Bibcode
Fiorentino, G.; Ferraro, Ivan; Iannicola, Giacinto; Bono, Giuseppe; Monelli, M.; Testa, Vincenzo; Arcidiacono, Carmelo; Faccini, Marco; Gilmozzi, Roberto; Xompero, Marco; Briguglio, Runa
Referencia bibliográfica
Proceedings of the SPIE, Volume 9148, id. 91483U 14 pp. (2014).
Advertised on:
8
2014
Número de citas
6
Número de citas referidas
3
Descripción
We present data collected using the camera PISCES coupled with the Firt
Light Adaptive Optics (FLAO) mounted at the Large Binocular Telescope
(LBT). The images were collected for two different pointings by using
two natural guide stars with an apparent magnitude of R ~< 13 mag.
During these observations the seeing was on average ~0.9 arcsec. The AO
performed very well, in fact the images display a mean FWHM of 0.05
arcsec and of 0.06 arcsec in the J- and in the Ks-band, respectively.
The Strehl ratio on the quoted images reaches 13-30% (J) and 50-65%
(Ks), in the off and in the central pointings respectively. On the basis
of this sample we have reached a J-band limiting magnitude of ~22.5 mag
and the deepest Ks-band limiting magnitude ever obtained in a crowded
stellar field: Ks ~23 mag. J-band images display a complex change in the
shape of the PSF when moving at larger radial distances from the natural
guide star. In particular, the stellar images become more elongated in
approaching the corners of the J-band images whereas the Ks-band images
are more uniform. We discuss in detail the strategy used to perform
accurate and deep photometry in these very challenging images. In
particular we will focus our attention on the use of an updated version
of ROMAFOT based on asymmetric and analytical Point Spread Functions.
The quality of the photometry allowed us to properly identify a feature
that clearly shows up in NIR bands: the main sequence knee (MSK). The
MSK is independent of the evolutionary age, therefore the difference in
magnitude with the canonical clock to constrain the cluster age, the
main sequence turn off (MSTO), provides an estimate of the absolute age
of the cluster. The key advantage of this new approach is that the error
decreases by a factor of two when compared with the classical one.
Combining ground-based Ks with space F606W photometry, we estimate the
absolute age of M15 to be 13.70+/- 0.80 Gyr.