No abstract available

No abstract available

No abstract available

No abstract available

[en] Quantum cascade lasers (QCLs) are becoming a key tool in several advanced experiments in the field of precision molecular spectroscopy and absolute frequency metrology. In view of this, a thorough control of their emission properties must be achieved, including a narrow linewidth as well as a high frequency-stability combined with referencing to a primary standard. Here, we give a review on recent developments and next perspectives in this scope, with particular regard to the use of QCLs in fundamental physics spectroscopic searches.

[en] We investigate the triggering and guiding of DC high-voltage discharges over a distance of 37 cm by filaments produced by ultraviolet (266 nm) laser pulses of 200 ps duration. The latter reduce the breakdown electric field by half and allow up to 80% discharge probability in an electric field of 920 kV m^–1. This high efficiency is not further increased by adding nanosecond pulses in the Joule range at 532 and at 1064 nm. However, the latter statistically increases the guiding length, thereby accelerating the discharge by a factor of 2. This effect is due both to photodetachment and to the heating of the plasma channel, that increases the efficiency of avalanche ionization and reduces electron attachment and recombination. (paper)

[en] A 7.5 square degree region of the galactic plane near 1 = 13⁰.5 has been surveyed with the Center for Astrophysics/University of Arizona's 1.02-m balloon-borne telescope and a 1 arcmin resolution 40 to 250 μm photometer. Forty-three far-infrared sources were detected. Comparison of the far-infrared and radio continuum flux densities of seventeen HII regions in the surveyed area indicates that dust does not significantly alter their ionization structure. Several of the far-infrared sources are not strong radio continuum sources and may be late-O or early-B stars embedded in the M17SW giant molecular cloud. The W33 complex has been resolved into several far-infrared sources for the first time. The far-infrared peak near the maser source W33A, a source probably at an early stage of stellar evolution, is unresolved with the 1 arcmin beam and has a bolometric luminosity of 1.6 x 10⁵L

[en] Results are presented of far-infrared photometric measurements of the Galactic Centre region. The observations have been made in two wavelength bands simultaneously, during balloon flights conducted in the south of France in 1972 and 1973 in co-operation with CNES

[en] The innermost tens of parsecs of our Galaxy are characterized by the presence of molecular cloud complexes surrounding Sgr A*, the radiative counterpart of the supermassive black hole (∼4 x 10⁶ M_sun) at the Galactic center. We seek to distinguish the different physical mechanisms that dominate the molecular clouds at the Galactic center, with special emphasis on the circumnuclear disk (CND). We also want to study the energy flow and model the variable emission of Sgr A*. Our study is based on NIR and submillimeter (sub-mm) observations. Using sub-mm maps, we describe the complex morphology of the molecular clouds and the circumnuclear disk, along with their masses (of order 10⁵-10⁶ M_sun), and derive also the temperature and spectral index maps of the regions under study. We conclude that the average temperature of the dust is 14 ± 4 K. The spectral index map shows that the 20 and 50 km s^-1 clouds are dominated by dust emission. Comparatively, in the CND and its surroundings the spectral indices decrease toward Sgr A* and range between about 1 and -0.6. These values are mostly explained with a combination of dust, synchrotron, and free-free emission in different ratios. The presence of non-thermal emission also accounts for the apparent low temperatures derived in these areas, indicating their unreliability. The Sgr A* light curves show significant flux density excursions in both the NIR and sub-mm domains. We have defined a classification system to account for the NIR variability of Sgr A*. Also, we have modeled on the NIR/sub-mm events. From our modeling results we can infer a sub-mm emission delay with respect to the NIR; we argue that the delay is due to the adiabatic expansion of the synchrotron source components.

[en] An optical vortex coronagraph has been implemented within the NIRC2 camera on the Keck II telescope and used to carry out on-sky tests and observations. The development of this new L ′-band observational mode is described, and an initial demonstration of the new capability is presented: a resolved image of the low-mass companion to HIP 79124, which had previously been detected by means of interferometry. With HIP 79124 B at a projected separation of 186.5 mas, both the small inner working angle of the vortex coronagraph and the related imaging improvements were crucial in imaging this close companion directly. Due to higher Strehl ratios and more relaxed contrasts in L ′ band versus H band, this new coronagraphic capability will enable high-contrast, small-angle observations of nearby young exoplanets and disks on a par with those of shorter-wavelength extreme adaptive optics coronagraphs.