An imaging polarimeter for the 4.2-m William Herschel Telescope at La Palma to image circumstellar environments (protoplanetary and debris disks and exoplanets).

Astronomical Results

Our ExPo (Extreme Polarimeter) instrument has been very successful. We had 8 separate observing runs with the instrument. This success has led to several grants for further instrumental developments related to circumstellar disks and exoplanets. With those additional funds we were able to build an adaptive optics (AO) system for ExPo and a polarimetric integral field unit (IFU), which greatly increased ExPo’s scientific capabilities, and hired a postdoc to help with the interpretation of the ExPo observations. In the following I will highlight some of the most interesting results.

The circumstellar environment of the young T Tauri star SU Aurigae

The circumstellar environments of classical T Tauri stars are challenging to directly image because of their high star-to-disk contrast ratio. We used imaging polarimetry where scattered and consequently polarised starlight from the star’s circumstellar disk can be separated from the unpolarised light of the central star. We obtained images of the circumstellar environment of SU Aur, a classical T Tauri star at the transition of T Tauri to Herbig stars. The images directly show that the disk extends out to 500 au with an inclination angle of ∼50 deg. Using interpretive models, we derived very small grains in the surface layers of its disk, with a very steep size- and surface-density distribution. Additionally, we resolved a large and extended nebulosity in our images that is most likely a remnant of the prenatal molecular cloud. The position angle of the disk, determined directly from our images, rules out a polar outflow or jet as the cause of this large-scale nebulosity.

Direct imaging of a massive dust cloud around R Coronae Borealis

We obtained 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 showed another scattering dust cloud at approximately 1.3 arcsec or 2000 AU from the star. We found 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 pproximately 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 μm) that 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.

Surprising detection of an equatorial dust lane on the AGB star IRC+10216

Low to intermediate mass stars evolve into large, luminous and cool giants at the end of their lives. Their sudden death is caused by intense mass-loss removing the star’s outer envelope and the fuel required for nuclear burning, leading to the formation of a planetary nebula. In contrast to the highly asymmetrical shapes of planetary nebulae, the circumstellar environments of their immediate progenitor stars on the asymptotic giant branch (AGB) appear to be spherically symmetric. Explaining how and when planetary nebulae are shaped remains elusive because of the difficulty in directly imaging the morphology of the preceding high mass-loss phase, as these stars are heavily enshrouded in an optically thick dusty envelope.

To understand the morphology of the circumstellar environments of AGB stars we observed the closest carbon-rich AGB star IRC+10216 in scattered light with our ExPo instrument at the William Herschel Telescope. When imaged in scattered light at optical wavelengths, IRC+10216 surprisingly shows a narrow equatorial density enhancement, in contrast to the large-scale spherical rings that have been imaged much further out. We used radiative transfer models to interpret this structure in terms of two models: firstly an equatorial density enhancement, commonly observed in the more evolved post-AGB stars, and secondly in terms of a dust ring model, where a local enhancement of mass-loss creates a spiral ring as the star rotates. We conclude that both models can be used to reproduce the dark lane in the scattered light images which is caused by an equatorially density enhancement formed by dense dust and not a bipolar outflow as previously thought. We are unable to place constraints on the formation of the equatorial density enhancement by a binary system.

Saturn in linear polarization (Keller and Stam 2012, Nature 483, 38)

Saturn in linear polarization. a Conventional image of Saturn. The planets’ rings are visible as a dark band because the rings were almost edge-on at the time of the observations. b, c, Linear-polarization images of the planet for two polarization orientations. Black and white indicate negative and positive polarization signals; grey corresponds to no polarization signal. Two features stand out in the polarization measurements: the polar regions and a band under the rings. The polar regions are known to have different atmospheric properties from the rest of the planet, but the band under the rings has so far eluded explanation.

Innovations

ExPo opened a new window for high-contrast polarimetry by combining ferro-electric liquid crystal polarization modulators, polarizing beamsplitters and a high-speed electron-multiplying CCD camera with advanced data reduction approaches. Particular innovations achieved with ExPo include:

• achieving a polarimetric sensitivity better than 10-4, about two orders of magnitude better than previous high-contrast imaging polarimeters
• a novel polarizing beamsplitter that combines two orthogonally polarized beams on a single camera with a broad, simultaneous wavelength coverage
• a laboratory test setup that closely mimics the optical setup at the telescope
• an approach to cosmic-ray suppression in high-dynamic range images
• a dual-beam polarimetry data reduction approach that eliminates flat-field errors and seeing-induced differential aberrations Many of the innovations are now used with great success in SPHERE and are fundamental in our ability to propose breakthrough instrument concepts for the E-ELT.

Funding provided by NWO/VICI