Speed of Meridional Flows and Magnetic Flux Transport on the Sun

We use the magnetic butterfly diagram to determine the speed of the magnetic flux transport on the solar surface towards the poles. The manifestation of the flux transport is clearly visible as elongated structures extended from the sunspot belt to the polar regions. The slopes of these structures are measured and interpreted as meridional magnetic flux transport speed. Comparison with the time-distance helioseismology measurements of the mean speed of the meridional flows at the depth of 3.5--12 Mm shows a generally good agreement, but the speeds of the flux transport and the meridional flow are significantly different in areas occupied by the magnetic field. The local circulation flows around active regions, especially the strong equatorward flows on the equatorial side of active regions affect the mean velocity profile derived by helioseismology, but do not influence the magnetic flux transport. The results show that the mean longitudinally averaged meridional flow measurements by helioseismology may not be used directly in solar dynamo models for describing the magnetic flux transport, and that it is necessary to take into account the longitudinal structure of these flows.Comment: 4 pages, 3 figures, accepted in ApJ Letter

Probability of Detecting a Planetary Companion during a Microlensing Event

The probability of detecting a planetary companion of a lensing star during a microlensing event toward the Galactic center, averaged over all relevant event and galactic parameters, when the planet-star mass ratio $q=0.001$ has a maximum exceeding 10% at an orbit semimajor axis $a$ near 1.5 AU for a uniform distribution of impact parameters. The maximum probability is raised to more than 20% for a distribution of source-lens impact parameters that is determined by the efficiency of event detection. The averaging procedures are carefully defined, and they determinine the dependence of the detection probabilities on several properties of the Galaxy. The probabilities scale approximately as $\sqrt{q}$. A planet is assumed detectable if the perturbation of the single lens light curve exceeds $2/(S/N)$ for at least 20 consecutive photometric points sometime during the event. Two meter telescopes with 60 second integrations in I-band with high time resolution photometry throughout the duration of an ongoing event are assumed. The probabilities are derived as a function of $a$, where they remain significant for $0.6<a<10$ AU. Dependence of the detection probabilities on the lens mass function, luminosity function of the source stars as modified by extinction, distribution of source-lens impact parameters, and the line of sight to the source are also determined, and the probabilities are averaged over the distribution of the projected planet position, the lens mass function, the distribution of impact parameters, the lens and source distances as weighted by their distributions along the line of sight and over the $I$-band apparent luminosity function of the sources. The extraction of the probabilility as a function of $a$ for a particular $q$ from empirical data is indicated.Comment: 32 pages, 20 figures, In Press, ApJ, Latex format with aas2pp4 forma

Composition-tuned magneto-optical Kerr effect in L10-MnxGa films with giant perpendicular anisotropy

We report the large polar magnetooptical Kerr effect in L10-MnxGa epitaxial films with giant perpendicular magnetic anisotropy in a wide composition range. The Kerr rotation was enhanced by a factor of up to 10 by decreasing Mn atomic concentration, which most likely arises from the variation of the effective spin-orbit coupling strength, compensation effect of magnetic moments at different Mn atom sites, and overall strain. The Kerr ellipticity and the magnitude of the complex Kerr angle is found to have more complex composition-dependence that varies with the photon energy. These L10-MnxGa films show large Kerr rotation of up to 0.10o, high reflectivity of 35%-55% in a wide wavelength range of 400~850 nm, and giant magnetic anisotropic field of up to 210 kOe, making them an interesting material system for emerging spintronics and terahertz modulator applications

A Model for Abundances in Metal-Poor Stars

It is argued that the abundances of r-process related elements in stars with -3<[Fe/H]<-1 can be explained by the contributions of three sources. The sources are: the first generations of very massive (>100 solar masses) stars that are formed from Big Bang debris and are distinct from SNII, and two types of SNII, the H and L events, which can occur only at [Fe/H]>-3. The H events are of high frequency and produce dominantly heavy (A>130) r-elements but no Fe (presumably leaving behind black holes). The L events are of low frequency and produce Fe and dominantly light (A<130) r-elements (essentially none above Ba). By using the observed abundances in two ultra-metal-poor stars and the solar r-abundances, the initial or prompt inventory of elements produced by the first generations of very massive stars and the yields of H and L events can be determined. The abundances of a large number of elements in a star can then be calculated from the model by using only the observed Eu and Fe abundances. To match the model results and the observational data for stars with -3<[Fe/H]<-1 requires that the solar r-abundances for Sr, Y, Zr, and Ba must be significantly increased from the standard values. Whether the solar r-components of these elements used here to obtain a fit to the stellar data can be reconciled with those obtained from solar abundances by subtracting the s-components calculated from models is not clear.Comment: 47 pages, 19 figures, to appear in Ap