Fragmentation and the formation of primordial protostars: the possible role of Collision Induced Emission

The mechanisms which could lead to chemo-thermal instabilities and fragmentation during the formation of primordial protostars are investigated analytically. We introduce approximations for H2 cooling rates bridging the optically thin and thick regimes. These allow us to discuss instabilities up to densities when protostars become optically thick to continuum radiation (n~10^16 cm^-3). During the collapse, instability arises at two different stages: at low density (n~10^8-10^11 cm^-3), it is due to fast 3-body reactions converting H into H2; at high density (n>10^13 cm^-3), it is due to Collisional Induced Emission (CIE). In agreement with the 3D simulations, we find that the instability at low densities cannot lead to fragmentation, because fluctuations do not survive turbulent mixing, and because their growth is slow. The situation at high density is similar. The CIE-induced instability is as weak as the low density one, with similar ratios of growth and dynamical time scales. Fluctuation growth time is longer than free fall time, and fragmentation seems unlikely. One then expects the first stars to be massive, not to form binaries nor harbour planets. Nevertheless, full 3D simulations are required. They could become possible using simplified estimates of radiative transfer effects, which we show to work very well in the 1D case. This indicates that the effects of radiative transfer during the initial stages of formation of primordial protostars can be treated as local corrections to cooling. (Abridged)Comment: 17 pages, 9 figures; accepted for publication in MNRA

Formation of Primordial Stars in a LCDM Universe

We study the formation of the first generation of stars in the standard cold dark matter model, using a very high-resolution hydordynamic simulations. Our simulation achieves a dynamic range of 10^{10} in length scale. With accurate treatment of atomic and molecular physics, it allows us to study the chemo-thermal evolution of primordial gas clouds to densities up to n = 10^{16}/cc without assuming any a priori equation of state; a six orders of magnitudes improvement over previous three-dimensional calculations. All the relevant atomic and molecular cooling and heating processes, including cooling by collision-induced continuum emission, are implemented. For calculating optically thick H2 cooling at high densities, we use the Sobolev method. To examine possible gas fragmentation owing to thermal instability, we compute explicitly the growth rate of isobaric perturbations. We show that the cloud core does not fragment in either the low-density or high-density regimes. We also show that the core remains stable against gravitational deformation and fragmentation. We obtain an accurate gas mass accretion rate within a 10 Msun innermost region around the protostar. The protostar is accreting the surrounding hot gas at a rate of 0.001-0.01 Msun/yr. From these findings we conclude that primordial stars formed in early minihalos are massive. We carry out proto-stellar evolution calculations using the obtained accretion rate. The resulting mass of the first star is M_ZAMS = 60-100 Msun, with the exact mass dependent on the actual accretion rate.Comment: 27 pages, 13 embedded figures. Revised versio

Thermal Evolution of Primordial Gas Clouds and Mass Range of Zero-Metal Stars

We investigate the evolution and fragmentation of a gas cloud with the primordial chemical composition. A hydrogen gas cloud collapses isothermally at 500-100C K when a low fraction of molecular hydrogen works as a coolant, and breaks into small subcondensations with mass less than 10 M_⦿ due to thermal instability associated with molecular dissociation. On the other hand a hydrogen gas cloud which contains no molecular hydrogen collapses isothermally at 6000-8000 K due to two-photon cooling of atomic hydrogen in a thermally stable condition, and enters the region where thermal energy exceeds radiation energy when thermal equilibrium between matter and radiation is achieved in the cloud. Consideration of energetics in the subsequent stage of the cloud evolution gives an estimate of the mass range 0.180 M_⦿ for stable nuclear-burning protostars of the first generation. The thermal behavior of a gas cloud in the regime of z (the ratio of heavy-element abundance to solar one) less than 10^ is essentially similar to that in case of no heavy elements, and the heavy-element cooling brings about thermal instability in a wide range of density and temperature in the regime of z greater than 10^. Linear perturbation analysis gives a growth time of the instability much shorter than the free-fall time, and suggests an efficient excitation of density fluctuations driven by thermal instability

