The Composition of Comets Cochran, Anita L.; Levasseur-Regourd, Anny-Chantal; Cordiner, Martin ...
Space science reviews,
12/2015, Letnik:
197, Številka:
1-4
Journal Article
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This paper is the result of the International Cometary Workshop, held in Toulouse, France in April 2014, where the participants came together to assess our knowledge of comets prior to the ESA ...Rosetta Mission. In this paper, we look at the composition of the gas and dust from the comae of comets. With the gas, we cover the various taxonomic studies that have broken comets into groups and compare what is seen at all wavelengths. We also discuss what has been learned from mass spectrometers during flybys. A few caveats for our interpretation are discussed. With dust, much of our information comes from flybys. They include in situ analyses as well as samples returned to Earth for laboratory measurements. Remote sensing IR observations and polarimetry are also discussed. For both gas and dust, we discuss what instruments the Rosetta spacecraft and Philae lander will bring to bear to improve our understanding of comet 67P/Churyumov-Gerasimenko as “ground-truth” for our previous comprehensive studies. Finally, we summarize some of the initial Rosetta Mission findings.
We report the first detection on Titan of the small cyclic molecule cyclopropenylidene (c-C3H2) from high-sensitivity spectroscopic observations made with the Atacama Large Millimeter/submillimeter ...Array. Multiple lines of cyclopropenylidene were detected in two separate data sets: ∼251 GHz in 2016 (Band 6) and ∼352 GHz in 2017 (Band 7). Modeling of these emissions indicates abundances of 0.50 0.14 ppb (2016) and 0.28 0.08 (2017) for a 350 km step model, which may either signify a decrease in abundance, or a mean value of 0.33 0.07 ppb. Inferred column abundances are (3-5) × 1012 cm−2 in 2016 and (1-2) × 1012 cm−2 in 2017, similar to photochemical model predictions. Previously the C3H ion has been measured in Titan's ionosphere by Cassini's Ion and Neutral Mass Spectrometer (INMS), but the neutral (unprotonated) species has not been detected until now, and aromatic versus aliphatic structure could not be determined by the INMS. Our work therefore represents the first unambiguous detection of cyclopropenylidene, the second known cyclic molecule in Titan's atmosphere along with benzene (C6H6) and the first time this molecule has been detected in a planetary atmosphere. We also searched for the N-heterocycle molecules pyridine and pyrimidine finding nondetections in both cases, and determining 2 upper limits of 1.15 ppb (c-C5H5N) and 0.85 ppb (c-C4H4N2) for uniform abundances above 300 km. These new results on cyclic molecules provide fresh constraints on photochemical pathways in Titan's atmosphere, and will require new modeling and experimental work to fully understand the implications for complex molecule formation.
Planck Galactic Cold Clumps (PGCCs) are considered to be the ideal targets to probe the early phases of star formation. We have conducted a survey of 72 young dense cores inside PGCCs in the Orion ...complex with the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.3 mm (band 6) using three different configurations (resolutions ∼0 35, 1 0, and 7 0) to statistically investigate their evolutionary stages and substructures. We have obtained images of the 1.3 mm continuum and molecular line emission (12CO, and SiO) at an angular resolution of ∼0 35 (∼140 au) with the combined arrays. We find 70 substructures within 48 detected dense cores with median dust mass ∼0.093 M and deconvolved size ∼0 27. Dense substructures are clearly detected within the central 1000 au of four candidate prestellar cores. The sizes and masses of the substructures in continuum emission are found to be significantly reduced with protostellar evolution from Class 0 to Class I. We also study the evolutionary change in the outflow characteristics through the course of protostellar mass accretion. A total of 37 sources exhibit CO outflows, and 20 (>50%) show high-velocity jets in SiO. The CO velocity extents (ΔVs) span from 4 to 110 km s−1 with outflow cavity opening angle width at 400 au ranging from Θobs400 ∼ 0 6-3 9, which corresponds to 33 4-125 7. For the majority of the outflow sources, the ΔVs show a positive correlation with Θobs400, suggesting that as protostars undergo gravitational collapse, the cavity opening of a protostellar outflow widens and the protostars possibly generate more energetic outflows.
