Showing papers by "Oliver Krause published in 2006"

Spitzer/mips infrared imaging of m31: further evidence for a spiral/ring composite structure

Abstract: New images of M31 at 24, 70, and 160 μm taken with the Multiband Imaging Photometer for Spitzer (MIPS) reveal the morphology of the dust in this galaxy. This morphology is well represented by a composite of two logarithmic spiral arms and a circular ring (radius ~10 kpc) of star formation offset from the nucleus. The two spiral arms appear to start at the ends of a bar in the nuclear region and extend beyond the star-forming ring. As has been found in previous work, the spiral arms are not continuous, but composed of spiral segments. The star-forming ring is very circular except for a region near M32 where it splits. The lack of well-defined spiral arms and the prominence of the nearly circular ring suggest that M31 has been distorted by interactions with its satellite galaxies. Using new dynamical simulations of M31 interacting with M32 and NGC 205, we find that, qualitatively, such interactions can produce an offset, split ring like that seen in the MIPS images.

Very Cold and Massive Cores near ISOSS J18364–0221: Implications for the Initial Conditions of High-Mass Star Formation

Abstract: We report the discovery of two very cold and massive molecular cloud cores in the region ISOSS J18364-0221. The object has been identified by a systematic search for very early evolutionary stages of high-mass stars using the 170 μm ISOPHOT Serendipity Survey (ISOSS). Submillimeter continuum and molecular line measurements reveal two compact cores within this region. The first core has a temperature of 16.5 K, shows signs of ongoing infall and outflows, has no near- or mid-infrared counterpart, and is massive enough (M ~ 75 M☉) to form at least one O star with an associated cluster. It is therefore considered a candidate for a genuine high-mass protostar and a high-mass analog to the Class 0 objects. The second core has an average gas and dust temperature of only ~12 K and a mass of M ~ 280 M☉. Its temperature and level of turbulence are below the values found for massive cores so far, and we suggest that this represents the initial conditions from which high-mass star formation occurs.

Star formation in the Cepheus Flare region: Implications from morphology and infrared properties of optically selected clouds

Abstract: Aims. A complex study of visual and infrared properties of ISM and an examination of cloud morphology was carried out using USNO, 2MASS, DIRBE, IRAS, and ISO data of an extended region in Cepheus to explore the conditions of cloud and star formation. Methods. We mapped the distribution of visual extinction in a 256 square degree area of Cepheus. We identified clouds and described their morphology quantitatively to find features such as globular and head-tail shape. We also characterized the region using infrared data and combined the results with those obtained from visual data. Results. Eight cloud complexes and four voids were identified and 208 dark clouds were localised, 86 of which had not been previously catalogued. The observed distribution of cloud axis ratios corresponds to near-prolate ellipsoidal clouds with random alignment. The cloud mass spectrum derived is dN cloud /dM cloud = 6.1 x 10 2 (M cloud /M o ) -1.70 M -1 o . We pointed out a linear relationship between FIR colour temperature and galactic latitude at intermediate galactic latitudes (3° < b < 18°) and a similar relationship for minimum cloud temperatures. The observed FIR colours of dark clouds were found to agree well with model calculations. We showed the signs of connection between the far-IR loop GIRL G109+11 and the active triggered star formation of adjacent cloud complexes. Relationships between the star-forming efficiency, peak extinction, and cloud mass were recognized and described with empirical formulae. YSOs were found in 7 of the 11 clouds with extinction above 4 mag. These 11 clouds represent 21% of the total cloud mass and contain 52% of all the YSO candidates. High star-forming efficiencies, 0.52% and 0.56%, were found in globular and head-tail clouds, respectively, relative to the mean efficiency for the clouds in the region (0.16%). Conclusions. Cloud mass and peak extinction were found to be the most important factors of star-forming efficiency besides cloud morphology. The estimated intrinsic axis-ratio distribution is consistent with cloud formation from large-scale external forcing.

The grating and filter wheels for the JWST NIRSpec instrument

Abstract: The Near-Infrared Spectrograph (NIRSpec) onboard the James Webb Space Telescope can be reconfigured in space for astronomical observation in a range of filter bands as well as spectral resolutions. This will be achieved using a Filter wheel (FWA) which carries 7 transmission filters and a Grating wheel (GWA) which carries six gratings and one prism. The large temperature shift between warm launch and cryogenic operation (30K) and high launch vibration loads on the one hand side and accurate positioning capability and minimum deformation of optical components on the other hand side must be consolidated into a single mechanical design which will be achieved using space-proven concepts derived from the successful ISO filter wheel mechanisms which were manufactured and tested by Carl Zeiss. Carl Zeiss Optronics has been selected by Astrium GmbH for the implementation of both NIRSpec wheel mechanisms. Austrian Aerospace and Max-Planck-Institut fur Astronomie Heidelberg (MPIA) will contribute major work shares to the project. The project was started in October 2005 and the preliminary designs have been finalized recently. Critical performance parameters are properly allocated to respective hardware components, procurements of long-lead items have been initiated and breadboard tests have started. This paper presents an overview of the mechanism designs, discusses its properties and the approach for component level tests.

The cold focal plane chopper of HERSCHEL's PACS instrument

Abstract: HERSCHEL’s 3.5 m primary mirror will be passively cooled to T ~ 80 K in the L2 orbit. In order to reduce the effects of the remaining high thermal background on the sensitive far in frared detectors (60..210 µm), a focal plane chopper is a vital element in the entrance optics of the imaging and spectroscopic instrument PACS. A gold coated 32 × 26 mm 2 plane mirror, suspended by two flexural pivots and driven by a linear motor, allows for precise square wave chopping with up to 9° throw at a frequency 10 Hz with a position ac curacy of 1 arcmin. The power required at T ~ 4 K is about 1 mW. The chopper has undergone an extensive qualification programme, including 650 million cold chop throws, 15 cold-warm-cold thermal cycles, 3-axis 26 G-vibration at T~4 K etc. Five models were built and thoroughly tested; the flight model of the chopper is now integrated into the flight model of PACS, ready for the HERSCHEL/PLANCK launch in 2008 by an ARIANE5 rocket and the following 5-year mission.

