Relación física entre el cúmulo abierto Hogg 15 y la estrella Wolf-Rayet WR 47

PIATTI, ANDRÉS E.; BICA, EDUARDO; SANTOS JR, JOÂO F.C.; CLARIA OLMEDO, JUAN JOSÉ

Buenos Aires

Congreso; Reunión Anual de la Asoc. Argentina de Astronomía; 2002

We revise the fundamental parameters of the faint open cluster Hogg 15, for which two recent colour-magnitude diagram (CMD) studies have obtained significantly different ages (Sagar et al. 2001, MNRAS, 327, 23; Piatti & Clariá 2001, A&A, 370, 931). In the present study, we combine a series of methods trying to constrain age, together with other fundamental parameters. We employ spatial extractions to construct the CMDs, and the cluster integrated spectrum to compare the latter with those of templates of known age. For Hogg 15 we derive an age of (20 ± 10) Myr, a reddening of (1.10 ± 0.05) mag, and a distance of (3.1 ± 0.5) kpc. We conclude that the estimation of an older age in the study of Piatti & Clariá can be accounted for in terms of main sequence/turnoff curvature being blurred in the CMDs, an effect mainly caused by field contamination. To clarify the issue of whether the Wolf-Rayet star HDE 311884 (WR 47), known to be a WN6 + O5V binary system, is associated or not with Hogg 15, we discuss its fundamental parameters, particularly its distance. Based on the WR 47 spectrum and available photometry, we assume that the underestimated distance implied by the Hipparcos parallax (216 pc) is affected by its binary character. By comparing the WR 47 spectrum with those of WR stars of a similar type, we conclude that WR 47 is not affected by a E(B-V) colour excess much higher than that associated with Hogg 15, namely E(B-V) = 1.10. On the basis of the WR 47's resulting distance of (5.2 ± 0.9) kpc, which largely surpasses that of the cluster, we conclude that WR 47 is not related to Hogg 15 from the point of view of origin, since the cluster and the star do not belong to the same formation event.