Molecular orientational dynamics of the endohedral fullerene Sc3_{3}N@C80_{80} as probed by 13^{13}C and 45^{45}Sc NMR

We measure 13C and 45Sc NMR lineshapes and spin-lattice relaxation times (T1) to probe the orientational dynamics of the endohedral metallofullerene Sc3N@C80. The measurements show an activated behavior for molecular reorientations over the full temperature range with a similar behavior for the temperature dependence of the 13C and 45Sc data. Combined with spectral data from Magic Angle Spinning (MAS) NMR, the measurements can be interpreted to mean the motion of the encapsulated Sc3N molecule is independent of that of the C80 cage, although this requires the similar temperature dependence of the 13C and 45Sc spin-lattice relaxation times to be coincidental. For the Sc3N to be fixed to the C80 cage, one must overcome the symmetry breaking effect this has on the Sc3N@C80 system since this would result in more than the observed two 13C lines.Comment: 6 pages, 5 figure

A storage and access architecture for efficient query processing in spatial database systems

Due to the high complexity of objects and queries and also due to extremely large data volumes, geographic database systems impose stringent requirements on their storage and access architecture with respect to efficient query processing. Performance improving concepts such as spatial storage and access structures, approximations, object decompositions and multi-phase query processing have been suggested and analyzed as single building blocks. In this paper, we describe a storage and access architecture which is composed from the above building blocks in a modular fashion. Additionally, we incorporate into our architecture a new ingredient, the scene organization, for efficiently supporting set-oriented access of large-area region queries. An experimental performance comparison demonstrates that the concept of scene organization leads to considerable performance improvements for large-area region queries by a factor of up to 150

The Multitude of Molecular Hydrogen Knots in the Helix Nebula

We present HST/NICMOS imaging of the H_2 2.12 \mu m emission in 5 fields in the Helix Nebula ranging in radial distance from 250-450" from the central star. The images reveal arcuate structures with their apexes pointing towards the central star. Comparison of these images with comparable resolution ground based images reveals that the molecular gas is more highly clumped than the ionized gas line tracers. From our images, we determine an average number density of knots in the molecular gas ranging from 162 knots/arcmin^2 in the denser regions to 18 knots/arcmin^2 in the lower density outer regions. Using this new number density, we estimate that the total number of knots in the Helix to be ~23,000 which is a factor of 6.5 larger than previous estimates. The total neutral gas mass in the Helix is 0.35 M_\odot assuming a mass of \~1.5x10^{-5} M_\odot for the individual knots. The H_2 intensity, 5-9x10^{-5} erg s^{-1} cm^{-2} sr^{-1}, remains relatively constant with projected distance from the central star suggesting a heating mechanism for the molecular gas that is distributed almost uniformly in the knots throughout the nebula. The temperature and H_2 2.12 \mu m intensity of the knots can be approximately explained by photodissociation regions (PDRs) in the individual knots; however, theoretical PDR models of PN under-predict the intensities of some knots by a factor of 10.Comment: 26 pages, 3 tables, 10 figures; AJ accepte

12^{12}C/13^{13}C ratio in planetary nebulae from the IUE archives

We investigated the abundance ratio of $^{12}$C/$^{13}$C in planetary nebulae by examining emission lines arising from \ion{C}{3} 2s2p ^3P_{2,1,0} \to 2s^2 ^1S_0. Spectra were retrieved from the International Ultraviolet Explorer archives, and multiple spectra of the same object were coadded to achieve improved signal-to-noise. The $^{13}$C hyperfine structure line at 1909.6 \AA was detected in NGC 2440. The $^{12}$C/$^{13}$C ratio was found to be $\sim4.4\pm$1.2. In all other objects, we provide an upper limit for the flux of the 1910 \AA line. For 23 of these sources, a lower limit for the $^{12}$C/$^{13}$C ratio was established. The impact on our current understanding of stellar evolution is discussed. The resulting high signal-to-noise \ion{C}{3} spectrum helps constrain the atomic physics of the line formation process. Some objects have the measured 1907/1909 flux ratio outside the low-electron density theoretical limit for $^{12}$C. A mixture of $^{13}$C with $^{12}$C helps to close the gap somewhat. Nevertheless, some observed 1907/1909 flux ratios still appear too high to conform to the presently predicted limits. It is shown that this limit, as well as the 1910/1909 flux ratio, are predominantly influenced by using the standard partitioning among the collision strengths for the multiplet $^1S_0$--$^3P_J$ according to the statistical weights. A detailed calculation for the fine structure collision strengths between these individual levels would be valuable.Comment: ApJ accepted: 19 pages, 3 Figures, 2 Table

Discrimination between the superconducting gap and the pseudo-gap in Bi2212 from intrinsic tunneling spectroscopy in magnetic field

Intrinsic tunneling spectroscopy in high magnetic field ($H$) is used for a direct test of superconducting features in a quasiparticle density of states of high-$T_c$ superconductors. We were able to distinguish with a great clarity two co-existing gaps: (i) the superconducting gap, which closes as $H \to H_{c2}(T)$ and $T\to T_c(H)$, and (ii) the $c$-axis pseudo-gap, which does not change neither with $H$, nor $T$. Strikingly different magnetic field dependencies, together with previously observed different temperature dependencies of the two gaps ~\cite{Krasnov}, speak against the superconducting origin of the pseudo-gap.Comment: 4 pages, 4 eps figure

