54 research outputs found
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
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
Multiplicity of Galactic Cepheids from long-baseline interferometry. II. The Companion of AX Circini revealed with VLTI/PIONIER
Aims: We aim at detecting and characterizing the main-sequence companion of
the Cepheid AX Cir ( 18 yrs). The long-term objective is
to estimate the mass of both components and the distance to the system.
Methods: We used the PIONIER combiner at the VLT Interferometer to obtain the
first interferometric measurements of the short-period Cepheid AX Cir and its
orbiting component. Results: The companion is resolved by PIONIER at a
projected separation mas and projection angle . We measured -band flux ratios between the companion and
the Cepheid of % and %, respectively at a
pulsation phase for the Cepheid and 0.48. The lower contrast at
is due to increased brightness of the Cepheid compared to the
. This gives an average apparent magnitude mag. The limb-darkened angular diameter of the Cepheid at the
two pulsation phases was measured to be
mas and mas, respectively at and 0.48. A lower limit on the total mass of the system was also derived
based on our measured separation, we found .Comment: Accepted for publication in Astronomy and Astrophysic
Investigating Cepheid Carinae's Cycle-to-cycle Variations via Contemporaneous Velocimetry and Interferometry
Baade-Wesselink-type (BW) techniques enable geometric distance measurements
of Cepheid variable stars in the Galaxy and the Magellanic clouds. The leading
uncertainties involved concern projection factors required to translate
observed radial velocities (RVs) to pulsational velocities and recently
discovered modulated variability. We carried out an unprecedented observational
campaign involving long-baseline interferometry (VLTI/PIONIER) and spectroscopy
(Euler/Coralie) to search for modulated variability in the long-period (P
35.5 d) Cepheid Carinae. We determine highly precise angular diameters
from squared visibilities and investigate possible differences between two
consecutive maximal diameters, . We characterize the
modulated variability along the line-of-sight using 360 high-precision RVs.
Here we report tentative evidence for modulated angular variability and confirm
cycle-to-cycle differences of Carinae's RV variability. Two successive
maxima yield = 13.1 0.7 (stat.) {\mu}as for
uniform disk models and 22.5 1.4 (stat.) {\mu}as (4% of the total angular
variation) for limb-darkened models. By comparing new RVs with 2014 RVs we show
modulation to vary in strength. Barring confirmation, our results suggest the
optical continuum (traced by interferometry) to be differently affected by
modulation than gas motions (traced by spectroscopy). This implies a previously
unknown time-dependence of projection factors, which can vary by 5% between
consecutive cycles of expansion and contraction. Additional interferometric
data are required to confirm modulated angular diameter variations. By
understanding the origin of modulated variability and monitoring its long-term
behavior, we aim to improve the accuracy of BW distances and further the
understanding of stellar pulsations.Comment: Accepted for publication in MNRAS. 19 pages, 13 figures, 10 table
The long-period Galactic Cepheid RS Puppis - III. A geometric distance from HST polarimetric imaging of its light echoes
As one of the most luminous Cepheids in the Milky Way, the 41.5-day RS Puppis
is an analog of the long-period Cepheids used to measure extragalactic
distances. An accurate distance to this star would therefore help anchor the
zero-point of the bright end of the period-luminosity relation. But, at a
distance of about 2 kpc, RS Pup is too far away for measuring a direct
trigonometric parallax with a precision of a few percent with existing
instrumentation. RS Pup is unique in being surrounded by a reflection nebula,
whose brightness varies as pulses of light from the Cepheid propagate outwards.
