Abstract
How universal is life? Were special conditions required for the formation of pre-biotic molecules in our Solar System, or are they a natural outcome of star-formation? These are fundamental questions at the intersection of astrophysics, chemistry, and Solar System studies that can be addressed through analysis of astrophysical ices using laboratory and chemical models. I present initial results from two JWST programs that reveal how the chemical environment and physical structure of cloud ices influences their inheritance by protoplanetary disks. These results provide additional complications in the community’s efforts to connect exoplanets’ atmospheric compositions with their formation histories.
1. Introduction
Volatile elements, like Carbon, Hydrogen, Oxygen, Nitrogen, and Sulfur, are critical to the detectability of planetary atmospheres and the origins of life as we know it. These elements are mostly confined to a thin layer on the surface of Earth, where they were likely delivered by icy planetesimals, like Comet 67P. These comets brought much of these elements in as ices, particularly simple ices like water, carbon monoxide, carbon dioxide, methane, and ammonia, but also as larger molecules, including complex organic molecules like methanol and ethanol, as well as higher temperature, refractory materials like PAHs. Cometary ices most likely originated in the Sun’s natal molecular cloud. However, the total amount and variety of ices inherited by planetary systems in this way is an open question.
In order to understand how common life could be in the universe, we need to quantify how much ice extrasolar comets and planets receive. These bodies are the final outcome of the star and planet formation process, which takes place over millions of years. Ices form at the beginning of this sequence in cold, dense molecular clouds, and they can be destroyed and refrozen multiple times as the icy grains infall from clouds towards disks around protostars, depending on the physical and chemical environments in the ices.
Therefore, we must understand the chemical and physical evolution of ices at each stage of the star formation process in order to understand the distribution of these elements in other planetary systems. We can break this problem down into three questions:
a) What are the chemical and physical properties of ice that forms in cold, dense molecular clouds?
b) How are ices chemically altered during the protostellar phase?
c) Which cloud ices are inherited by comet-forming regions of disks?
I summarize my teams’ new results on ice properties in clouds and disks, while protostars are covered in other papers within these proceedings (Y.-L. Yang and L. Tychoniec).
2. Overview of JWST observing programs
Ices are detectable through features seen in mid-infrared spectroscopy in ground-based laboratory experiments. Motions between constituent atoms within the icy matrix produce broad spectral features at frequencies characteristic of each different solid material, e.g. the O-H stretch water ice feature seen at 3 microns. Full spectral coverage between 3 and 15 microns is valuable, as many ices have weak and strong features over this wavelength region, enabling a chemical fingerprint of the ices with sufficient instrument spectral resolution [1]. These wavelengths are only partially visible from the ground, due to absorption by Earth’s atmosphere. Therefore, the new James Webb Space Telescope (JWST) is uniquely suited to capture such ice observations.
To answer the above questions, we designed two JWST observing programs in Cycle 1: the Early Release Science (ERS) program “Ice Age” and the GO program “Midplane Inclined Disk Astrochemistry Survey” (MIDAS). Ice Age maps ice evolution across each stage of star formation in a compact field within a single molecular cloud, Chameleon I. It uses a combination of NIRCam Wide Field Slitless Spectrograph to map the ices in the cloud in absorption against background star continuum spectra, with two targeted NIRSpec Fixed Slit and MIRI LRS spectra, respectively, to cover the full set of ice features for the two most extincted background stars in the cloud, near a young Class 0 protostar. Then we mapped an older Class I protostar and Class II edge-on disk using the highest spectral resolution IFU modes of NIRSpec and MIRI.
3. Comprehensive volatile budget for molecular clouds
Our first results paper was published in January 2023 on the two full NIRCam/NIRSpec/MIRI background star spectra [2]. In the NIRCam imaging (see Figure 1), we see a large number
of background stars, a jet from the hidden Class 0 protostar, the spectacular outflows from the older Class I protostar, and dozens of interloping galaxies. The targeted background stars are in a part of the cloud that is extremely cold, below 15 K, and the faint stars behind the cloud could not be detected previously without JWST’s high sensitivity.
In the NIRCam/NIRSpec/MIRI spectra, we can clearly see all the major ice features of water, CO2 and CO ice on the cold surfaces of the silicate dust grains within the molecular cloud. JWST’s high sensitivity and spectral resolution allows us to detect sulfur bearing ice species like carbonyl sufide and sulfur dioxide for the first time in pre-stellar clouds, at only 2% of the total sulfur budget: much less than we expected. Performing both global fits to the whole spectrum using the ENIIGMA fitting tool [3] and local fits to individual features, we determined column densities for the major and minor ice species. While there is more ice at higher extinctions, the ice looks remarkably similar between the two sources. After accounting for the remaining gas phase CO, we can calculate the fraction of each elemental budget that is locked up in ices (see Figure 2). For all four key elements, ices make up less than half of the elemental budget that we see. For carbon and oxygen, some of the remaining budget may be in CO gas that has yet to freeze out. Similarly, nitrogen could be carried by N2, which freezes out at colder temperatures than CO and has spectral features overlapping the strong CO2 band at 4.3 microns. However, the remaining fractions of each budget may be carried by refractory solid species, e.g. FeS, silicates, or soot that are difficult to observe. We confirm that the refractory carbon cannot be distributed into frozen PAHs, as we do not see their spectral signatures between 3.4-3.5 microns.
