# Observations of PAHs in the atmospheres of discs and exoplanets

Barbara Ercolano1,2, Christian Rab1,3, Karan Molaverdikhani1,2, Billy
Edwards4,5,6,7 E-mail<EMAIL_ADDRESS>Thomas Preibisch1, Leonardo Testi8,
Inga Kamp9, Wing-Fai Thi3
1 Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität
München, Scheinerstr. 1, 81679 München, Germany
2 Exzellenzcluster ‘Origins’, Boltzmannstr. 2, 85748 Garching, Germany
3 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1,
85748 Garching, Germany
4 Blue Skies Space Ltd., 69 Wilson Street, London, EC2A 2BB, UK
5 AIM, CEA, CNRS, Université Paris-Saclay, Université de Paris, F-91191 Gif-
sur-Yvette, France
6 Department of Physics and Astronomy, University College London, Gower
Street, London, WC1E 6BT, UK
7 Paris Region Fellow
8 European Southern Observatory (ESO), Karl-Schwarzschild-Str 2, D-85748
Garching, Germany
9 Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700
AV Groningen, The Netherlands

###### Abstract

Polycyclic aromatic hydrocarbons (PAHs) play a key role in the chemical and
hydrodynamical evolution of the atmospheres of exoplanets and planet-forming
discs. If they can survive the planet formation process, PAHs are likely to be
involved in pre-biotic chemical reactions eventually leading to more complex
molecules such as amino acids and nucleotides, which form the basis for life
as we know it. However, the abundance and specific role of PAHs in these
environments is largely unknown due to limitations in sensitivity and range of
wavelength of current and previous space-borne facilities. Upcoming infrared
space spectroscopy missions, such as Twinkle and Ariel, present a unique
opportunity to detect PAHs in the atmospheres of exoplanets and planet-forming
discs. In this work we present synthetic observations based on conservative
numerical modeling of typical planet-forming discs and a transiting hot
Saturnian planet around solar type star. Our models show that Twinkle and
Ariel might both be able to detect the 3.3 $\mu$m PAH feature within
reasonable observing time in discs and transiting planets, assuming that PAHs
are present with an abundance of at least one tenth of the interstellar medium
value.

###### keywords:

exoplanet atmospheres, protoplanetary discs, PAHs

##  1 Introduction

The chemical and hydrodynamical evolution of the gaseous atmospheres of young
exoplanets and planet-forming discs is a crucial step for the formation of
habitable worlds. Irradiation from the central star plays an important role by
kick-starting chemical reactions (see e.g. review from Henning & Semenov,
2013) and heating up the atmospheres of discs and planets, which under certain
circumstances will become gravitationally unbound and escape the system.
Thermally driven winds, also known as photoevaporative outflows, can disperse
the planet-forming material in discs and also remove substantial gas from the
atmospheres of young planets (see e.g. Ercolano & Pascucci, 2017; Owen, 2019,
for recent reviews).

In this context, large polyatomic molecules known as polycyclic aromatic
hydrocarbons (PAHs), play a crucial role in the thermo-chemo-dynamics of gas
atmospheres (e.g. Tielens, 2008). PAHs are constituted of multiple benzene
rings and, due to their high photoelectric yield (Bakes & Tielens, 1994), are
extremely efficient at converting incoming far-ultraviolet radiation (FUV)
into heating for the gas, thus having a strong influence on atmospheric loss
rates (Mitani et al., 2020; Gorti et al., 2009). PAHs dramatically affect the
ionisation level of the gas (e.g. Thi et al., 2019) and thus the coupling to
magnetic fields, thus also affecting the evolution of the gas in the
atmospheres. In the last few years PAHs have become the focus of significant
interest in the astrobiology community because of the likely role they play in
pre-biotic chemistry, representing an important step towards the formation of
amino-acids and nucleotides (Ehrenfreund et al., 2007). Despite the important
role PAHs play in the evolution of planetary and planet-forming disc
atmospheres, their abundance in these environments is largely unknown.

PAHs distinctive infrared features have been observed everywhere in the
interstellar medium (ISM) by space telescopes like the Infrared Space
Observatory (ISO) and the Spitzer Space Telescope, since the mid-nineties. The
abundance of PAHs (typical size of 50 carbon atoms) in the ISM is constrained
to be $\approx 3\times 10^{-7}$ relative to hydrogen nuclei (Tielens, 2008).
PAHs are also likely to exist in the atmospheres of exoplanets. In our solar
system, for example, PAHs have been detected in relatively high concentrations
(34 carbon atoms per PAH particle on average) in the fully evolved nitrogen-
rich atmosphere of Saturn’s moon Titan (Dinelli et al., 2013; López-Puertas et
al., 2013). However, no search for PAHs in the atmospheres of exoplanets has
been possible with current or past instrumentation.

Curiously, PAH detection remains elusive also in the planet-forming discs
around young solar-type stars (T-Tauri stars), while it is commonly observed
in discs around more massive stars (Herbig stars) (e.g. Seok & Li, 2017; Geers
et al., 2006; Visser et al., 2007). This prompts the question about whether
PAHs are destroyed in T Tauri discs, due to the strong X-ray irradiation of
their active central stars (Siebenmorgen & Krügel, 2010), while they can
survive in the atmospheres of discs around more massive stars that emit less
strongly at X-ray wavelengths (Lange et al., 2021). Another possibility, is
that PAHs are not detected in T Tauri discs because they have already been
processed into more complex organics through hydrogenation, oxygenation and
hydroxylation and have thus lost their infrared signature. The final
explanation could be that past and current observational facilities have not
had enough sensitivity or have not been able to observe at the appropriate
wavelength range in order to detect PAHs in T Tauri discs.

