The ubiquity of filamentary structure at various scales throughout 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 (Herschel Infrared Galactic Plane Survey) data complemented by spectral line cubes. We present a sample of the nine most prominent Herschel filaments, including six identified from a pilot search field plus three from outside the field. These filaments measure 37–99 pc long and 0.6–3.0 pc wide with masses (0.5–8.3) × 104 M⊙, and beam-averaged (28 arcsec, or 0.4–0.7 pc) peak H2 column densities of (1.7–9.3)× 1022 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 scaleheight and therefore are not simply part of a grander turbulent cascade.
ABSTRACT Large-scale gaseous filaments with lengths up to the order of 100 pc are on the upper end of the filamentary hierarchy of the Galactic interstellar medium (ISM). Their association with ...respect to the Galactic structure and their role in Galactic star formation are of great interest from both an 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 from being drawn. Here, we present (1) a new, automated method for identifying 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 , we have identified 54 large-scale filaments and derived mass ( ), length (10-276 pc), linear mass density (54-8625 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 20 pc from the Galactic mid-plane and 27% run in the center of spiral arms. 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 that can be useful for a quantitative comparison with spiral structures and numerical simulations.
Stars like our Sun form in self-gravitating dense and cold structures within interstellar clouds that are referred to as 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 Atacama Large Millimetre and submillimetre Array (ALMA) Band 6 observations (Atacama Compact Array and 12 m array) of the dust continuum emission of the 8 M pre-stellar core L1544, with an angular resolution of 2″ × 1 6 (linear resolution 270 au × 216 au). Within the primary beam, a compact region of 0.1 M , 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 × 106 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 magnetohydro-dynamical simulations of a contracting cloud core with a peak number density of 1 × 107 cm−3. Dense cores with lower central densities are completely filtered out when simulated 12 m 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.
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 H sub(2) ratio has to be around 1:1 or larger. The observed amount of water vapor within the core (about 1.5 x 10 super(-6) M sub(middot in circle)) can be maintained by far-UV photons locally produced by the impact of galactic cosmic rays with H sub(2) molecules. Such FUV photons irradiate the icy mantles, liberating water vapor 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.
Pre-stellar cores represent the initial conditions in the process of star and planet formation, therefore it is important to study their physical and chemical structure. Because of their volatility, ...nitrogen-bearing molecules are key to study the dense and cold gas present in pre-stellar cores. The NH3 rotational transition detected with Herschel-HIFI provides a unique combination of sensitivity and spectral resolution to further investigate physical and chemical processes in pre-stellar cores. Here we present the velocity-resolved Herschel-HIFI observations of the ortho-NH3(10 − 00) line at 572 GHz and study the abundance profile of ammonia across the pre-stellar core L1544 to test current theories of its physical and chemical structure. Recently calculated collisional coefficients have been included in our non-LTE radiative transfer code to reproduce Herschel observations. A gas-grain chemical model, including spin-state chemistry and applied to the (static) physical structure of L1544 is also used to infer the abundance profile of ortho-NH3. The hyperfine structure of ortho-NH3(10 − 00) is resolved for the first time in space. All the hyperfine components are strongly self-absorbed. The profile can be reproduced if the core is contracting in quasi-equilibrium, consistent with previous work, and if the NH3 abundance is slightly rising toward the core centre, as deduced from previous interferometric observations of para-NH3(1, 1). The chemical model overestimates the NH3 abundance at radii between ≃4000 and 15 000 AU by about two orders of magnitude and underestimates the abundance toward the core centre by more than one order of magnitude. Our observations show that chemical models applied to static clouds have problems in reproducing NH3 observations.
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 arrow right 0) and C super(18)O(J = 1 arrow right 0) lines with NANTEN, while its central region was mapped in CS(J = 2 arrow right 1 and J = 5 arrow right 4) with SEST, and in CS(J = 7 arrow right 6) and super(13)CO(J = 3 arrow right 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 3.2 x 10 super(3) M sub(middot in circle), molecular hydrogen density of 3.7 x 10 super(4) cm super(-3), dust temperature of 32 K, and integrated luminosity of 5.7 x 10 super(5) L sub(middot in circle), consistent with values found toward other massive star-forming dust clumps. The CS and super(13)CO spectra show the presence of two velocity components: a high-velocity component at ~ - 89 km s super(-1), seen toward four of the clumps, and a low-velocity component at ~ - 101 km s super(-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 + or - 0.2 and 1.2 + or - 0.2. A high-velocity molecular outflow is found at the center of the brightest clump, with a line width of 26 km s super(-1) (FWHM) in CS(J = 7 arrow right 6). Observations of SiO(J = 7 arrow right 6 and J = 8 arrow right 7), and SO(J sub(K) = 8 sub(8) arrow right 7 sub(7) and J sub(K) = 8 sub(7) arrow right 7 sub(6)) lines provide estimates of the gas rotational temperature toward this outflow >120 K and >75 K, respectively.
