122 research outputs found
A grid of 1D low-mass star formation collapse models
The current study was developed to provide a database of relatively simple
numerical simulations of protostellar collapse, as a template library for
observations of cores and very young protostars, and for researchers who wish
to test their chemical modeling under dynamic astrophysical conditions. It was
also designed to identify statistical trends that may appear when running many
models of the formation of low-mass stars by varying the initial conditions. A
large set of 143 calculations of the gravitational collapse of an isolated
sphere of gas with uniform temperature and a Bonnor-Ebert like density profile
was undertaken using a 1D fully implicit Lagrangian radiation hydrodynamics
code. The parameter space covered initial masses from 0.2 to 8 Msun,
temperatures of 5-30 K and radii between 3000 and 30,000 AU. A spread in the
thermal evolutionary tracks of the runs was found, due to differing initial
conditions and optical depths. Within less than an order of magnitude, all
first and second Larson cores had masses and radii independent of the initial
conditions. The time elapsed between the formation of the first and second
cores was found to strongly depend on the first core mass accretion rate, and
no first core in our grid of models lived for longer than 2000 years, before
the onset of the second collapse. The end product of a protostellar cloud
collapse, the second Larson core, is, at birth, a canonical object with a mass
and radius of about 3 Mjup and 8 Rjup, independent of its initial conditions.
The evolution sequence which brings the gas to stellar densities can however
proceed in a variety of scenarios, on different timescales, along different
isentropes, but each story line can largely be predicted by the initial
conditions. All the data from the simulations are publicly available at this
address: http://starformation.hpc.ku.dk/grid-of-protostars.Comment: 24 pages, 14 figures, accepted for publication in A&
On the role of the H2 ortho:para ratio in gravitational collapse during star formation
Hydrogen molecules (H2) come in two forms in the interstellar medium, ortho-
and para-hydrogen, corresponding to the two different spin configurations of
the two hydrogen atoms. The relative abundances of the two flavours in the
interstellar medium are still very uncertain, and this abundance ratio has a
significant impact on the thermal properties of the gas. In the context of star
formation, theoretical studies have recently adopted two different strategies
when considering the ortho:para ratio (OPR) of H2 molecules; the first
considers the OPR to be frozen at 3:1 while the second assumes that the species
are in thermal equilibrium. As the OPR potentially affects the protostellar
cores which form as a result of the gravitational collapse of a dense molecular
cloud, the aim of this paper is to quantify precisely what role the choice of
OPR plays in the properties and evolution of the cores. We used two different
ideal gas equations of state for a hydrogen and helium mix in a radiation
hydrodynamics code to simulate the collapse of a dense cloud and the formation
of the first and second Larson cores; the first equation of state uses a fixed
OPR of 3:1 while the second assumes thermal equilibrium. Simulations using an
equilibrium ratio collapse faster at early times and show noticeable
oscillations around hydrostatic equilibrium, to the point where the core
expands for a short time right after its formation before resuming its
contraction. In the case of a fixed 3:1 OPR, the core's evolution is a lot
smoother. The OPR was however found to have little impact on the size, mass and
radius of the two Larson cores. We conclude that if one is solely interested in
the final properties of the cores when they are formed, it does not matter
which OPR is used. On the other hand, if one's focus lies primarily in the
evolution of the first core, the choice of OPR becomes important.Comment: 9 pages, 5 figures. Accepted for publication in Astronomy &
Astrophysic
Protostellar birth with ambipolar and ohmic diffusion
The transport of angular momentum is capital during the formation of low-mass
stars; too little removal and rotation ensures stellar densities are never
reached, too much and the absence of rotation means no protoplanetary disks can
form. Magnetic diffusion is seen as a pathway to resolving this long-standing
problem. We investigate the impact of including resistive MHD in simulations of
the gravitational collapse of a 1 solar mass gas sphere, from molecular cloud
densities to the formation of the protostellar seed; the second Larson core. We
used the AMR code RAMSES to perform two 3D simulations of collapsing magnetised
gas spheres, including self-gravity, radiative transfer, and a non-ideal gas
equation of state to describe H2 dissociation which leads to the second
collapse. The first run was carried out under the ideal MHD approximation,
while ambipolar and ohmic diffusion was incorporated in the second calculation.
