In 2017, 1I/'Oumuamua was identified as the first known interstellar object in the Solar System1. Although typical cometary activity tracers were not detected2-6, 'Oumuamua showed a notable ...non-gravitational acceleration7. So far, there has been no explanation that can reconcile these constraints8. Owing to energetic considerations, outgassing of hyper-volatile molecules is favoured over heavier volatiles such as H2O and CO2 (ref. 9). However, there are theoretical and/or observational inconsistencies10 with existing models invoking the sublimation of pure H2 (ref. 9), N2 (ref. 11) and CO (ref. 12). Non-outgassing explanations require fine-tuned formation mechanisms and/or unrealistic progenitor production rates7,13-15. Here we report that the acceleration of 'Oumuamua is due to the release of entrapped molecular hydrogen that formed through energetic processing of an H2O-rich icy body. In this model, 'Oumuamua began as an icy planetesimal that was irradiated at low temperatures by cosmic rays during its interstellarjourney, and experienced warming during its passage through the Solar System. This explanation is supported by a large body of experimental work showing that H2 is efficiently and generically produced from H2O ice processing, and that the entrapped H2 is released over a broad range of temperatures during annealing of the amorphous water matrix16-22. We show that this mechanism can explain many of 'Oumuamua's peculiar properties without fine-tuning. This provides further support3 that 'Oumuamua originated as a planetesimal relic broadly similar to Solar System comets.
The nature and abundance of sulfur chemistry in protoplanetary disks (PPDs) may impact the sulfur inventory on young planets and therefore their habitability. PPDs also offer an interesting test bed ...for sulfur chemistry models, since each disk shows a diverse set of environments. In this context, we present new sulfur molecule observations in PPDs and new S-disk chemistry models. With the Atacama Large Millimeter/submillimeter Array we observed the CS 5-4 rotational transition toward five PPDs (DM Tau, DO Tau, CI Tau, LkCa 15, MWC 480) and the CS 6-5 transition toward three PPDs (LkCa 15, MWC 480, and V4046 Sgr). Across this sample, CS displays a range of radial distributions, from centrally peaked to gaps and rings. We also present the first detection in PPDs of 13CS 6-5 (LkCa 15 and MWC 480), C34S 6-5 (LkCa 15), and H2CS 817-716, 919-818, and 918-817 (MWC 480) transitions. Using LTE models to constrain column densities and excitation temperatures, we find that either 13C and 34S are enhanced in CS or CS is optically thick despite its relatively low brightness temperature. Additional lines and higher spatial resolution observations are needed to distinguish between these scenarios. Assuming that CS is optically thin, CS column density model predictions reproduce the observations within a factor of a few for both MWC 480 and LkCa 15. However, the model underpredicts H2CS by 1-2 orders of magnitude. Finally, comparing the H2CS/CS ratio observed toward the MWC 480 disk and toward different interstellar medium sources, we find the closest match with prestellar cores.
Abstract
The compositions of planet-forming disks are set by a combination of material inherited from the interstellar medium and material reprocessed during disk formation and evolution. Indeed, ...comets and primitive meteorites exhibit interstellar-like isotopic ratios and/or volatile compositions, supporting that some pristine material was incorporated intact into icy planetesimals in the solar nebula. To date, the survival of volatile interstellar material in the disk stage has not been modeled using realistic disk physics. Here, we present a modeling framework to track the destruction of interstellar ices on dust grains undergoing transport processes within a disk, with a particular focus on explaining the incorporation of pristine material into icy planetesimals. We find that it is difficult to explain inheritance through the local assembly of comets, as ice destruction is rapid for small (<10
μ
m) grains in the inner few tens of au. Instead, a plausible pathway to inheritance is to form pebbles at larger disk radii, which then drift inward to the comet-forming zone with their ices mostly preserved. Small grains beyond ∼100 au can experience ice photodissociation at the tens of percent level; however, little of the ice is actually lost from the grain, likely making this a robust site for in situ ice chemistry. Our models also indicate that many complex organic species should survive passage through the disk intact. This raises the possibility that organics synthesized in the interstellar medium can be delivered to terrestrial planets by icy-body impact and thus potentially participate in origins of life chemistry.
