We analyze new photometric data for the Herbig Be eclipsing binary TY CrA, which securely reveal the secondary eclipse, D0.03 mag deep in y. From the light-curve solution and our previous spectroscopic data, absolute dimensions of the primary and secondary stars are derived. The masses are found to be and the radii are and14 the luminosities are and and the e †ective temperaturesHere the uncertainties represent high-conÐdence limits, T 1 \ 12,000^500 T 2 \ 4900^400 not standard deviations. The secondary star is a preÈmain-sequence star located at the base of the Hayashi tracks. As such, it is the least evolved star with a dynamically measured mass. Given higher e †ective temperatures for the primary (e.g., 12,500 K), the solar-composition 1.64 evolutionary tracks M _ of Swenson et al., Claret, and DÏAntona & Mazzitelli are all consistent with the properties of the TY CrA secondary and suggest an age of order 3 Myr. The radius and projected rotational velocity of the secondary star are consistent with synchronous rotation. The primary star is located near the zero-age main sequence, which, for solar compositions, is consistent with an age of 3 Myr. However, the primary star is not well represented by any of the 3.16 evolutionary models, which predict somewhat higher M _ e †ective temperatures than observed.

We report the discovery of a double-lined, spectroscopic, eclipsing binary in the Orion star-forming region. We analyze the system spectroscopically and photometrically to empirically determine precise, distance-independent masses, radii, effective temperatures, and luminosities for both components. The measured masses for the primary and secondary, accurate to ∼ 1%, are 1.01 M ⊙ and 0.73 M ⊙ , respectively; thus the primary is a definitive pre-main-sequence solar analog, and the secondary is the lowestmass star yet discovered among pre-main-sequence eclipsing binary systems. We use these fundamental measurements to test the predictions of pre-main-sequence stellar evolutionary tracks. None of the models we examined correctly predict the masses of the two components simultaneously, and we implicate differences between the theoretical and empirical effective temperature scales for this failing. All of the models predict the observed slope of the mass-radius relationship reasonably well, though the observations tend to favor models with low convection efficiencies. Indeed, considering our newly determined mass measurements together with other dynamical mass measurements of pre-main-sequence stars in the literature, as well as measurements of Li abundances in these stars, we show that the data strongly favor evolutionary models with inefficient convection in the stellar interior, even though such models cannot reproduce the properties of the present-day Sun. 6 US Naval Observatory, Flagstaff Station, Flagstaff, AZ 1 Based on data collected with the Hobby-Eberly Telescope and the WIYN Telescope. 7 See http://www.ipac.caltech.edu/2mass for information about 2MASS 8 The WIYN Observatory is a joint facility of the University of Wisconsin-Madison, Indiana University, Yale University, and the National Optical Astronomy Observatories.10 The Hobby-Eberly Telescope is operated by McDonald Observatory on behalf of The University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen. The observations described here were obtained through community access made possible by NOAO. 11 More careful analysis of the V1174 Ori spectra after the observations were obtained indicates that the components of V1174 Ori have spectral types of K4.5 and ∼M1.5 (see §4.3.3), but this does not significantly affect our analysis.Fig. 3.-The O − C residuals for the solutions of Fig. 2 and Tables 8 and 9. The vertical bar at the left of each panel is the standard deviation of the residuals.

