120 research outputs found
Radiation-hydrodynamical simulations of massive star formation using Monte Carlo radiative transfer - I. Algorithms and numerical methods
Copyright © 2015 The Author Published by Oxford University Press on behalf of the Royal Astronomical SocietyWe present a set of new numerical methods that are relevant to calculating radiation pressure terms in hydrodynamics calculations, with a particular focus on massive star formation. The radiation force is determined from a Monte Carlo estimator and enables a complete treatment of the detailed microphysics, including polychromatic radiation and anisotropic scattering, in both the free-streaming and optically thick limits. Since the new method is computationally demanding we have developed two new methods that speed up the algorithm. The first is a photon packet splitting algorithm that enables efficient treatment of the Monte Carlo process in very optically thick regions. The second is a parallelization method that distributes the Monte Carlo workload over many instances of the hydrodynamic domain, resulting in excellent scaling of the radiation step. We also describe the implementation of a sink particle method that enables us to follow the accretion on to, and the growth of, the protostars. We detail the results of extensive testing and benchmarking of the new algorithms.Science & Technology Facilities Council (STFC
Spectropolarimetry of single and binary stars
Spectropolarimetry is a photon-hungry technique that will reach fruition in
the 8-m telescope age. Here I summarize some of the stellar spectropolarimetric
research that my collaborators and I have undertaken, with particular emphasis
on the circumstellar environment of massive stars, symbiotic binaries, and star
formation.Comment: 12 pages, 8 Figures, to appear in "Stellar Astrophysics with the
World's Largest Telescopes", J. Mikolajewska (ed), to be published by AI
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
Indirect Detection of Forming Protoplanets via Chemical Asymmetries in Disks
We examine changes in the molecular abundances resulting from increased
heating due to a self-luminous planetary companion embedded within a narrow
circumstellar disk gap. Using 3D models that include stellar and planetary
irradiation, we find that luminous young planets locally heat up the parent
circumstellar disk by many tens of Kelvin, resulting in efficient thermal
desorption of molecular species that are otherwise locally frozen out.
Furthermore, the heating is deposited over large regions of the disk, AU
radially and spanning azimuthally. From the 3D chemical
models, we compute rotational line emission models and full ALMA simulations,
and find that the chemical signatures of the young planet are detectable as
chemical asymmetries in observations. HCN and its isotopologues are
particularly clear tracers of planetary heating for the models considered here,
and emission from multiple transitions of the same species is detectable, which
encodes temperature information in addition to possible velocity information
from the spectra itself. We find submillimeter molecular emission will be a
useful tool to study gas giant planet formation in situ, especially beyond
AU.Comment: 14 pages, 14 figures, accepted for publication in Ap
Radiation hydrodynamics of triggered star formation: the effect of the diffuse radiation field
We investigate the effect of including diffuse field radiation when modelling
the radiatively driven implosion of a Bonnor-Ebert sphere (BES).
Radiation-hydrodynamical calculations are performed by using operator splitting
to combine Monte Carlo photoionization with grid-based Eulerian hydrodynamics
that includes self-gravity. It is found that the diffuse field has a
significant effect on the nature of radiatively driven collapse which is
strongly coupled to the strength of the driving shock that is established
before impacting the BES. This can result in either slower or more rapid star
formation than expected using the on-the-spot approximation depending on the
distance of the BES from the source object. As well as directly compressing the
BES, stronger shocks increase the thickness and density in the shell of
accumulated material, which leads to short, strong, photo-evaporative ejections
that reinforce the compression whenever it slows. This happens particularly
effectively when the diffuse field is included as rocket motion is induced over
a larger area of the shell surface. The formation and evolution of 'elephant
trunks' via instability is also found to vary significantly when the diffuse
field is included. Since the perturbations that seed instabilities are smeared
out elephant trunks form less readily and, once formed, are exposed to enhanced
thermal compression.Comment: Accepted for publication in MNRAS. 19 pages, 14 figures, 8 table
T Tauri stellar magnetic fields: He I measurements
We present measurements of the longitudinal magnetic field in the
circumstellar environment of seven classical T Tauri stars. The measurements
are based on high-resolution circular spectropolarimetry of the He I 5876
emission line, which is thought to form in accretion streams controlled by a
stellar magnetosphere. We detect magnetic fields in BP Tau, DF Tau and DN Tau,
and detect statistically significant fields in GM Aur and RW Aur A at one epoch
but not at others. We detect no field for DG Tau and GG Tau, with the caveat
that these objects were observed at one epoch only. Our measurements for BP Tau
and DF Tau are consistent, both in terms of sign and magnitude, with previous
studies, suggesting that the characteristics of T Tauri magnetospheres are
persistent over several years. We observed the magnetic field of BP Tau to
decline monotonically over three nights, and have detected a peak field of 4kG
in this object, the highest magnetic field yet observed in a T Tauri star. We
combine our observations with results from the literature in order to perform a
statistical analysis of the magnetospheric fields in BP Tau and DF Tau.
