On the Coupling between Helium Settling and Rotation-Induced Mixing in Stellar Radiative Zones: II- Application to light elements in population I main-sequence stars

In the two previous papers of this series, we have discussed the importance of t he $\mu$-gradients due to helium settling on rotation-induced mixing, first in a n approximate analytical way, second in a 2D numerical simulation. We have found that, for slowly rotating low mass stars, a process of ``creeping paralysis" in which the circulation and the diffusion are nearly frozen may take place below the convective zone. Here we apply this theory to the case of lithium and beryll ium in galactic clusters and specially the Hyades. We take into account the rota tional braking with rotation velocities adjusted to the present observations. We find that two different cells of meridional circulation appear on the hot side of the "lithium dip" and that the "creeping paralysis" process occurs, not dir ectly below the convective zone, but deeper inside the radiative zone, at the to p of the second cell. As a consequence, the two cells are disconnected, which ma y be the basic reason for the lithium increase with effective temperature on thi s side of the dip. On the cool side, there is just one cell of circulation and t he paralysis has not yet set down at the age of the Hyades; the same modelisatio n accounts nicely for the beryllium observations as well as for the lithium ones .Comment: 13 printed pages, 10 figures. ApJ, in press (April 20, 2003

The BHK Color Diagram: a New Tool to Study Young Stellar Populations

A new method to derive age differences between the various super star clusters observed in starburst galaxies using the two color diagram (B-H) vs (H-K) is presented. This method offers a quick and easy way to differentiate very young and intermediate age stellar populations even if data on extinction are unavailable. In this case, discrimination of regions younger and older than 4 Myr is feasible. With the availability of data on extinction, the time resolution can be improved significantly. The application of the method to the starbursting system Arp 299 is presented. The validity of the method is confirmed by comparing the equivalent width of the H-alpha line with the chronological map of the northern part of NGC 3690.Comment: 32 pages, 7 figures, 1 table, AJ accepte

The Chemical Evolution of Helium in Globular Clusters: Implications for the Self-Pollution Scenario

We investigate the suggestion that there are stellar populations in some globular clusters with enhanced helium (Y from 0.28 to 0.40) compared to the primordial value. We assume that a previous generation of massive Asymptotic Giant Branch (AGB) stars have polluted the cluster. Two independent sets of AGB yields are used to follow the evolution of helium and CNO using a Salpeter initial mass function (IMF) and two top-heavy IMFs. In no case are we able to produce the postulated large Y ~ 0.35 without violating the observational constraint that the CNO content is nearly constant.Comment: accepted for publication in Ap

Nilpotent orbits and codimension-two defects of 6d N=(2,0) theories

We study the local properties of a class of codimension-2 defects of the 6d N=(2,0) theories of type J=A,D,E labeled by nilpotent orbits of a Lie algebra \mathfrak{g}, where \mathfrak{g} is determined by J and the outer-automorphism twist around the defect. This class is a natural generalisation of the defects of the 6d theory of type SU(N) labeled by a Young diagram with N boxes. For any of these defects, we determine its contribution to the dimension of the Higgs branch, to the Coulomb branch operators and their scaling dimensions, to the 4d central charges a and c, and to the flavour central charge k.Comment: 57 pages, LaTeX2

X-ray Evidence of the Common Envelope Phase of V471 Tauri

Chandra Low Energy Transmission Grating Spectrograph observations of the pre-cataclysmic binary V471 Tau have been used to estimate the C/N abundance ratio of the K dwarf component for the first time. While the white dwarf component dominates the spectrum longward of 50 AA, at shorter wavelengths the observed X-ray emission is entirely due to coronal emission from the K dwarf. The H-like resonance lines of C and N yield an estimate of their logarithmic abundance ratio relative to the Sun of [C/N]=-0.38+/-0.15 - half of the currently accepted solar value. We interpret this result as the first clear observational evidence for the presumed common envelope phase of this system, during which the surface of the K dwarf was contaminated by CN-cycle processed material dredged up into the red giant envelope. We use the measured C/N ratio to deduce that 0.015-0.04 Msun was accreted by the K dwarf while engulfed, and show that this is consistent with a recent tentative detection of 13C in the K dwarf photosphere, and with the measured Li abundance in the scenario where the red giant companion was Li-rich during the common envelope phase.Comment: 6 pages, 2 figures, ApJL accepte

The tidal effects on the lithium abundance of binary systems with giant component

We analise the behavior of lithium abundance as a function of effective temperature, projected rotational velocity, orbital period and eccentricity for a sample of 68 binary systems with giant component and orbital period ranging from about 10 to 6400 days. For these binary systems the Li abundances show a gradual decrease with temperature, paralleling the well established result for single giants. We have also observed a dependence of lithium content on rotation. Binary systems with moderate to high rotation present also moderate to high Li content. This study shows also that synchronized binary systems with giant component seems to retain more of their original lithium than the unsynchronized systems. For orbital periods lower than 100 to 250 days, typically the period of synchronization for this kind of binary systems, lithium depleted stars seems to be unusual. The suggestion is made that there is an 'inhibited zone' in which synchronized binary systems with giant component having lithium abundance lower than a threshold level should be unusual.Comment: 6 pages, 3 Postscript figures, uses: aa.cls, psfig.st

