A new study of an old sink of sulfur in hot molecular cores: the sulfur residue

Sulfur appears to be depleted by an order of magnitude or more from its elemental abundance in star-forming regions. In the last few years, numerous observations and experiments have been performed in order to to understand the reasons behind this depletion without providing a satisfactory explanation of the sulfur chemistry towards high-mass star-forming cores. Several sulfur-bearing molecules have been observed in these regions, and yet none are abundant enough to make up the gas-phase deficit. Where, then, does this hidden sulfur reside? This paper represents a step forward in our understanding of the interactions among the various S-bearing species. We have incorporated recent experimental and theoretical data into a chemical model of a hot molecular core in order to see whether they give any indication of the identity of the sulfur sink in these dense regions. Despite our model producing reasonable agreement with both solid-phase and gas-phase abundances of many sulfur-bearing species, we find that the sulfur residue detected in recent experiments takes up only ~6 per cent of the available sulfur in our simulations, rather than dominating the sulfur budget.Comment: 13 pages, 6 colourful figures, accepted by MNRA

The Paleo-Environment Reconstruction on Mars: Focus Points for the next Astrobiology Missions

The reconstruction of past environmental conditions is important for estimation of the astrobiology potential and to support the targeting of future missions for Mars. Reviewing those observable features (e.g., paleo-environment indicators) that help such reconstruction is important, as they provide focus for current and future research, both in remote and in situ activities, as are Earth based laboratory analysis and simulation. In this work, these indicators are collected and characterized, focusing on the available data, uncertainties, missing information and targets of future research work. These indicators can be classified into groups of: surface morphological (kilometre spatial scale, water and ice related surface structures like fluvial channels, alluvial fans, banks, etc.); in situ m-µm spatial scale features (observable especially in vertical outcrops); and mineralogical (mineral types, OH content). Based on the collected indicators, good general (although not complete) knowledge is available on surface morphological features. Whereas information can be extrapolated from these data regarding the liquid water or frozen ice volume, and temperature, during the formation of certain features, information on pH and salt content, as well as temporal issues, can be only poorly estimated. Mineral paragenesis and different types of indicators should be used together to further support the interpretation. The missing data regarding certain environmental conditions represent the directions for future research

The fate of S-bearing species after ion irradiation of interstellar icy grain mantles

Context. Chemical models predict the presence of S-bearing molecules such as hydrogen sulfide (H2S) in interstellar icy grain mantles in dense molecular clouds. Up to now only two S-bearing molecules, namely sulfur dioxide (SO2) and carbonyl sulfide (OCS), have been detected in the solid phase towards young stellar objects (YSOs), while upper limits for solid H2S have been reported towards the same lines of sight. The estimated abundance of S-bearing molecules in icy grain mantles is not able to account for the cosmic S abundance. Aims. In this paper we studied the effects of ion irradiation on different icy targets formed by carbon monoxide (CO) and SO2 or H2S as mixtures and, for the first time, as layers. Methods. We carried out several irradiation experiments on ices containing SO2 or H2S mixed or layered with CO. The samples were irradiated with 200 keV protons in a high-vacuum chamber ($P < 10^{-7}$ mbar) at a temperature of 16–20 K. IR spectra of the samples were recorded after various steps of irradiation and after warm-up. Results. We have found that the column density of H2S and SO2, as well as CO, decreases after irradiation, and the formation of new molecular species is observed. In the case of CO:SO2 samples, OCS, sulfur trioxide (SO3), ozone (O3), and carbon dioxide (CO2) are the most abundant species formed. In the case of CO:H2S samples the most abundant species formed are OCS, SO2, carbon disulfide (CS2), hydrogen persulfide (H2S2), and CO2. The profile of the OCS band formed after irradiation of the CO:H2S mixture compares well with the profile of the OCS band detected towards the high mass YSO W33

The influence of temperature on the synthesis of molecules on icy grain mantles in dense molecular clouds

