2007A&A...465..899C -
Astronomy and Astrophysics, volume 465, 899-906 (2007/4-3)
Linear and circular polarisation of diffuse interstellar bands.
COX N.L.J., BOUDIN N., FOING B.H., SCHNERR R.S., KAPER L., NEINER C., HENRICHS H., DONATI J.-F. and EHRENFREUND P.
Abstract (from CDS):
The detection or absence of a polarisation signal in the diffuse interstellar band (DIB) profile can possibly give important clues on the identity of its carrier. For molecular gas-phase carriers the effect of polarisation on its electronic transitions depends, although uncertain how exactly, on its physical properties (e.g. molecular structure). On the other hand, if the carriers are grains, impurities or defects in grain mantles are also expected to show up in the polarisation of DIBs. Our aim is to detect a polarisation signal in a DIB profile or to derive stricter upper limits on the polarisation efficiency of DIB carriers. In order to detect and measure the linear and circular polarisation of the DIBs we observe reddened lines of sight showing continuum polarisation; for this study we select the stars
HD 21219,
HD 198478,
HD 197770,
HD 183143 and
HD 163472. We use spectropolarimetry in the wavelength range 4480 to 6620Å with the MuSiCoS echelle spectrograph mounted at the Telescope Bernard Lyot. Linear polarisation spectra are constructed in order to search for a polarisation signal in the DIB profiles. No significant change in the linear polarisation degree is found. We obtain a 2σ polarisation detection limit (per DIB FWHM) of 0.01-0.04% for
HD 21219, 0.04-0.14% for
HD 197770, 0.01-0.14% for
HD 183143 and 0.01-0.14% for
HD 198478, for the six investigated DIBs. We derive upper limits for the polarisation efficiency factor f for six strong narrow DIBs; f
max of 0.31, 0.44, 0.45, 0.18, 0.47 and 0.68 for the λλ 5780, 5797, 6196, 6284, 6379 and 6613 DIBs, respectively. The derived detection limits are similar to those derived by Adamson & Whittet (
1995ApJ...448L..49A), although for more lines of sight and more DIBs. Circular polarisation (Stokes V) spectra of high signal-to-noise have been obtained for the first time for the 5780, 5797, 6196, 6203, 6284, 6376, 6379 and 6613Å DIB profiles. No circular polarisation signal is detected. The 2σ
V (per 0.1Å) noise level limits are 1.0-2.5% for the DIBs in the line of sight towards
HD 197770 and 0.06-0.10% for the DIBs towards
HD 163472. The lack of polarisation of the DIB profiles is consistent with DIB carriers that are not directly related to solid features, like impurities in grain mantles. However, large (carbonaceous) gas phase molecules remain viable DIB carrier candidates.
Abstract Copyright:
∼
Journal keyword(s):
astrochemistry - polarisation - ISM: dust, extinction - ISM: lines and bands - ISM: molecules
CDS comments:
Abstract : HD 21219 is a probable misprint for HD 21291.
Simbad objects:
12
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