1,841 research outputs found
A fast solver for systems of reaction-diffusion equations
In this paper we present a fast algorithm for the numerical solution of
systems of reaction-diffusion equations, , , . Here,
is a vector-valued function, , is
large, and the corresponding system of ODEs, , is
stiff. Typical examples arise in air pollution studies, where is the given
wind field and the nonlinear function models the atmospheric chemistry.Comment: 8 pages, 3 figures, to appear in Proc. 13th Domain Decomposition
Conference, Lyon, October 200
The radial velocity curve of HD153919 (4U1700-37) revisited
We have re-analysed all available high-resolution ultraviolet IUE spectra of
the high-mass X-ray binary HD153919/4U1700-37. The radial velocity
semi-amplitude of 20.6 +/- 1.0 km/s and orbital eccentricity of 0.22 +/- 0.04
agree very well with the values obtained earlier from optical spectra. They
disagree with earlier conclusions for the same data reduced by Heap & Corcoran
(1992) and by Stickland & Lloyd (1993).Comment: 6 pages, latex, figure included, Astronomy & Astrophysics, in pres
The star formation history of RCW 36
Recent studies of massive-star forming regions indicate that they can contain
multiple generations of young stars. These observations suggest that star
formation in these regions is sequential and/or triggered by a previous
generation of (massive) stars. Here we present new observations of the star
forming region RCW 36 in the Vela Molecular Ridge, hosting a young cluster of
massive stars embedded in a molecular cloud complex. In the periphery of the
cluster several young stellar objects (YSOs) are detected which produce bipolar
jets (HH 1042 and HH 1043) demonstrating that these objects are still actively
accreting. The VLT/X-shooter spectrum of the jet structure of HH 1042 provides
detailed information on the physical conditions and kinematical properties of
the jet plasma. From this information the YSO's accretion history can be
derived. Combining the photometric and spectroscopic observations of RCW 36
gives insight into the formation process of individual stars and the star
formation history of this young massive-star forming region.Comment: 10 pages, 5 figures, to appear in the proceedings of the ESO workshop
"Circumstellar Dynamics at High Resolution", Foz do Iguacu (Br), Feb 2012,
eds. A. Carciofi and T. Riviniu
Stellar Wind Accretion in GX301-2: Evidence for a High-density Stream
The X-ray binary system GX301-2 consists of a neutron star in an eccentric
orbit accreting from the massive early-type star WRAY 977. It has previously
been shown that the X-ray orbital light curve is consistent with existence of a
gas stream flowing out from Wray 977 in addition to its strong stellar wind.
Here, X-ray monitoring observations by the Rossi X-ray Timing Explorer (RXTE)/
All-Sky-Monitor (ASM) and pointed observations by the RXTE/ Proportional
Counter Array (PCA) over the past decade are analyzed. We analyze both the flux
and column density dependence on orbital phase. The wind and stream dynamics
are calculated for various system inclinations, companion rotation rates and
wind velocities, as well as parametrized by the stream width and density. These
calculations are used as inputs to determine both the expected accretion
luminosity and the column density along the line-of-sight to the neutron star.
The model luminosity and column density are compared to observed flux and
column density vs. orbital phase, to constrain the properties of the stellar
wind and the gas stream. We find that the change between bright and medium
intensity levels is primarily due to decreased mass loss in the stellar wind,
but the change between medium and dim intensity levels is primarily due to
decreased stream density. The mass-loss rate in the stream exceeds that in the
stellar wind by a factor of 2.5. The quality of the model fits is significantly
better for lower inclinations, favoring a mass for WRAY 977 of 53 to 62 Msun.Comment: 19 pages, 6 figure
Testing Hydrodynamic Models of LMC X-4 with UV and X-ray Spectra
We compare the predictions of hydrodynamic models of the LMC X-4 X-ray binary
system with observations of UV P Cygni lines with the GHRS and STIS
spectrographs on the Hubble Space Telescope. The hydrodynamic model determines
density and velocity fields of the stellar wind, wind-compressed disk,
accretion stream, Keplerian accretion disk, and accretion disk wind. We use a
Monte Carlo code to determine the UV P Cygni line profiles by simulating the
radiative transfer of UV photons that originate on the star and are scattered
in the wind. The qualitative orbital variation predicted is similar to that
observed, although the model fails to reproduce the strong orbital asymmetry
(the observed absorption is strongest for phi>0.5). The model predicts a
mid-eclipse X-ray spectrum, due almost entirely to Compton scattering, with a
factor 4 less flux than observed with ASCA. We discuss how the model may need
to be altered to explain the spectral variability of the system.Comment: 11 figures, accepted by Ap
A dearth of short-period massive binaries in the young massive star forming region M17: Evidence for a large orbital separation at birth?
The formation of massive stars remains poorly understood and little is known
about their birth multiplicity properties. Here, we investigate the strikingly
low radial-velocity dispersion measured for a sample of 11 massive pre- and
near-main-sequence stars (sigma_rv = 5.6 +/- 0.2 km/s) in the young massive
star forming region M17 to obtain first constraints on the multiplicity
properties of young massive stellar objects. Methods: We compute the RV
dispersion of synthetic populations of massive stars for various multiplicity
properties and we compare the simulated sigma_rv distributions to the observed
value. We specifically investigate two scenarios: a low binary fraction and a
dearth of short-period binary systems. Results: Simulated populations with low
binary fractions (f_bin = 0.12_{-0.09}^{+0.16}) or with truncated period
distributions (P_cutoff > 9 months) are able to reproduce the low sigma_rv
observed within their 68%-confidence intervals. Parent populations with f_bin >
0.42 or P_cutoff < 47 d can however be rejected at the 5%-significance level.
