Optical Identification of ROSAT sources in M 67: activity in an old cluster

We present optical identification and high-resolution spectroscopy of ROSAT sources in the field of the old open cluster M 67. For the first time it is possible to analyze coronal and chromospheric activity of active stars in a solar-age cluster, and to compare it with field stars. ROSAT observed the high X-ray luminosity tail of the cluster sources. In agreement with what expected from studies of field stars, most of the detected X-ray sources are binaries, preferably with short periods and eccentric orbits. In addition, several of the M 67 ROSAT sources have peculiar locations in the cluster colour-magnitude diagram. This is most likely due to rather complex evolutionary histories, involving the presence of mass transfer or large mass losses. The X-ray luminosity of the sources does not scale with the stellar parameters in an obvious way. In particular, no relationship is found between coronal emission and stellar magnitude or binary period. The Ca II K chromospheric flux from most of the counterparts is in excess to that of single stars in the cluster by one order of magnitude. The X-ray luminosity of the sources in the old M 67 is one order of magnitude lower than the most active active binaries in the field, but comparable to that of the much younger binaries in the Hyades.Comment: 11 pages, 6 figures. Accepted for publication by Astronomy & Astrophysic

A view of the Galactic halo using beryllium as a time scale

Beryllium stellar abundances were suggested to be a good tracer of time in the early Galaxy. In an investigation of its use as a cosmochronometer, using a large sample of local halo and thick-disk dwarfs, evidence was found that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components. One is consistent with predictions of evolutionary models while the other is chemically indistinguishable from the thick-disk stars. This is interpreted as a difference in the star formation history of the two components and suggests that the local halo is not a single uniform population where a clear age-metallicity relation can be defined.Comment: To appear in Proceedings of the International Astronomical Union, IAU Symposium, Volume 265, Chemical abundances in the Universe: connecting first stars to planets, K. Cunha, M. Spite and B. Barbuy, eds. 2 Pages, 2 figure

Beryllium abundances along the evolutionary sequence of the open cluster IC 4651 - New test for hydrodynamical stellar models

[abridged] Previous analyses of lithium abundances in main sequence and red giant stars have revealed the action of mixing mechanisms other than convection in stellar interiors. Beryllium abundances in stars with lithium abundance determinations can offer valuable complementary information on the nature of these mechanisms. Our aim is to derive beryllium abundances along the whole evolutionary sequence of an open cluster, IC 4651. These Be abundances are used together with previously determined Li abundances, in the same sample stars, to investigate the mixing mechanisms in a range of stellar masses and evolutionary stages. New beryllium abundances are determined from high-resolution, high signal-to-noise UVES spectra using spectrum synthesis and model atmospheres. The careful synthetic modelling of the Be lines region is used to calculate reliable abundances in rapidly rotating stars. The observed behavior of Be and Li is compared to theoretical predictions from stellar models including rotation-induced mixing, internal gravity waves, atomic diffusion, and thermohaline mixing. Beryllium is detected in all the main sequence and turn-off sample stars, both slow- and fast-rotating stars, including the Li-dip stars, but was not detected in the red giants. Confirming previous results, we find that the Li dip is also a Be dip, although the depletion of Be is more modest than that of Li in the corresponding effective temperature range. For post-main-sequence stars, the Be dilution starts earlier within the Hertzsprung gap than expected from classical predictions as does the Li dilution. A clear dispersion in the Be abundances is also observed. Theoretical stellar models including the hydrodynamical transport processes mentioned above are able to reproduce well all the observed features.Comment: 12 pages, accepted for publication in A&A, revised final versio

Testing the LCDM model (and more) with the time evolution of the redshift

With the many ambitious proposals afoot for new generations of very large telescopes, along with spectrographs of unprecedented resolution, there arises the real possibility that the time evolution of the cosmological redshift may, in the not too distant future, prove to be a useful tool rather than merely a theoretical curiosity. Here I contrast this approach with the standard cosmological procedure based on the luminosity (or any other well-defined) distance. I then show that such observations would not only provide a direct measure of all the associated cosmological parameters of the LCDM model, but would also provide wide-ranging internal consistency checks. Further, in a more general context, I show that without introducing further time derivatives of the redshift one could in fact map out the dark energy equation of state should the LCDM model fail. A consideration of brane-world scenarios and interacting dark energy models serves to emphasize the fact that the usefulness of such observations would not be restricted to high redshifts.Comment: In final form as to appear in Physical Review D. 12 pages 6 figure

Beryllium abundances and the formation of the halo and the thick disk

The single stable isotope of beryllium is a pure product of cosmic-ray spallation in the ISM. Assuming that the cosmic-rays are globally transported across the Galaxy, the beryllium production should be a widespread process and its abundance should be roughly homogeneous in the early-Galaxy at a given time. Thus, it could be useful as a tracer of time. In an investigation of the use of Be as a cosmochronometer and of its evolution in the Galaxy, we found evidence that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components. One is consistent with predictions of evolutionary models while the other is chemically indistinguishable from the thick-disk stars. This is interpreted as a difference in the star formation history of the two components and suggests that the local halo is not a single uniform population where a clear age-metallicity relation can be defined. We also found evidence that the star formation rate was lower in the outer regions of the thick disk, pointing towards an inside-out formation.Comment: 6 pages, 5 figures, To appear in the Proceedings of IAU Symp. 268 - Light Elements in the Universe (C. Charbonnel, M. Tosi, F. Primas, C. Chiappini, eds

Beaming Binaries - a New Observational Category of Photometric Binary Stars

The new photometric space-borne survey missions CoRoT and Kepler will be able to detect minute flux variations in binary stars due to relativistic beaming caused by the line-of-sight motion of their components. In all but very short period binaries (P>10d), these variations will dominate over the ellipsoidal and reflection periodic variability. Thus, CoRoT and Kepler will discover a new observational class: photometric beaming binary stars. We examine this new category and the information that the photometric variations can provide. The variations that result from the observatory heliocentric velocity can be used to extract some spectral information even for single stars.Comment: 15 pages, 4 figures, accpeted for publication in The Astrophysical Journa