The First JWST View of a 30-Myr-old Protoplanetary Disk Reveals a Late-stage Carbon-
rich Phase
Feng Long
(
龙
凤
)
1
,
18
, Ilaria Pascucci
1
, Adrien Houge
2
, Andrea Banzatti
3
, Klaus M. Pontoppidan
4
, Joan Najita
5
,
Sebastiaan Krijt
6
, Chengyan Xie
1
, Joe Williams
6
, Gregory J. Herczeg
(
沈
雷
歌
)
7
,
8
, Sean M. Andrews
9
,
Edwin Bergin
10
, Geoffrey A. Blake
11
, María José Colmenares
10
, Daniel Harsono
12
, Carlos E. Romero-Mirza
9
,
Rixin Li
(
李
日新
)
13
,
19
, Cicero X. Lu
14
, Paola Pinilla
15
, David J. Wilner
9
, Miguel Vioque
16
, and Ke Zhang
17
the JDISCS collaboration
1
Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA;
fenglong@arizona.edu
2
Center for Star and Planet Formation, GLOBE Institute, University of Copenhagen, Øster Voldgade 5-7, DK-1350 Copenhagen, Denmark
3
Department of Physics, Texas State University, 749 North Comanche Street, San Marcos, TX 78666, USA
4
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
5
NSF's NOIRLab, 950 North Cherry Avenue, Tucson, AZ 85719, USA
6
School of Physics and Astronomy, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
7
Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People
’
s Republic of China
8
Department of Astronomy, Peking University, Beijing 100871, People
’
s Republic of China
9
Center for Astrophysics
|
Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
10
Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
11
Division of Geological & Planetary Sciences, MC 150-21, California Institute of Technology, Pasadena, CA 91125, USA
12
Institute of Astronomy, Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
13
Department of Astronomy, Theoretical Astrophysics Center, and Center for Integrative Planetary Science, University of California Berkeley, Berk
eley, CA 94720-
3411, USA
14
Gemini Observatory
/
NSF's NOIRLab, 670 North A
’
ohoku Place, Hilo, HI 96720, USA
15
Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
16
European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748, Garching bei Munchen, Germany
17
Department of Astronomy, University of Wisconsin
–
Madison, Madison, WI 53706, USA
Received 2024 October 02; revised 2024 November 14; accepted 2024 November 22; published 2025 January 6
Abstract
We present a JWST MIRI
/
MRS spectrum of the inner disk of WISE J044634.16
–
262756.1B
(
hereafter J0446B
)
,an
old
(
∼
34 Myr
)
M4.5 star but with hints of ongoing accretion. The spectrum is molecule-rich and dominated by
hydrocarbons. We detect 14 molecular species
(
H
2
,CH
3
,CH
4
,C
2
H
2
,
13
CCH
2
,C
2
H
4
,C
2
H
6
,C
3
H
4
,C
4
H
2
,C
6
H
6
,
HCN, HC
3
N, CO
2
,and
13
CO
2
)
and two atomic lines
(
[
Ne
II
]
and
[
Ar
II
]
)
, all observed for the
fi
rst time in a disk at this
age. The detection of spatially unresolved H
2
and Ne gas strongly supports that J0446B hosts a long-lived primordial
disk, rather than a debris disk. The marginal H
2
O detection and the high C
2
H
2
/
CO
2
column density ratio indicate that
the inner disk of J0446B has a very carbon-rich chemistry, with a gas-phase C
/
Oratio
2, consistent with what has
been found in most primordial disks around similarly low-mass stars. In the absence of signi
fi
cant outer disk dust
substructures, inner disks are expected to
fi
rst become water-rich due to the rapid inward drift of icy pebbles and
evolve into carbon-rich as outer disk gas
fl
ows inward on longer timescales. The faint millimeter emission in such
low-mass star disks implies that they may have depleted their outer icy pebble reservoir early and already passed the
water-rich phase. Models with pebble drift and volatile transport suggest that maintaining a carbon-rich chemistry for
tens of Myr likely requires a slowly evolving disk with
α
-viscosity
10
−
4
. This study represents the
fi
rst detailed
characterization of disk gas at
∼
30 Myr, strongly motivating further studies into the
fi
nal stages of disk evolution.