Both constraints are contrast with the high binary fraction and plethora of
short-period systems found in few Myr-old, OB-type populations. To explain the
difference, the first scenario requires a variation of the outcome of the
massive star formation process. In the the second scenario, compact binaries
must form later on, and the cut-off period may be related to physical
length-scales representative of the bloated pre-main-sequence stellar radii or
of their accretion disks. Conclusions: If the obtained constraints are
representative of the overall properties of massive young stellar objects, our
results may provide support to a formation process in which binaries are
initially formed at larger separations, then harden or migrate to produce the
typical (untruncated) power-law period distribution observed in few Myr-old OB
binaries.Comment: 5 pages; Accepted for publication in Astronomy and Astrophysics
Letter
The Dynamics of Hybrid Metabolic-Genetic Oscillators
The synthetic construction of intracellular circuits is frequently hindered
by a poor knowledge of appropriate kinetics and precise rate parameters. Here,
we use generalized modeling (GM) to study the dynamical behavior of topological
models of a family of hybrid metabolic-genetic circuits known as
"metabolators." Under mild assumptions on the kinetics, we use GM to
analytically prove that all explicit kinetic models which are topologically
analogous to one such circuit, the "core metabolator," cannot undergo Hopf
bifurcations. Then, we examine more detailed models of the metabolator.
Inspired by the experimental observation of a Hopf bifurcation in a
synthetically constructed circuit related to the core metabolator, we apply GM
to identify the critical components of the synthetically constructed
metabolator which must be reintroduced in order to recover the Hopf
bifurcation. Next, we study the dynamics of a re-wired version of the core
metabolator, dubbed the "reverse" metabolator, and show that it exhibits a
substantially richer set of dynamical behaviors, including both local and
global oscillations. Prompted by the observation of relaxation oscillations in
the reverse metabolator, we study the role that a separation of genetic and
metabolic time scales may play in its dynamics, and find that widely separated
time scales promote stability in the circuit. Our results illustrate a generic
pipeline for vetting the potential success of a potential circuit design,
simply by studying the dynamics of the corresponding generalized model
Sequential star formation in IRAS 06084-0611 (GGD 12-15): From intermediate-mass to high-mass stars
Context. The formation and early evolution of high- and intermediate-mass
stars towards the main sequence involves the interplay of stars in a clustered
and highly complex environment. To obtain a full census of this interaction,
the Formation and Early evolution of Massive Stars (FEMS) collaboration studies
a well-selected sample of 10 high-mass star-forming regions. Aims. In this
study we examine the stellar content of the high-mass star-forming region
centered on IRAS 06084-0611 in the Monoceros R2 cloud. Methods. Using the
near-infrared H- and K-band spectra from the VLT/SINFONI instrument on the ESO
Very Large Telescope (VLT)and photometric near-infrared NTT/SOFI, 2MASS and
Spitzer/IRAC data, we were able to determine the spectral types for the most
luminous stars in the cluster. Results. Two very young and reddened massive
stars have been detected by SINFONI: a massive Young Stellar Object (YSO) con-
sistent with an early-B spectral type and a Herbig Be star. Furthermore, stars
of spectral type G and K are detected while still in the Pre-Main Sequence
(PMS) phase. We derive additional properties such as temperatures, extinctions,
radii and masses. We present a Hertzsprung-Russell diagram and find most
objects having intermediate masses between \sim1.5-2.5 M\odot. For these stars
we derive a median cluster age of \sim4 Myr. Conclusions. Using Spitzer/IRAC
data we confirm earlier studies that the younger class 0/I objects are
centrally located while the class II objects are spread out over a larger area,
with rough scale size radii of \sim0.5 pc and \sim1.25 pc respectively.
Moreover, the presence of a massive YSO, an ultracompact H ii region and highly
reddened objects in the center of the cluster suggest a much younger age of < 1
Myr. A possible scenario for this observation would be sequential star
formation along the line of sight; from a cluster of intermediate-mass to
high-mass stars.Comment: 14 pages, 10 figures, 2 tables. Astronomy and Astrophysic
Quiescent X-ray variability in the neutron star Be/X-ray transient GRO J1750-27
The Be/X-ray transient GRO J1750-27 exhibited a type-II (giant) outburst in
2015. After the source transited to quiescence, we triggered our multi-year
Chandra monitoring programme to study its quiescent behaviour. The programme
was designed to follow the cooling of a potentially heated neutron-star crust
due to accretion of matter during the preceding outburst, similar to what we
potentially have observed before in two other Be/X-ray transients, namely 4U
0115+63 and V 0332+53. However, unlike for these other two systems, we do not
find any strong evidence that the neutron-star crust in GRO J1750-27 was indeed
heated during the accretion phase. We detected the source at a rather low X-ray
luminosity (~10^33 erg/s) during only three of our five observations. When the
source was not detected it had very low-luminosity upper limits (<10^32 erg/s;
depending on assumed spectral model). We interpret these detections and the
variability observed as emission likely due to very low-level accretion onto
the neutron star. We also discuss why the neutron-star crust in GRO J1750-27
might not have been heated while the ones in 4U 0115+63 and V 0332+53 possibly
were.Comment: 13 pages, 6 figures, 5 tables. Accepted for A&
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