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OBSERVING EXOPLANETS WITH HIGH-DISPERSION CORONAGRAPHY.
II. DEMONSTRATION OF AN ACTIVE SINGLE-MODE FIBER INJECTION UNIT
D. Mawet
1,2
, G. Ruane
1,3
, W. Xuan
1
, D. Echeverri
1
, N. Klimovich
1
, M. Randolph
1
, J. Fucik
1
, J.K. Wallace
2
,
J. Wang
1
, G. Vasisht
2
, R. Dekany
1
, B. Mennesson
2
, E. Choquet
2
, J.-R. Delorme
1
and E. Serabyn
2
Mawet et al. 2016, in preparation
ABSTRACT
High-dispersion coronagraphy (HDC) optimally combines high contrast imaging techniques such as
adaptive optics/wavefront control plus coronagraphy to high spectral resolution spectroscopy. HDC is
a critical pathway towards fully characterizing exoplanet atmospheres across a broad range of masses
from giant gaseous planets down to Earth-like planets. In addition to determining the molecular
composition of exoplanet atmospheres, HDC also enables Doppler mapping of atmosphere inhomo-
geneities (temperature, clouds, wind), as well as precise measurements of exoplanet rotational veloci-
ties. Here, we demonstrate an innovative concept for injecting the directly-imaged planet light into a
single-mode fiber, linking a high-contrast adaptively-corrected coronagraph to a high-resolution spec-
trograph (diffraction-limited or not). Our laboratory demonstration includes three key milestones:
close-to-theoretical injection efficiency, accurate pointing and tracking, on-fiber coherent modulation
and speckle nulling of spurious starlight signal coupling into the fiber. Using the extreme modal
selectivity of single-mode fibers, we also demonstrated speckle suppression gains that outperform
conventional image-based speckle nulling by at least two orders of magnitude.
Subject headings:
stars: brown dwarfs, stars: low-mass, stars: imaging, instrumentation: adaptive
optics, instrumentation: high angular resolution, instrumentation: spectrographs,
techniques: high angular resolution, techniques: spectroscopic
1.
INTRODUCTION
At the crossroads between planetary science and as-
tronomy, the field of exoplanet studies is undergoing
unprecedented growth. Aided by numerous dedicated
ground-based and space-based facilities and instruments,
thousands of new worlds have been discovered over the
past two decades. The vast majority of detections so far
have been through indirect measurements that take ad-
vantage of the gravitational influence of planets on their
host star, that of other stars on the space-time contin-
uum, or simply the photometric dimming of starlight as
the planet eclipses our line of sight. The techniques ex-
ploiting these effects, namely Doppler radial velocime-
try, micro-lensing, and transit photometry, are now rou-
tinely employed for exoplanet detection and have ushered
in a new era in planetary science called exoplanetology.
Exoplanetology has put the Solar System into a univer-
sal perspective, and finally provides an opportunity to
understand planet formation and evolution in statistical
terms.
Direct detection has eluded the exoplanet community
for many years, mainly due to the stark difficulty as-
sociated with disentangling the signal of an exoplanet
from its host star. The requirements to directly image
a planet stretch the limits of current facilities and in-
struments in all possible directions: angular resolution,
sensitivity, dynamic range, precision, and stability. The
advent of large ground-based and space-based telescopes,
dmawet@astro.caltech.edu
1
Department of Astronomy, California Institute of Technol-
ogy, 1200 E. California Blvd, MC 249-17, Pasadena, CA 91125
2
Jet Propulsion Laboratory, California Institute of Technol-
ogy, 4800 Oak Grove Drive, Pasadena, CA 91109
3
NSF Astronomy and Astrophysics Postdoctoral Fellow
adaptive optics, new infrared and optical detector tech-
nologies, and modern computing have admittedly done
little to overcome these challenges. A niche technology
borrowed from solar astronomy, namely coronagraphy,
once held the promise of revolutionizing the field, but
the long-awaited breakthrough is slow to unfold.
