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The stellar initial mass function (IMF) is an essential input for many astrophysical studies but only in a few cases it has been determined over the whole cluster mass range, limiting the conclusions about its nature. The 25 Orionis group (25 Ori) is an excellent laboratory to investigate the IMF across the entire mass range of the population, from planetary-mass objects to intermediate/high-mass stars. We combine new deep optical photometry with optical and near-infrared data from the literature to select 1687 member candidates covering a 1.1$^\circ$ radius area in 25 Ori. With this sample we derived the 25 Ori system IMF from 0.012 to 13.1 $M_\odot$. This system IMF is well described by a two-segment power-law with $\Gamma=-0.74\pm0.04$ for $m, Comment: Accepted for publication in MNRAS

The realistic simulation of variable star populations is fundamental to determine the selection function and contamination in existing and upcoming multi-epoch surveys. We present \ellisa, a simulator that produces an ensemble of mock light curves for a population of eclipsing binaries obtained from physical and orbital parameters consistent with different galactic populations, and which considers user-supplied time sampling and photometric errors to represent any given survey. We carried out a search for eclipsing binaries in the QUEST low-galactic latitude catalogue of variable stars, spanning an area of 476 sq. deg at $-25^\circ \la b \la 30^\circ$ and $190^ \circ \leqslant l \leqslant 230^ \circ$ towards the galactic anti-centre, and use \ellisa~to characterise the completeness of the resulting catalogue in terms of amplitudes and periods of variation as well as eclipsing binary type. The resulting catalogue consists of $1,125$ eclipsing binaries, out of which $179$, $60$ and $886$ are EA, EB and EW types respectively. We estimate, on average, $30\%$ completeness in the period range $0.25 \la P/d \la 1$ for EB+EW binaries and $15\%$ completeness for EA binaries with periods $2 \la P/d \la 10$, being the time sampling the primary factor determining the completeness of each type of eclipsing binary. This is one of few eclipsing binary catalogues reported with an estimate of the selection function. Mock eclipsing binary light curve libraries produced with \ellisa~can be used to estimate the selection function and optimise eclipsing binary searches in upcoming multi-epoch surveys such as Gaia, the Panoramic Survey Telescope and Rapid Response System, the Zwicky Transient Factory or the Large Synoptic Survey Telescope., Comment: 17 pages, 13 figures

En esta Tesis Doctoral estudiamos la formación, evolución y probabilidad de detección de planetas de tipo terrestre orbitando objetos de masa 0,08 M⊙ cercana al límite subestelar. Para estudiar el proceso de formación y evolución planetaria en tales sistemas, desarrollamos experimentos numéricos haciendo uso de un código de N-cuerpos preexistente, al cual modificamos incluyendo de manera secuencial efectos relevantes a los escenarios de trabajo. En una primera instancia, analizamos los procesos de formación y evolución planetaria posteriores a la disipación del gas en el disco. Para ello, se incluyó en el código de N-cuerpos los efectos de marea con la contracción y evolución del período de rotación del objeto central, y efectos de relatividad general. El resultado más importante es que ambos efectos juegan un rol primario en la evolución de los sistemas ya que permiten la supervivencia de planetas de tipo terrestre en órbitas compactas cercanas al objeto central e incluso dentro de la zona habitable. En una segunda instancia, analizamos la formación y evolución planetaria incluyendo etapas tempranas en las que está presente un disco de gas y etapas posteriores a la disipación del disco. Para la interacción entre los embriones planetarios y el gas del disco adoptamos dos prescripciones diferentes e independientes de la literatura que fueron incluidas en el código de N-cuerpos que previamente modificamos incluyendo los efectos de marea y relatividad general. La estructura del disco de gas es descripta a partir de perfiles radiales de densidad superficial, temperatura y altura. Los resultados indican que las propiedades físicas y orbitales de los planetas dependen fuertemente de la prescripción utilizada para modelar la interacción entre los embriones y el gas. Sólo una prescripción permite la supervivencia de planetas en la zona habitable y, además, da lugar a una distribución de cociente de períodos orbitales de planetas adyacentes que es consistente con la derivada a partir de exoplanetas de tipo terrestre observados alrededor de estrellas con masas M⋆ < 0,14 M⊙. Finalmente calculamos la probabilidad de detección de los sistemas simulados y de los pocos planetas terrestres observados en torno a objetos cercanos al límite subestelar. Para ello desarrollamos un código que simula tránsitos, cambios en velocidad radial y en movimiento propio de estrellas debido a la interacción con sus sistemas planetarios e incluyendo valores realistas de errores y muestreo temporal. Empleamos un análisis de frecuencias básico para la detección de planetas. Para estrellas a distancias d ≤ 12 pc los resultados muestran que el método de velocidad radial es el más eficiente y permite la detección de 100% de sistemas como los observados y de hasta 85% para los sistemas simulados. Con el método de tránsito se detecta 2% de los sistemas observados y 1% de los sistemas simulados. Los resultados muestran que el método de movimientos propios no es capaz todavía de detectar los sistemas de interés pero calculamos que lo será con los datos de Gaia DR5. Entender la formación y evolución de planetas terrestres en el límite subestelar es fundamental para estimar el número de planetas habitables en la Galaxia. Esta tesis aporta resultados relevantes para el tratamiento teórico de estos sistemas y ofrece estimaciones realistas sobre la detección de las contrapartes observacionales., Facultad de Ciencias Astronómicas y Geofísicas

