Quality Assurance of Spectral Ultraviolet Measurements in Europe Through the Development of a Transportable Unit (QASUME)

QASUME is a European Commission funded project that aims to develop and test a transportable unit for providing quality assurance to UV spectroradiometric measurements conducted in Europe. The comparisons will be performed at the home sites of the instruments, thus avoiding the risk of transporting instruments to participate in intercomparison campaigns. Spectral measurements obtained at each of the stations will be compared, following detailed and objective comparison protocols, against collocated measurements performed by a thoroughly tested and validated travelling unit. The transportable unit comprises a spectroradiometer, its calibrator with a set of calibration lamps traceable to the sources of different Standards Laboratories, and devices for determining the slit function and the angular response of the local spectroradiometers. The unit will be transported by road to about 25 UV stations over a period of about two years. The spectroradiometer of the transportable unit is compared in an intercomparison campaign with six instruments to establish a relation, which would then be used as a reference for its calibration over the period of its regular operation at the European stations. Different weather patterns (from clear skies to heavy rain) were present during the campaign, allowing the performance of the spectroradiometers to be evaluated under unfavourable conditions (as may be experienced at home sites) as well as the more desirable dry conditions. Measurements in the laboratory revealed that the calibration standards of the spectroradiometers differ by up to 10%. The evaluation is completed through comparisons with the same six instruments at their homes sites

X-Ray Bursts And Proton Captures Close To The Dripline

In low-mass binary systems involving a neutron star, proton-rich material from the atmosphere of the companion giant star can be accreted on the surface of the neutron star [1, 2, 3, 4]. (see also the review article [5]). Once a critical density (ß 10 6 g/cm 3 ) is reached within the accreted layer, thermonuclear fusion of hydrogen and helium is ignited and high temperatures are attained in an explosive runaway. Such thermonuclear flashes can be observed as so-called type I X-ray bursts. The energy is generated by hot hydrogen burning cycles and by burning helium in the 3ff reaction and a sequence of (ff,p) and (p,fl) reactions, which provide seed nuclei for the subsequent hydrogen burning in the rp process, consisting of rapid proton captures and fi \Gamma decays close to the proton dripline. The timescale of the rp<F33.

Explosive H-burning, the rp-process, and X-ray bursts

The major astrophysical events which involve explosive H-burning are novae and type I X-ray bursts. Both are related to binary stellar systems with hydrogen accretion from a binary companion onto a compact object, and the explosive ignition of the accreted H-layer. High densities cause the pressure to be dominated by the degenerate electron gas, preventing a stable and controlled burning. In the case of novae the compact object is a white dwarf, in the case of X-ray bursts it is a neutron star. Explosive II-burning in novae has been discussed in many recent articles [1, 2, 3, 4, 5]. Its processing is limited due to maximum temperatures of similar to 3X10(8)K. Only in X-ray bursts temperatures larger than 4x10(8)K are possible, which permit a break-out from the hot CNO-cycle, leading to a further temperature increase beyond 10(9)K, the onset of an rp-process (a sequence of proton captures and beta-decays) and burning of H and He to Fe/Ni and beyond. Here we investigate the rp-process by making use of a complete and updated nuclear reaction network from H to Sn. In particular we consider 2p-capture reactions that can bridge proton unbound nuclei and therefore accelerate the reaction flow. In a simplified one dimensional, one-cone X-ray burst model we find that for a 25 s burst the reaction flow reaches Cd. The consequences for energy production, final composition of the ashes, and fuel consumption are discussed

The astrophysical r-process: A comparison of calculations following adiabatic expansion with classical calculations based on neutron densities and temperatures

The rapid neutron-capture process (r-process) encounters unstable nuclei far from beta-stability. Therefore its observable features, like the abundances, witness (still uncertain) nuclear structure as well as the conditions in the appropriate astrophysical environment. With the remaining lack of a full understanding of its astrophysical origin, parameterized calculations are still needed. We consider two approaches: (1) the classical approach is based on (constant) neutron number densities n(n) and temperatures T over duration timescales tau; (2) recent investigations, motivated by the neutrino wind scenario from hot neutron stars after a supernova explosion, followed the expansion of matter with initial entropies S and electron fractions Y-e over expansion timescales tau. In the latter case the freezeout of reactions with declining temperatures and densities can be taken into account explicitly. We compare the similarities and differences between the two approaches with respect to resulting abundance features and their relation to solar r-process abundances, applying for the first time different nuclear mass models in entropy-based calculations. Special emphasis is given to the questions of (a) whether the same nuclear properties far from stability lead to similar abundance patterns and possible deficiencies in (1) and (2), and (b) whether some features can also provide clear constraints on the astrophysical conditions in terms of permitted entropies, Y-e values, and expansion timescales in (2). This relates mostly to the A > 110 mass range, where a fit to solar r-abundances in high-entropy supernova scenarios seems to be hard to attain. Possible low-entropy alternatives are presented

Explosive nucleosynthesis close to the drip lines

We give an overview of explosive burning and the role which neutron and/or proton separation energies play. We focus then on the rapid neutron capture process (r-process) which encounters unstable nuclei with neutron separation energies in the range 1-4 MeV, and the rapid proton capture process (rp-process), operating close to the proton drip-line. The site of the rp-process is related to hydrogen accreting neutron stars in binary stellar systems. Explosive II-burning produces nuclei as heavy as A=100, powering events observable as X-ray bursts. The r-process abundances witness nuclear structure far from beta-stability as well as the conditions in the appropriate astrophysical environment. But there is a remaining lack in the full understanding of its astrophysical origin, ranging from the high entropy neutrino wind, blown from hot neutron star surfaces after a supernova explosion, to low entropy "cold decompresssion" of neutron star matter ejected in mergers of binary neutron star systems.Peer reviewe