Diapositiva 1

1
Local stellar kinematic constraints on the
Galactic potential

• T. Antoja1 , F. Figueras1, B. Pichardo2, D.
  Fernández1, E. Moreno2, J. Torra1, O. Valenzuela2
• 1 Dep. dAstronomia i Meteorologia and IEEC-UB,
  Institut de Ciències del Cosmos de la Universitat
  de Barcelona
• 2 Instituto de Astronomia de la Universidad
  Nacional Autónoma de Mexico

Observations
Simulations
Data an extensive compilation of more than 24000
stars of the solar neighbourhood to characterise
these observed kinematic structures in the
U--V--age--Fe/H space.
Galactic Potential
Axisymmetric potential by Allen Santillán
(1991) spherical bulge disk halo

• Simulation Method
• The birth and evolution of test particles are
  simulated until the current moment (orbit
  computation)
• Varying the free parameters of the model, we
  obtain different phase space distributions for
  the test particles which can be directly compared
  to the real distributions in the solar
  neighbourhood.
• INITIAL CONDITIONS
• 300000 test particles
• positions at birth in a ring R 5.5, 11.5 kpc,
  z lt 50 pc
• Gaussian velocity distribution respect to the
  Regional Standard of Rest with ? 2 km/s in all 3
  directions
• Ages randomly distributed in the range 0, 2 Gyr
• NUMERICAL INTEGRATION Direct numerical
  integration of the test particle orbits under a
  certain model for the Galactic potential
• from t t (stars age) to t0 (present)
• RESULTS Once positions and residual velocities
  at t0 are computed, we study the final
  conditions of the stars
• in the solar ring 8,9 kpc by selecting stars
  in a box of 1 kpc size.

Results
Results
The WD denoising techniques definitely confirm
that branches connecting moving groups are the
dominant structures in the U-V plane. We confirm
the existence of the Sirius, Coma Berenices,
Hyades-Pleiades and Hercules branches. The first
three branches are spaced at intervals of
approximately 15 km s-1, with the Hercules branch
located about 30 km s-1 from the Hyades-Pleiades
branch. All branches present a negative slope of
about -16º in the U-V plane, lower than that
found by Skuljan el at. (1999). We have studied
the density drops in the U-V plane and in the
extremes of the branches. The existence of abrupt
edge lines is confirmed, which definitely rules
out the classic idea of a smooth velocity field
distribution.
Fig. ? Density field in the U-V plane for the
whole observational sample obtained by WD (left),
with the superposition of the classic moving
groups (top right). At bottom right, density
field for the Vrot component obtained integrating
de density in the Urot-Vrot plane.
We confirm an extended age distribution for all
the branches. For the youngest stars, only
Pleiades and Coma Berenices kinematic groups
appear. For stars between 0.1 and 0.5 Gyr, the
Hyades and Sirius structures have begun to turn
out. The extended branch-like shaped of Hercules
is detected in all subsamples with ages gt 2 Gyr
(much smaller than the age of gt 8 Gyr found by
Dehnen 2000 and Bobylev Bajkova 2007). We find
a periodicity in age of about 500-600 Myr in the
Hyades-Pleiades branch, maintained al least in
the 0-2.5 Gyr range. The Pleiades and Hyades
classic moving groups are definitely connected,
suggesting a common mechanism for their formation
in a unique branch.
Fig. ? Left Density field in the Vrot-age plane
of the stars with e_age lt 30 obtained by WD.
Right Integrated density of stars in each branch
worked out using the WD method as a function of
age for stars with e_age lt 30.
Fig. ? Density field in the age-Fe/H plane for
all stars with available metallicities and ages,
and for the different branches of Hercules,
Hyades-Pleiades, Coma Berenices and Sirius,
obtained by WD.
A wide range of metallicity is found for each
branch and a clear correlation between kinematics
and metallicity has been established. The
well-known difference in metallicity between the
Hyades and Pleiades moving groups is reflected in
the bimodality observed for this branch.
Concerning the age-metallicity relation (not
shown here), differences are observed among
branches. The periodic bumps in the age
distribution for the Hyades-Pleiades branch show
a decrease in metallicity with increasing age.
Fig. ? Density field in the Vrot-Fe/H plane
(rotation of -16º of the velocity components) for
the sample with available Fe/H obtained by WD.