Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003basbr..23...96c&link_type=abstract
Boletim da Sociedade Astronômica Brasileira (ISSN 0101-3440), vol.23, no.1, p. 96-97
Astronomy and Astrophysics
Astronomy
Scientific paper
The connection rotation-CaII emission flux-Rossby number is analyzed for a sample of bona fide 121 subgiant stars, with evolutionary status determined from HIPPARCOS trigonometric parallax measurements and from the Toulouse-Geneva code (do Nascimento, J.D.Jr., Charbonnel, C., Lèbre, A., de Laverny, P., De Medeiros, J.R. 2000, A&A 357, 931). The distribution of rotation and CaII emission flux as a function of effective temperature shows a discontinuity located rather around the same spectral type, F8IV. Blueward of this spectral type subgiants have a large spread of values of rotation and CaII flux, whereas stars redward of F8IV show essentially low rotation and low CaII flux. The Rossby number, Ro = Prot/tconv, is determined for the whole of the sample. The behavior of F(CaII) as a function of the Rossby number Ro points for two components of CaII emission flux. For stars with (B-V) > 0.55, namely G- and K- type subgiants, the CaII emission flux is strongly correlated with Ro, indicating that chromospheric activity in these spectral regions depends essentially on rotation and convection. Such a fact points clearly for a magnetic origin of CaII emission flux of G- and K-type subgiant stars. For subgiants with (B-V) < 0.55 the present analysis shows that the CaII emission flux is uniformly high and independent of Ro, pointing for a non-magnetic component of the chromospheric activity in this spectral region. In short, our results results confirm clearly that rotation is not the only parameter expected to influence stellar activity. In addition, we look for the extent to which rotation and convection can induce both helicity and differential rotation required for dynamo action in the subphotospheric region of subgiant stars.
Canto Martins B. L.
de Medeiros Jose Renan
do Nascimento D.. Jr. J.
Melo H. F. C.
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