Validation of magnetophonon spectroscopy as a tool for analyzing hot-electron effects in devices

It is shown that very high precision hot-electron magnetophonon experiments made on n+n−n+-GaAs sandwich device structures which are customized for magnetoresistance measurements can be very accurately modeled by a new Monte Carlo technique. The latter takes account of the Landau quantization and device architecture as well as material parameters. It is proposed that this combination of experiment and modeling yields a quantitative tool for the direct analysis of spatially localized very nonequilibrium electron distributions in small devices and low dimensional structures

A Training Framework and Follow-Up Observations for Multiculturally Inclusive Teaching: Is Believing That We are Emphasizing Diversity Enough?

The authors present a theoretically and empirically grounded training for multiculturally inclusive teaching for new instructors. After implementing this training, qualitative data were gathered from instructors to identify their experience of the training and concerns related to incorporating issues of diversity into their classrooms (Study 1). At the end of the semester immediately following the training, quantitative data were gathered from instructors and their students to examine the interaction between students’ and instructors’ perceived diversity emphasis (Study 2). When allowed to choose the extent to which they incorporated issues of diversity in their classes, the instructors differentially reported emphasizing diversity in class. In addition, results from multi-level linear modeling analyses demonstrated that instructors’ reported emphasis on diversity in the classroom did not predict students’ perceptions of the inclusion of issues of diversity. The authors discuss implications for the development of multiculturally supportive programs of learning at universities

Distribution ofWater Vapor in Molecular Clouds

We report the results of a large-area study of water vapor along the Orion Molecular Cloud ridge, the purpose of which was to determine the depth-dependent distribution of gas-phase water in dense molecular clouds. We find that the water vapor measured toward 77 spatial positions along the face-on Orion ridge, excluding positions surrounding the outflow associated with BN/KL and IRc2, display integrated intensities that correlate strongly with known cloud surface tracers such as CN, C2H, 13CO J = 5-4, and HCN, and less well with the volume tracer N2H+. Moreover, at total column densities corresponding to A V\u3c 15 mag, the ratio of H2O to C18O integrated intensities shows a clear rise approaching the cloud surface. We show that this behavior cannot be accounted for by either optical depth or excitation effects, but suggests that gas-phase water abundances fall at large A V. These results are important as they affect measures of the true water-vapor abundance in molecular clouds by highlighting the limitations of comparing measured water-vapor column densities with such traditional cloud tracers as 13CO or C18O. These results also support cloud models that incorporate freeze out of molecules as a critical component in determining the depth-dependent abundance of water vapor

First Passage Properties of the Erdos-Renyi Random Graph

We study the mean time for a random walk to traverse between two arbitrary sites of the Erdos-Renyi random graph. We develop an effective medium approximation that predicts that the mean first-passage time between pairs of nodes, as well as all moments of this first-passage time, are insensitive to the fraction p of occupied links. This prediction qualitatively agrees with numerical simulations away from the percolation threshold. Near the percolation threshold, the statistically meaningful quantity is the mean transit rate, namely, the inverse of the first-passage time. This rate varies non-monotonically with p near the percolation transition. Much of this behavior can be understood by simple heuristic arguments.Comment: 10 pages, 9 figures, 2-column revtex4 forma

Embedded Stellar Clusters in the W3/W4/W5 Molecular Cloud Complex

We analyze the embedded stellar content in the vicinity of the W3/W4/W5 HII regions using the FCRAO Outer Galaxy 12CO(J=1-0) Survey, the IRAS Point Source Catalog, published radio continuum surveys, and new near-infrared and molecular line observations. Thirty-four IRAS Point Sources are identified that have far-infrared colors characteristic of embedded star forming regions, and we have obtained K' mosaics and 13CO(J=1-0) maps for 32 of them. Ten of the IRAS sources are associated with an OB star and 19 with a stellar cluster, although three OB stars are not identified with a cluster. Half of the embedded stellar population identified in the K' images is found in just the 5 richest clusters, and 61% is contained in IRAS sources associated with an embedded OB star. Thus rich clusters around OB stars contribute substantially to the stellar population currently forming in the W3/W4/W5 region. Approximately 39% of the cluster population is embedded in small clouds with an average mass of ~130 Mo that are located as far as 100 pc from the W3/W4/W5 cloud complex. We speculate that these small clouds are fragments of a cloud complex dispersed by previous episodes of massive star formation. Finally, we find that 4 of the 5 known embedded massive star forming sites in the W3 molecular cloud are found along the interface with the W4 HII region despite the fact that most of the molecular mass is contained in the interior regions of the cloud. These observations are consistent with the classical notion that the W4 HII region has triggered massive star formation along the eastern edge of the W3 molecular cloud.Comment: to appear in ApJS, see http://astro.caltech.edu/~jmc/papers/w

