INVESTIGADORES
PIATTI Andres Eduardo
artículos
Título:
A New Giant Branch Clump Structure in the Large Magellanic Cloud
Autor/es:
PIATTI, ANDRÉS E.; GEISLER, DOUG; BICA, EDUARDO; CLARIA OLMEDO, JUAN JOSÉ; SANTOS JR, JOÂO F.C.; SARAJEDINI, A.; DOTTORI, HORACIO
Revista:
ASTRONOMICAL JOURNAL
Referencias:
Año: 1999 vol. 118 p. 2865 - 2874
ISSN:
0004-6256
Resumen:
We present Washington C and T1 CCD photometry of 21 fields
located in the northern part of the Large Magellanic Cloud (LMC), and
spread over a region of more than 2.5 deg2 approximately 6
deg from the bar. The surveyed areas were chosen on the basis of their
proximity to SL 388 and SL 509, whose fields showed the presence of a
secondary giant clump, observationally detected by Bica et al. We also
observed NGC 2209, located ~14 deg away from SL 509. From the collected
data, we found that most of the observed field CMDs do not show a
separate secondary clump, but rather reveal a continuous vertical
structure (VS), which is clearly seen for the first time. The VS also
appears in the field of NGC 2209. Its position and size are nearly the
same throughout the surveyed regions: it lies below the red giant clump
(RGC) and extends from the bottom of the RGC to ~0.45 mag fainter,
spanning the bluest color range of the RGC. In two fields in addition to
the NGC 2209 field the RGC is slightly tilted, following approximately
the reddening vector, while the VS maintains its verticality. We found
that the number of stars in the VS box defined by
Δ(C-T1)=1.45-1.55 mag and
ΔT1=18.75-19.15 mag has a strong spatial variation,
reaching the highest VS star density just northeast of SL 509. Moreover,
the more numerous the VS stars in a field, the larger the number of LMC
giants in the same zone. We also found that, in addition to SL 509, two
relatively massive star clusters, SL 515 and NGC 2209, separated by more
than 10 deg from each other, develop giant clumps with a considerable
number of VS stars. This result demonstrates that VS stars belong to the
LMC and are most likely the result of some kind of evolutionary process
in the LMC, particularly in those LMC regions with a noticeable large
giant population. Our results are successfully predicted by the models
of Girardi in the sense that a large proportion of 1-2 Gyr old stars
mixed with older stars and with metallicities higher than [Fe/H]~=-0.7
should result in a fainter and bluer secondary clump near the mass at
which degenerate core He burning takes place. However, our results
apparently suggest that in order to trigger the formation of VS stars,
there should be other conditions in addition to the appropriate age,
metallicity, and the necessary red giant star density. Indeed, stars
satisfying the requisites mentioned above are commonly found throughout
the LMC, but the VS phenomenon is only clearly seen in some isolated
regions. Finally, the fact that clump stars have an intrinsic luminosity
dispersion further constrains the use of the clump magnitude as a
reliable distance indicator.