Uni
fi
ed Astronomy Thesaurus concepts:
Protoplanetary disks
(
1300
)
;
Astrochemistry
(
75
)
;
Infrared astronomy
(
786
)
;
Circumstellar disks
(
235
)
1. Introduction
Gas in protoplanetary disks has a major impact on the
formation and evolution of planetary systems. The lifetime of the
gas disk directly constrains the timescale of giant planet
formation. The presence of disk gas can signi
fi
cantly alter
system architectures by driving planet migration and reshaping
orbital con
fi
gurations
(
see, e.g., S. Paardekooper et al.
2023
)
.
Additionally, the atmospheric compositions and potential habit-
ability of exoplanets are closely linked to the disk gas they
accrete. Observational and theoretical efforts have also been
made to link the C
/
O ratio of exoplanet atmospheres to disk
chemistry models to reveal the planet's formation location and
dynamic history
(
e.g., K. I. Öberg et al.
2011
;N.Madhusudhan
2012
;P.Mollièreetal.
2022
)
. Obtaining observational
constraints of the gas disk evolution and its chemical composi-
tion across disk radii is thus essential to develop a complete
model of planet formation.
Spectra taken with the Infrared Spectrograph
(
IRS
)
on
Spitzer with moderate resolution
(
R
∼
600
)
have revealed rich
volatile chemistry within the inner few au of young
protoplanetary disks and identi
fi
ed a series of water, OH,
CO
2
,C
2
H
2
, and HCN emission lines at
∼
5
–
38
μ
m
(
e.g.,
The Astrophysical Journal Letters,
978:L30
(
17pp
)
, 2025 January 10
https:
//
doi.org
/
10.3847
/
2041-8213
/
ad99d2
© 2025. The Author
(
s
)
. Published by the American Astronomical Society.
18
NASA Hubble Fellowship Program Sagan Fellow.
19
51 Pegasi b Fellow.
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.
1
J. S. Carr & J. R. Najita
2008
,
2011
; K. M. Pontoppidan et al.
2010
; C. Salyk et al.
2011
)
. Though faint disks around mid-to-
late M stars
(
with spectral type later than M3
)
were rarely
targeted with Spitzer, they were found to be different from their
solar analogs, showing brighter C
2
H
2
emission over HCN
(
I. Pascucci et al.
2009
)
and weaker H
2
O lines
(
I. Pascucci et al.
2013
)
. Comparison of the derived gas properties to thermo-
chemical models
(
e.g., J. R. Najita et al.
2011
)
has thus
suggested enhanced carbon chemistry with C
/
O ratios of
∼
1in
their disks
(
I. Pascucci et al.
2013
)
.
The Medium Resolution Spectrometer
(
MRS; M. Wells et al.
2015
)
on board the JWST Mid-Infrared Instrument
(
MIRI;
G. H. Rieke et al.
2015
)
now offers signi
fi
cantly improved
sensitivity and spectral resolution
(
R
∼
2000
−
4000; see Table
3 in K. M. Pontoppidan et al.
2024
)
, presenting new
opportunities to investigate chemical variations in disks around
different stellar types. Recent JWST observations of known
C-rich disks around very low-mass stars, such as 2MASS
J16053215
–
1933159
(
hereafter J160532; M5; B. Tabone et al.
2023
)
and ChaI-147
(
M5.5; A. M. Arabhavi et al.
2024
)
, have
further identi
fi
ed a large number of hydrocarbon molecules
(
see
Table
3
)
, including the
fi
rst detections of C
6
H
6
(
benzene
)
in
protoplanetary disks. In contrast, the MIRI
/
MRS spectrum of
Sz 114, a star with a similar spectral type as the two above, is
dominated by water emission
(
C. Xie et al.
2023
)
. This
suggests that factors beyond host star properties may play
important roles in disk chemical evolution. Theoretical models
by J. Mah et al.
