510 research outputs found
Constraining the Circumbinary Envelope of Z CMa via imaging polarimetry
Z CMa is a complex binary system, composed of a Herbig Be and an FU Ori star.
The Herbig star is surrounded by a dust cocoon of variable geometry, and the
whole system is surrounded by an infalling envelope. Previous
spectropolarimetric observations have reported a preferred orientation of the
polarization angle, perpendicular to the direction of a large, parsec-sized jet
associated with the Herbig star. The variability in the amount of polarized
light has been associated to changes in the geometry of the dust cocoon that
surrounds the Herbig star. We aim to constrain the properties of Z CMa by means
of imaging polarimetry at optical wavelengths. Using ExPo, a dual-beam imaging
polarimeter which operates at optical wavelengths, we have obtained imaging
(linear) polarimetric data of Z CMa. Our observations were secured during the
return to quiescence after the 2008 outburst. We detect three polarized
features over Z CMa. Two of these features are related to the two jets reported
in this system: the large jet associated to the Herbig star, and the micro-jet
associated to the FU Ori star. Our results suggest that the micro-jet extends
to a distance ten times larger than reported in previous studies. The third
feature suggests the presence of a hole in the dust cocoon that surrounds the
Herbig star of this system. According to our simulations, this hole can produce
a pencil beam of light that we see scattered off the low-density envelope
surrounding the system.Comment: Accepted for publication in A\&
The color dependent morphology of the post-AGB star HD161796
Context. Many protoplanetary nebulae show strong asymmetries in their
surrounding shell, pointing to asymmetries during the mass loss phase.
Questions concerning the origin and the onset of deviations from spherical
symmetry are important for our understanding of the evolution of these objects.
Here we focus on the circumstellar shell of the post-AGB star HD 161796. Aims.
We aim at detecting signatures of an aspherical outflow, as well as to derive
the properties of it. Methods. We use the imaging polarimeter ExPo (the extreme
polarimeter), a visitor instrument at the William Herschel Telescope, to
accurately image the dust shell surrounding HD 161796 in various wavelength
filters. Imaging polarimetry allows us to separate the faint, polarized, light
from circumstellar material from the bright, unpolarized, light from the
central star. Results. The shell around HD 161796 is highly aspherical. A clear
signature of an equatorial density enhancement can be seen. This structure is
optically thick at short wavelengths and changes its appearance to optically
thin at longer wavelengths. In the classification of the two different
appearances of planetary nebulae from HST images it changes from being
classified as DUPLEX at short wavelengths to SOLE at longer wavelengths. This
strengthens the interpretation that these two appearances are manifestations of
the same physical structure. Furthermore, we find that the central star is
hotter than often assumed and the relatively high observed reddening is due to
circumstellar rather than interstellar extinction.Comment: Accepted for publication in A&
The Extreme Polarimeter: Design, Performance, First Results & Upgrades
Well over 700 exoplanets have been detected to date. Only a handful of these
have been observed directly. Direct observation is extremely challenging due to
the small separation and very large contrast involved. Imaging polarimetry
offers a way to decrease the contrast between the unpolarized starlight and the
light that has become linearly polarized after scattering by circumstellar
material. This material can be the dust and debris found in circumstellar
disks, but also the atmosphere or surface of an exoplanet. We present the
design, calibration approach, polarimetric performance and sample observation
results of the Extreme Polarimeter, an imaging polarimeter for the study of
circumstellar environments in scattered light at visible wavelengths. The
polarimeter uses the beam-exchange technique, in which the two orthogonal
polarization states are imaged simultaneously and a polarization modulator
swaps the polarization states of the two beams before the next image is taken.
The instrument currently operates without the aid of Adaptive Optics. To reduce
the effects of atmospheric seeing on the polarimetry, the images are taken at a
frame rate of 35 fps, and large numbers of frames are combined to obtain the
polarization images. Four successful observing runs have been performed using
this instrument at the 4.2 m William Herschel Telescope on La Palma, targeting
young stars with protoplanetary disks as well as evolved stars surrounded by
dusty envelopes. In terms of fractional polarization, the instrument
sensitivity is better than 10^-4. The contrast achieved between the central
star and the circumstellar source is of the order 10^-6. We show that our
calibration approach yields absolute polarization errors below 1%
The inner environment of Z~CMa: High-Contrast Imaging Polarimetry with NaCo
Context. Z\,CMa is a binary composed of an embedded Herbig Be and an FU Ori
class star separated by au. Observational evidence indicate a complex
environment in which each star has a circumstellar disk and drives a jet, and
the whole system is embedded in a large dusty envelope.
Aims. We aim to probe the circumbinary environment of Z\,CMa in the inner 400
au in scattered light.
Methods. We use high contrast imaging polarimetry with VLT/NaCo at and
bands.
Results. The central binary is resolved in both bands. The polarized images
show three bright and complex structures: a common dust envelope, a sharp
extended feature previously reported in direct light, and an intriguing bright
clump located 0\farcs3 south of the binary, which appears spatially connected
to the sharp extended feature.
Conclusions.We detect orbital motion when compared to previous observations,
and report a new outburst driven by the Herbig star. Our observations reveal
the complex inner environment of Z\,CMa with unprecedented detail and contrast.Comment: Accepted for publication in A&A Letter
Constraining the mass of the planet(s) sculpting a disk cavity. The intriguing case of 2MASS J16042165-2130284
The large cavities observed in the dust and gas distributions of transition
disks may be explained by planet-disk interactions. At ~145 pc, 2MASS
J16042165-2130284 (J1604) is a 5-12 Myr old transitional disk with different
gap sizes in the mm- and m-sized dust distributions (outer edges at ~79
and at ~63 au, respectively). Its CO emission shows a ~30 au cavity.
