Splittings and C-complexes

The intersection pattern of the translates of the limit set of a quasi-convex subgroup of a hyperbolic group can be coded in a natural incidence graph, which suggests connections with the splittings of the ambient group. A similar incidence graph exists for any subgroup of a group. We show that the disconnectedness of this graph for codimension one subgroups leads to splittings. We also reprove some results of Peter Kropholler on splittings of groups over malnormal subgroups and variants of them.Comment: v2 final version incorporating referee's comment

Direct and indirect coupling of primary production and diel nitrate dynamics in a subtropical spring-fed river

We used high-frequency in situ measurements of nitrate (NO3-) and dissolved oxygen (DO) from the springfed Ichetucknee River, Florida, to derive multiple independent estimates of assimilatory nitrogen (N) demand, and to evaluate the short-term dependence of heterotrophic assimilation and dissimilation (e.g., denitrification) on gross primary productivity (GPP). Autotrophic N assimilation estimates derived from diel DO variability and GPP stoichiometry agreed closely with estimates based on integration of diel variation in NO3- concentration, although the correspondence of these metrics depended on the method used to estimate NO 3- baselines. In addition, day-to-day changes in nocturnal NO3- concentration maxima were strongly negatively correlated with day-to-day changes in GPP. Diel temperature variation in the Ichetucknee River indicated that this pattern could not be explained by hydrologic dispersion, while relationships between N assimilation and O 2 production at hourly intervals indicated minimal physiological lags. The estimated magnitude of heterotrophic assimilation was small, indicating that the relationship between changes in GPP and changes in nocturnal 3- maxima reflects sensitivity of denitrification to variation in exudation of labile organic matter by primary producers. We estimate that ~ 35% of denitrification may be fueled by the previous day's photosynthesis; this result is consistent with the broader hypothesis that the magnitude of autochthonous production in aquatic systems influences the fate of N via both direct and indirect mechanisms. © 2010, by the American Society of Limnology and Oceanography, Inc

Infrared photometry and mass loss rates for OBA supergiants and Of stars

We present infrared observations of 34 OBA supergiants and 10 Of and Oe stars. Excess free- free emission is detectable at 10 m but not at shorter wavelengths for the supergiants. From our 2.2 and 3.6 t m photometry of the supergiants, in conjunction with existing UBV data, we derive an invariant ratio of total to selective extinction, R = 3.10 + 0.03. Using published radio and infrared data, we derive the velocity law for the mass loss outflow from P Cygni, and we apply this velocity law to the measured 10 m excess fluxes from our sample of supergiants in order to derive mass loss rates. Our preliminary survey of Of and Oe stars shows excess free-free emission to be common at 3.6 m and sometimes detectable at 2.2 t m. Using our 2.2-10 m data, we derive mass loss rates for several 0 stars. Our results are consistent with radiation pressure being the driving mechanism for mass loss from OBA supergiants and Of stars. Subject headings: interstellar: matter - stars: circumstellar shells - stars: early-type - stars: mass loss - stars: Of-type - stars: supergiant

Infrared photometry of southern planetary nebulae and emission-line objects

Ten-micron photometry is presented of 23 planetary nebulae, seven very low excitation (VLE) nebulae, and 11 emission-line objects, of which more than half were also detected at 20 microns. The 10-micron behaviors of the various classes of objects are compared, and it is shown that planetary nebulae can easily be distinguished from VLE nebulae by virtue of the silicate emission displayed by the latter. A uniform sample of planetary nebulae for statistical analysis is created by combining the present sample with an earlier northern hemisphere survey. It is shown that L alpha photons are the dominant grain heating mechanism for nebular electron densities less than approximately 15,000/cu cm, whereas above this density direct dust absorption of stellar continuum photons dominates

Infrared Observations of Two Symmetric Nebulae

Infrared observations are presented of NGC 7635, IC 1470, and their early-type central stars. Large excess fluxes are found at both 10 and 20, which are interpreted as due to thermal emission by cool dust

Wolf-Rayet stars. VI. The nature of the optical and infrared continuum

Results of IR photometry between 1.6 and 11.3 microns are combined with results of scanner photometry between 3300 and 11,100 A to produce composite energy distributions for Wolf-Rayet stars. Reddening corrections are subtracted from the distributions to obtain IR-excess difference spectra which are fitted to either free-free emission or blackbody-like continua. WN stars are found to exhibit only free-free emission, while only WC stars show effects due to dust. Red and near-IR spectra are obtained for WC9 stars, and evidence is presented in favor of visual extinction by circumstellar dust around the star Ve2-45. The IR excesses in WC stars are interpreted as thermal emission by graphite grains; on this basis, radii and dust masses of circumstellar shells are estimated. Based on all these results, an evolutionary scheme is suggested for WC9 stars

