Using seismic inversions to obtain an internal mixing processes indicator for main-sequence solar-like stars

Determining accurate and precise stellar ages is a major problem in astrophysics. These determinations are either obtained through empirical relations or model-dependent approaches. Currently, seismic modelling is one of the best ways of providing accurate ages. However, current methods are affected by simplifying assumptions concerning mixing processes. In this context, providing new structural indicators which are less model-dependent and more sensitive to such processes is crucial. We build a new indicator for core conditions on the main sequence, which should be more sensitive to structural differences and applicable to older stars than the indicator t presented in a previous paper. We also wish to analyse the importance of the number and type of modes for the inversion, as well as the impact of various constraints and levels of accuracy in the forward modelling process that is used to obtain reference models for the inversion. First, we present a method to obtain new structural kernels and use them to build an indicator of central conditions in stars and test it for various effects including atomic diffusion, various initial helium abundances and metallicities, following the seismic inversion method presented in our previous paper. We then study its accuracy for 7 different pulsation spectra including those of 16CygA and 16CygB and analyse its dependence on the reference model by using different constraints and levels of accuracy for its selection We observe that the inversion of the new indicator using the SOLA method provides a good diagnostic for additional mixing processes in central regions of stars. Its sensitivity allows us to test for diffusive processes and chemical composition mismatch. We also observe that octupole modes can improve the accuracy of the results, as well as modes of low radial order.Comment: Accepted for publication in Astronomy and Astrophysic

Constraints on the structure of 16 Cyg A and 16 Cyg B using inversion techniques

Constraining mixing processes and chemical composition is a central problem in stellar physics as their impact on stellar age determinations leads to biases in our studies of stellar evolution, galactic history and exoplanetary systems. In two previous papers, we showed how seismic inversion techniques could offer strong constraints on such processes by pointing out weaknesses in theoretical models. We now apply our technique to the solar analogues 16CygA and 16CygB, being amongst the best targets in the Kepler field to test the diagnostic potential of seismic inversions. The combination of various seismic indicators helps to provide more constrained and accurate fundamendal parameters for these stars. We use the latest seismic, spectroscopic and interferometric observational constraints in the litterature for this system to determine reference models independently for both stars. We carry out seismic inversions of the acoustic radius, the mean density and a core conditions indicator. We note that a degeneracy exists for the reference models. Namely, changing the diffusion coefficient or the chemical composition within the observational values leads to 5% changes in mass, 3% changes in radius and up to 8% changes in age. We use acoustic radius and mean density inversions to improve our reference models then carry out inversions for a core conditions indicator. Thanks to its sensitivity to microscopic diffusion and chemical composition mismatches, we are able to reduce the mass dispersion to 2%, namely [0.96, 1.0] M_sun, the radius dispersion to 1%, namely [1.188, 1.200] R_sun and the age dispersion to 3%, namely [7.0, 7.4] Gy, for 16CygA. For 16CygB, we can check the consistency of the models but not reduce independently the age dispersion. Nonetheless, assuming consistency with the age of 16CygA helps to further constrain its mass and radius.Comment: Submitted to Astronomy and Astrophysic

Tethers in space handbook

The handbook provides a list and description of ongoing tether programs. This includes the joint U.S.-Italy demonstration project, and individual U.S. and Italian studies and demonstration programs. An overview of the current activity level and areas of emphasis in this emerging field is provided. The fundamental physical principles behind the proposed tether applications are addressed. Four basic concepts of gravity gradient, rotation, momentum exchange, and electrodynamics are discussed. Information extracted from literature, which supplements and enhances the tether applications is also presented. A bibliography is appended

A survey to test the story book knowledge of kindergarten children.

Thesis (Ed.M.)--Boston Universit

Photometric Decomposition of Barred Galaxies

We present a non-parametric method for decomposition of the light of disk galaxies into disk, bulge and bar components. We have developed and tested the method on a sample of 68 disk galaxies for which we have acquired I-band photometry. The separation of disk and bar light relies on the single assumption that the bar is a straight feature with a different ellipticity and position angle from that of the projected disk. We here present the basic method, but recognise that it can be significantly refined. We identify bars in only 47% of the more nearly face-on galaxies in our sample. The fraction of light in the bar has a broad range from 1.3% to 40% of the total galaxy light. If low-luminosity galaxies have more dominant halos, and if halos contribute to bar stability, the luminosity functions of barred and unbarred galaxies should differ markedly; while our sample is small, we find only a slight difference of low significance.Comment: Accepted to appear in AJ, 36 pages, 9 figures, full on-line figures available at http://www.physics.rutgers.edu/~sellwood/Reese.htm

