Astronomy and Astrophysics – Astrophysics
Scientific paper
2005-10-14
Astronomy and Astrophysics
Astrophysics
accepted
Scientific paper
10.1051/0004-6361:20053538
We used red clump stars to measure the surface mass density of the Galactic disk in the solar neighbourhood. High resolution spectra of red clump stars towards the NGP have been obtained with the ELODIE spectrograph at OHP for Tycho-2 selected stars, and nearby Hipparcos counterparts were also observed. We determined their distances, velocities, and metallicities to measure the gravitational force law perpendicular to the Galactic plane. As in most previous studies, we applied one-parameter models of the vertical gravitational potential. We obtained a disk surface mass density within 1.1kpc of the Galactic plane, Sigma_{1.1kpc}=64+/-5 Msun_pc^{-2}, with an excellent formal accuracy, however we found that such one-parameter models can underestimate the real uncertainties. Applying two-parameter models, we derived more realistic estimates of the total surface mass density within 800pc from the Galactic plane, Sigma_{0.8kpc}=57-66 Msun pc^{-2}, and within 1.1kpc, Sigma{1.1kpc}=57-79 Msun pc^{-2}. This can be compared to literature estimates of \sim40 Msun pc^{-2} in stars and to 13 Msun pc^{-2} in the less accurately measured ISM contribution. We conclude that there is no evidence of large amounts of dark matter in the disk and, furthermore, that the dark matter halo is round or not vey much flattened. A by-product of this study is the determination of the half period of oscillation by the Sun through the Galactic plane, 42+/-2Myr, which cannot be related to the possible period of large terrestrial impact craters \sim 33-37Myr.
Bienayme Olivier
Kovtyukh Valery V.
Mishenina Tamara V.
Siebert Arnaud
Soubiran Caroline
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