25,798 research outputs found
An HI Imaging Survey of Asymptotic Giant Branch Stars
We present an imaging study of a sample of eight asymptotic giant branch
(AGB) stars in the HI 21-cm line. Using observations from the Very Large Array,
we have unambiguously detected HI emission associated with the extended
circumstellar envelopes of six of the targets. The detected HI masses range
from M_HI ~ 0.015-0.055 M_sun. The HI morphologies and kinematics are diverse,
but in all cases appear to be significantly influenced by the interaction
between the circumstellar envelope and the surrounding medium. Four stars (RX
Lep, Y UMa, Y CVn, and V1942 Sgr) are surrounded by detached HI shells ranging
from 0.36 to 0.76 pc across. We interpret these shells as resulting from
material entrained in a stellar outflow being abruptly slowed at a termination
shock where it meets the local medium. RX Lep and TX Psc, two stars with
moderately high space velocities (V_space>56 km/s), exhibit extended gaseous
wakes (~0.3 and 0.6 pc in the plane of the sky), trailing their motion through
space. The other detected star, R Peg, displays a peculiar "horseshoe-shaped"
HI morphology with emission extended on scales up to ~1.7 pc; in this case, the
circumstellar debris may have been distorted by transverse flows in the local
interstellar medium. We briefly discuss our new results in the context of the
entire sample of evolved stars that has been imaged in HI to date.Comment: Accepted to AJ. A version with full resolution figures is available
at http://www.haystack.mit.edu/hay/staff/lmatthew/matthews_HI_survey.pd
Unveiling Sources of Heating in the Vicinity of the Orion BN/KL Hot Core as Traced by Highly Excited Inversion Transitions of Ammonia
Using the Expanded Very Large Array, we have mapped the vicinity of the Orion
BN/KL Hot Core with sub-arcsecond angular resolution in seven metastable
inversion transitions of ammonia: (J,K)=(6,6) to (12,12). This emission comes
from levels up to 1500 K above the ground state, enabling identification of
source(s) responsible for heating the region. We used this multi-transition
dataset to produce images of the rotational/kinetic temperature and the column
density of ammonia for ortho and para species separately and on a
position-by-position basis. We find rotational temperature and column density
in the range 160-490 K and (1-4)x10^17 cm^-2, respectively. Our
spatially-resolved images show that the highest (column) density and hottest
gas is found in a northeast-southwest elongated ridge to the southeast of
Source I. We have also measured the ortho-para ratio of ammonia, estimated to
vary in the range 0.9-1.6. Enhancement of ortho with respect to para and the
offset of hot ammonia emission peaks from known (proto)stellar sources provide
evidence that the ammonia molecules have been released from dust grains into
the gas-phase through the passage of shocks and not by stellar radiation. We
propose that the combined effect of Source I's proper motion and its
low-velocity outflow impinging on a pre-existing dense medium is responsible
for the excitation of ammonia and the Orion Hot Core. Finally, we found for the
first time evidence of a slow (5 km/s) and compact (1000 AU) outflow towards
IRc7.Comment: To appear in Astrophysical Journal Letters Special Issue on the EVLA.
8 pages, 4 figure
Coplanar Circumbinary Debris Disks
We present resolved Herschel images of circumbinary debris disks in the alpha
CrB (HD139006) and beta Tri (HD13161) systems. We find that both disks are
consistent with being aligned with the binary orbital planes. Though secular
perturbations from the binary can align the disk, in both cases the alignment
time at the distances at which the disk is resolved is greater than the stellar
age, so we conclude that the coplanarity was primordial. Neither disk can be
modelled as a narrow ring, requiring extended radial distributions. To satisfy
both the Herschel and mid-IR images of the alpha CrB disk, we construct a model
that extends from 1-300AU, whose radial profile is broadly consistent with a
picture where planetesimal collisions are excited by secular perturbations from
the binary. However, this model is also consistent with stirring by other
mechanisms, such as the formation of Pluto-sized objects. The beta Tri disk
model extends from 50-400AU. A model with depleted (rather than empty) inner
regions also reproduces the observations and is consistent with binary and
other stirring mechanisms. As part of the modelling process, we find that the
Herschel PACS beam varies by as much as 10% at 70um and a few % at 100um. The
70um variation can therefore hinder image interpretation, particularly for
poorly resolved objects. The number of systems in which circumbinary debris
disk orientations have been compared with the binary plane is now four. More
systems are needed, but a picture in which disks around very close binaries
(alpha CrB, beta Tri, and HD 98800, with periods of a few weeks to a year) are
aligned, and disks around wider binaries (99 Her, with a 50 yr period) are
misaligned, may be emerging. This picture is qualitatively consistent with the
