Line ratios and temperature structure in the deep photosphere

A program to monitor solar cycle variations of the solar flux by using suitable spectral line ratios is going on at Kitt Peak since 1976;the most sensitive to Teff variations are the ratios involving the C I 538.032 nm, whose formation depth is almost coincident with that of the continuum, and either the Fe I 537.958 or the Ti II 538.103. The temperature sensitivities of those line ratios have been empirically calibrated by observing the spectra of several solar-like stars by Gray and Livingston, while several attempts to obtain the same calibration theoretically, through Kurucz’s models of stellar atmospheres, showed difficulty in reproducing quantitatively the experimental results. Because the observed/computed ratio was approximately the same for both couples of lines, we argued that the problem was in the behaviour of C line, that is more affected than the others by the temperature structure of the deep photosphere, where it is formed. As, in these layers, the gradients of the average temperature are sensibly affected by different treatments of the convection, we compared, first of all, several theoretical models, distinguished from each other in including or not convective overshooting. Then we explored the effects due to variations of the value of the free parameter (α =�/HP ) and those ensued by different versions of the mixing-length theory

Is the solar spectrum latitude dependent? An investigation with SST/TRIPPEL

Context: In studies of the solar spectrum relative to spectra of solar twin stars, it has been found that the chemical composition of the Sun seems to depart systematically from those of the twins. One possible explanation is that the effect is due to the special aspect angle of the Sun when observed from Earth, as compared with the aspect angles of the twins. Thus, a latitude dependence of the solar spectrum, even with the heliocentric angle constant, could lead to effects of the type observed. Aim: We explore a possible variation in the strength of certain spectral lines, used in the comparisons between the composition of the Sun and the twins, at loci on the solar disk with different latitudes but at constant heliocentric angle. Methods: We use the TRIPPEL spectrograph at the Swedish 1-m Solar Telescope on La Palma to record spectra in five spectral regions in order to compare different locations on the solar disk at a heliocentric angle of 45 deg. Equivalent widths and other parameters are measured for fifteen different lines representing nine atomic species. Results: The relative variations in equivalent widths at the equator and at solar latitude 45 deg are found to be less than 1.5 % for all spectral lines studied. Translated to elemental abundances as they would be measured from a terrestrial and a hypothetical pole-on observer, the difference is estimated to be within 0.005 dex in all cases. Conclusion: It is very unlikely that latitude effects could cause the reported abundance difference between the Sun and the solar twins. The accuracy obtainable in measurements of small differences in spectral line strengths between different solar disk positions is very high.Comment: 9 pages, 10 figures, accepted by Astronomy & Astrophysic

Thermoeconomic cost assessment in future district heating networks

This paper aims at showing the capabilities of thermoeconomic analysis for solving cost assessments in district heating systems both at user and producer sides. In the near future it is expected that multiple producers are allowed to supply heat to the same district heating network, similarly to what happens in the case of the electric grid. Not only the amount of heat they may produce should be properly accounted, but also its quality, and also the pumping power that is requested to supply a unity of thermal energy to the endusers. Moreover, buildings equipped with low temperature heating system allow better use of the thermal energy vector, thus allowing larger efficiency of thermal plants. In the present work, the use of thermoeconomics for the analysis of these aspects is proposed. The approach allows one performing cost assessment in district heating, taking into account the effects of investment and operating costs and thermodynamic irreversibilities in the cost formation of heat from its production in the plants to its use in the buildings. Simple examples are analyzed in order to provide a quantitative evaluation of the various cost terms, depending on the operating conditions, topology and characteristics of the users/producers

UVES spectra of young brown dwarfs in Cha I: radial and rotational velocities

Based on high-resolution UVES spectra we found that the radial velocity (RV) dispersion of nine of twelve known young bona fide and candidate brown dwarfs in the Cha I dark cloud is 2.0 km/s, i.e. significantly smaller than the RV dispersion of T Tauri stars in Cha I (3.6 km/s) and only slightly larger than the dispersion of the surrounding molecular gas (1.2 km/s) (Mizuno et al. 1999). This result indicates that the majority of these brown dwarfs are not ejected with high velocity out of a dense region as proposed by some formation scenarios for brown dwarfs. The mean RV values are consistent with the objects being kinematic members of Cha I. The RV dispersion of the T Tauri stars confined to the Cha I region is based on a compilation of T Tauri stars with known RVs from the literature plus three T Tauri stars observed with UVES and unpublished RVs for nine T Tauri stars. Time-resolved spectroscopy revealed RV variations for five out of nine of the bona fide and candidate brown dwarfs in Cha I, which could be due to orbiting planets or surface features. Furthermore we derived rotational velocities vsin(i) and the Lithium 6708 \AA equivalent width.Comment: A&A Letter, in pres

