763 research outputs found
Non parametric reconstruction of distribution functions from observed galactic disks
A general inversion technique for the recovery of the underlying distribution
function for observed galactic disks is presented and illustrated. Under the
assumption that these disks are axi-symmetric and thin, the proposed method
yields the unique distribution compatible with all the observables available.
The derivation may be carried out from the measurement of the azimuthal
velocity distribution arising from positioning the slit of a spectrograph along
the major axis of the galaxy. More generally, it may account for the
simultaneous measurements of velocity distributions corresponding to slits
presenting arbitrary orientations with respect to the major axis. The approach
is non-parametric, i.e. it does not rely on a particular algebraic model for
the distribution function. Special care is taken to account for the fraction of
counter-rotating stars which strongly affects the stability of the disk. An
optimisation algorithm is devised -- generalising the work of Skilling & Bryan
(1984) -- to carry this truly two-dimensional ill-conditioned inversion
efficiently. The performances of the overall inversion technique with respect
to the noise level and truncation in the data set is investigated with
simulated data. Reliable results are obtained up to a mean signal to noise
ratio of~5 and when measurements are available up to . A discussion of
the residual biases involved in non parametric inversions is presented.
Prospects of application to observed galaxies and other inversion problems are
discussed.Comment: 11 pages, 13 figures; accepted for publication by MNRA
A certification method for the milling process of free-form surfaces using a test part
International audienceIt is generally admitted that the manufacturing of free-form surfaces requires the use of a CAD-CAM system. The toolpath accuracy and the dimensional quality of the final shape have to be in accordance with the geometrical specifications. But most of the time, the final parts present deviations from the expected shape. These deviations may be due to either the toolpath calculation (CAM system) or the cutting process itself. In the paper, we propose an analysis of the whole milling process to point out the possible sources of errors. These errors generally lead to geometrical deviations and the final part does not meet the required specifications. As the errors can be linked to geometrical particularities of the shape, we propose a test part associated with check means to bring out problems. The milling of this part using two different techniques of toolpath generation shows that obviously both toolpaths are not error-free and that errors result from different geometrical particularities of the part surfaces
Imaging reconstruction for infrared interferometry: first images of YSOs environment
The study of protoplanetary disks, where the planets are believed to form,
will certainly allow the formation of our Solar System to be understood. To
conduct observations of these objects at the milli-arcsecond scale, infrared
interferometry provides the right performances for T Tauri, FU Ori or Herbig
Ae/Be stars. However, the only information obtained so far are scarce
visibility measurements which are directly tested with models. With the outcome
of recent interferometers, one can foresee obtaining images reconstructed
independently of the models. In fact, several interferometers including IOTA
and AMBER on the VLTI already provide the possibility to recombine three
telescopes at once and thus to obtain the data necessary to reconstruct images.
In this paper, we describe the use of MIRA, an image reconstruction algorithm
developed for optical inter- ferometry data (squared visibilities and closure
phases) by E. Thiebaut. We foresee also to use the spectral information given
by AMBER data to constrain even better the reconstructed images. We describe
the use of MIRA to reconstruct images of young stellar objects out of actual
data, in particular the multiple system GW Orionis (IOTA, 2004), and discuss
the encountered difficulties.Comment: 10 pages, 6 figures, Proc. SPIE conference 7013 "Optical and Infrared
Interferometry" (Marseille 2008
High dynamic range imaging by pupil single-mode filtering and remapping
Because of atmospheric turbulence, obtaining high angular resolution images
with a high dynamic range is difficult even in the near infrared domain of
wavelengths. We propose a novel technique to overcome this issue. The
fundamental idea is to apply techniques developed for long baseline
interferometry to the case of a single-aperture telescope. The pupil of the
telescope is broken down into coherent sub-apertures each feeding a single-mode
fiber. A remapping of the exit pupil allows interfering all sub-apertures
non-redundantly. A diffraction-limited image with very high dynamic range is
reconstructed from the fringe pattern analysis with aperture synthesis
techniques, free of speckle noise. The performances of the technique are
demonstrated with simulations in the visible range with an 8 meter telescope.
