172 research outputs found
Ultra-low Amplitude Variables in the LMC -- Classical Cepheids, Pop. II Cepheids, RV Tau Stars and Binary Variables
A search for variable stars with ultra-low amplitudes (ULA), in the millimag
range, has been made in the combined MACHO and OGLE data bases in the broad
vicinity of the Cepheid instability strip in the HR diagram. A total of 25
singly periodic and 4 multiply periodic ULA objects has been uncovered. Our
analysis does not allow us to distinguish between pulsational and ellipsoidal
(binary) variability, nor between LMC and foreground objects. However, the
objects are strongly clustered and appear to be associated with the pulsational
instability strips of LMC Pop. I and II variables. When combined with the ULA
variables of Buchler et al (2005) a total of 20 objects fall close to the
classical Cepheid instability strip. However, they appear to fall on parallel
period-magnitude relations that are shifted to slightly higher magnitude which
would confer them a different evolutionary status. Low amplitude RV Tauri and
Pop. II Cepheids have been uncovered that do not appear in the MACHO or OGLE
catalogs. Interestingly, a set of binaries seem to lie on a PM relation that is
essentially parallel to that of the RV Tauri/Pop. II Cepheids.Comment: 13 pages, 13 (color) figures. Astrophysical Journal (accepted for
publlication
The impact of large-scale, long-term optical surveys on pulsating star research
The era of large-scale photometric variability surveys began a quarter of a century ago, when three microlensing projects - EROS, MACHO, and OGLE - started their operation. These surveys initiated a revolution in the field of variable stars and in the next years they inspired many new observational projects. Large-scale optical surveys multiplied the number of variable stars known in the Universe. The huge, homogeneous and complete catalogs of pulsating stars, such as Cepheids, RR Lyrae stars, or long-period variables, offer an unprecedented opportunity to calibrate and test the accuracy of various distance indicators, to trace the three-dimensional structure of the Milky Way and other galaxies, to discover exotic types of intrinsically variable stars, or to study previously unknown features and behaviors of pulsators. We present historical and recent findings on various types of pulsating stars obtained from the optical large-scale surveys, with particular emphasis on the OGLE project which currently offers the largest photometric database among surveys for stellar variability
The dynamical mass and evolutionary status of the type-II Cepheid in the eclipsing binary system OGLE-LMC-T2CEP-211 with a double-ring disk
We present the analysis of a peculiar W~Virginis (pWVir) type-II Cepheid,
OGLE-LMC-T2CEP-211 (), in a double-lined binary system
(), which shed light on virtually unknown evolutionary status
and structure of pWVir stars. The dynamical mass of the Cepheid (first ever for
a type-II Cepheid) is and the radius
. The companion is a massive ()
main-sequence star obscured by a disk. Such configuration suggests a mass
transfer in the system history. We found that originally the system
() was composed of and stars, with
the current Cepheid being more massive. The system age is now 200 My,
and the Cepheid is almost completely stripped of hydrogen, with helium mass of
of the total mass. It finished transferring the mass 2.5 My ago
and is evolving towards lower temperatures passing through the instability
strip. Comparison with observations indicate a reasonable
mass loss from the Cepheid. The companion is
most probably a Be main-sequence star with and .
