FURTHER CONSIDERATIONS
ON THE FORMATION OF THE
β PICTORIS MOVING GROUP
V.G. Ortega1 , R. de la Reza1 , E. Jilinski1,2 & B. Bazzanella1
1 Observatório Nacional/MCT, Rio de Janeiro, Brazil
2 Pulkovo Observatory, St. Petersburg, Russia
[email protected]
Abstract
1.
Some further developments concerning Ortega et al. (2002) work in the
β Pictoris Moving Group are reported. Results concerning the origin
and structure of that group are presented.
Introduction
Treatment of 3D orbits of stars in the Galaxy appears as a powerful
tool to determine the dynamical age and to investigate the formation
conditions of low mass Post-T Tauri stars (PTTS).
In our methodology we consider the present observed positions and
space velocities of all the members of a given group of PTTS and trace
back in time the motion of each star under the action of the general
Galactic potential.
The model of the Galactic potential that we used consists of three
components: disk (Miyamoto-Nagai), bulge (Plummer) and halo (logarithmic) (Hoogerwerf, de Bruijne & de Zeeuw (2001)).
The region in space where the 3D orbits have for the first time the
larger confinement is considered as the forming place of the group. In
other words it is the 3D size of the smallest resulting configuration.
The interval of time between the present observed configuration and
that of the minimum configuration is considered the dynamical age of
the group. Due to the relative short ages involved, we expect the stars
motions to be affected only by the Galactic potential.
1
2
This approach was applied to investigate the origin of the β Pictoris
moving group (BPMG) in Ortega et al.(2002). Here we present some
further developments of this work.
2.
β Pictoris and Sco-Cen subgroups
BPMG is the nearest PTTS association with a mean distance of ∼ 36
pc and is formed by 19 low mass star systems (Zuckerman et al. (2001)).
These authors find an evolutionary age of ∼ 12 Myr. Our dynamical
method gives an age of ∼ 11.5 Myr. This coincidence of two completely
different and independent age determinations is a strong indication that
BPMG is coeval and was formed as an unbound group.
A supernova explosion taking place at some critical distance between
10 and 100 pc from the natal cloud may be assumed as a probable trigger
mechanism of star formation.
In the case of BPMG, as may be also the case of the nearby association
TW Hya, such supernova events originate most probably in the nearby
Sco-Cen subgroups: Lower Centaurus Crux (LCC) and Upper Centaurus
Lupus (UCL). For a supernova to be the mechanism giving rise to star
formation in BPMG, the exploding O star must be placed at the critical
distance when it was ∼ 5 Myr old. This time interval is necessary for
the massive star to originate a supernova.
In order to estimate the critical distance we trace back the orbits of
the stars belonging to the LCC and UCL subgroups. To do that, we
take from de Zeeuw et al. (1999) compilation those stars with observed
radial velocity. The orbit back-tracing is performed down to HR ages
for LCC and UCL estimated to be between 16 − 20 Myr (Mamajek et
al. 2002, Sartori et al. 2003). In this way, we can follow the evolution
of the LCC and UCL centroids. Our calculations are consistent with a
supernova event taking place at the UCL subgroup as a probable source
of star formation in BPMG.
In the case of BPMG and considering the high quality of the observed
distances (all BPMG members were observed by Hipparcos) we explored
a new approach based on a closer examination of star orbits. Tracing
back the motions of nineteen BPMG star members shows a first more
concentrated structure formed by ten of them including the β Pictoris
star. It turns out that this concentration occurs at ∼ 11.2 Myr. A second
sparse region is occupied by the remaining nine stars. These do not
form any additional subgroup of different dynamical age but distribute
relative to the centroid of the first substructure. Such a structure can
be seen as a kind of “photography” of the star forming regions in the
BPMG natal cloud.
β Pictoris Moving Group (Ortega et al.)
3.
3
Conclusions
Following the same method adopted in Ortega et al. (2002) we have
shown that a supernova event, most probably in the Upper Centaurus
Lupus subgroup of Scorpio- Centaurus association, may be at the origin
of the β Pictoris moving group. We also show that this group presents a
structure in which a subgroup of ten stars are concentrated in a volume
of 18 pc radius at ∼ 11.2 Myr years while the other star systems form a
more extended region relative to the center of the previous grouping.
Acknowledgments
E.J. acknowledges financial support of FAPERJ.
References
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Vladimir Ortega (right), Evgueni Jilinski (center), Giovanni Pinzón (left).