Selección de objetos para los estudios de arqueología galáctica con el instrumento WEAVE

Abstract

After the success of the Gaia mission launched in 2013, which was intended to be operational for 5 years, providing measurements of parallax, positions and proper movements of stars and photometry; It was proposed to extend the mission time for another 5 years due to its excellent results. But this time the Gaia probe will not be alone in its mission, it is intended to use the data that Gaia provides so that the WEAVE instrument (WHT Enhanced Area Velocity Explorer), which is a multi-object spectrograph that is intended to be incorporated into the William Herschel telescope (WHT) from the Roque De Los Muchachos observatory located on the island of La Palma, supplement the measurements and information collected by Gaia and combine it with its own measurements. Four studies related to galactic archeology are planned with the WEAVE instrument, but the one related to this TFG is a low-resolution study of the high latitude of the Milky Way. In this mapping, the WEAVE instrument carries out a study of stars found in a certain latitude in the sky that corresponds to a field of view in which it is intended to have a view of the galactic halo and the thick disk, but eliminating the thin disk. On its own, Gaia is capable of measuring parallax to determine distances, proper movements to determine tangential velocities and low-resolution spectroscopy, it also has a radial velocity spectrometer that provides chemical information about stars and radial velocities as its name suggests. However, for magnitude intervals between 16 <~ G <~ 21 mag, the radial velocity spectrometer is unable to provide such information and this is where the WEAVE instrument comes into play, complementing Gaia with measurements of basic chemical information and radial velocity measurements (aside from the various studies that WEAVE plans to conduct, but not related to this work). However, the problem is that the WEAVE instrument is not capable of performing measurements for as many stars as Gaia so it is necessary to select exhaustively the elements to study. The following are chosen as targets for the study of the high latitude of the galaxy: red giants, candidates for extremely metal-poor stars, stars from the blue horizontal branch, and stars that have deviated from the main sequence. Within WEAVE, a selection has already been made for the giants using magnitude and color, as well as parallax and self-motion information provided by Gaia's astrometric instrument to reject local red main sequence stars from the selection. Although this selection is already good enough, there is still a fraction of unwanted stars that fall within the range of quite considerable selected parameters, so it is thought that this selection can be optimized. That is precisely the objective of this study, to optimize the selection of stars based on the data collected from Gaia's low-resolution spectrometer. In order to do this, we are going to start with a synthetic spectra that are used to simulate the data set that in advance will be supplied by Gaia's low resolution spectrograph (which are not yet available), and in this way to create a model that allows us, using these already known data (those from the synthetic spectrum), to obtain a relationship that works to a greater or lesser extent, to later be able to differentiate the two types of stars that are measured in the mission, which we call giants and dwarfs, requiring only flow measurements. To do this, in the first place, the data from the synthetic spectra library are transformed so that these are expressed in the same way that the Gaia spectrometer will provide its measurements, making transmitivity corrections, change of units, etc. To do this, we started from a code provided by the researcher Sergey Koposov, in which those transformations were carried out and from which the rest of the research and subsequent analysis could be developed, once the code was analyzed, the main task was understanding what was carried out in each part and looking if there was some kind of optimization flaw in it. In this study, the different properties of the set of synthetic "stars" provided by the synthetic spectra are analyzed, such as the metallicity and effective temperature distributions with respect to flow and logarithm of surface gravity, the distribution of fluxes with respect to wavelengths by means of percentiles or the various properties that highlight the differences in magnitude, among several others. The most promising method has seemed to be one based on comparisons of the different magnitude differences at different frequencies between the two groups of stars, reaching quite promising results with a selection of stars with a low percentage of contamination, however, the results they have turned out to be perhaps insufficient by themselves, although in collaboration with the WEAVE distinction method it could be quite useful as a purge in certain cases. Apart from this, several paths have been followed which, if they have not all reached the desired method, have provided us with very interesting information on the characteristics of the star distributions and are worthy of study and comment. From here on, the introduction section provides a scientific context about the project, as well as its motivation and some relevant data, the objectives section talks about the purpose of the work itself and what it is intended to obtain ; In the methodology section, it is explained how, starting from a series of synthetic data, we are going to operate with them to arrange them in a way that we can use them to investigate as if they were real measurements. In the different analyzes that are proposed, we will study the different methods followed and briefly discuss the results obtained with each of them, and finally, in the conclusions section, the information obtained and the result itself of the work carried out are discussed above all, finally several reasons are discussed why it would be interesting to continue investigating the method used.