107 research outputs found
HST observations of the limb polarization of Titan
Titan is an excellent test case for detailed studies of the scattering
polarization from thick hazy atmospheres. We present the first limb
polarization measurements of Titan, which are compared as a test to our limb
polarization models. Previously unpublished imaging polarimetry from the HST
archive is presented which resolves the disk of Titan. We determine
flux-weighted averages of the limb polarization and radial limb polarization
profiles, and investigate the degradation and cancelation effects in the
polarization signal due to the limited spatial resolution of our observations.
Taking this into account we derive corrected values for the limb polarization
in Titan. The results are compared with limb polarization models, using
atmosphere and haze scattering parameters from the literature.
In the wavelength bands between 250 nm and 2000 nm a strong limb polarization
of about 2-7 % is detected with a position angle perpendicular to the limb. The
fractional polarization is highest around 1 micron. As a first approximation,
the polarization seems to be equally strong along the entire limb. The detected
polarization is compatible with expectations from previous polarimetric
observations taken with Voyager 2, Pioneer 11, and the Huygens probe.
Our results indicate that ground-based monitoring measurements of the
limb-polarization of Titan could be useful for investigating local haze
properties and the impact of short-term and seasonal variations of the hazy
atmosphere of Titan. Planets with hazy atmospheres similar to Titan are
particularly good candidates for detection with the polarimetric mode of the
upcoming planet finder instrument at the VLT. Therefore, a good knowledge of
the polarization properties of Titan is also important for the search and
investigation of extra-solar planets.Comment: 13 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
Measurement of the earthshine polarization in the B, V, R, and I band as function of phase
The characterization of the polarimetric properties of the planet Earth is
important for the interpretation of expected observations and the planning of
future instruments. We present a multi-wavelengths and multi-phase set of
benchmark values for the polarization signal of the integrated light from the
planet Earth derived from new polarimetric observations of the earthshine
back-scattered from the Moon's dark side. Using a new, specially designed wide
field polarimeter we measured the fractional polarization of the earthshine in
the B, V, R and I filters for Earth phase angles alpha between 30{\deg} and
110{\deg}. The phase dependence of the earthshine polarization is fitted by a
function p x sin(alpha)^2. To determine the polarization of the planet Earth we
correct our earthshine measurements by a polarization efficiency function for
the lunar surface derived from measurements of lunar samples from the
literature. The polarization of the earthshine decreases towards longer
wavelengths and is about a factor 1.3 lower for the higher albedo highlands.
For mare regions the measured maximum polarization is about 13 % at quadrature
in the B band. The resulting fractional polarizations for Earth are 24.6 % for
the B band, 19.1 % for the V band, 13.5 % for the R band, and 8.3 % for the I
band. Together with literature values for the spectral reflectivity of Earth we
obtain a contrast between the polarized flux of the Earth and the (total) flux
of the Sun with an uncertainty of less than 20 % and we find that the best
phase to detect an Earth twin is around an Earth phase alpha=65{\deg}. The
polarimetric models of Earth-like planets from Stam (2008) are in qualitative
agreement with our results but there are also significant differences which
might guide more detailed computations.Comment: 14 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
Detection of scattered light from the hot dust in HD 172555
Debris disks or belts are important signposts for the presence of colliding
planetesimals and, therefore, for ongoing planet formation and evolution
processes in young planetary systems. Imaging of debris material at small
separations from the star is very challenging but provides valuable insights
into the spatial distribution of so-called hot dust produced by solid bodies
located in or near the habitable zone. We report the first detection of
scattered light from the hot dust around the nearby (d = 28.33 pc) A star HD
172555. We want to constrain the geometric structure of the detected debris
disk using polarimetric differential Imaging (PDI) with a spatial resolution of
25 mas and an inner working angle of about 0.1. We measured the polarized
light of HD 172555, with SPHERE-ZIMPOL, in the very broad band (VBB;
nm) filter for the projected separations between 0.08 (2.3
au) and 0.77 (22 au). We constrained the disk parameters by fitting models
for scattering of an optically thin dust disk taking the limited spatial
resolution and coronagraphic attenuation of our data into account. The
geometric structure of the disk in polarized light shows roughly the same
orientation and outer extent as obtained from thermal emission at 18 m.
