Stellar Systems Group

Buchholz, R. M.; Witzel, G.; Schödel, R.; Eckart, A.; Bremer, M.; Mužić, K.
Astronomy & Astrophysics, Volume 534, id.A117, 26 pp.
10/2011

ABSTRACT

Context. The Galactic center offers unique opportunities to study stellar and bow-shock polarization effects in a dusty environment.
Aims: The goals of this work are to provide near-infrared (NIR) polarimetry of the stellar sources in the central parsec at the resolution of an 8 m telescope for the first time, along with new insights into the nature of the known bright bow-shock sources.
Methods: We use adaptive-optics assisted observations obtained at the ESO VLT in the H- and Ks-band, applying both high-precision photometric methods specifically developed for crowded fields and a newly established polarimetric calibration for NACO to produce polarization maps of the central 3'' × 19'', in addition to spatially resolved polarimetry and a flux variability analysis on the extended sources in this region.
Results: We find foreground polarization mainly parallel to the Galactic plane, with average values of (4.6 ± 0.6)% at 26° ± 6° (Ks-band) and (9.3 ± 1.3)% at 20° ± 6° (H-band) in the center of the field-of-view (FOV). Farther away from the center, we find higher polarization degrees and steeper polarization angles: (7.5 ± 1.0)% at 11° ± 6° (Ks-band) and (12.1 ± 2.1)% at 13° ± 6° (H-band). p_H/p_Ks peaks at 1.9 ± 0.4, corresponding to a power law index for the wavelength dependency of α = 2.4 ± 0.7. These values also vary over the FOV, with higher values in the center. This is indicative of the influence of 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 mid-infrared (MIR) findings, both in total and spatially 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 allow us to separate the bow shock of IRS 21 from the central source for the first time in the Ks-band, finding the apex north of the central source and determining a standoff distance of ~400 AU, which matches previous estimates. This source also shows a ~50% increase in flux in the NIR over several years.

A table containing the polarization parameters of all reliably measured sources in the FOV is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strabg.fr/cgi-bin/qcat?J/A+A/534/A117