Context. Infrared observations show the presence of icy mantles along the line of sight toward young stellar objects (YSOs), where a temperature gradient is expected and indirectly observed. In this environment, icy mantles are affected by ion and UV irradiation. Laboratory experiments show that molecules are formed after irradiation of icy mixtures. However, most of the experiments done so far have been performed in the temperatures range of 10–20 K. Aims. To extend previous work we irradiated some icy mixtures, namely H2O:CO=10:1, H2O:CH4=4:1, and H2O:CO2=3:1 at two different temperatures (12 K and 40 or 60 K) to study the effects of temperature on the synthesis of molecules and the decrease in their parent species after ion irradiation. Methods. The experiments were performed in a high-vacuum chamber (P < 10-7 mbar), where icy samples were irradiated with 30 keV He+ ions and analyzed by a FTIR spectrophotometer. Infrared spectra of the samples were recorded after various steps of irradiation. Results. We found that the temperature affects the behavior of the volatile species (i.e., CO and CH4) during irradiation. As a consequence, the production of molecular species is generally more prevalent at 12 K than at either 40 or 60 K, while the decrease in their parent volatile species is faster at high temperature. Conclusions. We conclude that the behavior of each species depends on the value of its sublimation temperature with respect to the temperature of the sample. If this latter is higher than the sublimation temperature of a given species, then the effects of thermal desorption compete with those due to irradiation

A global response roadmap to the asteroid impact threat: The NEOShield perspective

Besides being of great scientific interest, near-Earth objects represent a well-founded threat to life on our planet. Nonetheless, up to now there has been no concerted international plan on how to deal with the impact threat, and how to prepare and implement mitigation measures. The NEOShield project is funded by the European Commission to address such issues, to investigate the feasibility of techniques to prevent a potentially catastrophic impact on Earth by an asteroid or a comet, and to develop detailed designs of appropriate space missions to test deflection techniques. In this work we present and discuss the scientific and strategic aspects of the asteroid impact threat, highlighting the necessary steps so as to be ready to react to future hazardous objects

Coexistence of Ferromagnetism and Superconductivity in Rapidly Quenched Ni_2NbSn Heusler Alloy

We present the study on production and structural, electric and magnetic properties of superconductive Ni₂NbSn Heusler alloy. The sample has been produced by melt-spinning method using tangential speed of copper wheel 20 m/s. Polycrystalline structure has been obtained showing single phase with B2 disorder with lattice constant a=6.1654 Å. Resistance measurement shows superconductive behavior with critical temperature close to 5 K. Magnetic measurements also exhibit diamagnetic contribution from superconductive phase. Additionally, the ferromagnetic state has been observed below 20 K, which points to the coexistence of magnetic and superconducting state

Laboratory experiments on the radiation astrochemistry of water ice phases

Water (H2O) ice is a ubiquitous component of the universe, having been detected in a variety of interstellar and Solar System environments where radiation plays an important role in its physico-chemical transformations. Although the radiation chemistry of H2O astrophysical ice analogues has been well studied, direct and systematic comparisons of different solid phases are scarce and are typically limited to just two phases. In this article, we describe the results of an in-depth study of the 2 keV electron irradiation of amorphous solid water (ASW), restrained amorphous ice (RAI) and the cubic (Ic) and hexagonal (Ih) crystalline phases at 20 K so as to further uncover any potential dependence of the radiation physics and chemistry on the solid phase of the ice. Mid-infrared spectroscopic analysis of the four investigated H2O ice phases revealed that electron irradiation of the RAI, Ic, and Ih phases resulted in their amorphization (with the latter undergoing the process more slowly) while ASW underwent compaction. The abundance of hydrogen peroxide (H2O2) produced as a result of the irradiation was also found to vary between phases, with yields being highest in irradiated ASW. This observation is the cumulative result of several factors including the increased porosity and quantity of lattice defects in ASW, as well as its less extensive hydrogen-bonding network. Our results have astrophysical implications, particularly with regards to H2O-rich icy interstellar and Solar System bodies exposed to both radiation fields and temperature gradients