6,147 research outputs found


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    The Objectives of this paper are: (1) to introduce the concept of spline functions; and (2) to account for complete and partial non-reversibility with spline functions. This paper illustrates this approach using a supply response example.Research Methods/ Statistical Methods,

    Baade-Wesselink distances to Galactic and Magellanic Cloud Cepheids and the effect of metallicity

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    (abridged) The aim of this paper is to investigate the metallicity dependence of the PL relation in V and K, based on a sample of 128 Galactic, 36 LMC, and 6 SMC Cepheids with individual Baade-Wesselink (BW) distances and individually determined metallicities from high-resolution spectroscopy. The p-relation finally adopted is 1.50 -0.24log P. The slope of this relation is based on the condition that the distance to the LMC does not depend on period or (V-K) colour and that the slope of the PL relation based on the BW distances agrees with that based on apparent magnitude. The zero point of the relation is tight to the Cepheids with HST and revised Hipparcos parallaxes as well as to Cepheids in clusters. The slope of the Galactic and LMC K-band relation formally agrees within the errors, and combining all Cepheids (including the SMC) results in a negligible metallicity dependence. A similar conclusion is found for the reddening-free Wesenheit relation. In the V-band the situation is more complex. The slope of the LMC and the Galactic PL relation differ at the 3sigma level. Combining the sample nevertheless results in a metallicity term significant at the 2sigma level. The details of the comparison of BW-based distances and Cepheids with HST and revised Hipparcos parallaxes also play a role. The method used by Storm et al. would lead to larger DM of 18.37 and 18.81 for the LMC and SMC, respectively. The LMC DM is shorter than the currently accepted value, which is in the range 18.42 to 18.55 (Walker 2012), and it is speculated that the p-factor may depend on metallicity.Comment: Accepted for A&

    Debye-Waller B values for some NaCl-type structures and interionic interaction

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    Debye-Waller B values of NaCl, KCl, MgO and AgCl are calculated from lattice dynamics based on a simple, rigid-ion type, interaction model. The parameters introduced in the model have intelligible significance. The present approach compares favourably with experimental evidence and previous physical calculations

    Eclipsing binaries in the Galactic Bulge: candidates for distance estimates

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    The 222~000 I-band light curves of variable stars detected by the OGLE-II survey in the direction of the Galactic Bulge have been searched for eclipsing binaries (EBs). A previously developed code to analyze lightcurve shapes and identify long period variables (LPVs) has been adapted to identify EBs. The parameters in the modified code have been optimised to recover a list of about 140 detached EBs in the Small Magellanic Cloud previously identified in the literature as particularly well suited for distance estimates (and wich have periods \more 0.85 days). The power of the code is demonstrated by identifying 16 and 178 previously uncatalogued EBs in the SMC and LMC, respectively. Among the 222~000 variable stars in the direction of the Galactic Bulge 3053 EBs have been identified. Periods and phased lightcurves are presented.Comment: accepted Astronomy and Astrophysic

    The photodissociation of CO in circumstellar envelopes

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    Carbon monoxide is the most abundant molecule after H2_2 and is important for chemistry in circumstellar envelopes around late-type stars. The size of the envelope is important when modelling low-J transition lines and deriving mass-loss rates from such lines. Now that ALMA is coming to full power the extent of the CO emitting region can be measured directly for nearby asymptotic giant branch (AGB) stars. In parallel, it has become obvious in the past few years that the strength of the interstellar radiation field (ISRF) can have a significant impact on the interpretation of the emission lines. In this paper an update and extension of the classical Mamon et al. (1988; ApJ 328, 797) paper is presented; these authors provided the CO abundance profile, described by two parameters, as a function of mass-loss rate and expansion velocity. Following recent work an improved numerical method and updated H2_2 and CO shielding functions are used and a larger grid is calculated that covers more parameter space, including the strength of the ISRF. The effect of changing the photodissociation radius on the low-J CO line intensities is illustrated in two cases.Comment: A&A in pres

    Irrelevance of atomic masses for Debye-Waller B values in the limit of high temperatures

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    An expression that approximates Debye-Waller B values by a sum of three terms is derived from the theory of lattice dynamics in the harmonic approximation. For cubic crystals (M is the mass of the th atom in the unit cell): B = T + 22h2/3k TM + /M2T3, where T D/2 and and are constants, depending on interatomic forces only. It is shown that for temperatures above the Debye temperature D of the lattice, the second and third terms in the above expression can be neglected. From this, it follows that above the Debye temperature Debye-Waller B values become independent of the atomic masses. Consequently, the heavier atoms in a lattice do not necessarily have the smaller B values