98 research outputs found
Nonlocal radiative coupling in non monotonic stellar winds
There is strong observational evidence of shocks and clumping in
radiation-driven stellar winds from hot, luminous stars. The resulting non
monotonic velocity law allows for radiative coupling between distant locations,
which is so far not accounted for in hydrodynamic wind simulations. In the
present paper, we determine the Sobolev source function and radiative line
force in the presence of radiative coupling in spherically symmetric flows,
extending the geometry-free formalism of Rybicki and Hummer (1978) to the case
of three-point coupling, which can result from, e.g., corotating interaction
regions, wind shocks, or mass overloading. For a simple model of an overloaded
wind, we find that, surprisingly, the flow decelerates at all radii above a
certain height when nonlocal radiative coupling is accounted for. We discuss
whether radiation-driven winds might in general not be able to re-accelerate
after a non monotonicity has occurred in the velocity law.Comment: accepted by A&A, 8 pages, 4 figure
New ATCA, ALMA and VISIR observations of the candidate LBV SK-67266 (S61): the nebular mass from modelling 3D density distributions
We present new observations of the nebula around the Magellanic candidate
Luminous Blue Variable S61. These comprise high-resolution data acquired with
the Australia Telescope Compact Array (ATCA), the Atacama Large
Millimetre/Submillimetre Array (ALMA), and VISIR at the Very Large Telescope
(VLT). The nebula was detected only in the radio, up to 17 GHz. The 17 GHz ATCA
map, with 0.8 arcsec resolution, allowed a morphological comparison with the
H Hubble Space Telescope image. The radio nebula resembles a spherical
shell, as in the optical. The spectral index map indicates that the radio
emission is due to free-free transitions in the ionised, optically thin gas,
but there are hints of inhomogeneities. We present our new public code RHOCUBE
to model 3D density distributions, and determine via Bayesian inference the
nebula's geometric parameters. We applied the code to model the electron
density distribution in the S61 nebula. We found that different distributions
fit the data, but all of them converge to the same ionised mass, ~0.1 , which is an order of magnitude smaller than previous estimates. We
show how the nebula models can be used to derive the mass-loss history with
high-temporal resolution. The nebula was probably formed through stellar winds,
rather than eruptions. From the ALMA and VISIR non-detections, plus the derived
extinction map, we deduce that the infrared emission observed by space
telescopes must arise from extended, diffuse dust within the ionised region.Comment: 17 pages, 9 figures. Authors list corrected. In press in MNRAS.
RHOCUBE code available online ( https://github.com/rnikutta/rhocube
ALMA observations of cool dust in a low-metallicity starburst, SBS0335-052
We present Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 0 Band 7
observations of an extremely metal-poor dwarf starburst galaxy in the Local
Universe, SBS0335-052 (12+log(O/H)~7.2). With these observations, dust is
detected at 870micron (ALMA Band 7), but 87% of the flux in this band is due to
free-free emission from the starburst. We have compiled a spectral energy
distribution (SED) of SBS0335-052 that spans almost 6 orders of magnitude in
wavelength and fit it with a spherical dust shell heated by a single-age
stellar population; the best-fit model gives a dust mass of (3.8+/-0.6)x10^4
Msun. We have also constructed a SED including Herschel archival data for
IZw18, another low-metallicity dwarf starburst (12+log(O/H)=7.17), and fit it
with a similar model to obtain a dust mass of (3.4+/-1.0)x10^2 Msun. Compared
with their atomic gas mass, the dust mass of SBS0335-052 far exceeds the
prediction of a linear trend of dust-to-gas mass ratio with metallicity, while
IZw18 falls far below. We use gas scaling relations to assess a putative
missing gas component in both galaxies and find that the missing, possibly
molecular, gas in SBS0335-052 is a factor of 6 times higher than the value
inferred from the observed HI column density; in IZw18 the missing component is
4 times smaller. Ultimately, despite their similarly low metallicity, the
differences in gas and dust column densities in SBS0335-052 and IZw18 suggest
that metal abundance does not uniquely define star-formation processes. At some
level, self-shielding and the survival of molecules may depend just as much on
gas and dust column density as on metallicity. The effects of low metallicity
may at least be partially compensated for by large column densities in the
interstellar medium.Comment: 15 pages, 11 figures, accepted for publication in A&
The Luminous Blue Variable RMC127 as seen with ALMA and ATCA
We present ALMA and ATCA observations of the luminous blue variable \rmc. The
radio maps show for the first time the core of the nebula and evidence that the
nebula is strongly asymmetric with a Z-pattern shape. Hints of this morphology
are also visible in the archival \emph{HST} image, which overall
resembles the radio emission. The emission mechanism in the outer nebula is
optically thin free-free in the radio. At high frequencies, a component of
point-source emission appears at the position of the star, up to the ALMA
frequencies. The rising flux density distribution () of this object suggests thermal emission from the ionized
stellar wind and indicates a departure from spherical symmetry with
. We examine different scenarios to explain this excess
of thermal emission from the wind and show that this can arise from a bipolar
outflow, supporting the suggestion by other authors that the stellar wind of
\rmc is aspherical. We fit the data with two collimated ionized wind models and
we find that the mass-loss rate can be a factor of two or more smaller than in
the spherical case. We also fit the photometry obtained by IR space telescopes
and deduce that the mid- to far-IR emission must arise from extended, cool
() dust within the outer ionized nebula. Finally we discuss two
possible scenarios for the nebular morphology: the canonical single star
expanding shell geometry, and a precessing jet model assuming presence of a
companion star.Comment: Accepted for publication in ApJ (minor revision included
AGN Dusty Tori: II. Observational Implications of Clumpiness
From extensive radiative transfer calculations we find that clumpy torus
models with \No \about 5--15 dusty clouds along radial equatorial rays
successfully explain AGN infrared observations. The dust has standard Galactic
composition, with individual cloud optical depth \tV \about 30--100 at visual.