In the ideal MHD simulation, the magnetic field dominates the energy budget
everywhere inside and around the first core, fueling interchange instabilities
and driving a low-velocity outflow. High magnetic braking removes essentially
all angular momentum from the second core. On the other hand, ambipolar and
ohmic diffusion create a barrier which prevents amplification of the magnetic
field beyond 0.1 G in the first Larson core which is now fully thermally
supported. A significant amount of rotation is preserved and a small
Keplerian-like disk forms around the second core. When studying the radiative
efficiency of the first and second core accretion shocks, we found that it can
vary by several orders of magnitude over the 3D surface of the cores. Magnetic
diffusion is a pre-requisite to star-formation; it enables the formation of
protoplanetary disks in which planets will eventually form, and also plays a
determinant role in the formation of the protostar itself.Comment: 18 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
Ambipolar diffusion in low-mass star formation. I. General comparison with the ideal MHD case
In this paper, we provide a more accurate description of the evolution of the
magnetic flux redistribution during prestellar core collapse by including
resistive terms in the magnetohydrodynamics (MHD) equations. We focus more
particularly on the impact of ambipolar diffusion. We use the adaptive mesh
refinement code RAMSES to carry out such calculations. The resistivities
required to calculate the ambipolar diffusion terms were computed using a
reduced chemical network of charged, neutral and grain species. The inclusion
of ambipolar diffusion leads to the formation of a magnetic diffusion barrier
in the vicinity of the core, preventing accumulation of magnetic flux in and
around the core and amplification of the field above 0.1G. The mass and radius
of the first Larson core remain similar between ideal and non-ideal MHD models.
This diffusion plateau has crucial consequences on magnetic braking processes,
allowing the formation of disk structures. Magnetically supported outflows
launched in ideal MHD models are weakened when using non-ideal MHD. Contrary to
ideal MHD misalignment between the initial rotation axis and the magnetic field
direction does not significantly affect the results for a given mu, showing
that the physical dissipation truly dominate over numerical diffusion. We
demonstrate severe limits of the ideal MHD formalism, which yield unphysical
behaviours in the long-term evolution of the system. This includes counter
rotation inside the outflow, interchange instabilities, and flux redistribution
triggered by numerical diffusion, none observed in non-ideal MHD. Disks with
Keplerian velocity profiles form in all our non-ideal MHD simulations, with
final mass and size which depend on the initial magnetisation. This ranges from
a few 0.01 solar masses and 20-30 au for the most magnetised case (mu=2) to 0.2
solar masses and 40-80 au for a lower magnetisation (mu=5).Comment: Accepted in A&A section 7 (on Wednesday, september the 16th, year
2015
On the role of the H2 ortho:para ratio in gravitational collapse during star formation
Hydrogen molecules (H2) come in two forms in the interstellar medium, ortho- and para-hydrogen, corresponding to the two different spin configurations of the two hydrogen atoms. The relative abundances of the two flavours in the interstellar medium are still very uncertain, and this abundance ratio has a significant impact on the thermal properties of the gas. In the context of star formation, theoretical studies have recently adopted two different strategies when considering the ortho:para ratio (OPR) of H2 molecules; the first considers the OPR to be frozen at 3:1 while the second assumes that the species are in thermal equilibrium. As the OPR potentially affects the protostellar cores which form as a result of the gravitational collapse of a dense molecular cloud, the aim of this paper is to quantify precisely what role the choice of OPR plays in the properties and evolution of the cores. We used two different ideal gas equations of state for a hydrogen and helium mix in a radiation hydrodynamics code to simulate the collapse of a dense cloud and the formation of the first and second Larson cores; the first equation of state uses a fixed OPR of 3:1 while the second assumes thermal equilibrium. Simulations using an equilibrium ratio collapse faster at early times and show noticeable oscillations around hydrostatic equilibrium, to the point where the core expands for a short