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Non-LTE line formation for Pr ii and Pr iii in A and Ap stars
Aims: Non-local thermodynamical equilibrium (non-LTE) line formation forsingly-ionized and doubly-ionized praseodymium is considered for a rangeof effective temperatures between 7250 K and 9500 K. We evaluate theinfluence of departures from LTE on Pr abundance determinations anddetermine a distribution of the Pr abundance in the atmosphere of theroAp star HD 24 712 from a non-LTE analysis of the Pr ii and Pr iiilines. Methods: A comprehensive model atom for Pr ii/iii ispresented based on the measured and the predicted energy levels, intotal, 6708 levels of Pr ii and Pr iii and the ground state of Pr iv.Calculations of the Pr ii energy levels and oscillator strengths for thetransitions in Pr ii and Pr iii are described. Results: Thedependence of non-LTE effects on the atmospheric parameters isdiscussed. At T_eff? 8000 K, departures from LTE lead to theoverionization of Pr ii and, therefore, to systematically depleted totalabsorption in the line and positive abundance corrections. At the lowertemperatures, different lines of Pr ii may be either weakened oramplified depending on the line strength. The non-LTE effects strengthenthe Pr iii lines and lead to negative abundance corrections. Non-LTEcorrections grow with effective temperature for the Pr ii lines, and, incontrast, they decline for the Pr iii lines. The Pr ii/iii model atom isapplied to determine the Pr abundance in the atmosphere of the roAp starHD 24 712 from the lines of two ionization stages. In the chemicallyuniform atmosphere with [Pr/H] = 3, the departures from LTE may explainonly a small part (approximately 0.3 dex) of the difference between theLTE abundances derived from the Pr ii and Pr iii lines (?2 dex). Wefind that the lines of both ionization stages are fitted for thevertical distribution of praseodymium, where the Pr enriched layer with[Pr/H] ? 4 exists in the outer atmosphere at log ?5000< -4. The departures from LTE for Pr ii/iii are strong in thestratified atmosphere and have the opposite sign for the Pr ii and Priii lines. The praseodymium stratification analysis of roAp stars has tobe performed based on non-LTE line formation. Using the revisedpartition function of Pr ii and experimental transition probabilities,we determine the solar non-LTE abundance to be log{(Pr/H)}_? =-11.15±0.08.Based on observations collected at the European Southern Observatory,Chile, 274.D-5011.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
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