232 research outputs found
Theory and laboratory tests of the multi-stage phase mask coronagraph
A large number of coronagraphs have been proposed to overcome the ratio that
exists between the star and its planet. The planet finder of the Extremely
Large Telescope, which is called EPICS, will certainly need a more efficient
coronagraph than the ones that have been developed so far. We propose to use a
combination of chromatic Four Quadrant Phase Mask coronagraph to achromatize
the dephasing of the device while maintaining a high rejection performance.
After describing this multi-stage FQPM coronagraph, we show preliminary results
of a study on its capabilities in the framework of the EPICS instrument, the
planet finder of the European Extremely Large Telescope. Eventually, we present
laboratory tests of a rough prototype of a multi-stage four-quadrant phase
mask. On one hand, we deduce from our laboratory data that a detection at the
10^-10 level is feasible in monochromatic light. On the other hand, we show the
detection of a laboratory companion fainter than 10^-8 with a spectral
bandwidth larger than 20%.Comment: 9 pages, 9 figures, To appear in SPIE proceeding- conference 7015
held in Marseille in June 200
Multi-stage four-quadrant phase mask: achromatic coronagraph for space-based and ground-based telescopes
Less than 3% of the known exoplanets were directly imaged for two main
reasons. They are angularly very close to their parent star, which is several
magnitudes brighter. Direct imaging of exoplanets thus requires a dedicated
instrumentation with large telescopes and accurate wavefront control devices
for high-angular resolution and coronagraphs for attenuating the stellar light.
Coronagraphs are usually chromatic and they cannot perform high-contrast
imaging over a wide spectral bandwidth. That chromaticity will be critical for
future instruments. Enlarging the coronagraph spectral range is a challenge for
future exoplanet imaging instruments on both space-based and ground-based
telescopes. We propose the multi-stage four-quadrant phase mask that associates
several monochromatic four-quadrant phase mask coronagraphs in series.
Monochromatic device performance has already been demonstrated and the
manufacturing procedures are well-under control since their development for
previous instruments on VLT and JWST. The multi-stage implementation simplicity
is thus appealing. We present the instrument principle and we describe the
laboratory performance for large spectral bandwidths and for both pupil shapes
for space- (off-axis telescope) and ground-based (E-ELT) telescopes. The
multi-stage four-quadrant phase mask reduces the stellar flux over a wide
spectral range (30%) and it is a very good candidate to be associated with a
spectrometer for future exoplanet imaging instruments in ground- and
space-based observatories.Comment: 7 pages, 11 figures, 4 tables, accepted in A&
The four-quadrant phase-mask coronagraph: white light laboratory results with an achromatic device
Achromatic coronagraphs are the subject of intensive research since they will be mandatory for many programs which aim at detecting and characterizing exoplanets. We report a laboratory experiment assessing the performance of the Four-Quadrant Phase-Mask coronagraph (FQPM) over a broadband wavelength range (Râ2). The achromatization of the FQPM is provided by achromatic halfwave plates (HWP). These phase shifters combine birefringent plates made of different materials with properly chosen thicknesses. The HWP thickness control is relaxed by two orders of magnitudes with respect to the classical (non-birefringent) dispersive plate approach. In our experiment we used a two stage stack of Quartz and MgF_2. This combination allows to cover a large spectral range in the visible (500-900nm) with a small phase error residual around Ï(â0.12 rad rms). With this achromatization, we obtained an attenuation of 755 on the white light PSF peak. This solution is directly applicable to ground-based telescopes using high order adaptive optics such as the ESO's VLT-Planet Finder project and could easily be transposed in the mid-infrared domain for future space-based missions like DARWIN/TPF
The Remarkable Be Star HD110432
HD110432 has gained considerable attention because it is a hard, variable
X-ray source similar to gamma Cas. From time-serial echelle data obtained over
two weeks during 2005 January and February, we find several remarkable
characteristics in the star's optical spectrum. The line profiles show rapid
variations on some nights which can be most likely be attributed to irregularly
occurring and short-lived migrating subfeatures. Such features have only been
observed to date in gamma Cas and AB Dor, two stars for which it is believed
magnetic fields force circumstellar clouds to corotate over the stellar
surface. The star's optical spectrum also exhibits a number of mainly FeII and
HeI emission features with profiles typical of an optically thin disk viewed
edge-on. Using spectral synthesis techniques, we find that its temperature is
9800K +/-300K, that its projected area is a remarkably large 100 stellar areas,
and its emitting volume resides at a distance of 1 AU from the star. We also
