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The gravitational term for clouds and cores entering in the virial theorem is usually assumed to be equal to the gravitational energy, since the contribution to the gravitational force from the mass distribution outside the volume of integration is assumed to be negligible. Such approximation may not be valid in the presence of an important external net potential. In the present work, we analyse the effect of an external gravitational field on the gravitational budget of a density structure. Our cases under analysis are (a) a giant molecular cloud (GMC) with different aspect ratios embedded within a galactic net potential, including the effects of gravity, shear and inertial forces, and (b) a molecular cloud (MC) core embedded within the gravitational potential of its parent MC. We find that for roundish GMCs, the tidal tearing due to the shear in the plane of the Galaxy is compensated by the tidal compression in the z-direction. The influence of the external effective potential on the total gravitational budget of these clouds is relatively small (up to ∼15–25 per cent), although not necessarily negligible. However, for more filamentary GMCs, elongated on the plane of the Galaxy, the external effective potential can be dominant and can even overwhelm self-gravity, regardless of whether its main effect on the cloud is to disrupt it or compress it. This may explain the presence of some GMCs with few or no signs of massive star formation, such as the Taurus or the Maddalena's clouds. In the case of dense cores embedded in their parent MC, we found that the gravitational content due to the external field may be more important than the gravitational energy of the cores themselves. This effect works in the same direction as the gravitational energy, that is favouring the collapse of cores. We speculate on the implications of these results for star formation models, in particular that apparently nearly magnetically critical cores may actually be supercritical due to the effect of the external potential. [ABSTRACT FROM AUTHOR]

The processing of dust in shocks in the interstellar medium can lead to changes in the dust to gas mass ratio and also to a radical evolution of the dust size distribution. Here we focus on the important results of numerical models that simulate the impact of shocks on the interstellar dust size distribution in molecular clouds and in the warm intercloud medium. We emphasize the necessity of detailed modelling of the grain charging processes and the grain inertia in order to unravel the complexities and consequences of the dust dynamics in shock waves. [ABSTRACT FROM AUTHOR]

Recent studies of the star formation region BBW 36 and associated molecular clouds are presented. The 12CO (1-0) observations, carried out with the 15-m SEST (Swedish-ESO) telescope (Cerro La Silla, Chile), revealed the existence of cloud a, connected with BBW 36 and of cloud b, having elongation in SE-NW direction. A red-shifted molecular outflow with velocity ∼+5 km/s (with respect to cloud a), having a direction parallel to the line of sight, was also observed. VLA observations showed the presence of a source VLA 2 at 3.6 cm with an elongation in the N-S direction. It is suggested that the VLA 2 source coincides with a dust disc (surrounding the object BBW 36). The star 3, which is one of the YSOs in the star-forming region BBW 36 and is connected with a bright comma-like nebula, can be the source of the molecular outflow. The star 3 has very high IR colors and is associated with an IRAS point source IRAS 07280-1829, which has IR colors, typical for an IRAS point source, connected with a water maser. On the 2MASS K image of BBW 36 we can see the existence of a bright nebula; a group of stars is embedded in that nebula, and among these stars there are stars with dust discs (or envelopes). On the 2MASS K image several spiral jets are also present, some of them with a condensation at the end. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports 13CO, C18O, HCO+ (J=1−0) spectral observations toward IRAS 23133+6050 with the 13.7m millimeter-wave telescope at Qinghai Station of PMO. Corresponding to the 13CO, C18O, HCO+ line emissions, the size of the observed molecular cloud core is 4.0pc, 2.1pc and 2.3pc, the virial mass is 2.7×103 M ⊙, 0.9×103 M ⊙ and 2.3×103 M ⊙, and the volume density of H2 is 2.7×103 cm−3, 5.1×103 cm−3 and 4.6×103 cm−3, respectively. Using the power-law function n(r) ∼r −p , the spatial density distribution of the cloud core was analyzed, the obtained exponent p is respectively 1.75, 1.56 and 1.48 for the 13CO, C18O and HCO+ cores, and it is found that the density distribution becomes gradually flatter from the outer region to the inner region of the core. The HCO+ abundance is 4.6×10−10, one order of magnitude less than the value for dark clouds, and slightly less than that for giant molecular clouds. The 13CO/C18O relative abundance ratio is 12.2, comparable with the value 11.8 for dark clouds, and the value 9.0∼15.6 for giant molecular clouds. A 13CO bipolar outflow is found in this region. The IRAS far-infrared luminosity and the virial masses give the luminosity-mass ratios 18.1, 51.1 and 21.2 from the three lines. [Copyright &y& Elsevier]

We examine the star-forming region in a molecular cloud with the coordinates l = 33.30°, b = 0.25° at a distance of 1.7 kpc, in which the nebula GM 2–30 is embedded. Apart from the previously known Herbig—Haro object HH 172, several new HH objects have been found, comprising at least two HH flows. The internal structure of these objects is studied. The relationship between the line emission and kinematic data obtained through multi-pupil spectroscopy suggest the presence of bipolar outflow associated with the nebula GM 2–30 and HH 172/HH 721, which show mirror symmetry according to a number of characteristics. No optical source of this flow is observed, although the source of energy of the second flow may be a weak star associated with HH 723. The linear dimensions of the flows (more than one parsec) suggest that they are the giant outflows. [ABSTRACT FROM AUTHOR]

In molecular clouds the dust charge can be determined by non-thermal electrons from cosmic-ray ionization. If the energy of the non-thermal electrons is E≥2 keV and their fraction is only 10-3-10-4 of the thermal electrons in a molecular gas, the grain charge can be as high as | z | ≈ 10 in the range of gas densities n≈ 105-106 cm-3. In the high-density range n>106 cm-3, the grain charge approaches the thermal limit | z | ≈ 0.3. [ABSTRACT FROM AUTHOR]

