Teramo, 6 Maggio 2008

1. Stelle pulsanti classiche: problemi aperti e prospettive futureMarcella MarconiINAF-Osservatorio Astronomico di Capodimonte Teramo, 6 Maggio 2008

2. OUTLINE • Pulsating stars: an introduction • Classical cepheids and RR Lyrae: why to continue studying them? • Problems and possible solutions Teramo, 6 Maggio 2008

3. NGC2419 Ripepi et al. 2007 Carina, Dall’Ora et al. 2003 They can be used as distance indicators NGC6822 Clementini et al. 2004 OGLE sample Pulsating stars κ and γ mechanisms are efficient in stars within the classical instability strip …but also as tracers of the properties of stellar populations Teramo, 6 Maggio 2008

4. Pulsation parameters are related to the intrinsic evolutionary ones P√ρ= costant→ Period is a function of mass, luminosity, effective temperature e.g. logP=11.497-3.48 logTe+0.84 logL-0.68 logM (Van Albada & Baker 1971) for RR Lyrae(F mode) For classical Cepheids by combining theP(M,L,Te)relation with aMLrelation (log L = a + b logM+ c logZ + d logY) predicted by stellar evolution provides the physical basis ofPeriod-Luminosity-Color (PLC) relations. In the case of statistically significant samples, by averaging the period at each fixed luminosity level gives the Period-Luminosity (PL) relation. Teramo, 6 Maggio 2008

5. Calibration of the extragalactic distance scale Input to pulsation models Theory versus observations H0 estimate (e.g. Freedman et al. 2001) Insight into evolutionary and pulsational physics Importance of Classical Cepheids Relevant role for the extragalactic distance scale and in stellar evolution studies They obey to a PL relation (usually the LMC PL is used) ML relations predicted by evolutionary calculations It is important to construct as accurate distance scale as possible that is independent of the CMB ! Teramo, 6 Maggio 2008

6. RR Lyrae stars • RR Lyrae are important tracers of chemical and dynamical properties of old stellar populations and their properties provide firm constraints on several important aspects of stellar evolution an cosmology. RR Lyrae stars are low mass He burning stars (Horizontal Branch) • RR Lyrae are important distance indicators for Pop II systems through: the Mv(RR)-[Fe/H] relation and also the existence of a Period-Luminosity relation in the Near-Infrared filters (J,H,K)

7. Classical Cepheids : what is still unclear… • Linearity of the PL over the whole observed period range 2) Dependence of Cepheid properties and PL on chemical composition 3) Origin of the mass discrepancy between evolutionary and pulsational estimates Teramo, 6 Maggio 2008

8. But linearity in the NIR bands! 10 d Marconi et al. 2005 Persson et al. 2004 Ngeow et al. 2005 Caputo, Marconi, Musella 2000 A&A On the linearity of Cepheid PL relations The Cepheid PL relation has long been considered to be a linear function of log(P) over the range 0.3 ≤ log(P) ≤ 2.0. (Madore & Freedman 1991, Tanvir 1997, Gieren et al. 1998, Udalski et al. 1999, Persson et al. 2004) Recent theoretical and empirical evidences in favour of a nonlinearity of PL relations, at least in the Large Magellanic Clouds and in the BVRI bands. (Bono et al. 1999, Caputo, Marconi, Musella 2000, Tammann & Reindl 2002, Kanbur &Ngeow 2004, Sandage et al. 2004, Marconi, Musella, Fiorentino 2005, Ngeow et al. 2005, 2008, Koen et al. 2007) Teramo, 6 Maggio 2008

9. Kanbur & Ngeow 2006 A possible physical explanation for this non-linearity is given by Kanbur et al. (2004), Kanbur & Ngeow (2006) and Kanbur et al. (2007) on the basis of Galactic, LMC and SMC Cepheid models respectively: The non-linearity is caused by the interaction of the Hydrogen ionization front and the photosphere and the way this interaction varies with period. L’effetto stimato di questa non linearità su H0 è di 1-2 % (Koen et al. 2007) Teramo, 6 Maggio 2008

10. Dependence on chemical composition The Cepheid PL relation is often assumed to be universal and the LMC PL is used to measure the distance to extragalactic Cepheids independently of their chemical composition (see e.g. the HST Key Project) A general consensus on the “universality” of the P-L relations, and in particular on their dependence on the Cepheid chemical composition has not been achieved yet. One of the key issues concerning the use of Cepheids as distance indicators is their dependence on chemical composition systematic effects on the extragalactic distance scale (and H0)!! Teramo, 6 Maggio 2008

