Line-of-Sight Reddening Predictions: Zero Points, Accuracies, the Interstellar Medium, and the Stellar Populations of Elliptical Galaxies

Revised (B-V)_0-Mg_2 data for 402 elliptical galaxies are given to test reddening predictions which can also tell us both what the intrinsic errors are in this relationship among gE galaxy stellar populations, as well as details of nearby structure in the interstellar medium (ISM) of our Galaxy and of the intrinsic errors in reddening predictions. Using least-squares fits, the explicit 1-sigma errors in the Burstein-Heiles (BH) and the Schlegel et al. (IR) predicted reddenings are calculated, as well as the 1-sigma observational error in the (B-V)_0-Mg_2 for gE galaxies. It is found that, in directions with E(B-V)<0.100 mag (where most of these galaxies lie), 1-sigma errors in the IR reddening predictions are 0.006 to 0.009 in E(B-V) mag, those for BH reddening prediction are 0.011 mag, and the 1-sigma agreement between the two reddening predictions is 0.007 mag. IR predictions have an accuracy of 0.010-0.011 mag in directions with E(B-V)>= 0.100 mag, significantly better than those of the BH predictions (0.024-0.025). Gas-to-dust variations that vary by a factor of 3, both high and low, exist along many lines-of-sight in our Galaxy. The approx 0.02 higher reddening zero point in E(B-V) previously determined by Schlegel et al. is confirmed, primarily at the Galactic poles. Despite this, both methods also predict many directions with E(B-V)<0.015 mag. Independent evidence of reddening at the North Galactic pole is reviewed, with the conclusion that there still exists directions at the NGP that have E(B-V)<<0.01. Two lines of evidence suggest that IR reddenings are overpredicted in directions with high gas-to-dust ratios. As high gas-to-dust directions in the ISM also include the Galactic poles, this overprediction is the likely cause of the E(B-V) = 0.02 mag larger IR reddening zero point.Comment: 5 figure

9286 Stars: An Agglomeration of Stellar Polarization Catalogs

This is a revision. The revisions are minor. The new version of the catalog should be used in preference to the old. The most serious error in the older version was that $\theta_diff$ was incorrect, being sometimes far too large, for Reiz and Franco entries; the correct values are all zero for that reference. We present an agglomeration of stellar polarization catalogs with results for 9286 stars. We have endeavored to eliminate errors, provide accurate (arcsecond) positions, sensibly weight multiple observations of the same star, and provide reasonable distances. This catalog is included as an ASCII file (catalog.txt) in the source of this submission.Comment: The most serious error in the older version was that $\theta_diff$ was incorrect, being sometimes far too large, for Reiz and Franco entries; the correct values are all zero for that reference. 11 pages, no figures. Accepted for Astronomical Journal. Catalog also available as an ASCII file by anonymous FTP from ftp://vermi.berkeley.edu/pub/polcat/p14.ou

Carbon Nanotubes as Schottky Barrier Transistors

We show that carbon nanotube transistors operate as unconventional "Schottky barrier transistors", in which transistor action occurs primarily by varying the contact resistance rather than the channel conductance. Transistor characteristics are calculated for both idealized and realistic geometries, and scaling behavior is demonstrated. Our results explain a variety of experimental observations, including the quite different effects of doping and adsorbed gases. The electrode geometry is shown to be crucial for good device performance.Comment: 4 pages, 5 figures, appears in Physical Review Letter

Lateral scaling in carbon nanotube field-effect transistors

We have fabricated carbon nanotube (CN) field-effect transistors with multiple, individually addressable gate segments. The devices exhibit markedly different transistor characteristics when switched using gate segments controlling the device interior versus those near the source and drain. We ascribe this difference to a change from Schottky barrier modulation at the contacts to bulk switching. We also find that the current through the bulk portion is independent of gate length for any gate voltage, offering direct evidence for ballistic transport in semiconducting CNs over at least a few hundred nanometers, even for relatively small carrier velocities.Comment: 4 pages, 4 figure

