We present high-spatial resolution HST and ground-based adaptive optics observations, and highsensitivity ISO (ISOCAM & ISOPHOT) observations of a sample of X-ray selected weak-line (WTTS) and post (PTTS) T Tauri stars located in the nearby Chamaeleon T and Scorpius-Centaurus OB associations. HST/NICMOS and adaptive optics observations aimed at identifying substellar companions (young brown dwarfs) at separations ≥ 30 A.U. from the primary stars. No such objects were found within 300 A.U. of any of the target stars, and a number of faint objects at larger separations can very likely be attributed to a population of field (background) stars. ISOCAM observations of 5 to 15 Myr old WTTS and PTTS in ScoCen reveal infrared excesses which are clearly above photospheric levels, and which have a spectral index intermediate between that of younger (1 to 5 Myr) T Tauri stars in Chamaeleon and that of pure stellar photospheres. The difference in the spectral index of the older PTTS in ScoCen compared to the younger classical and weak-line TTS in Cha can be attributed to a deficiency of smaller size (0.1 to 1 µm) dust grains relative to larger size (≈5 µm) dust grains in the disks of the PTTS. The lack of small dust grains is either due to the environment (effect of nearby O stars and supernova explosions) or due to disk evolution. If the latter is the case, it would hint that circumstellar disks start to get dust depleted at an age between 5 to 15 Myr. Dust depletion is very likely related to the build-up of larger particles (ultimately rocks and planetesimals) and thus an indicator for the onset of the period of planet formation.

This paper reports new exclusive cross sections for ep → e π + π − p using the CLAS detector at Jefferson Laboratory. These results are presented for the first time at photon virtualities 2.0 GeV 2 < Q 2 < 5.0 GeV 2 in the center-of-mass energy range 1.4 GeV < W < 2.0 GeV, which covers a large part of the nucleon resonance region. Using a model developed for the phenomenological analysis of electroproduction data, we see strong indications that the relative contributions from the resonant cross sections at W < 1.74 GeV increase with Q 2 . These data considerably extend the kinematic reach of previous measurements. Exclusive ep → e π + π − p cross section measurements are of particular importance for the extraction of resonance electrocouplings in the mass range above 1.6 GeV.

Results are presented for the first measurement of the double-polarization helicity asymmetry E for the eta photoproduction reaction gamma p -> eta p. Data were obtained using the FROzen Spin Target (FROST) with the CLAS spectrometer in Hall B at Jefferson Lab, covering a range of center-of-mass energy W from threshold to 2.15 GeV and a large range in center-of-mass polar angle. As an initial application of these data, the results have been incorporated into the Julich-Bonn model to examine the case for the existence of a narrow N* resonance between 1.66 and 1.70 GeV. The addition of these data to the world database results in marked changes in the predictions for the Eobservable from that model. Further comparison with several theoretical approaches indicates these data will significantly enhance our understanding of nucleon resonances. (C) 2016 Published by Elsevier B.V

Optical, near-infrared, and radio observations of the BL Lac object PKS 2155[304 were obtained simultaneously with a continuous UV/EUV/X-ray monitoring campaign in 1994 May. Further optical observations were gathered throughout most of 1994. The radio, millimeter, and near-infrared data show no strong correlations with the higher energies. The optical light curves exhibit Ñickering of 0.2È0.3 mag on timescales of 1È2 days, superposed on longer timescale variations. Rapid variations of D0.01 mag minute~1, if real, are the fastest seen to date for any BL Lac object. Small (0.2È0.3 mag) increases in the V and R bands occur simultaneously with a Ñare seen at higher energies. All optical wave bands (UBV RI) track each other well over the period of observation, with no detectable delay. For most of the period the average colors remain relatively constant, although there is a tendency for the colors (in particular, B[V) to vary more when the source fades. In polarized light, PKS 2155[304 showed strong color dependence (polarization increases toward the blue, and the highest optical polariza-P U /P I \ 1.31) tion (U \ 14.3%) ever observed for this source. The polarization variations trace the Ñares seen in the UV Ñux. For the fastest variability timescale observed, we estimate a central black hole mass of [1.5 ] 109(d/10) consistent with UV and X-ray constraints and smaller than previously calculated for M _ , this object.

Exclusive photoproduction cross sections have been measured for the process γp → pπ 0 [e + e − (γ )] with the Dalitz decay final state using tagged photon energies in the range of E γ = 1.275-5.425 GeV. The complete angular distribution of the final state π 0 , for the entire photon energy range up to large values of t and u, has been measured for the first time. The data obtained show that the cross section dσ/dt, at mid to large angles, decreases with energy as s −6.89±0.26 . This is in agreement with the perturbative QCD quark counting rule prediction of s −7 . Paradoxically, the size of angular distribution of measured cross sections is greatly underestimated by the QCD-based generalized parton distribution mechanism at highest available invariant energy s = 11 GeV 2 . At the same time, the Regge-exchange-based models for π 0 photoproduction are more consistent with experimental data.