Pore evolution in interstellar ice analogues: simulating the effects of temperature increase

Context. The level of porosity of interstellar ices - largely comprised of amorphous solid water (ASW) - contains clues on the trapping capacity of other volatile species and determines the surface accessibility that is needed for solid state reactions to take place. Aims. Our goal is to simulate the growth of amorphous water ice at low temperature (10 K) and to characterize the evolution of the porosity (and the specific surface area) as a function of temperature (from 10 to 120 K). Methods. Kinetic Monte Carlo simulations are used to mimic the formation and the thermal evolution of pores in amorphous water ice. We follow the accretion of gas-phase water molecules as well as their migration on surfaces with different grid sizes, both at the top growing layer and within the bulk. Results. We show that the porosity characteristics change substantially in water ice as the temperature increases. The total surface of the pores decreases strongly while the total volume decreases only slightly for higher temperatures. This will decrease the overall reaction efficiency, but in parallel, small pores connect and merge, allowing trapped molecules to meet and react within the pores network, providing a pathway to increase the reaction efficiency. We introduce pore coalescence as a new solid state process that may boost the solid state formation of new molecules in space and has not been considered so far.Comment: 9 pages, 8 figures Accepted for publication in A&

Porosity measurements of interstellar ice mixtures using optical laser interference and extended effective medium approximations

Aims. This article aims to provide an alternative method of measuring the porosity of multi-phase composite ices from their refractive indices and of characterising how the abundance of a premixed contaminant (e.g., CO2) affects the porosity of water-rich ice mixtures during omni-directional deposition. Methods. We combine optical laser interference and extended effective medium approximations (EMAs) to measure the porosity of three astrophysically relevant ice mixtures: H2O:CO2=10:1, 4:1, and 2:1. Infrared spectroscopy is used as a benchmarking test of this new laboratory-based method. Results. By independently monitoring the O-H dangling modes of the different water-rich ice mixtures, we confirm the porosities predicted by the extended EMAs. We also demonstrate that CO2 premixed with water in the gas phase does not significantly affect the ice morphology during omni-directional deposition, as long as the physical conditions favourable to segregation are not reached. We propose a mechanism in which CO2 molecules diffuse on the surface of the growing ice sample prior to being incorporated into the bulk and then fill the pores partly or completely, depending on the relative abundance and the growth temperature.Comment: 9 pages, 6 figures, 1 table. Accepted for publication in A&

Communications and sensing of illumination contributions in a power led lighting system

Abstract — In recent years, LED technology emerged as a prime candidate for the future illumination light source, due to high energy efficiency and long life time. In addition, LEDs offer a superior flexibility in terms of colors and shapes, which leads to a potentially infinite variety of available light patterns. In order to create these patterns via easy user interaction, we need to sense the local light contribution of each LED. This measurement could be enabled through tagging of the light of each LED with unique embedded IDs. To this end, we propose a new modulation and multiple access scheme, named as codetime division multiple access- pulse position modulation (CTDMA-PPM): a form of PPM which is keyed according to a spreading sequence, and in which the duty cycle is subject to pulse width modulation (PWM) according to the required lighting setting. Our scheme considers illumination constraints in addition to the communication requirements and, to our best knowledge, it has not been addressed by other optical modulation methods. Based on the proposed modulation method and multiple access schemes, we develop a system structure, which includes illumination sources, a sensor receiver and a control system. Illumination sources illuminate the environment and transmit information, simultaneously. According to our theoretical analysis, this system structure could support a number of luminaries equal to the size of the CDMA codebook times the dimming range. I

A Formal Study of the Privacy Concerns in Biometric-Based Remote Authentication Schemes

With their increasing popularity in cryptosystems, biometrics have attracted more and more attention from the information security community. However, how to handle the relevant privacy concerns remains to be troublesome. In this paper, we propose a novel security model to formalize the privacy concerns in biometric-based remote authentication schemes. Our security model covers a number of practical privacy concerns such as identity privacy and transaction anonymity, which have not been formally considered in the literature. In addition, we propose a general biometric-based remote authentication scheme and prove its security in our security model

Stretching Our Knowledge of the Electronic Ground State of C_3: the Spectroscopy of Stretching Modes of C_3

Laboratory astrophysics and astrochemistr

A Zero-Gravity Instrument to Study Low Velocity Collisions of Fragile Particles at Low Temperatures

We discuss the design, operation, and performance of a vacuum setup constructed for use in zero (or reduced) gravity conditions to initiate collisions of fragile millimeter-sized particles at low velocity and temperature. Such particles are typically found in many astronomical settings and in regions of planet formation. The instrument has participated in four parabolic flight campaigns to date, operating for a total of 2.4 hours in reduced gravity conditions and successfully recording over 300 separate collisions of loosely packed dust aggregates and ice samples. The imparted particle velocities achieved range from 0.03-0.28 m s^-1 and a high-speed, high-resolution camera captures the events at 107 frames per second from two viewing angles separated by either 48.8 or 60.0 degrees. The particles can be stored inside the experiment vacuum chamber at temperatures of 80-300 K for several uninterrupted hours using a built-in thermal accumulation system. The copper structure allows cooling down to cryogenic temperatures before commencement of the experiments. Throughout the parabolic flight campaigns, add-ons and modifications have been made, illustrating the instrument flexibility in the study of small particle collisions.Comment: D. M. Salter, D. Hei{\ss}elmann, G. Chaparro, G. van der Wolk, P. Rei{\ss}aus, A. G. Borst, R. W. Dawson, E. de Kuyper, G. Drinkwater, K. Gebauer, M. Hutcheon, H. Linnartz, F. J. Molster, B. Stoll, P. C. van der Tuijn, H. J. Fraser, and J. Blu