Coronagraphy was invented in the 1930s by French as-
tronomer Bernard Lyot (Lyot 1939) to observe and char-
acterize the solar corona without the need for natural
eclipses. The principle of coronagraphy is simple and
aims, by way of a device blocking the glare of the Sun,
at reducing the contrast of the scene to be within the dy-
namic range of the detectors. Coronagraphs now come
as standard equipment on any high-contrast imaging in-
strument, paired with wavefront control systems (adap-
tive optics), including deformable mirrors controlled in
closed loop via a series of dedicated wavefront sensors.
Downstream from the high-contrast equipment are clas-
sical imaging cameras, and/or low spectral resolution in-
tegral field spectrographs (IFS).
A key strategy to differentiate between planets and
leftover speckles of residual starlight is to modulate
the planet signal against the background of dynamic
and quasi-static speckles.
Many differential imag-
ing techniques have been devised to mitigate speckle
noise, such as: angular differential imaging (ADI), spec-
tral/simultaneous differential imaging (SDI), dual-band
differential imaging (DBI), reference star differential
imaging (RDI), polarization differential imaging (PDI),
coherent differential imaging (CDI), orbital differential
imaging (ODI) and binary differential imaging (BDI).
ADI and SDI are by far the most successful, but present
significant challenges at very small inner working an-
gles, owing to signal self-subtraction effects (Mawet et al.
arXiv:1703.00583v1 [astro-ph.EP] 2 Mar 2017
2
Mawet et al.
2012).
Here we propose and demonstrate a new concept
that optimally combines high-contrast imaging tech-
niques and high-resolution spectroscopy, called for the
sake of simplicity high-dispersion coronagraphy (HDC).
The promise of HDC is the cumulative gain in the per-
formance offered by each technique, as first suggested
by Riaud & Schneider (2007) and more recently refined
by Snellen et al. (2015). The reason being that high-
resolution spectroscopy sidesteps the problem of speckle
noise, since speckle noise has a low spectral resolution
signature (Krist et al. 2008) and is effectively part of the
continuum at high spectral resolution. Moreover, the
planet signal will be shifted in frequency (velocity) space
with respect to the star signal due to the Doppler effect
induced by the orbital motion of the planet around its
host star, enabling spectral lines to be disentangled from
one another. Thus, HDC is perhaps the only differential
method that will approach the photon noise limit.
In this paper, we present a new concept for feed-
ing a filtered beam of planet light to a high-resolution
spectrograph (Fig. 1). The framework of this proof-of-
concept is the Keck Planet Imager and Characterizer
project (KPIC), a planned upgrade to the W.M. Keck
Observatory adaptive optics system and high-contrast
instrument suite (Mawet et al. 2016). KPIC will serve
as a pathfinder for future high-contrast spectroscopic
instruments for large ground- and space-based facili-
ties: the Thirty Meter Telescope (TMT), the European-
Extremely Large Telescope (E-ELT), the Giant Mag-
ellan Telescope (GMT), NASA’s Habitability Explorer
(HabEx), and the Large UV Optical InfraRed (LUVOIR)
telescopes.
2.
HIGH-CONTRAST HIGH-RESOLUTION
SPECTROSCOPY OF EXOPLANETS
Now that thousands of exoplanets have been discov-
ered, detailed characterization of these planets is the log-
ical next step. The leading detection methods based on
radial velocities (RV) and transits provide only the mass
and/or size of the planet. With these measurements,
bulk density and chemical composition may be inferred
with exoplanet internal structure models. However, this
approach suffers from degeneracies, highlighting the need
for directly measuring their chemical compositions.
Detailed diagnoses of the chemical composition of
exoplanet atmospheres (see e.g. Barman et al. 2011;
Konopacky et al. 2013; Barman et al. 2015) remain a
challenge because of the small angular separation and
high contrast between exoplanets and their host stars.