expectation that the protoplanetary disks from which the debris emerged are
more likely to be aligned if their binaries have shorter periods.Comment: accepted to MNRA
Fluxon analogues and dark solitons in linearly coupled Bose-Einstein condensates
Two effectively one-dimensional parallel coupled Bose-Einstein condensates in
the presence of external potentials are studied. The system is modelled by
linearly coupled Gross-Pitaevskii equations. In particular, grey-soliton-like
solutions representing analogues of superconducting Josephson fluxons as well
as coupled dark solitons are discussed. Theoretical approximations based on
variational formulations are derived. It is found that the presence of a
magnetic trap can destabilize the fluxon analogues. However, stabilization is
possible by controlling the effective linear coupling between the condensates.Comment: 14 pages, 7 figures, The paper is to appear in Journal of Physics
HI Observations of the Asymptotic Giant Branch Star X Herculis: Discovery of an Extended Circumstellar Wake Superposed on a Compact High-Velocity Cloud
We report HI 21-cm line observations of the AGB star X Her obtained with the
Green Bank Telescope (GBT) and the Very Large Array (VLA). We have detected HI
emission totaling M_HI=2.1e-03 M_sun associated with the circumstellar envelope
of the star. The HI distribution exhibits a head-tail morphology, similar to
those previously observed around Mira and RS Cnc. The tail extends ~6.0' (0.24
pc) in the plane of the sky, along the direction of the star's space motion. We
also detect a velocity gradient of ~6.5 km/s across the envelope, consistent
with the HI tracing a turbulent wake that arises from the motion of a
mass-losing star through the ISM. GBT mapping of a 2x2deg region around X Her
reveals that the star lies (in projection) near the periphery of a much larger
HI cloud that also exhibits signatures of ISM interaction. The properties of
the cloud are consistent with those of compact high-velocity clouds. Using CO
observations, we have placed an upper limit on its molecular gas content of
N_H2<1.3e20 cm^-2. Although the distance to the cloud is poorly constrained,
the probability of a chance coincidence in position, velocity, and apparent
position angle of space motion between X Her and the cloud is extremely small,
suggesting a possible physical association. However, the large HI mass of the
cloud (~>2.4~M_sun) and the blueshift of its mean velocity relative to X Her
are inconsistent with an origin tied directly to stellar ejection. (abridged)Comment: Accepted to AJ; 47 pages, 15 figures; version with full resolution
figures available at
http://www.haystack.mit.edu/hay/staff/lmatthew/matthews_XHer.pd
A Documentary of High-Mass Star Formation: Probing the Dynamical Evolution of Orion Source I on 10-100 AU Scales using SiO Masers
A comprehensive picture of high-mass star formation has remained elusive, in
part because examples of high-mass YSOs tend to be relatively distant, deeply
embedded, and confused with other emission sources. These factors have impeded
dynamical investigations within tens of AU of high-mass YSOs--scales that are
critical for probing the interfaces where outflows from accretion disks are
launched and collimated. Using observations of SiO masers obtained with the VLA
and the VLBA, the KaLYPSO project is overcoming these limitations by mapping
the structure and dynamical/temporal evolution of the material 10-1000 AU from
the nearest high-mass YSO: Radio Source I in the Orion BN/KL region. Our data
include ~40 epochs of VLBA observations over a several-year period, allowing us
to track the proper motions of individual SiO maser spots and to monitor
changes in the physical conditions of the emitting material with time.
Ultimately these data will provide 3-D maps of the outflow structure over
approximately 30% of the outflow crossing time. Here we summarize recent
results from the KaLYPSO project, including evidence that high-mass star
formation is occurring via disk-mediated accretion.Comment: 5 pages; to appear in the proceedings of IAU Symposium 242,
Astrophysical Masers and their Environments, ed. J. Chapman & W. Baa
High temperature onset of field-induced transitions in the spin-ice compound Dy2Ti2O7
We have studied the field-dependent ac magnetic susceptibility of single
crystals of Dy2Ti2O7 spin ice along the [111] direction in the temperature
range 1.8 K - 7 K. Our data reflect the onset of local spin ice order in the
appearance of different field regimes. In particular, we observe a prominent
feature at approximately 1.0 T that is a precursor of the low-temperature
metamagnetic transition out of field-induced kagome ice, below which the
kinetic constraints imposed by the ice rules manifest themselves in a
substantial frequency-dependence of the susceptibility. Despite the relatively
high temperatures, our results are consistent with a monopole picture, and they
demonstrate that such a picture can give physical insight to the spin ice
systems even outside the low-temperature, low-density limit where monopole
excitations are well-defined quasiparticles
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