of 6
Astronomy
&
Astrophysics
A&A 670, A162 (2023)
https://doi.org/10.1051/0004-6361/202244485
© The Authors 2023
Planet search with the Keck/NIRC2 vortex coronagraph in the
M
s
band for Vega
Bin B. Ren (
)
1
,
2
,
3
, Nicole L. Wallack
4
,
5
, Spencer A. Hurt
6
, Dimitri Mawet
3
,
7
, Aarynn L. Carter
8
,
Daniel Echeverri
3
, Jorge Llop-Sayson
3
, Tiffany Meshkat
9
, Rebecca Oppenheimer
10
, Jonathan Aguilar
11
,
Eric Cady
7
, Élodie Choquet
12
, Garreth Ruane
7
, Gautam Vasisht
7
, and Marie Ygouf
7
1
Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, 06304 Nice, France
e-mail:
bin.ren@oca.eu
2
Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
3
Department of Astronomy, California Institute of Technology, MC 249-17, 1200 East California Boulevard, Pasadena CA 91125,
USA
4
Earth and Planets Laboratory, Carnegie Institution for Science, Washington DC 20015, USA
5
Division of Geological & Planetary Sciences, California Institute of Technology, MC 150-21, 1200 East California Boulevard,
Pasadena CA 91125, USA
6
Department of Earth Sciences, University of Oregon, Eugene OR 97403, USA
7
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena CA 91109, USA
8
Department of Astronomy & Astrophysics, University of California, Santa Cruz, 1156 High St, Santa Cruz CA 95064, USA
9
Infrared Processing and Analysis Center (IPAC), California Institute of Technology, MC 100-22, 1200 East California Boulevard,
Pasadena CA 91125, USA
10
American Museum of Natural History, New York NY 12345, USA
11
Space Telescope Science Institute (STScI), 3700 San Martin Drive, Baltimore MD 21218, USA
12
Aix-Marseille Univ., CNRS, CNES, LAM, Marseille, France
Received 12 July 2022 / Accepted 18 January 2023
ABSTRACT
Context.
Gaps in circumstellar disks can signal the existence of planetary perturbers, making such systems preferred targets for direct
imaging observations of exoplanets.
Aims.
Being one of the brightest and closest stars to the Sun, the photometric standard star Vega hosts a two-belt debris disk structure.
Together with the fact that its planetary system is being viewed nearly face-on, Vega has been one of the prime targets for planet
imaging efforts.
Methods.
Using the vector vortex coronagraph on Keck/NIRC2 in the
M
s
band at
4
.
67
μ
m, we report the planet detection limits from
1
au to
22
au for Vega with an on-target time of
1
.
8
h.
Results.
We reach a
3
M
Jupiter
limit outward of
12
au, which is nearly an order of magnitude deeper than for other existing studies.
Combining our observations with existing radial velocity studies, we can confidently rule out the existence of companions more than
8
M
Jupiter
from
22
au down to
0
.
1
au for Vega. Interior and exterior to
4
au, this combined approach reaches planet detection limits
down to
2
3
M
Jupiter
using radial velocity and direct imaging, respectively.
Conclusions.
By reaching multi-Jupiter mass detection limits, our results are expected to be complemented by the planet imaging of
Vega in the upcoming observations using the
James Webb
Space Telescope to obtain a more holistic understanding of the planetary
system configuration around Vega.
Key words.
planets and satellites: detection – techniques: high angular resolution – techniques: image processing –
planets and satellites: individual: Vega – stars: imaging
1. Introduction
Vega (
α
Lyrae,
Keoe
,
w
̄
aqi’
,
Zhinü
), one of the his-
torical photometric standard stars (Johnson & Morgan 1953),
is an A0 V star (e.g., Johnson & Morgan 1953) that is located
at
7
.
68
±
0
.
02
pc from the Solar System (van Leeuwen 2007).
Despite its proximity, Vega is not included in the
Gaia
Catalog
of Nearby Stars that are within
100
pc from the Sun, since its
brightness exceeds the
Gaia
limits (Gaia Collaboration 2021).
