Reconstruction of water ice: the neglected process OH + OH → H2O + O

Producción CientíficaContext. Although H2O is the most important molecular material found in the solid state in the interstellar medium, the chemical routes leading to ice through surface reactions are still a matter of discussion. Three reaction pathways proposed in the past are at the heart of current research: hydrogenation of atomic oxygen, molecular oxygen, and ozone. The reaction network finally leads to a small number of processes giving H2O: H + OH, H2 + OH, and H + H2O2. To these processes, OH + OH should be added. It is known to be efficient in atmospheric chemistry and takes the irradiations of the interstellar grains into account that, directly or indirectly, create a number of OH radicals on and in the icy mantles. Aims. We study the role of the existing ice in its own reconstruction after it is destroyed by the constant irradiation of interstellar grains and focus on the OH + OH reaction in the triplet state. Methods. We used numerical simulations with a high level of coupled cluster ab initio calculations for small water aggregates and methods relevant to density functional theory for extended systems, including a periodic description in the case of solid water of infinite dimensions. Results. OH + OH → H2O + O reaction profiles are reported that take the involvement of an increasing number of H2O support molecules into account. It is found that the top of the barrier opposing the reaction gradually decreases with the number of supporting H2O and falls below the level of the reactants for H2O layers or solid water. Conclusions. In contrast to the gas phase, the reaction is barrierless on water ice. By adding a reconstructed H2O molecule and a free oxygen atom at the surface of the remaining ice, this reaction leaves open the possibility of the ice reconstruction

A Spectroscopic Survey of Electronic Transitions of C6_6H, 13^{13}C6_6H, and C6_6D

Electronic spectra of C$_6$H are measured in the $18\,950-21\,100$ cm$^{-1}$ domain using cavity ring-down spectroscopy of a supersonically expanding hydrocarbon plasma. In total, 19 (sub)bands of C$_6$H are presented, all probing the vibrational manifold of the B$^2\Pi$ electronically excited state. The assignments are guided by electronic spectra available from matrix isolation work, isotopic substitution experiments (yielding also spectra for $^{13}$C$_6$H and C$_6$D), predictions from ab initio calculations as well as rotational fitting and vibrational contour simulations using the available ground state parameters as obtained from microwave experiments. Besides the $0_0^0$ origin band, three non-degenerate stretching vibrations along the linear backbone of the C$_6$H molecule are assigned: the $\nu_6$ mode associated with the C-C bond vibration and the $\nu_4$ and $\nu_3$ modes associated with C$\equiv$C triple bonds. For the two lowest $\nu_{11}$ and $\nu_{10}$ bending modes, a Renner-Teller analysis is performed identifying the $\mu^2\Sigma$($\nu_{11}$) and both $\mu^2\Sigma$($\nu_{10}$) and $\kappa^2\Sigma$($\nu_{10}$) components. In addition, two higher lying bending modes are observed, which are tentatively assigned as $\mu^2\Sigma$($\nu_9$) and $\mu^2\Sigma$($\nu_8$) levels. In the excitation region below the first non-degenerate vibration ($\nu_6$), some $^2\Pi-^{2}\Pi$ transitions are observed that are assigned as even combination modes of low-lying bending vibrations. The same holds for a $^2\Pi-^{2}\Pi$ transition found above the $\nu_6$ level. From these spectroscopic data and the vibronic analysis a comprehensive energy level diagram for the B$^2\Pi$ state of C$_6$H is derived and presented.Comment: Accepted for publication in The Journal of Physical Chemistry A (26 July 2016

