Franz Kerschbaum

The Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory

The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments on ESAs far infrared and submil- limetre observatory. It employs two Ge:Ga photoconductor arrays (stressed and unstressed) with 16 × 25 pixels, each, and two filled silicon bolometer arrays with 16 × 32 and 32 × 64 pixels, respectively, to perform integral-field spectroscopy and imaging photom- etry in the 60−210 μm wavelength regime. In photometry mode, it simultaneously images two bands, 60−85 μ mo r 85−125 μ ma nd 125−210 μm, over a field of view of ∼1.75 � × 3.5 � , with close to Nyquist beam sampling in each band. In spectroscopy mode, it images afi eld of 47 �� × 47 �� , resolved into 5 × 5 pixels, with an instantaneous spectral coverage of ∼ 1500 km s −1 and a spectral resolution of ∼175 km s −1 . We summarise the design of the instrument, describe observing modes, calibration, and data analysis methods, and present our current assessment of the in-orbit performance of the instrument based on the performance verification tests. PACS is fully operational, and the achieved performance is close to or better than the pre-launch predictions.

Mass loss rates of a sample of irregular and semiregular M-type AGB-variables

We have determined mass loss rates and gas expansion velocities for a sample of 69 M-type irregular (IRV; 22 objects) and semiregular (SRV; 47 objects) AGB-variables using a radiative transfer code to model their circumstellar CO radio line emission. We believe that this sample is representative for the mass losing stars of this type. The (molecular hydrogen) mass loss rate distribution has a median value of 2:0 10 7 M yr 1 , and a minimum of 2:0 10 8 M yr 1 and a maximum of 8 10 7 M yr 1 . M-type IRVs and SRVs with a mass loss rate in excess of 5 10 7 M yr 1 must be very rare, and among these mass losing stars the number of sources with mass loss rates below a few 10 8 M yr 1 must be small. We find no significant dierence between the IRVs and the SRVs in terms of their mass loss characteristics. Among the SRVs the mass loss rate shows no dependence on the period. Likewise the mass loss rate shows no correlation with the stellar temperature. The gas expansion velocity distribution has a median of 7.0 km s 1 , and a minimum of 2.2 km s 1 and a maximum of 14.4 km s 1 . No doubt, these objects sample the low gas expansion velocity end of AGB winds. The fraction of objects with low gas expansion velocities is very high, about 30% have velocities lower than 5 km s 1 , and there are objects with velocities lower than 3 km s 1 : V584 Aql, T Ari, BI Car, RX Lac, and L 2 Pup. The mass loss rate and the gas expansion velocity correlate well, a result in line with theoretical predictions for an optically thin, dust-driven wind. In general, the model produces line profiles which acceptably fit the observed ones. An exceptional case is R Dor, where the high-quality, observed line profiles are essentially flat-topped, while the model ones are sharply double-peaked. The sample contains four sources with distinctly double-component CO line profiles, i.e., a narrow feature centered on a broader feature: EP Aqr, RV Boo, X Her, and SV Psc. We have modelled the two components separately for each star and derive mass loss rates and gas expansion velocities. We have compared the results of this M-star sample with a similar C-star sample analysed in the same way. The mass loss rate characteristics are very similar for the two samples. On the contrary, the gas expansion velocity distributions are clearly dierent. In particular, the number of low-velocity sources is much higher in the M-star sample. We found no example of the sharply double-peaked CO line profile, which is evidence of a large, detached CO-shell, among the M-stars. About 10% of the C-stars show this phenomenon.

Unexpectedly large mass loss during the thermal pulse cycle of the red giant star R Sculptoris.