This question remains at the moment unanswered as most disc surveys (e.g. with
the Spitzer or Herschel satellites) have not covered a wavelength range
including the neutral PAH feature at $3.3\,\mathrm{\mu m}$. They mostly
targeted the PAH features at $6.2\,\mathrm{\mu m}$ and $7.6\,\mathrm{\mu m}$,
attributed to cations, which are unlikely to dominate in the atmospheres of T
Tauri discs. Furthermore, at longer wavelengths the emission is often
dominated by silicate emission or absorption, especially for T Tauri stars
(see Seok & Li 2017). This makes a clear identification PAH features at
$6.2\,\mathrm{\mu m}$ and $7.6\,\mathrm{\mu m}$ challenging and should be a
lesser issue at $3.3\,\mathrm{\mu m}$.

Attempts at detecting the $3.3\,\mathrm{\mu m}$ feature from the ground (e.g.
with the Very Large Telescope, VLT) have returned limited success (see the
high noise level of these observations reported by Seok & Li (2017), as the
Earth atmosphere poses significant limitations at this wavelength. In
principle, the $3.3\;\mathrm{\mu m}$ feature could have been detected by the
Infrared Space Observatory (ISO) SWS instrument (in orbit for 28 months from
November 1995), but the sensitivity was too low to detect it or to set
meaningful upper limits for discs around T Tauri stars. Fortunately, this
situation is likely to change in the near future with the launch of the James
Webb Space Telescope111https://www.cosmos.esa.int/web/jwst (JWST) and new
facilities like Twinkle222www.twinkle-spacemission.co.uk (Edwards et al.,
2018) and Ariel333https://arielmission.space (Tinetti et al., 2018, 2021),
which will be able to observe in the near-infrared with unprecedented
sensitivity. In this work we especially focus on Twinkle and Ariel, because
their capabilities for observations of PAHs is unexplored and these missions
will have large, dedicated programs for extrasolar science. Furthermore
Twinkle and Ariel can provide data complementary to JWST in the mid-infrared,
but also in the near-infrared where many planet-forming discs might be too
bright for JWST.

The aim of this paper is to present a first investigation of the observability
of the 3.3 $\mu$m PAH feature in the atmospheres of nearby planet-forming
discs and exoplanets. Through conservative modelling, convolved with
instrumental responses of Twinkle and Ariel space telescopes, we obtain
synthetic observations of (i) PAH emission spectra in a typical T Tauri disc
and (ii) PAH absorption features in transit data of hot-Saturn orbiting G-type
star (V=9, K=7.5) for different assumptions of PAH abundance. Our methods and
assumptions are described in Section 2. Section 3 presents our results and
discusses them in terms of observability. Finally, Section 4 contains a short
summary of our results.

##  2 Methods

###  2.1 PAH cross-sections

We calculate the PAH cross-sections following Li & Draine (2001) and including
updates from Draine & Li (2007) for circumcoronene PAHs consisting of 54
carbon and 16 hydrogen atoms for both neutral and ionized PAHs (for further
details also see Woitke et al. 2016). The resulting cross-sections are shown
in Figure 1 for the neutral and ionized species and, for a mixture assuming a
charge fraction of 1%. These cross-sections represent optical-properties of
”astro-PAHs” and are consistent with astronomical observations in the ISM (see
Draine & Li, 2007, for details). As seen from Fig. 1 the strength of the
$3.3\,\mathrm{\mu m}$ PAH feature is relatively independent of the charge-
fraction as the cross-sections for the neutrals and ions are similar, hence we
do not expect a strong dependence of the resulting emission on the actual
charge fraction of the gas. We use those cross-sections for both the disc and
the exoplanet atmosphere models.

Figure 1: Employed PAH cross-sections for neutral PAHs (blue solid line) and
positively charged PAHs (orange solid line). The dashed black line shows the
total cross-section assuming a charge fraction of 1%. The annotated (light)
gray bars show the wavelength coverage of the Twinkle, Ariel, JWST/NIRSpec and
JWST/MIRI spectrographs. The dotted vertical line marks the $3.3\,\mathrm{\mu
m}$ PAH feature.

###  2.2 Calculations of PAH emission spectra from T Tauri discs

We model the PAH emission at 3.3 $\mu$m from a disc around a T Tauri star with
the radiation thermo-chemical code ProDiMo (PROtoplanetary DIsc MOdel, e.g.
Woitke et al. 2009; Kamp et al. 2010; Thi et al. 2011; Rab et al. 2018, 2020).
ProDiMo solves consistently the continuum radiative transfer, the gas and dust
thermal balance and the chemical abundances assuming a fixed 2D (radius and
height) axisymmetric disc structure for the gas and the dust. The chemical
network is described in Kamp et al. (2017) (235 chemical species and 2840
chemical reactions) and includes PAH charge chemistry and adsorption of PAHs
onto dust grains.

For the fiducial T Tauri disc model we assume a parameterized flared disc
structure with a total disc mass of $M_{\mathrm{disc}}=0.02\,M_{\odot}$, a
gas-to-dust mass-ratio of 100, an inner radius of
$R_{\mathrm{in}}=0.07\,\mathrm{au}$, a characteristic radius (tapered-edge) of
$R_{\mathrm{tap}}=100\,\mathrm{au}$ (gives an outer disc radius of about
$600\,\mathrm{au}$). The scale height of the disc is given by a power-law with
a reference scale-height of $10\,\mathrm{au}$ at a radius of
$100\,\mathrm{au}$ and a flaring power-law index of 1.15. For further details
on this fiducial T Tauri disc model we refer to Woitke et al. (2016).

Figure 2: Input stellar spectra for the three discs models with varying
accretion luminosity $L_{\mathrm{accr}}$, representing three mass-accretion
rates of $\dot{M}_{\mathrm{accr}}\approx 10^{-9},\,10^{-8}$ and
$10^{-7}\,M_{\sun}\mathrm{yr}^{-1}$.

