833 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
Coronal activity cycles in nearby G and K stars - XMM-Newton monitoring of 61 Cygni and Alpha Centauri
We use X-ray observations of the nearby binaries 61 Cyg A/B (K5V and K7V) and
Alpha Cen A/B (G2V and K1V) to study the long-term evolution of magnetic
activity in weakly to moderately active G + K dwarfs over nearly a decade.
Specifically we search for X-ray activity cycles and related coronal changes
and compare them to the solar behavior. For 61 Cyg A we find a regular coronal
activity cycle analog to its 7.3 yr chromospheric cycle. The X-ray brightness
variations are with a factor of three significantly lower than on the Sun, yet
the changes of coronal properties resemble the solar behavior with larger
variations occurring in the respective hotter plasma components. 61 Cyg B does
not show a clear cyclic coronal trend so far, but the X-ray data matches the
more irregular chromospheric cycle. Both Alpha Cen stars exhibit significant
long-term X-ray variability. Alpha Cen A shows indications for cyclic
variability of an order of magnitude with a period of about 12-15 years; the
Alpha Cen B data suggests an X-ray cycle with an amplitude of about six to
eight and a period of 8-9 years. The sample stars exhibit X-ray luminosities
ranging between Lx < 1x10^26 - 3x10^27 erg s^-1 in the 0.2-2.0 keV band and
have coronae dominated by cool plasma with variable average temperatures of
around 1.0-2.5 MK. We find that coronal activity cycles are apparently a common
phenomenon in older, slowly rotating G and K stars. The spectral changes of the
coronal X-ray emission over the cycles are solar-like in all studied targets.Comment: 11 pages, 9 figures, accepted by Astronomy and Astrophysic
High resolution spectroscopy for Cepheids distance determination. V. Impact of the cross-correlation method on the p-factor and the gamma-velocities
The cross correlation method (hereafter CC) is widely used to derive the
radial velocity curve of Cepheids when the signal to noise of the spectra is
low. However, if it is used with the wrong projection factor, it might
introduce some biases in the Baade-Wesselink (hereafter BW) methods of
determining the distance of Cepheids. In addition, it might affect the average
value of the radial velocity curve (or gamma-velocity) important for Galactic
structure studies. We aim to derive a period-projection factor relation
(hereafter Pp) appropriate to be used together with the CC method. Moreover, we
investigate whether the CC method can explain the misunderstood previous
calculation of the K-term of Cepheids. We observed eight galactic Cepheids with
the HARPS spectrograph. For each star, we derive an interpolated CC radial
velocity curve using the HARPS pipeline. The amplitudes of these curves are
used to determine the correction to be applied to the semi-theoretical
projection factor derived in Nardetto et al. (2007). Their average value (or
gamma-velocity) are also compared to the center-of-mass velocities derived in
Nardetto et al. (2008). The correction in amplitudes allows us to derive a new
Pp relation: p = [-0.08+-0.05] log P +[1.31+-0.06]. We also find a negligible
wavelength dependence (over the optical range) of the Pp relation. We finally
show that the gamma-velocity derived from the CC method is systematically
blue-shifted by about 1.0 +- 0.2km/s compared to the center-of-mass velocity of
the star. An additional blue-shift of 1.0km/s is thus needed to totally explain
the previous calculation of the K-term of Cepheids (around 2km/s). The new Pp
relation we derived is a solid tool for the distance scale calibration
(abridged).Comment: Comments : 9 pages, 3 Postscript figures, 5 Tables, accepted for
publication in A&
Integrated optics for astronomical interferometry - VI. Coupling the light of the VLTI in K band
Our objective is to prove that integrated optics (IO) is not only a good
concept for astronomical interferometry but also a working technique with high
performance. We used the commissioning data obtained with the dedicated K-band
integrated optics two-telescope beam combiner which now replaces the fiber
coupler MONA in the VLTI/VINCI instrument. We characterize the behaviour of
this IO device and compare its properties to other single mode beam combiner
like the previously used MONA fiber coupler. The IO combiner provides a high
optical throughput, a contrast of 89% with a night-to-night stability of a few
percent. Even if a dispersive phase is present, we show that it does not bias
the measured Fourier visibility estimate. An upper limit of 0.005 for the
cross-talk between linear polarization states has been measured. We take
advantage of the intrinsic contrast stability to test a new astronomical
prodecure for calibrating diameters of simple stars by simultaneously fitting
the instrumental contrast and the apparent stellar diameters. This method
reaches an accuracy with diameter errors of the order of previous ones but
without the need of an already known calibrator. These results are an important
step of integrated optics and paves the road to incoming imaging interferometer
projects
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
Searching for visual companions of close Cepheids. VLT/NACO lucky imaging of Y~Oph, FF~Aql, X~Sgr, W~Sgr and ~Aql
Aims: High-resolution imaging in several photometric bands can provide color
and astrometric information of the wide-orbit component of Cepheid stars. Such
measurements are needed to understand the age and evolution of pulsating stars.
In addition, binary Cepheids have the potential to provide direct and
model-independent distances and masses. Methods: We used the NAOS-CONICA
adaptive optics instrument (NACO) in the near-infrared to perform a deep search
for wide components around the classical Cepheids, Y~Oph, FF~Aql, X~Sgr, W~Sgr,
and ~Aql, within a field of view (FoV) of ( for ~Aql). Results: We were able to reach contrast -8\,mag and -7\,mag in the radius range ,
which enabled us to constrain the presence of wide companions. For Y~Oph,
FF~Aql, X~Sgr, W~Sgr, and ~Aql at , we ruled out the presence
of companions with a spectral type that is earlier than a B7V, A9V, A9V, A1V,
and G5V star, respectively. For , no companions earlier than
O9V, B3V, B4V, B2V, and B2V star, respectively, are detected. A component is
detected close to ~Aql at projected separation \,mas and a position angle . We estimated its
dereddened apparent magnitude to be and derived a
spectral type that ranges between an F1V and F6V star. Additional photometric
and astrometric measurements are necessary to better constrain this star and
check its physical association to the ~Aql system.Comment: Accepted for publication in Astronomy and Astrophysic
A List of Bright Interferometric Calibrators measured at the ESO VLTI
In a previous publication (Richichi & Percheron 2005) we described a program
of observations of candidate calibrator stars at the ESO Very Large Telescope
Interferometer (VLTI), and presented the main results from a statistical point
of view. In the present paper, we concentrate on establishing a new homogeneous
group of bright interferometric calibrators, based entirely on publicly
available K-band VLTI observations carried out with the VINCI instrument up to
July 2004. For this, we have defined a number of selection criteria for the
quality and volume of the observations, and we have accordingly selected a list
of 17 primary and 47 secondary calibrators. We have developed an approach to a
robust global fit for the angular diameters using the whole volume of
quality-controlled data, largely independent of a priori assumptions. Our
results have been compared with direct measurements, and indirect estimates
based on spectrophotometric methods, and general agreement is found within the
combined uncertainties. The stars in our list cover the range K=-2.9 to +3.0
mag in brightness, and 1.3 to 20.5 milliarcseconds in uniform-disk diameter.
The relative accuracy of the angular diameter values is on average 0.4% and 2%
for the primary and secondary calibrators respectively. Our calibrators are
well suited for interferometric observations in the near-infrared on baselines
between ~20m and ~200m, and their accuracy is superior, at least for the
primary calibrators, to other similar catalogues. Therefore, the present list
of calibrators has the potential to lead to significantly improved
interferometric scientific results
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&
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