76 research outputs found
Increasing the raw contrast of VLT/SPHERE with the dark-hole technique. II. On-sky wavefront correction and coherent differential imaging
Context. Direct imaging of exoplanets takes advantage of state-of-the-art
adaptive optics (AO) systems, coronagraphy, and post-processing techniques.
Coronagraphs attenuate starlight to mitigate the unfavorable flux ratio between
an exoplanet and its host star. AO systems provide diffraction-limited images
of point sources and minimize optical aberrations that would cause starlight to
leak through coronagraphs. Post-processing techniques then estimate and remove
residual stellar speckles such as noncommon path aberrations (NCPAs) and
diffraction from telescope obscurations. Aims. We aim to demonstrate an
efficient method to minimize the speckle intensity due to NCPAs during an
observing night on VLT/SPHERE. Methods. We implement an iterative dark-hole
(DH) algorithm to remove stellar speckles on-sky before a science observation.
It uses a pair-wise probing estimator and a controller based on electric field
conjugation. This work presents the first such on-sky minimization of speckles
with a DH technique on SPHERE. Results. We show the standard deviation of the
normalized intensity in the raw images is reduced by a factor of up to 5 in the
corrected region with respect to the current calibration strategy under median
conditions for VLT. This level of contrast performance obtained with only 1 min
of exposure time reaches median performances on SPHERE that use post-processing
methods requiring 1h-long sequences of observations. We also present an
alternative calibration method that takes advantage of the starlight coherence
and improves the post-processed contrast levels rms by a factor of about 3.
Conclusions. This on-sky demonstration represents a decisive milestone for the
future design, development, and observing strategy of the next generation of
ground-based exoplanet imagers for 10m to 40m telescope.Comment: Accepted in Astronomy & Astrophysic
Observing Strategies for the NICI Campaign to Directly Image Extrasolar Planets
We discuss observing strategy for the Near Infrared Coronagraphic Imager
(NICI) on the 8-m Gemini South telescope. NICI combines a number of techniques
to attenuate starlight and suppress superspeckles: 1) coronagraphic imaging, 2)
dual channel imaging for Spectral Differential Imaging (SDI) and 3) operation
in a fixed Cassegrain rotator mode for Angular Differential Imaging (ADI). NICI
will be used both in service mode and for a dedicated 50 night planet search
campaign. While all of these techniques have been used individually in large
planet-finding surveys, this is the first time ADI and SDI will be used with a
coronagraph in a large survey. Thus, novel observing strategies are necessary
to conduct a viable planet search campaign.Comment: 12 pages, 10 figures, submitted to Proceedings of the SPI
Fundamental limitations of high contrast imaging set by small sample statistics
In this paper, we review the impact of small sample statistics on detection
thresholds and corresponding confidence levels (CLs) in high contrast imaging
at small angles. When looking close to the star, the number of resolution
elements decreases rapidly towards small angles. This reduction of the number
of degrees of freedom dramatically affects CLs and false alarm probabilities.
Naively using the same ideal hypothesis and methods as for larger separations,
which are well understood and commonly assume Gaussian noise, can yield up to
one order of magnitude error in contrast estimations at fixed CL. The
statistical penalty exponentially increases towards very small inner working
angles. Even at 5-10 resolution elements from the star, false alarm
probabilities can be significantly higher than expected. Here we present a
rigorous statistical analysis which ensures robustness of the CL, but also
imposes a substantial limitation on corresponding achievable detection limits
(thus contrast) at small angles. This unavoidable fundamental statistical
effect has a significant impact on current coronagraphic and future high
contrast imagers. Finally, the paper concludes with practical recommendations
to account for small number statistics when computing the sensitivity to
companions at small angles and when exploiting the results of direct imaging
planet surveys.Comment: 12 pages, 10 figures, accepted to Ap
Early Results from VLT-SPHERE: Long-Slit Spectroscopy of 2MASS 0122-2439B, a Young Companion Near the Deuterium Burning Limit
We present 0.95-1.80 m spectroscopy of the 12-27
companion orbiting the faint (13.6), young (120 Myr) M-dwarf
2MASS J01225093--2439505 ("2M0122--2439 B") at 1.5 arcsecond separation (50
AU). Our coronagraphic long-slit spectroscopy was obtained with the new high
contrast imaging platform VLT-SPHERE during Science Verification. The unique
long-slit capability of SPHERE enables spectral resolution an order of
magnitude higher than other extreme AO exoplanet imaging instruments. With a
low mass, cool temperature, and very red colors, 2M0122-2439 B occupies a
particularly important region of the substellar color-magnitude diagram by
bridging the warm directly imaged hot planets with late-M/early-L spectral
types (e.g. Pic b and ROXs 42Bb) and the cooler, dusty objects near the
L/T transition (e.g. HR 8799bcde and 2MASS 1207b). We fit BT-Settl atmospheric
models to our 350 spectrum and find =1600100 K
and =4.50.5 dex. Visual analysis of our 2M0122-2439 B spectrum
suggests a spectral type L3-L4, and we resolve shallow -band alkali lines,
confirming its low gravity and youth. Specifically, we use the Allers & Liu
(2013) spectral indices to quantitatively measure the strength of the FeH, VO,
KI, spectral features, as well as the overall -band shape. Using these
indices, along with the visual spectral type analysis, we classify 2M0122-2439
B as an intermediate gravity (INT-G) object with spectral type L3.71.0.Comment: Accepted to ApJ Letters, 8 pages, 4 figures, some minor typographical
issues were fixe