SWAS Observations of Water in Molecular Outflows

We present SWAS detections of the ground-state 1(10)-1(01) transition of o-H2O at 557 GHz in 18 molecular outflows. These results are combined with ground-based observations of the J=1-0 transitions of 12CO and 13CO obtained at the FCRAO and, for a subset of the outflows, data from ISO. Assuming the SWAS water line emission originates from the same gas traced by CO emission, we find that the outflowing gas in most outflows has an o-H2O abundance relative to H2 of between 10(-7) and 10(-6). Analysis of the water abundance as a function of outflow velocity reveals a strong dependence. The water abundance increases with velocity, and at the highest outflow velocities some outflows have relative o-H2O abundances of order 10(-4). However the mass of gas with such elevated water abundances represents less that 1% of the total outflow gas mass. The ISO LWS observations of high-J rotational lines of CO and the 179.5 micron transition of o-H2O provide evidence for a warmer outflow component than required to produce either the SWAS or FCRAO lines. The mass associated with the ISO emission is similar to that responsible for the highest velocity water emission detected by SWAS. The bulk of the outflowing gas has an abundance of o-H2O well below what would be expected if the gas has passed through a C-shock with shock velocities greater than 10 km/s. Gas-phase water can be depleted in the post-shock gas due to freeze-out onto grain mantles, however the rate of freeze-out is too slow to explain our results. Therefore we believe that only a small fraction of the outflowing molecular gas has passed through shocks strong enough to fully convert the gas-phase oxygen to water. This result has implications for the acceleration mechanism of the molecular gas in these outflows.Comment: Accepted for publication in Ap.J., 51 pages including 4 pages of figure

A Search for Small-Scale Clumpiness in Dense Cores of Molecular Clouds

We have analyzed HCN(1-0) and CS(2-1) line profiles obtained with high signal-to-noise ratios toward distinct positions in three selected objects in order to search for small-scale structure in molecular cloud cores associated with regions of high-mass star formation. In some cases, ripples were detected in the line profiles, which could be due to the presence of a large number of unresolved small clumps in the telescope beam. The number of clumps for regions with linear scales of ~0.2-0.5 pc is determined using an analytical model and detailed calculations for a clumpy cloud model; this number varies in the range: ~2 10^4-3 10^5, depending on the source. The clump densities range from ~3 10^5-10^6 cm^{-3}, and the sizes and volume filling factors of the clumps are ~(1-3) 10^{-3} pc and ~0.03-0.12. The clumps are surrounded by inter-clump gas with densities not lower than ~(2-7) 10^4 cm^{-3}. The internal thermal energy of the gas in the model clumps is much higher than their gravitational energy. Their mean lifetimes can depend on the inter-clump collisional rates, and vary in the range ~10^4-10^5 yr. These structures are probably connected with density fluctuations due to turbulence in high-mass star-forming regions.Comment: 23 pages including 4 figures and 4 table

Submillimeter Wave Astronomy Satellite observations of comet 9P/Tempel 1 and Deep Impact

On 4 July 2005 at 5:52 UT the Deep Impact mission successfully completed its goal to hit the nucleus of 9P/Tempel 1 with an impactor, forming a crater on the nucleus and ejecting material into the coma of the comet. NASA's Submillimeter Wave Astronomy Satellite (SWAS) observed the 1(10)-1(01) ortho-water ground-state rotational transition in comet 9P/Tempel 1 before, during, and after the impact. No excess emission from the impact was detected by SWAS and we derive an upper limit of 1.8e7 kg on the water ice evaporated by the impact. However, the water production rate of the comet showed large natural variations of more than a factor of three during the weeks before and after the impact. Episodes of increased activity with Q(H2O)~1e28 molecule/s alternated with periods with low outgassing (Q(H2O)<~5e27 molecule/s). We estimate that 9P/Tempel 1 vaporized a total of N~4.5e34 water molecules (~1.3e9 kg) during June-September 2005. Our observations indicate that only a small fraction of the nucleus of Tempel 1 appears to be covered with active areas. Water vapor is expected to emanate predominantly from topographic features periodically facing the Sun as the comet rotates. We calculate that appreciable asymmetries of these features could lead to a spin-down or spin-up of the nucleus at observable rates.Comment: 38 pages, 2 tables, 7 figures; Icarus, in pres

One-carbon metabolism in cancer

Cells require one-carbon units for nucleotide synthesis, methylation and reductive metabolism, and these pathways support the high proliferative rate of cancer cells. As such, anti-folates, drugs that target one-carbon metabolism, have long been used in the treatment of cancer. Amino acids, such as serine are a major one-carbon source, and cancer cells are particularly susceptible to deprivation of one-carbon units by serine restriction or inhibition of de novo serine synthesis. Recent work has also begun to decipher the specific pathways and sub-cellular compartments that are important for one-carbon metabolism in cancer cells. In this review we summarise the historical understanding of one-carbon metabolism in cancer, describe the recent findings regarding the generation and usage of one-carbon units and explore possible future therapeutics that could exploit the dependency of cancer cells on one-carbon metabolism

Water Abundance in Molecular Cloud Cores

We present Submillimeter Wave Astronomy Satellite (SWAS) observations of the 1_{10}-1_{01} transition of ortho-water at 557 GHz toward 12 molecular cloud cores. The water emission was detected in NGC 7538, Rho Oph A, NGC 2024, CRL 2591, W3, W3(OH), Mon R2, and W33, and was not detected in TMC-1, L134N, and B335. We also present a small map of the water emission in S140. Observations of the H_2^{18}O line were obtained toward S140 and NGC 7538, but no emission was detected. The abundance of ortho-water relative to H_2 in the giant molecular cloud cores was found to vary between 6x10^{-10} and 1x10^{-8}. Five of the cloud cores in our sample have previous water detections; however, in all cases the emission is thought to arise from hot cores with small angular extents. The water abundance estimated for the hot core gas is at least 100 times larger than in the gas probed by SWAS. The most stringent upper limit on the ortho-water abundance in dark clouds is provided in TMC-1, where the 3-sigma upper limit on the ortho-water fractional abundance is 7x10^{-8}.Comment: 5 pages, 3 Postscript figures, uses aastex.cls, emulateapj5.sty (included), and apjfonts.sty (included