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Aspects of Stellar Atmospheres and Star Formation |
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Advisor: Luiz Paulo Ribeiro Vaz |
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Co-advisor: Gibor Basri |
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May 2000 |
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Abstract: We initially investigate some processes that affect the stellar atmosphere, such as the gravity-brightening and limb-darkening, and the influence of the reflection effect on them. We study the gravity-brightening exponents using the Uppsala Model Atmospheres. In non-illuminated convective atmospheres the exponent value (beta=0.32) proposed by Lucy was confirmed only for T_eff close to 6500 K. The exponent depends upon T_eff, being rather insensitive to variations of the mixing-length parameter, the stellar mass and the use of grey or non-grey atmospheres. We demonstrate the influence of the mutual illumination in a close binary on the gravity-brightening exponent. The external illumination increases the value of beta, the larger the amount of incident flux the larger the value of the exponent. This effect is caused by the ``quenching'' of convection as the external illumination heats the surface layers of the illuminated star, bringing it closer to radiative equilibrium, where beta is close to unity. We determine monochromatic, bolometric and passband-specific limb-darkening coefficients for illuminated atmospheres. Our results show that illuminated coefficients are significantly different from the non-illuminated ones. We test different methods to calculated the coefficients and showed that in the illuminated case the method proposed by Van Hamme is recommended in order to obtain coefficients that preserve the total emergent flux from the atmosphere. In the second part of this work we study Classical T Tauri stars. We discuss the many processes acting in the forming star-disk system and test the predictions of magnetospheric accretion models. We present a spectral analysis of 30 T Tauri stars observed with the Hamilton echelle spectrograph over more than a decade. We discuss the relation between different line-forming regions and search for good accretion rate indicators. We confirm several important points of the models, such as the correlation between accretion and outflow, broad emission components that are mostly central or slightly blueshifted, and only the occasional presence of redshifted absorption. We also show, however, that the broad emission components supposedly formed in the magnetospheric accretion flow only partially support the models. Unlike the predictions, they are sometimes redshifted and are mostly found to be symmetric. The published theoretical profiles do not have a strong resemblance to our observed ones. We also present the analysis of 103 spectra of the Classical T Tauri star DR Tau. The star exhibits strong emission lines that show a wide variety and variability in profile shapes. These lines show both outflow and infall signatures which change on a variety of timescales. The system shows quasiperiodic variations in line intensities and position, but a unique period that describes all of the data could not be found. The Balmer line profiles are generally strongly peaked in the red, and do not resemble published theoretical magnetospheric accretion profiles. We suggest that the system is seen nearly pole-on. This can explain the highly asymmetric Balmer lines if the line emissivity increases strongly near the star. The Ca II and He I emission line components are found to be very symmetric and that they could be produced by magnetic turbulence. |
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