PDS70b Shows Stellar-like Carbon-to-oxygen Ratio
Chih-Chun Hsu
1
, Jason J. Wang
(
王
劲
飞
)
1
,
2
, Geoffrey A. Blake
3
, Jerry W. Xuan
4
, Yapeng Zhang
4
,
Jean-Baptiste Ruf
fi
o
5
, Katelyn Horstman
4
, Julianne Cronin
1
,
2
, Ben Sappey
5
, Yinzi Xin
4
, Luke Finnerty
6
,
Daniel Echeverri
4
, Dimitri Mawet
4
,
7
, Nemanja Jovanovic
4
, Clarissa R. Do Ó
5
, Ashley Baker
4
, Randall Bartos
7
,
Benjamin Calvin
4
,
6
, Sylvain Cetre
8
, Jacques-Robert Delorme
8
, Gregory W. Doppmann
8
, Michael P. Fitzgerald
6
,
Joshua Liberman
9
, Ronald A. López
6
, Evan Morris
10
, Jacklyn Pezzato-Rovner
4
, Tobias Scho
fi
eld
4
, Andrew Skemer
10
,
J. Kent Wallace
7
, and Ji Wang
(
王
吉
)
11
1
Center for Interdisciplinary Exploration and Research in Astrophysics
(
CIERA
)
, Northwestern University, 1800 Sherman Ave., Evanston, IL 60201, USA;
chsu@northwestern.edu
2
Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Rd., Evanston, IL 60208, USA
3
Division of Geological & Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
4
Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
5
Department of Astronomy & Astrophysics, University of California, San Diego, La Jolla, CA 92093, USA
6
Department of Physics & Astronomy, 430 Portola Plaza, University of California, Los Angeles, CA 90095, USA
7
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr.,Pasadena, CA 91109, USA
8
W. M. Keck Observatory, 65-1120 Mamalahoa Hwy, Kamuela, HI, USA
9
James C. Wyant College of Optical Sciences, University of Arizona, Meinel Building 1630 E. University Blvd., Tucson, AZ 85721, USA
10
Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA
11
Department of Astronomy, The Ohio State University, 100 W 18th Ave., Columbus, OH 43210, USA
Received 2024 October 30; revised 2024 November 20; accepted 2024 November 21; published 2024 December 18
Abstract
The
~
5 Myr PDS 70 is the only known system with protoplanets residing in the cavity of the circumstellar disk
from which they formed, ideal for studying exoplanet formation and evolution within its natal environment. Here,
we report the
fi
rst spin constraint and C
/
O measurement of PDS 70b from Keck
/
KPIC high-resolution
spectroscopy. We detected CO
(
3.8
σ
)
and H
2
O
(
3.5
σ
)
molecules in the PDS 70b atmosphere via cross correlation,
with a combined CO and H
2
O template detection signi
fi
cance of 4.2
σ
. Our forward-model
fi
ts, using BT-Settl
model grids, provide an upper limit for the spin rate of PDS 70b
(
<
29 km s
−
1
)
. The atmospheric retrievals
constrain the PDS 70b C
/
O ratio to
0
.28
0.12
0.20
-
+
(
<
0.63 under 95% con
fi
dence level
)
and a metallicity
[
C
/
H
]
of
0.2
0.5
0.8
-
-
+
dex, consistent with that of its host star. The following scenarios can explain our measured C
/
Oof
PDS 70b in contrast with that of the gas-rich outer disk
(
for which C
/
O
1
)
. First, the bulk composition of
PDS 70b might be dominated by dust
+
ice aggregates rather than disk gas. Another possible explanation is that the
disk became carbon enriched
after
PDS 70b was formed, as predicted in models of disk chemical evolution and as
observed in both very low-mass stars and older disk systems with JWST
/
MIRI. Because PDS 70b continues to
accrete and its chemical evolution is not yet complete, more sophisticated modeling of the planet and the disk, and
higher-quality observations of PDS 70b
(
and possibly PDS 70c
)
, are necessary to validate these scenarios.
Uni
fi
ed Astronomy Thesaurus concepts:
Exoplanet atmospheres
(
487
)
;
Exoplanet formation
(
492
)
;
High resolution
spectroscopy
(
2096
)
;
High angular resolution
(
2167
)
1. Introduction
Since the discovery of the
fi
rst exoplanet around a Sun-like
star
(
M. Mayor & D. Queloz
1995
)
,
>
5700 exoplanets are now
known.
