Low-mass Mode Research Articles

PAHs and fullerenes as structural and compositional motifs tracing and distinguishing organic carbon from soot

Examining the features distinguishing organic carbon from soot is crucial for understanding the source, the effect on the environment and their respective role in aerosol chemistry and soot formation. Beside to the obvious PAH picking-out in the low-mass mode (C number < 40), the challenging identification of PAHs in the high-mass mode (C number > 40) of organic carbon, separated by carbon particulate matter extraction from young and mature soot thermophoretically sampled in premixed flames, was done by laser-desorption-time-of-flight mass spectrometry, exploiting the laser power increase. The perusal of organic carbon mass spectra through mathematical tools in comparison to aromatic and alkyl-substituted PAH-laden samples and the persistence of high-mass mode at high laser power led to exclude the contribution of dimers and alkyl-bridged PAHs attributing the second mode to both fully-benzenoid and cyclopenta-PAHs.Profound differences between mass spectra of organic carbon and soot were noticed as neither molecules nor radicals of PAHs could be drawn out from soot, even at high laser power, and only small radicals and carbon clusters like fullerenes were observed, especially for young soot. These inferences evidenced the importance of analysing separately organic carbon and soot especially if insights into soot particles nucleation are to be obtained. In the case of benzene flame, already at the inception, soot consists of strongly tangled aromatic motifs crosslinked each other, presumably deriving from reactive coagulation/clustering of relatively small aromatic hydrocarbons/radicals early formed. In methane and ethylene flames, coalesced liquid-like material composed of soot and PAHs is formed and transformed later on undergoing some carbonization and molecular growth, respectively.

Characterization of thin film tandem solar cells by radiofrequency pulsed glow discharge – Time of flight mass spectrometry

Beside low production costs and the use of nontoxic and abundant raw materials, silicon based thin-film solar cells have the advantage to be built up as multi junction devices like tandem or triple junction solar cells. Silicon thin film modules made of tandem cells with hydrogenated amorphous silicon (a-Si:H) top cell and microcrystalline (μc) Si:H bottom cell are available on the market. In this work, the analytical potential of state-of-the art radiofrequency (rf) pulsed glow discharge (PGD) time of flight mass spectrometry (TOFMS) commercial instrumentation is investigated for depth profiling analysis of tandem-junctions solar cells on 2mm thick glass substrate with 1µm thick ZnO:Al. Depth profile characterization of two thin film tandem photovoltaic devices was compared using millisecond and sub-millisecond rf-PGD regimes, as well as the so-called “low mass mode” available in the commercial instrument used. Two procedures for sample preparation, namely using flat or rough cell substrates, were compared and the distribution of dopant elements (phosphorous, boron and germanium) was investigated in both cases. Experimental results obtained by rf-PGD-TOFMS as well as electrical measurements of the samples showed that a worse depth resolution of dopant elements in the silicon layers (e.g. distribution of boron in a thicker region that suggests a diffusion of this dopant in the coating of the sample) found using a rough sample substrate was related to a higher power conversion efficiency.

FRAGMENTATION IN DUSTY LOW-METALLICITY STAR-FORMING HALOS

The first stars in the universe, termed Population III, are thought to have been very massive compared to the stars that form in the present epoch. As feedback from the first generation of stars altered the contents of the interstellar medium, the universe switched to a low-mass mode of star formation, which continues in the high metallicity stars formed in the present era. Several studies have investigated the transition between metal-free and metal-enriched star formation, with tentative evidence being found for a metallicity threshold near 10^-3.5 Z_sun due to atomic and molecular transitions and another threshold near 10^-5.5 Z_sun due to dust. In this work, we simulate the formation of stars in idealized low-metallicity halos using the AMR code Enzo. We conduct several simulations of 10^6 M_sun and 10^7 M_sun halos in which the metal content, initial rotation, and degree of turbulence are varied in order to study the effect of these properties on gas fragmentation over a range of densities. We find tentative support for the idea of a critical metallicity, but the effect of varying metallicity on the gas we observe is not as dramatic as what has been reported in earlier studies. We find no clear relation between the initial spin or the initial level of turbulence in the halo and the final properties of the gas contained therein. Additionally, we find that the degree to which the Jeans length is refined, the initial density profile of the gas, and the inclusion of deuterium chemistry each have a significant effect on the evolution and fragmentation of the gas in the halo - in particular, we find that at least 64 grid cells are needed to cover the Jeans length in order to properly resolve the fragmentation.

