285 research outputs found
H2 distribution during 2-phase Molecular Cloud Formation
We performed high-resolution, 3D MHD simulations and we compared to
observations of translucent molecular clouds. We show that the observed
populations of rotational levels of H2 can arise as a consequence of the
multi-phase structure of the ISM.Comment: 2 pages, 1 figure. Due to appear in the proceedings of the 6th
Zermatt ISM Symposium: "Conditions and Impact of Star Formation: From Lab to
Space
Temporal evolution of magnetic molecular shocks II. Analytics of the steady state and semi-analytical construction of intermediate ages
In the first paper of this series (Paper I) we computed time dependent
simulations of multifluid shocks with chemistry and a transverse magnetic field
frozen in the ions, using an adaptive moving grid. In this paper, we present
new analytical results on steady-state molecular shocks. Relationships between
density and pressure in the neutral fluid are derived for the cold magnetic
precursor, hot magnetic precursor, adiabatic shock front, and the following
cooling layer. The compression ratio and temperature behind a fully
dissociative adiabatic shock is also derived. To prove that these results may
even hold for intermediate ages, we design a test to locally characterise the
validity of the steady state equations in a time-dependent shock simulation.
Applying this tool to the results of Paper I, we show that most of these shocks
(all the stable ones) are indeed in a quasi-steady state at all times, i.e. : a
given snapshot is composed of one or more truncated steady shock. Finally, we
use this property to produce a construction method of any intermediate time of
low velocity shocks (u < 20 km/s) with only a steady-state code. In particular,
this method allows one to predict the occurrence of steady CJ-type shocks more
accurately than previously proposed criteria.Comment: A&A in pres
Temporal evolution of magnetic molecular shocks I. Moving grid simulations
We present time-dependent 1D simulations of multifluid magnetic shocks with
chemistry resolved down to the mean free path. They are obtained with an
adaptive moving grid implemented with an implicit scheme. We examine a broad
range of parameters relevant to conditions in dense molecular clouds, with
preshock densities between 10^3 and 10^5 cm-3, velocities between 10 and 40
km/s, and three different scalings for the transverse magnetic field: B=0,0.1,1
\mu G \sqrt{n.cm3}. We first use this study to validate the results of
Chi\`eze, Pineau des For\^ets & Flower (1998), in particular the long delays
necessary to obtain steady C-type shocks, and we provide evolutionary
time-scales for a much greater range of parameters. We also present the first
time-dependent models of dissociative shocks with a magnetic precursor,
including the first models of stationary CJ shocks in molecular conditions. We
find that the maximum speed for steady C-type shocks is reached before the
occurrence of a sonic point in the neutral fluid, unlike previously thought. As
a result, the maximum speed for C-shocks is lower than previously believed.
Finally, we find a large amplitude bouncing instability in J-type fronts near
the H2 dissociation limit (u ~ 25-30 km/s), driven by H2
dissociation/reformation. At higher speeds, we find an oscillatory behaviour of
short period and small amplitude linked to collisional ionisation of H. Both
instabilities are suppressed after some time when a magnetic field is present.
In a companion paper, we use the present simulations to validate a new
semi-analytical construction method for young low-velocity magnetic shocks
based on truncated steady-state models.Comment: A&A in pres
Neutronization During Type Ia Supernova Simmering
Prior to the incineration of a white dwarf (WD) that makes a Type Ia
supernova (SN Ia), the star "simmers" for ~1000 years in a convecting, carbon
burning region. We have found that weak interactions during this time increase
the neutron excess by an amount that depends on the total quantity of carbon
burned prior to the explosion. This contribution is in addition to the
metallicity (Z) dependent neutronization through the 22Ne abundance (as studied
by Timmes, Brown, & Truran). The main consequence is that we expect a floor to
the level of neutronization that dominates over the metallicity contribution
when Z/Z_\odot<2/3, and it can be important for even larger metallicities if
substantial energy is lost to neutrinos via the convective Urca process. This
would mask any correlations between SN Ia properties and galactic environments
at low metallicities. In addition, we show that recent observations of the
dependences of SNe Ia on galactic environments make it clear that metallicity
alone cannot provide for the full observed diversity of events.Comment: Accepted for publication in The Astrophysical Journal, 5 pages, 4
figure
A two-dimensional mixing length theory of convective transport
The helioseismic observations of the internal rotation profile of the Sun
raise questions about the two-dimensional (2D) nature of the transport of
angular momentum in stars. Here we derive a convective prescription for
axisymmetric (2D) stellar evolution models. We describe the small scale motions
by a spectrum of unstable linear modes in a Boussinesq fluid. Our saturation
prescription makes use of the angular dependence of the linear dispersion
relation to estimate the anisotropy of convective velocities. We are then able
to provide closed form expressions for the thermal and angular momentum fluxes
with only one free parameter, the mixing length.