To model the irradiation of the disc by the star in the center we use a
PHOENIX stellar spectrum (Brott & Hauschildt, 2005) for a young solar-like
star (age $\approx\\!1\\!-\\!3\,\mathrm{Myr}$) with
$M_{*}=0.7\,M_{\odot},L_{*}=1\,L_{\odot}$ and
$T_{\mathrm{eff}}=4000\,\mathrm{K}$. To simulate the radiation produced by
mass accretion onto the star we use a black body component with
$T=12000\,\mathrm{K}$ and consider three different accretion luminosities
$L_{\mathrm{accr}}$ of $0.01,0.1$ and $1.0\times$ the bolometric luminosity
$L_{\mathrm{bol}}$. The full input spectra are then given by the sum of the
accretion component and the photospheric spectrum, and are shown in Figure 2.
The chosen accretion luminosities correspond to mass accretion rates444We
assume
$L_{\mathrm{accr}}=\frac{\mathrm{G}M_{*}{\dot{M}_{\mathrm{accr}}}}{R_{*}}$
(e.g. Hartmann et al., 1997; Marleau et al., 2017) onto the star of
$\dot{M}_{\mathrm{accr}}\approx 10^{-9},10^{-8}$ and
$10^{-7}\,M_{\sun}\mathrm{yr}^{-1}$ and far-UV (ultraviolet) luminosities
(definition from Yang et al. 2012; Aresu et al. 2014) of $\approx 6\times
10^{-4},6\times 10^{-3}$, and $6\times 10^{-2}\,L_{\mathrm{\odot}}$ and are
representative of observed mass accretion rates/far-UV luminosities of T Tauri
stars (e.g. Yang et al., 2012; Alcalá et al., 2017; Manara et al., 2020). We
vary the accretion luminosity because it is especially the far-UV radiation
that heats the gas and PAHs.

We use circumcoronene PAHs (see Sect. 2.1) and assume a total PAH abundance in
the disc of 0.1 times the typical PAH abundance in the ISM of $3\times
10^{-7}$ per hydrogen nuclei (Tielens, 2008). The total PAH abundance is fixed
in the model and we do not include any formation or destruction processes for
the PAH molecules. To calculate the PAH emission, the PAHs are included in the
wavelength dependent radiative transfer assuming radiative equilibrium to
determine the separate PAH and dust temperatures. This is a fast and
reasonably accurate method, compared to the quantum heating formalism with
stochastic PAH temperature distribution, as shown in Woitke et al. (2016).

The resulting SEDs (spectral energy distribution), including the PAH features,
are produced via ray tracing assuming a disc inclination of $i=45^{\circ}$ and
a distance to the target of $140\,\mathrm{pc}$. For the PAH cross-sections,
used in the radiative-transfer, we assume a fixed charge fraction of 1% (see
Figure 1). For a T Tauri disc model like presented here a significantly higher
charge fraction is not expected (Thi et al., 2019), and furthermore the
$3.3\,\mathrm{\mu m}$ feature does not strongly dependent on the charge
fraction (see Sect. 2.1). The modelled synthetic spectra are used as input for
the Twinkle/Ariel pipeline for a realistic simulation of the observations (see
Sect. 2.4).

###  2.3 Calculations of PAH signal in exoplanet transits

PAHs are believed to form over a wide range of temperatures (e.g. López-
Puertas et al., 2013; Zhao et al., 2018). However, the existance of these
molecules is not confirmed on exoplanets. To investigate the feasibility of
observing PAHs by the upcoming space facilities, we model the synthetic
transmission spectra of a generic exoplanet with the radiative transfer code
petitRADTRANS (Mollière et al., 2019, 2020). As a test case, we assume a
typical hot Saturnian planet ($R=0.5\,R_{\mathrm{J}}$, log(g)=3.0,
$T_{\mathrm{eq}}=1200\,\mathrm{K}$; see e.g. Molaverdikhani et al. 2019a)
orbiting a solar type star (V=9, K=7.5). In order to isolate the effect of PAH
opacity in the atmosphere we assume a hydrogen/helium dominant atmosphere and
include the PAHs as the only other opacity source in the atmosphere. For this
purpose, no Collision-induced absorption (CIA) or Rayleigh scattering due to
H/He was included. For these calculations, we use the same PAH cross-sections
as for the disc model (see Sect. 2.1 and Figure 1). On the Earth, atmospheric
PAHs are known to vary in size; ranging from 0.1 to 20 $\mu$m but mostly with
a size of a few $\mu$m (e.g. Lv et al., 2016). Such size distribution hints
for the importance of Mie scattering contribution at NIR/IR, where the current
and future facilities such as JWST, Ariel, and Twinkle will focus on.
Consequently, the effect of scattering were also considered using the standard
Mie formalism.

###  2.4 Synthetic observations with Twinkle and Ariel

To access the detectability of features due to PAHs in both discs and
exoplanetary atmospheres, we simulated observations for both Twinkle and
Ariel. In each case, we utilised radiometric models which account for a
variety of noise sources including shot noise, instrument emission, dark
current, read noise and zodiacal light. The instrument simulators are
described in depth in Edwards & Stotesbury (2021) and Mugnai et al. (2020) for
Twinkle and Ariel, respectively. Additionally, for both observatories, noise
due to pointing inaccuracies (i.e. jitter) was imported from dynamical
modelling with ExoSim (Sarkar et al., 2021). Also the relative calibration
error across the $3.3\,\mathrm{\mu m}$ PAH feature is included in the
instrument simulations.

