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Search Results: 1 - 10 of 219519 matches for " C. Leinert "
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Unidentified Infrared Bands in the Interstellar Medium across the Galaxy
J. Kahanpaa,K. Mattila,K. Lehtinen,C. Leinert,D. Lemke
Physics , 2003, DOI: 10.1051/0004-6361:20030440
Abstract: We present a set of 6-12 micron ISOPHOT-S spectra of the general interstellar medium of the Milky Way. This part of the spectrum is dominated by a series of strong, wide emission features commonly called the Unidentified Infrared Bands. The sampled area covers the inner Milky Way from l = -60 degr to +60 degr with a ten-degree step in galactic longitude and nominal latitudes 0,+-1 degr. For each grid position the actual observed direction was selected from IRAS 100 micron maps to minimize contamination by point sources and molecular clouds. All spectra were found to display the same spectral features. Band ratios are independent of band strength and Galactic coordinates. A comparison of total observed flux in band features and IRAS 100 micron emission, a tracer for large interstellar dust grains, shows high correlation at large as well as small 1-arcmin scales. This implies a strong connection between large dust grains and the elusive band carriers; the evolutionary history and heating energy source of these populations must be strongly linked. The average mid-infrared spectrum of the Milky Way is found to be similar to the average spectrum of spiral galaxy NGC 891 and the spectra of other spirals. The common spectrum can therefore be used as a template for the 6-12 micron emission of late-type spiral galaxies. Finally, we show that interstellar extinction only weakly influences the observed features even at lambda = 10 microns, where the silicate absorption feature is strongest.
Determination of the cosmic far-infrared background level with the ISOPHOT instrument
M. Juvela,K. Mattila,D. Lemke,U. Klaas,C. Leinert,Cs. Kiss
Physics , 2009, DOI: 10.1051/0004-6361/200811351
Abstract: The cosmic infrared background (CIRB) consists mainly of the integrated light of distant galaxies. In the far-infrared the current estimates of its surface brightness are based on the measurements of the COBE satellite. Independent confirmation of these results is still needed from other instruments. In this paper we derive estimates of the far-infrared CIRB using measurements made with the ISOPHOT instrument aboard the ISO satellite. The results are used to seek further confirmation of the CIRB levels that have been derived by various groups using the COBE data. We study three regions of very low cirrus emission. The surface brightness observed with the ISOPHOT instrument at 90, 150, and 180 um is correlated with hydrogen 21 cm line data from the Effelsberg radio telescope. Extrapolation to zero hydrogen column density gives an estimate for the sum of extragalactic signal plus zodiacal light. The zodiacal light is subtracted using ISOPHOT data at shorter wavelengths. Thus, the resulting estimate of the far-infrared CIRB is based on ISO measurements alone. In the range 150 to 180 um, we obtain a CIRB value of 1.08+-0.32+-0.30 MJy/sr quoting statistical and systematic errors separately. In the 90 um band, we obtain a 2-sigma upper limit of 2.3 MJy/sr. The estimates derived from ISOPHOT far-infrared maps are consistent with the earlier COBE results.
The structure of disks around intermediate-mass young stars from mid-infrared interferometry. Evidence for a population of group II disks with gaps
J. Menu,R. van Boekel,Th. Henning,Ch. Leinert,C. Waelkens,L. B. F. M. Waters
Physics , 2015, DOI: 10.1051/0004-6361/201525654
Abstract: The disks around Herbig Ae/Be stars are commonly divided into group I and group II based on their far-infrared spectral energy distribution, and the common interpretation for that is flared and flat disks. Recent observations suggest that many flaring disks have gaps, whereas flat disks are thought to be gapless. The different groups of objects can be expected to have different structural signatures in high-angular-resolution data. Over the past 10 years, the MIDI instrument on the Very Large Telescope Interferometer has collected observations of several tens of protoplanetary disks. We model the large set of observations with simple geometric models. A population of radiative-transfer models is synthesized for interpreting the mid-infrared signatures. Objects with similar luminosities show very different disk sizes in the mid-infrared. Restricting to the young objects of intermediate mass, we confirm that most group I disks are in agreement with being transitional. We find that several group II objects have mid-infrared sizes and colors overlapping with sources classified as group I, transition disks. This suggests that these sources have gaps, which has been demonstrated for a subset of them. This may point to an intermediate population between gapless and transition disks. Flat disks with gaps are most likely descendants of flat disks without gaps. Gaps, potentially related to the formation of massive bodies, may therefore even develop in disks in a far stage of grain growth and settling. The evolutionary implications of this new population could be twofold. Either gapped flat disks form a separate population of evolved disks, or some of them may further evolve into flaring disks with large gaps. The latter transformation may be governed by the interaction with a massive planet, carving a large gap and dynamically exciting the grain population in the disk.
The mid-infrared diameter of W Hydrae
R. Zhao-Geisler,A. Quirrenbach,R. Koehler,B. Lopez,C. Leinert
Physics , 2011, DOI: 10.1051/0004-6361/201016310
Abstract: Mid-infrared (8-13 microns) interferometric data of W Hya were obtained with MIDI/VLTI between April 2007 and September 2009, covering nearly three pulsation cycles. The spectrally dispersed visibility data of all 75 observations were analyzed by fitting a circular fully limb-darkened disk (FDD) model to all data and individual pulsation phases. Asymmetries were studied with an elliptical FDD. Modeling results in an apparent angular FDD diameter of W Hya of about (80 +/- 1.2) mas (7.8 AU) between 8 and 10 microns, which corresponds to an about 1.9 times larger diameter than the photospheric one. The diameter gradually increases up to (105 +/- 1.2) mas (10.3 AU) at 12 microns. In contrast, the FDD relative flux fraction decreases from (0.85 +/- 0.02) to (0.77 +/- 0.02), reflecting the increased flux contribution from a fully resolved surrounding silicate dust shell. The asymmetric character of the extended structure could be confirmed. An elliptical FDD yields a position angle of (11 +/- 20) deg and an axis ratio of (0.87 +/- 0.07). A weak pulsation dependency is revealed with a diameter increase of (5.4 +/- 1.8) mas between visual minimum and maximum, while detected cycle-to-cycle variations are smaller. W Hya's diameter shows a behavior that is very similar to the Mira stars RR Sco and S Ori and can be described by an analogous model. The constant diameter part results from a partially resolved stellar disk, including a close molecular layer of H2O, while the increase beyond 10 microns can most likely be attributed to the contribution of a spatially resolved nearby Al2O3 dust shell.