This radial structure suggests that giant planets are sculpting this disk. We
aim to constrain the masses and locations of plausible giant planets around
J1604. We observed J1604 with the Spectro-Polarimetric High-contrast Exoplanet
REsearch (SPHERE) at the Very Large Telescope (VLT), in IRDIFS\_EXT,
pupil-stabilized mode, obtaining YJH- band images with the integral field
spectrograph (IFS) and K1K2-band images with the Infra-Red Dual-beam Imager and
Spectrograph (IRDIS). The dataset was processed exploiting the angular
differential imaging (ADI) technique with high-contrast algorithms. Our
observations reach a contrast of ~12 mag from 0.15" to 0.80"
(~22 to 115 au), but no planet candidate is detected. The disk is directly
imaged in scattered light at all bands from Y to K, and it shows a red color.
This indicates that the dust particles in the disk surface are mainly
m-sized grains. We confirm the sharp dip/decrement in
scattered light in agreement with polarized light observations. Comparing our
images with a radiative transfer model we argue that the southern side of the
disk is most likely the nearest. This work represents the deepest search yet
for companions around J1604. We reach a mass sensitivity of from ~22 to ~115 au according to a hot start scenario. We propose
that a brown dwarf orbiting inside of ~15 au and additional Jovian planets at
larger radii could account for the observed properties of J1604 while
explaining our lack of detection.Comment: 10 pages, 7 Figures. Accepted for publication in A&A . Abridged
abstrac
Resistance of uveal melanoma to the interstrand cross-linking agent mitomycin C is associated with reduced expression of CYP450R
background: Uveal melanoma (UM) is the most common primary intraocular tumour of adults, frequently metastasising to the liver. Hepatic metastases are difficult to treat and are mainly unresponsive to chemotherapy. To investigate why UM are so chemo-resistant we explored the effect of interstrand cross-linking agents mitomycin C (MMC) and cisplatin in comparison with hydroxyurea (HU).
methods: Sensitivity to MMC, cisplatin and HU was tested in established UM cell lines using clonogenic assays. The response of UM to MMC was confirmed in MTT assays using short-term cultures of primary UM. The expression of cytochrome P450 reductase (CYP450R) was analysed by western blotting, and DNA cross-linking was assessed using COMET analysis supported by γ-H2AX foci formation.
results: Both established cell lines and primary cultures of UM were resistant to the cross-linking agent MMC (in each case P<0.001 in Student's t-test compared with controls). In two established UM cell lines, DNA cross-link damage was not induced by MMC (in both cases P<0.05 in Students's t-test compared with damage induced in controls). In all, 6 out of 6 UMs tested displayed reduced expression of the metabolising enzyme CYP450R and transient expression of CYP450R increased MMC sensitivity of UM.
conclusion: We suggest that reduced expression of CYP450R is responsible for MMC resistance of UM, through a lack of bioactivation, which can be reversed by complementing UM cell lines with CYP450R
Direct imaging of a massive dust cloud around R Coronae Borealis
We present recent polarimetric images of the highly variable star R CrB using
ExPo and archival WFPC2 images from the HST. We observed R CrB during its
current dramatic minimum where it decreased more than 9 mag due to the
formation of an obscuring dust cloud. Since the dust cloud is only in the
line-of-sight, it mimics a coronograph allowing the imaging of the star's
circumstellar environment. Our polarimetric observations surprisingly show
another scattering dust cloud at approximately 1.3" or 2000 AU from the star.
We find that to obtain a decrease in the stellar light of 9 mag and with 30% of
the light being reemitted at infrared wavelengths (from R CrB's SED) the grains
in R CrB's circumstellar environment must have a very low albedo of
approximately 0.07%. We show that the properties of the dust clouds formed
around R CrB are best fitted using a combination of two distinct populations of
grains size. The first are the extremely small 5 nm grains, formed in the low
density continuous wind, and the second population of large grains (~0.14
{\mu}m) which are found in the ejected dust clouds. The observed scattering
cloud, not only contains such large grains, but is exceptionally massive
compared to the average cloud.Comment: 8 pages, 7 figures published in A&
The effects of disk and dust structure on observed polarimetric images of protoplanetary disks
Imaging polarimetry is a powerful tool for imaging faint circumstellar
material. For a correct analysis of observations we need to fully understand
the effects of dust particle parameters, as well as the effects of the
telescope, atmospheric seeing, and assumptions about the data reduction and
processing of the observed signal. Here we study the major effects of dust
particle structure, size-dependent grain settling, and instrumental properties.
We performed radiative transfer modeling using different dust particle models
and disk structures. To study the influence of seeing and telescope diffraction
we ran the models through an instrument simulator for the ExPo dual-beam
imaging polarimeter mounted at the 4.2m William Herschel Telescope (WHT).
Particle shape and size have a strong influence on the brightness and
detectability of the disks. In the simulated observations, the central
resolution element also contains contributions from the inner regions of the
protoplanetary disk besides the unpolarized central star. This causes the
central resolution element to be polarized, making simple corrections for
instrumental polarization difficult. This effect strongly depends on the
spatial resolution, so adaptive optics systems are needed for proper
polarization calibration. We find that the commonly employed homogeneous sphere
model gives results that differ significantly from more realistic models. For a
proper analysis of the wealth of data available now or in the near future, one
must properly take the effects of particle types and disk structure into
account. The observed signal depends strongly on the properties of these more
realistic models, thus providing a potentially powerful diagnostic. We conclude
that it is important to correctly understand telescope depolarization and
calibration effects for a correct interpretation of the degree of polarization.Comment: Accepted for publication in A&
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