The asymptotic-giant-branch star R Sculptoris is surrounded by a detached shell of dust and gas. The shell originates from a thermal pulse during which the star underwent a brief period of increased mass loss. It has hitherto been impossible to constrain observationally the timescales and mass-loss properties during and after a thermal pulse—parameters that determine the lifetime of the asymptotic giant branch and the amount of elements returned by the star. Here we report observations of CO emission from the circumstellar envelope and shell around R Sculptoris with an angular resolution of 1.3″. What was previously thought to be only a thin, spherical shell with a clumpy structure is revealed to also contain a spiral structure. Spiral structures associated with circumstellar envelopes have been previously seen, leading to the conclusion that the systems must be binaries. Combining the observational data with hydrodynamic simulations, we conclude that R Sculptoris is a binary system that underwent a thermal pulse about 1,800 years ago, lasting approximately 200 years. About 3 × 10−3 solar masses of material were ejected at a velocity of 14.3 km s−1 and at a rate around 30 times higher than the pre-pulse mass-loss rate. This shows that about three times more mass was returned to the interstellar medium during and immediately after the pulse than previously thought.

Infrared optical properties of spinels A study of the carrier of the 13, 17 and 32m emission features observed in ISO-SWS spectra of oxygen-rich AGB stars ?

In a previous paper, we have proposed magnesium aluminium spinel to be the carrier of the 13 and 17 m band features observed in the ISO spectra of some red giants. The IR optical properties of spinel strongly depend on its chemistry and its internal structure. To study the dependence of spinels IR-spectra on its aluminium content, we have synthesized a number of crystals with dierent Al/Mg-ratios. Additionally, we performed an annealing experiment to investigate the phase transition between ordered and partially disordered spinel taking place at about 1200 K. We derived sets of optical constants of our natural, annealed and synthetic spinels in order to calculate the absorption eciencies of small (sub-m-sized) spherical particles. Thereby, it turned out that natural as well as near-stoichiometric synthetic spinel can indeed be considered as a suitable candidate for the carrier of the 13m feature observed in the spectra of some oxygen rich circumstellar shells. To illustrate this, we reinvestigated the mean prole of the residual dust emission in the 12{18m wavelength range (i.e., in the so-called trough region between the two silicate bands). The reality of the emission feature at 16.8m is conrmed by our new investigation. We demonstrated that it is not an instrumental artifact since it is not present in ISO spectra of K-stars. In the course of our laboratory work, we found a third prominent emissivity maximum of spinel at 32m. This feature could also be detected in the spectra of the brightest 13m band emission sources; we derived its mean band prole, too. The new sets of optical constants here presented have been made available for public access in the electronic database http://www.astro.uni-jena.de.

Long period variables detected by ISO in the Small Magellanic Cloud

This article presents the study of the light-curves extracted from the MACHO database of a sample of stars observed by the Infrared Space Observatory in the Small Magellanic Cloud. These stars belong to the ISO-Mini-Survey catalogue of the Magellanic Clouds (ISO-MCMS, Loup et al. in preparation). Most of them are in the asymptotic giant branch (AGB) and supergiant phases. The dominant period and amplitude of pulsation have been derived and the stars have been classified as Mira or Semi-Regular pulsators. Furthermore, the cross-identification with near-infrared DENIS and 2MASS magnitudes available within the ISO-MCMS allowed us: (i) to investigate the properties of these stars in the combined near- and mid-infrared colour-magnitude diagrams, (ii) to derive the bolometric magnitude by integrating the spectral energy distribution and (iii) to estimate the mass-loss rate. The stars have been divided into carbon- (C-) and oxygen-rich (O-rich) using the (J − KS , KS ) colour-magnitude diagram and their period and amplitude distributions have been compared. C-rich AGB stars have a sharp peak in their period distribution at about 250 days and have on average a larger amplitude than O-rich AGB stars. This effect, not previously detected from the study of similar stars in the Large Magellanic Cloud and in the Baades window, might be closely related to the metallicity of the environment in which the stars have formed.

A census of AGB stars in Local Group galaxies - II. NGC 185 and NGC 147

We present results of our ongoing photometric survey of Local Group galaxies, using a four filter technique based on the method of Wing (1971) to identify and characterise the late-type stellar content. Two narrow band filters centred on spectral features of TiO and CN allow us to distinguish between AGB stars of different chemistries (M-type (O-rich) and C-type (C-rich)). The major parts of two dwarf galaxies of the M 31 subgroup - NGC 185 and NGC 147 - were observed. From photometry in V and i we estimate the tip of the RGB, and derive distance moduli respectively. With additional photometric data in the narrow band filters TiO and CN we identify 154 new AGB carbon stars in NGC 185 and 146 in NGC 147. C/M ratios are derived, as well as mean absolute magnitudesMi� , bolometric magnitudes Mbol, luminosity functions, and the spatial/radial distributions of the C stars in both galaxies.

