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Context. Circumstellar discs are ubiquitous around young stars, but rapidly dissipate their gas and dust on timescales of a few Myr. The Herschel Space Observatory allows for the study of the warm disc atmosphere, using far-infrared spectroscopy to measure gas content and excitation conditions, and far-IR photometry to constrain the dust distribution.Aims. We aim to detect and characterize the gas content of circumstellar discs in four targets as part of the Herschel science demonstration phase.Methods. We carried out sensitive medium resolution spectroscopy and high sensitivity photometry at gimel similar to 60-190 mu m using the Photodetector Array Camera and Spectrometer instrument on the Herschel Space Observatory.Results. We detect [OI] 63 mu m emission from the young stars HD 169142, TW Hydrae, and RECX 15, but not HD 181327. No other lines, including [CII] 158 and [OI] 145, are significantly detected. All four stars are detected in photometry at 70 and 160 mu m. Extensive models are presented in associated papers.

A large proportion of northern peatlands consists of Sphagnum-dominated ombrotrophic bogs. In these bogs, peat mosses (Sphagnum) and vascular plants occur in an apparent stable equilibrium, thereby sustaining the carbon sink function of the bog ecosystem. How global warming and increased nitrogen (N) deposition will affect the species composition in bog vegetation is still unclear. We performed a transplantation experiment in which mesocosms with intact vegetation were transplanted southward from north Sweden to north-east Germany along a transect of four bog sites, in which both temperature and N deposition increased. In addition, we monitored undisturbed vegetation in control plots at the four sites of the latitudinal gradient. Four growing seasons after transplantation, ericaceous dwarf shrubs had become much more abundant when transplanted to the warmest site which also had highest N deposition. As a result ericoid aboveground biomass in the transplanted mesocosms increased most at the southernmost site, this site also had highest ericoid biomass in the undisturbed vegetation. The two dominant Sphagnum species showed opposing responses when transplanted southward; Sphagnum balticum height increment decreased, whereas S. fuscum height increment increased when transplanted southward. Sphagnum production did not differ significantly among the transplanted mesocosms, but was lowest in the southernmost control plots. The dwarf shrub expansion and increased N concentrations in plant tissues we observed, point in the direction of a positive feedback toward vascular plant-dominance suppressing peat-forming Sphagnum in the long term. However, our data also indicate that precipitation and phosphorus availability influence the competitive balance between Sphagnum, dwarf shrubs and graminoids.

The far-infrared spectra of circumstellar envelopes around various oxygen-rich stars were observed using the ISO Long Wavelength Spectrometer (LWS). We have examined high signal-to-noise ISO LWS observations of the luminous supergiant star, VY CMa, with the aim of identifying all of the spectral lines. By paying particular attention to water lines, we aim to separate the lines due to other species, in particular, to prepare for forthcoming observations that will cover the same spectral range using Herschel PACS and at higher spectral resolution using Herschel HIFI and SOFIA. We have developed a fitting method to account for blended water lines using a simple weighting scheme to distribute the flux. We have applied this approach to several other stars which we compare with VY CMa. We present line fluxes for the unblended H2O and CO lines, and present detections of several possible nu_2=1 vibrationally excited water lines. We also identify blended lines of OH, one unblended and several blended lines of NH3, and one possible detection of H3O+., 15 pages, 6 figures, accepted for publication in A&A

Context: Red super-giant (RSG) stars exhibit significant mass loss through a slow and dense wind. They are often considered to be the more massive counter parts of Asymptotic Giant Branch (AGB) stars. While the AGB mass loss is linked to their strong pulsations, the RSG are often only weakly variable. Aim: To study the conditions at the base of the wind, by determining the dust composition in a sample of RSG. The dust composition is thought to be sensitive to the density, temperature and acceleration at the base of the wind. Method: We compile a sample of 27 RSG infrared spectra (ISO-SWS) and supplement these with photometric measurements to obtain the full spectral energy distribution (SED). These data are modelled using a dust radiative transfer code. The results are scrutinised for correlations. Results: We find (1) strong correlations between dust composition, mass-loss rate and stellar luminosity, roughly in agreement with the theoretical dust condensation sequence, (2) the need for a continuous (near-)IR dust opacity and tentatively propose amorphous carbon, and (3) significant differences with AGB star winds: presence of PAHs, absence of 'the' 13 micron band, and a lack of strong water bands. Conclusions: Dust condensation in RSG is found to experience a similar freeze-out process as in AGB stars. Together with the positive effect of the stellar luminosity on the mass-loss rate, this suggests that radiation pressure on dust grains is an important ingredient in the driving mechanism. Still, differences with AGB stars are manifold and thus the winds of RSG deserve separate studies., Comment: 12 pages, 9 figures, accepted for publication in A&A

Several observational pieces of evidence indicate that specific evolutionary channels that involve Roche lobe overflow are not correctly accounted for by the classical Roche model. We generalise the concept of the Roche lobe in the presence of extra forces (caused by radiation pressure or pulsations). By computing the distortion of the equipotential surfaces, we are able to evaluate the impact of these perturbing forces on the stability of Roche lobe overflow (RLOF). Radiative forces are parametrised through the constant reduction factor that they impose on the gravitational force from the radiating star (neglecting any shielding in the case of high optical thickness). Forces imparted by pulsation are derived from the velocity profile of the wind that they trigger. We provide analytical expressions to compute the generalised Roche radius. Depending on the extra force, the Roche lobe radius may either stay unchanged, become smaller, or even become meaningless (in the presence of a radiatively- or pulsation-driven wind). There is little impact on the RLOF stability., info:eu-repo/semantics/published

