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Register a new userThe intrinsic and interstellar broadband linear polarization of nearby FGK dwarfs
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Daniel V. Cotton Dr
Publication date
2017
fields
Physics
and research's language is
English
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We present linear polarization measurements of nearby FGK dwarfs to parts-per-million (ppm) precision. Before making any allowance for interstellar polarization, we found that the active stars within the sample have a mean polarization of 28.5 +/- 2.2 ppm while the inactive stars have a mean of 9.6 +/- 1.5 ppm. Amongst inactive stars we initially found no difference between debris disk host stars (9.1 +/- 2.5 ppm) and the other FGK dwarfs (9.9 +/- 1.9 ppm). We develop a model for the magnitude and direction of interstellar polarization for nearby stars. When we correct the observations for the estimated interstellar polarization we obtain 23.0 +/-2.2 ppm for the active stars, 7.8 +/- 2.9 ppm for the inactive debris disk host stars and 2.9 +/- 1.9 ppm for the other inactive stars. The data indicates that whilst some debris disk host stars are intrinsically polarized most inactive FGK dwarfs have negligible intrinsic polarization, but that active dwarfs have intrinsic polarization at levels ranging up to ~45 ppm. We briefly consider a number of mechanisms, and suggest differential saturation of spectral lines in the presence of magnetic fields is the best able to explain the polarization seen in active dwarfs. The results have implications for current attempts to detect polarized reflected light from hot Jupiters by looking at the combined light of the star and planet.
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The aim of the present study is to determine the Li abundances for a large set of the FGK dwarfs and to analyse the connections between the Li content, stellar parameters, and activity. Atmospheric parameters, rotational velocities and Li abundances were determined from a homogeneous collection of the echelle spectra with high resolution and high signal-to-noise ratio. Rotational velocities vsini were determined by calibrating the cross-correlation function. Effective temperatures Teff were estimated by the line-depth ratio method. Surface gravities log g were computed by two methods: iron ionization balance and parallax. LTE Li abundances were computed using the synthetic spectrum method. The behaviour of the Li abundance was examined in correlation with Teff, [Fe/H], vsini and level of activity in three stellar groups of different temperatures. The stellar parameters and Li abundances are presented for 150 slow rotating stars of the lower part of MS. The studied stars show a decline in the Li abundance with decreasing temperature Teff and a significant spread, which should be due to the differences of age. A correlation between Li abundance, vsini and level of chromospheric activity is seen for stars with 6000>Teff>5700 K, and it is tighter for stars with 5700>Teff>5200 K. Stars with Teff<5200 K do not show any correlation between log A(Li) and vsini. The relationship between chromospheric and coronal fluxes in active stars with detected Li as well as in less active stars gives a hint that there exist different conditions in the action of the dynamo mechanism in those stars. We found that the Li-activity correlation is evident only in a restricted temperature range and the Li abundance spread seems to be present in a group of low chromospheric activity stars that also show a broad spread in chromospheric vs coronal activity.
We report on near-infrared J- and H-band linear polarimetric photometry of eight ultracool dwarfs (two late-M, five L0-L7.5, and one T2.5) with known evidence for photometric variability due to dust clouds, anomalous red infrared colors, or low-gravity atmospheres. The polarimetric data were acquired with the LIRIS instrument on the William Herschel Telescope. We also provide mid-infrared photometry in the interval 3.4-24 micron for some targets obtained with Spitzer and WISE, which has allowed us to confirm the peculiar red colors of five sources in the sample. We can impose modest upper limits of 0.9% and 1.8% on the linear polarization degree for seven targets with a confidence of 99%. Only one source, 2MAS, J02411151-0326587 (L0), appears to be strongly polarized (P ~ 3%) in the J-band with a significance level of P/sigma_P ~ 10. The likely origin of its linearly polarized light and rather red infrared colors may reside in a surrounding disk with an asymmetric distribution of grains. Given its proximity (66 +/- 8 pc), this object becomes an excellent target for the direct detection of the disk.
We present the first linear polarimetric survey of white dwarfs (WDs). Our sample consists of WDs of DA and DC spectral types in the SDSS r magnitude range from 13 to 17. We performed polarimetric observations with the RoboPol polarimeter attached to the 1.3-m telescope at the Skinakas Observatory. We have 74 WDs in our sample, of which almost all are low polarized WDs with polarization degree (PD) smaller than 1%, while only 2 have PD higher than 1%. There is an evidence that on average the isolated WDs of DC type have higher PD (with median PD of 0.78%) than the isolated DA type WDs (with median PD of 0.36%). On the other hand, the median PD of isolated DA type WDs is almost the same, i.e. 0.36% as the median PD of DA type white dwarfs in binary systems with red dwarfs (dM type), i.e. 0.33%. This shows, as expected, that there is no contribution to the PD from the companion if the WD companion is the red dwarf, which is the most common situation for WDs binary systems. We do not find differences in the polarization degree between magnetic and non-magnetic WDs. Because 97% of WDs in our sample have PD lower than 1%, they can be used as faint zero--polarized standard star in the magnitude range from 13 up to 17 of SDSS r filter. They cover the Northern sky between 13 hour to 23 hour in right ascension and from -11 degrees to 78 degrees in declination. Additionally, we found that for low extinction values (< 0.04) the best model that describes the dependence of PD on E(B-V) is given by the equation: PD_{max, ISM}[%] = 0.65 x E(B-V)^{0.12}.
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