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Register a new userEvryscope-South Survey of Upper- and Pre-main Sequence Solar Neighborhood Stars
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Using photometric data collected by Evryscope-South, we search for nearby young variable systems on the upper-main sequence (UMS) and pre-main sequence (PMS). The Evryscopes are all-sky high-cadence telescope arrays operating in the Northern and Southern hemispheres. We base our search on a Gaia-selected catalog of young neighborhood upper- and pre-main sequence stars which were chosen through both astrometric and photometric criteria. We analyze 44,971 Evryscope-South light curves in search of variability. We recover 615 variables, with 378 previously known, and 237 new discoveries including 84 young eclipsing binary (EB) candidates. We discover a new highly eccentric binary system and recover a further four previously known systems, with periods ranging from 299 to 674 hr. We find 158 long-period (>50 hr) candidate EB systems, 9 from the PMS and 149 from the UMS, which will allow constraints on the mass-radius-age relation. These long-period EBs include a 179.3 hr PMS system and a 867.8 hr system from the UMS. For PMS variable candidates we estimate system ages, which range from 1 to 23 Myr for non-EBs and from 2 to 17 Myr for EBs. Other non-EB discoveries that show intrinsic variability will allow relationships between stellar rotation rates, ages, activity, and mass to be characterized.
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The bulk of X-ray emission from pre-main-sequence (PMS) stars is coronal in origin. We demonstrate herein that stars on Henyey tracks in the Hertzsprung-Russell diagram have lower $log(L_X/L_ast)$, on average, than stars on Hayashi tracks. This effect is driven by the decay of $L_X$ once stars develop radiative cores. $L_X$ decays faster with age for intermediate mass PMS stars, the progenitors of main sequence A-type stars, compared to those of lower mass. As almost all main sequence A-type stars show no detectable X-ray emission, we may already be observing the loss of their coronae during their PMS evolution. Although there is no direct link between the size or mass of the radiative core and $L_X$, the longer stars have spent with partially convective interiors, the weaker their X-ray emission becomes. This conference paper is a synopsis of Gregory, Adams and Davies (2016).
We present initial result of a large spectroscopic survey aimed at measuring the timescale of mass accretion in young, pre-main-sequence stars in the spectral type range K0 - M5. Using multi-object spectroscopy with VIMOS at the VLT we identified the fraction of accreting stars in a number of young stellar clusters and associations of ages between 1 - 50 Myr. The fraction of accreting stars decreases from ~60% at 1.5 - 2 Myr to ~2% at 10 Myr. No accreting stars are found after 10 Myr at a sensitivity limit of $10^{-11}$ Msun yr-1. We compared the fraction of stars showing ongoing accretion (f_acc) to the fraction of stars with near-to-mid infrared excess (f_IRAC). In most cases we find f_acc < f_IRAC, i.e., mass accretion appears to cease (or drop below detectable level) earlier than the dust is dissipated in the inner disk. At 5 Myr, 95% of the stellar population has stopped accreting material at a rate of > 10^{-11} Msun yr-1, while ~20% of the stars show near-infrared excess emission. Assuming an exponential decay, we measure a mass accretion timescale (t_acc) of 2.3 Myr, compared to a near-to-mid infrared excess timescale (t_IRAC) of 2.9 Myr. Planet formation, and/or migration, in the inner disk might be a viable mechanism to halt further accretion onto the central star on such a short timescale.
Aims: We study the influence of rotation and disc lifetime on lithium depletion of pre-main sequence (PMS) solar-type stars. Methods: The impact of rotational mixing and of the hydrostatic effects of rotation on lithium abundances are investigated by computing non-rotating and rotating PMS models that include a comprehensive treatment of shellular rotation. The influence of the disc lifetime is then studied by comparing the lithium content of PMS rotating models experiencing different durations of the disc-locking phase between 3 and 9 Myr. Results: The surface lithium abundance at the end of the PMS is decreased when rotational effects are included. During the beginning of the lithium depletion phase, only hydrostatic effects of rotation are at work. This results in a decrease in the lithium depletion rate for rotating models compared to non-rotating ones. When the convective envelope recedes from the stellar centre, rotational mixing begins to play an important role due to differential rotation near the bottom of the convective envelope. This mixing results in a decrease in the surface lithium abundance with a limited contribution from hydrostatic effects of rotation, which favours lithium depletion during the second part of the PMS evolution. The impact of rotation on PMS lithium depletion is also found to be sensitive to the duration of the disc-locking phase. When the disc lifetime increases, the PMS lithium abundance of a solar-type star decreases owing to the higher efficiency of rotational mixing in the radiative zone. A relationship between the surface rotation and lithium abundance at the end of the PMS is then obtained: slow rotators on the zero-age main sequence are predicted to be more lithium-depleted than fast rotators due to the increase in the disc lifetime.
Pre-main sequence (PMS) stars evolve into main sequence (MS) phase over a period of time. Interestingly, we found a scarcity of studies in existing literature that examines and attempts to better understand the stars in PMS to MS transition phase. The purpose of the present study is to detect such rare stars, which we named as Transition Phase (TP) candidates - stars evolving from the PMS to the MS phase. We identified 98 TP candidates using photometric analysis of a sample of 2167 classical Be (CBe) and 225 Herbig Ae/Be (HAeBe) stars. This identification is done by analyzing the near- and mid-infrared excess and their location in the optical color-magnitude diagram. The age and mass of 58 of these TP candidates are determined to be between 0.1-5 Myr and 2-10.5 M$_odot$, respectively. The TP candidates are found to possess rotational velocity and color excess values in between CBe and HAeBe stars, which is reconfirmed by generating a set of synthetic samples using the machine learning approach.
Magnetic fields are at the heart of the observed stellar activity in late-type stars, and they are presumably generated by a dynamo mechanism at the interface layer between the radiative and the convective stellar regions. Since dynamo models are based on the interaction between differential rotation and convective motions, the introduction of rotation in the ATON 2.3 stellar code allows for explorations regarding a physically consistent treatment of magnetic effects in stellar structure and evolution, even though there are formidable mathematical and numerical challenges involved. As examples, we present theoretical estimates for both the local (tau_c) and global (tau_g) convective turnover times for rotating pre-main sequence solar-type stars, based on up-to-date input physics for stellar models. Our theoretical predictions are compared with the previous ones available in the literature. In addition, we investigate the dependence of the convective turnover time on convection regimes, the presence of rotation and atmospheric treatment. Those estimates, this quantities can be used to calculate the Rossby number, Ro, which is related to the magnetic activity strength in dynamo theories and, at least for main-sequence stars, shows an observational correlation with stellar activity. More important, they can also contribute for testing stellar models against observations. Our theoretical values of tau_c, tau_g and Ro qualitatively agree with those published by Kim & Demarque (1996). By increasing the convection efficiency, tau_g decreases for a given mass. FST models show still lower values. The presence of rotation shifts tau_g towards slightly higher values when compared with non-rotating models. The use of non-gray boundary conditions in the models yields values of tau_g smaller than in the gray approximation.
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