Discovery of Reflection Nebulosity Around Five Vega-like Stars

Coronagraphic optical observations of six Vega-like stars reveal reflection nebulosities, five of which were previously unknown. The nebulosities illuminated by HD 4881, HD 23362, HD 23680, HD 26676, and HD 49662 resemble that of the Pleiades, indicating an interstellar origin for dust grains. The reflection nebulosity around HD 123160 has a double-arm morphology, but no disk-like feature is seen as close as 2.5 arcsec from the star in K-band adaptive optics data. We demonstrate that uniform density dust clouds surrounding HD 23362, HD 23680 and HD 123160 can account for the observed 12-100 micron spectral energy distributions. For HD 4881, HD 26676, and HD 49662 an additional emission source, such as from a circumstellar disk or non-equilibrium grain heating, is required to fit the 12-25 micron data. These results indicate that in some cases, particularly for Vega-like stars located beyond the Local Bubble (>100 pc), the dust responsible for excess thermal emission may originate from the interstellar medium rather than from a planetary debris system.Comment: The Astrophysical Journal, in press for March, 2002 (32 pages, 13 figures

Vector wind, horizontal divergence, wind stress and wind stress curl from SEASAT-SASS at one degree resolution

Conventional data obtained in 1983 are contrasted with SEASAT-A scatterometer and scanning multichannel microwave radiometer (SMMR) data to show how observations at a single station can be extended to an area of about 150,000 square km by means of remotely sensed data obtained in nine minutes. Superobservations at a one degree resolution for the vector winds were estimated along with their standard deviations. From these superobservations, the horizontal divergence, vector wind stress, and the curl of the wind stress can be found. Weather forecasting theory is discussed and meteorological charts of the North Pacific Ocean are presented. Synoptic meteorology as a technique is examined

Halting Planet Migration in the Evacuated Centers of Protoplanetary Disks

Precise Doppler searches for extrasolar planets find a surfeit of planets with orbital periods of 3-4 days, and no planets with orbital periods less than 3 days. The circumstellar distance, R_0, where small grains in a protoplanetary disk reach sublimation temperature (~1500 K) corresponds to a period of ~6 days. Interior to R_0, turbulent accretion due to magneto-rotational instability may evacuate the disk center. We suggest that planets with orbital periods of 3-4 days are so common because migrating planets halt once this evacuated region contains the sites of their exterior 2:1 Lindblad resonances.Comment: 9 pages, 1 figure, to appear in ApJ letter

Synoptic scale wind field properties from the SEASAT SASS

Dealiased SEASAT SEASAT A Scatterometer System SASS vector winds obtained during the Gulf Of Alaska SEASAT Experiment GOASEX program are processed to obtain superobservations centered on a one degree by one degree grid. The grid. The results provide values for the combined effects of mesoscale variability and communication noise on the individual SASS winds. These superobservations winds are then processed further to obtain estimates of synoptic scale vector winds stress fields, the horizontal divergence of the wind, the curl of the wind stress and the vertical velocity at 200 m above the sea surface, each with appropriate standard deviations of the estimates for each grid point value. They also explain the concentration of water vapor, liquid water and precipitation found by means of the SMMR Scanning Multichannel Microwave Radiometer at fronts and occlusions in terms of strong warm, moist air advection in the warm air sector accompanied by convergence in the friction layer. Their quality is far superior to that of analyses based on conventional data, which are shown to yield many inconsistencies

Hypersymmetry: a Z_3-graded generalization of supersymmetry

We propose a generalization of non-commutative geometry and gauge theories based on ternary Z_3-graded structures. In the new algebraic structures we define, we leave all products of two entities free, imposing relations on ternary products only. These relations reflect the action of the Z_3-group, which may be either trivial, i.e. abc=bca=cab, generalizing the usual commutativity, or non-trivial, i.e. abc=jbca, with j=e^{(2\pi i)/3}. The usual Z_2-graded structures such as Grassmann, Lie and Clifford algebras are generalized to the Z_3-graded case. Certain suggestions concerning the eventual use of these new structures in physics of elementary particles are exposed

Imaging Polarimetric Observations of a New Circumstellar Disk System

Few circumstellar disks have been directly observed. Here we use sensitive differential polarimetric techniques to overcome atmospheric speckle noise in order to image the circumstellar material around HD 169142. The detected envelope or disk is considerably smaller than expectations based on the measured strength of the far-IR excess from this system

NICMOS Observations of the Pre-Main-Sequence Planetary Debris System HD 98800

Spectral energy distributions (SEDs) from 0.4 to 4.7 microns are presented for the two principal stellar components of HD~98800, A and B. The third major component, an extensive planetary debris system (PDS), emits > 20% of the luminosity of star B in a blackbody SED at 164 +/- 5K extending from mid-IR to millimeter-wavelengths. At 0.95 microns a preliminary upper limit of < 0.06 is obtained for the ratio of reflected light to the total from star B. This result limits the albedo of the PDS to < 0.3. Values are presented for the temperature, luminosity, and radius of each major systemic component. Remarkable similarities are found between the PDS and the interplanetary debris system around the Sun as it could have appeared a few million years after its formation.Comment: LaTeX, 9 pages with 1 encapsulated postscript figure and one specially formatted Table which is rendered as a postscript file and included as a figure. Accepted for publication in Astrophysical Journal Letter