The spectral resolution in the simulations of Twinkle and Ariel are
$\Delta\lambda\approx 0.089\,\mathrm{\mu m}$ and $\Delta\lambda\approx
0.033\,\mathrm{\mu m}$ at $3.3\,\mathrm{\mu m}$. For comparison the predicted
full-width-half-maximum of the $3.3\,\mathrm{\mu m}$ feature in the model is
$\approx 0.045\,\mathrm{\mu m}$, but has quite broad wings of
$\Delta\lambda\approx 0.1\,\mathrm{\mu m}$ at 10% level of the peak emission
(see also Fig. 1). To improve on the limited spectral resolution of the
instruments we also explore a method that allows to finely sample the region
around the PAH feature of interest; the details of this method are described
in Appendix C.

For each mission, we simulated observations after one hour of integration for
the disc model (see Sect. 2.2), and the uncertainties on the transit depth of
a hot-Saturn orbiting the G-type star described in Section 2.3 after three
observation sequences. We find that both observatories are generally within
the photon noise limited regime for our simulations, although, for Twinkle,
the dark current is a major noise source in the exoplanet observations
considered here.

##  3 Results

For both the disc case and the planet case we can show that, under the
assumptions described in the previous sections, PAHs might be detectable at
$3.3\,\mathrm{\mu m}$ by Twinkle and Ariel if they are present at one tenth
ISM abundance level ($\approx 3\times 10^{-8}$ per hydrogen nuclei) in the
atmospheres of typical T Tauri discs or at a mass fraction of $10^{-7}$
(abundance of $\approx 3\times 10^{-10}$ per hydrogen nuclei) for irradiated
planets.

Figure 3: Synthetic disc spectra for the full Twinkle wavelength regime for
the three disc models with varying accretion luminosity (solid colored lines).
The black dotted line shows the results for a model with
$L_{\mathrm{accr}}=1\,L_{\mathrm{bol}}$ (where $L_{\mathrm{bol}}$ is the
bolometric luminosity) but without any PAHs in the disc. The grey thin line
shows the stellar spectrum for the model with
$L_{\mathrm{accr}}=1\,L_{\mathrm{bol}}$ as it would be observed if there is no
disc.

###  3.1 Observability of the $\mathbf{3.3\,\mathbf{\mu m}}$ PAH feature in T
Tauri discs

Figure 3 shows the SED for the T Tauri disc model from $0.5-4.5\,\mathrm{\mu
m}$ (i.e. the Twinkle wavelength regime) as modeled by ProDiMo (see Sect.
2.2), containing PAHs with one tenth of the ISM level abundance. The signal
depends strongly on the accretion luminosity in particular the strength of the
$3.3\,\mathrm{\mu m}$ PAH feature, which becomes clearly visible for the high
accretion luminosity model. Figure 3 also shows that there are no other easily
detectable PAH features, especially at shorter wavelengths the spectral
features are mostly dominated by the stellar emission. We therefore focus for
the rest of the discussion on the $3.3\,\mathrm{\mu m}$ feature as it is the
most promising feature for detection of PAHs in discs with Twinkle/Ariel. For
reference we also show the PAH emission at longer wavelengths (e.g. for
JWST/MIRI) as predicted by our model in Appendix A.

Figure 4: Twinkle and Ariel simulations for the three discs models (colored
lines) using spectral bin centering and double sampling (for details see Sect.
8). From top to bottom we show the results for Twinkle, Ariel with centered
bins (post-fix $\mathtt{cen.}$) at the $3.3\,\mathrm{\mu m}$ PAH feature and
with additional double sampling (post-fix $\mathtt{cen.\,ds}$). Left column:
the simulated spectra for each model (solid colored lines) and the
corresponding continuum fit (dashed lines). Right column: continuum subtracted
spectra (observed flux divided by the continuum fit). The grey area indicates
$\pm 1\%$ of the continuum level, for reference. The predicted uncertainties
of the instrument simulations for the flux are included, but are not visible
as they are smaller than the line thickness. The fluctuations around
$\mathrm{flux/continuum\,fit}=1$ (grey solid line) is a result of the
continuum subtraction and not the instrument noise. The vertical dotted line
marks the wavelength of $3.3\,\mathrm{\mu m}$ in all panels, for reference.

In Figure 4 we show the results from the Twinkle/Ariel simulations (see Sect.
2.4) for the $3.3\,\mathrm{\mu m}$ PAH feature using different spectral
observing setups. To extract the $3.3\,\mathrm{\mu m}$ PAH feature, we fitted
a third order polynomial function around the feature to remove the continuum.
The extracted feature is shown in the second column of Figure 4. The shaded
area in this panel indicates $\pm 1\%$ of the continuum level. For the models
presented here any signal above the 1% level corresponds to a $10\sigma$
detection of the $3.3\,\mathrm{\mu m}$ PAH feature as long as the signal-to-
noise ratio $\mathrm{SNR}\\!\gtrsim\\!1000$. This is achievable for both
Twinkle and Ariel with observing times less than one hour (see Appendix B) for
the disc model presented here.

The first two rows of Figure 4 show the results of the Twinkle/Ariel
simulations for the native spectral resolution. We note that we extracted the
spectral bins such that, for each instrument, one is positioned at the peak
flux of the feature. The last two rows show the expected spectra with a
special setup where we use a method we dub “double sampling”, where the
datasets are split into two half an hour integrations with different placing
of the spectral bins (see Appendix C). With this setup the spectral data has a
lower SNR but the PAH feature is better sampled and therefore the PAH feature
peak-flux to continuum ratio is increased. Such a setup is beneficial for
extracting the feature, enhancing the chances for a detection and also
enabling the observer to ensure any peak in the signal seen is at the expected
wavelength. The importance of the spectral resolution and spectral sampling is
also apparent from the comparison of the results from Twinkle and Ariel
(Figure 4): Ariel has an approximately two times higher native spectral
resolution than Twinkle and thus more easily resolves the feature. While
Twinkle time will be dedicated to the study of discs, the bulk of Ariel’s will
be used specifically for the study of exoplanetary atmospheres. Nonetheless,
Ariel will have around 10% of the mission time set aside for complementary
science and thus we hope to motivate the study of discs as part of this
program.

