Spectroscopy in Open Cluster Remnants

SOLIVELLA, G. R.; GIORGI, E. E. S.; BAUME, G. L.; VÁZQUEZ, R. A.

REV. MEX. ASTRON. ASTR.

Instituto de Astronomía-UNAM

Lugar: Distrito Federal, Mexico; Año: 2009 vol. 35 p. 306 - 306

1405-2059

The nature of NGC 1557, NGC 1963 y NGC 2017, catalogued as posssible clusters remnants (Bica et al. 2001, A&A, 366, 827) has been investigated combining infrared photometry and spectral classification. Another object, NGC 1520, an open cluster candidate (Dias et al. 2002, A&A, 389, 871), was also investigated. The four objects are located in poorly populated fields at galactic latitudes |b|>15°. Spectra were obtained at CASLEO (Argentina), using a REOSC-DS spectrograph, during several runs from 2004 to 2006. The spectra cover a range from 3900 to 5500Å. These observations were complemented with JHK photometry and proper motions from the 2MASS and UCAC 2 (Zacharias et al. 2003, IAUJD, 16, 45) catalogues respectively. We obtained spectral classification for 36 stars in the four fields. It allows us to clarify the nature of these objects, in the sense that no one of them is a real open cluster remnant. The available proper motion data and infrared color-magnitude diagrams analysis are not enough to give another interpretation when both are combined with spectral classification. In fact, the spectrophotometric distances derived for the bright stars in each region suggest that we are looking at just a handful of evolved stars superimposed along the line of sight. This is revealing that the simple strategy of superimposing isochrones to apparent main sequences and ilusory red giant branches can lead to wrong conclutions in the computation of survival rates of open clusters.