The First VLT FORS1 spectra of Lyman-break candidates in the HDF-S and AXAF Deep Field

We report on low-resolution multi-object spectroscopy of 30 faint targets (R \~ 24-25) in the HDF-S and AXAF deep field obtained with the VLT Focal Reducer/low dispersion Spectrograph (FORS1). Eight high-redshift galaxies with 2.75< z < 4 have been identified. The spectroscopic redshifts are in good agreement with the photometric ones with a dispersion $\sigma_z = 0.07$ at z<2 and $\sigma_z = 0.16$ at z>2. The inferred star formation rates of the individual objects are moderate, ranging from a few to a few tens solar masses per year. Five out of the eight high-z objects do not show prominent emission lines. One object has a spectrum typical of an AGN. In the AXAF field two relatively close pairs of galaxies have been identified, with separations of 8.7 and 3.1 proper Mpc and mean redshifts of 3.11 and 3.93, respectively.Comment: 5 pages Latex, with 2 PostScript figures. Astronomy and Astrophysics, in pres

The Araucaria Project: the Local Group Galaxy WLM--Distance and metallicity from quantitative spectroscopy of blue Supergiants

The quantitative analysis of low resolution spectra of A and B supergiants is used to determine a distance modulus of 24.99 +/- 0.10 mag (995 +/- 46 Kpc) to the Local Group galaxy WLM. The analysis yields stellar effective temperatures and gravities, which provide a distance through the Flux weighted Gravity--Luminosity Relationship (FGLR). Our distance is 0.07 mag larger than the most recent results based on Cepheids and the tip of the RGB. This difference is within the 1-sigma overlap of the typical uncertainties quoted in these photometric investigations. In addition, non-LTE spectral synthesis of the rich metal line spectra (mostly iron, chromium and titanium) of the A supergiants is carried out, which allows the determination of stellar metallicities. An average metallicity of -0.87 +/- 0.06 dex with respect to solar metallicity is found.Comment: 46 pages, 14 figures (2 low resolution color figures). Accepted for publication by Ap

On the stability of very massive primordial stars

The stability of metal-free very massive stars ($Z$ = 0; M = 120 - 500 \msol) is analyzed and compared with metal-enriched stars. Such zero-metal stars are unstable to nuclear-powered radial pulsations on the main sequence, but the growth time scale for these instabilities is much longer than for their metal-rich counterparts. Since they stabilize quickly after evolving off the ZAMS, the pulsation may not have sufficient time to drive appreciable mass loss in Z = 0 stars. For reasonable assumptions regarding the efficiency of converting pulsational energy into mass loss, we find that, even for the larger masses considered, the star may die without losing a large fraction of its mass. We find a transition between the $\epsilon$- and $\kappa$-mechanisms for pulsational instability at Z\sim 2\E{-4} - 2\E{-3}. For the most metal-rich stars, the $\kappa$-mechanism yields much shorter $e$-folding times, indicating the presence of a strong instability. We thus stress the fundamental difference of the stability and late stages of evolution between very massive stars born in the early universe and those that might be born today.Comment: 7 pages, 5 figures. Minor changes, more results given in Table 1, accepted for publication in Ap

Unexpected Scaling of the Performance of Carbon Nanotube Transistors

We show that carbon nanotube transistors exhibit scaling that is qualitatively different than conventional transistors. The performance depends in an unexpected way on both the thickness and the dielectric constant of the gate oxide. Experimental measurements and theoretical calculations provide a consistent understanding of the scaling, which reflects the very different device physics of a Schottky barrier transistor with a quasi-one-dimensional channel contacting a sharp edge. A simple analytic model gives explicit scaling expressions for key device parameters such as subthreshold slope, turn-on voltage, and transconductance.Comment: 4 pages, 4 figure