Peculiar stars spectra

B and A stars with peculiar Mn and Hg Lines
Since 1931 W.W. Morgan, at Yerkes observatory, recognized a peculiar group of B8-9 stars that show strong Manganese and Mercury lines (Hg is almost unknown in other stellar spectra). High resolution observations of Hg 3984 ┼ line show shifts of 0.2 ┼ from star to star, attributed to a different isotopes mixing.
These stars were classified originally as Ap (peculiar A stars) but later it was shown that they do not have a magnetic field, they do not show spectral lines variability and they have no abundancies of rare earth.
The origin of their spectra is still unknown and probably correlated with the behaviour of some hot B stars that show He lines fainter than what could be expected since their temperature. Some stars that belong to this class are: a And, c Lupi, c Cancri and 53 Tauri.

Picture 1: A part of the spectrum of Alfa And, a 2nd magnitude star shared in common between Andromeda and Pegasus square, recorded with 0.7 ┼/pixel dispersion and 5 min exposition (Cassegrain F/20 diam 60 cm). Blue color shows Mn lines while green shows Hg lines.

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