Issue |
A&A
Volume 426, Number 3, November II 2004
|
|
---|---|---|
Page(s) | 1021 - 1034 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361:20040568 | |
Published online | 18 October 2004 |
The age-activity-rotation relationship in solar-type stars*
1
European Southern Observatory, Karl Schwarzschild Strasse 2, 85748, Garching bei München, Germany e-mail: gpace@eso.org
2
Dipartimento di Astronomia, Università di Trieste, via GB Tiepolo 11, 34131 Trieste, Italy
Received:
31
March
2004
Accepted:
27
June
2004
We present Ca K line chromospheric fluxes measured from high-resolution spectra in 35 G dwarf stars of 5 open clusters to determine the age-activity-rotation relationship
from the young Hyades and
Praesepe (0.6 Gyr) to the old M 67 (~4.5 Gyr) through the two intermediate age clusters IC 4651 and NGC 3680 (~1.7 Gyr).
The full amplitude of the activity index within a cluster is
slightly above 60 % for all clusters but one, NGC 3680, in which
only two stars were observed. As a comparison, the same Solar
Ca
index varies by ~
during a solar cycle.
Four of our clusters (Hyades and Praesepe, IC 4651 and NGC 3680) are pairs of
twins as far as age is concerned; the Hyades have the same chromospheric-activity
level as Praesepe, at odds
with early claims based on X-ray observations.
Both stars in NGC 3680 are indistinguishable, as far as chromospheric activity is concerned,
from those in the coeval IC 4651. This is a validation of the existence of an age-activity relationship.
On the other hand, the two intermediate age clusters have the same activity level as
the much older M 67 and the Sun. Our data therefore shows that a dramatic decrease in
chromospheric activity takes place in solar stars between the Hyades and the
IC 4651 age, of about 1 Gyr. Afterwards, activity remains
virtually constant for more than 3 Gyr.
We have also measured
for all of our stars and the average rotational velocity shows the same trend
as the chromospheric-activity index.
We briefly investigate the impact of this result on the age determinations of field G dwarfs in
the solar neighborhood; the two main
conclusions are that a consistent group of “young” stars (i.e. as active as Hyades stars) is present,
and that it is virtually impossible to give accurate chromospheric ages for
stars older than ~2 Gyr. The observed abrupt decline in activity explains very well the Vaughan-Preston gap.
Key words: stars: late-type / stars: activity / stars: chromosphere / stars: rotation
Observations collected at the ESO VLT. Some data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.
© ESO, 2004
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