With an age of
445
±
13
Myr (Yoon et al. 2010), Vega shows
a prototypical mid-infrared excess in the Infrared Astronomical
Contrast curve is only available at the CDS via anonymous
ftp to
cdsarc.cds.unistra.fr
(
130.79.128.5
) or via
https://
cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/670/A162
Satellite observations, situating it in an evolutionary stage that
is between star formation and our Solar System (Aumann et al.
1984).
The two-belt debris disk system around Vega may result
from planet-disk interactions under various planetary configu-
rations (e.g., Matrà et al. 2020). In fact, most existing directly
imaged planets are found in bright debris disk systems (e.g.,
HR 8799: Marois et al. 2008,
β
Pic: Lagrange et al. 2009,
51 Eri: Macintosh et al. 2015), making it more likely to find
giant planets in debris disk systems than around stars without
disks (Meshkat et al. 2017). Combined with the observational
fact that giant planets are more likely to exist at
1
10
au from
their host stars in both direct imaging and radial velocity sur-
veys (Nielsen et al. 2019; Fulton et al. 2021), the proximity
A162, page 1 of 6
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A&A 670, A162 (2023)
of Vega makes it one of the best systems for giant planet
searches.
Before the resolved imaging of debris belts around Vega, dust
structures around Vega have suggested the existence of poten-
tial planetary perturbers (e.g., Holland et al. 1998; Wilner et al.
2002) and called for the deep imaging of them. Combining
the Gemini Altar adaptive optics (AO) system and the Gemini
Near-infrared Imager (NIRI), Marois et al. (2006) obtained
5
σ
detection limits of better than 18 Mag at
3
′′
10
′′
in the off-
methane
1
.
58
μ
m
6
.
5%
filter, or
3
M
Jupiter
at
8
′′
. Using the
MMT Observatory AO and the Clio camera, Hinz et al. (2006)
and Heinze et al. (2008) obtained
10
σ
limits of better than
12
Mag at
2
′′
11
′′
at the
M
band, or
10
M
Jupiter
.
Recent observational studies on the Vega planetary sys-
tem are in direct imaging, transiting, radial velocity, and
(sub)millimeter interferometric imaging. In direct imaging,
Meshkat et al. (2018) presented observations from the coro-
nagraphic integral field spectrograph P1640 at the Palomar
Observatory in
J
and
H
bands. Despite a nondetection of plan-
ets, they obtained
5
σ
planet detection limits from
0
.
′′
25
to
2
′′
,
reaching a best sensitivity of
20
M
Jupiter
at
1
.
′′
5
or
12
au. From
a complimentary approach, using the Atacama Large Millime-
ter/submillimeter Array (ALMA), Matrà et al. (2020) observed
and modeled the outer belt from
60
au to
200
au. To explain
the observed disk architecture, Matrà et al. (2020) discussed
three mechanisms which include a single giant planet, multi-
ple low-mass planets, and no outer planets. Combining a decade
of Tillinghast Refector Echelle Spectrograph (TRES) spectra for
radial velocity and two sectors of Transiting Exoplanet Survey
Satellite (TESS) photometry, and under the scenario that the
planetary orbits have inclinations between
1
.
5
and
11
.
5
, Hurt
et al. (2021) obtained a nondetection of
1
10
M
Jupiter
planets
within
1
10
au, while reporting a candidate Jovian signal with a
period of
2
.
43
days in radial velocity measurements. Neverthe-
less, depending on the orbit orientations of the planets, the mass
limit can vary by up to a factor of
10
in Fig. 6 of Hurt et al.
(2021).
To improve existing high-contrast imaging limits, test differ-
ent mechanisms for the formation of the Vega debris disk system
observed in ALMA, and explore beyond the nearly edge-on limi-
tations from radial velocity and transit studies, we observed Vega
in the
M
s
band using the vortex coronagraph on Keck/NIRC2. In
the
M
s
band, planets have relatively larger planet-to-star bright-
ness ratios than in the
J
or
H
band (e.g., Spiegel & Burrows
2012; Skemer et al. 2014), and the advantage of the
M
s
band
over shorter-wavelength bands improves the intermediate system
ages for a planet of a given mass (e.g., Currie et al. 2022), both
enabling us to explore around Vega for possible cooler and less
massive planets than existing studies.