(
2023
)
indicate that, under the combined effects
of icy pebble drift and gas accretion, the inner disk would
initially experience a drop in gas C
/
O ratio due to ice
sublimation, followed by an increase from the outer gas in
fl
ow.
The transition point is expected to occur earlier in disks around
lower-mass stars due to their closer-in ice lines and shorter
viscous timescales. Additionally, the properties of dust traps
could substantially in
fl
uence the chemical evolution pathways
(
e.g., A. Kalyaan et al.
2023
; J. Mah et al.
2024
)
.
Although typical disk lifetimes are known to be only a few
Myr
(
e.g., J. Hernández et al.
2008
; Á. Ribas et al.
2014
)
,recent
studies have identi
fi
ed a number of accreting disks surrounding
very low-mass stars with ages of 30
–
50 Myr
(
e.g., A. Boucher
2016
;S.J.Murphyetal.
2018
; S. M. Silverberg et al.
2020
)
.
This old sample thus provides a unique prospect to enable gas
evolution studies across tens of Myr. Here we present JWST
/
MIRI observations for one such old accreting disk around the
M4.5 star WISE J044634.16
–
262756.1B
(
hereafter J0446B;
Gaia DR3 coordinate 04:46:34.25
–
26:27:55.57; Gaia Collabora-
tion
2022
)
. This represents the
fi
rst study of gas-rich disks at
∼
30 Myr old, including the
fi
rst detections of H
2
,
[
Ne
II
]
,and
large numbers of hydrocarbons at such old ages. The Letter is
structured as follows. In Section
2
, we describe our target and
the JWST observations. The resulting spectrum and slab model
fi
ts for identi
fi
ed lines are presented in Section
3
.Wethen
discuss the implications of these line detections for disk chemical
evolution and planet formation in Section
4
and summarize the
key points in Section
5
.
2. The Target and Observations
2.1. J0446B: An Old Disk with Hints of Accretion
WISE J044634.16
–
262756.1 was identi
fi
ed as a source with
strong infrared excess through the Disk Detective citizen science
project, with a high probability of membership in the
4
2
4
6
-
+
Myr-
old Columba association
(
S. M. Silverberg et al.
2020
)
.Images
from Pan-STARRS and Gaia revealed it as consisting of two
stars
(
J0446A, the SW component, and J0446B, the NE
component
)
separated by 2
.
3, corresponding to
∼
189 au at a
distance of 82 pc
(
Gaia Collaboration
2022
)
. Recently,
K. L. Luhman
(
2024
)
reassessed the memberships and ages of
nearby young moving groups based on Gaia DR3 and assigned
J0446A and J0446B to
χ
1
For, a region physically related to
Columba with an age of
3
3.7
1.9
2.
0
-
+
Myr. In this Letter, we adopt
this new age determination based on lithium depletion models.
The absence of lithium in the Gemini
/
GMOS spectra of J0446A
and J0446B also aligns with such an old age
(
S. M. Silverberg
et al.
2020
)
. Evidence of accretion was suggested in both disks
by S. M. Silverberg et al.
(
2020
)
, though the H
α
line widths are
at the borderline of separating accretion from chromospheric
activity. J0446B shows stronger and broader H
α
line emission
than J0446A; for the latter, our photospheric-like MIRI spectra
(
see Appendix
A
)
are more in agreement with H
α
arising from
chromospheric activity. In this Letter, we thus focus on the gas-
rich disk around J0446B, which has an estimated mass accretion
rate of 2.5
×
10
−
11
M
e
yr
−
1
(
S. M. Silverberg et al.
2020
)
. This
rate falls at the lower end of the reported range for young 0.1
–
0.2
M
e
stars of 1
–
10 Myr
(
see C. F. Manara et al.
2023
for a
compilation and references therein
)
.
The M6 spectral type
(
2800 K
)
measured by S. M. Silverberg
et al.
