299 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
The relative and absolute ages of old globular clusters in the LCDM framework
Old Globular Clusters (GCs) in the Milky Way have ages of about 13 Gyr,
placing their formation time in the reionization epoch. We propose a novel
scenario for the formation of these systems based on the merger of two or more
atomic cooling halos at high-redshift (z>6). First generation stars are formed
as an intense burst in the center of a minihalo that grows above the threshold
for hydrogen cooling (halo mass M_h~10^8 Msun) by undergoing a major merger
within its cooling timescale (~150 Myr). Subsequent minor mergers and sustained
gas infall bring new supply of pristine gas at the halo center, creating
conditions that can trigger new episodes of star formation. The dark-matter
halo around the GC is then stripped during assembly of the host galaxy halo.
Minihalo merging is efficient only in a short redshift window, set by the LCDM
parameters, allowing us to make a strong prediction on the age distribution for
old GCs. From cosmological simulations we derive an average merging redshift
=9 and narrow distribution Dz=2, implying average GC age =13.0+/-0.2
Gyr including ~0.2 Gyr of star formation delay. Qualitatively, our scenario
reproduces other general old GC properties (characteristic masses and number of
objects, metallicity versus galactocentric radius anticorrelation, radial
distribution), but unlike age, these generally depend on details of baryonic
physics. In addition to improved age measurements, direct validation of the
model at z~10 may be within reach of ultradeep gravitationally lensed
observations with the James Webb Space Telescope.Comment: ApJL accepted, minor content changes to highlight the robust
predictions for GC ages; for an animated version of Fig. 1 (minihalo mergers
movie) see
http://www.ph.unimelb.edu.au/~mtrenti/trenti_padoan_jimenez_2015_fig1_GC_movie.gi
- …