Búsqueda de tránsitos planetarios alrededor de enanas ultrafrías.

Abstract

Questions about the existence of other worlds similar to ours begin in the Ancient Greece with Epicuro, reappeared in the 16th century with Giordano Bruno, and in the 18th century with Isaac Newton until the first evidence of a extrasolar planet in 20th century. But Exoplanets search starts to be very popular since the first discovery in 1995 OF 51Pegb, a hot Jupiter around a solar type star, by Michel Mayor y Didier Queloz. Since then, the field of exoplanets has had an important progress that has allowed the discovery of many exoplanets in our galaxy. In this work we will use one of the most popular methods to find a planet: the Transit Method. This method consists in the detection of the deep in the flux of the star produced when a exoplanet goes before the star covering up an area of it. We use it to study two M dwarfs, two stars classified as intermediate M dwarfs, that have a low temperature and they are named ultracool. Our data will consist in photometric values obtained using the instrument MuSCAT2 at the Telescopio Carlos Sánchez. At the moment, there are more than four thousand of exoplanets discovered through different methods. These planets can be of different types, from planets smaller than Earth to more massive than Jupiter and they can be orbiting different stars. But one of the more interesting stars are the M dwarfs, where new studies seem to indicate that exoplanets around them are more likely to be rocky like the Earth. Furthermore, their low brightness and small size allow to detect planets smaller and with closer orbit to the star planets easier and it is possible to characterize the atmosphere of the exoplanet due to the good the relative higher signal to noise of the planet in in this type of star. There are a lot of methods to detect new exoplanets. The most used are Transit, Radial Velocity, Gravitational Microlensing and Direct Image. These methods have produced the 99% of all the exoplanet discoveries, being the transit method the best with a 77% of them. Using transit technique we can calculate the radio of the exoplanet, directly related to transit depth, and if we know its mass (using the Radial Velocity method), we could calculate its density. Moreover, we can use the secondary transit to obtain the equilibrium temperature, and if we used spectroscopic instruments, we could analyze the atmospheric composition using the called Transit Transmission Spectroscopy. The most successful application of this method is by space missions where the data is not affected by atmospheric disturbances. Space missions work with two principal methods, Transit and Radial Velocity, being the first the most common. Nowadays, there are only two missions whose main objective is the search of exoplanets, ASTERIA and TESS, both using the transits method. TESS is the most important and expected to find around twenty thousand new exoplanets . In the next decade, there are several new missions planned, most using transits too, where one of the most important will be the PLATO mission. It is necessary to work in ground based projects that carried out a follow up of the exoplanet candidates from space missions. There are so much ground based projects, where the most used method is the Radial Velocity, but there are also many which use the transit method . Among them, projects ESPRESSO, TRAPPIST and CARMENES should be noted. Our instrument, MuSCAT2, belongs to the ground based projects that work with transits. MuSCAT2 is a recent instrument that was installed in 2018 in the Observatorio del Teide. MuSCAT2 have a system of dichroics that allow simultaneous multi-color observations in the Sloan filters g, r, i and zs. MuSCAT2 obtained data of our two targets: GJ 3396 and 1RXSJ1733, two variable stars classified like M dwarfs. We are working with 12 nights of GJ 3396 and 40 of 1RXSJ1733, having photometric data for both of them. We have worked with the pipeline of the instrument to get the light curve of each night, modelled the variable contribution of the star calculating its period, subtracted this contribution by a own method (smooth fits) or by a pipeline of MuSCAT2, and to carried out the search of the transit in the “flat curve”. T We have worked in the two targets following the same process: First, we have used the different versions of the pipeline that produce the light curve using the photometric data. Second, we had to use the Lomb-Scargle Periodogram, a tool that allowed us to create a periodogram of the data and obtain the period of the variability of the target. Then, we used a smooth fit that produced a model of this variability so we can subtract it and to produce flattened data where we could see a transit. There is a mathematical algorithm, the Box Least-Square, that helps us to analyze the results and model a transit if it exists. But the nature of the data, which is grouped together because MuSCAT2 observes from night to night, produces an error in the algorithm giving bad results. Finally, we will make a visual analysis of the data. After all this, we have not found a signal of a transit in either target, which indicates that there are no exoplanets around this M dwarfs, or that the exoplanets do not transit between us and the star, or that the exoplanet is small and we cannot detect it. Finally, we do a fast study of the stellar activity of one of our targets, 1RXSJ1733, due to its high activity. This target presents a big number of flares, an explosive rise of the flux, that produces some effects in our data.