Optcal polarization observations in Hogg22 and NGC 6204

R MARTINEZ; M. M. VERGNE; C. FEINSTEIN

ASTRONOMY AND ASTROPHYSICS

EDP Science

Lugar: Paris; Año: 2004 vol. 419 p. 965 - 973

0004-6361

We present new (UBVRI) multicolor linear polarimetric data for 22 of the brighteststars in the area of the open clusters Hogg 22 and NGC 6204 to study the properties of the ISM (interstellar medium) toward these clusters and between them.The new data were incorporated in our data set of previous observations (Waldhausen et al. 1999), resulting the in observed stars in the region.Our data yield for NGC 6204 a mean polarization percentage of $P_{lambda_{max}}sim1.8%$, close to the polarizationvalue produced by the ISM with normal efficiency ($P_{lambda_{max}} sim 5 E_{B-V}$) with a color excess of  $E_{B-V} =0.51$. Meanwhile for Hogg 22, located behind NGC 6204, the mean polarization  is $P_{lambda_{max}}sim 2.15%$,  lower than the expected value for the observed color excess of  $E_{B-V} =0.68$ (Forbes et al., 1996) and the average efficiency of polarization for the interstellar dust. The mean angle of the polarization vectors of Hogg 22 is $ heta=44^{circ}.9$, which agrees  with the expected angle produce by dust particles aligned in the direction of the Galactic Plane ($ heta=48^{circ}$), while for NGC 6204 a lower value,  $ heta=33^{circ}.7$,  was found.Therefore, we believe that Hogg 22 is depolarized by the same dust that is polarizing NGC 6204, due to different orientations of the dust particles (and magnetic fields) that polarize the starlight.