8,322 research outputs found
The occurrence of classical Cepheids in binary systems
Classical Cepheids, like binary stars, are laboratories for stellar evolution
and Cepheids in binary systems are especially powerful ones. About one-third of
Galactic Cepheids are known to have companions and Cepheids in eclipsing binary
systems have recently been discovered in the Large Magellanic Cloud. However,
there are no known Galactic binary Cepheids with orbital periods less than one
year. We compute population synthesis models of binary Cepheids to compare to
the observed period and eccentricity distributions of Galactic Cepheids as well
as to the number of observed eclipsing binary Cepheids in the LMC. We find that
our population synthesis models are consistent with observed binary properties
of Cepheids. Furthermore, we show that binary interaction on the red giant
branch prevents some red giant stars from becoming classical Cepheids. Such
interactions suggest that the binary fraction of Cepheids should be
significantly less than that of their main-sequence progenitors, and that
almost all binary Cepheids have orbital periods longer than one year. If the
Galactic Cepheid spectroscopic binary fraction is about 35%, then the
spectroscopic binary fraction of their intermediate mass main sequence
progenitors is about 40-45%.Comment: 7 pages, 3 figures, resubmitted to A&
Using Ultra Long Period Cepheids to Extend the Cosmic Distance Ladder to 100 Mpc and Beyond
We examine the properties of 17 long period (80-180 days) and very luminous
(median absolute magnitude of M_I= -7.93 and M_V= -7.03) Cepheids to see if
they can serve as an useful distance indicator. We find that these Ultra Long
Period (ULP) Cepheids have a relatively shallow Period-Luminosity (PL)
relation, so in fact they are more "standard candle"-like than classical
Cepheids. In the reddening-free Wesenheit index, the slope of the ULP PL
relation is ~10 times less steep than the standard PL relation for the SMC
Cepheids. The scatter of our sample about the W_I PL relation is 0.22 mag,
approaching that of classical Cepheids and Type Ia Supernovae. We expect this
scatter to decrease as bigger and more uniform samples of ULP Cepheids are
obtained. We also measure a non-zero period derivative for one ULP Cepheid (SMC
HV829) and use the result to probe evolutionary models and mass loss of massive
stars. ULP Cepheids main advantage over classical Cepheids is that they are
more luminous, and as such show great potential as stellar distance indicators
to galaxies up to 100 Mpc and beyond.Comment: Accepted for Publication in ApJ. 11 pages, 8 figure
Ultra-Low Amplitude Cepheids in the Large Magellanic Cloud
The MACHO variables of LMC Field 77 that lie in the vicinity of the Cepheid
instability strip are reexamined. Among the 144 variables that we identify as
Cepheids we find 14 that have Fourier amplitudes <0.05 mag in the MACHO red
band, of which 7 have an amplitude <0.006 mag : we dub the latter group of
stars ultra-low amplitude (ULA) Cepheids. The variability of these objects is
verified by a comparison of the MACHO red with the MACHO blue lightcurves and
with those of the corresponding OGLE LMC stars. The occurrence of ULA Cepheids
is in agreement with theory. We have also discovered 2 low amplitude variables
whose periods are about a factor of 5--6 smaller than those of F Cepheids of
equal apparent magnitude. We suggest that these objects are Cepheids undergoing
pulsations in a surface mode and that they belong to a novel class of Strange
Cepheids (or Surface Mode Cepheids) whose existence was predicted by Buchler et
al. (1997).Comment: 4 pages, 5 figures, slightly revised, to appear in ApJ Letter
The Baade-Wesselink p-factor applicable to LMC Cepheids
Context. Recent observations of LMC Cepheids bring new constraints on the
slope of the period-projection factor relation (hereafter Pp relation) that is
currently used in the Baade-Wesselink (hereafter BW) method of distance
determination. The discrepancy between observations and theoretical analysis is
particularly significant for short period Cepheids Aims. We investigate three
physical effects that might possibly explain this discrepancy: (1) the
spectroscopic S/N that is systematically lower for LMC Cepheids (around 10)
compared to Galactic ones (up to 300), (2) the impact of the metallicity on the
dynamical structure of LMC Cepheids, and (3) the combination of infrared
photometry/interferometry with optical spectroscopy. Methods. To study the S/N
we use a very simple toy model of Cepheids. The impact of metallicity on the
projection factor is based on the hydrodynamical model of delta Cep already
described in previous studies. This model is also used to derive the position
of the optical versus infrared photospheric layers. Results. We find no
significant effect of S/N, metallicity, and optical-versus-infrared
observations on the Pp relation. Conclusions. The Pp relation of Cepheids in
the LMC does not differ from the Galactic relation. This allows its universal
application to determine distances to extragalactic Cepheids via BW analysis.Comment: accepted in A&A LETTER
M31 PAndromeda Cepheid sample observed in four HST bands
Using the M31 PAndromeda Cepheid sample and the HST PHAT data we obtain the
largest Cepheid sample in M31 with HST data in four bands. For our analysis we
consider three samples: A very homogeneous sample of Cepheids based on the
PAndromeda data, the mean magnitude corrected PAndromeda sample and a sample
complementing the PAndromeda sample with Cepheids from literature. The latter
results in the largest catalog with 522 fundamental mode (FM) Cepheids and 102
first overtone (FO) Cepheids with F160W and F110W data and 559 FM Cepheids and
111 FO Cepheids with F814W and F475W data. The obtained dispersion of the
Period-Luminosity relations (PLRs) is very small (e.g. 0.138 mag in the F160W
sample I PLR). We find no broken slope in the PLRs when analyzing our entire
sample, but we do identify a subsample of Cepheids that causes the broken
slope. However, this effect only shows when the number of this Cepheid type
makes up a significant fraction of the total sample. We also analyze the sample
selection effect on the Hubble constant.Comment: 32 pages, 19 figures, 9 tables, accepted for publication in ApJ,
electronic data will be available on CD
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