Extinction Variations in the H II Regions Sharpless 156 and 162

Accurate spectrophotometric observations of the nebulae Sh 156 and Sh 162 and of their exciting stars, combined with new high resolution radio maps by Israel, enable the derivation of Av towards 15 points in these nebulae. These values are compared with other values of Av for the entire nebulae and their exciting stars

Discovery of an Edge-on Dust Disk around the [WC10] Central Star CPD –56°8032

We present Hubble Space Telescope ultraviolet and optical Space Telescope Imaging Spectrograph spectroscopy of the [WCL] planetary nebula central star CPD -56°8032, obtained during its latest light-curve minimum. The UV spectrum shows the central star's continuum light distribution to be split into two bright peaks separated by 0farcs10. We interpret this finding as due to an edge-on disk or torus structure that obscures direct light from the star, which is seen primarily via its light scattered from the disk's rims or lobes. CPD -56°8032 is an archetype of dual dust chemistry [WCL] planetary nebulae, which exhibit strong infrared emission features from both carbon-rich and oxygen-rich materials, and for which the presence of a disk harboring the O-rich grains had been suggested. Our direct observation of an edge-on occulting dust structure around CPD -56°8032 provides strong support for such a model and for binary interactions being responsible for the correlation between the dual dust chemistry phenomenon in planetary nebulae and the presence of a hydrogen-deficient [WCL] Wolf-Rayet central star

Inference of riverine nitrogen processing from longitudinal and diel variation in dual nitrate isotopes

Longitudinal and diel measurements of dual isotope composition (δ¹⁵N and δ¹⁸O) in nitrate (NO<inf>3</inf>-N) were made in the Ichetucknee River, a large (∼8m³ s ^-1), entirely spring-fed river in North Florida, to determine whether isotopic variation can deconvolve assimilatory and dissimilatory removal. Comparing nitrate concentrations and isotope composition during the day and night we predicted (1) daytime declines in total fractionation due to low assimilatory fractionation and (2) diurnal variation in dual isotope coupling between 1:1 (assimilation) and 2:1 (denitrification). Five daytime longitudinal transects comprising 10 sampling stations showed consistent NO<inf>3</inf>-N removal (25-35% of inputs) and modest fractionation (¹⁵ε <inf>total</inf> between -2 and -6‰, enriching the residual nitrate pool). Lower fractionation (by ∼1‰) during two nighttime transects, suggests higher fractionation due to assimilation than denitrification. Total fractionation was significantly negatively associated with discharge, input [NO<inf>3</inf>-N], N mass removal, and fractional water loss. Despite well-constrained mass balance estimates that denitrification dominated total N removal, isotope coupling was consistently 1:1, both for longitudinal and diel sampling. Hourly samples on two dates at the downstream location showed significant diel variation in concentration ([NO<inf>3</inf>-N] amplitude = 60 to 90 μg N L^-1) and isotope composition (δ¹⁵N amplitude = -0.7‰ to -1.6‰). Total fractionation differed between day and night only on one date but estimated assimilatory fractionation assuming constant denitrification was highly variable and implausibly large (for N, ¹⁵ε = -2 to -25‰), suggesting that fractionation and removal due to denitrification is not diurnally constant. Pronounced counterclockwise hysteresis in the relationship between [NO<inf>3</inf>-N] and δ¹⁵N suggests diel variation in N isotope dynamics. Together, low fractionation, isotope versus concentration hysteresis, and consistent 1:1 isotope coupling suggests that denitrification is controlled by NO <inf>3</inf>^- diffusion into the benthic sediments, the length of which is mediated by riverine oxygen dynamics. While using dual isotope behavior to deconvolve removal pathways was not possible, isotope measurements did yield valuable information about riverine N cycling and transformations. Copyright © 2012 by the American Geophysical Union

The dual dust chemistries of planetary nebulae with [WCL] central stars

The rather rare class of central stars of planetary nebulae that show very low-excitation Wolf–Rayet spectra has been a subject of great interest, particularly in the infrared, since its discovery in the late 1960s. Further peculiarities have been found with the advent of infrared spectroscopy from ISO. Notably, these objects simultaneously betray the presence of regions of carbon-rich and oxygen-rich dust chemistry. We compare and contrast complete ISO spectra between 2 and 200 μm of a sample of six [WC8] to [WC11] central stars, finding many similarities. Among this sample, one star provides strong evidence of quasi-periodic light variations, suggestive of a dust cloud orbiting in a plane from which we view the system