find that the star's absorption profiles extend to +/-1000 km/s, a fact which
we cannot explain. Otherwise, HD110432 and gamma Cas share similarly peculiar
X-ray and optical characteristics such as high X-ray temperature, erratic X-ray
variability on timescales of a few hours, optical emission lines, and
submigrating features in optical line profiles. Because of these similarities,
we suggest that this star is a new member of a select class of "gamma Cas
analogs."Comment: 31 pages, 9 figures, accepted by ApJ (3/20/06
Magnetic topology and surface differential rotation on the K1 subgiant of the RS CVn system HR 1099
We present here spectropolarimetric observations of the RS CVn system HR 1099
(V711 Tau) secured from 1998 February to 2002 January with the
spectropolarimeter MuSiCoS at the Telescope Bernard Lyot (Observatoire du Pic
du Midi, France). We apply Zeeman-Doppler Imaging and reconstruct brightness
and magnetic surface topologies of the K1 primary subgiant of the system, at
five different epochs. We confirm the presence of large, axisymmetric regions
where the magnetic field is mainly azimuthal, providing further support to the
hypothesis that dynamo processes may be distributed throughout the whole
convective zone in this star. We study the short-term evolution of surface
structures from a comparison of our images with observations secured at
close-by epochs by Donati et al. (2003) at the Anglo-Australian Telescope. We
conclude that the small-scale brightness and magnetic patterns undergo major
changes within a timescale of 4 to 6 weeks, while the largest structures remain
stable over several years. We report the detection of a weak surface
differential rotation (both from brightness and magnetic tracers) indicating
that the equator rotates faster than the pole with a difference in rotation
rate between the pole and the equator about 4 times smaller than that of the
Sun. This result suggests that tidal forces also impact the global dynamic
equilibrium of convective zones in cool active stars.Comment: accepted by MNRA
Rotational modulation of the photospheric and chromospheric activity in the young, single K2-dwarf PW And
High resolution echelle spectra of PW And (HD~1405) have been taken during
eight observing runs from 1999 to 2002. The detailed analysis of the spectra
allow us to determine its spectral type (K2V), mean heliocentric radial
velocity (V_hel = -11.15 km/s) rotational velocity (vsin{i} = 22.6 km/s), and
equivalent width of the lithium line 6707.8 AA (EW(LiI) = 273 mAA). The
kinematic (Galactic Velocity (U, V, W)) confirms its membership of the Local
Association moving group, in agreement with the age (30 to 80 Myrs) inferred
from the color magnitude diagram and the lithium equivalent width. Photospheric
activity (presence of cool spots that disturb the profiles of the photospheric
lines) has been detected as changes in the the bisectors of the cross
correlation function (CCF) resulting of cross-correlate the spectra of PW And
with the spectrum of a non active star of similar spectral type. These
variations of the CCF bisectors are related to the variations in the measured
radial velocities and are modulated with a period similar to the photometric
period of the star. At the same time, chromospheric activity has been analyzed,
using the spectral subtraction technique and simultaneous spectroscopic
observations of the H_alpha, H_beta, NaI D_1 and D_2$, HeI D_3, MgI b triplet,
CaII H&K, and CaII infrared triplet lines. A flare was observed during the last
observing run of 2001, showing an enhancement in the observed chromospheric
lines. A less powerful flare was observed on 2002 August 23. The variations of
the chromospheric activity indicators seem to be related to the photospheric
activity. A correlation between radial velocity, changes in the CCF bisectors
and equivalent width of different chromospheric lines is observed with a
different behaviour between epochs 1999, 2001 and 2002.Comment: Latex file with 20 pages, 21 figures tar'ed gzip'ed. Full postscript
(text, figures and tables) available at
http://www.ucm.es/info/Astrof/users/dmg/pub_dmg.html Accepted for publication
in: Astronomy & Astrophysics (A&A
Starspots on the fastest rotators in the Beta Pic moving group
Aims: We carried out high-resolution spectroscopy and BV(I)_C photometric
monitoring of the two fastest late-type rotators in the nearby Beta Pictoris
moving group, HD199143 (F7V) and CD-641208 (K7V). The motivation for this work
is to investigate the rotation periods and photospheric spot patterns of these
very young stars, with a longer term view to probing the evolution of rotation
and magnetic activity during the early phases of main-sequence evolution. We
also aim to derive information on key physical parameters, such as rotational
velocity and rotation period. Methods: We applied maximum entropy (ME) and
Tikhonov regularizing (TR) criteria to derive the surface spot map
distributions of the optical modulation observed in HD199143 (F7 V) and
CD-641208 (K7V). We also used cross-correlation techniques to determine stellar
parameters such as radial velocities and rotational velocities. Lomb-Scargle
periodograms were used to obtain the rotational periods from differential
