719 research outputs found
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
Probing the Interstellar Medium using HI absorption and emission towards the W3 HII region
HI spectra towards the W3 HII complex are presented and used to probe the
Galactic structure and interstellar medium conditions between us and this
region. The overall shape of the spectra is consistent with the predictions of
the Two-Arm Spiral Shock model wherein the gas found in the -40 km/s to -50
km/s range has been accelerated by some 20 km/s from its rotation curve
velocity. Spin temperatures of ~100 K are derived for the Local Arm gas, lower
than found in a previous, similar study towards DR 7. For the interarm region,
values on the order of 300 K are found, implying a negligible filling factor
for the Cold Neutral Medium (<< 1%). Some of the absorbing gas at velocities
near -40 km/s is confirmed to be associated with the HII regions.Comment: 23 pages, 6 figures, accepted for publication in the Astronomical
Journa
Deriving Telescope Mueller Matrices Using Daytime Sky Polarization Observations
Telescopes often modify the input polarization of a source so that the
measured circular or linear output state of the optical signal can be
signficantly different from the input. This mixing, or polarization
"cross-talk", is defined by the optical system Mueller matrix. We describe here
an efficient method for recovering the input polarization state of the light
and the full 4 x 4 Mueller matrix of the telescope with an accuracy of a few
percent without external masks or telescope hardware modification. Observations
of the bright, highly polarized daytime sky using the Haleakala 3.7m AEOS
telescope and a coude spectropolarimeter demonstrate the technique.Comment: Accepted for publication in PAS
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
Aromatic Features in AGN: Star-Forming Infrared Luminosity Function of AGN Host Galaxies
We describe observations of aromatic features at 7.7 and 11.3 um in AGN of
three types including PG, 2MASS and 3CR objects. The feature has been
demonstrated to originate predominantly from star formation. Based on the
aromatic-derived star forming luminosity, we find that the far-IR emission of
AGN can be dominated by either star formation or nuclear emission; the average
contribution from star formation is around 25% at 70 and 160 um. The
star-forming infrared luminosity functions of the three types of AGN are
flatter than that of field galaxies, implying nuclear activity and star
formation tend to be enhanced together. The star-forming luminosity function is
also a function of the strength of nuclear activity from normal galaxies to the
bright quasars, with luminosity functions becoming flatter for more intense
nuclear activity. Different types of AGN show different distributions in the
level of star formation activity, with 2MASS> PG> 3CR star formation rates.Comment: Accepted for publication in ApJ, 24 pages, 13 figure
Accurate Wavenumbers for Mid-Infrared Fine-Structure Lines
We present accurate new wavenumbers for a set of 13 mid-infrared
fine-structure lines. The wavenumbers were determined from observations of the
planetary nebula NGC 7027 and of the red supergiant Alpha Scorpii. Most of the
new wavenumbers are good to within 0.0025%, or 8 km/s. We provide details on
the measurements and present an analysis of the errors. In addition, we present
the first observations of hyperfine splitting in the [Na IV] 1106 cm-1 line.Comment: 12 pages text, 2 postscript figures, uses AASTeX macros, figures are
gzipped and uuencode
A search for broad infrared recombination lines in NGC 1068
We report infrared spectroscopy of the prototypical Seyfert 2 galaxy NGC
1068, aiming at detection of broad components of hydrogen recombination lines
that originate in the obscured broad-line region. Using the Short Wavelength
Spectrometer on board the Infrared Space Observatory, we have observed for the
first time the regions of Brackett beta 2.626um and Pfund alpha 7.460um, and
present improved data for Brackett alpha 4.052um. No significant broad
components are detected, implying an equivalent visual extinction to the
broad-line region of at least 50 magnitudes and an obscuring column density of
at least 10^23 cm^-2. While consistent with a highly obscured broad-line
region, as required by the classical unified scenario, these limits are not yet
significant enough to discriminate strongly between different torus models or
to constrain properties of the gas causing the very large X-ray obscuration. We
discuss the systematic limitations of infrared broad-line region searches and
suggest that Brackett alpha may often be the most favorable transition for
future searches.Comment: aastex (V4), 4 eps figures. Accepted by Ap
Unidentified Infrared Emission Bands in the Diffuse Interstellar Medium
Using the Mid-Infrared Spectrometer on board the Infrared Telescope in Space
and the low-resolution grating spectrometer (PHT-S) on board the Infrared Space
Observatory, we obtained 820 mid-infrared (5 to 12 m) spectra of the
diffuse interstellar medium (DIM) in the Galactic center, W51, and Carina
Nebula regions. These spectra indicate that the emission is dominated by the
unidentified infrared (UIR) emission bands at 6.2, 7.7, 8.6, and 11.2 m.
The relative band intensities (6.2/7.7 m, 8.6/7.7 m, and 11.2/7.7
m) were derived from these spectra, and no systematic variation in these
ratios was found in our observed regions, in spite of the fact that the
incident radiation intensity differs by a factor of 1500. Comparing our results
with the polycyclic aromatic hydrocarbons (PAHs) model for the UIR band
carriers, PAHs in the DIM have no systematic variation in their size
distribution, their degree of dehydrogenation is independent of the strength of
UV radiation field, and they are mostly ionized. The latter finding is
incompatible with past theoretical studies, in which a large fraction of
neutral PAHs is predicted in this kind of environment. A plausible resolution
of this discrepancy is that the recombination coefficients for electron and
large PAH positive ion are by at least an order of magnitude less than those
adopted in past theoretical studies. Because of the very low population of
neutral state molecules, photoelectric emission from interstellar PAHs is
probably not the dominant source of heating of the diffuse interstellar gas.
The present results imply constant physical and chemical properties of the
carriers of the UIR emission bands in the DIM.Comment: 13 pages, 6 figures. Accepted for publication in Ap
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
Carbon Recombination Lines from the Galactic Plane at 34.5 & 328 MHz
We present results of a search for carbon recombination lines in the Galaxy
at 34.5 MHz (C) made using the dipole array at Gauribidanur near
Bangalore. Observations made towards 32 directions, led to detections of lines
in absorption at nine positions. Followup observations at 328 MHz
(C) using the Ooty Radio Telescope detected these lines in emission.
A VLA D-array observation of one of the positions at 330 MHz yielded no
detection implying a lower limit of 10' for the angular size of the line
forming region.
The longitude-velocity distribution of the observed carbon lines indicate
that the line forming region are located mainly between 4 kpc and 7 kpc from
the Galactic centre. Combining our results with published carbon recombination
line data near 76 MHz (\nocite{erickson:95} Erickson \et 1995) we obtain
constraints on the physical parameters of the line forming regions. We find
that if the angular size of the line forming regions is , then
the range of parameters that fit the data are: \Te K, \ne \cm3 and pathlengths pc which may correspond to thin
photo-dissociated regions around molecular clouds. On the other hand, if the
line forming regions are in extent, then warmer gas (\Te K) with lower electron densities (\ne \cm3) extending
over several tens of parsecs along the line of sight and possibly associated
with atomic \HI gas can fit the data. Based on the range of derived parameters,
we suggest that the carbon line regions are most likely associated with
photo-dissociation regions.Comment: To appear in Journal of Astrophysics & Astronomy, March 200
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