The Jupiter-family comet (JFC) 46P/Wirtanen passed the Earth at a distance of 0.077 au on 2018 December 16 UT, presenting a rare opportunity to study the chemical structure of its coma. With the ...James Clerk Maxwell Telescope we achieved a resolution of 800 km, which is smaller than the scale lengths of some distributed cometary molecules at the comet's heliocentric distance of 1 au. Spectroscopic observations of the J = 4 − 3 transition of HCN showed generally uniform levels of outgassing activity during the observing period, 2018 December 14-20. Gas expansion velocities were ∼0.6 km s−1, and the derived average HCN production rate was 7.4 × 1024 mol s−1. HCN and CH3OH emissions were detected at least 30″ (1600 km) from the nucleus, and the abundances of these species were typical for a JFC. The radial distribution of CH3OH is consistent with an extended source of sublimation such as a population of icy grains-cometary halo ice primaries, or CHIPs-as has been invoked previously to explain hyperactivity in comets. The abundance of H2CO is normal if it is a daughter species. HNC and CO were not detected, but a sensitive nondetection of CS implies an unusually low CS:H2O ratio of <0.02%. The peak brightness of the 850 m continuum emissions from icy coma dust particles of size ∼1 mm fell from (52 6) to (40 3) mJy beam−1 through the week, while the size of the dust coma remained essentially constant, with dust extending to ∼1000 km from the nucleus. The total mass of those particles was ∼2 × 108 kg.
We report the detection of four new hot corino sources, G211.47-19.27S, G208.68-19.20N1, G210.49-19.79W, and G192.12-11.10, from a survey study of Planck Galactic Cold Clumps in the Orion Molecular ...Cloud Complex with the Atacama Compact Array. Three sources had been identified as low-mass Class 0 protostars in the Herschel Orion Protostar Survey. One source in the λ Orionis region is first reported as a protostellar core. We have observed abundant complex organic molecules (COMs), primarily methanol but also other oxygen-bearing COMs (in G211.47-19.27S and G208.68-19.20N1) and the molecule of prebiotic interest NH2CHO (in G211.47-19.27S), signifying the presence of hot corinos. While our spatial resolution is not sufficient to resolve most of the molecular emission structure, the large line width and high rotational temperature of COMs suggest that they likely reside in the hotter and innermost region immediately surrounding the protostar. In G211.47-19.27S, the D/H ratio of methanol (CH2DOH/CH3OH) and the 12C/13C ratio of methanol (CH3OH/13CH3OH) are comparable to those of other hot corinos. Hydrocarbons and long-carbon-chain molecules such as c-C3H2 and HCCCN are also detected in the four sources, likely tracing the outer and cooler molecular envelopes.
Abstract
The presence of complex organic molecules (COMs) in the interstellar medium is of great interest since it may link to the origin and prevalence of life in the universe. Aiming to investigate ...the occurrence of COMs and their possible origins, we conducted a chemical census toward a sample of protostellar cores as part of the Atacama Large Millimeter/submillimeter Array Survey of Orion Planck Galactic Cold Clumps project. We report the detection of 11 hot corino sources, which exhibit compact emissions from warm and abundant COMs, among 56 Class 0/I protostellar cores. All of the hot corino sources discovered are likely Class 0, and their sizes of the warm region (>100 K) are comparable to 100 au. The luminosity of the hot corino sources exhibits positive correlations with the total number of methanol and the extent of its emissions. Such correlations are consistent with the thermal desorption picture for the presence of hot corinos and suggest that the lower-luminosity (Class 0) sources likely have a smaller region with COM emissions. With the same sample selection method and detection criteria being applied, the detection rates of the warm methanol in the Orion cloud (15/37) and the Perseus cloud (28/50) are statistically similar when the cloud distances and the limited sample size are considered. Observing the same set of COM transitions will bring a more informative comparison between the cloud properties.
Abstract
The internal structure of the prestellar core G208.68-19.02-N2 (G208-N2) in the Orion Molecular Cloud 3 (OMC-3) region has been studied with the Atacama Large Millimeter/submillimeter Array. ...The dust continuum emission revealed a filamentary structure with a length of ∼5000 au and an average H
2
volume density of ∼6 × 10
7
cm
−3
. At the tip of this filamentary structure, there is a compact object, which we call a
nucleus
, with a radius of ∼150–200 au and a mass of ∼0.1
M
⊙
. The nucleus has a central density of ∼2 × 10
9
cm
−3
with a radial density profile of
r
−1.87±0.11
. The density scaling of the nucleus is ∼3.7 times higher than that of the singular isothermal sphere (SIS). This as well as the very low virial parameter of 0.39 suggests that the gravity is dominant over the pressure everywhere in the nucleus. However, there is no sign of CO outflow localized to this nucleus. The filamentary structure is traced by the N
2
D
+
3–2 emission, but not by the C
18
O 2–1 emission, implying the significant CO depletion due to high density and cold temperature. Toward the nucleus, the N
2
D
+
also shows the signature of depletion. This could imply either the depletion of the parent molecule, N
2
, or the presence of the embedded very-low luminosity central source that could sublimate the CO in the very small area. The nucleus in G208-N2 is considered to be a prestellar core on the verge of first hydrostatic core (FHSC) formation or a candidate for the FHSC.