The high expected SNRs for both Ariel and Twinkle would in principle allow to
extract the $3.3\,\mathrm{\mu m}$ PAH feature for peak-to-continuum flux
ratios $<1.01$ (i.e. at 1% of the continuum level) with reasonable observing
times (i.e. $\approx 1\,\mathrm{h}$). This implies that the $3.3\,\mathrm{\mu
m}$ feature can be detected with both Ariel and Twinkle with more than
$10\sigma$ significance for the models with $L_{\mathrm{accr}}\gtrsim
0.1\,L_{\mathrm{bol}}$. In the model with
$L_{\mathrm{accr}}=0.01\,L_{\mathrm{bol}}$ the PAH signal is limited to the
central spectral bin (especially for Twinkle), which does not allow for a
conclusive identification the feature. It is out of the scope of this paper to
fully optimize the extraction of the feature as this work is an exploratory
study to assess the capability of Twinkle/Ariel to observe PAHs in T Tauri
discs. However, our results indicate that the limiting factor for those
telescopes is not the SNR but the spectral resolution (i.e. the feature is
smeared out). However, our models allowed to identify a good strategy for
placing spectral bins and sampling of the spectrum, which is a promising
method to compensate for the expected limited spectral resolution of Twinkle
and Ariel.

The NIRSpec555https://www.cosmos.esa.int/web/jwst-nirspec (Near-InfRared
Spectrometer, Bagnasco et al. 2007) instrument on board of the JWST (James
Webb Space Telescope) is also capable of observing the $3.3\,\mathrm{\mu m}$
PAH feature. However, the saturation limit for NIRSpec at $3.3\,\mathrm{\mu
m}$ is $\approx\\!4.9\times 10^{-14}\,\mathrm{Wm^{-2}\mu m^{-1}}$ (full-frame
readout, 2-group ramps666https://www.stsci.edu/jwst/science-planning/proposal-
planning-toolbox/sensitivity-and-saturation-limits) which is lower than the
fluxes of $\approx 1-2\times 10^{-13}\,\mathrm{Wm^{-2}\mu m^{-1}}$ for the
fiducial T Tauri disc model presented here, indicating that JWST/NIRSpec might
not be the ideal instrument for ”typical” T-Tauri discs and might be more
useful for lower-mass stars or brown dwarfs (a detailed investigation of this
aspect is out of the scope of this paper). Twinkle/Ariel will therefore be
especially useful for bright targets, in particular targets with high
accretion rates and hence strong UV radiation (i.e. high $L_{\mathrm{accr}}$).
Those targets are especially interesting as their strong UV radiation
efficiently heats the PAH molecules and therefore strongly enhances the
chances of detection of the $3.3\,\mathrm{\mu m}$ PAH feature. Observations
with Twinkle/Ariel will also allow to study long/mid-term variability of this
interesting near-infrared spectral region due to their long baseline lifetimes
(7 and 4 years, respectively) and because they are launched after JWST
(expected launch dates are 2021, 2024 and 2029 for JWST, Twinkle and Ariel,
respectively).

Disc observations with Twinkle/Ariel will also provide complementary data to
mid-IR observations of discs with the Low Resolution Spectrometer (LRS) of the
Mid-Infrared Instrument
(MIRI777https://www.jwst.nasa.gov/content/observatory/instruments/miri.html,
Kendrew et al. 2015) of JWST, where the saturation issue is less severe. Discs
observed with MIRI will therefore be ideal targets for follow-up observations
with Twinkle and Ariel. Such complementary data will be most relevant for
detailed PAH studies (e.g. abundance, composition, sizes) in discs. For
example the $3.3\,\mathrm{\mu m}$ PAH feature is likely produced by the C–H
stretching modes, whereas the features at mid-infrared wavelengths (see Fig.
1) are produce by C–H bending and C–C stretching and bending modes (Draine &
Li, 2007). Furthermore, detecting PAH emission at various wavelengths opens up
the possibility to study PAH abundance as function of disk radius even with
spatially unresolved observations, as the short-wavelength emission is
expected to emit closer to the star compared to longer wavelengths.

Figure 5:  Top) PAHs signatures in the synthetic transmission spectra of a hot
Saturnian (R=0.5$\rm R_{J}$, log(g)=3.0, $\rm T_{eq}$=1200 K) around a G5 type
star. The spectra are calculated for charged (solid red) and neutral (solid
blue) PAHs and at two mass fractions ($f=10^{-3}$ and $10^{-7}$, which
correspond to abundances of $\approx 3\times 10^{-6}$ and $\approx 3\times
10^{-10}$ per hydrogen nuclei). The effect of scattering is also shown (dotted
lines) for these four cases. As a reference, uncertainties are shown for both
Twinkle (blue) and Ariel (black) only in the case of neutral PAHs without
scattering (blue error bars), assuming 3 transits with a duration of 3 hr and
a stellar V-mag=9. Hubble Space Telescope’s STIS and WFC3 wavelength coverage
spans over the diagnostic feature of charged PAHs at around 1 $\mathrm{\mu m}$
(gray shaded regions). The diagnostic feature at 3.3 $\mathrm{\mu m}$ is a
prominent feature in both charged and neutral PAHs (green shaded region) and
hence counts as a promising feature to be investigated by Twinkle. Bottom:
zoomed in the top plot around the 3.3 $\mathrm{\mu m}$ feature.

###  3.2 Observability of the $\mathbf{3.3\,\mathbf{\mu m}}$ PAH feature in
exoplanets transits

In Figure 5 we show the expected spectroscopic signatures in terms of transit
depth for charged (solid red) and neutral PAHs (solid blue), and at two mass
fractions ($f=10^{-3}$ and $10^{-7}$; abundance of $\approx 3\times 10^{-6}$
and $\approx 3\times 10^{-10}$ per hydrogen nuclei). The effect of Mie
scattering is also shown with dotted lines for these four cases.

