PAH Strength and the Interstellar Radiation Field around the Massive Young Cluster NGC3603

We present spatial distribution of polycyclic aromatic hydrocarbons and ionized gas within the Galactic giant HII region NGC3603. Using the IRS instrument on board the Spitzer Space Telescope, we study in particular the PAH emission features at ~5.7, 6.2, 7.7, 8.6, and 11.3um, and the [ArII] 6.99um, [NeII] 12.81um, [ArIII] 8.99um, and [SIV] 10.51um forbidden emission lines. The observations probe both ionized regions and photodissociation regions. Silicate emission is detected close to the central cluster while silicate absorption is seen further away. We find no significant variation of the PAH ionization fraction across the whole region. The emission of very small grains lies closer to the central stellar cluster than emission of PAHs. The PAH/VSG ratio anticorrelates with the hardness of the interstellar radiation field suggesting a destruction mechanism of the molecules within the ionized gas, as shown for low-metallicity galaxies by Madden et al. (2006).Comment: Accepted for publication in ApJ. Corrected typo

Atmospheric refractivity effects on mid-infrared ELT adaptive optics

We discuss the effect of atmospheric dispersion on the performance of a mid-infrared adaptive optics assisted instrument on an extremely large telescope (ELT). Dispersion and atmospheric chromaticity is generally considered to be negligible in this wavelength regime. It is shown here, however, that with the much-reduced diffraction limit size on an ELT and the need for diffraction-limited performance, refractivity phenomena should be carefully considered in the design and operation of such an instrument. We include an overview of the theory of refractivity, and the influence of infrared resonances caused by the presence of water vapour and other constituents in the atmosphere. `Traditional' atmospheric dispersion is likely to cause a loss of Strehl only at the shortest wavelengths (L-band). A more likely source of error is the difference in wavelengths at which the wavefront is sensed and corrected, leading to pointing offsets between wavefront sensor and science instrument that evolve with time over a long exposure. Infrared radiation is also subject to additional turbulence caused by the presence of water vapour in the atmosphere not seen by visible wavefront sensors, whose effect is poorly understood. We make use of information obtained at radio wavelengths to make a first-order estimate of its effect on the performance of a mid-IR ground-based instrument. The calculations in this paper are performed using parameters from two different sites, one `standard good site' and one `high and dry site' to illustrate the importance of the choice of site for an ELT.Comment: 11 pages, to be published in SPIE Proceedings vol. 7015, Adaptive Optics Systems, eds. N. Hubin, C.E. Max and P.L. Wizinowich, 200

NGC 3603 - a Local Template for Massive Young Clusters

We present a study of the star cluster associated with the massive Galactic HII region NGC3603 based on near-IR broad-- and narrowband observations taken with ISAAC/VLT under excellent seeing conditions (<0.4''). We discuss color-color diagrams and address the impact of the high UV flux on the disk evolution of the low-mass stars.Comment: 3 pages, 3 figures. To appear in the Proceedings of IAU Symposium 207 "Extragalactic Star Clusters", eds. E. Grebel, D. Geisler and D. Minitt

Elemental Abundances of Blue Compact Dwarfs from mid-IR Spectroscopy with Spitzer

We present a study of elemental abundances in a sample of thirteen Blue Compact Dwarf (BCD) galaxies, using the $\sim$10--37$\mu$m high resolution spectra obtained with Spitzer/IRS. We derive the abundances of neon and sulfur for our sample using the infrared fine-structure lines probing regions which may be obscured by dust in the optical and compare our results with similar infrared studies of starburst galaxies from ISO. We find a good correlation between the neon and sulfur abundances, though sulfur is under-abundant relative to neon with respect to the solar value. A comparison of the elemental abundances (neon, sulfur) measured from the infrared data with those derived from the optical (neon, sulfur, oxygen) studies reveals a good overall agreement for sulfur, while the infrared derived neon abundances are slightly higher than the optical values. This indicates that either the metallicities of dust enshrouded regions in BCDs are similar to the optically accessible regions, or that if they are different they do not contribute substantially to the total infrared emission of the host galaxy.Comment: 11 pages, 6 figures, accepted by Ap

Modifizierung der Struktur und optoelektronischen Eigenschaften durch Mischungsbildung und Druck

An kolumnar-diskotischen Triphenylenderivaten wurde der Einfluß von hydrostatischem Druck und binärer Mischungsbildung auf Struktur und optoelektronischen Eigenschaften der flüssigkristallinen Phase untersucht. Dazu wurden Messungen mittels DSC, Polarisationsmikroskopie,Röntgenstrukturanalyse mit Synchrotronstrahlung, UV-VIS-Spektroskopie unter Druck und an orientierten Proben und DK-Messungen durchgeführt. Die Kompressibilitäten der flüssigkristallinen Phase wurden aus dem Experiment berechnet. Unter anderem hatte die binäre Mischungsbildung einen Einfluß auf die Morphologie und die Kristallisation der Triphenylene

Chemical composition and mixing in giant HII regions: NGC3603, 30Doradus, and N66

We investigate the chemical abundances of NGC3603 in the Milky Way, of 30Doradus in the Large Magellanic Cloud, and of N66 in the Small Magellanic Cloud. Mid-infrared observations with the Infrared Spectrograph onboard the Spitzer Space Telescope allow us to probe the properties of distinct physical regions within each object: the central ionizing cluster, the surrounding ionized gas, photodissociation regions, and buried stellar clusters. We detect [SIII], [SIV], [ArIII], [NeII], [NeIII], [FeII], and [FeIII] lines and derive the ionic abundances. Based on the ionic abundance ratio (NeIII/H)/(SIII/H), we find that the gas observed in the MIR is characterized by a higher degree of ionization than the gas observed in the optical spectra. We compute the elemental abundances of Ne, S, Ar, and Fe. We find that the alpha-elements Ne, S, and Ar scale with each other. Our determinations agree well with the abundances derived from the optical. The Ne/S ratio is higher than the solar value in the three giant HII regions and points toward a moderate depletion of sulfur on dust grains. We find that the neon and sulfur abundances display a remarkably small dispersion (0.11dex in 15 positions in 30Doradus), suggesting a relatively homogeneous ISM, even though small-scale mixing cannot be ruled out.Comment: Accepted for submission to ApJ. The present version replaces the submitted one. Changes: new title, new figure, the text was modified in the discussio

Spin Disorder and Magnetic Anisotropy in Fe3O4 Nanoparticles

We have studied the magnetic behavior of dextran-coated magnetite (Fe$_3$O$_4$) nanoparticles with median particle size \left=8 $nm$. Magnetization curves and in-field M\"ossbauer spectroscopy measurements showed that the magnetic moment $M_S$ of the particles was much smaller than the bulk material. However, we found no evidence of magnetic irreversibility or non-saturating behavior at high fields, usually associated to spin canting. The values of magnetic anisotropy $K_{eff}$ from different techniques indicate that surface or shape contributions are negligible. It is proposed that these particles have bulk-like ferrimagnetic structure with ordered A and B sublattices, but nearly compensated magnetic moments. The dependence of the blocking temperature with frequency and applied fields, $T_B(H,\omega)$, suggests that the observed non-monotonic behavior is governed by the strength of interparticle interactions.Comment: 11 pages, 7 figures, 3 Table