WIYN/Hydra Detection of Lithium Depletion in F Stars of the Young Open Cluster M35 and Implications for the Development of the Lithium Gap

We report discovery of significant depletion of Li on the surfaces of F dwarf stars in the 150-Myr-old open cluster M35, analagous to a feature in the 700-Myr-old Hyades cluster that has been referred to as the ``Li gap.'' We have caught the gap in the act of forming: using high resolution, high S/N, WIYN/Hydra observations, we detect Li in all but a few M35 F stars; the maximum depletion lies at least 0.6-0.8 dex below minimally depleted (or undepleted) stars. The M35 Li depletion region, a) is quite wide, with clear depletion seen from 6000K to 6700K or hotter; b) shows a significant dispersion in Li abundance at all T_eff, even with stars of the same T_eff; and c) contains undepleted stars (as well as depleted ones) in the (narrow) classical Hyades gap region, which itself shows no undepleted stars. All of these M35 Li depletion properties support rotationally-induced slow mixing as the primary physical mechanism that forms the gap, and argues against other proposed mechanisms, particularly diffusion and steady main sequence mass loss. When viewed in the context of the M35 Li depletion properties, the Hyades Li gap may well be wider than is usually recognized.Comment: 14 Pages, 3 figures. Accepted to ApJ Letter

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

3He-Driven Mixing in Low-Mass Red Giants: Convective Instability in Radiative and Adiabatic Limits

We examine the stability and observational consequences of mixing induced by 3He burning in the envelopes of first ascent red giants. We demonstrate that there are two unstable modes: a rapid, nearly adiabatic mode that we cannot identify with an underlying physical mechanism, and a slow, nearly radiative mode that can be identified with thermohaline convection. We present observational constraints that make the operation of the rapid mode unlikely to occur in real stars. Thermohaline convection turns out to be fast enough only if fluid elements have finger-like structures with a length to diameter ratio l/d > 10. We identify some potentially serious obstacles for thermohaline convection as the predominant mixing mechanism for giants. We show that rotation-induced horizontal turbulent diffusion may suppress the 3He-driven thermohaline convection. Another potentially serious problem for it is to explain observational evidence of enhanced extra mixing. The 3He exhaustion in stars approaching the red giant branch (RGB) tip should make the 3He mixing inefficient on the asymptotic giant branch (AGB). In spite of this, there are observational data indicating the presence of extra mixing in low-mass AGB stars similar to that operating on the RGB. Overmixing may also occur in carbon-enhanced metal-poor stars.Comment: 25 pages, 6 figures, modified version, accepted by Ap

Identification of gravity waves in hydrodynamical simulations

The excitation of internal gravity waves by an entropy bubble oscillating in an isothermal atmosphere is investigated using direct two-dimensional numerical simulations. The oscillation field is measured by a projection of the simulated velocity field onto the anelastic solutions of the linear eigenvalue problem for the perturbations. This facilitates a quantitative study of both the spectrum and the amplitudes of excited g-modes.Comment: 12 pages, 11 figures, Appendices only available onlin

Single-fluorophore orientation determination with multiview polarized illumination : modeling and microscope design

Author Posting. © Optical Society of America, 2017. This article is posted here by permission of Optical Society of America for personal use, not for redistribution. The definitive version was published in Optics Express 25 (2017): 31309-31325, doi:10.1364/OE.25.031309.We investigate the use of polarized illumination in multiview microscopes for determining the orientation of single-molecule fluorescence transition dipoles. First, we relate the orientation of single dipoles to measurable intensities in multiview microscopes and develop an information-theoretic metric—the solid-angle uncertainty—to compare the ability of multiview microscopes to estimate the orientation of single dipoles. Next, we compare a broad class of microscopes using this metric—single- and dual-view microscopes with varying illumination polarization, illumination numerical aperture (NA), detection NA, obliquity, asymmetry, and exposure. We find that multi-view microscopes can measure all dipole orientations, while the orientations measurable with single-view microscopes is halved because of symmetries in the detection process. We also find that choosing a small illumination NA and a large detection NA are good design choices, that multiview microscopes can benefit from oblique illumination and detection, and that asymmetric NA microscopes can benefit from exposure asymmetry.National Institute of Health (NIH) (R01GM114274, R01EB017293)

Histoire et dynamique des forêts de l'ubac du massif des Maures au cours des derniers 8000 ans.

Le passé nous aide souvent à éclairer le présent. Ceci est également vrai dans le domaine des écosystèmes forestiers. L'article qui suit retrace l'histoire de la végétation de l'ubac du massif des Maures : quelles étaient les espèces présentes il y a 8000 ans ; comment la végétation naturelle a été transformée suite aux usages, notamment la mise en place du pâturage et de la suberaie productive ; comment se caractérise la dynamique en cours ces 40 dernières années..

