# A Mini-Neptune from _TESS_ and _CHEOPS_ Around the 120 Myr Old AB Dor member
HIP 94235

George Zhou University of Southern Queensland, Centre for Astrophysics, West Street, Toowoomba, QLD 4350 Australia  Christopher P. Wirth Harvard University, Cambridge, MA 02138, USA.  Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA.  Chelsea X. Huang University of Southern Queensland, Centre for Astrophysics, West Street, Toowoomba, QLD 4350 Australia  Alexander Venner Aberdeen, UK  Kyle Franson  Department of Astronomy, The University of Texas at Austin, TX 78712, USA  Samuel N. Quinn Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA.  L. G. Bouma 51 Pegasi b Fellow  Cahill Center for Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA  Adam L. Kraus Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA  Andrew W. Mann Department of Physics and Astronomy, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA  Elisabeth.R. Newton Department of Physics and Astronomy, Dartmouth College, Hanover, NH 03755, USA  Diana Dragomir  Department of Physics and Astronomy, University of New Mexico, 210 Yale Blvd NE, Albuquerque, NM 87106, USA  Alexis Heitzmann University of Southern Queensland, Centre for Astrophysics, West Street, Toowoomba, QLD 4350 Australia  Nataliea Lowson University of Southern Queensland, Centre for Astrophysics, West Street, Toowoomba, QLD 4350 Australia  Stephanie T. Douglas  Department of Physics, Lafayette College, 730 High St., Easton, PA 18042, USA  Matthew Battley Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK  Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK  Edward Gillen Winton Fellow  Astronomy Unit, Queen Mary University of London, Mile End Road, London E1 4NS, UK  Astrophysics Group, Cavendish Laboratory, J.J. Thomson Avenue, Cambridge CB3 0HE, UK  Amaury Triaud  School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK  David W. Latham Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA.  Steve B. Howell NASA Ames Research Center, Moffett Field, CA 94035, USA  J. D. Hartman Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08540, USA  Benjamin M. Tofflemire Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA  Robert A. Wittenmyer University of Southern Queensland, Centre for Astrophysics, West Street, Toowoomba, QLD 4350 Australia  Brendan P. Bowler  Department of Astronomy, The University of Texas at Austin, TX 78712, USA  Jonathan Horner University of Southern Queensland, Centre for Astrophysics, West Street, Toowoomba, QLD 4350 Australia  Stephen R. Kane Department of Earth and Planetary Sciences, University of California, Riverside, CA 92521, USA  John Kielkopf Department of Physics and Astronomy, University of Louisville, Louisville, KY 40292, USA  Peter Plavchan George Mason University, 4400 University Drive MS 3F3, Fairfax, VA 22030, USA  Duncan J. Wright University of Southern Queensland, Centre for Astrophysics, West Street, Toowoomba, QLD 4350 Australia  Brett C. Addison University of Southern Queensland, Centre for Astrophysics, West Street, Toowoomba, QLD 4350 Australia  Matthew W. Mengel University of Southern Queensland, Centre for Astrophysics, West Street, Toowoomba, QLD 4350 Australia  Jack Okumura  University of Southern Queensland, Centre for Astrophysics, West Street, Toowoomba, QLD 4350 Australia  George Ricker  Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA  Roland Vanderspek  Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA  Sara Seager Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA  Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA  Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA  Jon M. Jenkins NASA Ames Research Center, Moffett Field, CA 94035, USA  Joshua N. Winn Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA  Tansu Daylan Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA  Department of Astrophysical Sciences, Princeton University, Peyton Hall, Princeton, NJ 08544  Michael Fausnaugh Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA  Michelle Kunimoto Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139

###### Abstract

The _TESS_ mission has enabled discoveries of the brightest transiting planet
systems around young stars. These systems are the benchmarks for testing
theories of planetary evolution. We report the discovery of a mini-Neptune
transiting a bright star in the AB Doradus moving group. HIP 94235 (TOI-4399,
TIC 464646604) is a $V_{\mathrm{mag}}=8.31$ G-dwarf hosting a
$3.00_{-0.28}^{+0.32}\,R_{\oplus}$ mini-Neptune in a 7.7 day period orbit. HIP
94235 is part of the AB Doradus moving group, one of the youngest and closest
associations. Due to its youth, the host star exhibits significant photometric
spot modulation, lithium absorption, and X-ray emission. Three 0.06% transits
were observed during Sector-27 of the _TESS_ Extended Mission, though these
transit signals are dwarfed by the 2% peak-to-peak photometric variability
exhibited by the host star. Follow-up observations with _CHEOPS_ confirmed the
transit signal and prevented the erosion of the transit ephemeris. HIP 94235
is part of a 50 AU G-M binary system. We make use of diffraction limited
observations spanning 11 years, and astrometric accelerations from
_Hipparchos_ and _Gaia_ , to constrain the orbit of HIP 94235 B. HIP 94235 is
one of the tightest stellar binaries to host an inner planet. As part of a
growing sample of bright, young planet systems, HIP 94235 b is ideal for
follow-up transit observations, such as those that investigate the evaporative
processes driven by high-energy radiation that may sculpt the valleys and
deserts in the Neptune population.

planetary systems — stars: individual (HIP 94235) techniques: spectroscopic,
photometric

††journal: AJ††software: emcee (Foreman-Mackey et al., 2013), batman
(Kreidberg, 2015), astropy (Astropy Collaboration et al., 2018), PyAstronomy
(Czesla et al., 2019), comove (Tofflemire et al., 2021)

##  1 Introduction

Young planets offer a time lapse view of the construction of the exoplanet
demographics. Planetary systems are thought to undergo rapid evolution within
the first hundreds of millions of years after their formation. Follow-up
characterization of small young planets helps to test our models for the
contraction and mass loss processes that they undergo during this time frame.

Thousands of close-in Neptunes and super-Earths were discovered by the primary
_Kepler_ mission (e.g. Petigura et al., 2013; Burke et al., 2015; Zhu et al.,
2018). The mechanisms that sculpted the period-radius distribution of these
planets can shed light on the early formation and evolution of planetary
systems. The evaporation of primordial Hydrogen and Helium envelopes, driven
by UV and X-ray radiation from young stars, can reproduce the sub-Neptune
desert and radius valley (e.g. Lopez et al., 2012; Owen & Wu, 2013, 2017).
These processes act on rapid timescales because the high-energy fluxes from
young stars rapidly decline over time. Mass loss can also be driven by heat
leaking out from a planet’s deep interior, a process that can last hundreds of
millions of years as the planets cool down after accretion (e.g. Lopez &
Fortney, 2013; Ginzburg et al., 2018; Gupta & Schlichting, 2021). Giant
impacts within compact super-Earth and Neptune systems may erode the envelopes
of some planets, occurring after disk dissipation and before the systems
dyanmically cool within the first hundred million years (e.g. Marcus et al.,
2009; Inamdar & Schlichting, 2015). As their primordial gaseous envelopes are
stripped away, it is possible that some planets lying in less energetic
environments may be replenished by secondary atmospheres (e.g. Kite & Barnett,
2020). Other mechanisms, such as in-situ formation in gas-poor disks, may
naturally carve out the current period-radius distribution of small planets
(Lee & Connors, 2021; Lee et al., 2022), and the radii of small planets may
evolve much slower than predicted from run-away mass loss models (David et
al., 2021).

Young planets can help establish the time-scales for the evolution of the
period-radius distribution of the super-Earth and Neptune populations. For
some planets, mass loss through photoevaporation occurs throughout their
lifetime without significantly changing their radii. Extended atmospheres have
been observed for Neptune sized planets about older field stars in X-ray
(Ehrenreich et al., 2012), Lyman-$\alpha$ (Kulow et al., 2014; Ehrenreich et
al., 2015; Lavie et al., 2017; Bourrier et al., 2018), and He I (Spake et al.,
2018; Allart et al., 2019; Kirk et al., 2020). However, for planets between
2-4 $R_{\oplus}$, run-away evaporation may occur. This process strips away the
outer primordial envelope of these Neptunes and super-Earths, leaving behind
rocky cores. Real-time measurements of photoevaporation for younger systems
can help establish the timescale for this process and its influence on the
radius evolution of planets. Zhang et al. (2022b) detected the Lyman-$\alpha$
transit of the outer planet HD 63433 c in a 400 Myr old planetary system. They
also reported the lack of a detectable escaping hydrogen atmosphere for the
inner planet, suggesting that it may have already undergone run-away
evaporation and lost its primordial atmosphere. Rockcliffe et al. (2021)
reported a non-detection of escaping hydrogen for the 650 Myr old K2-25b, with
one possibility for the non-detection being factors related to the star’s
youth. Zhang et al. (2022a) reported escaping He I for the 500 Myr old mini-
Neptune HD 73583b, showing an excess 0.68% absorption for the He I 10830 Å
line. Tentative detections of atmospheric escape have been reported for
planets in the V1298 Tau system in Ca II, H-$\alpha$ (Feinstein et al., 2021),
and He I (Gaidos et al., 2022)

The _K2_ mission (Howell et al., 2014) yielded some of the first young
transiting planets (e.g. Mann et al., 2016a, b; David et al., 2016; Vanderburg
et al., 2018; Rizzuto et al., 2018; David et al., 2019b, a; Livingston et al.,
2019; Barragán et al., 2019), and provided insight into the radius
distribution of these planets compared to those about older stars (e.g.
Rizzuto et al., 2017; David et al., 2021). The _TESS_ mission (Ricker et al.,
2015) is finding young planets that are more suitable for in-depth
characterization: DS Tuc A (Newton et al., 2019), HD 63433 (Mann et al.,
2020), and AU Mic (Plavchan et al., 2020) are amongst the brightest planet-
hosting stars known. These are the best targets for in-depth follow-up studies
with the suite of new astronomical observatories coming online this decade.

In this paper, we present the discovery of a mini-Neptune sized planet
transiting the $V=8.3$ star HIP 94235, a member of the $\sim 120$ Myr old AB
Doradus moving group, one of the youngest and closest stellar associations.
HIP 94235 was identified as a planet host through a search for planets around
active stars (Zhou et al., 2021). The star’s youth is confirmed from its
spectroscopic and photometric characteristics, such as its rapid rotation and
significant photometric modulation, strength of lithium absorption, and X-ray
emission. The kinematics and independent age estimation of HIP 94235 agree
with that of members of the AB Doradus moving group. The shallow 600 ppm
transits of HIP 94235 b were identified in the single sector of observations
obtained by _TESS_ during the first sector of its ongoing Extended Mission.
Subsequent observations via the _CHEOPS_ space telescope made it possible to
confirm the existence of the transits and improved our ability to predict
future transit times for follow-up studies.