11. The synthetic linear PL relations get shallower as the metallicity increases. For the B, V, I bands the slope decreases by 29 %, 15 % and 8% respectively, as Z increases from Z=0.004 to Z=0.02 Caputo, Marconi, Musella 2000 A&A On the theoretical side…… Linear nonadiabatic models mostly suggest negligible effects (see e.g. Alibert et al. 1999, Saio & Gautschy 1998, Sandage et al. 1999) Nonlinear convective pulsation models (Bono, Marconi & Stellingwerf 1999, Fiorentino et al. 2002, Marconi, Musella, Fiorentino 2005) predict a metallicity effect on the predicted PL relations depending on the adopted photometric bands. Metal-rich pulsators with periods longer than five days present fainter optical magnitudes than the metal-poor ones. Teramo, 6 Maggio 2008

12. Y also plays a role at the highest metallicities (Z ≥ 0.02) The slope decreases as Z increases at fixed ∆Y/∆Z The slope increases as Y increases at fixed Z Fiorentino, Caputo, Marconi, Musella 2002 ApJ Teramo, 6 Maggio 2008

13. Turnover of the metallicity correction at Z≈0.02 Use of LMC-calibrated VI PL relations justified for P ≤ 10 d and/or ∆Y/∆Z ≤ 2.0 Marconi, Musella, Fiorentino 2005, ApJ For P ≥ 20 d and [O/H] ≥ 0.2 as measured in several spiral galaxies observed by the HST Key Project the average predictedmetallicity correction varies from ~ -0.2 mag to ~ +0.25 mag as ∆Y/∆Z varies from 2 to 3.5 Teramo, 6 Maggio 2008

14. LMC SMC Sasselov et al. 1997 Sandage et al. 2004 Empirical results on the PL metallicity dependence Some empirical tests seem to suggest that metal-rich Cepheids are, at fixed period, brighter than metal poor ones, either over the entire period range or at least for periods shorter than ≈ 25 d Typically one refers to γ=δµ0/δlogZ, with δµ0 quantifying the metallicity correction and δlogZ=logZLMC-logZCeph According to these empirical studies γ is negative up to -0.4 mag dex-1 with an average value of ≈ -0.25 mag dex-1 (Sakai et al. 2004 and references therein) Teramo, 6 Maggio 2008

15. In particular Sakai et al. 2004 find γ=-0.25 mag dex-1 from thecomparison of distances based on Cepheids and the TRGB Bono et al. 2008 ApJ But this result was questioned by Rizzi et al. (2007 ApJ), who provided revised TRGB distances. Teramo, 6 Maggio 2008

16. Spectroscopic [Fe/H] measurements of Galactic Cepheids indicate that the visual PL relation depends on the metal content in agreement with model predictions. Correzione teorica Romaniello et al. (2005) Teramo, 6 Maggio 2008

17. Direct empirical tests of the metallicity effects ∆[O/H]=0.7 dex γ=-0.27 mag dex-1 ∆[O/H]~0.5 dex γ=-0.29 mag dex-1 (Macri et al. 2006) But both the comparison with pulsation models and the most recent HII abundance measurements (Diaz et al. 2000) suggest a rather constant LMC-like metal-content for the Cepheids observed in the two fields of NGC4258 (Bono et al. 2008 ApJ in press). 1) Cepheid observations in the outer and inner field of M101 (Kennicutt et al. 1998, 2003) But blended Cepheids could affect this results (see e.g. Macri et al. 2006) Blended (inner field)→ appear brighter → distance underestimated 2) BVI observations of a large Cepheid sample in two field of the galaxy NGC4258 If confirmed this occurrnce would prevent any reliable differential determination of the PL metallicity dependence! Teramo, 6 Maggio 2008

18. Distance estimates based on the “near-infrared surface brightness” indicate a vanishing metallicity effect between Galactic and Magellanic Cepheids (Gieren et al. 2005, Fouquè et al. 2007) The p factor and its possible dependence on the pulsation period is still debated in the literature (Nardetto et al. 2007, 2008 and references therein) The results based on the infrared surface brightness technique rely on the assumption on the p factor used to convert the radial velocity measurement into the pulsation velocity Teramo, 6 Maggio 2008

19. New optical and NIR PL relations for Galactic Cepheids, that are found to have slopes similar to the LMC counterparts. Accurate trigonometric parallaxes for ten Galactic Cepheids (Benedict et al. 2007) using the Fine Guide Sensor available on board of the HST. Teramo, 6 Maggio 2008

20. →18.43 →29.33 Bono et al. 2008 ApJ Very good agreement with the EB and maser estimates if the metallicity correction predicted by models is adopted. The absolute WVI functions of these Galactic variables, as based on the absolute magnitudes obtained from the HST parallaxes, can be used to infer the distance to LMC and NGC4258 µ0,VI(LMC)= 18.45±0.09 mag (EBs →18.41±0.09mag Guinan et al. 2004) µ0,VI(4258)=29.35±0.12mag (maser→29.29±0.15mag Herrnstein et al. 1999) But the metal abundances of LMC and NGC4258 Cepheids are lower than the Galactic ones →a metallicity correction should be applied ! Teramo, 6 Maggio 2008

