Context.
The detection and characterization of Earth-like exoplanets is one of the major science drivers for the next generation of telescopes. Direct imaging of the planets will play a major role in ...observations. Current direct imaging instruments are limited by evolving non-common path aberrations (NCPAs). The NCPAs have to be compensated for by using the science focal-plane image. A promising sensor is the self-coherent camera (SCC). An SCC adds a pinhole to the Lyot stop in the coronagraph to introduce a probe electric field. The pinhole has to be separated by at least 1.5 times the pupil size to separate the NCPA speckles from the probe electric field. However, such a distance lets through very little light, which makes it difficult to use an SCC at high speed or on faint targets.
Aims.
A spectrally modulated self-coherent camera (SM-SCC) is proposed as a solution to the throughput problem. The SM-SCC uses a pinhole with a spectral filter and a dichroic beam splitter, which creates images with and without the probe electric field. This allows the pinhole to be placed closer to the pupil edge and increases the throughput. Combining the SM-SCC with an integral field unit (IFU) can be used to apply more complex modulation patterns to the pinhole and the Lyot stop. A modulation scheme with at least three spectral channels can be used to change the pinhole to an arbitrary aperture with higher throughput. This adds an additional degree of freedom in the design of the SM-SCC.
Methods.
The performance of the SM-SCC is investigated analytically and through numerical simulations.
Results.
Numerical simulations show that the SM-SCC increases the pinhole throughput by a factor of 32, which increases the wavefront sensor sensitivity by a factor of 5.7. The reconstruction quality of the sensor is tested by varying the central wavelength of the spectral channels. A smaller separation between the wavelength channels leads to better results. The SM-SCC reaches a contrast of 1 × 10
−9
for bright targets in closed-loop control with the presence of photon noise, phase errors, and amplitude errors. The contrast floor on fainter targets is photon-noise-limited and reaches 1 × 10
−7
. The SM-SCC with an IFU can handle randomly generated reference field apertures. For bright targets, the SM-SCC-IFU reaches a contrast of 3 × 10
−9
in closed-loop control with photon noise, amplitude errors, and phase errors.
Conclusions.
The SM-SCC is a promising focal-plane wavefront sensor for systems that use multiband observations, either through integral field spectroscopy or dual-band imaging.
Fourier-based wavefront sensors, such as the Pyramid Wavefront Sensor (PWFS), are the current preference for high contrast imaging due to their high sensitivity. However, these wavefront sensors have ...intrinsic nonlinearities that constrain the range where conventional linear reconstruction methods can be used to accurately estimate the incoming wavefront aberrations. We propose to use Convolutional Neural Networks (CNNs) for the nonlinear reconstruction of the wavefront sensor measurements. It is demonstrated that a CNN can be used to accurately reconstruct the nonlinearities in both simulations and a lab implementation. We show that solely using a CNN for the reconstruction leads to suboptimal closed loop performance under simulated atmospheric turbulence. However, it is demonstrated that using a CNN to estimate the nonlinear error term on top of a linear model results in an improved effective dynamic range of a simulated adaptive optics system. The larger effective dynamic range results in a higher Strehl ratio under conditions where the nonlinear error is relevant. This will allow the current and future generation of large astronomical telescopes to work in a wider range of atmospheric conditions and therefore reduce costly downtime of such facilities.
Current wavefront sensors for high resolution imaging have either a large dynamic range or a high sensitivity. A new kind of wavefront sensor is developed which can have both: the Generalised Optical ...Differentiation wavefront sensor. This new wavefront sensor is based on the principles of optical differentiation by amplitude filters. We have extended the theory behind linear optical differentiation and generalised it to nonlinear filters. We used numerical simulations and laboratory experiments to investigate the properties of the generalised wavefront sensor. With this we created a new filter that can decouple the dynamic range from the sensitivity. These properties make it suitable for adaptive optic systems where a large range of phase aberrations have to be measured with high precision.