4. Chemical environments of cloud ice formation
The formation sequence of ices may determine their chemical properties, through the layering of different species. Fortunately, the ice keeps a record of its history through the profile of each ice feature, and with JWST’s high spectral resolution, we can resolve these profile variations to study how the ices formed.
Traditionally, ice has been thought to form in a sequence in which water forms first on grains, with other ices (CO2, NH3, and CH4) forming in small amounts within the water matrix. Then CO gas catastrophically freezes out, allowing more complex species like methanol and ethanol to form by hydrogenation of CO. However, interestingly when we look at methanol, we see evidence for both a CO-rich component and water-rich component [2]. The former can be explained by traditional hydrogenation of CO, but the water-rich component suggests that methanol and methane may be formed early by the H-abstraction processes investigated in recent works [4,5]. Ice Age also detected more complex species, including ethanol, for the first time in molecular cloud ices. They were best-fit by laboratory ice spectra mixed with water ice, suggesting that they, too, formed early before CO froze out. Such early formation is predicted by laboratory experiments and chemical models [6,7]. Similar complex species have been identified at later protostellar
Class 0/I stages of evolution by JWST studies [8].
5. Physical cloud ice environment
JWST’s superior sensitivity and spectral resolution also provide insight into the physical structure of the icy grains. Weak dangling OH features are detected towards eight lines of sight in the NIRCam WFSS data at 2.7 microns for the first time [2,9]. These features indicate either that the ices are porous or result from substantial mixing of other ices into the water ice matrix. Since the latter is indicated by the main ice feature profiles, both processes could occur simultaneously. In addition to these potential modifications of the ice matrix structure, distortions in the H2O, CO2, and CO ice profiles suggest that the ice grains have grown in these dense clouds, relative to their sizes in more diffuse clouds. Detailed radiative transfer fitting suggests the maximum grain size is nearly four times that of the ISM [10]. This is consistent with the growth predictions of dynamical models at timescales of 1 Myr and densities consistent with this part of the cloud. Larger grains have less bulk surface for chemical reactions and absorb less radiation. The combination of large, mixed ice grains may preserve more volatile ice species intact during the hot protostellar infall phase, leading to more cloud inheritance by protoplanetary disks.
6. Protoplanetary disk CHONS inventories
The first spectra of edge-on protoplanetary disks, from both the Ice Age and MIDAS programs, show absorption signatures of the major ice species, H2O, CO2, and CO, in addition to spatially resolved PAH emission signatures [11,12,13]. Since these disks are around low-mass T Tauri stars, with minimal external UV radiation that could excite the PAHs, this suggests that the UV produced by the stellar accretion shocks is sufficient to excite a thin layer of PAHs in the upper layers of the disk [12]. There is also spatially resolved jet emission seen via atomic lines in the IFU data; the same data is able to provide spatially resolved ice absorption spectra as well [11]. The vertical distribution of ices in particular shows a surprisingly large amount of CO ice at higher elevations in the disk than it would be expected to exist in the solid state. This could suggest that CO ice is trapped within either a H2O or CO2 ice matrix. Such trapping has been observed in laboratory studies [14]. Whether this CO is inherited from the cloud or produced in the disk is still an open question.
Comparing the depth of the ice features with those in the molecular cloud, at first these ice absorptions appear weaker, suggesting that ice has been lost to sublimation. However, the ice is locally saturated by scattered dust continuum emission. This prevents a straightforward conversion from optical depth into true column densities; using CO2 isotopologues, the true column of CO2 is a factor of 10 larger than the naive optical depths would suggest [11]. This result indicates that radiative transfer models are necessary in order to retrieve abundances [15].
The effects of ice mixing and scattering can be seen by combining radiative transfer models with updated optical constants for mixed ices. Grain surface models for mixed and layered ices can be used with these optical constants to produce new opacities for radiative transfer models. These models reveal that the ice feature profiles are strongly impacted by the degree of ice species mixing versus layering. Our team shows that it is not possible to have layers containing pure CO ice; rather CO must be trapped with CO2 and H2O [16]. Additionally, these disks appear to have enhanced ice abundances relative to the ISM, which could explain the pattern of enhanced ice giant atmospheric carbon abundances seen in our Solar System.
7. Conclusions
We have demonstrated JWST’s capabilities for measuring the elemental budgets and chemical and physical environment of ices in molecular clouds and protoplanetary disks. The mixing of ices emerges as a key result, both in clouds and disks. In clouds, it allows greater chemical complexity at earlier stages, using up a larger fraction of the CHONS volatile elemental budget, and possibly aids in the survival of the most volatile ices as they heat up when entering the disk. Additionally, the potentially large refractory solids fraction of the CHONS budget in clouds could increase CHONS solids survival during infall. Within the disk, taken to the extreme, the trapping of CO in mixed ice matrices could allow carbon-rich ices to survive closer to the star, potentially erasing the presence of a CO snowline. Ultimately this should impact the compositions of planets forming in these disks.
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