magnitude time series. Results: We find periods and inclinations of 0.356 days
and 21.5deg for HD199143, and 0.355 days and 50.1deg for CD-641208. The spot
maps of HD199143 obtained from the ME and TR methods are very similar, although
the latter gives a smoother distribution of the filling factor. Maps obtained
at two different epochs three weeks apart show a remarkable increase in spot
coverage amounting to ~7% of the surface of the photosphere over a time period
of only ~20 days. The spot maps of CD-641208 from the two methods show good
longitudinal agreement, whereas the latitude range of the spots is extended to
cover the whole visible hemisphere in the TR map. The distributions obtained
from the first light curve of HD199143 show the presence of an extended and
asymmetric active longitude with the maximum filling factor at longitude
~325degree.Comment: Accepted by A&A. 13 pages, 13 figures (4 online included), 5 Table
No detection of large-scale magnetic fields at the surfaces of Am and HgMn stars
We investigate the magnetic dichotomy between Ap/Bp and other A-type stars by
carrying out a deep spectropolarimetric study of Am and HgMn stars. Using the
NARVAL spectropolarimeter at the Telescope Bernard Lyot (Observatoire du Pic du
Midi, France), we obtained high-resolution circular polarisation spectroscopy
of 12 Am stars and 3 HgMn stars. Using Least Squares Deconvolution (LSD), no
magnetic field is detected in any of the 15 observed stars. Uncertaintiies as
low as 0.3 G (respectively 1 G) have been reached for surface-averaged
longitudinal magnetic field measurements for Am (respectively HgMn) stars.
Associated with the results obtained previously for Ap/Bp stars, our study
confirms the existence of a magnetic dichotomy among A-type stars. Our data
demonstrate that there is at least one order of magnitude difference in field
strength between Zeeman detected stars (Ap/Bp stars) and non Zeeman detected
stars (Am and HgMn stars). This result confirms that the
spectroscopically-defined Ap/Bp stars are the only A-type stars harbouring
detectable large-scale surface magnetic fields.Comment: 6 pages, 3 figures, accepted for publication in A&
Detailed analysis of Balmer lines in cool dwarf stars
An analysis of H alpha and H beta spectra in a sample of 30 cool dwarf and
subgiant stars is presented using MARCS model atmospheres based on the most
recent calculations of the line opacities. A detailed quantitative comparison
of the solar flux spectra with model spectra shows that Balmer line profile
shapes, and therefore the temperature structure in the line formation region,
are best represented under the mixing length theory by any combination of a low
mixing-length parameter alpha and a low convective structure parameter y. A
slightly lower effective temperature is obtained for the sun than the accepted
value, which we attribute to errors in models and line opacities. The programme
stars span temperatures from 4800 to 7100 K and include a small number of
population II stars. Effective temperatures have been derived using a
quantitative fitting method with a detailed error analysis. Our temperatures
find good agreement with those from the Infrared Flux Method (IRFM) near solar
metallicity but show differences at low metallicity where the two available
IRFM determinations themselves are in disagreement. Comparison with recent
temperature determinations using Balmer lines by Fuhrmann (1998, 2000), who
employed a different description of the wing absorption due to self-broadening,
does not show the large differences predicted by Barklem et al. (2000). In
fact, perhaps fortuitously, reasonable agreement is found near solar
metallicity, while we find significantly cooler temperatures for low
metallicity stars of around solar temperature.Comment: 17 pages, 9 figures, to appear in A&
EK Eridani: the tip of the iceberg of giants which have evolved from magnetic Ap stars
We observe the slowly-rotating, active, single giant, EK Eri, to study and
infer the nature of its magnetic field directly. We used the spectropolarimeter
NARVAL at the Telescope Bernard Lyot, Pic du Midi Observatory, and the Least
Square Deconvolution method to create high signal-to-noise ratio Stokes V
profiles. We fitted the Stokes V profiles with a model of the large-scale
magnetic field. We studied the classical activity indicators, the CaII H and K
lines, the CaII infrared triplet, and H\alpha line. We detected the Stokes V
signal of EK Eri securely and measured the longitudinal magnetic field Bl for
seven individual dates spanning 60% of the rotational period. The measured
longitudinal magnetic field of EK Eri reached about 100 G and was as strong as
fields observed in RSCVn or FK Com type stars: this was found to be
extraordinary when compared with the weak fields observed at the surfaces of
slowly-rotating MS stars or any single red giant previously observed with
NARVAL. From our modeling, we infer that the mean surface magnetic field is
about 270 G, and that the large scale magnetic field is dominated by a poloidal
component. This is compatible with expectations for the descendant of a
strongly magnetic Ap star.Comment: 8 pages, 6 figures. Accepted for publication in A&
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