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.
Mass Loss History of Evolved Stars
We provide high-resolution maps of the circumstellar dust shells of several dozen Asymptotic Giant Branch (AGB) stars using data from the Spitzer space telescope Multiband Imaging Photometer (MIPS) imaged at 70μm. AGB stars are the major contributors of chemical elements such as carbon, oxygen and silicon, which are essential to the existence of life in the universe, through mass loss processes that take place at the surface of the star. We probe the spatial distribution of cold (∼40 K) dust grains in order to trace the history of mass loss from the observed radial density variation in these shells. Our images illustrate that different morphologies may evolve because AGB stars can already interact with the interstellar medium (ISM) that surrounds them even at these early stages of the AGB mass loss history. Relative motion of the star as well as the ISM with respect to the local environment may be of particular importance for shell structure evolution.
Photoreflectance characterization of vanadium-doped GaAs layers grown by metalorganic vapor phase epitaxy
Abstract We characterized the vanadium-doped (GaAs:V) layers grown on GaAs substrate by metalorganic vapor phase epitaxy. The conductivity of the samples is checked by the Hall effect measurements, and it is n type with electrons concentration ranging from 2×10 13 to 2×10 17  cm −3 . We used the photoreflectance technique (PR) to study the effect of vanadium doping on the optical properties of GaAs. Our experimental results show that the built-in electric field F and the broadening parameter Γ 0 increase with V-doping concentration. Furthermore, the temperature dependence of the built-in electric field and the broadening parameter of GaAs:V samples have been investigated by PR. It has been observed that these parameters increase with increasing temperature. The decrease of the built-in electric field with decreasing temperature is due to the photovoltage effect.We characterized the vanadium-doped GaAs (GaAs:V) layers grown on GaAs substrate by metalorganic vapor phase epitaxy.