Interstellar neutral hydrogen (HI) emission spectra manifest several families of linewidths whose numerical values (34, 13, and 6 km/s) appear to be related to the critical ionization velocities (CIVs) of the most abundant interstellar atomic species. Extended new analysis of HI emission profiles shows that the 34-km/s-wide component, which probably corresponds to the CIV for helium, is pervasive. The 34-km/s-wide linewidth family is found in low-velocity (local) HI profiles and in the so-called high-velocity clouds (HVCs). In addition, published studies of HI linewidths found in the Magellanic Stream, Very High Velocity Clouds, and Compact HVCs, all of which are believed to be intergalactic, have noted that the typical values are of the same order. If the CIV effect does play a role in interstellar space, it may be expected to produce locally enhanced electron densities where rapidly moving neutral gas masses interact with the surrounding plasma. Evidence that suggests that this phenomenon is occurring in interstellar space is presented. It manifests as a spatial association between peaks in HI structure offset with respect to peaks in high-frequency radio continuum data obtained with the Wilkinson Microwave Anisotropy Probe. [ABSTRACT FROM AUTHOR]

Cosmic-ray intensity data recorded with the ground-based neutron monitor at Deep River have been investigated taking into account the associated interplanetary magnetic field and solar-wind plasma data during 1981 – 1994. A large number of days having abnormally high or low amplitudes for five or more successive days as compared to the annual average amplitude of diurnal anisotropy have been taken as high- or low-amplitude anisotropic wave-train events. The amplitude of the diurnal anisotropy of these events is found to increase on days with a magnetic cloud as compared to the days prior to the event, and it is found to decrease during the later period of the event as the cloud passes the Earth. The high-speed solar-wind streams do not play any significant role in causing these types of events. However, corotating solar-wind streams produce significant deviations in cosmic-ray intensity during high- and low-amplitude events. The interplanetary disturbances (magnetic clouds) are also effective in producing cosmic-ray decreases. Hα solar flares have a good positive correlation with both the amplitude and direction of the anisotropy for high-amplitude events, while the principal magnetic storms have a good positive correlation with both amplitude and direction of the anisotropy for low-amplitude events. The source responsible for these unusual anisotropic wave trains in cosmic rays has been proposed. [ABSTRACT FROM AUTHOR]

Interplanetary magnetic clouds (MCs) are one of the main sources of large non-recurrent geomagnetic storms. With the aid of a force-free flux rope model, the dependence of the intensity of geomagnetic activity (indicated by Dst index) on the axial orientation (denoted by θ and φ in GSE coordinates) of the magnetic cloud is analyzed theoretically. The distribution of the Dst values in the ( θ, φ) plane is calculated by changing the axial orientation for various cases. It is concluded that (i) geomagnetic storms tend to occur in the region of θ<0°, especially in the region of θ≲−45°, where larger geomagnetic activity could be created; (ii) the intensity of geomagnetic activity varies more strongly with θ than with φ; (iii) when the parameters B 0 (the magnetic field strength at the flux rope axis), R 0 (the radius of the flux rope), or V (the bulk speed) increase, or | D| (the shortest distance between the flux rope axis and the x-axis in GSE coordinates) decreases, a flux rope not only can increase the intensity of geomagnetic activity, but also is more likely to create a storm, however the variation of n (the density) only has a little effect on the intensity; (iv) the most efficient orientation (MEO) in which a flux rope can cause the largest geomagnetic activity appears at φ∼0° or ∼ 180°, and some value of θ which depends mainly on D; (v) the minimum Dst value that could be caused by a flux rope is the most sensitive to changes in B 0 and V of the flux rope, and for a stronger and/or faster MC, a wider range of orientations will be geoeffective. Further, through analyzing 20 MC-caused moderate to large geomagnetic storms during 1998 – 2003, a long-term prediction of MC-caused geomagnetic storms on the basis of the flux rope model is proposed and assessed. The comparison between the theoretical results and the observations shows that there is a close linear correlation between the estimated and observed minimum Dst values. This suggests that using the ideal flux rope to predict practical MC-caused geomagnetic storms is applicable. The possibility of the long-term prediction of MC-caused geomagnetic storms is discussed briefly. [ABSTRACT FROM AUTHOR]

Abstract: High-velocity clouds (HVCs) are defined by their high radial velocities which are incompatible with a participation in the normal rotation of our Galaxy. They are observed mainly in the 21cm line of neutral hydrogen. HVCs can be found all over the sky in the form of large complexes as well as compact, isolated clouds. To substantiate the concept of a circum-galactic population of HVCs we carried out an HI blind survey of a large area around the Andromeda Galaxy (M31) with the 100m radio telescope at Effelsberg. We detected several HVCs near the disk of M31, but not a single HVC was found beyond a projected distance of about 50kpc, indicating that HVCs are located in the vicinity of the large galaxies. Follow-up synthesis observations with the Westerbork synthesis radio telescope show that the detected HVCs have diameters of the order of 1kpc and HI masses of typically a few times 105 M ⊙. Some of the HVCs near M31 are likely associated with the recently discovered giant stellar stream. Another HVC near NGC 205 may also be of tidal origin, whereas other HVCs around M31 are isolated and may be considered candidates for the primordial, gaseous dark matter satellites predicted by CDM structure formation scenarios. [Copyright &y& Elsevier]

We address the hypothesis that high-velocity clouds correspond to the ‘missing’ dwarf galaxies of the Local Group predicted by cosmological simulations. To this end, we present optical and near-infrared photometry of five additional high-velocity clouds, one of which produces Lyman series absorption on the sight line towards the quasar Ton S210, with sufficient resolution and sensitivity to enable the detection of an associated stellar content. We do not detect significant stellar populations intrinsic to any of the five clouds. In combination with the results from our Paper I, which had yielded non-detections of stellar content in another five cases, we find that there is a 50 per cent chance of getting a null result in 10 trials if fewer than 7 per cent of all high-velocity clouds contain stars. We conclude that the population of high-velocity clouds is an unlikely repository for the ‘missing’ dwarfs of the Local Group. [ABSTRACT FROM AUTHOR]

