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Current burning issues in stellar physics, for both hot and cool stars, concern their magnetism. In hot stars, stable magnetic fields of fossil origin impact their stellar structure and circumstellar environment, with a likely major role in stellar evolution. However, this role is complex and thus poorly understood as of today. It needs to be quantified with high-resolution UV spectropolarimetric measurements. In cool stars, UV spectropolarimetry would provide access to the structure and magnetic field of the very dynamic upper stellar atmosphere, providing key data for new progress to be made on the role of magnetic fields in heating the upper atmospheres, launching stellar winds, and more generally in the interaction of cool stars with their environment (circumstellar disk, planets) along their whole evolution. UV spectropolarimetry is proposed on missions of various sizes and scopes, from POLLUX on the 15-m telescope LUVOIR to the Arago M-size mission dedicated to UV spectropolarimetry.
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Current burning issues in stellar physics, for both hot and cool stars, concern their magnetism. In hot stars, stable magnetic fields of fossil origin impact their stellar structure and circumstellar environment, with a likely major role in stellar evolution. However, this role is complex and thus poorly understood as of today. It needs to be quantified with high-resolution UV spectropolarimetric measurements. In cool stars, UV spectropolarimetry would provide access to the structure and magnetic field of the very dynamic upper stellar atmosphere, providing key data for new progress to be made on the role of magnetic fields in heating the upper atmospheres, launching stellar winds, and more generally in the interaction of cool stars with their environment (circumstellar disk, planets) along their whole evolution.
Stellar spectropolarimetry is a relatively new remote sensing tool for exploring stellar atmospheres and circumstellar environments. We present the results of our HiVIS survey and a multi-wavelength ESPaDOnS follow-up campaign showing detectable linear polarization signatures in many lines for most obscured stars. This survey shows polarization at and below 0.1% across many lines are common in stars with often much larger H-alpha signatures. These smaller signatures are near the limit of typical systematic errors in most night-time spectropolarimeters. In an effort to increase our precision and efficiency for detecting small signals we designed and implemented the new HiVIS bi-directionally clocked detector synchronized with the new liquid-crystal polarimeter package. We can now record multiple independent polarized spectra in a single exposure on identical pixels and have demonstrated 10^-4 relative polarimetric precision. The new detector allows for the movement of charge on the device to be synchronized with phase changes in the liquid-crystal variable retarders at rates of >5Hz. It also allows for more efficient observing on bright targets by effectively increasing the pixel well depth. With the new detector, low and high resolution modes and polarization calibrations for the instrument and telescope, we substantially reduce limitations to the precision and accuracy of this new spectropolarimetric tool.
Observations of the Earthshine off the Moon allow for the unique opportunity to measure the large-scale Earth atmosphere. Another opportunity is realized during a total lunar eclipse which, if seen from the Moon, is like a transit of the Earth in front of the Sun. We thus aim at transmission spectroscopy of an Earth transit by tracing the solar spectrum during the total lunar eclipse of January 21, 2019. Time series spectra of the Tycho crater were taken with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope (LBT) in its polarimetric mode in Stokes IQUV at a spectral resolution of 130000 (0.06 AA). In particular, the spectra cover the red parts of the optical spectrum between 7419-9067 AA . The spectrographs exposure meter was used to obtain a light curve of the lunar eclipse. The brightness of the Moon dimmed by 10.75 mag during umbral eclipse. We found both branches of the O$_2$ A-band almost completely saturated as well as a strong increase of H$_2$O absorption during totality. The deep penumbral spectra show significant excess absorption from the NaI 5890 AA doublet, the CaII infrared triplet around 8600 AA, and the KI line at 7699 AA in addition to several hyper-fine-structure lines of MnI and even from BaII. The detections of the latter two elements are likely due to an untypical solar center-to-limb effect rather than Earths atmosphere. The absorption in CaII and KI remained visible throughout umbral eclipse. A small continuum polarization of the O$_2$ A-band of 0.12% during umbral eclipse was detected at 6.3$sigma$. No line polarization of the O$_2$ A-band, or any other spectral-line feature, is detected outside nor inside eclipse. It places an upper limit of $approx$0.2% on the degree of line polarization during transmission through Earths atmosphere and magnetosphere.
Magnetic bright points are small-scale magnetic elements ubiquitous across the solar disk, with the prevailing theory suggesting that they form due to the process of convective collapse. Employing a unique full Stokes spectropolarimetric data set of a quiet Sun region close to disk centre obtained with the Swedish Solar Telescope, we look at general trends in the properties of magnetic bright points. In total we track 300 MBPs in the data set and we employ NICOLE
Context. Remote sensing of weak and small-scale solar magnetic fields is of utmost relevance for a number of important open questions in solar physics. This requires the acquisition of spectropolarimetric data with high spatial resolution (0.1 arcsec) and low noise (1e-3 to 1e-5 of the continuum intensity). The main limitations to obtain these measurements from the ground, are the degradation of the image resolution produced by atmospheric seeing and the seeing-induced crosstalk (SIC). Aims. We introduce the prototype of the Fast Solar Polarimeter (FSP), a new ground-based, high-cadence polarimeter that tackles the above-mentioned limitations by producing data that are optimally suited for the application of post-facto image restoration, and by operating at a modulation frequency of 100 Hz to reduce SIC. Results. The pnCCD camera reaches 400 fps while keeping a high duty cycle (98.6 %) and very low noise (4.94 erms). The modulator is optimized to have high (> 80%) total polarimetric efficiency in the visible spectral range. This allows FSP to acquire 100 photon-noise-limited, full-Stokes measurements per second. We found that the seeing induced signals present in narrow-band, non-modulated, quiet-sun measurements are (a) lower than the noise (7e-5) after integrating 7.66 min, (b) lower than the noise (2.3e-4) after integrating 1.16 min and (c) slightly above the noise (4e-3) after restoring case (b) by means of a multi-object multi-frame blind deconvolution. In addition, we demonstrate that by using only narrow-band images (with low SNR of 13.9) of an active region, we can obtain one complete set of high-quality restored measurements about every 2 s.
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