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ilustraciones, graficas, tablas En este trabajo se presentan los resultados de un completo estudio sobre la evolución rotacional de las estrellas de baja masa en su etapa evolutiva previa a la secuencia principal. Para el estudio se realiza una extensión del modelo rotacional de Matt [Matt et al., 2012], el cual predice la velocidad ecuatorial de rotación para una estrella del tipo solar durante los primeros tres millones de años, época en la cual la estrella experimenta torques de acreción y de viento estelar estimulado por la acreción. El modelo incorpora cambios en el momento de inercia de la estrella, en la intensidad del campo magnético, pérdida de momento angular por vientos estelares estimulados por acreción y una disminución exponencial de la tasa de acreción. Se asume que la acreción estelar decae exponencialmente con el tiempo y que el disco es permeado por un campo magnético estelar dipolar y con intensidad constante. La extensión del modelo, se realiza hasta la edad del Sol, incluyendo efectos de pérdida de momento angular a través de vientos magnetizados de tipo Kawaler [Kawaler, 1988] una vez la estrella se ubica en la secuencia principal. La calibración de la evolución rotacional se realiza en términos de la rotación solar hoy y a través del análisis de una muestra representativa de estrellas jóvenes con velocidad rotacional reportada en la literatura. Esta muestra consiste en 231 estrellas pertenecientes a 8 asociaciones estelares con edades entre 8Myr y 600Myr. Para altos rotadores (V > 150km/s) se encuentra que la constante Kw que de fine la intensidad del torque de viento magnetizado es igual a 6,018X10^47 gr^{3/2} cm^{1/2} s^{-1} mientras que para bajos rotadores (V < 10km=s) la constante Kw es igual a 5.988X10^47 gr^{3/2} cm^{1/2} s^{-1}. Finalmente, se concluye que la escala de tiempo característica para el decaimiento exponencial de la acreción que mejor ajusta la muestra observada es t_a = 8Myr para un tiempo de vida de disco t_D = 4Myr para un campo magnético de 2kG y un tiempo de vida de disco t_D = 2Myr para un campo magnético de 0.5kG. (Texto tomado de la fuente) This paper presents the results of a complete study on the rotational evolution of low-mass stars in their evolutionary stage prior to the main sequence in the color-magnitude diagram. For the study, an extension of the rotational model of Matt [Matt et al., 2012] is carried out, which predicts the equatorial speed of rotation for a star of the solar type during the rest three million years, when the star experiences accretion torques and stellar wind stimulated by accretion. The model incorporates changes in the star's moment of inertia, in the intensity of the magnetic eld, loss of angular momentum by accretion-stimulated stellar winds, and an exponential decrease in the accretion rate. It is assumed that stellar accretion decays exponentially with time and that the disk is permeated by a constant intensity dipole stellar magnetic eld. The extension of the model is carried out until the age of the Sun, including the e ects of loss of angular momentum through magnetized winds of the Kawaler type [Kawaler, 1988] once the star is located in the main sequence. The calibration of the rotational evolution is carried out in terms of the solar rotation today and through the analysis of a representative sample of young stars with rotational speed reported in the literature. This sample consists of 231 stars belonging to 8 stellar associations with ages between 8Myr and 600Myr. For high rotators (V > 150km=s) it is found that the constant Kw that de nes the intensity of the magnetized wind torque is equal to 6.018X10^47 gr^{3/2} cm^{1/2} s^{-1} while for low rotators (V < 10km=s) the constant Kw is equal to 5.988X10^47 gr^{3/2} cm^{1/2} s^{-1}. Finally, it's concluded that the character 'i stic time scale for the exponential decay of the accretion o n that best ts the observed sample is ta = 8Myr for a disk lifetime tD = 4Myr for a magnetic eld of 2kG and a disk lifetime of tD = 2Myr for a magnetic eld of 0.5kG. (Text taken from source) Maestría Magíster en Ciencias - Astronomía Astrofísica Estelar

4 pages, 3 fig. This research has made use of the SIMBAD database, operated at the CDS, Strasbourg, France, and of NASA’s Astrophysics Data System Abstract Service., TZ Fornacis is an evolved binary system with an orbital period of 75.7 days and a circular orbit. The two stars have similar masses (2.045 ± 0.055 and 1.945 ± 0.027 M ) but very different radii: 8.33 ± 0.12 and 3.966 ± 0.088 R . One of its most interesting characteristics is that while the primary rotates synchronously with the mean orbital angular velocity, the secondary is rotating 16 times faster than this reference value. The stellar and tidal evolution of such a system was investigated in the past using some simplifications: integration of the time scales for circularization and synchronization that are valid only for low eccentricities and small departures from the synchronism, which are not fully valid for the case of TZ Fornacis. Another detected problem is the inconsistency between the observed levels of circularization/synchronization and the theoretical critical times and the inferred age (the time of synchronization of the secondary was found to be shorter than its age). The main goal of the present paper is to advance a little more in our understanding of the stellar and tidal evolution of TZ For. Methods. In order to improve our understanding of the tidal evolution of TZ For, we adopt new absolute dimensions and compute specific stellar evolutionary models for the precise observed masses. We explicitly integrate the differential equations that govern the tidal evolution (eccentricity, angular velocities and orbital period) by using these stellar models. Results. The stellar models indicate that there are two possibilities for the position of the primary on the HR diagram: case A (primary on the clump) and case B (primary on the first ascendent branch). The integration of the complete differential equations of tidal evolution was crucial to shed some light on the tidal evolution of TZ For. The observed levels of circularization/synchronization are now compatible with the inferred age and with the times of circularization/synchronization in both cases. The mentioned inconsistency is no longer detected. In particular, we have found that the primary is synchronized with the orbital period, while the secondary is rotating 17.6 (or 16.2) times faster depending on the selected model, which agrees well with the observations., MEC (AYA2006-06375, AYA2009-14000-C03-01)