Detection of Molecular Oxygen in Orion: An Astrochemical Dilemma
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astrochemistry↗
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Infrared Spectra and Optical Constants of Elusive Amorphous Methane
New and accurate laboratory results are reported for amorphous methane (CH4) ice near 10 K for the study of the interstellar medium (ISM) and the outer Solar System. Near- and mid-infrared (IR) data, including spectra, band strengths, absorption coefficients, and optical constants, are presented for the first time for this seldom-studied amorphous solid. The apparent IR band strength near 1300 cm(exp −1) (7.69 micrometer) for amorphous CH4 is found to be about 33% higher than the value long used by IR astronomers to convert spectral observations of interstellar CH4 into CH4 abundances. Although CH4 is most likely to be found in an amorphous phase in the ISM, a comparison of results from various laboratory groups shows that the earlier CH4 band strength at 1300 cm(exp −1) (7.69 micrometer) was derived from IR spectra of ices that were either partially or entirely crystalline CH4 Applications of the new amorphous-CH4 results are discussed, and all optical constants are made available in electronic form.
ISM: abundances↗
Probing the Effects of External Irradiation on Low-Mass Protostars Through Unbiased Line Surveys
The envelopes of molecular gas around embedded low-mass protostars show different chemistries, which can be used to trace their formation history and physical conditions. The excitation conditions of some molecular species can also be used to trace these physical conditions, making it possible to constrain for instance sources of heating and excitation. Aims. We study the range of influence of an intermediate-mass Herbig Be protostar. We also study the effect of feedback from the environment on the chemical and physical properties of embedded protostars. Methods. We followed up on an earlier line survey of the Class 0I source R CrA IRS7B in the 0.8 mm window with an unbiased line survey of the same source in the 1.3 mm window using the Atacama Pathfinder Experiment (APEX) telescope. We also studied the excitation of the key species H2CO, CH3OH, and c-C3H2 in a complete sample of the 18 embedded protostars in the Corona Australis star-forming region. Radiative transfer models were employed to establish abundances of the molecular species. Results. We detect line emission from 20 molecular species (32 including isotopologues) in the two surveys. The most complex species detected are CH3OH, CH3CCH, CH3CHO, and CH3CN (the latter two are only tentatively detected). CH3CN and several other complex organic molecules are significantly under-abundant in comparison with what is found towards hot corino protostars. The H2CO rotational temperatures of the sources in the region decrease with the distance to the Herbig Be star R CrA, whereas the c-C3H2 temperatures remain constant across the star-forming region.Conclusions. The high H2CO temperatures observed towards objects close to R CrA suggest that this star has a sphere of influence of several 10 000 AU in which it increases the temperature of the molecular gas to 3050 K through irradiation. The chemistry in the IRS7B envelope differs significantly from many other embedded protostars, which could be an effect of the external irradiation from R CrA.
ISM: individual objects↗
High-Resolution IR Absorption Spectroscopy of Polycyclic Aromatic Hydrocarbons: The Realm of Anharmonicity
We report on an experimental and theoretical investigation of the importance of anharmonicity in the 3 micrometers CH stretching region of Polycyclic Aromatic Hydrocarbon (PAH) molecules. We present mass-resolved, high-resolution spectra of the gas-phase cold ((is) approximately 4K) linear PAH molecules naphthalene, anthracene, and tetracene. The measured IR spectra show a surprisingly high number of strong vibrational bands. For naphthalene, the observed bands are well separated and limited by the rotational contour, revealing the band symmetries. Comparisons are made to the harmonic and anharmonic approaches of the widely used Gaussian software. We also present calculated spectra of these acenes using the computational program SPECTRO, providing anharmonic predictions enhanced with a Fermi-resonance treatment that utilizes intensity redistribution. We demonstrate that the anharmonicity of the investigated acenes is strong, dominated by Fermi resonances between the fundamental and double combination modes, with triple combination bands as possible candidates to resolve remaining discrepancies. The anharmonic spectra as calculated with SPECTRO lead to predictions of the main modes that fall within 0.5% of the experimental frequencies. The implications for the Aromatic Infrared Bands, specifically the 3-m band are discussed.
