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CONTEXT: Accurate mass, radius, and abundance determinations from binaries provide important information on stellar evolution, fundamental to central fields in modern astrophysics and cosmology. AIMS: Within the long-term Copenhagen Binary Project, we aim to obtain high-quality light curves and standard photometry for double-lined detached eclipsing binaries with late A, F, and G type main-sequence components, needed for the determination of accurate absolute dimensions and abundances, and for detailed comparisons with results from recent stellar evolutionary models. METHODS: Between March 1985 and July 2007, we carried out photometric observations of AD Boo, HW CMA, SW CMa, V636 Cen, VZ Hya, and WZ Oph at the Str"omgren Automatic Telescope at ESO, La Silla. RESULTS: We obtained complete uvby light curves, ephemerides, and standard uvby\beta indices for all six systems.For V636 Cen and HW CMa, we present the first modern light curves, whereas for AD Boo, SW CMa, VZ Hya, and WZ Oph, they are both more accurate and more complete than earlier data. Due to a high orbital eccentricity (e = 0.50), combined with a low orbital inclination (i = 84.7), only one eclipse, close to periastron, occurs for HW CMa. For the two other eccentric systems, V636 Cen (e = 0.134) and SW CMa (e = 0.316), apsidal motion has been detected with periods of 5270 +/- 335 and 14900 +/- 3600 years, respectively.

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. — —

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 star-forming 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 disk 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., Corrected typo in author name

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.

We present 4179 differential observations in the V filter measured by a robotic telescope, as well as 25 pairs of radial velocities from high-resolution spectroscopic observations, of the detached, EB-type, 1.69 day period double-lined eclipsing binary star V885 Cyg. Absolute dimensions of the components are determined with high precision (better than 1.5% in the masses and radii) for the purpose of testing various aspects of theoretical modeling. We obtain 2.005 ± 0.029 M⊙ and 2.345 ± 0.012 R⊙ for the hotter, but smaller, less massive and less luminous photometric primary (star A), and 2.234 ± 0.026 M⊙ and 3.385 ± 0.026 R⊙ for the cooler, larger, more massive and more luminous photometric secondary (star B). The effective temperatures and interstellar reddening of the stars are accurately determined from uvbyβ photometry: 8375 ± 150 K for the primary, 8150 ± 150 K for the secondary—corresponding to spectral types of A3m and A4m—and 0.058 mag for Eb-y. The metallic-lined character of the stars is revealed by high-resolution spectroscopy and uvbyβ photometry. The orbits are circular, and spectral line widths give observed rotational velocities that are synchronous with the orbital motion for both components. The components of V885 Cyg are main-sequence stars with an age of about 500 Myr according to models. Our estimate of the age of this system would seem to favor the hydrodynamic damping formalism of Tassoul & Tassoul in this particular case, since both the components' spins are synchronous and the orbit is circular.

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 Msun and 0.73 Msun, 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 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.

We present a deep survey of H? emission-line stars in the NGC 2264 region using wide-field objective prism films. We find 357 H? emission-line stars, of which 113 were previously detected, within an area of 3? ? 3? centered on the Cone Nebula, with a majority of stars being concentrated in a dense cluster at the center of the region. We present a large-scale CO map of NGC 2264 and find a strong correlation between the cluster of H? emission-line stars and the most massive core in the cloud complex. A more extended halo of stars poorly correlated with the gas may represent stars that have drifted away from their birth place, suggesting that star formation has taken place in NGC 2264 for several million years.

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.

In the last decades, the early pre main sequence stellar rotational evolution picture has been constrained by studies targeting different young regions at a variety of ages. Observational studies suggest a mass-rotation dependence, and for some mass ranges a connection between rotation and the presence of a circumstellar disk. Not still fully explored, though, is the role of environmental conditions on the rotational regulation. We investigate the rotational properties of candidate members of the young massive association Cygnus OB2. The Stetson variability index, Lomb-Scargle periodogram, Saunders statistics, string/rope length method, and visual verification of folded light curves were applied to select 1224 periodic variable stars. Completeness and contamination of the periodic sample was derived from Monte Carlo simulations, out of which 894 periods were considered reliable. Our study was considered reasonably complete for periods from 2 to 30 days. The general rotational scenario seen in other young regions is confirmed by Cygnus OB2 period distributions, with disked stars rotating on average slower than non-disked stars. A mass-rotation dependence was also verified, but as in NGC 6530, lower mass stars are rotating on average slower than higher mass stars, with an excess of slow rotators among the lower mass population. The effect of the environment on the rotational properties of the association was investigated by re-analysing the results while taking into account the incident UV radiation arising from O stars in the association. Results compatible with the disk-locking scenario were verified for stars with low UV incidence, but no statistical significant relation between rotation and disk presence was verified for stars with high UV incidence suggesting that massive stars can have an important role on regulating the rotation of nearby low mass stars., Submitted on December 23, 2016

The absolute dimensions of the eclipsing pair in the triple system TY CrA provide constraints on the evolutionary models for both main sequence and pre-main sequence phases.