Abstract
Phosphorus is a necessary element for life on Earth, but at present, we have limited constraints on its chemistry in star- and planet-forming regions; to date, phosphorus carriers have only ...been detected toward a few low-mass protostars. Motivated by an apparent association between phosphorus molecule emission and outflow shocking, we used the IRAM 30 m telescope to target PN and PO lines toward seven solar-type protostars with well-characterized outflows and firmly detected phosphorus molecules in three new sources. This sample, combined with archival observations of three additional sources, enables the first exploration of the demographics of phosphorus chemistry in low-mass protostars. The sources with PN detections show evidence for strong outflow shocks based on their H
2
O 1
10
–1
01
fluxes. On the other hand, no protostellar properties or bulk outflow mechanical properties are found to correlate with the detection of PN. This implies that gas-phase phosphorus is specifically linked to shocked gas within the outflows. Still, the PN and PO line kinematics suggest an emission origin in postshocked gas rather than directly shocked material. Despite sampling a wide range of protostellar properties and outflow characteristics, we find a fairly narrow range of source-averaged PO/PN ratios (0.6–2.2) and volatile P abundances as traced by (PN+PO)/CH
3
OH (∼1%–3%). Spatially resolved observations are needed to further constrain the emission origins and environmental drivers of the phosphorus chemistry in these sources.
The organic content of protoplanetary disks sets the initial compositions of planets and comets, thereby influencing subsequent chemistry that is possible in nascent planetary systems. We present ...observations of the complex nitrile-bearing species CH3CN and HC3N toward the disks around the T Tauri stars AS 209, IM Lup, LkCa 15, and V4046 Sgr as well as the Herbig Ae stars MWC 480 and HD 163296. HC3N is detected toward all disks except IM Lup, and CH3CN is detected toward V4046 Sgr, MWC 480, and HD 163296. Rotational temperatures derived for disks with multiple detected lines range from 29 to 73 K, indicating emission from the temperate molecular layer of the disk. V4046 Sgr and MWC 480 radial abundance profiles are constrained using a parametric model; the gas-phase CH3CN and HC3N abundances with respect to HCN are a few to tens of percent in the inner 100 au of the disk, signifying a rich nitrile chemistry at planet- and comet-forming disk radii. We find consistent relative abundances of CH3CN, HC3N, and HCN between our disk sample, protostellar envelopes, and solar system comets; this is suggestive of a robust nitrile chemistry with similar outcomes under a wide range of physical conditions.
Abstract
Observations of protoplanetary disks have revealed them to be complex and dynamic, with vertical and radial transport of gas and dust occurring simultaneously with chemistry and planet ...formation. Previous models of protoplanetary disks focused primarily on chemical evolution of gas and dust in a static disk, or dynamical evolution of solids in a chemically passive disk. In this paper, we present a new 1D method for modeling pebble growth and chemistry simultaneously. Gas and small dust particles are allowed to diffuse vertically, connecting chemistry at all elevations of the disk. Pebbles are assumed to form from the dust present around the midplane, inheriting the composition of ices at this location. We present the results of this model after 1 Myr of disk evolution around a 1
M
⊙
star at various locations both inside and outside the CO snowline. We find that for a turbulent disk (
α
= 10
−3
), CO is depleted from the surface layers of the disk by roughly 1–2 orders of magnitude, consistent with observations of protoplanetary disks. This is achieved by a combination of ice sequestration and decreasing UV opacity, both driven by pebble growth. Further, we find the selective removal of ice species via pebble growth and sequestration can increase gas phase C/O ratios to values of approximately unity. However, our model is unable to produce C/O values of ∼1.5–2.0 inferred from protoplanetary disk observations, implying selective sequestration of ice is not sufficient to explain C/O ratios >1.
Molecular lines observed toward protoplanetary disks carry information about physical and chemical processes associated with planet formation. We present ALMA Band 6 observations of C2H, HCN, and ...C18O in a sample of 14 disks spanning a range of ages, stellar luminosities, and stellar masses. Using C2H and HCN hyperfine structure fitting and HCN/H13CN isotopologue analysis, we extract optical depth, excitation temperature, and column density radial profiles for a subset of disks. C2H is marginally optically thick (τ ∼ 1-5) and HCN is quite optically thick (τ ∼ 5-10) in the inner 200 au. The extracted temperatures of both molecules are low (10-30 K), indicative of either subthermal emission from the warm disk atmosphere or substantial beam dilution due to chemical substructure. We explore the origins of C2H morphological diversity in our sample using a series of toy disk models and find that disk-dependent overlap between regions with high UV fluxes and high atomic carbon abundances can explain a wide range of C2H emission features (e.g., compact versus extended and ringed versus ringless emission). We explore the chemical relationship between C2H, HCN, and C18O and find a positive correlation between C2H and HCN fluxes but no relationship between C2H or HCN with C18O fluxes. We also see no evidence that C2H and HCN are enhanced with disk age. C2H and HCN seem to share a common driver; however, more work remains to elucidate the chemical relationship between these molecules and the underlying evolution of C, N, and O chemistries in disks.
