2,274 research outputs found
Radial and rotational velocities of young brown dwarfs and very low-mass stars in the Upper Scorpius OB association and the rho Ophiuchi cloud core
We present the results of a radial velocity (RV) survey of 14 brown dwarfs
(BDs) and very low-mass (VLM) stars in the Upper Scorpius OB association
(UScoOB) and 3 BD candidates in the rho Ophiuchi dark cloud core. We obtained
high-resolution echelle spectra at the Very Large Telescope using Ultraviolet
and Visual Echelle Spectrograph (UVES) at two different epochs for each object,
and measured the shifts in their RVs to identify candidates for binary/multiple
systems in the sample. The average time separation of the RV measurements is
21.6d, and our survey is sensitive to the binaries with separation < 0.1 au. We
found that 4 out of 17 objects (or 24^{+16}_{-13} per cent by fraction) show a
significant RV change in 4-33d time scale, and are considered as
binary/multiple `candidates.' We found no double-lined spectroscopic binaries
in our sample, based on the shape of cross-correlation curves. The RV
dispersion of the objects in UScoOB is found to be very similar to that of the
BD and VLM stars in Chamaeleon I (Cha I). We also found the distribution of the
mean rotational velocities (v sin i) of the UScoOB objects is similar to that
of the Cha I, but the dispersion of v sin i is much larger than that of the Cha
I objects.Comment: 10 pages, 5 figures, accepted for publication in MNRA
Three-dimensional simulations of rotationally-induced line variability from a Classical T Tauri star with a misaligned magnetic dipole
We present three-dimensional (3-D) simulations of rotationally induced line
variability arising from complex circumstellar environment of classical T Tauri
stars (CTTS) using the results of the 3-D magnetohydrodynamic (MHD) simulations
of Romanova et al., who considered accretion onto a CTTS with a misaligned
dipole magnetic axis with respect to the rotational axis. The density, velocity
and temperature structures of the MHD simulations are mapped on to the
radiative transfer grid, and corresponding line source function and the
observed profiles of neutral hydrogen lines (H-beta, Pa-beta and Br-gamma) are
computed using the Sobolev escape probability method. We study the dependency
of line variability on inclination angles (i) and magnetic axis misalignment
angles (Theta). By comparing our models with the Pa-beta profiles of 42 CTTS
observed by Folha & Emerson, we find that models with a smaller misaligngment
angle (Theta<~15 deg.) are more consistent with the observations which show
that majority of Pa-beta are rather symmetric around the line centre. For a
high inclination system with a small dipole misalignment angle (Theta ~ 15
deg.), only one accretion funnel (on the upper hemisphere) is visible to an
observer at any given rotational phase. This can cause an anti-correlation of
the line equivalent width in the blue wing (v0)
over a half of a rotational period, and a positive correlation over other half.
We find a good overall agreement of the line variability behaviour predicted by
our model and those from observations. (Abridged)Comment: 15 pages, 13 figures. Accepted for publication in MNRAS. A version
with full resolution figures can be downloaded from
http://www.physics.unlv.edu/~rk/preprint/inclined_dipole.pd
Micro-crystalline inclusions analysis by PIXE and RBS
A characteristic feature of the nuclear microprobe using a 3 MeV proton beam
is the long range of particles (around 70 \mu m in light matrices). The PIXE
method, with EDS analysis and using the multilayer approach for treating the
X-ray spectrum allows the chemistry of an intra-crystalline inclusion to be
measured, provided the inclusion roof and thickness at the impact point of the
beam (Z and e, respectively) are known (the depth of the inclusion floor is Z +
e). The parameter Z of an inclusion in a mineral can be measured with a
precision of around 1 \mu m using a motorized microscope. However, this value
may significantly depart from Z if the analyzed inclusion has a complex shape.
