439 research outputs found
Measuring helium abundance difference in giants of NGC 2808
Multiple populations have been detected in several globular clusters (GC)
that do not display a spread in metallicity. Unusual features of their CMD can
be interpreted in terms of differences in the Helium content of the stars
belonging to the sub-populations. Differences in He abundance have never been
directly observed. We attempt to measure these differences in two giant stars
of NGC 2808 with very similar parameters but different Na and O abundances,
hence that presumably belong to different sub-populations, by directly
comparing their He I 10830 {\AA} lines. The He 10830 {\AA} line forms in the
upper chromosphere. Our detailed models derive the chromospheric structure
using the Ca II and H, and simulate the corresponding He I 10830 line
profiles. We show that, at a given value of He abundance, the He I 10830
equivalent width cannot significantly change without a corresponding much
larger change in the Ca II lines. We have used the VLT-CRIRES to obtain
high-resolution spectra in the 10830 {\AA} region, and the VLT-UVES to obtain
spectra of the Ca II and H lines of our target stars. The two target
stars have very similar Ca II and H lines, but different appearances in
the He region. One line, blueshifted by 17 km s with respect to the He
10830 rest wavelength, is detected in the spectrum of the Na-rich star, whereas
the Na-poor star spectrum is consistent with a non-detection. The difference in
the spectra is consistent and most closely explained by an He abundance
difference between the two stars of 0.17.We provide direct
evidence of a significant He line strength difference in giant stars of NGC
2808 belonging to different sub-populations, which had been previously detected
by other photometric and spectroscopic means.Comment: Accepted for publication in A&
Single standard calibration and data processing in flow injection titration based on concentration gradients
This paper describes use of gradients of concentration generated in
flow injection (FI) systems to perform determinations based on points where the concentration of titrant and analyte are at stoichiometric ratio. Two procedures were developed. In one procedure the titrant is injected in a FI manifold and merges with
the sample which is continuously pumped towards the detector. In the other procedure the sample is injected and merged with the titrant which is continuously pumped. Both techniques make use of concentration gradients of the sample or titrant generated in FI
manifolds that contain a mixing chamber. This gradient is calibrated employing only one standard solution (usually the titrant) in order to convert any detector signal, obtained in the
elapsed time after injection, to instantaneous concentration values. The flow system is microcomputer controlled and data are treated to locate points where the concentration of titrant and analyte are at the stoichiometric ratio. These points are found in abrupt changes
of the signal × concentration curves obtained in the presence of the reaction. The method has been evaluated for determination of Fe(II) and acetic acid by spectrophotometric and conductimetric detection, respectively. Results show a mean relative standard deviation lower than 1%, an average accuracy of 1% and a high
sampling processing capability (40 to 60 samples per hour)
Helium abundance difference within globular clusters: NGC 2808
Multiple populations have been recently detected in most Galactic globular clusters,
even with no significant spread in metallicity. Unusual features of the observed colourmagnitude diagrams can be explained by differences in the He content of the stars belonging to the sub-populations. We report on empirical evidence of He abundance spread in a few globular clusters, with particular attention to NGC 2808, where He abundance variation has been measured in a pair of otherwise identical red giant stars using the HeI 1083 nm line.A quantitative estimate of this difference has been derived by appropriate chromospheric modelling, in very good agreement with stellar evolution requirements.Fil: Cacciari, C.. INAF, Observatorio Astronomico di Bologna; ItaliaFil: Pasquini, L.. ESO; AlemaniaFil: Valenti, E.. ESO; AlemaniaFil: Käufl, H. U.. ESO; AlemaniaFil: Mauas, Pablo Jacobo David. Consejo Nacional de Investigaciónes CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de AstronomÃa y FÃsica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de AstronomÃa y FÃsica del Espacio; Argentin
Virtual Compton Scattering off the Pseudoscalar Meson Octet
We present a calculation of the virtual Compton scattering amplitude for the
pseudoscalar meson octet in the framework of chiral perturbation theory at
. We calculate the electromagnetic generalized polarizabilities
and compare the results in the real Compton scattering limit to available
experimental values. Finally, we give predictions for the differential cross
section of electron-meson bremsstrahlung.Comment: 9 pages, Latex, uses cjp3.sty (included), 4 eps figures, to be
published in the proceedings of the 13th Indian-Summer School "Understanding
the Structure of Hadrons," August 28 - September 1, 2000, Prague, Czech
Republi
Concept and optical design of the cross-disperser module for CRIRES
This is the peer reviewed version of the following article: Oliva, Ernesto, A. Tozzi, D. Ferruzzi, L. Origlia, A. Hatzes, R. Follert, T. Loewinger et al. "Concept and optical design of the cross-disperser module for CRIRES+." In SPIE Astronomical Telescopes+ Instrumentation, pp. 91477R-91477R. International Society for Optics and Photonics, 2014, which has been published in final form at 10.1117/12.2054381
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