Can the James Webb Space Telescope detect isolated population III stars?

Isolated population III stars are postulated to exist at approximately z=10-30 and may attain masses up to a few hundred solar masses. The James Webb Space telescope (JWST) is the next large space based infrared telescope and is scheduled for launch in 2014. Using a 6.5 meter primary mirror, it will probably be able to detect some of the first galaxies forming in the early Universe. A natural question is whether it will also be able to see any isolated population III stars. Here, we calculate the apparent broadband AB-magnitudes for 300 solar masses population III stars in JWST filters at z=10-20. Our calculations are based on realistic stellar atmospheres and take into account the potential flux contribution from the surrounding HII region. The gravitational magnification boost achieved when pointing JWST through a foreground galaxy cluster is also considered. Using this machinery, we derive the conditions required for JWST to be able to detect population III stars in isolation. We find that a detection of individual population III stars with JWST is unlikely at these redshifts. However, the main problem is not necessarily that these stars are too faint, once gravitational lensing is taken into account, but that their surface number densities are too low.Comment: 6 pages, 3 figures, proceedings of CRF2010, DESY Hamburg, Nov 9-12 201

Mitofusins Mfn1 and Mfn2 coordinately regulate mitochondrial fusion and are essential for embryonic development

Mitochondrial morphology is determined by a dynamic equilibrium between organelle fusion and fission, but the significance of these processes in vertebrates is unknown. The mitofusins, Mfn1 and Mfn2, have been shown to affect mitochondrial morphology when overexpressed. We find that mice deficient in either Mfn1 or Mfn2 die in midgestation. However, whereas Mfn2 mutant embryos have a specific and severe disruption of the placental trophoblast giant cell layer, Mfn1-deficient giant cells are normal. Embryonic fibroblasts lacking Mfn1 or Mfn2 display distinct types of fragmented mitochondria, a phenotype we determine to be due to a severe reduction in mitochondrial fusion. Moreover, we find that Mfn1 and Mfn2 form homotypic and heterotypic complexes and show, by rescue of mutant cells, that the homotypic complexes are functional for fusion. We conclude that Mfn1 and Mfn2 have both redundant and distinct functions and act in three separate molecular complexes to promote mitochondrial fusion. Strikingly, a subset of mitochondria in mutant cells lose membrane potential. Therefore, mitochondrial fusion is essential for embryonic development, and by enabling cooperation between mitochondria, has protective effects on the mitochondrial population

On the discrete Peyrard-Bishop model of DNA: stationary solutions and stability

As a first step in the search of an analytical study of mechanical denaturation of DNA in terms of the sequence, we study stable, stationary solutions in the discrete, finite and homogeneous Peyrard-Bishop DNA model. We find and classify all the stationary solutions of the model, as well as analytic approximations of them, both in the continuum and in the discrete limits. Our results explain the structure of the solutions reported by Theodorakopoulos {\em et al.} [Phys. Rev. Lett. {\bf 93}, 258101 (2004)] and provide a way to proceed to the analysis of the generalized version of the model incorporating the genetic information.Comment: 15 pages, 12 figure

Variation in African American parents' use of early childhood physical discipline

Physical discipline is endorsed by a majority of adults in the U.S. including African American (AA) parents who have high rates of endorsement. Although many studies have examined physical discipline use among AA families, few have considered how early childhood physical discipline varies within the population. Individuals within a cultural group may differ in their engagement in cultural practices (Rogoff, 2003). Furthermore, AA families’ characteristics and their contexts, which are shaped by the interaction of social position, racism, and segregation (García Coll et al., 1996), likely influence how AA families physically discipline their young children. This study examined variation in early childhood physical discipline among AA families living in low-income communities and relations with demographic and contextual factors. Year 1 data from 310 AA parents living in three regionally distinct low-income communities were used from a sequential longitudinal intervention program study of the development and prevention of conduct disorder. Latent class analyses were conducted using parents’ responses on a measure, of the frequency of overall physical discipline, spanking, and hitting during prekindergarten and kindergarten. The associations between latent classes and six demographic and contextual factors were examined using the Bolck, Croon, and Hagenaars (BCH) method. The factors were: child gender (59% male); marital status (51% never married); parental education (66% high school graduates and beyond); income (mean = $16.66K, S.D. = 12.50), family stress, and perception of neighborhood safety. Measures included the Family Information Form, Life Changes, and the Neighborhood Questionnaire. After considering two to seven class solutions, five physical discipline classes or sub-groups were identified. Classes were defined by discipline frequency (‘Infrequent’, ‘Weekly’, ‘Monthly’, ‘Almost-Every-Day’ and ‘Weekly-All’) as well as by discipline type (only parents in the ‘Weekly-All’ class hit their children). Significant associations were found between class membership, and child gender, marital status, income, and perception of neighborhood safety. Girls were more likely to be physically disciplined infrequently, χ2(4, N = 310) = 11.88, p = .05. The ‘Weekly’ class had significantly fewer married parents than all classes except ‘Almost-Every-Day’, χ2(4, N = 310) = 21.56, p < .001. Parents in the ‘Almost-Every-Day’ class had a significantly lower income than parents in all other classes except “Weekly-All”, χ2(4, N = 310) = 10.88, p = .03. Finally, parents in the “Almost-Every-Day” class perceived their neighborhood as significantly less safe compared to those in all other classes except the ‘Weekly-All’ class, χ2(4, N = 310) = 14.13 p = .01. These findings suggest that AA families vary in physical discipline during early childhood; this variation may result in sub-groups with different demographic characteristics. Associations between frequent discipline classes and perceptions of neighborhood safety implies that some AA parents may use physical discipline to protect their children from being harmed if they believe their communities are unsafe. Future research should qualitatively examine how AA parents respond to unsafe neighborhoods in their parenting behaviors, including physical discipline

Models for application of radiation boundary condition for MHD waves in collapse calculations

The problem of reflection of magnetohydrodynamic (MHD) waves at the boundary of a numerical grid has to be resolved in order to obtain reliable results for the end state of the (isothermal) collapse of a rotating, magnetic protostellar cloud. Since the goal of investigating magnetic braking in collapse simulations is to see if the transport of angular momentum via alfven waves is large enough to solve the angular momentum problem an approximation that artificially suppresses large amplitudes in the MHD waves can be self-defeating. For this reason, four alternate methods of handling reflected waves where no assumptions are made regarding the amplitudes of the waves were investigated. In order to study this problem (of reflection) without interference from other effects these methods were tried on two simpler cases. The four methods are discussed