12
Among these advances, directly imaged companions
offer a unique laboratory to study exoplanet formation and
evolution
(
B. P. Bowler
2016
; T. Currie et al.
2023
)
. These
imaged planets are mostly young and warm
(
<
500 Myr
)
, thus
bright and well separated from their host stars. Myriad
protoplanetary disks have been characterized extensively in
the millimeter band, but PDS 70 remains the only system that
has con
fi
rmed protoplanets b and c residing in the cavity of
their natal, gas-rich disk from which they formed
(
M. Keppler
et al.
2018
; A. Müller et al.
2018
; S. Y. Haffert et al.
2019
)
,
13
serving as a unique opportunity to study exoplanet formation
and evolution in situ.
PDS 70 A is a K7 T Tauri star in the Upper Centaurus Lupus
association at 112 pc
(
M. J. Pecaut & E. E. Mamajek
2016
;
M. Keppler et al.
2018
; Gaia Collaboration et al.
2023
)
, with a
slightly subsolar metallicity
(
[
Fe
/
H
]
=
−
0.11
±
0.1; M. Steinmetz
et al.
2020b
)
,aC
/
O ratio of 0.44
±
0.19
(
A. J. Cridland et al.
2023
)
, and an age of 5.4
±
1.0 Myr
(
P. Riaud et al.
2006
;
M. J. Pecaut & E. E. Mamajek
2016
)
.
14
PDS 70b has a mass of
2
–
4
M
Jup
, a semimajor axis of 20.8
0.7
0.
6
-
+
au, and a nonzero orbital
eccentricity of 0.17
±
0.06; while PDS 70 c has a mass of
1
–
3
M
Jup
with a circular orbit
(
eccentricity 0.037
0.02
5
0.041
-
+
)
at
34.3
1.8
2.2
-
+
au
(
J. J. Wang et al.
2020
,
2021c
)
. PDS 70b and c
The Astrophysical Journal Letters,
977:L47
(
9pp
)
, 2024 December 20
https:
//
doi.org
/
10.3847
/
2041-8213
/
ad95e8
© 2024. The Author
(
s
)
. Published by the American Astronomical Society.
Original content from this work may be used under the terms
of the
Creative Commons Attribution 4.0 licence
. Any further
distribution of this work must maintain attribution to the author
(
s
)
and the title
of the work, journal citation and DOI.
12
NASA Exoplanet Archive compilation on 2024 October 7;
https:
//
exoplanetarchive.ipac.caltech.edu
/
.
13
AB Aur b is a con
fi
rmed protoplanet with its disk
(
T. Currie et al.
2022
)
.
See also other candidates summarized in T. Currie et al.
(
2023
)
.
14
We note that PDS 70 A might be a member in the subgroup
ν
Cen of the
Upper Centaurus Lupus, with reported ages between
~
9 and 16 Myr
(
S. Rat-
zenböck et al.
2023a
,
2023b
)
, while most disk lifetimes are less than 10 Myr
(
S. Pfalzner & F. Dincer
2024
)
.
1
show clear signs of H
α
(
S. Y. Haffert et al.
2019
)
, indicative of
ongoing accretion, but no detections of H
β
were reported in
J. Hashimoto et al.
(
2020
)
.
The PDS 70 outer disk has a gas-phase C
/
O ratio
1as
inferred from Atacama Large Millimeter
/
submillimeter Array
(
ALMA
)
spectroscopy
(
S. Facchini et al.
2021
; A. J. Cridland
et al.
2023
; C. J. Law et al.
2024
)
, and the PDS 70 inner disk
was detected in H
2
O and CO
2
with JWST
/
MIRI
(
G. Perotti
et al.
2023
)
. Both PDS 70b and c lie within the CO ice line
(
at
56
–
85 au; A. J. Cridland et al.
2023
; C. J. Law et al.
2024
)
. The
measurements of the C
/
O ratio of PDS 70b and c are crucial
for constraining the formation and evolution of the planetary
system, but no clear molecular detections have been reported
prior to this work, despite attempts such as the one using the
SINFONI integral
fi
eld spectrograph
(
R
~
5000
)
at the Very
Large Telescope
(
VLT; G. Cugno et al.
2021
)
. The PDS 70 A
and b properties relevant to this work are summarized in
Table
1
.
