J/A+A/709/A277 Large Interstellar Polarisation Survey. III (Siebenmorgen+, 2026)
Large Interstellar Polarisation Survey. III.
Observational constraints on the structure of grains
Siebenmorgen R., Bagnulo S., Fanciullo L., Vannieuwenhuyse T., Guillet V.
<Astron. Astrophys. 709, A277 (2026)>
=2026A&A...709A.277S 2026A&A...709A.277S (SIMBAD/NED BibCode)
ADC_Keywords: Polarization ; Interstellar medium ; Spectra, optical ;
Spectra, infrared
Keywords: dust, extinction
Abstract:
Our understanding of dust in the diffuse interstellar medium remains
incomplete with regard to the structure, composition, size
distribution, and alignment properties of its grains. Joint
observations of reddening, starlight polarisation spectra, and
polarised dust emission for individual sightlines provide essential
constraints on such properties. We study a far-UV selected sample of
96 reddening curves, for which optical linear polarisation spectra
were obtained with FORS at the VLT as part of the Large Interstellar
Polarisation Survey (LIPS). Starlight polarisation spectra for 60
stars are presented in this work. These data are combined with Gaia
distance estimates and Planck thermal dust emission. A three-component
dust model is made publicly available. It consists of nanoparticles,
amorphous grains, and micrometre-sized dust agglomerates, varying
axial ratios, porosities, sizes, element abundances, and alignment
efficiencies to match the observations. The diversity of reddening and
polarisation spectra is well reproduced by prolate grains with typical
axial ratios of two, a porosity of 10%, and high alignment
efficiencies. Such efficiencies can be achieved with radiative torque
alignment theory (RAT), but not with imperfect Davis-Greenstein (IDG)
alignment, except when adjusting the magnetic-field orientation to
maximise the polarisation. Micrometre-sized dust contributes
wavelength-independent grey extinction in the optical, accounts for
about one-third of the visual extinction, and carries one-third of the
dust mass. A follow-up submillimetre survey with high-resolution
polarimetry will further constrain grain shapes and alignment physics.
Description:
FORS spectropolarimetric observations of selected stars used in the
study "LIPS III. Observational constraints on the structure of
grains". The data include flux spectra and linear polarisation
parameters obtained with the FORS instrument using the 300V and 300I
grisms.
Data are distributed as compressed tar archives.
Individual tar files are named as follows:
YYYY-MM-DDSTARNAMEFILTER.tar.gz
where:
YYYY-MM-DD observation date
STARNAME name of the observed star
FILTER grism identifier:
300V V band
300I I band
Files corresponding to the same star and observing date, but different
filters and bin sizes, are also grouped into combined archives named:
STARNAME_TYYYY-MM-DD.tar.gz
Within each tar archive, spectra are provided as ASCII files named:
STARNAMEFILTERTYYYY-MM-DD_sxxxxnm.dat
where:
xxxxnm indicates the approximate wavelength bin size in nanometres.
Each archive contains spectra rebinned to approximately the following
bin widths:
0.16, 0.32, 0.65, 1.29, 2.58, 5.17, 10.3, 20.7, 41.3nm
The spectra are provided as fixed-format ASCII tables.
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
list.dat 82 103 List of studied stars
sp/* . 103 Individual tar files of spectra
data.zip 512 59054 Compressed tar archive of spectra
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See also:
J/A+A/608/A146 : Southern Hemisphere ISM FORS2 spectropolarimetry
(Bagnulo+, 2017)
Byte-by-byte Description of file: list.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 --- Name Star name
13- 14 I2 h RAh Simbad right ascension (J2000)
16- 17 I2 min RAm Simbad right ascension (J2000)
19- 23 F5.2 s RAs Simbad right ascension (J2000)
25 A1 --- DE- Simbad declination sign (J2000)
26- 27 I2 deg DEd Simbad declination (J2000)
29- 30 I2 arcmin DEm Simbad declination (J2000)
32- 35 F4.1 arcsec DEs Simbad declination (J2000)
37- 46 A10 "date" Obs.date Observation date
48- 51 A4 --- Grim Grims (300I or 300V)
53- 82 A30 --- FileName Name of the tar file with spectra in
subdirectory sp
--------------------------------------------------------------------------------
Description of file: (STARNAMEFILTERTYYYY-MM-DD_sxxxxnm.dat)
Byte-by-byte Description of file:
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
4- 17 E14.9 nm lambda Wavelength
21- 34 E14.9 --- Flux Flux in ADU (1)
38- 51 E14.9 --- e_Flux RMS uncertainty on Flux, computed as the square
root of the electron counts (in ADU) (2)
54- 68 E15.9 --- Q/I Q/I Stokes parameter
71- 85 E15.9 --- e_Q/I RMS uncertainty on Q/I due to photon noise
88-102 E15.9 --- U/I U/I Stokes parameter
106-119 E14.9 --- e_U/I RMS uncertainty on U/I (2)
123-136 E14.9 --- Pl Fraction of linear polarisation
139-153 E15.9 deg theta Polarisation position angle
157-171 E15.9 deg e_theta RMS uncertainty on theta
--------------------------------------------------------------------------------
Note (1): Fluxes are given in analogue-to-digital units (ADU).
Note (2): Uncertainties correspond to photon-noise estimates.
--------------------------------------------------------------------------------
Acknowledgements:
Ralf Siebenmorgen, Ralf.Siebenmorgen(at)eso.org
License: CC-BY-4.0
References:
Bagnulo et al., Paper I 2017A&A...608A.146B 2017A&A...608A.146B Cat. J/A+A/608/A146
Siebenmorgen et al., Paper II 2018A&A...611A...5S 2018A&A...611A...5S
(End) Patricia Vannier [CDS] 17-Apr-2026