We present experimental constraints on the insertion of oxygen atoms into methane to form methanol in astrophysical ice analogs. In gas-phase and theoretical studies this process has previously been ...demonstrated to have a very low or nonexistent energy barrier, but the energetics and mechanisms have not yet been characterized in the solid state. We use a deuterium UV lamp filtered by a sapphire window to selectively dissociate O2 within a mixture of O2:CH4 and observe efficient production of CH3OH via O(1D) insertion. CH3OH growth curves are fit with a kinetic model, and we observe no temperature dependence of the reaction rate constant at temperatures below the oxygen desorption temperature of 25 K. Through an analysis of side products we determine the branching ratio of ice-phase oxygen insertion into CH4: ∼65% of insertions lead to CH3OH, with the remainder leading instead to H2CO formation. There is no evidence for CH3 or OH radical formation, indicating that the fragmentation is not an important channel and that insertions typically lead to increased chemical complexity. CH3OH formation from O2 and CH4 diluted in a CO-dominated ice similarly shows no temperature dependence, consistent with expectations that insertion proceeds with a small or nonexistent barrier. Oxygen insertion chemistry in ices should therefore be efficient under low-temperature ISM-like conditions and could provide an important channel to complex organic molecule formation on grain surfaces in cold interstellar regions such as cloud cores and protoplanetary disk midplanes.
Abstract
HCN is among the most commonly detected molecules in star- and planet-forming regions. It is of broad interest as a tracer of star formation physics, a probe of nitrogen astrochemistry, and ...an ingredient in prebiotic chemical schemes. Despite this, one of the most fundamental astrochemical properties of HCN remains poorly characterized: its thermal desorption behavior. Here, we present a series of experiments to characterize the thermal desorption of HCN in astrophysically relevant conditions, with a focus on predicting the HCN sublimation fronts in protoplanetary disks. We derive HCN–HCN and HCN–H
2
O binding energies of 3207 ± 197 and 4192 ± 68 K, which translate to disk midplane sublimation temperatures around 85 and 103 K. For a typical midplane temperature profile, HCN should only begin to sublimate ∼1–2 au exterior to the H
2
O snow line. Additionally, in H
2
O-dominated mixtures (20:1 H
2
O:HCN), we find that the majority of HCN remains trapped in the ice until H
2
O crystallizes. Thus, HCN may be retained in disk ices at almost all radii where H
2
O-rich planetesimals form. This implies that icy body impacts to planetary surfaces should commonly deliver this potential prebiotic ingredient. A remaining unknown is the extent to which HCN is pure or mixed with H
2
O in astrophysical ices, which impacts the HCN desorption behavior as well as the outcomes of ice-phase chemistry. Pure HCN and HCN:H
2
O mixtures exhibit distinct IR bands, raising the possibility that the James Webb Space Telescope will elucidate the mixing environment of HCN in star- and planet-forming regions and address these open questions.
Abstract
Complex organic molecules (COMs) have been observed toward several low-mass young stellar objects (LYSOs). Small and heterogeneous samples have so far precluded conclusions on typical COM ...abundances, as well as the origin(s) of abundance variations between sources. We present observations toward 16 deeply embedded (Class 0/I) low-mass protostars using the IRAM 30 m telescope. We detect CH
2
CO, CH
3
CHO, CH
3
OCH
3
, CH
3
OCHO, CH
3
CN, HNCO, and HC
3
N toward 67%, 37%, 13%, 13%, 44%, 81%, and 75% of sources, respectively. Median column densities derived using survival analysis range between 6.0 × 10
10
cm
−2
(CH
3
CN) and 2.4 × 10
12
cm
−2
(CH
3
OCH
3
), and median abundances range between 0.48% (CH
3
CN) and 16% (HNCO) with respect to CH
3
OH. Column densities for each molecule vary by about one order of magnitude across the sample. Abundances with respect to CH
3
OH are more narrowly distributed, especially for oxygen-bearing species. We compare observed median abundances with a chemical model for low-mass protostars and find fair agreement, although some modeling work remains to bring abundances higher with respect to CH
3
OH. Median abundances with respect to CH
3
OH in LYSOs are also found to be generally comparable to observed abundances in hot cores, hot corinos, and massive YSOs. Compared with comets, our sample is comparable for all molecules except HC
3
N and CH
2
CO, which likely become depleted at later evolutionary stages.
PDF
