Testing Spallation Processes With Beryllium and Boron

The nucleosynthesis of Be and B by spallation processes provides unique insight into the origin of cosmic rays. Namely, different spallation schemes predict sharply different trends for the growth of LiBeB abundances with respect to oxygen. ``Primary'' mechanisms predict BeB $\propto$ O, and are well motivated by the data if O/Fe is constant at low metallicity. In contrast, ``secondary'' mechanisms predict BeB $\propto$ O$^2$ and are consistent with the data if O/Fe increases towards low metallicity as some recent data suggest. Clearly, any primary mechanism, if operative, will dominate early in the history of the Galaxy. In this paper, we fit the BeB data to a two-component scheme which includes both primary and secondary trends. In this way, the data can be used to probe the period in which primary mechanisms are effective. We analyze the data using consistent stellar atmospheric parameters based on Balmer line data and the continuum infrared flux. Results depend sensitively on Pop II O abundances and, unfortunately, on the choice of stellar parameters. When using recent results which show O/Fe increasing toward lower metallicity, a two-component Be-O fits indicates that primary and secondary components contribute equally at [O/H]$_{eq}$ = -1.8 for Balmer line data; and [O/H]$_{eq}$ = -1.4 to -1.8 for IRFM. We apply these constraints to recent models for LiBeB origin. The Balmer line data does not show any evidence for primary production. On the other hand, the IRFM data does indicate a preference for a two-component model, such as a combination of standard GCR and metal-enriched particles accelerated in superbubbles. These conclusions rely on a detailed understanding of the abundance data including systematic effects which may alter the derived O-Fe and BeB-Fe relations.Comment: 40 pages including 11 ps figures. Written in AASTe

A Spectroscopic Study of the Ancient Milky Way: F- and G-Type Stars in the Third Data Release of the Sloan Digital Sky Survey

(Abridged) We perform an analysis of spectra and photometry for 22,770 stars included in the third data release (DR3) of the SDSS. We measure radial velocities and, based on a model-atmosphere analysis, derive estimates ofthe atmospheric parameters (effective temperature, surface gravity, and [Fe/H]) for each star. Stellar evolution models are then used to estimate distances. The SDSS sample covers a range in stellar brightness of 14 < V < 22, and comprises large numbers of F- and G-type stars from the thick-disk and halo populations (up to 100 kpc from the galactic plane), therefore including some of the oldest stars in the Milky Way. In agreement with previous results from the literature, we find that halo stars exhibit a broad range of iron abundances, with a peak at [Fe/H] ~ -1.4. This population exhibits essentially no galactic rotation. Thick-disk G-dwarf stars at distances from the galactic plane in the range 1<|z|<3 kpc show a much more compact metallicity distribution, with a maximum at [Fe/H] ~ -0.7, and a median galactic rotation velocity at that metallicity of 157 +/- 4 km/s (a lag relative to the thin disk of 63 km/s). A comparison of color indices and metal abundances with isochrones indicates that no significant star formation has taken place in the halo in the last ~ 11 Gyr, but there are thick-disk stars which are at least 2 Gyr younger. We find the metallicities of thick-disk stars to be nearly independent of galactocentric distance between 5 and 14 kpc, in contrast with the marked gradients found in the literature for the thin disk. No vertical metallicity gradient is apparent for the thick disk, but we detect a gradient inits rotational velocity of -16 +/- 4 km/s/kpc between 1 and 3 kpc from the plane.Comment: 18 pages, 16 figures; accepted for publication in the ApJ; also available from http://hebe.as.utexas.edu

