379 research outputs found
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
Large Magellanic Cloud Distance from Cepheid Variables using Least Squares Solutions
Distance to the Large Magellanic Cloud (LMC) is determined using the Cepheid
variables in the LMC. We combine the individual LMC Cepheid distances obtained
from the infrared surface brightness method and a dataset with a large number
of LMC Cepheids. Using the standard least squares method, the LMC distance
modulus can be found from the ZP offsets of these two samples. We have adopted
both a linear P-L relation and a ``broken'' P-L relation in our calculations.
The resulting LMC distance moduli are 18.48+-0.03 mag and 18.49+-0.04 mag
(random error only), respectively, which are consistent to the adopted 18.50
mag in the literature.Comment: 2 pages and 1 figure, to appear in proceeding of "Galaxies in the
Local Volume" Sydney 8-13 July 200
Identifying Contaminated K-band Globular Cluster RR Lyrae Photometry
Acquiring near-infrared K-band (2.2 um) photometry for RR Lyrae variables in
globular clusters and nearby galaxies is advantageous since the resulting
distances are less impacted by reddening and metallicity. However, K-band
photometry for RR Lyrae variables in M5, Reticulum, M92, omega Cen, and M15
display clustercentric trends. HST ACS data imply that multiple stars in close
proximity to RR Lyrae variables located near the cluster core, where the
stellar density increases markedly, are generally unresolved in ground-based
images. RR Lyrae variables near the cluster cores appear to suffer from
photometric contamination, thereby yielding underestimated cluster distances
and biased ages. The impact is particularly pernicious since the contamination
propagates a systematic uncertainty into the distance scale, and hinders the
quest for precision cosmology. The clustercentric trends are probably
unassociated with variations in chemical composition since an empirical K-band
period-magnitude relation inferred from Araucaria/VLT data for RR Lyrae
variables in the Sculptor dSph exhibits a negligible metallicity dependence:
(0.059+-0.095)[Fe/H], a finding that supports prior observational results. A
future multi-epoch high-resolution near-infrared survey, analogous to the
optical HST ACS Galactic Globular Cluster Survey, may be employed to establish
K-band photometry for the contaminating stars discussed here.Comment: To appear in PAS
The Araucaria Project: the Local Group Galaxy WLM--Distance and metallicity from quantitative spectroscopy of blue Supergiants
The quantitative analysis of low resolution spectra of A and B supergiants is
used to determine a distance modulus of 24.99 +/- 0.10 mag (995 +/- 46 Kpc) to
the Local Group galaxy WLM. The analysis yields stellar effective temperatures
and gravities, which provide a distance through the Flux weighted
Gravity--Luminosity Relationship (FGLR). Our distance is 0.07 mag larger than
the most recent results based on Cepheids and the tip of the RGB. This
difference is within the 1-sigma overlap of the typical uncertainties quoted in
these photometric investigations. In addition, non-LTE spectral synthesis of
the rich metal line spectra (mostly iron, chromium and titanium) of the A
supergiants is carried out, which allows the determination of stellar
metallicities. An average metallicity of -0.87 +/- 0.06 dex with respect to
solar metallicity is found.Comment: 46 pages, 14 figures (2 low resolution color figures). Accepted for
publication by Ap
Photometric standard stars in the BVI system in a wide field centered on the spiral galaxy NGC 300
Based on 13 nights of observations of four fields in NGC 300, we have set up
an extensive sequence of stars with accurate BVI photometry covering a
relatively large (25 x 25 arcmin) region centered on this galaxy. This sequence
of standard stars is very useful for calibrating the photometry of variable
stars and other objects in NGC 300 and other galaxies obtained from wide field
mosaic images. Our standard star list contains B, V and I measurements for 390
stars. The accuracy of the zero points in the V filter and B-V color is better
than 0.02 mag, and about 0.03 mag for the V-I color. We found very good
agreement between our measurements and those previously obtained by Walker for
26 stars near NGC 300.Comment: 12 pages, Latex, 1 Figure in gif format, 1 Table as postscript file.
P.A.S.P. in pres
Extended envelopes around Galactic Cepheids IV. T Monocerotis and X Sagittarii from mid-infrared interferometry with VLTI/MIDI
Aims. We study the close environment of nearby Cepheids using high spatial
resolution observations in the mid-infrared with the VLTI/MIDI instrument, a
two-beam interferometric recombiner. Methods. We obtained spectra and
visibilities for the classical Cepheids X Sgr and T Mon. We fitted the MIDI
measurements, supplemented by B, V, J, H, K literature photometry, with the
numerical transfer code DUSTY to determine the dust shell parameters. We used a
typical dust composition for circumstellar environments. Results. We detect an
extended dusty environment in the spectra and visibilities for both stars,
although T Mon might suffer from thermal background contamination. We attribute
this to the presence of a circumstellar envelope (CSE) surrounding the
Cepheids. This is optically thin for X Sgr (tau(0.55microns) = 0.008), while it
appears to be thicker for T Mon (tau(0.55micron) = 0.15). They are located at
about 15-20 stellar radii. Following our previous work, we derived a likely
period-excess relation in the VISIR PAH1 filter, f(8.6micron)[%]=
0.81(+/-0.04)P[day]. We argue that the impact of CSEs on the mid-IR
period-luminosity (P-L) relation cannot be negligible because they can bias the
Cepheid brightness by up to about 30 %. For the K-band P-L relation, the CSE
contribution seems to be lower (< 5 %), but the sample needs to be enlarged to
firmly conclude that the impact of the CSEs is negligible in this band.Comment: Accepted for publication in Astronomy and Astrophysic
Binary Cepheids from optical interferometry
Classical Cepheid stars have been considered since more than a century as
reliable tools to estimate distances in the universe thanks to their
Period-Luminosity (P-L) relationship. Moreover, they are also powerful
astrophysical laboratories, providing fundamental clues for studying the
pulsation and evolution of intermediate-mass stars. When in binary systems, we
can investigate the age and evolution of the Cepheid, estimate the mass and
distance, and constrain theoretical models. However, most of the companions are
located too close to the Cepheid (1-40 mas) to be spatially resolved with a
10-meter class telescope. The only way to spatially resolve such systems is to
use long-baseline interferometry. Recently, we have started a unique and
long-term interferometric program that aims at detecting and characterizing
physical parameters of the Cepheid companions, with as main objectives the
determination of accurate masses and geometric distances.Comment: 8 pages, Proceeding of the conference "Setting a new standard in the
analysis of binary stars", September 2013, Leuven, Belgiu
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