The eect of mutual illumination in a close bi- nary on the limb-darkening coecients is studied using the UMA (Uppsala Model Atmosphere) code in convective line-blanketed atmospheres illuminated by line-blanketed fluxes, for 3 700 K

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.

We analyze new photometric data for the Herbig Be eclipsing binary TY CrA, which securely reveal the secondary eclipse, ~0.03 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 M1 = 3.16 ± 0.02 M☉ and M2 = 1.64 ± 0.01 M☉, the radii are R1 = 1.80 ± 0.10 R☉ and R2 = 2.08 ± 0.14 R☉, the luminosities are L1 = 67 ± 12 L☉ and L2 = 2.4 ± 0.8 L☉, and the effective temperatures are T1 = 12,000 ± 500 K and T2 = 4900 ± 400 K. Here the uncertainties represent high-confidence limits, not standard deviations. The secondary star is a pre–main-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 effective temperatures for the primary (e.g., 12,500 K), the solar-composition 1.64 M☉ evolutionary tracks of Swenson et al., Claret, and D'Antona & 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 M☉ evolutionary models, which predict somewhat higher effective temperatures than observed.

The observational data of the Orion Nebula Cluster (ONC) is reanalyzed by means of new sets of pre-main sequence evolutionary tracks including rotation, non-gray boundary conditions and either low (LCE) or high convection efficiency (HCE). Our goal is to get a better understanding of the appropriate physical constraints for the rotational evolution of the stars within the ONC, with particular emphasis on the choice of their initial angular momentum and its variation with time. The role played by convection is a key aspect of our analysis, since there are conflicting results from theory and observations. For example, 2-D radiation hydrodynamic simulations predict HCE in the pre-main sequence, while semi-empirical calibrations either seem to require that convection is less efficient in pre-MS than in the following main-sequence phase. We derived stellar masses and ages for the ONC by using both LCE and HCE. Another relevant aspect that we considered was the role of non-gray atmospheres. Our results show that the resulting mass distribution for the bulk of the ONC population is in the range 0.2−0.4M for our non-gray models, and in the range 0.1-0.3M for gray models. In agreement with previous works, we found that a large percentage (∼70%) of low-mass stars (M≤Mtr, where Mtr is a transition mass) in the ONC appears to be fast rotators (P

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 M1 = 1.417 +/- 0.021 MSun, M2 = 1.346 +/- 0.024 MSun, R1 = 2.451 +/- 0.027 RSun, and R2 = 2.281 +/- 0.014 RSun. 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., Comment: 12 pages in emulateapj format, including all tables and figures. To appear in The Astronomical Journal (accepted)

The standard cosmological model suggests that after the “Big Bang”, 14 billion of years ago, the universe entered a period of expansion and cooling. In the first one millionth of a second appear quarks, glúons, electrons and neutrinos, followed by the appearance of protons and neutrons. In this paper, we describe the “cosmic battle” between gravitation and energy, responsible for the lighter chemical elements and the formation of the stars. We describe the thermodynamics of irreversible processes of systems which are far away from equilibrium, a route that is followed by the universe, seen as a living system.

At least during the last ten years, the Brazilian astronomical community has been asking for an echelle spectrograph for the 1.6 m telescope installed at Pico dos Dias Observatory (Brazopolis, MG, Brazil, OPD/MCTI/LNA). Among the scientific cases are topics related to the chemical evolution of the Galaxy, asteroseismology, chemical composition and chromospheric activities of solar type stars and the relations between solar analogues and terrestrial planets. During 2009 the project finally got started. The called ECHARPE spectrograph (Espectrografo ECHelle de Alta Resolucao para o telescopio Perkin-Elmer) is being projected to offer a spectral resolution of R ~ 50000, in the range 390-900 nm and with a single exposition. It will be a bench spectrograph with two channels: blue and red, fed by two optical fibers (object, sky or calibration) with aperture of 1.5 or 2.0 arcseconds. The instrument will be placed in one of the telescope pillar ramification, in the originals installations of a Coude spectrograph and in a specially created environment controlled room. In this work we will present the scientific motivations, the conceptual optical design, the expected performance of the spectrograph, and the status of its development. ECHARPE is expected to be delivered to the astronomical community in 2014, fully prepared and optimized for remote operations.

