Power-2 limb-darkening coefficients for the uvby, UBVRIJHK, SDSS ugriz, Gaia, Kepler, TESS, and CHEOPS photometric systems II. PHOENIX spherically symmetric stellar atmosphere models

The Spanish MEC (AYA2015-71718-R, ESP2017-87676-C5-2-R, PID2019-107061GB-C64, and PID2019-109522GB-C52) is gratefully acknowledged for its support during the development of this work. A.C. acknowledges financial support from the grant CEX2021-001131-S funded by MCIN/AEI/ 10.13039/501100011033. This research has made use of the SIMBAD database, operated at the CDS, Strasbourg, France, of NASA’s Astrophysics Data System Abstract Service and of SVO Filter Profile supported from the Spanish MINECO through grant AYA2017-84089.Context. The phenomenon of limb-darkening is relevant to many topics in astrophysics, including the analysis of light curves of eclipsing binaries, optical interferometry, measurement of stellar diameters, line profiles of rotating stars, gravitational microlensing, and transits of extrasolar planets Aims. Multiple parametric limb-darkening laws have been presented, and there are many available sources of theoretical limb-darkening coefficients (LDCs) calculated using stellar model atmospheres. The power-2 limb-darkening law allows a very good representation of theoretically predicted intensity profiles, but few LDCs are available for this law from spherically symmetric model atmospheres. We therefore present such coefficients in this work. Methods. We computed LDCs for the space missions Gαiα, Kepler, TESS, and CHEOPS and for the passbands uνby, UBVRIJHK, and SDSS ugriz, using the PHOENIX-COND spherical models. We adopted two methods to characterise the truncation point, which sets the limb of the star: the first (M1) uses the point where the derivative dI(r)/dr is at its maximum - where I(r) is the specific intensity as a function of the normalised radius r - corresponding to μcri, and the second (M2) uses the midpoint between the point μcri and the point located at μcn-1. The LDCs were computed adopting the Levenberg-Marquardt least-squares minimisation method, with a resolution of 900 equally spaced μ points, and covering 823 model atmospheres for a solar metallicity, effective temperatures of 2300-12000 K, log g values from 0.0 to 6.0, and microturbulent velocities of 2 km s-1. As our previous calculations of LDCs using spherical models included only 100 μ points, we also updated the calculations for the four-parameter law for the passbands listed above, and compared them with those from the power-2 law. Results. Comparisons between the quality of the fits provided by the power-2 and four-parameter laws show that the latter presents a lower merit function, X2, than the former for both cases (M1 and M2). This is important when choosing the best approach for a particular science goal.The Spanish MEC (AYA2015-71718-R, ESP2017-87676-C5-2-R, PID2019-107061GB-C64, PID2019-109522GB-C52)MCIN/AEI/ 10.13039/501100011033 CEX2021-001131-SCDSNASA’s Astrophysics Data System Abstract ServiceThe Spanish MINECO AYA2017-8408

MECI: A Method for Eclipsing Component Identification

We describe an automated method for assigning the most probable physical parameters to the components of an eclipsing binary, using only its photometric light curve and combined colors. With traditional methods, one attempts to optimize a multi-parameter model over many iterations, so as to minimize the chi-squared value. We suggest an alternative method, where one selects pairs of coeval stars from a set of theoretical stellar models, and compares their simulated light curves and combined colors with the observations. This approach greatly reduces the parameter space over which one needs to search, and allows one to estimate the components' masses, radii and absolute magnitudes, without spectroscopic data. We have implemented this method in an automated program using published theoretical isochrones and limb-darkening coefficients. Since it is easy to automate, this method lends itself to systematic analyses of datasets consisting of photometric time series of large numbers of stars, such as those produced by OGLE, MACHO, TrES, HAT, and many others surveys.Comment: 25 pages, 7 figures, accepted for publication in Ap

Probing the Stellar Surface of HD 209458 from Multicolor Transit Observations

Multicolor photometric observations of a planetary transit in the system HD 209458 are analyzed. The observations, made in the Stromgren photometric system, allowed a recalculation of the basic physical properties of the star-planet system. This includes derivation of linear limb-darkening values of HD 209458, which is the first time that a limb-darkening sequence has observationally been determined for a star other than the Sun. As the derived physical properties depend on assumptions that are currently known with limited precision only, scaling relations between derived parameters and assumptions are given. The observed limb-darkening is in good agreement with theoretical predictions from evolutionary stellar models combined with ATLAS model atmospheres, verifying these models for the temperature (Teff ~ 6000K), surface gravity (log g ~ 4.3) and mass (~ 1.2 Msol) of HD 209458.Comment: 16 pages, 8 figures, uses elsart.cls, accepted for New Astronom