Astrochemistry↗
Ice Chemistry in Interstellar Dense Molecular Clouds, Protostellar Disks, and Comets
Despite the low temperatures (T less than 20K), low pressures, and low molecular densities found in much of the cosmos, considerable chemistry is expected to occur in many astronomical environments. Much of this chemistry happens in icy grain mantles on dust grains and is driven by ionizing radiation. This ionizing radiation breaks chemical bonds of molecules in the ices and creates a host of ions and radicals that can react at the ambient temperature or when the parent ice is subsequently warmed. Experiments that similar these conditions have demonstrated a rich chemistry associated with these environments that leads to a wide variety of organic products. Many of these products are of considerable interest to astrobiology. For example, the irradiation of simple ices has been shown to abiotically produce amino acids, nucleobases, quinones, and amphiphiles, all compounds that play key roles in modern biochemistry. This suggests extraterrestrial chemistry could have played a role in the origin of life on Earth and, by extension, do so on planets in other stellar systems.
Ice Irradiation↗
Sugar and Sugar Derivatives in Residues Produced from the UV Irradiation of Astrophysical Ice Analogs
A large variety and number of organic compounds of prebiotic interest are known to be present in carbonaceous chondrites. Among them, one sugar (dihydroxyacetone) as well as several sugar acids, sugar alcohols, and other sugar derivatives have been reported in the Murchison and Murray meteorites. Their presence, along with amino acids, amphiphiles, and nucleobases strongly suggests that molecules essential to life can form abiotically under astrophysical conditions. This hypothesis is supported by laboratory studies on the formation of complex organic molecules from the ultraviolet (UV) irradiation of simulated astrophysical ice mixtures consisting of H2O, CO, CO2, CH3OH, CH4, NH3, etc., at low temperature. In the past 15 years, these studies have shown that the organic residues recovered at room temperature contain amino acids, amphiphiles, nucleobases, as well as other complex organics. However, no systematic search for the presence of sugars and sugar derivatives in laboratory residues have been reported to date, despite the fact that those compounds are of primary prebiotic significance. Indeed, only small (up to 3 carbon atoms) sugar derivatives including glycerol and glyceric acid have been detected in residues so far.
Ice Irradiation↗
The Role of Extraterrestrial Materials in the Origin of Life
It has been well established for some time now that C-rich organic materials are relatively common in a number of environments in space. This is known through the telescopic detection of these materials using spectroscopy techniques in the infrared and sub-millimeter wavelength ranges and through the identification of organics in extraterrestrial materials. Extraterrestrial materials in which organics have been found include collected meteorites and interplanetary dust particles, and samples returned by NASA spacecraft from comets. These organics are produced by a variety of astrochemical processes. Despite their abiotic origins, these organic materials of are considerable interest to astrobiology for several reasons. First, organic materials of any composition are important as a means of delivering the elements C, H, O, and N to the surfaces of newly formed planets, and these elements are likely critical to the origin and subsequent evolution of life (certainly for life as we know it). In addition, it is clear that at least a portion of the organics found in space are in the form of molecules that play important roles in modern biology - for example, molecules like amino acids, amphiphiles, quinones, etc. Thus, the delivery of extraterrestrial organics to planetary surfaces brings not only bulk C, H, O, and N, but also molecular complexity in forms that are potentially useful for the origin and early evolution of life. This suggests that the production and delivery of cosmic organic compounds may have played key roles in the origin of life on Earth and, by extension, on other planets in the universe.
Astrochemistry↗
The Formation of Nucleobases from the Irradiation of Purine in Astophysical Ices and Comparisons with Meteorites.