HIP96515A is a double-lined spectroscopic binary with a visual companion (HIP96515B) at 8.6 arcsec. It is included in the SACY catalog as a potential young star and classified as an eclipsing binary in the ASAS Catalog. We have analyzed spectroscopic and photometric observations of the triple system. The high-resolution optical spectrum of HIP96515A has been used to derive a mass ratio, M_2/M_1, close to 0.9, with the SB2 components showing spectral types of M1 and M2. The ASAS and Hipparcos light-curves of HIP96515A show periodic variations with P=2.3456 days, confirming that HIP96515A is an eclipsing binary with preliminary parameters of i=89, M_Aa=0.59+-0.03 Msun and M_Ab=0.54+-0.03 Msun, for the primary and secondary, respectively, at an estimated distance of 42+-3 pc. This is a new eclipsing binary with component masses below 0.6 Msun. Multi-epoch observations of HIP 96515 A&B show that the system is a common proper motion pair. The optical spectrum of HIP 96515B is consistent with a pure helium atmosphere (DB) white dwarf. We estimate a total age (main-sequence lifetime plus cooling age) of 400 Myr for the white dwarf. If HIP 96515 A&B are coeval, and assuming a common age of 400 Myr, the comparison of the masses of the eclipsing binary members with evolutionary tracks shows that they are underestimated by ~15% and ~10%, for the primary and secondary, respectively., Accepted for publication in A&A, 9 pages, 9 figures

We present the result of a survey for Herbig-Haro objects in the “Gulf of Mexico”, a part of the dark cloud L935. Images taken through Hα and [S ii] filters at SUBARU Suprime Cam covering an area of 34 ′ ×27 ′ reveal 35 new Herbig-Haro objects in this active star forming region, little studied so far.

We present new V-band differential brightness measurements as well as new radial-velocity measurements of the detached, circular, 0.84-day period, double-lined eclipsing binary system CV Boo. These data along with other observations from the literature are combined to derive improved absolute dimensions of the stars for the purpose of testing various aspects of theoretical modeling. Despite complications from intrinsic variability we detect in the system, and despite the rapid rotation of the components, we are able to determine the absolute masses and radii to better than 1.3% and 2%, respectively. We obtain M(A) = 1.032 +/- 0.013 M(Sun) and R(B) = 1.262 +/- 0.023 R(Sun) for the hotter, larger, and more massive primary (star A), and M(B) = 0.968 +/- 0.012 M(Sun) and R(B) = 1.173 +/- 0.023 R(Sun) for the secondary. The estimated effective temperatures are 5760 +/- 150 K and 5670 +/- 150 K. The intrinsic variability with a period about 1% shorter than the orbital period is interpreted as being due to modulation by spots on one or both components. This implies that the spotted star(s) must be rotating faster than the synchronous rate, which disagrees with predictions from current tidal evolution models according to which both stars should be synchronized. We also find that the radius of the secondary is larger than expected from stellar evolution calculations by about 10%, a discrepancy also seen in other (mostly lower-mass and active) eclipsing binaries. We estimate the age of the system to be approximately 9 Gyr. Both components are near the end of their main-sequence phase, and the primary may have started the shell hydrogen-burning stage., 17 pages in emulateapj format, including figures and tables. To appear in The Astronomical Journal

Rotational evolution in young stars is described by pMS evolutionary tracks including rotation, conservation of angular momentum (AM), and simulations of disk-locking (DL). By assuming that DL is the regulation mechanism for the stellar angular velocity during the early stages of pMS, we use our models and observational data to constrain disk lifetimes (Tdisk) of a sample of low-mass stars in the ONC and NGC2264. The period distributions of the ONC and NGC2264 are bimodal and depend on the stellar mass. To follow the rotational evolution of these two clusters' stars, we generated some sets of evolutionary tracks. We assumed that the evolution of fast rotators can be modeled by considering conservation of AM during all stages and of moderate rotators by considering conservation of angular velocity during the first stages of evolution. With these models we estimate a mass and an age for all stars. For the ONC, we assume that the secondary peak in the period distribution is due to high-mass objects locked in their disks, with a locking period (Plock) of ~8 days. For NGC2264 we make two hypotheses: (1) the stars in the secondary peak are locked with Plock=5 days, and (2) NGC2264 is in a later stage in the rotational evolution (this implies in a DL scenario with Plock=8 days, a Tdisk of 1 Myr and, after that, constant AM evolution). We simulated the period distribution of NGC2264 when its mean age was 1 Myr. Dichotomy and bimodality appear in the simulated distribution, presenting one peak at 2 days and another one at 5-7 days, indicating that the assumption of Plock=8 days is plausible. Our hypotheses are compared with observational disk diagnoses available in the literature. DL models with Plock=8 days and 0.2 Myr, 20 pages and 24 figures

