284 research outputs found
Singular limits in phase dynamics with physical viscosity and capillarity
Following pioneering work by Fan and Slemrod who studied the effect of artificial viscosity terms, we consider the system of conservation laws arising in liquid-vapor phase dynamics with physical viscosity and capillarity effects taken into account. Following Dafermos we consider self-similar solutions to the Riemann problem and establish uniform total variation bounds, allowing us to deduce new existence results. Our analysis cover both the hyperbolic and the hyperbolic-elliptic regimes and apply to arbitrarily large Riemann data. The proofs rely on a new technique of reduction to two coupled scalar equations associated with the two wave fans of the system. Strong L1 convergence to a weak solution of bounded variation is established in the hyperbolic regime, while in the hyperbolicelliptic regime a stationary singularity near the axis separating the two wave fans, or more generally an almost-stationary oscillating wave pattern (of thickness depending upon the capillarityviscosity ratio) are observed which prevent the solution to have globally bounded variation
Boundary layers in weak solutions of hyperbolic conservation laws. III. Vanishing relaxation limits
This is the third part of a series concerned with boundary layers in solutions of nonlinear hyperbolic systems of conservation laws. We consider here self-similar solutions of the Riemann problem, following a pioneering idea by Dafermos. The system under study is strictly hyperbolic but no assumption of genuine nonlinearity is made. The boundary is possibly characteristic, that the sign of the characteristic speed near the boundary is not known a priori. We investigate the effect of vanishing relaxation terms on the solutions of the Riemann problem. We show that the boundary Riemann problem with relaxation admits continuous solutions that remain uniformly bounded in the total variation norm. Following the second part of this series, we derive the necessary uniform estimates near the boundary which allow us to describe the structure of the boundary layer even when the boundary is characteristic. Our analysis provides still a new approach to the existence of Riemann solutions for systems of conservation laws
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
Radiation Driven Implosion and Triggered Star Formation
We present simulations of initially stable isothermal clouds exposed to
ionizing radiation from a discrete external source, and identify the conditions
that lead to radiatively driven implosion and star formation. We use the
Smoothed Particle Hydrodynamics code SEREN (Hubber et al. 2010) and the
HEALPix-based photoionization algorithm described in Bisbas et al. (2009). We
find that the incident ionizing flux is the critical parameter determining the
evolution: high fluxes simply disperse the cloud, whereas low fluxes trigger
star formation. We find a clear connection between the intensity of the
incident flux and the parameters of star formation.Comment: 4 pages, 2 figures, conference proceedings, IAU Symposium 270 (eds.
Alves, Elmegreen, Girart, Trimble
Near-arcsecond resolution observations of the hot corino of the solar type protostar IRAS 16293-2422
Complex organic molecules have previously been discovered in solar type
protostars, raising the questions of where and how they form in the envelope.
Possible formation mechanisms include grain mantle evaporation, interaction of
the outflow with its surroundings or the impact of UV/X-rays inside the
cavities. In this Letter we present the first interferometric observations of
two complex molecules, CH3CN and HCOOCH3, towards the solar type protostar
IRAS16293-2422. The images show that the emission originates from two compact
regions centered on the two components of the binary system. We discuss how
these results favor the grain mantle evaporation scenario and we investigate
the implications of these observations for the chemical composition and
physical and dynamical state of the two components.Comment: 5 pages (apjemulate), 2 figures; accepted by ApJ
Broad N2H+ emission towards the protostellar shock L1157-B1
We present the first detection of N2H+ towards a low-mass protostellar
outflow, namely the L1157-B1 shock, at about 0.1 pc from the protostellar
cocoon. The detection was obtained with the IRAM 30-m antenna. We observed
emission at 93 GHz due to the J = 1-0 hyperfine lines. The analysis of the
emission coupled with the HIFI CHESS multiline CO observations leads to the
conclusion that the observed N2H+(1-0) line originates from the dense (> 10^5
cm-3) gas associated with the large (20-25 arcsec) cavities opened by the
protostellar wind. We find a N2H+ column density of few 10^12 cm-2
corresponding to an abundance of (2-8) 10^-9. The N2H+ abundance can be matched
by a model of quiescent gas evolved for more than 10^4 yr, i.e. for more than
the shock kinematical age (about 2000 yr). Modelling of C-shocks confirms that
the abundance of N2H+ is not increased by the passage of the shock. In summary,
N2H+ is a fossil record of the pre-shock gas, formed when the density of the
gas was around 10^4 cm-3, and then further compressed and accelerated by the
shock.Comment: ApJ, in pres
Smooth Gowdy symmetric generalized Taub-NUT solutions
We study a class of S3 Gowdy vacuum models with a regular past Cauchy horizon
which we call smooth Gowdy symmetric generalized Taub-NUT solutions. In
particular, we prove existence of such solutions by formulating a singular
initial value problem with asymptotic data on the past Cauchy horizon. The
result of our investigations is that a future Cauchy horizon exists for generic
asymptotic data. Moreover, we derive an explicit expression for the metric on
the future Cauchy horizon in terms of the asymptotic data on the past horizon.
