Università degli Studi di Roma “Tor Vergata” Facoltà di Scienze Matematiche, Fisiche e Naturali. Dottorato di ricerca in Astronomia – XVII ciclo The RR Lyrae distance scale from Near-Infrared photometry Massimo Dall’Ora Coordinatore Relatore Prof. Roberto Buonanno Prof. Roberto Buonanno Tutore Prof. Giuseppe Bono Contents Abstract 1 1 The Scientific Problem 7 1.1 RR Lyrae stars as distance indicators 7 1.2 The PhD project 8 2 RR Lyrae stars 12 2.1 Observational properties 12 2.2 Evolutionary properties 15 2.3 Pulsation Physics 16 2.3.1 Generalities 16 2.3.2 Kappa and Gamma mechanisms 19 2.4 Pulsation Periods 21 2.5 Oosterhoff dichotomy 22 2.6 The Blazhko effect 24 2.7 Mean magnitudes 25 2.8 RR Lyrae stars as distance indicators: the MV −[ Fe / H ] relation 25 2.8.1 Pros and Cons 25 2.8.2 Outline of the current calibrations 28 2.9 RR Lyrae stars as distance indicators: the FOBE method 30 2.10 RR Lyrae stars as distance indicators: the PLK relation 31 2.10.1 Empirical evidence and theory 31 2.10.2 PLK relation: fine tuning 37 2.10.3 The MK −[ FeH / ] − log P relation 40 1 3 Data reduction 42 3.1 Near-infrared arrays 42 3.2 Array operation 43 3.3 The sky in the infrared 44 3.4 SOFI – Son OF ISAAC 46 3.4.1 Optical arrangement 46 3.4.2 The Detector 49 3.5 Observational techniques in the IR 49 3.6 Data pre-reduction 50 3.7 Photometric reduction 52 3.7.1 Overview 52 3.7.2 Calculus of the PSF – DAOPHOT 53 3.7.3 Photometry – ALLFRAME 54 3.7.4 The photometric calibration 56 4 Observations and Color Magnitude Diagrams 61 4.1 The LMC Cluster Reticulum 61 4.1.1 Introduction 61 4.1.2 Observations and data reduction 61 4.1.3 Color-Magnitude Diagram 65 4.2 The Galactic Globular Cluster M68 66 4.2.1 Introduction 66 4.2.2 Observations and data reduction 67 4.2.3 Color-Magnitude Diagram 70 4.3 The Galactic Globular Cluster NGC 3201 71 4.3.1 Introduction 71 4.3.2 Observations and data reductions 73 4.3.3 Color-Magnitude Diagram 75 2 5 Discussion of the results: distance estimates 77 5.1 The distance to Reticulum 77 5.2 The distance to LMC 84 5.3 The distance to M68 86 5.4 The distance to NGC 3201 91 5.5 The RGB Bump 95 Conclusions 97 Bibliography 101 Acknowledgments 107 3 4 Abstract The work carried out in this thesis is part of a long-term project aimed at measuring accurate near-infrared magnitudes of RR Lyrae stars in a sample of Galactic globular clusters that cover a wide metallicity range ( −≤2.3 [Fe / H ] ≤− 1.3 ) and host a sizable population of RR Lyrae stars ( NRR ≥ 10 ). In this thesis we will present the result obtained for the Galactic globular clusters M68 and NGC3201, and for the Large Magellanic Cloud cluster Reticulum. The results for these clusters are all based on the observations carried out with SOFI/NTT. The data collected allowed us to provide accurate estimates of J, K mean magnitudes for a large portion of the cluster RR Lyrae variables, and in turn to derive in a homogeneous context the slope of the K-band Period-Luminosity relation ( PLK ). The slope of the observed PLK relation agrees quite well with theoretical pulsational predictions. On the basis of this agreement between our empirical PLK relations and the theoretical ones, we adopted the latter for estimating the absolute distances to the target clusters. The distances obtained for M68, NGC 3201, and Reticulum disclose a fairly good agreement with the most recent and accurate independent estimates (Baade-Wesselink calibration, FOBE method, δ Scuti Period-Luminosity relation). Current data, together with near-infrared data already collected, will allow us to accomplish the following goals: • to derive, on the basis of the new empirical calibration of the PLK , absolute distances for field RR Lyrae stars for which are available mean K magnitudes and to compare them with distances based on Baade-Wesselink method; • to provide, on the basis of predicted and observed PL(J-K) relations, independent estimates of the reddening toward the target clusters; • to supply an independent test on a wide metallicity range of the accuracy of both evolutionary and pulsational predictions adopted to construct the PLK and the PL(J-K) relations; • to obtain accurate optical/near-infrared Color-Magnitude diagrams (CMDs) for the target clusters by adopting the new data and the optical data available in the literature. 5 These CMDs will allow us to compare theory and observations by using color indexes (V-J), ( V-K), which are only marginally affected by systematic uncertainties; • to use ( V-K) colors to derive accurate estimates of the effective temperatures of the globular clusters RR Lyrae stars, which in turn can be used to provide an estimate of the helium abundance. This result is particularly interesting, because globular clusters are among the oldest objects that can be found in the Galaxy, and therefore the estimated helium estimates can give a hint on the primordial helium abundance. 