Stefanie N. Milam

The 12C/13C Isotope Gradient Derived from Millimeter Transitions of CN: The Case for Galactic Chemical Evolution

New measurements of 12 C/ 13 C ratios in Galactic molecular clouds have been conducted using the N ¼ 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 clouds, and they include sources that lie in the range of 0.09‐16.41 kpc in distance from the Galactic center (DGC). The ratios derived from CN indicate a gradient with Galactic distance of 12 C/ 13 C ¼ 6:01DGC þ 12:28. This gradient agrees rather closely with those derived from measurements of CO and H2CO. The least-squares fit to all data points for the three molecules is 12 C/ 13 C ¼ 6:21DGC þ 18:71. CO, CN, and H2CO 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 ¼ 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.

The impact and recovery of asteroid 2008 TC 3

In the absence of a firm link between individual meteorites and their asteroidal parent bodies, asteroids are typically characterized only by their light reflection properties, and grouped accordingly into classes. On 6 October 2008, a small asteroid was discovered with a flat reflectance spectrum in the 554–995 nm wavelength range, and designated 2008 TC3 (refs 4–6). It subsequently hit the Earth. Because it exploded at 37 km altitude, no macroscopic fragments were expected to survive. Here we report that a dedicated search along the approach trajectory recovered 47 meteorites, fragments of a single body named Almahata Sitta, with a total mass of 3.95 kg. Analysis of one of these meteorites shows it to be an achondrite, a polymict ureilite, anomalous in its class: ultra-fine-grained and porous, with large carbonaceous grains. The combined asteroid and meteorite reflectance spectra identify the asteroid as F class, now firmly linked to dark carbon-rich anomalous ureilites, a material so fragile it was not previously represented in meteorite collections.

Chemical complexity in the winds of the oxygen-rich supergiant star VY Canis Majoris

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.

Formation of Uracil from the Ultraviolet Photo-Irradiation of Pyrimidine in Pure H2O Ices

The detection of nucleobases in carbonaceous chondrites such as Murchison supports the scenario in which extraterrestrial organic molecules could have contributed to the origin of life on Earth. However, such large molecules have not been observed to date in astrophysical environments, in particular, comets and the interstellar medium (ISM). The physico-chemical conditions under which nucleobases and, more generally, N-heterocycles were formed are unknown, as are their mechanisms of formation. In this work, H2O:pyrimidine ice mixtures were irradiated with UV photons under interstellar/cometary-relevant conditions to study the formation of pyrimidine derivatives, including the nucleobase uracil. Liquid and gas chromatography analyses of the samples produced in our experiments revealed the presence of numerous photoproducts among which 4(3H)-pyrimidone and uracil could be conclusively identified. The photostability of pyrimidine against UV photons was also studied, and we showed that it would survive from the ISM to the solar nebula if formed and preserved in ice mantles on the surface of cold grains. We propose pathways for the formation of 4(3H)-pyrimidone and uracil under astrophysically relevant conditions and discuss the possibility for such molecules to survive from the ISM to their delivery to Earth and other Solar System bodies.

CIRCUMSTELLAR 12C/13C ISOTOPE RATIOS FROM MILLIMETER OBSERVATIONS OF CN AND CO: MIXING IN CARBON- AND OXYGEN-RICH STARS

A survey of the 12C/13C ratio toward circumstellar envelopes has been conducted at millimeter wavelengths using the facilities of the Arizona Radio Observatory (ARO). The ratios were obtained for a sample of local C- and O-rich asymptotic giant branch and supergiant stars from observations of the 12C and 13C isotopologues of CO and CN, respectively. The J = 1 → 0 transitions of both molecules were observed at λ = 3 mm using the ARO 12 m telescope, while the J = 2 → 1 lines of the two species were measured using the ARO Sub-Millimeter Telescope (SMT) at λ = 1 mm. The 12C/13C ratios were determined from the CO data by modeling both transitions simultaneously with a circumstellar radiative transfer code, which can account for the high opacities present in the emission from this species. In the case of CN, the hyperfine structure was used to evaluate opacity effects. Ratios obtained independently from CO and CN are in good agreement. For the C-rich envelopes, the ratios fall in the range 12C/13C ~ 25–90, while the O-rich shells have values of 10-35. Ratios of 12C/13C ~ 3–14 are found for the supergiant stars, with the exception of VY CMa, where the values lie in the range 25–46. All ratios obtained in this study are ≤ 89, the solar value, suggesting that substantial carbon-13 enrichment may be currently occurring in the local interstellar medium. A qualitative model was constructed based on first and third dredge-up convective mixing that can reproduce the observed ratios. Substantial mixing of H-burning products must occur to explain the ratios in the O-rich objects, while a wide range of 12C/13C values can be generated by only a few percent mixing of He-burning ashes in the C-rich case. The 12C/13C ratios obtained in this study should help improve stellar yield models and contribute to the understanding of Galactic chemical evolution.

