1,377 research outputs found
Effelsberg Observations of Excited-State (6.0 GHz) OH in Supernova Remnants and W3(OH)
While masers in the 1720 MHz transition of OH are detected toward many
supernova remnants (SNRs), no other OH transition is seen as a maser in SNRs.
We present a search for masers at 6049 MHz, which has recently been predicted
to produce masers by pure collisional excitation at conditions similar to that
required for 1720 MHz masing. The Effelsberg 100 m telescope was used to
observe the excited-state 6016, 6030, 6035, and 6049 MHz lines of OH toward
selected SNRs, most of which have previously-detected bright 1720 MHz masers.
No excited-state masers are found toward SNRs, consistent with previous
observations of the 6049 MHz and other excited-state transitions. We do not see
clear evidence of absorption toward SNR target positions, although we do see
evidence of absorption in the molecular cloud at +50 km/s near Sgr A East. Weak
absorption is detected at 6016 MHz toward W3(OH), while stronger, narrower
emission is seen at 6049 MHz, suggesting that the 6049 MHz emission is a
low-gain maser. We conclude that conditions in SNRs are not conducive to
excited-state maser emission, especially in excited-state satellite lines.Comment: 4 pages using emulateapj.cls including 2 tables and 1 figure,
accepted to ApJ
Dynamics of cholesteric structures in an electric field
Motivated by Lehmann-like rotation phenomena in cholesteric drops we study
the transverse drift of two types of cholesteric fingers, which form rotating
spirals in thin layers of cholesteric liquid crystal in an ac or dc electric
field. We show that electrohydrodynamic effects induced by Carr-Helfrich charge
separation or flexoelectric charge generation can describe the drift of
cholesteric fingers. We argue that the observed Lehmann-like phenomena can be
understood on the same basis.Comment: 4 pages, 4 figures, submitted to PR
EVLA Observations of OH Masers in ON 1
This Letter reports on initial Expanded Very Large Array (EVLA) observations
of the 6035 MHz masers in ON 1. The EVLA data are of good quality, lending
confidence in the new receiver system. Nineteen maser features, including six
Zeeman pairs, are detected. The overall distribution of 6035 MHz OH masers is
similar to that of the 1665 MHz OH masers. The spatial resolution is sufficient
to unambiguously determine that the magnetic field is strong (~ -10 mG) at the
location of the blueshifted masers in the north, consistent with Zeeman
splitting detected in 13441 MHz OH masers in the same velocity range. Left and
right circularly polarized ground-state features dominate in different regions
in the north of the source, which may be due to a combination of magnetic field
and velocity gradients. The combined distribution of all OH masers toward the
south is suggestive of a shock structure of the sort previously seen in W3(OH).Comment: 4 pages using emulateapj.cls including 2 tables and 2 color figure
Structure of W3(OH) from Very High Spectral Resolution Observations of 5 Centimeter OH Masers
Recent studies of methanol and ground-state OH masers at very high spectral
resolution have shed new light on small-scale maser processes. The nearby
source W3(OH), which contains numerous bright masers in several different
transitions, provides an excellent laboratory for high spectral resolution
techniques. We present a model of W3(OH) based on EVN observations of the
rotationally-excited 6030 and 6035 MHz OH masers taken at 0.024 km/s spectral
resolution. The 6.0 GHz masers are becoming brighter with time and show
evidence for tangential proper motions. We confirm the existence of a region of
magnetic field oriented toward the observer to the southeast and find another
such region to the northeast in W3(OH), near the champagne flow. The 6.0 GHz
masers trace the inner edge of a counterclockwise rotating torus feature.