In this thesis, I present the properties of IRAS 19306+1407 central source and its sur- rounding circumstellar envelope (CSE), from the analysis of near-infrared (near-IR) polarimetry and integral field spectroscopy (IFS), with supporting archived HST im- ages and sub-millimetre (sub-mm) photometry. This is supported by axi-symmetric light scattering (ALS), axi-symmetric radiative transfer (DART) and molecular hy-drogen (H2) shock models. The polarimetric images show that IRAS 19306+1407 has a dusty torus, which deviates from axisymmetry and exhibits a ‘twist’ feature. The DART and ALS modelling shows that the CSE consists of Oxygen-rich sub-micrometre dust grains, with a range in temperature from 130±30 to 40±20 K at the inner and outer radius, respectively, with inner and outer radii of 1.9±0.1×1014 and 2.7±0.1×1015 m. The CSE detached 400±10 years ago and the mass loss lasted 5700±160 years, assuming a constant asymptotic giant branch (AGB) outflow speed of 15 km s−1. The dust mass and total mass of the CSE is 8.9±5×10−4 and 1.8±1.0×10 −1 M⊙, assuming a gas-to-dust ratio of 200. The mass loss rate was 3.4±2.1×10−5 M⊙ year−1. The central source is consistent with a B1I-type star with a radius of 3.8 ± 0.6 R⊙, luminosity of 4500 ± 340 L⊙ at a distance of 2.7 ± 0.1 kpc. A purpose built idl package (fus) was developed and used in the SINFONI IFS data critical final reduction steps. It also produced emission line, kinematic and line ratio images. The IFS observations show that H2 is detected throughout the CSE, located in bright arcs and in the bipolar lobes. The velocity of the H2 is greatest at the end of the lobes. Brγ emission originates from, or close, to the central source – produced by a fast jump (J) shock or photo-ionised atomic gas. The 1-0 S(1)/2- 1 S(1) and 1-0 S(1)/3-2 S(3) ratios were used as a diagnostic and determined that H2 was excited by bow shaped shocks; however, these shock models could not wholly explain the observed rotational and vibrational temperatures. The CDR values were fitted by combining continuous (C) or J-bow shock and fluorescence models, with a contribution from the latter, observed throughout the CSE (5–77 per cent). The majority of shock can be described by a C-bow shock model with B = 0.02 to 1.28 mG. Shocks are predominately seen in the equatorial regions. Polarimetry and IFS highlight a ‘twist’ feature, which could be due to an episodic jet undergoing a recent change in the outflow direction. The sub-arcsecond IFS observations reveal a flocculent structure in the south- east bright arc, consisting of several clumps interpreted as a fast-wind eroding an equatorial torus, possibly forming H2 knots seen in (some) evolved planetary nebulae (PNe). My analysis has effectively constrained the following: spectral type, stellar radius, luminosity and distance, chemistry, dust grain properties, geometry, age, mass loss, excitation mechanism and evolutionary state of the post-AGB star and its surrounding CSE. I conclude that IRAS 19306+1407 is a post-AGB object on the verge becoming a PN.

We observed the young planetary nebula NGC 7027 with the HST Imaging Spectrograph (STIS) in long-slit mode at five slit positions along the major axis of the nebula, over the wavelength range from 3000 to similar to 10,000 angstrom. We used dereddened line ratios to determine line-of-sight average values for (1) He(++) and He(+) ionization fractions from He II lambda 4686 and He I lambda 5876; (2) electron density from both [S II] (lambda 6716/lambda 6731) and [Ar IV] (lambda 4711/lambda 4740); and (3) temperature from [O III] (lambda 4959 + lambda 5007)/lambda 4363. The He(++)/H(+) ratio varies across the nebula, with values up to 0.06 over the central parts, falling off to 0.03 at offsets of 5 ''-6 '' from the central star. The line-of-sight mean densities vary by factors of similar to 5 on angular scales as small as similar to 1 ''. The average electron temperatures are generally consistent with a constant = 13,800 +/- 2000 K within the uncertainties, except for similar to 2% of measured pixels where T(e) > 18,000 K. The variations of density along the slits are not obviously correlated with variations in extinction. The ionization structure of the nebula varies on both large and small scales. [O I] lambda 6300, H alpha, and He II lambda 4686 intensity profiles along each slit appear to trace ionization gradients at H alpha peaks, with the highest ionization on the side toward the central star and lowest ionization on the far side. These structures may result from effects of stellar radiation, shocks from a fast stellar wind, or some combination of the two processes.

"The X-ray Universe 2008" Symposium held in Granada, Spain, 27-30 May, 2008; Published online at https://www.cosmos.esa.int/web/xmm-newton/2008-symposium, id.56. We present an analysis of archival RXTE data of the X- ray binary source 4U 1538−52/QV Nor. The RXTE ob- servatory made one complete binary cycle observation on January 1997 and 2001, respectively. The X-ray con- tinuum data are well described by an absorbed Nega- tive Positive power laws Exponentials (NPEX) compo- nent modified by an iron emission line at 6.4 keV and a cyclotron absorption line at 20 keV. Using a simple spher- ically symmetric wind model to describe the X-ray ab- sorption variations as a function of orbital phase, we in- ferred a wind mass-loss rate from the companion star of (1.3 − 2.5) × 10−6 M⊙/yr. Our results are consistent with those obtained by the Ginga X-ray observatory. We have also analyzed X-ray flux variations over the binary orbit and we have found a sharp increase at orbital phase 0.34 in both set of data. However, the increase at orbital phase 0.66 is significantly different in the two observa- tions.