Cryogenic filter- and spectrometer wheels for the Mid Infrared Instrument (MIRI) of the James Webb Space Telescope (JWST)

Abstract: Following a warm launch in 2013 the MIRI instrument aboard JWST will be operated for a lifetime of 5-10 years in the L2-orbit at a temperature of ~6 K. The main requirements for its three wheel mechanisms include: (1) reliability, (2) optical precision, (3) low power dissipation, (4) high vibration capability, (5) functionality at 4 < T < 300 K. The filter wheel carries broad and narrow band spectral filters, coronographic masks and a prism on its 18 positions. Each of the two spectrometer wheels is equipped with two disks on both sides of a central torque motor, one of them carries 6 gratings, the other a dichroic/mirror arrangement. The optical positions are defined by a ratchet mechanism. No closed loop control is required; therefore the long time average heat dissipation is negligible. A new ratchet mechanism had to be developed to satisfy a 120° increment of only three positions for the spectrometer wheels. Extensive cold and warm tests were performed on the development models of the filter and spectrometer wheels at MPIA. These results stimulated numerous improvements in the mechanical and thermal design which are now to be implemented in the qualification and flight models developed jointly with Carl Zeiss. Synergies are expected from a similar development of the NIRSPEC wheels, in which MPIA and Carl Zeiss are involved.

Spitzer/MIPS Infrared Imaging of M31: Further Evidence for a Spiral/Ring Composite Structure

Abstract: New images of M31 at 24, 70, and 160 micron taken with the Multiband Imaging Photometer for Spitzer (MIPS) reveal the morphology of the dust in this galaxy. This morphology is well represented by a composite of two logarithmic spiral arms and a circular ring (radius ~10 kpc) of star formation offset from the nucleus. The two spiral arms appear to start at the ends of a bar in the nuclear region and extend beyond the star forming ring. As has been found in previous work, the spiral arms are not continuous but composed of spiral segments. The star forming ring is very circular except for a region near M32 where it splits. The lack of well defined spiral arms and the prominence of the nearly circular ring suggests that M31 has been distorted by interactions with its satellite galaxies. Using new dynamical simulations of M31 interacting with M32 and NGC 205 we find that, qualitatively, such interactions can produce an offset, split ring like that seen in the MIPS images.

Laboratory simulation of cosmic radiation effects on stressed Ge:Ga detectors at L2 and curing

Abstract: The Photodetector Array Camera and Spectrometer (PACS) is one of three science instruments on board the Herschel space observatory to be launched in 2008. It will perform imaging photometry and spectroscopy in the wavelength range from 57 μm to 210 μm. The integral field spectrometer contains two 25 x 16 pixel cameras of Gallium doped Germanium crystals (Ge:Ga). By stressing these crystals, cutoff wavelengths of 127 μm (low-stressed, 200 N) and 205 μm (high-stressed, 800 N) are reached. The characterization of these detectors (responsivity, noise equivalent power (NEP), dark current,...) is carried out at the Max-Planck-Institutes for Astronomy (MPIA, Heidelberg) and Extraterrestrial Physics (MPE, Garching). Both test facilities allow simulation of the in-flight operational conditions of the arrays and provide accurate IR fluxes by means of external/internal black bodies and calibrated cold attenuation filters. A radioactive 137Cs source is used at MPIA to simulate the steady cosmic radiation impact on the photoconductor arrays in order to study the radiation induced changes in responsivity, noise, and the transient behavior. The goal is to determine the optimal operating parameters (temperature, bias, integration time,..) for the operation at the L2 orbit, the best curing method, curing frequency and calibration procedure for high photometric accuracy. The "lessons learned" on operating, curing, deglitching and calibrating stressed Ge:Ga detectors during the ISO mission are applied as well as the relevant reports from IRAS and Spitzer.

A test setup for the characterization of far-infrared filters under cryogenic conditions

Abstract: The characterization and calibration of far-infrared (FIR) detectors is a delicate task that requires good knowledge of the incident flux and its spectral composition. In many test setups the FIR flux to the detectors is provided by means of an external or internal black body and a set of cold attenuation, band pass, and blocking filters. For scientific instruments (e.g. PACS aboard ESA's Herschel satellite) band pass and blocking filters are used to achieve the desired spectral throughput either as order sorting filters in spectrometers or for selecting a wavelength range in imaging cameras. In all cases a detailed knowledge of the spectral transmittance of the used filters is mandatory for an accurate calibration of the system. We have build a test platform that allows to measure the transmission of cold (T ~ 4K) filters in the far-infrared. The setup uses a dual grating monochromator with excellent spectral purity and a resolution up to 800, which is operated under a dry nitrogen atmosphere to eliminate water vapor absorption bands. An Si-bolometer is used as detector and is read out by a cryogenic low noise trans-impedance amplifier circuit with common mode rejection and a warm electronics using a lock-in amplifier and a 22 bit analog-to-digital converter. A cryogenic filter slider in the setup allows for differential measurements between filters and the use of cold order sorting filters. We present initial results for FIR cut-on and attenuation filters, demonstrating that our setup is suited to measure transmissions as low as 10 -4 over the covered wavelength range.