N-heterocycles have been identified in meteorites and their extraterrestrial origins are suggested by isotopic ratio measurements. Although small N- heterocycles have not been detected in the interstellar medium (ISM), recent experiments in our lab have shown that the irradiation of the aromatic molecules like benzene (C6H6) and naphthalene (C10H8) in mixed molecular ices leads to the formation of O- and N-heterocyclic molecules. Among the class of N-heterocycles are the nucleobases, which are of astrobiological interest because they are the information bearing units of DNA and RNA. Nucleobases have been detected in meteorites [3-5], with isotopic signatures that are also consistent with an extraterrestrial origin. Three of the biologically relevant nucleobases (uracil, cytosine, and guanine) have a pyrimidine core structure while the remaining two (adenine and guanine) possess a purine core. Previous experiments in our lab have demonstrated that all of the bio-logical nucleobases (and numerous other molecules) with a pyrimidine core structure can be produced by irradiating pyrimidine in mixed molecular ices of several compositions [6-8]. In this work, we study the formation of purine-based molecules, including the nucleobases adenine, and guanine, from the ultraviolet (UV) irradiation of purine in ices consisting mixtures of H2O and NH3 at low temperature. The experiments are designed to simulate the astrophysical conditions under which these species may be formed in dense molecular clouds, protoplanetary disks, or on the surfaces of icy bodies in planetary systems.
Astrochemistry↗
High-Resolution IR Absorption Spectroscopy of Polycyclic Aromatic Hydrocarbons in the 3-micrometers Region: Role of Periphery
In this work we report on high-resolution IR absorption studies that provide a detailed view on how the peripheral structure of irregular polycyclic aromatic hydrocarbons (PAHs) affects the shape and position of their 3-micrometers absorption band. To this purpose we present mass-selected, high-resolution absorption spectra of cold and isolated phenanthrene, pyrene, benz[a]antracene, chrysene, triphenylene, and perylene molecules in the 2950-3150 per cm range. The experimental spectra are compared with standard harmonic calculations, and anharmonic calculations using a modified version of the SPECTRO program that incorporates a Fermi resonance treatment utilizing intensity redistribution. We show that the 3-micrometers region is dominated by the effects of anharmonicity, resulting in many more bands than would have been expected in a purely harmonic approximation. Importantly, we find that anharmonic spectra as calculated by SPECTRO are in good agreement with the experimental spectra. Together with previously reported high-resolution spectra of linear acenes, the present spectra provide us with an extensive dataset of spectra of PAHs with a varying number of aromatic rings, with geometries that range from open to highly-condensed structures, and featuring CH groups in all possible edge configurations. We discuss the astrophysical implications of the comparison of these spectra on the interpretation of the appearance of the aromatic infrared 3-micrometers band, and on features such as the two-component emission character of this band and the 3-micrometers emission plateau.
techniques: spectroscopic↗
Sugars and Sugar Derivatives in Residues Produced from the UV Irradiation of Astrophysical Ice Analogs
Carbonaceous chondrites contain a large variety of organic compounds of prebiotic interest, which include amino acids, amphiphiles, nucleobases, and sugar derivatives. The presence of these compounds strongly suggests that molecules essential to life can form abiotically under astrophysical conditions. Among the sugar derivatives reported in the Murchison and Murray meteorites, only one sugar (dihydroxyacetone) was found, together with a variety of sugar alcohols and sugar acids containing up to 6 carbon atoms, including sugar acid derivatives of the biological sugars ribose and glucose. On the other hand, laboratory studies on the formation of complex organic molecules from the ultraviolet (UV) irradiation of simulated astrophysical ice mixtures consisting of H2O, CO, CO2, CH3OH, CH4, NH3, etc., at low temperature have been routinely carried out in the past 15 years. These studies have shown that the organic residues recovered at room temperature contain amino acids, amphiphiles, nucleobases, as well as other complex organics, supporting a scenario in which molecules of prebiotic interest can form in extra-terrestrial environments.
Astrochemistry↗
Formation and Recondensation of Complex Organic Molecules During Protostellar Luminosity Outbursts
During the formation of stars, the accretion of surrounding material toward the central object is thought to undergo strong luminosity outbursts followed by long periods of relative quiescence, even at the early stages of star formation when the protostar is still embedded in a large envelope. We investigated the gas-phase formation and recondensation of the complex organic molecules (COMs) di-methyl ether and methyl formate, induced by sudden ice evaporation processes occurring during luminosity outbursts of different amplitudes in protostellar envelopes. For this purpose, we updated a gas-phase chemical network forming COMs in which ammonia plays a key role. The model calculations presented here demonstrate that ion-molecule reactions alone could account for the observed presence of di-methyl ether and methyl formate in a large fraction of protostellar cores without recourse to grain-surface chemistry, although they depend on uncertain ice abundances and gas-phase reaction branching ratios. In spite of the short outburst timescales of about 100 years, abundance ratios of the considered species higher than 10% with respect to methanol are predicted during outbursts due to their low binding energies relative to water and methanol which delay their recondensation during cooling. Although the current luminosity of most embedded protostars would be too low to produce complex organics in the hot-core regions that are observable with current sub-millimetric interferometers, previous luminosity outburst events would induce the formation of COMs in extended regions of protostellar envelopes with sizes increasing by up to one order of magnitude.