Las supergigantes de tipo-A son las estrellas intrínsecamente más brillantes en el visible y, por lo tanto, uno de los mejores indicadores de distancias extragalácticas en potencia. Estas estrellas presentan procesos físicos muy complejos en sus atmósferas con un campo de radiación muy energético y de alta densidad de momento, en combinación con una atmósfera extendida y tenue y un viento estelar. Uno de los mejores trazadores de estas envolturas extensas son las l ' ineas UV de diferentes iones (principalmente Mg I1, Al I1, Si II, e II and Fe II). Los pocos trabajos sobre el espectro UV de estas estrellas muestran que la espectroscopía UV de alta resolución y un análisis de variabilidad son cruciales para progresar en el conocimiento de las atmósferas de las supergigantes de tipo-A. Una cantidad importante de cuestiones permanecen abiertas. Aquí resumimos los resultados más relevantes encontrados en el rango UV y las cuestiones que se derivan para el viento de las supergigantes-A. Simultáneamente se argumenta lo esencial de una misión UV., A-type supergiants are intrinsically the brightest stars at visual wavelengths, and therefore one of the best potential extragalactic distance indicators. These stars present a complex atmospheric physics with a large energy and momentum density of the radiation field, in combination with an extended and tenous atmosphere and a stellar wind. One of the best tracers of these expanding envelopes are the UV lines of different ions (mainly Mg I1, Al I1, Si I1, e II and Fe n). The scarce works on the UV spectrum of these stars show that high resolution UV spectroscopy and variability analysis are crucial to make progress in the understanding of A-type supergiants atmospheres. A number of important issues remain open. We summarize here the major findings on the winds of A-supergiants in the UV range and the derived open questions. Simultaneously, we address the critical importance of a UV mission to solve them.

In the generally accepted, but poorly documented model, silicate J-type C-stars are binary objects for which the silicate emission originates from a circumbinary or a circumcompanion disc. We aim at testing this hypothesis by a thorough spectral and spatial observational study of one object: IRAS18006-3213. We obtained, analysed and modeled high spatial resolution interferometric VLTI/MIDI observations on multiple baselines ranging from 45 m to 100 m. All observations resolved the object and show the very compact nature of the N-band emission (~30 mas). In addition, the highest spatial resolution data show a significant differential phase jump around 8.3 micron. This demonstrates the asymmetric nature of the N-band emission. Moreover, the single telescope N-band spectrum shows the signature of highly processed silicate grains. These data are used to confirm the model on silicate J-type C-stars for IRAS18006-3213. We show that the most favourable model of the dust geometry is a stable circumbinary disc around the system, seen under an intermediate inclination. The data presented on the silicate J-type C-star IRAS18006-3213 provide evidence that the oxygen rich dust is trapped in a circumbinary disc. The formation of this disc is probably linked to the binary nature of the central star., Comment: 11 pages, 11 figures, accepted for publication in A&A

Peat bogs have historically represented exceptional carbon (C) sinks because of their extremely low decomposition rates and consequent accumulation of plant remnants as peat. Among the factors favoring that peat accumulation, a major role is played by the chemical quality of plant litter itself, which is poor in nutrients and characterized by polyphenols with a strong inhibitory effect on microbial breakdown. Because bogs receive their nutrient supply solely from atmospheric deposition, the global increase of atmospheric nitrogen (N) inputs as a consequence of human activities could potentially alter the litter chemistry with important, but still unknown, effects on their C balance. Here we present data showing the decomposition rates of recently formed litter peat samples collected in nine European countries under a natural gradient of atmospheric N deposition from ≈0.2 to 2 g·m −2 ·yr −1 . We found that enhanced decomposition rates for material accumulated under higher atmospheric N supplies resulted in higher carbon dioxide (CO 2 ) emissions and dissolved organic carbon release. The increased N availability favored microbial decomposition ( i ) by removing N constraints on microbial metabolism and ( ii ) through a chemical amelioration of litter peat quality with a positive feedback on microbial enzymatic activity. Although some uncertainty remains about whether decay-resistant Sphagnum will continue to dominate litter peat, our data indicate that, even without such changes, increased N deposition poses a serious risk to our valuable peatland C sinks.

We report infrared observations of the planetary nebula Hen 2-113 obtained with VLT/NACO, VLTI/MIDI, VLT/ISAAC and TIMMI at the ESO 3.6m. Hen 2-113 exhibits a clear ring-like structure superimposed to a more diffuse environment visible in the L' (3.8$\mu$m), M' (4.78$\mu$m) and 8.7$\mu$m bands. No clear core at 8.7$\mu$m and no fringes through the N band could be detected for this object with MIDI. A qualitative interpretation of the object structure is proposed using a diabolo-like geometrical model. The PAH content of the nebula was also studied with ISAAC and TIMMI observations. This indicates that the PAHs are mostly concentrated towards the lobes of the diabolo and the bipolar lobes of the nebula. In L' band, a void $0.3\arcsec$ in diameter was discovered with NACO around the central source. The L' and M' fluxes from the central source were derived from NACO data indicating an important infrared excess with respect to the expected stellar emission based on stellar models and short wavelength data. The observed flux from this source in the L' and M' is about 300 and 800 times respectively than those expected from a model including only the central star. Moreover, the central object appears resolved in L' band with measured FWHM about 155 mas. This infrared excess can be explained by emission from a cocoon of hot dust (T$\sim$1000K) with a total mass $\sim10^{-9}$M$\_{\odot}$., Comment: 11 pages, 7 figures, accepted for publication in A&A

