Entire Clouds Research Articles

Survey of the Magellanic Clouds at 4.8 and 8.64 GHz

AbstractDetailed 4.8 and 8.64 GHz radio images of the entire Large and Small Magellanic Clouds with half-power beamwidths of 35″ at 4.8 GHz and 22″ at 8.64 GHz have been obtained using the Australia Telescope Compact Array. Full polarimetric observations were made. Several thousand mosaic positions were used to cover an area of 6° on a side for the LMC and 4.5° for the SMC. These images have sufficient spatial resolution (~ 8 and 5 pc, respectively) and sensitivity (3σ of 1.5 mJy beam−1) to identify most of the individual supernova remnants and H ii regions and also, in combination with available data from the Parkes 64-m telescope, the structure of the smooth emission in these galaxies. We have recently revised the early data analysis (Dickel et al. 2005) by increasing the CLEAN cutoff limit to recover more intermediate-spacing data and thus present more accurate brightnesses for extended sources. In addition, limited data using the sixth antenna at 4.5 – 6 km baselines are available to distinguish bright point sources (< 3″ and 2″, respectively) and to help estimate sizes of individual sources smaller than the resolution of the full survey. The resulting database will be valuable for statistical studies and comparisons with X-ray, optical, and infrared surveys of the LMC with similar resolution.

Rethinking the N(H2)/I(CO) conversion factor

Consequences of an improved formulation, proposed in an earlier paper, for the N(H2)/I(CO) conversion factor are examined. That formulation quantified the statement that the velocity-integrated radiation temperature of the 12CO J= 1 0 line, I(12CO), ‘counts’ optically thick clumps. The clump effective optical depth was approximated as a power law of the clump's optical depth on its central sightline; the power-law index, e, had values between zero and unity. Assuming virialization of the clumps yielded an expression for the N(H2)/I(CO) conversion factor, or X-factor, whose dependence on physical parameters like density and temperature was ‘softened’ by power-law indices of less than unity that depend on the parameter e. One important suggestion of this formulation is that virialization of entire clouds is irrelevant. The densities required to give reasonable values of Xf are consistent with those found in cloud clumps (i.e. ∼103 H2 cm−3). Thus virialization of clumps, rather than of entire clouds, is consistent with the observed values of Xf. And even virialization of clumps is not strictly required; only a relationship between clump velocity width and column density similar to that of virialization can still yield reasonable values of the X-factor. The underlying physics is now at the scale of cloud clumps, implying that the X-factor can probe subcloud structure. The properties of real clumps in real molecular clouds can be used to estimate the X-factor within these clouds and then be compared with the observationally determined X-factor. This yields X-factor values that are within a factor of 2 of the observed values. This is acceptable for the first attempt, but reducing this discrepancy will require improving the formulation. While this formulation improves upon the earlier explanation given about 20 yr ago, it has shortcomings of its own. These include uncertainties as to why e seems to be constant from cloud to cloud, uncertainties in defining the average clump density and neglecting certain complications, such as non-local thermodynamic equilibrium effects, magnetic fields, turbulence, etc. Despite these shortcomings, the proposed formulation represents the first major improvement in understanding the X-factor because it is the first formulation to include radiative transfer.

Star Formation Efficiency in Driven, Supercritical, Turbulent Clouds

We present measurements of the star formation efficiency (SFE) in three-dimensional numerical simulations of driven turbulence in supercritical, ideal-MHD, and nonmagnetic regimes characterized by a mean normalized mass-to-flux ratio μ, all with 64 Jeans masses and similar rms Mach numbers (~10). In most cases, the moderately supercritical runs with μ = 2.8 have significantly lower SFEs than the nonmagnetic cases, being comparable to observational estimates for entire molecular clouds (≲5% over 4 Myr). Also, as the mean field is increased, the number of collapsed objects decreases and the median mass of the collapsed objects increases. However, the largest collapsed-object masses systematically occur in the weak-field case, μ = 8.8. The high-density tails of the density histograms in the simulations are depressed as the mean magnetic field strength is increased. This suggests that the smaller numbers and larger masses of the collapsed objects in the magnetic cases may be due to a greater scarcity and lower mean densities (implying larger Jeans masses) of the collapse candidates. In this scenario, the effect of a weak field is to reduce the probability of a core reaching its thermal Jeans mass, even if it is supercritical. We thus suggest that the SFE may be monotonically reduced as the field strength increases from zero to subcritical values, rather than there being a discontinuous transition between the sub- and supercritical regimes, and that a crucial question to address is whether the turbulence in molecular clouds is driven or decaying, with current observational and theoretical evidence favoring (albeit inconclusively) the driven regime.

