Aqueously altered igneous rocks sampled on the floor of Jezero crater, Mars

The Perseverance rover landed in Jezero crater, Mars, to investigate ancient lake and river deposits. We report observations of the crater floor, below the crater's sedimentary delta, finding the floor consists of igneous rocks altered by water. The lowest exposed unit, informally named Séítah, is a coarsely crystalline olivine-rich rock, which accumulated at the base of a magma body. Fe-Mg carbonates along grain boundaries indicate reactions with CO₂-rich water, under water-poor conditions. Overlying Séítah is a unit informally named Máaz, which we interpret as lava flows or the chemical complement to Séítah in a layered igneous body. Voids in these rocks contain sulfates and perchlorates, likely introduced by later near-surface brine evaporation. Core samples of these rocks were stored aboard Perseverance for potential return to Earth.

Perseverance rover reveals an ancient delta-lake system and flood deposits at Jezero crater, Mars

[Figure: see text].

Dynamics of cluster-forming hub-filament systems: The case of the high-mass star-forming complex Monoceros R2

CONTEXT

High-mass stars and star clusters commonly form within hub-filament systems. Monoceros R2 (hereafter Mon R2), at a distance of 830 pc, harbors one of the closest such systems, making it an excellent target for case studies.

AIMS

We investigate the morphology, stability and dynamical properties of the Mon R2 hub-filament system.

METHODS

We employ observations of the 13CO and C18O 1→0 and 2→1 lines obtained with the IRAM-30m telescope. We also use H2 column density maps derived from Herschel dust emission observations.

RESULTS

We identified the filamentary network in Mon R2 with the DisPerSE algorithm and characterized the individual filaments as either main (converging into the hub) or secondary (converging to a main filament) filaments. The main filaments have line masses of 30-100 M ⊙ pc-1 and show signs of fragmentation, while the secondary filaments have line masses of 12-60 M ⊙ pc-1 and show fragmentation only sporadically. In the context of Ostriker's hydrostatic filament model, the main filaments are thermally supercritical. If non-thermal motions are included, most of them are trans-critical. Most of the secondary filaments are roughly transcritical regardless of whether non-thermal motions are included or not. From the morphology and kinematics of the main filaments, we estimate a mass accretion rate of 10-4-10-3 M ⊙ yr-1 into the central hub. The secondary filaments accrete into the main filaments with a rate of 0.1-0.4×10-4 M ⊙ yr-1. The main filaments extend into the central hub. Their velocity gradients increase towards the hub, suggesting acceleration of the gas.We estimate that with the observed infall velocity, the mass-doubling time of the hub is ~ 2:5 Myr, ten times larger than the free-fall time, suggesting a dynamically old region. These timescales are comparable with the chemical age of the Hii region. Inside the hub, the main filaments show a ring- or a spiral-like morphology that exhibits rotation and infall motions. One possible explanation for the morphology is that gas is falling into the central cluster following a spiral-like pattern.

Structure of photodissociation fronts in star-forming regions revealed by observations of high-J CO emission lines with Herschel

CONTEXT

In bright photodissociation regions (PDRs) associated to massive star formation, the presence of dense "clumps" that are immersed in a less dense interclump medium is often proposed to explain the difficulty of models to account for the observed gas emission in high-excitation lines.

AIMS

We aim at presenting a comprehensive view of the modeling of the CO rotational ladder in PDRs, including the high-J lines that trace warm molecular gas at PDR interfaces.

METHODS

We observed the 12CO and 13CO ladders in two prototypical PDRs, the Orion Bar and NGC 7023 NW using the instruments onboard Herschel. We also considered line emission from key species in the gas cooling of PDRs (C+, O, H2) and other tracers of PDR edges such as OH and CH+. All the intensities are collected from Herschel observations, the literature and the Spitzer archive and are analyzed using the Meudon PDR code.

RESULTS

A grid of models was run to explore the parameter space of only two parameters: thermal gas pressure and a global scaling factor that corrects for approximations in the assumed geometry. We conclude that the emission in the high-J CO lines, which were observed up to J up =23 in the Orion Bar (J up =19 in NGC 7023), can only originate from small structures of typical thickness of a few 10-3 pc and at high thermal pressures (Pth ~ 108 K cm-3).

