J/A+A/615/A42 Anomalous extinction sightlines stars polarisation (Cikota+, 2018)
Spectropolarimetry of Galactic stars with anomalous extinction sightlines.
Cikota A., Hoang T., Taubenberger S., Patat F., Mazzei P., Cox N.L.J.,
Zelaya P., Cikota S., Tomasella L., Benetti S., Rodeghiero G.
<Astron. Astrophys. 615, A42 (2018)>
=2018A&A...615A..42C 2018A&A...615A..42C (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; Stars, nearby ; Polarization ; Extinction
Keywords: polarization - ISM: general - dust, extinction -
supernovae: general - galaxies: ISM
Abstract:
Highly reddened type Ia supernovae (SNe Ia) with low
total-to-selective visual extinction ratio values, RV, also show
peculiar linear polarization wavelength dependencies with peak
polarizations at short wavelengths (λmax≲0.4um). It is not
clear why sightlines to SNe Ia display such different continuum
polarization profiles from interstellar sightlines in the Milky Way
with similar RV values. We investigate polarization profiles of a
sample of Galactic stars with low RV values, along anomalous
extinction sightlines, with the aim to find similarities to the
polarization profiles that we observe in SN Ia sightlines. We
undertook spectropolarimetry of 14 stars, used archival data for 3
additional stars, and ran dust extinction and polarization simulations
(by adopting the picket-fence alignment model) to infer a simple dust
model (size distribution, alignment) that can reproduce the observed
extinction and polarization curves. Our sample of Galactic stars with
low RV values and anomalous extinction sightlines displays normal
polarization profiles with an average λmax∼0.53um, and is
consistent within 3σ to a larger coherent sample of Galactic
stars from the literature. Despite the low RV values of dust toward
the stars in our sample, the polarization curves do not show any
similarity to the continuum polarization curves observed toward SNe Ia
with low RV values. There is a correlation between the best-fit
Serkowski parameters K and λmax, but we did not find any
significant correlation between RV and λmax. Our simulations
show that the K-λmax relationship is an intrinsic property
of polarization. Furthermore, we have shown that in order to reproduce
polarization curves with normal λmax and low RV values, a
population of large (a ≥0.1um) interstellar silicate grains must be
contained in the dust composition.
Description:
Given are final polarization curves of stars with anomalous extinction
sightlines, observed with FORS2, AFOSC, and CAFOS, and shown in Fig.
1. The filename consists of the instrument name with which the star was
observed (FORS2, AFOSC, or CAFOS), and the star name, in the following
form: instrument_star.dat
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
sources.dat 92 14 List of studied stars
files/* . 20 Individual files with final polarization curves
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Byte-by-byte Description of file: sources.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 --- Name Star name
12- 13 I2 h RAh Right ascension (J2000)
15- 16 I2 min RAm Right ascension (J2000)
18- 22 F5.2 s RAs Right ascension (J2000)
24 A1 --- DE- Declination sign (J2000)
25- 26 I2 deg DEd Declination (J2000)
28- 29 I2 arcmin DEm Declination (J2000)
31- 34 F4.1 arcsec DEs Declination (J2000)
36- 53 A18 --- AFOSC Name of the file with AFOSC data
in subdirectory files
55- 73 A19 --- CAFOS Name of the file with CAFOS data
in subdirectory files
75- 92 A18 --- FORS2 Name of the file with FORS2 data
in subdirectory files
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Byte-by-byte Description of file: files/*
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Bytes Format Units Label Explanations
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1- 4 I4 0.1nm lambda Wavelength in Angstroem
6- 20 E15.9 % Q Stokes Q in percent
22- 35 E14.9 % e_Q Error of Stokes Q in percent
37- 51 E15.9 % U Stokes U in percent
53- 66 E14.9 % e_U Error of Stokes U in percent
68- 81 E14.9 % Pol Degree of polarization in percent
83- 96 E14.9 % e_Pol Error of degree of polarization in percent
98-111 E14.9 deg PolPA Polarization angle in degree
113-126 E14.9 deg e_PolPA Error of polarization angle in degree
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Acknowledgements:
Aleksandar Cikota, acikota(at)eso.org
(End) Patricia Vannier [CDS] 21-Mar-2018