##  2 Observations

Figure 1: The field around HIP 94235 from Left Deep Sky Survey R’ band, Middle
_TESS_ , and Right _CHEOPS_. The images have approximately the same field
sizes (5′) and orientations.

###  2.1 Candidate identification with TESS

The Transiting Exoplanet Survey Satellite (_TESS_ , Ricker et al., 2015)
performed photometric measurements of HIP 94235 in its Sector 27 Camera 2
observations between 2020-07-04 and 2020-07-30. HIP 94235 was observed at two
minute cadence via target pixel stamp observations, and was also included as a
target of multiple _TESS_ Guest Investigator Programs (G03251, D. Huber;
G03272, J. Burt). We make use of Simple Aperture Photometry light curves
(Twicken et al., 2010; Morris et al., 2020) from the Science Processing
Observation Center (SPOC, Jenkins et al., 2016), extracted from the two minute
target pixel files for subsequent analyses.

HIP 94235 was identified as a planet candidate by a dedicated search for
planets around young stars (Zhou et al., 2021). We first identified HIP 94235
as a possible young star via its rotationally modulated light curve. Three
transits were identified using a Box-fitting Least Squares search (Kovács et
al., 2002) on the light curve after detrending using a high order spline-
fitting procedure (Vanderburg & Johnson, 2014). HIP 94235 was also identified
as a threshold crossing event in both the SPOC pipeline (Jenkins et al., 2016)
and the MIT QuickLook Pipeline (Huang et al., 2020a). The SPOC threshold
crossing event diagnostic tests indicate the transit is on target to within
$5.3\pm 2.5\arcsec$. However, it did not survive the TOI vetting process
initially because of the large amplitude of the residuals in the detrended
light curves, which is due to the intrinsic variability of the star. The
candidate was promoted to become a _TESS_ Object of Interest (TOI) after the
confirmation of the photometric transit signals using the _CHEOPS_
observations (Section 2.2).

To account for the instrumental noise due to spacecraft motion and stellar
variability in the light curves, we perform a simultaneous detrending using
the spacecraft quarternions and basis splines following a similar process to
that described in Vanderburg et al. (2019). We iteratively fit for linear
coefficients of the mean, standard deviation, and skew terms of the three
quarternions, together with a spline matrix created by the lightkurve package
(Lightkurve Collaboration et al., 2018). The detrended light curve is adopted
for our subsequent analyses in Section 4.

The corrected _TESS_ light curve from Sector-27 is shown in Figure 2. Figure 3
shows close-ups of individual transits of HIP 94235 b in the raw and detrended
_TESS_ light curves. Figure 4 shows the phase-folded _TESS_ transit light
curve and its best fit model from our global modeling (Section 4).

Figure 2: Sector 27 2 min cadence _TESS_ photometry of the HIP 94235 system.
Top: Simple Aperture _TESS_ light curve of HIP 94235. The three detected
transits of HIP 94235 b are indicated by the arrow marks. Bottom: The custom
detrended _TESS_ light curve of HIP 94235 are shown, with the best fit model
overlaid in red. Figure 3: Individual transits of HIP 94235 b from _TESS_
Sector-27. The top row shows the transits pre-detrending, the bottom row shows
the same transits post-detrending. The best fit transit model is overlaid in
red. Figure 4: Phase folded transit light curves from _TESS_ (Top) and
_CHEOPS_ (Bottom). The black points show the dataset binned at 30 minute
intervals. The red lines show the respective models from our global analysis.

###  2.2 Follow-up photometry with _CHEOPS_

To confirm the existence and constrain the transit ephemeris of HIP 94235 b,
we obtained space-based photometric observations using the ESA’s
CHaracterising ExOPlanets Satellite (_CHEOPS_ Benz et al., 2021), through the
_CHEOPS_ Guest Observers Programme (AO2-005). _CHEOPS_ is a 0.32 m Ritchey-
Chrétien telescope in a nadir-locked 700 km low-Earth orbit, along the Earth’s
day/night terminator, with a period of 98.725 minutes. The telescope has a
field of view of $17\arcmin\times 17\arcmin$, and is instrumentally defocused
with a point-spread function of $16\arcsec$ at a plate scale of
$1\arcsec\mathrm{pixel}^{-1}$.

A transit of HIP 94235 b was observed by _CHEOPS_ between 2021-08-19 23:59 to
2021-08-20 07:33 UTC (visit ID 1568350). The visit consists of five orbits,
with an exposure time of 17 s coadded onboard to a cadence of 34 s. The visit
had an observing efficiency of 61% (fraction of time on target), with
interruptions primarily due to Earth stray light. A total of 498 exposures
were obtained, of which, nine exposures were affected by stray light and Earth
occultation, eight by South Atlantic Anomaly crossings. The observations were
reduced by the _CHEOPS_ Data Reduction Pipeline v13.1.0 (Hoyer et al., 2020),
accounting for bias, dark current, flat fielding, bad pixel correction, smear
contamination and linearization. We adopt the optimal aperture light curve,
with a circular aperture of 31 pixels, for our analysis. Aperture
contamination by background stars is estimated to be at the $6\times 10^{-4}$
level, and is accounted for via the simulations from the data reduction
pipeline.

The _CHEOPS_ light curve exhibits a smooth hours-long trend that can be
attributed to the spot modulated rotational variations of HIP 94235. Shorter
timescale instrumental variations associated with the spacecraft roll angle
are seen on the orbital timescales (Maxted et al., 2021). We model the spot
modulation over the five orbits of observations with a 4th order polynomial
after removal of the transit model. We also fit for a 5th order correlation
between the spacecraft roll angle and the resulting light curve residuals.
This modeling is performed simultaneous to the global fit of the _TESS_ light
curve and associated parameters, such that the uncertainties from this
detrending process are fully accounted for in our results (Section 4). The raw
and corrected _CHEOPS_ light curves are shown in Figure 5, and the binned and
corrected phase-folded data is compared with that from _TESS_ in Figure 4.

The _CHEOPS_ observation was most crucial in refining the ephemeris of HIP
94235 b. With a transit depth of 600 ppm, detecting the photometric transit
event with ground-based facilities is difficult. With only three transits
available during the single sector of _TESS_ observations over the entire
primary and first extended mission, the _TESS_ ephemeris would have quickly
become stale. The ephemeris uncertainty using _TESS_ observations alone would
have been 5.2 hours after five years, making any transit follow-up studies
more difficult to schedule. The single _CHEOPS_ transit reduced the five-year
transit timing uncertainty to 8 minutes. Figure 6 illustrates the reduction in
transit ephemeris uncertainty enabled by the _CHEOPS_ observation.

Figure 5: Transit of HIP 94235 b as captured by _CHEOPS_ on 2021-08-19 over
five _CHEOPS_ orbits. The first panel panel shows the pre-detrending light
curve extracted at an optimal aperture of 31 pixels. The large scale smooth
variation is induced by the spot modulation of the target star, and is
accounted for in our model via a fourth order polynomial. Orbital-timescale
variations are also present, and are accounted for via a 5th order polynomial
fit between the spacecraft roll angle and the light curve residuals, after
subtraction of the transit model and the spot modulation trend. The second
panel shows the detrended _CHEOPS_ light curve. The third panel shows the
residuals after subtraction of the transit, spot modulation signal, and
instrumental systematics models. The fourth panel shows the correlation
between the light curve, after removal of the transit and spot modulation
signals, and the spacecraft roll angle, with the best fit model overlaid.
Figure 6: Transit timing uncertainty for HIP 94235 b with and without follow-
up _CHEOPS_ observations. With only the single sector of _TESS_ observations,
the timing uncertainty erodes by $\sim 1\,\mathrm{hr\,yr}^{-1}$, quickly
making targeted transit follow-up difficult. The _CHEOPS_ transit allowed the
transit ephemeris to be preserved.

###  2.3 Clearing of nearby eclipsing binaries from Las Cumbres Observatory

The large _TESS_ and _CHEOPS_ point-spread functions allow for multiple stars
within a photometric aperture to be the source of a photometric transit
detection. We obtained a transit observation of HIP 94235 b with the Las
Cumbres Observatory Global Network (LCOGT Brown et al., 2013) 1 m telescope at
Las Campanas Observatory on 2021-05-26 UTC. The observations were performed in
the $g^{\prime}$ band, and were defocused to a full width at half maximum of
$7\arcsec$ due to the brightness of the target star. The observations showed
that no nearby stars visually separable with HIP 94235 within $1\arcmin$
exhibited eclipsing or transiting events during the predicted transit of HIP
94235 b. Though the transit of HIP 94235 b was not detected due to its shallow
depth, this LCOGT observation cleared nearby stars of being stellar eclipsing
binaries.

###  2.4 Reconnaissance Spectroscopy

To characterize the target star and confirm its youth spectroscopically, we
obtained three observations of HIP 94235 with the High Resolution Spectrograph
(HRS Crause et al., 2014) on the Southern African Large Telescope (SALT;
Buckley et al., 2006) in October 2020. HRS is a fibre fed echelle spectrograph
with a resolving power of $R\sim 65,000$ over the wavelength range of
$3700-8900\,\AA$. Spectral extraction was performed via the MIDAS pipeline
(Kniazev et al., 2016,
2017)111http://www.saao.ac.za/~akniazev/pub/HRS_MIDAS/HRS_pipeline.pdf. These
observations confirmed the presence of the lithium 6708 Å doublet and strong
chromospheric emission in the Calcium H & K line cores. The lack of
significant radial velocity variations or secondary sets of spectral lines
indicated the transiting candidate was not an obvious blended eclipsing
stellar binary system.

We obtained 16 observations of HIP 94235 using the CHIRON fiber fed cross-
dispersed echelle spectrometer at the SMARTS 1.5-meter telescope located at
Cerro Tololo Inter-American Observatory, Chile (Tokovinin et al., 2013).
CHIRON has a spectral resolving power of $R=80,000$ over the wavelength range
of 4100 to $8700$ Å. Spectra from CHIRON were reduced as per Paredes et al.
(2021). Radial velocities were measured following the procedure from Zhou et
al. (2021) via a least-squares deconvolution of each observation against a
synthetic non-rotating template generated from the ATLAS9 model atmospheres
(Castelli & Kurucz, 2004). The radial velocity measurements are presented in
Table 1.

