24 research outputs found
Prestellar Core Formation, Evolution, and Accretion from Gravitational Fragmentation in Turbulent Converging Flows
We investigate prestellar core formation and accretion based on
three-dimensional hydrodynamic simulations. Our simulations represent local
pc regions within giant molecular clouds where a supersonic turbulent
flow converges, triggering star formation in the post-shock layer. We include
turbulence and self-gravity, applying sink particle techniques, and explore a
range of inflow Mach number . Two sets of cores are identified
and compared: -cores are identified of a time snapshot in each simulation,
representing dense structures in a single cloud map; -cores
are identified at their individual time of collapse, representing the initial
mass reservoir for accretion. We find that cores and filaments form and evolve
at the same time. At the stage of core collapse, there is a well-defined,
converged characteristic mass for isothermal fragmentation that is comparable
to the critical Bonner-Ebert mass at the post-shock pressure. The core mass
functions (CMFs) of -cores show a deficit of high-mass cores
() compared to the observed stellar initial mass function
(IMF). However, the CMFs of -cores are similar to the observed CMFs and
include many low-mass cores that are gravitationally stable. The difference
between -cores and -cores suggests that the full sample
from observed CMFs may not evolve into protostars. Individual sink particles
accrete at a roughly constant rate throughout the simulations, gaining one
-core mass per free-fall time even after the initial mass
reservoir is accreted. High-mass sinks gain proportionally more mass at late
times than low-mass sinks. There are outbursts in accretion rates, resulting
from clumpy density structures falling into the sinks
The X_CO conversion factor from galactic multiphase ISM simulations
CO(J=1-0) line emission is a widely used observational tracer of molecular
gas, rendering essential the X_CO factor, which is applied to convert CO
luminosity to H_2 mass. We use numerical simulations to study how X_CO depends
on numerical resolution, non-steady-state chemistry, physical environment, and
observational beam size. Our study employs 3D magnetohydrodynamics (MHD)
simulations of galactic disks with solar neighborhood conditions, where star
formation and the three-phase interstellar medium (ISM) are self-consistently
regulated by gravity and stellar feedback. Synthetic CO maps are obtained by
post-processing the MHD simulations with chemistry and radiation transfer. We
find that CO is only an approximate tracer of H_2. On parsec scales, W_CO is
more fundamentally a measure of mass-weighted volume density, rather than H_2
column density. Nevertheless, consistent with
observations, insensitive to the evolutionary ISM state or radiation field
strength if steady-state chemistry is assumed. Due to non-steady-state
chemistry, younger molecular clouds have slightly lower X_CO and flatter
profiles of X_CO versus extinction than older ones. The CO-dark H_2 fraction is
26-79 %, anti-correlated with the average extinction. As the observational beam
size increases from 1 pc to 100 pc, X_CO increases by a factor of ~ 2. Under
solar neighborhood conditions, X_CO in molecular clouds is converged at a
numerical resolution of 2 pc. However, the total CO abundance and luminosity
are not converged even at the numerical resolution of 1 pc. Our simulations
successfully reproduce the observed variations of X_CO on parsec scales, as
well as the dependence of X_CO on extinction and the CO excitation temperature.Comment: accepted by Ap
Impact of magneto-rotational instability on grain growth in protoplanetary disks: I. Relevant turbulence properties
Turbulence in the protoplanetary disks induces collisions between dust
grains, and thus facilitates grain growth. We investigate the two fundamental
assumptions of the turbulence in obtaining grain collisional velocities -- the
kinetic energy spectrum and the turbulence autocorrelation time -- in the
context of the turbulence generated by the magneto-rotational instability
(MRI). We carry out numerical simulations of the MRI as well as driven
turbulence, for a range of physical and numerical parameters. We find that the
convergence of the turbulence -parameter does not necessarily imply the
convergence of the energy spectrum. The MRI turbulence is largely solenoidal,
for which we observe a persistent kinetic energy spectrum of . The
same is obtained for solenoidal driven turbulence with and without magnetic
field, over more than 1 dex near the dissipation scale. This power-law slope
appears to be converged in terms of numerical resolution, and to be due to the
bottleneck effect. The kinetic energy in the MRI turbulence peaks at the
fastest growing mode of the MRI. In contrast, the magnetic energy peaks at the
dissipation scale. The magnetic energy spectrum in the MRI turbulence does not