We present observations of the famous OH/IR star OH26.5+0.6 obtained using the Mid-Infrared Interferometric Instrument MIDI at the European Southern Observatory (ESO) Very Large Telescope Interferometer VLTI. The emission of the dusty envelope, spectrally dispersed at a resolution of 30 from 8 to 13.5 micron, appears resolved by a single dish UT telescope. In particular the angular diameter increases strongly within the silicate absorption band. Moreover an acquisition image taken at 8.7 micron exhibits, after deconvolution, a strong asymmetry. The axis ratio is 0.75+/-0.07 with the FWHM of the major and minor axis which are 286mas and 214mas respectively. The measured PA angle, 95 degrees +/-6 degrees is reminiscent of the asymmetry in the OH maser emission detected at 1612MHz by Bowers & Johnston (1990) for this star. In interferometric mode the UT1-UT3 102m baseline was employed to detect the presence of the star. No fringes have been found with a detection threshold estimated to be of the order of 1% of the total flux of the source, i.e. 5-8 Jy. These observations were carried out during the phase of maximum luminosity of the star, when the dust shell is more diluted and therefore the chance to detect the central source maximized. We modeled the dusty environment based on the work of Justannont et al. (1996). In particular, the failure to detect fringes provides strong constraints on the opacities in the inner regions of the dust shell or in the close vicinity of the star., Accepted in A&A

First mid-infrared images of a sample of AGB and post-AGB carbon stars (V Hya, IRC +10216, CIT 6 and Roberts 22) obtained at La Silla Observatory (ESO, Chile) are reported. CIT 6 presents a cometary-like feature clearly seen in the 9.7$\mu$m image, Roberts 22 shows an envelope slightly elongated in the north-east direction while images of V Hya and IRC+10216 are roughly spherically symmetric. Using inversion technique, the dust emissivity was derived from the observed intensity profiles, allowing a determination of the grain temperature and density distributions inside the envelope for these stars. Dust masses and mass-loss rates were estimated for V Hya and IRC +10216. Our results are comparable to those obtained in previous studies if dust grains have dimensions in the range $\sim$ 0.01 - 0.2 $\mu$m. Color maps suggest the presence of temperature inhomogeneities in the central regions of the dust envelopes. In the case of V Hya, an eccentric hot point, which direction coincides with the jet previously seen in [SII] emission, suggest that we are observing a material ejected in a previous mass-loss event. Bipolar lobes are clearly seen in the color maps of Roberts 22 and IRC +10216., Comment: accepted for publication in Astronomy and Astrophysics

Calcium (Ca) is an important element for neutralizing soil acidity in temperate forests. The immediate availability of Ca in forested acid soils is largely dependent on mineralization of organic Ca, which may differ significantly among tree species. I estimated net Ca mineralization in the forest floor and upper 15 cm of mineral soil beneath six tree species in a mixed-species forest in northwestern Connecticut, using the buried bag method. Net Ca mineralization in the forest floor was significantly correlated with mass loss of the decomposing forest floor litter. Higher mass loss fractions during the summer and in forest floors beneath sugar maple (Acer saccharum) and white ash (Fraxinus americana) coincided with higher net Ca mineralization rates. More Ca was released per unit mass loss of forest floor beneath sugar maple and white ash (362 and 390 mmol kg−1, respectively) than beneath American beech (Fagus grandifolia), red maple (Acer rubrum), red oak (Quercus rubra) and hemlock (Tsuga canadensis) (183, 133, 147, and 190 mmol kg−1, respectively). Due to the high forest floor mass beneath red maple, beech, red oak, and hemlock, net Ca mineralization in the forest floor per unit area beneath these tree species did not differ significantly from sugar maple and white ash (ranging between 80 mmol m−2 per year for beech and 141 mmol m−2 per year for sugar maple). Net Ca mineralization in the mineral soil was significantly larger beneath sugar maple (142 mmol m−2 per year) than beneath the other tree species (ranging between −10 mmol m−2 per year for beech and 55 mmol m−2 per year for white ash). These results show that Ca mineralization rates are large and differ significantly among tree species, affecting the spatial pattern of soil acidity and Ca availability in a mixed-species forest stand.

In many bird species early breeders have higher reproductive performance than late breeders from the same population. This could be caused by a reduction in environmental factors related to date per se (Date Hypothesis), or because poorer performers nest later (Parent Quality Hypothesis). We manipulated hatch date of Tree Swallows Tachycineta bicolor by switching clutches with different lay dates, generating broods with advanced or delayed timing, and assessed the impact of the experiment on nestling mass. The Date Hypothesis better explained the decline in nestling mass in the first half of the season, while the Parent Quality Hypothesis was supported in the second half. We also found that female mass loss was unintentionally reduced in advanced females and suggest that such impacts of the experiment on parent quality, or correlations between nestlings and their actual parents via heritability or maternal effects, could bias hatch-date manipulation experiments towards supporting the Date Hypothesis. Differential costs of incubation, either due to naturally low temperatures early in the season, or due to the unintentional manipulation of female incubation costs, appear to have driven support for the Date Hypothesis early in the season.