Both constraints are mitigated by a high-contrast imag-
ing system, which usually consists of an extreme adaptive
optics (AO) system and a coronagraph. Current state-of-
the-art high-contrast imaging systems such as the Gemini
Planet Imager at the Gemini South telescope (Macintosh
et al. 2015) and SPHERE at the Very Large Telescope
(Beuzit et al. 2008) are able to achieve 10
3
to 10
4
raw
starlight suppression levels at a few tenths of an arcsec-
ond, allowing detections and very low-resolution spec-
troscopy (spectral resolution R
'
50) of gas giant planets
and brown dwarfs orbiting nearby young stars.
Riaud & Schneider (2007) and Snellen et al. (2015)
suggested that contrast sensitivity may be further im-
proved by coupling a high-dispersion spectrograph with
a high-contrast imaging system. In this scheme, the high-
contrast imaging system serves as a spatial filter to sep-
arate the light from the star and the planet, and the
high-dispersion spectrograph serves as a spectral filter
taking advantage of differences between the stellar and
planetary spectra, including absorption lines and radial
velocities (see Fig. 1).
Using high-dispersion spectroscopy as a way to spec-
trally isolate the planet signal has been successfully
demonstrated by a number of integrated light studies. In-
deed, high-resolution transmission spectroscopy has been
used to detect molecular gas in the atmosphere of tran-
siting planets (Snellen et al. 2010; Birkby et al. 2013;
de Kok et al. 2013). At a high spectral resolution, re-
solved molecular lines may be used to study day-to-night
side wind velocity (Snellen et al. 2010) and verify 3D ex-
oplanet atmospheric circulation models (Kempton et al.
2014). The spectral lines of a planet may also be sepa-
rated from stellar lines with sufficient differences in radial
velocities (
>
50 km/s), breaking the degeneracy between
the true planet mass and orbital inclination (Brogi et al.
2012, 2013, 2014; Lockwood et al. 2014). Moreover, high-
resolution spectroscopy has led to the first measurement
of planet rotational velocity (Snellen et al. 2014). While
not yet feasible on exoplanets yet, high-resolution spec-
troscopy has helped generate the first global cloud map
of brown dwarf Luhman 16 B via the Doppler imaging
technique (Crossfield et al. 2014).
High-resolution spectroscopy is poised to become even
more powerful when combined with high-contrast imag-
ing. The signal-to-noise ratio (SNR) achieved by an HDC
instrument is to first order (Snellen et al. 2015):
S/N
=
ηS
planet
S
star
/K
+
σ
2
bg
+
σ
2
rn
+
σ
2
dark
N
lines
,
(1)
where
S
planet
is the planet signal making it to the spec-
trograph with efficiency
η
,
S
star
is the signal from the
star (both in units of photo-electrons per pixel),
K
is the
suppression factor of the star at the planet’s position,
and
σ
2
bg
,
σ
2
rn
, and
σ
2
dark
are the photon shot noise from
the sky and telescope background, the readout noise, and
the dark current noise, respectively.
N
lines
is a multi-
plication factor that takes into account the number and
strength of the individual planet lines targeted, which
is a defining strength of high-resolution spectroscopy
(Snellen et al. 2015).
The planet/star contrast sensitivity achieved by
integrated-light high-dispersion spectroscopy is currently
demonstrated at the 10
4
level, which corresponds to the
stellar photon noise limit (Snellen et al. 2015). When
coupled with a state-of-the-art high-contrast imaging
system with a raw starlight suppression of 10
3
or bet-
ter, a high-contrast high-dispersion spectroscopy instru-
ment can potentially exceed 10
7
planet/star contrast,
providing superior sensitivity than a high-contrast imag-
ing system or a high-dispersion spectrograph alone. This
would allow the physical and chemical processes taking
place on an exoplanet to be studied in unprecedented
details.
It is important to note that high-spectral-resolution
observations of a single spatial resolution element render
spatial speckle variations (spatial speckle noise) irrele-