4,598 research outputs found
PEPSI deep spectra. I. The Sun-as-a-star
As part of the first Potsdam Echelle Polarimetric and Spectroscopic
Instrument (PEPSI) key-science project, we aim to provide well-exposed (viz.
deep) high-resolution spectra of representative stellar targets. These spectra
will be made available in form of (electronic) atlases. The first star in this
series of papers is our Sun. It also acts as a system-performance cornerstone.
The deep spectra in this paper are the results of combining up to 100
consecutive exposures per wavelength setting and are compared with other solar
flux atlases. Our software for the optimal data extraction and reduction of
PEPSI spectra is described and verified with the solar data. Three deep solar
flux spectra with a spectral resolution of up to 270,000, a continuous
wavelength coverage from 383 nm to 914 nm, and a photon signal to noise ratio
(S/N) of between 2,000-8,000:1 depending on wavelength are presented.
Additionally, a time-series of 996 high-cadence spectra in one cross disperser
is used to search for intrinsic solar modulations. The wavelength calibration
based on Th-Ar exposures and simultaneous Fabry-Perot combs enables an absolute
wavelength solution within 10 m/s (rms) with respect to the HARPS laser-comb
solar atlas and a relative rms of 1.2 m/s for one day. For science
demonstration, we redetermined the disk-average solar Li abundance to
1.09+/-0.04 dex on the basis of 3D NLTE model atmospheres. We detected
disk-averaged p-mode RV oscillations with a full amplitude of 47 cm/s at 5.5
min. Comparisons with two solar FTS atlases, as well as with the HARPS solar
atlas, validate the PEPSI data product. Now, PEPSI/SDI solar-flux spectra are
being taken with a sampling of one deep spectrum per day, and are supposed to
continue a full magnetic cycle of the Sun.Comment: in press, 13 pages, 8 figures, data available from pepsi.aip.d
Quantum Phase Transition of Ground-state Entanglement in a Heisenberg Spin Chain Simulated in an NMR Quantum Computer
Using an NMR quantum computer, we experimentally simulate the quantum phase
transition of a Heisenberg spin chain. The Hamiltonian is generated by a
multiple pulse sequence, the nuclear spin system is prepared in its
(pseudo-pure) ground state and the effective Hamiltonian varied in such a way
that the Heisenberg chain is taken from a product state to an entangled state
and finally to a different product state.Comment: 5 pages, 5 eps figures. Accepted in Phys. Rev.
Isotope Spectroscopy
The measurement of isotopic ratios provides a privileged insight both into
nucleosynthesis and into the mechanisms operating in stellar envelopes, such as
gravitational settling. In this article, we give a few examples of how isotopic
ratios can be determined from high-resolution, high-quality stellar spectra. We
consider examples of the lightest elements, H and He, for which the isotopic
shifts are very large and easily measurable, and examples of heavier elements
for which the determination of isotopic ratios is more difficult. The presence
of 6Li in the stellar atmospheres causes a subtle extra depression in the red
wing of the 7Li 670.7 nm doublet which can only be detected in spectra of the
highest quality. But even with the best spectra, the derived Li abundance
can only be as good as the synthetic spectra used for their interpretation. It
is now known that 3D non-LTE modelling of the lithium spectral line profiles is
necessary to account properly for the intrinsic line asymmetry, which is
produced by convective flows in the atmospheres of cool stars, and can mimic
the presence of 6Li. We also discuss briefly the case of the carbon isotopic
ratio in metal-poor stars, and provide a new determination of the nickel
isotopic ratios in the solar atmosphere.Comment: AIP Thinkshop 10 "High resolution optical spectroscopy", invited
talk, AN in pres
Bremsstrahlung from a microscopic model of relativistic heavy ion collisions
We compute bremsstrahlung arising from the acceleration of individual charged baryons and mesons during the time evolution of high-energy Au+Au collisions at the Relativistic Heavy Ion Collider using a microscopic transport model. We elucidate the connection between bremsstrahlung and charge stop- ping by colliding artificial pure proton on pure neutron nuclei. From the inten- sity of low energy bremsstrahlung, the time scale and the degree of stopping could be accurately extracted without measuring any hadronic observables. PACS: 25.75.-q, 13.85.Q
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