The eclipsing triple system RV Crt shows a composite spectrum dominated by the sharp-lined late-F type spectrum of the tertiary. The spectral lines of the close binary are considerably broadened, since both components rotate synchronously with the orbit. The Doppler shifts and the component spectra are successfully recovered by application of the spectral disentangling technique. The cooler, less massive component of the close binary is larger than the hotter one. The hotter component of the close binary is very similar to the tertiary. The system is probably still on its way to the zero-age main sequence

Aims. In order to increase the limited number of B-stars with accurately known dimensions, and also the number of well studied eclipsing binaries in open clusters, we have undertaken observations and studies of four southern double-lined eclipsing B-type binaries; DWCar, BF Cen, ACVel, and NSV 5783.\ud Methods. Complete uvby light curves were observed between January 1982 and April 1991 at the Danish 0.5 m telescope at ESO La Silla, since 1985 known as the Strömgren Automatic Telescope (SAT). Standard indices for the systems and the comparison stars,as well as additional minima observations for ACVel, have been obtained later at SAT. For DWCar and ACVel, high-resolution spectra for definitive spectroscopic orbits have also been obtained; they are presented as part of the detailed analyses of these systems.\ud A few spectra of NSV 5783 are included in the present paper.\ud Results. For all four systems, the first modern accurate light curves have been established. DWCar is a detached system consisting of two nearly identical components. It is member of the young open cluster Cr228. A detailed analysis, based on the new light curves and 29 high-resolution spectra, is published separately. BFCen is semidetached and is member of NGC 3766. Modern spectra are\ud needed for a detailed study. ACVel is a detached system with at least one more star. A full analysis, based on the new light curves and 18 high-resolution spectra, is published separately. NSV 5783 is discovered to be an eclipsing binary consisting of two well-detached\ud components in an 11-day period eccentric (e = 0.18) orbit. Secondary eclipse is practically total. From the light curves and a few high-resolution spectra, accurate photometric elements and preliminary absolute dimensions have been determined. The quite similar components have masses of about 5 M and radii of about 3.5 R, and they seem to have evolved just slightly off the ZAMS. The\ud measured rotational velocities (≈150 km s−1) are about 6 times those corresponding to pseudosynchronization.

We derive the orbital period and monitor in detail the eclipses of two deep-southern double-lined eclipsing binaries of considerable interest. We present new uvby photometry over more than 100 nights for V883 Cen, scan for the first time the eclipses of η Mus and analyse the data for these two systems together with earlier published photometry. We also add information from high-dispersion spectra. V883 Cen consists of two sharp-lined, mid-B-type stars in an orbit of 35.44733 d. The slower rotating component, Prot= 55.215 d, is a chemically peculiar star of the type He-weak SrTi that is partially eclipsed by its slightly more massive companion. Owing to the high eccentricity of the orbit and its orientation, only one eclipse occurs. The orbital period of η Mus is 57.5 h, rather than the 20 d quoted up to the present in the literature. That close binary, consisting of two late-B-type stars, shows identical partial eclipses. Interest in the close binary is triggered by the pre-main-sequence companion at 2.7 arcsec. HD 118978, one of the comparison stars used for V883 Cen, is identified as a new variable. This fast-rotating B9 (sub)giant belongs presumably to the class of slowly pulsating B-type stars.

Complete uvby light curves of the detached triple-lined late B-type eclipsing binary V906 Scorpii, secured from 1987 to 1991, are presented. A detailed photometric analysis based on these observations and on new spectroscopic material yields accurate masses and radii (errors ≲ 2%) for the components, confirms that the system is a member of NGC 6475 (Messier 7), and is published separately (Alencar et al. 1997).

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. 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 gives us the opportunity to begin explorations regarding a physically consistent treatment of magnetic effects in stellar structure and evolution, in spite of the formidable mathematical and numerical challenges involved.As an example of such explorations, we present theoretical estimates of the convective turnover time τc for rotating pre‐main sequence solar‐type stars, based on up‐to‐date input physics for stellar models. Those estimates, as opposed to the use of empirically derived values of τc for such matters, can be used to calculate the Rossby number Ro, which is related to the magnet...

We report our present efforts for introducing magnetic fields in the ATON stellar evolution code code, which now evolved to truly modifying the stellar structure equations so that they can incorporate the effects of an imposed, large-scale magnetic field. Preliminary results of such an approach, as applied to low-mass stellar models, are presented and discussed.