Masers at 6030 MHz are usually a factor of a few weaker than at 6035 MHz but
trace the same material. Velocity gradients of nearby Zeeman components are
much more closely correlated than in the ground state, likely due to the
smaller spatial separation between Zeeman components. Hydroxyl maser peaks at
very long baseline interferometric resolution appear to have structure on
scales both smaller than that resolvable as well as on larger scales.Comment: 21 pages using emulateapj.cls including 16 figures and 2 tables,
accepted to Ap
Full-Polarization Observations of OH Masers in Massive Star-Forming Regions: I. Data
We present full-polarization VLBA maps of the ground-state, main-line, 2 Pi
3/2, J = 3/2 OH masers in 18 Galactic massive star-forming regions. This is the
first large polarization survey of interstellar hydroxyl masers at VLBI
resolution. A total of 184 Zeeman pairs are identified, and the corresponding
magnetic field strengths are indicated. We also present spectra of the NH3
emission or absorption in these star-forming regions. Analysis of these data
will be presented in a companion paper.Comment: 111 pages, including 42 figures and 21 tables, to appear in ApJ
5cm OH masers as diagnostics of physical conditions in star-forming regions
We demonstrate that the observed characteristics of the 5 cm OH masers in
star-forming regions can be explained with the same model and the same
parameters as the 18 cm and the 6 cm OH masers. In our already published study
of the 18 cm and the 6 cm OH masers in star-forming regions we had examined the
pumping of the 5 cm masers, but did not report the results we had found because
of some missing collision rate coefficients, which in principle could be
important. The recently published observations on the 5 cm masers of OH
encourage us to report our old calculations along with some new ones that we
have performed. These calculations, in agreement with the observations, reveal
the main lines at 5 cm as strong masers, the 6049 MHz satellite line as a weak
maser, and the 6017 MHz satellite line as never inverted for reasonable values
of the parametersComment: TeX 15 pages, 30 postscript figures, accepted by Ap
ALMA observations of TiO around VY Canis Majoris
Titanium dioxide, TiO, is a refractory species that could play a crucial
role in the dust-condensation sequence around oxygen-rich evolved stars. To
date, gas phase TiO has been detected only in the complex environment of
the red supergiant VY CMa. We aim to constrain the distribution and excitation
of TiO around VY CMa in order to clarify its role in dust formation. We
analyse spectra and channel maps for TiO extracted from ALMA science
verification data. We detect 15 transitions of TiO, and spatially resolve
the emission for the first time. The maps demonstrate a highly clumpy,
anisotropic outflow in which the TiO emission likely traces gas exposed to
the stellar radiation field. A roughly east-west oriented, accelerating
bipolar-like structure is found, of which the blue component runs into and
breaks up around a solid continuum component. A distinct tail to the south-west
is seen for some transitions, consistent with features seen in the optical and
near-infrared. We find that a significant fraction of TiO remains in the
gas phase outside the dust-formation zone and suggest that this species might
play only a minor role in the dust-condensation process around extreme
oxygen-rich evolved stars like VY CMa.Comment: Accepted for publication in Astronomy & Astrophysics, 25 pages, 20
figure
Disentangling effects of abiotic factors and biotic interactions on cross-taxon congruence in species turnover patterns of plants, moths and beetles
High cross-taxon congruence in species diversity patterns is essential for the use of surrogate taxa in biodiversity conservation, but presence and strength of congruence in species turnover patterns, and the relative contributions of abiotic environmental factors and biotic interaction towards this congruence, remain poorly understood. In our study, we used variation partitioning in multiple regressions to quantify cross-taxon congruence in community dissimilarities of vascular plants, geometrid and arciinid moths and carabid beetles, subsequently investigating their respective underpinning by abiotic factors and biotic interactions. Significant cross-taxon congruence observed across all taxon pairs was linked to their similar responses towards elevation change. Changes in the vegetation composition were closely linked to carabid turnover, with vegetation structure and associated microclimatic conditions proposed causes of this link. In contrast, moth assemblages appeared to be dominated by generalist species whose turnover was weakly associated with vegetation changes. Overall, abiotic factors exerted a stronger influence on cross-taxon congruence across our study sites than biotic interactions. The weak congruence in turnover observed particularly between plants and moths highlights the importance of multi-taxon approaches based on groupings of taxa with similar turnovers, rather than the use of single surrogate taxa or environmental proxies, in biodiversity assessments
Maser emission from SiO isotopologues traces the innermost 100 AU around Radio Source I in Orion BN/KL
We have used the Very Large Array (VLA) at 7 mm wavelength to image five
rotational transitions (J=1-0) from three SiO isotopologues towards Orion
BN/KL: 28SiO v=0,1,2; and 29SiO and 30SiO v=0. For the first time, we have
mapped the 29SiO and 30SiO J=1-0 emission, established the maser nature of the
emission, and confirmed association with the deeply embedded high-mass young
stellar object commonly denoted radio Source I. The 28SiO v=0 maser emission
shows a bipolar structure that extends over ~700 AU along a northeast-southwest
axis, and we propose that it traces a bipolar outflow driven by Source I. The
high-brightness isotopic SiO maser emission imaged with a ~0.2 arcsec
resolution has a more compact distribution, generally similar to that of the
28SiO v=1,2 emission, and it probably traces bulk gas flows in a region of
diameter <100 AU centered on Source I. On small scales of <10 AU, however,
compact 29SiO/30SiO v=0 and 28SiO v=1,2 emission features may be offset from
one another in position and line-of-sight velocity.
From a radiative transfer analysis based on a large velocity gradient (LVG)
pumping model, we derive similar temperatures and densities for the optimum
excitation of both 29SiO/30SiO v=0 and 28SiO v=1,2 masers, significantly higher
than required for 28SiO v=0 maser excitation. In order to account for the
small-scale differences among the isotopologues (v=0) and the main species
(v=1,2), follow-up radiative transfer modeling that incorporates non-local line
overlap among transitions of all SiO isotopic species may be required.Comment: 10 pages, 3 figures, accepted for publication by Ap
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