astrochemistry↗
Infrared Spectra and Band Strengths of CH3SH, an Interstellar Molecule
Three solid phases of CH3SH (methanethiol or methyl mercaptan) have been prepared and their mid-infrared spectra recorded at 10-110 degrees Kelvin, with an emphasis on the 17-100 degrees Kelvin region. Refractive indices have been measured at two temperatures and used to estimate ice densities and infrared band strengths. Vapor pressures for the two crystalline phases of CH3SH at 110 degrees Kelvin are estimated. The behavior of amorphous CH3SH on warming is presented and discussed in terms of Ostwald's step rule. Comparisons to CH3OH under similar conditions are made, and some inconsistencies and ambiguities in the CH3SH literature are examined and corrected.
Astrochemistry↗
Photo-Induced Deuterium Enrichment in Residues Produced from the UV Irradiation of Pyrimidine in H2O and H2O+NH3 Ices
Organic compounds found in meteorites often show isotopic signatures of their interstellar/protosolar heritage as enrichments in D and 15N. Meteoritic organics found to be enriched in D include amino acids, hydroxy and dicarboxylic acids, as well as polycyclic aromatic hydrocarbons (PAHs). Processes that can produce isotopic enrichments in presolar/protosolar materials include gas-phase ion-molecule reactions, gas-grain surface reactions, and unimolecular photo-dissociation reactions involving PAHs. Because many molecules in interstellar clouds are enriched in D, the presence of D anomalies in meteorites is thought to originate from preserved or slightly altered interstellar/protostellar materials. However, the link between isotopic enrichments seen in space and those in meteoritic compounds and their relationship remain unclear. In this work, we present results of hydrogen isotopic fractionation for compounds in organic residues produced from the UV irradiation using an H2- discharge UV lamp of H2O:pyrimidine = 20:1 and H2O:NH3:pyrimidine = 20:2:1 ice mixtures at low temperature (is less than 20 K). After irradiation, the resulting residues are dissolved in H2O and analyzed with gas chromatography-mass spectrometry coupled with isotope ratio mass spectrometry (GC-MS/IRMS) [1], following a protocol similar to that used for previous analyses of comparable samples [2,3]. We used this technique to measure compound-specific D/H isotopic ratios for the initial pyrimidine and for two photo-products present in the residues, namely, 2,2'-bipyrimidine and an unidentified bipyrimidine isomer [2-4]. Measuring D enrichments in bipyrimidines has the advantage that the H atoms on these molecules are not easily exchangeable with other compounds, in particular the H2O and NH3 present in the ices or the solvents used to extract the samples for GC-MS/IRMS measurements. The δD value for the initial pyrimidine, measured with a high-temperature conversion elemental analyzer connected to the IRMS, was found to be -30% per mille. Preliminary measurements made on a residue produced from the UV irradiation of an H2O:NH3:pyrimidine = 20:2:1 ice mixture indicate δD values of +118% per mille for 2,2'- bipyrimidine and +92% per mille for the other bipyrimidine isomer, and therefore show a significant D enrichment during the photo-processing and warm-up that lead to their formation [5]. New measurements are currently being performed on a number of residues produced from simpler H2O:pyrimidine = 20:1 ice mixtures under different experimental conditions and will be presented here.
