187 research outputs found
Calibration of the Distance Scale from Cepheids
We have used the infrared surface brightness technique to obtain a new
absolute calibration of the Cepheid PL relation in optical and near-infrared
bands from improved data on Galactic stars. The infrared surface brightness
distances to the Galactic variables are consistent with direct interferometric
Cepheid distance measurements, and with the PL calibration coming from
Hipparcos parallaxes of nearby Cepheids, but are more accurate than these
determinations. We find that in all bands, the Galactic Cepheid PL relation
appears to be slightly, but significantly steeper than the corresponding
relation defined by the LMC Cepheids. Since the slope of our LMC Cepheid sample
is clearly better defined than the one of the much smaller Galactic sample, we
fit the LMC slopes to our Galactic calibrating Cepheid sample (which introduces
only a small uncertainty) to obtain our final, adopted and improved absolute
calibrations of the Cepheid PL relations in the VIWJHK bands. Comparing the
absolute magnitudes of 10-day period Cepheids in both galaxies which are only
slightly affected by the different Galactic and LMC slopes of the PL relation,
we derive values for the LMC distance modulus in all these bands which can be
made to agree extremely well under reasonable assumptions for both, the
reddening law, and the adopted reddenings of the LMC Cepheids. This yields, as
our current best estimate from Cepheid variables, a LMC distance modulus of
18.55 +- 0.06.Comment: to be published in: "Stellar Candles", Lecture Notes in Physics
(http://link.springer.de/series/lnpp
Very accurate Distances and Radii of Open Cluster Cepheids from a Near-Infrared Surface Brightness Technique
We have obtained the radii and distances of 16 galactic Cepheids supposed to
be members in open clusters or associations using the new optical and
near-infrared calibrations of the surface brightness (Barnes-Evans) method
given by Fouque & Gieren (1997). We discuss in detail possible systematic
errors in our infrared solutions and conclude that the typical total
uncertainty of the infrared distance and radius of a Cepheid is about 3 percent
in both infrared solutions, provided that the data are of excellent quality and
that the amplitude of the color curve used in the solution is larger than ~0.3
mag.
We compare the adopted infrared distances of the Cepheid variables to the
ZAMS-fitting distances of their supposed host clusters and associations and
find an unweighted mean value of the distance ratio of 1.02 +- 0.04. A detailed
discussion of the individual Cepheids shows that the uncertainty of the
ZAMS-fitting distances varies considerably from cluster to cluster. We find
clear evidence that four Cepheids are not cluster members (SZ Tau, T Mon, U Car
and SV Vul) while we confirm cluster membership for V Cen and BB Sgr for which
the former evidence for cluster membership was only weak. After rejection of
non-members, we find a weighted mean distance ratio of 0.969 +- 0.014, with a
standard deviation of 0.05, which demonstrates that both distance indicators
are accurate to better than 5%, including systematic errors, and that there is
excellent agreement between both distance scales.Comment: LaTeX, 11 Figures, 5 Tables, to be published in The Astrophysical
Journal, Oct. 10, 1997 issu
The Araucaria Project. The Distance to the Small Magellanic Cloud from Near-Infrared Photometry of RR Lyrae Variables
We have obtained deep infrared J and K band observations of nine 4.9x4.9
arcmin fields in the Small Magellanic Cloud (SMC) with the ESO New Technology
Telescope equipped with the SOFI infrared camera. In these fields, 34 RR Lyrae
stars catalogued by the OGLE collaboration were identified. Using different
theoretical and empirical calibrations of the infrared
period-luminosity-metallicity relation, we find consistent SMC distance moduli,
and find a best true distance modulus to the SMC of 18.97 +/- 0.03
(statistical) +/- 0.12 (systematic) mag which agrees well with most independent
distance determinations to this galaxy, and puts the SMC 0.39 mag more distant
than the LMC for which our group has recently derived, from the same technique,
a distance of 18.58 mag.Comment: AJ submitted and accepte
The Araucaria Project: The effect of blending on the Cepheid distance to NGC 300 from Advanced Camera for Surveys images
We have used the Advanced Camera for Surveys aboard the Hubble Space
Telescope to obtain F435W, F555W and F814W single-epoch images of six fields in
the spiral galaxy NGC 300. Taking advantage of the superb spatial resolution of
these images, we have tested the effect that blending of the Cepheid variables
studied from the ground with close stellar neighbors, unresolved on the
ground-based images, has on the distance determination to NGC 300. Out of the
16 Cepheids included in this study, only three are significantly affected by
nearby stellar objects. After correcting the ground-based magnitudes for the
contribution by these projected companions to the observed flux, we find that
the corresponding Period-Luminosity relations in V, I and the Wesenheit
magnitude W_I are not significantly different from the relations obtained
without corrections. We fix an upper limit of 0.04 magnitudes to the systematic
effect of blending on the distance modulus to NGC 300.
