85 research outputs found
The circumstellar envelopes of the Cepheids L Car and RS Pup - Comparative study in the infrared with Spitzer, VLT/VISIR and VLTI/MIDI
19 pages, 16 figuresInternational audience[Abridged] - Context: Circumstellar envelopes (CSEs) around Cepheids are particularly interesting as they could impact the Cepheid distance scale, and imply stellar mass loss. Aims: Our goal is to establish the spatial and spectral properties of the CSEs of L Car and RS Pup. This is done through a parametrization of the envelopes in terms of fractional flux (with respect to the star) and angular size. - Methods: We retrieved archival Spitzer images of the two stars (3.5-70 mic), and obtained new imaging with the VLT/VISIR camera in BURST mode (8.6-11.9 mic), as well as interferometry with VLTI/MIDI (8-13 mic). This combination allows us to probe the envelopes over arcminute to milliarcsecond scales. - Results: The CSE of RS Pup is resolved at 24 and 70 mic by Spitzer, and around 10 mic by MIDI and VISIR. The compact envelope of L Car is resolved only in the VISIR and MIDI observations. However, the properties of the IR excesses differ considerably : a warm component is present around both stars at a spatial scale of a few 100 to a few 1 000 AU, but RS Pup presents in addition a large (several 100 000 AU) and cold (~40 K) dusty envelope. - Conclusions: We propose that the reflection nebula surrounding RS Pup has an interstellar origin, while the warm CSEs of both stars were created by ongoing stellar mass loss. Such warm circumstellar envelopes are probably common around Cepheids
Observational calibration of the projection factor of Cepheids I. The Type II Cepheid kappa Pavonis
The distances of pulsating stars, in particular Cepheids, are commonly
measured using the parallax of pulsation technique. The different versions of
this technique combine measurements of the linear diameter variation (from
spectroscopy) and the angular diameter variation (from photometry or
interferometry) amplitudes, to retrieve the distance in a quasi-geometrical
way. However, the linear diameter amplitude is directly proportional to the
projection factor (hereafter p-factor), which is used to convert spectroscopic
radial velocities (i.e., disk integrated) into pulsating (i.e., photospheric)
velocities. The value of the p-factor and its possible dependence on the
pulsation period are still widely debated. Our goal is to measure an
observational value of the p-factor of the type-II Cepheid kappa Pavonis, whose
parallax was measured with an accuracy of 5% using HST/FGS. We used this
parallax as a starting point to derive the p-factor of kappa Pav, using the
SPIPS technique, which is a robust version of the parallax-of-pulsation method
that employs radial velocity, interferometric and photometric data. We applied
this technique to a combination of new VLTI/PIONIER optical interferometric
angular diameters, new CORALIE and HARPS radial velocities, as well as
multi-colour photometry and radial velocities from the literature. We obtain a
value of p = 1.26 +/- 0.07 for the p-factor of kappa Pav. This result agrees
with several of the recently derived Period-p-factor relationships from the
literature, as well as previous observational determinations for Cepheids.
Individual estimates of the p-factor are fundamental to calibrating the
parallax of pulsation distances of Cepheids. Together with previous
observational estimates, the projection factor we obtain points to a weak
dependence of the p-factor on period.Comment: 8 pages, 6 figures, accepted in A&
Spatially extended emission around the Cepheid RS Puppis in near-infrared hydrogen lines. Adaptive optics imaging with VLT/NACO
It has been recently discovered that Cepheids harbor circumstellar envelopes
(CSEs). RS Pup is the Cepheid that presents the most prominent circumstellar
envelope known, the origin of which is not yet understood. Our purpose is to
estimate the flux contribution of the CSE around RS Pup at the one arcsecond
scale (~2000 AU) and to investigate its geometry, especially regarding
asymmetries, to constrain its physical properties. We obtained near-infrared
images in two narrow band filters centered on \lambda = 1.644 and 2.180 \mu m
(NB 1.64 and IB 2.18, respectively) that comprise two recombination lines of
hydrogen: the 12-4 and 7-4 (Brackett \gamma) transitions, respectively. We used
NACO's cube mode observations in order to improve the angular resolution with
the shift-and-add technique, and to qualitatively study the symmetry of the
spatially extended emission from the CSE with a statistical study of the
speckle noise. We probably detect at a 2\sigma level an extended emission with
a relative flux (compared with the star in the same filter) of 38 17% in
the NB 1.64 filter and 24 11% in the IB 2.18 filter. This emission is
centered on RS Pup and does not present any detectable asymmetry. We attribute
the detected emission to the likely presence of an hydrogen envelope
surrounding the star
An edge-on translucent dust disk around the nearest AGB star L2 Puppis - VLT/NACO spectro-imaging from 1.04 to 4.05 microns and VLTI interferometry
As the nearest known AGB star (d=64pc) and one of the brightest (mK-2), L2
Pup is a particularly interesting benchmark object to monitor the final stages
of stellar evolution. We report new lucky imaging observations of this star
with the VLT/NACO adaptive optics system in twelve narrow band filters covering
the 1.0-4.0 microns wavelength range. These diffraction limited images reveal
an extended circumstellar dust lane in front of the star, that exhibits a high
opacity in the J band and becomes translucent in the H and K bands. In the L
band, extended thermal emission from the dust is detected. We reproduce these
observations using Monte-Carlo radiative transfer modeling of a dust disk with
the RADMC-3D code. We also present new interferometric observations with the