This complements earlier results about S2xS1 Gowdy models.Comment: 56 pages, 1 figure. The new version contains a detailed explanation
of the Fuchsian method on the 2-spher
Triggered Star Formation in the Orion Bright-Rimmed Clouds
We have developed an empirical and effective set of criteria, based on the
2MASS colors, to select candidate classical T Tauri stars (CTTS). This provides
a useful tool to study the young stellar population in star-forming regions.
Here we present our analysis of the bright-rimmed clouds (BRCs) B 35, B 30, IC
2118, LDN 1616, LDN 1634, and Orion East to show how massive stars interact
with molecular clouds to trigger star formation. Our results support the
radiation-driven implosion model in which the ionization fronts from OB stars
compress a nearby cloud until the local density exceeds the critical value,
thereby inducing the cloud to collapse to form stars. We find that only BRCs
associated with strong IRAS 100 micron emission (tracer of high density) and
H-alpha emission (tracer of ionization fronts) show signs of ongoing star
formation. Relevant timescales, including the ages of O stars, expanding HII
regions, and the ages of CTTS, are consistent with sequential star formation.
We also find that CTTS are only seen between the OB stars and the BRCs, with
those closer to the BRCs being progressively younger. There are no CTTS leading
the ionization fronts, i.e., within the molecular clouds. All these provide
strong evidence of triggered star formation and show the major roles massive
stars play in sustaining the star-forming activities in the region.Comment: To appear in Ap
Dzyaloshinskii-Moriya interaction in transport through single molecule transistors
The Dzyaloshinskii-Moriya interaction is shown to result in a canting of
spins in a single molecule transistor. We predict non-linear transport
signatures of this effect induced by spin-orbit coupling for the generic case
of a molecular dimer. The conductance is calculated using a master equation and
is found to exhibit a non-trivial dependence on the magnitude and direction of
an external magnetic field. We show how three-terminal transport measurements
allow for a determination of the coupling-vector characterizing the
Dzyaloshinskii-Moriya interaction. In particular, we show how its orientation,
defining the intramolecular spin chirality, can be probed with ferromagnetic
electrodes
Diagnosing shock temperature with NH and HO profiles
In a previous study of the L1157 B1 shocked cavity, a comparison between
NH(1-) and HO(1--1) transitions showed a
striking difference in the profiles, with HO emitting at definitely higher
velocities. This behaviour was explained as a result of the high-temperature
gas-phase chemistry occurring in the postshock gas in the B1 cavity of this
outflow. If the differences in behaviour between ammonia and water are indeed a
consequence of the high gas temperatures reached during the passage of a shock,
then one should find such differences to be ubiquitous among chemically rich
outflows. In order to determine whether the difference in profiles observed
between NH and HO is unique to L1157 or a common characteristic of
chemically rich outflows, we have performed Herschel-HIFI observations of the
NH(1-0) line at 572.5 GHz in a sample of 8 bright low-mass outflow
spots already observed in the HO(1--1) line within
the WISH KP. We detected the ammonia emission at high-velocities at most of the
outflows positions. In all cases, the water emission reaches higher velocities
than NH, proving that this behaviour is not exclusive of the L1157-B1
position. Comparisons with a gas-grain chemical and shock model confirms, for
this larger sample, that the behaviour of ammonia is determined principally by
the temperature of the gas.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Societ
- âŠ