UIT Detection of Hot Stars in the Globular Cluster NGC362

We used the Ultraviolet Imaging Telescope during the March 1995 Astro-2 mission to obtain a deep far-UV image of the globular cluster NGC 362, which was formerly thought to have an almost entirely red horizontal branch (HB). 84 hot (T_eff > 8500 K) stars were detected within a radius of 8'.25 of the cluster center. Of these, 43 have FUV magnitudes consistent with HB stars in NGC 362, and at least 34 are cluster members. The number of cluster members is made uncertain by background contamination from blue stars in the Small Magellanic Cloud (SMC). There are six candidate supra-HB stars which have probably evolved from the HB. We discuss the implications of these results for the production of hot blue stars in stellar populations.Comment: 10 pages AASLaTeX including one postscript figure and one compressed bitmap, .jpg format. To appear in Ap. J. Letters. Postscript version also available at http://www.astro.virginia.edu/~bd4r

Can Extra Mixing in RGB and AGB Stars Be Attributed to Magnetic Mechanisms?

It is known that there must be some weak form of transport (called cool bottom processing, or CBP) acting in low mass RGB and AGB stars, adding nuclei, newly produced near the hydrogen-burning shell, to the convective envelope. We assume that this extra-mixing originates in a stellar dynamo operated by the differential rotation below the envelope, maintaining toroidal magnetic fields near the hydrogen-burning shell. We use a phenomenological approach to the buoyancy of magnetic flux tubes, assuming that they induce matter circulation as needed by CBP models. This establishes requirements on the fields necessary to transport material from zones where some nuclear burning takes place, through the radiative layer, and into the convective envelope. Magnetic field strengths are determined by the transport rates needed by CBP for the model stellar structure of a star of initially 1.5 solar mass, in both the AGB and RGB phases. The field required for the AGB star in the processing zone is B_0 ~ 5x10^6 G; at the base of the convective envelope this yields an intensity B_E < 10^4 G (approximately). For the RGB case, B_0 ~ 5x10^4 to 4x10^5 G, and the corresponding B_E are ~ 450 to 3500 G. These results are consistent with existing observations on AGB stars. They also hint at the basis for high field sources in some planetary nebulae and the very large fields found in some white dwarfs. It is concluded that transport by magnetic buoyancy should be considered as a possible mechanism for extra mixing through the radiative zone, as is required by both stellar observations and the extensive isotopic data on circumstellar condensates found in meteorites.Comment: 26 pages, 4 figures, accepted by Astrophysical Journa

Geometrical Effects of Baryon Density Inhomogeneities on Primordial Nucleosynthesis

We discuss effects of fluctuation geometry on primordial nucleosynthesis. For the first time we consider condensed cylinder and cylindrical-shell fluctuation geometries in addition to condensed spheres and spherical shells. We find that a cylindrical shell geometry allows for an appreciably higher baryonic contribution to be the closure density (\Omega_b h_{50}^2 \la 0.2) than that allowed in spherical inhomogeneous or standard homogeneous big bang models. This result, which is contrary to some other recent studies, is due to both geometry and recently revised estimates of the uncertainties in the observationally inferred primordial light-element abundances. We also find that inhomogeneous primordial nucleosynthesis in the cylindrical shell geometry can lead to significant Be and B production. In particular, a primordial beryllium abundance as high as [Be] = 12 + log(Be/H) $\approx -3$ is possible while still satisfying all of the light-element abundance constraints.Comment: Latex, 20 pages + 11 figures(not included). Entire ps file with embedded figures available via anonymous ftp at ftp://genova.mtk.nao.ac.jp/pub/prepri/bbgeomet.ps.g

Asymptotic distribution of quasi-normal modes for Kerr-de Sitter black holes

We establish a Bohr-Sommerfeld type condition for quasi-normal modes of a slowly rotating Kerr-de Sitter black hole, providing their full asymptotic description in any strip of fixed width. In particular, we observe a Zeeman-like splitting of the high multiplicity modes at a=0 (Schwarzschild-de Sitter), once spherical symmetry is broken. The numerical results presented in Appendix B show that the asymptotics are in fact accurate at very low energies and agree with the numerical results established by other methods in the physics literature. We also prove that solutions of the wave equation can be asymptotically expanded in terms of quasi-normal modes; this confirms the validity of the interpretation of their real parts as frequencies of oscillations, and imaginary parts as decay rates of gravitational waves.Comment: 66 pages, 6 figures; journal version (to appear in Annales Henri Poincar\'e