Our image indicates the presence of a strongly inclined (),
roughly axisymmetric dust belt with an outer radius in the range between
0.3 (8.5 au) and 0.4 (11.3 au). We derive a lower limit for the
polarized flux contrast ratio for the disk of in the VBB filter. This ratio is small, only
9 %, when compared to the fractional infrared flux excess (). The model simulations show that more polarized light could be
produced by the dust located inside 2 au, which cannot be detected with the
instrument configuration used.Comment: 16 pages, 10 figure
Prospects of detecting the polarimetric signature of the Earth-mass planet α Centauri B b with SPHERE/ZIMPOL
Context. Over the past five years, radial-velocity and transit techniques have revealed a new population of Earth-like planets with masses of a few Earth masses. Their very close orbit around their host star requires an exquisite inner working angle to be detected in direct imaging and sets a challenge for direct imagers that work in the visible range, such as SPHERE/ZIMPOL.
Aims. Among all known exoplanets with less than 25 Earth masses we first predict the best candidate for direct imaging. Our primary objective is then to provide the best instrument setup and observing strategy for detecting such a peculiar object with ZIMPOL. As a second step, we aim at predicting its detectivity.
Methods. Using exoplanet properties constrained by radial velocity measurements, polarimetric models and the diffraction propagation code CAOS, we estimate the detection sensitivity of ZIMPOL for such a planet in different observing modes of the instrument. We show how observing strategies can be optimized to yield the best detection performance on a specific target.
Results. In our current knowledge of exoplanetary systems, α Centauri B b is the most promising target with less than 25 Earth masses for ZIMPOL. With a gaseous Rayleigh-scattering atmosphere and favorable inclinations, the planet could be detected in about four hours of observing time, using the four-quadrant phase-mask coronograph in the I band. However, if α Centauri B b should display unfavorable polarimetric and reflective properties similar to that of our Moon, it is around 50 times fainter than the best sensitivity of ZIMPOL.
Conclusions. α Centauri B is a primary target for SPHERE. Dedicated deep observations specifically targeting the radial velocity-detected planet can lead to a detection if the polarimetric properties of the planet are favorable
Exploring dust around HD142527 down to 0.025" / 4au using SPHERE/ZIMPOL
We have observed the protoplanetary disk of the well-known young Herbig star
HD 142527 using ZIMPOL Polarimetric Differential Imaging with the VBB (Very
Broad Band, ~600-900nm) filter. We obtained two datasets in May 2015 and March
2016. Our data allow us to explore dust scattering around the star down to a
radius of ~0.025" (~4au). The well-known outer disk is clearly detected, at
higher resolution than before, and shows previously unknown sub-structures,
including spirals going inwards into the cavity. Close to the star, dust
scattering is detected at high signal-to-noise ratio, but it is unclear whether
the signal represents the inner disk, which has been linked to the two
prominent local minima in the scattering of the outer disk, interpreted as
shadows. An interpretation of an inclined inner disk combined with a dust halo
is compatible with both our and previous observations, but other arrangements
of the dust cannot be ruled out. Dust scattering is also present within the
large gap between ~30 and ~140au. The comparison of the two datasets suggests
rapid evolution of the inner regions of the disk, potentially driven by the
interaction with the close-in M-dwarf companion, around which no polarimetric
signal is detected.Comment: 11 pages, 7 figures, accepted for publication in A
Shadows cast on the transition disk of HD 135344B. Multiwavelength VLT/SPHERE polarimetric differential imaging
The protoplanetary disk around the F-type star HD 135344B (SAO 206462) is in
a transition stage and shows many intriguing structures both in scattered light
and thermal (sub-)millimeter emission which are possibly related to planet
formation processes. We study the morphology and surface brightness of the disk
in scattered light to gain insight into the innermost disk regions, the
formation of protoplanets, planet-disk interactions traced in the surface and
midplane layers, and the dust grain properties of the disk surface. We have
carried out high-contrast polarimetric differential imaging (PDI) observations
with VLT/SPHERE and obtained polarized scattered light images with ZIMPOL in R-
and I-band and with IRDIS in Y- and J-band. The scattered light images reveal
with unprecedented angular resolution and sensitivity the spiral arms as well
as the 25 au cavity of the disk. Multiple shadow features are discovered on the
outer disk with one shadow only being present during the second observation
epoch. A positive surface brightness gradient is observed in the stellar
irradiation corrected images in southwest direction possibly due to an
azimuthally asymmetric perturbation of the temperature and/or surface density
by the passing spiral arms. The disk integrated polarized flux, normalized to
the stellar flux, shows a positive trend towards longer wavelengths which we
attribute to large aggregate dust grains in the disk surface. Part of the the
non-azimuthal polarization signal in the Uphi image of the J-band observation
could be the result of multiple scattering in the disk. The detected shadow
features and their possible variability have the potential to provide insight
into the structure of and processes occurring in the innermost disk regions.Comment: Accepted for publication in A&A, 20 pages, 15 figure
First light of the VLT planet finder SPHERE. II. The physical properties and the architecture of the young systems PZ Tel and HD 1160 revisited
[Abridged] Context. The young systems PZ Tel and HD 1160, hosting known
low-mass companions, were observed during the commissioning of the new planet
finder SPHERE with several imaging and spectroscopic modes. Aims. We aim to
refine the physical properties and architecture of both systems. Methods. We
use SPHERE commissioning data and REM observations, as well as literature and
unpublished data from VLT/SINFONI, VLT/NaCo, Gemini/NICI, and Keck/NIRC2.
