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

Abundances of planetary nebulae in the Galactic bulge

Context. Planetary nebulae (PNe) abundances are poorly known for those nebulae in the Galactic bulge. This is because of the high and uneven extinction in the bulge which makes visual spectral measurements difficult. In addition, the extinction corrections may be unreliable. Elements considered are O, N, Ne, S, Ar, and Cl. Aims. We determine the abundances in 19 PNe, 18 of which are located in the bulge. This doubles the number of PNe abundance determinations in the bulge. The Galactic abundance gradient is discussed for five elements. Methods. The mid-infrared spectra measured by the Spitzer Space Telescope are used to determine the abundances. This part of the spectrum is little affected by extinction for which an uncertain correction is no longer necessary. In addition the connection with the visible and ultraviolet spectrum becomes simpler because hydrogen lines are observed both in the infrared and in the visible spectra. In this way we more than double the number of PNe with reliable abundances. Results. Reliable abundances are obtained for O, N, Ne, S, and Ar for Galactic bulge PNe. Conclusions. The Galactic abundance gradient is less steep than previously thought. This is especially true for oxygen. The sulfur abundance is reliable because all stages of ionization expected have been measured. It is not systematically low compared to oxygen as has been found for some Galactic PNe

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

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

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

High-velocity collimated outflows in planetary nebulae: NGC 6337, He 2-186, and K 4-47

We have obtained narrow-band images and high-resolution spectra of the planetary nebulae NGC 6337, He 2-186, and K 4-47, with the aim of investigating the relation between their main morphological components and several low-ionization features present in these nebulae. The data suggest that NGC 6337 is a bipolar PN seen almost pole on, with polar velocities higher than 200 km/s. The bright inner ring of the nebula is interpreted to be the "equatorial" density enhancement. It contains a number of low-ionization knots and outward tails that we ascribe to dynamical instabilities leading to fragmentation of the ring or transient density enhancements due to the interaction of the ionization front with previous density fluctuations in the ISM. The lobes show a pronounced point-symmetric morphology and two peculiar low-ionization filaments whose nature remains unclear. The most notable characteristic of He 2-186 is the presence of two high-velocity (higher than 135 km/s) knots from which an S-shaped lane of emission departs toward the central star. K 4-47 is composed of a compact core and two high-velocity, low-ionization blobs. We interpret the substantial broadening of line emission from the blobs as a signature of bow shocks, and using the modeling of Hartigan, Raymond, & Hartman (1987), we derive a shock velocity of 150 km/s and a mild inclination of the outflow on the plane of the sky. We discuss possible scenarios for the formation of these nebulae and their low-ionization features. In particular, the morphology of K 4-47 hardly fits into any of the usually adopted mass-loss geometries for single AGB stars. Finally, we discuss the possibility that point-symmetric morphologies in the lobes of NGC 6337 and the knots of He 2-186 are the result of precessing outflows from the central stars.Comment: 16 pages plus 7 figures, ApJ accepted. Also available at http://www.iac.es/publicaciones/preprints.htm

Morphology and Composition of the Helix Nebula

We present new narrow-band filter imagery in H-alpha and [N II] 6584 along with UV and optical spectrophotometry measurements from 1200 to 9600 Angstroms of NGC 7293, the Helix Nebula, a nearby, photogenic planetary nebula of large diameter and low surface brightness. Detailed models of the observable ionized nebula support the recent claim that the Helix is actually a flattened disk whose thickness is roughly one-third its diameter with an inner region containing hot, highly ionized gas which is generally invisible in narrow-band images. The outer visible ring structure is of lower ionization and temperature and is brighter because of a thickening in the disk. We also confirm a central star effective temperature and luminosity of 120,000K and 100L_sun, and we estimate a lower limit to the nebular mass to be 0.30M_sun. Abundance measurements indicate the following values: He/H=0.12 (+/-0.017), O/H=4.60x10^-4 (+/-0.18), C/O=0.87 (+/-0.12), N/O=0.54 (+/-0.14), Ne/O=0.33 (+/-0.04), S/O=3.22x10^-3 (+/-0.26), and Ar/O=6.74x10^-3 (+/-0.76). Our carbon abundance measurements represent the first of their kind for the Helix Nebula. The S/O ratio which we derive is anomalously low; such values are found in only a few other planetary nebulae. The central star properties, the super-solar values of He/H and N/O, and a solar level of C/O are consistent with a 6.5M_sun progenitor which underwent three phases of dredge-up and hot bottom burning before forming the planetary nebula.Comment: 50-page manuscript plus 11 postscript figures. This revised version corrects a typo in earlier submission. Nothing else has changed. Accepted for publication in the Astrophysical Journa

The physical parameters, excitation and chemistry of the rim, jets and knots of the planetary nebula NGC 7009

We present long-slit optical spectra along the major axis of the planetary nebula NGC 7009. These data allow us to discuss the physical, excitation and chemical properties of all the morphological components of the nebula, including its remarkable systems of knots and jets. The main results of this analysis are the following: i) the electron temperature throughout the nebula is remarkably constant, T_e[OIII] = 10200K; ii) the bright inner rim and inner pair of knots have similar densities of N_e = 6000cm^{-3}, whereas a much lower density of N_e = 1500cm^{-3} is derived for the outer knots as well as for the jets; iii) all the regions (rim, inner knots, jets and outer knots) are mainly radiatively excited; and iv) there are no clear abundance changes across the nebula for He, O, Ne, or S. There is a marginal evidence for an overabundance of nitrogen in the outer knots (ansae), but the inner ones (caps) and the rim have similar N/H values that are at variance with previous results. Our data are compared to the predictions of theoretical models, from which we conclude that the knots at the head of the jets are not matter accumulated during the jet expansion through the circumstellar medium, neither can their origin be explained by the proposed HD or MHD interacting-wind models for the formation of jets/ansae, since the densities as well as the main excitation mechanisms of the knots, disagree with model predictions.Comment: Figure 1 was changed because features were misidentified in the previous version. 17 pages including 5 figures and 3 tables. ApJ in press. Also available at http://www.iac.es/galeria/denise

Far-UV Spectroscopic Analyses of Four Central Stars of Planetary Nebulae

We analyze the Far-UV/UV spectra of four central stars of planetary nebulae with strong wind features -- NGC 2371, Abell 78, IC 4776 and NGC 1535, and derive their photospheric and wind parameters by modeling high-resolution FUSE (Far-Ultraviolet Spectroscopic Explorer) data in the Far-UV and HST-STIS and IUE data in the UV with spherical non-LTE line-blanketed model atmospheres. Abell 78 is a hydrogen-deficient transitional [WR]-PG 1159 object, and we find NGC 2371 to be in the same stage, both migrating from the constant-luminosity phase to the white dwarf cooling sequence with Teff ~= 120 kK, Mdot ~= 5x10^-8 Msun/yr. NGC 1535 is a ``hydrogen-rich'' O(H) CSPN, and the exact nature of IC 4776 is ambiguous, although it appears to be helium burning. Both objects lie on the constant-luminosity branch of post-AGB evolution and have Teff ~= 65 kK, Mdot ~= 1x10^-8 Msun/yr. Thus, both the H-rich and H-deficient channels of PN evolution are represented in our sample. We also investigate the effects of including higher ionization stages of iron (up to FeX) in the model atmosphere calculations of these hot objects (usually neglected in previous analyses), and find iron to be a useful diagnostic of the stellar parameters in some cases. The Far-UV spectra of all four objects show evidence of hot (T ~ 300 K) molecular hydrogen in their circumstellar environments.Comment: 38 pages, 8 figures (6 color). Accepted for publication in Ap

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