Infrared Properties of Solid Titanium Oxides: Exploring Potential Primary Dust Condensates

We present optical constants and opacities of solid TiO2 ,T i 2O3, magnesium and calcium titanates, largely based on laboratory measurements. These dust species deserve interest as potential primary condensates in oxygen-rich dusty environments. Of the three known solid TiO2 phases, only one (rutile) has been extensively studied so far with respect to its mid-IR optical properties. We compare these with our measurements of the optical constants of anatase, brookite, and CaTiO3. Furthermore, for several Mg-Ti-oxides, powder transmission spectra are shown. While all known TiO2 modifications have their strongest bands between 13 and 13.5 lm (for spherical particles), CaTiO3, MgTiO3, and other Mg titanates have principal maxima of their absorption coefficients between 14 and 19 lm. This makes a spectroscopic identification of circumstellar Ti oxide particles rather difficult, because both the 13 and the 14–19 lm region are crowded with other features in the spectra of oxygen-rich circumstellar shells. Subject headings: circumstellar matter — methods: laboratory — molecular data — solar system: formation — stars: AGB and post-AGB — stars: mass loss

Multi-colour light variation of AGB stars observed with ISO

New visual light curves and infrared multi-epoch photometry are presented for a sample of AGB- stars spectroscopically observed with ISO. While the ISO work is or will be presented elsewhere, the aim of this paper is to give an overview of the properties of the light change of the objects. This information is crucial for the interpretation of the valuable ISO material. Using the University of Vienna Twin Automatic Photoelectric Telescope (APT) we monitored the stars of our sample in the photometric bands V and IC. We present the light curve of each object and derive the parameters of the current light change, such as period(s) and amplitude. Furthermore we give V IC colours and colour variations for these objects. Our results allow us to derive some general results on semiregular and irregular variables. Only in less than 50% of the cases could we conrm the GCVS period. Moreover, we did not nd any pronounced dierence between SRb and Lb variables in the regularity of the light curve. The existence of rapid oscillations indicated by Hipparcos data could not be conrmed. In addition to the visual light changes we present new near infrared photometry data. Although typically only few data points are available, they can be viewed relative to the better-monitored visual light curves providing information on possible phase shifts and dierences in amplitude in dierent parts of the spectrum. Furthermore, multi-epoch photometry allows us to derive mean colours for these objects.

The SAFARI Imaging Spectrometer for the SPICA space observatory

The Japanese SPace Infrared telescope for Cosmology and Astrophysics, SPICA, will provide astronomers with a long awaited new window on the universe. Having a large cold telescope cooled to only 6K above absolute zero, SPICA will provide a unique environment where instruments are limited only by the cosmic background itself. A consortium of European and Canadian institutes has been established to design and implement the SpicA FAR infrared Instrument SAFARI, an imaging spectrometer designed to fully exploit this extremely low far infrared background environment provided by the SPICA observatory. SAFARI’s large instantaneous field of view combined with the extremely sensitive Transition Edge Sensing detectors will allow astronomers to very efficiently map large areas of the sky in the far infrared – in a square degree survey of a 1000 hours many thousands of faint sources will be detected, and a very large fraction of these sources will be fully spectroscopically characterised by the instrument. Efficiently obtaining such a large number of complete spectra is essential to address several fundamental questions in current astrophysics: how do galaxies form and evolve over cosmic time?, what is the true nature of our own Milky Way?, and why and where do planets like those in our own solar system come into being?

An independent distance estimate to CW Leonis

CW Leo has been observed six times between October 2009 and June 2012 with the SPIRE instrument on board the Herschel satellite. Variability has been detected in the flux emitted by the central sta ...