We explore magnetohydrodynamic (MHD) solutions for envelope expansions with core collapse (EECC) with isothermal MHD shocks in a quasi-spherical symmetry, and outline potential astrophysical applications of such magnetized shock flows. By including a random magnetic field in a gas medium, we further extend the recent isothermal shock results of Bian & Lou who have unified earlier similarity isothermal shock solutions of Tsai & Hsu, of Shu et al. and of Shen & Lou in a more general framework. MHD shock solutions are divided into three classes according to the downstream characteristics near the core. Class I solutions are those characterized by free-fall collapses towards the core downstream of an MHD shock, while Class II solutions are those characterized by Larson–Penston (LP) type near the core downstream of an MHD shock. Class III solutions are novel, sharing both features of Class I and II solutions with the presence of a sufficiently strong magnetic field as a prerequisite. Various MHD processes may occur within the regime of these isothermal MHD shock similarity solutions, such as submagnetosonic oscillations, free-fall core collapses, radial contractions and expansions. Both possibilities of perpendicular and oblique MHD shocks are analysed. Under the current approximation of MHD EECC (MEECC) solutions, only perpendicular shocks are systematically calculated. These similar MHD shocks propagate at either submagnetosonic or supermagnetosonic constant speeds. We can construct Class I, II and III MHD shocks matching with an isothermal magnetostatic outer envelope or an MHD breeze. We can also construct families of twin MHD shock solutions as well as an ‘isothermal MHD shock’ separating two magnetofluid regions of two different yet constant temperatures. The versatile behaviours of such MHD shock solutions may be utilized to model a wide range of astrophysical problems, including star formation in magnetized molecular clouds, ‘MHD champagne flows’ in H ii regions around luminous massive OB stars, MHD link between the asymptotic giant branch (AGB) phase and the protoplanetary nebula (pPN) phase with a hot central magnetized white dwarf, relativistic MHD pulsar winds in supernova remnants (SNRs), radio afterglows of soft gamma-ray repeaters (SGRs) and so forth. [ABSTRACT FROM AUTHOR]

We present Ca ii interstellar observations towards 20 early-type stars, to place lower distance limits to intermediate- and high-velocity clouds (IHVCs) in their lines of sight. The spectra are also employed to estimate the Ca abundance in the low-velocity gas towards these objects, when combined with Leiden–Dwingeloo 21-cm H i survey data of spatial resolution . Nine of the stars, which lie towards IHVC complexes H, K and gp, were observed with the intermediate dispersion spectrograph on the Isaac Newton Telescope at a resolution of 9000 (∼33 km s−1) and signal-to-noise ratio (S/N) per pixel of 75–140. A further nine objects were observed with the Utrecht Echelle Spectrograph on the William Herschel Telescope at and S/N per pixel of 10–25. Finally, two objects were observed in both Ca ii K and Na i D lines using the 2D COUDÉ spectrograph on the McDonald 2.7-m telescope at . The abundance of Ca ii plotted against H i column density for the objects in the current sample with heights above the Galactic plane ( z) exceeding 1000 pc is found to obey the Wakker & Mathis (2000) relation. Also, the reduced column density of Ca ii K as function of z is consistent with the larger sample taken from Smoker et al. (2003) . Higher S/N observations than those previously taken towards HVC complex H stars HD 13256 and HILT 190 reinforce the assertion that this lies at a distance exceeding 4000 pc. No obvious absorption is detected in observations of ALS 10407 and HD 357657 towards IVC complex gp. The latter star has a spectroscopically estimated distance of ∼2040 pc, although this was derived assuming the star lies on the main sequence and without any reddening correction being applied. Finally, no Ca ii K absorption is detected towards two stars along the line of sight to complex K, namely PG 1610+529 and PG 1710+490. The latter is at a distance of ∼700 pc, hence placing a lower distance limit to this complex, where previously only an upper distance limit of 6800 pc was available. [ABSTRACT FROM AUTHOR]

In our model describing the leakage of ionizing radiation from the Galactic disk into the halo, disk stars can contribute substantially to the ionization of halo objects such as high-velocity clouds and the Magellanic stream. This ionization is produced by a relatively hard radiation field, which can maintain its ionizing effect even at a considerable distance from the plane of the disk. [ABSTRACT FROM AUTHOR]

The electromagnetic instability of an interstellar medium with an arbitrary velocity distribution is examined over the large scale lengths typical of gas-dust clouds without a significant magnetic field. It is shown that over a moderate time scale (months and years) these instabilities can develop and that the requirement of stability is satisfied by a narrow class of distributions that are close to spherical. [ABSTRACT FROM AUTHOR]

Gentle protein electrospray ionization is achieved using the high-velocity gas flow of an air amplifier to improve desolvation in conventional ESI and generate intact folded protein ions in the gas phase. Comparisons are made between the ESI spectra of a number of model proteins, including ubiquitin, cytochrome c, lysozyme, and myoglobin, over a range of pH values under optimized conditions, with and without using an air amplifier to achieve high-velocity gas flow. Previously reported increased ion signals are confirmed. In addition, the peaks recorded using the air amplifier are shown to be narrower, corresponding to more complete desolvation. Significant changes in the charge-state distribution also are observed, with a shift to lower charge state at high-velocity flow. The relationship between the observed charge-state distribution and protein conformation was explored by comparing the charge-state shifts and the distributions of charge states for proteins that are or are not stable in their native conformations in low pH solutions. The data suggest retention of native or nativelike protein conformations using the air amplifier in all cases examined. This is explained by a mechanism in which the air amplifier rapidly creates small droplets from the original large ESI droplets and these microdroplets then desolvate without a significant decrease in pH, resulting in retention of the folded protein conformations. Furthermore, the holoform of ionized myoglobin is visible at pH 3.5, a much lower value than the minimum needed to see this form in conventional ESI. These results provide evidence for the importance of the conditions used in the desolvation process for the preservation of the protein conformation and suggest that the conditions achieved when using high-velocity gas flows to assist droplet evaporation and ion desolvation are much gentler than those in conventional ESI experiments. [ABSTRACT FROM AUTHOR]

We have mapped 16 molecular clouds toward a new OB association in the Pup-CMa region to derive their physical properties. The observations were carried out in the 12CO ( J = 1 – 0) line with the Southern millimetre-wave Telescope at Cerro Tololo, Chile. Distances have been determined kinematically using the rotation curve of Brand with R⊙ = 8.5 kpc and V⊙ = 220 km/s. Masses have been derived adopting a CO luminosity to H2 conversion factor X = 3.8 . 1020 molecules cm-2 (K km/s)-1. The observed mean radial velocity of the clouds is comparable with the mean radial velocity of stars composing an OB association in Pup-CMa; it is in favor of the close connection of clouds with these stars. [ABSTRACT FROM AUTHOR]

