Distance to the Large Magellanic Cloud

People involved at OAB: Bragaglia, Clementini, Maio.

The luminosities of RR Lyrae and clump stars in the bar of the LMC and their role in measuring distances have been investigated in collaboration with Carretta, Gratton (INAF-Padova Obs.) and Di Fabrizio (INAF-TNG).

$B$, $V$, $I$ light curves have been obtained for 129 RR Lyrae stars, 4 candidate Anomalous Cepheids, 11 Classical Cepheids, 11 eclipsing binaries, and one $\delta$ Scuti star in two fields located close to the bar of the LMC. The complete catalogue of the individual photometric measurements and light curves is provided in Di Fabrizio et al. (2004). A very accurate estimate of the average apparent luminosity of the LMC RR Lyrae and clump stars and independent estimate of the local LMC reddening have been obtained from these photometric data. The dereddened apparent average luminosity of the LMC RR Lyrae stars defined by our study has been combined with a number of independent determinations of the absolute luminosity of RR Lyrae stars, to obtain estimates of the distance to the LMC. The derived values are only 1 $\sigma$ shorter than provided by the Population I distance indicators and make it possible to reconcile the short- and long-distance scales on a common value for the distance modulus of the LMC of $\mu_{\rm LMC} = 18.515 \pm 0.085$ mag (Clementini et al. 2003a).

In collaboration with M. Marconi (INAF-Napoli Obs.) the extensive and detailed grid of nonlinear convective pulsation models of RR Lyrae stars computed by Bono et al. (2001) and Marconi et al. (2004) has been used to reproduce the observed $V,B$ light curves of 7 ab- and 7 c-type pulsators in our LMC RR Lyrae sample. The method performs the theoretical reproduction of both the general features and detailed morphology of the observed light curves and provides in turn an independent estimate of the distance to the LMC that is fully consistent with the long distance scale (Marconi & Clementini 2004, in preparation).

In collaboration with McNamara (Brigham Young Univ.) we have used the improved version of the period-luminosity relation for $\delta$ Scuti stars defined by McNamara (2003) to derive an independent estimate of the distance to the LMC based on the $\delta$ Scuti star identified in one of our LMC fields (Clementini & McNamara 2004, in preparation).