Complete depletion in prestellar cores Walmsley, C. M.; Flower, D. R.; G. Pineau des Forêts
Astronomy and astrophysics (Berlin),
05/2004, Letnik:
418, Številka:
3
Journal Article
Recenzirano
Odprti dostop
We have carried out calculations of ionization equilibrium and deuterium fractionation for conditions appropriate to a completely depleted, low mass pre-protostellar core, where heavy elements such ...as C, N, and O have vanished from the gas phase and are incorporated in ice mantles frozen on dust grain surfaces. We put particular emphasis on the interpretation of recent observations of H2D+ towards the centre of the prestellar core L 1544 (Caselli et al. CITE) and also compute the ambipolar diffusion timescale. We consider explicitly the ortho and para forms of H2, H$_{3}^{+}$, and H2D+. Our results show that the ionization degree under such conditions depends sensitively on the grain size distribution or, more precisely, on the mean grain surface area per hydrogen nucleus. Depending upon this parameter and upon density, the major ion may be H+, H$_{3}^{+}$, or D$_{3}^{+}$. We show that the abundance of ortho-H2D+ observed towards L 1544 can be explained satisfactorily in terms of a complete depletion model and that this species is, as a consequence, an important tracer of the kinematics of prestellar cores.
Aims.We have studied the chemistry of nitrogen-bearing species during the initial stages of protostellar collapse, with a view to explaining the observed longevity of N2H+ and NH3 and the high levels ...of deuteration of these species. Methods.We followed the chemical evolution of a medium comprising gas and dust as it underwent free-fall gravitational collapse. Chemical processes which determine the relative populations of the nuclear spin states of molecules and molecular ions were included explicitly, as were reactions which lead ultimately to the deuteration of the nitrogen-containing species N2H+ and NH3. The freeze-out of “heavy” molecules on to dust grains was taken into account. Results.We found that the timescale required for the nitrogen-containing species to attain their steady-state values was much larger than the free-fall time and even comparable with the probable lifetime of the precursor molecular cloud. However, it transpires that the chemical evolution of the gas during gravitational collapse is insensitive to its initial composition. If we suppose that the grain-sticking probabilities of atomic nitrogen and atomic oxygen are both less than unity ($S \lesssim 0.3$), we find that the observed differential freeze-out of nitrogen- and carbon-bearing species can be reproduced by the model of free-fall collapse when a sufficiently large grain radius ($a_{\rm g} \approx 0.50$ μm) is adopted. Furthermore, the results of our collapse model are consistent with the high levels of deuteration of N2H+ and NH3 which have been observed in L1544, for example, providing that $0.5 \lesssim a_{\rm g} \lesssim 1.0$ μm. We note that the ortho:para H2D+ ratio, and fractional abundance of ortho-H2D+, which is the observed form of H2D+, should be largest where ND3 is most abundant.
Aims. We studied the abundance of HCN, H13CN, and HN13C in a sample of prestellar cores, in order to search for species associated with high density gas. Methods. We used the IRAM 30 m radiotelescope ...to observe along the major and the minor axes of L1498, L1521E, and TMC 2, three cores chosen on the basis of their CO depletion properties. We mapped the J = 1 → 0 transition of HCN, H13CN, and HN13C towards the source sample plus the J = 1 → 0 transition of N2H+ and the J = 2 → 1 transition of C18O in TMC 2. We used two different radiative transfer codes, making use of recent collisional rate calculations, in order to determine more accurately the excitation temperature, leading to a more exact evaluation of the column densities and abundances. Results. We find that the optical depths of both H13CN(1−0) and HN13C(1−0) are non-negligible, allowing us to estimate excitation temperatures for these transitions in many positions in the three sources. The observed excitation temperatures are consistent with recent computations of the collisional rates for these species and they correlate with hydrogen column density inferred from dust emission. We conclude that HCN and HNC are relatively abundant in the high density zone, n(H2) ~ 105 cm-3, where CO is depleted. The relative abundance HNC/HCN differs from unity by at most 30% consistent with chemical expectations. The three hyperfine satellites of HCN(1−0) are optically thick in the regions mapped, but the profiles become increasingly skewed to the blue (L1498 and TMC 2) or red (L1521E) with increasing optical depth suggesting absorption by foreground layers.
We have studied the ortho, para, and, in the case of D$_3^+$, meta forms of the multiply-deuterated isotopes of H$_3^+$, under physical conditions believed to be appropriate to pre-protostellar ...cores. As deuterons have integral nuclear spin, $I = 1$, Bose-Einstein statistical laws apply. Having extended the network of chemical reactions used in our previous study (Walmsley et al. CITE), we have calculated the population densities of ortho- and para-D2H+ and of ortho- and meta-D$_3^+$. In the former case, comparison is made with the recent observations of para-D2H+ in the prestellar core 16293E (Vastel et al. CITE). Using radiative transition probabilities computed by Ramanlal & Tennyson (CITE), we have predicted the intensities of the near infrared vibrational transitions of the deuterated isotopes of H$_3^+$. Many of these transitions can be observed, in absorption, only from above the Earth's atmosphere, but some might be detectable through atmospheric windows.
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