Abstract
The formation and evolution of (exo-)planets is one of the central questions in astronomy, and in the context of the JWST mission. The earliest stages of these processes take place in protoplanetary disks of gas and dust around young stars. It has become clear that most of these disks are born in the midst of a cluster of stars and are thus submitted to strong ultraviolet (UV) radiation. Despite its relevance to planet formation theories (our proto-Solar System disk formed in a cluster) very little is known about the role of this external UV radiation in the formation, evolution, and chemical composition of embryonic planetary systems. The unprecedented capabilities of JWST combined to ground based facilities now enables us to characterize protoplanetary disks irradiated by UV from the cluster.
1 Introduction
One of the most fundamental questions of astronomy concerns the origin of our planet: how it formed and how life emerged. In this context, special interest is directed toward the earliest stages of this emergence, specifically the point at which an interstellar cloud succumbs to gravity, giving rise to a young star encircled by an accretion disk of gas and dust. It is within these disks that planets take shape. These initial phases of evolution occur over a relatively brief period, just a few million years, in contrast to the potential longevity of a planetary system, which can persist for up to ten billion years. Nevertheless, these early stages hold paramount importance as they establish the initial conditions governing the physical properties (such as planetary size), chemical compositions (as the molecular content of planetary atmospheres and cores), and the potential for the emergence of life in the developing planetary system.
To study this early stage, astrophysicists observe the evolution of proto-planetary disks (PPDs) in star-forming clouds within our Galaxy. This field of research is one of the most active in astrophysics, as it addresses many fundamental questions: how does the gas and dust of PPDs lead to planet formation? Which fractions of the disk mass are accreted on the central star, transformed into planets, or blown away back to the interstellar medium by winds/jets? And, last but not least, what is the chemical composition of nascent planets and how is it acquired?
New generation telescopes, in particular the ALMA and NOEMA radio interferometers, and the JWST [1] are revolutionizing our understanding of the architecture, physical properties, and chemical composition of PPDs, enabling the direct observation of planet formation “in action”. The vast majority of studies dedicated to PPDs are targeting nearby low-mass star-forming regions (Taurus, Ophiucus, and Lupus, at distances of less than 150 pc). This is, however, a significant observational bias, as the majority of low-mass stars form within stellar clusters that contain one or more massive stars [2]. These massive (OB-type) stars, being on a faster track formation path than solar-type stars, light up first and emit strong ultraviolet (UV) radiation: ionizing extreme ultraviolet (EUV; E > 13.6 eV) and dissociating far ultraviolet (FUV; ∼ 6 eV < E < 13.6 eV) photons. Hence, in environments where most low-mass stars and planets form, EUV and FUV photons are expected to be ubiquitous and thus to have a major impact on the physics and chemistry in these environments [3]. Such an impact is also applicable to the Solar System, which is known to have formed near massive stars [4, 5]. Despite the importance that UV may have during planet formation, there have been very limited studies focusing on its role in the chemical inventory at the moment when planets form. This is nonetheless of paramount importance, since the chemistry occurring during the early steps of planet formation determines the composition of planetary atmospheres, in the Solar System and in exoplanets.
FUV photons also heat the gas such that the thermal velocity of the particles exceeds the liberation velocity, creating winds which remove material from the disk [6]. This process, called “photoevaporation”, can be fundamental in PPD evolution. The evaporation of disk material can modify the mass of gas available to form giant planets and blow away the small grains reducing the capability of the disk to form rocky bodies. The evaporation rate can be large enough to suppress planet formation [7].
The importance of UV-triggered evaporation in PPDs was first driven home by the discovery of almost two hundred irradiated disks, aka “proplyds”, in the Hubble Space Telescope images of the Orion Nebula cluster [8, 9]. Images of these systems have been improved using VLT instruments [10, 11]. These visible-light images are mostly sensitive to the hot uppermost atomic and ionized disk gas layers affected by EUV but prevent any detailed analysis of the bulk disk chemical composition and its dynamics. A major limitation stems from the large distances to the planet-forming disk present in large clusters containing massive stars, such as Orion, which is at 390 pc (according to the latest GAIA measurements [12]), that is about three times farther than Taurus or Ophiucus. This has made it more difficult to obtain spectroscopic observations of the various disk constituents (molecular gas, dust grains, and ice) in cluster environments and thus to characterize their disk physical properties and chemical composition.
2 JWST observations of protoplanetary disks within stellar clusters
Recent JWST observations conducted in the context of the PDRs4All[1] Early Release Science program [13] of Orion have uncovered a “Rosetta stone” of irradiated disks: the d203-506 system (Fig. 1, [14]). This is an example of a PPD (of ∼ 10 Jupiter masses) around a low-mass star externally irradiated solely by FUV from nearby massive stars. The FUV photon flux is about 104 times higher than the radiation field around the profusely studied isolated disks in Taurus-like clouds, thus d203-506 is more representative of the irradiation conditions of the proto-Solar System disk [5]. Analysis of the H2 line emission detected with JWST in d203-506 using the meudon pdr code (Fig. 2, [15]) allowed the first direct determination of the mass-loss rate associated with external FUV photoevaporation, about 10−7 solar masses per year [14]. This high rate has the ability to inhibit giant planet formation and illustrates the crucial role that external FUV can play in the context of planet formation. So far, however, this kind of detailed analysis has been conducted only towards this object, and additional studies are required to obtain a more general picture regarding the effect of FUV photons on planet formation.
