Micron-sized forsterite grains in the pre-planetary nebula of IRAS 17150-3224 - Searching for clues on the mysterious evolution of massive AGB stars

We study the grain properties and location of the forsterite crystals in the circumstellar environment of the pre-planetary nebula (PPN) IRAS 17150-3224 in order to learn more about the as yet poorly understood evolutionary phase prior to the PPN. We use the best-fit model for IRAS 17150-3224 of Meixner et al. (2002) and add forsterite to this model. We investigate different spatial distributions and grain sizes of the forsterite crystals in the circumstellar environment. We compare the spectral bands of forsterite in the mid-infrared and at 69 micrometre in radiative transport models to those in ISO-SWS and Herschel/PACS observations. We can reproduce the non-detection of the mid-infrared bands and the detection of the 69 micrometre feature with models where the forsterite is distributed in the whole outflow, in the superwind region, or in the AGB-wind region emitted previous to the superwind, but we cannot discriminate between these three models. To reproduce the observed spectral bands with these three models, the forsterite crystals need to be dominated by a grain size population of 2 micrometre up to 6 micrometre. We hypothesise that the large forsterite crystals were formed after the superwind phase of IRAS 17150-3224, where the star developed an as yet unknown hyperwind with an extremely high mass-loss rate (10^-3 Msol/yr). The high densities of such a hyperwind could be responsible for the efficient grain growth of both amorphous and crystalline dust in the outflow. Several mechanisms are discussed that might explain the lower-limit of 2 micrometre found for the forsterite grains, but none are satisfactory. Among the mechanisms explored is a possible selection effect due to radiation pressure based on photon scattering on micron-sized grains.Comment: Accepted by A&

Photoprotection in intertidal benthic diatoms = Fotoprotectie bij intertidale benthische kiezelwieren

Benthic diatoms are dominant primary producers in intertidal marine sediments and are roughly divided into two main growth forms: the epipelon comprises mainly larger raphid motile diatoms and dominates silty sediments, whereas the epipsammon mainly consists of small motile and non-motile species that live in close association with single sand particles. As intertidal sediments are characterized by rapidly fluctuating and often extreme light conditions, benthic diatoms display behavioural as well as physiological photoprotection mechanisms. Vertical migration into the sediment (behavioural photoprotection), however, is largely restricted to epipelic diatoms, whereas epipsammic diatoms have to undergo changes in light conditions. As in natural communities (in situ studies) it is hard to characterize the photoprotective strategies of diatom growth forms (as natural communities can contain both epipelic and epipsammic growth forms), we studied the photoprotection capacity of unialgal isolates belonging to the main growth forms under controlled lab conditions.v One of the major physiological photoprotection mechanisms is to dissipate excess light energy as heat which can be measured as Non-Photochemical Quenching (NPQ). The capacity of this mechanism is mainly defined by the xanthophyll cycle (XC) pigment diatoxanthin and Light-harvesting Complex X (LHCX) proteins. We show that epipelic and epipsammic diatoms show fundamentally different photoprotective responses: epipsammic diatoms have a higher NPQ and associated XC capacities compared to epipelic diatoms. In the latter group, the behavioural response (vertical migration) is more important which may alleviate the need for strong physiological photoprotection. The regulation and performance of NPQ was further studied using model representatives of each functional group during and after exposure to high light. The epipsammic species Opephora guenter-grassii could rapidly switch NPQ on and off by relying on fast XC kinetics. This species also demonstrated high de novo synthesis of xanthophylls within a relatively short period of time (1 h), including significant amounts of zeaxanthin, a feature not observed before in other diatoms. In contrast, the epipelic representative Seminavis robusta showed slower NPQ and associated XC kinetics, partly relying on NPQ conferred by de novo synthetized diatoxanthin molecules and synthesis of Light-Harvesting Complex X (LHCX) isoforms. The genome of S. robusta contains fourteen LHCX genes. For eight LHCX genes we could show distinct upregulation during (strong) light exposure. While overall our results support the a trade-off between behavioural and physiological photoprotection mechanisms other factors besides growth form, such as environmental factors, cell size, substrate type and photoacclimation, may influence photoprotective strategies and explain species-specific photoregulation traits in intertidal benthic diatoms

The problematically short superwind of OH/IR stars - Probing the outflow with the 69 {\mu}m spectral band of forsterite

Spectra of OH/IR stars show prominent spectral bands of crystalline olivine (Mg$_{(2-2x)}$Fe$_{(2x)}$SiO$_{4}$). To learn more about the timescale of the outflows of OH/IR stars, we study the spectral band of crystalline olivine at 69 {\mu}m. The 69 {\mu}m band is of interest because its width and peak wavelength position are sensitive to the grain temperature and to the exact composition of the crystalline olivine. With Herschel/PACS, we observed the 69 {\mu}m band in the outflow of 14 OH/IR stars. By comparing the crystalline olivine features of our sample with those of model spectra, we determined the size of the outflow and its crystalline olivine abundance. The temperature indicated by the observed 69 {\mu}m bands can only be reproduced by models with a geometrically compact superwind ($R_{\rm{SW}}\lesssim$ 2500 AU = 1400 R$_{*}$).This means that the superwind started less than 1200 years ago (assuming an outflow velocity of 10 km/s). The small amount of mass lost in one superwind and the high progenitor mass of the OH/IR stars introduce a mass loss and thus evolutionary problem for these objects, which has not yet been understood.Comment: Accepted by A&

Understanding AGB evolution in Galactic bulge stars from high-resolution infrared spectroscopy

An analysis of high-resolution near-infrared spectra of a sample of 45 asymptotic giant branch (AGB) stars towards the Galactic bulge is presented. The sample consists of two subsamples, a larger one in the inner and intermediate bulge, and a smaller one in the outer bulge. The data are analysed with the help of hydrostatic model atmospheres and spectral synthesis. We derive the radial velocity of all stars, and the atmospheric chemical mix ([Fe/H], C/O, $^{12}$C/$^{13}$C, Al, Si, Ti, and Y) where possible. Our ability to model the spectra is mainly limited by the (in)completeness of atomic and molecular line lists, at least for temperatures down to $T_{\rm eff}\approx3100$ K. We find that the subsample in the inner and intermediate bulge is quite homogeneous, with a slightly sub-solar mean metallicity and only few stars with super-solar metallicity, in agreement with previous studies of non-variable M-type giants in the bulge. All sample stars are oxygen-rich, C/O$<$1.0. The C/O and carbon isotopic ratios suggest that third dredge-up (3DUP) is absent among the sample stars, except for two stars in the outer bulge that are known to contain technetium. These stars are also more metal-poor than the stars in the intermediate or inner bulge. Current stellar masses are determined from linear pulsation models. The masses, metallicities and 3DUP behaviour are compared to AGB evolutionary models. We conclude that these models are partly in conflict with our observations. Furthermore, we conclude that the stars in the inner and intermediate bulge belong to a more metal-rich population that follows bar-like kinematics, whereas the stars in the outer bulge belong to the metal-poor, spheroidal bulge population.Comment: 21 pages, 13 figures, 6 tables (incl. appendix), years of work, published in MNRA