The parameter e can hardly be measured optically. By using combined RBS and
PIXE measurements, it is possible to obtain the geometrical information needed
for quantitative elemental analysis. This paper will present measurements on
synthetic samples to investigate the advantages of the technique, and also on
natural solid and fluid inclusions in quartz. The influence of the geometrical
parameters will be discussed with regard to the concentration determination by
PIXE. In particular, accuracy of monazite micro-inclusion dating by coupled
PIXE-RBS will be presented
An algorithm for Monte-Carlo time-dependent radiation transfer
A new Monte-Carlo algorithm for calculating time-dependent radiative-transfer
under the assumption of LTE is presented. Unlike flux-limited diffusion the
method is polychromatic, includes scattering, and is able to treat the
optically thick and free-streaming regimes simultaneously. The algorithm is
tested on a variety of 1-d and 2-d problems, and good agreement with benchmark
solutions is found. The method is used to calculate the time-varying spectral
energy distribution from a circumstellar disc illuminated by a protostar whose
accretion luminosity is varying. It is shown that the time lag between the
optical variability and the infrared variability results from a combination of
the photon travel time and the thermal response in the disc, and that the lag
is an approximately linear function of wavelength.Comment: 10 pages, 10 figures, accepted for publication by MNRA
Accretion dynamics in the classical T Tauri star V2129 Oph
We analyze the photometric and spectroscopic variability of the classical T
Tauri star V2129 Oph over several rotational cycles to test the dynamical
predictions of magnetospheric accretion models. The photometric variability and
the radial velocity variations in the photospheric lines can be explained by
rotational modulation due to cold spots, while the radial velocity variations
of the He I (5876 \AA) line and the veiling variability are due to hot spot
rotational modulation. The hot and cold spots are located at high latitudes and
about the same phase, but the hot spot is expected to sit at the chromospheric
level, while the cold spot is at the photospheric level. Using the
dipole+octupole magnetic-field configuration previously proposed in the
literature for the system, we compute 3D MHD magnetospheric simulations of the
star-disk system. We use the simulation's density, velocity and scaled
temperature structures as input to a radiative transfer code, from which we
calculate theoretical line profiles at all rotational phases. The theoretical
profiles tend to be narrower than the observed ones, but the qualitative
behavior and the observed rotational modulation of the H\alpha and H\beta
emission lines are well reproduced by the theoretical profiles. The
spectroscopic and photometric variability observed in V2129 Oph support the
general predictions of complex magnetospheric accretion models with
non-axisymmetric, multipolar fields.Comment: Accepted by Astronomy and Astrophysic
Tolerance and adaptive evolution of triacylglycerol-producing Rhodococcus opacus to lignocellulose-derived inhibitors
Background: Lignocellulosic biomass has been investigated as a renewable non-food source for production of biofuels. A significant technical challenge to using lignocellulose is the presence of microbial growth inhibitors generated during pretreatment processes. Triacylglycerols (TAGs) are potential precursors for lipid-based biofuel production. Rhodococcus opacus MITXM-61 is an oleaginous bacterium capable of producing large amounts of TAGs on high concentrations of glucose and xylose present in lignocellulosic hydrolysates. However, this strain is sensitive to ligonocellulose-derived inhibitors. To understand the toxic effects of the inhibitors in lignocellulosic hydrolysates, strain MITXM-61 was examined for tolerance toward the potential inhibitors and was subjected to adaptive evolution for the resistance to the inhibitors.
Results: We investigated growth-inhibitory effects by potential lignocellulose-derived inhibitors of phenols (lignin, vanillin, 4-hydroxybenzaldehyde (4-HB), syringaldehyde), furans (furfural and 5-hydroxymethyl-2-furaldehyde), and organic acids (levulinic acid, formic acid, and acetic acid) on the growth and TAG production of strain MITXM-61. Phenols and furans exhibited potent inhibitory effects at a concentration of 1 g L−1, while organic acids had insignificant impacts at concentrations of up to 2 g L−1. In an attempt to improve the inhibitor tolerance of strain MITXM-61, we evaluated the adaptation of this strain to the potential inhibitors. Adapted mutants were generated on defined agar media containing lignin, 4-HB, and syringaldehyde. Strain MITXM-61SHL33 with improved multiple resistance of lignin, 4-HB, and syringaldehyde was constructed through adaptive evolution-based strategies. The evolved strain exhibited a two- to threefold increase in resistance to lignin, 4-HB, and syringaldehyde at 50% growth-inhibitory concentrations, compared to the parental strain. When grown in genuine lignocellulosic hydrolysates of corn stover, wheat straw, and hardwood containing growth inhibitors, strain MITXM-61SHL33 exhibited a markedly shortened lag phase in comparison with that of strain MITXM-61.
