Time-Series Ensemble Photometry and the Search for Variable Stars in the Open Cluster M11

This work presents the first large-scale photometric variability survey of the intermediate age (~200 Myr) open cluster M11. Thirteen nights of data over two observing seasons were analyzed (using crowded field and ensemble photometry techniques) to obtain high relative precision photometry. In this study we focus on the detection of candidate member variable stars for follow-up studies. A total of 39 variable stars were detected and can be categorized as follows: 1 irregular (probably pulsating) variable, 6 delta Scuti variables, 14 detached eclipsing binary systems, 17 W UMa variables, and 1 unidentified/candidate variable. While previous proper motion studies allow for cluster membership determination for the brightest stars, we find that membership determination is significantly hampered below V=15,R=15.5 by the large population of field stars overlapping the cluster MS. Of the brightest detected variables that have a high likelihood of cluster membership, we find five systems where further work could help constrain theoretical stellar models, including one potential W UMa member of this young cluster.Comment: 38 pages, 13 figures, accepted for December 2005 AJ, high-resolution version available upon reques

The Higgs decay rate to two photons in a model with two fermiophobic-Higgs doublets

We consider a three Higgs doublet model with an $S_3$ symmetry in which beside the SM-like doublet there are two fermiophobic doublets. Due to the new charged scalars there is an enhancement in the two-photon decay while the other channels have the same decay widths that the SM neutral Higgs. The fermiophobic scalars are mass degenerated unless soft terms breaking the $S_3$ symmetry are added.Comment: typos were corrected, the figures have been modified and the conclusions were increased. Still contains 15 pages, 2 figure

Low ordered magnetic moment by off-diagonal frustration in undoped parent compounds to iron-based high-Tc superconductors

A Heisenberg model over the square lattice recently introduced by Si and Abrahams to describe local-moment magnetism in the new class of Fe-As high-Tc superconductors is analyzed in the classical limit and on a small cluster by exact diagonalization. In the case of spin-1 iron atoms, large enough Heisenberg exchange interactions between neighboring spin-1/2 moments on different iron 3d orbitals that frustrate true magnetic order lead to hidden magnetic order that violates Hund's rule. It accounts for the low ordered magnetic moment observed by elastic neutron diffraction in an undoped parent compound to Fe-As superconductors. We predict that low-energy spin-wave excitations exist at wavenumbers corresponding to either hidden Neel or hidden ferromagnetic order.Comment: 7 pages, 6 figures, version published in Physical Review Letter

Partitioning Schemes and Non-Integer Box Sizes for the Box-Counting Algorithm in Multifractal Analysis

We compare different partitioning schemes for the box-counting algorithm in the multifractal analysis by computing the singularity spectrum and the distribution of the box probabilities. As model system we use the Anderson model of localization in two and three dimensions. We show that a partitioning scheme which includes unrestricted values of the box size and an average over all box origins leads to smaller error bounds than the standard method using only integer ratios of the linear system size and the box size which was found by Rodriguez et al. (Eur. Phys. J. B 67, 77-82 (2009)) to yield the most reliable results.Comment: 10 pages, 13 figure

Nearby Young, Active, Late-type Dwarfs in Gaia's First Data Release

The Galex Nearby Young Star Survey (GALNYSS) has yielded a sample of $\sim$2000 UV-selected objects that are candidate nearby ($D \stackrel{<}{\sim}$150 pc), young (age $\sim$10--100 Myr), late-type stars. Here, we evaluate the distances and ages of the subsample of (19) GALNYSS stars with Gaia Data Release 1 (DR1) parallax distances $D \le 120$ pc. The overall youth of these 19 mid-K to early-M stars is readily apparent from their positions relative to the loci of main sequence stars and giants in Gaia-based color-magnitude and color-color diagrams constructed for all Galex- and WISE-detected stars with parallax measurements included in DR1. The isochronal ages of all 19 stars lie in the range $\sim$10--100 Myr. Comparison with Li-based age estimates indicates a handful of these stars may be young main-sequence binaries rather than pre-main sequence stars. Nine of the 19 objects have not previously been considered as nearby, young stars, and all but one of these are found at declinations north of $+$30$^\circ$. The Gaia DR1 results presented here indicate that the GALNYSS sample includes several hundred nearby, young stars, a substantial fraction of which have not been previously recognized as having ages $\stackrel{<}{\sim}$100 Myr.Comment: 30 pages, 4 tables, 7 figures; to appear in The Astrophysical Journal; 1st replacement to correct typos/omissions in Table 3 and acknowledgments; 2nd replacement to incorporate corrections to ApJ proof