117 research outputs found
The Araucaria Project. Population effects on the V and I band magnitudes of red clump stars
We present measurements of the V and I band magnitudes of red clump stars in
15 nearby galaxies obtained from recently published homogenous HST photometry.
Supplementing these results with similar data for another 8 galaxies available
in the literature the populational effects on the V and I band magnitudes of
red clump stars were investigated. Comparing red clump magnitudes with the
I-band magnitude of the TRGB in a total sample of 23 galaxies possessing very
different environments we demonstrate that population effects strongly affect
both the V and I band magnitude of red clump stars in a complex way. Our
empirical results basically confirm the theoretical results of Girardi and
Salaris, and show that optical (VI) photometry of red clump stars is not an
accurate method for the determination of distances to nearby galaxies at the
present moment, as long as the population effects are not better calibrated,
both empirically and theoretically. Near infrared photometry is a much better
way to measure galaxy distances with red clump stars given its smaller
sensitivity to population effects.Comment: AJ in pres
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 Distance to the Sculptor dwarf spheroidal galaxy from infrared photometry of RR Lyrae stars
We have obtained single-phase near-infrared magnitudes in the J and K bands
for a sample of 78 RR Lyrae stars in the Sculptor dSph galaxy. Applying
different theoretical and empirical calibrations of the
period-luminosity-metallicity relation for RR Lyrae stars in the infrared, we
find consistent results and obtain a true, reddening-corrected distance modulus
of 19.67 0.02 (statistical) 0.12 (systematic) mag for Sculptor from
our data. This distance value is consistent with the value of 19.68 0.08
mag which we obtain from earlier V-band data of RR Lyrae stars in Sculptor, and
the V magnitude-metallicity calibration of Sandage (1993). It is also in a very
good agreement with the results obtain by Rizzi (2002) based on tip of the red
giant branch (TRGB, 19.64 0.08 mag) and horizontal branch (HB, 19.66
0.15 mag).Comment: AJ in pres
The Araucaria Project. First Cepheid Distance to the Sculptor Group Galaxy NGC 7793 from Variables discovered in a Wide-Field Imaging Survey
We have detected, for the first time, Cepheid variables in the Sculptor Group
spiral galaxy NGC 7793. From wide-field images obtained in the optical V and I
bands on 56 nights in 2003-2005, we have discovered 17 long-period (24-62 days)
Cepheids whose periods and mean magnitudes define tight period-luminosity
relations. We use the (V-I) Wesenheit index to determine a reddening-free true
distance modulus to NGC 7793 of 27.68 +- 0.05 mag (internal error) +- 0.08 mag
(systematic error). The comparison of the reddened distance moduli in V and I
with the one derived from the Wesenheit magnitude indicates that the Cepheids
in NGC 7793 are affected by an average total reddening of E(B-V)=0.08 mag, 0.06
of which is produced inside the host galaxy. As in the earlier Cepheid studies
of the Araucaria Project, the reported distance is tied to an assumed LMC
distance modulus of 18.50. The quoted systematic uncertainty takes into account
effects like blending and possible inhomogeneous filling of the Cepheid
instability strip on the derived distance. The reported distance value does not
depend on the (unknown) metallicity of the Cepheids according to recent
theoretical and empirical results. Our Cepheid distance is shorter, but within
the errors consistent with the distance to NGC 7793 determined earlier with the
TRGB and Tully-Fisher methods. The NGC 7793 distance of 3.4 Mpc is almost
identical to the one our project had found from Cepheid variables for NGC 247,
another spiral member of the Sculptor Group located close to NGC 7793 on the
sky. Two other conspicuous spiral galaxies in the Sculptor Group, NGC 55 and
NGC 300, are much nearer (1.9 Mpc), confirming the picture of a very elongated
structure of the Sculptor Group in the line of sight put forward by Jerjen et
al. and others.Comment: AJ in pres
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 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
Synthetic Population of Binary Cepheids. II. The effect of companion light on the extragalactic distance scale
Because of their period-luminosity relation (PLR), classical Cepheids play a
key role in the calibration of the extragalactic distance scale and the
determination of the Hubble-Lema\^{i}tre constant . Recent findings show
that the majority of classical Cepheids should be in binary or multiple
systems, which might undermine their accuracy, as the extra -- and unaccounted
for -- light from the companions of Cepheids causes a shift in the PLR. We
quantify this shift using synthetic populations of binary Cepheids that we
developed for this purpose, as described in Paper I of this series. We find
that while all PLRs are shifted toward brighter values due to the excess light
from the companions, the bias in the relative distance modulus between two
galaxies hosting binary Cepheids can be either positive or negative, depending
on the percentage of binary Cepheids in them. If the binarity percentage in the
two galaxies is similar, the effect of binarity is canceled. Otherwise, it
introduces a shift in the distance modulus of the order of millimags in the
near-infrared passbands and Wesenheit indices, and tens of millimags in the
visual domain; its exact value depends on the variant of the synthetic
population (a unique combination of metallicity, star formation history, shape
and location of the instability strip, and initial parameter distributions).
Such shifts in distance moduli to type Ia supernova host galaxies introduce an
additional statistical error on , which however does not prevent measuring
with a precision of 1%.Comment: 16 pages, 11 figures, accepted for publication in The Astrophysical
Journa
Synthetic population of binary Cepheids. I. The effect of metallicity and initial parameter distribution on characteristics of Cepheids' companions
The majority of classical Cepheids are binary stars, yet the contribution of
companions' light to the total brightness of the system has been assumed
negligible and lacked a thorough, quantitative evaluation. We present an
extensive study of synthetic populations of binary Cepheids, that aims to
characterize Cepheids' companions (e.g. masses, evolutionary and spectral
types), quantify their contribution to the brightness and color of Cepheid
binaries, and assess the relevance of input parameters on the results. We
introduce a collection of synthetic populations, which vary in metal content,
initial parameter distribution, location of the instability strip edges, and
star formation history. Our synthetic populations are free from the selection
bias, while the percentage of Cepheid binaries is controlled by the binarity
parameter. We successfully reproduce recent theoretical and empirical results:
the percentage of binary Cepheids with main sequence (MS) companions, the
contrast-mass ratio relation for binary Cepheids with MS companions, the
appearance of binary Cepheids with giant evolved companions as outlier data
points above the period-luminosity relation. Moreover, we present the first
estimation of the percentage of binary Cepheids in the Large Magellanic Cloud
and announce the quantification of the effect of binarity on the slope and
zero-point of multiband period-luminosity relations, which will be reported in
the next paper of this series.Comment: 29 pages, 19 figures (5 in the Appendix), accepted for Ap
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