CONCLUSIONS

Compiling data from the literature, we found that the gas thermal pressure increases with the intensity of the UV radiation field given by G0, following a trend in line with recent simulations of the photoevaporation of illuminated edges of molecular clouds. This relation can help rationalising the analysis of high-J CO emission in massive star formation and provides an observational constraint for models that study stellar feedback on molecular clouds.

Spatial distribution of FIR rotationally excited CH+ and OH emission lines in the Orion Bar PDR

CONTEXT

The methylidyne cation (CH⁺) and hydroxyl (OH) are key molecules in the warm interstellar chemistry, but their formation and excitation mechanisms are not well understood. Their abundance and excitation are predicted to be enhanced by the presence of vibrationally excited H₂ or hot gas (~500-1000 K) in photodissociation regions with high incident FUV radiation field. The excitation may also originate in dense gas (> 10⁵ cm^-3) followed by nonreactive collisions with H₂, H, and electrons. Previous observations of the Orion Bar suggest that the rotationally excited CH⁺ and OH correlate with the excited CO, a tracer of dense and warm gas, and formation pumping contributes to CH⁺ excitation.

AIMS

Our goal is to examine the spatial distribution of the rotationally excited CH⁺ and OH emission lines in the Orion Bar in order to establish their physical origin and main formation and excitation mechanisms.

METHODS

We present spatially sampled maps of the CH⁺ J=3-2 transition at 119.8 µm and the OH Λ-doublet at 84 µm in the Orion Bar over an area of 110″×110″ with Herschel (PACS). We compare the spatial distribution of these molecules with those of their chemical precursors, C⁺, O and H₂, and tracers of warm and dense gas (high-J CO). We assess the spatial variation of CH⁺ J=2-1 velocity-resolved line profile at 1669 GHz with Herschel HIFI spectrometer observations.

RESULTS

The OH and especially CH⁺ lines correlate well with the high-J CO emission and delineate the warm and dense molecular region at the edge of the Bar. While notably similar, the differences in the CH⁺ and OH morphologies indicate that CH⁺ formation and excitation are strongly related to the observed vibrationally excited H₂. This, together with the observed broad CH⁺ line widths, indicates that formation pumping contributes to the excitation of this reactive molecular ion. Interestingly, the peak of the rotationally excited OH 84 µm emission coincides with a bright young object, proplyd 244-440, which shows that OH can be an excellent tracer of UV-irradiated dense gas.

CONCLUSIONS

The spatial distribution of CH⁺ and OH revealed in our maps is consistent with previous modeling studies. Both formation pumping and nonreactive collisions in a UV-irradiated dense gas are important CH⁺ J=3-2 excitation processes. The excitation of the OH Λ-doublet at 84 µm is mainly sensitive to the temperature and density.

The first CO+ image: I. Probing the HI/H2 layer around the ultracompact HII region Mon R2

The CO⁺ reactive ion is thought to be a tracer of the boundary between a HII region and the hot molecular gas. In this study, we present the spatial distribution of the CO⁺ rotational emission toward the Mon R2 star-forming region. The CO⁺ emission presents a clumpy ring-like morphology, arising from a narrow dense layer around the HII region. We compare the CO⁺ distribution with other species present in photon-dominated regions (PDR), such as [CII] 158 µm, H₂ S(3) rotational line at 9.3 µm, polycyclic aromatic hydrocarbons (PAHs) and HCO⁺. We find that the CO⁺ emission is spatially coincident with the PAHs and [CII] emission. This confirms that the CO⁺ emission arises from a narrow dense layer of the HI/H₂ interface. We have determined the CO⁺ fractional abundance, relative to C⁺ toward three positions. The abundances range from 0.1 to 1.9 ×10^-10 and are in good agreement with previous chemical model, which predicts that the production of CO⁺ in PDRs only occurs in dense regions with high UV fields. The CO⁺ linewidth is larger than those found in molecular gas tracers, and their central velocity are blue-shifted with respect to the molecular gas velocity. We interpret this as a hint that the CO⁺ is probing photo-evaporating clump surfaces.