Separable $C^{\ast}$-Algebras and weak$^{\ast}$-fixed point property
Gero Fendler,Michael Leinert
Mathematics , 2013,
Abstract: It is shown that the dual $\hat{A}$ of a separable $C^{\ast}$-algebra $A$ is discrete if and only if its Banach space dual has the weak$^{\ast}$-fixed point property. We prove further that these properties are equivalent to the uniform weak$^{\ast}$ Kadec-Klee property of $A^{\ast}$ and to the coincidence of the weak$^{\ast}$ topology with the norm topology on the pure states of $A$.
Approximation in $AC(σ)$
Ian Doust,Michael Leinert
Mathematics , 2013,
Abstract: In order to extend the theory of well-bounded operators to include operators with nonreal spectrum, Ashton and Doust introduced definitions for two new algebras of functions defined on a nonempty compact subset $\sigma$ of the plane. These are the functions of bounded variation and the absolutely continuous functions on $\sigma$. Proofs involving absolutely continuous functions usually require that one first works with elements of a dense subset and then take limits. In this paper we present some new theorems about approximating absolutely continuous functions as well as providing missing proofs for some important earlier results.
Isomorphisms of $AC(σ)$ spaces
Ian Doust,Michael Leinert
Mathematics , 2013,
Abstract: Analogues of the classical Banach-Stone theorem for spaces of continuous functions are studied in the context of the spaces of absolutely continuous functions introduced by Ashton and Doust. We show that if $AC(\sigma_1)$ is algebra isomorphic to $AC(\sigma_2)$ then $\sigma_1$ is homeomorphic to $\sigma_2$. The converse however is false. In a positive direction we show that the converse implication does hold if the sets $\sigma_1$ and $\sigma_2$ are confined to a restricted collection of compact sets, such as the set of all simple polygons.
Inverse-Closed Subalgebras of Noncommutative Tori
Karlheinz Gr?chenig,Michael Leinert
Mathematics , 2012,
Abstract: We give a systematic construction of inverse-closed (Banach) subalgebras in general higher-dimensional non-commutative tori
Mid-Infrared Spectral Variability Atlas of Young Stellar Objects
á. Kóspál,P. ábrahám,J. A. Acosta-Pulido,C. P. Dullemond,Th. Henning,M. Kun,Ch. Leinert,A. Moór,N. J. Turner
Physics , 2012, DOI: 10.1088/0067-0049/201/2/11
Abstract: Optical and near-infrared variability is a well-known property of young stellar objects. However, a growing number of recent studies claim that a considerable fraction of them also exhibit mid-infrared flux changes. With the aim of studying and interpreting variability on a decadal timescale, here we present a mid-infrared spectral atlas containing observations of 68 low- and intermediate mass young stellar objects. The atlas consists of 2.5-11.6 um low-resolution spectra obtained with the ISOPHOT-S instrument on-board the Infrared Space Observatory (ISO) between 1996 and 1998, as well as 5.2-14.5 um low-resolution spectra obtained with the IRS instrument on-board the Spitzer Space Telescope between 2004 and 2007. The observations were retrieved from the ISO and Spitzer archives and were post-processed interactively by our own routines. For those 47 objects where multi-epoch spectra were available, we analyze mid-infrared spectral variability on annual and/or decadal timescales. We identify 37 variable candidate sources. Many stars show wavelength-independent flux changes, possibly due to variable accretion rate. In several systems, all exhibiting 10 um silicate emission, the variability of the 6-8 um continuum and the silicate feature exhibit different amplitudes. A possible explanation is variable shadowing of the silicate emitting region by an inner disk structure of changing height or extra silicate emission from dust clouds in the disk atmosphere. Our results suggest that mid-infrared variability, in particular the wavelength-dependent changes, are more ubiquitous than was known before. Interpreting this variability is a new possibility to explore the structure of the disk and its dynamical processes.
The very low mass multiple system LHS\,1070 -- a testbed for model atmospheres for the lower end of the main sequence
A. S. Rajpurohit,C. Reyle,M. Schultheis,Ch. Leinert,F. Allard,D. Homeier,T. Ratzka,P. Abraham,B. Moster,S. Witte,N. Ryde
Physics , 2012, DOI: 10.1051/0004-6361/201219029
Abstract: LHS1070 is a nearby multiple system of low mass stars. It is an important source of information for probing the low mass end of the main sequence, down to the hydrogen-burning limit. The primary of the system is a mid-M dwarf and two components are late-M to early L dwarfs, at the star-brown dwarf transition. Hence LHS1070 is a valuable object to understand the onset of dust formation in cool stellar atmospheres.This work aims at determining the fundamental stellar parameters of LHS1070 and to test recent model atmospheres: BT-Dusty,BT-Settl, DRIFT, and MARCS models.Unlike in previous studies, we have performed a chi^2-minimization comparing well calibrated optical and infrared spectra with recent cool star synthetic spectra leading to the determination of the physical stellar parameters Teff, radius, and log g for each of the three components of LHS1070.
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