We present an analysis of 32 high-resolution echelle spectra of the premain sequence spectroscopic binary AK Sco obtained during 1998 and 2000, as well as a total of 71 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. 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 and obtain physical parameters for the two stars.

The most important ingredient of a stellar model is its initial mass. Hence, real stars with known masses are important test objects for stellar models. Initial chemical composition is the other important starting parameter and should therefore also be known (Clausen, 1992). Finally, the most direct indicator of the evolution of a star is its radius. Together, these parameters determine the effective temperature and luminosity of the model, to be tested against a real star.

We have redetermined the absolute dimensions of the mid B-type eclipsing binary U Oph from new light and radial-velocity curves, accounting for both the apsidal motion and the light-time orbit around the third star. The stars in U Oph have masses of 5.27 and 4.74 M⊙(±1.5%) and are located in the middle of the main-sequence band for an an age of ∼50 Myr. U Oph and three other systems (V760 Sco, MU, Cas and DI Her) all have components within 10% of 5M⊙ and ages below 100 Myr; we find significant heavy-element abundance differences between these young nearby stars.

In order to detect variable stars in the well known star forming region, the Orion Nebula Cluster, a series of 22 exposures taken from November 1996 to October 1998, using the ESO 100/152cm Schmidt telescope, covering a field of 5° × 5° was analyzed. The films (Kodak Tech-Pan 4415 emulsions, effective spectral range from ~630 nm to 690 nm) were digitized by the SuperCOSMOS machine, the measurements calibrated to the R magnitude of the USNO B1.0 catalogue and differential photometry was performed throughout the whole field. In the process, a set of 260 stars that remained constant in the 22 films and were well distributed over the field was selected and used as comparison stars for the differential photometry of all the other stars in the field. Diverse statistical studies were performed in order to characterize the type and degree of variability of the objects. The 22 films, all exposed for 30 minutes each, were stacked together at our request by the SuperCOSMOS team, producing perhaps the deepest wide field image of M42 ever taken.This database (>150 000 objects, mostly stars and ~2% galaxies) is going to be used as a starting point for the Variable Young Stellar Object Survey (VYSOS) project, which consists of 2 fully automated robotic telescopes of 41cm each, one installed on Mauna Loa (Hawaii, USA) and the other at Cerro Armazones (Chile), both using the Pan STARRS set of photometric filters. We discovered thousands of new variable stars within the 5° × 5° region studied. We have variability statistics for all objects and are classifying the variable stars according to the variability type and amplitude. We intend to make this database available via the WEB.

The correlations angular momentaL to massesM are studied for different types of spectroscopic binaries. The functionsL=AMb have the coefficientb with the values expected from a Keplerian mechanics, but the valuesA(q, T), A(q, a), A(q, v), associated tob=5/3, 3/2, and 2, respectively, are given (statistically speaking) by multiples or submultiples of discrete values of: the mass ratiosq, the semi-major axesa, periodsT, and velocitiesv of the reduced mass. This indicates the existence of a discrete unit of actionL=(1/2π)×potential energy xperiod. Postulates about equivalent states of angular momenta for different orbital parameters are introduced, being this coherent with the analysis of the up-to-date data. Among other examples of the application of such equivalence postulates, we haveL(M) (W-type of the WUMa systems)≈L(M) (main group of the Algol binaries). The quantum units of action seen here are equivalent to those seen in the solar system in one of our previous works. From comparisons with galaxies and single stars, it is evidence that there is not an unique universal functionL=AMb, when the fine structure of the relation is analysed: each type of object has its own coefficients,A, b. It sems to be that there are an upper and a lower limit for all the possible functions. The upper limit isL=AgM5/3, withAg≈1 associated to periodsT Hubble time, and the lower limit isL=γGM2/c, with γ≤1. The existence of the upper limit can be investigated with studies of pairs of galaxies, and the lower limit can be tested with analysis of single G, K, M stars. The quantical hypothesis introduced here can be checked definitely, when available larger samples of data with low errors, with similar quality as the selected list of almost 80 eclipsing binaries (mainly detached systems) analysed here.

The weakest point in the modern models of eclipsing binary systems (EBS) is the treatment given to the effects of mutual irradiation. In this review, which does not have a similar one in the literature, I tried to collect all the work done on the irradiation problem until the middle of 1984, in order to make possible an evaluation of the present status of problem. Special emphasis is given to the applicability of the results to the analysis of EBS. The treatment given to the effect by the early studies as well as by practically all the modern models of EBS is described, and special attention is given to works analysing the problem using stellar model atmospheres. It turns out that the effect is more complex than suspected earlier, but that significant progress has been made recently.