of 21
Validation of Elemental and Isotopic Abundances in Late-M Spectral Types with the
Benchmark HIP 55507 AB System
Jerry W. Xuan
1
, Jason Wang
2
, Luke Finnerty
3
, Katelyn Horstman
1
, Simon Grimm
4
, Anne E. Peck
5
, Eric Nielsen
5
,
Heather A. Knutson
6
, Dimitri Mawet
1
,
7
, Howard Isaacson
8
,
9
, Andrew W. Howard
1
, Michael C. Liu
10
, Sam Walker
10
,
Mark W. Phillips
10
, Geoffrey A. Blake
6
, Jean-Baptiste Ruf
fi
o
1
, Yapeng Zhang
1
, Julie Inglis
6
, Nicole L. Wallack
11
,
Aniket Sanghi
1
, Erica J. Gonzales
12
, Fei Dai
1
,
6
, Ashley Baker
1
, Randall Bartos
7
, Charlotte Z. Bond
13
, Marta L. Bryan
14
,
Benjamin Calvin
1
,
3
, Sylvain Cetre
15
, Jacques-Robert Delorme
15
, Greg Doppmann
15
, Daniel Echeverri
1
,
Michael P. Fitzgerald
3
, Nemanja Jovanovic
1
, Joshua Liberman
1
,
16
, Ronald A. López
3
, Emily C. Martin
12
,
Evan Morris
12
, Jacklyn Pezzato
1
, Garreth Ruane
7
, Ben Sappey
17
, Tobias Scho
fi
eld
1
, Andrew Skemer
12
,
Taylor Venenciano
18
, J. Kent Wallace
7
, Ji Wang
19
, Peter Wizinowich
15
, Yinzi Xin
1
, Shubh Agrawal
20
,
Clarissa R. Do Ó
17
, Chih-Chun Hsu
21
, and Caprice L. Phillips
19
1
Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA;
jxuan@astro.caltech.edu
2
Center for Interdisciplinary Exploration and Research in Astrophysics
(
CIERA
)
and Department of Physics and Astronomy, Northwestern University, Evanston, IL
60208, USA
3
Department of Physics & Astronomy, 430 Portola Plaza, University of California, Los Angeles, CA 90095, USA
4
Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
5
Department of Astronomy, New Mexico State University, P.O. Box 30001, MSC 4500, Las Cruces, NM 88003, USA
6
Division of Geological & Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
7
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
8
Department of Astronomy, University of California at Berkeley, CA 94720, USA
9
Centre for Astrophysics, University of Southern Queensland, Toowoomba, QLD, Australia
10
Institute for Astronomy, University of Hawai
i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
11
Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC 20015, USA
12
Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA
13
UK Astronomy Technology Centre, Royal Observatory, Edinburgh EH9 3HJ, UK
14
David A. Dunlap Institute Department of Astronomy & Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada
15
W.M. Keck Observatory, 65-1120 Mamalahoa Highway, Kamuela, HI, USA
16
James C. Wyant College of Optical Sciences, University of Arizona, Meinel Building, 1630 East University Boulevard, Tucson, AZ 85721, USA
17
Center for Astrophysics and Space Sciences, University of California, San Diego, La Jolla, CA 92093, USA
18
Physics and Astronomy Department, Pomona College, 333 North College Way, Claremont, CA 91711, USA
19
Department of Astronomy, The Ohio State University, 100 West 18th Avenue, Columbus, OH 43210, USA
20
Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA
21
Center for Interdisciplinary Exploration and Research in Astrophysics
(
CIERA
)
, Northwestern University, 1800 Sherman, Evanston, IL 60201, USA
Received 2023 October 6; revised 2023 November 30; accepted 2023 December 3; published 2024 February 1
Abstract
M dwarfs are common host stars to exoplanets but often lack atmospheric abundance measurements. Late-M dwarfs are
also good analogs to the youngest substellar companions, which share similar
T
eff
2300
2800 K. We present
atmospheric analyses for the M7.5 companion HIP 55507 B and its K6V primary star with Keck
/
KPIC high-resolution
(
R
35,000
)
K
-band spectroscopy. First, by includ
ing KPIC relative radial velocitie
s between the primary and secondary
in the orbit
fi
t, we improve the dynamical mass precision by 60% and
fi
nd
=
-
+
MM
88.0
B
3.2
3.4
Jup
, putting HIP 55507 B
above the stellar
substellar boundary. We also
fi
nd that HIP 55507 B orbits its K6V primary star with
=
-
+
a
38
3
4
au and
e
=
0.40
±
0.04. From atmospheric retrievals of HIP 55507 B, we measure
[
C
/
H
]
=
0.24
±
0.13,
[
O
/
H
]
=
0.15
±
0.13,
and C
/
O
=
0.67
±
0.04. Moreover, we strongly detect
13
CO
(
7.8
σ
signi
fi
cance
)
and tentatively detect
H
O
2
18
(
3.7
σ
signi
fi
cance
)
in the companion
s atmosphere and measure
=
-
+
CO CO 98
12
13
22
28
and
=
-
+
H
O H O 240
2
16
2
18
80
14
5
after
accounting for systematic errors. From a simpli
fi
ed retrieval analysis of HIP 55507 A, we measure
=
-
+
CO CO 79
12
13
16
21
and
=
-
+
C
O C O 288
16
18
70
12
5
for the primary star. These results demonstrate that HIP 55507 A and B have consistent
12
C
/
13
Cand
16
O
/
18
Otothe
<
1
σ
level, as expected for a chemically homoge
neous binary system. Given the similar
fl
ux
ratios and separations between HIP 55507 AB and systems with young substellar companions, our results open the door
to systematically measuring
13
CO and
H
O
2
18
abundances in the atmospheres of substellar or even planetary-mass
companions with similar spectral types.