In many bird species, females undergo a marked decline in body condition during the first days of the nestling period. This decline may be because brooding young chicks reduces the time available for foraging. Alternatively, it might be viewed as an adaptive way to reduce flight costs when the food demand of the brood is highest. To test these hypotheses we modified the brooding commitment of House Sparrows Passer domesticus by manipulating brood size to see if changes in time spent brooding affects adult body condition. During the nestling period, females provided on average three times as much brooding as males. Reduced broods received 14% more brooding than large broods and time spent brooding declined with brood size and chick age according to an exponential decay function. Male body condition was unaffected by brood size and remained stable throughout the reproductive period. Body condition of females with enlarged broods decreased gradually during the nestling period, whereas that of females tending reduced broods dropped abruptly and significantly upon hatching. This resulted in females with reduced broods having lower body condition during the first half of the nestling period than those with enlarged broods. The sharp drop in body condition of females with reduced broods coincided with the period that brooding was most intensive. Indeed, female body condition at the end of the nestling period was negatively correlated with the proportion of time they spent brooding during the first half of the nestling period. Thus, the probable lower homeothermic capacities of reduced broods implies a higher brooding commitment for female House Sparrows that, in turn, may reduce their opportunity to forage and consequently also their body condition.

Context: The principle definition of habitability for exoplanets is whether they can sustain liquid water on their surfaces, i.e. that they orbit within the habitable zone. However, the planet's magnetosphere should also be considered, since without it, an exoplanet's atmosphere may be eroded away by stellar winds. Aims: The aim of this paper is to investigate magnetospheric protection of a planet from the effects of stellar winds from solar-mass stars. Methods: We study hypothetical Earth-like exoplanets orbiting in the host star's habitable zone for a sample of 124 solar-mass stars. These are targets that have been observed by the Bcool collaboration. Using two wind models, we calculate the magnetospheric extent of each exoplanet. These wind models are computationally inexpensive and allow the community to quickly estimate the magnetospheric size of magnetised Earth-analogues orbiting cool stars. Results: Most of the simulated planets in our sample can maintain a magnetosphere of ~5 Earth radii or larger. This suggests that magnetised Earth analogues in the habitable zones of solar analogues are able to protect their atmospheres and is in contrast to planets around young active M dwarfs. In general, we find that Earth-analogues around solar-type stars, of age 1.5 Gyr or older, can maintain at least a Paleoarchean Earth sized magnetosphere. Our results indicate that planets around 0.6 - 0.8 solar-mass stars on the low activity side of the Vaughan-Preston gap are the optimum observing targets for habitable Earth analogues., Comment: 8 pages, 3 figures, accepted to Astronomy and Astrophysics

This thesis investigates the Baldwin-effect in Wolf-Rayet (WR) stars. The Baldwin-effect is a correlation with negative slope between the equivalent width of spectral emission lines and the monochromatic luminosity of the underlying continuum at the line wavelength. This effect has been known to exist in spectra of Active Galactic Nuclei and WR stars. The Baldwin effect in WR stars may function as a distance estimator, as it may provide a method to derive the stellar luminosity directly from the spectrum. Distances of WR stars are hard to derive. These stars lose matter in the form of a strong stellar wind, which hides the underlying star from view. The dense stellar wind is the reason that absolute visual magnitudes of WR stars do not correlate well with the star's temperature or intrinsic colours. Therefore, the derivation of the distance of a WR star through observational properties is not an easy task to perform. The research of this thesis confirms previous thoughts that the Baldwin effect in WR stars is mainly caused by differences in radius among stars, reflecting differences in wind density. Furthermore, it is found that the spread in the equivalent width-luminosity relations is caused by diffences in mass-loss rate and terminal wind velocity among stars and, to a lesser extend, differences in stellar temperature. This theoretical explanation of the Baldwin-effect is done by comparing simple semi-analytical expressions with the results of model stellar atmosphere calculations. For the application of the Baldwin-effect to estimate WR star distances, it is found that a reasonably accurate estimate of the mass-loss rate is required. However, the derived mass-loss rates of WR stars generally depend on an assumed distance of the star. Therefore it is tried to combine several mass-loss studies and find typical values of the mass-loss rate as function of stellar subtype. After adopting a value of the mass-loss rate for its spectral subtype, the distance of the star can be estimated through the Baldwin-effect. The performance of the Baldwin-effect as a distance estimator is tested by concentrating at WR stars with well known distances, such as those in the Large Magellanic Cloud or WR stars that are a member of an open star cluster or OB association. It is concluded that estimating WR star distances by means of this method does give a reasonable indication of the stellar distance, but often not better than existing methods.

