Accretion Centers Induced in a Molecular Cloud Core After a Penetrating Collision - Selected Topics of Computational and Experimental Fluid Mechanics - page 482

which are within the sphere of radius r acc , and whose center is the accretion center
itself. All those particles will give up their masses and momenta to the accretion
center. We then change the Gadget 2 type for all those particles, and therefore, they
will not be advanced in time anymore.
We run our implementation to locate accretion centers in the Gadget 2 code by
using only one value for the threshold density, that is cm 3 .

Finally, let us mention that in order to take into account the increase in temperature
due to non-adiabatic core contraction during the gravitational collapse, in this paper
we carry out the simulations using the barotropic equation of state:

10 − 14 g ˁ s = 5 . 0 × /

1

ʳ − 1

ˁ
ˁ crit

c 0 ˁ

p = + , (4)

as proposed by Boss et al. ( 2000 ), where ʳ ≡ 5 / 3 and for the critical density we

10 − 14 gcm − 3 . assume the value ˁ crit = 5 . 0 ×

4 Results

Although the penetrating collisions are clearly a 3 D phenomenon, we here show the
main simulation results using 2 D iso-density plots for a thin slice of matter parallel
to the XY plane. A color scale to distinguish the iso-density regions is set once
the SPH particles defining the slice have been selected. Two numerical values to
illustrate the different stages of the evolution process are included at the bottom of
each iso-density panel: the time t and corresponding peak density

ˁ max . It should be
noted that there is no relation between the density colors associated with different
panels, not even in the same figure.
The collision between the bullet and the target initially generates an agglomerate of
dense particles in the contact region. As the bullet penetrates into the target core, this
agglomerate is thickened by a snowplow effect, as seen as a small filament oriented
perpendicular to the pre-collision velocity direction. At the ends of the filament, two
gas arms develop due to the flow of particles emitted by the collision front, see for
instance the first panels of Figs. 2 , 3 , and 4 .
The depth of penetration of the bullet is proportional to the pre-collision velocity.
For example, in model P 3, the target core gets evenly separated in two parts, as can
be seen in Fig. 4 . However, we note that in models P 1, P 2, and P 3, the viscosity
and self-gravity manage to slow down the penetration and to re-start the gravitational
collapse of the entire collided gas system. We then note that the densest gas region
of the collided system takes the form of a bending filament, which is closely aligned
with the pre-collision velocity direction, see the last panels of Figs. 2 , 3 , and 4 .It
is then likely that these models will still end up as a binary or multiple system of
proto-stars.