753 research outputs found
Deep into the Water Fountains: The case of IRAS 18043-2116
(Abridged) The formation of large-scale (hundreds to few thousands of AU)
bipolar structures in the circumstellar envelopes (CSEs) of post-Asymptotic
Giant Branch (post-AGB) stars is poorly understood. The shape of these
structures, traced by emission from fast molecular outflows, suggests that the
dynamics at the innermost regions of these CSEs does not depend only on the
energy of the radiation field of the central star. Deep into the Water
Fountains is an observational project based on the results of programs carried
out with three telescope facilities: The Karl G. Jansky Very Large Array
(JVLA), The Australia Telescope Compact Array (ATCA), and the Very Large
Telescope (SINFONI-VLT). Here we report the results of the observations towards
the WF nebula IRAS 180432116: Detection of radio continuum emission in the
frequency range 1.5GHz - 8.0GHz; HO maser spectral features and radio
continuum emission detected at 22GHz, and H ro-vibrational emission lines
detected at the near infrared. The high-velocity HO maser spectral
features, and the shock-excited H emission detected could be produced in
molecular layers which are swept up as a consequence of the propagation of a
jet-driven wind. Using the derived H column density, we estimated a
molecular mass-loss rate of the order of Myr. On the
other hand, if the radio continuum flux detected is generated as a consequence
of the propagation of a thermal radio jet, the mass-loss rate associated to the
outflowing ionized material is of the order of 10Myr.
The presence of a rotating disk could be a plausible explanation for the
mass-loss rates estimated.Comment: 10 pages, 5 figures. Accepted for publication in A&
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&
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
Spatially Resolved Magnetic Field Structure in the Disk of a T Tauri Star
Magnetic fields in accretion disks play a dominant role during the star
formation process but have hitherto been observationally poorly constrained.
Field strengths have been inferred on T Tauri stars themselves and possibly in
the innermost part of the accretion disk, but the strength and morphology of
the field in the bulk of the disk have not been observed. Unresolved
measurements of polarized emission (arising from elongated dust grains aligned
perpendicular to the field) imply average fields aligned with the disks.
Theoretically, the fields are expected to be largely toroidal, poloidal, or a
mixture of the two, which imply different mechanisms for transporting angular
momentum in the disks of actively accreting young stars such as HL Tau. Here we
report resolved measurements of the polarized 1.25 mm continuum emission from
HL Tau's disk. The magnetic field on a scale of 80 AU is coincident with the
major axis (~210 AU diameter) of the disk. From this we conclude that the
magnetic field inside the disk at this scale cannot be dominated by a vertical
component, though a purely toroidal field does not fit the data well either.
The unexpected morphology suggests that the magnetic field's role for the
accretion of a T Tauri star is more complex than the current theoretical
understanding.Comment: Accepted for publication in Natur
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
Charting Circumstellar Chemistry of Carbon-rich AGB Stars: I. ALMA 3 mm spectral surveys
AGB stars are major contributors to the chemical enrichment of the ISM
through nucleosynthesis and extensive mass loss. Most of our current knowledge
of AGB atmospheric and circumstellar chemistry, in particular in a C-rich
environment, is based on observations of the carbon star IRC+10216. We aim to
obtain a more generalised understanding of the chemistry in C-rich AGB CSEs by
studying a sample of three carbon stars, IRAS15194-5115, IRAS15082-4808, and
IRAS07454-7112, and test the archetypal status often attributed to IRC+10216.
We performed spatially resolved, unbiased spectral surveys in ALMA Band 3. We
identify a total of 132 rotational transitions from 49 molecular species. There
are two main morphologies of the brightness distributions: centrally-peaked
(e.g. HCN) and shell-like (e.g. CH). We estimated the sizes of the
molecular emitting regions using azimuthally-averaged radial profiles of the
line brightness distributions, and derived abundance estimates. Of the shell
distributions, the cyanopolyynes peak at slightly smaller radii than the
hydrocarbons, and CN and HNC show the most extended emission. The emitting
regions for each species are the smallest for IRAS07454-7112. We find that,
within the uncertainties of the analysis, the three stars present similar
abundances for most species, also compared to IRC+10216. We find that SiO is
more abundant in our three stars compared to IRC+10216. Our estimated isotopic
ratios match well the literature values for the sources. The observed
circumstellar chemistry appears very similar across our sample and compared to
that of IRC+10216, both in terms of the relative location of the emitting
regions and molecular abundances. This implies that, to a first approximation,
the chemical models tailored to IRC+10216 are able to reproduce the observed
chemistry in C-rich envelopes across roughly an order of magnitude in wind
density.Comment: 21 pages. 13 figures and 8 tables in the main text. 5 appendices
contain additional figures and tables. Appendix tables are available in
electronic form at the CDS, along with the reprocessed ALMA cubes and
spectra, at http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ or via anonymous
ftp to cdsarc.u-strasbg.fr(130.79.128.5). Accepted for publication in
Astronomy & Astrophysic
GASKAP -- The Galactic ASKAP Survey
A survey of the Milky Way disk and the Magellanic System at the wavelengths
of the 21-cm atomic hydrogen (HI) line and three 18-cm lines of the OH molecule
will be carried out with the Australian Square Kilometre Array Pathfinder
telescope. The survey will study the distribution of HI emission and absorption
with unprecedented angular and velocity resolution, as well as molecular line
thermal emission, absorption, and maser lines. The area to be covered includes
the Galactic plane (|b|< 10deg) at all declinations south of delta = +40deg,
spanning longitudes 167deg through 360deg to 79deg at b=0deg, plus the entire
area of the Magellanic Stream and Clouds, a total of 13,020 square degrees. The
brightness temperature sensitivity will be very good, typically sigma_T ~ 1 K
at resolution 30arcsec and 1 km/s. The survey has a wide spectrum of scientific
goals, from studies of galaxy evolution to star formation, with particular
contributions to understanding stellar wind kinematics, the thermal phases of
the interstellar medium, the interaction between gas in the disk and halo, and
the dynamical and thermal states of gas at various positions along the
Magellanic Stream.Comment: 45 pages, 8 figures, Pub. Astron. Soc. Australia (in press
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