Origin of molecular oxygen in Comet 67P/Churyumov-Gerasimenko

Molecular oxygen has been detected in the coma of comet 67P/Churyumov-Gerasimenko with abundances in the 1-10% range by the ROSINA-DFMS instrument on board the Rosetta spacecraft. Here we find that the radiolysis of icy grains in low-density environments such as the presolar cloud may induce the production of large amounts of molecular oxygen. We also show that molecular oxygen can be efficiently trapped in clathrates formed in the protosolar nebula, and that its incorporation as crystalline ice is highly implausible because this would imply much larger abundances of Ar and N2 than those observed in the coma. Assuming that radiolysis has been the only O2 production mechanism at work, we conclude that the formation of comet 67P/Churyumov-Gerasimenko is possible in a dense and early protosolar nebula in the framework of two extreme scenarios: (1) agglomeration from pristine amorphous icy grains/particles formed in ISM and (2) agglomeration from clathrates that formed during the disk's cooling. The former scenario is found consistent with the strong correlation between O2 and H2O observed in 67P/C-G's coma while the latter scenario requires that clathrates formed from ISM icy grains that crystallized when entering the protosolar nebula.Comment: The Astrophysical Journal Letters, in pres

Revealing the mid-infrared emission structure of IRAS 16594-4656 and IRAS 07027-7934

TIMMI2 diffraction-limited mid-infrared images of a multipolar proto-planetary nebula IRAS 16594-4656 and a young [WC] elliptical planetary nebula IRAS 07027-7934 are presented. Their dust shells are for the first time resolved (only marginally in the case of IRAS 07027-7934) by applying the Lucy-Richardson deconvolution algorithm to the data, taken under exceptionally good seeing conditions (<0.5"). IRAS 16594-4656 exhibits a two-peaked morphology at 8.6, 11.5 and 11.7 microns which is mainly attributed to emission from PAHs. Our observations suggest that the central star is surrounded by a toroidal structure observed edge-on with a radius of 0.4" (~640 AU at an assumed distance of 1.6 kpc) with its polar axis at P.A.~80 degrees, coincident with the orientation defined by only one of the bipolar outflows identified in the HST optical images. We suggest that the material expelled from the central source is currently being collimated in this direction and that the multiple outflow formation has not been coeval. IRAS 07027-7934 shows a bright, marginally extended emission (FWHM=0.3") in the mid-infrared with a slightly elongated shape along the N-S direction, consistent with the morphology detected by HST in the near-infrared. The mid-infrared emission is interpreted as the result of the combined contribution of small, highly ionized PAHs and relatively hot dust continuum. We propose that IRAS 07027-7934 may have recently experienced a thermal pulse (likely at the end of the AGB) which has produced a radical change in the chemistry of its central star.Comment: 35 pages, 8 figures (figures 1, 2, 4 and 6 are in low resolution) accepted for publication in Ap

Infrared Emission from Interstellar Dust. II. The Diffuse Interstellar Medium

We present a quantitative model for the infrared emission from dust in the diffuse interstellar medium. The model consists of a mixture of amorphous silicate grains and carbonaceous grains, each with a wide size distribution ranging from molecules containing tens of atoms to large grains > 1 um in diameter. We assume that the carbonaceous grains have polycyclic aromatic hydrocarbon (PAH)-like properties at very small sizes, and graphitic properties for radii a > 50 A. On the basis of recent laboratory studies and guided by astronomical observations, we propose "astronomical" absorption cross sections for use in modeling neutral and ionized PAHs from the far ultraviolet to the far infrared. We also propose modifications to the far-infrared emissivity of "astronomical silicate". We calculate energy distribution functions for small grains undergoing "temperature spikes" due to stochastic absorption of starlight photons, using realistic heat capacities and optical properties. Using a grain size distribution consistent with the observed interstellar extinction, we are able to reproduce the near-IR to submillimeter emission spectrum of the diffuse interstellar medium, including the PAH emission features at 3.3, 6.2, 7.7, 8.6, and 11.3um. The model is compared with the observed emission at high Galactic latitudes as well as in the Galactic plane, as measured by COBE and IRTS. We calculate infrared emission spectra for our dust model heated by a range of starlight intensities, and we provide tabulated dust opacities (extended tables available at http://www.astro.princeton.edu/~draine/dust/dustmix.html)Comment: Final version published in ApJ, 554, 778 but with factor 1.086 error in Table 6 and Fig. 16 corrected. Main change from astro-ph version 1 is correction of typographical errors in Table 1, and correction of typo in eq. (A2). 51 pages, 16 figures, Late

