Faint companions in the close environment of star-forming dwarf galaxies: possible overlooked starburst triggers?
Using the NASA Extragalactic Database, we have searched the close environment of 98 star– forming dwarf galaxies (SFDGs) from field– and low density environments for companion galaxies. Most of the found companions are dwarf galaxies, previously disregarded in environmental studies of SFDGs. Using a subsample at low redshifts, cz < 2000 kms 1 , i.e. less biased against dwarf companions, we find that 30% of the SFDGs have close companions within a projected ]
Chemical Evolution of Dwarf Irregular Galaxies - chemodynamical models and the effect of gas infall
Because of their low gravitational energies dwarf galaxies are greatly exposed to energetical influences by the interstellar medium, like e.g. stellar radiation, winds or explosions, or by their environment. While the metallicity depletion in dwarf galaxies can be explained in general by supernova-driven galactic winds, the reason for their low N/O ratios at low O abundance is not yet completely understood. Stellar yields enrich the different gas phases with elements that are characteristic for their stellar progenitors. Gas-phase transitions are necessary for a mixing of elements, but depend sensitively on the thermal and dynamical state of the interstellar medium. Models of chemical evolution start usually with a high N/O ratio at low O abundance according to a metal enrichment by ancient stellar populations with traditional yields, but can only reproduce the N/O-O peculiarity by the stepwise element release and mostly by the application of multiple starbursts in order to account also for a selective element reduction by galactic winds. Chemodynamical models of dwarf galaxies, however, demonstrate that strong evaporation of clouds by the hot supernova gas leads to an almost perfect mixing of the interstellar gas and a large-scale homogenization of abundance ratios. In addition, even with star-formation self-regulation these models can successfully account for the observed N/O-O values in a self-consistent way. Although new stellar yields have been taken into account which provide mainly secondary N production from massive stars, a significant discrepancy remains between the chemodynamical dwarf galaxy models and closed-box models that take the chemodynamical treatment into account. In this paper we therefore discuss to what amount gas infall is responsible to affect the N/O ratio.
Thermally Induced Dynamics of Satellite Solar Panels
Thermally induced structural motions are known to affect the attitude dynamics of low Earth orbiting satellites during eclipse transitions. Motions of e exible appendages such as solar panels lead to rigid body rotations of the entire satellite because the total angular momentum of the system is conserved. These potentially large attitude disturbances may violate pointing accuracy and jitter requirements. One type of thermally induced dynamics exhibited by solar panels is thermal snap. The Upper Atmosphere Research Satellite is a prominent example of a satellite that experiences thermal snap disturbances during eclipse transitions. This paper describes recent studies of thermally induced dynamicsof solarpanels, including an analysisofsatelliteattitudedynamics resulting from thermally induced structural motions and a laboratory investigation of the thermal-structural performance of a satellite solar panel. Analytical and experimental results demonstrate thermal bending deformations with acceleration transients that have characteristic thermal snap disturbance histories in response to rapid changes in heating. The studies show that solar panel thermal snap disturbances are caused by through-the-thickness temperature differences that vary at a nonconstant rate. Finite element analysis correctly predicts the thermal snap phenomenon observed in the solar panel experiments.