In addition, we follow Zhou et al. (2021) and measure the spectroscopic
atmospheric parameters of HIP 94235 from the CHIRON spectra. We match the
spectrum of HIP 94235 against an interpolated library of $\sim 10,000$
observed spectra pre-classified by the Spectroscopic Classification Pipeline
(Buchhave et al., 2012), finding that HIP 94235 has an effective temperature
of $5991\pm 50$ K, surface gravity of $4.46\pm 0.02$ dex, and metallicity of
$\mathrm{[M/H]}=-0.05\pm 0.10$ dex. We adopt the spectroscopic effective
temperature as a prior in the global modeling described in Section 4.

We also used the MINERVA-Australis telescope array for further reconnaissance
of HIP 94235. MINERVA-Australis is an array of four identical 0.7 m CDK700
telescopes located at Mt Kent Observatory, Australia (Addison et al., 2019).
The telescopes feed into a single KiwiSpec echelle spectrograph with a
spectral resolving power of $R\approx 80,000$ over the wavelength region of
$4800-6200$ Å. The instrument is environmentally controlled inside a vacuum
chamber, and simultaneous wavelength calibration is provided by two
calibration fibers that bracket the science fibers on the detector, each fed
from a quartz lamp via an iodine cell. Radial velocities are measured from the
extracted spectra as per our CHIRON analyses described above, via a least-
squares deconvolution between the observations and the synthetic non-rotating
template. The radial velocities are provided in Table 1.

Table 1:  Radial Velocity Measurements of HIP 94235  BJD | RV | RV Error | Instrument
---|---|---|---
| (km s-1) | (km s-1) |
2459305.91348 | 8.131 | 0.100 | CHIRON
2459309.87630 | 8.211 | 0.134 | CHIRON
2459311.89601 | 8.220 | 0.073 | CHIRON
2459320.89052 | 8.152 | 0.082 | CHIRON
2459327.86151 | 8.246 | 0.079 | CHIRON
2459335.86210 | 8.340 | 0.120 | CHIRON
2459337.89892 | 8.123 | 0.099 | CHIRON
2459339.86391 | 8.144 | 0.087 | CHIRON
2459459.55982 | 8.069 | 0.087 | CHIRON
2459461.57939 | 8.255 | 0.083 | CHIRON
2459463.56573 | 8.285 | 0.072 | CHIRON
2459464.59690 | 8.273 | 0.068 | CHIRON
2459465.56939 | 8.271 | 0.282 | CHIRON
2459467.53083 | 8.394 | 0.092 | CHIRON
2459478.48380 | 8.459 | 0.098 | CHIRON
2459479.48429 | 8.123 | 0.119 | CHIRON
2459480.51450 | 8.355 | 0.093 | CHIRON
2459481.51666 | 8.466 | 0.096 | CHIRON
2459482.55682 | 8.350 | 0.068 | CHIRON
2459485.52901 | 8.485 | 0.062 | CHIRON
2459506.52501 | 8.237 | 0.071 | CHIRON
2459508.52808 | 8.107 | 0.102 | CHIRON
2459319.14408 | 8.841 | 0.104 | MINERVA-Australis Tel 3
2459324.21159 | 8.964 | 0.175 | MINERVA-Australis Tel 3
2459332.10157 | 8.914 | 0.178 | MINERVA-Australis Tel 3
2459348.10572 | 9.096 | 0.165 | MINERVA-Australis Tel 3
2459452.00282 | 9.152 | 0.143 | MINERVA-Australis Tel 3
2459453.04237 | 8.976 | 0.150 | MINERVA-Australis Tel 3
2459453.99779 | 9.120 | 0.140 | MINERVA-Australis Tel 3
2459456.02452 | 9.171 | 0.139 | MINERVA-Australis Tel 3
2459476.98480 | 8.961 | 0.109 | MINERVA-Australis Tel 3
2459319.14408 | 9.208 | 0.161 | MINERVA-Australis Tel 4
2459324.21159 | 8.766 | 0.101 | MINERVA-Australis Tel 4
2459348.10572 | 8.995 | 0.167 | MINERVA-Australis Tel 4
2459452.00282 | 9.177 | 0.194 | MINERVA-Australis Tel 4
2459453.04237 | 9.017 | 0.146 | MINERVA-Australis Tel 4
2459453.99779 | 9.265 | 0.148 | MINERVA-Australis Tel 4
2459476.98480 | 9.121 | 0.179 | MINERVA-Australis Tel 4
2459319.14408 | 8.955 | 0.176 | MINERVA-Australis Tel 5
2459324.21159 | 9.085 | 0.137 | MINERVA-Australis Tel 5
2459332.10157 | 8.741 | 0.123 | MINERVA-Australis Tel 5
2459348.10572 | 9.077 | 0.181 | MINERVA-Australis Tel 5
Figure 7: Radial velocities for HIP 94235 from CHIRON (red) and MINERVA-
Australis (black), with errorbars representing the quadrature addition of the
observational uncertainties and the best fit jitter. No orbital variations
were detected, as expected for a small planet about an active, rapidly
rotating star. The velocity orbit upper limits at
$1\sigma=0.35\,M_{\mathrm{jup}}$ and $3\sigma=1.27\,M_{\mathrm{jup}}$ are
shown in the solid and dashed red curves respectively.

##  3 Age and membership

HIP 94235 shares space velocities with the AB Doradus moving group (Section
3.1.1). The age of the group has been estimated to be between 50-150 Myr. The
group has also been linked to be co-evolving with the Pleiades. We adopt an AB
Doradus age of 120 Myr for the remainder of this discussion. In addition, a
neighborhood search for tangentially co-moving stars with HIP 94235 reveals a
tentative population of stars, with an age of $\sim 120$ Myr, determined by
their rotation periods. The following sections examine the kinematic
properties of HIP 94235 and the association, as well as the photometric and
spectroscopic characteristics of HIP 94235 that identify its youth independent
of the kinematics.

As accretion dwindles, young stars spin up as they conserve their angular
momentum while they contract in radius. The spin up leads to increased
magnetic activity, large photospheric spots, and increased chromospheric
activity. Both rotation and activity then decay with time as angular momentum
is lost through the stellar wind over the main sequence lifetime of the star.
The _TESS_ light curve of HIP 94235 exhibits significant rotational spot
modulation, which first drew our attention to its potential youth. The
presence of X-ray emission and lithium absorption for the Sun-like host star
confirm its youth. A flare event was also observed during the second orbit of
the _TESS_ sector, consistent with behavior expected for a young star.

###  3.1 Kinematics

####  3.1.1 The AB Doradus moving group

Accurate estimation of the age of a single star is notoriously difficult
(Soderblom, 2010). Stars in clusters and associations have accurate age
estimates since the population can be assessed as a whole, the stars can be
seen to be co-evolving based on their color-magnitude, rotation, and lithium
abundance distributions. Planets found in co-evolving populations offer much
more stringent tests on the temporal evolution of planet properties.

Zuckerman et al. (2004) identified a set of stars within $\sim 50$ pc co-
moving with AB Doradus. The star AB Doradus itself is amongst the closest (15
pc) and most well studied young stars. Membership of the group has been
revised based on chemo-kinematic analyses of the homogeneity of the stars (da
Silva et al., 2009) and updated kinematics from new missions (e.g. Malo et
al., 2013; Gagné et al., 2018). Today, dozens of bona-fide members define the
extent and characteristics of the group, and its age has been estimated to
range from 50 Myr (Zuckerman et al., 2004), $\sim 100-120$ Myr (Luhman et al.,
2005), $>110$ Myr (Barenfeld et al., 2013), to $\sim 150$ Myr (Bell et al.,
2015).

Figure 8 shows the color-magnitude and space-motion of HIP 94235 alongside the
AB Doradus moving group and the Pleiades cluster. HIP 94235 shares kinematic
properties with the moving group, and has been classified as a bona-fide
member via _Hipparcos_ (Malo et al., 2013) and _Gaia_ (Gagné et al., 2018;
Ujjwal et al., 2020) space-motions.

The group has long been linked to Pleiades cluster due to their shared
kinematic velocities (Luhman et al., 2005; Ortega et al., 2007). Recent
mapping of new low density moving groups from Gaia (Kounkel & Covey, 2019)
suggest that the AB Doradus moving group, alongside newly identified Theia 301
and Theia 369 associations, form a long tidal tail streaming away from the
Pleiades (Gagné et al., 2021).

Though more distant (130 pc), the Pleiades cluster contains $\sim 1000$
members, and its age has been more thoroughly investigated than AB Doradus.
Estimates converge to $\sim 120$ Myr via non-rotating isochrones (e.g. Meynet
et al., 1993), lithium depletion boundary (e.g. Stauffer et al., 1998; Barrado
y Navascués et al., 2004), and 3D rotational isochrones (e.g. Brandt & Huang,
2015). The spectroscopic and photometric characteristics of HIP 94235 agree
with members of the Pleiades, reaffirming our adopted age of $\sim 120$ Myr
for the system.

Figure 8: HIP 94235 can be placed kinematically in the $\sim 120$ Myr old AB
Doradus moving group. The figure shows the photometric and space motion
properties of stars in AB Doradus and Pleiades from Gagné et al. (2018). Both
groups have ages of $\sim 120$ Myr, and are thought to be co-moving (Luhman et
al., 2005; Ortega et al., 2007; Kounkel & Covey, 2019). The top left panel
shows the _Gaia_ color magnitude diagram for the two clusters. The remaining
panels show the galactic positions $X,Y,Z$ and motions $u,v,w$ of the
selection. HIP 94235 is marked by the red star in all panels. The local
standard of rest has not been corrected for in the velocities.

####  3.1.2 An independent rotation sequence for co-moving and co-evolving
stars

The sparsity and spread of the AB Doradus moving group makes it difficult to
securely identify true members of the group. The canonical membership list has
been evolving with each new chemo-kinematic dataset since it was originally
defined in Zuckerman et al. (2004). Upcoming _Gaia_ releases with refined
radial velocities for fainter distributions of cool stars may help redefine
groups like AB Doradus, and its links with other associations.

To ensure that our age estimation does not hinge on literature classifications
of HIP 94235, we also independently search for a co-moving population that may
confirm its youth.

