Stelle pulsanti classiche: problemi aperti e prospettive future Marcella Marconi INAF-Osservatorio Astronomico di Capodimonte Teramo, 6 Maggio 2008

Stelle pulsanti classiche: problemi aperti e prospettive future

Marcella Marconi INAF-Osservatorio Astronomico di Capodimonte

Teramo, 6 Maggio 2008

OUTLINE

•Pulsating stars: an introduction

•Classical cepheids and RR Lyrae: why to continue studying them?

•Problems and possible solutions


Pulsating stars


…but also as tracers of the properties of stellar populations

κ and γ mechanismsare efficient in stars within the classical instability strip

They can be used as distance indicators

OGLE sample

Carina, Dall’Ora et al. 2003 NGC2419 Ripepi et al. 2007

NGC6822 Clementini et al. 2004

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 the P(M,L,Te) relation with a ML relation (log L = a + b logM + c logZ + d logY) predicted by stellar evolution provides the physical basis of Period-Luminosity-Color (PLC) relations.


In the case of statistically significant samples, by averaging the period at each fixed luminosity levelgives the Period-Luminosity (PL) relation.

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)

It is important to construct as accurate distance scaleas possible that is independent of the CMB !

Calibration of the extragalactic distance scale

H0 estimate(e.g. Freedman et al. 2001)

ML relations predicted byevolutionary calculations

Input to pulsation models

Theory versus observations

Insight into evolutionary and pulsational physics

RR Lyrae stars

RR Lyrae stars are low mass He burning stars (Horizontal Branch)

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 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)

Classical Cepheids : what is still unclear…

1) 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

On the linearity of Cepheid PL relations


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)

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)

Ngeow et al. 2005

10 d

But linearity in the NIR bands!

Persson et al. 2004

Marconi et al. 2005

Caputo, Marconi, Musella 2000 A&A


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.

Kanbur & Ngeow 2006

L’effetto stimato di questa non linearità su H0 è di 1-2 % (Koen et al. 2007)

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)


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)!!

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.


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.

Caputo, Marconi, Musella 2000 A&A

The synthetic linear PL relations get shallower as the metallicity increases.

For the B, V, I bands the slopedecreases by 29 %, 15 % and 8% respectively, as Z increases from Z=0.004 to Z=0.02

Metal-rich pulsators with periods longer than five days present fainter optical magnitudes than the metal-poor ones.


Y also plays a role at the highest metallicities (Z ≥ 0.02)

The slope decreases as Z increasesat fixed ∆Y/∆Z

The slope increases as Y increasesat fixed Z

Fiorentino, Caputo, Marconi, Musella 2002 ApJ


Use of LMC-calibrated VI PL relations justified for P ≤ 10 d and/or ∆Y/∆Z ≤ 2.0

Marconi, Musella, Fiorentino 2005, ApJ

Turnover of the metallicity correction at Z≈0.02

For P ≥ 20 d and [O/H] ≥ 0.2 as measured in several spiral galaxies observed by the HST Key Project the average predicted metallicity correction varies from ~ -0.2 mag to ~ +0.25 mag as ∆Y/∆Z varies from 2 to 3.5

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)

Sasselov et al. 1997

LMC

SMC

Sandage et al. 2004


In particular Sakai et al. 2004 find γ=-0.25 mag dex-1 from the comparison of distances based on Cepheids and the TRGB

But this result was questioned by Rizzi et al. (2007 ApJ), who provided revised TRGB distances.

Bono et al. 2008 ApJ

Teramo, 6 Maggio 2008Romaniello et al. (2005)

Correzione teorica

Spectroscopic [Fe/H] measurements of Galactic Cepheids indicate that the visual PL relation depends on the metal content in agreement with model predictions.


Direct empirical tests of the metallicity effects

1) Cepheid observations in the outer and inner field of M101 (Kennicutt et al. 1998, 2003)

2) BVI observations of a large Cepheid sample in two field of thegalaxy NGC4258

∆[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 blended Cepheids could affect this results (see e.g. Macri et al. 2006)Blended (inner field)→ appear brighter → distance underestimated

If confirmed this occurrnce would prevent any reliable differential determination of the PL metallicity dependence!

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).

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 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

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)

Accurate trigonometric parallaxes for ten Galactic Cepheids (Benedict et al. 2007) using the Fine Guide Sensor available on board of the HST.


New optical and NIR PL relations for Galactic Cepheids, that arefound to have slopes similar to the LMC counterparts.

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±0.09 mag Guinan et al. 2004)

µ0,VI(4258)=29.35±0.12 mag(maser→ 29.29±0.15 mag 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 !

Bono et al. 2008 ApJ

→18.43

→29.33

Very good agreement with the EB and maser estimates if the metallicity correction predicted by models is adopted.

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)

Pulsation velocity integration

Interferometric measurements angular diameter variation over the pulsation cycle

Stellar radius variation over the pulsation cycle

radial velocity from spectral line profiles+ projection (p) factor

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

Kervella et al. 2004

Distances to Galactic Cepheids up to 1Kpc

The p factor is related to

limb-darkening effects

velocity gradient

dynamical structure of theCepheid atmosphere

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)

Direct estimate of the p factor for δ Cephei (p=1.27±0.06) by Merand et al.(2005) using the HST parallax.

The angular diameter measurements are also affected by the presence of circumstellar envelopes around Cepheids (Merand et al. 2007)


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.

Constraints on the stellar mass and test of evolutionary predictions: mass loss and/or overshooting ?

Fit soddisfacente della variazione fotometrica, della velocità radiale e del raggio, con p=1.28


Observations and modeling of Cepheids in metal poor galaxies

The case of IzW18: saturation of the metallicity effect toward lower abundances?

Aloisi et al. 2007 ApJL

Marconi et al. 2008


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!

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.

The MV-[Fe/H] relation

Their intrinsic luminosity provides fundamental 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)

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)

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?)

Non linearity of the MV-[Fe/H] relation

Bono, Capto, Di Criscienzo 2007

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 metallicity2) smaller pulsation amplitude → reliable mean magnitudes3) the existence of a PL relation

Sollima et al. 2006Del principe et al. 2006

The PLK for RR Lyrae: theoretical constraints


Bono et al. 2003 MNRAS)

Application to the prototype RR Lyr

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

Marconi & Clementini 2008

Suk-Jin et al. 2008Ripepi et al. 2007 ApJL

NGC2419

Oo I

Gap

Oo II

Variable stars in the field and GCs of M31Variable stars in the field and GCs of M31

Pilot study based onHST – WFPC2 archive observations ofG11, G33, G64, G32225 candidate RR Lyrae starsClementini et al. 2001, ApJ559, L109

HST Cycle 15 Program 11081 PI G. Clementini78 orbits with WPC2observations ofG11, G33, G76, G105, G322, B514 June – September 2007Results for B514

LBT approved SDT program PI G. Clementiniobservations of4 LBC@LBT fileds on the haloand stream of M31.Field S2 observed in October 2007

Documents

Stelle pulsanti classiche: problemi aperti e prospettive future Marcella Marconi INAF-Osservatorio Astronomico di Capodimonte Teramo, 6 Maggio 2008