Theoretical isochrones for the Delta a photometric system

We have calculated theoretical isochrones for the photometric Delta a system to derive astrophysical parameters such as the age, reddening and distance modulus for open clusters. The Delta a system samples the flux depression at 520 nm which is highly efficient to detect chemically peculiar (CP) objects of the upper main sequence. The evolutionary status of CP stars is still a matter of debate and very important to test, for example, the dynamo and diffusion theories. In fact, the dynamo or fossil origin of the magnetic fields present in this kind of stars it still not clear. Using the stellar evolutionary models by Claret (1995), a grid of isochrones with different initial chemical compositions for the Delta a system was generated. The published data of 23 open clusters were used to fit these isochrones with astrophysical parameters (age, reddening and distance modulus) from the literature. As an additional test, isochrones with the same parameters for Johnson UBV data of these open clusters were also considered. The fits show a good agreement between the observations and the theoretical grid. We find that the accuracy of fitting isochrones to Delta a data without the knowledge of the cluster parameters is between 5 and 15%.Comment: 5 pages, 4 figures, accepted by A&

Power-2 limb-darkening coefficients for the uvby, UBVRIJHK, SDSS ugriz, Gaia, Kepler, and TESS photometric systems: I. ATLAS stellar atmosphere models

Context. Limb darkening is an important stellar phenomenon and must be accounted for in the study of stellar spectra, eclipsing binaries, transiting planetary systems, and microlensing events. The power-2 limb-darkening law provides a good match to the specific intensities predicted by stellar atmosphere models: it is better than other two-parameter laws and is only surpassed by the four-parameter law. Aims. Predictions of the limb-darkening coefficients for the power-2 law are not widely available. We therefore compute them, using stellar atmosphere models generated by the ATLAS (plane-parallel) code. Methods. Limb-darkening coefficients were computed for the space missions Gaia, Kepler, and TESS as well as for the photometric systems teuby, UBVRIJHK, and SDSS ugriz. The calculations were performed by adopting the Levenberg-Marquardt least-squares minimisation method and were computed with a resolution of 100 equally spaced viewing angles. We used 9586 model atmospheres covering 19 metallicities, effective temperatures of 3500-50 000 K, log g values from 0.0 to 5.0, and microturbulent velocities of 0, 1, 2, 4, and 8 km s(-1). Results. We confirm the superiority of the power-2 law, in terms of the quality of the fits, over other two-parameter laws. This is particularly relevant for the quadratic law, which is widely used. Conclusions. We recommend the use of the power-2 law in cases where a two-parameter law is needed.Spanish Government AYA2015-71718-R ESP2017-87676-C5-2-R PID2019-107061GB-C64 PID2019-109522GB-C52State Agency for Research of the Spanish MCIU SEV-2017-0709Spanish Government AYA2017-8408

Exploratory scenarios for the differential nuclear and tidal evolution of TZ Fornacis

Aims. TZ Fornacis is a double-lined eclipsing binary system with similar masses (2.057 +/- 0.001 and 1.958 +/- 0.001 M-circle dot) but characterized by very different radii (8.28 +/- 0.22 and 3.94 +/- 0.17 R-circle dot). This similarity in terms of mass makes it possible to study the system's differential stellar evolution as well as some aspects of its tidal evolution. With regard to its orbital elements, it was recently confirmed that its orbit is circular with an orbital period of 75.7 days. The less massive component rotates about 17 times faster than the primary one, which is synchronized with the mean orbital angular velocity. Our main objective in this work is to study both the nuclear and the tidal evolution of the system. Methods. To model the TZ For system, we used the MESA package, computing the grids using the exact observed masses, radii, and effective temperatures as input, and then varying the metallicity, the core overshooting amount, and the mixing-length parameter. A chi(2) statistic was used to infer the optimal values of the core overshooting and the mixing-length parameters. The same procedure was used to generate rotating models with the GRANADA code. The respective errors in the average age of TZ For were less than 5%. On the other hand, the differential equations that govern the tidal evolution were integrated using the fifth-order Runge-Kutta method, ith a tolerance of 1 x 10(-7). Results. We explored two scenarios regarding the initial eccentricities: a high one (0.30) and a case of an initial circular orbit. A good agreement has been found between the observational values of the eccentricity, synchronism levels, and orbital period with the values predicted by the integration of the tidal evolution equations. The influence of the friction timescale on the evolution of the orbital elements of TZ For is also studied here. The orbital elements most affected by the uncertainties in the friction timescale are the synchronism levels of the two components. On the other hand, we used the properties of the rotating models generated by the GRANADA code as the initial angular velocities instead of using trial values. In this case, comparisons between the theoretical values of the orbital elements and their observed counterparts also lead to a good interagreement.Spanish Government ESP2017-87676-C5-2-R PID2019-107061GB-C64 PID2019-109522GB-C52State Agency for Research of the Spanish MCIU SEV-2017-070