Figure 9: The distribution of stars sharing tangential velocities of HIP 94235
identified in our neighborhood search. Top left We find 140 stars to exhibit
clear rotational signatures in their _TESS_ light curves. The blue points are
located within 5 $\rm km\,s^{-1}$ of HIP 94235 in $U$, $V$, and $W$. These
blue points form rotation sequence that agrees with the expected age of HIP
94235 at 120 Myr. The Pleiades sequence from Rebull et al. (2016) is shown in
yellow. The remaining panels show the $U$, $V$, and $W$ velocity distribution
of the tangentially co-moving stars. A clear group can be found around HIP
94235, and is marked out by the 5 $\rm km\,s^{-1}$ boundary in grey.

We follow Tofflemire et al. (2021) and search for stars within 50 pc of HIP
94235 that share its tangential velocity to within $5\,\mathrm{km\,s}^{-1}$
using the comove package222https://github.com/adamkraus/Comove. In this
search, we assume the radial velocity of HIP 94235 for all neighborhood stars.
The search returns a set of $\sim 2000$ stars between $3<G<20$ in magnitude.
We then query for _TESS_ MIT QLP light curves, returning 300 matches up to
$T_{\mathrm{mag}}=13.5$. We apply a Lomb Scargle period search for each light
curve, with an upper limit on the rotation period of $P_{\mathrm{rot}}<13$.
After manual examination, we find 140 stars that exhibit rotational modulation
in their light curves with secure periods.

Figure 9 shows the rotation distribution of these 140 stars relative to the
Pleiades. The vast majority of these stars are relatively bright, and have
well measured radial velocities from _Gaia_. We find 21 stars with space
motion velocities within 5 $\rm km\,s^{-1}$of HIP 94235, these are coloured in
blue for clarity. This population forms a rotation sequence that agrees with
HIP 94235. This set of co-evolving stars are presented in Table 2.

To estimate the age of the distribution, we fit its color and rotation period
with a rotation-age relationship. We select eight F, G, K stars
($0.1<B-V<0.8$) within our sample that lie on the slow sequence of the
rotation sequence, and model their rotation periods with the age-color-
rotation relationship from Mamajek & Hillenbrand (2008). The posterior age
distribution for these eight stars, and their joint posterior, is shown in
Figure 10. We find that the color-rotation distribution of this subsample can
be described by the slow sequence with an age of $118_{-16}^{+18}$ Myr.

We obtained a small number of spectra of this subsample using the Las Cumbres
Observatory (Brown et al., 2013) Robotic Echelle Spectrographs (NRES)
facilities. The stars that were observed exhibit lithium absorption as
expected for their youth. The lithium $6708\,\AA$ equivalent width has been
noted in Table 2 where available. Future works examining the lithium
absorption of this subsample may yield a lithium depletion boundary to confirm
this age estimate.

Figure 10: The age posterior of a subsample of F,G,K stars that are co-moving
with HIP 94235. We fit the age relationship from Mamajek & Hillenbrand (2008)
to this population. The age posterior of each individual star is shown in
grey, and the joint posterior in blue. We find a best-fit age of
$118_{-16}^{+18}$ Myr to the distribution.

Of these 21 co-moving stars, only one is a canonical member of AB Doradus
(Gagné et al., 2018). It is likely that the AB Doradus moving group itself is
dispersed and ill defined, and our subset of co-moving and potentially co-
evolving stars form part of the extended AB Doradus group.

Table 2:  Candidate co-moving and co-evolving stars with HIP 94235  TIC | RA (deg) | DEC (deg) | U ($\rm km\,s^{-1}$) | V ($\rm km\,s^{-1}$) | W ($\rm km\,s^{-1}$) | B (mag) | V (mag) | T (mag) | K (mag) |  $P_{\mathrm{rot}}$ (d) | Li EW $(\AA)$
---|---|---|---|---|---|---|---|---|---|---|---
71314712 | 330.63261 | -47.67748 | -4.0 | -26.0 | -4.0 | 9.3 | 8.8 | 8.2 | 7.4 | 2.4 |
91231096 | 310.66707 | -46.67197 | -1.9 | -26.3 | -1.9 | 11.0 | 10.0 | 9.1 | 7.6 | 4.6 |
93839949 | 176.15935 | -49.41756 | -7.3 | -24.7 | -7.3 | 9.9 | 8.9 | 8.0 | 6.5 | 1.9 |
101403239 | 300.79541 | -52.96796 | -1.7 | -26.0 | -1.7 | 12.8 | 11.9 | 10.9 | 9.4 | 12.7 |
108272865 | 285.27521 | -28.71442 | -5.2 | -27.4 | -5.2 | 9.0 | 8.5 | 8.0 | 7.2 | 2.4 |
142144969 | 99.73088 | -74.42526 | -5.2 | -26.1 | -5.2 | 10.5 | 9.7 | 9.0 | 7.9 | 7.0 |
197597944 | 327.20235 | -39.48626 | -8.5 | -28.1 | -8.5 | 10.3 | 9.7 | 9.0 | 8.0 | 4.1 |
206603521 | 21.80129 | -57.29366 | -7.9 | -26.8 | -7.9 | 14.5 | 13.0 | 11.1 | 8.8 | 3.8 |
234299476 | 358.66836 | -60.85971 | -8.3 | -27.2 | -8.3 | 11.1 | 10.0 | 9.0 | 7.5 | 9.0 |
270200832 | 331.81221 | -74.08641 | -3.0 | -26.4 | -3.0 | 9.5 | 9.0 | 8.5 | 7.8 | 2.0 |
270259954 | 359.04562 | -39.05314 | -7.5 | -27.7 | -7.5 | 9.2 | 8.2 | 7.3 | 5.9 | 7.8 |
270377865 | 345.08089 | -26.15444 | -3.1 | -26.3 | -3.1 | 8.1 | 7.5 | 6.9 | 5.9 | 3.6 |
278271178 | 112.29658 | -82.20387 | -4.4 | -24.9 | -4.4 | 14.3 | 13.1 | 11.1 | 8.9 | 10.9 |
280683734 | 342.73660 | -79.16594 | -7.5 | -27.0 | -7.5 | 9.6 | 9.0 | 8.4 | 7.6 | 2.6 | 0.136
290081380 | 319.52174 | -29.52160 | -7.0 | -29.4 | -7.0 | 15.1 | 13.9 | 11.5 | 9.3 | 10.3 |
341498715 | 318.52194 | -63.70066 | -6.9 | -27.1 | -6.9 | 10.9 | 10.1 | 9.4 | 8.2 | 5.9 |
357709300 | 189.06702 | -79.52628 | -7.3 | -26.8 | -7.3 | 12.2 | 11.0 | 10.2 | 8.7 | 7.0 |
369897885 | 171.32165 | -84.95449 | -8.4 | -25.4 | -9.6 | 8.1 | 7.6 | 7.2 | 6.5 | 1.5 | 0.030
389660501 | 321.33728 | -43.51207 | -3.5 | -25.8 | -3.5 | 13.4 | 12.1 | 11.0 | 9.1 | 7.1 |
405077613 | 112.74683 | -84.32411 | -8.6 | -27.2 | -8.6 | 10.8 | 10.0 | 9.2 | 7.9 | 4.9 | 0.200
409141582 | 312.08079 | -28.02455 | -5.7 | -27.3 | -5.7 | 13.7 | 13.6 | 11.4 | 9.1 | 2.7 |
409141582 | 312.08194 | -28.02453 | -9.2 | -25.0 | -9.2 | 13.7 | 13.6 | 11.4 | 9.1 | 2.7 |
439417806 | 301.15247 | -35.21447 | -3.7 | -26.2 | -3.7 | 9.4 | 8.9 | 12.2 | 7.7 | 1.4 |
464646604 | 287.74114 | -60.27264 | -5.7 | -27.0 | -5.7 | 8.9 | 8.3 | 7.8 | 6.9 | 2.2 | 0.141
466277708 | 305.70238 | -65.25823 | -6.9 | -27.0 | -6.9 | 10.3 | 9.6 | 9.0 | 8.1 | 3.8 |

###  3.2 X-ray

The rapid rotation in young stars leads to increased chromospheric activity.
As a result, young stars often exhibit higher X-ray and UV emissions than
their slowly rotating older counterparts. HIP 94235 is cataloged in the Second
ROSAT All-sky Bright Source Catalog (Boller et al., 2016). HIP 94235 matches
with 2RXS J191057.9-601611, with a count rate of $0.1766\pm 0.0425$
counts/second and a hardness ratio of $-0.124\pm 0.176$. Using the calibration
from Fleming et al. (1995), we find an X-ray luminosity for HIP 94235 of
$\log(L_{X}/L_{\mathrm{bol}})=-3.93\pm 0.13$.

The strength of the X-ray activity can help yield a qualitative age estimate
for a single star. We adopt Equation A3 from Mamajek & Hillenbrand (2008) to
find an approximate age of $50-130$ Myr $(1\sigma)$ for HIP 94235 from its
X-ray luminosity, consistent with the expected age of the AB Doradus group.

###  3.3 Lithium

Stars undergo lithium depletion during their main-sequence evolution. Lithium
is depleted during proton collisions in the cores of stars. Convective mixing
between the envelope and the core leads to a gradual depletion of lithium
absorption in the observed spectra of Sun-like stars. As such, the lithium
abundance of a star can be a tracer for its youth, though direct age estimates
from the lithium absorption strength is only qualitative, as is the case for
any age indicator when interpreting single stars. Our CHIRON spectra of HIP
94235 reveal significant absorption about the 6708 Å Li doublet. We model the
doublet and the nearby Fe I line simultaneously, measuring a Li equivalent
width of $0.1413\pm 0.0092\,\AA$ for HIP 94235.

Figure 11 compares the lithium 6708 Å equivalent width against distributions
from known AB Doradus members (da Silva et al., 2009), Pleiades, and Praesepe
clusters (Quinn et al., 2012, 2014; Zhou et al., 2021). HIP 94235 is
consistent with that of other known members of AB Doradus and Pleiades, and
exhibits convincingly stronger lithium absorption than members of the 600 Myr
Praesepe cluster, in agreement with a 120 Myr age estimate.

Figure 11: Age indicators of HIP 94235 are consistent with that from AB
Doradus and Pleiades members. AB Doradus members are shown in open orange
circles, Pleiades members in closed orange circles, and Praesepe members in
grey. Top panel shows the distribution of Li 6708 Å doublet equivalent widths
for AB Doradus (da Silva et al., 2009), Pleiades and Praesepe members (Quinn
et al., 2012, 2014; Zhou et al., 2021). Bottom panel shows the distribution of
rotation periods for the same set of stars.