We integrate the Vlasov–Poisson equations giving the evolution of a dynamical system in phase space using a continuous set of local basis functions. In practice, the method decomposes the density in phase space into small smooth units having compact support. We call these small units‘clouds’ and choose them to be Gaussians of elliptical support. Fortunately, the evolution of these clouds in the local potential has an analytical solution that can be used to evolve the whole system during a significant fraction of dynamical time. In the process, the clouds, initially round, change shape and become elongated. At some point, the system needs to be remapped on round clouds once again. This remapping can be performed optimally using a small number of Lucy iterations. The remapped solution can be evolved again with the cloud method, and the process can be iterated a large number of times without showing significant diffusion. Our numerical experiments show that it is possible to follow the two-dimensional phase-space distribution during a large number of dynamical times with excellent accuracy. The main limitation to this accuracy is the finite size of the clouds, which results in coarse-graining the structures smaller than the clouds and induces small aliasing effects at these scales. However, it is shown in this paper that this method is consistent with an adaptive refinement algorithm which allows one to track the evolution of the finer structure in phase space. It is also shown that the generalization of the cloud method to four-dimensional and six-dimensional phase space is quite natural. [ABSTRACT FROM AUTHOR]

ABSTRACT The standard treatment of cooling in cold dark matter haloes assumes that all of the gas within a 'cooling radius' cools and contracts monolithically to fuel galaxy formation. Here we take into account the expectation that the hot gas in galactic haloes is thermally unstable and prone to fragmentation during cooling and we show that the implications are more far-reaching than previously expected: allowing multiphase cooling fundamentally alters expectations about gas infall in galactic haloes and naturally gives rise to a characteristic upper limit on the masses of galaxies, as observed. Specifically, we argue that cooling should proceed via the formation of high-density, ∼104 K clouds, pressure-confined within a hot gas background. The background medium that emerges has a low density, and can survive as a hydrostatically stable corona with a long cooling time. The fraction of halo baryons contained in the residual hot core component grows with halo mass because the cooling density increases with gas temperature, and this leads to an upper-mass limit in quiescent, non-merged galaxies of ∼1011 M⊙. In this scenario, galaxy formation is fuelled by the infall of pressure-supported clouds. For Milky-Way-size systems, clouds of mass ∼5 × 106 M⊙ that formed or merged within the last several Gyr should still exist as a residual population in the halo, with a total mass in clouds of ∼2 × 1010 M⊙. The baryonic mass of the Milky Way galaxy is explained naturally in this model, and is a factor of 2 smaller than would result in the standard treatment without feedback. We expect clouds in galactic haloes to be ∼1 kpc in size and to extend ∼150 kpc from galactic centres. The predicted properties of Milky Way clouds match well the observed radial velocity distribution, angular sizes, column densities and velocity widths of high-velocity clouds around our Galaxy. The clouds we predict are also of the type needed to explain high-ion absorption systems at z < 1, and the predicted covering factor around external galaxies is consistent with observations. [ABSTRACT FROM AUTHOR]

The limitations imposed by the shear instability on the formation of gigantic molecular clouds in the central regions of spiral galaxies are examined. The criteria obtained here are illustrated using the example of six galaxies for which the detailed rotation curves are known. The different mechanisms for formation of molecular clouds which apply in the central and edge regions of disk galaxies are evaluated. [ABSTRACT FROM AUTHOR]

Astrophysical and cosmochemical data show that many kinds of hydrocarbons are widespread in space, including giant molecular clouds, diffuse interstellar medium, comets, interplanetary dust particles, and carbonaceous meteorites. Here an effort is made to show the close relation between high-molecular weight hydrocarbons observed in space and existing on Earth. Results of astrochemical modelling of dust grains in dense collapsing cores of giant molecular clouds are also presented. They show that about 10% of the total abundance of dust grains may be the result of aliphatic hydrocarbons. This dust serves as initial material for comets, formed in protosolar nebula. The problem of survival of cometary organics during impact onto the Earth is discussed, and it is shown that the so-called “soft-landing comet“ hypothesis may explain the accumulation of complex hydrocarbons on the Earth's surface. We conclude that a significant fraction of terrestrial prebiotic petroleum was delivered by extraterrestrial matter. [ABSTRACT FROM AUTHOR]

We have mapped the neutral gas content of the surroundings of three galaxies on the outskirts of the Local Group – Wolf–Lundmark–Melotte (WLM), NGC 1313 and Sextans A – at high velocity resolution and brightness sensitivity. We present high-quality H i spectra and parameters for the target galaxies, and find no unknown companion H i clouds nearer than ∼100 kpc to the targets, despite reaching a sensitivity of for narrow-linewidth objects. [ABSTRACT FROM AUTHOR]

We present an X-ray absorption analysis of the high-velocity system (HVS) in NGC 1275 using results from a deep 200-ks Chandra observation. We are able to describe the morphology of the HVS in more detail than ever before. We present a Hubble Space Telescope image for comparison, and note close correspondence between the deepest X-ray absorption and the optical absorption. A column density map of the HVS shows an average column density NH of with a range from to . From the NH map we calculate a total mass for the absorbing gas in the HVS of at solar abundance. 75 per cent of the absorbing mass is contained in the four regions of deepest absorption. We examine temperature maps produced by spectral fitting and find no direct evidence for shocked gas in the HVS. Using deprojection methods and the depth of the observed absorption, we are able to put a lower limit on the distance of the HVS from the nucleus of 57 kpc, showing that the HVS is quite separate from the body of NGC 1275. [ABSTRACT FROM AUTHOR]