Abstract.We present an analysis of 32 high-resolution echelle spectra of the pre-main sequence spectroscopic binary AK Sco obtained during 1998 and 2000, as well as a total of 72 photoelectric radial-velocity observations from the period 1986-1994. These data allow considerable improvement of the period and other orbital parameters of AK Sco. Our analysis also includes eight series of photometric observations in the uvby and Geneva seven-color systems from 1987, 1989, 1990, 1992, 1994 and 1997. No eclipses or other periodic variations are seen in the photometry, but the well-determined HIPPARCOS parallax allows us to constrain the orbital inclination of the system to the range 65• , leading to the following physical parameters for the two near-identical stars: M = 1.35 ± 0.07 M , R = 1.59 ± 0.35 R , and v sin i = 18.5 ± 1.0 km s −1 . Disk models have been fit to the spectral energy distribution of AK Sco from 350 nm to 1100 µm. The above stellar parameters permit a consistent solution with an inner rim temperature of 1250 K, instead of the usual 1500 K corresponding to the dust evaporation temperature. Dynamical effects due to tidal interaction of the binary system are supposed to be responsible for pushing the inner disk radius outwards. Combining simultaneous photometric and spectroscopic data sets allows us to compute the dust obscuration in front of each star at several points over the orbit. The results demonstrate the existence of substructure at scales of just a single stellar diameter, and also that one side of the orbit is more heavily obscured than the other. The spectrum of AK Sco exhibits emission and absorption lines that show substantial variety and variability in shape. The accretion-related lines may show both outflow and infall signatures. The system displays variations at the binary orbital period in both the photospheric and accretion-related line intensities and equivalent widths, although with appreciable scatter. The periodic variations in the blue and red wing of Hβ are almost 180• out of phase. We find no evidence of enhanced accretion near the periastron passage in AK Sco as expected theoretically and observed previously in DQ Tau, a similarly young binary system with a mass ratio near unity and an eccentric orbit. The Hα equivalent width displays rather smooth variations at the stellar period, peaking around phases 0.6-0.7, far away from periastron where theory expects the maximum accretion rate to occur.

Using the United Kingdom Infrared Telescope on Mauna Kea, we have carried out a new near-infrared J, H, K monitoring survey of almost a square degree of the starforming Orion Nebula Cluster with observations on 120 nights over three observing seasons, spanning a total of 894 days. We monitored ∼ 15,000 stars down to J ≈ 20 using the WFCAM instrument, and have extracted 1203 significantly variable stars from our data. By studying variability in young stellar objects (YSOs) in the H − K, K color-magnitude diagram, we are able to distinguish between physical mechanisms of variability. Many variables show color behavior indicating either dust-extinction or disk/accretion activity, but we find that when monitored for longer periods of time, a number of stars shift between these two variability mechanisms. Further, we show that the intrinsic timescale of disk/accretion variability in young stars is longer than that of dust-extinction variability. We confirm that variability amplitude is statistically correlated with evolutionary class in all bands and colors.Our investigations of these 1203 variables have revealed 73 periodic AA Tau type variables, many large-amplitude and long-period (P > 15 day) YSOs, including three stars showing widely-spaced periodic brightening events consistent with circumbinary arXiv:1505.01495v2 [astro-ph.SR] 15 May 2015 -2disk activity, and four new eclipsing binaries. These phenomena and others indicate the activity of long-term disk/accretion variability processes taking place in young stars. We have made the light curves and associated data for these 1203 variables available online.

Context. Rotational evolution in the pre-main sequence is described with new sets of pre-MS evolutionary tracks including rotation, non-gray boundary conditions (BCs) and either low (LCE) or high convection efficiency (HCE). Aims. Using observational data and our theoretical predictions, we aim at constraining (1) the differences obtained for the rotational evolution of stars within the ONC by means of these different sets of new models; (2) the initial angular momentum of low mass stars, by means of their templates in the ONC. Methods. We discuss the reliability of current stellar models for the pre-MS. While the 2D radiation hydrodynamic simulations predict HCE in pre-MS, semi-empirical calibrations either seem to require that convection is less efficient in pre-MS than in the following MS phase (lithium depletion) or are still contradictory (binary masses). We derive stellar masses and ages for the ONC by using both LCE and HCE. Results. The resulting mass distribution for the bulk of the ONC population is in the range 0.2−0.4 M for our new non-gray models and, as in previous analyse, in the range 0.1−0.3 M for models having gray BCs. In agreement with Herbst et al. (2002) we find that a large percentage (∼70%) of low-mass stars (M < ∼ 0.5 M for LCE; M < ∼ 0.35 M for HCE) in the ONC appears to be fast rotators (P < 4 days). Three possibilities are open: 1) ∼70% of the ONC low mass stars lose their disk at early evolutionary phases; 2) their "locking period" is shorter; 3) the period evolution is linked to a different morphology of the magnetic fields of the two groups of stars. We also estimate the range of initial angular momentum consistent with the observed periods. Conclusions. The comparisons made indicate that a second parameter is needed to describe convection in the pre-MS, possibly related to the structural effect of a dynamo magnetic field.