A DETACHED DUST SHELL SURROUNDING THE J-TYPE CARBON STAR Y CANUM VENATICORUM
This paper reports the first clear detection of a de- tached dust shell surrounding the prototypical J-type carbon star Y CVn in 90 m and 160 m maps taken with the ISO/ISOPHOT 1 . The projected inner radius of the shell is 180 -190 , corresponding to (6.4-7.1) 10 17 cm at a distance of 250 pc. The shell thickness is obtained to be (2-5) 10 17 cm. The mass-loss rate at the formation of the shell is estimated to be in the range (7-20) 10 6 M yr 1 , about two orders of magni- tude higher than the present-day mass-loss rate derived from CO gas observations. The obtained mass in the shell is (4-14) 10 2 M. It is concluded that the mass-loss rate decreased by two orders of magnitude on a short time scale 1.4 10 4 years ago as- suming an average shell expansion velocity of 15 km s 1 ,a nd that Y CVn has been staying at the low mass-loss state. The duration of the previous higher mass-loss phase would be at most 2 10 4 years even if the asymmetry in the shell geometry is taken into account. The evolutionary status of Y CVn is also discussed.
Bow-shock chemistry in the interstellar medium
Stars are bad neighbors: they often disturb their surroundings. They sometimes travel very fast through the interstellar medium (ISM). They frequently undergo violent ejection events which leave an imprint on their neighborhood (jets, winds, supernovae). These supersonic flows generate shocks both in the ejected material (bow shock) and in the stellar environment (termination shock). The global purpose of this thesis is to link the properties of stars and the ISM by studying these shocks. We model them with the Paris-Durham planar shock code, which incorporates a wealth of micro-physics and chemical processes relevant to the magnetized ISM. In the first part, we model 3D magnetized axisymmetric bow shock by a statistical distribution of 1D planar shocks computed with the Paris-Durham model. For the first time, we examine systematically the effect of the geometry, age, and various other parameters on the H2 excitation diagram and emission line profiles. Then, we will show that this 3D bow shock model unprecedentedly improves interpretations of the H2 observations in Orion BNKL and BHR71, and show how spectrally resolved H2 line emission profiles on the Herbig-Haro object HH54 can be used to extract a wealth of dynamical information. In the second part, we model 1D steady stellar winds from AGB stars, which trigger the termination shock, by including relevant physical and chemical ingredients in the Paris-Durham code. For the first time, we examine the timedependent conversion of atomic to molecular hydrogen along the wind trajectories of “hot” and “cold” AGB stars. We suggest that the low abundance of HI inferred from observations is due to hydrogen locked in its molecular form. Then, we try to reproduce HI 21-cm line profile in a “hot” AGB (called Y Cvn) and a “cold” AGB (called CW Leo).
rPET detectors design and data processing
Small animal PET systems based on rotating planar detectors posses some interesting advantages for high sensitivity, high resolution imaging. We have designed the rPET detectors based on MLS crystals assembled on a 30times30 matrix optically coupled to a flat-panel PS-PMT. Weighted position readout circuits pre-process the 64 signals from the 8times8 anodes matrix, which are digitized using a charge-integrating converter. The amplification electronics, including the trigger output for coincidence detection, and the high voltage supply are integrated in a three PCBs stack that forms the base attached to the back of the PMT. The whole assembly is enclosed in a light tight, lead (Pb) shielded aluminum box. The detectors are mounted on a rotating gantry with more than 180 degrees rotation span. The digitized events are screened and histogramed, and a modified center of gravity algorithm removes from the position calculation those signals with poor signal to noise ratio. Apparent mean crystal size on the 511 keV field-flood images is 0.6 mm, mean peak-to-valley ratio is better than 8, and intrinsic resolution is 1.5 mm at the central row, with the energy window wide open. Sensitivity (CPS) for a pair of these detectors set in coincidence at 160 mm distance is 1%