Context. Telescopes like the Extremely Large Telescope (ELT) and the Giant Magellan Telescope (GMT) will be used together with extreme adaptive optics (AO) instruments to directly image Earth-like ...planets. The AO systems will need to perform at the fundamental limit in order to image Earth twins. A crucial component is the wavefront sensor. Interferometric wavefront sensors, such as the Zernike wavefront sensor (ZWFS), have been shown to perform close to the fundamental sensitivity limit. However, sensitivity comes at the cost of linearity; the ZWFS has strong nonlinear behavior. Aims. The aim of this work is to increase the dynamic range of Zernike-like wavefront sensors by using nonlinear reconstruction algorithms combined with phase sorting interferometry (PSI) and multi-wavelength measurements. Methods. The response of the ZWFS is explored analytically and numerically. Results. The proposed iterative (non)linear reconstructors reach the machine precision for small aberrations (<0.25 rad rms). Coupling the nonlinear reconstruction algorithm with PSI increases the dynamic range of the ZWFS by a factor of three to about 0.75 rad rms. Adding multiple wavebands doubles the dynamic range again, to 1.4 radians rms. Conclusions. The ZWFS is one of the most sensitive wavefront sensors, but has a limited dynamic range. The ZWFS will be an ideal second-stage wavefront sensor if it is combined with the proposed nonlinear reconstruction algorithm.
Context.
Accreting planetary-mass objects have been detected at H
α
, but targeted searches have mainly resulted in non-detections. Accretion tracers in the planetary-mass regime could originate from ...the shock itself, making them particularly susceptible to extinction by the accreting material. High-resolution (
R
> 50 000) spectrographs operating at H
α
should soon enable one to study how the incoming material shapes the line profile.
Aims.
We calculate how much the gas and dust accreting onto a planet reduce the H
α
flux from the shock at the planetary surface and how they affect the line shape. We also study the absorption-modified relationship between the H
α
luminosity and accretion rate.
Methods.
We computed the high-resolution radiative transfer of the H
α
line using a one-dimensional velocity–density–temperature structure for the inflowing matter in three representative accretion geometries: spherical symmetry, polar inflow, and magnetospheric accretion. For each, we explored the wide relevant ranges of the accretion rate and planet mass. We used detailed gas opacities and carefully estimated possible dust opacities.
Results.
At accretion rates of
Ṁ
≲ 3 × 10
−6
M
J
yr
−1
, gas extinction is negligible for spherical or polar inflow and at most
A
H
α
≲ 0.5 mag for magnetospheric accretion. Up to
Ṁ
≈ 3 × 10
−4
M
J
yr
−1
, the gas contributes
A
H
α
≲ 4 mag. This contribution decreases with mass. We estimate realistic dust opacities at H
α
to be
κ
~ 0.01–10 cm
2
g
−1
, which is 10–10
4
times lower than in the interstellar medium. Extinction flattens the
L
H
α
–
Ṁ
relationship, which becomes non-monotonic with a maximum luminosity
L
H
α
~ 10
−4
L
⊙
towards
Ṁ
≈ 10
−4
M
J
yr
−1
for a planet mass ~10
M
J
. In magnetospheric accretion, the gas can introduce features in the line profile, while the velocity gradient smears them out in other geometries.
Conclusions.
For a wide part of parameter space, extinction by the accreting matter should be negligible, simplifying the interpretation of observations, especially for planets in gaps. At high
Ṁ
, strong absorption reduces the H
α
flux, and some measurements can be interpreted as two
Ṁ
values. Highly resolved line profiles (
R
~ 10
5
) can provide (complex) constraints on the thermal and dynamical structure of the accretion flow.
Context.
Jets and outflows are thought to play important roles in regulating star formation and disk evolution. An important question is how the jets are launched. HD 163296 is a well-studied Herbig ...Ae star that hosts proto-planet candidates, a protoplanetary disk, a protostellar jet, and a molecular outflow, which makes it an excellent laboratory for studying jets.
Aims.
We aim to characterize the jet at the inner regions and check if there are large differences with the features at large separations. A secondary objective is to demonstrate the performance of Multi Unit Spectroscopic Explorer (MUSE) in high-contrast imaging of extended line emission.
Methods.
MUSE in the narrow field mode (NFM) can provide observations at optical wavelengths with high spatial (∼75 mas) and medium spectral (
R
∼ 2500) resolution. With the high-resolution spectral differential imaging technique, we can characterize the kinematic structures and physical conditions of jets down to 100 mas.
Results.