2. Observation and data reduction
We observed Vega using the Keck/NIRC2 vortex coronagraph
in the
M
s
band during two individual nights using the narrow
camera with a pixel size of
9
.
942
mas (e.g., Service et al. 2016;
Mawet et al. 2019). The first observation was on UT 2018 August
30 under program C314 (PI: D. Mawet), with the total integra-
tion time being
1991
s (=0.181 s
×
100 coadds
×
110
frames),
and the parallactic angle change being
74
.
9
. The second obser-
vation was on UT 2019 August 20 under program N097 (PI:
T. Meshkat), with the total integration time being
4500
s
(=0.25 s
×
150
coadds
×
120
frames), and the parallactic angle
change being
81
.
6
. We list the observation details in Table 1.
Table 1.
Keck/NIRC2 vortex coronagraph observation log.
Target
Vega
filter
M
s
band
UT date
2018 Aug. 30
2019 Aug. 20
UT start
05:15:26.35
06:35:47.62
UT end
07:15:00.52
10:12:23.95
Airmass
(
a
)
1
.
066
±
0
.
009 1
.
159
±
0
.
101
DIMM seeing
(
a
,
b
)
0
.
′′
42
±
0
.
′′
11 1
.
′′
18
±
0
.
′′
55
MASS seeing
(
a
,
b
)
0
.
′′
16
±
0
.
′′
06 0
.
′′
18
±
0
.
′′
08
Precipitable water vapor level
7
mm
2
mm
Parallactic angle change
74
.
9
81
.
6
Single integration time
0
.
181
s
0
.
25
s
Coadd for single frame
100
150
Total frame count
110
120
On-target integration time
1991
s
4500
s
Total on-target time
1
.
803
h
Pixel scale
9
.
942
mas
Notes.
(
a
)
The uncertainties in this paper are
1
σ
unless otherwise spec-
ified.
(
b
)
Calculated during this observation using
http://mkwc.ifa.
hawaii.edu/current/seeing/index.cgi
.
Faint planetary signals are often overwhelmed by the stellar
point spread function (PSF) of the central star. In order to remove
the stellar PSF and reveal faint surrounding signals, we first pre-
processed the data using the
VIP
package (Gomez Gonzalez
et al. 2017), which was further customized for NIRC2 vortex
observations by performing flat-fielding, bad pixel and back-
ground removal, and image centering (Xuan et al. 2018). We then
reduced the preprocessed data using the Karhunen–Loève image
projection (KLIP) algorithm (Soummer et al. 2012; Amara &
Quanz 2012), which performs principal component analysis to
capture the stellar PSF and speckles in the observation. For an
image, we removed its stellar PSF and speckles by first projecting
it to the KLIP components, and then subtracting the projection
from the original image to obtain the residuals. Astrophysical
signals including planets and disks thus reside in these residu-
als. To obtain the final image for each observation, we obtained
the residual maps for each individual readout, then rotated and
median combined them. We present the combined two-epoch
result using
10%
of the KLIP components with angular differen-
tial imaging (ADI: Marois et al. 2006) in Fig. 1 for demonstration
purposes, in which we did not identify point sources that are
beyond
5
σ
of the noise that is within the same angular separation
from the stars.
Using
VIP
and taking into account self-subtraction and over-
fitting with ADI while using KLIP, we generated the
5
σ
contrast
limits for each observation following Xuan et al. (2018) by
varying the reduction parameters to obtain the corresponding
detection limits while performing corrections for small sample
statistics in Mawet et al. (2014). Specifically,
VIP
performs injec-
tion recovery for companions at different locations to measure
the throughput from ADI and KLIP (Gomez Gonzalez et al.