show a clear power-law range, and is almost constant over approximately 1 dex
near the dissipation scale. The turbulence autocorrelation time is nearly
constant at large scales, limited by the shearing timescale, and shows a
power-law drop close to at small scales, with a slope steeper than
that of the eddy crossing time. The deviation from the standard picture of the
Kolmogorov turbulence with the injection scale at the disk scale height can
potentially have a significant impact on the grain collisional velocities.Comment: Accepted by Ap
Photochemistry and Heating/Cooling of the Multiphase Interstellar Medium with UV Radiative Transfer for Magnetohydrodynamic Simulations
We present an efficient heating/cooling method coupled with chemistry and
ultraviolet (UV) radiative transfer, which can be applied to numerical
simulations of the interstellar medium (ISM). We follow the time-dependent
evolution of hydrogen species (H, H, H), assume carbon/oxygen species
(C, C, CO, O, and O) are in formation-destruction balance given the
non-steady hydrogen abundances, and include essential heating/cooling processes
needed to capture thermodynamics of all ISM phases. UV radiation from discrete
point sources and the diffuse background is followed through adaptive ray
tracing and a six-ray approximation, respectively, allowing for H
self-shielding; cosmic ray (CR) heating and ionization are also included. To
validate our methods and demonstrate their application for a range of density,
metallicity, and radiation field, we conduct a series of tests, including the
equilibrium curves of thermal pressure vs. density, the chemical and thermal
structure in photo-dissociation regions, H I-to-H transitions, and the
expansion of H II regions and radiative supernova remnants. Careful treatment
of photochemistry and CR ionization is essential for many aspects of ISM
physics, including identifying the thermal pressure at which cold and warm
neutral phases co-exist. We caution that many current heating and cooling
treatments used in galaxy formation simulations do not reproduce the correct
thermal pressure and ionization fraction in the neutral ISM. Our new model is
implemented in the MHD code Athena and incorporated in the TIGRESS simulation
framework, for use in studying the star-forming ISM in a wide range of
environments.Comment: 57 pages, 22 figures; accepted for publication in ApJ
Coagulation-Fragmentation Equilibrium for Charged Dust: Abundance of Submicron Grains Increases Dramatically in Protoplanetary Disks
Dust coagulation in protoplanetary disks is not straightforward and is
subject to several slow-down mechanisms, such as bouncing, fragmentation and
radial drift to the star. Furthermore, dust grains in UV-shielded disk regions
are negatively charged due to collisions with the surrounding electrons and
ions, which leads to their electrostatic repulsion. For typical disk
conditions, the relative velocities between micron-size grains are small and
their collisions are strongly affected by the repulsion. On the other hand,
collisions between pebble-size grains can be too energetic, leading to grain
fragmentation. The aim of the present paper is to study a combined effect of
the electrostatic and fragmentation barriers on dust evolution. We numerically
solve the Smoluchowski coagulation-fragmentation equation for grains whose
charging occurs under conditions typical for the inner disk regions, where
thermal ionization operates. We find that dust fragmentation efficiently
resupplies the population of small grains under the electrostatic barrier. As a
result, the equilibrium abundance of sub-micron grains is enhanced by several
orders of magnitude compared to the case of neutral dust. For some conditions
with fragmentation velocities m s, macroscopic grains are
completely destroyed.Comment: accepted for publication in Ap
Introducing TIGRESS-NCR: I. Co-Regulation of the Multiphase Interstellar Medium and Star Formation Rates
Massive, young stars are the main source of energy that maintains multiphase
structure and turbulence in the interstellar medium (ISM), and without this
"feedback" the star formation rate (SFR) would be much higher than is observed.
Rapid energy loss in the ISM and efficient energy recovery by stellar feedback
lead to co-regulation of SFRs and the ISM state. Realistic approaches to this
problem should solve the dynamical evolution of the ISM, including star
formation, and the input of feedback energy self-consistently and accurately.
Here, we present the TIGRESS-NCR numerical framework, in which UV radiation,
supernovae, cooling and heating processes, and gravitational collapse are
modeled explicitly. We use an adaptive ray tracing method for UV radiation
transfer from star clusters represented by sink particles, accounting for
attenuation by dust and gas. We solve photon-driven chemical equations to
determine the abundances of H (time-dependent) and C/O-bearing species
(steady-state), which then set cooling and heating rates self-consistently.
