Computer Science
Scientific paper
Jun 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011phdt........60m&link_type=abstract
Ph.D. thesis, Universidad Complutense de Madrid, Spain, 2011
Computer Science
Stars: Activity, Atmospheres, Chromospheres, Flare, Late-Type, Rotation, Exoplanets
Scientific paper
This dissertation has investigated the chromospheric activity and rotation of nearby cool stars, which can potentially host exoplanetary systems. 1. High-resolution echelle spectra have been obtained for 565 nearby (d ≤ 25 pc) cool (spectral types F to M) stars. The observations were taken using high resolution echelle optical spectrographs. The observations were designed to ensure a spectral coverage including all the optical magnetic activity indicator lines: from the Ca II H & K lines to the Ca II IRT, including all the Balmer lines Hα, Hβ, Hγ, Hδ, and H?. This fact has ensured a simultaneous analysis of the magnetic activity using different diagnostics. The spectral coverage of the spectra has also permitted a precise analysis of the stellar properties as well as rotational and radial velocities. 2. The suitability of the stars as targets in exoplanetary search surveys has been analysed using the results obtained in the spectroscopic survey. Using the measured chromospheric activity in the optical spectrum, activity-induced RV jitter has been calculated for the active stars in the sample. Although the intrinsic variability of stellar activity makes it impossible to directly subtract the computed values from the RV signal, it provides an estimation of the activity-related noise. Therefore, this values can be used to set the minimum detectable mass for a planet orbiting the star or to determine the minimal amplitude variation that could indicate the existence of a planet. The compilation of the activity, rotation and predicted activity-induced RV jitter build up into a catalogue that determines the suitability of the stars as targets in exoplanet search surveys. 3. The relationship between pairs of excess surface flux in different activity diagnostics has been analysed using the results from the spectroscopic survey. The results show a clear correlation between the activity measured in different optical indicators. This fact confirms previous findings and contributes additional evidence that indicates that the slope of the logarithmic flux-flux relationships increases with increasing height of the formation region of the line. Although most of the relationships are found to be independent on the spectral type and the luminosity class, as previous studies had pointed out, a clear dichotomy is found in the relationships between the emission in the Hα line and any calcium indicator (Ca II H, Ca II K or Ca II IRT).When those activity indices are compared, there are clearly two different linear relationships. When comparing the behaviour of Hα as a chromospheric activity indicator and X-ray emission as a coronal one, we were able to prove that those stars deviating from the general flux-flux relationships are those with X-ray emission in the saturation regime. Besides, these stars have been found to be late-K or M stars, which are either young or flare stars. This fact suggests that their "peculiar" Hα emission is related to a different magnetic field generation mechanism. 4. The stars in the Herschel's approved Open Time Key Programme DUNES have been spectroscopically characterized. Using high resolution spectroscopic data several stellar parameters and properties have been derived. In particular, we have measured rotational velocities, lithium abundances and chromospheric activity in all the optical indicators. These results will be essential for the success of the project because it will permit to ensure the correct identification of debris discs and the correlation of the discs and the stellar properties.
PhD Thesis Advisor: David Montes (Universidad Complutense de Madrid, UCM)
DUNES (Dust Around Nearby Stars): http://www.mpia-hd.mpg.de/DUNES/
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