2017). While measuring our throughput, we injected companions
along three radial branches spread throughout the image (origi-
nating from the masked center) where the averaged throughput
at each radial location in the image was determined from these
multiple estimates of the throughput at different branches (Xuan
et al. 2018). We computed the contrast for the entire image for
each combination of inner and outer mask size and number of
A162, page 2 of 6
B. B. Ren et al.: Vega in
M
-band
-15 au
-10 au
-5 au
0 au
5 au
10 au
15 au
-2
′′
-1
′′
.5
-1
′′
-0
′′
.5
0
′′
0
′′
.5
1
′′
1
′′
.5
2
′′
RA
-2
′′
-1
′′
.5
-1
′′
-0
′′
.5
0
′′
0
′′
.5
1
′′
1
′′
.5
2
′′
Decl.
+
1
′′
5 au
N
E
10
6
10
5
10
4
10
3
10
2
contrast
Fig. 1.
Combined two-epoch NIRC2 image of Vega in the
M
s
band
using ADI with
10%
of the KLIP components for demonstration pur-
poses. We did not identify point sources that were more than
5
σ
levels
beyond the noise of a similar angular separation from the star. The pixel
values correspond to lower limits of contrast values due to over- and
self-subtraction with KLIP and ADI, respectively.
principal components, where we computed up to 30 principal
components. The ADI reduction was performed with no rotation
gap; for one combination of inner and outer radii from Table 1
of Xuan et al. (2018), the full-frame reduction was performed on
the annulus zone with the region inward of the outer radius or
outward of the inner radius included. For example, for an outer
radius of
0
.
′′
5
and an inner radius of
0
.
′′
08
, all pixels with radial
separations between
8
pixels and
50
pixels from the center of the
image, or between
0
.
′′
08
and
0
.
′′
5
, were included in the reduction
(see the reduction details in Xuan et al. 2018). While we did not
use annular ADI, we did utilize the best contrast achieved from
our full-frame ADI at each one-pixel annulus.
To obtain the final detection limit for each angular sepa-
ration from the star with a step size of 1 pixel, we compared
the detection limits from different combinations of reduction
parameters. The reduction parameters include the frame size
(i.e., algorithmic inner and outer radii) and the number of prin-
cipal components (see Sect. 2 and Table 1 of Xuan et al. (2018)
for details on computing most optimal contrasts using five dif-
ferent frame size combinations). Therefore, while our optimal
contrast is the combination of frames processed using different
parameters, we were optimizing at each radial location. For our
observations, the number of principal components adopted for
the final contrast curve ranges from 8 to 29, with a median of 15
and a standard deviation of 7.4.
The faintest companion that can be detected from such com-
binations at
5
σ
level was adopted as our final detection limit.
For the 2019 data, the exposures that were used to image the
unblocked central source were saturated, we thus fit the unsatu-
rated first Airy ring to that of a theoretical model of the vortex
stellar PSF (while taking into account the PSF broadening effects
due to weather by convolving a 2D Gaussian distribution), and
used the best-fit model to generate the corresponding contrast
curve. We then combined our detection limits from the two
observations, and present them in Fig. 2. Although the 2018
observation has a shorter total on-target exposure time than
the 2019 one, their total parallactic angle change difference is
only
6
.
7
. With the Differential Image Motion Monitor (DIMM)
seeing of
0
.
′′
41
±
0
.
′′
11
in 2018 being more stable than that of
1
.
′′
18
±
0
.
′′
55
in 2019, the 2018 data dominate the detection limits
in the combined dataset.
3. Analysis
3.1. Mass detection limits
3.1.1. NIRC2 imaging
Being the photometric standard, Vega’s apparent magnitude is
defined as
0
in the
M
s
band. Adopting a distance of
7
.
68
±
0
.
02
pc
in van Leeuwen (2007), the absolute magnitude for Vega in the
M
s
band was
0
.
573
±
0
.
006
.
