716 research outputs found
From the ashes: JVLA observations of water fountain nebula candidates show the rebirth of IRAS 18455+0448
[abridged] The class of water fountain nebulae is thought to represent the
stage of the earliest onset of collimated bipolar outflows during the
post-Asymptotic Giant Branch phase. They thus play a crucial role in the study
of the formation of bipolar Planetary Nebulae (PNe). To date, 14 water fountain
nebulae have been identified. The identification of more sources in this unique
stage of stellar evolution will enable us to study the origin of bipolar PNe
morphologies in more detail. We present the results of seven sources observed
with the JVLA that were identified as water fountain candidates in an
Effelsberg 100m telescope survey of 74 AGB and early post-AGB stars. We find
that our sample of water fountain candidates displays strong variability in
their 22 GHz H2O maser spectra. The JVLA observations show an extended bipolar
H2O maser outflow for one source, the OH/IR star IRAS 18455+0448. This source
was previously classified as a 'dying' OH/IR star based on the exponential
decrease of its 1612 MHz OH maser and the lack of H2O masers. We therefore also
re-observed the 1612, 1665, and 1667 MHz OH masers. We confirm that the 1612
MHz masers have not reappeared and find that the 1665/1667 MHz masers have
decreased in strength by several orders of magnitude during the last decade.
The OH/IR star IRAS 18455+0448 is confirmed to be a new addition to the class
of water fountain nebulae. Its kinematic age is approximately 70 yr, but could
be lower, depending on the distance and inclination. Previous observations
indicate, with significant uncertainty, that IRAS 18455+0448 has a surprisingly
low mass compared to available estimates for other water fountain nebulae. The
available historical OH maser observations make IRAS 18455+0448 unique for the
study of water fountain nebulae and the launch of post-AGB bipolar outflows...Comment: 8 pages, 5 figures, accepted for publication in A&A (revised minor
typos
Polarisation of molecular lines in the circumstellar envelope of the post-Asymptotic Giant Branch star OH 17.7-2.0
(abridged) The role of magnetic field in the shaping of Planetary Nebulae
(PNe), either directly or indirectly after being enhanced by binary
interaction, has long been a topic of debate. Large scale magnetic fields
around pre-PNe have been inferred from polarisation observations of masers.
However, because masers probe very specific regions, it is still unclear if the
maser results are representative of the intrinsic magnetic field in the
circumstellar envelope (CSE). Molecular line polarisation can provide important
information about the magnetic field. A comparison between the field morphology
determined from maser observations and that observed in the more diffuse CO
gas, can reveal if the two tracers probe the same magnetic field. We compare
observations taken with ALMA of molecular line polarisation around the
post-Asymptotic Giant Branch)/pre-PNe star OH~17.7-2.0 with previous
observations of polarisation in the 1612~MHz OH maser region. We detect
CO~ molecular line polarisation at a level of that displays an
ordered linear polarisation structure. We find that, correcting for Faraday
rotation of the OH~maser linear polarisation vectors, the OH and CO linearly
polarised emission trace the same large scale magnetic field. A structure
function analysis of the CO linear polarisation reveals a plane-of-the-sky
magnetic field strength of ~mG in the CO region, consistent with
previous OH Zeeman observations. The consistency of the ALMA CO molecular line
polarisation with maser observations indicate that both can be used to
determine the magnetic field in CSEs. The existence of a strong, ordered,
magnetic-field around OH 17.7-2.0 indicates that magnetic fields are likely
involved in the formation of this bipolar pre-PNe.Comment: 13 pages, 9 figures; accepted for publication in A&
Submillimeter H2O masers in water-fountain nebulae
We report the first detection of submillimeter water maser emission toward
water-fountain nebulae, which are post-AGB stars that exhibit high-velocity
water masers. Using APEX we found emission in the ortho-H2O (10_29-9_36)
transition at 321.226 GHz toward three sources: IRAS 15445-5449, IRAS
18043-2116 and IRAS 18286-0959. Similarly to the 22 GHz masers, the
submillimeter water masers are expanding with a velocity larger than that of
the OH masers, suggesting that these masers also originate in fast bipolar
outflows. In IRAS 18043-2116 and IRAS 18286-0959, which figure among the
sources with the fastest water masers, the velocity range of the 321 GHz masers
coincides with that of the 22 GHz masers, indicating that they likely coexist.