Uni
fi
ed Astronomy Thesaurus concepts:
Atmospheric composition
(
2120
)
;
Stellar atmospheres
(
1584
)
;
Isotopic
abundances
(
867
)
;
Radial velocity
(
1332
)
1. Introduction
The elemental abundances of exoplanets and substellar
companions encode their accretion history, providing valuable
insights into planet and star formation mechanisms. It is now
well recognized that measuring abundance ratios besides C
/
O
The Astrophysical Journal,
962:10
(
21pp
)
, 2024 February 10
https:
//
doi.org
/
10.3847
/
1538-4357
/
ad1243
© 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.
1
are crucial for breaking degeneracies and providing a more
complete picture of substellar atmospheres
(
e.g., Cridland et al.
2020
; Turrini et al.
2021
; Mollière et al.
2022
; Chachan et al.
2023
)
when compared to abundance measurements of their
host stars. Recently, isotopologue ratios have also emerged as
an observable in substellar atmospheres
(
Molliere & Snel-
len
2019
; Morley et al.
2019
)
. Zhang et al.
(
2021a
)
measured
=
-
+
CO CO 31
12
13
1
0
17
for the young super-Jupiter TYC 8998-
760-1 b, while Line et al.
(
2021
)
reported
12
CO
/
13
CO
=
10.2
42.6 for the hot Jupiter WASP-77 Ab. Finnerty et al.
(
2023
)
also reported a tentative
13
CO enrichment for WASP-
33 b, although higher signal-to-noise ratio
(
S
/
N
)
data are
needed to con
fi
rm this result. On the other hand, Zhang et al.
(
2021b
)
reported
=
-
+
CO CO 97
12
13
17
25
for an isolated brown
dwarf. These results potentially indicate that the varying
12
C
/
13
C of these objects can be used to constrain their
formation histories. However, more analysis and measurements
are required to bolster our con
fi
dence in these results
(
Line
et al.
2021
)
.
There are abundant measurements of isotopologues in the
stellar literature, especially for giant stars. More recently,
studies have measured isotopologue ratios in dwarf stars
(
e.g.,
Cross
fi
eld et al.
2019
; Botelho et al.
2020
; Coria et al.
2023
)
,
which are thought to better preserve the initial isotopic
abundances in their envelopes compared to giant stars and
are therefore useful for constraining galactic chemical evol-
ution
(
Romano et al.
2017
)
. For context, the Sun has
12
C
/
13
C
=
93.5
±
3.1 and
16
O
/
18
O
=
525
±
21
(
Lyons et al.
2018
)
, while the average local interstellar medium values are
12
C
/
13
C
=
69
±
6 and
16
O
/
18
O
=
557
±
30
(
Wilson
1999
)
.In
circumstellar disks, the relative isotopic abundances can differ
from the inherited interstellar medium values due to processes
such as self-shielding. For example, Calahan et al.
(
2022
)
showed that in certain regions of the inner disk, self-shielding
of CO and C
18
O and UV shielding of H
2
O can result in an
enhanced
H
O
2
18
abundance at the expense of C
18
O. In Zhang
et al.
(
2021a
)
, the authors proposed that ices beyond the CO
snow line may be
13
CO-rich, so if a planet accreted a
signi
fi
cant amount of ice beyond the CO snow line it may
exhibit a lower
12
CO
/
13
CO value compared to its host star.
