Luiz Paulo Ribeiro Vaz

Dynamical Mass Constraints on Low-Mass Pre-Main-Sequence Stellar Evolutionary Tracks: An Eclipsing Binary in Orion with a 1.0 M☉ Primary and a 0.7 M☉ Secondary*

Abstract : 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 approx. 1%, are 1.01 and 0.73 M(solar), respectively; thus, the primary is a definitive pre-main-sequence solar analog, and the secondary is the lowest-mass star yet discovered among pre-main-sequence eclispsing binary systems. We use these fundamental measurements to test the predictions of per-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 ell 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.

The pre-main sequence spectroscopic binary AK Scorpii revisited ?;??

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 < i< 70, leading to the following physical parameters for the two near-identical stars: M= 1:35 0:07 M, R= 1:59 0:35 R ,a ndv sin i= 18:5 1: 0k m s 1 . Disk models have been fit to the spectral energy distribution of AK Sco from 350 nm to 1100 m. 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 eects 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 H 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 H 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.

ABSOLUTE PROPERTIES OF THE ECLIPSING BINARY STAR RW LACERTAE

We present 3004 differential observations in the V bandpass measured by a robotic telescope, as well as 36 pairs of radial velocities from high-resolution spectroscopic observations, of the detached, eccentric, EA-type, 10.37 day period, double-lined eclipsing binary star RW Lac. Absolute dimensions of the components are determined with excellent precision (better than 0.7% in the masses and 0.5% in the radii) for the purpose of testing various aspects of theoretical modeling. We obtain 0.928 ± 0.006 M⊙ and 1.186 ± 0.004 R⊙ for the hotter, larger, more massive, and more luminous photometric primary (star A) and 0.870 ± 0.004 M⊙ and 0.964 ± 0.004 R⊙ for the cooler, smaller, less massive, and less luminous photometric secondary (star B). A faint, third component contributes 2.6% of the light in V but is not detected in our spectrograms. The effective temperatures and interstellar reddening of the stars are accurately determined from UBV and uvbyβ photometry and from analysis of the spectrograms: 5760 ± 100 K for the primary and 5560 ± 150 K for the secondary, corresponding to spectral types of G5 and G7, and 0.050 mag for interstellar reddening Eb-y. The orbits are slightly eccentric, and spectral line widths give observed rotational velocities that are not significantly different from synchronous for both components. The components of RW Lac are old, somewhat metal-deficient, low-mass, main-sequence stars with an age of about 11 Gyr, according to models.

Theoretical values of convective turnover times and Rossby numbers for solar-like, pre-main sequence stars

Context. 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 (tachocline) between the radiative core and the base of the convective envelope. Aims. Since dynamo models are based on the interaction between differential rotation and convective motions, the introduction of rotation in the ATON 2. 3 stellar evolutionary 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. Methods. As examples of such explorations, we present theoretical estimates for both the local convective turnover time (τ c ,), and global convective times (τ g ) 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. Results. Those estimates, as opposed to the use of empirically derived values of τ c for such matters, 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. Conclusions. Our theoretical values of τ c , τ g and Ro qualitatively agree with those published by Kim & Demarque (1996, ApJ, 457, 340). By increasing the convection efficiency, T g decreases for a given mass. FST models show still lower values. The presence of rotation shifts τ g towards slightly higher values when compared with non-rotating models. The use of non-gray boundary conditions in the models yields values of τ g smaller than in the gray approximation.

Combined effects of tidal and rotational distortions on the equilibrium configuration of low-mass, pre-main sequence stars

Context. In close binary systems, the axial rotation and the mutual tidal forces of the component stars deform each other and destroy their spherical symmetry by means of the respective disturbing potentials. Aims. We present new models for low-mass, pre-main sequence stars that include the combined distortion effects of tidal and rotational forces on the equilibrium configuration of stars. Using our theoretical results, we aim at investigating the effects of interaction between tides and rotation on the stellar structure and evolution. Methods. The Kippenhahn & Thomas (1970) approximation, along with the Clairaut-Legendre expansion for the gravitational potential of a self-gravitating body, is used to take the effects of tidal and rotational distortions on the stellar configuration into account. Results. We obtained values of internal structure constants for low-mass, pre-main sequence stars from stellar evolutionary models that consider the combined effects of rotation and tidal forces due to a companion star. We also derived a new expression for the rotational inertia of a tidally and rotationally distorted star. Our values corresponding to standard models (with no distortions) are compatible with those available in literature. Our distorted models were successfully used to analyze the eclipsing binary system EK Cep, reproducing the stellar radii, effective temperature ratio, lithium depletion, rotational velocities, and the apsidal motion rate in the age interval of 15.5-16.7 Myr. Conclusions. In the low-mass range, the assumption that harmonics greater than j=2 can be neglected seems not to be fully justified, although it is widely used when analyzing the apsidal motion of binary systems. The non-standard evolutionary tracks are cooler than the standard ones, mainly for low-mass stars. Distorted models predict more massconcentrated stars at the zero-age main-sequence than standard models.