Magnetic field independence of the spin gap in YBa_2Cu_3O_{7-delta}

We report, for magnetic fields of 0, 8.8, and 14.8 Tesla, measurements of the temperature dependent ^{63}Cu NMR spin lattice relaxation rate for near optimally doped YBa_2Cu_3O_{7-delta}, near and above T_c. In sharp contrast with previous work we find no magnetic field dependence. We discuss experimental issues arising in measurements of this required precision, and implications of the experiment regarding issues including the spin or pseudo gap.Comment: 4 pages, 3 figures, as accepted for publication in Physical Review Letter

A New Way to Detect Massive Black Holes in Galaxies: The Stellar Remnants of Tidal Disruption

We point out that the tidal disruption of a giant may leave a luminous (10^35-10^39 ergs/s), hot (10-100 eV) stellar core. The ``supersoft'' source detected by Chandra at the center of M31 may be such a core; whether or not it is, the observations have shown that such a core is detectable, even in the center of a galaxy. We therefore explore the range of expected observational signatures and how they may be used to (1) test the hypothesis that the M31 source is a remnant of tidal stripping and (2) discover evidence of black holes and disruption events in other galaxies.Comment: Four pages with 1 figure. Appeared in ApJL (2001, 551, L37

Kinematic and morphological modeling of the bipolar nebula Sa2-237

We present [OIII]500.7nm and Halpha+[NII] images and long-slit, high resolution echelle spectra in the same spectral regions of Sa2--237, a possible bipolar planetary nebula. The image shows a bipolar nebula of about 34" extent, with a narrow waist, and showing strong point symmetry about the central object, indicating it's likely binary nature. The long slit spectra were taken over the long axis of the nebula, and show a distinct ``eight'' shaped pattern in the velocity--space plot, and a maximum projected outflow velocity of V=106km/s, both typical of expanding bipolar planetary nebulae. By model fitting the shape and spectrum of the nebula simultaneously, we derive the inclination of the long axis to be 70 degrees, and the maximum space velocity of expansion to be 308 km/s. Due to asymmetries in the velocities we adopt a new value for the system's heliocentric radial velocity of -30km/s. We use the IRAS and 21cm radio fluxes, the energy distribution, and the projected size of Sa2-237 to estimate it's distance to be 2.1+-0.37kpc. At this distance Sa2-237 has a luminosity of 340 Lsun, a size of 0.37pc, and -- assuming constant expansion velocity -- a nebular age of 624 years. The above radial velocity and distance place Sa2--237 in the disk of the Galaxy at z=255pc, albeit with somewhat peculiar kinematics.Comment: 10pp, 4 fig

A New Look At Carbon Abundances In Planetary Nebulae. IV. Implications For Stellar Nucleosynthesis

This paper is the fourth and final report on a project designed to study carbon abundances in a sample of planetary nebulae representing a broad range in progenitor mass and metallicity. We present newly acquired optical spectrophotometric data for three Galactic planetary nebulae IC 418, NGC 2392, and NGC 3242 and combine them with UV data from the IUE Final Archive for identical positions in each nebula to determine accurate abundances of He, C, N, O, and Ne at one or more locations in each object. We then collect abundances of these elements for the entire sample and compare them with theoretical predictions of planetary nebula abundances from a grid of intermediate mass star models. We find some consistency between observations and theory, lending modest support to our current understanding of nucleosynthesis in stars below 8 M_o in birth mass. Overall, we believe that observed abundances agree with theoretical predictions to well within an order of magnitude but probably not better than within a factor of 2 or 3. But even this level of consistency between observation and theory enhances the validity of published intermediate-mass stellar yields of carbon and nitrogen in the study of the abundance evolution of these elements.Comment: 41 pages, 11 figures. Accepted for publication in the Astrophysical Journa

X-ray Imaging of Planetary Nebulae with Wolf-Rayet-type Central Stars: Detection of the Hot Bubble in NGC 40

We present the results of Chandra X-ray Observatory (CXO) observations of the planetary nebulae (PNs) NGC 40 and Hen 2-99. Both PNs feature late-type Wolf-Rayet central stars that are presently driving fast ~1000 km/s, massive winds into denser, slow-moving (~10 km/s) material ejected during recently terminated asymptotic giant branch (AGB) evolutionary phases. Hence, these observations provide key tests of models of wind-wind interactions in PNs. In NGC 40, we detect faint, diffuse X-ray emission distributed within a partial annulus that lies nested within a ~40'' diameter ring of nebulosity observed in optical and near-infrared images. Hen 2-99 is undetected. The inferred X-ray temperature (T_X ~10^6 K) and luminosity (L_X ~ 2 X 10^30 ergs/s) of NGC 40 are the lowest measured thus far for any PN displaying diffuse X-ray emission. These results, combined with the ring-like morphology of the X-ray emission from NGC 40, suggest that its X-ray emission arises from a ``hot bubble'' that is highly evolved and is generated by a shocked, quasi-spherical fast wind from the central star, as opposed to AGB or post-AGB jet activity. In constrast, the lack of detectable X-ray emission from Hen 2-99 suggests that this PN has yet to enter a phase of strong wind-wind shocks.Comment: 15 pages, 5 figures to appear in The Astrophysical Journa