The effects of ambipolar diffusion on the linear stability of weakly ionized accretion discs are examined. Earlier work on this topic has focused on axial magnetic fields and perturbation wavenumbers. We consider here more general field and wavenumber geometries, and find that qualitatively new results are obtained. Provided a radial wavenumber and azimuthal field are present along with their axial counterparts, ambipolar diffusion will always be destabilizing, with unstable local modes appearing at well-defined wavenumber bands. The wavenumber corresponding to the maximum growth rate need not, in general, lie along the vertical axis. Growth rates become small relative to the local angular velocity when the ion–neutral collision time exceeds the orbital time. In common with Hall electromotive forces, ambipolar diffusion destabilizes both positive and negative angular velocity gradients. In at least some cases, therefore, uniformly rotating molecular cloud cores may reflect the marginally stable state of the ambipolar magnetorotational instability. [ABSTRACT FROM AUTHOR]

I summarize the new developments in HVC research that have occurred in the past few years. These include a) the determination of approximate distances to some IVCs and HVC complex A; b) the measurement of the metallicity of the Magellanic Stream and the conclusion that tidal forces are dominant in creating the Stream; c) the derivation that the metallicity of complex C is 0.1 solar, with some evidence for metallicity variations; d) the result that NIO in complex C is 0.2 times the solar ratio, a value otherwise only seen in dwarf galaxies and Damped Lyα systems; e) the discovery of high-velocity 0 VI absorption, both associated with previously known H I HVCs and in directions without a H I HVC; f) the proposal that HVCs form a population of objects permeating the Local Group. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigate the hypothesis that compact high-velocity clouds (CHVC) are the ‘missing’ dwarf galaxies of the Local Group, by searching them for populations of resolved stars. To this end we conducted two distinct tests based on optical and near-infrared single-star photometry. The optical and the near-infrared experiments complement one another; the optical data help us to rule out distant populations but they are restricted to the central regions of the gas distributions, whereas the near-infrared photometry allows us to set limits on nearby populations spread over the typical cloud size. First, we discuss deep optical single-star photometry of five CHVCs in the V and I filters, obtained with the FORS instrument at the Very Large Telescope (VLT). We find that their optical colour–magnitude diagrams are indistinguishable from that of a population of Galactic stars, and attribute all of the resolved stars to the Galactic foreground. We present simulations that address the question of how much of a ‘normal’ dwarf-galaxy-type population we might hide in the data. A Kolmogorov–Smirnov test allows us to set very stringent limits on the absence of a resolved stellar population in CHVCs. Secondly, we also culled near-infrared single-star photometry in the J, H, and K[sub S] bands for four of the CHVCs from the Two Micron All Sky Survey (2MASS). The infrared data do not reveal any stellar content in the CHVCs which resembles that of nearby dwarf galaxies either, and are explained with Galactic foreground as well. We interpret our null detections as indicating that the five CHVCs investigated by us do not host an associated stellar content which is similar to that of the known dwarf galaxies of the Local Group. These CHVCs are very likely to be pure hydrogen clouds in which no star formation has taken place over cosmic time. [ABSTRACT FROM AUTHOR]

It has been suggested that some high-velocity hydrogen clouds, rather than being associated with the Galaxy, may be dispersed throughout the Local Group. If true these hydrogen clouds may be the 'debris' left over from the formation of the Local Group predicted by numerical simulations. Some of these clouds have measured H I column densities higher than that normally assumed to be the threshold for star formation, and they are metal-enriched. We have carried out V- and/-band CCD observations of three compact high-velocity hydrogen clouds from the sample of Braun & Burton in an attempt to detect resolved giant branch tip stars and/or diffuse emission. We detect no stars that can be associated with the hydrogen clouds down to a magnitude limit of m[SUBV] = 23.3 and m[SUBI] = 22.0. We find no diffuse emission down to a central surface brightness limit of ≈25.0μV. [ABSTRACT FROM AUTHOR]

We report the discovery of high-velocity dense gas from a bipolar outflow source near NGC 2068 in the L1630 giant molecular cloud. CO and HCO[sup +] J=3→2 line wings have a bipolar distribution in the vicinity of LBS 17-H with the flow orientated roughly east–west and perpendicular to the elongation of the submillimetre dust continuum emission. The flow is compact (total extent ∼0.2 pc) and contains of the order of 0.1 M[sub ⊙] of swept-up gas. The high-velocity HCO[sup +] emission is distributed over a somewhat smaller area <0.1 pc in extent. A map of C[sup 18]O J=2→1 emission traces the LBS 17 core and follows the ambient HCO[sup +] emission reasonably well, with the exception of the direction towards LBS 17-H where there is a significant anticorrelation between the C[sup 18]O and HCO[sup +]. A comparison of beam-matched C[sup 18]O and dust-derived H[sub 2] column densities suggests that CO is depleted by up to a factor of ∼50 at this position if the temperature is as low as 9 K, although the difference is substantially reduced if the temperature is as high as 20 K. Chemical models of collapsing clouds can account for this discrepancy in terms of different rates of depletion on to dust grains for CO and HCO[sup +]. LBS 17-H has a previously known water maser coincident with it but there are no known near-infrared, IRAS or radio continuum sources associated with this object, leading to the conclusion that it is probably very young. A greybody fit to the continuum data gives a luminosity of only 1.7 L[sub ⊙] and a submillimetre-to-bolometric luminosity ratio of 0.1, comfortably satisfying the criteria for classification as a class 0 protostar candidate. [ABSTRACT FROM AUTHOR]

High-velocity submicrometre-sized dust particles expelled from the jovian system have been identified by dust detectors on board several spacecraft. On the basis of periodicities in the dust impact rate, Jupiter's moon Io was found to be the dominant source of the streams. The grains become positively charged within the plasma environment of Jupiter's magnetosphere, and gain energy from its co-rotational electric field. Outside the magnetosphere, the dynamics of the grains are governed by the interaction with the interplanetary magnetic field that eventually forms the streams. A similar process was suggested for Saturn. Here we report the discovery by the Cassini spacecraft of bursts of high-velocity dust particles (=100?km?s-1) within~70 million kilometres of Saturn. Most of the particles detected at large distances appear to originate from the outskirts of Saturn's outermost main ring. All bursts of dust impacts detected within 150 Saturn radii are characterized by impact directions markedly different from those measured between the bursts, and they clearly coincide with the spacecraft's traversals through streams of compressed solar wind. [ABSTRACT FROM AUTHOR]

