273 research outputs found
The environment of the fast rotating star Achernar - Thermal infrared interferometry with VLTI/MIDI and SIMECA modeling
Context: As is the case of several other Be stars, Achernar is surrounded by
an envelope, recently detected by near-IR interferometry.
Aims: We search for the signature of circumstellar emission at distances of a
few stellar radii from Achernar, in the thermal IR domain.
Methods: We obtained interferometric observations on three VLTI baselines in
the N band (8-13 mic), using the MIDI instrument.
Results: From the measured visibilities, we derive the angular extension and
flux contribution of the N band circumstellar emission in the polar direction
of Achernar. The interferometrically resolved polar envelope contributes 13.4
+/- 2.5 % of the photospheric flux in the N band, with a full width at half
maximum of 9.9 +/- 2.3 mas (~ 6 Rstar). This flux contribution is in good
agreement with the photometric IR excess of 10-20% measured by fitting the
spectral energy distribution. Due to our limited azimuth coverage, we can only
establish an upper limit of 5-10% for the equatorial envelope. We compare the
observed properties of the envelope with an existing model of this star
computed with the SIMECA code.
Conclusions: The observed extended emission in the thermal IR along the polar
direction of Achernar is well reproduced by the existing SIMECA model. Already
detected at 2.2mic, this polar envelope is most probably an observational
signature of the fast wind ejected by the hot polar caps of the star.Comment: A&A Letter, in pres
First direct detection of a Keplerian rotating disk around the Be star Arae using the VLTI/AMBER instrument
Aims. We aim to study the geometry and kinematics of the disk around the Be
star Arae as a function of wavelength, especially across the
Br emission line. The main purpose of this paper is to answer the
question about the nature of the disk rotation around Be stars. Methods. We use
the VLTI/AMBER instrument operating in the K band which provides a gain by a
factor 5 in spatial resolution compared to previous VLTI/MIDI observations.
Moreover, it is possible to combine the high angular resolution provided with
the (medium) spectral resolution of AMBER to study the kinematics of the inner
part of the disk and to infer its rotation law. Results. We obtain for the
first time the direct evidence that the disk is in keplerian rotation,
answering a question that occurs since the discovery of the first Be star
Cas by father Secchi in 1866. We also present the global geometry of
the disk showing that it is compatible with a thin disk + polar enhanced winds
modeled with the SIMECA code. We found that the disk around Arae is
compatible with a dense equatorial matter confined in the central region
whereas a polar wind is contributing along the rotational axis of the central
star. Between these two regions the density must be low enough to reproduce the
large visibility modulus (small extension) obtained for two of the four VLTI
baselines. Moreover, we obtain that Arae is rotating very close to its
critical rotation. This scenario is also compatible with the previous MIDI
measurements.Comment: 15 page
Fast ray-tracing algorithm for circumstellar structures (FRACS). II. Disc parameters of the B[e] supergiant CPD-57° 2874 from VLTI/MIDI data
B[e] supergiants are luminous, massive post-main sequence stars exhibiting
non-spherical winds, forbidden lines, and hot dust in a disc-like structure.
The physical properties of their rich and complex circumstellar environment
(CSE) are not well understood, partly because these CSE cannot be easily
resolved at the large distances found for B[e] supergiants (typically \ga
1~kpc). From mid-IR spectro-interferometric observations obtained with
VLTI/MIDI we seek to resolve and study the CSE of the Galactic B[e] supergiant
CPD-57\degr\,2874. For a physical interpretation of the observables
(visibilities and spectrum) we use our ray-tracing radiative transfer code
(FRACS), which is optimised for thermal spectro-interferometric observations.
Thanks to the short computing time required by FRACS (~s per monochromatic
model), best-fit parameters and uncertainties for several physical quantities
of CPD-57\degr\,2874 were obtained, such as inner dust radius, relative flux
contribution of the central source and of the dusty CSE, dust temperature
profile, and disc inclination. The analysis of VLTI/MIDI data with FRACS
allowed one of the first direct determinations of physical parameters of the
dusty CSE of a B[e] supergiant based on interferometric data and using a full
model-fitting approach. In a larger context, the study of B[e] supergiants is
important for a deeper understanding of the complex structure and evolution of
hot, massive stars
Microstructural dynamics of motor learning and sleep-dependent consolidation: A diffusion imaging study
Memory consolidation can benefit from post-learning sleep, eventually leading to long-term microstructural brain modifications to accommodate new memory representations. Non-invasive diffusion-weighted magnetic resonance imaging (DWI) allows the observation of (micro)structural brain remodeling after time-limited motor learning. Here, we combine conventional diffusion tensor imaging (DTI) and neurite orientation dispersion and density imaging (NODDI) that allows modeling dendritic and axonal complexity in gray matter to investigate with improved specificity the microstructural brain mechanisms underlying time- and sleep-dependent motor memory consolidation dynamics. Sixty-one young healthy adults underwent four DWI sessions, two sequential motor trainings, and a night of total sleep deprivation or regular sleep distributed over five days. We observed rapid-motor-learning-related remodeling in occipitoparietal, temporal, and motor-related subcortical regions, reflecting temporary dynamics in learning-related neuronal brain plasticity processes. Sleep-related consolidation seems not to exert a detectable impact on diffusion parameters, at least on the timescale of a few days
