350 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
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
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
The Araucaria Project. An improved distance to the Sculptor spiral galaxy NGC 300 from its Cepheid variables
In a previous paper, we reported on the discovery of more than a hundred new
Cepheid variables in the Sculptor Group spiral NGC 300 from wide-field images
taken in the B and V photometric bands at ESO/La Silla. In this paper, we
present additional VI data, derive improved periods and mean magnitudes for the
variables, and construct period-luminosity relations in the V, I and the
reddening-independent (V-I) Wesenheit bands using 58 Cepheid variables with
periods between 11 and 90 days. We obtain tightly defined relations, and by
fitting the slopes determined for the LMC Cepheids by the OGLE II Project we
obtain reddening-corrected distances to the galaxy in all bands. We adopt as
our best value the distance derived from the reddening-free Wesenheit
magnitudes, which is 26.43 0.04 (random) 0.05 (systematic) mag. We
argue that our current distance result for NGC 300 is the most accurate which
has so far been obtained using Cepheid variables, and that it is largely free
from systematic effects due to metallicity, blending, and sample selection. It
agrees very well with the recent distance determination from the tip of the red
giant branch method obtained from HST data by Butler et al. (2004), and it is
consistent with the Cepheid distance to NGC 300 which was derived by Freedman
et al. (2001) from CCD photometry of a smaller sample of stars.Comment: Latex, Astronomical Journal in pres
The long-period Galactic Cepheid RS Puppis - II. 3D structure and mass of the nebula from VLT/FORS polarimetry
The long-period Cepheid RS Pup is surrounded by a large dusty nebula
reflecting the light from the central star. Due to the changing luminosity of
the central source, light echoes propagate into the nebula. This remarkable
phenomenon was the subject of Paper I.The origin and physical properties of the
nebula are however uncertain: it may have been created through mass loss from
the star itself, or it could be the remnant of a pre-existing interstellar
cloud. Our goal is to determine the 3D structure of the nebula, and estimate
its mass. Knowing the geometrical shape of the nebula will also allow us to
retrieve the distance of RS Pup in an unambiguous manner using a model of its
light echoes (in a forthcoming work). The scattering angle of the Cepheid light
in the circumstellar nebula can be recovered from its degree of linear
polarization. We thus observed the nebula surrounding RS Pup using the
polarimetric imaging mode of the VLT/FORS instrument, and obtained a map of the
degree and position angle of linear polarization. From our FORS observations,
we derive a 3D map of the distribution of the dust, whose overall geometry is
an irregular and thin layer. The nebula does not present a well-defined
symmetry. Using a simple model, we derive a total dust mass of M(dust) = 2.9
+/- 0.9 Msun for the dust within 1.8 arcmin of the Cepheid. This translates
into a total mass of M(gas+dust) = 290 +/- 120 Msun, assuming a dust-to-gas
ratio of 1.0 +/- 0.3 %. The high mass of the dusty nebula excludes that it was
created by mass-loss from the star. However, the thinness nebula is an
indication that the Cepheid participated to its shaping, e.g. through its
radiation pressure or stellar wind. RS Pup therefore appears as a regular
long-period Cepheid located in an exceptionally dense interstellar environment.Comment: 14 pages, 21 figures. Accepted for publication in A&
Improved Baade-Wesselink surface-brightness relations
Recent, and older accurate, data on (limb-darkened) angular diameters is
compiled for 221 stars, as well as BVRIJK[12][25] magnitudes for those objects,
when available. Nine stars (all M-giants or supergiants) showing excess in the
[12-25] colour are excluded in the analysis as this may indicate the present of
dust influencing the optical and near-infrared colours as well. Based on this
large sample, Baade-Wesselink surface-brightness (SB) relations are presented
for dwarfs, giants, supergiants and dwarfs in the optical and near-infrared.
M-giants are found to follow different SB-relations from non-M giants, in
particular in V-(V-R). The preferred relation for non-M giants are compared to
earlier relation by Fouque & Gieren (1997, based on 10 stars) and Nordgren et
al. (2002, based on 57 stars). Increasing the sample size does not lead to a
lower rms value. It is shown that the residuals do not correlate with
metallicity at a significant level. The finally adopted observed angular
diameters are compared to those predicted by Cohen et al. (1999) for 45 stars
in common, and there is reasonable overall, to good agreement when \theta <6
mas. Finally, I comment on the common practice in the literature to average,
and then fix, the zero point of the V-(V-K), V-(V-R) and K-(J-K) relations, and
then re-derive the slopes. Such a common zero point at zero colour is not
expected from model atmospheres for the (V-R) colour and depends on gravity.
