Modeling disk fragmentation and multiplicity in massive star formation

Context. There is growing evidence that massive stars grow by disk accretion in a similar way to their low-mass counterparts. Early in evolution, these disks can achieve masses that are comparable to the current stellar mass, and therefore the forming disks are highly susceptible to gravitational fragmentation. Aims. We investigate the formation and early evolution of an accretion disk around a forming massive protostar, focussing on its fragmentation physics. To this end, we follow the collapse of a molecular cloud of gas and dust, the formation of a massive protostar, the formation of its circumstellar disk, and the formation and evolution of the disk fragments. Methods. We used a grid-based, self-gravity radiation hydrodynamics code including a sub-grid module for stellar evolution and dust evolution. We purposely do not use a sub-grid module for fragmentation such as sink particles to allow for all paths of fragment formation and destruction, but instead we keep the spatial grid resolution high enough to properly resolve the physical length scales of the problem, namely the pressure scale height and Jeans length of the disk. Simulations are performed on a grid in spherical coordinates with a logarithmic spacing of the grid cells in the radial direction and a cosine distribution of the grid cells in the polar direction, focusing the spatial resolution on the disk midplane. As a consequence, roughly 25% of the total number of grid cells, corresponding to ~26 million grid cells, are used to model the disk physics. These constitute the highest resolution simulations performed up to now on disk fragmentation around a forming massive star with the physics considered here. For a better understanding of the effects of spatial resolution and to compare our high-resolution results with previous lower resolution studies in the literature, we perform the same simulation at five different resolutions, each run differing in resolution from its predecessor by a factor of two. Results. The cloud collapses and a massive (proto)star is formed in its center surrounded by a fragmenting Keplerian-like accretion disk with spiral arms. The fragments have masses of ~1 M⊙, and their continuous interactions with the disk, spiral arms, and other fragments result in eccentric orbits. Fragments form hydrostatic cores surrounded by secondary disks with spiral arms that also produce new fragments. We identified several mechanisms of fragment formation, interaction, and destruction. Central temperatures of the fragments can reach the hydrogen dissociation limit, form second Larson cores, and evolve into companion stars. Based on this, we study the multiplicity predicted by the simulations and find approximately six companions at different distances from the primary: from possible spectroscopic multiples, to companions at distances between 1000 and 2000 au.

Limited Proteolysis of Cyclooxygenase-2 Enhances Cell Proliferation

Accumulating evidence suggests that the cyclooxygenase-2 (COX-2) enzyme has additional catalytic-independent functions. Here we show that COX-2 appears to be cleaved in mouse and human tumors, which led us to hypothesize that COX-2 proteolysis may play a role in cell proliferation. The data presented herein show that a K598R point mutation at the carboxyl-terminus of COX-2 causes the appearance of several COX-2 immunoreactive fragments in nuclear compartments, and significantly enhances cell proliferation. In contrast, insertion of additional mutations at the border of the membrane-binding and catalytic domains of K598R COX-2 blocks fragment formation and prevents the increase in proliferation. Transcriptomic analyses show that K598R COX-2 significantly affects the expression of genes involved in RNA metabolism, and subsequent proteomics suggest that it is associated with proteins that regulate mRNA processing. We observe a similar increase in proliferation by expressing just that catalytic domain of COX-2 (ΔNT- COX-2), which is completely devoid of catalytic activity in the absence of its other domains. Moreover, we show that the ΔNT- COX-2 protein also interacts in the nucleus with β-catenin, a central regulator of gene transcription. Together these data suggest that the cleavage products of COX-2 can affect cell proliferation by mechanisms that are independent of prostaglandin synthesis.

On the influence of water on fragmentation of the amino acid L-threonine

The present study describes the fragmentation of the amino acid L-threonine (C4H9NO3) with and without the inclusion of water influence. The fragmentation of the L-threonine molecule was theoretically studied using the Becke’s three-parameter hybrid functional method by applying the non-local correlation provided by Lee, Yang and Parr (B3LYP) with the correlation consistent triple zeta basis (cc-pVTZ). The polarizable continuum model (PCM) was used to evaluate the influence of water. Fragments were selected based on data from mass spectrometry experiments. The chemical compositions of fragments were identified, and the appearance energy was calculated. Based on the obtained results, we can conclude that water affects the appearance energy, the fragment structure and the fragment formation processes.

Phase transition dynamics in hot nuclei and N/Z influence

An abnormal production of events with almost equal-sized fragments was theoretically proposed as a signature of spinodal instabilities responsible for nuclear multifragmentation in the Fermi energy domain. On the other hand finite size effects are predicted to strongly reduce this extra production. High statistics quasifusion hot nuclei produced in central collisions between Xe and Sn isotopes at 32 and 45 MeV per nucleon incident energies have been used to definitively establish, through the experimental measurement of charge correlations, the presence of spinodal instabilities. N/Z influence was also studied. The nature of the phase transition dynamics i.e. the fragment formation was the last missing piece of the puzzle concerning the liquidgas transition in nuclei.

Масс-спектрометрия молекулы ксилита

The technique and results of a mass spectrometric study on the yield of positive ions formed upon ionization of xylitol molecules by electron impact are discussed. The mass spectra of xylitol are studied in the range of mass numbers of 10–160 Da and in the range of bombarding electrons energies of 5–80 eV. Based on the analysis of the measured mass spectra, a scheme of xylitol fragmentation is proposed, which contains the most probable channels of formation of ion-fragments upon electron impact, when the energy of the incident electrons significantly exceeds the ionization potential of the molecule. The appearance energies of ionized fragments of xylitol are determined for the first time from the energy dependences of the effective cross-section formation of the ions upon electron impact. The dynamics of ionized fragment formation is studied in the temperature range of the initial substance evaporation of 340–410 K. The behavior of the temperature dependence indicates that the formation processes of water and oxonium cations compete with each other.