Discovery of a Magnetic DZ White Dwarf with Zeeman-Split Lines of Heavy Elements

A spectroscopic survey of previously-unstudied Luyten Half Second proper motion stars has resulted in the discoveries of two new cool magnetic white dwarfs. One (LHS 2273) is a routine DA star, T= 6,500K, with Zeeman-split H alpha and H beta, for which a simple model suggests a polar field strength of 18.5 MG viewed close to equator-on. However, the white dwarf LHS 2534 proves to be the first magnetic DZ showing Zeeman-split Na I and Mg I components, as well as Ca I and Ca II lines for which Zeeman components are blended. The Na I splittings result in a mean surface field strength estimate of 1.92 MG. Apart from the magnetic field, LHS 2534 is one of the most heavily-blanketed and coolest DZ white dwarfs at T ~ 6,000K.Comment: 7 pages, Astrophysical Journal (Letters), in pres

Sistema ubiquo applicato all'assistenza a persone con problemi di memoria

Il modello di computazione Ubiquitous Computing sarà di basilare importanza per ogni sistema futuro nell'ambito dell'IT. L'aumento della capacità di interagire tra dispositivi diversi e tra reti diverse, per creare una rete diffusa, in cui l'utente è immerso in essa, è l'obiettivo principale di tale modello. In questa tesi tale paradigma è stato calato nel contesto dell'assistenza alle persone, soprattutto anziane, definendo un sistema che rispondesse adeguatamente alle esigenze degli utenti con lievi problemi di memoria. Per “lievi problemi” si intende gli effetti causati dal naturale decadimento delle facoltà mnemoniche, e non dovute a patologie o traumi. Dall'analisi degli ultimi lavori sul tema, si è giunti a definire un'architettura di sistema distribuita aperta, peer-to-peer, in completa controtendenza delle soluzioni attuali, che puntano ad un modello centralizzato. Si sono definiti più servizi, che rispondono ad esigenze diverse dell'utente : dall'assunzione dei medicinali ad un'agenda degli impegni, da un sistema trova oggetti smarriti al controllo remoto della casa. Utilizzando la tecnologia Java per sistemi distribuiti (Jini) si è implementata un'infrastruttura (che abbiamo chiamato “Reminder”) che si basa sulle funzioni del sistema descritto. Tale infrastruttura analizza tutte le operazioni principali dei servizi precedentemente definiti : dalla gestione con il database, all'interazione tra le entità di Jini, fino all'interfacciamento con i dispositivi esterni.ope

Image acquisition system for the Italian panoramic monochromator of the THEMIS telescope

We describe the image acquisition system of the italian panoramic monochromator which will be in operation at the focus F2 of the THEMIS solar telescope to acquire images of the Sun in the visible spectrum (between 420 and 700 nm) with a spectral resolving power of about 300 000. The system consists of two CCD cameras (using either 512-512 or 1024- 1024 square-pixel sensors) controlled and readout by dedicated electronics and a personal computer. The data transmission between the sensors and the computer is performed by means of an optical link

A multiwavelength radial velocity search for planets around the brown dwarf LP 944-20

The nearby brown dwarf LP 944-20 has been monitored for radial velocity variability at optical and near-infrared wavelengths using the VLT/UVES and the Keck/NIRSPEC spectrographs, respectively. The UVES radial velocity data obtained over 14 nights spanning a baseline of 841 days shows significant variability with an amplitude of 3.5 km s$^{-1}$. The periodogram analysis of the UVES data indicates a possible period between 2.5 hours and 3.7 hours, which is likely due to the rotation of the brown dwarf. However, the NIRSPEC data obtained over 6 nights shows an rms dispersion of only 0.36 km s$^{-1}$ and do not follow the periodic trend. These results indicate that the variability seen with UVES is likely to be due to rotationally modulated inhomogeneous surface features. We suggest that future planet searches around very low-mass stars and brown dwarfs using radial velocities will be better conducted in the near-infrared than in the optical.Comment: accepted by ApJ Letter

Evaluating the stability of atmospheric lines with HARPS

Context: In the search for extrasolar systems by radial velocity technique, a precise wavelength calibration is necessary for high-precision measurements. The choice of the calibrator is a particularly important question in the infra-red domain, where the precision and exploits still fall behind the achievements of the optical. Aims: We investigate the long-term stability of atmospheric lines as a precise wavelength reference and analyze their sensitivity to different atmospheric and observing conditions. Methods: We use HARPS archive data on three bright stars, Tau Ceti, Mu Arae and Epsilon Eri, spanning 6 years and containing high-cadence measurements over several nights. We cross-correlate this data with an O2 mask and evaluate both radial velocity and bisector variations down to a photon noise of 1 m/s. Results: We find that the telluric lines in the three data-sets are stable down to 10 m/s (r.m.s.) over the 6 years. We also show that the radial velocity variations can be accounted for by simple atmospheric models, yielding a final precision of 1-2 m/s. Conclusions: The long-term stability of atmospheric lines was measured as being of 10 m/s over six years, in spite of atmospheric phenomena. Atmospheric lines can be used as a wavelength reference for short-time-scales programs, yielding a precision of 5 m/s "out-of-the box". A higher precision, down to 2 m/s can be reached if the atmospheric phenomena are corrected for by the simple atmospheric model described, making it a very competitive method even on long time-scales.Comment: 7 pages, accepted for publication in A&