The α CrB system is among the brightest known eclipsing binaries in an eccentric orbit. In such systems, the periods between successive primary ( P prim ) and secondary minima ( P sec ) typically differ, and the difference between the two periods provides information on apsidal motion in the system. However, with a binary period of 17.36 days and eclipse durations of more than 8 h, these eclipses are difficult to observe from the ground. We present new photometric observations of primary and secondary eclipses of α CrB obtained with the TESS satellite and we re-examine the available X-ray observations of α CrB at the time of secondary optical minimum. We also combine the optical and X-ray data to determine precise values of P prim and P sec to explore the consequences for apsidal motion in the α CrB system. We have determined an internal structure constant that is in good agreement with theoretical expectations and we show that there is no need to invoke any substantial misalignment between the rotation and orbital axes of the primary.

Stellar activity is ubiquitous in late-type stars. The special geometry of eclipsing binary systems is particularly advantageous to study the stellar surfaces and activity. We present a detailed study of the 145 d CoRoT light curve of the short-period (2.17 d) eclipsing binary CoRoT 105895502. By means of light-curve modeling with Nightfall, we determine the orbital period, effective temperature, Roche-lobe filling factors, mass ratio, and orbital inclination of CoRoT 105895502 and analyze the temporal behavior of starspots in the system. Our analysis shows one comparably short-lived (≈40 d) starspot, remaining quasi-stationary in the binary frame, and one starspot showing prograde motion at a rate of 2.3° day−1, whose lifetime exceeds the duration of the observation. In the CoRoT band, starspots account for as much as 0.6% of the quadrature flux of CoRoT 105895502, however we cannot attribute the spots to individual binary components with certainty. Our findings can be explained by differential rotation, asynchronous stellar rotation, or systematic spot evolution.

Context. Parallaxes for 331 classical Cepheids, 31 Type II Cepheids, and 364 RR Lyrae stars in common between Gaia and the Hipparcos and Tycho-2 catalogues are published in Gaia Data Release 1 (DR1) as part of the Tycho-Gaia Astrometric Solution (TGAS).Aims. In order to test these first parallax measurements of the primary standard candles of the cosmological distance ladder, which involve astrometry collected by Gaia during the initial 14 months of science operation, we compared them with literature estimates and derived new period-luminosity (PL), period-Wesenheit (PW) relations for classical and Type II Cepheids and infrared PL, PL-metallicity (PLZ), and optical luminosity-metallicity (M-V-[Fe/H]) relations for the RR Lyrae stars, with zero points based on TGAS.Methods. Classical Cepheids were carefully selected in order to discard known or suspected binary systems. The final sample comprises 102 fundamental mode pulsators with periods ranging from 1.68 to 51.66 days (of which 33 with sigma(pi)/pi < 0.5). The Type II Cepheids include a total of 26 W Virginis and BL Herculis stars spanning the period range from 1.16 to 30.00 days (of which only 7 with sigma(pi)/pi < 0.5). The RR Lyrae stars include 200 sources with pulsation period ranging from 0.27 to 0.80 days (of which 112 with sigma(pi)/pi < 0.5). The new relations were computed using multi-band (V,I,J,K-s) photometry and spectroscopic metal abundances available in the literature, and by applying three alternative approaches: (i) linear least-squares fitting of the absolute magnitudes inferred from direct transformation of the TGAS parallaxes; (ii) adopting astrometry-based luminosities; and (iii) using a Bayesian fitting approach. The last two methods work in parallax space where parallaxes are used directly, thus maintaining symmetrical errors and allowing negative parallaxes to be used. The TGAS-based PL,PW,PLZ, and M-V- [Fe/H] relations are discussed by comparing the distance to the Large Magellanic Cloud provided by different types of pulsating stars and alternative fitting methods.Results. Good agreement is found from direct comparison of the parallaxes of RR Lyrae stars for which both TGAS and HST measurements are available. Similarly, very good agreement is found between the TGAS values and the parallaxes inferred from the absolute magnitudes of Cepheids and RR Lyrae stars analysed with the Baade-Wesselink method. TGAS values also compare favourably with the parallaxes inferred by theoretical model fitting of the multi-band light curves for two of the three classical Cepheids and one RR Lyrae star, which were analysed with this technique in our samples. The K-band PL relations show the significant improvement of the TGAS parallaxes for Cepheids and RR Lyrae stars with respect to the Hipparcos measurements. This is particularly true for the RR Lyrae stars for which improvement in quality and statistics is impressive.Conclusions. TGAS parallaxes bring a significant added value to the previous Hipparcos estimates. The relations presented in this paper represent the first Gaia-calibrated relations and form a work-in-progress milestone report in the wait for Gaia-only parallaxes of which a first solution will become available with Gaia Data Release 2 (DR2) in 2018.