With an age of
445
±
13
Myr (Yoon et al. 2010) and adopting
the AMES-Cond evolutionary models (Baraffe et al. 2003), we
converted the contrast to
5
σ
mass detection limits in Fig. 2. We
reached a detection limit of less than
5
M
Jupiter
beyond
9
au, and
3
M
Jupiter
beyond
14
au.
3.1.2. NIRC2 imaging and TRES radial velocity
Combining the 2018 NIRC2 results with the radial velocity data
from TRES in Hurt et al. (2021), we followed Hurt et al. (2021)
to obtain the mass limits assuming the planetary orbits are well
aligned with the spin axis of Vega. To explore the detectability
of companions from both direct imaging and radial velocity mea-
surements, we randomly generated
10
6
radial velocity samples of
companion orbits following Hurt et al. (2021). The semi-major
axis follows a log-uniform distribution ranging from
0
.
1
au to
22
au. The companion mass follows a log-uniform distribution
ranging from
0
.
1
to
100
M
Jupiter
. The sine value of orbital inclina-
tion follows a uniform distribution from
1
.
5
to
11
.
5
. The orbital
eccentricity follows a beta distribution described in Kipping
(2013). The argument of periastron follows a uniform distribu-
tion ranging from
0
to
2
π
radian. The time of periastron passage
follows a uniform distribution which was determined by the
orbital period. The stellar mass follows a Gaussian distribution
using the measurements from Monnier et al. (2012). In each
radial velocity sample, we scaled Gaussian noise according to the
uncertainties of the TRES measurements in Hurt et al. (2021).
We fit a flat line to each synthetic radial velocity curve using
RadVal
(Fulton et al. 2018).
For all simulated radial velocity samples, we considered a
synthetic signal to be detectable in radial velocity when its
p
-
value was
<
0
.
001
(i.e.,
3
.
3
σ
), while ignoring correlated noise
(Hurt et al. 2021). At a specific radial separation from the star,
we further required that a companion be detectable when its mass
is above the detection limit in Fig. 2.
We present the detection probability of companions from the
above injection-recovery procedure in Fig. 3. The detectability
of companions from combined radial velocity and direct imag-
ing follows two trends as a function of stellocentric separation.
Interior to
4
au, the detectable planets increases with radial sep-
aration, approaching planets with
8
M
Jupiter
down to
1
M
Jupiter
at
0
.
1
au. Exterior to
4
au, the detectability is dominated by
NIRC2 imaging, reaching down to
2
M
Jupiter
at
22
au. Under
the Hurt et al. (2021) framework, there is a possible nonabso-
lute detection of companions near
22
au, which is limited by
the NIRC2 field of view in our study, since the sampled orbital
eccentricity adopted from Kipping (2013) can position planets
with a semi-major axis less than
22
au beyond the
22
au angular
radius.
A162, page 3 of 6
A&A 670, A162 (2023)
0 au
5 au
10 au
15 au
20 au
0
′′
0.
′′
5
1
′′
1.
′′
5
2
′′
2.
′′
5
Separation
10
6
10
5
10
4
10
3
5 Contrast
2018
2019
Combined
NIRCam F444W
6
8
10
12
14
M
s
0 au
5 au
10 au
15 au
20 au
0
′′
0.
′′
5
1
′′
1.
′′
5
2
′′
2.
′′
5
Separation
1
10
10
2
5 Mass Limit (
M
Jupiter
)
2018
2019
Combined
NIRCam F444W
Fig. 2.
Detection limits of point sources around Vega in the
M
s
band. Left:
5
σ
contrast, in comparison with the JWST/NIRCam F444W on-sky
contrast calculated from Carter et al. (2022). Right:
5
σ
mass detection limit, and expected mass detection limit with NIRCam F444W. We note that
the NIRCam F444W values have an assumed identical instrument performance in the JWST/ERS-1386 program (see Sect. 3.3).
0.
′′
1
1
′′
0.1 au
1 au
5 au
10 au
20 au
Separation
0.1
1
10
100
Mass (M
Jupiter
)
10%
50%
80%
95%
99%
Fig. 3.