We present new polarimetric imaging of the nebula obtained with HST/ACS. The
derived map of the degree of linear polarization pL allows us to reconstruct
the three-dimensional structure of the dust distribution. To retrieve the
scattering angle from the pL value, we consider two different polarization
models, one based on a Milky Way dust mixture and one assuming Rayleigh
scattering. Considering the derived dust distribution in the nebula, we adjust
a model of the phase lag of the photometric variations over selected nebular
features to retrieve the distance of RS Pup. We obtain a distance of 1910 +/-
80 pc (4.2%), corresponding to a parallax of 0.524 +/- 0.022 mas. The agreement
between the two polarization models we considered is good, but the final
uncertainty is dominated by systematics in the adopted model parameters. The
distance we obtain is consistent with existing measurements from the
literature, but light echoes provide a distance estimate that is not subject to
the same systematic uncertainties as other estimators (e.g. the Baade-Wesselink
technique). RS Pup therefore provides an important fiducial for the calibration
of systematic uncertainties of the long-period Cepheid distance scale.Comment: 14 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
Cepheid distances from the SpectroPhoto-Interferometry of Pulsating Stars (SPIPS) - Application to the prototypes delta Cep and eta Aql
The parallax of pulsation, and its implementations such as the
Baade-Wesselink method and the infrared surface bright- ness technique, is an
elegant method to determine distances of pulsating stars in a quasi-geometrical
way. However, these classical implementations in general only use a subset of
the available observational data. Freedman & Madore (2010) suggested a more
physical approach in the implementation of the parallax of pulsation in order
to treat all available data. We present a global and model-based
parallax-of-pulsation method that enables including any type of observational
data in a consistent model fit, the SpectroPhoto-Interferometric modeling of
Pulsating Stars (SPIPS). We implemented a simple model consisting of a
pulsating sphere with a varying effective temperature and a combina- tion of
atmospheric model grids to globally fit radial velocities, spectroscopic data,
and interferometric angular diameters. We also parametrized (and adjusted) the
reddening and the contribution of the circumstellar envelopes in the
near-infrared photometric and interferometric measurements. We show the
successful application of the method to two stars: delta Cep and eta Aql. The
agreement of all data fitted by a single model confirms the validity of the
method. Derived parameters are compatible with publish values, but with a
higher level of confidence. The SPIPS algorithm combines all the available
observables (radial velocimetry, interferometry, and photometry) to estimate
the physical parameters of the star (ratio distance/ p-factor, Teff, presence
of infrared excess, color excess, etc). The statistical precision is improved
(compared to other methods) thanks to the large number of data taken into
account, the accuracy is improved by using consistent physical modeling and the
reliability of the derived parameters is strengthened thanks to the redundancy
in the data.Comment: 10 pages, 4 figures, A&A in pres
Observational calibration of the projection factor of Cepheids
Context. The distance to pulsating stars is classically estimated using the parallax-of-pulsation (PoP) method, which combines spec- troscopic radial velocity (RV) measurements and angular diameter (AD) estimates to derive the distance of the star. A particularly important application of this method is the determination of Cepheid distances in view of the calibration of their distance scale. How- ever, the conversion of radial to pulsational velocities in the PoP method relies on a poorly calibrated parameter, the projection factor (p-factor). Aims. We aim to measure empirically the value of the p-factors of a homogeneous sample of nine bright Galactic Cepheids for which trigonometric parallaxes were measured with the Hubble Space Telescope (HST) Fine Guidance Sensor by Benedict et al. (2007). Methods. We use the SPIPS algorithm, a robust implementation of the PoP method that combines photometry, interferometry, and radial velocity measurements in a global modeling of the pulsation of the star. We obtained new interferometric angular diameter mea- surements using the PIONIER instrument at the Very Large Telescope Interferometer (VLTI), completed by data from the literature. Using the known distance as an input, we derive the value of the p-factor of the nine stars of our sample and study its dependence with the pulsation period. Results. We find the following p-factors: p = 1.20 ± 0.12 for RT Aur, p = 1.48 ± 0.18 for T Vul, p = 1.14 ± 0.10 for FF Aql, p = 1.31 ± 0.19 for Y Sgr, p = 1.39 ± 0.09 for X Sgr, p = 1.35 ± 0.13 for W Sgr, p = 1.36 ± 0.08 for β Dor, p = 1.41 ± 0.10 for ζ Gem, and p = 1.23 ± 0.12 for ` Car. Conclusions. The values of the p-factors that we obtain are consistently close to p = 1.324 ± 0.024. We observe some dispersion around this average value, but the observed distribution is statistically consistent with a constant value of the p-factor as a function of the pulsation period (χ2 = 0.669). The error budget of our determination of the p-factor values is presently dominated by the uncertainty on the parallax, a limitation that will soon be waived by Gaia
The association between physical activity and healthcare costs in children – results from the GINIplus and LISAplus cohort studies
Wages, Employment and Futures Markets
This paper places the competitive firm under output price uncertainty in a standard efficiency wage model, wherein the work effort of labor depends on the wage rate set by the firm. Irrespective of the availability of a commodity futures market, we show that the Solow condition holds in that the equilibrium effort-wage elasticity is unity. The optimal wage rate is preference-free and independent of the underlying output price uncertainty under the efficiency wage hypothesis. Furthermore, we show that the introduction of the commodity futures market induces the firm to hire more labor and thereby produce more output if the firm is sufficiently risk averse. (JEL: D21; J31)link_to_OA_fulltex
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