Towards IRAS 15445-5449 the submillimeter masers appear in a different velocity
range, indicating that they are tracing different regions. The intensity of the
submillimeter masers is comparable to that of the 22 GHz masers, implying that
the kinetic temperature of the region where the masers originate should be Tk >
1000 K. We propose that the passage of two shocks through the same gas can
create the conditions necessary to explain the presence of strong high-velocity
321 GHz masers coexisting with the 22 GHz masers in the same region.Comment: 4 pages, 1 figure. Accepted for publication in A&A Letter
Kinematics of the Outflow From The Young Star DG Tau B: Rotation in the vicinities of an optical jet
We present CO(2-1) line and 1300 m continuum observations made
with the Submillimeter Array (SMA) of the young star DG Tau B. We find, in the
continuum observations, emission arising from the circumstellar disk
surrounding DG Tau B. The CO(2-1) line observations, on the other hand,
revealed emission associated with the disk and the asymmetric outflow related
with this source. Velocity asymmetries about the flow axis are found over the
entire length of the flow. The amplitude of the velocity differences is of the
order of 1 -- 2 km s over distances of about 300 -- 400 AU. We interpret
them as a result of outflow rotation. The sense of the outflow and disk
rotation is the same. Infalling gas from a rotating molecular core cannot
explain the observed velocity gradient within the flow. Magneto-centrifugal
disk winds or photoevaporated disk winds can produce the observed rotational
speeds if they are ejected from a keplerian disk at radii of several tens of
AU. Nevertheless, these slow winds ejected from large radii are not very
massive, and cannot account for the observed linear momentum and angular
momentum rates of the molecular flow. Thus, the observed flow is probably
entrained material from the parent cloud. DG Tau B is a good laboratory to
model in detail the entrainment process and see if it can account for the
observed angular momentum.Comment: Accepted to Ap
HD101584: Circumstellar characteristics and evolutionary status
We have performed a study of the characteristics of the circumstellar
environment of the binary object HD101584, that provides information on a
likely evolutionary scenario. We have obtained and analysed ALMA observations,
complemented with observations using APEX, of a large number of molecular
lines. An analysis of the spectral energy distribution has also been performed.
Emissions from 12 molecular species (not counting isotopologues) have been
observed, and most of them mapped with angular resolutions in the range 0.1" to
0.6". Four circumstellar components are identified: i) a central compact source
of size 0.15", ii) an expanding equatorial density enhancement (a flattened
density distribution in the plane of the orbit) of size 3", iii) a bipolar
high-velocity outflow (150 km/s), and iv) an hourglass structure. The outflow
is directed almost along the line of sight. There is evidence of a second
bipolar outflow. The mass of the circumstellar gas is 0.5[D/1 kpc]^2 Msun,
about half of it lies in the equatorial density enhancement. The dust mass is
0.01[D/1 kpc]^2 Msun, and a substantial fraction of this is in the form of
large-sized, up to 1 mm, grains. The estimated kinetic age of the outflow is
770[D/1 kpc] yr. The kinetic energy and the scalar momentum of the accelerated
gas are estimated to be 7x10^(45)[D/1 kpc]^2 erg and 10^(39)[D/1 kpc]^2 g cm/s,
respectively. We provide good evidence that the binary system HD101584 is in a
post-common-envelope-evolution phase, that ended before a stellar merger.
Isotope ratios combined with stellar mass estimates suggest that the primary
star's evolution was terminated already on the first red giant branch (RGB).
Most of the energy required to drive the outflowing gas was probably released
when material fell towards the companion.Comment: Accepted for publication in A&
Evolution of the Outflow in the Water Fountain Source IRAS 18043-2116
We present the spectral and spatial evolution of H2O masers associated with the water fountain source IRAS 18043-2116, found in observations with the Nobeyama 45 m Telescope and the Australia Telescope Compact Array. We have found new highest-velocity components of the H2O masers (at the redshifted side V (LSR) similar or equal to 376 km s(-1) and at the blueshifted side V (LSR) similar or equal to -165 km s(-1)), and the resulting velocity spread of similar or equal to 540 km s(-1) breaks the speed record of fast jets/outflows in this type of sources. The locations of those components have offsets from the axis joining the two major maser clusters, indicating a large opening angle of the outflow (similar to 60 degrees). The evolution of the maser cluster separation of similar to 2.9 mas yr(-1) and the compact (similar to 0.\u27\u27 2) CO emission source mapped with the Atacama Large Millimeter-submillimeter Array suggest a very short (similar to 30 yr) timescale of the outflow. We also confirmed an increase in the flux density of the 22 GHz continuum source. The properties of the jet and the continuum sources and their possible evolution in the transition to the planetary nebula phase are further discussed
Synthesis of a Graphene-Encapsulated Fe 3 C/Fe Catalyst Supported on Sporopollenin Exine Capsules and Its Use for the Reverse Water–Gas Shift Reaction
Bioderived materials have emerged as sustainable catalyst supports for several heterogeneous reactions owing to their naturally occurring hierarchal pore size distribution, high surface area, and thermal and chemical stability. We utilize sporopollenin exine capsules (SpECs), a carbon-rich byproduct of pollen grains, composed primarily of polymerized and cross-linked lipids, to synthesize carbon-encapsulated iron nanoparticles via evaporative precipitation and pyrolytic treatments. The composition and morphology of the macroparticles were influenced by the precursor iron acetate concentration. Most significantly, the formation of crystalline phases (Fe3C, α-Fe, and graphite) detected via X-ray diffraction spectroscopy showed a critical dependence on iron loading. Significantly, the characteristic morphology and structure of the SpECs were largely preserved after high-temperature pyrolysis. Analysis of Brunauer–Emmett–Teller surface area, the D and G bands from Raman spectroscopy, and the relative ratio of the C═C to C–C bonding from high-resolution X-ray photoelectron spectroscopy suggests that porosity, surface area, and degree of graphitization were easily tuned by varying the Fe loading. A mechanism for the formation of crystalline phases and meso-porosity during the pyrolysis process is also proposed. SpEC-Fe10% proved to be highly active and selective for the reverse water–gas shift reaction at high temperatures (>600 °C)
