The New Horizons spacecraft flew past the Kuiper Belt object (486958) Arrokoth (also known as 2014 MU69) in January 2019. Because of the great distance to the outer Solar System and limited ...bandwidth, it will take until late 2020 to downlink all the spacecraft's observations back to Earth. Three papers in this issue analyze recently downlinked data, including the highest-resolution images taken during the encounter (see the Perspective by Jewitt). Spencer et al. examined Arrokoth's geology and geophysics using stereo imaging, dated the surface using impact craters, and produced a geomorphological map. Grundy et al. investigated the composition of the surface using color imaging and spectroscopic data and assessed Arrokoth's thermal emission using microwave radiometry. McKinnon et al. used simulations to determine how Arrokoth formed: Two gravitationally bound objects gently spiraled together during the formation of the Solar System. Together, these papers determine the age, composition, and formation process of the most pristine object yet visited by a spacecraft.
Abstract
We have obtained images of the center of the star-forming cluster IC 348 with the James Webb Space Telescope and have identified brown dwarf candidates based on their photometry and ...point-like flux profiles. Low-resolution spectroscopy has been performed on four promising candidates, three of which have molecular absorption bands that indicate late spectral types. Among those late-type objects, the brightest is similar to known young L dwarfs while the other two show the so-called 3.4
μ
m feature that has been previously observed in the diffuse interstellar medium and in the atmospheres of Saturn and Titan, which has been attributed to an unidentified aliphatic hydrocarbon. Those two objects also exhibit features between 1.1 and 2.6
μ
m that we identify as the overtone and combination bands for that hydrocarbon. After accounting for the hydrocarbon bands, the remaining spectral features are consistent with youth and inconsistent with field dwarfs. Based on the low extinctions of those objects and the strengths of the overtone and combination bands, we conclude that the hydrocarbon resides in their atmospheres rather than in foreground material. Thus, our detections of the 3.4
μ
m feature are the first in atmospheres outside of the solar system. The presence of this hydrocarbon is not predicted by any atmospheric models of young brown dwarfs. Based on its luminosity and evolutionary models, the faintest new member of IC 348 has an estimated mass of 3–4
M
Jup
, making it a strong contender for the least massive free-floating brown dwarf that has been directly imaged to date.
Abstract We present JWST MIRI MRS observations of the edge-on protoplanetary disk around the young subsolar-mass star Tau 042021, acquired as part of the Cycle 1 GO program “Mapping Inclined Disk ...Astrochemical Signatures.” These data resolve the mid-IR spatial distributions of H 2 , revealing X-shaped emission extending to ∼200 au above the disk midplane with a semiopening angle of 35° ± 5°. We do not velocity-resolve the gas in the spectral images, but the measured semiopening angle of the H 2 is consistent with a magnetohydrodynamic wind origin. A collimated, bipolar jet is seen in forbidden emission lines from Ne ii , Ne iii , Ni ii , Fe ii , Ar ii , and S iii . Extended H 2 O and CO emission lines are also detected, reaching diameters of ∼90 and 190 au, respectively. Hot molecular emission is not expected at such radii, and we interpret its extended spatial distribution as scattering of inner disk molecular emission by dust grains in the outer disk surface. H i recombination lines, characteristic of inner disk accretion shocks, are similarly extended and are likely also scattered light from the innermost star–disk interface. Finally, we detect extended polycyclic aromatic hydrocarbon (PAH) emission at 11.3 μ m cospatial with the scattered-light continuum, making this the first low-mass T Tauri star around which extended PAHs have been confirmed, to our knowledge. MIRI MRS line images of edge-on disks provide an unprecedented window into the outflow, accretion, and scattering processes within protoplanetary disks, allowing us to constrain the disk lifetimes and accretion and mass-loss mechanisms.
Context. The abundance and distribution of ice in protoplanetary disks is critical for an understanding of the link between the composition of circumstellar matter and the composition of exoplanets. ...Edge-on protoplanetary disks are a useful tool for constraining this ice composition and its location in the disk because the spectral signatures of the ice can be observed in absorption against the continuum emission that arises from the warmer regions in the central disk. Aims. The aim of this work is to model ice absorption features in protoplanetary disks and to determine how well the abundance of the main ice species throughout the disk can be determined within the uncertainty of the physical parameter space. The edge-on proto-planetary disk around HH 48 NE, a target of the James Webb Space Telescope Early Release program Ice Age, is used as a reference system. Methods. We used the full anisotropic scattering capabilities of the radiative transfer code RADMC-3D to ray-trace the mid-infrared continuum. Using a constant parameterized ice abundance, we added ice opacities to the dust opacity in regions in which the disk was cold enough for the main carbon, oxygen, and nitrogen carriers to freeze out. Results. The global abundance relative to the dust content of the main ice carriers in HH 48 NE can be determined within a factor of 3 when the uncertainty of the physical parameters is taken into account. Ice features in protoplanetary disks can be saturated at an optical depth of ≲1 due to local saturation. Ices are observed at various heights in the disk model, but in this model, spatial information is lost for features at wavelengths >7 µm when observing with James Webb Space Telescope because the angular resolution decreases towards longer wavelengths. Spatially observed ice optical depths cannot be directly related to column densities, as would be the case for direct absorption against a bright continuum source, because of radiative transfer effects. Vertical snowlines will not be a clear transition because the height of the snow surface increases radially, but their location may be constrained from observations using radiative transfer modeling. Radial snowlines are not really accessible. Not only the ice abundance, but also the inclination, the settling, the grain size distribution, and the disk mass have a strong impact on the observed ice absorption features in disks. Relative changes in the ice abundance can only be inferred from observations if the source structure is well constrained.