6 Chapter 1 The Scientific Problem 1.1 RR Lyrae stars as distance candles Since the first trigonometric distance measurement performed by Bessel on the star P Cygni in 1838, one of the major problems in the Galactic and in the extragalactic astronomy is the development of robust methods to provide accurate estimates of the cosmic distances. Together with the development of the spectroscopic observations, Astronomy became Astrophysics with the advent of distance measurements, the third dimension in our vision of the sky. Indeed, the knowledge of the physics of the celestial objects, and of the Universe as a whole, is tied to the knowledge of the absolute distances. Connected to the distance scale problem is the building of a comprehensive framework of the formation and evolution of the galaxies, and first of all of the Milky Way. Indeed, the calibration of robust standard candles allows us to trace the 3-D structure of our system and of the group of galaxies to which the Galaxy belongs, the Local Group. Moreover, correct estimates of the stellar distances allow us, via a comparison with the theoretical models of stellar evolution, to give an idea of the ages of the stellar populations and, together with chemical abundances, to trace back the history of their formation and evolution. In this context, RR Lyrae stars are particularly important, since they are standard candles that can be observed in all the old components of the stellar systems: Galactic and extragalactic globular clusters, Galactic halo, Galactic Bulge, ancient populations of the nearby galaxies, such as the Magellanic Clouds and the dwarf spheroidal galaxies of the Local Group. These systems were among the first formed in the Universe, and therefore it is crucial to have accurate estimates 7 of their distances and ages, in order have strong constraints on the formation and evolution of the structures in the Universe. Without entering into a detailed discussion on the calibration of the standard candles, here we mention only the fact that the use of the RR Lyrae stars as distance indicators is still hampered by several uncertainties, both on the empirical and on the theoretical side, mainly connected to the dependence of their absolute V-band magnitude on the chemical composition and on the uncertainty on the evolutionary status. Current uncertainties on the absolute visual magnitude of the RR Lyrae stars are of the order of 0.2-0.3 mag. Details will be given in chapter 2. At the same time, RR Lyrae stars in the photometric K-band (centered on the effective wavelength λeff = 2.2 µ m ) do obey to a tight linear period-luminosity relation ( PLK ). This relation offers several advantages, both theoretical and observational, when compared to other techniques. Once calibrated, this relation could be a powerful and sound tool to gain accurate and homogeneous distances for old stellar components. 1.2 The PhD Project The main goal of this thesis project is to substantially improve the distance estimates to the globular clusters with the PLK relation. Results have been produced for two Galactic globular clusters, namely M68 and NGC 3201, and for the Large Magellanic Cloud cluster Reticulum. Data presented in this thesis work are part of a more general project, which has a threefold objective: • we still lack an extensive K-band observational database for RR Lyrae stars. Indeed, empirical estimates of the PLK relation have been published only for nine Galactic globular clusters: M3, M4, M5, M15, M107, NGC 3201, NGC 5466, Ω Cen (Longmore et al. 1990), and IC 4499 (Storm 2004); M3 was observed also by Butler (2003). Unfortunately, the Longmore et al. (1990) measurements are heavily affected by observational uncertainties such as crowding, since their photometry was carried out with a fixed-aperture photometer and the derived PLK for the single clusters have uncertainty of the order of 10-15% on the slope. Butler (2003) observed only seven RR Lyrae stars in the innermost regions of the Galactic globular cluster M3. Storm used single-epoch observations to estimate a distance to IC4499, but the measurements are affected, on 8 average, by an error of the order of 10%. The fist objective of our project is therefore to supply a homogeneous photometric database to derive an accurate estimate of the empirical PLK ; • theory predicts a non-negligible dependence of the PLK on the chemical composition, which translates in a different zero-point of the relation.