###  3.4 Rotation

The _TESS_ observations show that HIP 94235 exhibits significant photometric
variability at the 2% level, consistent with the semi-periodic signature of
spot modulation. Figure 12 shows the periodicity of HIP 94235 via a Lomb-
Scargle periodogram, with a peak rotation period of $2.24\pm 0.11$ days. We
adopt the width of the rotational peak of the periodogram as the uncertainty
on the measured period. The rotational modulation is clear in phase-folded
light curve in the right panel of Figure 12, and the evolution of starspots
over successive rotations can also be seen. Additionally, we modeled the light
curve via a stochastically-driven damped harmonic oscillator through a
Gaussian process using the celerite package (Foreman-Mackey et al., 2017).
Modeling the posterior via a Markov chain through emcee (Foreman-Mackey et
al., 2013), we found a posterior distribution for the frequency term
$\log\omega_{0}$ to be $-0.791_{-0.045}^{+0.047}$, corresponding to a period
of $2.20\pm 0.10$ days.

Figure 11 shows the rotation periods of AB Doradus, as well as the Pleiades
and Praesepe cluster members. We adopt the membership list for AB Doradus from
Gagné et al. (2018), and derived rotation periods for stars with available
_TESS_ light curves from the MIT FFI QLP library (Huang et al., 2020b) for
stars with available light curves. Rotation periods are measured via a Lomb-
Scargle period search, and down-selected by hand to remove stars that do not
show unambiguous rotational signatures. By-hand corrections of period aliases
were also applied. Of the 66 members listed in Gagné et al. (2018), 55 were
bright enough and had accessible _TESS_ QLP light curves, and 37 yielded
convincing detections of a rotation signal. Rotation periods for Pleiades
members were adopted from Rebull et al. (2016), and for Praesepe from Rebull
et al. (2017). The rotation of HIP 94235 agrees with the Pleiades and AB
Doradus distribution, in agreement with its kinematic age estimates.

Figure 12: Left: Lomb-Scargle periodogram of the HIP 94235 from its _TESS_ light curve. The red dashed line denotes the adopted rotation period, while the dotted purple lines denote the aliases at $P/2$ and $2P$. The orbital period of the planet is marked in orange. Right: Phase-folded TESS light curve, with color gradient applied such that later periods are a lighter blue. Table 3:  Properties of HIP 94235  Parameter | Value | Source |
---|---|---|---
Astrometry
Right Ascension | 19:10:57.87 | Gaia Collaboration et al. (2021) |
Declination |  $-$60:16:21.49 | Gaia Collaboration et al. (2021) |
Parallax (mas) | $17.061\pm 0.037$ | Gaia Collaboration et al. (2021) |
Proper Motion
Gaia (2016.1) RA Proper Motion (mas yr-1) | $11.632\pm 0.025$ | Gaia Collaboration et al. (2021) |
Gaia (2016.1) Dec Proper Motion (mas yr-1) | $-100.836\pm 0.025$ | Gaia Collaboration et al. (2021) |
Hipparcos (1991.1) RA Proper Motion (mas yr-1) | $12.755\pm 0.890$ | 199 (1997) |
Hipparcos (1991.4) Dec Proper Motion (mas yr-1) | $-99.617\pm 0.796$ | 199 (1997) |
Hipparcos-Gaia Average RA Proper Motion (mas yr-1) | $11.509\pm 0.025$ | Brandt (2021) |
Hipparcos-Gaia Dec Average Proper Motion (mas yr-1) | $-100.628\pm 0.019$ | Brandt (2021) |
Photometry
TESS (mag) | $7.758\pm 0.006$ | Stassun et al. (2018) |
B (mag) | $8.943\pm 0.027$ | Henden et al. (2016) |
V (mag) | $8.31\pm 0.03$ | Henden et al. (2016) |
J (mag) | $7.201\pm 0.023$ | Skrutskie et al. (2006) |
H (mag) | $6.966\pm 0.023$ | Skrutskie et al. (2006) |
K (mag) | $6.881\pm 0.027$ | Skrutskie et al. (2006) |
 _Gaia_ (mag) | $8.173207\pm 0.00031$ | Gaia Collaboration et al. (2021) |
$\mathrm{\emph{Gaia}}_{\mathrm{BP}}$ (mag) | $8.4634\pm 0.0013$ | Gaia Collaboration et al. (2021) |
$\mathrm{\emph{Gaia}}_{\mathrm{RP}}$ (mag) | $7.70637\pm 0.00087$ | Gaia Collaboration et al. (2021) |
WISE W1 (mag) | $6.841\pm 0.066$ | Cutri & et al. (2012) |
WISE W2 (mag) | $6.823\pm 0.02$ | Cutri & et al. (2012) |
WISE W3 (mag) | $6.836\pm 0.016$ | Cutri & et al. (2012) |
WISE W4 (mag) | $6.728\pm 0.067$ | Cutri & et al. (2012) |
Kinematics and Position
$U$ ($\rm km\,s^{-1}$) | $-5.61\pm 0.34$ | Derived |
$V$ ($\rm km\,s^{-1}$) | $-27.03\pm 0.17$ | Derived |
$W$ ($\rm km\,s^{-1}$) | $-11.82\pm 0.18$ | Derived |
Distance (pc) | $58.54_{-0.07}^{+0.08}$ | Derived |
Physical Properties
$M_{\star}$ ($M_{\odot}$) | $1.094_{-0.007}^{+0.024}$ | Fitted (Uniform Prior) |
$R_{\star}$ ($R_{\odot}$) | $1.08_{-0.10}^{+0.11}$ | Fitted (Uniform Prior) |
Effective Temperature $T_{\mathrm{eff}}$ (K) | $5991\pm 50$ | This paper |
Surface Gravity $\log g_{\star}$ (cgs) | $4.460\pm 0.05$ | This paper |
[m/H] | 0.0 | (Barenfeld et al., 2013) |
$v\sin I_{\star}$ $(\rm km\,s^{-1})$ | $24.4\pm 1.0$ | This paper |
$I_{\star}\,(^{\circ})$ |  $>70$ $(3\sigma)$ | Calculated as per Masuda & Winn (2020) |
Age (Myr) | 50-150 | Zuckerman et al. (2004) |
|  | Luhman et al. (2005) |
|  | Bell et al. (2015) |
Limb darkening coefficients (TESS) | (0.17, 0.41) | Claret (2017) |
Limb darkening coefficients (CHEOPS) | (0.27, 0.29) | Claret (2017) |
Activity indicators
$P_{rot}$ (days) | $2.24\pm 0.11$ | This paper |
Lithium $6708\,\AA$ Equivalent Width $(\AA)$ | $0.1413\pm 0.0092$ | This paper |
X-Ray luminosity $\log(L_{X}/L_{\mathrm{bol}})$ | $-3.93\pm 0.13$ | (Boller et al., 2016) |

##  4 Global model

To best estimate the stellar and planetary properties of the HIP 94235 system,
we perform a global modeling incorporating the _TESS_ and _CHEOPS_ transits,
radial velocities, and photometric and spectroscopic properties of HIP 94235.

The transit models were computed as per Mandel & Agol (2002), implemented via
batman (Kreidberg, 2015). The free parameters for this model are the stellar
mass and radius, the time of transit center $T_{0},$ the orbital period $P$,
the planetary radius ratio $R_{p}/R_{\star}$, and the eccentricity parameters
$\sqrt{e}\cos{\omega}$ and $\sqrt{e}\sin{\omega}$, where $e$ is the
eccentricity and $\omega$ is the longitude of periastron.

We model the _CHEOPS_ transit and its associated stellar variability and
instrumental characteristics simultaneous to the global modeling. The transit
model is computed as per Mandel & Agol (2002). The hours-long rotational
modulation signal is modeled via a 4th degree polynomial with respect to time.
The correlation between the light curve and the spacecraft motion is modeled
via a 5th degree polynomial against the roll angle. Figure 5 shows the
_CHEOPS_ light curve before and after the removal of the best fit stellar
variability and instrumental model.

The out-of-transit Keplerian radial velocity was modeled with additional
parameters describing the systemic velocity $\gamma$, planetary mass $M_{p}$,
and a jitter term for each instrument.

The stellar mass and radius were modeled using the MIST isochrones (Dotter,
2016), and constrained by their photometric magnitudes and parallax priors
from _Gaia_ _G_ , Bp, Rp (Gaia Collaboration et al., 2018), _Hipparcos_ TYCHO
$B$ and $V$ bands (Perryman et al., 1997), 2MASS $J$, $H$, and $Ks$ bands
(Skrutskie et al., 2006). Additionally, the age was restricted to be $120\pm
50$ Myr as per the age of the AB Dor moving group, and the limb darkening
coefficients were fixed to theoretically interpolated values (Claret &
Bloemen, 2011; Claret, 2017). All other parameters were assigned uniform
priors with physically motivated boundaries.

The modeling was performed simultaneously for all parameters using Monte Carlo
Markov Chain (MCMC) analysis, making use of the emcee package (Foreman-Mackey
et al., 2013). Results are listed in Tables 3 and 4, the best fit light curve
model is shown in Figure 2, and the best fit radial velocity model is shown in
Figure 7. The spectral energy distribution of HIP 94235 is shown in Figure 13,
along with the template ATLAS9 model spectrum (Castelli & Kurucz, 2004)
computed at the best fit stellar parameters of HIP 94235. We note that the
mid-infrared WISE magnitudes show no excess that might be indicative of a
remnant debris disk around the young star.

