554 research outputs found
Nonlinear hyperbolic systems: Non-degenerate flux, inner speed variation, and graph solutions
We study the Cauchy problem for general, nonlinear, strictly hyperbolic
systems of partial differential equations in one space variable. First, we
re-visit the construction of the solution to the Riemann problem and introduce
the notion of a nondegenerate (ND) system. This is the optimal condition
guaranteeing, as we show it, that the Riemann problem can be solved with
finitely many waves, only; we establish that the ND condition is generic in the
sense of Baire (for the Whitney topology), so that any system can be approached
by a ND system. Second, we introduce the concept of inner speed variation and
we derive new interaction estimates on wave speeds. Third, we design a wave
front tracking scheme and establish its strong convergence to the entropy
solution of the Cauchy problem; this provides a new existence proof as well as
an approximation algorithm. As an application, we investigate the
time-regularity of the graph solutions introduced by the second author,
and propose a geometric version of our scheme; in turn, the spatial component
of a graph solution can be chosen to be continuous in both time and space,
while its component is continuous in space and has bounded variation in
time.Comment: 74 page
First detection of CF+ towards a high-mass protostar
We report the first detection of the J = 1 - 0 (102.6 GHz) rotational lines
of CF+ (fluoromethylidynium ion) towards CygX-N63, a young and massive
protostar of the Cygnus X region. This detection occurred as part of an
unbiased spectral survey of this object in the 0.8-3 mm range, performed with
the IRAM 30m telescope. The data were analyzed using a local thermodynamical
equilibrium model (LTE model) and a population diagram in order to derive the
column density. The line velocity (-4 km s-1) and line width (1.6 km s-1)
indicate an origin from the collapsing envelope of the protostar.
We obtain a CF+ column density of 4.10e11 cm-2. The CF+ ion is thought to be
a good tracer for C+ and assuming a ratio of 10e-6 for CF+/C+, we derive a
total number of C+ of 1.2x10e53 within the beam. There is no evidence of carbon
ionization caused by an exterior source of UV photons suggesting that the
protostar itself is the source of ionization. Ionization from the protostellar
photosphere is not efficient enough. In contrast, X-ray ionization from the
accretion shock(s) and UV ionization from outflow shocks could provide a large
enough ionizing power to explain our CF+ detection.
Surprisingly, CF+ has been detected towards a cold, massive protostar with no
sign of an external photon dissociation region (PDR), which means that the only
possibility is the existence of a significant inner source of C+. This is an
important result that opens interesting perspectives to study the early
development of ionized regions and to approach the issue of the evolution of
the inner regions of collapsing envelopes of massive protostars. The existence
of high energy radiations early in the evolution of massive protostars also has
important implications for chemical evolution of dense collapsing gas and could
trigger peculiar chemistry and early formation of a hot core.Comment: 6 page
The L1157-B1 astrochemical laboratory: testing the origin of DCN
L1157-B1 is the brightest shocked region of the large-scale molecular
outflow, considered the prototype of chemically rich outflows, being the ideal
laboratory to study how shocks affect the molecular gas. Several deuterated
molecules have been previously detected with the IRAM 30m, most of them formed
on grain mantles and then released into the gas phase due to the shock. We aim
to observationally investigate the role of the different chemical processes at
work that lead to formation the of DCN and test the predictions of the chemical
models for its formation. We performed high-angular resolution observations
with NOEMA of the DCN(2-1) and H13CN(2-1) lines to compute the deuterated
fraction, Dfrac(HCN). We detected emission of DCN(2-1) and H13CN(2-1) arising
from L1157-B1 shock. Dfrac(HCN) is ~4x10 and given the uncertainties, we
did not find significant variations across the bow-shock. Contrary to HDCO,
whose emission delineates the region of impact between the jet and the ambient
material, DCN is more widespread and not limited to the impact region. This is
consistent with the idea that gas-phase chemistry is playing a major role in
the deuteration of HCN in the head of the bow-shock, where HDCO is undetected
as it is a product of grain-surface chemistry. The spectra of DCN and H13CN
match the spectral signature of the outflow cavity walls, suggesting that their
emission result from shocked gas. The analysis of the time dependent gas-grain
chemical model UCL-CHEM coupled with a C-type shock model shows that the
observed Dfrac(HCN) is reached during the post-shock phase, matching the
dynamical timescale of the shock. Our results indicate that the presence of DCN
in L1157-B1 is a combination of gas-phase chemistry that produces the
widespread DCN emission, dominating in the head of the bow-shock, and
sputtering from grain mantles toward the jet impact region.Comment: Accepted for publication in A&A. 7 pages, 5 Figures, 1 Tabl
Triggered massive-star formation on the borders of Galactic HII regions. III. Star formation at the periphery of Sh2-219
Context. Massive-star formation triggered by the expansion of HII regions.
