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Mass Function of the Tidal Tails

Technical requirements: V, I images down to a limiting magnitude of $V\sim27.5$ ( $I\sim 25.5$) in a $2\times2$ square degrees field.

Recent extensive campaigns conducted to directly measure the tidal radius of globular clusters (GCs) using their radial surface density profiles have lead to the discovery of large tidal tails of stars around GGCs (Grillmair et al. 1995, AJ, 109, 2553; Zaggia et al. 1995, Mem. Sait, 441, 667; Zaggia, Piotto & Capaccioli 1997, A&A, 327, 1004; Lehman & Scholz 1997, A&AS, ). These tidal tails are present as departures in the surface density profiles at large radii, with a break from a King (1966, AJ, 71, 64) profile at the tidal radius followed by a power law decline which varies from cluster to cluster. Evidence of tidal tails has been found in 16 Galactic GGCs out of 21 surveyed (Grillmair 1998, ASP Conf. Ser., Galactic Halos, astro-ph/9711223), which means that this is a common feature of GGCs. These tails are a consequence of the interaction of the GGCs with the Galactic gravitational potential.

A wide field imager is of paramount importance for:

For the first program, an 8m class telescope is NOT needed.

A prime focus imager at LBT would be of fundamental importance for the second study.

The study of the stellar population in the tidal tail can be directly used to gather information on the internal and externally induced dynamical evolution of the GGCs. In the last few years, observational evidences have been collected by our group (and others) that, due to the evaporation of stars and the interaction with the gravitational potential of the Galaxy, GGCs loose stars (Capaccioli, Piotto, Stiavelli 1993, MNRAS, 261, 819; Piotto, Cool, King 1997, AJ, 113,1345 and references therein). This star loss is enhanced by the tidal shocks experienced by the cluster when it passes through the disk and/or the bulge of the Galaxy. In combination with the internal mass segregation, and as a consequence of the fact that the stars closer to the tidal radius are preferentially lost, the star loss is selective, in the sense that the low mass stars are more easily escaping from the cluster (Capaccioli, Piotto, Stiavelli 1993). This selective loss of stars modifies the mass function, and if we do not know how the stars are lost, there is no hope to have information on the initial mass function (which is what we want to have) from the present day mass function.

Modeling these dynamical effects is almost hopeless (Vesperini and Heggie 1997, MNRAS, 289, 898). However, a wide field imager offers a unique opportunity to directly investigate both the internal mass segregation and the star loss rate as a function of the stellar mass. This kind of investigation can be carried out with the LBT prime focus on the sample of GGCs for which a tidal tail has already been identified (see the list in Grillmair 1998) and mapped (i.e. we know exactly where the extra tidal radius stars are). The problem of the mass function of the tidal tail in GGCs has never been studied, as in their original work Grillmair et al. (1995) used photographic plates. An example of the kind of work that can be done with the LBT wide field imager can be found in Zaggia, Piotto, and Capaccioli (1997), Saviane et al. (1998, A&A, in publication), Veronesi et al. (1996, ASP CF, vol. 92, 301), where we used a mosaic of NTT+EMMI images to cover M55, N1851, and E3, respectively. In these works we limited ourselves to study the internal structure and mass segregation in these clusters: a mosaic of the tails was impossible with the relatively small field of EMMI. The tails discovered by Grillmair et al. (1995) cover a field of 1-2 square degrees. A $25\times25$ square arcmin imager at LBT would give both the field size and limiting magnitude necessary in order to extend the same type of work to the external tails.

The large field coverage will be used not only to map the tail but also to properly map the foreground/background stellar population, a problem of fundamental importance in this project. The tidal tails stars will be identified using the color selection technique, which consist in using stars on the fiducial sequence of the observed GGC in order to eliminate non-members. This technique reduces the foreground/background contribution by at least $90\%$, depending on the precision of the photometry (Zaggia, Piotto, Capaccioli 1997).

The foreseen performances of the LBT wide imager Vlim=27.7 in 104 sec would allow to get luminosity functions down to MV>12for all the clusters with (m-M)V<15.7 in a reasonable time. This would enable us to get extra tidal radius MFs down to $m<0.15m_\odot$for the same clusters.

The same kind of observations, would allow to directly observe the effects of:

next up previous contents
Next: Mass segregation in globular Up: Stellar populations Previous: Globular Clusters
Guido Buscema