We detect multiple atomic lines in two new knots, B3 and A4, at distances of < 4″ from the host star with MUSE. The derived
Ṁ
jet
/
Ṁ
acc
is about 0.08 and 0.06 for knots B3 and A4, respectively. The observed Ca II/S II ratios indicate that there is no sign of dust grains at distances of < 4″. Assuming the A4 knot traced the streamline, we can estimate a jet radius at the origin by fitting the half width half maximum of the jet, which sets an upper limit of 2.2 au on the size of the launching region. Although MUSE has the ability to detect the velocity shifts caused by high- and low-velocity components, we found no significant evidence of velocity decrease transverse to the jet direction in our 500 s MUSE observation.
Conclusions.
Our work demonstrates the capability of using MUSE NFM observations for the detailed study of stellar jets in the optical down to 100 mas. The derived
Ṁ
jet
/
Ṁ
acc
, no dust grain, and jet radius at the star support the magneto-centrifugal models as a launching mechanism for the jet.
Context.
Protoplanetary disks contain structures such as gaps, rings, and spirals, which are thought to be produced by the interaction between the disk and embedded protoplanets. However, only a few ...planet candidates are found orbiting within protoplanetary disks, and most of them are being challenged as having been confused with disk features.
Aims.
The VLT/MUSE discovery of PDS 70 c demonstrated a powerful way of searching for still-forming protoplanets by targeting accretion signatures with medium-resolution integral field spectroscopy. We aim to discover more proto-planetary candidates with MUSE, with a secondary aim of improving the high-resolution spectral differential imaging (HRSDI) technique by analyzing the instrumental residuals of MUSE.
Methods.
We analyzed MUSE observations of five young stars with various apparent brightnesses and spectral types. We applied the HRSDI technique to perform high-contrast imaging. The detection limits were estimated using fake planet injections.
Results.
With a 30 min integration time, MUSE can reach 5
σ
detection limits in apparent H
α
line flux down to 10
−14
and 10
−15
erg s
−1
cm
−2
at 0.075′′ and 0.25′′, respectively. In addition to PDS 70 b and c, we did not detect any clear accretion signatures in PDS 70, J1850-3147, and V1094 Sco down to 0.1′′. MUSE avoids the small sample statistics problem by measuring the noise characteristics in the spatial direction at multiple wavelengths. We detected two asymmetric atomic jets in HD 163296 with a very high spatial resolution (down to 8 au) and medium spectral resolution (
R
~ 2500).
Conclusions.
The HRSDI technique when applied to MUSE data allows us to reach the photon noise limit at small separations (i.e., <0.5′′). With the combination of high-contrast imaging and medium spectral resolution, MUSE can achieve fainter detection limits in apparent line flux than SPHERE/ZIMPOL by a factor of ~5. MUSE has some instrumental issues that limit the contrast that appear in cases with strong point sources, which can be either a spatial point source due to high Strehl observations or a spectral point source due to a high line-to-continuum ratio. We modified the HRSDI technique to better handle the instrumental artifacts and improve the detection limits. To avoid the instrumental effects altogether, we suggest faint young stars with relatively low H
α
line-to-continuum ratio to be the most suitable targets for MUSE to search for potential protoplanets.
Detecting and monitoring gas species is an important part of remote sensing because the state of the environment can be retrieved from the state of the gas species. This can be used to track ...temperature and pressure structures in the atmosphere for weather predictions, or monitor the air quality. Discriminating different species is easier at higher spectral resolution when the spectral lines are clearly resolved. The need to do this at high spatial resolution and over large fields of view leads to a trade-off between spectral and spatial resolution and spectral bandwidth. We propose to use a highly multiplexed Bragg grating that can optically combine the relevant information from the spectrum without the need to disperse the whole spectrum. This allows us to circumvent the spatial and spectral trade-off and therefore substantially increase the field of view compared to conventional hyperspectral imagers. A dynamic implementation based on acousto-optical filters that can be adapted on the fly is discussed as an easy and flexible way to create the multiplexed gratings. We describe the details of multiplexed Bragg gratings and show that we can retrieve the spatial distribution of individual species abundances in gas mixtures, and we show that we can even do this for the atmospheres of exoplanets orbiting far-away stars.
PDF
