Issue |
A&A
Volume 493, Number 2, January II 2009
|
|
---|---|---|
Page(s) | 471 - 479 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:200810550 | |
Published online | 06 November 2008 |
New period-luminosity and period-color relations of classical Cepheids
III. Cepheids in SMC
1
The Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101, USA
2
Department of Physics and Astronomy, Univ. of Basel Klingelbergstrasse 82, 4056 Basel, Switzerland e-mail: g-a.tammann@unibas.ch
Received:
9
July
2008
Accepted:
13
October
2008
The photometric data for 460 classical, fundamental-mode Cepheids in
the SMC with measured by Udalski et al. have been
analyzed for their period-color (P-C) and period-luminosity (P-L)
relations, and for the variation in amplitude across the instability
strip in a similar way to what was done in Papers I and II of this
series.
The SMC Cepheids are bluer in
at a given period
than for both the Galaxy and the LMC. Their P-C relation in
is best fit by two lines intersecting at
days. Their break must necessarily exist also in the P-L
relations in B and/or V, but it remains hidden in the magnitude
scatter. An additional pronounced break of the P-L relations in B,
V, and I occurs at
days.
The observed slope of the lines of constant period in the HR diagram
agrees with the theoretical expectation from the pulsation equation.
The largest amplitude Cepheids for periods less than 13 days occur
near the blue edge of the instability strip. The sense is reversed in
the period interval from 13 to 20 days, as in the Galaxy and the LMC.
The SMC P-L relation is significantly flatter than that for the
Galaxy, NGC 3351, NGC 4321, M31, all of which have nearly the
same steep slope. The SMC P-L slope is intermediate between that
of these steep slope cases and the very shallow slope of Cepheids
in the lower metallicity galaxies of NGC 3109 and Sextans A/B,
consistent with the premise that the Cepheid P-L relation varies
from galaxy-to-galaxy as a function of metallicity.
Failure to take the slope differences in the P-L relation into account
as a function of metallicity using Cepheids as distance indicators
results in incorrect Cepheid distances. Part of the 15% difference
between our long distance scale – now independently supported by tip
of the red-giant branch (TRGB) distances – and that of the HST Key
Project short scale is due to the effect of using an inappropriate P-L
relation.
Key words: stars: variables: Cepheids / galaxies: Magellanic Clouds / cosmology: distance scale
© ESO, 2009
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