Applying these methods, we present high-resolution magnetohydrodynamics
simulations of differentially rotating local galactic disks representing
typical conditions of nearby star-forming galaxies. We analyze ISM properties
and phase distributions and show good agreement with existing multiwavelength
galactic observations. We measure midplane pressure components (turbulent,
thermal, and magnetic) and the weight, demonstrating that vertical dynamical
equilibrium holds. We quantify the ratios of pressure components to the SFR
surface density, which we call the feedback yields. The TIGRESS-NCR framework
will allow for a wide range of parameter exploration, including low metallicity
system.Comment: ApJ submitted. 28 pages, 13 figures excluding Appendi
The environmental dependence of the X_CO conversion factor
CO is the most widely used observational tracer of molecular gas. The
observable CO luminosity is translated to H_2 mass via a conversion factor,
X_CO, which is a source of uncertainty and bias. Despite variations in X_CO,
the empirically-determined solar neighborhood value is often applied across
different galactic environments. To improve understanding of X_CO, we employ 3D
magnetohydrodynamics simulations of the interstellar medium (ISM) in galactic
disks with a large range of gas surface densities, allowing for varying
metallicity, far-ultraviolet (FUV) radiation, and cosmic ray ionization rate
(CRIR). With the TIGRESS simulation framework we model the three-phase ISM with
self-consistent star formation and feedback, and post-process outputs with
chemistry and radiation transfer to generate synthetic CO(1--0) and (2--1)
maps. Our models reproduce the observed CO excitation temperatures,
line-widths, and line ratios in nearby disk galaxies. X_CO decreases with
increasing metallicity, with a power-law slope of -0.8 for the (1--0) line and
-0.5 for the (2--1) line. X_CO also decreases at higher CRIR, and is
insensitive to the FUV radiation. As density increases, X_CO first decreases
due to increasing excitation temperature, and then increases when the emission
is fully saturated. We provide fits between X_CO and observable quantities such
as the line ratio, peak antenna temperature, and line brightness, which probe
local gas conditions. These fits, which allow for varying beam size, may be
used in observations to calibrate out systematic biases. We also provide
estimates of the CO-dark H_2 fraction at different gas surface densities,
observational sensitivities, and beam sizes.Comment: Accepted by Ap
Dust grains cannot grow to millimeter sizes in protostellar envelopes
A big question in the field of star and planet formation is the time at which
substantial dust grain growth occurs. The observed properties of dust emission
across different wavelength ranges have been used as an indication that
millimeter-sized grains are already present in the envelopes of young
protostars. However, this interpretation is in tension with results from
coagulation simulations, which are not able to produce such large grains in
these conditions. In this work, we show analytically that the production of
millimeter-sized grains in protostellar envelopes is impossible under the
standard assumptions about the coagulation process. We discuss several
possibilities that may serve to explain the observed dust emission in the
absence of in-situ grain growth to millimeter sizes.Comment: Accepted to Ap
Herschel/PACS View Of Disks Around Low-Mass Stars And Brown Dwarfs In The TW Hydrae Association
We conducted Herschel/PACS observations of five very low-mass stars or brown dwarfs located in the TW Hya association with the goal of characterizing the properties of disks in the low stellar mass regime. We detected all five targets at 70 mu m and 100 mu m and three targets at 160 mu m. Our observations, combined with previous photometry from 2MASS, WISE, and SCUBA-2, enabled us to construct spectral energy distributions (SEDs) with extended wavelength coverage. Using sophisticated radiative transfer models, we analyzed the observed SEDs of the five detected objects with a hybrid fitting strategy that combines the model grids and the simulated annealing algorithm and evaluated the constraints on the disk properties via the Bayesian inference method. The modeling suggests that disks around low-mass stars and brown dwarfs are generally flatter than their higher mass counterparts, but the range of disk mass extends to well below the value found in T Tauri stars, and the disk scale heights are comparable in both groups. The inferred disk properties (i.e., disk mass, flaring, and scale height) in the low stellar mass regime are consistent with previous findings from large samples of brown dwarfs and very low-mass stars. We discuss the dependence of disk properties on their host stellar parameters and find a significant correlation between the Herschel far-IR fluxes and the stellar effective temperatures, probably indicating that the scaling between the stellar and disk masses (i.e., M-disk proportional to M-star) observed mainly in low-mass stars may extend down to the brown dwarf regime.Natural Science Foundation of Jiangsu Province of China BK20141046Youth Qianren Program of the National Science Foundation of ChinaNational Aeronautics and Space AdministrationStrategic Priority Research Program >The Emergence of Cosmological Structures> of the Chinese Academy of Sciences XDB09000000Astronom
Herschel/PACS view of disks around low-mass stars and brown dwarfs in the TW Hya association
We conducted Herschel/PACS observations of five very low-mass stars or brown
dwarfs located in the TW Hya association with the goal of characterizing the
properties of disks in the low stellar mass regime. We detected all five
targets at and and three targets at
. Our observations, combined with previous photometry from
2MASS, WISE, and SCUBA-2, enabled us to construct SEDs with extended wavelength
coverage. Using sophisticated radiative transfer models, we analyzed the
observed SEDs of the five detected objects with a hybrid fitting strategy that
combines the model grids and the simulated annealing algorithm and evaluated
the constraints on the disk properties via the Bayesian inference method. The
modelling suggests that disks around low-mass stars and brown dwarfs are
generally flatter than their higher mass counterparts, but the range of disk
mass extends to well below the value found in T Tauri stars, and the disk scale
heights are comparable in both groups. The inferred disk properties (i.e., disk
mass, flaring, and scale height) in the low stellar mass regime are consistent
with previous findings from large samples of brown dwarfs and very low-mass
stars. We discuss the dependence of disk properties on their host stellar
parameters and find a significant correlation between the Herschel far-IR
fluxes and the stellar effective temperatures, probably indicating that the
scaling between the stellar and disk masses (i.e., ) observed mainly in low-mass stars may extend down to the brown
dwarf regime.Comment: Accepted for publication in A&