Rotationally and tidally distorted compact stars A theoretical approach to the gravity-darkening exponents for white dwarfs

To the best of our knowledge, there are no specific calculations of gravity-darkening exponents for white dwarfs in the literature. On the other hand, the number of known eclipsing binaries whose components are tidally and/or rotationally distorted white dwarfs is increasing year on year. Our main objective is to present the first theoretical approaches to the problem of the distribution of temperatures on the surfaces of compact stars distorted by rotation and/or tides in order to compare with relevant observational data. We find discrepancies between the gravity-darkening exponents calculated with our methods and the predictions of the von Zeipel theorem, particularly in the cases of cold white dwarfs; although the discrepancy also applies to higher effective temperatures under determined physical conditions. We find physical connections between the gravity-darkening exponents calculated using our modified method of triangles strategy with the convective efficiency (defined here as the ratio of the convective to the total flux). A connection between the entropy and the gravity-darkening coefficients is also found: variations of the former cause changes in the way the temperature is distributed on distorted stellar surfaces. On the other hand, we have generalised the von Zeipel theorem for the case of hot white dwarfs. Such a generalisation allows us to predict that, under certain circumstances, the value of the gravity-darkening exponent may be smaller than 1.0, even in the case of high effective temperatures.Comment: Accepted publication Astronomy&Astrophysic

On the Selection of Photometric Planetary Transits

We present a new method for differentiating between planetary transits and eclipsing binaries based on the presence of the ellipsoidal light variations. These variations can be used to detect stellar secondaries with masses ~0.2 M_sun orbiting sun-like stars at a photometric accuracy level which has already been achieved in transit surveys. By removing candidates exhibiting this effect it is possible to greatly reduce the number of objects requiring spectroscopic follow up with large telescopes. Unlike the usual candidate selection method, which are primarily based on the estimated radius of the orbiting object, this technique is not biased against bona-fide planets and brown dwarfs with large radii, because the amplitude of the effect depends on the transiting object's mass and orbital distance. In many binary systems, where a candidate planetary transit is actually due to the partial eclipse of two normal stars, the presence of flux variations due to the gravity darkening effect will show the true nature of these systems. We show that many of the recent OGLE-III photometric transit candidates exhibit the presence of significant variations in their light curves and are likely to be due to stellar secondaries. We find that the light curves of white dwarf transits will generally not mimic those of small planets because of significant gravitationally induced flux variations. We discuss the relative merits of methods used to detect transit candidates which are due to stellar blends rather than planets. We outline how photometric observations taken in two bands can be used to detect the presence of stellar blends.Comment: ApJ, 11 pages, 2 figures, 1 table, replaced with accepted versio

Análisis cinemático de robots manipuladores redundantes: Aplicación a los robots Kuka LWR 4+ y ABB Yumi

En este trabajo se presenta un análisis cinemático aplicado a dos manipuladores serie redundantes: el Kuka LWR 4+ y el ABB Yumi. En particular, se deriva la cinemática directa para ambos manipuladores y se resuelve el problema de la cinemática inversa. Para el Kuka LWR 4+ dicha solución se obtiene en forma analítica, mientras que para el ABB Yumi se sigue un enfoque analítico y numérico. Además, se calculan simbólicamente tanto las singularidades del Kuka LWR 4+ como las direcciones singulares asociadas a éstas. Este estudio contribuye al conocimiento cinemático de dos manipuladores redundantes de gran actualidad e interés para la comunidad robótica, y proporciona información útil para el diseño de diferentes algoritmos y leyes de control.Postprint (author's final draft