Primordial Gas Clouds Research Articles

Formation of Primordial Stars in a ΛCDM 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.

Early structure formation in quintessence models and its implications for cosmic reionization from first stars

We present the first hydrodynamic N-body simulations of primordial gas clouds responsible for the reionization process in dark energy cosmologies. We compare the cosmological constant scenario with a SUGRA quintessence model with marked dynamics in order to highlight effects due to the different acceleration histories imposed by the dark energy. We show that both the number density of gas clouds and their clumpiness keep a record of the expansion rate during evolution, similar to the non-linear dark matter profile at virialization, as was recently demonstrated by Dolag et al. Varying the shape of the primordial power spectrum, we show how this effect is mitigated by a running spectral index decreasing the power at small scales. Our results demonstrate that, in order to constrain the dark energy from large-scale structures, one must track its effects down to the distribution of luminous matter.

The First Jets in the Universe: Protostellar Jets from the First Stars

The protostellar jets driven by the formation of the first stars are studied by using three-dimensional MHD nested grid simulations. Starting from a slowly rotating spherical cloud of 5.1 × 104 M☉ permeated by a uniform magnetic field, we follow the evolution from the central number density nc = 103 cm-3 to nc 1023 cm-3. Protostars of 10-3 M☉ are formed at nc 1022 cm-3, and the magnetic flux density is amplified by 10 orders of magnitude from the initial value. Consequently, the formed protostar has a magnetic field of ~106 G, which is much larger than the flux density of the present-day counterparts, reflecting the fact that the dissipation of a magnetic field is ineffective in primordial gas clouds. If the initial magnetic field B > 10-9(nc/103 cm-3)2/3 G, a protostellar jet is launched whose velocity reaches ~70 km s-1. As a result, a fraction (3%-10%) of the accreting matter is blown off from the central region. If this jet continues to sweep out the surrounding gas that otherwise accretes onto the central star or circumstellar disk, the final mass of the first star can be substantially reduced. In addition, dense postshock regions behind the bow shocks are expected to promote chemical reactions, and this provides possible environments for subsequent low-mass star formation in the early universe.

The Polytropic Equation of State of Primordial Gas Clouds

The polytropic equation of state (EOS) of primordial gas clouds with modest enrichment is computed, motivated by the recent observations of very Fe-deficient stars, [Fe/H] ~ 10-3.5 to 10-5, such as HE 0107-5240 and CS 29498-043. These stars are overabundant, relative to Fe, in C and O. We assume that the observed abundances of species like C, O, Si, and Fe are representative of the gas from which the currently observed metal-deficient stars formed. Under this assumption, we find that this primordial metal abundance pattern has profound consequences for the thermal balance and chemical composition of the gas and hence for the EOS of the parental cloud. The polytropic EOS is soft for low, [O/H] 10-2 because of the large opacity in the CO and H2O cooling lines. It is further found that a regulating role is played by the presence and temperature of the dust, even when the overall carbon abundance is only [C/H] ~ 10-2. When the dust is warmer than the gas, a region with γ ~ 1.2 results around a density of ~104 cm-3. When the dust is colder than the gas, a region with γ ~ 0.8 is found for a density of ~106 cm-3. Implications for the primordial initial mass function (IMF) as well as the IMF in starburst galaxies, for which the metallicity is supersolar, are explored and related to processes that influence the temperature of the ambient dust.

Dissipation of Magnetic Flux in Primordial Gas Clouds

We report the strength of seed magnetic flux of accretion disk surrounding the PopIII stars. The magnetic field in accretion disk might play an important role in the transport of angular momentum because of the turbulence induced by Magneto-Rotational Instability (MRI). On the other hand, since the primordial star-forming clouds contain no heavy elements and grains, they experience much different thermal history and different dissipation history of the magnetic field in the course of their gravitational contraction, from those in the present-day star-forming molecular clouds. In order to assess the magnetic field strength in the accretion disk of PopIII stars, we calculate the thermal history of the primordial collapsing clouds, and investigate the coupling of magnetic field with primordial gas. As a result, we find that the magnetic field strongly couple with primordial gas cloud throughout the collapse, i.e. the magnetic field are frozen to the gas as far as initial field strength satisfies $ B\lesssim 10^{-5}(n_{\rm H}/10^3{\rm cm^{-3}})^{0.55}{\rm G}$.