Limits on the Contribution of Endogenic Radiolysis to the Presence of Molecular Oxygen in Comet 67P/Churyumov-Gerasimenko

Radiolytic production has been proposed as a potential source for the molecular oxygen observed in comet 67P/Churyumov-Gerasimenko. Radiolysis can be exogenic or endogenic, the latter due to radionuclides present in the dust constitutive of the comet nucleus. We investigated the possibility of forming a significant amount of molecular oxygen through endogenic radiolysis. We applied a model of radiolytic production, developed for an Earth rock-water mixture, and improved it to account for the effect of the size of a radionuclide-bearing grain on the net radiation deposited in its ice mantle. We calculated the possible production of molecular oxygen considering the available experimental values of radiolytic yields. We found that endogenic radiolysis cannot account for the totality of the 3.8% (relative to water) O2abundance derived from the ROSINA observations, with an end member case of our model producing at most a 1% abundance. By contrast, we predict H2O2production leads to an abundance up to two orders of magnitude above observed values

Outflows from Massive YSOs as Seen with the Infrared Array Camera

The bipolar outflow from the massive star forming cluster in DR21 is one of the most powerful known, and in IRAC images the outflow stands out by virtue of its brightness at 4.5 um (Band 2). Indeed, IRAC images of many galactic and extragalactic star formation regions feature prominent Band 2 morphologies. We have analyzed archival ISOSWS spectra of the DR21 outflow, and compare them to updated H2 shocked and UV-excitation models. We find that H2 line emission contributes about 50% of the flux of the IRAC bands at 3.6 um, 4.5 um , and 5.8 um, and is a significant contributor to the 8.0 um band as well, and confirm that the outflow contains multiple excitation mechanisms. Other potentially strong features, in particular Br alpha and CO emission, have been suggested as contributing to IRAC fluxes in outflows, but they are weak or absent in DR21; surprisingly, there also is no evidence for strong PAH emission. The results imply that IRAC images can be a powerful detector of, and diagnostic for, outflows caused by massive star formation activity in our galaxy, and in other galaxies as well. They also suggest that IRAC color-color diagnostic diagrams may need to take into account the possible influence of these strong emission lines. IRAC images of the general ISM in the region, away from the outflow, are in approximate but not precise agreement with theoretical models.Comment: Accepted for publication in the Astrophysical Journal; 32 pages; 7 figure

ISOCAM Mid-infrared spectroscopy and NIR photometry of the HII complex N4 in LMC

[Abridged] We present the analysis of ISOCAM-CVF and NIR photometry data of the HII region complex N4 in LMC. The aim is twofold: 1) to study the connection between the ISM and the star content of this region; 2)to investigate the effects of the lower than galactic metallicity on dust properties. A dust features -- gas lines -- continuum fitting technique on the data, allows the production of images in each single emission and the detailed analysis of dust, and ionized gas. The NIR photometry provides, for the first time, information on the stellar content of N4. The images in single dust feature bands and gas lines clearly show that the HII region core is completely devoid of the carriers responsible for the Aromatic Features (AFs). On the other hand, the ionized gas arises almost completely in this dust cavity, where also the two main exciting stars of N4 are located. We find evidences that the effect of lower than Galactic metallicity on the carriers responsible for the AFs, is not to prevent their formation or to modify their chemical properties, but to enhance their destruction by the high and hard ISRF. We show that this mechanism is more efficient on smaller dust particles/molecules thus affecting the dust-size distribution. We argue that effects on dust--size distribution, rather than thedifferent dust properties due to a lower metallicity, should be taken into account when analyzing more distant relatively low metallicity galaxies. Finally, the analysis of the stellar content of N4 reveals 7 stars: 4 reddened O MS stars and 3 stars with envelopes. In particular, one of these, seems to be an Ultra Compact HII region containing an embedded YSO.Comment: 14 pages including 16 figures, Accepted for publication in A&

The composition of the protosolar disk and the formation conditions for comets

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.Comment: To appear in Space Science Reviews. The final publication is available at Springer via http://dx.doi.org/10.1007/s11214-015-0167-