time right after its formation before resuming its contraction. In the case of a fixed 3:1 OPR, the core's evolution is a lot smoother. The OPR was however found to have little impact on the size, mass and radius of the two Larson cores. We conclude that if one is solely interested in the final properties of the cores when they are formed, it does not matter which OPR is used. On the other hand, if one's focus lies primarily in the evolution of the first core, the choice of OPR becomes important.The research leading to these results has received funding from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007−2013 Grant Agreement No. 247060). K.T. is supported by Japan Society for the Promotion of Science (JSPS) Postdoctoral Fellowship for Research Abroad. The authors would also like to thank the anonymous referee for useful comments
Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: III. X-ray spectral modelling
Following the Swift X-ray observations of the 2006 outburst of the recurrent
nova RS Ophiuchi, we developed hydrodynamical models of mass ejection from
which the forward shock velocities were used to estimate the ejecta mass and
velocity. In order to further constrain our model parameters, here we present
synthetic X-ray spectra from our hydrodynamical calculations which we compare
to the Swift data. An extensive set of simulations was carried out to find a
model which best fits the spectra up to 100 days after outburst. We find a good
fit at high energies but require additional absorption to match the low energy
emission. We estimate the ejecta mass to be in the range (2-5) x 10^{-7} solar
masses and the ejection velocity to be greater than 6000 km/s (and probably
closer to 10,000 km/s). We also find that estimates of shock velocity derived
from gas temperatures via standard model fits to the X-ray spectra are much
lower than the true shock velocities.Comment: 13 pages, 5 figures, Accepted for publication in Ap
Simulations of protostellar collapse using multigroup radiation hydrodynamics. I. The first collapse
Radiative transfer plays a major role in the process of star formation. Many
simulations of gravitational collapse of a cold gas cloud followed by the
formation of a protostellar core use a grey treatment of radiative transfer
coupled to the hydrodynamics. However, dust opacities which dominate extinction
show large variations as a function of frequency. In this paper, we used
frequency-dependent radiative transfer to investigate the influence of the
opacity variations on the properties of Larson's first core. We used a
multigroup M1 moment model in a 1D radiation hydrodynamics code to simulate the
spherically symmetric collapse of a 1 solar mass cloud core. Monochromatic dust
opacities for five different temperature ranges were used to compute Planck and
Rosseland means inside each frequency group. The results are very consistent
with previous studies and only small differences were observed between the grey
and multigroup simulations. For a same central density, the multigroup
simulations tend to produce first cores with a slightly higher radius and
central temperature. We also performed simulations of the collapse of a 10 and
0.1 solar mass cloud, which showed the properties of the first core to be
independent of the initial cloud mass, with again no major differences between
grey and multigroup models. For Larson's first collapse, where temperatures
remain below 2000 K, the vast majority of the radiation energy lies in the IR
regime and the system is optically thick. In this regime, the grey
approximation does a good job reproducing the correct opacities, as long as
there are no large opacity variations on scales much smaller than the width of
the Planck function. The multigroup method is however expected to yield more
important differences in the later stages of the collapse when high energy (UV
and X-ray) radiation is present and matter and radiation are strongly
decoupled.Comment: 9 pages, 5 figures, accepted for publication in A&
Protostellar birth with ambipolar and ohmic diffusion