A Potential Pollen Proxy for ENSO Derived From the Sajama Ice Core

An annually resolved pollen record spanning a 39-year period ( 1958 - 1996) from the Sajama Ice Cap, located on the western Bolivian Altiplano, reveals significant interannual variations in both pollen concentration and composition. The pollen assemblages within the annual layers are dominated by typical Altiplano taxa, especially Poaceae and Asteraceae. On an annual basis the pollen concentrations are strongly negatively correlated (Pearson\u27s r = - 0.716) with the Southern Oscillation Index (SOI). Studies from Sajama and other tropical ice caps have shown that during El Nino years, the weather on the Altiplano is decidedly warmer and drier, which enhances ablation on tropical ice caps through increased sublimation. This process results in the concentration of pollen within an annual layer, and thus provides a mechanism to reconstruct past El Nino events, so long as annual resolution is obtainable within the ice core

CRYSTAL AND MOLECULAR-STRUCTURE OF 2',3',5'-TRI-O-ACETYL-6-O-(MESITYLENESULPHONYL)GUANOSINE

The structure of a 6-O-mesitylenesulphonyl derivative of 2',3',5'-tri-O-acetylguanosine, C 24 H 29 N50 10 S, has been determined by X-ray diffraction. Crystals are monoclinic, a = 26.370 (4), b = 8.200 (2), c = 17.991 (3) A, fl = 132.77 (4) o. The solution of the structure in space group C2 was not straightforward and is described in detail. Refinement converged at R = 0.110 for 1102 observed reflections. The guanine base displays some deviations from its usual geometry due to the loss of C(6)-O(6) double-bond character. The ribose sugar is C(2')-endo puckered

Formation Rate, Evolving Luminosity Function, Jet Structure, and Progenitors for Long Gamma-Ray Bursts

We constrain the isotropic luminosity function (LF) and formation rate of long gamma-ray bursts (GRBs) by fitting models 'jointly' to both the observed differential peak flux and redshift distributions. We find evidence supporting an evolving LF, where the luminosity scales as (1+z)^delta with an optimal delta of 1.0\pm 0.2. For a single power-law LF, the best slope is ~ -1.57 with an upper luminosity of 10^53.3 erg/s, while the best slopes for a double power-law LF are approximately -1.6 and -2.6 with a break luminosity of 10^52.7 erg/s. Our finding implies a jet model intermediate between the universal structured epsilon(theta) proportional to theta^-2 model and the quasi-universal Gaussian structured model. For the uniform jet model our result is compatible with an angle distribution between 2 and 15 grades. Our best constrained GRB formation rate histories increase from z=0 to z=2 by a factor of ~30 and then continue increasing slightly. We connect these histories to that of the cosmic star formation history, and compare with observational inferences up to z~6. GRBs could be tracing the cosmic rates of both the normal and obscured star formation regimes. We estimate a current GRB event rate in the Milky Way of ~5 10^-5 yr^-1, and compare it with the birthrate of massive close WR+BH binaries with orbital periods of hours. The agreement is rather good suggesting that these systems could be the progenitors of the long GRBs.Comment: ApJ accepted. Goodness-of-fit tests and small corrections to the text were introduced, the number of figures reduced to 4 out of 6, new references were added. Main results and conclusions remain the sam

Process and System for Establishing a Moving Target Connection for Secure Communications in Client/Server Systems

A system and method performs a moving target blind rendezvous by exchanging data through a distributed hash table. The system allows users to securely send small pieces of information over a network while only requiring an exchange of public keys ahead of time. The system relies on the size and resilience of the BitTorrent Distributed Hash Table and the security properties of cryptographic constructions such as Elliptic Curve Diffie-Hellman key exchange and secure one-way hash functions.https://digitalcommons.usmalibrary.org/patents/1000/thumbnail.jp

Angular momentum distribution of hot gas and implications for disk galaxy formation

We study the angular momentum profiles both for dark matter and for gas within virialized halos, using a statistical sample of halos drawn from cosmological hydrodynamics simulations. Three simulations have been analyzed, one is the ``non-radiative'' simulation, and the other two have radiative cooling. We find that the gas component on average has a larger spin and contains a smaller fraction of mass with negative angular momentum than its dark matter counterpart in the non-radiative model. As to the cooling models, the gas component shares approximately the same spin parameter as its dark matter counterpart, but the hot gas has a higher spin and is more aligned in angular momentum than dark matter, while the opposite holds for the cold gas. After the mass of negative angular momentum is excluded, the angular momentum profile of the hot gas component approximately follows the universal function originally proposed by Bullock et al. for dark matter, though the shape parameter $\mu$ is much larger for hot gas and is comfortably in the range required by observations of disk galaxies. Since disk formation is related to the distribution of hot gas that will cool, our study may explain the fact that the disk component of observed galaxies contains a smaller fraction of low angular momentum material than dark matter in halos.Comment: 30 pages, 12 figures, 4 tables, accepted for publication in Ap