The models naturally explain the observed behavior of the 10\mic silicate
feature, in particular the lack of deep absorption features in AGN of any type.
The weak 10\mic emission feature tentatively detected in type 2 QSO can be
reproduced if in these sources \No drops to \about 2 or \tV exceeds \about 100.
The clouds angular distribution must have a soft-edge, e.g., Gaussian profile,
the radial distribution should decrease as or . Compact tori can
explain all observations, in agreement with the recent interferometric evidence
that the ratio of the torus outer to inner radius is perhaps as small as \about
5--10. Clumpy torus models can produce nearly isotropic IR emission together
with highly anisotropic obscuration, as required by observations. In contrast
with strict variants of unification schemes where the viewing-angle uniquely
determines the classification of an AGN into type 1 or 2, clumpiness implies
that it is only a probabilistic effect; a source can display type 1 properties
even from directions close to the equatorial plane. The fraction of obscured
sources depends not only on the torus angular thickness but also on the cloud
number \No. The observed decrease of this fraction at increasing luminosity can
be explained with a decrease of either torus angular thickness or cloud number,
but only the latter option explains also the possible emergence of a 10\mic
emission feature in QSO2.Comment: To appear in ApJ September 20, 200
Near-Infrared Polarimetric Adaptive Optics Observations of NGC 1068: A torus created by a hydromagnetic outflow wind
We present J' and K' imaging linear polarimetric adaptive optics observations
of NGC 1068 using MMT-Pol on the 6.5-m MMT. These observations allow us to
study the torus from a magnetohydrodynamical (MHD) framework. In a 0.5" (30 pc)
aperture at K', we find that polarisation arising from the passage of radiation
from the inner edge of the torus through magnetically aligned dust grains in
the clumps is the dominant polarisation mechanism, with an intrinsic
polarisation of 7.0%2.2%. This result yields a torus magnetic field
strength in the range of 482 mG through paramagnetic alignment, and
139 mG through the Chandrasekhar-Fermi method. The measured
position angle (P.A.) of polarisation at K is found to be similar to the
P.A. of the obscuring dusty component at few parsec scales using infrared
interferometric techniques. We show that the constant component of the magnetic
field is responsible for the alignment of the dust grains, and aligned with the
torus axis onto the plane of the sky. Adopting this magnetic field
configuration and the physical conditions of the clumps in the MHD outflow wind
model, we estimate a mass outflow rate 0.17 M yr at 0.4
pc from the central engine for those clumps showing near-infrared dichroism.
The models used were able to create the torus in a timescale of 10
yr with a rotational velocity of 1228 km s at 0.4 pc. We conclude
that the evolution, morphology and kinematics of the torus in NGC 1068 can be
explained within a MHD framework.Comment: 14 pages, 4 figures, Accepted by MNRA
Mid-infrared imaging- and spectro-polarimetric subarcsecond observations of NGC 1068
We present sub-arcsecond 7.513 m imaging- and spectro-polarimetric
observations of NGC 1068 using CanariCam on the 10.4-m Gran Telescopio
CANARIAS. At all wavelengths, we find:
(1) A 90 60 pc extended polarized feature in the northern ionization
cone, with a uniform 44 polarization angle. Its polarization
arises from dust and gas emission in the ionization cone, heated by the active
nucleus and jet, and further extinguished by aligned dust grains in the host
galaxy. The polarization spectrum of the jet-molecular cloud interaction at
24 pc from the core is highly polarized, and does not show a silicate
feature, suggesting that the dust grains are different from those in the
interstellar medium.
(2) A southern polarized feature at 9.6 pc from the core. Its
polarization arises from a dust emission component extinguished by a large
concentration of dust in the galaxy disc. We cannot distinguish between dust
emission from magnetically aligned dust grains directly heated by the jet close
to the core, and aligned dust grains in the dusty obscuring material
surrounding the central engine. Silicate-like grains reproduce the polarized
dust emission in this feature, suggesting different dust compositions in both
ionization cones.
