HI ``Tails'' from Cometary Globules in IC1396

IC 1396 is a relatively nearby (750 pc), large (>2 deg), HII region ionized by a single O6.5V star and containing bright-rimmed cometary globules. We have made the first arcmin resolution images of atomic hydrogen toward IC 1396, and have found remarkable ``tail''-like structures associated with some of the globules and extending up to 6.5 pc radially away from the central ionizing star. These HI ``tails'' may be material which has been ablated from the globule through ionization and/or photodissociation and then accelerated away from the globule by the stellar wind, but which has since drifted into the ``shadow'' of the globules. This report presents the first results of the Galactic Plane Survey Project recently begun by the Dominion Radio Astrophysical Observatory.Comment: 11 pages, 5 postscript figures, uses aaspp4.sty macros, submitted in uuencoded gzipped tar format, accepted for publication in Astrophysical Journal Letters, colour figures available at http://www.drao.nrc.ca/~schieven/news_sep95/ic1396.htm

Spectral Index of the Diffuse Radio Background Measured From 100 to 200 MHz

The mean absolute brightness temperature of the diffuse radio background was measured as a function of frequency in a continuous band between 100 and 200 MHz over an effective solid angle of ~pi str at high Galactic latitude. A spectral brightness temperature index of beta = 2.5 +/- 0.1 (alpha_s = 0.5) was derived from the observations, where the error limits are 3-sigma and include estimates of the instrumental systematics. Zenith drift scans with central declinations of -26.5 degrees and spanning right ascensions 0 to 10 hours yielded little variation in the mean spectral index. The mean absolute brightness temperature at 150 MHz was found to reach a minimum of T = 237 +/- 10 K at a right ascension of 2.5 hours. Combining these measurements with those of Haslam et al. 1982 yields a spectral index of beta = 2.52 +/- 0.04 between 150 and 408 MHz.Comment: 8 pages including 7 figures and 4 tables. Accepted by A

G55.0+0.3: A Highly Evolved Supernova Remnant

Multi-frequency analysis has revealed the presence of a new supernova remnant, G55.0+0.3, in the Galactic plane. A kinematic distance of 14 kpc has been measured from HI spectral line data. The faint, clumpy half-shell is non-thermal and has a physical radius of 70 pc. Using an evolutionary model, the age of the remnant is estimated to be on the order of one million years, which exceeds conventional limits by a factor of five. The remnant may be associated with the nearby pulsar J1932+2020, which has a spin-down age of 1.1 million years. This work implies that the radiative lifetimes of remnants could be much longer than previously suggested.Comment: 27 pages, 7 figures in 9 files (figures 1 and 2 require 2 files each), Accepted for publication in The Astrophysical Journal (Jan. 20, 1998 volume

Exciton binding energies in carbon nanotubes from two-photon photoluminescence

One- and two-photon luminescence excitation spectroscopy showed a series of distinct excitonic states in single-walled carbon nanotubes. The energy splitting between one- and two-photon-active exciton states of different wavefunction symmetry is the fingerprint of excitonic interactions in carbon nanotubes. We determine exciton binding energies of 0.3-0.4 eV for different nanotubes with diameters between 0.7 and 0.9 nm. Our results, which are supported by ab-initio calculations of the linear and non-linear optical spectra, prove that the elementary optical excitations of carbon nanotubes are strongly Coulomb-correlated, quasi-one dimensionally confined electron-hole pairs, stable even at room temperature. This alters our microscopic understanding of both the electronic structure and the Coulomb interactions in carbon nanotubes, and has direct impact on the optical and transport properties of novel nanotube devices.Comment: 5 pages, 4 figure

Scaling of the conductance in gold nanotubes

A new form of gold nanobridges has been recently observed in ultrahigh-vacuum experiments, where the gold atoms rearrange to build helical nanotubes, akin in some respects to carbon nanotubes. The good reproducibility of these wires and their unexpected stability will allow for conductance measurements and make them promising candidates for future applications . We present here a study of the transport properties of these nanotubes in order to understand the role of chirality and of the different orbitals in quantum transport observables. The conductance per atomic row shows a light decreasing trend as the diameter grows, which is also shown through an analytical formula based on a one-orbital model.Comment: 5 pages, 6 figure