VLTI/VINCI and MIDI instruments. We measure in the K band an upper limit to the
limb-darkened angular diameter of theta_LD = 17.9 +/- 1.6 mas, converting to a
maximum linear radius of R = 123 +/- 14 Rsun. Considering the geometry of the
extended K band emission in the NACO images, this upper limit is probably close
to the actual angular diameter of the star. The position of L2 Pup in the
Herzsprung-Russell diagram indicates that this star has a mass around 2 Msun
and is probably experiencing an early stage of the asymptotic giant branch. We
do not detect any stellar companion of L2 Pup in our adaptive optics and
interferometric observations, and we attribute its apparent astrometric wobble
in the Hipparcos data to variable lighting effects on its circumstellar
material. We however do not exclude the presence of a binary companion, as the
large loop structure extending to more than 10 AU to the North-East of the disk
in our L band images may be the result of interaction between the stellar wind
of L2 Pup and a hidden secondary object. The geometric configuration that we
propose, with a large dust disk seen almost edge-on, appears particularly
favorable to test and develop our understanding of the formation of bipolar
nebulae.Comment: 16 pages, 15 figure
Observational calibration of the projection factor of Cepheids. II. Application to nine Cepheids with HST/FGS parallax measurements
The distance to pulsating stars is classically estimated using the
parallax-of-pulsation (PoP) method, which combines spectroscopic radial
velocity measurements and angular diameter estimates to derive the distance of
the star. An important application of this method is the determination of
Cepheid distances, in view of the calibration of their distance scale. However,
the conversion of radial to pulsational velocities in the PoP method relies on
a poorly calibrated parameter, the projection factor (p-factor). We aim to
measure empirically the value of the p-factors of a homogeneous sample of nine
Galactic Cepheids for which trigonometric parallaxes were measured with the
Hubble Space Telescope Fine Guidance Sensor. 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. We obtained new interferometric angular diameters using the PIONIER
instrument at the Very Large Telescope Interferometer, completed by data from
the literature. Using the known distance as an input, we derive the value of
the p-factor and study its dependence with the pulsation period. We find the
following p-factors: 1.20 0.12 for RT Aur, 1.48 0.18 for T Vul,
1.14 0.10 for FF Aql, 1.31 0.19 for Y Sgr, 1.39 0.09 for X
Sgr, 1.35 0.13 for W Sgr, 1.36 0.08 for Dor, 1.41
0.10 for Gem, and 1.23 0.12 for Car. These values 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.
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.Comment: 18 pages, 13 figure
Toward a renewed Galactic Cepheid distance scale from Gaia and optical interferometry
Through an innovative combination of multiple observing techniques and mod-
eling, we are assembling a comprehensive understanding of the pulsation and
close environment of Cepheids. We developed the SPIPS modeling tool that
combines all observables (radial velocimetry, photometry, angular diameters
from interferometry) to derive the relevant physical parameters of the star
(effective temperature, infrared ex- cess, reddening,...) and the ratio of the
distance and the projection factor d/p. We present the application of SPIPS to
the long-period Cepheid RS Pup, for which we derive p = 1.25 +/- 0.06. The
addition of this massive Cepheid consolidates the existing sample of p-factor
measurements towards long-period pulsators. This allows us to conclude that p
is constant or mildly variable around p = 1.29 +/- 0.04 (+/-3%) as a function
of the pulsation period. The forthcoming Gaia DR2 will provide a considerable
improvement in quantity and accuracy of the trigonometric parallaxes of
Cepheids. From this sample, the SPIPS modeling tool will enable a robust
calibration of the Cepheid distance scale.Comment: 5 pages, 4 figures, proceedings of the 22nd Los Alamos Stellar
Pulsation Conference "Wide-field variability surveys: a 21st-century
perspective" held in San Pedro de Atacama, Chile, Nov. 28-Dec. 2, 201
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
Multiplicity of Galactic Cepheids and RR Lyrae stars from Gaia DR2. I. Binarity from proper motion anomaly
Context. Classical Cepheids (CCs) and RR Lyrae stars (RRLs) are important classes of variable stars used as standard candles to estimate galactic and extragalactic distances. Their multiplicity is imperfectly known, particularly for RRLs. Astoundingly, to date only one RRL has convincingly been demonstrated to be a binary, TU UMa, out of tens of thousands of known RRLs. Aims: Our aim is to detect the binary and multiple stars present in a sample of Milky Way CCs and RRLs. Methods: In the present article, we combine the HIPPARCOS and Gaia DR2 positions to determine the mean proper motion of the targets, and we search for proper motion anomalies (PMa) caused by close-in orbiting companions. Results: We identify 57 CC binaries from PMa out of 254 tested stars and 75 additional candidates, confirming the high binary fraction of these massive stars. For 28 binary CCs, we determine the companion mass by combining their spectroscopic orbital parameters and astrometric PMa. We detect 13 RRLs showing a significant PMa out of 198 tested stars, and 61 additional candidates. Conclusions: We determine that the binary fraction of CCs is likely above 80%, while that of RRLs is at least 7%. The newly detected systems will be useful to improve our understanding of their evolutionary states. The discovery of a significant number of RRLs in binary systems also resolves the long-standing mystery of their extremely low apparent binary fraction. Full Tables A.1 and A.3 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/623/A11
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