Relations derived in this way may be biased.Comment: accepted for publication in the MNRA
EUCLID : Dark Universe Probe and Microlensing planet Hunter
There is a remarkable synergy between requirements for Dark Energy probes by
cosmic shear measurements and planet hunting by microlensing. Employing weak
and strong gravitational lensing to trace and detect the distribution of matter
on cosmic and Galactic scales, but as well as to the very small scales of
exoplanets is a unique meeting point from cosmology to exoplanets. It will use
gravity as the tool to explore the full range of masses not accessible by any
other means. EUCLID is a 1.2m telescope with optical and IR wide field imagers
and slitless spectroscopy, proposed to ESA Cosmic Vision to probe for Dark
Energy, Baryonic acoustic oscillation, galaxy evolution, and an exoplanet hunt
via microlensing. A 3 months microlensing program will already efficiently
probe for planets down to the mass of Mars at the snow line, for free floating
terrestrial or gaseous planets and habitable super Earth. A 12+ months survey
would give a census on habitable Earth planets around solar like stars. This is
the perfect complement to the statistics that will be provided by the KEPLER
satellite, and these missions combined will provide a full census of extrasolar
planets from hot, warm, habitable, frozen to free floating.Comment: 6 pages 3 figures, invited talk in Pathways towards habitable
planets, Barcelona, Sept 200
V371 Per - A Thick-Disk, Short-Period F/1O Cepheid
V371 Per was found to be a double-mode Cepheid with a fundamental mode period
of 1.738 days, the shortest among Galactic beat Cepheids, and an unusually high
period ratio of 0.731, while the other Galactic beat Cepheids have period
ratios between 0.697 and 0.713. The latter suggests that the star has a
metallicity [Fe/H] between -1 and -0.7. The derived distance from the Galactic
Plane places it in the Thick Disk or the Halo, while all other Galactic beat
Cepheids belong to the Thin Disk. There are indications from historical data
that both the fundamental and first overtone periods have lengthened.Comment: Accepted for publication in MNRA
Towards A Census of Earth-mass Exo-planets with Gravitational Microlensing
Thirteen exo-planets have been discovered using the gravitational
microlensing technique (out of which 7 have been published). These planets
already demonstrate that super-Earths (with mass up to ~10 Earth masses) beyond
the snow line are common and multiple planet systems are not rare. In this
White Paper we introduce the basic concepts of the gravitational microlensing
technique, summarise the current mode of discovery and outline future steps
towards a complete census of planets including Earth-mass planets. In the
near-term (over the next 5 years) we advocate a strategy of automated follow-up
with existing and upgraded telescopes which will significantly increase the
current planet detection efficiency. In the medium 5-10 year term, we envision
an international network of wide-field 2m class telescopes to discover
Earth-mass and free-floating exo-planets. In the long (10-15 year) term, we
strongly advocate a space microlensing telescope which, when combined with
Kepler, will provide a complete census of planets down to Earth mass at almost
all separations. Such a survey could be undertaken as a science programme on
Euclid, a dark energy probe with a wide-field imager which has been proposed to
ESA's Cosmic Vision Programme.Comment: 10 pages. White Paper submission to the ESA Exo-Planet Roadmap
Advisory Team. See also "Inferring statistics of planet populations by means
of automated microlensing searches" by M. Dominik et al. (arXiv:0808.0004
Pinning down the ram-pressure-induced halt of star formation in the Virgo cluster spiral galaxy NGC 4388. A joint inversion of spectroscopic and photometric data
In a galaxy cluster, the evolution of spiral galaxies depends on their
cluster environment. Ram pressure due to the rapid motion of a spiral galaxy
within the hot intracluster medium removes the galaxy's interstellar medium
from the outer disk. Once the gas has left the disk, star formation stops. The
passive evolution of the stellar populations should be detectable in optical
spectroscopy and multi-wavelength photometry. The goal of our study is to
recover the stripping age of the Virgo spiral galaxy NGC 4388, i.e. the time
elapsed since the halt of star formation in the outer galactic disk using a
combined analysis of optical spectra and photometry. We performed VLT FORS2
long-slit spectroscopy of the inner star-forming and outer gas-free disk of NGC
4388. We developed a non-parametric inversion tool that allows us to
reconstruct the star formation history of a galaxy from spectroscopy and
photometry. The tool was tested on a series of mock data using Monte Carlo
simulations. The results from the non-parametric inversion were refined by
applying a parametric inversion method. The star formation history of the
unperturbed galactic disk is flat. The non-parametric method yields a rapid
decline of star formation < 200 Myr ago in the outer disk. The parametric
method is not able to distinguish between an instantaneous and a long-lasting
star formation truncation. The time since the star formation has dropped by a
factor of two from its pre-stripping value is 190 +- 30 Myr. We are able to
give a precise stripping age that is consistent with revised dynamical models.Comment: 12 pages, 10 figures, accepted for publication in A&
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