Figure 13: Spectral energy distribution of HIP 94235. Magnitudes from Tycho $B$, $V$, Gaia $Bp$, $G$, $Rp$, 2MASS $J$, $H$, $Ks$, and WISE $W1$, $W2$, $W3$,$W4$ are plotted. We note that WISE $W1$, $W2$, $W3$,$W4$ were not used in the global modeling, and are marked by a lighter color compared to the bluer bands. The template spectrum from ATLAS9 model atmospheres (Castelli & Kurucz, 2004) is plotted for reference. Table 4:  Derived parameters for HIP 94235 b  Parameter | Joint model | Priors |
---|---|---|---
Fitted Parameters
$T_{0}$ (BJD) | $2459037.8704_{-0.0022}^{+0.0011}$ | Uniform |
$P$ (days) | $7.713057_{-0.000021}^{+0.000021}$ | Uniform |
$R_{p}/R_{\star}$ | $0.0253_{-0.00059}^{+0.00075}$ | Uniform |
$i$ (deg) | $87.14_{-0.17}^{+0.16}$ | Uniform |
$\sqrt{e}\cos{\omega}$ | $0.07_{-0.54}^{+0.50}$ | Uniform |
$\sqrt{e}\sin{\omega}$ | $0.26_{-0.27}^{+0.20}$ | Uniform |
$M_{p}$ ($M_{\earth}$) |  $<379$ $(3\sigma)$ | Uniform |
$\gamma_{\mathrm{MINERVA-Australis3}}$ | $9013_{-70}^{+58}$ | Uniform |
$\gamma_{\mathrm{MINERVA-Australis5}}$ | $8960_{-126}^{+123}$ | Uniform |
$\gamma_{\mathrm{MINERVA-Australis6}}$ | $9050_{-81}^{+88}$ | Uniform |
$\gamma_{\mathrm{CHIRON}}$ | $8264_{-30}^{+31}$ | Uniform |
RV Jitter MINERVA-Australis 3 | $80_{-56}^{+73}$ | Uniform |
RV Jitter MINERVA-Australis 5 | $154_{-98}^{+210}$ | Uniform |
RV Jitter MINERVA-Australis 6 | $140_{-79}^{+113}$ | Uniform |
RV Jitter CHIRON | $100_{-28}^{+35}$ | Uniform |
Inferred parameters
$e$ | $0.32_{-0.20}^{+0.20}$ | Derived |
$\omega$ (deg) | $17_{-92}^{+67}$ | Derived |
$R_{p}$ ($R_{\oplus}$) | $3.00_{-0.28}^{+0.32}$ | Derived |
$a/R_{\star}$ | $15.7_{-1.5}^{+1.6}$ | Derived |
$a$ ($AU$) | $0.07870_{-0.00017}^{+0.00056}$ | Derived |
Transit duration (days) | $0.103_{-0.018}^{+0.009}$ | Derived |
$K_{\mathrm{rv}}$ $(\rm m\,s^{-1})$ |  $<150$ $(3\sigma)$ | Derived |
$T_{\mathrm{eq}}$ (K) | $1060\pm 50$ | Derived |

##  5 HIP 94235 B: A 60 AU M-dwarf companion

Table 5:  Diffraction limited measurements of HIP 94235 B  Instrument | Epoch | Separation | Position Angle | $\Delta m$ | Reference
---|---|---|---|---|---
|  | (mas) | (∘) | (mag) |
VLT-NaCo | 2010-07-30 | $506\pm 7$ | $150.6\pm 0.8$ |  $3.8\pm 0.3$ (H band) |  Chauvin et al. (2015), Desidera et al. (2015)
Gemini-Zorro | 2021-07-23 | $596\pm 5$ | $162.87\pm 0.48$ | 5.84 (832 nm) | This Work
Gemini-Zorro | 2021-10-22 | $600\pm 8$ | $161.73\pm 0.75$ | 5.31 (832 nm) | This Work

Diffraction limited observations revealed a bound M-dwarf companion to HIP
94235. The companion was identified in speckle imaging observations of HIP
94235 during the candidate vetting process, and also identified from archival
adaptive optics observations searching for wide Jovian companions to young
stars 11 years prior. We describe the observations below, and perform compute
astrometric orbital solutions for HIP 94235B.

We obtained high-contrast imaging at 562nm and 832 nm with the Zorro speckle
imager on the 8 m Gemini South Observatory on 2021-07-23 and 2021-10-22, which
revealed a faint stellar companion to HIP 94235. Zorro is a dual-channel
speckle imager with a pixel scale of $0.01\arcsec\mathrm{pixel}^{-1}$ and an
approximate full width at half maximum of $0.02\arcsec$. Data reduction and
analysis were performed as per Howell et al. (2011) and Howell et al. (2016).
Figure 14 shows the Zorro image at 562nm and 832 nm of HIP 94235 on
2021-10-22. The 832 nm observation achieved a contrast ratio of $\Delta
m=6.68$ at $0.5\arcsec$ separation. A companion of $\Delta m=5.3$ was
identified at a separation of $0.6\arcsec$ in the red band. No companions with
contrasts brighter than $\Delta m=4.7$ were detected in the blue arm.
Observations from 2021-07-23 also identified the same companion in the 832 nm
observations, though the blue arm was not functional at the time.

Figure 14: Gemini South Zorro speckle observations of HIP 94235 on 2021-10-22.
The speckle auto cross correlation functions are shown in the top row, with
the image from the Blue camera at 562nm on the left, and the Red camera at
832nm on the right. A companion is detected in the red arm with a contrast of
$\Delta m=5.3$ at a separation of $0.6\arcsec$. No companions were detected on
the blue arm with a limit of $\Delta m>4.7$. The companion is marked in the
red arm image.

Adaptive optics imaging of HIP 94235 was also carried out as part of a large
program to characterize the occurrence rates of giant planets at large
separations by Chauvin et al. (2015). These observations, using the NaCo high
contrast Adaptive Optics (AO) imager on VLT-UT4, were obtained on 2010-07-30.
They revealed the stellar companion at a separation of $0.5\arcsec$ with a
contrast of $\Delta H=3.8\pm 0.3$. The positional information from the
diffraction limited imaging observations are listed in Table 5.

Based on the _Gaia_ and _Hipparcos_ proper motions measurements, the
accumulated motion of HIP 94235 over the 11 year interval is $1.1\arcsec$. In
contrast, the relative motion between the imaged companion and the target star
is $0.1\arcsec$, which strongly suggests the pair is a a bound stellar binary
with a projected separation of 31 AU. We henceforth refer to this companion as
HIP 94235 B.

To determine the approximate properties of HIP 94235 B, we adopt the 100 Myr
MIST isochrones (Choi et al., 2016) to model its 832 nm and $H$-band
magnitudes. We approximate the 832nm band with the $I$ band magnitudes from
MIST, and adopt magnitudes for the companion of $I=10.76\pm 0.3$ and
$H=13.05\pm 0.4$. We adopt a fixed metallicity of $[\mathrm{M/H}]=0$ as per
that for the AB Doradus moving group (da Silva et al., 2009), and adopt a
strong Gaussian prior for the distance to HIP 94235 B as per its _Gaia_
parallax. We find the companion is an M-dwarf of mass $0.26\pm
0.04\,M_{\odot}$ and radius $0.31\pm 0.03\,R_{\odot}$. The companion is
incapable of hosting the transiting companion responsible for the planetary
transit signal detected in _TESS_. To check if this companion is capable of
being the source of the transit signal, we deblend and refit for the transit
about the M-dwarf companion. We find a best fit radius ratio of
$R_{2}/R_{1}=0.46\pm 0.01$ for such a system, with a V-shaped transit that is
incompatible with the observed light curve. A model comparison yields a
Bayesian information criterion difference of $\Delta\mathrm{BIC}=65.9$ between
the best fit model of a transit about the companion M-dwarf and that of our
nominal planetary transit scenario. We therefore rule out the companion being
the source of the observed transit signal.

Although the available imaging detections cover only a small part of the
orbital arc of HIP 94235 B, it is possible in principle that the binary orbit
could be constrained. Several authors (e.g. Calissendorff & Janson, 2018;
Snellen & Brown, 2018; Brandt, 2018; Kervella et al., 2019) have demonstrated
that it is possible to combine proper motion measurements from _Gaia_ and
_Hipparcos_ to produce long-timescale astrometric data that can be used to
detect the reflex orbital motion of orbiting companions. Brandt et al. (2019)
extended this technique further by jointly fitting Hipparcos-Gaia astrometry
with radial velocities and relative astrometry from direct imaging, and found
that the orbits of massive companions can be precisely constrained even when
the orbital periods are much longer than the observational duration. This
method has been profitably applied to many further systems; Bowler et al.
(2021), for example, were able to extract precise orbital parameters and
masses for two white dwarfs with orbital periods in excess of $\gtrsim 200$
yrs despite possessing no more than $\sim 30$ yrs of observational data in
both cases.

Inspection the Hipparcos-Gaia Catalog of Accelerations (Brandt, 2021) shows
that HIP 94235 displays a statistically significant astrometric acceleration
of 0.24 mas yr-1 ($\chi^{2}=58.5$) between the _Gaia_ and _Hipparcos-Gaia_
proper motions, equivalent to a drift in the stellar tangential velocity of
$\approx 70~{}\rm m\,s^{-1}$, which can plausibly be attributed to HIP 94235
B. This motivates us to attempt an orbital fit for HIP 94235 B based on the
available data.

Table 6:  Orbital parameters of HIP 94235 B  Parameter | Median $\pm$ 1$\sigma$ | Mode | Priors
---|---|---|---
Informed Priors
Parallax (mas) | $17.061\pm 0.037$ | - | Gaussian
$M_{\mathrm{A}}$ ($M_{\odot}$) | $1.094\pm 0.05$ | - | Gaussian
$M_{\mathrm{B}}$ ($M_{\odot}$) | $0.26\pm 0.04$ | - | Gaussian
Fitted Parameters
$a$ (AU) | $56^{+9}_{-7}$ | $54$ | Log-uniform
$\sqrt{e}\sin{\omega}$ | $0.35^{+0.14}_{-0.19}$ | $0.40$ | Uniform
$\sqrt{e}\cos{\omega}$ | $0.20^{+0.48}_{-0.34}$ | $0.48$ | Uniform
Mean anomaly at BJD $=2457000$ (deg) | $170^{+110}_{-80}$ | $130$ | Uniform
$i$ (deg) | $67.8^{+2.7}_{-2.9}$ | $67.4$ | $\sin{i}$
$\Omega$ (deg) | $20^{+11}_{-7}$ | $18$ | Uniform
Barycentric RA proper motion (mas yr-1) | $10.50\pm 0.24$ | $10.46$ | Uniform
Barycentric declination proper motion (mas yr-1) | $-102.99\pm 0.35$ | $102.91$ | Uniform
Derived Parameters
$P$ (years) | $365^{+92}_{-69}$ | $362$ | Derived
$e$ | $0.25^{+0.22}_{-0.14}$ | $0.19$ | Derived
$\omega$ (deg) | $300^{+30}_{-80}$ | $320$ | Derived
Periastron distance (AU) | $43^{+11}_{-15}$ | $47$ | Derived
Time of periastron (years CE) | $2184^{+107}_{-74}$ | $2164$ | Derived

Figure 15: Keplerian orbital model to the proper motion of HIP 94235 (top left
in right ascension, top right in declination) and relative astrometry of HIP
94235 B (bottom left in separation, bottom right in position angle). The best-
fit model is shown in black, while the orbits in gray are drawn randomly from
the posteriors. In the fit to the proper motions, all values are normalised to
the proper motion of the system barycenter. Figure 16: The projected sky orbit
of HIP 94235 B. The data format is as in Figure 15. The dotted line marks the
intersection of the planes of the orbit and of the sky and the black arrowhead
indicates the direction of motion. Despite the short arc of observations our
joint fit allows us to robustly constrain several orbital parameters,
including strong constraints on the orbital inclination
($i~{}=~{}67.8^{+2.7}_{-2.9}$ deg) and semi-major axis ($a~{}=~{}56^{+9}_{-7}$
AU) of the binary.