In this Letter, we report the
fi
rst abundance measurement of
the protoplanet PDS 70b using atmospheric molecular line
detections of CO and H
2
O from Keck
/
Keck Planet Imager and
Characterizer
(
KPIC
)
high-resolution spectroscopy. In Section
2
,
we describe our KPIC observation and data reduction. Section
3
presents our detection of CO and H
2
O, while Sections
4
and
5
show our forward-modeling results using a self-consistent
modeling and retrieval framework, respectively. In Section
6
,
we discuss possible interpretations of our measured C
/
OofPDS
70b with the host star and disk measurements in the literature.
We summarize our
fi
ndings in Section
7
.
2. Observations and Data Reduction
The KPIC connects the Keck II Telescope's adaptive optics
system to the infrared high-resolution spectrograph NIRSPEC
(
I. S. McLean et al.
1998
,
2000
; E. C. Martin et al.
2018
)
using
single-mode
fi
bers
(
D. Mawet et al.
2016
,
2017
; J.-R. Delorme
et al.
2021
)
, enabling diffraction-limited high-resolution
spectroscopy
(
R
∼
35,000
)
. KPIC underwent a series of
upgrades in 2024 April
(
D. Echeverri et al.
2024
;K.A.Horstman
et al.
2024
; N. Jovanovic et al. 2024, submitted
)
.
We observed PDS 70b on 2024 May 23
(
UT
)
under clear
and stable weather conditions, with a natural seeing of
0
.65
.
We used KPIC
fi
bers 2 and 4
(
out of the four KPIC
fi
bers
)
. The
total exposure time on PDS 70b was limited to 140 minutes
(
fourteen 600 s exposures
)
due to the source decl. and the
Nasmyth platform limit of the Keck II telescope. KPIC
observations require an orbital prediction of the planet to
offset from the host star, and we used the astrometry solutions
in J. J. Wang et al.
(
2021c
)
and
whereistheplanets
(
J. J. Wang et al.
2021a
)
15
and offset to the separation of
148.76 mas and position angle of 128.28 mas relative to the
host star PDS 70 A during our observations. We also observed
PDS 70 A and used on-axis star spectra as an empirical star
template to model the starlight contribution injected into the
fi
ber pointed at the planet. The exposure time on PDS 70 A was
taken as six 180 s exposures using KPIC
fi
bers 2 and 4, for a
total of 18 minutes, roughly before and after a set of
40
–
60 minute exposures on PDS 70b. For the spectral trace
identi
fi
cations and telluric calibrator, we used the A0V star
HD 118214
(
A. Cowley et al.
1969
)
, with two exposures of
30 s for each
fi
ber. For the wavelength calibration, we used
early M giant star
(
C-R4IIIb; P. C. Keenan
1993
)
HIP 62944,
with two exposures of 30 s for each
fi
ber. HIP 62944 has
narrow spectral lines and telluric lines that enable precise
wavelength calibration.
Our KPIC data were reduced using the
KPIC Data
Reduction Pipeline
,
16
with the procedures detailed in
J. J. Wang et al.
(
2021b
)
and C.-C. Hsu et al.
(
2024b
)
, using
background subtraction and optimal source extraction
(
K. Horne
1986
)
. The PDS 70 A and PDS 70b spectra were
background subtracted using the thermal background frames of
the corresponding integration times
(
180 and 600 s for PDS 70
A and b, respectively
)
.
3. Cross-correlation Function Detection
Our detection of PDS 70b uses the forward-modeling cross-
correlation function
(
CCF
)
technique. We refer readers to
Table 1
PDS 70 A and B Properties
Property
(
unit
)
Value
References
PDS 70 A
R.A.
(
J2000
)
14:08:10.15
(
1
)
decl..
(
J2000
)
−
41:23:52.57
(
1
)
μ
α
(
mas yr
−
1
)
−
29.70
±
0.02
(
1
)
μ
δ
(
mas yr
−
1
)
−
24.04
±
0.02
(
1
)
Mass
(
M
e
)
0.88
±
0.02
(
2
)
Age
(
Myr
)
~
5
(
3
)
SpT
K7IVe
(
4
)
Gaia
G
11.606
±
0.004
(
1
)
J
MKO
(
mag
)
9.55
±
0.02
(
5
)
H
MKO
(
mag
)
8.82
±
0.04
(
5
)
K
S,MKO
(
mag
)
8.54
±
0.02
(
5
)
π
(
mas
)
8.898
±
0.019
(
1
)
distance
(
pc
)
112.4
±
0.2
(
1
)
RV
(
km s
−
1
)
a
6.65
0.22
0.1
4
-
+
(
9
)
vi
sin
(
km s
−
1
)
17.3
0.3
0.