Detection probability of point sources as a function of the semi-
major axis of point sources for Vega, using a combination of the 2018
NIRC2 observation and the TRES radial velocity measurements from
Hurt et al. (2021) (see Sect. 3.1.2). We can reach an
8
M
Jupiter
detection
limit at
4
au, while probing down to
2
3
M
Jupiter
at
0
.
1
au and
22
au,
respectively.
3.2. Disk formation from mass limits
The ALMA observation in Matrà et al. (2020) resolves the outer
dust belt of Vega extending from
60
au beyond
150
au. To
explain the observed planetary system architecture, the authors
have analyzed the two scenarios that involve planets: either a
chain of small planets within
70
au with mass
6
M
, or a sole
5
M
Jupiter
at
50
60
au.
The combined Keck/NIRC2
M
s
-band and TRES result can
exclude the existence of
8
M
Jupiter
planets from
0
.
1
au to
22
au.
Despite the fact that with an age of
400
Myr there is no clear
brightness difference between the hot-start and cold-start models
(e.g., Spiegel & Burrows 2012), the observations presented here
cannot rule out the sole giant planet which resides at
50
60
au
in Matrà et al. (2020) for Vega.
For future exploration of the proposed sole giant planet at
50
au to
60
au using Keck/NIRC2 in the
M
s
band, a field of
view that is between
6
.
′′
5
and
7
.
′′
8
is needed. With a pixel scale of
9
.
942
mas, this corresponds to a half width between
654
pix-
els and
785
pixels, which exceeds the current
512
pixel half
width of the narrow camera for NIRC2 and such an observa-
tion is thus not achievable. Alternatively, NIRC2 offers sampling
setups that have pixel sizes of
20
mas or
40
mas, yet these setups
are not feasible for
M
s
-band imaging due to the corresponding
increased sky background. Nevertheless, the planned upgrades
of the NIRC2 detector electronics may permit faster readouts to
enable
M
s
-band imaging for such purposes.
To explore far separation imaging for Vega using the exist-
ing confirguration of NIRC2, either offsetting the vortex center
away from the center (e.g., the observation of HR 8799 b using
the Gemini Planet Imager in Wang et al. 2018) of the NIRC2
narrow camera, or executing observations without the vortex
coronagraph, could enable a half width of
785
pixels to test the
sole giant planet scenario. With this
1024
×
1024
pixel field of
view, the minimum permitted readout time is
0
.
18
s
1
, which
could enable a nearly identical exposure sequence in the 2018
dataset presented here. Given that extra parallactic angle change
is needed to cover the entire field of view for these setups, they
are beyond the scope of this study.
1
https://www2.keck.hawaii.edu/inst/nirc2/
ObserversManual.html#Section2.4
A162, page 4 of 6
B. B. Ren et al.: Vega in
M
-band
3.3. Implications for JWST observations
At an age of
445
±
13
Myr for Vega (Yoon et al. 2010), giant
planets with several Jupiter masses do not have clear brightness
distinction between different formation models in the
M
band
(e.g., Fig. 7 of Spiegel & Burrows 2012). For planets with less
than
5
M
Jupiter
, their brightness is expected to peak at
4
μ
m to
6
μ
m (e.g., Fig. 6 of Spiegel & Burrows 2012).
To image planets that are brightest at these wavelengths, we
applied the Keck/NIRC2
M
s
band which operates at a central
wavelength of
4
.
67
μ
m with a bandpass of
0
.
24
μ
m
2
. In compar-
ison, the NIRCam instrument onboard the
James Webb
Space
Telescope (JWST) can cover nearly half the expected brightest
wavelengths with its F444W filter (
4
μ
m to
5
μ
m)
3
. By reaching
a multi-Jupiter mass limit for
10
au to
20
au within this study, and
one Jupiter mass or better beyond
20
au with NIRCam (Meshkat
et al. 2018, Fig. 5 therein), we can combine Keck/NIRC2 and
JWST/NIRCam to reach the deepest planet detection limit to
investigate the planetary architecture for the Vega system.