(
2020
)
results in optical emission that is much fainter than
observed
(
for both stars
)
. We determined a spectral type of M4.5
by comparing the Gaia XP spectra of J0446B
(
G. Busso et al.
2022
)
to XP spectra of objects in the TW Hya association
(
as
described in K. L. Luhman
2023
, see Figure
9
in Appendix
A
)
.
This corresponds to an effective temperature of
∼
3100 K
(
M. J. Pecaut & E. E. Mamajek
2013
)
, which provides a
much-improved
fi
t to the broadband spectral energy distribution
(
SED
)
. The stellar luminosity of J0446B is 0.016
L
e
, measured
from the VISTA
J
-band magnitude of 11.828, the bolometric
correction from M. J. Pecaut & E. E. Mamajek
(
2013
)
, and the
zero-point
fl
ux of 3.013
×
10
35
erg s
−
1
. Based on G. Somers
et al.
(
2020
)
models, we estimate the stellar mass to be 0.13
–
0.22
M
e
, depending on whether age is
fi
xed or not. The dust
disk of J0446B was detected but unresolved with Atacama Large
Millimeter
/
submillimeter Array
(
ALMA
)
observations
(
with a
beam size of
∼
0
.
6
)
, yielding a total
fl
ux of 1.2 mJy at 0.9 mm
(
K. Flaherty, private communication
)
, which corresponds to a
dust mass of only 0.1
M
⊕
, assuming optically thin emission and
adopting the same opacity as used in I. Pascucci et al.
(
2016
)
and
a dust temperature of 20 K. This is about 1 order of magnitude
lower than the typical dust mass in disks around young stars of
similar types
(
C. F. Manara et al.
2023
)
.
2.2. JWST Observations and Data Reduction
J0446B was observed with MIRI
/
MRS on 2024 January 29
as part of the JWST Cycle 2 program GO-3153
(
PI: F. Long
)
,
which was designed to reveal the nature of this class of old
accreting disks. The observation started with the target
acquisition procedure using a neutral density
fi
lter, which
placed the brighter IR source
(
i.e., J0446B in the binary
system
)
in the center of the
fi
eld of view. The four-point dither
pattern
(
optimized for point sources
)
was used for thermal
background subtraction. To ensure a high signal-to-noise ratio
of
∼
100 at the longest wavelengths without saturation, each
subband was integrated with 60 groups per ramp in the
FASTR1 mode with a total integration time of
∼
16 minutes.
2
The Astrophysical Journal Letters,
978:L30
(
17pp
)
, 2025 January 10
Long et al.
All three subbands were selected to cover the full wavelength
range from 4.9 to 28.6
μ
m.
As part of the JWST Disk Infrared Spectral Chemistry
Survey
(
the JDISCS collaboration
)
, our data reduction follows
the procedure established in K. M. Pontoppidan et al.
(
2024
)
.
Brie
fl
y, individual cubes were built for each exposure, channel,
and subband using
callwebb_step2
of the JWST Calibra-
tion Pipeline version 1.15.0 and Calibration Reference Data
System context 1254. After background subtraction using the
two opposite dither positions, a 1D spectrum was extracted
with an aperture that increased linearly with wavelength within
each subband. We note that J0446A and J0446B are well
separated in every wavelength channel; thus, the extracted
spectrum is not contaminated by the companion, nor is it
affected by the background subtraction. Lastly, the relative
spectral response functions derived from observations of
asteroids
(
GO-1549 and GO-3034
)
were applied to our data
spectrum to remove remaining fringe patterns and provide
accurate absolute spectrophotometric calibration.
3. Analysis and Results
3.1. Spectrum Overview and Continuum Subtraction
The full SED of J0446B, including the JWST MIRI
/
MRS
spectrum, is shown in Figure
1
. The wavelength range short of
∼
7
μ
m in our MIRI spectrum well aligns with the stellar
photosphere model of 3100 K, suggesting that the region very
close to the central star is depleted of dust materials
(
see the
review of C. Espaillat et al.
