RT info:eu-repo/semantics/doctoralThesis
T1 Search and characterization of exo-earths participation in the development of the espresso spectrograph for the VLT
A1 Suárez Mascareño, Alejandro
A2 Programa  de Doctorado en Astrofísica
K1 Astronomía óptica
K1 Planetas
AB The search for exo-Earths using Doppler spectroscopy faces nowadays two maindifficulties. The first is a technical limitation. The precision of the instrumentslimits the measurable amplitudes to signals produced by super-Earths orbitingclose to their host stars. Terrestrial planets orbiting solar-type stars inducevariations smaller than 1 m s−1except when orbiting very close to the star.The arrival of ESPRESSO and the Laser Frequency Comb as calibration unitwill palliate this limitation and move the measurement precision to the rangeof 5 cm s−1. So far the search in solar-Type stars is limited to super-Earths.The alternative is to find terrestrial planets in the habitable zone is the searcharound M-type stars. Their lower mass makes the signals of terrestrial planetsdetectable by today’s instruments.The second issue is the presence of radial velocity variations induced by thehomogeneities of the stellar surface. The changes in the stellar surface and thestellar rotation induce periodic signals that go from less than 1 m s−1in thequietest stars to dozens of m s−1in active stars and might be easily mistakenfor signals induced by rocky planets. In the case of M-dwarfs the need ofperforming the analysis of the stellar induced signals is even more important.Signals produced by early M-dwarfs have periods compatible with the habitablezone of those stars and amplitudes similar to those of super-Earths. The rise inprecision expected for the next years will make this kind of signals visible evenin the quietest stars making even more important to understand and correctlymodel them.Using chromospheric indicators from high resolution spectroscopic and photometric time series we study both long term and short term variability (solarlike cycles and rotation) of a sample of 176 low magnetic activity stars withspectral types going from late F-type to mid M-type stars. We provide newmeasurements of magnetic cycles for 105 stars, and rotation periods for 123stars. We study the distribution of rotation periods, finding that there is anevolution in the rotation period lengths, getting longer for later type stars, thatsaturates when reaching K-type stars and begins to increase again at types laterthan M2. We study the distribution of magnetic cycles, finding no clear difference among different spectral types.We study the relationships between the measured periods and the chromospheric activity level of the stars, measured as the log10(R0HK). To do so weextend the log10(R0HK) calibration to cover stars up to B-V ∼ 2.0. We find astrong correlation between the measured rotation period and the mean levelof chromospheric activity that allows to predict the rotation periods of mainsequence stars from G-type to mid M-type with an accuracy of the ∼ 20% of the period.We analyse the radial velocity time series of 133 stars searching both foractivity induce signals and planetary signals. We detect rotation induced signals in 46 stars, with amplitudes going from 0.3 to 16.4 ms−1. We study therelationship between the amplitude of the induced signals and the activity indicators, finding a correlation between the mean level of chromospheric activitylog10(R0HK) and the measured amplitude of the signal. This relationship showsthat FGK stars show a similar behaviour, which is different for M-type stars.We find also a relationship between the amplitude of the modulation in theMount Wilson S index and the amplitude of the induced radial velocity signal. This relationship behaves differently for two populations of stars, with abreaking point around mid-K stars.After cleaning the radial velocity series form stellar activity induced signalswe performed a search for dynamical signals in the same sample of stars. Werecovered most of the previously published planet candidates, except for a fewcases where we attribute a stellar origin to the previously assumed planetarycompanion. We present the discovery of six new planets around the stars HD1581, HD 161098, HD 176986, GJ 536 and GJ 3998, with minimum masses inM⊕ units of 5.87 ± 0.99, 26.60 ± 4.39, 4.86 ± 0.66 and 5.47 ± 0.84, 2.35 ±0.42 and 6.12 ± 1.11 respectively located at 0.321, 2.503, 0.061, 0.066, 0.029and 0.089 au from their parent stars. The latter two have been reported byAffer et al. (2016). Five of them are presumably rocky planets. We suggestthe presence of a seventh one in the star HD 161098 with a minimum mass of5.30 ± 1.22 M⊕ located at 0.317 au of the star, which could be habitable giventhe appropriate atmospheric conditions. We suggest also the presence of fiveadditional candidates that still require further analysis and observations.
YR 2016
FD 2016
LK http://riull.ull.es/xmlui/handle/915/26025
UL http://riull.ull.es/xmlui/handle/915/26025
LA en
DS Repositorio institucional de la Universidad de La Laguna
RD 27-sep-2024