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Cool Stars (cool + star)

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Physics and Astronomy100%

Selected Abstracts

Testing the accuracy of synthetic stellar libraries

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
Lucimara P. Martins
ABSTRACT One of the main ingredients of stellar population synthesis models is a library of stellar spectra. Both empirical and theoretical libraries are used for this purpose, and the question about which one is preferable is still debated in the literature. Empirical and theoretical libraries are being improved significantly over the years, and many libraries have become available lately. However, it is not clear in the literature what are the advantages of using each of these new libraries, and how far behind models are compared to observations. Here we compare in detail some of the major theoretical libraries available in the literature with observations, aiming at detecting weaknesses and strengths from the stellar population modelling point of view. Our test is twofold: we compared model predictions and observations for broad-band colours and for high-resolution spectral features. Concerning the broad-band colours, we measured the stellar colour given by three recent sets of model atmospheres and flux distributions, and compared them with a recent UBVRIJHK calibration which is mostly based on empirical data. We found that the models can reproduce with reasonable accuracy the stellar colours for a fair interval in effective temperatures and gravities. The exceptions are (1) the U,B colour, where the models are typically redder than the observations, and (2) the very cool stars in general (V,K, 3). Castelli & Kurucz is the set of models that best reproduce the bluest colours (U,B, B,V) while Gustafsson et al. and Brott & Hauschildt more accurately predict the visual colours. The three sets of models perform in a similar way for the infrared colours. Concerning the high-resolution spectral features, we measured 35 spectral indices defined in the literature on three high-resolution synthetic libraries, and compared them with the observed measurements given by three empirical libraries. The measured indices cover the wavelength range from ,3500 to ,8700 Å. We found that the direct comparison between models and observations is not a simple task, given the uncertainties in parameter determinations of empirical libraries. Taking that aside, we found that in general the three libraries present similar behaviours and systematic deviations. For stars with Teff, 7000 K, the library by Coelho et al.is the one with best average performance. We detect that lists of atomic and molecular line opacities still need improvement, specially in the blue region of the spectrum, and for the cool stars (Teff, 4500 K). [source]

Experimental Ti i oscillator strengths and their application to cool star analysis

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
R. J. Blackwell-Whitehead
ABSTRACT We report experimental oscillator strengths for 88 Ti i transitions covering the wavelength range 465,3892 nm, 67 of which had no previous experimental values. Radiative lifetimes for 13 energy levels, including the low energy levels 3d2(3F) 4s4p (3P) z 5D°j, have been measured using time-resolved laser-induced fluorescence. Intensity-calibrated Ti i spectra have been measured using Fourier transform spectroscopy to determine branching fractions for the decay channels of these levels. The branching fractions are combined with the radiative lifetimes to yield absolute transition probabilities and oscillator strengths. Our measurements include 50 transitions in the previously unobserved infrared region , > 1.0 ,m, a region of particular interest to the analysis of cool stars and brown dwarfs. [source]

Recent observations of AB Dor and interpretation

ASTRONOMISCHE NACHRICHTEN, Issue 4 2009
E. Budding
Abstract We use minimal empirical modelling techniques to interpret recent (2006,2007) photometry and spectroscopy of AB Dor. We compare, in particular, broadband (B and V) maculation effects with emission features in high-resolution Ca II K-line spectroscopy. We also compare emission effects in the Ca II Kand H, lines observed at different rotational phases. We refer to a broader multiwavelength campaign, of which these optical data were a part, involving X-ray and microwave observations to be published later. The broadband light curves are characterized by one outstanding macula, whereas the emission lines suggest 4 possible main chromospheric activity sites (,faculae'). These appear at a similar latitude and with comparable size to the main umbra, but there are significant displacements in longitude. However, one strong facular concentration near phase zero may have a physical relationship to the main macula. The derived longitudes of these features would have been affected by differential rotation operating over the several months between the spectroscopic and photometric observations, but the difference of at least ,30° between facula and umbra appears too great to allow their coincidence. The possibility of a large bipolar surface structure is considered, keeping in mind the bipolar character of solar activity centres: the activity of rapidly rotating cool stars being generally compared with that of the Sun, scaled up by a few orders of magnitude. Observed microwave activity may link to this same main photospheric and chromospheric centre picked up by the optical analysis. Characterization of macular and facular contributions in stellar activity sites would be improved with a closer timing of observations and higher signal to noise ratios in emission line data (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]