Atmospheres of exoplanets have been successfully characterized by the Hubble
Space Telescope (HST). In particular, STIS and WFC3 instruments on board HST
have revealed variety of atmospheric constituents on these planets, including
water, haze and clouds. Prominent features of the charged PAHs between the 1
to $1.5\,\mathrm{\mu m}$ range fit nicely with the wavelengths coverage of
these instruments (shaded gray regions in Figure 5). Therefore, they should be
already visible in current observation, if they exist in the atmospheres of
exoplanets. Our initial inspection of some of the high S/N space-based spectra
of exoplanets (e.g. Sing et al., 2016; Tsiaras et al., 2018) hints against the
existence of charged PAHs in the observed exoplanets. Further in-depth
retrieval analysis would be needed for setting an upper limit on the charged
PAH abundances in exoplanetary atmospheres, which is beyond the scope of
current work.

There are a number of features visible in the transit depth of atmospheres
containing neutral PAHs. First of all there is a well defined drop-off at
approximately $0.6\,\mathrm{\mu m}$ in the absorption cross-section of neutral
PAHs. This should be the strongest feature accessible by the current
facilities, including HST. Again an initial inspection of some of the best
currently available observations reveals otherwise. However, this is expected
since scattering effects the shape of spectra and severely washes out this
feature as shown by the dotted line in Figure 5. The effect of scattering are
less prominent at longer wavelengths and hence near and mid-infrared
facilities would be geared up with more appropriate tools to investigate PAHs.

The neutral PAH feature at 3.3$\,\mu$m is an example of such near-infrared
features. The effect of scattering on this feature is almost negligible and
its detection might be feasible through the observation of 3 transits. Due to
the magnitude of this feature, even a tracing amount of neutral PAHs in
exoplanet atmospheres, e.g. $f=10^{-7}$, might be accessible by the upcoming
facilities such as JWST, Twinkle and Ariel. Note that the feature strength
changes only by a factor of few between $f=10^{-3}$ and $10^{-7}$ due to the
nature of the transit geometry.

The Twinkle mission has dedicated time to constrain PAH abundances in the
atmosphere of exoplanets and discs. Uncertainty estimations shown in Figure 5
suggest a detectable signal should be reachable with 3 transits if the
atmosphere contains neutral PAHs, regardless of scattering and for PAHs mass
fraction larger than $10^{-7}$. Assuming a transit duration of 3 hours, which
is appropriate for a typical hot Jupiter (e.g. HD 209458 b), and allocating 3
hours of out-of-transit baseline, 18 hours observations would be sufficient to
achieve the signal shown.

Uncertainties in the transmission spectra are also estimated for the Ariel
instruments using the same set of assumptions as described above (these are
shown as black errorbars in Figure 5). Unsurprisingly, when assuming the same
number of observations for each telescope, Ariel provides a better opportunity
to explore the noted $3.3\,\mathrm{\mu m}$ feature given the larger collecting
area of its primary mirror, which is particularly evident in the noise
simulations given the margins currently held on Twinkle’s performance. Ariel’s
extended wavelength coverage to the longer wavelengths would also aid with
constraining the features located at wavelengths longer than $5\,\mathrm{\mu
m}$. However, Twinkle will provide a better spectral resolution at shorter
wavelengths ($<$1.95 $\mu$m) and thus could facilitate the study of possible
temporal variations in PAHs signals.

Note that in order to investigate PAH signals, we isolated them in our
simulations by excluding all other opacity sources. In reality, additional
opacity sources should be expected, e.g. water or methane, which might weaken
or mute other molecular features like PAHs (e.g. Molaverdikhani et al.,
2019a). Moreover, a variety of atmospheric processes, such as disequilibrium
chemistry, might change molecular abundances in the atmosphere of exoplanets
and hence spectral signatures (e.g. Molaverdikhani et al., 2019b). Such
processes might affect even the hottest exoplanets, where thermochemical
equilibrium is expected (e.g. Molaverdikhani et al., 2020a). Clouds might also
affect these signatures through their radiative or chemical feedbacks (e.g.
Kreidberg et al., 2014; Molaverdikhani et al., 2020b). Further investigations
would be required to quantitatively assess the effect of these processes on
the detectability of PAHs in the spectra of exoplanets and under different
conditions.

##  4 Conclusions

We have performed numerical models of the infrared PAH emission from a typical
planet-forming disc and a transiting hot Saturnian planet around a solar-type
star. Post-processing our models to obtain synthetic observations with Twinkle
and Ariel, we can motivate an observational strategy targeting the
$3.3\,\mathrm{\mu m}$ feature, which is typical of neutral PAHs.

We have shown that assuming a PAH abundance as small as one tenth of the ISM
value, both Twinkle and Ariel should be able to detect the $3.3\,\mathrm{\mu
m}$ feature in T Tauri discs for typical central star luminosities and
accretion properties, within less than one hour observation.

The 3.3 $\mu$m feature from neutral PAHs should also be detectable in some
transiting exoplanets, for abundances one tenth of the ISM value. As an
example we have modeled a hot Saturnian planet transiting a G-type star and
have shown that the signal to noise ratio required for a firm detection could
be made with Twinkle or Ariel within 3 transits which would require fewer than
20 hours of observing time, if we assume a transit duration of 3 hours and 3
hours of out-of-transit baseline.

We note that even non-detections would be extremely valuable as they would
allow to constrain stringent upper limits to the PAH abundance in the
atmospheric gas, thus providing insights on the relevance of PAH destruction
processes in these environments.

## Data Availability

The data underlying this article are available from the authors upon request.

