278 research outputs found
The observable prestellar phase of the IMF
The observed similarities between the mass function of prestellar cores (CMF)
and the stellar initial mass function (IMF) have led to the suggestion that the
IMF is already largely determined in the gas phase. However, theoretical
arguments show that the CMF may differ significantly from the IMF. In this
Letter, we study the relation between the CMF and the IMF, as predicted by the
IMF model of Padoan and Nordlund. We show that 1) the observed mass of
prestellar cores is on average a few times smaller than that of the stellar
systems they generate; 2) the CMF rises monotonically with decreasing mass,
with a noticeable change in slope at approximately 3-5 solar masses, depending
on mean density; 3) the selection of cores with masses larger than half their
Bonnor-Ebert mass yields a CMF approximately consistent with the system IMF,
rescaled in mass by the same factor as our model IMF, and therefore suitable to
estimate the local efficiency of star formation, and to study the dependence of
the IMF peak on cloud properties; 4) only one in five pre-brown-dwarf core
candidates is a true progenitor to a brown dwarf.Comment: ApJ Letters, accepte
Turbulence-Induced Relative Velocity of Dust Particles II: The Bidisperse Case
We extend our earlier work on turbulence-induced relative velocity between
equal-size particles (Pan and Padoan, Paper I) to particles of arbitrarily
different sizes. The Pan and Padoan (PP10) model shows that the relative
velocity between different particles has two contributions, named the
generalized shear and acceleration terms, respectively. The generalized shear
term represents the particles' memory of the spatial flow velocity difference
across the particle distance in the past, while the acceleration term is
associated with the temporal flow velocity difference on individual particle
trajectories. Using the simulation of Paper I, we compute the root-mean-square
relative velocity, ^1/2, as a function of the friction times, tau_p1 and
tau_p2, of the two particles, and show that the PP10 prediction is in
satisfactory agreement with the data, confirming its physical picture. For a
given tau_p1 below the Lagrangian correlation time of the flow, T_L, ^1/2
as a function of tau_p2 shows a dip at tau_p2~tau_p1, indicating tighter
velocity correlation between similar particles. Defining a ratio
f=tau_pl/tau_ph, with tau_pl and tau_ph the friction times of the smaller and
larger particles, we find that ^1/2 increases with decreasing f due to the
generalized acceleration contribution, which dominates at f<1/4. At a fixed f,
our model predicts that ^1/2 scales as tau_ph^1/2 for tau_p,h in the
inertial range of the flow, stays roughly constant for T_L <tau_ph < T_L/f, and
finally decreases as tau_ph^-1/2 for tau_ph>>T_L/f. The acceleration term is
independent of the particle distance, r, and thus reduces the r-dependence of
^1/2 in the bidisperse case.Comment: 23 pages, 12 figures, Accepted to Ap
On star formation in primordial protoglobular clouds
Using a new physical model for star formation (Padoan 1995) we have tested
the possibility that globular clusters (GCs) are formed from primordial mass
fluctuations, whose mass scale ( - M) is selected out of
a CDM spectrum by the mechanism of non-equilibrium formation of . We show
that such clouds are able to convert about 0.003 of their total mass into a
bound system (GC) and about 0.02 into halo stars. The metal enriched gas is
dispersed away from the GC by supernova explosions and forms the galactic disk.
These mass ratios between GCs, halo and disk depend on the predicted IMF which
is a consequence of the universal statistics of fluid turbulence. They also
depend on the ratio of baryonic over non-baryonic mass ,, and are
comparable with the values observed in typical spiral galaxies for . The computed mass and radius for a GC ( M
and 30 pc) are in good agreement with the average values in the Galaxy. The
model predicts an exponential cut off in the stellar IMF below 0.1 M
in GCs and 0.6 M in the halo. The quite massive star formation in
primordial clouds leads to a large number of supernovae and to a high blue
luminosity during the first two Gyr of the life of every galaxy
Infall-Driven Protostellar Accretion and the Solution to the Luminosity Problem
We investigate the role of mass infall in the formation and evolution of
protostars. To avoid ad hoc initial and boundary conditions, we consider the
infall resulting self-consistently from modeling the formation of stellar
clusters in turbulent molecular clouds. We show that infall rates in turbulent
clouds are comparable to accretion rates inferred from protostellar
luminosities or measured in pre-main-sequence stars. They should not be
neglected in modeling the luminosity of protostars and the evolution of disks,
even after the embedded protostellar phase. We find large variations of infall
rates from protostar to protostar, and large fluctuations during the evolution
of individuals protostars. In most cases, the infall rate is initially of order
10\msun\ yr, and may either decay rapidly in the formation of
low-mass stars, or remain relatively large when more massive stars are formed.
The simulation reproduces well the observed characteristic values and scatter
of protostellar luminosities and matches the observed protostellar luminosity
function. The luminosity problem is therefore solved once realistic
protostellar infall histories are accounted for, with no need for extreme
accretion episodes. These results are based on a simulation of randomly-driven
magneto-hydrodynamic turbulence on a scale of 4pc, including self-gravity,
adaptive-mesh refinement to a resolution of 50AU, and accreting sink particles.
The simulation yields a low star formation rate, consistent with the
observations, and a mass distribution of sink particles consistent with the
observed stellar initial mass function during the whole duration of the
simulation, forming nearly 1,300 sink particles over 3.2 Myr.Comment: 21 pages, 16 figures, accepted for publication in Ap
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