4
-
+
(
9
)
[
Fe
/
H
]
(
dex
)
−
0.11
±
0.1
(
6
)
C
/
O
0.44
±
0.19
(
7
)
PDS 70b
Mass
(
M
Jup
)
2
–
4
(
7
)
Radius
(
R
Jup
)
2
.7
0.3
0.
4
-
+
(
8
)
a
(
au
)
b
20.8
0.7
0.6
-
+
(
2
)
e
b
0.17
±
0.06
(
2
)
i
(
deg
)
b
130.5
2.4
2.5
-
+
(
2
)
T
eff
(
K
)
1204
53
5
2
-
+
(
8
)
L
′
(
mag
)
14.64
±
0.18
(
8
)
vi
sin
(
km s
−
1
)
<
29
c
(
9
)
RV
(
km s
−
1
)
a
1.7
5.2
3.
4
-
-
+
(
9
)
[
C
/
H
]
(
dex
)
0.2
0.5
0.8
-
-
+
(
9
)
C
/
O
0.28
0.12
0.2
0
-
+
(
9
)
Notes.
a
Barycentric RV on MJD 60453.27578.
b
Dynamically stable solutions in J. J. Wang et al.
(
2021c
)
.
c
Upper limit at 95% con
fi
dence level.
References
.
(
1
)
Gaia Collaboration et al.
(
2023
)
;
(
2
)
J. J. Wang et al.
(
2021c
)
;
(
3
)
A. Müller et al.
(
2018
)
;
(
4
)
M. J. Pecaut & E. E. Mamajek
(
2016
)
;
(
5
)
R. M. Cutri et al.
(
2003
)
;
(
6
)
M. Steinmetz et al.
(
2020b
)
;
(
7
)
A. J. Cridland
et al.
(
2023
)
;
(
8
)
J. J. Wang et al.
(
2020
)
;
(
9
)
This work.
15
http:
//
whereistheplanet.com
/
16
https:
//
github.com
/
kpicteam
/
kpic_pipeline
2
The Astrophysical Journal Letters,
977:L47
(
9pp
)
, 2024 December 20
Hsu et al.
J. J. Wang et al.
(
2021b
)
, J. W. Xuan et al.
(
2022
)
, and C.-
C. Hsu et al.
(
2024b
)
for the formulation and implementation of
our methods.
In short, we jointly
fi
t for the host star contribution, the
planet template with various combinations of molecules, and
the telluric and instrument response using the least-squares-
fi
t-
based CCF. The CCF signal of a given velocity shift was
obtained by optimizing the amplitude of the host star
contribution
(
from the observed on-axis star spectra
)
and the
planet templates. Our planet templates including only CO or
H
2
O along with combined CO
+
H
2
O opacities are derived
from Sonora Bobcat models
(
M. Marley et al.
2018
;
M. S. Marley et al.
2021
)
at effective
T
eff
=
1200 K and a
surface gravity of
g
log
=
4.0 cgs dex
(
J. J. Wang et al.
2021b
)
,
consistent with the PDS 70b parameters derived in J. J. Wang
et al.
(
2021c
)
. We ran the velocity from
−
1000 to
+
1000 km s
−
1
and used the last
±
800 km s
−
1
CCF wings to
compute the noise and estimate the signal-to-noise ratio
(
SNR
)
.
We found detections of CO
(
CCF SNR
~
3.8
)
,H
2
O
(
CCF
SNR
~
3.5
)
, and CO
+
H
2
O
(
CCF SNR
~
4.2
)
in our CCF
curves, as shown in Figure
1
. We further constrain the physical
parameters of PDS 70b in Sections
4
and
5
.
4. Bulk Parameters of PDS 70 A and B
4.1. PDS 70 A
If distinct,
fi
ts of the radial velocity
(
RV
)
of PDS 70 A
and 70 b on the same night can help us validate our
protoplanet detection. We forward-modeled NIRSPEC order
33
(
2.29
–
2.34
μ
m
)
of our PDS 70 A spectra using the
SMART
package
(
C.-C. Hsu et al.
2021a
,
2021b
)
, with the procedures
detailed in C.-C. Hsu et al.
(
2021a
,
2023
)
.