The JWST (NIRSpec+MIRI) spectrum of d203-506 also reveals a rich photochemistry (Fig. 3). CH+, a key intermediate in gas-phase UV-driven organic chemistry, which has been searched for many years, was detected for the first time in this object [16]. This discovery expands the possible routes available to build complexity in the early stages of planet formation. The detection of CH+ relied on the unique quality of the JWST spectrum combined with major efforts from scientists in the field of quantum chemistry and laboratory spectroscopy. This synergy has led to a spectacular match between the JWST spectrum and predictions for CH+, providing new constraints on the molecular physics of this species [17]. OH arising from the photodissociation of a hidden reservoir of water vapor has also been detected in this source (Fig. 3, [18]), highlighting how FUV photons truly govern the chemistry in the upper layers of this externally illuminated disk. The chemical signatures observed in d203-506, dominated by molecular ions and radicals, differ radically from what is so far observed with JWST in isolated disks (Fig. 3) where neutral species (H2O, C2H2, etc.) dominate the emission [19, 20, 21].
It is however difficult to truly rationalize things in terms of types of environments. For instance, recent JWST results show that tracers of UV chemistry such as CH+ can be observed in isolated disk (at least one, i.e., TW Hya, see contribution by I. Kamp in this volume). This may indicate an active photochemistry in this disk, perhaps driven by FUV photons from the central source. Conversely, a PPD in the NGC 6357 massive star cluster has been found to show emission compatible with the inner disk chemistry of isolated disks [22]. Therefore, while the results of PDRs4All on d203-506 do point to a change of paradigm with respect to the origin of chemical complexity in planet-forming systems (organic molecules can form in situ, triggered by the presence of FUV radiation, and not only be inherited as part of the ice grain-mantles formed in previous evolutive pre-stellar cloud stages) a lot remains to be done to disentangle the role of several chemical and physical processes involved in the formation planetary systems near massive stars within stellar cluster, and elsewhere.
Several irradiated disks are bright in the NIRCam images of the Orion Nebula [23, 24], hence future spectroscopy with JWST should clearly help unravel their physical and chemical properties. This may also help uncover an additional blind spot regarding the question of the presence and composition of ices in these targets. So far, even with the sensitivity of JWST, we have not been able to detect their absorption clearly in d203-506 or other UV irradiated disks. Specific observations with JWST should probably be designed, following what has been successfully achieved by the Ice Age ERS program [25] to detect these ices and determine their composition.
Another intriguing topic concerns the presence/absence of polycyclic aromatic hydrocarbons (PAHs) inside those disks or their winds. While PAHs are ubiquitous in the Orion Nebula [26, 27], they have only been detected clearly in one externally illuminated disk [28]. There are some hints of their emission at 3.3 µm in d203-506 (Fig. 3), however this will require a careful extraction including background subtraction, or dedicated observations with NIRSpec. In the context of the study of externally illuminated disks, radio interferometers such as ALMA are also critical tools: they provide the complementary data to probe the disk kinematics and colder phase chemistry [29, 30], and may also be used to search for the products of the FUV-driven chemistry revealed by the JWST. Ground based observations in the visible, using adaptive optics, also provide spectacular images allowing to probe the hot neutral and ionized layers of those disks [11].
Of course, the interpretation of these data will require models. Precursor models of irradiated disks, focusing on the dynamics, have been based at first on analytical expressions [4], and still continue to rely on semi-analytical approximations for some key processes (see e.g., [31]). Other disk models consider the impact of external FUV illumination on the disk chemical structure [32], but without including the photoevaporation dynamics and adopting a simplified radiative transfer and thermo-chemistry. While these studies have been key in starting to chart this new domain, their lack of detailed modeling of the interaction of external FUV photons with the disk material makes them inappropriate to directly interpret the JWST observations of numerous molecular and atomic lines. Conversely, 1d models such as the Meudon pdr code include details of the molecular physics necessary to successfully interpret the observations (Fig. 2) and provide the assessment of gas density and temperature, but do not relate at all to the local dynamics. Therefore, self-consistent models including dynamics, chemistry, and radiative transfer combined (in more than one dimension) will be required in the future.
Acknowledgements
Much of the work presented in this extended abstract was done as part of the PDRs4All ERS program whose PIs are Olivier Berné, Els Peeters, Emilie Habart. The data was obtained with support of the data reduction team of PDRs4All: Felipe Alarcón, Amelie Canin, Ryan Chown, Olga Kannavou, Ameek Sidhu, Ilane Schroetter, Boris Trahin, Dries van de Putte. Fig. 1 was made by O. Berné et Ilane Schroetter. Fig. 2 was made by E. Bron and published in [14]. Authorization of reuse license 5756960878235 provided by AAAS. Fig. 3 was made by I. Schroetter. I thank J. Goicoechea, E. Bron, and F. Le Petit for their help writing parts of this text.
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[1] pdrs4all.org