The Gemini NICI Planet-Finding Campaign: The Offset Ring of HR 4796 A
We present J, H, CH_4 short (1.578 micron), CH_4 long (1.652 micron) and
K_s-band images of the dust ring around the 10 Myr old star HR 4796 A obtained
using the Near Infrared Coronagraphic Imager (NICI) on the Gemini-South 8.1
meter Telescope. Our images clearly show for the first time the position of the
star relative to its circumstellar ring thanks to NICI's translucent focal
plane occulting mask. We employ a Bayesian Markov Chain Monte Carlo method to
constrain the offset vector between the two. The resulting probability
distribution shows that the ring center is offset from the star by 16.7+/-1.3
milliarcseconds along a position angle of 26+/-3 degrees, along the PA of the
ring, 26.47+/-0.04 degrees. We find that the size of this offset is not large
enough to explain the brightness asymmetry of the ring. The ring is measured to
have mostly red reflectivity across the JHK_s filters, which seems to indicate
micron-sized grains. Just like Neptune's 3:2 and 2:1 mean-motion resonances
delineate the inner and outer edges of the classical Kuiper Belt, we find that
the radial extent of the HR 4796 A and Fomalhaut rings could correspond to the
3:2 and 2:1 mean-motion resonances of hypothetical planets at 54.7 AU and 97.7
AU in the two systems, respectively. A planet orbiting HR 4796 A at 54.7 AU
would have to be less massive than 1.6 Mjup so as not to widen the ring too
much by stirring.Comment: Accepted to A&A for publication on April 23, 2014 (15 pages, 9
figures, 4 tables
NICI: combining coronagraphy, ADI, and SDI
The Near-Infrared Coronagraphic Imager (NICI) is a high-contrast AO imager at
the Gemini South telescope. The camera includes a coronagraphic mask and dual
channel imaging for Spectral Differential Imaging (SDI). The instrument can
also be used in a fixed Cassegrain Rotator mode for Angular Differential
Imaging (ADI). While coronagraphy, SDI, and ADI have been applied before in
direct imaging searches for exoplanets. NICI represents the first time that
these 3 techniques can be combined. We present preliminary NICI commissioning
data using these techniques and show that combining SDI and ADI results in
significant gains.Comment: Proc. SPIE, Vol. 7014, 70141Z (2008
A Direct Imaging Survey of Spitzer detected debris disks: Occurrence of giant planets in dusty systems
We describe a joint high contrast imaging survey for planets at Keck and VLT
of the last large sample of debris disks identified by the Spitzer Space
Telescope. No new substellar companions were discovered in our survey of 30
Spitzer-selected targets. We combine our observations with data from four
published surveys to place constraints on the frequency of planets around 130
debris disk single stars, the largest sample to date. For a control sample, we
assembled contrast curves from several published surveys targeting 277 stars
which do not show infrared excesses. We assumed a double power law distribution
in mass and semi-major axis of the form f(m,a) = , where
we adopted power law values and logarithmically flat values for the mass and
semi-major axis of planets. We find that the frequency of giant planets with
masses 5-20 and separations 10-1000 AU around stars with debris
disks is 6.27% (68% confidence interval 3.68 - 9.76%), compared to 0.73% (68%
confidence interval 0.20 - 1.80%) for the control sample of stars without
disks. These distributions differ at the 88% confidence level, tentatively
suggesting distinctness of these samples.Comment: Accepted to A
The Spitzer c2d Survey of Nearby Dense Cores: III: Low Mass Star Formation in a Small Group, L1251B
We present a comprehensive study of a low-mass star-forming region,L1251B, at
wavelengths from the near-infrared to the millimeter. L1251B, where only one
protostar, IRAS 22376+7455, was known previously, is confirmed to be a small
group of protostars based on observations with the Spitzer Space Telescope. The
most luminous source of L1251B is located 5" north of the IRAS position. A
near-infrared bipolar nebula, which is not associated with the brightest object
and is located at the southeast corner of L1251B, has been detected in the IRAC
bands. OVRO and SMA interferometric observations indicate that the brightest
source and the bipolar nebula source in the IRAC bands are deeply embedded disk
sources.Submillimeter continuum observations with single-dish telescopes and
the SMA interferometric observations suggest two possible prestellar objects
with very high column densities. Outside of the small group, many young stellar
object candidates have been detected over a larger region of 12' x 12'.
Extended emission to the east of L1251B has been detected at 850 micron; this
"east core" may be a site for future star formation since no point source has
been detected with IRAC or MIPS. This region is therefore a possible example of
low-mass cluster formation, where a small group of pre- and protostellar
objects (L1251B) is currently forming, alongside a large starless core (the
east core).Comment: 35 pages, 15 figures, accepted for publication in ApJ, for the full
resolution paper, visit
"http://peggysue.as.utexas.edu/SIRTF/PAPERS/pap27.pub.pdf
Performance of the Near-infrared coronagraphic imager on Gemini-South
We present the coronagraphic and adaptive optics performance of the
Gemini-South Near-Infrared Coronagraphic Imager (NICI). NICI includes a
dual-channel imager for simultaneous spectral difference imaging, a dedicated
85-element curvature adaptive optics system, and a built-in Lyot coronagraph.
It is specifically designed to survey for and image large extra-solar gaseous
planets on the Gemini Observatory 8-meter telescope in Chile. We present the
on-sky performance of the individual subsystems along with the end-to-end
contrast curve. These are compared to our model predictions for the adaptive
optics system, the coronagraph, and the spectral difference imaging.Comment: Proc. SPIE, Vol. 7015, 70151V (2008
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