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Register a new userThe 5200A flux depression of chemically peculiar stars: I. Synthetic Delta a photometry - the normality line
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F. Kupka Astronomy Unit
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The Delta a photometric system provides an efficient observational method to identify and distinguish magnetic and several other types of chemically peculiar (CP) stars of spectral types B to F from other classes of stars in the same range of effective temperatures. We have developed a synthetic photometric system which can be used to explore the capability of model atmospheres with individual element abundances to predict photometric Delta a magnitudes which measure the extent of the flux depression around 5200A found in different types of CP stars. In this first paper, we confirm the observed dependency of the a-index as a function of various colour indices sensitive to the effective temperature of stars as well as its average scatter expected from surface gravity variations within the main sequence band. The behaviour of the so-called normality line of Delta a systems used in photometric observations of CP stars is well reproduced. The metallicity dependence of the normality line of the Delta a system was computed for several grids of model atmospheres where the abundances of elements heavier than He had been scaled +-0.5 dex with respect to the solar value. We estimate a lowering of Delta a magnitudes for CP stars within the Magellanic Clouds by approximately -3 mmag relative to those in the solar neighbourhood assuming an average metallicity of [Fe/H]= -0.5 dex. Using these results on the metallicity bias of the Delta a system we find the observational systems in use suitable to identify CP stars in other galaxies or distant regions of our own galaxy and capable to provide data samples on a statistically meaningful basis. In turn, the synthetic system is suitable to test the performance of model atmospheres for CP stars. This work will be presented in follow-up papers of this series.
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Magnetic chemically peculiar (mCP) stars are important to astrophysics because their complex atmospheres lend themselves perfectly to the investigation of the interplay between such diverse phenomena as atomic diffusion, magnetic fields, and stellar rotation. The present work is aimed at identifying new mCP stars using spectra collected by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). Suitable candidates were selected by searching LAMOST DR4 spectra for the presence of the characteristic 5200A flux depression. Spectral classification was carried out with a modified version of the MKCLASS code and the accuracy of the classifications was estimated by comparison with results from manual classification and the literature. Using parallax data and photometry from Gaia DR2, we investigated the space distribution of our sample stars and their properties in the colour-magnitude diagram. Our final sample consists of 1002 mCP stars, most of which are new discoveries (only 59 previously known). Traditional mCP star peculiarities have been identified in all but 36 stars, highlighting the efficiency of the codes peculiarity identification capabilities. The derived temperature and peculiarity types are in agreement with manually derived classifications and the literature. Our sample stars are between 100 Myr and 1 Gyr old, with the majority having masses between 2M(Sun) and 3M(Sun). Our results could be considered as strong evidence for an inhomogeneous age distribution among low-mass (M < 3M(Sun)) mCP stars. We identified several astrophysically interesting objects: two mCP stars have distances and kinematical properties in agreement with halo stars; an eclipsing binary system hosting an mCP star component; and an SB2 system likely comprising of an mCP star and a supergiant component.
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