A Consistency Test of Spectroscopic Gravities for Late-Type Stars

Chemical analyses of late-type stars are usually carried out following the classical recipe: LTE line formation and homogeneous, plane-parallel, flux-constant, and LTE model atmospheres. We review different results in the literature that have suggested significant inconsistencies in the spectroscopic analyses, pointing out the difficulties in deriving independent estimates of the stellar fundamental parameters and hence,detecting systematic errors. The trigonometric parallaxes measured by the HIPPARCOS mission provide accurate appraisals of the stellar surface gravity for nearby stars, which are used here to check the gravities obtained from the photospheric iron ionization balance. We find an approximate agreement for stars in the metallicity range -1 <= [Fe/H] <= 0, but the comparison shows that the differences between the spectroscopic and trigonometric gravities decrease towards lower metallicities for more metal-deficient dwarfs (-2.5 <= [Fe/H] <= -1.0), which casts a shadow upon the abundance analyses for extreme metal-poor stars that make use of the ionization equilibrium to constrain the gravity. The comparison with the strong-line gravities derived by Edvardsson (1988) and Fuhrmann (1998a) confirms that this method provides systematically larger gravities than the ionization balance. The strong-line gravities get closer to the physical ones for the stars analyzed by Fuhrmann, but they are even further away than the iron ionization gravities for the stars of lower gravities in Edvardsson's sample. The confrontation of the deviations of the iron ionization gravities in metal-poor stars reported here with departures from the excitation balance found in the literature, show that they are likely to be induced by the same physical mechanism(s).Comment: AAS LaTeX v4.0, 35 pages, 10 PostScript files; to appear in The Astrophysical Journa

Full-Stokes polarimetry with circularly polarized feeds - Sources with stable linear and circular polarization in the GHz regime

We present a pipeline that allows recovering reliable information for all four Stokes parameters with high accuracy. Its novelty relies on the treatment of the instrumental effects already prior to the computation of the Stokes parameters contrary to conventional methods, such as the M\"uller matrix one. The instrumental linear polarization is corrected across the whole telescope beam and significant Stokes $Q$ and $U$ can be recovered even when the recorded signals are severely corrupted. The accuracy we reach in terms of polarization degree is of the order of 0.1-0.2 %. The polarization angles are determined with an accuracy of almost 1$^{\circ}$. The presented methodology was applied to recover the linear and circular polarization of around 150 Active Galactic Nuclei. The sources were monitored from July 2010 to April 2016 with the Effelsberg 100-m telescope at 4.85 GHz and 8.35 GHz with a cadence of around 1.2 months. The polarized emission of the Moon was used to calibrate the polarization angle. Our analysis showed a small system-induced rotation of about 1$^{\circ}$ at both observing frequencies. Finally, we identify five sources with significant and stable linear polarization; three sources remain constantly linearly unpolarized over the period we examined; a total of 11 sources have stable circular polarization degree $m_\mathrm{c}$ and four of them with non-zero $m_\mathrm{c}$. We also identify eight sources that maintain a stable polarization angle over the examined period. All this is provided to the community for polarization observations reference. We finally show that our analysis method is conceptually different from the traditionally used ones and performs better than the M\"uller matrix method. Although it was developed for a system equipped with circularly polarized feeds it can easily be modified for systems with linearly polarized feeds as well.Comment: 19 pages, 17 figures, accepted for publication in Astronomy & Astrophysics on May 30, 201

Hadronic shift in pionic hydrogen

The hadronic shift in pionic hydrogen has been redetermined to be $\epsilon_{1s}=7.086\,\pm\,0.007(stat)\,\pm\,0.006(sys)$\,eV by X-ray spectroscopy of ground state transitions applying various energy calibration schemes. The experiment was performed at the high-intensity low-energy pion beam of the Paul Scherrer Institut by using the cyclotron trap and an ultimate-resolution Bragg spectrometer with bent crystals.Comment: 10 pages, 6 figure

Correlation entropy of synaptic input-output dynamics

The responses of synapses in the neocortex show highly stochastic and nonlinear behavior. The microscopic dynamics underlying this behavior, and its computational consequences during natural patterns of synaptic input, are not explained by conventional macroscopic models of deterministic ensemble mean dynamics. Here, we introduce the correlation entropy of the synaptic input-output map as a measure of synaptic reliability which explicitly includes the microscopic dynamics. Applying this to experimental data, we find that cortical synapses show a low-dimensional chaos driven by the natural input pattern.Comment: 7 pages, 6 Figures (7 figure files