Young stellar objects studied at X-ray and optical wavelengths

Available from TIB Hannover: RR 8257(51) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman

Detection of planetary spectral features through circumstellar dust - a Monte Carlo simulation

One of the methods envisaged to detect planets outside of our solar system assumes that certain spectroscopic features at 10 and below 20 #mu#m wavelength are typical of planets. Their detection would then be an unambiguous sign of the presence of Earth-like planets other than ours. In these spectral regions, there might be interference with other features being generally typical of circumstellar material: the silicate features of dust. In order to get an estimate of this effect, we used our 3D Monte Carlo radiation transport code to model the spectrum of a Keplerian disk with embedded artificial planets encircling a central star of solar luminosity and temperature. The disk structure and the grain model resemble real properties in protoplanetary disks. The model was calculated for a set of different optical depths at various positions of the observer in respect to the configuration, the detectability of the embedded planetary sources is discussed. (orig.)SIGLEAvailable from TIB Hannover: RR 8257(59) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman

Aluminum oxide and the opacity of oxygen-rich circumstellar dust in the 12-17-#mu#m range

Amorphous alumina (Al_2O_3) was produced by a sol-gel technique in order to make available its optical constants for possible astrophysical applications. Gradual annealing showed that the X-ray amorphousness of alumina ended somewhere between 723 and 873 K. Above this transition point, the structure changes into disordered #gamma#-Al_2O_3. At T>1273 K crystalline #alpha#-Al_2O_3 (corundum) is formed. Mie calculations show that amorphous alumina exhibits a wide Al-O vibrational band, peaking at 11.5-11.8 #mu#m and having a steep 'blue' and an extended 'red' wing. It may be an important contributor to the continuous opacity between the silicate bands in oxygen-rich circumstellar envelopes, whereas it is ruled out for the explanation of the 13-#mu#m band. An average 13-#mu#m band profile was derived from 51 IRAS LRS spectra of bright Mira stars and semiregular variables. Its shape, which is satifactorily represented by a Lorentz profile, can be reproduced by Mie calculations with the data of #alpha#-Al_2O_3, but not with those of #gamma#-Al_2O_3. The calculations show that the 13-#mu#m band profile of #alpha#-Al_2O_3 is sensitive to grain shape. If #alpha#-Al_2O_3 is the absorber, a second band should be present at 21 #mu#m. A close correlation was found between the strengths of the 13-#mu#m band and the 10-#mu#m silicate band. It suggests that the 13-#mu#m band carrier could also be somehow connected with silicate dust. Experimental arguments supporting this attribution are presented. (orig.)SIGLEAvailable from TIB Hannover: RR 8257(63) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman

The outburst of the T Tauri star EX Lupi in 1994

We have observed an outburst of the T Tauri star EX Lup in March 1994. We present both photometric (BVR) and spectroscopic (low and medium resolution) observations carried out during the decline after outburst. The star appears much bluer during outburst due to an increased emission of a hot continuum. This is accompanied by a strong increase of the veiling of photospheric lines. We observe inverse P Cygni profiles of many emission lines over a large brightness range of EX Lup. We briefly discuss these features towards the model of magnetospherically supported accretion of disk material. (orig.)SIGLEAvailable from TIB Hannover: RR 8257(43) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany); Deutsche Forschungsgemeinschaft (DFG), Bonn (Germany)DEGerman

Hydrodynamical models and synthetic spectra of circumstellar dust shells around AGB stars. Pt. 1 Stationary solutions

We present a sample of hydrodynamical steady state models of circumstellar gas/dust shells around late type giants together with computed spectral energy distributions (SEDs). In these models, the stellar wind is driven by radiation pressure on dust grains and subsequent momentum transfer to the gas molecules via collisions. Given the fundamental stellar parameters (M_*, L_*, T_e_f_f), the mass loss rate (M), and the dust properties, a self-consistent physical model of the circumstellar gas/dust shell is obtained from the numerical solution of the coupled equations of hydrodynamics and radiative transfer. The computed outflow velocities and infrared fluxes of the circumstellar envelopes can be compared directly with the observed properties of stars on asymptotic giant branch. Plotting the positions of our steady state models in different IRAS two-color-diagrams, we confirm that, for fixed dust properties, all models fall on a simple color-color relation with M (or optical depth) as the only parameter. Surprisingly, we find a good agreement between the synthetic spectra resulting from the self-consistent hydrodynamical approach and those obtained from much simpler models based on a constant outflow velocity and ignoring drift of dust relative to the gas. Our models are compared with the results of similar calculations by Netzer and Elitzur (1993). We find significant differences which are probably the result of some unrealistic approximations in the treatment of radiative transfer underlying the model calculations of Netzer and Elitzur. Moreover, our results demonstrate that, in general, gas pressure cannot be neglected for winds with relatively low expansion velocities (u_e < 30 km/s). For given stellar parameters and dust properties, the theoretical minimum (maximum) mass loss rate decreases (increases) significantly when gas pressure is taken into account. (orig.)Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Through the upper AGB towards a planetary: A hydrodynamical simulation. -Time-dependent hydrodynamical models of circumstellar dust shells around carbon-and oxygen-rich AGB stars

Available from TIB Hannover: RR 7310(96-17)+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDeutsche Agentur fuer Raumfahrtangelegenheiten (DARA) GmbH, Bonn (Germany)DEGerman