This is the final version of the article. Available from EDP Sciences via the DOI in this record.The transport of angular momentum is capital during the formation of low-mass stars; too little removal and rotation ensures stellar densities are never reached, too much and the absence of rotation means no protoplanetary disks can form. Magnetic diffusion is seen as a pathway to resolving this long-standing problem. We investigate the impact of including resistive MHD in simulations of the gravitational collapse of a 1 solar mass gas sphere, from molecular cloud densities to the formation of the protostellar seed; the second Larson core. We used the AMR code RAMSES to perform two 3D simulations of collapsing magnetised gas spheres, including self-gravity, radiative transfer, and a non-ideal gas equation of state to describe H2 dissociation which leads to the second collapse. The first run was carried out under the ideal MHD approximation, while ambipolar and ohmic diffusion was incorporated in the second calculation. In the ideal MHD simulation, the magnetic field dominates the energy budget everywhere inside and around the first core, fueling interchange instabilities and driving a low-velocity outflow. High magnetic braking removes essentially all angular momentum from the second core. On the other hand, ambipolar and ohmic diffusion create a barrier which prevents amplification of the magnetic field beyond 0.1 G in the first Larson core which is now fully thermally supported. A significant amount of rotation is preserved and a small Keplerian-like disk forms around the second core. When studying the radiative efficiency of the first and second core accretion shocks, we found that it can vary by several orders of magnitude over the 3D surface of the cores. Magnetic diffusion is a pre-requisite to star-formation; it enables the formation of protoplanetary disks in which planets will eventually form, and also plays a determinant role in the formation of the protostar itself.We are indebted to the anonymous referee for his/her insightful
comments that have vastly improved the solidity of our study, with no
stones left unturned. We also thank Troels Haugbølle for very useful discussions
during the writing of this paper. NV gratefully acknowledges support from the
European Commission through the Horizon 2020 Marie Skłodowska-Curie Actions
Individual Fellowship 2014 programme (Grant Agreement no. 659706).
The research leading to these results has also received funding from the European
Research Council under the European Community’s Seventh Framework
Programme (FP7/2007-2013 Grant Agreement no. 247060). We acknowledge
financial support from "Programme National de Physique Stellaire" (PNPS) of
CNRS/INSU, CEA and CNES, France. This work was granted access to the
HPC resources of CINES (Occigen) under the allocation 2016-047247 made by
GENCI. We also made use of the astrophysics HPC facility at the University of
Copenhagen, which is supported by a research grant (VKR023406) from Villum
Fonden. In addition, we thank the Service d’Astrophysique, IRFU, CEA Saclay,
and the Laboratoire Astrophysique Instrumentation Modélisation, France, for
granting us access to the supercomputer IRFUCOAST where the groundwork with
many test calculations were performed. All the figures were created using the
OSIRIS8 visualization package for RAMSES, except Fig. 4 which was rendered
with the PARAVIEW9 software
Multigroup radiation hydrodynamics with flux-limited diffusion and adaptive mesh refinement
International audienceContext. Radiative transfer plays a crucial role in the star formation process. Because of the high computational cost, radiation-hydrodynamics simulations performed up to now have mainly been carried out in the grey approximation. In recent years, multifrequency radiation-hydrodynamics models have started to be developed in an attempt to better account for the large variations in opacities as a function of frequency.Aims. We wish to develop an efficient multigroup algorithm for the adaptive mesh refinement code RAMSES which is suited to heavy proto-stellar collapse calculations.Methods. Because of the prohibitive timestep constraints of an explicit radiative transfer method, we constructed a time-implicit solver based on a stabilized bi-conjugate gradient algorithm, and implemented it in RAMSES under the flux-limited diffusion approximation.Results. We present a series of tests that demonstrate the high performance of our scheme in dealing with frequency-dependent radiation-hydrodynamic flows. We also present a preliminary simulation of a 3D proto-stellar collapse using 20 frequency groups. Differences between grey and multigroup results are briefly discussed, and the large amount of information this new method brings us is also illustrated.Conclusions. We have implemented a multigroup flux-limited diffusion algorithm in the RAMSES code. The method performed well against standard radiation-hydrodynamics tests, and was also shown to be ripe for exploitation in the computational star formation context
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