Effects of Heavy Element Abundance on Evolution of Supernova Remnants

The evolution of supernova remnants is investigated numerically for the various abundances of the ambient medium in order to see the effects of a supernova explosion on the interstellar gas in various stages of the galactic evolution. It is shown that the time-evolution of a supernova remnant is delayed with the decrease in the heavy element abundance because of the suppressed cooling efficiency in the gas, and that the dynamical behavior of a remnant in the regime of z (the ratio of heavy element abundance to cosmic one) less than 10^ is essentially the same as that in the case of no heavy element. The heavy element cooling also affects properties of dense shell behind the shock front, and the kinetic energy and the radius of a remnant. A discussion is given that the pressure in the hot gas, produced by supernova explosions, could be sufficiently high to support against the gravitational contraction in the initial phase of the Galaxy

The mass spectrum of metal-free Stars resulting from photodissociation feedback: A scenario for the formation of low-mass population III stars

The initial mass function (IMF) of metal-free stars that form in the initial starburst of massive (virial temperatures >10^4K) metal-free protogalaxies is studied. In particular, we focus on the effect of H2 photodissociation by pre-existing stars on the fragmentation mass scale, presumedly determined by the Jeans mass at the end of the initial free-fall phase, i.e., at the so-called ``loitering phase,'' characterized by the local temperature minimum. Photodissociation diminishes the Jeans mass at the loitering phase, thereby reducing the fragmentation mass scale of primordial clouds. Thus, in a given cloud, far ultraviolet (FUV) radiation from the first star, which is supposedly very massive (about 10^3Msun), reduces the mass scale for subsequent fragmentation. Through a series of similar processes the IMF for metal-free stars is established. If FUV radiation exceeds a threshold level, the star-forming clumps collapse solely through atomic cooling. Correspondingly, the fragmentation scale drops discontinuously from a few time 10Msun to sub-solar scales. In compact clouds (>1.6kpc for clouds of gas mass 10^8Msun), this level of radiation field is attained, and sub-solar mass stars are formed even in a metal-free environment. Consequently, the IMF becomes bi-modal, with peaks at a few tenths of Msun and a few times 10Msun. The high-mass portion of the IMF is found to be a very steep function of the stellar mass, xi_high(m) being proportinal to m^{-5}. Therefore, the typical mass scale of metal-free stars is significantly smaller than that of the very first stars. Also we study the thermal instability in collapsing primordial prestellar clumps, and discuss why the thermal instability occuring during the three-body H2 formation does not appear to manifest itself in causing further fragmentation of such clumps.Comment: 34 pages, 6 figures, ApJ accepte

The Formation of the First Stars in the Universe

In this review, I survey our current understanding of how the very first stars in the universe formed, with a focus on three main areas of interest: the formation of the first protogalaxies and the cooling of gas within them, the nature and extent of fragmentation within the cool gas, and the physics -- in particular the interplay between protostellar accretion and protostellar feedback -- that serves to determine the final stellar mass. In each of these areas, I have attempted to show how our thinking has developed over recent years, aided in large part by the increasing ease with which we can now perform detailed numerical simulations of primordial star formation. I have also tried to indicate the areas where our understanding remains incomplete, and to identify some of the most important unsolved problems.Comment: 74 pages, 4 figures. Accepted for publication in Space Science Review

The role of tomosynthesis compared to magnetic resonance imaging in breast diagnostics - a study comparison

