Estudo espectroscópico da atividade cromosférica em estrelas do tipo solar: a busca por caracterizar estrelas análogas em mínimo de Maunder

Abstract

Since the first observations of sunspots made by Thomas Harriot and subsequently by Galileo and several other scientists, astronomers looked for methods to better understand the magnetic phenomena in the Sun and in solar-type stars. It is well established that the first indicators used to characterize magnetic activity were sunspots. It is also worth highlighting one of the most intriguing phenomena of the evolution of solar magnetism known as Maunder’s minimum, which occurred specifically between 1645 and 1715. What makes this minimum of activity enigmatic is the registration of very few sunspots and practically no variability. over 70 years, remaining to this day without a consolidated explanation. In the twentieth century, with the advancement of observational techniques related to spectroscopy, astronomers began to make observations of solar/stellar activity also through spectra. The solar/stellar chromospheric activity has been the subject of exhaustive treatment over the last six decades in the literature. The main spectroscopic indicators come from the Ca II H&K resonant lines, particularly the indicators SMW and the index with photosphere correction R0 HK. Spectroscopic measurements of the activity of solar-type stars have been carried out since 1960, in an attempt to answer the question of how unique the magnetic evolution of the Sun is in relation to other solar-type stars. All this background has allowed to determine that stars of solar type have activity cycles, in the case of the Sun a primary cycle of 11 years is well established and a possible secondary cycle of 88 years may exist. The goal of this doctoral thesis is the observational investigation of the chromospheric activity of solar-type stars that are often cited in the literature as Maunder minimum candidates. In our work we obtained as main results: 1) The characterization and determination of the activity cycle of the solar analog HD43587 Pcyc = 10, 44 ± 3, 03 yrs, as well as estimates of its rotational period Prot = 22, 6 ± 1, 9d, and characterization of its magnetic field; 2) Development of theoretical tools to estimate the observations of stellar spots globally in other stars of the solar type; In addition we estimated the activity proxies to the solar Maunder minimum, i.e. hSMW i = 0.1625 ± 0.0003, log hR0 HKi = −4.933 ± 0.002 and the variabilities σSMW = 1.1 × 10−7 and σlog R0 HK= 2.7 × 10−6; 3) Analysis and determination of the activity cycle of 28 stars of the solar type indicated in the literature as in the state of Maunder minimum; 4) Calibration of activity indexes based on the Ca II H&K lines using the Coudé spectrograph located at the Pico dos Dias Observatory.