In regions where stars form, variations in density and temperature can cause gas to freeze out onto dust grains forming ice mantles, which influences the chemical composition of a cloud. The aim of this paper is to understand in detail the depletion (and desorption) of CO on (from) interstellar dust grains. Experimental simulations were performed under two different (astrophysically relevant) conditions. In parallel, Kinetic Monte Carlo simulations were used to mimic the experimental conditions. In our experiments, CO molecules accrete onto water ice at temperatures below 27 K, with a deposition rate that does not depend on the substrate temperature. During the warm-up phase, the desorption processes do exhibit subtle differences, indicating the presence of weakly bound CO molecules, therefore highlighting a low diffusion efficiency. IR measurements following the ice thickness during the TPD confirm that diffusion occurs at temperatures close to the desorption. Applied to astrophysical conditions, in a pre-stellar core, the binding energies of CO molecules, ranging between 300 and 850 K, depend on the conditions at which CO has been deposited. Because of this wide range of binding energies, the depletion of CO as a function of A V is much less important than initially thought. The weakly bound molecules, easily released into the gas phase through evaporation, change the balance between accretion and desorption, which result in a larger abundance of CO at high extinctions. In addition, weakly bound CO molecules are also more mobile, and this could increase the reactivity within interstellar ices.

Aims. We propose a method for measuring the projected rotational velocity v sin i with high precision even in spectra with blended lines. Though not automatic, our method is designed to be applied systematically to large numbers of objects without excessive computational requirement. Methods. We calculated the cross correlation function (CCF) of the object spectrum against a zero-rotation template and used the Fourier transform (FT) of the CCF central maximum to measure the parameter v sin i taking the limb darkening effect and its wavelength dependence into account. The procedure also improves the definition of the CCF base line, resulting in errors related to the continuum position under 1% even for v sin i = 280 km s −1 . Tests with high-resolution spectra of F-type stars indicate that an accuracy well below 1% can be attained even for spectra where most lines are blended. Results. We have applied the method to measuring v sin i in 251 A-type stars. For stars with v sin i over 30 km s −1 (2-3 times our spectra resolution), our measurement errors are below 2.5% with a typical value of 1%. We compare our results with Royer et al. (2002a) using 155 stars in common, finding systematic differences of about 5% for rapidly rotating stars.

This paper contains the second part of a study on a smart panel with five decentralized velocity feedback control units using proof mass electrodynamic actuators [Gonzalez Diaz et al., J. Acoust. Soc. Am. 124, 886 (2008)]. The implementation of five decentralized control loops is analyzed, both theoretically and experimentally. The stability properties of the five decentralized control units have been assessed with the generalized Nyquist criterion by plotting the loci of the eigenvalues of the fully populated matrix of frequency response functions between the five error signals and five input signals to the amplifiers driving the actuators. The control performance properties have been assessed in terms of the spatially averaged response of the panel measured with a scanning laser vibrometer and the total sound power radiated measured in an anechoic room. The two analyses have shown that reductions of up to 10 dB in both vibration response and sound radiation are measured at low audio frequencies, below about 250 Hz.

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This paper presents a simulation study on the downscaling of multiple electrodynamic proof mass actuators for the implementation of decentralized velocity feedback control loops on a thin panel. The system is conceived to reduce the panel response and sound radiation at low resonance frequencies. In the first part of the paper, the principal downscaling laws of a single proof mass actuator are revisited. In particular, the scaling laws are given for: (a) the fundamental natural frequency, (b) the damping factor, (c) the static displacement, (d) the maximum current that can be fed back to the actuator, (e) the maximum stroke of the proof mass and (f) the maximum control force that can be produced by the actuator. The second part of the paper presents a numerical study concerning the control performance produced by decentralized control systems with an increasing number of control units, which are scaled down in such a way as to keep the total base surface occupied by the actuators constant. This study shows that the control performance tends to rise as the number of control units is increased. However, this trend is reversed for large arrays of small scale actuators since the gain margin of the feedback loops tends to decrease with downscaling and incrementation of the actuators density.