This thesis investigates the Baldwin-effect in Wolf-Rayet (WR) stars. The Baldwin-effect is a correlation with negative slope between the equivalent width of spectral emission lines and the monochromatic luminosity of the underlying continuum at the line wavelength. This effect has been known to exist in spectra of Active Galactic Nuclei and WR stars. The Baldwin effect in WR stars may function as a distance estimator, as it may provide a method to derive the stellar luminosity directly from the spectrum. Distances of WR stars are hard to derive. These stars lose matter in the form of a strong stellar wind, which hides the underlying star from view. The dense stellar wind is the reason that absolute visual magnitudes of WR stars do not correlate well with the star's temperature or intrinsic colours. Therefore, the derivation of the distance of a WR star through observational properties is not an easy task to perform. The research of this thesis confirms previous thoughts that the Baldwin effect in WR stars is mainly caused by differences in radius among stars, reflecting differences in wind density. Furthermore, it is found that the spread in the equivalent width-luminosity relations is caused by diffences in mass-loss rate and terminal wind velocity among stars and, to a lesser extend, differences in stellar temperature. This theoretical explanation of the Baldwin-effect is done by comparing simple semi-analytical expressions with the results of model stellar atmosphere calculations. For the application of the Baldwin-effect to estimate WR star distances, it is found that a reasonably accurate estimate of the mass-loss rate is required. However, the derived mass-loss rates of WR stars generally depend on an assumed distance of the star. Therefore it is tried to combine several mass-loss studies and find typical values of the mass-loss rate as function of stellar subtype. After adopting a value of the mass-loss rate for its spectral subtype, the distance of the star can be estimated through the Baldwin-effect. The performance of the Baldwin-effect as a distance estimator is tested by concentrating at WR stars with well known distances, such as those in the Large Magellanic Cloud or WR stars that are a member of an open star cluster or OB association. It is concluded that estimating WR star distances by means of this method does give a reasonable indication of the stellar distance, but often not better than existing methods.

We report the detection of infrared fine-structure lines of [Fe II] and [Si II] in the ISO-SWS spectra of two supergiants, alpha Ori and alpha Sco. From the observed intensities we infer that the emitting regions have temperatures in the range 1200-1800 K and are located within about 20 R* from the star. This is interior to the region where dust is supposed to condense in the expanding envelope so that Fe and Si are not yet incorporated into the dust. The gas density required to thermalize these lines is of order 10(6) cm(-3) consistent with the expected density in the outflow at the adopted location. The total mass of gas being cooled through these atomic fine structure lines is of the order of 10(-4) Mo. Our calculation shows that cooling through the [O I] 63 mu m line extends over a larger radius than for both [Fe II] and [Si II].

We have studied the absorption bands around 14 mum in the spectra of 11 carbon stars with mass-loss rates ranging from 10(-8) to 10(-4) M-circle dot yr(-1), based on data obtained with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). All stars clearly show a C2H2 absorption band peaking at 13.7 mum, while the contribution from HCN molecules is small in this wavelength region. A simple plane-parallel LTE model with two layers at different temperatures is used to derive the C2H2 abundances in the outer photosphere and in the circumstellar envelope. We find that (1) the column density of the hot-layer, placed at about 3R* with a temperature of 1400 K is roughly the same for all stars regardless of the mass-loss rate, and (2) the contribution of cool molecules in the circumstellar envelope increases with the dust mass-loss rate, (3) the abundance of C2H2 in the two layers is about the same, i.e. no obvious depletion of C2H2 molecules seems to occur in the circumstellar envelope.

We have studied the absorption bands around 14 mum in the spectra of 11 carbon stars with mass-loss rates ranging from 10(-8) to 10(-4) M-circle dot yr(-1), based on data obtained with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). All stars clearly show a C2H2 absorption band peaking at 13.7 mum, while the contribution from HCN molecules is small in this wavelength region. A simple plane-parallel LTE model with two layers at different temperatures is used to derive the C2H2 abundances in the outer photosphere and in the circumstellar envelope. We find that (1) the column density of the hot-layer, placed at about 3R* with a temperature of 1400 K is roughly the same for all stars regardless of the mass-loss rate, and (2) the contribution of cool molecules in the circumstellar envelope increases with the dust mass-loss rate, (3) the abundance of C2H2 in the two layers is about the same, i.e. no obvious depletion of C2H2 molecules seems to occur in the circumstellar envelope.

We present 10 micron, ISO-SWS and Australia Telescope Compact Array observations of the region in the cluster Wd1 in Ara centred on the B[e] star Ara C. An ISO-SWS spectrum reveals emission from highly ionised species in the vicinity of the star, suggesting a secondary source of excitation in the region. We find strong radio emission at both 3.5cm and 6.3cm, with a total spatial extent of over 20 arcsec. The emission is found to be concentrated in two discrete structures, separated by 14''. The westerly source is resolved, with a spectral index indicative of thermal emission. The easterly source is clearly extended and nonthermal (synchrotron) in nature. Positionally, the B[e] star is found to coincide with the more compact radio source, while the southerly lobe of the extended source is coincident with Ara A, an M2 I star. Observation of the region at 10micron reveals strong emission with an almost identical spatial distribution to the radio emission. Ara C is found to have an extreme radio luminosity in comparison to prior radio observations of hot stars such as O and B supergiants and Wolf-Rayet stars, given the estimated distance to the cluster. An origin in a detatched shell of material around the central star is therefore suggested; however given the spatial extent of the emission, such a shell must be relatively young (10^3 yrs). The extended non thermal emission associated with the M star Ara A is unexpected; to the best of our knowledge this is a unique phenomenon. SAX (2-10keV) observations show no evidence of X-ray emission, which might be expected if a compact companion were present., Comment: 5 pages including encapsulated figures, figure 3 separate. Accepted for MNRAS pink pages