Resonant infrared irradiation of CO and CH3OH interstellar ices

Solid-phase photo-processes involving icy dust grains greatly affect the chemical evolution of the interstellar medium by leading to the formation of complex organic molecules and by inducing photodesorption. So far, the focus of laboratory studies has been mainly on the impact of energetic ultraviolet (UV) photons on ices, but direct vibrational excitation by infrared (IR) photons is expected to influence the morphology and content of interstellar ices as well. However, little is still known about the mechanisms through which this excess vibrational energy is dissipated, and its implications on the structure and ice photochemistry. In this work, we present a systematic investigation of the behavior of interstellar relevant CO and CH3OH ice analogues upon resonant excitation of vibrational modes using tunable infrared radiation, leading to both the quantification of infrared-induced photodesorption and insights in the impact of vibrational energy dissipation on ice morphology. We utilize an ultrahigh vacuum setup at cryogenic temperatures to grow pure CO and CH3OH ices, as well as mixtures of the two. We expose the ices to intense, near-monochromatic mid-infrared free-electron-laser radiation to selectively excite the species. The dissipation of vibrational energy is observed to be highly dependent on the excited mode and the chemical environment of the ice. All amorphous ices undergo some degree of restructuring towards a more organized configuration upon on-resonance irradiation. Moreover, IR-induced photodesorption is observed to occur for both pure CO and CH3OH ices, with interstellar photodesorption efficiencies of the order of 10 molecules cm-2 s-1 (i.e., comparable to or higher than UV-induced counterparts). Indirect photodesorption of CO upon vibrational excitation of CH3OH in ice mixtures is also observed to occur, particularly in environments rich in methanol.Comment: Accepted for publication in A&A. 19 pages, 14 figures, 2 table

Intramembranous processing by γ-secretase regulates reverse signaling of ephrin-B2 in migration of microglia

The Eph-ephrin system plays pivotal roles in cell adhesion and migration. The receptor-like functions of the ephrin ligands allow the regulation of intracellular processes via reverse signaling. γ-Secretase mediated processing of ephrin-B has previously been linked to activation of Src, a kinase crucial for focal adhesion and podosome phosphorylation. Here, we analyzed the role of γ-secretase in the stimulation of reverse ephrin-B2 signaling in the migration of mouse embryonic stem cell derived microglia. The proteolytic generation of the ephrin-B2 intracellular domain (ICD) by γ-secretase stimulates Src and focal adhesion kinase (FAK). Inhibition of γ-secretase decreased the phosphorylation of Src and FAK, and reduced cell motility. These effects were associated with enlargement of the podosomal surface. Interestingly, expression of ephrin-B2 ICD could rescue these effects, indicating that this proteolytic fragment mediates the activation of Src and FAK, and thereby regulates podosomal dynamics in microglial cells. Together, these results identify γ-secretase as well as ephrin-B2 as regulators of microglial migration

Laboratory evidence for efficient water formation in interstellar ices

Even though water is the main constituent in interstellar icy mantles, its chemical origin is not well understood. Three different formation routes have been proposed following hydrogenation of O, O2, or O3, but experimental evidence is largely lacking. We present a solid state astrochemical laboratory study in which one of these routes is tested. For this purpose O2 ice is bombarded by H- or D-atoms under ultra high vacuum conditions at astronomically relevant temperatures ranging from 12 to 28 K. The use of reflection absorption infrared spectroscopy (RAIRS) permits derivation of reaction rates and shows efficient formation of H2O (D2O) with a rate that is surprisingly independent of temperature. This formation route converts O2 into H2O via H2O2 and is found to be orders of magnitude more efficient than previously assumed. It should therefore be considered as an important channel for interstellar water ice formation as illustrated by astrochemical model calculations.Comment: 15 pages, 4 figures. ApJ, in pres

Complex organic molecules in low-mass protostars on Solar System scales -- II. Nitrogen-bearing species

The chemical inventory of planets is determined by the physical and chemical processes that govern the early phases of star formation. The aim is to investigate N-bearing complex organic molecules towards two Class 0 protostars (B1-c and S68N) at millimetre wavelengths with ALMA. Next, the results of the detected N-bearing species are compared with those of O-bearing species for the same and other sources. ALMA observations in Band 6 ($\sim$ 1 mm) and Band 5 ($\sim$ 2 mm) are studied at $\sim$ 0.5" resolution, complemented by Band 3 ($\sim$ 3 mm) data in a $\sim$ 2.5" beam. NH2CHO, C2H5CN, HNCO, HN13CO, DNCO, CH3CN, CH2DCN, and CHD2CN are identified towards the investigated sources. Their abundances relative to CH3OH and HNCO are similar for the two sources, with column densities that are typically an order of magnitude lower than those of O-bearing species. The largest variations, of an order of magnitude, are seen for NH2CHO abundance ratios with respect to HNCO and CH3OH and do not correlate with the protostellar luminosity. In addition, within uncertainties, the N-bearing species have similar excitation temperatures to those of O-bearing species ($\sim$ 100 $\sim$ 300 K). The similarity of most abundances with respect to HNCO, including those of CH2DCN and CHD2CN, hints at a shared chemical history, especially the high D/H ratio in cold regions prior to star formation. However, some of the variations in abundances may reflect the sensitivity of the chemistry to local conditions such as temperature (e.g. NH2CHO), while others may arise from differences in the emitting areas of the molecules linked to their different binding energies in the ice. The two sources discussed here add to the small number of sources with such a detailed chemical analysis on Solar System scales. Future JWST data will allow a direct comparison between the ice and gas abundances of N-bearing species.Comment: Accepted to A&A, 41 pages, 37 figure