The Angular Momentum Evolution of Very Low Mass Stars

We present theoretical models of the angular momentum evolution of very low mass stars (0.1 - 0.5 M_sun) and solar analogues (0.6 - 1.1 M_sun). We investigate the effect of rotation on the effective temperature and luminosity of these stars. We find that the decrease in T_eff and L can be significant at the higher end of our mass range, but becomes negligible below 0.4 M_sun. Formulae for relating T_eff to mass and v_rot are presented. We compare our models to rotational data from young open clusters of different ages to infer the rotational history of low mass stars, and the dependence of initial conditions and rotational evolution on mass. We find that the qualitative conclusions for stars below 0.6 M_sun do not depend on the assumptions about internal angular momentum transport, which makes these low mass stars ideal candidates for the study of the angular momentum loss law and distribution of initial conditions. We find that neither models with solid body nor differential rotation can simultaneously reproduce the observed stellar spin down in the 0.6 to 1.1 M_sun mass range and for stars between 0.1 and 0.6 M_sun. The most likely explanation is that the saturation threshold drops more steeply at low masses than would be predicted with a simple Rossby scaling. In young clusters there is a systematic increase in the mean rotation rate with decreased temperature below 3500 K (0.4 M_sun). This suggests either inefficient angular momentum loss or mass-dependent initial conditions for stars near the fully convective boundary. (abridged)Comment: To appear in the May 10, 2000 Ap

Seismic diagnostics for transport of angular momentum in stars 1. Rotational splittings from the PMS to the RGB

Rotational splittings are currently measured for several main sequence stars and a large number of red giants with the space mission Kepler. This will provide stringent constraints on rotation profiles. Our aim is to obtain seismic constraints on the internal transport and surface loss of angular momentum of oscillating solar-like stars. To this end, we study the evolution of rotational splittings from the pre-main sequence to the red-giant branch for stochastically excited oscillation modes. We modified the evolutionary code CESAM2K to take rotationally induced transport in radiative zones into account. Linear rotational splittings were computed for a sequence of $1.3 M_{\odot}$ models. Rotation profiles were derived from our evolutionary models and eigenfunctions from linear adiabatic oscillation calculations. We find that transport by meridional circulation and shear turbulence yields far too high a core rotation rate for red-giant models compared with recent seismic observations. We discuss several uncertainties in the physical description of stars that could have an impact on the rotation profiles. For instance, we find that the Goldreich-Schubert-Fricke instability does not extract enough angular momentum from the core to account for the discrepancy. In contrast, an increase of the horizontal turbulent viscosity by 2 orders of magnitude is able to significantly decrease the central rotation rate on the red-giant branch. Our results indicate that it is possible that the prescription for the horizontal turbulent viscosity largely underestimates its actual value or else a mechanism not included in current stellar models of low mass stars is needed to slow down the rotation in the radiative core of red-giant stars.Comment: 15 pages, 13 figures, accepted for publication in A&

Spindown of massive rotating stars

Models of rapidly rotating massive stars at low metallicities show significantly different evolution and higher metal yields compared to non-rotating stars. We estimate the spin-down time-scale of rapid rotating non-convective stars supporting an alpha-Omega dynamo. The magnetic dynamo gives rise to mass loss in a magnetically controlled stellar wind and hence stellar spin down owing to loss of angular momentum. The dynamo is maintained by strong horizontal rotation-driven turbulence which dominates over the Parker instability. We calculate the spin-down time-scale and find that it could be relatively short, a small fraction of the main-sequence lifetime. The spin-down time-scale decreases dramatically for higher surface rotations suggesting that rapid rotators may only exhibit such high surface velocities for a short time, only a small fraction of their main-sequence lifetime.Comment: Accepted by MNRA

Stellar Pollution in the Solar Neighborhood

We study spectroscopically determined iron abundances of 642 solar-type stars to search for the signature of accreted iron-rich material. We find that the metallicity [Fe/H] of a subset of 466 main sequence stars, when plotted as a function of stellar mass, mimics the pattern seen in lithium abundances in open clusters. Using Monte Carlo models we find that, on average, these stars have accreted about 0.4 Earth masses of iron while on the main sequence. A much smaller sample of 19 stars in the Hertzsprung gap, which are slightly evolved and whose convection zones are significantly more massive, have lower average [Fe/H], and their metallicity shows no clear variation with stellar mass. These findings suggest that terrestrial-type material is common around solar type stars.Comment: 33 pages, 11 figures. Submitted to Ap