A peek behind the dusty curtain: K_S-band polarization photometry and bow shock models of the Galactic center source IRS 8^⋆

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
e-mail: rauch@ph1.uni-koeln.de; eckart@ph1.uni-koeln.de; buchholz@ph1.uni-koeln.de; maca@ph1.uni-koeln.de, sabha@ph1.uni-koeln.de; cstraubm@ph1.uni-koeln.de; mvalencias@ph1.uni-koeln.de; syazici@ph1.uni-koeln.de
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, 19001 Santiago, Chile
e-mail: kmuzic@eso.org

Received: 22 June 2012
Accepted: 3 December 2012

Abstract

Context. IRS 8 offers the possibility to study unique bow shock properties and polarization features in the dusty environment at the Galactic center.

Aims. We aim to gain insight into the properties of the surrounding material of the bow shock source IRS 8, based on the measured polarization distribution along its extended feature. Bow shock models enable a relative positioning of its central star with respect to the northern arm and Sagittarius A*.

Methods. The analysis is based on polarimetric K_S-band and nonpolarimetric L′- and N-band data obtained with the adaptive-optics assisted instrument NAOS/CONICA (K_S- and L′-band) and the mid-infrared imager/spectrograph VISIR (N-band) at the ESO Very Large Telescope. A polarization map covering an area of ~31.9′′ × 8.2′′ at the position of IRS 8 was obtained using high-precision photometric methods, together with a polarimetric calibration specified for NACO. In addition, simulated L′-band bow shock models are presented, which allow a positioning of IRS 8 with respect to the interaction medium of the northern arm.

Results. This work presents interstellar dust properties for the northern arm in the vicinity of a bow shock, as well as a three-dimensional positioning of IRS 8 with respect to the northern arm and Sagittarius A*. The presented polarization maps for the Galactic center source IRS 8 show a total intrinsic polarization degree of 3.3% at 13° east of north, measured on nondeconvolved images, and 4.3% at 19° on Lucy-Richardson deconvolved images. Analysis of the polarization distribution along the bow shock reveals a polarization pattern as it would be produced by absorption on aligned grains and shows a decreasing polarization degree towards the apex of the bow shock. Current literature provides a mean electron density for IRS 8. The comparison of this value with properties of simulated polarization maps in a circumstellar environment, which are capable of multiple scattering, leads to the conclusion that multiple scattering events reduce the degree of polarization at the apex of the bow shock. Additionally, bow shock models that allow relative positioning of IRS 8 with respect to the northern arm and Sagittarius A* are presented. The results indicate that its central star is located closer towards the observer than the northern arm.