Raw dynamic ranges of 1: can be obtained in only a few tens of seconds of
integration time for bright objects.Comment: 5 pages, 3 figures. accepted for publication in MNRA
Dynamical Masses of Young Stars in Multiple Systems
We present recent measurements of the orbital motion in the young binaries DF
Tau and ZZ Tau, and the hierarchical triple Elias 12, that were obtained with
the Fine Guidance Sensors on the HST and at the Keck Observatory using adaptive
optics. Combining these observations with previous measurements from the
literature, we compute preliminary orbital parameters for DF Tau and ZZ Tau. We
find that the orbital elements cannot yet be determined precisely because the
orbital coverage spans only about 90 degr in position angle. Nonetheless, the
range of possible values for the period and semi-major axis already defines a
useful estimate for the total mass in DF Tau and ZZ Tau, with values of
0.90{+0.85}{-0.35} M_sun and 0.81{+0.44}{-0.25} M_sun, respectively, at a
fiducial distance of 140 pc.Comment: 26 pages, 9 figures, accepted for publication in A
Imaging the spotty surface of Betelgeuse in the H band
This paper reports on H-band interferometric observations of Betelgeuse made
at the three-telescope interferometer IOTA. We image Betelgeuse and its
asymmetries to understand the spatial variation of the photosphere, including
its diameter, limb darkening, effective temperature, surrounding brightness,
and bright (or dark) star spots. We used different theoretical simulations of
the photosphere and dusty environment to model the visibility data. We made
images with parametric modeling and two image reconstruction algorithms: MIRA
and WISARD. We measure an average limb-darkened diameter of 44.28 +/- 0.15 mas
with linear and quadratic models and a Rosseland diameter of 45.03 +/- 0.12 mas
with a MARCS model. These measurements lead us to derive an updated effective
temperature of 3600 +/- 66 K. We detect a fully-resolved environment to which
the silicate dust shell is likely to contribute. By using two imaging
reconstruction algorithms, we unveiled two bright spots on the surface of
Betelgeuse. One spot has a diameter of about 11 mas and accounts for about 8.5%
of the total flux. The second one is unresolved (diameter < 9 mas) with 4.5% of
the total flux. Resolved images of Betelgeuse in the H band are asymmetric at
the level of a few percent. The MOLsphere is not detected in this wavelength
range. The amount of measured limb-darkening is in good agreement with model
predictions. The two spots imaged at the surface of the star are potential
signatures of convective cells.Comment: 10 pages, 10 figures, accepted for publication in A&A, references
adde
A Test of Pre-Main Sequence Evolutionary Models Across the Stellar/Substellar Boundary Based on Spectra of the Young Quadruple GG Tau
We present spatially separated optical spectra of the components of the young
hierarchical quadruple GG Tau. Spectra of GG Tau Aa and Ab (separation 0".25 ~
35 AU) were obtained with the Faint Object Spectrograph aboard the Hubble Space
Telescope. Spectra of GG Tau Ba and Bb (separation 1".48 ~ 207 AU) were
obtained with both the HIRES and the LRIS spectrographs on the W. M. Keck
telescopes. The components of this mini-cluster, which span a wide range in
spectral type (K7 - M7), are used to test both evolutionary models and the
temperature scale for very young, low mass stars under the assumption of coeval
formation. Of the evolutionary models tested, those of Baraffe et al. (1998,
A&A, 337, 403) yield the most consistent ages when combined with a temperature
scale intermediate between that of dwarfs and giants. The version of the
Baraffe et al. models computed with a mixing length nearly twice the pressure
scale height is of particular interest as it predicts masses for GG Tau Aa and
Ab that are in agreement with their dynamical mass estimate.
Using this evolutionary model and a coeval (at 1.5 Myrs) temperature scale,
we find that the coldest component of the GG Tau system, GG Tau Bb, is
substellar with a mass of 0.044 +/- 0.006 Msun. This brown dwarf companion is
especially intriguing as it shows signatures of accretion, although this
accretion is not likely to alter its mass significantly. GG Tau Bb is currently
the lowest mass, spectroscopically confirmed companion to a T Tauri star, and
is one of the coldest, lowest mass T Tauri objects in the Taurus-Auriga star
forming region.Comment: 25 pages, 6 figures, accepted for publication in The Astrophysical
Journa
Phase Referencing in Optical Interferometry
One of the aims of next generation optical interferometric instrumentation is
to be able to make use of information contained in the visibility phase to
construct high dynamic range images. Radio and optical interferometry are at
the two extremes of phase corruption by the atmosphere. While in radio it is
possible to obtain calibrated phases for the science objects, in the optical
this is currently not possible. Instead, optical interferometry has relied on
closure phase techniques to produce images. Such techniques allow only to
achieve modest dynamic ranges. However, with high contrast objects, for faint
targets or when structure detail is needed, phase referencing techniques as
used in radio interferometry, should theoretically achieve higher dynamic
ranges for the same number of telescopes. Our approach is not to provide
evidence either for or against the hypothesis that phase referenced imaging
gives better dynamic range than closure phase imaging. Instead we wish to
explore the potential of this technique for future optical interferometry and
also because image reconstruction in the optical using phase referencing
techniques has only been performed with limited success. We have generated
simulated, noisy, complex visibility data, analogous to the signal produced in
radio interferometers, using the VLTI as a template. We proceeded with image
reconstruction using the radio image reconstruction algorithms contained in
AIPS IMAGR (CLEAN algorithm). Our results show that image reconstruction is
successful in most of our science cases, yielding images with a 4
milliarcsecond resolution in K band. (abridged)Comment: 11 pages, 36 figure
Observations of T-Tauri Stars using HST-GHRS: I. Far Ultraviolet Emission Lines
We have analyzed GHRS data of eight CTTS and one WTTS. The GHRS data consists
of spectral ranges 40 A wide centered on 1345, 1400, 1497, 1550, and 1900 A.
These UV spectra show strong SiIV, and CIV emission, and large quantities of
sharp (~40 km/s) H2 lines. All the H2 lines belong to the Lyman band and all
the observed lines are single peaked and optically thin. The averages of all
the H2 lines centroids for each star are negative which may indicate that they
come from an outflow. We interpret the emission in H2 as being due to
fluorescence, mostly by Ly_alpha, and identify seven excitation routes within 4
A of that line. We obtain column densities (10^12 to 10^15 cm^-2) and optical
depths (~1 or less) for each exciting transition. We conclude that the
populations are far from being in thermal equilibrium. We do not observe any
lines excited from the far blue wing of Ly_alpha, which implies that the
molecular features are excited by an absorbed profile. SiIV and CIV (corrected
for H2 emission) have widths of ~200 km/s, and an array of centroids
(blueshifted lines, centered, redshifted). These characteristics are difficult
to understand in the context of current models of the accretion shock. For DR
Tau we observe transient strong blueshifted emission, perhaps the a result of
reconnection events in the magnetosphere. We also see evidence of multiple
emission regions for the hot lines. While CIV is optically thin in most stars
in our sample, SiIV is not. However, CIV is a good predictor of SiIV and H2
emission. We conclude that most of the flux in the hot lines may be due to
accretion processes, but the line profiles can have multiple and variable
components.Comment: 67 pages, 19 figures, Accepted in Ap
- âŠ