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Revista mexicana de astronomía y astrofísica
Print version ISSN 0185-1101
Rev. mex. astron. astrofis vol.44 n.2 Ciudad de México Oct. 2008
A new search for variable stars in the globular cluster NGC 63661
A. Arellano Ferro,2 Sunetra Giridhar,3 V. Rojas López,2 R. Figuera,4 D. M. Bramich,5 and P. Rosenzweig4
2 Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70264, 04510 México, D. F., México. (armando@astroscu.unam.mx, victoria@astroscu.unam.mx).
3 Indian Institute of Astrophysics, Bangalore, India.
4 Grupo de Astrofísica Teórica, Facultad de Ciencias, Universidad de Los Andes, Mérida, Venezuela. (rfiguera@ula.ve, patricia@ula.ve).
5 Isaac Newton Group of Telescopes, Apdo. Postal 321, E38700 Santa Cruz de la Palma, Canary Islands, Spain. (dmb@ing.iac.es).
Received 2008 April 28
Accepted 2008 July 28
RESUMEN
A través de fotometría CCD de NGC 6366 se han descubierto nuevas estrellas variables. Se descubrieron: dos posibles Cefeidas Anómalas (o Cefeidas de Población II), tres gigantes de largo período, una SX Phe y una binaria eclipsante. También se reporta una lista de 10 posibles variables. La curva de luz de la estrella RRab, V1, fue descompuesta en sus armónicos de Fourier y éstos se emplearon para estimar la metalicidad y la distancia de la estrella; [Fe/H] = 0.87 ±0.14 y d = 3.2 ±0.1 kpc. Se argumenta que la estrella V1 podría no ser miembro del cúmulo, sino un objeto más distante. Si es así, se puede emplear V1 para calcular un límite superior para la distancia del cúmulo de 2.8 ± 0.1 kpc. La relación P L para estrellas tipo SX Phe y los modos de pulsación identificados en la SX Phe descubierta, V6, permiten una determinación independiente de la distancia d = 2.7 ± 0.1 kpc. Se discute el caso de V1 en el marco de la relación MV [Fe/H] para estrellas RR Lyrae.
ABSTRACT
New CCD photometry of NGC 6366 has lead to the discovery of some variable stars. Two possible Anomalous Cepheids (or Pop II Cepheids), three long period variables, one SX Phe and one eclipsing binary have been found. Also, a list of 10 candidate variables is reported. The light curve of the RRab star, V1, has been decomposed into its Fourier harmonics, and the Fourier parameters were used to estimate the star's metallicity and distance; [Fe/H] = 0.87±0.14 and d = 3.2±0.1 kpc. It is argued that V1 may not be a member of the cluster but rather a more distant object. If this is so, an upper limit for the distance to the cluster of 2.8 ±0.1 kpc can be estimated. The P L relationship for SX Phe stars and the identified modes in the newly discovered SX Phe variable, V6, allow yet another independent determination of the distance to the cluster, d = 2.7 ± 0.1 kpc. The MV [Fe/H] relationship for RR Lyrae stars is addressed and the case of V1 is discussed.
Key Words: globular clusters: general globular clusters: individual (NGC 6366) stars: variables: other stars: variables: RR Lyrae.
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1 Based on observations collected at the Indian Astrophysical Observatory, Hanle, India.