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
Teramo, 6 Maggio 2008
Carina, Dall’Ora et al. 2003
NGC2419 Ripepi et al. 2007
Pulsating stars
κ and γ mechanisms
are efficient in stars within
the classical instability strip
They can be used as
distance
NGC6822
Clementiniindicators
et al. 2004
…but also as tracers of
the properties of
stellar populations
OGLE sample
Teramo, 6 Maggio 2008
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
by significant
combining samples,
the P(M,L,Te)
In the
case ofCepheids
statistically
by
relation
ML relation
L luminosity
= a + b logM
+c
averagingwith
the aperiod
at each(log
fixed
level
logZ
d logY)
predicted by stellar
evolution
gives +the
Period-Luminosity
(PL) relation.
provides the physical basis of Period-LuminosityColor (PLC) relations.
Teramo, 6 Maggio 2008
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)
Calibration of the
extragalactic
distance scale
ML relations predicted by
evolutionary calculations
Input to pulsation models
Theory versus observations
H0 estimate
(e.g. Freedman et al. 2001)
It is important to construct as
accurate distance scale
as possible that is independent of
the CMB !
Insight into evolutionary and
pulsational physics
Teramo, 6 Maggio 2008
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
Teramo, 6 Maggio 2008
On the linearity of Cepheid PL relations
But linearity in the NIR bands!
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
10 d 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)
Marconi et al. 2005
Persson et al. 2004
Ngeow et al. 2005
Caputo, Marconi, Musella 2000 A&A
Teramo, 6 Maggio 2008
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)
Kanbur & Ngeow 2006
Teramo, 6 Maggio 2008
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
the “universality”
the P-L relations,
One
of theconsensus
key issueson
concerning
the use ofofCepheids
as distance
and
in particular
their dependence
on the
Cepheid chemical
indicators
is theiron
dependence
on chemical
composition

composition
has not
achieved yet.distance scale (and H0)!!
systematic effects
on been
the extragalactic
Teramo, 6 Maggio 2008
On the theoretical side……
Linear nonadiabatic models mostly
effects
Thesuggest
syntheticnegligible
linear PL relations
(see e.g. Alibert et al. 1999, Saio & Gautschy
1998, Sandage
al. 1999)
get shallower
as the et
metallicity
increases.
Nonlinear convective pulsation models (Bono, Marconi &
the B, V, I bands the slope
Stellingwerf 1999, Fiorentino et al. For
2002,
Marconi, Musella,
decreases by 29 %, 15 % and 8%
Fiorentino 2005) predict a metallicity
effect on
predicted
respectively,
as the
Z increases
fromPL
relations depending on the adoptedZ=0.004
photometric
bands.
to Z=0.02
Metal-rich pulsators with periods
longer than five days present
fainter optical magnitudes than
the metal-poor ones.
Caputo, Marconi, Musella 2000 A&A
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
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
Marconi, Musella, Fiorentino 2005, ApJ
Teramo, 6 Maggio 2008
Empirical results on the PL metallicity dependence
Some empirical tests seem to suggest that metal-rich Cepheids
are, at fixed period, brighter LMC
than metal poor ones, either over
the entire period range or at least for periods shorter than ≈ 25 d
SMC
Typically one refers to γ=δµ0/δlogZ, with δµ0 quantifying the
metallicity correction and δlogZ=logZLMC-logZCeph
According to these empirical
γ is negative up to -0.4 mag
Sasselovstudies
et al. 1997
dex-1 with an average value of ≈ -0.25 mag dex-1 (Sakai et al.
2004 and references therein)
Sandage et al. 2004
Teramo, 6 Maggio 2008
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 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
1) Cepheid observations in the outer and inner field of M101
(Kennicutt et al. 1998, 2003)
∆[O/H]=0.7 dex
γ=-0.27 mag dex-1
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
∆[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
measurements
(Diaz et
al. 2000)
a rather
constant
LMC-like
Ifabundance
confirmed this
occurrnce would
prevent
anysuggest
reliable
differential
determination
for the
Cepheids observed in the two fields of NGC4258 (Bono et
ofmetal-content
the PL metallicity
dependence!
al. 2008 ApJ in press).
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 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)
Teramo, 6 Maggio 2008
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 are
found to have slopes similar to the LMC counterparts.
Teramo, 6 Maggio 2008
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 →18.43
(EBs →18.41±0.09 mag Guinan et al. 2004)
µ0,VI(4258)=29.35±0.12 mag
→29.33
(maser→ 29.29±0.15 mag Herrnstein et al. 1999)
Bono et al. 2008 ApJ
But the metal abundances of LMC and NGC4258 Cepheids are lower than the
Galactic ones →a metallicity correction should be applied !
Very good agreement with the EB and maser estimates if the metallicity
correction predicted by models is adopted.
Teramo, 6 Maggio 2008
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
angular diameter variation over
the pulsation cycle
Interferometric measurements
radial velocity from spectral line profiles
+ projection (p) factor
Pulsation velocity integration
Stellar radius variation over
the pulsation cycle
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
Teramo, 6 Maggio 2008
limb-darkening effects
The p factor is related to
velocity gradient
dynamical structure of the
Cepheid atmosphere
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, spectroand differential- interferometry) is needed to efficiently constrain the physical
parameters of the Cepheid atmosphere and the p factor (Nardetto et al. 2007, 2008)
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.
Fit soddisfacente della
variazione
fotometrica,
Constraints on the stellar mass and test of evolutionary
predictions:
della velocità radiale e
mass loss and/or overshooting ?
del raggio, con p=1.28
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
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
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)
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?)
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 metallicity
2) smaller pulsation amplitude → reliable mean magnitudes
3) the existence of a PL relation
Del principe et al. 2006
Sollima et al. 2006
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
RR Lyrae stars as stellar population tracers:
new interesting issues
RR Lyrae in “special” globular clusters
Oo II
RR Lyrae as tracers of stellar streams
Gap
NGC2419
ExtraglacticOoRR
I Lyrae
Marconi & Clementini 2008
Suk-Jin et al. 2008Ripepi et al. 2007 ApJL
Teramo, 6 Maggio 2008
Variable stars in the field and GCs of M31
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
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