We have investigated the spectro-photometric properties of the Asymptotic Giant Branch (AGB) stars and their contribution to the integrated infrared emission in simple stellar populations (SSP). Adopting analytical relations describing the evolution of these stars in the HR diagram and empirical relations for the mass-loss rate and the wind terminal velocity, we were able to model the effects of the dusty envelope around these stars, with a minimal number of parameters. We computed isochrones at different age and initial metal content. We compare our models with existing infrared colors of M giants and Mira stars and with IRAS PSC data. Contrary to previous models, in the new isochrones the mass-loss rate, which establishes the duration of the AGB phase, also determines the spectral properties of the stars. The contribution of these stars to the integrated light of the population is thus obtained in a consistent way. We find that the emission in the mid infrared is about one order of magnitude larger when dust is taken into account in an intermediate age population, irrespective of the particular mixture adopted. The dependence of the integrated colors on the metallicity and age is discussed, with particular emphasis on the problem of age-metallicity degeneracy. We show that, contrary to the case of optical or near infrared colors, the adoption of a suitable pass-band in the mid infrared allows a fair separation of the two effects. We suggest intermediate redshift elliptical galaxies as possible targets of this method of solving the age-metallicity dilemma. The new SSP models constitute a first step in a more extended study aimed at modelling the spectral properties of the galaxies from the ultraviolet to the far infrared., 16 pages, 10 figures, to appear in A&A

We have conducted a survey of CN N=2-1 and N=1-0 line emission in the envelopes of evolved stars. The sample consists of 42 objects, including C-rich and O-rich envelopes, S-stars, detached envelopes, and proto-planetary nebulae. Confident detections have been achieved in 30 objects. Both CN lines are bright in C-rich envelopes, and the 2-1 line has been detected in 5 O-rich objects (previously, CN had been detected in only one O-rich envelope). The excitation temperature T-rot, evaluated from the 2-1/1-0 intensity ratio, is similar to 3-6 K in most carbon stars, and greater than or equal to 10-20 K in O-rich envelopes. We find that the CN spectra display anomalies in the rotational, fine, and hyperfine line ratios. Anomalies in the rotational excitation appear in W Ori and UU Aur, two stars which are known to present HCN upsilon=0 J=1-0 masers. The excitation of the CN 2-1 line is unusually high in both objects, and UU Aur may present a weak maser effect in this line. Anomalies are also observed in the intensity ratios of the fine and hyperfine components. If such anomalies were due to the envelope thickness, the required line opacities would be excessively high, in particular for low mass-loss rate objects. We thus suggest that the observed anomalies are the result of an anomalous excitation. Pumping through the optical and near-IR bands seems to play a dominant role in the CN excitation. A comparison with previously published HCN data shows that the CN/HCN ratio of the total numbers of molecules in C-rich stars tends to be larger in the objects with lower mass-loss rate, supporting the idea that CN is mainly formed from the photodissociation of HCN. The average peak abundance of CN is similar to 1.9 10(-5) in C-rich objects, and is about 300 times smaller (similar to 6.6 10(-8)) in O-rich envelopes. The CN/HCN peak abundance ratio is similar to 0.45 in C-rich stars, in agreement with photodissociation chemical models, and similar to 0.04 in O-rich objects. This last value is about two orders of magnitude smaller than the predictions of standard chemical models, and suggest that CN is destroyed by additional mechanisms than photodissociation in O-rich envelopes.

Many glaciers along the southeast and northwest coasts of Greenland have accelerated, increasing the ice sheet's contribution to global sea-level rise. In this article, we map elevation changes on Upernavik Isstrom (UI), West Greenland, during 2003 to 2009 using high-resolution ice, cloud and land elevation satellite laser altimeter data supplemented with altimeter surveys from NASA's Airborne Topographic Mapper during 2002 to 2010. To assess thinning prior to 2002, we analyze aerial photographs from 1985. We document at least two distinct periods of dynamically induced ice loss during 1985 to 2010 characterized by a rapid retreat of the calving front, increased ice speed, and lowering of the ice surface. The first period occurred before 1991, whereas the latter occurred during 2005 to 2009. Analyses of air and sea-surface temperature suggest a combination of relatively warm air and ocean water as a potential trigger for the dynamically induced ice loss. We estimate a total catchment-wide ice-mass loss of UI caused by the two events of 72.3 +/- 15.8 Gt during 1985 to 2010, whereas the total melt-induced ice-mass loss during this same period is 19.8 +/- 2.8 Gt. Thus, 79% of the total ice-mass loss of the UI catchment was caused by ice dynamics, indicating the importance of including dynamically induced ice loss in the total mass change budget of the Greenland ice sheet.

We study the circumstellar environment of the M-type AGB star RT Vir using mid-infrared high spatial resolution observations from the ESO-VLTI focal instrument MIDI. The aim of this study is to provide observational constraints on theoretical prediction that the winds of M-type AGB objects can be driven by photon scattering on iron-free silicate grains located in the close environment (about 2 to 3 stellar radii) of the star. We interpreted spectro-interferometric data, first using wavelength-dependent geometric models. We then used a self-consistent dynamic model atmosphere containing a time-dependent description of grain growth for pure forsterite dust particles to reproduce the photometric, spectrometric, and interferometric measurements of RT Vir. Since the hydrodynamic computation needs stellar parameters as input, a considerable effort was first made to determine these parameters. MIDI differential phases reveal the presence of an asymmetry in the stellar vicinity. Results from the geometrical modeling give us clues to the presence of aluminum and silicate dust in the close circumstellar environment (< ~5 stellar radii). Comparison between spectro-interferometric data and a self-consistent dust-driven wind model reveals that silicate dust has to be present in the region between 2 to 3 stellar radii to reproduce the 59 and 63 m baseline visibility measurements around 9.8 micron. This gives additional observational evidence in favor of winds driven by photon scattering on iron-free silicate grains located in the close vicinity of an M-type star. However, other sources of opacity are clearly missing to reproduce the 10-13 micron visibility measurements for all baselines. This study is a first attempt to understand the wind mechanism of M-type AGB stars by comparing photometric, spectrometric, and interferometric measurements with state-of-the-art, self-consistent dust-driven wind models. The agreement of the dynamic model atmosphere with interferometric measurements in the 8-10 micron spectral region gives additional observational evidence that the winds of M-type stars can be driven by photon scattering on iron-free silicate grains. Finally, a larger statistical study and progress in advanced self-consistent 3D modeling are still required to solve the remaining problems., Astronomy and Astrophysics (2013)