Absolute properties of the eclipsing binary system AQ Serpentis: A stringent test of convective core overshooting in stellar evolution models

We report differential photometric observations and radial-velocity measurements of the detached, 1.69 day period, double-lined eclipsing binary AQ Ser. Accurate masses and radii for the components are determined to better than 1.8% and 1.1%, respectively, and are M 1 = 1.417 ± 0.021 M ☉, M 2 = 1.346 ± 0.024 M ☉, R 1 = 2.451 ± 0.027 R ☉, and R 2 = 2.281 ± 0.014 R ☉. The temperatures are 6340 ± 100 K (spectral type F6) and 6430 ± 100 K (F5), respectively. Both stars are considerably evolved, such that predictions from stellar evolution theory are particularly sensitive to the degree of extra mixing above the convective core (overshoot). The component masses are different enough to exclude a location in the H-R diagram past the point of central hydrogen exhaustion, which implies the need for extra mixing. Moreover, we find that current main-sequence models are unable to match the observed properties at a single age even when allowing the unknown metallicity, mixing length parameter, and convective overshooting parameter to vary freely and independently for the two components. The age of the more massive star appears systematically younger. AQ Ser and other similarly evolved eclipsing binaries showing the same discrepancy highlight an outstanding and largely overlooked problem with the description of overshooting in current stellar theory.

Non-gray rotating stellar models and the evolutionary history of the Orion Nebular Cluster

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 effi cient 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.

V907 Scorpii: A Remarkable Binary Star Whose Eclipses Turn On and Off and On and Off

V907 Scorpii, near the open cluster M7, and possibly a member, is unique among all known eclipsing binary stars because its eclipses have turned on and off twice within modern history. By using all available photometric and spectroscopic data, we have discovered that it is at least a triple star and possibly a quadruple star system consisting of a visual binary with a very long orbital period, the brighter member of which is itself the triple star. The triple star contains an eclipsing binary star (B9.5 V) with an orbital period of 3.78 days and a faint, distant companion (late K, or perhaps a white dwarf) with an orbital period of 99.3 days around the center of mass of the triple star system. Radial velocity measurements allow the masses to be estimated. Because the orbital planes of the eclipsing binary and its triple companion are not coplanar, the orbital plane of the eclipsing binary shows nodal regression with a period of 68 yr. For about one-third of this time, the close binary is eclipsing; the rest of the time the inclination is too small for eclipses to occur. The earliest observations of the system in the year 1899 show eclipses; the eclipses stopped about 1918, started again about 1963, and stopped again in about 1986. We predict that the eclipses should start occurring once again in the year 2030 ± 5.

An analysis of the light curve of the post common envelope binary MT Serpentis

Photometric observations of MT Ser, the central star of the planetary nebula Abell 41 are presented. The periodic modulations detected by Grauer & Bond (1983) are conrmed, thus rmly establishing the binary nature of MT Ser. The signicantly enlarged time base permits us to derive more accurate ephemeris. The orbital period is either P1 =0 :113226533 days or twice that value, P2 =0 :226453066 days. We analyze the light curve (after a careful subtraction of the nebular contribution) with the Wilson-Devinney light curve synthesis routine. Since it is not a priori clear which is the true orbital period of MT Ser, two radically dierent models, one based on P1 the other on P2 are considered: (1) A low temperature component orbiting around a hot sub-dwarf. The variability is then due to a reflection eect together with ellipsoidal variations of one or both components. (2) Two hot sub- dwarfs of similar temperature and luminosity, partially eclipsing each other and exhibiting ellipsoidal variations. In both models, the primary as well as the secondary component are required to almost ll their respective Roche lobes. A contact conguration is possible. Pros and cons can be found for either of the two models. A nal decision between them has to await the observations of a radial velocity curve. The orbital period is currently decreasing at a rate of _ P=P = 1:15 10 9 d 1 . Interpreting this as due to mass loss via a stellar winds permits us to

Absolute dimensions of eclipsing binaries - XXV. U Ophiuchi and the evolution and composition of 5 M

Context. Precise stellar masses and radii provide unique information on stellar evolution. In a Galactic context, they may also provide information on the evolution of the Solar neighbourhood. Aims. We aim to determine absolute dimensions for the mid B-type eclipsing binary U Ophiuchi and compare the inferred ages and chemical compositions to those of other binary stars with masses near 5 M� . Methods. We determine masses, radii, log g ,l ogTeff, and luminosities for the stars in U Oph from new radial velocities and uvby light curves. By improving the Wilson-Devinney code, we also derive precise apsidal-motion and light-time orbits of this triple system, using 353 times of minimum over 120 years. Finally, we compare the data for U Oph and three similar systems with the predictions of stellar models. — —