36,911 research outputs found
Chemical evolution in the early phases of massive star formation II: Deuteration
The chemical evolution in high-mass star-forming regions is still poorly
constrained. Studying the evolution of deuterated molecules allows to
differentiate between subsequent stages of high-mass star formation regions due
to the strong temperature dependence of deuterium isotopic fractionation. We
observed a sample of 59 sources including 19 infrared dark clouds, 20 high-mass
protostellar objects, 11 hot molecular cores and 9 ultra-compact HII regions in
the (3-2) transitions of the four deuterated molecules, DCN, DNC, DCO+ and N2D+
as well as their non-deuterated counterpart. The overall detection fraction of
DCN, DNC and DCO+ is high and exceeds 50% for most of the stages. N2D+ was only
detected in a few infrared dark clouds and high-mass protostellar objects. It
can be related to problems in the bandpass at the frequency of the transition
and to low abundances in the more evolved, warmer stages. We find median D/H
ratios of ~0.02 for DCN, ~0.005 for DNC, ~0.0025 for DCO+ and ~0.02 for N2D+.
While the D/H ratios of DNC, DCO+ and N2D+ decrease with time, DCN/HCN peaks at
the hot molecular core stage. We only found weak correlations of the D/H ratios
for N2D+ with the luminosity of the central source and the FWHM of the line,
and no correlation with the H2 column density. In combination with a previously
observed set of 14 other molecules (Paper I) we fitted the calculated column
densities with an elaborate 1D physico-chemical model with time-dependent
D-chemistry including ortho- and para-H2 states. Good overall fits to the
observed data have been obtained the model. It is one of the first times that
observations and modeling have been combined to derive chemically based
best-fit models for the evolution of high-mass star formation including
deuteration.Comment: 26 pages, 16 figures, accepted at A&
Supernova Simulations from a 3D Progenitor Model -- Impact of Perturbations and Evolution of Explosion Properties
We study the impact of large-scale perturbations from convective shell
burning on the core-collapse supernova explosion mechanism using
three-dimensional (3D) multi-group neutrino hydrodynamics simulations of an 18
solar mass progenitor. Seed asphericities in the O shell, obtained from a
recent 3D model of O shell burning, help trigger a neutrino-driven explosion
330ms after bounce whereas the shock is not revived in a model based on a
spherically symmetric progenitor for at least another 300ms. We tentatively
infer a reduction of the critical luminosity for shock revival by ~20% due to
pre-collapse perturbations. This indicates that convective seed perturbations
play an important role in the explosion mechanism in some progenitors. We
follow the evolution of the 18 solar mass model into the explosion phase for
more than 2s and find that the cycle of accretion and mass ejection is still
ongoing at this stage. With a preliminary value of 0.77 Bethe for the
diagnostic explosion energy, a baryonic neutron star mass of 1.85 solar masses,
a neutron star kick of ~600km/s and a neutron star spin period of ~20ms at the
end of the simulation, the explosion and remnant properties are slightly
atypical, but still lie comfortably within the observed distribution. Although
more refined simulations and a larger survey of progenitors are still called
for, this suggests that a solution to the problem of shock revival and
explosion energies in the ballpark of observations are within reach for
neutrino-driven explosions in 3D.Comment: 23 pages, 22 figures, accepted for publication in MNRA
Instability of a stalled accretion shock: evidence for the advective-acoustic cycle
We analyze the linear stability of a stalled accretion shock in a perfect gas
with a parametrized cooling function L ~ rho^{beta-alpha} P^alpha. The
instability is dominated by the l=1 mode if the shock radius exceeds 2-3 times
the accretor radius, depending on the parameters of the cooling function. The
growth rate and oscillation period are comparable to those observed in the
numerical simulations of Blondin & Mezzacappa (2006). The instability mechanism
is analyzed by separately measuring the efficiencies of the purely acoustic
cycle and the advective-acoustic cycle. These efficiencies are estimated
directly from the eigenspectrum, and also through a WKB analysis in the high
frequency limit. Both methods prove that the advective-acoustic cycle is
unstable, and that the purely acoustic cycle is stable. Extrapolating these