## Acknowledgements

We acknowledge the support of the DFG priority program SPP 1992 “Exploring the
Diversity of Extrasolar Planets” (DFG PR 569/13-1, ER 685/7-1) & the Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation) - 325594231. This
reasearch was supported by the Excellence Cluster ORIGINS which is funded by
the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under
Germany’s Excellence Strategy - EXC-2094 - 390783311. The simulations have
been partly carried out on the computing facilities of the Computational
Center for Particle and Astrophysics (C2PAP). CHR is grateful for support from
the Max Planck Society. BE is the Project Scientist of the Twinkle space
mission at Blue Skies Space Ltd and is a Laureate of the Paris Region
fellowship programme which is supported by the Ile-de-France Region and has
received funding under the Horizon 2020 innovation framework programme and the
Marie Sklodowska-Curie grant agreement no. 945298.

## References

  * Alcalá et al. (2017) Alcalá J. M., et al., 2017, A&A, 600, A20
  * Aresu et al. (2014) Aresu G., et al., 2014, A&A, 566, A14
  * Bagnasco et al. (2007) Bagnasco G., et al., 2007, in Heaney J. B., Burriesci L. G., eds, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series Vol. 6692, Cryogenic Optical Systems and Instruments XII. p. 66920M, doi:10.1117/12.735602
  * Bakes & Tielens (1994) Bakes E. L. O., Tielens A. G. G. M., 1994, ApJ, 427, 822
  * Brott & Hauschildt (2005) Brott I., Hauschildt P. H., 2005, in Turon C., O’Flaherty K. S., Perryman M. A. C., eds, ESA Special Publication Vol. 576, The Three-Dimensional Universe with Gaia. p. 565 (arXiv:astro-ph/0503395)
  * Dinelli et al. (2013) Dinelli B. M., López-Puertas M., Adriani A., Moriconi M. L., Funke B., García-Comas M., D’Aversa E., 2013, Geophys. Res. Lett., 40, 1489
  * Draine & Li (2007) Draine B. T., Li A., 2007, ApJ, 657, 810
  * Edwards & Stotesbury (2021) Edwards B., Stotesbury I., 2021, AJ, 161, 266
  * Edwards et al. (2018) Edwards B., et al., 2018, Experimental Astronomy
  * Ehrenfreund et al. (2007) Ehrenfreund P., Ruiterkamp R., Peeters Z., Foing B., Salama F., Martins Z., 2007, Planet. Space Sci., 55, 383
  * Ercolano & Pascucci (2017) Ercolano B., Pascucci I., 2017, Royal Society Open Science, 4, 170114
  * Geers et al. (2006) Geers V. C., et al., 2006, A&A, 459, 545
  * Gorti et al. (2009) Gorti U., Dullemond C. P., Hollenbach D., 2009, ApJ, 705, 1237
  * Hartmann et al. (1997) Hartmann L., Cassen P., Kenyon S. J., 1997, ApJ, 475, 770
  * Henning & Semenov (2013) Henning T., Semenov D., 2013, Chemical Reviews, 113, 9016
  * Kamp et al. (2010) Kamp I., Tilling I., Woitke P., Thi W.-F., Hogerheijde M., 2010, A&A, 510, A18
  * Kamp et al. (2017) Kamp I., Thi W.-F., Woitke P., Rab C., Bouma S., Ménard F., 2017, A&A, 607, A41
  * Kendrew et al. (2015) Kendrew S., et al., 2015, PASP, 127, 623
  * Kreidberg et al. (2014) Kreidberg L., et al., 2014, Nature, 505, 69
  * Lange et al. (2021) Lange K., Dominik C., Tielens A. G. G. M., 2021, A&A, 653, A21
  * Li & Draine (2001) Li A., Draine B. T., 2001, ApJ, 554, 778
  * López-Puertas et al. (2013) López-Puertas M., et al., 2013, The Astrophysical Journal, 770, 132
  * Lv et al. (2016) Lv Y., Li X., Xu T. T., Cheng T. T., Yang X., Chen J. M., Iinuma Y., Herrmann H., 2016, Atmospheric Chemistry and Physics, 16, 2971
  * Manara et al. (2020) Manara C. F., et al., 2020, A&A, 639, A58
  * Marleau et al. (2017) Marleau G.-D., Klahr H., Kuiper R., Mordasini C., 2017, ApJ, 836, 221
  * Mitani et al. (2020) Mitani H., Nakatani R., Yoshida N., 2020, arXiv e-prints, p. arXiv:2005.08676
  * Molaverdikhani et al. (2019a) Molaverdikhani K., Henning T., Mollière P., 2019a, ApJ, 873, 32
  * Molaverdikhani et al. (2019b) Molaverdikhani K., Henning T., Mollière P., 2019b, ApJ, 883, 194
  * Molaverdikhani et al. (2020a) Molaverdikhani K., Helling C., Lew B. W. P., MacDonald R. J., Samra D., Iro N., Woitke P., Parmentier V., 2020a, A&A, 635, A31
  * Molaverdikhani et al. (2020b) Molaverdikhani K., Henning T., Mollière P., 2020b, ApJ, 899, 53
  * Mollière et al. (2019) Mollière P., Wardenier J. P., van Boekel R., Henning T., Molaverdikhani K., Snellen I. A. G., 2019, A&A, 627, A67
  * Mollière et al. (2020) Mollière P., et al., 2020, A&A, 640, A131
  * Mugnai et al. (2020) Mugnai L. V., Pascale E., Edwards B., Papageorgiou A., Sarkar S., 2020, Experimental Astronomy, 50, 303
  * Owen (2019) Owen J. E., 2019, Annual Review of Earth and Planetary Sciences, 47, 67
  * Rab et al. (2018) Rab C., Güdel M., Woitke P., Kamp I., Thi W. F., Min M., Aresu G., Meijerink R., 2018, A&A, 609, A91
  * Rab et al. (2020) Rab C., Kamp I., Dominik C., Ginski C., Muro-Arena G. A., Thi W. F., Waters L. B. F. M., Woitke P., 2020, A&A, 642, A165
  * Sarkar et al. (2021) Sarkar S., Pascale E., Papageorgiou A., Johnson L. J., Waldmann I., 2021, Experimental Astronomy,
  * Seok & Li (2017) Seok J. Y., Li A., 2017, ApJ, 835, 291
  * Siebenmorgen & Krügel (2010) Siebenmorgen R., Krügel E., 2010, A&A, 511, A6
  * Sing et al. (2016) Sing D. K., et al., 2016, Nature, 529, 59
  * Thi et al. (2011) Thi W.-F., Woitke P., Kamp I., 2011, MNRAS, 412, 711
  * Thi et al. (2019) Thi W. F., Lesur G., Woitke P., Kamp I., Rab C., Carmona A., 2019, A&A, 632, A44
  * Tielens (2008) Tielens A. G. G. M., 2008, ARA&A, 46, 289
  * Tinetti et al. (2018) Tinetti G., et al., 2018, Experimental Astronomy, 46, 135
  * Tinetti et al. (2021) Tinetti G., et al., 2021, arXiv e-prints, p. arXiv:2104.04824
  * Tsiaras et al. (2018) Tsiaras A., et al., 2018, AJ, 155, 156
  * Visser et al. (2007) Visser R., Geers V. C., Dullemond C. P., Augereau J. C., Pontoppidan K. M., van Dishoeck E. F., 2007, A&A, 466, 229
  * Woitke et al. (2009) Woitke P., Kamp I., Thi W.-F., 2009, A&A, 501, 383
  * Woitke et al. (2016) Woitke P., et al., 2016, A&A, 586, A103
  * Yang et al. (2012) Yang H., et al., 2012, ApJ, 744, 121
  * Zhao et al. (2018) Zhao L., et al., 2018, Nature Astronomy, 2, 973