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of 1.1 mm dust continuum and CO 2–1 emission toward six dense cores within the Ophiuchus molecular cloud. We detect compact, subarcsecond continuum structures toward three targets, two of which (Oph A N6 and SM1) are located in the Ophiuchus A ridge. Two targets, SM1 and GSS 30, contain two compact sources within the ALMA primary beam. We argue that several of the compact structures are small (R ≲ 80 au) accretion disks around young protostars, based on their resolved, elongated structures, coincident radio and X-ray detections, or bipolar outflow detections. While CO line wings extend to ±10–20 km s−1 for the more evolved sources GSS 30 IRS3 and IRS1, CO emission toward other sources, where detected, only extends a few km s−1 from the cloud vLSR. The dust spectral index toward the compact objects suggests either that the disks are optically thick at 1.1 mm or that significant grain growth has already occurred. We identify, for the first time, a single compact continuum source (R ∼ 100 au) toward N6 embedded within a larger continuum structure. SM1N is extended in the continuum but is highly centrally concentrated, with a density profile that follows a r−1.3 power law within 200 au and additional structure suggested by the uv-data. Both N6 and SM1N show no clear bipolar outflows with velocities greater than a few km s−1 from the cloud velocity. These sources are candidates to be the youngest protostars or first hydrostatic cores in the Ophiuchus molecular cloud. [ABSTRACT FROM AUTHOR]

We present the results of a single dish interstellar maser survey toward ∼200 molecular clouds (MCs) in the extreme outer Galaxy (EOG) possibly lying in the Outer Scutum-Centaurus (OSC) arm. The observations resulted in the detections of two new 22.2 GHz water masers, two new 6.7 GHz class II methanol masers, and one new 4765 MHz excited-state OH maser. These new detections include one high Galactic latitude (b > 2°) methanol maser, one farthest Galactic methanol maser, and one farthest Galactic OH maser from the Sun. The velocities of the masers and the CO emission are in agreement with each other, which strongly indicates an association between the maser and the EOG cloud. The presence of the masers provides reliable evidence for ongoing massive star-forming activity in the OSC arm. The EOG clouds with detected interstellar masers are all associated with young stellar objects. Masers are primarily detected toward the brighter and more massive MCs. Because of the large distances involved, the “faint” masers with luminosity below 10−6L☉ are not accessible by our current observations. The low number of masers in the EOG regions supports the scenario of decreasing metallicity and decreasing star formation rate/efficiency at large Galactocentric radii. Our new maser detections are valuable targets for future VLBI parallax and proper motion measurements. [ABSTRACT FROM AUTHOR]

We observed five giant molecular clouds (GMCs) in the Large Magellanic Cloud (LMC) in the 12CO J = 1–0 line using the Atacama Large Millimeter/submillimeter Array (ALMA). The sample includes four GMCs with some signs of star formation—either YSOs, H ii regions, and/or young clusters—and one quiescent GMC without any sign of massive star formation. The data from the ALMA 12 m, 7 m, and Total-Power arrays are jointly deconvolved to obtain high-fidelity images at high spatial resolution (3″ = 0.7 pc). The four star-forming GMCs show very complex structures with clumps and filaments. The quiescent GMC shows a relatively diffuse, extended emission distribution without prominent clumps or filaments. This difference is similar to that between structured molecular gas in Milky Way spiral arms and unstructured gas in the inter-arm regions. We characterize the difference with the brightness distribution function and brightness distribution index. In conjunction with other ALMA studies of GMCs in the LMC, the five GMCs tentatively form an evolutionary trend: from less structured, quiescent GMCs to more structured, actively star-forming GMCs. A future ALMA study will be able to map molecular clouds over the LMC and reveal the evolutionary sequence of molecular clouds. [ABSTRACT FROM AUTHOR]

We predict the UV emission line intensities produced by turbulent mixing layers (TMLs) between hot and warm gas and in high-velocity clouds (HVCs) traveling through hot gas such as that in the extended Galactic halo, circumgalactic medium, and intergalactic space. The calculations include the effects of hydrodynamic mixing, cooling, and non-equilibrium ionization and were done with the FLASH code and the Raymond and Smith code. A variety of mixing-layer and cloud conditions were simulated, resulting in 13 individual models. The intensities of 21 UV emission lines or doublets of 14 ions of carbon, nitrogen, oxygen, and silicon in TMLs and HVCs are tabulated. These ions are C ii, C iii, C iv, N ii, N iii, N iv, N v, O iii, O iv, O v, O vi, Si ii, Si iii, and Si iv. Owing to the relatively small assumed pressures and depths, our predicted intensities are small. However, when properly scaled to account for the pressures and depths of such gas in other places, the predicted intensities can be used to estimate the sensitivity needed for observations of the diffuse UV emission or compared with observations of a variety of astrophysical structures. The ratios of the predicted Si iv, C iv, N v, and O vi intensities are expected to remain invariant when the predictions are scaled to correspond to denser regions. The ratios of the predicted Si iv, C iv, and O vi intensities are compared with those of regions in our Galaxy that were observed by SPEAR/FIMS and the Far Ultraviolet Spectroscopic Explorer and with those predicted for other models, including an analytical model of TMLs and a model with a magnetized thermal conduction front. [ABSTRACT FROM AUTHOR]

We present the discovery of expanding spherical shells around low- to intermediate-mass young stars in the Orion A giant molecular cloud using observations of (1–0) and (1–0) from the Nobeyama Radio Observatory 45 m telescope. The shells have radii from 0.05 to 0.85 pc and expand outward at 0.8–5 km s−1. The total energy in the expanding shells is comparable to protostellar outflows in the region. Together, shells and outflows inject enough energy and momentum to maintain the cloud turbulence. The mass-loss rates required to power the observed shells are two to three orders of magnitude higher than predicted for line-driven stellar winds from intermediate-mass stars. This discrepancy may be resolved by invoking accretion-driven wind variability. We describe in detail several shells in this paper and present the full sample in the online journal. [ABSTRACT FROM AUTHOR]

