Issue |
Astron. Astrophys. Suppl. Ser.
Volume 127, Number 2, January II 1998
|
|
---|---|---|
Page(s) | 277 - 294 | |
DOI | https://doi.org/10.1051/aas:1998350 | |
Published online | 15 January 1998 |
Are metallic A-F giants evolved Am stars? Rotation and rate of binaries among giant F stars *,**
Institut d'Astronomie de l'Université de Lausanne, CH-1290 Chavannes-des-Bois, Switzerland
Send offprint request to: P. North
Received:
30
October
1996
Accepted:
9
May
1997
We test the hypothesis of Berthet (1992) which foresees that Am
stars become giant metallic A and F stars (defined by an enhanced value of
the blanketing parameter of the Geneva photometry) when they
evolve.
If this hypothesis is right, Am and metallic A-FIII stars need to have the
same rate of binaries and a similar distribution of
. From our new
spectroscopic data and from
and radial velocities in the literature,
we show that it is not the case. The metallic giant stars are often fast
rotators with
larger than 100 km s-1, while the maximum rotational
velocity for Am stars is about 100 km s-1. The rate of tight binaries with
periods less than 1000 days is less than 30% among metallic giants, which is
incompatible with the value of 75% for Am stars (Abt & Levy
1985). Therefore, the simplest way to explain the existence of giant
metallic F stars is to suggest that all normal A and early F stars might go
through a short "metallic" phase when they are finishing their life on the main sequence.
Besides, it is shown that only giant stars with spectral type comprised
between F0 and F6 may have a really enhanced
value, while all A-type
giants seem to be normal.
Key words: stars: binaries: spectroscopic / stars: chemically peculiar / stars: evolution / stars: rotation / stars: δ Scuti
© European Southern Observatory (ESO), 1998