An Independent Calibration of Stellar Ages: HST Observations of White Dwarfs at V=25

The white dwarf luminosity function of a stellar cluster will have a sharp truncation at a luminosity which is determined by the time since formation of the first white dwarfs in that cluster. Calculation of the dependence of this limiting luminosity on age requires relatively well-understood physics and is independent of stellar evolutionary models. Thus, measurement of the termination of the white dwarf luminosity function provides an independent method to determine the age of a cluster, and thereby to calibrate stellar evolutionary ages. We have obtained HST WFPC2 data in two open clusters, identified the white dwarf sequence, and proved the feasibility of this approach, by detecting white dwarfs to V=25. Much deeper data are feasible. From our present limited data, we show that degenerate cooling ages are not consistent with some published isochrone ages for clusters with ages of order 1Gyr.Comment: 5 pages plus 3 figures ps format, paper in press in MNRAS: previous attempt lost the tex

Non-Adiabatic Spin Transfer Torque in Real Materials

The motion of simple domain walls and of more complex magnetic textures in the presence of a transport current is described by the Landau-Lifshitz-Slonczewski (LLS) equations. Predictions of the LLS equations depend sensitively on the ratio between the dimensionless material parameter $\beta$ which characterizes non-adiabatic spin-transfer torques and the Gilbert damping parameter $\alpha$. This ratio has been variously estimated to be close to 0, close to 1, and large compared to 1. By identifying $\beta$ as the influence of a transport current on $\alpha$, we derive a concise, explicit and relatively simple expression which relates $\beta$ to the band structure and Bloch state lifetimes of a magnetic metal. Using this expression we demonstrate that intrinsic spin-orbit interactions lead to intra-band contributions to $\beta$ which are often dominant and can be (i) estimated with some confidence and (ii) interpreted using the "breathing Fermi surface" model.Comment: 18 pages, 9 figures; submitted to Phys. Rev.

Constraining the Distribution of L- & T-Dwarfs in the Galaxy

We estimate the thin disk scale height of the Galactic population of L- & T-dwarfs based on star counts from 15 deep parallel fields from the Hubble Space Telescope. From these observations, we have identified 28 candidate L- & T- dwarfs based on their (i'-z') color and morphology. By comparing these star counts to a simple Galactic model, we estimate the scale height to be 350+-50 pc that is consistent with the increase in vertical scale with decreasing stellar mass and is independent of reddening, color-magnitude limits, and other Galactic parameters. With this refined measure, we predict that less than 10^9 M_{sol} of the Milky Way can be in the form L- & T- dwarfs, and confirm that high-latitude, z~6 galaxy surveys which use the i'-band dropout technique are 97-100% free of L- & T- dwarf interlopers.Comment: 4 pages, 4 figures, accepted to ApJ

Kinematics of Nearby Subdwarf Stars

We present an analysis of the space motions of 742 subdwarf stars based on the sample of Carney et al. (1994, CLLA). Hipparcos parallaxes, TYC2+HIP proper motions and Tycho2 proper motions were combined with radial velocities and metallicities from CLLA. The kinematical behavior is discussed in particular in relation to their metallicities. The majority of these sample stars have metal abundances of [Fe/H] >-1 and represent the thick disk population. The halo component, with [Fe/H] <-1.6, is characterized by a low mean rotation velocity and a radially elongated velocity ellipsoid. In the intermediate metallicity range (-1.6 < [Fe/H] <-1), we find a significant number of subdwarfs with disklike kinematics. We interpret this population of stars as a metal-weak thick disk population.Comment: 6 pages, 7 figures, accepted by Astronomy & Astrophysic