As part of our HST imaging program, we present improved photometry for 40
blue supergiants in NGC 300.Comment: To be published in the Astrophysical Journa
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 Impact of Contaminated RR Lyrae/Globular Cluster Photometry on the Distance Scale
RR Lyrae variables and the stellar constituents of globular clusters are
employed to establish the cosmic distance scale and age of the universe.
However, photometry for RR Lyrae variables in the globular clusters M3, M15,
M54, M92, NGC2419, and NGC6441 exhibit a dependence on the clustercentric
distance. For example, variables and stars positioned near the crowded
high-surface brightness cores of the clusters may suffer from photometric
contamination, which invariably affects a suite of inferred parameters (e.g.,
distance, color excess, absolute magnitude, etc.). The impetus for this study
is to mitigate the propagation of systematic uncertainties by increasing
awareness of the pernicious impact of contaminated and radial-dependent
photometry.Comment: To appear in ApJ
Evidence for a Universal Slope of the Period-Luminosity Relation from Direct Distances to Cepheids in the LMC
We have applied the infrared surface brightness (ISB) technique to derive
distances to 13 Cepheid variables in the LMC which have periods from 3-42 days.
The corresponding absolute magnitudes define PL relations in VIWJK bands which
agree exceedingly well with the corresponding Milky Way relations obtained from
the same technique, and are in significant disagreement with the observed LMC
Cepheid PL relations, by OGLE-II and Persson et al., in these bands. Our data
uncover a systematic error in the p-factor law which transforms Cepheid radial
velocities into pulsational velocities. We correct the p-factor law by
requiring that all LMC Cepheids share the same distance. Re-calculating all
Milky Way and LMC Cepheid distances with the revised p-factor law, we find that
the PL relations from the ISB technique both in LMC and in the Milky Way agree
with the OGLE-II and Persson et al. LMC PL relations, supporting the conclusion
of no metallicity effect on the slope of the Cepheid PL relation in
optical/near infrared bands.Comment: 4 pages, to appear in the proceedings of the "Stellar Pulsation and
Evolution" conference, Monte Porzio Catone, June 200
Infrared Surface Brightness Distances to Cepheids: a comparison of Bayesian and linear-bisector calculations
We have compared the results of Bayesian statistical calculations and
linear-bisector calculations for obtaining Cepheid distances and radii by the
infrared surface brightness method. We analyzed a set of 38 Cepheids using a
Bayesian Markov Chain Monte Carlo method that had been recently studied with a
linear-bisector method. The distances obtained by the two techniques agree to
1.5 \pm 0.6% with the Bayesian distances being larger. The radii agree to 1.1%
\pm 0.7% with the Bayesian determinations again being larger. We interpret this
result as demonstrating that the two methods yield the same distances and
radii. This implies that the short distance to the LMC found in recent
linear-bisector studies of Cepheids is not caused by deficiencies in the
mathematical treatment. However, the computed uncertainties in distance and
radius for our dataset are larger in the Bayesian calculation by factors of
1.4-6.7. We give reasons to favor the Bayesian computations of the
uncertainties. The larger uncertainties can have a significant impact upon
interpretation of Cepheid distances and radii obtained from the infrared
surface brightness method.Comment: 27 pages with 9 figure
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