883 research outputs found

    Rotation of Horizontal Branch Stars in Globular Clusters

    Get PDF
    The rotation of horizontal branch stars places important constraints on angular momentum evolution in evolved stars and therefore rotational mixing on the giant branch. Prompted by new observations of rotation rates of horizontal branch stars, we calculate simple models for the angular momentum evolution of a globular cluster giant star from the base of the giant branch to the star's appearance on the horizontal branch. We include mass loss, and infer the accompanied loss of angular momentum for each of four assumptions about the internal angular momentum profile. These models are compared to observations of horizontal branch rotation rates in M13. We find that rapid rotation on the horizontal branch can be reconciled with slow solid body main sequence rotation if giant branch stars have differential rotation in their convective envelopes and a rapidly rotating core, which is then followed by a redistribution of angular momentum on the horizontal branch. We discuss the physical reasons why these very different properties relative to the solar case may exist in giants. Rapid rotation in the core of the main sequence precursors of the rapidly rotating horizontal branch star, or an angular momentum source on the giant branch is required for all cases if the rotational velocity of turnoff stars is less than 4 km s−1^{-1}. We suggest that the observed range in rotation rates on the horizontal branch is caused by internal angular momentum redistribution which occurs on a timescale comparable to the evolution of the stars on the horizontal branch. The apparent lack of rapid horizontal branch rotators hotter than 12 000 K in M13 could be a consequence of gravitational settling, which inhibits internal angular momentum transport. Alternative explanations and observational tests are discussed.Comment: 32 pages, 7 figures, submitted to the Astrophysical Journa

    The Lithium-Rotation Correlation in the Pleiades Revisited

    Get PDF
    The dispersion in lithium abundance at fixed effective temperature in young cool stars like the Pleiades has proved a difficult challenge for stellar evolution theory. We propose that Li abundances relative to a mean temperature trend, rather than the absolute abundances, should be used to analyze the spread in abundance. We present evidence that the dispersion in Li equivalent widths at fixed color in cool single Pleiades stars is at least partially caused by stellar atmosphere effects (most likely departures from ionization predictions of model photospheres) rather than being completely explained by genuine abundance differences. We find that effective temperature estimates from different colors yield systematically different values for active stars. There is also a strong correlation between stellar activity and Li excess, but not a one-to-one mapping between unprojected stellar rotation (from photometric periods) and Li excess. Thus, it is unlikely that rotation is the main cause for the dispersion in the Li abundances. Finally, there is a strong correlation between detrended Li excess and potassium excess but not calcium-- perhaps supporting incomplete radiative transfer calculations (and overionization effects in particular) as an important source of the Li scatter. Other mechanisms, such as very small metallicity variations and magnetic fields, which influence PMS Li burning may also play a role. Finally, we find no statistical evidence for a decrease in dispersion in the coolest Pleiades stars, contrary to some previous work

    Angular Momentum Evolution of Stars in the Orion Nebula Cluster

    Full text link
    (Abridged) We present theoretical models of stellar angular momentum evolution from the Orion Nebula Cluster (ONC) to the Pleiades and the Hyades. We demonstrate that observations of the Pleiades and Hyades place tight constraints on the angular momentum loss rate from stellar winds. The observed periods, masses and ages of ONC stars in the range 0.2--0.5 M⊙_\odot, and the loss properties inferred from the Pleiades and Hyades stars, are then used to test the initial conditions for stellar evolution models. We use these models to estimate the distribution of rotational velocities for the ONC stars at the age of the Pleiades (120 Myr). The modeled ONC and observed Pleiades distributions of rotation rates are not consistent if only stellar winds are included. In order to reconcile the observed loss of angu lar momentum between these two clusters, an extrinsic loss mechanism such as protostar-accretion disk interaction is required. Our model, which evolves the ONC stars with a mass dependent saturation threshold normalized such that ωcrit=5.4ω⊙\omega_{crit} = 5.4 \omega_\odot at 0.5 \m, and which includes a distribution of disk lifetimes that is uniform over the range 0--6 Myr, is consistent with the Pleiades. This model for disk-locking lifetimes is also consistent with inferred disk lifetimes from the percentage of stars with infrared excesses observed in young clusters. Different models, using a variety of initial period distributions and different maximum disk lifetimes, are also compared to the Pleiades. For disk-locking models that use a uniform distribution of disk lifetimes over the range 0 to τmax\tau_{max}, the acceptable range of the maximum lifetime is 3.5<τmax<8.53.5 < \tau_{max} < 8.5 Myr.Comment: 21 pages, 7 figures, submitted to Ap
    • …
    corecore