A spectroscopic study of the nearby late-type stellar population: Properties of members of stellar kinematic groups and stars with circumstellar dusty debris discs.
Creators: name: Maldonado, J.
Publisher: name: Zenodo
Year: 2012
Subjects: Open clusters and associations: general - Stars: abundances - Stars: activity - Stars: age - Stars: circumstellar matter - Stars: planetary systems - Stars: late-type - Technique: spectroscopy

description: Understanding how stars and planetary systems form and evolve is one of the major goals of modern Astrophysics. Given that their stellar properties resemble those of our Sun, main-sequence, late-type stars usually constitute primary targets in exo-planet search programs, as well as surveys seeking for dusty debris discs similar to the Solar System’s zodiacal light and the Edgeworth Kuiper Belt. Nearby stars have the additional advantage that high-resolution observations can be obtained for large samples, allowing a precise determination of their properties. In this dissertation, the results of a high-resolution (λ/∆λ ∼ 57000), optical spectra study of a sample of nearby (distance less than 25 pc), main-sequence, late-type (spectral type FGK) stars are presented. The specific objective of this project is the spectroscopic characterisation of the nearby late-type stellar population, in particular the stars hosting debris discs. In order to analyse the temporal evolution of debris discs, stellar ages as accurate as possible are required. However, stellar age is one of the most difficult parameters to determine, in particular for solar-type field stars. Therefore, in the first part of this dissertation a comparison between different methods to derive ages is performed. Young stars are identified on the basis of their kinematics and plausible membership to stellar kinematic groups. Radial velocities are derived by using the cross-correlation technique and galactic spatial-velocity components (U, V, W) are computed. The young nature of the candidates selected by their kinematics is confirmed or rejected by analysing their lithium abundances, rotation periods, and levels of chromospheric and coronal emission. Our results show that roughly 25% of the stars analysed show a kinematics in agreement with some of the known kinematic groups. Nevertheless, when age diagnostics are considered, most of these stars are too old to be members of kinematic groups, and only 25.5% of the candidates (6.5% of the total stars) can be considered as probable members of a kinematic group. The results of this study are published by Maldonado et al. (2010, Chapter 3). In the second part of this dissertation, we study whether the presence or absence of debris discs correlate with the stellar properties of the host stars, in particular with the stellar age and metallicity, since the presence of planets (giant-planets) is known to correlate with the metal content of the host star. Fundamental stellar parameters, T<sub>eff</sub> , log g, and [Fe/H] are calculated by measuring the equivalent width of a sample of isolated Fe I and Fe II lines and applying the iron excitation and ionisation equilibrium conditions. Data from the literature for those stars with debris disc not included in our spectroscopic observations are also used. We have found a smooth transition toward higher metallicities from stars with neither debris disc nor planets to stars hosting giant-planets. Stars with debris discs show a similar metallicity distribution to that of stars without discs. Stars harbouring both debris discs and planets show the same metallicity behaviour as stars hosting planets. These results can be explained by core-accretion models of planet formation. In addition, we have also found that planets orbiting around stars with debris discs are usually cool. The data suggest that stars hosting debris and gas-giant planets tend to have low-dust luminosities, and that there seems to be an anticorrelation between the dust luminosity and the innermost planet eccentricity. These trends could be explained by recent simulations of dynamical instabilities produced by eccentric giant planets. This study is published by Maldonado et al. (2012, Chapter 4). The confirmation or rejection of the apparent trends suggested in Chapter 4 requires the analysis of extended samples, i.e., the detection of new debris discs around stars with planets. The possibility of finding colder and fainter debris discs using the new capabilities offered by the Herschel Space Telescope is discussed in Chapter 5. This work was motivated as a preparatory activity for the DUNES (DUst around NEarby Stars, Principal Investigator: Carlos Eiroa) project, a Herschel Open Time Key Project aimed to detect circumstellar debris discs with IR excesses as low as the one of the Edgeworth-Kuiper belt. The work addresses how stellar photospheres are predicted at far-IR wavelengths, analysing the different sources of uncertainties and their influence in the detection of a debris discs: i) choice of the stellar atmosphere model (ATLAS9, PHOENIX, MARCS); ii) stellar parameters; and iii) choice of the photometric points used to match the models with the observed spectral energy distribution. This work played a central role to accurately fix the exposure time of each of the DUNES targets, achieving a compromise between the exposure time of each target and optimising, as much as possible, the total observing time granted for DUNES. This work has been submitted for publication (Chapter 5).