Context. Parallaxes for 331 classical Cepheids, 31 Type II Cepheids, and 364 RR Lyrae stars in common between Gaia and the H ipparcos and Tycho -2 catalogues are published in Gaia Data Release 1 (DR1) as part of the Tycho - Gaia Astrometric Solution (TGAS). Aims. In order to test these first parallax measurements of the primary standard candles of the cosmological distance ladder, which involve astrometry collected by Gaia during the initial 14 months of science operation, we compared them with literature estimates and derived new period-luminosity ( PL ), period-Wesenheit ( PW ) relations for classical and Type II Cepheids and infrared PL , PL -metallicity ( PLZ ), and optical luminosity-metallicity ( M V -[Fe / H]) relations for the RR Lyrae stars, with zero points based on TGAS. Results. Good agreement is found from direct comparison of the parallaxes of RR Lyrae stars for which both TGAS and HST measurements are available. Similarly, very good agreement is found between the TGAS values and the parallaxes inferred from the absolute magnitudes of Cepheids and RR Lyrae stars analysed with the Baade-Wesselink method. TGAS values also compare favourably with the parallaxes inferred by theoretical model fitting of the multi-band light curves for two of the three classical Cepheids and one RR Lyrae star, which were analysed with this technique in our samples. The K -band PL relations show the significant improvement of the TGAS parallaxes for Cepheids and RR Lyrae stars with respect to the H ipparcos measurements. This is particularly true for the RR Lyrae stars for which improvement in quality and statistics is impressive. Conclusions. TGAS parallaxes bring a significant added value to the previous H ipparcos estimates. The relations presented in this paper represent the first Gaia -calibrated relations and form a work-in-progress milestone report in the wait for Gaia -only parallaxes of which a first solution will become available with Gaia Data Release 2 (DR2) in 2018.

Gaia is a very ambitious mission of the European Space Agency. At the heart of Gaia lie the measurements of the positions, distances, space motions, brightnesses and astrophysical parameters of stars, which represent fundamental pillars of modern astronomical knowledge. We provide a brief description of the Gaia mission with an emphasis on binary stars. In particular, we summarize results of simulations, which estimate the number of binary stars to be processed to several tens of millions. We also report on the catalogue release scenarios. In the current proposal, the first results for binary stars will be available in 2017 (for a launch in 2013).

The Kepler mission has led to the serendipitous discovery of a significant number of "super flares" - white light flares with energies between 10(33) erg and 10(36) erg - on solar-type stars. It has been speculated that these could be "freak" events that might happen on the Sun, too. We have started a programme to study the nature of the stars on which these super flares have been observed. Here we present high-resolution spectroscopy of 11 of these stars and discuss our results. We find that several of these stars are very young, fast-rotating stars where high levels of stellar activity can be expected, but for some other stars we do not find a straightforward explanation for the occurrence of super flares.

We report on a search for the Fe xiii forbidden coronal line at 3388.1 Å in a sample of 15 M-type dwarf stars covering the whole spectral class as well as different levels of activity. A clear detection was achieved for LHS 2076 during a major flare and for CN Leo, where the line had been discovered before. For some other stars the situation is not quite clear. For CN Leo we investigated the timing behaviour of the Fe xiii line and report a high level of variability on a timescale of hours which we ascribe to microflare heating.

In the course of an all-sky survey for young stars in the solar neighbourhood, we have found a tight kinematic group of 10 F-G type zero-age main-sequence stars in the field. Here we discuss the origin of these stars. Backtracking the space motions of these stars, we argue that likely candidates for the parent association are the Perseus OB3 (Per OB3), Upper Centaurus-Lupus (UCL) and Lower Centaurus-Crux (LCC) associations, and that we are witnessing the ongoing diffusion of (at least one of) these associations into the field.

In the course of surveying a large number of nearby (approximate to 50 pc or less) stars for indicators of youth, we have discovered a number of hitherto unknown spectroscopic binaries. Here we present a list of these new binaries with pertinent data. Two of these (HD 143705 and HD 89959) have been observed six times each, and their respective radial velocity curves are discussed.

