On the connection between the magneto-elliptic and magneto-rotational instabilities

It has been recently suggested that the magneto-rotational instability (MRI) is a limiting case of the magneto-elliptic instability (MEI). This limit is obtained for horizontal modes in the presence of rotation and an external vertical magnetic field, when the aspect ratio of the elliptic streamlines tends to infinite. In this paper we unveil the link between these previously unconnected mechanisms, explaining both the MEI and the MRI as different manifestations of the same Magneto-Elliptic-Rotational Instability (MERI). The growth rates are found and the influence of the magnetic and rotational effects is explained, in particular the effect of the magnetic field on the range of negative Rossby numbers at which the horizontal instability is excited. Furthermore, we show how the horizontal rotational MEI in the rotating shear flow limit links to the MRI by the use of the local shearing box model, typically used in the study of accretion discs. In such limit the growth rates of the two instability types coincide for any power-type background angular velocity radial profile with negative exponent corresponding to the value of the Rossby number of the rotating shear flow. The MRI requirement for instability is that the background angular velocity profile is a decreasing function of the distance from the centre of the disk which corresponds to the horizontal rotational MEI requirement of negative Rossby numbers. Finally a physical interpretation of the horizontal instability, based on a balance between the strain, the Lorentz force and the Coriolis force is given.Comment: 15 pages, 3 figures. Accepted for publication in the Journal of Fluid Mechanic

The Optical Gravitational Lensing Experiment. Is Interstellar Extinction Toward the Galactic Center Anomalous?

Photometry of the Galactic bulge, collected during the OGLE-II microlensing search, indicates high and non-uniform interstellar extinction toward the observed fields. We use the mean I-band magnitude and V-I color of red clump stars as a tracer of interstellar extinction toward four small regions of the Galactic bulge with highly variable reddening. Similar test is performed for the most reddened region observed in the LMC. We find that the slope of the location of red clump stars in the color-magnitude diagrams (CMDs) in the Galactic bulge is significantly smaller than the slope of the reddening line following the standard extinction law (R_V=3.1) for approximations of the extinction curve by both Cardelli, Clayton and Mathis (1989, CCM89) and Fitzpatrick (1999, F99). The differences are much larger for the CCM89 approximation which, on the other hand, indicates the same slopes for the control field in the LMC, contrary to the F99 approximation. We discuss possible systematic effects that could cause the observed discrepancy. Anomalous extinction toward the Galactic bulge seems to be the most natural explanation. Our data indicate that, generally, the ratio of the total to selective absorption, R_VI, is much smaller toward the Galactic bulge than the value corresponding to the standard extinction curve (R_V=3.1). However, R_VI varies from one line-of-sight to another. Our results explain why the red clump and RR Lyr stars in the Baade's window dereddened with standard value of R_VI are redder compared to those of the local population.Comment: 16 pages. Accepted for publication in ApJ. Major changes include: comparison of the OGLE-II photometry with other data, additional comparison of the observed reddening line with that resulting from approximation of the standard extinction curve by Fitzpatrick (1999

Boomerang effects of gambling warnings exposed to non-problem gamblers

Gambling is the most popular form of entertainment in most markets where it is legal. Theindustry has embraced technology and is a growing category online and through mobileplatforms. Governments throughout the world worry about the product when gambling ismore widely available and more private to play. Warnings for problem gambling have longbeen used in land-based gambling venues but online gambling often does not have thisremedy. In addition, non-problem gamblers make up about 99% of gamblers but littleresearch has tested their reaction to warnings. An online casino was developed to testwarnings and found that a significant proportion of non-problem gamblers gambled morefrequently after exposure to the warnings. Because increased frequency of gambling is one symptom of problem gambling, the implications of these findings are discussed in terms of future remedies for consumers that have problems with gambling products

Pulsational and evolutionary analysis of the double-mode RR Lyrae star BS Com

We derive the basic physical parameters of the field double-mode RR Lyrae star BS Com from its observed periods and the requirement of consistency between the pulsational and evolutionary constraints. By using the current solar-scaled horizontal branch evolutionary models of Pietrinferni et al. (2004) and our linear non-adiabatic purely radiative pulsational models, we get M/M(Sun) = 0.698 +/- 0.004, log(L/L(Sun)) = 1.712 +/- 0.005, T(eff) = 6840 +/- 14 K, [Fe/H] = -1.67 +/- 0.01, where the errors are standard deviations assuming uniform age distribution along the full range of uncertainty in age. The last two parameters are in a good agreement with the ones derived from the observed BVIc colours and the updated ATLAS9 stellar atmosphere models. We get T(eff) = 6842 +/- 10 K, [Fe/H] = -1.58 +/- 0.11, where the errors are purely statistical ones. It is remarkable that the derived parameters are nearly independent of stellar age at early evolutionary stages. Later stages, corresponding to the evolution toward the asymptotic giant branch are most probably excluded because the required high temperatures are less likely to satisfy the constraints posed by the colours. We also show that our conclusions are only weakly sensitive to nonlinear period shifts predicted by current hydrodynamical models.Comment: Accepted for publication by MNRAS on 2008 February 01. The paper contains 4 figures and 8 table

Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy

We review HB stars in a broad astrophysical context, including both variable and non-variable stars. A reassessment of the Oosterhoff dichotomy is presented, which provides unprecedented detail regarding its origin and systematics. We show that the Oosterhoff dichotomy and the distribution of globular clusters (GCs) in the HB morphology-metallicity plane both exclude, with high statistical significance, the possibility that the Galactic halo may have formed from the accretion of dwarf galaxies resembling present-day Milky Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the second-parameter problem is presented. A technique is proposed to estimate the HB types of extragalactic GCs on the basis of integrated far-UV photometry. The relationship between the absolute V magnitude of the HB at the RR Lyrae level and metallicity, as obtained on the basis of trigonometric parallax measurements for the star RR Lyrae, is also revisited, giving a distance modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are studied. Finally, the conductive opacities used in evolutionary calculations of low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and Space Scienc

Binary Planetary Nebulae Nuclei towards the Galactic Bulge. I. Sample Discovery, Period Distribution and Binary Fraction

Binarity has been hypothesised to play an important, if not ubiquitous, role in the formation of planetary nebulae (PNe). Yet there remains a severe paucity of known binary central stars required to test the binary hypothesis and to place strong constraints on the physics of the common-envelope (CE) phase of binary stellar evolution. Large photometric surveys offer an unrivalled opportunity to efficiently discover many binary central stars. We have combined photometry from the OGLE microlensing survey with the largest sample of PNe towards the Galactic Bulge to systematically search for new binaries. A total of 21 periodic binaries were found thereby more than doubling the known sample. The orbital period distribution was found to be best described by CE population synthesis models when no correlation between primary and secondary masses is assumed for the initial mass ratio distribution. A comparison with post-CE white dwarf binaries indicates both distributions are representative of the true post-CE period distribution with most binaries exhibiting periods less than one day. An estimated close binary fraction of 12--21% is derived and is the first robust and independent validation of the prevailing 10--15% fraction estimated by Bond (2000). This suggests that binarity is not a precondition for the formation of PNe and that close binaries do not play a dominant role in the shaping of nebular morphologies. Systematic effects and biases of the survey are discussed with implications for future photometric surveys.Comment: Accepted for publication in A&