Laboratory Astrochemistry↗
Chemical Complexity Induced by Efficient Ice Evaporation in the Barnard 5 Molecular Cloud
Cold gas-phase water has recently been detected in a cold dark cloud, Barnard 5 located in the Perseus complex, by targeting methanol peaks as signposts for ice mantle evaporation. Observed morphology and abundances of methanol and water are consistent with a transient non-thermal evaporation process only affecting the outermost ice mantle layers, possibly triggering a more complex chemistry. Here we present the detection of the complex organic molecules (COMs) acetaldehyde (CH3CHO) and methyl formate (CH3OCHO), as well as formic acid (HCOOH) and ketene (CH2CO), and the tentative detection of di-methyl ether (CH3OCH3) towards the ''methanol hotspot'' of Barnard 5 located between two dense cores using the single dish OSO 20 m, IRAM 30 m, and NRO 45 m telescopes. The high energy cis-conformer of formic acid is detected, suggesting that formic acid is mostly formed at the surface of interstellar grains and then evaporated. The detection of multiple transitions for each species allows us to constrain their abundances through LTE and non-LTE methods. All the considered COMs show similar abundances between approx. 1 and approx. 10% relative to methanol depending on the assumed excitation temperature. The non-detection of glycolaldehyde, an isomer of methyl formate, with a [glycolaldehyde]/[methyl formate] abundance ratio lower than 6%, favours gas phase formation pathways triggered by methanol evaporation. According to their excitation temperatures derived in massive hot cores, formic acid, ketene, and acetaldehyde have been designated as ''lukewarm'' COMs whereas methyl formate and di-methyl ether were defined as ''warm'' species. Comparison with previous observations of other types of sources confirms that lukewarm and warm COMs show similar abundances in low-density cold gas whereas the warm COMs tend to be more abundant than the lukewarm species in warm protostellar cores. This abundance evolution suggests either that warm COMs are indeed mostly formed in protostellar environments and/or that lukewarm COMs are efficiently depleted by increased hydrogenation efficiency around protostars.
ISM: molecules↗
Interstellar Ices and Radiation-Induced Oxidations of Alcohols
Infrared spectra of ices containing alcohols that are known or potential interstellar molecules are examined before and after irradiation with 1 MeV protons at ~20 K. The low-temperature oxidation (hydrogen loss) of six alcohols is followed, and conclusions are drawn based on the results. The formation of reaction products is discussed in terms of the literature on the radiation chemistry of alcohols and a systematic variation in their structures. The results from these new laboratory measurements are then applied to a recent study of propargyl alcohol. Connections are drawn between known interstellar molecules, and several new reaction products in interstellar ices are predicted.
astrochemistry – infrared: ISM – ISM: molecule↗
Observational Results of a Multi-Telescope Campaign in Search of Interstellar Urea [(NH2)2CO]
In this paper, we present the results of an observational search for gas phase urea [(NH2)2CO] observed toward the Sgr B2(N-LMH) region. We show data covering urea transitions from ∼100 GHz to 250 GHz from five different observational facilities: the Berkeley-Illinois-Maryland-Association (BIMA) Array, the Combined Array for Research in Millimeter-wave Astronomy (CARMA), the NRAO 12 m telescope, the IRAM 30 m telescope, and the Swedish-ESO Submillimeter Telescope (SEST). The results show that the features ascribed to urea can be reproduced across the entire observed bandwidth and all facilities by best-fit column density, temperature, and source size parameters which vary by less than a factor of two between observations merely by adjusting for telescope-specific parameters. Interferometric observations show that the emission arising from these transitions is cospatial and compact, consistent with the derived source sizes and emission from a single species. Despite this evidence, the spectral complexity of both (NH2)2CO and of Sgr B2(N) makes the definitive identification of this molecule challenging. We present observational spectra, laboratory data, and models, and discuss our results in the context of a possible molecular detection of urea.
clouds - ISM↗
Interstellar Ices and Radiation-induced Oxidations of Alcohols
Infrared spectra of ices containing alcohols that are known or potential interstellar molecules are examined before and after irradiation with 1 MeV protons at ~20 K. The low-temperature oxidation (hydrogen loss) of six alcohols is followed, and conclusions are drawn based on the results. The formation of reaction products is discussed in terms of the literature on the radiation chemistry of alcohols and a systematic variation in their structures. The results from these new laboratory measurements are then applied to a recent study of propargyl alcohol. Connections are drawn between known interstellar molecules, and several new reaction products in interstellar ices are predicted.
astrochemistry – infrared: ISM – ISM: molecule↗