A Resolved Molecular Gas Disk around the Nearby A Star 49 Ceti

The A star 49 Ceti, at a distance of 61 pc, is unusual in retaining a substantial quantity of molecular gas while exhibiting dust properties similar to those of a debris disk. We present resolved observations of the disk around 49 Ceti from the Submillimeter Array in the J=2-1 rotational transition of CO with a resolution of 1.0x1.2 arcsec. The observed emission reveals an extended rotating structure viewed approximately edge-on and clear of detectable CO emission out to a distance of ~90 AU from the star. No 1.3 millimeter continuum emission is detected at a 3-sigma sensitivity of 2.1 mJy/beam. Models of disk structure and chemistry indicate that the inner disk is devoid of molecular gas, while the outer gas disk between 40 and 200 AU from the star is dominated by photochemistry from stellar and interstellar radiation. We determine parameters for a model that reproduces the basic features of the spatially resolved CO J=2-1 emission, the spectral energy distribution, and the unresolved CO J=3-2 spectrum. We investigate variations in disk chemistry and observable properties for a range of structural parameters. 49 Ceti appears to be a rare example of a system in a late stage of transition between a gas-rich protoplanetary disk and a tenuous, virtually gas-free debris disk.Comment: 11 pages, 6 figures, accepted for publication in Ap

Development of ‘RL Plus’: winter wheat variety performance in relation to site characteristics (HGCA Project Report No. 365)

This project developed RL Plus, an augmented version of the Recommended Lists for Cereals and Oilseeds, published by the HGCA on CD and the internet (http://www.hgca.com) to provide the cereals and oilseeds industry with means of interrogating and analysing data from HGCA-funded variety trials. Given that RL Plus is fully documented and published in electronic form, it is not described further here. This report is confined to additional research analysing relative variety performance of winter wheat in terms of site characteristics. A spreadsheet was constructed of treated yields of winter wheat from 43 varieties across 506 trials (Recommended List, National List or BSPB trials) in the UK from harvests during 1992-2002. This included site information collected from the trials, and supplemented with the site location (OS coordinates), soil types, meteorological data and drought index derived during the project. Complete, or nearcomplete, data existed for 249 trials from harvests during 1993-2003. These data were used to investigate factors associated with site variation in variety yields. Data-mining techniques were used to identify site variables that explained variation in variety yields between sites. This information was used to build models to describe and predict patterns of variety variability due to site differences. Variation in variety yields could be modelled in terms of overall variety differences (43% variety variation accounted for), and variety interactions with large-scale trend due to geographic location (general climate, 16%), small-scale location trend specific to years (micro-climates, 14%), expected site yield (2%), late sown crops (crops sown on/after 30 October, 0.4%), sites with sandy or shallow soil (0.5%), sites with low soil K index (0.4%), differences between years (4.5%), differences between sites (unexplained by site variables, 2%), and other unexplained variation (18%). Further investigation suggested that other site variables, such as previous cropping, might also influence variety variability but that the relationship was local (differed between regions). The results of the statistical analysis can be used to optimise use of the ‘Varieties on your Farm’ module of RL Plus. In general, geographic location appears to be the most important site variable influencing variation in variety yields across the UK. However, for particular varieties, the expected site yield, soil type or soil K index may be equally important

Sub-millimeter images of a dusty Kuiper belt around eta Corvi

We present sub-millimeter and mid-infrared images of the circumstellar disk around the nearby F2V star eta Corvi. The disk is resolved at 850um with a size of ~100AU. At 450um the emission is found to be extended at all position angles, with significant elongation along a position angle of 130+-10deg; at the highest resolution (9.3") this emission is resolved into two peaks which are to within the uncertainties offset symmetrically from the star at 100AU projected separation. Modeling the appearance of emission from a narrow ring in the sub-mm images shows the observed structure cannot be caused by an edge-on or face-on axisymmetric ring; the observations are consistent with a ring of radius 150+-20AU seen at 45+-25deg inclination. More face-on orientations are possible if the dust distribution includes two clumps similar to Vega; we show how such a clumpy structure could arise from the migration over 25Myr of a Neptune mass planet from 80-105AU. The inner 100AU of the system appears relatively empty of sub-mm emitting dust, indicating that this region may have been cleared by the formation of planets, but the disk emission spectrum shows that IRAS detected an additional hot component with a characteristic temperature of 370+-60K (implying a distance of 1-2AU). At 11.9um we found the emission to be unresolved with no background sources which could be contaminating the fluxes measured by IRAS. The age of this star is estimated to be ~1Gyr. It is very unusual for such an old main sequence star to exhibit significant mid-IR emission. The proximity of this source makes it a perfect candidate for further study from optical to mm wavelengths to determine the distribution of its dust.Comment: 22 pages, 4 figures. Scheduled for publication in ApJ 10 February 2005 issu