Conclusion: This study provides important clues to overcome the negative effects of inhibitors in lignocellulosic hydrolysates on TAG production of R. opacus cells. The findings can contribute to significant progress in detoxified pretreatment of hydrolysates and development of more efficient strains for industrial TAG fermentations of R. opacus using lignocellulosic biomass
Dynamics of Rotating Accretion Flows Irradiated by a Quasar
We study the axisymmetric, time-dependent hydrodynamics of rotating flows
that are under the influence of supermassive black hole gravity and radiation
from an accretion disk surrounding the black hole. This work is an extension of
the earlier work presented by Proga, where nonrotating flows were studied.
Here, we consider effects of rotation, a position-dependent radiation
temperature, density at large radii, and uniform X-ray background radiation. As
in the non-rotating case, the rotating flow settles into a configuration with
two components (1) an equatorial inflow and (2) a bipolar inflow/outflow with
the outflow leaving the system along the pole. However, with rotation the flow
does not always reach a steady state. In addition, rotation reduces the outflow
collimation and the outward flux of mass and kinetic energy. Moreover rotation
increases the outward flux of the thermal energy and can lead to fragmentation
and time-variability of the outflow. We also show that a position-dependent
radiation temperature can significantly change the flow solution. In
particular, the inflow in the equatorial region can be replaced by a thermally
driven outflow. Generally, as it have been discussed and shown in the past, we
find that self-consistently determined preheating/cooling from the quasar
radiation can significantly reduce the rate at which the central BH is fed with
matter. However, our results emphasize also a little appreciated feature.
Namely, quasar radiation drives a non-spherical, multi-temperature and very
dynamic flow. These effects become dominant for luminosities in excess of 0.01
of the Eddington luminosity.Comment: accepted for publication in Ap
Modelling circumstellar discs with 3D radiation hydrodynamics
We present results from combining a grid-based radiative transfer code with a
Smoothed Particle Hydrodynamics code to produce a flexible system for modelling
radiation hydrodynamics. We use a benchmark model of a circumstellar disc to
determine a robust method for constructing a gridded density distribution from
SPH particles. The benchmark disc is then used to determine the accuracy of the
radiative transfer results. We find that the SED and the temperature
distribution within the disc are sensitive to the representation of the disc
inner edge, which depends critically on both the grid and SPH resolution. The
code is then used to model a circumstellar disc around a T-Tauri star. As the
disc adjusts towards equilibrium vertical motions in the disc are induced
resulting in scale height enhancements which intercept radiation from the
central star. Vertical transport of radiation enables these perturbations to
influence the mid-plane temperature of the disc. The vertical motions decay
over time and the disc ultimately reaches a state of simultaneous hydrostatic
and radiative equilibrium.Comment: MNRAS accepted; 15 pages; 17 figures, 4 in colou
Facing the wind of the pre-FUor V1331 Cyg
The mass outflows in T Tauri stars (TTS) are thought to be an effective
mechanism to remove angular momentum during the pre-main-sequence contraction
of a low-mass star. The most powerful winds are observed at the FUor stage of
stellar evolution. V1331 Cyg has been considered as a TTS at the pre-FUor
stage. We analyse high-resolution spectra of V1331 Cyg collected in 1998-2007
and 20-d series of spectra taken in 2012. For the first time the photospheric
spectrum of the star is detected and stellar parameters are derived: spectral
type G7-K0 IV, mass 2.8 Msun, radius 5 Rsun, vsini < 6 km/s. The photospheric
spectrum is highly veiled, but the amount of veiling is not the same in
different spectral lines, being lower in weak transitions and much higher in
strong transitions. The Fe II 5018, Mg I 5183, K I 7699 and some other lines of
metals are accompanied by a `shell' absorption at radial velocity of about -240
km/s. We show that these absorptions form in the post-shock gas in the jet,
i.e. the star is seen though its jet. The P Cyg profiles of H-alpha and H-beta
indicate the terminal wind velocity of about 500 km/s, which vary on
time-scales from several days to years. A model of the stellar wind is
developed to interpret the observations. The model is based on calculation of
hydrogen spectral lines using the radiative transfer code TORUS. The observed
H-alpha and H-beta line profiles and their variability can be well reproduced
with a stellar wind model, where the mass-loss rate and collimation (opening
angle) of the wind are variable. The changes of the opening angle may be
induced by small variability in magetization of the inner disc wind. The
mass-loss rate is found to vary within (6-11)x10^{-8} Msun/yr, with the
accretion rate of 2.0x10^{-6} Msun/yr.Comment: 11 pages, 12 figures; accepted for publication in MNRAS.
Typographical errors have been corrected after the proof stag
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