Results. We derive new photometry and confirm the nearly daily photometric
variability of PZ Tel A. Using literature data spanning 38 yr, we show that the
star also exhibits a long-term variability trend. The 0.63-3.8 mic SED of PZ
Tel B allows us to revise its properties: spectral type M7+/-1, Teff=2700+/-100
K, log(g)<4.5 dex, log(L/L_Sun)=-2.51+/-0.10 dex, and mass 38-72 MJ. The 1-3.8
mic SED of HD 1160 B suggests a massive brown dwarf or a low-mass star with
spectral type M5.5-7.0, Teff=3000+/-100 K, [M/H]=-0.5-0.0 dex,
log(L/L_Sun)=-2.81+/-0.10 dex, and mass 39-168 MJ. We confirm the deceleration
and high eccentricity (e>0.66) of PZ Tel B. For e<0.9, the inclination,
longitude of the ascending node, and time of periastron passage are well
constrained. The system is seen close to an edge-on geometry. We reject other
brown dwarf candidates outside 0.25" for both systems, and massive giant
planets (>4 MJ) outside 0.5" for the PZ Tel system. We also show that K1-K2
color can be used with YJH low-resolution spectra to identify young L-type
companions, provided high photometric accuracy (<0.05 mag) is achieved.
Conclusions. SPHERE opens new horizons in the study of young brown dwarfs and
giant exoplanets thanks to high-contrast imaging capabilities at optical and
near-infrared wavelengths, as well as high signal-to-noise spectroscopy in the
near-infrared from low (R~30-50) to medium resolutions (R~350).Comment: 25 pages, 23 figures, accepted for publication in A&A on Oct. 13th,
2015; version including language editing. Typo on co-author name on astroph
page corrected, manuscript unchange
Post conjunction detection of Pictoris b with VLT/SPHERE
With an orbital distance comparable to that of Saturn in the solar system,
\bpic b is the closest (semi-major axis \,9\,au) exoplanet that has
been imaged to orbit a star. Thus it offers unique opportunities for detailed
studies of its orbital, physical, and atmospheric properties, and of
disk-planet interactions. With the exception of the discovery observations in
2003 with NaCo at the Very Large Telescope (VLT), all following astrometric
measurements relative to \bpic have been obtained in the southwestern part of
the orbit, which severely limits the determination of the planet's orbital
parameters. We aimed at further constraining \bpic b orbital properties using
more data, and, in particular, data taken in the northeastern part of the
orbit.
We used SPHERE at the VLT to precisely monitor the orbital motion of beta
\bpic b since first light of the instrument in 2014. We were able to monitor
the planet until November 2016, when its angular separation became too small
(125 mas, i.e., 1.6\,au) and prevented further detection. We redetected \bpic b
on the northeast side of the disk at a separation of 139\,mas and a PA of
30 in September 2018. The planetary orbit is now well constrained.
With a semi-major axis (sma) of au (1 ), it
definitely excludes previously reported possible long orbital periods, and
excludes \bpic b as the origin of photometric variations that took place in
1981. We also refine the eccentricity and inclination of the planet. From an
instrumental point of view, these data demonstrate that it is possible to
detect, if they exist, young massive Jupiters that orbit at less than 2 au from
a star that is 20 pc away.Comment: accepted by A&
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