Isotopic Anomalies in Primitive Solar System Matter: Spin-State-Dependent Fractionation of Nitrogen and Deuterium in Interstellar Clouds

Organic material found in meteorites and interplanetary dust particles is enriched in D and 15N. This is consistent with the idea that the functional groups carrying these isotopic anomalies, nitriles and amines, were formed by ion-molecule chemistry in the protosolar nebula. Theoretical models of interstellar fractionation at low temperatures predict large enrichments in both D and 15N and can account for the largest isotopic enrichments measured in carbonaceous meteorites. However, more recent measurements have shown that, in some primitive samples, a large 15N enrichment does not correlate with one in D, and that some D-enriched primitive material displays little, if any, 15N enrichment. By considering the spin-state dependence in ion-molecule reactions involving the ortho and para forms of H2, we show that ammonia and related molecules can exhibit such a wide range of fractionation for both 15N and D in dense cloud cores. We also show that while the nitriles, HCN and HNC, contain the greatest 15N enrichment, this is not expected to correlate with extreme D enrichment. These calculations therefore support the view that solar system 15N and D isotopic anomalies have an interstellar heritage. We also compare our results to existing astronomical observations and briefly discuss future tests of this model.

Nucleobases and Prebiotic Molecules in Organic Residues Produced from the Ultraviolet Photo-Irradiation of Pyrimidine in NH3 and H2O+NH3 Ices

Although not yet identified in the interstellar medium (ISM), N-heterocycles including nucleobases-the information subunits of DNA and RNA-are present in carbonaceous chondrites, which indicates that molecules of biological interest can be formed in non-terrestrial environments via abiotic pathways. Recent laboratory experiments and ab initio calculations have already shown that the irradiation of pyrimidine in pure H(2)O ices leads to the formation of a suite of oxidized pyrimidine derivatives, including the nucleobase uracil. In the present work, NH(3):pyrimidine and H(2)O:NH(3):pyrimidine ice mixtures with different relative proportions were irradiated with UV photons under astrophysically relevant conditions. Liquid- and gas-chromatography analysis of the resulting organic residues has led to the detection of the nucleobases uracil and cytosine, as well as other species of prebiotic interest such as urea and small amino acids. The presence of these molecules in organic residues formed under abiotic conditions supports scenarios in which extraterrestrial organics that formed in space and were subsequently delivered to telluric planets via comets and meteorites could have contributed to the inventory of molecules that triggered the first biological reactions on their surfaces.

Ethyl alcohol and sugar in comet C/2014 Q2 (Lovejoy).

First detections of ethyl alcohol and glycolaldehyde in a comet provide new constraints on solar system chemical complexity. The presence of numerous complex organic molecules (COMs; defined as those containing six or more atoms) around protostars shows that star formation is accompanied by an increase of molecular complexity. These COMs may be part of the material from which planetesimals and, ultimately, planets formed. Comets represent some of the oldest and most primitive material in the solar system, including ices, and are thus our best window into the volatile composition of the solar protoplanetary disk. Molecules identified to be present in cometary ices include water, simple hydrocarbons, oxygen, sulfur, and nitrogen-bearing species, as well as a few COMs, such as ethylene glycol and glycine. We report the detection of 21 molecules in comet C/2014 Q2 (Lovejoy), including the first identification of ethyl alcohol (ethanol, C2H5OH) and the simplest monosaccharide sugar glycolaldehyde (CH2OHCHO) in a comet. The abundances of ethanol and glycolaldehyde, respectively 5 and 0.8% relative to methanol (0.12 and 0.02% relative to water), are somewhat higher than the values measured in solar-type protostars. Overall, the high abundance of COMs in cometary ices supports the formation through grain-surface reactions in the solar system protoplanetary disk.

