Astronomischer Jahresbericht, Band XVII

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Thermal Equilibration of 176-Lu via K-Mixing

In astrophysical environments, the long-lived (\T_1/2 = 37.6 Gy) ground state of 176-Lu can communicate with a short-lived (T_1/2 = 3.664 h) isomeric level through thermal excitations. Thus, the lifetime of 176-Lu in an astrophysical environment can be quite different than in the laboratory. We examine the possibility that the rate of equilibration can be enhanced via K-mixing of two levels near E_x = 725 keV and estimate the relevant gamma-decay rates. We use this result to illustrate the effect of K-mixing on the effective stellar half-life. We also present a network calculation that includes the equilibrating transitions allowed by K-mixing. Even a small amount of K-mixing will ensure that 176-Lu reaches at least a quasi-equilibrium during an s-process triggered by the 22-Ne neutron source.Comment: 9 pages, 6 figure

The problem of the Pleiades distance. Constraints from Stromgren photometry of nearby field stars

The discrepancy between the Pleiades cluster distance based on Hipparcos parallaxes and main sequence fitting is investigated on the basis of Stromgren photometry of F-type stars. Field stars with the same metallicity as the Pleiades have been selected from the m1 index and a technique has been developed to locate the ZAMS of these field stars in color-magnitude diagrams based on the color/temperature indices b-y, v-y, and beta. Fitting the Pleiades to these ZAMS relations results in a distance modulus of 5.61+/-0.03 mag in contrast to the Hipparcos modulus of 5.36+/-0.06 mag. Hence, we cannot confirm the recent claim by Grenon (1999) that the distance problem is solved by adopting a low metallicity of the Pleiades ([Fe/H]=-0.11) as determined from Geneva photometry. The metallicity sensitivity of the ZAMS determined by the field stars is investigated, and by combining this sensitivity in all three color/temperature indices b-y, v-y, and beta we get a independent test of the Pleiades distance modulus which support our value of 5.61 mag. Furthermore, the field star sample used for the comparison is tested against theoretical isochrones of different ages to show that evolutionary effects in the field star sample are not biasing our distance modulus estimate significantly. Possible explanations of the Pleiades distance problem are discussed and it is suggested that the discrepancy in the derived moduli may be linked to a non-spherical shape of the cluster.Comment: 11 pages, 5 figures, accepted for publication in A&

A Stromgren view of the multiple populations in globular clusters

We discuss a variety of photometric indices assembled from the uvby Stromgren system. Our aim is to examine the pros and cons of the various indices to find the most suitable one(s) to study the properties of multiple populations in globular clusters (GCs) discovered by spectroscopy. We explore in particular the capabilities of indices like m_1 and c_y at different metallicities. We define a new index delta_4=(u-v)-(b-y) to separate first and second stellar generations in GCs of any metal abundance, since it keeps the sensitivity to multiple stellar populations over all the metallicity range and at the same time minimizes the sensitivity to photometric errors. We detecte clear differences in the red giant branches of the GCs examined, like skewness or bi/multi-modality in color distribution. We connect the photometric information with the spectroscopic results on O, Na abundances we obtained in our survey of GCs. Finally, we compute the effects of different chemical composition on the Stromgren filters and indices using synthetic spectra.Comment: Accepted for publication on Astronomy and Astrophysics. Figures 1,3,5 degraded in resolutio

Investigating star formation in the young open cluster NGC 6383

By studying young open clusters, the mechanisms important for star formation over several Myr can be examined. For example, accretion rate as a function of rotational velocity can be investigated. Similarly, sequential star formation triggered by massive stars with high mass-loss rates can be studied in detail. We identified and characterized probable members of NGC 6383, as well as determined cluster parameters. New Stromgren uvby CCD photometry, obtained by us, is presented. This new data, together with Johnson UBV and 2MASS data in the NIR, was used to investigate characteristics of pre- as well as zero age main sequence cluster members. We present Stromgren uvby CCD photometry for 272 stars in the field of NGC 6383 and derive its reddening, E(b-y)=0.21(4)mag, as well as distance, d=1.7(3)kpc from the Sun. Several stars with NIR excess and objects in the domain of the classical Herbig Ae/Be and T Tauri stars were detected. Two previously known variables were identified as rapidly-rotating PMS stars. The field population is clearly separated from the probable members in the color-magnitude diagram. NGC 6383 is a young open cluster, with an age of less than 4 Myr, undergoing continuous star formation. True pre-main sequence members might be found down to absolute magnitudes of +6mag, with a variety of rotational velocities and stellar activities.Comment: 7 pages, 6 figures, accepted by A&

Propagation of ionizing radiation in HII regions: the effects of optically thick density fluctuations

The accepted explanation of the observed dichotomy of two orders of magnitude between in situ measurements of electron density in HII regions, derived from emission line ratios, and average measurements based on integrated emission measure, is the inhomogeneity of the ionized medium. This is expressed as a "filling factor", the volume ratio of dense to tenuous gas, measured with values of order 10^-3. Implicit in the filling factor model as normally used, is the assumption that the clumps of dense gas are optically thin to ionizing radiation. Here we explore implications of assuming the contrary: that the clumps are optically thick. A first consequence is the presence within HII regions of a major fraction of neutral hydrogen. We estimate the mean H^o/H^+ ratio for a population of HII regions in the spiral galaxy NGC 1530 to be the order of 10, and support this inference using dynamical arguments. The optically thick clumpy models allow a significant fraction of the photons generated by the ionizing stars to escape from their HII region. We show, by comparing model predictions with observations, that these models give an account at least as good as, and probably better than that of conventional models, of the radial surface brightness distribution and of selected spectral line diagnostics for physical conditions within HII regions. These models explain how an HII region can appear, from its line ratios, to be ionization bounded, yet permit a major fraction of its ionizing photons to escape.Comment: 9 pages, 7 figures (2 of them in colours), accepted for publication in A&