To initialize our model we first used orbitize! (Blunt et al., 2020) to fit
the relative astrometry of HIP 94235 B assuming system total system mass of
$1.09+0.26=1.35~{}M_{\odot}$. orbitize! makes use of the Orbits For The
Impatient (OFTI) Bayesian sampling method (Blunt et al., 2017) that is well-
suited to fitting the orbits of directly detected companions with orbital
periods much longer than the observational span such as HIP 94235 B. Next we
extracted the posteriors from the orbitize! fit and calculated the
corresponding $\chi^{2}$ value for a fit to the _Hipparcos-Gaia_ astrometry
for each set of orbital parameters assuming stellar masses of
$M_{\mathrm{A}}=1.09$ $M_{\odot}$, $M_{\mathrm{B}}=0.26$ $M_{\odot}$. This
allowed us to identify a tightly constrained initial parameter space for most
orbital parameters.

To model the orbit of HIP 94235 B, we run a joint fit to the astrometry based
on that of Brandt et al. (2019). For modelling the _Hipparcos-Gaia_ astrometry
we use the equations described in Venner et al. (2021b), while the
corresponding expressions for the relative astrometry can be found in Pourbaix
(1998). As in Venner et al. (2021a, b), we resample the _Hipparcos_ and _Gaia_
proper motions using the observational epochs recorded in the Hipparcos
Intermediate Astrometric Data for the former and the Gaia Observation Forecast
Tool333https://gaia.esac.esa.int/gost/ for the latter. To explore the model
parameter space we use the differential evolution MCMC sampler
edmcmc444https://github.com/avanderburg/edmcmc (Vanderburg, 2021).

A total of 11 parameters are used for the model: the system parallax $\varpi$,
the primary mass $M_{A}$, the secondary mass $M_{B}$, the semi-major axis $a$,
the eccentricity $e$ and argument of periastron $\omega$ parameterized as
$\sqrt{e}\sin\omega$ and $\sqrt{e}\cos\omega$, the mean anomaly at an
arbitrary reference epoch $\text{BJD}=2457000$, the orbital inclination $i$,
the longitude of node $\Omega$, and finally two terms for the proper motion of
the system barycenter.

The radial velocity trend generated by HIP 94235 B is too small to be detected
in the available data so we do not make use of RV data in the joint fit. The
lack of radial velocity information in the fit results in the classical
180-degree degeneracy in the longitude of node $\Omega$ and argument of
periastron $\omega$; following convention we report the solution with $\Omega$
in the range $[0,180]$ deg. The argument of periastron used in our model is
that of the primary’s orbit rather than that of the companion.

Of the 11 parameters used in the joint model, the parallax $\varpi$ was
assigned a Gaussian prior of $17.061\pm 0.037$ mas based on the _Gaia_ EDR3
astrometric solution while the primary mass $M_{A}$ was given a Gaussian prior
of $1.094\pm 0.05$ $M_{\odot}$, with an inflated uncertainty as compared to
the value in Table 3 to avoid unduly biasing the model. Initial trials of the
orbital fit were run without informed priors on the secondary mass $M_{B}$,
however these runs tended to produce results skewed towards implausibly small
values for this parameter ($M_{B}<0.1~{}M_{\odot}$), which in turn resulted in
excessively broad distributions in the orbital parameters. It was therefore
deemed prudent to adopt an informed prior of $M_{B}=0.26\pm 0.04~{}M_{\odot}$
for the final model.

The results of our joint model for the orbit of HIP 94235 B are presented in
Table 6. Despite the span of observations being much shorter than the orbital
period of the binary, we are able to robustly constrain most orbital
parameters. We measure a semi-major axis of $a~{}=~{}56^{+9}_{-7}$ AU for HIP
94235 B, approximately $\approx 80\%$ larger than the projected separation,
corresponding to an orbital period of $365^{+92}_{-69}$ yr. Our model shows a
preference for relatively low orbital eccentricities ($0.25^{+0.22}_{-0.14}$,
$e<0.61$ at $95\%$ confidence), resulting in a periastron separation of
$43^{+11}_{-15}$ AU for the binary. We obtain a tightly constrained orbital
inclination of $67.8^{+2.7}_{-2.9}$ degrees and a precise longitude of node of
$20^{+11}_{-7}$ degrees, while our measurements of $\omega$ and the time of
periastron for HIP 94235 B are comparatively imprecise ($300^{+30}_{-80}$ deg,
$2184^{+107}_{-74}$ CE).

Our fit to the astrometric data is shown in Figures 15 and 16. In the fit to
the absolute astrometry it can be observed that the _Hipparcos_ proper motion
measurement is too imprecise to significantly detect the astrometric signal.
Conversely, the _Gaia_ and averaged _Hipparcos-Gaia_ proper motions are so
precise that their uncertainties are not visible in the figure, and it is
these measurements that drive the detection of the astrometric reflex signal
of HIP 94235 B. The importance of the $>10$-year timespan of relative
astrometry is evident from Figures 15 and 16, as were it not for the detection
of HIP 94235 B in an archival NaCo observation in 2010 it would not be
possible to derive such strong constraints on the binary orbit.

We note that the astrometric orbit solution is dependent on the priors we
adopt for the system. For example, when the mass prior for HIP 94235 B is
removed, the orbital semi-major axis becomes degenerate with eccentricity. As
such, future diffraction limited observations remain important in validating
and refining the orbital parameters of the system.

Based on our results, we predict the magnitude of the radial velocity trend on
HIP 94235 generated by HIP 94235 B to be $9.6\pm 1.3~{}\rm
m\,s^{-1}~{}yr^{-1}$. This trend is not detectable in our RV data, but it is
possible that future high-precision radial velocity measurements will be able
to detect this acceleration. If so, the radial velocity information could then
be used to improve the constraints on the orbit of HIP 94235 B.

##  6 Summary and Discussion

We report the discovery and statistical validation of a mini-Neptune around a
bright young Sun-like star. HIP 94235 b is a
$3.00_{-0.28}^{+0.32}\,R_{\oplus}$ planet in a 7.7 day period orbit around its
$V_{\mathrm{mag}}=8.31$ host star. Based on its kinematics, HIP 94235 can be
placed in the AB Doradus moving group, with an age of $\sim 120$ Myr. The
kinematics age is in agreement with that expected from the stellar rotation
rate, lithium abundance, and X-ray emission intensity.

The 600 ppm transits of HIP 94235 b were identified at a signal-to-noise of
9.7 from _TESS_ Extended Mission Section 27 observations. However, the 2-day
periodicity 2% stellar variability led to the planetary signal slipping
through the _TESS_ official planet selection processes. The detection of HIP
94235 b demonstrated the necessity of a dedicated search for planets around
noisy, active young stars. Confirmation of transits came from the _CHEOPS_
mission. We obtained five orbits of observations with _CHEOPS_ to recover the
transit of HIP 94235 b. Such follow-up would have been difficult to schedule
with ground-based facilities due to the shallow transit.

Figure 6 demonstrates the importance of CHEOPS follow-up in preserving the
transit timing ephemeris. Without such observations, the timing derived from
the three transits observed in the single sector _TESS_ observation would have
eroded by $\sim 1$ hour per year. If a follow-up confirmation was not obtained
within the first year after discovery, targeted transit observations would
have been difficult to schedule and the recovery of such a small planet
challenging (see Dragomir et al., 2020).

HIP 94235 b is one of the smallest planets that has been found transiting a
young star. Figure 17 shows the position of HIP 94235 b in the distribution of
small planets around young stars. Most _TESS_ planets that transit young
stars, including HIP 94235 b, have radii that place them above the radius
valley delineated by Fulton et al. (2017) and Owen & Wu (2017). HIP 94235 b is
at the edge of detectability for the _TESS_ light curve of HIP 94235, and
selection biases likely shape the current _TESS_ distribution of young
planets. However, Rizzuto et al. (2017) found young clusters and associations
surveyed by _K2_ hosted planets larger than the equivalent distribution about
field stars. A similar holistic study of young stars surveyed by _TESS_ may
elucidate the radius evolution timescale for young planets.

Interestingly, despite continuous monitoring from the _K2_ mission of $\sim
1000$ Pleiades members, no confirmed planets have yet been found in the $125$
Myr old cluster (e.g. Hartman et al., 2010; Rizzuto et al., 2017). Systems
such as TOI-451 (Newton et al., 2021) in the Pisces-Eridanus stream (Meingast
et al., 2019; Curtis et al., 2019), and HIP 94235 in AB Doradus, may help
constrain the occurrence rates and radius properties of planets at the $\sim
100$ Myr age range. These can help infer if the absence of planets in the
Pleiades is due to detection biases, or if other astrophysical mechanisms may
be at play.

|
---|---
Figure 17: HIP 94235 b amongst the distribution of small planets. The left
panel shows the radius and equilibrium temperature distribution of planets
from Fulton et al. (2017). The sizes of each point mark the $V$ band magnitude
of each host star, with the scale defined by the left panel. Planets around
known young stars marked by the individual points. HIP 94235 b lies near the
edge of the mini-Neptune desert. The right panel compares the planetary radius
and host star magnitude of HIP 94235 b compared to other systems. Young
planets are highlighted. The cyan points mark other transiting systems from
the NASA Exoplanet Archive (January 2021). Young systems included in the
figure are TOI-1227 (Mann et al., 2021), V1298 Tau (David et al., 2019a, b),
K2-33 (Mann et al., 2016b; David et al., 2016), DS Tuc A (Newton et al., 2019;
Benatti et al., 2019), Kepler-63 (Sanchis-Ojeda et al., 2013), HIP 67522
(Rizzuto et al., 2020), HD63433 (Mann et al., 2020), TOI-451 (Newton et al.,
2021), AU Mic (Plavchan et al., 2020), TOI-251 (Zhou et al., 2021), TOI-942
(Zhou et al., 2021; Carleo et al., 2021), K2-284 (David et al., 2018a),
TOI-837 (Bouma et al., 2020), TOI-1098 (Tofflemire et al., 2021), K2-233
(David et al., 2018b), K2-77 (Gaidos et al., 2017), K2-95, K2-100, K2-101,
K2-102, K2-104, EPIC-211822797 (Mann et al., 2017), TOI-1807, TOI-2076 (Hedges
et al., 2021), Kepler 1627A (Bouma et al., 2021), TOI-1268 (Dong et al.,
2022).

