Astrometric signal profile fitting for Gaia

A tool for representation of the one-dimensional astrometric signal of Gaia is described and investigated in terms of fit discrepancy and astrometric performance with respect to number of parameters required. The proposed basis function is based on the aberration free response of the ideal telescope and its derivatives, weighted by the source spectral distribution. The influence of relative position of the detector pixel array with respect to the optical image is analysed, as well as the variation induced by the source spectral emission. The number of parameters required for micro-arcsec level consistency of the reconstructed function with the detected signal is found to be 11. Some considerations are devoted to the issue of calibration of the instrument response representation, taking into account the relevant aspects of source spectrum and focal plane sampling. Additional investigations and other applications are also suggested.Comment: 13 pages, 21 figures, Accepted by MNRAS 2010 January 29. Received 2010 January 28; in original form 2009 September 3

Dipole Symmetry Near Threshold

In celebrating Iachello's 60th birthday we underline many seminal contributions for the study of the degrees of freddom relevant for the structure of nuclei and other hadrons. A dipole degree of freedom, well described by the spectrum generating algebra U(4) and the Vibron Model, is a most natural concept in molecular physics. It has been suggested by Iachello with much debate, to be most important for understanding the low lying structure of nuclei and other hadrons. After its first observation in $^{18}O$ it was also shown to be relevant for the structure of heavy nuclei (e.g. $^{218}Ra$). Much like the Ar-benzene molecule, it is shown that molecular configurations are important near threshold as exhibited by states with a large halo and strong electric dipole transitions. The cluster-molecular Sum Rule derived by Alhassid, Gai and Bertsch (AGB) is shown to be a very useful model independent tool for examining such dipole molecular structure near thereshold. Accordingly, the dipole strength observed in the halo nuclei such as $^6He, ^{11}Li, ^{11}Be, ^{17}O$, as well as the N=82 isotones is concentrated around threshold and it exhausts a large fraction (close to 100%) of the AGB sum rule, but a small fraction (a few percent) of the TRK sum rule. This is suggested as an evidence for a new soft dipole Vibron like oscillations in nuclei.Comment: Presented at Iachello's Fest, Symmetry in Physics, Erice, March 23-30, 2003. Supported by USDOE Grant No. DE-FG02-94ER4087

Open Questions in Stellar Helium Burning Addressed With Real Photons

The outcome of helium burning is the formation of the two elements, carbon and oxygen. The ratio of carbon to oxygen at the end of helium burning is crucial for understanding the final fate of a progenitor star and the nucleosynthesis of heavy elements in Type II supernova, with oxygen rich star predicted to collapse to a black hole, and a carbon rich star to a neutron star. Type Ia supernovae (SNeIa) are used as standard candles for measuring cosmological distances with the use of an empirical light curve-luminosity stretching factor. It is essential to understand helium burning that yields the carbon/oxygen white dwarf and thus the initial stage of SNeIa. Since the triple alpha-particle capture reaction, $^{8}Be(\alpha,\gamma)^{12}C$, the first burning stage in helium burning, is well understood, one must extract the cross section of the $^{12}C(\alpha,\gamma)^{16}O$ reaction at the Gamow window (300 keV) with high accuracy of approximately 10% or better. This goal has not been achieved despite repeated strong statements that appeared in the literature. In particular constraint from the beta-delayed alpha-particle emission of $^{16}N$ were shown to not sufficiently restrict the p-wave cross section factor; e.g. a low value of $S_{E1}(300)$ can not be ruled out. Measurements at low energies, are thus mandatory for determining the elusive cross section factor for the $^{12}C(\alpha,\gamma)^{16}O$ reaction. We are constructing a Time Projection Chamber (TPC) for use with high intensity photon beams extracted from the HI$\gamma$S/TUNL facility at Duke University to study the $^{16}O(\gamma,\alpha)^{12}C$ reaction, and thus the direct reaction at energies as low as 0.7 MeV. This work is in progress.Comment: Int. Conf. Fission and Neutron-Rich Nuclei, Sanibel, Florida, Nov 6-9, 200