Issue |
A&A
Volume 476, Number 2, December III 2007
|
|
---|---|---|
Page(s) | 863 - 879 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20077587 | |
Published online | 02 October 2007 |
Classical Cepheid pulsation models
XI. Effects of convection and chemical composition on the period-luminosity and period-Wesenheit relations
1
INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy e-mail: giuliana.fiorentino@oabo.inaf.it
2
INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy e-mail: caputo@mporzio.astro.it
3
INAF – Osservatorio Astronomico Di Capodimonte, via Moiariello 16, 131 Napoli, Italy e-mail: [marcella;ilaria]@na.astro.it
Received:
2
April
2007
Accepted:
8
July
2007
In spite of the relevance of classical Cepheids as
primary distance indicators, a general consensus on the dependence
of the period-luminosity (PL) relation on the Cepheid chemical
composition has not yet been achieved. From the theoretical point
of view, our previous investigations were able to reproduce some
empirical tests for suitable assumptions on the helium-to-metal
relative enrichment, but these results relied on specific
assumptions concerning the mass-luminosity relation and the
efficiency of the convective transfer in the pulsating envelopes.
In this paper, we investigate the effects of the assumed value of
the mixing-length parameter on the pulsation properties
and we release the assumption of a fixed mass-luminosity relation.
To this purpose, new nonlinear convective fundamental
pulsation models have been computed for various chemical
compositions (
, 0.008, 0.01 and 0.02) and adopting
, which is larger than that (1.5) used in our
previous papers. From the extended model set, synthetic PL
relations in the various photometric bands are derived using the
predicted instability strip together with recent evolutionary
tracks.
We show that as the
value increases the pulsation region
gets narrower, mostly due to the blueward shift of the red edge
for fundamental pulsation, with the effect becoming stronger at
the higher metal contents (
). However, the comparison
of the new models with previously computed models shows that the
variation has no consequence on the predicted
period-Wesenheit (PW) relations, which instead are influenced by
the pulsator metal content. On this basis, we present a
straightforward way to infer the distance and metal content of
variables with observed BVI or BVK magnitudes. As for the PL
relations, we show that either the zero-point and the slope are
very slightly modified by the
variation, at constant
chemical composition. We also confirm that: (1) moving from visual
to longer wavelengths, the predicted period-magnitude distribution
for a given metal content becomes narrower and its slope becomes
steeper; (2) decreasing the metal content, the PL relations
become steeper and brighter, with the amount of this metallicity
effect decreasing from optical to near-infrared bands.
Overall, we show that our pulsation relations appear fully
consistent with the observed properties of Galactic and Magellanic
Cloud Cepheids, supporting the predicted steepening and
brightening of the PL relations when moving from metal-rich to
metal-poor variables. Moreover, we show that the distances
inferred by the predicted PW relations agree with recently
measured trigonometric parallaxes, whereas they suggest a
correction to the values based on the Infrared Surface Brightness
technique, as already found from an independent method. Finally,
also the pulsation metal contents suggested by the predicted PW
relations appear in statistical agreement with spectroscopic
[Fe/H] measurements.
Key words: cosmology: distance scale / stars: variables: Cepheids
© ESO, 2007
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