Planets like HIP 94235 b, lying near the edge of the sub-Neptune valley, can
provide key observational tests for the mechanisms of mass loss in young
planets. HIP 94235 b and other recently discovered planets around young stars
are subjected to significant high-energy radiation, which can be a dominant
driver for rapid mass loss within the first hundreds of millions of years post
formation (Owen & Jackson, 2012). To estimate the mass evolution of HIP 94235
b, we adopt the analytical approach from Owen & Wu (2017). We find that we can
replicate the current radius of HIP 94235 b via a planet model that has a high
initial envelope mass fraction, and undergoes rapid mass loss with a time
scale of $\sim$ 100-200 Myr (Figure 18). HIP 94235 b has a current energy-
limited mass-loss rate of $\sim 5\,M_{\oplus}\,\mathrm{Gyr}^{-1}$. At the end
of this process, we expect the envelope mass fraction to reduce from 10% to
$\sim 1$% of the total planet mass. Such mass and radius evolution is expected
for many close-in Neptunes and super-Earths around Sun-like stars.

Figure 18: At 100 Myrs, HIP 94235 is undergoing run-away mass-loss evolution.
The top shows the mass evolution of primordial envelope for the planet as per
Owen & Wu (2017). Within the first $\sim 100$ Myr, HIP 94235 b is expected to
lose most of its primordial envelope. The bottom panel shows the corresponding
radius evolution expected for HIP 94235 b over the next few hundred million
years.

Figure 19 shows the X-ray irradiation experienced by HIP 94235 b compared to
other systems about X-ray sources identified in the Second ROSAT Point Source
Catalog (Boller et al., 2016). To convert ROSAT count rates to fluxes, we
adopt the calibration in Fleming et al. (1995) and parallaxes made available
from Gaia Collaboration et al. (2018). The majority of planetary systems
around ROSAT X-ray sources are young, and the recent _TESS_ discoveries of
nearby planet-hosting young stars are the most suitable targets for follow-up
X-ray and UV observations. AU Mic (Plavchan et al., 2020) has the highest
ROSAT count rate for any planet hosting star. Similarly, other nearby systems
such as the planets around V1298 Tau (David et al., 2019b, a), the ultra-short
period super-Earth TOI-1807b (Hedges et al., 2021), DS Tuc Ab (Newton et al.,
2019), and HIP 94235 b mark the inner boundary of X-ray irradiation for small
planets. The only planets around stars not identified as young in literature
residing in more energetic environments are the hot Jupiter NGTS-16b (Tilbrook
et al., 2021) and the Earth-sized inner planet in the Kepler-1514 system
(Dalba et al., 2021). Neither have radii susceptible to significant
modification by photoevaporation. Recent observations by Zhang et al. (2022b)
detected the Lyman-alpha transit of HD63433c. Similar observations in the
X-ray for young active stars have the potential of anchoring the
photoevaporation models.

Figure 19: A large fraction of host stars that exhibit significant X-ray
emission are known young stars. The X-ray irradiation received by all known
transiting planets systems that have X-ray counterparts in the second ROSAT
point source catalog (Boller et al., 2016) are shown. We see a paucity of
planets with radii between $2-10\,R_{\oplus}$ in energetic environments. Older
systems are marked by open light blue points, planets around known young stars
or association and cluster members are marked by closed points, with older
systems in light blue, younger systems in red. For brevity, names with ‘TOI’
have been truncated to their TOI numbers only, and the V1298 Tau system are
denoted by ‘Vb’, ‘Vc’, ‘Vd’, ‘Ve’.

Other avenues of mass loss are also possible. Atmospheric erosion from giant
impact events can occur even more quickly, before compact super-Earth systems
settle dynamically, acting on the tens of millions of years time scale post
disk dispersal (e.g. Izidoro et al., 2017). Core-powered mass loss may also
reproduce the observed radius distribution independent of the host star
irradiation (Lopez & Fortney, 2013; Ginzburg et al., 2018). By inferring the
ages of _Kepler_ systems through numerous indicators, David et al. (2021)
found the radius gap may form at longer timescales. As such, it is unclear
which mechanism dominates in shaping our current planet distribution.

At 50 AU, HIP 94235 is one of the tightest stellar binaries (Su et al., 2021).
Like the DS Tuc AB system (Newton et al., 2019), the orbit of the binary is
aligned with that of the planetary orbit. This follows the trend from
Christian et al. (2022) that wide transiting planet-hosting binaries with
separations between 100-700 AU are preferentially found in edge-on orbits.
Christian et al. (2022) suggest that such trends may be due to the companions
being formed from disk fragmentation, or the realignment of the inner disk by
the perturbing outer companion. The efforts by _TESS_ follow-up teams to
provide diffraction limited imaging of a majority of planet candidates, such
as HIP 94235, will help probe the continuation of this trend to $<100$ AU
separations.

The brightness of HIP 94235 makes the system suitable for follow-up
atmospheric characterization with the next generation of space and ground-
based facilities. Adopting the mass-radius relationship from Wolfgang et al.
(2016), HIP 94235 b has a predicted mass of $11.2\pm 1.4\,M_{\oplus}$,
yielding a transmission spectroscopic metric of $96\pm 25$. As such, HIP 94235
b ranks amongst the top dozen known planets between $1.6-4\,R_{\oplus}$ in its
suitability for follow-up transmission observations. In the era of JWST, we
can compare the atmosphere of HIP 94235 b against planets of similar radii
about older stars. We may find that young planets host primarily primordial
atmospheres dominated by hydrogen and helium, with older planets hosting
heavier water-rich atmospheres, or that some highly energetic environments may
never allow secondary atmospheres to form. Orbital obliquities of young
planets can help constrain the timescales of migration for planets that may
have formed further out in their planetary systems. We expect a $\sim
10\,\mathrm{m\,s}^{-1}$ Rossiter-McLaughlin signal if HIP 94235 b is in a well
aligned projected orbit. Stellar activity will be a limiting factor in
achieving a secure detection of the spectroscopic transit, though past works
have shown that this can be mitigated on transit-timescales due to the
smoothly varying nature of the rotational modulated velocity noise (e.g. Palle
et al., 2020; Benatti et al., 2021).

We respectfully acknowledge the traditional custodians of all lands throughout
Australia, and recognise their continued cultural and spiritual connection to
the land, waterways, cosmos, and community. We pay our deepest respects to all
Elders, ancestors and descendants of the Giabal, Jarowair, and Kambuwal
nations, upon whose lands the MINERVA-Australis facility at Mt Kent is
situated. GZ thanks the support of the ARC DECRA program DE210101893. CW and
GZ thank the support of the TESS Guest Investigator Program G03007. CH thanks
the support of the ARC DECRA program DE200101840. EG gratefully acknowledges
support from the David and Claudia Harding Foundation in the form of a Winton
Exoplanet Fellowship. CHEOPS is an ESA mission in partnership with Switzerland
with important contributions to the payload and the ground segment from
Austria, Belgium, France, Germany, Hungary, Italy, Portugal, Spain, Sweden and
the United Kingdom. We thank support from the CHEOPS GO Programme and Science
Operations Centre for help in the preparation and analysis of the CHEOPS
observations. This research has used data from the CTIO/SMARTS 1.5m telescope,
which is operated as part of the SMARTS Consortium by RECONS This study was
based in part on observations made using the Las Cumbres Observatory global
telescope network, using time allocated by the National Science Foundation’s
NOIRLab (NOIRLab Prop. ID NOAO2021A-009; principal investigator: J. Hartman).
Some of the observations reported in this paper were obtained with the
Southern African Large Telescope (SALT). Some of the observations in the paper
made use of the High-Resolution Imaging instrument Zorro obtained under Gemini
LLP Proposal Number: GN/S-2021A-LP-105. Zorro was funded by the NASA Exoplanet
Exploration Program and built at the NASA Ames Research Center by Steve B.
Howell, Nic Scott, Elliott P. Horch, and Emmett Quigley. Zorro was mounted on
the Gemini North (and/or South) telescope of the international Gemini
Observatory, a program of NSF’s OIR Lab, which is managed by the Association
of Universities for Research in Astronomy (AURA) under a cooperative agreement
with the National Science Foundation. on behalf of the Gemini partnership: the
National Science Foundation (United States), National Research Council
(Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio
de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência,
Tecnologia, Inovações e Comunicações (Brazil), and Korea Astronomy and Space
Science Institute (Republic of Korea). This research has made use of the NASA
Exoplanet Archive, which is operated by the California Institute of
Technology, under contract with the National Aeronautics and Space
Administration under the Exoplanet Exploration Program. Funding for the TESS
mission is provided by NASA’s Science Mission directorate. We acknowledge the
use of public TESS Alert data from pipelines at the TESS Science Office and at
the TESS Science Processing Operations Center. This research has made use of
the Exoplanet Follow-up Observation Program website, which is operated by the
California Institute of Technology, under contract with the National
Aeronautics and Space Administration under the Exoplanet Exploration Program.
This paper includes data collected by the TESS mission, which are publicly
available from the Mikulski Archive for Space Telescopes (MAST). Resources
supporting this work were provided by the NASA High-End Computing (HEC)
Program through the NASA Advanced Supercomputing (NAS) Division at Ames
Research Center for the production of the SPOC data products. MINERVA-
Australis is supported by Australian Research Council LIEF Grant LE160100001,
Discovery Grants DP180100972 and DP220100365, Mount Cuba Astronomical
Foundation, and institutional partners University of Southern Queensland, UNSW
Sydney, MIT, Nanjing University, George Mason University, University of
Louisville, University of California Riverside, University of Florida, and The
University of Texas at Austin. D. D. acknowledges support from the TESS Guest
Investigator Program grants 80NSSC21K0108 and 80NSSC22K0185. TESS, CHEOPS,
Exoplanet Archive, CTIO 1.5 m, LCOGT, Gemini:Zorro, SALT, MINERVA-Australis

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