The exploration of our solar system is being radically changed since the beginning of operations of the James Webb Space Telescope (JWST) in mid 2022. JWST’s extraordinary sensitivity and ...instrumentation allow for sensitive searches for the building blocks of life and to test for habitability, also enabling new discoveries on small bodies to giant planets across our solar system and beyond.
New measurements of (12)C/(13)C ratios in Galactic molecular clouds have been conducted using the N = 1 1 0 transition of the CN radical. This species is unique in that it has extensive hyperfine ...structure that can be accurately used to correct for line saturation effects. Combined with the past observations of Savage and coworkers, the ratios derived from CN are the most extensive data set to date for molecular zclouds, and they include sources that lie in the range of 0.09-16.41 kpc in distance from the Galactic center (D(GC)). The ratios derived from CN indicate a gradient with Galactic distance of (12)C/(13)C = 6.01D(GC) + 12.28. This gradient agrees rather closely with those derived from measurements of CO and H(2)CO. The least-squares fit to all data points for the three molecules is (12)C/(13)C = 6.21D(GC) + 18.71. CO, CN, and H(2)CO are synthesized from quite varied reactions, and any (13)C fractionation must follow different pathways for these three species. The relatively good agreement between the (12)C/(13)C ratios of the three molecules, as well as their lack of correlation with gas kinetic temperature, suggests that chemical fractionation and isotope-selective photodissociation both do not play a substantial role in influencing such ratios. Therefore, the (12)C/(13)C gradient found in the Galaxy is a true indicator of Galactic chemical evolution. The apparent discrepancy between the solar system ((12)C/(13)C = 89) and local interstellar medium values ((12)C/(13)C - 68) of this ratio may be a result of (13)C enrichment since the formation of the solar system, as predicted by recent models.
Abstract
The ortho-to-para ratio (OPR) of formaldehyde (H
2
CO) has been used as a probe to estimate the formation temperature of molecules in interstellar, circumstellar, and cometary environments, ...relying on the assumption that nuclear spin conversion is extremely slow, preserving the OPR from molecular formation. An OPR for H
2
CO less than 3 corresponds to a spin temperature below 30 K and has been proposed to result from formation at low temperatures within an ice, whereas an OPR of 3 is interpreted as arising from warmer formation in the gas phase. In spite of this common assumption, there is no laboratory evidence in the literature to date in support of it. Here, in the first study of its kind for H
2
CO, we report rotational spectroscopy measurements of the OPR of H
2
CO sublimated after its formation in methanol (CH
3
OH) ice samples that were photolyzed by ultraviolet light at 10, 15, 20, and 40 K. None of the measured OPR values correlated with the ice formation temperature.
Results are presented from the first cometary observations using the Atacama Large Millimeter/Submillimeter Array (ALMA), including measurements of the spatially resolved distributions of HCN, HNC, H ...sub(2)CO, and dust within the comae of two comets: C/2012 F6 (Lemmon) and C/2012 S1 (ISON), observed at heliocentric distances of 1.5 AU and 0.54 AU, respectively. These observations (with angular resolution approximate0".5), reveal an unprecedented level of detail in the distributions of these fundamental cometary molecules, and demonstrate the power of ALMA for quantitative measurements of the distributions of molecules and dust in the inner comae of typical bright comets. In both comets, HCN is found to originate from (or within a few hundred kilometers of) the nucleus, with a spatial distribution largely consistent with spherically symmetric, uniform outflow. By contrast, the HNC distributions are clumpy and asymmetrical, with peaks at cometocentric radii ~500-1000 km, consistent with release of HNC in collimated outflow(s). Compared to HCN, the H sub(2)CO distribution in comet Lemmon is very extended. The interferometric visibility amplitudes are consistent with coma production of H sub(2)CO and HNC from unidentified precursor material(s) in both comets. Adopting a Haser model, the H sub(2)CO parent scale length is found to be a few thousand kilometers in Lemmon and only a few hundred kilometers in ISON, consistent with the destruction of the precursor by photolysis or thermal degradation at a rate that scales in proportion to the solar radiation flux.
Gas-phase molecules in cometary atmospheres (comae) originate primarily from (1) outgassing by the nucleus, (2) sublimation of icy grains in the near-nucleus coma, and (3) coma (photo)chemical ...processes. However, the majority of cometary gases observed at radio wavelengths have yet to be mapped, so their production/release mechanisms remain uncertain. Here we present observations of six molecular species toward comet 46P/Wirtanen, obtained using the Atacama Large Millimeter/submillimeter Array during the comet's unusually close (∼0.1 au) approach to Earth in 2018 December. Interferometric maps of HCN, CH3OH, CH3CN, H2CO, CS, and HNC were obtained at an unprecedented sky-projected spatial resolution of up to 25 km, enabling the nucleus and coma sources of these molecules to be accurately quantified. The HCN, CH3OH, and CH3CN spatial distributions are consistent with production by direct outgassing from (or very close to) the nucleus, with a significant proportion of the observed CH3OH originating from sublimation of icy grains in the near-nucleus coma (at a scale length Lp = 36 ± 7 km). On the other hand, H2CO, CS, and HNC originate primarily from distributed coma sources (with Lp values in the range 550–16,000 km), the identities of which remain to be established. The HCN, CH3OH, and HNC abundances in 46P are consistent with the average values previously observed in comets, whereas the H2CO, CH3CN, and CS abundances are relatively low.
