2,614 research outputs found
A Direct Distance to the LMC Cepheid HV 12198 from the Infrared Surface Brightness Technique
We report on a first application of the infrared surface brightness technique
on a Cepheid in the Large Magellanic Cloud, the variable HV 12198 in the young
globular cluster NGC 1866. From this one star, we determine a distance modulus
of 18.42 +- 0.10 (random and systematic uncertainty) to the cluster. When the
results on further member Cepheids in NGC 1866 become available, we expect to
derive the distance to the LMC with a +- 3-4 percent accuracy, including
systematic errors, from this technique.Comment: 4 pages, 4 figures, accepted in ApJ Letter
BVRIJK light curves and radial velocity curves for selected Magellanic Cloud Cepheids
We present high precision and well sampled BVRIJK light curves and radial
velocity curves for a sample of five Cepheids in the SMC. In addition we
present radial velocity curves for three Cepheids in the LMC. The low
metallicity (Fe/H ~ -0.7) SMC stars have been selected for use in a
Baade-Wesselink type analysis to constrain the metallicity effect on the
Cepheid Period-Luminosity relation. The stars have periods of around 15 days so
they are similar to the Cepheids observed by the Extragalactic Distance Scale
Key Project on the Hubble Space Telescope. We show that the stars are
representative of the SMC Cepheid population at that period and thus will
provide a good sample for the proposed analysis. The actual Baade-Wesselink
analysis are presented in a companion paper.Comment: Accepted for publication in A&A, 23 pages, 10 figures, data tables
will be made available electronically from the CD
Distances to six Cepheids in the LMC cluster NGC1866 from the near-IR surface-brightness method
We derive individual distances to six Cepheids in the young populous star
cluster NGC1866 in the Large Magellanic Cloud employing the near-IR surface
brightness technique. With six stars available at the exact same distance we
can directly measure the intrinsic uncertainty of the method. We find a
standard deviation of 0.11 mag, two to three times larger than the error
estimates and more in line with the estimates from Bayesian statistical
analysis by Barnes et al. (2005). Using all six distance estimates we determine
an unweighted mean cluster distance of 18.30+-0.05. The observations indicate
that NGC1866 is close to be at the same distance as the main body of the LMC.
If we use the stronger dependence of the p-factor on the period as suggested by
Gieren et al. (2005) we find a distance of 18.50+-0.05 (internal error) and the
PL relations for Galactic and MC Cepheids are in very good agreement.Comment: Presented at the conference "Stellar Pulsation and Evolution" in
Monte Porzio Catone, June 2005. To appear in Mem. Soc. Ast. It. 76/
Direct Distances to Cepheids in the Large Magellanic Cloud: Evidence for a Universal Slope of the Period-Luminosity Relation up to Solar Abundance
We have applied the infrared surface brightness (ISB) technique to derive
distances to 13 Cepheids in the LMC which span a period range from 3 to 42
days. From the absolute magnitudes of the variables calculated from these
distances, we find that the LMC Cepheids define tight period-luminosity
relations in the V, I, W,
J and K bands which agree exceedingly well with the corresponding Galactic PL
relations derived from the same technique, and are significantly steeper than
the LMC PL relations in these bands observed by the OGLE-II Project in V, I and
W, and by Persson et al. in J and K. We find that the tilt-corrected true
distance moduli of the LMC Cepheids show a significant dependence on period,
which hints at a systematic error in the ISB technique related to the period of
the stars. We identify as the most likely culprit the p-factor which converts
the radial into pulsational velocities; our data imply a much steeper period
dependence of the p-factor than previously thought, and we derive p=1.58
(+/-0.02) -0.15 (+/-0.05) logP as the best fit from our data, with a zero point
tied to the Milky Way open cluster Cepheids. Using this revised p-factor law,
the period dependence of the LMC Cepheid distance moduli disappears, and at the
same time the Milky Way and LMC PL relations agree among themselves, and with
the directly observed LMC PL relations, within the 1 sigma uncertainties. Our
main conclusion is that the previous, steeper Galactic PL relations were caused
by an erroneous calibration of the p-factor law, and that there is now evidence
that the slope of the Cepheid PL relation is independent of metallicity up to
solar metallicity, in both optical, and near-infrared bands.Comment: ApJ accepte
Evaluation of SIR-A space radar for geologic interpretation: United States, Panama, Colombia, and New Guinea
Comparisons between LANDSAT MSS imagery, and aircraft and space radar imagery from different geologic environments in the United States, Panama, Colombia, and New Guinea demonstrate the interdependence of radar system geometry and terrain configuration for optimum retrieval of geologic information. Illustrations suggest that in the case of space radars (SIR-A in particular), the ability to acquire multiple look-angle/look-direction radar images of a given area is more valuable for landform mapping than further improvements in spatial resolution. Radar look-angle is concluded to be one of the most important system parameters of a space radar designed to be used for geologic reconnaissance mapping. The optimum set of system parameters must be determined for imaging different classes of landform features and tailoring the look-angle to local topography
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
Universal algebraic relaxation of velocity and phase in pulled fronts generating periodic or chaotic states
We investigate the asymptotic relaxation of so-called pulled fronts propagating into an unstable state, and generalize the universal algebraic velocity relaxation of uniformly translating fronts to fronts that generate periodic or even chaotic states. A surprising feature is that such fronts also exhibit a universal algebraic phase relaxation. For fronts that generate a periodic state, like those in the Swift-Hohenberg equation or in a Rayleigh-BĂ©nard experiment, this implies an algebraically slow relaxation of the pattern wavelength just behind the front, which should be experimentally testable
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