The Large‐ScaleJ = 3 → 2 andJ = 2 → 1 CO Emission from M17 and Its Implications for Extragalactic CO Observations

We observed a 10x20 pc region of the molecular cloud M17 in the 12CO and 13CO J=3-2 and J=2-1 transitions to determine their global behavior and to assess the reliability of using ratios of CO line intensities integrated over an entire cloud to determine the physical conditions within the cloud. Both the 12CO/13CO J=2-1 and J=3-2 line ratios correlate with the 13CO integrated intensity, with smaller line ratios observed at locations with large integrated intensities. This correlation is likely due to variations in the column density from one position to another within M17. The 12CO and 13CO (J=3-2/J=2-1) line ratios show no significant variation from place to place within M17, even on the peak of the photon-dominated region. A Large Velocity Gradient analysis of globally averaged line ratios gives results in reasonable agreement with the results obtained for individual lines-of-sight through the cloud, which suggests that the typical physical conditions in a molecular cloud can be determined using CO line ratios integrated over the entire cloud. There appears to be a clear trend of increasing 12CO/13CO J=2-1 and J=3-2 line ratios as one moves from Galactic molecular cloud cores to entire Galactic molecular clouds to normal galaxies. The most likely explanation of the high line ratios for normal galaxies is a significant contribution to the CO emission by low column density material, such as diffuse molecular clouds or the outer envelopes of giant molecular clouds.

Learning attractors in an asynchronous, stochastic electronic neural network.

LANN27 is an electronic device implementing in discrete electronics a fully connected (full feedback) network of 27 neurons and 351 plastic synapses with stochastic Hebbian learning. Both neurons and synapses are dynamic elements, with two time constants--fast for neurons and slow for synapses. Learning, synaptic dynamics, is analogue and is driven in a Hebbian way by neural activities. Long-term memorization takes place on a discrete set of synaptic efficacies and is effected in a stochastic manner. The intense feedback between the nonlinear neural elements, via the learned synaptic structure, creates in an organic way a set of attractors for the collective retrieval dynamics of the neural system, akin to Hebbian learned reverberations. The resulting structure of the attractors is a record of the large-scale statistics in the uncontrolled, incoming flow of stimuli. As the statistics in the stimulus flow changes significantly, the attractors slowly follow it and the network behaves as a palimpsest--old is gradually replaced by new. Moreover, the slow learning creates attractors which render the network a prototype extractor: entire clouds of stimuli, noisy versions of a prototype, used in training, all retrieve the attractor corresponding to the prototype upon retrieval. Here we describe the process of studying the collective dynamics of the network, before, during and following learning, which is rendered complex by the richness of the possible stimulus streams and the large dimensionality of the space of states of the network. We propose sampling techniques and modes of representation for the outcome.

Learning attractors in an asynchronous, stochastic electronic neural network

LANN27 is an electronic device implementing in discrete electronics a fully connected (full feedback) network of 27 neurons and 351 plastic synapses with stochastic Hebbian learning. Both neurons and synapses are dynamic elements, with two time constants—fast for neurons and slow for synapses. Learning, synaptic dynamics, is analogue and is driven in a Hebbian way by neural activities. Long-term memorization takes place on a discrete set of synaptic efficacies and is effected in a stochastic manner. The intense feedback between the nonlinear neural elements, via the learned synaptic structure, creates in an organic way a set of attractors for the collective retrieval dynamics of the neural system, akin to Hebbian learned reverberations. The resulting structure of the attractors is a record of the large-scale statistics in the uncontrolled, incoming flow of stimuli. As the statistics in the stimulus flow changes significantly, the attractors slowly follow it and the network behaves as a palimpsest—old is gradually replaced by new. Moreover, the slow learning creates attractors which render the network a prototype extractor: entire clouds of stimuli, noisy versions of a prototype, used in training, all retrieve the attractor corresponding to the prototype upon retrieval.Here we describe the process of studying the collective dynamics of the network, before, during and following learning, which is rendered complex by the richness of the possible stimulus streams and the large dimensionality of the space of states of the network. We propose sampling techniques and modes of representation for the outcome.