However, more detailed modeling work is needed to under-
stand the details of isotopic composition and fractionation
chemistry in circumstellar disks
(
Öberg et al.
2023
)
.
In this work, we study the HIP 55507 AB system, which
consists of a M7.5 companion that orbits
40 au from its K6V
primary star. The M dwarf companion was initially identi
fi
ed
from a radial velocity
(
RV
)
trend and later con
fi
rmed by
adaptive optics imaging
(
Gonzales et al.
2020
)
. Using
K
-band
high-resolution
(
R
35,000
)
spectra from Keck
/
KPIC, we
carry out an atmospheric retrieval analysis of HIP 55507 B to
measure the C
/
O,
[
C
/
H
]
,
12
CO
/
13
CO, and
H
OHO
2
16
2
18
in its
atmosphere. In addition, we analyze the KPIC spectra of the
primary star, HIP 55507 A, to measure its
12
CO
/
13
CO and
C
16
O
/
C
18
O using a simpli
fi
ed version of the same framework.
From the high-resolution spectra, we also measure the RVs
of both stars to compute their relative RV. Relative RV data
have been shown to improve orbital constraints for directly
imaged companions, especially when the other data only
sparsely cover the orbital period
(
Schwarz et al.
2016
; Ruf
fi
o
et al.
2019
; Do Ó et al.
2023
)
. We include the KPIC relative
RVs in orbit
fi
ts to measure the companion
s orbital parameters
and dynamical mass.
This paper is organized as follows. In Section
2
, we describe
the properties of HIP 55507 A, including an estimate of its age.
The Keck
/
HIRES, Keck
/
NIRC2, and Keck
/
KPIC observa-
tions and data reduction are detailed in Section
3
. In Section
4
,
we summarize the orbit
fi
ts for HIP 55507 B. Section
5
lays out
our spectral analysis framework for both HIP 55507 A and B,
including the retrieval setup. Section
6
describes the lessons
from our injection-recovery tests for atmospheric retrievals of
HIP 55507 B. The main results of our spectral analysis are
described in Section
7
, with our conclusions in Section
8
.
2. Primary Star Properties
HIP 55507 A is a K6V star located at 25.41 pc with
M
=
0.67
±
0.02
M
e
and
T
eff
=
4250
±
90 K
(
Yee et al.
2017
;
Stassun et al.
2019
; Sebastian et al.
2021
; Anders et al.
2022
)
.
By comparing the star
s Keck
/
HIRES optical spectra with an
empirical spectral library using the
SpecMatch-Emp
tool
(
Yee et al.
2017
)
, we obtain
[
Fe
/
H
]
=
0.02
±
0.09 for the
star.
22
We tabulate the literature properties of HIP 55507 A in
Table
1
. HIP 55507 A hosts a low-mass companion
fi
rst
detected from RV and direct imaging as part of the TRENDS
survey
(
Gonzales et al.
2020
)
.
We estimated the age of HIP 55507 A in two ways. First, we
searched for lithium with the
ARC Echelle Spectrograph
(
Wang
et al.
2003
)
at the Apache Point Observatory 3.5 m on 2023 April
30. The spectrum was reduced with
pyvista
.
23
The spectrum is
placed at rest wavelengths by applying a barycentric correction
and removing the RV measured by Gaia DR2
(
Gaia
Collaboration et al.
2018
)
. No Li absorption is visible at
6707.79
Å
above the noise, and we determine an upper limit of
20 m
Å
on the lithium equivalent width
(
EW
)
by constructing a
series of Li lines with Gaussian pro
fi
les of varying EWs. With
this EW upper limit, we place a lower limit on the stellar age
using BAFFLES
(
Stanford-Moore et al.
2020
)
, which uses a
Bayesian framework to calculate probability distributions on
stellar age for single stars based on Li EW measurements of
stars in stellar associations with robust ages. BAFFLES can
derive a probability distribution function for a
fi
eld star given
an upper limit on Li EW by using
fi
ts to the median Li EW as a
function of
B
V
for each cluster and the scatter about those
relations. Given a Li EW upper limit of 20 m
Å
and
B
V
=
1.24 for HIP 55507 A, we
fi
nd 2
σ
and 3
σ
lower age limits of
838 and 286 Myr, respectively
(
see Figure
1
)
.
We also searched TESS light curves for rotational modula-
tion using the
lightkurve
package
(
Lightkurve Collabora-
tion et al.
2018
)
. HIP 55507 A was observed over two
consecutive TESS sectors covering a baseline of 57 days.
