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Issue A&A
Volume 419, Number 3, June I 2004
Page(s) 965 - 973
Section Interstellar and circumstellar matter
DOI 10.1051/0004-6361:20035635



A&A 419, 965-973 (2004)
DOI: 10.1051/0004-6361:20035635

Optical polarization observations in Hogg 22 and NGC 6204

R. Martínez, M. M. Vergne and C. Feinstein

Facultad de Ciencias Astronómicas y Geofísicas, Observatorio Astronómico, Paseo del Bosque, 1900 La Plata, Argentina Instituto de Astrofísica de La Plata, CONICET, Argentina
(Received 5 November 2003 / Accepted 2 December 2003 )

Abstract
We present new (UBVRI) multicolor linear polarimetric data for 22 of the brightest stars 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 in 28 observed stars in the region. Our data yield for NGC 6204 a mean polarization percentage of $P_{\lambda_{{\rm max}}}\sim1.8\%$, close to the polarization value produced by the ISM with normal efficiency ( $P_{\lambda_{{\rm max}}} \sim 5 E_$) with a color excess of EB-V =0.51. Meanwhile for Hogg 22, located behind NGC 6204, the mean polarization is $P_{\lambda_{{\rm max}}}\sim 2.15\%$, lower than the expected value for the observed color excess of EB-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 $\theta=44\fdg9$, which agrees with the expected angle produce by dust particles aligned in the direction of the Galactic Plane ( $\theta=48^{\circ}$), while for NGC 6204 a lower value, $\theta=33\fdg7$, 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.


Key words: ISM: dust, extinction -- Galaxy: open clusters and associations: individual: NGC 6204 -- Galaxy: open clusters and associations: individual: Hogg 22

Offprint request: C. Feinstein, cfeinstein@fcaglp.edu.ar

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