2014
)
. Within this wavelength
range, we also see clear stellar absorption features of CO and
H
2
O
(
see Figure
10
in Appendix
B
)
. At longer wavelengths,
both the 10 and 20
μ
m silicate features are visible. Addition-
ally, we
fi
nd two other bumps around 7.5 and 14
μ
m, which are
likely due to a pseudo-continuum produced by optically thick
molecular emission
(
B. Tabone et al.
2023
)
.
The MIRI
/
MRS spectrum of J0446B shows rich molecular
line emission, particularly within the wavelength range of
12
–
16
μ
m
(
Figure
2
)
. Initial inspection of the spectrum based
on the HITRAN database
(
I. E. Gordon et al.
2022
)
and the
iSLAT
tool
(
E. G. Jellison et al.
2024
)
indicates that the inner
disk of J0446B contains a large number of hydrocarbon
molecules, including CH
4
(
peaking at 7.65
μ
m
)
,C
2
H
4
(
10.53
μ
m
)
,C
2
H
6
(
12.17
μ
m
)
,C
2
H
2
(
13.69
μ
m
)
,
13
CCH
2
(
13.73
μ
m
)
, and C
4
H
2
(
15.92
μ
m
)
. The bright emission at
14.85
μ
m corresponds to the Q branch of the hot bending mode
ν
4 of benzene, C
6
H
6
, as described in B. Tabone et al.
(
2023
)
,
which has a line peak
fl
ux comparable to C
2
H
2
. Following the
recent study of a carbon-rich disk around ChaI-147
(
A. M. Arabhavi et al.
2024
)
, we also
fi
nd emission lines of
C
3
H
4
and CH
3
around the corresponding wavelengths of 15.80
and 16.48
μ
m, respectively. Two nitrogen-bearing molecules,
HCN and HC
3
N
(
15.08
μ
m
)
, are clearly detected, and the HCN
emission is heavily blended with C
2
H
2
.CO
2
(
14.98 and
16.18
μ
m
)
, along with its isotopologue
13
CO
2
(
15.41
μ
m
)
,is
the only robustly identi
fi
ed oxygen-bearing molecule in the
spectrum of J0446B. The H
2
O emission, though widely
detected in T Tauri disks
(
see the review of K. M. Pontoppidan
et al.
2014
)
, is very weak and marginally detected in this disk.
Multiple molecular hydrogen
(
H
2
)
lines and two atomic lines
(
[
Ne
II
]
and
[
Ar
II
]
)
are also identi
fi
ed and will be discussed in
Section
3.3
.
To facilitate further analysis o
f the molecular lines, the dust
continuum emission needs to be removed from the observed
spectrum. Following the procedure outlined in K. M. Pontoppidan
et al.
(
2024
)
,we
fi
rst computed the underlying continuum using
an iterative median
fi
lter applied over
fi
ve rounds, with a
window of 65 and
∼
95 wavelength channels for long and short
wavelengths, respectively. We excluded wavelength ranges of
7.1
–
8.5 and 12.0
–
16.5
μ
m, where optically thick C
2
H
2
emission
could produce a pseudo-continuum. At wavelengths
>
16.5
μ
m,
Figure 1.
The SED of J0446B including the JWST MIRI
/
MRS spectrum in blue
(
the noisy long-wavelength range is marked with a lighter color
)
. The gray curve
shows a stellar photospheric model with
T
eff
=
3100 K
(
F. Allard et al.
2012
)
. The four Wide-
fi
eld Infrared Survey Explorer band photometry encompassed both
stellar components in the binary system of J0446; the triangles represent the contribution of J0446B to the W1 and W2 bands assuming the same
fl
ux ratio as the
Ks
band. The inset panel highlights the 10
μ
m silicate features, where the orange curve marks the identi
fi
ed continuum, two dashed lines represent emissivity curves of
amorphous silicates
(
olivine
)
for a single grain size of 0.1 and 2.5
μ
m
(
C. Jaeger et al.
1994
)
, and the dotted line represents that for crystalline silicate of forsterite
(
C. Koike et al.
2003
)
.
3
The Astrophysical Journal Letters,
978:L30
(
17pp
)
, 2025 January 10
Long et al.