##  Appendix A PAH features in the mid-infrared

Figure 6: Continuum subtracted PAH spectrum showing also the emission features
in the mid-infrared range. This Figure can be directly compared to the results
of Seok & Li (2017).

For reference we show in Fig. 6 the predicted PAH-emission spectrum ranging
from $2$ to $14\,\mathrm{\mu m}$ for our three disk models (see Sect. 2.2). In
the mid-infrared the continuum subtraction is more challenging than in at
$3.3\,\mathrm{\mu m}$ and furthermore the PAH emission features can be
dominated or polluted by silicate emission or absorption (see Seok & Li 2017).
We therefore chose a simply approach and use the continuum level from models
that are identical but do not include any PAHs. We also did not apply any
post-processing for simulating a particular instrument as the main purpose is
to show that our model predictions in the mid-infrared wavelength regime can
be compared to the results presented in Seok & Li (2017).

In Seok & Li (2017) detailed modeling of PAH emission observations of a large
sample of Herbig Ae/BE and a few T Tauri stars is done. Seok & Li (2017) uses
very similar PAH cross-section as are used in this work, but uses more
detailed PAH modeling (e.g. varying PAH sizes) and Monte Carlo radiative
transfer. The comparison of their model results for T Tauri stars to our Fig.
6 shows that our model predictions are in good agreement with the range of the
PAH feature strengths of the the models and the observational data presented
in Seok & Li (2017).

##  Appendix B Signal to Noise ratios for the disc models

In Figure 7 we show the expected signal to noise ratios (SNR) for Twinkle and
Ariel for one hour integration time for two of our disc models. The expected
SNR are more than sufficient to detect the $3.3\,\mathrm{\mu m}$ PAH feature
as simulated in our disc model, and likely shorter observing times are
possible (depending on the target). High SNRs are required to extract weak
features on top of a strong continuum, which is usually the case in planet-
forming discs. Although Twinkle/Ariel were not primarily designed to observe
planet-forming discs, the expected high SNRs makes them very interesting
options for disc observations in the near-infrared, despite their limited
spectral resolution.

Figure 7: Expected signal to noise ratios (SNR) for Twinkle (dots) and Ariel
(crosses) for two different disc models (colors). These are reference values
for one hour integration time.

##  Appendix C Instrument spectral setups

To improve the peak flux to continuum ratio for the PAH feature, the
theoretical PAH spectrum can be used to optimize the placement of the
extracted spectral bins of Twinkle and Ariel. In Figure 8 we show an example
for placing the center of one spectral bin at the wavelength of the expected
peak flux of the feature (here $\approx 3.3\,\mathrm{\mu m}$) and for double
sampling the feature. For double sampling, two observational setups are used
with the dataset split into two (i.e. each used for half of the total
observing time). For each dataset, the spectral bins were sampled differently,
where one sampling should be centered at the peak of the feature and the other
offset by half the bandwidth of the bins. The double sampled spectrum is shown
by the varying colours in Figure 8 and is used in this work for the disc
models (Sect. 3.1). For reference, we also show in Figure 9 the results using
the native spectral setup of the instruments, and with applying double
sampling (model post-fix $\mathtt{ds}$), without centering the bins on the PAH
feature at 3.3 $\mu$m.

Figure 8: Illustration of the double sampling method. The grey solid lines
always shows the same model spectrum (before post-processing with any
simulator) and are just shifted for clarity. The errorbars show the results
from the instrument simulations. The standard cases (Ariel: blue, Twinkle:
red) show the results without any special treatment (i.e. centering). The
results using centering (i.e. placing the spectral bin at the peak of the
expected feature) and double sampling ($\mathtt{cen.\,ds}$) are shown in two
different colors, where each color represents one sampling (i.e. two observing
setups with different bin placings are required). Figure 9: Same as Figure 4
but without using centering of the spectral bins. The top two rows show the
results for the native instrument setups of Twinkle and Ariel; the bottom
using double sampling without centering (i.e. for cases where the peak of a
feature cannot be determined accurately enough).