E. Irene Vega,^2,4 Ana María Orsatti,^3,4 and Ruben E. Martínez^2,3

² Facultad de Ciencias Astronómicas y Geofísicas, Observatorio Astronómico, La Plata, Universidad Nacional La Plata, C.P. 1900, La Plata, Argentina (irene@fcaglp.unlp.edu.ar) (ruben@fcaglp.unlp.edu.ar) .

³ Instituto de Astrofísica de La Plata (UNLP- CONICET), Fac. de Cs. Astronómicas y Geofísicas, Universidad Nacional La Plata, C.P. 1900, La Plata, Argentina (ana_orsatti@hotmail.com).

⁴ Member of the Carrera del Investigador Científico, CONICET, Universidad Nacional La Plata, C.P. 1900, La Plata, Argentina.

Received April 1 2014. ]]> Accepted September 2 2014.

RESUMEN

Se presentan observaciones polarimétricas en las bandas U BV RI de 66 estrellas situadas en la dirección del cúmulo NGC 4755, incluyendo once β Cephei. Casi la mitad de estas variables muestran indicios de polarización intrínseca, pero según la teoría ninguna clase de pulsación puede originarla. Del análisis de las principales características de sus curvas P_v vs. λ_v hemos encontrado posibles orígenes de la dispersión de la luz en atmósferas extendidas, así como sospechas de binaridad en algunos casos. La polarización media para los miembros de NGC 4755 es P_v = 2.76%, con θ_v = 76.°6. Las partículas tienen un tamaño medio normal para el ISM, y la eficiencia es elevada. Con el uso exclusivo de herramientas polarimétricas se han identificado un total de 25 estrellas miembro.

ABSTRACT

We present U BV RI polarimetric observations of 66 stars in the direction of NGG 4755, including eleven β cephei stars. About half of these variables show indications of intrinsic polarization, but according to the theory neither radial nor non-radial pulsations could cause such polarizations. From the main characteristics of their P_v vs. λ_v curves we found as a possible origin, in some cases, scattering of light in extended atmospheres and suspected binarity, in some cases. The parameters of the interstellar medium towards the cluster are P_v = 2.76 ± 0.13(%) and θ_v = 76.°6 ± 0.°9; the mean wavelength of maximum polarization amounts to 0.56 ± 0.04 µm. The value of polarization efficiency is higher than the mean, indicating a good alignment of the dust particles. A total of 25 stars were identified as members of NGG 4755; the identification task was improved through the use of polarimetric tools.

Key Words: ISM: dust, extinction — open clusters and associations: NGG 4755; stars: variables: β Cephei.

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ACKNOWLEDGMENT

We acknowledge the excellent recommendations of an unknown referee, which certainly improved the final version. The use of NASA's Sky View facility (http://skywiew.gsfc.nasa.gov) located at NASA Goddard Space Flight Center, and the gallery image of NOAO (http://www.noao.edu/image_gallery/html/im0423.html), together with the technical support and hospitality at GASLEO during the observing runs, are also acknowledged. Special thanks go to Dr. Hugo G. Marraco for his useful comments.

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Nota

¹ Based on observations obtained at 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.