Using the on-sky JWST/NIRCam F444W contrast curve
from Carter et al. (2022) in the JWST ERS-1386 program
(Hinkley et al. 2022), we calculated the expected on-sky contrast
for Vega with NIRCam F440W in Fig. 2 for JWST GTO-1193
observations as follows. Assuming an identical instrument per-
formance, which is an optimistic estimation given that JWST
GTO-1193 uses MASK430R since it has a larger inner work-
ing angle – and thus lower throughput – than MASK335R in the
JWST ERS-1386 observations in Carter et al. (2022), we scaled
the exposure times with the SUB320 subarray (i.e., Observa-
tions 35 and 36) from the JWST GTO-1193 observations and
we recalculated the contrast based on the exposure time differ-
ence. We then converted the JWST/NIRCam F444W contrast to
point source mass following Carter et al. (2021) for Vega while
adopting the same age and apparent magnitude as for NIRC2.
In terms of reaching nominal contrast, the Keck/NIRC2
M
S
-
band observations perform better than NIRCam F440W within
1
′′
and reach a similar quality in exterior regions. In terms
of mass detection limits, the Keck/NIRC2
M
S
-band observa-
tions perform systematically better than the NIRCam F444W
under the abovementioned optimistic assumptions. In addition,
the degradation of contrast close to the transmission near the
edges of the coronagraphic masks
4
from MASK335R in JWST
ERS-1386 to MASK430R in JWST GTO-1193 has been ignored.
All three aspects demonstrate that the NIRC2
M
S
-band observa-
tions presented here establish the deepest high-contrast imaging
exploration of planetary companions for Vega in the probed
regions. Nevertheless, we emphasize that NIRCam should be
better than NIRC2 for most other systems, since the study pre-
sented here should be the best-case scenario for ground-based
M
S
-band imaging due to the brightness of Vega; for other targets
that are fainter than Vega, they should have less favorable con-
trasts due to the relative background levels on the ground from
NIRC2.
3.4. Planet detection toward 0.1 arcsec
To obtain a more general understanding of planetary existence,
the detection of intermediate separation planets near
0
.
′′
1
are
2
https://www2.keck.hawaii.edu/inst/nirc2/filters.html
3
https://jwst-docs.stsci.edu/jwst-near-infrared-
camera/nircam-instrumentation/nircam-filters
4
Fig.
3
of
https://jwst-docs.stsci.edu/jwst-near-
infrared-camera/nircam-instrumentation/nircam-
coronagraphic-occulting-masks-and-lyot-stops
expected to yield the most thorough discovery of planets (e.g.,
Nielsen et al. 2019; Fulton et al. 2021). In fact, for the detection of
far-separation planets, existing direct imaging surveys equipped
with extreme AO systems have experienced a degradation of
contrast close to the central sources (e.g., Nielsen et al. 2019;
Vigan et al. 2021; Xie et al. 2022). In comparison, for the detec-
tion of close-in planets, existing radial velocity surveys have less
completeness for long orbital period planets (e.g., Wittenmyer
et al. 2020; Fulton et al. 2021). As a result, in the detection prob-
ability map of companions for Vega in Fig. 3, there is a clear gap
for middling separations.
To fill the gap, the concept of the vortex fiber nuller (VFN;
Ruane et al. 2019; Echeverri et al. 2020) is designed to detect and
characterize planets near and within
0
.
′′
1
. Using nulling inter-
ferometry in the near-infrared, VFN suppresses on-axis starlight
while retaining off-axis companion light with acceptable loss,
which increases the signal-to-noise ratio for companions and
thus better enables their detection and characterization. The VFN
has already been installed for the phase 2 development of the
Keck Planet Imager and Characterizer (KPIC; Jovanovic et al.
2020, Echeverri et al., in prep.) on the Keck Observatory, pro-
viding access to planets between
30
and
90
mas in the
K
band.