results to low frequency leads us to interpret the dominant mode as an
advective-acoustic instability, different from the purely acoustic
interpretation of Blondin & Mezzacappa (2006). A simplified characterization of
the instability is proposed, based on an advective-acoustic cycle between the
shock and the radius r_nabla where the velocity gradients of the stationary
flow are strongest. The importance of the coupling region in this mechanism
calls for a better understanding of the conditions for an efficient
advective-acoustic coupling in a decelerated, nonadiabatic flow, in order to
extend these results to core-collapse supernovae.Comment: 29 pages, 18 figures, to appear in ApJ (1 new Section, 2 new Figures
Rotation-supported Neutrino-driven Supernova Explosions in Three Dimensions and the Critical Luminosity Condition
We present the first self-consistent, three-dimensional (3D) core-collapse
supernova simulations performed with the Prometheus-Vertex code for a rotating
progenitor star. Besides using the angular momentum of the 15 solar-mass model
as obtained in the stellar evolution calculation with an angular frequency of
about 0.001 rad/s (spin period of more than 6000 s) at the Si/Si-O interface,
we also computed 2D and 3D cases with no rotation and with a ~300 times shorter
rotation period and different angular resolutions. In 2D, only the nonrotating
and slowly rotating models explode, while rapid rotation prevents an explosion
within 500 ms after bounce because of lower radiated neutrino luminosities and
mean energies and thus reduced neutrino heating. In contrast, only the fast
rotating model develops an explosion in 3D when the Si/Si-O interface collapses
through the shock. The explosion becomes possible by the support of a powerful
SASI spiral mode, which compensates for the reduced neutrino heating and pushes
strong shock expansion in the equatorial plane. Fast rotation in 3D leads to a
"two-dimensionalization" of the turbulent energy spectrum (yielding roughly a
-3 instead of a -5/3 power-law slope at intermediate wavelengths) with enhanced
kinetic energy on the largest spatial scales. We also introduce a
generalization of the "universal critical luminosity condition" of Summa et al.
(2016) to account for the effects of rotation, and demonstrate its viability
for a set of more than 40 core-collapse simulations including 9 and 20
solar-mass progenitors as well as black-hole forming cases of 40 and 75
solar-mass stars to be discussed in forthcoming papers.Comment: 24 pages, 19 figures; refereed version with additional section on
resolution dependence; accepted by Ap
A two-parameter criterion for classifying the explodability of massive stars by the neutrino-driven mechanism
Thus far, judging the fate of a massive star (either a neutron star (NS) or a
black hole) solely by its structure prior to core collapse has been ambiguous.
Our work and previous attempts find a non-monotonic variation of successful and
failed supernovae with zero-age main-sequence mass, for which no single
structural parameter can serve as a good predictive measure. However, we
identify two parameters computed from the pre-collapse structure of the
progenitor, which in combination allow for a clear separation of exploding and
non-exploding cases with only few exceptions (~1-2.5%) in our set of 621
investigated stellar models. One parameter is M4, defining the normalized
enclosed mass for a dimensionless entropy per nucleon of s=4, and the other is
mu4 = d(m/M_sun)/d(r/1000 km) at s=4, being the normalized mass-derivative at
this location. The two parameters mu4 and M4*mu4 can be directly linked to the
mass-infall rate, Mdot, of the collapsing star and the electron-type neutrino
luminosity of the accreting proto-NS, L_nue ~ M_ns*Mdot, which play a crucial
role in the "critical luminosity" concept for the theoretical description of
neutrino-driven explosions as runaway phenomenon of the stalled accretion
shock. All models were evolved employing the approach of Ugliano et al. for
simulating neutrino-driven explosions in spherical symmetry. The neutrino
emission of the accretion layer is approximated by a gray transport solver,
while the uncertain neutrino emission of the 1.1 M_sun proto-NS core is
parametrized by an analytic model. The free parameters connected to the
core-boundary prescription are calibrated to reproduce the observables of
Supernova 1987A for five different progenitor models.Comment: 23 pages, 12 figures; accepted by ApJ; revised version considerably
enlarged (Fig. 7 and Sect.3.6 added
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