High velocity gas that does not conform to Galactic rotation is observed throughout the Galaxy’s halo. One component of this gas, H i high velocity clouds (HVCs), have attracted attention since their discovery in the 1960s and remain controversial in terms of their origins, largely due to the lack of reliable distance estimates. The recent discovery of enhanced magnetic fields toward HVCs has encouraged us to explore their connection to cloud evolution, kinematics, and survival as they fall through the magnetized Galactic halo. For a reasonable model of the halo magnetic field, most infalling clouds see transverse rather than radial field lines. We find that significant compression (and thereby amplification) of the ambient magnetic field occurs in front of the cloud and in the tail of material stripped from the cloud. The compressed transverse field attenuates hydrodynamical instabilities. This delays cloud destruction, though not indefinitely. The observed field compression is related to the cloud’s distance from the Galactic plane. As a result, the observed rotation measure provides useful distance information on a cloud’s location. [ABSTRACT FROM AUTHOR]

High-velocity clouds (HVCs) hydrodynamically interact with their surroundings. In scenarios with small clouds, the net result is erosion of the HVC, but in scenarios with large, massive clouds, it is capture and cooling of environmental gas by the cloud. In order to examine these effects over long periods of time, we made detailed three-dimensional hydrodynamic FLASH simulations of massive HVCs (1.35 × 105M☉ to 1.35 × 108M☉) traveling through hot, low-density media like that in the extended Galactic halo, circumgalactic media, and intergalactic space for at least 200 Myr. By setting the metallicity of the clouds to differ from that of the ambient gas, we were able to track the transfer of material between the two media. We found that massive clouds condense substantial amounts of ambient gas, up to 100% of their initial mass in certain cases. This gas cools to temperatures below 1.0 × 104 K, but retains some high ions. The amount of condensed gas depends on the cloud mass, the ambient density, and the cloud density and temperature, but apparently not on the velocity. We discuss the ramifications for cloud survivability and for the transport of halo, circumgalactic, and intergalactic gas to the disk of the Galaxy aboard massive HVCs. [ABSTRACT FROM AUTHOR]

We have combined observations of Galactic high-velocity H i from two surveys: a very sensitive survey from the Green Bank 140 ft Telescope with limited sky coverage, and the less sensitive but complete Galactic All Sky Survey from the 64 m Parkes Radio Telescope. The two surveys preferentially detect different forms of neutral gas due to their sensitivity. We adopt a machine learning approach to divide our data into two populations that separate across a range in column density: (1) a narrow line-width population typical of the majority of bright high velocity cloud components, and (2) a fainter, broad line-width population that aligns well with that of the population found in the Green Bank survey. We refer to these populations as dense and diffuse gas, respectively, and find that diffuse gas is typically located at the edges and in the tails of high velocity clouds, surrounding dense components in the core. A fit to the average spectrum of each type of gas in the Galactic All Sky Survey data reveals the dense population to have a typical line width of ∼20 km s−1 and brightness temperature of ∼0.3 K, while the diffuse population has a typical line width of ∼30 km s−1 and a brightness temperature of ∼0.2 K. Our results confirm that most surveys of high velocity gas in the Milky Way halo are missing the majority of the ubiquitous diffuse gas, and that this gas is likely to contribute at least as much mass as the dense gas. [ABSTRACT FROM AUTHOR]

We analyze the physical properties and energy balance of density enhancements in two SPH simulations of the formation, evolution, and collapse of giant molecular clouds. In the simulations, no feedback is included, so all motions are due either to the initial decaying turbulence or to gravitational contraction. We define clumps as connected regions above a series of density thresholds. The resulting full set of clumps follows the generalized energy equipartition relation, , where is the velocity dispersion, R is the “radius,” and Σ is the column density. We interpret this as a natural consequence of gravitational contraction at all scales rather than virial equilibrium. Nevertheless, clumps with low Σ tend to show a large scatter around equipartition. In more than half of the cases, this scatter is dominated by external turbulent compressions that assemble the clumps rather than by small-scale random motions that would disperse them. The other half does actually disperse. Moreover, clump sub-samples selected by means of different criteria exhibit different scalings. Sub-samples with narrow Σ ranges follow Larson-like relations, although characterized by their respective values of Σ. Finally, we find that (i) clumps lying in filaments tend to appear sub-virial, (ii) high-density cores ( cm3) that exhibit moderate kinetic energy excesses often contain sink (“stellar”) particles and the excess disappears when the stellar mass is taken into account in the energy balance, and (iii) cores with kinetic energy excess but no stellar particles are truly in a state of dispersal. [ABSTRACT FROM AUTHOR]

We present new, wide, and deep images in the AzTEC/ASTE 1.1 mm continuum and the 12CO (J = 1-0) emission toward the north- ern part of the Orion-A GMC. We have found evidence for interactions between molecular clouds and the external forces that may trigger star formation. Two types of possible triggers were revealed: (1) Collisions of the diffuse gas on the cloud surface, particularly at the eastern side of the OMC-2/3 region, and (2) Irradiation of UV on the pre-existing filaments and dense molecular cloud cores. Our wide-field and high-sensitivity imaging has provided the first comprehensive view of the potential sites of triggered star formation in the Orion-A GMC. [ABSTRACT FROM AUTHOR]

The Rosette molecular cloud has a characteristic morphology, where the central OB cluster NGC 2244 has blown a circularshaped cavity into the cloud and the expanding HII-region now inter- acts with the cloud. We use continuum data obtained with the PACS (70 and 160 µm) and SPIRE instruments (250, 350, 500 µm) of the Herschel telescope to investigate the spatial distribution of cold and warm gas and to locate all star-forming sites in Rosette. We detected a clear negative temperature gradient and a positive density gradient (running from the HII-region/molecular cloud interface into the cloud), and an age-sequence (from more evolved to younger) with increasing distance to the cluster NGC 2244. The existence of such temperature and density gradients and the observed age-sequence imply that star formation in Rosette may indeed be influenced by the radiative impact of the central NGC 2244 cluster. [ABSTRACT FROM AUTHOR]