Observations of eruption clouds from Sakura-zima volcano, Kyushu, Japan from Skylab 4

Abstract Hasselblad and Nikon stereographic photographs taken from Skylab between 9 June 1973 and 1 February 1974 give synoptic plan views of several entire eruption clouds emanating from Sakura-zima volcano in Kagoshima Bay, Kyushu, Japan. Analytical plots of these stereographic pairs, studied in combination with meteorological data, indicate that the eruption clouds did not penetrate the tropopause and thus did not create a stratospheric dust veil of long residence time. A horizontal eddy diffusivity of the order of 10 6 cm 2 s −1 and a vertical eddy diffusivity of the order of 10 5 cm 2 s −1 were calculated from the observed plume dimensions and from available meteorological data. These observations are the first, direct evidence that explosive eruption at an estimated energy level of about 10 18 ergs per paroxysm may be too small under atmospheric conditions similar to those prevailing over Sakura-zima for volcanic effluents to penetrate low-level tropospheric temperature inversions and, consequently, the tropopause over northern middle latitudes. Maximum elevation of the volcanic clouds was determined to be 3.4 km. The cumulative thermal energy release in the rise of volcanic plumes for 385 observed explosive eruptions was estimated to be 10 20 to 10 21 ergs (10 13 to 10 14 J), but the entire thermal energy release associated with pyroclastic activity may be of the order of 2.5 × 10 22 ergs (2.5 × 10 15 J). Estimation of the kinetic energy component of explosive eruptions via satellite observation and meteorological consideration of eruption clouds is thus useful in volcanology as an alternative technique to confirm the kinetic energy estimates made by ground-based geological and geophysical methods, and to aid in construction of physical models of potential and historical tephra-fallout sectors with implications for volcano-hazard prediction.

Molecular Clouds in W49 and W51

view Abstract Citations (95) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Molecular Clouds in W49 and W51 Scoville, N. Z. ; Solomon, P. M. Abstract Radio observations of six molecular lines have been obtained in the W49 and W51 H ii region sources as part of an investigation of the physical conditions in molecular clouds and the relationship of these clouds to the H ii regions. The principal observations are maps with 4' spacing of the 6-cm formaldehyde (H2CO) absorption and strip maps with 1' spacing of carbon monoxide (CO, J = 1 0) emission at 2.6 mm. A few selected positions were also observed in 13CO and C18O as well as the carbon monosulfide lines (CS, 3 2 and 2 1) at 2 and 3 mm. Seven distinct clouds are found, and five of these are associated with or near H ii regions. The molecular clouds are all much larger than the H ii regions; one near W51 (at 65 km i) extends over an area containing at least three H ii regions and appears as a self-absorption feature in CO. The mass of three of these molecular clouds is estimated to be at least 10 -10 M0, 10 times as great as the ionized regions. Average hydrogen molecule densities, over the entire clouds, are about 20() 1 000 cm - 3; but near the H ii regions there are probably compact, high-density (> 106 cm-3) molecular regions which are responsible for the high-excitation lines of CS. The excitation temperature of the 6-cm H2CO transition in two clouds in the direction of W49A is estimated to be 1.76 t 1.2 K. The large velocity difference between these clouds and the H ii region and the narrowness of the lines suggest that the clouds are dark nebulae and unrelated to W49A; however, there is H2O maser emission at the velocities of the dark cloud lines suggesting that some of the maser emission may be from foreground objects. Subject headings: molecules, interstellar - nebulae - radio lines - star formation Publication: The Astrophysical Journal Pub Date: February 1973 DOI: 10.1086/151940 Bibcode: 1973ApJ...180...31S full text sources ADS |