Keck Imaging of Binary L Dwarfs

We present Keck near-infrared imaging of three binary L dwarf systems, all of which are likely to be sub-stellar. Two are lithium dwarfs, and a third exhibits an L7 spectral type, making it the coolest binary known to date. All have component flux ratios near 1 and projected physical separations between 5 and 10 AU, assuming distances of 18 to 26 pc from recent measurements of trigonometric parallax. These surprisingly similar binaries represent the sole detections of companions in ten L dwarf systems which were analyzed in the preliminary phase of a much larger dual-epoch imaging survey. The detection rate prompts us to speculate that binary companions to L dwarfs are common, that similar-mass systems predominate, and that their distribution peaks at radial distances in accord both with M dwarf binaries and with the radial location of Jovian planets in our own solar system. To fully establish these conjectures against doubts raised by biases inherent in this small preliminary survey, however, will require quantitative analysis of a larger volume-limited sample which has been observed with high resolution and dynamic range.Comment: LaTex manuscript in 13 pages, 3 postscript figures, Accepted for publication in the Letters of the Astrophysical Journal; Postscript pre-print version available at: http://www.hep.upenn.edu/PORG/papers/koerner99a.p

Time-resolved gas thermometry by laser-induced grating spectroscopy with a high-repetition rate laser system

Thermometry using Laser Induced Grating Spectroscopy (LIGS) is reported using a high– repetition rate laser system, extending the technique to allow time–resolved measurements of gas dynamics. LIGS signals were generated using the second harmonic output at 532 nm of a commercially available high– repetition rate Nd:YAG laser with nitrogen dioxide as molecular seed. Measurements at rates up to 10 kHz were demonstrated under static cell conditions. Transient temperature changes of the same gas contained in a cell subjected to rapid compression by injection of gas were recorded at 1 kHz to derive the temperature evolution of the compressed gas showing temperature changes of 50 K on a time scale of 0.1 s with a measurement precision of 1.4 %. The data showed good agreement with an analytical thermodynamic model of the compression process

Effects of intake-port throttling on combustion behaviour in diesel low-temperature combustion

This article describes the effects of intake-port throttling on diesel low-temperature combustion at a low and medium load condition. These conditions were known for their characteristically high hydrocarbon emissions predominantly from over-mixed and under-mixed mixture zones, respectively. The investigation was carried out to supplement current findings in the literature with valuable information on the formation of high hydrocarbon emissions with increasing swirl levels generated by intake-port throttling. This was achieved through the use of cycle-resolved high hydrocarbon measurements in addition to cycle averaged emissions and in-cylinder pressure-derived metrics. While there was negligible overall effect at the moderately dilute low-load conditions, increasing swirl has been shown to be beneficial to premixing efficacy under highly dilute conditions with extended ignition delay. This potential advantage was found to be nullified by the swirl-induced confinement of fuel and combustion products to the central region of the cylinder leading to poor late cycle burn rates and increased smoke emissions. High hydrocarbon emissions from the squish and head quench regions were reduced by an increase in swirl ratio

Multi-model assessment identifies livestock grazing as a major contributor to variation in European Union land and water footprints

Food systems are the largest users of land and water resources worldwide. Using a multi-model approach to track food through the global trade network, we calculated the land footprint (LF) and water footprint (WF) of food consumption in the European Union (EU). We estimated the EU LF as 140–222 Mha yr−1 and WF as 569–918 km3 yr−1. These amounts are 5–7% of the global LF and 6–10% of the global WF of agriculture, with the EU representing 6% of the global population. We also calculated the global LF of livestock grazing, accounting only for grass eaten, to be 1,411–1,657 Mha yr−1, and the global LF of agriculture to be 2,809–3,014 Mha yr−1, which is about two-thirds of what the Food and Agriculture Organization Statistics (FAOSTAT) database reports. We discuss here the different methods for calculating the LF for livestock grazing, underscoring the need for a consistent methodology when monitoring the food LF and WF reduction goals set by the EU’s Farm To Fork Strategy.</p

Surface Gravities for 228 M, L, and T Dwarfs in the NIRSPEC Brown Dwarf Spectroscopic Survey