(3) An upper limit of polarization degree of 0.3 per cent in the core. Based
on our polarization model, the expected polarization of the obscuring dusty
material is 0.1 per cent in the 813 m wavelength range. This
low polarization may be arising from the passage of radiation through aligned
dust grains in the shielded edges of the clumps.Comment: 17 pages, 10 figures, accepted for publication at MNRA
Counter-rotation and High-velocity Outflow in the Parsec-scale Molecular Torus of NGC 1068
We present 1.4 pc resolution observations of 256 GHz nuclear radio continuum
and HCN () in the molecular torus of NGC 1068. The integrated radio
continuum emission has a flat spectrum consistent with free-free emission and
resolves into an X-shaped structure resembling an edge-brightened bicone. HCN
is detected in absorption against the continuum, and the absorption spectrum
shows a pronounced blue wing that suggests a high-velocity molecular outflow
with speeds reaching 450 km/s. Analysis of the off-nucleus emission line
kinematics and morphology reveals two nested, rotating disk components. The
inner disk, inside pc, has kinematics consistent with the nearly
edge-on, geometrically thin water megamaser disk in Keplerian rotation around a
central mass of 1.66\times 10^7\,\mbox{M}_\odot. The outer disk, which
extends to ~pc radius, counter-rotates relative to the inner disk. The
rotation curve of the outer disk is consistent with rotation around the same
central mass as the megamaser disk but in the opposite sense. The morphology of
the molecular gas is asymmetric around the nuclear continuum source. We
speculate that the outer disk formed from more recently introduced molecular
gas falling out of the host galaxy or from a captured dwarf satellite galaxy.
In NGC 1068, we find direct evidence that the molecular torus consists of
counter-rotating and misaligned disks on parsec scales.ERC grant 695671 'QUENCH
HIGH-VELOCITY BIPOLAR MOLECULAR EMISSION from AN AGN TORUS
We have detected in ALMA observations CO J = 6 - 5 emission from the nucleus
of the Seyfert galaxy NGC 1068. The low-velocity (up to +/- 70 km/s relative to
systemic) CO emission resolves into a 12x7 pc structure, roughly aligned with
the nuclear radio source. Higher-velocity emission (up to +/- 400 km/s) is
consistent with a bipolar outflow in a direction nearly perpendicular (roughly
80 degrees) to the nuclear disk. The position-velocity diagram shows that in
addition to the outflow, the velocity field may also contain rotation about the
disk axis. These observations provide compelling evidence in support of the
disk-wind scenario for the AGN obscuring torus.FONDECYT (Grant ID: 3140436), Science and Technology Facilities CouncilThis is the author accepted manuscript. The final version is available from Institute of Physics Publishing via http://dx.doi.org/10.3847/2041-8205/829/1/L
The Dusty Tori of Nearby QSOs as Constrained by High-Resolution Mid-IR Observations
We present mid-infrared (MIR; 7.5–13.5 μm) imaging and spectroscopy observations obtained with the CanariCam (CC) instrument on the 10.4-m Gran Telescopio CANARIAS for a sample of 20 nearby, MIR bright and X-ray luminous quasi-stellar objects (QSOs). We find that for the majority of QSOs the MIR emission is unresolved at angular scales ∼0.3 arcsec, corresponding to physical scales ≲ 600 pc. We find that the higher-spatial resolution CC spectra have similar shapes to those obtained with Spitzer/IRS, and hence we can assume that the spectra are not heavily contaminated by extended emission in the host galaxy. We thus take advantage of the higher signal-to-noise ratio Spitzer/IRS spectra, as a fair representation of the nuclear emission, to decompose it into a combination of active galactic nuclei (AGN), polycyclic aromatic hydrocarbon (PAH) and stellar components. In most cases, the AGN is the dominant component, with a median contribution of 85 per cent of the continuum light at MIR (5–15 μm) within the IRS slit. This IR AGN emission is well reproduced by clumpy torus models. We find evidence for significant differences in the parameters that describe the dusty tori of QSOs when compared with the same parameters of Seyfert 1 and 2 nuclei. In particular, we find a lower number of clouds (N0 ≲ 12), steeper radial distribution of clouds (q ∼ 1.5–3.0) and clouds that are less optically thick (τV ≲ 100) than in Seyfert 1, which could be attributed to dusty structures that have been partially evaporated and piled up by the higher radiation field in QSOs. We find that the combination of the angular width σtorus, viewing angle i, and number of clouds along the equatorial line, N0, produces large escape probabilities (Pesc \u3e 2 per cent) and low geometrical covering factors (f2 ≲ 0.6), as expected for AGN with broad lines in their optical spectra
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