In this paper a first step is made to search for hot post-AGB stars in the IRAS Point Source Catalog. In order to find objects that evolved off the AGE a longer time ago than post-AGB objects discussed in the literature, objects that were not detected at 12 mu m by IRAS were selected. The selection yielded 15 objects, of which 12 have spectral type B, and would appear to be efficient in finding hot objects. However, this result does not necessarily mean that these are old evolved objects. Some stars are associated with dense galactic cirrus and are probably normal massive supergiants, and a number of stars are good post-AGB candidates because of their galactic latitudes and their cool, extended infrared emission. The possibility remains that these objects are normal supergiants heating the local interstellar medium. Finally, three remaining objects appear to be genuine post-AGB (candidate) stars, as is attested by their spectral and photometric properties. The main conclusion derived from modelling their spectral energy distributions is that the objects evolved off the AGE about 1000 years ago, significantly longer than what is found for other post-AGB stars. Emission lines have appeared in the spectrum of one of these objects, SAO 243756, over the last 20 years. SAO 243756 bears close resemblance to other objects that have been proposed to have entered the Planetary Nebula phase recently. Several aspects of the problem of selecting post-AGB stars in general are discussed.

The results of a medium resolution near-infrared spectral survey of 18 post-AGB candidate stars are presented. Most of the stars have near-infrared hydrogen lines in absorption, which is normal for their spectral types. Three stars, HD 101584, HD 179821 and HD 170756 have the CO first overtone bands in emission; in two of these the emission is variable. It is suggested that the CO emission is the result of post-AGB mass loss. HD 52961 shows the rarely occurring 3.5 mu m emission feature and is only the fourth object in which this feature has been detected.

We present theoretical H alpha and HI infrared recombination line calculations for low-density discs around B stars. Such a disc shows no visible emission in H alpha, while the HI IR recombination lines are in emission. This phenomenon has been found in the spectrum of the B0.2V star, tau Sco and could be simulated with a simple disc model. As an extension of that particular case we calculate the entire IR HI line spectrum of a normal B star surrounded by a low-density disc with a theoretical curve of growth for HI IR line fluxes, which we introduce as a tool for studying low-density discs. We find that IR emission lines may be detectable for densities up to about 10(-14) gcm(-3) which is a factor 10(2) - 10(3) lower than typically found in Be stars. For different spectral types, B0, B2, B5 and B8 we determined the density range for which emission is prominent in the IR recombination lines but not in Ha alpha.

We present the astrometric VLBI observations of water masers associated with IRAS 22480+6002 (IRC+60370, hereafter I22480) with the VLBI Exploration of Radio Astrometry (VERA). The stellar type of I22480 looks unusual as a stellar maser source and has been debated since the 1970s. We successfully determined the annual parallax of a group of the water maser spots, pi=0.400+/-0.025 mas, corresponding to a distance to I22480 of D=2.50 /+0.17/-0.15 kpc. This suggests that the estimated bolometric luminosity of I22480 should be revised to 35 000 L_sun, favoring a K-type supergiant rather than an RV Tau-type variable star previously suggested. Although the spectral type is unusual as a stellar maser source, the internal motions of the water maser features suggest that the water masers are associated with the circumstellar envelope of this star. Taking into account a possible stellar motion with respect to the maser feature motions, we derived a secular proper motion of I22480, (mu_alpha, mu_delta)=(-2.58+/-0.33, -1.91+/-0.17) [mas yr^-1]. The derived motion of I22480 in the Milky Way has a deviation by ~ -30 km/s in the galactic azimuthal direction from a circular motion estimated from the galactocentric distance to I22480 and assumption of a flat Galactic rotation curve. This peculiar motion is still comparable to those typically seen in the water maser sources located in the Perseus spiral arm. Taking into account the peculiar motion and the proximity to the Galactic midplane (z ~60 pc), I22480 may be a member of the Galactic thin disk.

We have investigated the report of Tamura and Takeuti that the H alpha line of the F-type post-AGB star HD56126 is variable on time-scales of minutes. To this end, HD 56126 was observed on two occasions with the William Herschel Telescope. 17 and 30 spectra (first and second runs, respectively) were taken within time-spans of 1.5 h in order to detect any short-term variations. We find that the H alpha line profile changed strongly over the two-month interval, but no evidence is found for shortterm variability. The variability Tamura and Takeuti claim to find is probably due to the low signal-to-noise ratio in their spectra.

An analysis of the measurements of mid-eclipse times of AT Peg has been presented. It indicates a period decrease rate of dP/dt = −4.2 × 10−7 d/yr, which can be interpreted in terms of mass loss from the system via stellar wind with a rate between (1 and 2) × 10−8 Mʘ/yr. The O–C diagram shows a growing sine wave covering two different cycles of 13 yr and 31.9 yr with amplitudes equal to 0.026 and 0.032 day, respectively. These unequal durations of the cycles may be explained by magnetic activity cycling variations due to star spots. The obtained characteristics of the second cycle are consistent with similar systems when applying Applegate’s mechanism.