self-gravitating molecular clouds. In order to study the influence of the magnetic field, we performed both hydrodynamical and magnetohydrodynamical simulations. We also ran a series of …
Using JCMT Gould Belt Survey data from CO J=3-2 isotopologues, we present a meta-analysis of the outflows and energetics of star-forming regions in several Gould Belt clouds. The majority of the regions are strongly gravitationally bound. There is evidence that molecular outflows transport large quantities of momentum and energy. Outflow energies are at least 20 per cent of the total turbulent kinetic energies in all of the regions studied and greater than the turbulent energy in half of the regions. However, we find no evidence that outflows increase levels of turbulence, and there is no corr…
[Abridged] Theoretical and numerical studies of star formation have shown that magnetic field (B) has a strong influence on both disk formation and fragmentation; even a relatively low B can prevent these processes. However, very few studies investigated the combined effects of B and turbulence. We study the effects of turbulence in magnetized core collapse, focusing on the magnetic diffusion, the orientation of the angular momentum (J) of the protostellar core, and on its consequences on disk formation, fragmentation and outflows. We perform 3D, AMR, MHD simulations of magnetically supercriti…
The Central Molecular Zone (CMZ) contains most of the mass of our Galaxy but its star formation rate is one order of magnitude lower than in the Galactic disc. This is likely related to the fact that the bulk of the gas in the CMZ is in a warm ($>$100 K) and turbulent phase with little material in the pre-stellar phase. We present in this Letter observations of deuterium fractionation (D/H ratios) of HCN, HNC, HCO$^{+}$, and N$_{2}$H$^{+}$ towards the CMZ molecular cloud G+0.693-0.027. These observations clearly show, for the first time, the presence of a colder, denser, and less turbulent nar…
We study the influence of gas metallicity, turbulence, and non-equilibrium chemistry on the evolution of the two-phase interstellar medium (warm and cold atomic phases), and thereby constrain the initial conditions for star formation prevailing in turbulent gas. We perform high-resolution simulations in three dimensions, including a realistic non-equilibrium treatment of the ionization state of the gas, and examine both driven and decaying turbulence. This allows us to explore variations in the metallicity Z. In this paper, we study solar metallicity, Z=Z_sun, and low metallicity, Z=1d-3 Z_sun…