CARBON CHEMISTRY IN THE ENVELOPE OF VY CANIS MAJORIS: IMPLICATIONS FOR OXYGEN-RICH EVOLVED STARS

Observations of the carbon-bearing molecules CO, HCN, CS, HNC, CN, and HCO+ have been conducted toward the circumstellar envelope of the oxygen-rich red supergiant star, VY Canis Majoris (VY CMa), using the Arizona Radio Observatory (ARO). CO and HCN were also observed toward the O-rich shells of NML Cyg, TX Cam, IK Tau, and W Hya. Rotational transitions of these species at 1 mm, 0.8 mm, and 0.4 mm were measured with the ARO Submillimeter Telescope, including the J = 6 → 5 line of CO at 691 GHz toward TX Cam and W Hya. The ARO 12 m was used for 2 mm and 3 mm observations. Four transitions were observed for HCO+ in VY CMa, the first definitive identification of this ion in a circumstellar envelope. Molecular line profiles from VY CMa are complex, indicating three separate outflows: a roughly spherical flow and separate red- and blueshifted winds, as suggested by earlier observations. Spectra from the other sources appear to trace a single outflow component. The line data were modeled with a radiative transfer code to establish molecular abundances relative to H2 and source distributions. Abundances for CO derived for these objects vary over an order of magnitude, f ~ 0.4-5 × 10–4, with the lower values corresponding to the supergiants. For HCN, a similar range in abundance is found (f ~ 0.9-9 × 10–6), with no obvious dependence on the mass-loss rate. In VY CMa, HCO+ is present in all three outflows with f ~ 0.4-1.6 × 10–8 and a spatial extent similar to that of CO. HNC is found only in the red- and blueshifted components with [HCN]/[HNC] ~ 150-190, while [CN]/[HCN] ~ 0.01 in the spherical flow. All three velocity components are traced in CS, which has a confined spatial distribution and f ~ 2-6 × 10–7. These observations suggest that carbon-bearing molecules in O-rich shells are produced by a combination of photospheric shocks and photochemistry. Shocks may play a more prominent role in the supergiants because of their macroturbulent velocities.

Formaldehyde in Comets C/1995 O1 (Hale-Bopp), C/2002 T7 (LINEAR), and C/2001 Q4 (NEAT): Investigating the Cometary Origin of H2CO

Observations offormaldehyde (H2CO) have been conducted toward comets C/1995 O1 (Hale-Bopp), C/2001 Q4 (NEAT), and C/2002 T7 (LINEAR) using the Arizona Radio Observatory (ARO) 12 m telescope at 1.2 and 2 mm. Aperture synthesis maps of H2CO at 3 mm were made using the Berkeley-Illinois-Maryland Association (BIMA) interferometer toward comet Hale-Bopp. These data indicate that the production rate of H2CO is � 3:7 ; 10 28 s � 1 at � 1 AU in comet Hale-Bopp, using a simple Monte Carlo model, if a nuclear origin for the molecule is assumed. However, maps of H2CO in Hale-Bopp, in comparison with CO, show an extended distribution (rs � 15,000 km) with small-scale structure oriented roughly along the comet-Sun direction. This result suggests a source of H2CO other than the comet nucleus. The extended source of formaldehyde is probably grains composed of a mixture of silicates and organicmaterial.Theproductionrate forH2CO increasestoQ � 1:4 ; 10 29 s � 1 assuming suchanextended grain source. This value implies a Q/Q(H2O) � 1:4%, which is similar to the production rate ratio of Q/Q(H2O) � 4% derived from in situ measurements of H2CO in comet Halley. Production rates for H2CO toward comets C/2002 T7 (LINEAR) and C/2001 Q4 (NEAT) are 1:4 ; 10 27 and 5:6 ; 10 26 s � 1 , respectively, modeled using the extended grain source. The spectra of H2CO measured toward comet C/2002 T7 (LINEAR) show evidence for a second velocity component, most likely arising from comet fragmentation. Subject headingg astrobiology — comets: individual (Hale-Bopp (C/1995 O1), NEAT (C/2001 Q4), LINEAR (C/2002 T7)) — radio lines: solar system — techniques: interferometric