Critical study of the distribution of rotational velocities of Be stars; II: Differential rotation and some hidden effects interfering with the interpretation of the Vsin i parameter
We assume that stars may undergo surface differential rotation to study its
impact on the interpretation of and on the observed distribution
of ratios of true rotational velocities u=V/V_\rm c (V_\rm c is
the equatorial critical velocity). We discuss some phenomena affecting the
formation of spectral lines and their broadening, which can obliterate the
information carried by concerning the actual stellar rotation. We
studied the line broadening produced by several differential rotational laws,
but adopted Maunder's expression
as an attempt to account for
all of these laws with the lowest possible number of free parameters. We
studied the effect of the differential rotation parameter on the
measured parameter and on the distribution of ratios
u=V/V_\rm c. We conclude that the inferred is smaller than
implied by the actual equatorial linear rotation velocity V_\rm eq if the
stars rotate with . For a
given the deviations of are larger when . If
the studied Be stars have on average , the number of rotators with
V_\rm eq\simeq0.9V_\rm c is larger than expected from the observed
distribution ; if these stars have on average , this number
is lower than expected. We discuss seven phenomena that contribute either to
narrow or broaden spectral lines, which blur the information on the rotation
carried by and, in particular, to decide whether the Be phenomenon
mostly rely on the critical rotation. We show that two-dimensional radiation
transfer calculations are needed in rapid rotators to diagnose the stellar
rotation more reliably.Comment: To appear in A&
GI2T/REGAIN spectro-interferometry with a new infrared beam combiner
We have built an infrared beam combiner for the GI2T/REGAIN interferometer of
the Observatoire de la Cote d'Azur. The beam combiner allows us to record
spectrally dispersed Michelson interference fringes in the near-infrared J-, H-
or K-bands. The beam combiner has the advantage that Michelson interferograms
can simultaneously be recorded in about 128 different spectral channels. The
tilt of the spectrally dispersed fringes is a measure of the instantaneous
optical path difference. We present the optical design of the beam combiner and
GI2T/REGAIN observations of the Mira star R Cas with this beam combiner in the
spectral range of 2.00 micron - 2.18 micron (observations on 22 and 25 August
1999; variability phase 0.08; V-magnitude approx. 6; seven baselines between
12m and 24m; reference stars Vega and Beta Peg). The spectrograph of the beam
combiner consists of an anamorphotic cylindrical lens system, an image plane
slit, and a grism. A system of digital signal processors calculates the
ensemble average power spectrum of the spectrally dispersed Michelson
interferograms and the instantaneous optical path difference error in real
time. From the observed R Cas visibilities at baselines 12.0m, 13.8m and 13.9m,
a 2.1 micron uniform-disk diameter of 25.3mas +/-3.3mas was derived. The
unusually high visibility values at baselines >16m show that the stellar
surface of R Cas is more complex than previously assumed. The visibility values
at baselines >16m can be explained by high-contrast surface structure on the
stellar surface of R Cas or other types of unexpected center-to-limb
variations. The R Cas observations were compared with theoretical Mira star
models yielding a linear Rosseland radius of 276Rsun +/-66Rsun and an effective
temperature of 2685K+/-238K for R Cas at phase 0.08.Comment: 10 pages, 6 figures, see also
http://www.mpifr-bonn.mpg.de/div/speckle, SPIE conf 4006 "Interferometry in
Optical Astronomy", in pres
Constraining Disk Parameters of Be Stars using Narrowband H-alpha Interferometry with the NPOI
Interferometric observations of two well-known Be stars, gamma Cas and phi
Per, were collected and analyzed to determine the spatial characteristics of
their circumstellar regions. The observations were obtained using the Navy
Prototype Optical Interferometer equipped with custom-made narrowband filters.
The filters isolate the H-alpha emission line from the nearby continuum
radiation, which results in an increased contrast between the interferometric
signature due to the H-alpha-emitting circumstellar region and the central
star. Because the narrowband filters do not significantly attenuate the
continuum radiation at wavelengths 50 nm or more away from the line, the
interferometric signal in the H-alpha channel is calibrated with respect to the
continuum channels. The observations used in this study represent the highest
spatial resolution measurements of the H-alpha-emitting regions of Be stars
obtained to date. These observations allow us to demonstrate for the first time
that the intensity distribution in the circumstellar region of a Be star cannot
be represented by uniform disk or ring-like structures, whereas a Gaussian
intensity distribution appears to be fully consistent with our observations.Comment: 23 pages, 14 figures, accepted for publication in A
Grown-up stars physics with MATISSE
MATISSE represents a great opportunity to image the environment around
massive and evolved stars. This will allow one to put constraints on the
circumstellar structure, on the mass ejection of dust and its reorganization ,
and on the dust-nature and formation processes. MATISSE measurements will often
be pivotal for the understanding of large multiwavelength datasets on the same
targets collected through many high-angular resolution facilities at ESO like
sub-millimeter interferometry (ALMA), near-infrared adaptive optics (NACO,
SPHERE), interferometry (PIONIER, GRAVITY), spectroscopy (CRIRES), and
mid-infrared imaging (VISIR). Among main sequence and evolved stars, several
cases of interest have been identified that we describe in this paper.Comment: SPIE, Jun 2016, Edimbourgh, Franc
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