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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.

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Stelle pulsanti classiche: problemi aperti e prospettive futureMarcella MarconiINAF-Osservatorio Astronomico di Capodimonte

Teramo, 6 Maggio 2008

  • Pulsating stars: an introduction
  • Classical cepheids and RR Lyrae: why to continue studying them?
  • Problems and possible solutions

Teramo, 6 Maggio 2008

pulsating stars

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

pulsation parameters are related to the intrinsic evolutionary ones
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

importance of classical cepheids

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

rr lyrae stars
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)
classical cepheids what is still unclear
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

on the linearity of cepheid pl relations

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


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

dependence on chemical composition
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


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


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


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

empirical results on the pl metallicity dependence



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


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


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


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


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


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




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)


(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


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


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)


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


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


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



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


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)


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?)


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


Application to the prototype RR Lyr

The PLK for RR Lyrae: theoretical constraints

Bono et al. 2003 MNRAS)

Teramo, 6 Maggio 2008





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


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


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