Adaptive-optics assisted near-infrared polarization measurements of sources in the Galactic center

Abstract

The goals of this work are to provide NIR polarimetry of the stellar sources in the central pc at the resolution of an 8m telescope for the first time, along with new insights into the nature of the known bright bow-shock sources. We use AO assisted observations with the ESO VLT in the H- and Ks-band, applying high precision photometric methods developed for crowded fields and a new polarimetric calibration for NACO to produce polarization maps of the central 3"x19", in addition to spatially resolved polarimetry and a variability analysis on the extended sources. We find foreground polarization parallel to the Galactic plane, with averages of (4.6+/-0.6)% at 26{\deg}+/-6{\deg} (Ks) and (9.3+/-1.3)% at 20{\deg}+/-6{\deg} (H) in the center. At larger distances from the center, we find different polarization parameters: (7.5+/-1.0)% at 11{\deg}+/-6{\deg} (Ks) and (12.1+/-2.1)% at 13{\deg}+/-6{\deg} (H). p_H/p_Ks peaks at 1.9+/-0.4, with a power law index for the wavelength dependency of alpha = 2.4+/-0.7. This also varies over the FOV, with higher values in the center, indicating local effects on the total polarization, possibly dichroic extinction by Northern Arm dust. The two extended sources IRS 21 and 1W show similar intrinsic polarization degrees of 6.1% resp. 7.8% (Ks) and 6.9% (H, only 1W) at polarization angles coincident with previous NIR and MIR findings, both in total and resolved. The spatial polarization pattern of both sources points to scattering on aligned elongated dust grains as the major source of intrinsic polarization, and matches the known orientation of the magnetic field. Our data also allows us to separate the bow shock of IRS 21 from the central source for the first time in the Ks-band, with the apex north of the center and a standoff distance of ~400 AU, matching previous estimates. This source also shows a ~50% increase in flux in the NIR over several years.