Contraction of broken symmetries via Kac-Moody formalism

I investigate contractions via Kac-Moody formalism. In particular, I show how the symmetry algebra of the standard 2-D Kepler system, which was identified by Daboul and Slodowy as an infinite-dimensional Kac-Moody loop algebra, and was denoted by ${\mathbb H}_2$, gets reduced by the symmetry breaking term, defined by the Hamiltonian $$H(\beta)= \frac 1 {2m} (p_1^2+p_2^2)- \frac \alpha r - \beta r^{-1/2} \cos ((\phi-\gamma)/2).$$ For this $H (\beta)$ I define two symmetry loop algebras ${\mathfrak L}_{i}(\beta), i=1,2$, by choosing the `basic generators' differently. These ${\mathfrak L}_{i}(\beta)$ can be mapped isomorphically onto subalgebras of ${\mathbb H}_2$, of codimension 2 or 3, revealing the reduction of symmetry. Both factor algebras ${\mathfrak L}_i(\beta)/I_i(E,\beta)$, relative to the corresponding energy-dependent ideals $I_i(E,\beta)$, are isomorphic to ${\mathfrak so}(3)$ and ${\mathfrak so}(2,1)$ for $E0$, respectively, just as for the pure Kepler case. However, they yield two different non-standard contractions as $E \to 0$, namely to the Heisenberg-Weyl algebra ${\mathfrak h}_3={\mathfrak w}_1$ or to an abelian Lie algebra, instead of the Euclidean algebra ${\mathfrak e}(2)$ for the pure Kepler case. The above example suggests a general procedure for defining generalized contractions, and also illustrates the {\em `deformation contraction hysteresis'}, where contraction which involve two contraction parameters can yield different contracted algebras, if the limits are carried out in different order.Comment: 21 pages, 1 figur

Deep Luminosity Functions of Old and Intermediate-Age Globular Clusters in NGC 1316: Evidence for Dynamical Evolution of Second-Generation Globular Clusters

The Advanced Camera for Surveys on board the Hubble Space Telescope has been used to obtain deep high-resolution images of the giant early-type galaxy NGC 1316 which is an obvious merger remnant. These observations supersede previous, shallower observations which revealed the presence of a population of metal-rich globular clusters of intermediate age (~ 3 Gyr). We detect a total of 1496 cluster candidates, almost 4 times as many as from the previous WFPC2 images. We confirm the bimodality of the color distribution of clusters, even in V-I, with peak colors 0.93 and 1.06. The large number of detected clusters allows us to evaluate the globular cluster luminosity functions as a function of galactocentric radius. We find that the luminosity function of the inner 50% of the intermediate-age, metal-rich (`red') population of clusters differs markedly from that of the outer 50%. In particular, the luminosity function of the inner 50% of the red clusters shows a clear flattening consistent with a turnover that is about 1.0 mag fainter than the turnover of the blue clusters. This constitutes the first direct evidence that metal-rich cluster populations formed during major mergers of gas-rich galaxies can evolve dynamically (through disruption processes) into the red, metal-rich cluster populations that are ubiquitous in `normal' giant ellipticals.Comment: Accepted for publication in ApJ Letters; 4 pages in emulateapj style. 3 figure

Regional gray matter growth, sexual dimorphism, and cerebral asymmetry in the Neonatal Brain

journal articleAlthough there has been recent interest in the study of childhood and adolescent brain development, very little is known about normal brain development in the first few months of life. In older children, there are regional differences in cortical gray matter development, whereas cortical gray and white matter growth after birth has not been studied to a great extent. The adult human brain is also characterized by cerebral asymmetries and sexual dimorphisms, although very little is known about how these asymmetries and dimorphisms develop. We used magnetic resonance imaging and an automatic segmentation methodology to study brain structure in 74 neonates in the first few weeks after birth. We found robust cortical gray matter growth compared with white matter growth, with occipital regions growing much faster than prefrontal regions. Sexual dimorphism is present at birth, with males having larger total brain cortical gray and white matter volumes than females. In contrast to adults and older children, the left hemisphere is larger than the right hemisphere, and the normal pattern of fronto-occipital asymmetry described in older children and adults is not present. Regional differences in cortical gray matter growth are likely related to differential maturation of sensory and motor systems compared with prefrontal executive function after birth. These findings also indicate that whereas some adult patterns of sexual dimorphism and cerebral asymmetries are present at birth, others develop after birth

Rethinking neo-Salafism through an Emerging Fiqh of Citizenship: The Changing Status of Minorities in the Discourse of Yusuf al-Qaradawi and the ‘School of the Middle Way’

This quick study will trace on-going evolutions in the thought of the Egyptian Shaykh Yusuf al-Qaradawi and his “School of the Middle Way” or al-Madrasa al-Wasaṭiyya over the period 1985 to 2010. While al-Qaradawi’s early work on the citizenship status of non-Muslims was neo-traditionalist in that it advocated retaining the dhimma system, a minority citizenship model based on differential rights and responsibilities which emphasises communal autonomy for minorities within a sharīʿa regime for the Muslim majority, we will show that he has since moved away from this position and is actively engaged in the process of developing an innovative and inclusive theory of “Islamic Citizenship” that endows non-Muslims with equal civil and political rights and responsibilities