The fairly recent detection of a variety of anions in the interstellar molecular clouds have underlined the importance of realistically modelling the processes governing their abundance. To pursue this task, our earlier calculations for the radiative electron attachment (REA) rates for C4H−, C6H−, and C8H− are employed in the present work, within a broad network of other concurrent reactions, to generate the corresponding column density ratios of anion/neutral (A/N) relative abundances. The latter are then compared with those obtained in recent years from observational measurements. The calculations involved the time-dependent solutions of a large network of chemical processes over an extended time interval and included a series of runs in which the values of REA rates were repeatedly scaled over several orders of magnitude. Macroscopic parameters for the Clouds’ modelling were also varied to cover a broad range of physical environments. It was found that, within the range and quality of the processes included in the present network,and selected from state-of-the-art astrophysical databases, the REA values required to match the observed A/N ratios needed to be reduced by orders of magnitude for C4H− case, while the same rates for C6H− and C8H− only needed to be scaled by much smaller factors. The results suggest that the generally proposed formation of interstellar anions by REA mechanism is overestimated by current models for the C4H− case, for which is likely to be an inefficient path to formation. This path is thus providing a rather marginal contribution to the observed abundances of C4H−, the latter being more likely to originate from other chemical processes in the network, as we discuss in some detail in the present work. Possible physical reasons for the much smaller differences against observations found instead for the values of the (A/N) ratios in two other, longer members of the series are put forward and analysed within the evolutionary modelling discussed in the present work. [ABSTRACT FROM AUTHOR]

We present distributions of two molecular clouds having velocities of 2 and 14 km s−1 toward RCW 38, the youngest super star cluster in the Milky Way, in the 12CO J = 1–0 and 3–2 and 13CO J = 1–0 transitions. The two clouds are likely physically associated with the cluster as verified by the high intensity ratio of the J = 3–2 emission to the J = 1–0 emission, the bridging feature connecting the two clouds in velocity, and their morphological correspondence with the infrared dust emission. The velocity difference is too large for the clouds to be gravitationally bound. We frame a hypothesis that the two clouds are colliding with each other by chance to trigger formation of the ∼20 O stars that are localized within ∼0.5 pc of the cluster center in the 2 km s−1 cloud. We suggest that the collision is currently continuing toward part of the 2 km s−1 cloud where the bridging feature is localized. This is the third super star cluster alongside Westerlund 2 and NGC 3603 where cloud–cloud collision has triggered the cluster formation. RCW 38 is the youngest super star cluster in the Milky Way, holding a possible sign of on-going O star formation, and is a promising site where we may be able to witness the moment of O star formation. [ABSTRACT FROM AUTHOR]

Protoplanetary disks originate in the collapse of molecular cloud cores. The formation and evolution of disks are influenced by the properties of molecular cloud cores. In this paper we investigate the dependence of disk lifetimes and accretion rates on cloud core properties. We find that the lifetime increases as the angular velocities and the mass of cloud cores increase and that the lifetime decreases as the core temperature increases. We have calculated the distribution of disk lifetimes and disk fractions with stellar age. Our calculations show that the lifetime is in the range of 1–15 Myr and that the typical lifetime is 1–3 Myr. There are a few disks with lifetimes greater than 10 Myr and of the disks have lifetimes less than 1 Myr. We also fit the disk fraction by an exponential decay curve with characteristic time ∼3.7 Myr. Our results explain the observations of disk lifetimes. We also find that the accretion rate does not change significantly with ω and generally decreases with . At the early evolution of the disks, the relation is about . Since the effects of the photoevaporation are weak at this stage, this relation is the consequence of the cloud core properties. At the late evolution of the disks, the relation is about . For low accretion rates at this stage, the relation results from the effects of X-ray photoevaporation. The calculated relations are consistent with the observations. [ABSTRACT FROM AUTHOR]

The article focuses on the measurements of high-resolution cloud and boundary layer on high seas. The observations during the Rain in Cumulus over the Ocean (RICO) represent the most comprehensive set of cloud structure, cloud field, surface flux and boundary layer structure measurements. It was found out that three radars operating from the ship characterized the structures of the clouds. The cloud liquid water path and water vapor path was estimated using a microwave radiometer. Moreover, the wide range of cloud and precipitation conditions were clearly within the range of conditions expected in scientific objectives.

We present predictions of the Si iv ions in turbulent mixing layers (TMLs) between hot and cool gas and in cool high-velocity clouds (HVCs) that travel through a hot halo, complementing the C iv, N v, and O vi predictions in Kwak & Shelton, Kwak et al., and Henley et al. We find that the Si iv ions are most abundant in regions where the hot and cool gases first begin to mix or where the mixed gas has cooled significantly. The predicted column densities of high velocity Si iv and the predicted ratios of Si iv to C iv and O vi found on individual sightlines in our HVC simulations are in good agreement with observations of high velocity gas. Low velocity Si iv is also seen in the simulations, as a result of decelerated gas in the case of the HVC simulations and when looking along directions that pass perpendicular to the direction of motion in the TML simulations. The ratios of low velocity Si iv to C iv and O vi in the TML simulations are in good agreement with those recorded for Milky Way halo gas, while the ratio of Si iv to O vi from the decelerated gas in the HVC simulations is lower than that observed at normal velocity in the Milky Way halo. We attribute the shortfall of normal velocity Si iv to not having modeled the effects of photoionization and, following Henley et al., consider a composite model that includes decelerated HVC gas, supernova remnants, galactic fountain gas, and the effect of photoionization. [ABSTRACT FROM AUTHOR]

W49N is a mini-starburst in the Milky Way and is thus an ideal laboratory for high-mass star formation studies. Due to its large distance ( kpc), the kinematics inside and between the dense molecular clumps in W49N are far from well-understood. The Submillimeter Array observations resolved the continuum emission into two clumps. The molecular line observation of SO2 (–) suggests that the two clumps have a velocity difference of ∼7 km s−1. The eastern clump is very close to two radio sources “G1” and “G2,” and the western clump coincides with a radio source “B.” The HCN (3–2) line reveals an extremely energetic outflow, which is among the most energetic molecular outflows in the Milky Way. This is the first report of high-velocity molecular outflow detection in W49N. The outflow jet might be in precession, which could account for the distribution, velocity, and rotation of water maser spots. Three absorption systems are identified in (3–2) spectra. The absorption features are blueshifted with respect to the emission of SO2 (–) lines, indicating that a cold layer is expanding in front of the warm gas. Further analysis indicates that the expansion is decelerated from the geometric expansion centers. [ABSTRACT FROM AUTHOR]