People involved at OAB: Bellazzini, Cacciari, Clementini, Baldacci,
Maio.
In merging scenarios the Galaxy halo was made up of accreted dwarf Spheroidal Galaxies (dSph's) similar to the known satellite of the Milky Way (MW). A number of GGCs thus may originate from dSph's that were accreted by the MW and should have properties that were directly inherited from their ``ancestors". A puzzling feature of the GGCs is that they sharply divide into two distinct Oostheroff types according to the mean periods of their RR Lyrae stars and the relative proportions of fundamental and first overtone-mode pulsators (Oosterhoff 1939). In the MW there are no clusters filling the gap between Oostheroff type I and type II objects. This is not true of the LMC clusters, which instead have mean periods that fill the Oosterhoff gap. The Galaxy cannot have been assembled from LMC-like protogalactic fragments.
High quality BVI photometric observations were taken in November 2001 of the clusters NGC362 and NGC1904 for a detailed study of their RR Lyrae variables and the second-parameter effect. These data are presently being analysed, in collaboration with M. Catelan (Pontificia Universidad Catolica de Chile).
A very detailed analysis of the RR Lyrae stars in M3 is presently being performed in collaboration with Corwin (Univ. of North Carolina). Based on very accurate multi-colour photometry, this study is showing the impact and characteristics of the Blazhko stars, the presence of a good number of evolved stars and their properties, as well as the characteristics of the main variable star population.
time series photometry reaching half a magnitude below the
TO of the remote
galactic globular cluster NGC2419 was obtained with DOLORES@TNG.
Study of the light curves and pulsation
characteristics of the variables is in progress and will allow to check
whether this cluster could be the relict of an extragalactic system
accreted by the Milky Way.
This work is in collaboration with Marconi, Musella, Ripepi (INAF-Napoli Obs.),
Di Fabrizio (INAF-TNG).
We are undertaking a systematic study of the variable stars in the 5 globular
clusters of the Fornax galaxy
to determine their Oosterhoff types and test whether the MW could
have accreted
GCs from Fornax-like protogalactic systems.
27 RR Lyrae (among which 9 double
mode pulsators),
2 Anomalous and 2 Dwarf Cepheids
have been identified in Fornax 3 (Clementini et al. 2004a).
Examples of
their light curves
are shown in the right panel of Fig. 5.
time series photometry of clusters #2, #4 and #5 was
obtained with the 6.5m Magellan/Clay telescope.
Figure 6 shows the
instrumental color-magnitude diagram of Fornax 5, reaching about
2 mag below the cluster TO.
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Double-mode RR Lyrae stars (RRd's) pulsate simultaneously in the fundamental and in the first-overtone radial modes, while evolving across the HB instability strip. They provide information on the mass, mass-metallicity relation and on the direction and rate of HB evolution. Re-analysis of Corwin & Carney (2001) time series data for the GC M3 using the image subtraction method (Alard 2000), has led to the discovery of three new double-mode RR Lyrae stars in this cluster. Two of them lie in the Petersen diagram (Petersen 1973), in positions implying large spread in mass and/or heavy element mass fraction among the M3 HB stars. Three of the M3 RRd's have changed their dominant pulsations mode in a one year time-span, thus suggesting that they are undergoing a rapid evolutionary phase, and that both redward and blueward evolution may take place among HB stars. Full discussion of the results is presented in Clementini et al. (2004b). This study is in collaboration with Corwin (Univ. of North Carolina at Charlotte), and Carney (Univ. of North Carolina at Chapel Hill).