Furthermore, the limitation of VFN in localizing the companions
in their orbit will be further resolved with the concept of pho-
tonic lantern nuller (PLN; e.g., Xin et al. 2022). We expect that
the application of VFN and PLN will fill the planet detection and
characterization gap between direct imaging and radial velocity.
For Vega, they will better explore possible hidden planets toward
0
.
′′
1
in Fig. 3. Nevertheless, the limitation of VFN in the
K
band
is that it is less sensitive to evolved and cooler giant planets due
to the drop in planet brightness in shorter wavelengths.
4. Summary
We report
1
.
8
h of
M
s
-band imaging observations of Vega using
the Keck/NIRC2 vortex coronagraph. Despite a nondetection of
companions, we have pushed the mass detection limits from
existing high-contrast imaging observations in Meshkat et al.
(2018) by nearly an order of magnitude smaller (see Fig. 2).
Combining the NIRC2 results with an existing radial velocity
study using TRES in Hurt et al. (2021), we can confidently rule
out companions more massive than
8
M
Jupiter
from
0
.
1
au to
22
au for Vega. Within this range, NIRC2 can reach planets that
are less massive than
5
M
Jupiter
beyond
9
au.
While the NIRC2 observations presented here are sensitive
to planets of
5
M
Jupiter
at
9
au down to
3
M
Jupiter
at
22
au, it
is limited by the field of view and thus cannot be used to test
the scenario in Matrà et al. (2020) that one multi-Jupiter mass
planet at
50
au to
60
au is responsible for the planetary system
architecture. Alternatively, observing without using the vortex
coronagraph while performing multipoint dithering
5
, combined
with extra parallactic angle change, may provide enough sensi-
tivity to image such a perturber for the Vega system by pushing
deeper than Heinze et al. (2008) for these separations.
Combining Keck/NIRC2 in the
M
s
band and NIRCam in its
F444W filter assuming identical performance with JWST from
ERS-1386 observations (Hinkley et al. 2022; Carter et al. 2022),
we can reach comparable detection limits of companions inte-
rior to
20
au. Interior to
10
au, although companion imaging
limits increase from
5
M
Jupiter
at
10
au to
100
M
Jupiter
at
2
au
in Fig. 2, this region is where radial velocity measurements can
5
https://www2.keck.hawaii.edu/inst/nirc2/
ObserversManual.html#Section3.2.10
A162, page 5 of 6
A&A 670, A162 (2023)
provide multi-Jupiter detection limits or better as shown in Hurt
et al. (2021). Future works following Mawet et al. (2019) and
Llop-Sayson et al. (2021) in combining measurements includ-
ing direct imaging here and radial velocity in Hurt et al. (2021),
as well as upcoming imaging observations with JWST GTO-
1193, in addition to the VFN and PLN concepts to detect planets
toward
0
.
′′
1
, could allow us to obtain the most holistic under-
standing for the planetary system of this historical photometric
standard star – Vega.
Acknowledgements.
We thank the anonymous referee for their constructive
comments that increased the clarity and reproducibility of this paper. This
research is partially supported by NASA ROSES XRP, award 80NSSC19K0294.
B.B.R. has received funding from the European Research Council (ERC) under
the European Union’s Horizon 2020 research and innovation programme (PRO-
TOPLANETS, grant agreement No. 101002188). É.C. has received funding from
the European Research Council (ERC) under the European Union’s Horizon
Europe research and innovation programme (ESCAPE, grant agreement No
101044152). Some of the data presented herein were obtained at the W.M. Keck
Observatory, which is operated as a scientific partnership among the California
Institute of Technology, the University of California and the National Aeronautics
and Space Administration. The Observatory was made possible by the generous
financial support of the W.M. Keck Foundation. The authors wish to recognize
and acknowledge the very significant cultural role and reverence that the summit
of Maunakea has always had within the indigenous Hawaiian community. We are
most fortunate to have the opportunity to conduct observations from this moun-
tain. Part of the computations presented here was conducted in the Resnick High
Performance Computing Center, a facility supported by Resnick Sustainability
Institute at the California Institute of Technology.
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