THREE MODES OF METAL-ENRICHED STAR FORMATION IN THE EARLY UNIVERSE

Simulations of the formation of Population III (Pop III) stars suggest that they were much more massive than the Pop II and Pop I stars observed today. This is due to the collapse dynamics of metal-free gas, which is regulated by the radiative cooling of molecular hydrogen. We study how the collapse of gas clouds is altered by the addition of metals to the star-forming environment by performing a series of simulations of pre-enriched star formation at various metallicities. For metallicities below the critical metallicity, Z_cr, collapse proceeds similarly to the metal-free case, and only massive objects form. For metallicities well above Z_cr, efficient cooling rapidly lowers the gas temperature to the temperature of the CMB. The gas is unable to radiatively cool below the CMB temperature, and becomes thermally stable. For high metallicities, Z >= 10^-2.5 Zsun, this occurs early in the evolution of the gas cloud, when the density is still relatively low. The resulting cloud-cores show little or no fragmentation, and would most likely form massive stars. If the metallicity is not vastly above Z_cr, the cloud cools efficiently but does not reach the CMB temperature, and fragmentation into multiple objects occurs. We conclude that there were three distinct modes of star formation at high redshift (z >= 4): a `primordial' mode, producing massive stars (10s to 100s Msun) at very low metallicities (Z <= 10^-3.75 Zsun); a CMB-regulated mode, producing moderate mass (10s of Msun) stars at high metallicites (Z >= 10^-2.5 Zsun at redshift z ~ 15-20); and a low-mass (a few Msun) mode existing between those two metallicities. As the universe ages and the CMB temperature decreases, the range of the low mass mode extends to higher metallicities, eventually becoming the only mode of star formation. (Abridged)

Importance of Thermodynamics for Fragmentation and Star Formation

We discuss results from numerical simulations of star formation under various environmental conditions ranging from the turbulent interstellar medium to low-mass halos in the early universe. The thermodynamic behavior of the star-forming gas plays a crucial role in fragmentation and determines the stellar mass function as well as the dynamic properties of the nascent stellar cluster. The thermodynamic state of the gas is a result of the balance between heating and cooling processes, which in turn are determined by atomic and molecular physics and by chemical abundances. Features in the effective equation of state of the gas, such as a transition from a cooling to a heating regime, define a characteristic mass scale for fragmentation and so set the peak of the initial mass function of stars (IMF). As it is based on fundamental physical quantities and constants, this is an attractive approach to explain the apparent universality of the IMF in the solar neighborhood as well as the transition from purely primordial high-mass star formation to the low-mass mode observed today.

Formation of Stellar Clusters and the Importance of Thermodynamics for Fragmentation

AbstractWe discuss results from numerical simulations of star cluster formation in the turbulent interstellar medium (ISM). The thermodynamic behavior of the star-forming gas plays a crucial role in fragmentation and determines the stellar mass function as well as the dynamic properties of the nascent stellar cluster. This holds for star formation in molecular clouds in the solar neighborhood as well as for the formation of the very first stars in the early universe. The thermodynamic state of the ISM is a result of the balance between heating and cooling processes, which in turn are determined by atomic and molecular physics and by chemical abundances. Features in the effective equation of state of the gas, such as a transition from a cooling to a heating regime, define a characteristic mass scale for fragmentation and so set the peak of the initial mass function of stars (IMF). As it is based on fundamental physical quantities and constants, this is an attractive approach to explain the apparent universality of the IMF in the solar neighborhood as well as the transition from purely primordial high-mass star formation to the more normal low-mass mode observed today.

The Transition from the First Stars to the Second Stars in the Early Universe

We observe a sharp transition from a singular, high-mass mode of star formation to a low-mass-dominated mode, in numerical simulations, at a metallicity of 10-3 Z☉. We incorporate a new method for including the radiative cooling from metals into adaptive mesh refinement hydrodynamic simulations. Our results illustrate how metals, produced by the first stars, led to a transition from the high-mass star formation mode of Population III stars to the low-mass mode that dominates today. We ran hydrodynamic simulations with cosmological initial conditions in the standard ΛCDM model, with metallicities, from zero to 10-2 Z☉, beginning at redshift z = 99. The simulations were run until a dense core forms at the center of a 5 × 105 M☉ dark matter halo, at z ~ 18. Analysis of the central 1 M☉ core reveals that the two simulations with the lowest metallicities, Z = 0 and 10-4 Z☉, contain one clump with 99% of the mass, while the two with metallicities Z = 10-3 and 10-2 Z☉ each contain two clumps that share most of the mass. The Z = 10-3 Z☉ simulation also produced two low-mass protostellar objects with masses between 10-2 and 10-1 M☉. Gas with Z ≥ 10-3 Z☉ is able to cool to the temperature of the cosmic microwave background (CMB), which sets a lower limit to the minimum fragmentation mass. This suggests that the second-generation stars produced a spectrum of lower mass stars but were still more massive, on average, than stars formed in the local universe.

Bimodal Star Formation and Remnant-Dominated Galactic Models

Current data on the luminosity function of nearby stars allow the possibility that the stellar initial mass function (IMF) is double-peaked and that the star formation rate (SFR) has decreased substantially with time. It is then possible to account for all of the unseen mass in the solar vicinity as stellar remnants. A model for the solar neighborhood has been constructed in which the IMF is bimodal, the SFR is constant for the low-mass mode and strongly decreasing for the high-mass mode, and the mass in remnants is equal to the column density of unseen matter; this model is found to be consistent with all of the available constraints on the evolution and stellar content of the solar neighborhood. In particular, the observed chemical evolution is satisfactorily reproduced without infall. The total SFR in the model decreases roughly with the 1.4 power of the gas content, which is more plausible than the nearly constant SFR required by models with a monotonic IMF.