Assuming a dipolar field, we determine a polar field of ~3kG and a dipole
offset of 40deg for BP Tau, while DF Tau's field is consistent with a polar
field of ~-4.5kG and a dipole offset of 10deg. We conclude that many classical
T Tauri stars have circumstellar magnetic fields that are both strong enough
and sufficiently globally-ordered to sustain large-scale magnetospheric
accretion flows.Comment: 8 pages, 3 figures. Accepted by MNRAS. Corrected typo
What can the SEDs of first hydrostatic core candidates reveal about their nature?
The first hydrostatic core (FHSC) is the first stable object to form in
simulations of star formation. This stage has yet to be observed definitively,
although several candidate FHSCs have been reported. We have produced synthetic
spectral energy distributions (SEDs) from 3D hydrodynamical simulations of
pre-stellar cores undergoing gravitational collapse for a variety of initial
conditions. Variations in the initial rotation rate, radius and mass lead to
differences in the location of the SED peak and far-infrared flux. Secondly, we
attempt to fit the SEDs of five FHSC candidates from the literature and five
newly identified FHSC candidates located in the Serpens South molecular cloud
with simulated SEDs. The most promising FHSC candidates are fitted by a limited
number of model SEDs with consistent properties, which suggests the SED can be
useful for placing constraints on the age and rotation rate of the source. The
sources we consider most likely to be in FHSC phase are B1-bN, CB17-MMS,
Aqu-MM1 and Serpens South candidate K242. We were unable to fit SerpS-MM22,
Per-Bolo 58 and Chamaeleon-MMS1 with reasonable parameters, which indicates
that they are likely to be more evolved.Comment: 26 pages, 28 figures. Accepted for publication in MNRA
Evidence for high accretion-rates in Weak-Line T Tauri stars?
We have discovered T Tauri stars which show startling spectral variability
between observations seperated by 20 years. In spectra published by Bouvier &
Appenzeller (1992) these objects showed very weak H-alpha emission, broad CaII
absorption and so called ``composite spectra'', where the spectral type
inferred from the blue region is earlier than that inferred from the red. We
present here new spectroscopy which shows that all four stars now exhibit
strong H-alpha emission, narrow CaII emission and a spectral type which is
consistent at all wavelengths.
We propose a scheme to understand these changes whereby the composite spectra
of these stars can be explained by a period of active accretion onto the
central, young star. In this scheme the composite spectrum consists of a
contribution from the stellar photosphere and a contribution from a hot,
optically thick, accretion component. The optically thick nature of the
accretion flow explains the weakness of the H-alpha emission during this phase.
Within this scheme, the change to a single spectral type at all wavelengths and
emergence of strong H-alpha emission are consistent with the accretion columns
becoming optically thin, as the accretion rate drops. There is a strong analogy
here with the dwarf novae class of interacting binaries, which show similar
behaviour during the decline from outbursts of high mass-transfer rate.
The most important consequence of this interpretation is that these objects
bring into question the association of Weak-Line T Tauri stars (WTTs) with
non-accreting or discless objects. In light of this result we consider the
justification for this paradigm.Comment: 6 pages, 3 figures. Accepted for publication in MNRA
- …