Abstract
Starless cores represent the initial stage of evolution toward (proto)star formation, and a subset of them, known as prestellar cores, with high density (∼ 10
6
cm
−3
or higher) and being ...centrally concentrated are expected to be embryos of (proto)stars. Determining the density profile of prestellar cores therefore provides an important opportunity to gauge the initial conditions of star formation. In this work, we perform rigorous modeling to estimate the density profiles of three nearly spherical prestellar cores among a sample of five highly dense cores detected by our recent observations. We employed multiscale observational data of the (sub)millimeter dust continuum emission, including those obtained by SCUBA-2 on the James Clerk Maxwell Telescope with a resolution of ∼ 5600 au and by multiple Atacama Large Millimeter/submillimeter Array observations with a resolution as high as ∼ 480 au. We are able to consistently reproduce the observed multiscale dust continuum images of the cores with a simple prescribed density profile, which bears an inner region of flat density and an
r
−2
profile toward the outer region. By utilizing the peak density and the size of the inner flat region as a proxy for the dynamical stage of the cores, we find that the three modeled cores are most likely unstable and prone to collapse. The sizes of the inner flat regions, as compact as ∼ 500 au, signify them as being the highly evolved prestellar cores rarely found to date.
Abstract
Protostellar outflows and jets play a vital role in star formation as they carry away excess angular momentum from the inner disk surface, allowing the material to be transferred toward the ...central protostar. Theoretically, low-velocity and poorly collimated outflows appear from the beginning of the collapse at the first hydrostatic core (FHSC) stage. With growing protostellar core mass, high-density jets are launched, entraining an outflow from the infalling envelope. Until now, molecular jets have been observed at high velocity (≳100 km s
−1
) in early Class 0 protostars. We, for the first time, detect a dense molecular jet in SiO emission with low velocity (∼4.2 km s
−1
, deprojected ∼24 km s
−1
) from source G208.89–20.04Walma (hereafter G208Walma) using ALMA Band 6 observations. This object has some characteristics of FHSCs, such as a small outflow/jet velocity, extended 1.3 mm continuum emission, and
N
2
D
+
line emission. Additional characteristics, however, are typical of early protostars: collimated outflow and SiO jet. The full extent of the outflow corresponds to a dynamical timescale of ∼
930
−
100
+
200
yr. The spectral energy distribution also suggests a very young source having an upper limit of
T
bol
∼ 31 K and
L
bol
∼ 0.8
L
⊙
. We conclude that G208Walma is likely in the transition phase from FHSC to protostar, and the molecular jet has been launched within a few hundred years of initial collapse. Therefore, G208Walma may be the earliest object discovered in the protostellar phase with a molecular jet.
Abstract
Hot corinos are of great interest due to their richness in interstellar complex organic molecules (COMs) and the consequent potential prebiotic connection to solar-like planetary systems. ...Recent surveys have reported an increasing number of detected hot corinos in Class 0/I protostars; however, the relationships between their physical properties and the hot-corino signatures remain elusive. In this study, our objective is to establish a general picture of the detectability of hot corinos by identifying the origins of the hot-corino signatures in the sample of young stellar objects (YSOs) obtained from the Atacama Large Millimeter/submillimeter Array Survey of Orion Planck Galactic Cold Clumps project. We apply spectral energy distribution modeling to our sample and identify the physical parameters of the modeled YSOs directly, linking the detection of hot-corino signatures to the envelope properties of the YSOs. Imaging simulations of the methanol emission further support this scenario. We therefore posit that the observed COM emission originates from the warm inner envelopes of the sample YSOs, based on both the warm region size and the envelope density profile. The former is governed by the source luminosity and is additionally affected by the disk and cavity properties, while the latter is related to the evolutionary stages. This scenario provides a framework for detecting hot-corino signatures toward luminous Class 0 YSOs, with fewer detections being observed toward similarly luminous Class I sources.
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