Astron. Astrophys. Suppl. Ser. 144, 195-202

Dust properties in NGC 6611

A.M. Orsatti ¹²⁴⁵ - E.I. Vega ¹²³⁴⁵ - H.G. Marraco ¹⁴

Send offprint request: A.M. Orsatti, e-mail: aorsatti@fcaglp.fcaglp.unlp.edu.ar

1 - Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900, La Plata, Argentina
2 - Programa de Fotometría y Estructura Galáctica, PROFOEG, CONICET, Paseo del Bosque s/n, 1900, La Plata, Argentina
3 - Instituto de Astronomía y Física del Espacio, C. C. 67 Suc. 28, 1428, Buenos Aires, Argentina
4 - Member of the Carrera del Investigador Científico of CONICET
5 - Visiting Astronomer, Complejo Astronómico El Leoncito, operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina and the Universities of La Plata, Córdoba, and San Juan

Received July 28, 1999; accepted February 25, 2000

Abstract:

An investigation on dust properties in the NW portion of NGC 6611 has been conducted polarimetrically to test the existence of abnormal extinction in that section of the open cluster in an independent way. As previously suggested for the $\eta$ Carinae nebula by Tapia et al. ([1988b]) and subsequently confirmed by Marraco et al. ([1993]), the canonical relation between E _V-K/E _B-V and $\lambda _{\rm max}$ (the wavelength of maximun interstellar polarization) is not valid for stars belonging to dusty H II regions, as is the case for the observed portion of M 16. This may arise mainly from the presence of silicate grains of a slightly larger size than the standard ISM and also from a considerable increase in mean graphite grain size, according to previous results from Chini & Wargau ([1990]).

About 50$\%$ of the observed stars in NGC 6611 present indications of intrinsic polarization in their measurements; a similar percentage was found in IC 2944 (Vega et al. [1994]), another young open cluster in close relation with an H II region.

Key words: ISM: dust, extinction -- Galaxy: open clusters and associations: individual (NGC 6611) -- ISM: HII regions -- ISM: magnetic fields