VLTI/PIONIER reveals the close environment of the evolved system HD101584
Context: The observed orbital characteristics of post-asymptotic giant branch
(post-AGB) and post-red giant branch (post-RGB) binaries are not understood. We
suspect that the missing ingredients to explain them probably lie in the
continuous interaction of the central binary with its circumstellar
environment. Aims: We aim at studying the circumbinary material in these
complex systems by investigating the connection between the innermost and
large-scale structures. Methods: We perform high-angular resolution
observations in the near-infrared continuum of HD101584, which has a complex
structure as seen at millimeter wavelengths with a disk-like morphology and a
bipolar outflow due to an episode of strong binary interaction. To account for
the complexity of the target we first perform an image reconstruction and use
this result to fit a geometrical model to extract the morphological and thermal
features of the environment. Results: The image reveals an unexpected
double-ring structure. We interpret the inner ring to be produced by emission
from dust located in the plane of the disk and the outer ring to be produced by
emission from dust that is located 1.6[D/1kpc] au above the disk plane. The
inner ring diameter (3.94[D/1kpc] au), and temperature (T=154010K) are
compatible with the dust sublimation front of the disk. The origin of the
out-of-plane ring (with a diameter of 7.39[D/1kpc] au and a temperature of
1014K) could be due to episodic ejection or a dust condensation front in
the outflow. Conclusion: The observed outer ring is possibly linked with the
blue-shifted side of the large scale outflow seen by ALMA and is tracing its
launching location to the central star. Such observations give morphological
constraints on the ejection mechanism. Additional observations are needed to
constrain the origin of the out-of-plane structure.Comment: Accepted to A&A. 14 pages, 13 figure
Magnetically aligned dust and SiO maser polarisation in the envelope of the red supergiant VY Canis Majoris
International audienceWe use Atacama Large Millimeter/submillimeter Array Band 5 science verification observations of the red supergiant VY CMa to study the polarization of SiO thermal/masers lines and dust continuum at ~1.7 mm wavelength. We analyse both linear and circular polarization and derive the magnetic field strength and structure, assuming the polarization of the lines originates from the Zeeman effect, and that of the dust originates from aligned dust grains. We also discuss other effects that could give rise to the observed polarization. We detect, for the first time, significant polarization (~3%) of the circumstellar dust emission at millimeter wavelengths. The polarization is uniform with an electric vector position angle of . Varying levels of linear polarization are detected for the J=4-3 28SiO v=0, 1, 2, and 29SiO v=0, 1 lines, with the strongest polarization fraction of ~30% found for the 29SiO v=1 maser. The linear polarization vectors rotate with velocity, consistent with earlier observations. We also find significant (up to ~1%) circular polarization in several lines, consistent with previous measurements. We conclude that the detection is robust against calibration and regular instrumental errors, although we cannot yet fully rule out non-standard instrumental effects. Emission from magnetically aligned grains is the most likely origin of the observed continuum polarization. This implies that the dust is embedded in a magnetic field >13 mG. The maser line polarization traces the magnetic field structure. The magnetic field in the gas and dust is consistent with an approximately toroidal field configuration, but only higher angular resolution observations will be able to reveal more detailed field structure. If the circular polarization is due to Zeeman splitting, it indicates a magnetic field strength of ~1-3 Gauss, consistent with previous maser observations
The ALMA Survey of 70 μm Dark High-mass Clumps in Early Stages (ASHES). VI. The Core-scale CO Depletion
Studying the physical and chemical properties of cold and dense molecular clouds is crucial for the understanding of how stars form. Under the typical conditions of infrared dark clouds, CO is removed from the gas phase and trapped onto the surface of dust grains by the so-called depletion process. This suggests that the CO-depletion factor (f D ) can be a useful chemical indicator for identifying cold and dense regions (i.e., prestellar cores). We have used the 1.3 mm continuum and C18O (2-1) data observed at the resolution of ∼5000 au in the ALMA Survey of 70 μm Dark High-mass Clumps in Early Stages (ASHES) to construct averaged maps of f D in 12 clumps to characterize the earliest stages of the high-mass star formation process. The average f D determined for 277 of the 294 ASHES cores follows an unexpected increase from the prestellar to the protostellar stage. If we exclude the temperature effect due to the slight variations in the NH3 kinetic temperature among different cores, we explain this result as a dependence primarily on the average gas density, which increases in cores where protostellar conditions prevail. This shows that f D determined in high-mass star-forming regions at the core scale is insufficient to distinguish among prestellar and protostellar conditions for the individual cores and should be complemented by information provided by additional tracers. However, we confirm that the clump-averaged f D values correlate with the luminosity-to-mass ratio of each source, which is known to trace the evolution of the star formation process
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