Conditions in the protosolar nebula have left their mark in the composition of cometary volatiles, thought to be some of the most pristine material in the solar system. Cometary compositions ...represent the end point of processing that began in the parent molecular cloud core and continued through the collapse of that core to form the protosun and the solar nebula, and finally during the evolution of the solar nebula itself as the cometary bodies were accreting. Disentangling the effects of the various epochs on the final composition of a comet is complicated. But comets are not the only source of information about the solar nebula. Protostellar disks around young stars similar to the protosun provide a way of investigating the evolution of disks similar to the solar nebula while they are in the process of evolving to form their own solar systems. In this way we can learn about the physical and chemical conditions under which comets formed, and about the types of dynamical processing that shaped the solar system we see today.
This paper summarizes some recent contributions to our understanding of both cometary volatiles and the composition, structure and evolution of protostellar disks.
Editor’s summaryEuropa, an icy moon of Jupiter, has a subsurface ocean beneath a crust of water ice. Solid carbon dioxide (CO2) has previously been observed on its surface, but the source was ...unknown. Two teams analyzed infrared spectroscopy of Europa from the James Webb Space Telescope to investigate the CO2 source. Trumbo and Brown found that the CO2 is concentrated in a region with geology that indicates transport of material to the surface from within the moon, and they discuss the implications for the composition of Europa’s internal ocean. Villanueva et al. also identified an internal origin of the CO2 and measured its 12C/13C isotope ratio. They searched for plumes of volatile material breaching the surface but found a lower activity than earlier observations. Together, these studies demonstrate that there is a source of carbon within Europa, probably in its ocean. —Keith T. Smith
Submillimeter/far-IR spectroscopy was used to detect and quantify organic molecules sublimated after the
ultraviolet photolysis (at 12 K) and warm-up (up to 300 K) of a methanol (CH3OH) ice sample. ...Eleven sublimated
photoproducts were uniquely identified: carbon monoxide (CO), formaldehyde (H2CO), ketene (C2H2O),
acetaldehyde (CH3CHO), ethylene oxide (CH2OCH2), vinyl alcohol (CH2CHOH), ethanol (CH3CH2OH),
dimethyl ether (CH3OCH3), methyl formate (HCOOCH3), glycolaldehyde (HOCH2CHO), and acetone
((CH3)2CO). Two additional products were detected in the photolyzed ice by Fourier-transform infrared (FTIR)
spectroscopy: carbon dioxide (CO2) and methane (CH4). The rotational temperatures and gas densities were
calculated for the organics containing two or more C atoms via a rotation diagram analysis, and the gas-phase
submillimeter/far-IR technique was used in tandem with mass spectrometry and FTIR spectroscopy of the ice
during photolysis. The abundance ratios of the sublimated species (normalized to methanol) were compared to
those observed in hot cores (Orion-KL, Sagittarius B2(N), and IRAS 16293-2422(B)) and in comets C/2014 Q2
(Lovejoy) and 67P/Churyumov–Gerasimenko.
The interstellar medium is enriched primarily by matter ejected from old, evolved stars. The outflows from these stars create spherical envelopes, which foster gas-phase chemistry. The chemical ...complexity in circumstellar shells was originally thought to be dominated by the elemental carbon to oxygen ratio. Observations have suggested that envelopes with more carbon than oxygen have a significantly greater abundance of molecules than their oxygen-rich analogues. Here we report observations of molecules in the oxygen-rich shell of the red supergiant star VY Canis Majoris (VY CMa). A variety of unexpected chemical compounds have been identified, including NaCl, PN, HNC and HCO+. From the spectral line profiles, the molecules can be distinguished as arising from three distinct kinematic regions: a spherical outflow, a tightly collimated, blue-shifted expansion, and a directed, red-shifted flow. Certain species (SiO, PN and NaCl) exclusively trace the spherical flow, whereas HNC and sulphur-bearing molecules (amongst others) are selectively created in the two expansions, perhaps arising from shock waves. CO, HCN, CS and HCO+ exist in all three components. Despite the oxygen-rich environment, HCN seems to be as abundant as CO. These results suggest that oxygen-rich shells may be as chemically diverse as their carbon counterparts.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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