21. angular diameter variation over the pulsation cycle radial velocity from spectral line profiles + projection (p) factor Stellar radius variation over the pulsation cycle Kervella et al. 2004 New interesting results are expected from……. Long-baseline interferometers currently provide a new, quasi-geometric way to calibrate the Cepheid PL relation → Interferometric Baade Wesselink (IBW) Distances to Galactic Cepheids up to 1Kpc Interferometric measurements Pulsation velocity integration Angular and linear diameters have to correspond to the same physical layer in the star to correctly estimate the distance. The p factor is currently the most important limiting quantity of the IBW method Teramo, 6 Maggio 2008

22. limb-darkening effects velocity gradient dynamical structure of the Cepheid atmosphere The angular diameter measurements are also affected by the presence of circumstellar envelopes around Cepheids (Merand et al. 2007) The p factor is related to Direct estimate of the p factor for δ Cephei (p=1.27±0.06) by Merand et al.(2005) using the HST parallax. The combination of different techniques (high resolution spectroscopy, spectro- and differential- interferometry) is needed to efficiently constrain the physical parameters of the Cepheid atmosphere and the p factor(Nardetto et al. 2007, 2008)

23. Model fitting of light, radial velocity and radius curves of Cepehids The case of δ Cephei (Natale, Marconi, Bono 2008 ApJL) The fitting of the radius (angular diameter) curve has the advantage of being independent of both the interstellar extinction and the p factor. Fit soddisfacente della variazione fotometrica, della velocità radiale e del raggio, con p=1.28 Constraints on the stellar mass and test of evolutionary predictions: mass loss and/or overshooting ? Teramo, 6 Maggio 2008

24. Observations and modeling of Cepheids in metal poor galaxies The case of IzW18: saturation of the metallicity effect toward lower abundances? Marconi et al. 2008 Aloisi et al. 2007 ApJL Teramo, 6 Maggio 2008

25. Waiting for GAIA and SIM………… Significant improvement in geometric parallaxes for Cepheids will come from the space-based, all-sky astrometry mission GAIA (Mignard 2005) and Space Interferometry Mission PlanetQuest (Unwin 2005) with ~ 10 µas precision parallaxes. Final results expected by the end of the next decade! Teramo, 6 Maggio 2008

26. Conclusions There are exciting and debated problems in the study of stellar pulsation with important implications for stellar evolution and cosmology. Conclusive results are expected from new promising observational and theoretical techniques, as well as from the futures space missions GAIA and SIM. Teramo, 6 Maggio 2008

28. Their intrinsic luminosity providesfundamental constraints to the distance to the galactic center, GGCs, nearby galaxies (Magellanic Clouds, M31, dwarf spheroidal galaxies) and a calibration for secondary distance indicators such as the GCLF (Di Criscienzo et al. 2006) Mv(RR) = a + b [Fe/H] Long debate on the values of a and b…..(see e.g.Cacciari & Clementini 2003) The MV-[Fe/H] relation Only recently the estimates of a seems to be converging taowards ~ 0.20, as supported by studies of field RR Lyrae in the Milky Way (Fernley et al. 1998, Chaboyer 1999) and in the LMC (Gratton et al. 2004) and in M31 GCs (Rich et al.2005)

29. Bono, Capto, Di Criscienzo 2007 Non linearity of the MV-[Fe/H] relation There are empirical and theoretical evidences for a nonlinearity of the above relation (Bono et al. 2007, Caputo et al. 2000, Di Criscienzo et al. 2004) Mv(HB) depends on both metallicity and HB morphology (Oo type?)

30. Del principe et al. 2006 Sollima et al. 2006 The K band PL relation for RR Lyrae Observations of RR Lyrae in the NIR bands have several advantages: 1) a smaller dependence on interstellar extinction and metallicity 2) smaller pulsation amplitude → reliable mean magnitudes 3) the existence of a PL relation Teramo, 6 Maggio 2008

31. Application to the prototype RR Lyr The PLK for RR Lyrae: theoretical constraints Bono et al. 2003 MNRAS) Teramo, 6 Maggio 2008

32. Oo II Gap NGC2419 Oo I Marconi & Clementini 2008 Ripepi et al. 2007 ApJL Suk-Jin et al. 2008 RR Lyrae stars as stellar population tracers: new interesting issues RR Lyrae in “special” globular clusters RR Lyrae as tracers of stellar streams Extraglactic RR Lyrae Teramo, 6 Maggio 2008

33. Variable stars in the field and GCs of M31 LBT approved SDT program PI G. Clementini observations of 4 LBC@LBT fileds on the halo and stream of M31. Field S2 observed in October 2007 Pilot study based on HST – WFPC2 archive observations of G11, G33, G64, G322 25 candidate RR Lyrae stars Clementini et al. 2001, ApJ 559, L109 HST Cycle 15 Program 11081 PI G. Clementini 78 orbits with WPC2 observations of G11, G33, G76, G105, G322, B514 June – September 2007 Results for B514