V4743 Sagittarii (Nova Sgr 2002 No. 3) was discovered on 2002 September 20. We obtained a 5 ks ACIS-S spectrum in 2002 November and found that the nova was faint in X-rays. We then obtained a 25 ks Chandra Low Energy Transmission Grating Spectrometer (LETGS) observation on 2003 March 19. By this time, it had evolved into the supersoft X-ray phase exhibiting a continuous spectrum with deep absorption features. The light curve from the observation showed large-amplitude oscillations with a period of 1325 s (22 minutes) followed by a decline in the total count rate after ~13 ks of observations. The count rate dropped from ~40 counts s-1 to practically zero within ~6 ks and stayed low for the rest of the observation (~6 ks). The spectral hardness ratio changed from maxima to minima in correlation with the oscillations and then became significantly softer during the decay. Strong H-like and He-like lines of oxygen, nitrogen, and carbon were found in absorption during the bright phase, indicating temperatures between 1 and 2 MK, but they were shifted in wavelength corresponding to a Doppler velocity of -2400 km s-1. The spectrum obtained after the decline in count rate showed emission lines of C VI, N VI, and N VII, suggesting that we were seeing expanding gas ejected during the outburst, probably originating from CNO-cycled material. An XMM-Newton Target of Opportunity observation, obtained on 2002 April 4 and a later LETGS observation from 2003 July 18 also showed oscillations, but with smaller amplitudes.

V4743 Sgr (Nova Sgr 2002 No. 3) was discovered on 20 September 2002. We obtained a 5ks ACIS-S spectrum in November 2002 and found that the nova was faint in X-rays. We then obtained a 25ks CHANDRA LETGS observation on 19 March 2003. By this time, it had evolved into the Super Soft X-ray phase exhibiting a continuous spectrum with deep absorption features. The light curve from the observation showed large amplitude oscillations with a period of 1325s (22min) followed by a decline in total count rate after ~13ks of observations. The count rate dropped from ~40cts/s to practically zero within ~6ks and stayed low for the rest of the observation (~6ks). The spectral hardness ratio changed from maxima to minima in correlation with the oscillations, and then became significantly softer during the decay. Strong H-like and He-like lines of oxygen, nitrogen, and carbon were found in absorption during the bright phase, indicating temperatures between 1-2MK, but they were shifted in wavelength corresponding to a Doppler velocity of -2400km/s. The spectrum obtained after the decline in count rate showed emission lines of CVI, NVI, and NVII suggesting that we were seeing expanding gas ejected during the outburst, probably originating from CNO-cycled material. An XMM-Newton ToO observation, obtained on 4 April 2003 and a later LETGS observation from 18 July 2003 also showed oscillations, but with smaller amplitudes.

We present the results of an extensive all-sky survey of nearby stars of spectral type F8 or later in a systematic search of young (zero-age main sequence) objects. Our sample has been derived by cross-correlating the ROSAT All-Sky Survey and the TYCHO catalogue, yielding a total of 754 candidates distributed more or less randomly over the sky. Follow-up spectroscopy of these candidate objects has been performed on 748 of them. We have discovered a tight kinematic group of ten stars with extremely high lithium equivalent widths that are presumably younger than the Pleiades, but again distributed rather uniformly over the sky. Furthermore, about 43 per cent of our candidates have detectable levels of lithium, thus indicating that these are relatively young objects with ages not significantly above the Pleiades age.

This is an overview of the current knowledge on dispersed young stellar populations. It is discussed whether the existence of significant numbers of dispersed young stars can be expected, how they can be detected, and what progress has been made in recent years to actually discover such populations.

All ‘solar-like’ stars1 are surrounded by coronae2, which contain magnetically confined plasma at temperatures above 106 K. (Until now, only the Sun's corona could be observed in the optical—as a shimmering envelope during a total solar eclipse.) As the underlying stellar ‘surfaces’—the photospheres—are much cooler, some non-radiative process must be responsible for heating the coronae. The heating mechanism is generally thought to be magnetic in origin, but is not yet understood even for the case of the Sun. Ultraviolet emission lines first led to the discovery of the enormous temperature of the Sun's corona3,4, but thermal emission from the coronae of other stars has hitherto been detectable only from space, at X-ray wavelengths. Here we report the detection of emission from highly ionized iron (Fe XIII at 3,388.1 Å) in the corona of the red-dwarf star CN Leonis, using a ground-based telescope. The X-ray flux inferred from our data is consistent with previously measured X-ray fluxes, and the non-thermal line width of 18.4 km s-1 indicates great similarities between solar and stellar coronal heating mechanisms. The accessibility and spectral resolution (45,000) of the ground-based instrument are much better than those of X-ray satellites, so a new window to the study of stellar coronae has been opened.

We report the first spectroscopic discovery of an extragalactic bona-fide T Tauri star. The object, LTS J054427-692659, is a low-mass, late-type star located within the LMC dark cloud Hodge II 139. It shows H α emission with an equivalent width of 78 A, in line with galactic T Tauri stars, but in excess of any main-sequence dwarf star. The only known plausible interpretation of LTS J054427-692659 is a LMC T Tauri star.