Polycyclic aromatic hydrocarbons and molecular hydrogen in oxygen-rich planetary nebulae: the case of NGC 6720

Evolved stars are primary sources for the formation of polycyclic aromatic hydrocarbons (PAHs) and dust grains. Their circumstellar chemistry is usually designated as either oxygen-rich or carbon-rich, although dual-dust chemistry objects, whose infrared spectra reveal both silicate- and carbon-dust features, are also known. The exact origin and nature of this dual-dust chemistry is not yet understood. Spitzer-IRS mid-infrared spectroscopic imaging of the nearby, oxygen-rich planetary nebula NGC 6720 reveals the presence of the 11.3 μm aromatic (PAH) emission band. It is attributed to emission from neutral PAHs, since no band is observed in the 7-8 μm range. The spatial distribution of PAHs is found to closely follow that of the warm clumpy molecular hydrogen emission. Emission from both neutral PAHs and warm H2 is likely to arise from photo-dissociation regions associated with dense knots that are located within the main ring. The presence of PAHs together with the previously derived high abundance of free carbon (relative to CO) suggest that the local conditions in an oxygen-rich environment can also become conducive to in-situ formation of large carbonaceous molecules, such as PAHs, via a bottom-up chemical pathway. In this scenario, the same stellar source can enrich the interstellar medium with both oxygen-rich dust and large carbonaceous molecules.

Mixed aliphatic and aromatic composition of evaporating very small grains in NGC 7023 revealed by the 3.4/3.3 μm ratio

CONTEXT

A chemical scenario was proposed for photon-dominated regions (PDRs) according to which UV photons from nearby stars lead to the evaporation of very small grains (VSGs) and the production of gas-phase polycyclic aromatic hydrocarbons (PAHs).

AIMS

Our goal is to achieve better insight into the composition and evolution of evaporating very small grains (eVSGs) and PAHs through analyzing the infrared (IR) aliphatic and aromatic emission bands.

METHODS

We combined spectro-imagery in the near- and mid-IR to study the spatial evolution of the emission bands in the prototypical PDR NGC 7023. We used near-IR spectra obtained with the IRC instrument onboard AKARI to trace the evolution of the 3.3 μm and 3.4 μm bands, which are associated with aromatic and aliphatic C-H bonds on PAHs. The spectral fitting involved an additional broad feature centered at 3.45 μm that is often referred to as the plateau. Mid-IR observations obtained with the IRS instrument onboard the Spitzer Space Telescope were used to distinguish the signatures of eVSGs and neutral and cationic PAHs. We correlated the spatial evolution of all these bands with the intensity of the UV field given in units of the Habing field G0 to explore how their carriers are processed.

RESULTS

The intensity of the 3.45 μm plateau shows an excellent correlation with that of the 3.3 μm aromatic band (correlation coefficient R = 0.95) and a relatively poor correlation with the aliphatic 3.4 μm band (R=0.77). This indicates that the 3.45 μm feature is dominated by the emission from aromatic bonds. We show that the ratio of the 3.4 μm and 3.3 μm band intensity (I3.4/I3.3) decreases by a factor of 4 at the PDR interface from the more UV-shielded layers (G0 ~ 150, I3.4/I3.3 = 0.13) to the more exposed layers (G0 > 1 × 104, I3.4/I3.3 = 0.03). The intensity of the 3.3 μm band relative to the total neutral PAH intensity shows an overall increase with G0, associated with an increase of both the hardness of the UV field and the H abundance. In contrast, the intensity of the 3.4 μm band relative to the total neutral PAH intensity decreases with G0, showing that their carriers are actively destroyed by UV irradiation and are not efficiently regenerated. The transition region between the aliphatic and aromatic material is found to correspond spatially with the transition zone between neutral PAHs and eVSGs.

CONCLUSIONS

We conclude that the photo-processing of eVSGs leads to the production of PAHs with attached aliphatic sidegroups that are revealed by the 3.4 μm emission band. Our analysis provides evidence for the presence of very small grains of mixed aromatic and aliphatic composition in PDRs.