From the light curves, we found a clear periodic signal of
15.8 days
(
see Figure
1
)
. Nearby stars within 15
do not exhibit
similar modulation, suggesting the modulation likely originates
from HIP 55507 A. If we attribute the periodic signal to the
stellar rotation period, a Lomb
Scargle analysis of the two
TESS sectors yields a period of 15.8
±
1.8 days. Given
T
eff
=
4250
±
90 K, we use the gyrochronology tool from
Bouma et al.
(
2023
)
to derive an age of
-
+
1.7
0.7
0.4
Gyr. Therefore,
both the lack of Li and relatively slow rotation point to an age
of
1
2 Gyr for HIP 55507 A.
22
The error bar of 0.09 dex comes from the rms difference between the
measured
[
Fe
/
H
]
of stars in the spectral library and their derived
[
Fe
/
H
]
from
SpecMatch-Emp
. It is the recommended uncertainty to adopt when using
SpecMatch-Emp
(
Yee et al.
2017
)
.
23
https:
//
pyvista.readthedocs.io
/
en
/
latest
/
index.html
2
The Astrophysical Journal,
962:10
(
21pp
)
, 2024 February 10
Xuan et al.
3. Observations and Data Reduction
3.1. Keck
/
HIRES
We collected spectra of HIP 55507 A from 2009 April to 2023
June using the High Resolu
tion Echelle Spectrometer
(
HIRES;
R
60,000; Vogt et al.
1994
)
at the W.M. Keck Observatory. The
data from 2009 to 2015 were collected as part of the M2K
program
(
Gaidos et al.
2013
)
. The observation setup is the same as
that used by the California Planet Search
(
Howard et al.
2010
)
.
The wavelength calibration was co
mputedusinganiodinegascell
in the light path. A iodine-free template spectrum bracketed by
observations of rapidly rotating B-type stars was used to
deconvolve the stellar spectrum from the spectrograph point-
spread function
(
PSF
)
. We then forward model the spectra taken
with the iodine cell using the deconvolved template spectra, the
PSF model, and the iodine cell line atlas
(
Butler et al.
1996
)
.The
Keck
/
HIRES RVs are presented in Appendix
A
and show a long-
term trend with curvature, which is induced by HIP 55507 B
(
Appendix
B
)
.
3.2. Keck
/
NIRC2
We observed HIP 55507 B in the
¢
L
band on UT 2021 May
19 and the
K
and
M
s
bands on UT 2022 June 9 using Keck
/
NIRC2. We did not use a focal plane mask but observed in
pupil-tracking mode to exploit sky rotation for angular
differential imaging
(
ADI; Liu
2004
; Marois et al.
2006
)
.
HIP 55507 B was also imaged with Keck
/
NIRC2 on UT 2012
January 7 and 2015 May 29
(
PI: Justin Crepp
)
as part of the
TRENDS survey
(
Gonzales et al.
2020
)
. The astrometry from
Gonzales et al.
(
2020
)
shows a
100
°
discrepancy in position
angle
(
PA
)
compared to calibrated images in the Keck
Observatory Archive,
24
which could be caused by a mismatch
between the pupil- and
fi
eld-tracking modes used in each
observation
(
E. Gonzales 2023, private communication
)
.
Therefore, we reanalyzed the archival NIRC2 data from
Gonzales et al.
(
2020
)
to update the astrometry. Finally, we
include a single astrometric epoch from UT 2021 December 21
reported in Franson et al.
(
2023
)
.
We
fi
rst preprocess the data using the Vortex Imaging
Processing software package
(
Gomez Gonzalez et al.
2017
;
Christiaens et al.
2023
)
. We perform
fl
at-
fi
elding and bad-pixel
removal and correct for geometric distortions by applying the
solution in Service et al.
(
2016
)
for observations after the
NIRC2 camera and adaptive optics system were realigned on
UT 2015 April 13 and the solution from Yelda et al.
(
2010
)
for
the archival 2012 observation. Then, we perform sky
subtraction following the procedure described in Xuan et al.
(
2018
)
. To register the HIP 55507 B frames, we identify the
star
ʼ
s position by
fi
tting a 2D Gaussian to the stellar PSF in
each frame.
After obtaining the preprocessed cubes, we extracted the
astrometry and photometry of the companion using
pyKLIP
(
Wang et al.
2015
)
, which models a stellar PSF with
Karhunen
Loève image processing following the framework
in Soummer et al.
(
2012
)
and Pueyo
(
2016
)
. We used ADI to
subtract the stellar PSF and tested various model choices to
minimize the residuals after stellar PSF subtraction while
preserving the companion signal, following guidelines in Redai
et al.
