Publicação
Hunting for Brown Dwarfs in Corona Australis
| Resumo: | Brown dwarfs are substellar objects with masses ranging from 0.08 Solar masses at the high end, down to those overlapping with masses of giant exoplanets. The study of young brown dwarfs is key to understanding both planet- and star-formation processes. It helps constrain the low-mass regime of the Initial Mass Function which, for nearby star-forming regions, is well characterized down to ∼ 10 − 20 Jupiter masses but becomes loosely defined for lower mass values. Having in mind that young brown dwarfs are very faint sources, a complete census of brown dwarfs in nearby star-forming clouds, where they can be more easily detected, is imperative to improve our knowledge on both planetary and stellar formation models, as well as the shape of the substellar Initial Mass Function. The Corona Australis cloud is one of the closest star-forming regions to the Solar system, at a distance of ∼ 150 parsecs. In 2017, it was observed using the Suprime-Cam instrument at the Subaru telescope. These observations have a field-of-view of ∼ 0.255 deg2 , and result in the deepest optical photometric catalog of the region with magnitudes ranging down to 23 mags in the I-band, which is equivalent to ∼ 3 Jupiter masses at a distance of 150 pc (with no extinction) using the AMES-COND model. The work of this thesis consists of the data reduction of this dataset and of the selection of a list of candidate sources for future spectroscopic observations in Corona Australis. The PSF photometry has been performed using the Source-Extractor and PSFEx software and calibrated with the help of the DENIS I-band photometry, and colours were obtained by cross-matching the dataset with the VISTA Hemisphere Survey catalog. This resulted in an IJKs catalog of 21 133 sources. After photometry, we selected sources from our catalog which presented colours consistent with those of young objects. From the resulting list of sources, we made two further selections. One for the sources present in the Gaia EDR3 catalog, selecting those with proper motions and parallaxes similar to spectroscopically confirmed members of Corona Australis. This selection produced a list of 15 objects. For the other selection, below the Gaia limit, we use only colours. Most of the sources we are interested in are substellar objects and are, hence, very faint. Because of this, they were not detected by Gaia. As such, we selected sources which are below the Gaia limit (I ∼ 19 mag) and above 22 mag. This method resulted in a list of 313 sources, where 145 objects present colours for masses below 5 Jupiter masses, and with most of our candidates residing in the planetary-mass regime (if indeed confirmed as members of Corona Australis) according to the AMES-COND models. It should be stressed, however, that this selection method is expected to produce a large number of contaminants and therefore requires further confirmation through spectroscopy observations. The total list of candidates for follow up spectroscopy observations is then composed of 328 sources. During the Subaru observations, an Hα catalog was also produced. Hα information from this catalog was used to flag possible active accretors in our list of candidates, although only 3 sources from our list of 328 candidates present bright Hα emission. Using empirical models from Pecaut and Mamajek (2013), we built an extinction map of the observed field. This extinction map was used alongside the Besançon galaxy model to estimate the number of contaminants in our list of candidates for follow-up spectroscopy observations. We found a contamination rate of ∼ 90%, meaning we expect to confirm ∼ 30 new planetary mass brown dwarfs. Having only ∼ 100 of these objects been identified so far, our spectroscopy efforts may increase the current budget of known free-floating planetary mass objects by ∼ 30%. The preparatory work for the follow-up spectroscopy observations is also developed here. These observations will be done using the K-band Multi Object Spectrograph at the Very Large Telescope. Seven different fields are proposed encompassing 219 sources from our selection list. Using the Exposure Time Calculator, KMOS-dedicated software and the observation preparation tool p2, we estimate a full exposure time of six hours and twenty-five minutes. |
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| Autores principais: | Baptista, André Gonçalo Costa |
| Assunto: | Anãs Castanhas Corona Australis Fotometria Redução de Dados Subaru Teses de mestrado - 2022 |
| Ano: | 2022 |
| País: | Portugal |
| Tipo de documento: | dissertação de mestrado |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade de Lisboa |
| Idioma: | inglês |
| Origem: | Repositório da Universidade de Lisboa |
| Resumo: | Brown dwarfs are substellar objects with masses ranging from 0.08 Solar masses at the high end, down to those overlapping with masses of giant exoplanets. The study of young brown dwarfs is key to understanding both planet- and star-formation processes. It helps constrain the low-mass regime of the Initial Mass Function which, for nearby star-forming regions, is well characterized down to ∼ 10 − 20 Jupiter masses but becomes loosely defined for lower mass values. Having in mind that young brown dwarfs are very faint sources, a complete census of brown dwarfs in nearby star-forming clouds, where they can be more easily detected, is imperative to improve our knowledge on both planetary and stellar formation models, as well as the shape of the substellar Initial Mass Function. The Corona Australis cloud is one of the closest star-forming regions to the Solar system, at a distance of ∼ 150 parsecs. In 2017, it was observed using the Suprime-Cam instrument at the Subaru telescope. These observations have a field-of-view of ∼ 0.255 deg2 , and result in the deepest optical photometric catalog of the region with magnitudes ranging down to 23 mags in the I-band, which is equivalent to ∼ 3 Jupiter masses at a distance of 150 pc (with no extinction) using the AMES-COND model. The work of this thesis consists of the data reduction of this dataset and of the selection of a list of candidate sources for future spectroscopic observations in Corona Australis. The PSF photometry has been performed using the Source-Extractor and PSFEx software and calibrated with the help of the DENIS I-band photometry, and colours were obtained by cross-matching the dataset with the VISTA Hemisphere Survey catalog. This resulted in an IJKs catalog of 21 133 sources. After photometry, we selected sources from our catalog which presented colours consistent with those of young objects. From the resulting list of sources, we made two further selections. One for the sources present in the Gaia EDR3 catalog, selecting those with proper motions and parallaxes similar to spectroscopically confirmed members of Corona Australis. This selection produced a list of 15 objects. For the other selection, below the Gaia limit, we use only colours. Most of the sources we are interested in are substellar objects and are, hence, very faint. Because of this, they were not detected by Gaia. As such, we selected sources which are below the Gaia limit (I ∼ 19 mag) and above 22 mag. This method resulted in a list of 313 sources, where 145 objects present colours for masses below 5 Jupiter masses, and with most of our candidates residing in the planetary-mass regime (if indeed confirmed as members of Corona Australis) according to the AMES-COND models. It should be stressed, however, that this selection method is expected to produce a large number of contaminants and therefore requires further confirmation through spectroscopy observations. The total list of candidates for follow up spectroscopy observations is then composed of 328 sources. During the Subaru observations, an Hα catalog was also produced. Hα information from this catalog was used to flag possible active accretors in our list of candidates, although only 3 sources from our list of 328 candidates present bright Hα emission. Using empirical models from Pecaut and Mamajek (2013), we built an extinction map of the observed field. This extinction map was used alongside the Besançon galaxy model to estimate the number of contaminants in our list of candidates for follow-up spectroscopy observations. We found a contamination rate of ∼ 90%, meaning we expect to confirm ∼ 30 new planetary mass brown dwarfs. Having only ∼ 100 of these objects been identified so far, our spectroscopy efforts may increase the current budget of known free-floating planetary mass objects by ∼ 30%. The preparatory work for the follow-up spectroscopy observations is also developed here. These observations will be done using the K-band Multi Object Spectrograph at the Very Large Telescope. Seven different fields are proposed encompassing 219 sources from our selection list. Using the Exposure Time Calculator, KMOS-dedicated software and the observation preparation tool p2, we estimate a full exposure time of six hours and twenty-five minutes. |
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