154 research outputs found
Spatial polarization modulators: distinguishing diffraction effects from spatial polarization modulation
Are we alone? In our quest to find life beyond Earth, we use our own planet to develop and verify new methods and techniques to remotely detect life. Our Life Signature Detection polarimeter (LSDpol), a snapshot full-Stokes spectropolarimeter to be deployed in the field and in space, looks for signals of life on Earth by sensing the linear and circular polarization states of reflected light. Examples of these biosignatures are linear polarization resulting from O2-A band and vegetation, e.g. the Red edge and the Green bump, as well as circular polarization resulting from the homochirality of biotic molecules. LSDpol is optimized for sensing circular polarization. To this end, LSDpol employs a spatial light modulator in the entrance slit of the spectrograph, a liquid-crystal quarter-wave retarder where the fast axis rotates as a function of slit position. The original design of LSDpol implemented a dual-beam spectropolarimeter by combining a quarter-wave plate with a polarization grating. Unfortunately, this design causes significant linear-to-circular cross-talk. In addition, it revealed spurious polarization modulation effects. Here, we present numerical simulations that illustrate how Fresnel diffraction effects can create these spurious modulations. We verified the simulations with accurate polarization state measurements in the lab using 100% linearly and circularly polarized light.Instrumentatio
Magnetism, chemical spots, and stratification in the HgMn star Ï• Phoenicis
Context. Mercury-manganese (HgMn) stars have been considered as non-magnetic and non-variable chemically peculiar (CP) stars
for a long time. However, recent discoveries of the variability in spectral line profiles have suggested an inhomogeneous surface
distribution of chemical elements in some HgMn stars. From the studies of other CP stars it is known that magnetic field plays a key
role in the formation of surface spots. All attempts to find magnetic fields in HgMn stars have yielded negative results.
Aims. In this study, we investigate the possible presence of a magnetic field in Ï• Phe (HD 11753) and reconstruct surface distribution
of chemical elements that show variability in spectral lines.We also test a hypothesis that a magnetic field is concentrated in chemical
spots and look into the possibility that some chemical elements are stratified with depth in the stellar atmosphere.
Methods. Our analysis is based on high-quality spectropolarimetric time-series observations, covering a full rotational period of
the star. Spectra were obtained with the HARPSpol at the ESO 3.6-m telescope. To increase the sensitivity of the magnetic field
search, we employed the least-squares deconvolution (LSD) technique. Using Doppler imaging code INVERS10, we reconstructed
surface chemical distributions by utilising information from multiple spectral lines. The vertical stratification of chemical elements
was calculated with the DDAFit program.
Results. Combining information from all suitable spectral lines, we set an upper limit of 4 G on the mean longitudinal magnetic field.
For chemical spots, an upper limit on the longitudinal field varies between 8 and 15 G. We confirmed the variability of Y, Sr, and Ti
and detected variability in Cr lines. Stratification analysis showed that Y and Ti are not concentrated in the uppermost atmospheric
layers.
Conclusions. Our spectropolarimetric observations rule out the presence of a strong, globally-organised magnetic field in Ï• Phe.
This implies an alternative mechanism of spot formation, which could be related to a non-equilibrium atomic diffusion. However, the
typical time scales of the variation in stratification predicted by the recent time-dependent diffusion models exceed significantly the
spot evolution time-scale reported for Ï• Phe
Observing the Earth as an exoplanet with LOUPE, the Lunar Observatory for Unresolved Polarimetry of Earth
The detections of small, rocky exoplanets have surged in recent years and
will likely continue to do so. To know whether a rocky exoplanet is habitable,
we have to characterise its atmosphere and surface. A promising
characterisation method for rocky exoplanets is direct detection using
spectropolarimetry. This method will be based on single pixel signals, because
spatially resolving exoplanets is impossible with current and near-future
instruments. Well-tested retrieval algorithms are essential to interpret these
single pixel signals in terms of atmospheric composition, cloud and surface
coverage. Observations of Earth itself provide the obvious benchmark data for
testing such algorithms. The observations should provide signals that are
integrated over the Earth's disk, that capture day and night variations, and
all phase angles. The Moon is a unique platform from where the Earth can be
observed as an exoplanet, undisturbed, all of the time. Here, we present LOUPE,
the Lunar Observatory for Unresolved Polarimetry of Earth, a small and robust
spectropolarimeter to observe our Earth as an exoplanet.Comment: 14 pages, 3 figures, submitted in special Issue of Planetary and
Space Science on Scientific Preparations for Lunar Exploratio
Magnetically Controlled Accretion on the Classical T Tauri Stars GQ Lupi and TW Hydrae
Stars and planetary system
LOUPE: Observing Earth from the Moon to prepare for detecting life on Earth-like exoplanets
LOUPE, the Lunar Observatory for Unresolved Polarimetry of the Earth, is a
small, robust spectro-polarimeter with a mission to observe the Earth as an
exoplanet. Detecting Earth-like planets in stellar habitable zones is one of
the key challenges of modern exoplanetary science. Characterising such planets
and searching for traces of life requires the direct detection of their
signals. LOUPE provides unique spectral flux and polarisation data of sunlight
reflected by the Earth, the only planet known to harbor life. This data will be
used to test numerical codes to predict signals of Earth-like exoplanets, to
test algorithms that retrieve planet properties, and to fine-tune the design
and observational strategies of future space observatories. From the Moon,
LOUPE will continuously see the entire Earth, enabling it to monitor the signal
changes due to the planet's daily rotation, weather patterns, and seasons,
across all phase angles. Here, we present both the science case and the
technology behind LOUPE's instrumental and mission design.Comment: 13 pages, 5 figures. Accepted for publication in Royal Society
Philosophical Transactions A. Corrected typos in v
Comment on "Resolving the 180-deg Ambiguity in Solar Vector Magnetic Field Data: Evaluating the Effects of Noise, Spatial Resolution, and Method Assumptions"
In a recent paper, Leka at al. (Solar Phys. 260, 83, 2009)constructed a
synthetic vector magnetogram representing a three-dimensional magnetic
structure defined only within a fraction of an arcsec in height. They rebinned
the magnetogram to simulate conditions of limited spatial resolution and then
compared the results of various azimuth disambiguation methods on the resampled
data. Methods relying on the physical calculation of potential and/or
non-potential magnetic fields failed in nearly the same, extended parts of the
field of view and Leka et al. (2009) attributed these failures to the limited
spatial resolution. This study shows that the failure of these methods is not
due to the limited spatial resolution but due to the narrowly defined test
data. Such narrow magnetic structures are not realistic in the real Sun.
Physics-based disambiguation methods, adapted for solar magnetic fields
extending to infinity, are not designed to handle such data; hence, they could
only fail this test. I demonstrate how an appropriate limited-resolution
disambiguation test can be performed by constructing a synthetic vector
magnetogram very similar to that of Leka et al. (2009) but representing a
structure defined in the semi-infinite space above the solar photosphere. For
this magnetogram I find that even a simple potential-field disambiguation
method manages to resolve the ambiguity very successfully, regardless of
limited spatial resolution. Therefore, despite the conclusions of Leka et al.
(2009), a proper limited-spatial-resolution test of azimuth disambiguation
methods is yet to be performed in order to identify the best ideas and
algorithms.Comment: Solar Physics, in press (19 pp., 5 figures, 2 tables
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