We combine 131 new medium-resolution (R~2000) J-band spectra of M, L, and T dwarfs from the Keck NIRSPEC Brown Dwarf Spectroscopic Survey (BDSS) with 97 previously published BDSS spectra to study surface-gravity-sensitive indices for 228 low-mass stars and brown dwarfs spanning spectral types M5-T9. Specifically, we use an established set of spectral indices to determine surface gravity classifications for all M6-L7 objects in our sample by measuring equivalent widths (EW) of the K I lines at 1.1692, 1.1778, 1.2529 um, and the 1.2 um FeHJ absorption index. Our results are consistent with previous surface gravity measurements, showing a distinct double peak - at ~L5 and T5 - in K I EW as a function of spectral type. We analyze K I EWs of 73 objects of known ages and find a linear trend between log(Age) and EW. From this relationship, we assign age ranges to the very low gravity, intermediate gravity, and field gravity designations for spectral types M6-L0. Interestingly, the ages probed by these designations remain broad, change with spectral type, and depend on the gravity sensitive index used. Gravity designations are useful indicators of the possibility of youth, but current datasets cannot be used to provide a precise age estimate.Comment: 33 pages, 13 figures, ApJ in pres

Effects of hydrogen addition on high-pressure nonpremixed natural gas combustion

The effects of hydrogen addition on the ignition and combustion of a high-pressure methane jet in a quiescent charge of high-temperature, medium-pressure air were investigated numerically and experimentally. Subsequently, the results of these two fundamental studies were applied to the interpretation of combustion and emissions measurements from a pilot-ignited natural gas engine fueled with similar fuels. Whereas, under quiescent conditions, the influence of hydrogen addition on the autoignition delay time of the gaseous jet was small, a markedly greater effect was observed in the more complex environment of the research engine. Similarly, in the two fundamental studies, the addition of hydrogen to the methane fuel resulted in a reduction of NOx emissions, whereas increased levels of NOx emissions were observed from the engine, highlighting the difference between the autoignition and pilot-ignition process

Comparison of the microalgal community within fast ice at two sites along the Ross Sea coast, Antarctica

Diverse microbial communities survive within the sea ice matrix and are integral to the energy base of the Southern Ocean. Here we describe initial findings of a four season survey (between 1999-2004) of community structure and biomass of microalgae within the sea ice and in the underlying water column at Cape Evans and Cape Hallett, in the Ross Sea, Antarctica as part of the Latitudinal Gradient Project. At Cape Evans, bottom-ice chlorophyll a levels ranged from 4.4 to 173 mg Chl a m-2. Dominant species were Nitzschia stellata, N. lecointei, and Entomoneis kjellmanii, while the proportion of Berkeleya adeliensis increased steadily during spring. Despite being obtained later in the season, the Cape Hallett data show considerably lower standing stocks of chlorophyll ranging from 0.11 to 36.8 mg Chl a m-2. This difference was attributed to a strong current, which may have ablated much of the bottom ice biomass and provided biomass to the water below. This loss of algae from the bottom of the ice may explain why the ice community contributed only 2% of the standing stock in the total water column. Dominant species at Cape Hallett were Nitzschia stellata, Fragilariopsis curta and Cylindrotheca closterium. The low biomass at Cape Hallett and the prevalence of smaller-celled diatoms in the bottom ice community indicate that the ice here is more typical of pack ice than fast ice. Further data will allow us to quantify and model the extent to which ice-driven dynamics control the structure and function of the sea ice ecosystem and to assess its resilience to changing sea ice conditions. © Antarctic Science Ltd

Tracking of spectral lines in an ARCAP time-frequency representation

International audienceARCAP time-frequency representation of narrow-band signals are madde of instantaneous characteristics (frequencies and amplitudes), without any time li,ks. In order to extract the frequency modulations (or spectral trajectoeries), we propose to re-create them on the basis of ballistic integrator models. The analytic expression of the corresponding asymptotic Kalman filter gains allows a very simple implementation of asociation procedures including trajectory birth or death. The points being associated, a Fraser filtering leads to the smoothed spectral trajectories

Tracking of spectral lines in an ARCAP time-frequency representation

International audienceARCAP time-frequency representation of narrow-band signals are madde of instantaneous characteristics (frequencies and amplitudes), without any time li,ks. In order to extract the frequency modulations (or spectral trajectoeries), we propose to re-create them on the basis of ballistic integrator models. The analytic expression of the corresponding asymptotic Kalman filter gains allows a very simple implementation of asociation procedures including trajectory birth or death. The points being associated, a Fraser filtering leads to the smoothed spectral trajectories