Variable stars in nearby galaxies

People involved at OAB: Clementini, Contreras, Federici, Tosi

The detection and study of the pulsating variables in a number of LG galaxies (Fornax, Ursa Minor, Small Magellanic Cloud, M31), as well as in several of the new ultra-faint satellites recently discovered around the Milky Way and Andromeda galaxies, and in most distant systems like the blue compact galaxy (BCD) IZw18, is being carried out in collaboration with E. Held, M. Gullieuszik and L. Rizzi (INAF-Padova Obs.), E. Poretti (INAF-Brera Obs.), M. Moretti (Univ. Bologna), M. Marconi, I. Musella, V. Ripepi, M. Dall'Ora (INAF-Napoli Obs.), L. Di Fabrizio (INAF-TNG), C. Greco (UniGe), G. Fiorentino (Groningen), H. Smith (MSU, USA), M. Catelan (PUC, Chile), B. Pritzl (Macalester Univ., USA), A. Nota (STScI, USA), J. Gallagher (Univ. Wisconsin, USA), E. Grebel (ARI, Heidelberg, Germany), K. Kinemuchi (Univ. of Florida, USA), T. Beers (MSU, USA), A. Aloisi, R. van der Marel (STSci, USA), F. Annibali (INAF-Padova Obs.), A. Saha (NOAO, USA).

The Fornax Project
The Fornax project (Clementini et al. 2007) is an international collaboration set up to make a comprehensive and deep ($V \leq 26$ mag) study of the variable star population in the field and globular clusters of the Fornax dSph. To this end we have carried out a wide-area ($\sim 1$ deg$^2$) time series photometric survey of the galaxy field, as well as obtaining high spatial resolution photometry of the Fornax GCs. In 2009 we have published results from the study of the variable stars in cluster Fornax 5 (Greco et al. 2009). A paper presenting the study of the variable star population in cluster Fornax 3 is in preparation (Clementini et al. 2010, in preparation). According to the pulsation properties of the RR Lyrae stars, the Fornax GCs have been found to belong to an Oosterhoff-intermediate class and to fill the Oosterhoff-gap defined by the Galactic GCs.

This study is in collaboration with E. Held, M. Gullieuszik (INAF-Padova Obs.), E. Poretti (INAF-Brera Obs.), M. Catelan (PUC, Chile), H. Smith (MSU, USA), B. Pritzl (Macalester Univ., USA). This research was partly supported by MIUR-PRIN04 grant n. CRA 1.06.02.03, and by PRIN-INAF06 grant n. CRA 1.06.09.15.

The ``Small Magellanic Cloud in Space and Time'' and ``The VISTA near-infrared $YJK_s$ survey of the Magellanic System, VMC''
As part of large international collaborations we are making a detailed and comprehensive study of the field and cluster stellar populations, structure and evolution of the entire Magellanic Cloud System (MCS, see also Sect. ). The MCS pulsating variable stars are key tools of these studies (see Clementini 2009). In 2009 we published the structural parameters of six SMC clusters (Glatt et al. 2009) among which the only globular in the SMC: NGC121. Study of the NGC121 variable stars is in progress. Guaranteed time observations to study constant and variable star populations of the SMC body, Bridge and Stream down to the turn-off of the oldest stars, are planned for OmegaCAM@VST (the STEP@VST survey, PI: V. Ripepi, CoIs: members of the OAB; Ripepi et al. 2006). The MC project was extended to the near-infrared in the framework of the approved VMC@VISTA (The VISTA near-infrared $YJK_s$ survey of the Magellanic System, PI: M. Cioni, CoIs: members of the OAB) ESO public survey (see Cioni et al. 2008). The VMC observations started in fall 2009 and are now complete for a number of LMC fields. A full description of the survey and results from these first data have been published in Cioni et al. (2010, A&A submitted).

This study is part of the PhD Thesis of M. Moretti (Univ. Bologna), and is in collaboration with V. Ripepi, M. Marconi, I. Musella, E. Cappellaro (INAF-Napoli Obs.), A. Nota, M. Sirianni (STSCI, USA), J. Gallagher (Univ. Wisconsin, USA), M.R. Cioni (Univ. Hertfordshire, UK), and E. Grebel (ARI, Heidelberg, Germany). This research is partially supported by the PRIN INAF 2008.

Stellar Archeology in the Milky Way halo: variable stars and stellar populations in the newly discovered Milky Way satellites
Our team is systematically studying the variable star content as well as the stellar populations of the ultra-faint dwarf (UFDs) satellites recently discovered around the Milky Way and M31. In 2009 we published results for 3 Milky Way UFDs, namely: Coma (Musella et al. 2009), Leo IV (Moretti et al. 2009) and UMa II (Dall'Ora et al. 2010, submitted), while analysis is in progress on Hercules, UMa I, Leo T, and for the M31 UFDs And XI, XII, XIII, and And XXI. Figure  shows the CMDs and maps of the Coma dSph from our study published in Musella et al. (2009).

This study is part of the PhD Thesis of M. Moretti, and of the Master Thesis of A. Garofalo, and is in collaboration with, V. Ripepi, M. Marconi, I. Musella, M. Dall'Ora (INAF-Napoli Obs.), L. Di Fabrizio (INAF-TNG), H. Smith (MSU, USA), M. Catelan (PUC, Chile), B. Pritzl (Macalester Univ., USA), K. Kinemuchi (Univ. of Florida, USA), T. Beers (MSU, USA).

IZw18
Variable stars were identified for the first time in the very metal-poor BCD IZw18, using proprietary ACS@HST time-series photometry (Aloisi et al. 2007). They include two long-period variables and 3 Classical Cepheids, the lowest metallicity ones found so far. We have studied these variable stars in detail and derived from the Classical Cepheids a new distance to IZw18 (19 Mpc), which is much more reliable than that obtained from other indicators (Fiorentino et al. 2010). We have exploited the unique value of this metal-poor sample of Cepheids to constrain theoretical models for pulsation (Marconi et al. 2010) and extend the calibration of the Cepheid extragalactic distance scale to low metallicity regimes. This study is in collaboration with G. Fiorentino (Groningen, NL), M. Marconi and I. Musella (INAF-Napoli Obs.), F. Annibali (INAF-Padova Obs.), A. Aloisi and R. van der Marel (STScI, USA), A. Saha (NOAO, USA) and is partially funded by ASI-Cofis-2007.

Figure: Upper panels: $V, B-V$(left) and $V, V-I$(right) CMDs of the ultra-faint Milky Way dwarf spheroidal satellite Coma (Musella et al. 2009). Solid (blue) and dashed (black) lines are the ridgelines of the Galactic globular clusters M68 and M3, respectively. Red and cyan dots are stars respectively within $\pm$ 0.06 mag in $B-V$ and $\pm$ 0.05 mag in $V-I$, and from $\pm$ 0.05/0.06 to $\pm$ 0.1 mag from the ridgelines of M68. Blue dots are nonvariable stars on the galaxy HB, green dots are stars in the blue stragglers' region. Variable stars are marked by filled triangles, cyan: RRc star, magenta: RRab star, blue: short period variable. Open circles mark 24 spectroscopically confirmed members of the Coma dSph. Black dots are galaxies of cluster MaxBCG J 186.85861+2380004. Lower panels: maps of objects most likely belonging to the Coma dSph, colour-coded and with the same symbols as in the top panels of the figure. HB stars are outlined by squares. The large circles show the galaxy half-light radius $r_h$(exponential$)=5.9$ arcmin. The dashed circles show the position of the galaxy cluster.