Our results yield a good agreement with a pulsation theory model for a
hydrogen-deficient pulsator, confirming the described evolutionary scenario. We
detected a two-ring disk () and a shell
() around the companion, that is probably a
combination of the matter from the past mass transfer, the mass being lost by
the Cepheid due to wind and pulsations, and a decretion disk around a rapidly
rotating secondary. Our study together with observational properties of pWVir
stars suggests that their majority are products of a similar binary evolution
interaction.Comment: 21 pages, 14 figures, 6 tables, accepted for publication in Ap
The Araucaria Project. The Distance to the Local Group Galaxy WLM from Near-Infrared Photometry of Cepheid Variables
We have obtained deep images in the near-infrared J and K filters for several
fields in the Local Group galaxy WLM. We report intensity mean magnitudes for
31 Cepheids located in these fields which we previously discovered in a
wide-field optical imaging survey of WLM. The data define tight
period-luminosity relations in both near-infrared bands which we use to derive
the total reddening of the Cepheids in WLM and the true distance modulus of the
galaxy from a multiwavelength analysis of the reddened distance moduli in the
VIJK bands. From this, we obtain the values E(B-V) = 0.082 0.02, and
mag, with a systematic uncertainty in the
distance of about 3%. This Cepheid distance agrees extremely well with
the distance of WLM determined from the I-band TRGB method by ourselves and
others. Most of the reddening of the Cepheids in WLM (0.06 mag) is produced
inside the galaxy, demonstrating again the need for an accurate determination
of the total reddening and/or the use of infrared photometry to derive Cepheid
distances which are accurate to 3% or better, even for small irregular galaxies
like WLM.Comment: latex, 6 figures. accepted to Ap
The Araucaria Project. Near-Infrared Photometry of Cepheid Variables in the Sculptor Galaxy NGC 55
We have obtained deep images in the near-infrared J and K filters of four
fields in the Sculptor Group spiral galaxy NGC 55 with the ESO VLT and ISAAC
camera. For 40 long-period Cepheid variables in these fields which were
recently discovered by Pietrzy{\'n}ski et al., we have determined mean J and K
magnitudes from observations at two epochs, and derived distance moduli from
the observed PL relations in these bands. Using these values together with the
previously measured distance moduli in the optical V and I bands, we have
determined a total mean reddening of the NGC 55 Cepheids of E(B-V)=0.127
0.019 mag, which is mostly produced inside NGC 55 itself. For the true distance
modulus of the galaxy, our multiwavelength analysis yields a value of 26.434
0.037 mag (random error), corresponding to a distance of 1.94 0.03
Mpc. This value is tied to an adopted true LMC distance modulus of 18.50 mag.
The systematic uncertainty of our derived Cepheid distance to NGC 55 (apart
from the uncertainty on the adopted LMC distance) is 4%, with the main
contribution likely to come from the effect of blending of some of the Cepheids
with unresolved companion stars. The distance of NGC 55 derived from our
multiwavelength Cepheid analysis agrees within the errors with the distance of
NGC 300, strengthening the case for a physical association of these two
Sculptor Group galaxies.Comment: latex. ApJ accepte
The Araucaria Project. An Accurate Distance to the Local Group Galaxy NGC 6822 from Near-Infrared Photometry of Cepheid Variables
We have measured near-infrared magnitudes in the J and K bands for 56 Cepheid
variables in the Local Group galaxy NGC 6822 with well-determined periods and
optical light curves in the V and I bands. Using the template light curve
approach of Soszynski, Gieren and Pietrzynski, accurate mean magnitudes were
obtained from these data which allowed us to determine with unprecedented
accuracy the distance to NGC 6822 from a multi-wavelength period-luminosity
solution in the VIJK bands. From our data, we obtain a distance to NGC 6822 of
(m-M)_{0} = 23.312 +- 0.021 (random error) mag, with an additional systematic
uncertainty of about 3 %. This distance value is tied to an assumed LMC
distance modulus of 18.50. From our multiwavelength approach, we find for the
total (average) reddening to the NGC 6822 Cepheids E(B-V) = 0.356 +- 0.013 mag,
which is in excellent agreement with a previous determination of McGonegal et
al. from near-infrared photometry and implies significant internal reddening of
the Cepheids in NGC 6822. Our present, definitive distance determination of NGC
6822 from Cepheids agrees within 2 % with the previous distance we had derived
from optical photometry alone, but has significantly reduced error bars.
Our Cepheid distance to NGC 6822 is in excellent agreement with the recent
independent determination of Cioni and Habing from the I-band magnitude of the
tip of the red giant branch. It also agrees well, within the errors, with the
early determination of McGonegal et al. (1983) from random-phase H-band
photometry of nine Cepheids.Comment: Accepted to be published in the Ap
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
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