(
2023
)
. The 2015 observations for HIP 55507 used
fi
eld-
tracking mode, so we used a least-squares minimization code to
compute the astrometry. We note that our measured astrometry
from the archival Gonzales et al.
(
2020
)
data agree at the
<
1
σ
level with those reported in Franson et al.
(
2023
)
, who also
reanalyzed these data.
From
pyKLIP
forward modeling
(
Wang et al.
2016
)
,we
obtain the
fl
ux ratio between the star and companion for each
photometric band, which we convert to apparent and absolute
magnitudes. For the
¢
L
and
M
s
bands, we scale the
fl
ux ratios to
the primary star
s
W
1 and
W
2 mag, respectively.
25
We convert
the Two Micron All Sky Survey
(
2MASS
)
K
into Mauna Kea
Observatories
(
MKO
)
K
for HIP 55507 A using the color
Table 1
Properties of HIP 55507 AB
Property
Value
References
HIP 55507 A
α
2000.0
11:22:05.75
(
1
)
δ
2000.0
+
46:54:30.2
(
1
)
π
a
(
mas
)
39.35
±
0.015
(
1
)
Distance
(
pc
)
25.41
±
0.02
(
1
)
m
d
a
cos
(
mas yr
1
)
197.49
±
0.01
(
1
)
μ
δ
(
mas yr
1
)
134.78
±
0.01
(
1
)
SpT
K6V
(
4
)
Gaia
G
(
mag
)
9.271
±
0.003
(
1
)
J
(
mag
)
7.367
±
0.021
(
6
)
H
(
mag
)
6.760
±
0.042
(
6
)
K
s
(
mag
)
6.613
±
0.021
(
6
)
W
1
(
mag
)
6.544
±
0.075
(
7
)
W
2
(
mag
)
6.553
±
0.023
(
7
)
Age
(
Gyr
)
-
+
1
.7
0.7
0.
4
This paper
Mass
b
(
M
e
)
0.67
±
0.02
(
2
)
,
(
3
)
,
(
4
)
,
(
5
)
Literature
T
eff
(
K
)
4250
±
90
(
2
)
,
(
3
)
,
(
5
)
Literature
g
log
(
dex
)
4.58
±
0.06
(
3
)
,
(
4
)
,
(
5
)
,
(
8
)
Literature
vi
sin
(
km s
1
)
3.0
±
1.0
(
5
)
T
eff
(
K
)
4200
±
50
This paper
g
log
(
dex
)
4.40
±
0.25
This paper
P
rot
(
days
)
15.8
±
1.8
This paper
[
Fe
/
H
]
0.02
±
0.09
This paper
12
CO
/
13
CO
-
+
7
9
1
6
21
This paper
C
16
O
/
C
18
O
-
+
2
88
70
125
This paper
HIP 55507 B
SpT
M7.5
This paper
Mass
(
M
Jup
)
-
+
88.0
3.2
3.4
This paper
vi
sin
(
km s
1
)
5.50
±
0.25
This paper
[
C
/
H
]
0.24
±
0.13
This paper
[
O
/
H
]
0.15
±
0.13
This paper
C
/
O
0.67
±
0.04
This paper
12
CO
/
13
CO
-
+
9
8
22
2
8
This paper
H
OH
O
2
16
2
18
-
+
2
40
80
145
This paper
Notes.
a
We correct for the DR3 parallax zero-point following the guidelines in
Lindegren et al.
(
2021
)
.
b
The literature values for the stellar mass,
T
eff
, and log
g
agree reasonably well,
so we take the weighted average from the more recent papers and adopt the
standard deviation of the different values as the uncertainty for each parameter.
References.
(
1
)
Gaia Collaboration et al.
(
2023
)
,
(
2
)
Sebastian et al.
(
2021
)
,
(
3
)
Stassun et al.
(
2019
)
,
(
4
)
Petigura
(
2015
)
,
(
5
)
Anders et al.
(
2022
)
,
(
6
)
Cutri et al.
(
2003
)
,
(
7
)
Cutri et al.
(
2021
)
,
(
8
)
Fouesneau et al.
(
2022
)
,
(
9
)
Yee et al.
(
2017
)
.
24
https:
//
koa.ipac.caltech.edu
/
cgi-bin
/
KOA
/
nph-KOAlogin
25
We assume the stars have
¢
L
W
1
=
0 and
¢
M
s
W
2
=
0, as these
photometric bands are in the Rayleigh
Jeans tail of the spectral energy
distribution for HIP 55507 A.
3
The Astrophysical Journal,
962:10
(
21pp
)
, 2024 February 10
Xuan et al.