The forward method
fi
ts the stellar spectra using the BT-Settl
CIFIST models
(
I. Baraffe et al.
2015
)
, along with a telluric
model as a function of air mass and precipitable water vapor
(
S. Moehler et al.
2014
)
. The best-
fi
t parameters were derived
using the Markov Chain Monte Carlo
(
MCMC
)
package
emcee
(
D. Foreman-Mackey et al.
2013
)
. We incorporated a
fringe model determined outside of MCMC, using a formula-
tion similar to B. Cale et al.
(
2019
; see also K. A. Horstman
et al.
2024
; W. Xuan et al.
2024b
)
. There are nine parameters in
our MCMC forward-model
fi
t: effective temperature
(
T
eff
)
,
surface gravity
(
g
log
)
, RV, projected rotational velocity
(
vi
sin
)
, air mass, precipitable water vapor, and three nuisance
parameters
(
fl
ux and wavelength offsets, and a noise in
fl
ation
scale factor
)
. We used 100 chains and 10,000 steps, with a
burn-in of the
fi
rst 2000 steps. Convergence occurred within
the
fi
rst 1000 steps veri
fi
ed by visual inspection of the chain
evolutions.
Our best-
fi
t values are
T
eff
=
3849
39
108
-
+
K,
g
log
=
5.20
±
0.04 cm s
−
2
dex,
vi
sin
=
17.3
0.3
0.4
-
+
km s
−
1
, and
(
barycentric
velocity
–
corrected
)
RV
=
6.65
0.22
0.14
-
+
km s
−
1
, while our mea-
sured
vi
sin
is consistent with the literature measurement of
17.16
±
0.16 km s
−
1
(
T. Thanathibodee et al.
2020
; C. Swastik
et al.
2021
; B. P. Bowler et al.
2023
)
. Our RV is consistent with
lower-resolution spectroscopic measurements such as RAVE
(
4
±
7kms
−
1
; M. Steinmetz et al.
2020a
)
, Gaia
(
3.1
±
1.4 km s
−
1
; Gaia Collaboration et al.
2018
)
, and HARPS
RV
=+
6.0
±
1.5 km s
−
1
(
T. Thanathibodee et al.
2020
)
. Our
derived RV of PDS 70 A is also consistent with the optimal RV
of 4.8 km s
−
1
for belonging to the
ν
Cen membership reported
in S. Ratzenböck et al.
(
2023a
,
2023b
)
, while their RV scatters
in the subgroup are typically a few km s
−
1
. When determined
solely through high-resolution spectroscopy over narrow
wavelength ranges,
T
eff
and
g
log
measurements can have
signi
fi
cant uncertainties. We therefore stress that our RV and
vi
sin
constraints, which are our targeted physical parameters
in this work, are largely insensitive to the true values of
T
eff
and
g
log
(
see discussions and justi
fi
cations in C.-C. Hsu et al.
2021a
,
2024a
and C. A. Theissen et al.
2022
)
.
4.2. PDS 70b
To determine the atmospheric parameters of PDS 70b, we
forward-modeled the observed KPIC spectra of PDS 70b by
simultaneously
fi
tting stellar, which is the diffracted starlight
that leaks into the
fi
ber pointed at the planet, and planet
fl
ux
contributions. The
fi
tting procedures are detailed in C.-C. Hsu
et al.
(
2024b
)
. For the wavelength range of our analysis, we
used the NIRSpec orders 31
–
33
(
2.29
–
2.49
μ
m
)
, as these
contain CO, H
2
O, and CH
4
features and provided the best
wavelength calibration. We used the on-axis KPIC spectra of
PDS 70 A to serve as an empirical template and the BT-Settl
CIFIST model grids
(
I. Baraffe et al.
2015
)
for the planet
model. In short, we
fi
tted 17 parameters in total, including
T
eff
,
Figure 1.
Cross-correlation functions
(
CCFs
)
of our PDS 70b KPIC spectra against the molecular templates derived from the Sonora Bobcat models. Left: CCF
(
red
solid line
)
of our KPIC spectra for the CO molecular templates. The stellar barycentric
–
included RV is depicted by the gray vertical dashed line. The auto-correlation
function
(
ACF
)
of the CO molecular templates, normalized to the peak of CCF, is plotted as a dashed red line. The ACF serves as a guide to detections of given
molecular templates. Middle: same as the left panel for the H
2
O molecular templates in blue. Right: same as the upper panel for the CO
+
H
2
O molecular templates in
magenta.
3
The Astrophysical Journal Letters,
977:L47
(
9pp
)
, 2024 December 20
Hsu et al.