Young stellar aggregates are the laboratories where we study the process by which almost every star in the Galaxy has formed. Most of the young stellar groups break up in at most a few tens of Myr and only a few of them survive as gravitational bound entities to become open clusters. This phenomenon is usually understood as a rapid evolution of young stellar systems when the parental molecular gas is expelled, however, there is also evidence suggesting that these unbound stellar associations are formed in a structured process. To understand how stars are formed and how young stellar systems evolve, it is essential to estimate the total mass of the association, which can be achieved through the analysis of the stellar initial mass function (IMF). Despite many contributions to the study of the IMF, only a few of them over the whole cluster mass range, it is still unclear how it depends on environmental conditions and/or time. Due to its closeness, low extinction, stellar density and evolutionary status, an excellent place to study the IMF, from planetary-mass objects to intermediate/high-mass stars, and the early evolution of a young stellar system that just emerged from its embedded phase, is the 25 Orionis stellar group (25 Ori). Combining new deep optical photometry from DECam with optical and NIR data from the literature, we selected 1687 member candidates of 25 Ori with\({I_c}\)-band magnitudes between 5 and 23.3 mag in an area of 1.1\({^\circ}\) radius. With this sample we derived the system IMF of 25 Ori from 0.012 to 13.1 \({M_\odot}\), which is one of the few IMFs across the entire mass range of a stellar association. The resultant system IMF is well-described by a two-part power-law function and by a tapered power-law form. We also report its best lognormal parameterization. This system IMF do not present significant variations within a radius of about 7 pc, which indicates that the substellar and stellar objects in 25 Ori do not have any preferential spatial distribution. We compared the reported system IMF as well as the BD/ratio ratio with those of a large diversity of stellar populations and did not find any significant discrepancies, which strongly supports the hypothesis that the star formation mechanism is largely insensitive to environmental conditions. In order to confirm the membership of each candidate in our sample, we have an ongoing spectroscopic survey using several world-wide facilities. We have obtained high-resolution (\({R\sim22000}\)) spectra of 77 intermediate/high-mass (1.3-11 \({M_\odot}\)) candidates with OAN-SPM/MES and of 1185 intermediate-mass (0.3-5.2 \({M_\odot}\)) candidates with SDSS-IV/APOGEE-2. Additionally, we have low-resolution (\({R\approx1000-2000}\)) spectra of 400 low-mass (0.25-0.8 \({M_\odot}\)) candidates with MMT/Hectospec, of 172 low-mass (0.09-0.7 \({M_\odot}\)) candidates with SDSS-III/BOSS and of 66 brown dwarf (0.01-0.08 \({M_\odot}\)) candidates with GTC/OSIRIS. After applying diverse membership criteria, we have so far confirmed 530 members from the spectroscopic sample, out of which 290 lie inside the 25 Ori area and 208 of them are confirmed for the first time. With this sample of confirmed member, plus those in the literature, we estimated that 25 Ori is a 6.5\({\pm}\)2.5 Myr old population located at 356\({\pm}\)47 pc and presenting a low extinction of 0.29\({\pm}\)0.26 mag. Also, we estimated that the 25 Ori mean radial velocity is 20.9\({\pm}\)2.0 km s\({^{-1}}\). Using these parameters we found that 25 Ori is a dynamically young group without time enough to be relaxed and confirmed that it is, in fact, a gravitationally unbound association that will be part of the Galactic Disk population. Considering the observed and confirmed members as well as those already confirmed in the literature, the spectroscopic follow-up is \({\sim}\)75% complete, with most of the remaining candidates to be observed with estimated masses around the substellar mass limit. We have ongoing observations to complete the spectroscopic survey of 25 Ori.