Aims. To understand if sequential star formation is taking place at the
periphery of the HII region Sh2-219. Methods. We present 12CO(2-1) line
observations of this region, obtained at the IRAM 30-m telescope (Pico Veleta,
Spain). Results. In the optical, Sh2-219 is spherically symmetric around its
exciting star; furthermore it is surrounded along three quarters of its
periphery by a ring of atomic hydrogen. This spherical symmetry breaks down at
infrared and millimetre wavelengths. A molecular cloud of about 2000\msol lies
at the southwestern border of Sh2-219, in the HI gap. Two molecular
condensations, elongated along the ionization front, probably result from the
interaction between the expanding HII region and the molecular cloud. In this
region of interaction there lies a cluster containing many highly reddened
stars, as well as a massive star exciting an ultracompact HII region. More
surprisingly, the brightest parts of the molecular cloud form a `chimney',
perpendicular to the ionization front. This chimney is closed at its south-west
extremity by H-alpha walls, thus forming a cavity. The whole structure is 7.5
pc long. A luminous H-alpha emission-line star, lying at one end of the chimney
near the ionization front, may be responsible for this structure. Confrontation
of the observations with models of HII region evolution shows that Sh2-219 is
probably 10^5 yr old. The age and origin of the near-IR cluster observed on the
border of Sh2-219 remain unknown.Comment: 11 pages, 10 figures. To be published in A&
High spectral resolution observations of HNC3 and HCCNC in the L1544 prestellar core
HCCNC and HNC3 are less commonly found isomers of cyanoacetylene, HC3N, a
molecule that is widely found in diverse astronomical sources. We want to know
if HNC3 is present in sources other than the dark cloud TMC-1 and how its
abundance is relative to that of related molecules. We used the ASAI unbiased
spectral survey at IRAM 30m towards the prototypical prestellar core L1544 to
search for HNC3 and HCCNC which are by-product of the HC3NH+ recombination,
previously detected in this source. We performed a combined analysis of
published HNC3 microwave rest frequencies with thus far unpublished millimeter
data because of issues with available rest frequency predictions. We determined
new spectroscopic parameters for HNC3, produced new predictions and detected it
towards L1544. We used a gas-grain chemical modelling to predict the abundances
of N-species and compare with the observations. The modelled abundances are
consistent with the observations, considering a late stage of the evolution of
the prestellar core. However the calculated abundance of HNC3 was found 5-10
times higher than the observed one. The HC3N, HNC3 and HCCNC versus HC3NH+
ratios are compared in the TMC-1 dark cloud and the L1544 prestellar core.Comment: Accepted in MNRAS letters. 5 pages plus 2 additional pages for the
on-line materia
The CHESS spectral survey of star forming regions: Peering into the protostellar shock L1157-B1 - II. Shock dynamics
Context. The outflow driven by the low-mass class 0 protostar L1157 is the prototype of the so-called chemically active outflows. The bright bowshock B1 in the southern outflow lobe is a privileged testbed of magneto-hydrodynamical (MHD) shock models, for which dynamical and chemical processes are strongly interdependent.
Aims. We present the first results of the unbiased spectral survey of the L1157-B1 bowshock, obtained in the framework of the key program âChemical HErschel Surveys of star forming regionsâ (CHESS). The main aim is to trace the warm and chemically enriched gas and to infer the excitation conditions in the shock region.
Methods. The CO 5-4 and o-H2_O 1_(10)â1_(01) lines have been detected at high-spectral resolution in the unbiased spectral survey of the HIFI-band 1b spectral window (555â636 GHz), presented by Codella et al. in this volume. Complementary ground-based observations in the submm window help establish the origin of the emission detected in the main-beam of HIFI and the physical conditions in the shock.
Results. Both lines exhibit broad wings, which extend to velocities much higher than reported up to now. We find that the molecular emission arises from two regions with distinct physical conditions : an extended, warm (100 K), dense (3 Ă 10^5 cm^(-3)) component at low-velocity, which dominates the water line flux in Band 1; a secondary component in a small region of B1 (a few arcsec) associated with high-velocity, hot (>400 K) gas of moderate density ((1.0â3.0) Ă 10^4 cm^(-3)), which appears to dominate the flux of the water line at 179ÎŒm observed with PACS. The water abundance is enhanced by two orders of magnitude between the low- and the high-velocity component, from 8 Ă 10^(-7) up to 8 Ă 10^(-5). The properties of the high-velocity component agree well with the predictions of steady-state C-shock models
Effect of collagen network orientation on the cervical intervertebral disc response to flexion load
National audienc
Windows through the Dusty Disks Surrounding the Youngest Low Mass Protostellar Objects
The formation and evolution of young low mass stars are characterized by
important processes of mass loss and accretion ocurring in the innermost
regions of their placentary circumstellar disks. Because of the large
obscuration of these disks at optical and infrared wavelengths in the early
protostellar stages (Class 0 Sources), they were previously detected only at
radio wavelengths using interferometric techniques. We have detected with the
Infrared Space Observatory (ISO) the mid-infrared emission associated with the
Class 0 protostar VLA1 in the HH1-2 region located in the Orion nebula. The
emission arises in the three wavelength windows at 5.3, 6.6 and 7.5 micras
where the absorption due to ices and silicates has a local minimum that exposes
the central parts of the youngest protostellar systems to mid-infrared
investigations. The mid-infrared emission arises from a central source with 4
AU diameter at an averaged temperature of 700 K, deeply embedded in a dense
region with a visual extinction of Av=80-100mag.Comment: The article is here and on pres
Water and acetaldehyde in HH212: The first hot corino in Orion
Aims: Using the unprecedented combination of high resolution and sensitivity
offered by ALMA, we aim to investigate whether and how hot corinos,
circumstellar disks, and ejected gas are related in young solar-mass
protostars. Methods: We observed CHCHO and deuterated water (HDO)
high-excitation ( up to 335 K) lines towards the Sun-like protostar
HH212--MM1. Results: For the first time, we have obtained images of CHCHO
and HDO emission in the inner 100 AU of HH212. The multifrequency line
analysis allows us to contrain the density ( 10 cm),
temperature ( 100 K), and CHCHO abundance ( 0.2--2
10) of the emitting region. The HDO profile is asymmetric at low
velocities ( 2 km s from ). If the HDO line is
optically thick, this points to an extremely small ( 20--40 AU) and dense
( 10 cm) emitting region. Conclusions: We report the first
detection of a hot corino in Orion. The HDO asymmetric profile indicates a
contribution of outflowing gas from the compact central region, possibly
associated with a dense disk wind.Comment: Astronomy & Astrophysics Letter, in pres
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