We illustrate our prescription for slow rotation, to first order in the
rotation rate. In this limit, the thermodynamical variables are spherically
symetric, while the angular momentum depends both on radius and latitude. We
obtain a closed set of equations for stellar evolution, with a self-consistent
description for the transport of angular momentum in convective regions. We
derive the linear coefficients which link the angular momentum flux to the
rotation rate (- effect) and its gradient (-effect). We
compare our results to former relevant numerical work.Comment: MNRAS accepted, 10 pages, 1 figure, version prior to language editio
The C-flash and the ignition conditions of type Ia supernovae
Thanks to a stellar evolution code able to compute through the
C-flash we link the binary population synthesis of single degenerate
progenitors of type Ia supernovae (SNe Ia) to their physical condition at the
time of ignition. We show that there is a large range of possible ignition
densities and we detail how their probability distribution depends on the
accretion properties. The low density peak of this distribution qualitatively
reminds of the clustering of the luminosities of Branch-normal SNe Ia. We
tighten the possible range of initial physical conditions for explosion models:
they form a one-parameter family, independent of the metallicity. We discuss
how these results may be modified if we were to relax our hypothesis of a
permanent Hachisu wind or if we were to include electron captures.Comment: 10 pages, 14 figures, MNRAS accepte
Dense molecular globulettes and the dust arc towards the runaway O star AE Aur (HD 34078)
Some runaway stars are known to display IR arc-like structures around them,
resulting from their interaction with surrounding interstellar material. The
properties of these features as well as the processes involved in their
formation are still poorly understood. We aim at understanding the physical
mechanisms that shapes the dust arc observed near the runaway O star AEAur
(HD34078). We obtained and analyzed a high spatial resolution map of the
CO(1-0) emission that is centered on HD34078, and that combines data from both
the IRAM interferometer and 30m single-dish antenna. The line of sight towards
HD34078 intersects the outer part of one of the detected globulettes, which
accounts for both the properties of diffuse UV light observed in the field and
the numerous molecular absorption lines detected in HD34078's spectra,
including those from highly excited H2 . Their modeled distance from the star
is compatible with the fact that they lie on the 3D paraboloid which fits the
arc detected in the 24 {\mu}m Spitzer image. Four other compact CO globulettes
are detected in the mapped area. These globulettes have a high density and
linewidth, and are strongly pressure-confined or transient. The good spatial
correlation between the CO globulettes and the IR arc suggests that they result
from the interaction of the radiation and wind emitted by HD 34078 with the
ambient gas. However, the details of this interaction remain unclear. A wind
mass loss rate significantly larger than the value inferred from UV lines is
favored by the large IR arc size, but does not easily explain the low velocity
of the CO globulettes. The effect of radiation pressure on dust grains also
meets several issues in explaining the observations. Further observational and
theoretical work is needed to fully elucidate the processes shaping the gas and
dust in bow shocks around runaway O stars. (Abridged)Comment: Accepted for publication in Astronomy & Astrophysic
Uncertainties and robustness of the ignition process in type Ia supernovae
It is widely accepted that the onset of the explosive carbon burning in the
core of a CO WD triggers the ignition of a SN Ia. The features of the ignition
are among the few free parameters of the SN Ia explosion theory. We explore the
role for the ignition process of two different issues: firstly, the ignition is
studied in WD models coming from different accretion histories. Secondly, we
estimate how a different reaction rate for C-burning can affect the ignition.
Two-dimensional hydrodynamical simulations of temperature perturbations in the
WD core ("bubbles") are performed with the FLASH code. In order to evaluate the
impact of the C-burning reaction rate on the WD model, the evolution code
FLASH_THE_TORTOISE from Lesaffre et al. (2006) is used. In different WD models
a key role is played by the different gravitational acceleration in the
progenitor's core. As a consequence, the ignition is disfavored at a large
distance from the WD center in models with a larger central density, resulting
from the evolution of initially more massive progenitors. Changes in the C
reaction rate at T < 5e8 K slightly influence the ignition density in the WD
core, while the ignition temperature is almost unaffected. Recent measurements
of new resonances in the C-burning reaction rate (Spillane et al. 2007) do not
affect the core conditions of the WD significantly. This simple analysis,
performed on the features of the temperature perturbations in the WD core,
should be extended in the framework of the state-of-the-art numerical tools for
studying the turbulent convection and ignition in the WD core. Future
measurements of the C-burning reactions cross section at low energy, though
certainly useful, are not expected to affect dramatically our current
understanding of the ignition process.Comment: 7 pages, 5 figures, A&A accepte
Effects of turbulent diffusion on the chemistry of diffuse clouds
Aims. We probe the effect of turbulent diffusion on the chemistry at the
interface between a cold neutral medium (CNM) cloudlet and the warm neutral
medium (WNM). Methods. We perform moving grid, multifluid, 1D, hydrodynamical
simulations with chemistry including thermal and chemical diffusion. The
diffusion coefficients are enhanced to account for turbulent diffusion. We
post-process the steady-states of our simulations with a crude model of
radiative transfer to compute line profiles. Results. Turbulent diffusion
spreads out the transition region between the CNM and the WNM. We find that the
CNM slightly expands and heats up: its CH and H content decreases due to
the lower density. The change of physical conditions and diffusive transport
increase the H content in the CNM which results in increased OH and HO.
Diffusion transports some CO out of the CNM. It also brings H into contact
with the warm gas with enhanced production of CH, H, OH and HO at
the interface. O lines are sensitive to the spread of the thermal profile in
the intermediate region between the CNM and the WNM. Enhanced molecular content
at the interface of the cloud broadens the molecular line profiles and helps
exciting transitions of intermediate energy. The relative molecular yield are
found higher for bigger clouds. Conclusions. Turbulent diffusion can be the
source of additional molecular production and should be included in chemical
models of the interstellar medium (ISM). It also is a good candidate for the
interpretation of observational problems such as warm H, CH formation
and presence of H.Comment: 13 pages, 23 figures, A&A accepte
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