Periodic orbits in the restricted three-body problem and Arnold's J+J^+-invariant

We apply Arnold's theory of generic smooth plane curves to Stark-Zeeman systems. This is a class of Hamiltonian dynamical systems that describes the dynamics of an electron in an external electric and magnetic field, and includes many systems from celestial mechanics. Based on Arnold's $J^+$-invariant, we introduce invariants of periodic orbits in planar Stark-Zeeman systems and study their behaviour.Comment: 36 Pages, 16 Figure

The Stellar Composition of the Star Formation Region CMa R1 -- III. A new outburst of the Be star component in Z CMa

We report on a recent event in which, after more than a decade of slowly fading, the visual brightness of the massive young binary Z CMa suddenly started to rise by about 1 magnitude in December 1999, followed by a rapid decline to its previous brightness over the next six months. This behaviour is similar to that exhibited by this system around its eruption in February 1987. A comparison of the intrinsic luminosities of the system with recent evolutionary calculations shows that Z CMa may consist of a 16 M_sun B0 IIIe primary star and a ~ 3 M_sun FUOr secondary with a common age of ~ 3 x 10^5 yr. We also compare new high-resolution spectra obtained in Jan. and Feb. 2000, during the recent rise in brightness, with archive data from 1991 and 1996. The spectra are rich in emission lines, which originate from the envelope of the early B-type primary star. The strength of these emission lines increased strongly with the brightness of Z CMa. We interpret the collected spectral data in terms of an accretion disc with atmosphere around the Herbig B0e component of Z CMa, which has expanded during the outbursts of 1987 and 2000. A high resolution profile of the 6300 A [O I] emission line, obtained by us in March 2002 shows an increase in flux and a prominent blue shoulder to the feature extending to ~ -700 km/s, which was much fainter in the pre-outburst spectra. We propose that this change in profile is a result of a strong change in the collimation of a jet, as a result of the outburst at the start of this century.Comment: 22 pages, 12 figures, accepted for publication in MNRA

An Improved Metallicity Calibration with UBV Photometry

We used the data of 701 stars covering the colour index interval 0.32<B-V<=1.16, with metallicities -1.76<=[Fe/H]<=+0.40 dex, which were taken from PASTEL catalogue and estimated metallicity dependent guillotine factors which provide a more accurate metallicity calibration. We reduced the metallicities of 11 authors to the metallicities of Valenti & Fischer (2005), thus obtained a homogeneous set of data which increased the accuracy of the calibration, i.e. [Fe/H]=-14.316*delta^{2}_{0.6}-3.557*delta_{0.6}+0.105. Comparison of the metallicity residuals, for two sets of data, based on the metallicity dependent guillotine factors with the ones obtained via metal free guillotine factors, shows that metallicities estimated by means of new guillotine factors are more accurate than the other ones. This advantage can be used in the metallicity gradient investigation of the Galactic components, i.e. thin disc, thick disc and halo.Comment: 12 pages, including 10 figures and 6 tables, accepted for publication in PAS

The Rest-frame Optical Colors of 99,000 SDSS Galaxies

We synthesize the rest-frame Stroemgren colors using SDSS spectra for 99,088 galaxies selected from Data Release 1. This narrow-band ~200 AA photometric system (uz, vz, bz, yz), first designed for the determination of effective temperature, metallicity and gravity of stars, measures the continuum spectral slope of galaxies in the rest-frame 3200-5800 AA wavelength range. Galaxies form a remarkably narrow locus (~0.03 mag) in the resulting color-color diagram. The Bruzual & Charlot population synthesis models suggest that the position of a galaxy along this locus is controlled by a degenerate combination of metallicity and age of the dominant stellar population. Galaxy distribution along the locus is bimodal, with the local minimum corresponding to an ~1 Gyr old single stellar population. The position perpendicular to the locus is independent of metallicity and age, and reflects the galaxy's dust content, as implied by both the models and the statistics of IRAS detections. A comparison of this locus with the galaxy locus in the H_delta-D_n(4000) diagram, utilized by Kauffmann et al. (2003) to estimate stellar masses, reveals a tight correlation, although the two analyzed spectral ranges barely overlap. Overall, the galaxy spectral energy distribution in the entire UV to near-IR range can be described as a single-parameter family with an accuracy of 0.1 mag, or better. This nearly one-dimensional distribution of galaxies in the multi-dimensional space of measured parameters strongly supports the conclusion of Yip et al. (2004), based on a principal component analysis, that SDSS galaxy spectra can be described by a small number of eigenspectra. Apparently, the contributions of stellar populations that dominate the optical emission from galaxies are combined in a simple and well-defined way.Comment: Accepted for publication in MNRAS; 19 pages, 28 color figure