We study the changes in physical and chemical conditions during the early stages of collapse of a pre-protostellar core, starting from initial conditions appropriate to a dense molecular cloud and ...proceeding to the “completely depleted” limit. We allow for molecular desorption from the grain surfaces and follow the evolution of the ionization degree and the ionic composition as functions of time and density. The timescale for collapse is treated as a parameter and taken equal to either the free-fall or the ambipolar diffusion time. The processes of freeze-out on to the dust grains and of coagulation of the grains were treated simultaneously with the chemical evolution of the medium in the course of its collapse. When proceeding at close to its maximum rate, coagulation has important consequences for the degree of ionization and the ionic composition of the medium, but its effect on the freeze-out of the neutral species is modest. An innovation of our study is to calculate the grain charge distribution; this is done in parallel with the chemistry and the dynamics. The grain charge distribution is significant because H+ ions recombine predominantly on the surfaces of negatively charged grains. We have also attempted to reproduce with our models the observational result that nitrogen-containing species, such as NH3 and N2H+, remain in the gas phase at densities for which CO and other C-containing molecules appear to have frozen on to grain surfaces. We conclude that recent measurements of the adsorption energies of N2 and CO invalidate the interpretation of these observations in terms of the relative volatilities of N2 and CO. We consider an alternative explanation, in terms of low sticking coefficients for either molecular or atomic N; but this hypothesis requires experimental confirmation. We find that, irrespective of the nitrogen chemistry, the main gas phase ion is either H+ or H$_3^+$ (and its deuterated isotopes) at densities above 105 cm-3; whether H+ or H$_3^+$ predominates depends sensitively on the rate of increase in grain size (decrease in grain surface area per unit volume of gas) during core contraction. Our calculations show that H+ will predominate if grain coagulation proceeds at close to its maximum rate, and H$_3^+$ otherwise.
Aims.We have studied the mechanisms which govern the degree of ionization of the gas in molecular clouds and prestellar cores, with a view to interpreting the relative abundances of the carbon-chain ...species C nH and their negative ions, CnH-. Methods.We followed the chemical evolution of a medium comprising gas and dust as it evolves towards its steady-state composition. Various assumptions were made concerning the grain size-distribution and the fraction of very small grains (in practice, PAH), as well as the cosmic ray ionization rate. Particular attention was paid to reactions which determine the fractional ionization of the gas and the charge of the grains. Results.We found that the abundance ratio n(C6H-)/n(C6H) is determined essentially by the ratio of the free electron density to the density of atomic hydrogen. A model with a high fractional abundance of PAH and a low fractional abundance of electrons yields agreement to a factor of 2 with the value of the ratio C6H-:C6H observed recently in TMC-1. However, the fractional abundances of the molecular ions HCO+ and DCO+ are then higher than observed. The best overall fit with the observations of TMC-1 is obtained when the cosmic ray ionization rate is reduced, together with the rate of removal of atomic hydrogen from the gas phase (owing to adsorption on to grains).
The ubiquity of filamentary structure at various scales through out the Galaxy has triggered a renewed interest in their formation, evolution, and role in star formation. The largest filaments can ...reach up to Galactic scale as part of the spiral arm structure. However, such large scale filaments are hard to identify systematically due to limitations in identifying methodology (i.e., as extinction features). We present a new approach to directly search for the largest, coldest, and densest filaments in the Galaxy, making use of sensitive Herschel Hi-GAL data complemented by spectral line cubes. We present a sample of the 9 most prominent Herschel filaments, including 6 identified from a pilot search field plus 3 from outside the field. These filaments measure 37-99 pc long and 0.6-3.0 pc wide with masses (0.5-8.3)\(\times10^4 \, M_\odot\), and beam-averaged (\(28"\), or 0.4-0.7 pc) peak H\(_2\) column densities of (1.7-9.3)\(\times 10^{22} \, \rm{cm^{-2}}\). The bulk of the filaments are relatively cold (17-21 K), while some local clumps have a dust temperature up to 25-47 K. All the filaments are located within <~60 pc from the Galactic mid-plane. Comparing the filaments to a recent spiral arm model incorporating the latest parallax measurements, we find that 7/9 of them reside within arms, but most are close to arm edges. These filaments are comparable in length to the Galactic scale height and therefore are not simply part of a grander turbulent cascade.
We report molecular line and dust continuum observations toward the high-mass star forming region G331.5-0.1, one of the most luminous regions of massive star-formation in the Milky Way, located at ...the tangent region of the Norma spiral arm, at a distance of 7.5 kpc. Molecular emission was mapped toward the G331.5-0.1 GMC in the CO (J=1-0) and C18O (J=1-0) lines with NANTEN, while its central region was mapped in CS (J=2-1 and J=5-4) with SEST, and in CS (J=7-6) and 13CO (J=3-2) with ASTE. Continuum emission mapped at 1.2 mm with SIMBA and at 0.87 mm with LABOCA reveal the presence of six compact and luminous dust clumps, making this source one of the most densely populated central regions of a GMC in the Galaxy. The dust clumps are associated with molecular gas and they have the following average properties: size of 1.6 pc, mass of 3.2x10^3 Msun, molecular hydrogen density of 3.7x10^4 cm^{-3}, dust temperature of 32 K, and integrated luminosity of 5.7x10^5 Lsun, consistent with values found toward other massive star forming dust clumps. The CS and 13CO spectra show the presence of two velocity components: a high-velocity component at ~ -89 km s^{-1}, seen toward four of the clumps, and a low-velocity component at ~ -101 km s^{-1} seen toward the other two clumps. Radio continuum emission is present toward four of the molecular clumps, with spectral index estimated for two of them of 0.8+-0.2 and 1.2+-0.2. A high-velocity molecular outflow is found at the center of the brightest clump, with a line width of 26 km s^{-1} (FWHM) in CS (J=7-6). Observations of SiO (J=7-6 and J=8-7), and SO (J_K=8_8-7_7 and J_K=8_7-7_6) lines provide estimates of the gas rotational temperature toward this outflow >120 K and >75 K, respectively.
