Phase space characteristics of fragmenting nuclei described as excited disordered systems

We investigate the thermodynamical content of a cellular model which describes nuclear fragmentation as a process taking place in an excited disordered system. The model which reproduces very well the size distribution of fragments does not show the existence of a first order phase transition.Comment: 14 pages, TeX type, 7 figure

Influence of Logging on Douglas Fir Beetle Populations

All species of bark beetles of economic importance prefer to attack freshly-killed host material. Logging slash, wind-throw, and fire-killed timber provide ideal breeding grounds for bark beetles. A few species, mostly in the Dendroctonus group, are able to kill living trees. When beetles in the group, raised in preferred host material, cannot find any or enough freshly-killed trees, logs, or slash to enter, they may attack living trees. In the interior of British Columbia, infestations of the Douglas fir beetle can often be traced to logging disturbance

Photoinjector-generation of a flat electron beam with transverse emittance ratio of 100

The generation of a flat electron beam directly from a photoinjector is an attractive alternative to the electron damping ring as envisioned for linear colliders. It also has potential applications to light sources such as the generation of ultra-short x-ray pulses or Smith-Purcell free electron lasers. In this Letter, we report on the experimental generation of a flat-beam with a measured transverse emittance ratio of $100\pm 20.2$ for a bunch charge of $\sim 0.5$ nC; the smaller measured normalized root-mean-square emittance is $\sim 0.4$ $\mu$m and is limited by the resolution of our experimental setup. The experimental data, obtained at the Fermilab/NICADD Photoinjector Laboratory, are compared with numerical simulations and the expected scaling laws.Comment: 5 pages, 3 figure

Benchmarking of 3D space charge codes using direct phase space measurements from photoemission high voltage DC gun

We present a comparison between space charge calculations and direct measurements of the transverse phase space for space charge dominated electron bunches after a high voltage photoemission DC gun followed by an emittance compensation solenoid magnet. The measurements were performed using a double-slit setup for a set of parameters such as charge per bunch and the solenoid current. The data is compared with detailed simulations using 3D space charge codes GPT and Parmela3D with initial particle distributions created from the measured transverse and temporal laser profiles. Beam brightness as a function of beam fraction is calculated for the measured phase space maps and found to approach the theoretical maximum set by the thermal energy and accelerating field at the photocathode.Comment: 11 pages, 23 figures. submitted to Phys Rev ST-A

Modeling the color evolution of luminous red galaxies - improvements with empirical stellar spectra

Predicting the colors of Luminous Red Galaxies (LRGs) in the Sloan Digital Sky Survey (SDSS) has been a long-standing problem. The g,r,i colors of LRGs are inconsistent with stellar population models over the redshift range 0.1<z<0.7. The g-r colors in the models are on average redder than the data while the r-i colors in the models are bluer towards low redshift. Beyond redshift 0.4, the predicted r-i color becomes instead too red, while the predicted g-r agrees with the data. We provide a solution to this problem, through a combination of new astrophysics and a fundamental change to the stellar population modeling. We find that the use of the empirical library of Pickles (1998) instead of theoretical spectra modifies the predicted colors exactly in the way suggested by the data. The reason is a lower flux in the empirical libraries, with respect to the theoretical ones, in the wavelength range 5500-6500 AA. The discrepancy increases with decreasing effective temperature independently of gravity. This result has general implications for a variety of studies from globular clusters to high-redshift galaxies. The astrophysical part of our solution regards the composition of the stellar populations of these massive Luminous Red Galaxies. We find that on top of the previous effect one needs to consider a model in which ~3% of the stellar mass is in old metal-poor stars. Other solutions such as substantial blue Horizontal Branch at high metallicity or young stellar populations can be ruled out by the data. Our new model provides a better fit to the g-r and r-i colors of LRGs and gives new insight into the formation histories of these most massive galaxies. Our model will also improve the k- and evolutionary corrections for LRGs which are critical for fully exploiting present and future galaxy surveys.Comment: Submitted to ApJ Letters. High resolution version available at http://www.maraston.eu/Maraston_etal_2008.pd

The problem of the Pleiades distance. Constraints from Stromgren photometry of nearby field stars

The discrepancy between the Pleiades cluster distance based on Hipparcos parallaxes and main sequence fitting is investigated on the basis of Stromgren photometry of F-type stars. Field stars with the same metallicity as the Pleiades have been selected from the m1 index and a technique has been developed to locate the ZAMS of these field stars in color-magnitude diagrams based on the color/temperature indices b-y, v-y, and beta. Fitting the Pleiades to these ZAMS relations results in a distance modulus of 5.61+/-0.03 mag in contrast to the Hipparcos modulus of 5.36+/-0.06 mag. Hence, we cannot confirm the recent claim by Grenon (1999) that the distance problem is solved by adopting a low metallicity of the Pleiades ([Fe/H]=-0.11) as determined from Geneva photometry. The metallicity sensitivity of the ZAMS determined by the field stars is investigated, and by combining this sensitivity in all three color/temperature indices b-y, v-y, and beta we get a independent test of the Pleiades distance modulus which support our value of 5.61 mag. Furthermore, the field star sample used for the comparison is tested against theoretical isochrones of different ages to show that evolutionary effects in the field star sample are not biasing our distance modulus estimate significantly. Possible explanations of the Pleiades distance problem are discussed and it is suggested that the discrepancy in the derived moduli may be linked to a non-spherical shape of the cluster.Comment: 11 pages, 5 figures, accepted for publication in A&