DA495 - an aging pulsar wind nebula

We present a radio continuum study of the pulsar wind nebula (PWN) DA 495 (G65.7+1.2), including images of total intensity and linear polarization from 408 to 10550 MHz based on the Canadian Galactic Plane Survey and observations with the Effelsberg 100-m Radio Telescope. Removal of flux density contributions from a superimposed \ion{H}{2} region and from compact extragalactic sources reveals a break in the spectrum of DA 495 at 1.3 GHz, with a spectral index ${\alpha}={-0.45 \pm 0.20}$ below the break and ${\alpha}={-0.87 \pm 0.10}$ above it (${S}_\nu \propto{\nu^{\alpha}}$). The spectral break is more than three times lower in frequency than the lowest break detected in any other PWN. The break in the spectrum is likely the result of synchrotron cooling, and DA 495, at an age of $\sim$20,000 yr, may have evolved from an object similar to the Vela X nebula, with a similarly energetic pulsar. We find a magnetic field of $\sim$1.3 mG inside the nebula. After correcting for the resulting high internal rotation measure, the magnetic field structure is quite simple, resembling the inner part of a dipole field projected onto the plane of the sky, although a toroidal component is likely also present. The dipole field axis, which should be parallel to the spin axis of the putative pulsar, lies at an angle of {\sim}50\degr east of the North Celestial Pole and is pointing away from us towards the south-west. The upper limit for the radio surface brightness of any shell-type supernova remnant emission around DA 495 is $\Sigma_{1 GHz} \sim 5.4 \times 10^{-23}$ OAWatt m$^{-2}$ Hz$^{-1}$ sr$^{-1}$ (assuming a radio spectral index of $\alpha = -0.5$), lower than the faintest shell-type remnant known to date.Comment: 25 pages, accepted by Ap

TRIS III: the diffuse galactic radio emission at δ=+42∘\delta=+42^{\circ}

We present values of temperature and spectral index of the galactic diffuse radiation measured at 600 and 820 MHz along a 24 hours right ascension circle at declination $\delta = +42^{\circ}$. They have been obtained from a subset of absolute measurements of the sky temperature made with TRIS, an experiment devoted to the measurement of the Cosmic Microwave Background temperature at decimetric-wavelengths with an angular resolution of about $20^{\circ}$. Our analysis confirms the preexisting picture of the galactic diffuse emission at decimetric wavelength and improves the accuracy of the measurable quantities. In particular, the signal coming from the halo has a spectral index in the range $2.9-3.1$ above 600 MHz, depending on the sky position. In the disk, at TRIS angular resolution, the free-free emission accounts for the 11% of the overall signal at 600 MHz and 21% at 1420 MHz. The polarized component of the galactic emission, evaluated from the survey by Brouw and Spoelstra, affects the observations at TRIS angular resolution by less than 3% at 820 MHz and less than 2% at 600 MHz. Within the uncertainties, our determination of the galactic spectral index is practically unaffected by the correction for polarization. Since the overall error budget of the sky temperatures measured by TRIS at 600 MHz, that is 66 mK(systematic)$+$18 mK (statistical), is definitely smaller than those reported in previous measurements at the same frequency, our data have been used to discuss the zero levels of the sky maps at 150, 408, 820 and 1420 MHz in literature. Concerning the 408 MHz survey, limiting our attention to the patch of sky corresponding to the region observed by TRIS, we suggest a correction of the base-level of $(+3.9\pm 0.6)$K.Comment: Accepted for publication in the Astrophysical Journa

Gamma-ray Flares and VLBI Outbursts of Blazars

A model is developed for the time dependent electromagnetic - radio to gamma-ray - emission of active galactic nuclei, specifically, the blazars, based on the acceleration and creation of leptons at a propagating discontinuity or {\it front} of a Poynting flux jet. The front corresponds to a discrete relativistic jet component as observed with very-long-baseline-interferometry (VLBI). Equations are derived for the number, momentum, and energy of particles in the front taking into account synchrotron, synchrotron-self-Compton (SSC), and inverse-Compton processes as well as photon-photon pair production. The apparent synchrotron, SSC, and inverse-Compton luminosities as functions of time are determined. Predictions of the model are compared with observations in the gamma, optical and radio bands. The delay between the high-energy gamma-ray flare and the onset of the radio is explained by self-absorption and/or free-free absorption by external plasma. Two types of gamma-ray flares are predicted depending on pair creation in the front.Comment: 11 pages, submitted to ApJ. 10 figures can be obtained from R. Lovelace by sending postal address to [email protected]