Water is a crucial molecule in molecular astrophysics as it controls much of the gas/grain chemistry, including the formation and evolution of more complex organic molecules in ices. Pre-stellar ...cores provide the original reservoir of material from which future planetary systems are built, but few observational constraints exist on the formation of water and its partitioning between gas and ice in the densest cores. Thanks to the high sensitivity of the Herschel Space Observatory, we report on the first detection of water vapor at high spectral resolution toward a dense cloud on the verge of star formation, the pre-stellar core L1544. The line shows an inverse P-Cygni profile, characteristic of gravitational contraction. To reproduce the observations, water vapor has to be present in the cold and dense central few thousand AU of L1544, where species heavier than Helium are expected to freeze-out onto dust grains, and the ortho:para H2 ratio has to be around 1:1 or larger. The observed amount of water vapor within the core (about 1.5x10^{-6} Msun) can be maintained by Far-UV photons locally produced by the impact of galactic cosmic rays with H2 molecules. Such FUV photons irradiate the icy mantles, liberating water wapor in the core center. Our Herschel data, combined with radiative transfer and chemical/dynamical models, shed light on the interplay between gas and solids in dense interstellar clouds and provide the first measurement of the water vapor abundance profile across the parent cloud of a future solar-type star and its potential planetary system.
Stars like our Sun form in self-gravitating dense and cold structures within interstellar clouds, called pre-stellar cores. Although much is known about the physical structure of dense clouds just ...before and soon after the switch-on of a protostar, the central few thousand astronomical units (au) of pre-stellar cores are unexplored. It is within these central regions that stellar systems assemble and fragmentation may take place, with the consequent formation of binaries and multiple systems. We present ALMA Band 6 observations (ACA and 12m array) of the dust continuum emission of the 8 Msun pre-stellar core L1544, with angular resolution of 2'' x 1.6'' (linear resolution 270 au x 216 au). Within the primary beam, a compact region of 0.1 Msun, which we call a "kernel", has been unveiled. The kernel is elongated, with a central flat zone with radius Rker ~ 10'' (~ 1400 au). The average number density within Rker is ~1 x 10^6 cm^{-3}, with possible local density enhancements. The region within Rker appears to have fragmented, but detailed analysis shows that similar substructure can be reproduced by synthetic interferometric observations of a smooth centrally concentrated dense core with a similar central flat zone. The presence of a smooth kernel within a dense core is in agreement with non-ideal magneto-hydro-dynamical simulations of a contracting cloud core with a peak number density of 1 x 10^7 cm^{-3}. Dense cores with lower central densities are completely filtered out when simulated 12m-array observations are carried out. These observations demonstrate that the kernel of dynamically evolved dense cores can be investigated at high angular resolution with ALMA.
Large-scale gaseous filaments with length up to the order of 100 pc are on
the upper end of the filamentary hierarchy of the Galactic interstellar medium.
Their association with respect to the ...Galactic structure and their role in
Galactic star formation are of great interest from both observational and
theoretical point of view. Previous "by-eye" searches, combined together, have
started to uncover the Galactic distribution of large filaments, yet inherent
bias and small sample size limit conclusive statistical results to be drawn.
Here, we present (1) a new, automated method to identify large-scale
velocity-coherent dense filaments, and (2) the first statistics and the
Galactic distribution of these filaments. We use a customized minimum spanning
tree algorithm to identify filaments by connecting voxels in the
position-position-velocity space, using the Bolocam Galactic Plane Survey
spectroscopic catalog. In the range of $7.^{\circ}5 \le l \le 194^{\circ}$, we
have identified 54 large-scale filaments and derived mass ($\sim 10^3 - 10^5 \,
M_\odot$), length (10-276 pc), linear mass density (54-8625 $M_\odot \,
\rm{pc}^{-1}$), aspect ratio, linearity, velocity gradient, temperature,
fragmentation, Galactic location and orientation angle. The filaments
concentrate along major spiral arms. They are widely distributed across the
Galactic disk, with 50% located within $\pm$20 pc from the Galactic mid-plane
and 27% run in the center of spiral arms (aka "bones"). An order of 1% of the
molecular ISM is confined in large filaments. Massive star formation is more
favorable in large filaments compared to elsewhere. This is the first
comprehensive catalog of large filaments useful for a quantitative comparison
with spiral structures and numerical simulations.
PDF