Die Mammadiagnostik ist ein Teilbereich im Berufsalltag von Radiologietechnolog*innen. Daher ist es relevant, mögliche Vorteile bzw Nachteile von Tomosythese und MRT (= Magnetresonanztomographie) im Bezug auf Spezifität und Sensitivität zu kennen. Das Ziel dieser Arbeit ist es, den aktuellen Stellenwert der Tomosynthese in der Mammadiagnostik im Vergleich zu der Brust- MRT darzustellen. Daher konzentriert sich diese Arbeit einerseits auf Unterschiede, Vorteile und Nutzen der beiden bildgebenden Verfahren im Vergleich. Es wird erarbeitet, welche Untersuchungsmodalität der anderen bei gewissen Indikationen überlegen ist. Es werden die Sensitivität und Spezifität der Tomosynthese im Vergleich zur Brust-MRT gegenübergestellt. Die Methodik beruht auf Studien, die ein besseres Verständnis zu diesem Themengebiet liefern. Unter anderem wird Fachliteratur aus der Bibliothek der FH Campus Wien miteinbezogen, um genügend themenspezifisches Recherchematerial zu sammeln. Die Datenbanken, die zur Suche der Studien gewählt werden, sind ScienceDirect, PubMed, Medline und Cochrane Library, welche durch den Fernzugriff der FH Campus Wien im Datenbankinformationssystem (=DBIS) zugänglich sind. Um die Suche einzugrenzen, werden Studien in englischer und deutscher Sprache verwendet, die im Zeitraum von 2016 und 2021 publiziert wurden. Zusammenfassend lässt sich sagen, dass die Auswahl der Bildgebung bei der Mamma-Untersuchung eine zentrale Rolle bei der Erkennung und Einteilung von Brustkrebs spielt und bei der Entscheidungsfindung der Behandlung hilft. Die Tomosynthese liefert gegenüber der Mamma-MRT eine höhere Spezifität jedoch eine niedrigere Sensitivität hier ist der Brust-MRT überlegen. Die Kombination mehrerer unterschiedlicher Bildgebungen liefert ein optimales Ergebnis im Bereich der Mammadiagnostik.Breast diagnostic is part of the daily routine of radiology technologists. Hence, it is important to know the advantages and disadvantages of different technologies such as tomosynthesis and magnetic resonance imaging (MRI), especially concerning specificity and sensitivity. The aim of this work is to show the current status of tomosynthesis in breast diagnostics compared to breast MRI. This work concentrates on the differences, advantages, and benefits of the two imaging methods. Furthermore, it investigates which examination modality is superior to the other for certain indications. The sensitivity and specificity of tomosynthesis are compared to breast MRI. The methodology mainly involved reviewing studies that provided insights about this topic. Additionally, literature from the library of the FH Campus Wien is included in order to collect sufficient topic-specific research material. The databases that were considered to search for the studies were ScienceDirect, PubMed, Medline, and Cochrane Library, which can be accessed remotely by the FH Campus Wien in the database information system (DBIS). The investigation was limited to English and German studies that were published between 2016 and 2021. In conclusion, the choice of imaging in the breast exam plays a central role in the detection and classification of breast cancer and helps in making treatment decisions. Compared to breast MRI, tomosynthesis provides a higher specificity, but a lower sensitivity, which is superior to breast MRI. The combination of several different imaging systems deliver optimal results in the field of breast diagnostics

Understanding the Experiences of Students Identified as Uninvolved at the University of San Diego

The purpose of this study was to explore and understand why students are not involved at the University of San Diego (USD). My research question is: How can I help the Student Leadership Involvement and Changemaking (SLIC) better understand the experiences of students who are uninvolved at USD? Participants for this study included involved and uninvolved students at USD. After conducting this research and learning more about students’ involvement experiences, I have strengthened my professional practice in designing, programming and/or changing a structure of our department to make it more engaging and welcoming to students. Understanding some of the students’ involvement experiences at USD helped create recommendations for restructuring involvement initiatives such as the Involvement Consultant (IC) program, the Alcalá Bazaar, as well as the promotion of events and extracurricular activities around campus

Star Formation Driven by Thermal-Chemical Instability in a Pre-Galactic Gas Cloud

The hydrogen molecule would work as an efficient coolant in a pre-galactic medium which is free from heavy elements. It is shown by linear perturbation analysis that molecular reactions lead to a thermal instability of condensation mode (Sabano and Yoshii 1977, Yoshii and Sabano 1979, Silk 1983). In the present paper we study the nonlinear growth of perturbations by a one-dimensional simulation of gas dynamics, which includes molecular reactions as well as an energy equation. We numerically follow the time-evolution of a spherically symmetric perturbation, which is superposed on initially uniform medium under free-fall contraction, by a Lagrangian hydrodynamical programme with an artificial viscosity (Richtmyer and Morton 1967).</jats:p