Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the absolute age of the metal-rich globular M71 (NGC 6838): I.
  optical photometry   We investigated the absolute age of the Galactic globular cluster M71 (NGC
6838) by using optical ground-based images (u',g',r',i',z') collected with the
MegaCam camera at the Canada-France-Hawaii-Telescope (CFHT). We performed a
robust selection of field and cluster stars by applying a new method based on
the 3D (r',u'-g',g'-r') Color-Color-Magnitude-Diagram. The comparison between
the Color-Magnitude-Diagram of the candidate cluster stars and a new set of
isochrones, at the locus of the Main Sequence Turn Off (MSTO), suggests an
absolute age of 12+/-2 Gyr. The absolute age was also estimated using the
difference in magnitude between the MSTO and the so-called main sequence knee,
a well defined bending occurring in the lower main sequence. This feature was
originally detected in the near-infrared (NIR) bands and explained as a
consequence of an opacity mechanism (collisionally induced absorption of
molecular hydrogen) in the atmosphere of cool low-mass stars (Bono et al.
2010). The same feature was also detected in the r',u'-g' and in the r',g'-r'
CMD, thus supporting previous theoretical predictions by Borysow et al. (1997).
The key advantage in using the Delta^(Knee)_(TO) as an age diagnostic is that
it is independent of uncertainties affecting the distance, the reddening and
the photometric zero-point. We found an absolute age of 12+/-1 Gyr that agrees,
within the errors, with similar age estimates, but the uncertainty is on
average a factor of two smaller. We also found that the Delta^(Knee)_(TO) is
more sensitive to the metallicity than the MSTO, but the dependence becomes
vanishing using the difference in color between the MSK and the MSTO.

12
The categories of this paper are: nucl-th
13
The categories of this paper are: quant-ph
15
The categories of this paper are: nucl-ex
24
The categories of this paper are: dg-ga
26
The categories of this paper are: bayes-an
23
The categories of this paper are: solv-int
30
The categories of this paper are: q-alg
33
The categories of this paper are: plasm-ph
2
The categories of this paper are: math
9
The categories of this paper are: nlin
16
The categories of this paper are: stat
28
The categories of this paper are: alg-geom
3
The categories of this paper are: physics
27
The categories of this paper are: comp-gas
1
The categories of this paper are: cs
31
The categories of this paper are: ao-sci