Ices are the main carriers of volatiles in protoplanetary disks and are crucial to our understanding of the protoplanetary disk chemistry that ultimately sets the organic composition of planets. The ...Director’s Discretionary-Early Release Science (DD-ERS) program Ice Age on the
James Webb
Space Telescope (JWST) follows the ice evolution through all stages of star and planet formation. JWST’s exquisite sensitivity and angular resolution uniquely enable detailed and spatially resolved inventories of ices in protoplanetary disks. JWST/NIRSpec observations of the edge-on Class II protoplanetary disk HH 48 NE reveal spatially resolved absorption features of the major ice components H
2
O, CO
2
, and CO, and multiple weaker signatures from less abundant ices NH
3
, OCN
−
, and OCS. Isotopologue
13
CO
2
ice has been detected for the first time in a protoplanetary disk. Since multiple complex light paths contribute to the observed flux, the ice absorption features are filled in by ice-free scattered light. This implies that observed optical depths should be interpreted as lower limits to the total ice column in the disk and that abundance ratios cannot be determined directly from the spectrum. The
12
CO
2
/
13
CO
2
integrated absorption ratio of 14 implies that the
12
CO
2
feature is saturated, without the flux approaching zero, indicative of a very high CO
2
column density on the line of sight, and a corresponding abundance with respect to hydrogen that is higher than interstellar medium values by a factor of at least a few. Observations of rare isotopologues are crucial, as we show that the
13
CO
2
observation allowed us to determine the column density of CO
2
to be at least 1.6 × 10
18
cm
−2
, which is more than an order of magnitude higher than the lower limit directly inferred from the observed optical depth. Spatial variations in the depth of the strong ice features are smaller than a factor of two. Radial variations in ice abundance, for example snowlines, are significantly modified since all observed photons have passed through the full radial extent of the disk. CO ice is observed at perplexing heights in the disk, extending to the top of the CO-emitting gas layer. Although poorly understood radiative transfer effects could contribute to this, we argue that the most likely interpretation is that we observed some CO ice at high temperatures, trapped in less volatile ices such as H
2
O and CO
2
. Future radiative transfer models will be required to constrain the physical origin of the ice absorption and the implications of these observations for our current understanding of disk physics and chemistry.
Infrared photometry and spectroscopy (1-25 mu m) of background stars reddened by the Lupus molecular cloud complex are used to determine the properties of grains and the composition of ices before ...they are incorporated into circumstellar envelopes and disks. H sub(2)O ices form at extinctions of A sub(K) = 0.25 + or - 0.07 mag (A sub(V) = 2.1 + or - 0.6). Such a low ice formation threshold is consistent with the absence of nearby hot stars. Overall, the Lupus clouds are in an early chemical phase. The low solid CH sub(3) OH abundance (<3%-8% relative to H sub(2)O) indicates a low gas phase H/CO ratio, which is consistent with the observed incomplete CO freeze out. Furthermore it is found that the grains in Lupus experienced growth by coagulation. This process is likely related to grain growth by coagulation, as traced by the A sub(7.4)/A sub(K) continuum extinction ratio, but not to ice mantle formation. Conversely, grains acquire ice mantles before the process of coagulation starts.
Abstract
The properties of dust change during the transition from diffuse to dense clouds as a result of ice formation and dust coagulation, but much is still unclear about this transformation. We ...present 2–20
μ
m spectra of 49 field stars behind the Perseus and Serpens Molecular Clouds and establish relationships between the near-infrared continuum extinction (
A
K
) and the depths of the 9.7
μ
m silicate (
τ
9.7
) and 3.0
μ
m H
2
O ice (
τ
3.0
) absorption bands. The
τ
9.7
/
A
K
ratio varies from large, diffuse interstellar medium-like values (∼0.55), to much lower ratios (∼0.26). Above extinctions of
A
K
∼ 1.2 (
A
V
∼ 10; Perseus, Lupus, dense cores) and ∼2.0 (
A
V
∼ 17; Serpens), the
τ
9.7
/
A
K
ratio is lowest. The
τ
9.7
/
A
K
reduction from diffuse to dense clouds is consistent with a moderate degree of grain growth (sizes up to ∼0.5
μ
m), increasing the near-infrared color excess (and thus
A
K
), but not affecting the ice and silicate band profiles. This grain growth process seems to be related to the ice column densities and dense core formation thresholds, highlighting the importance of density. After correction for Serpens foreground extinction, the H
2
O ice formation threshold is in the range of
A
K
= 0.31–0.40 (
A
V
= 2.6–3.4) for all clouds, and thus grain growth takes place after the ices are formed. Finally, abundant CH
3
OH ice (∼21% relative to H
2
O) is reported for 2MASSJ18285266+0028242 (Serpens), a factor of >4 larger than for the other targets.
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