Rate Coefficient for H + + H 2 ( X [FORMULA][F][SUP]1[/SUP][SUP]+[/SUP][INF]g[/INF][/F][/FORMULA], = 0, J = 0) H(1 s ) + [FORMULA][F][RM]H[/RM][SUP]+[/SUP][INF]2[/INF][/F][/FORMULA] Charge Transfer and Some Cosmological Implications

Krstic has carried out the first quantum mechanical calculations near threshold for the charge transfer (CT) process H^+ + H_2(X ^1Sigma_g^+, nu=0, J=0) --> H(1s) + H_2^+. These results are relevant for models of primordial galaxy and first star formation that require reliable atomic and molecular data for obtaining the early universe hydrogen chemistry. Using the results of Krstic, we calculate the relevant CT rate coefficient for temperatures between 100 and 30,000 K. We also present a simple fit which can be readily implemented into early universe chemical models. Additionally, we explore how the range of previously published data for this reaction translates into uncertainties in the predicted gas temperature and H_2 relative abundance in a collapsing primordial gas cloud. Our new data significantly reduce these cosmological uncertainties that are due to the uncertainties in the previously published CT rate coefficients.

Radiative Shock–induced Collapse of Intergalactic Clouds

Accumulating observational evidence for a number of radio galaxies suggests an association between their jets and regions of active star formation. The standard picture is that shocks generated by the jet propagate through an inhomogeneous medium and trigger the collapse of overdense clouds, which then become active star-forming regions. In this contribution, we report on recent hydrodynamic simulations of radiative shock-cloud interactions using two different cooling models: an equilibrium cooling-curve model assuming solar metallicities and a nonequilibrium chemistry model appropriate for primordial gas clouds. We consider a range of initial cloud densities and shock speeds in order to quantify the role of cooling in the evolution. Our results indicate that for moderate cloud densities (≳1 cm-3) and shock Mach numbers (≲20), cooling processes can be highly efficient and result in more than 50% of the initial cloud mass cooling to below 100 K. We also use our results to estimate the final H2 mass fraction for the simulations that use the nonequilibrium chemistry package. This is an important measurement, since H2 is the dominant coolant for a primordial gas cloud. We find peak H2 mass fractions of ≳10-2 and total H2 mass fractions of ≳10-5 for the cloud gas, consistent with cosmological simulations of first star formation. Finally, we compare our results with the observations of jet-induced star formation in "Minkowski's Object," a small irregular starburst system associated with a radio jet in the nearby cluster of galaxies Abell 194. We conclude that its morphology, star formation rate (~0.3 M☉ yr-1) and stellar mass (~1.2 × 107 M☉) can be explained by the interaction of a ~9 × 104 km s-1 jet with an ensemble of moderately dense (~10 cm-3), warm (104 K) intergalactic clouds in the vicinity of its associated radio galaxy at the center of the galaxy cluster.

Simulations of Early Structure Formation: Primordial Gas Clouds

We use cosmological simulations to study the origin of primordial star-forming in a ΛCDM universe, by following the formation of dark matter halos and the cooling of within them. To model the physics of chemically pristine gas, we employ a nonequilibrium treatment of the chemistry of nine species (e−, H, H+, He, He+, He++, H2, H, H−) and include cooling by molecular hydrogen. By considering cosmological volumes, we are able to study the statistical properties of primordial halos, and the high resolution of our simulations enables us to examine these objects in detail. In particular, we explore the hierarchical growth of bound structures forming at redshifts z ≈ 25-30 with total masses in the range ≈105-106 M☉. We find that when the amount of molecular hydrogen in these objects reaches a critical level, cooling by rotational line emission is efficient, and dense clumps of cold form. We identify these gas clouds as sites for primordial star formation. In our simulations, the threshold for cloud formation by molecular cooling corresponds to a critical halo mass of ≈5 × 105 h-1 M☉, in agreement with earlier estimates, but with a weak dependence on redshift in the range z > 16. The complex interplay between the gravitational formation of dark halos and the thermodynamic and chemical evolution of the compromises analytic estimates of the critical H2 fraction. Dynamical heating from mass accretion and mergers opposes relatively inefficient cooling by molecular hydrogen, delaying the production of star-forming in rapidly growing halos. We also investigate the effect of photodissociating ultraviolet radiation on the formation of primordial clouds. We consider two extreme cases, first by including a uniform radiation field in the optically thin limit and second by accounting for the maximum effect of self-shielding in virialized regions. For radiation with Lyman-Werner band flux J > 10-23 ergs s-1 cm-2 Hz-1 sr-1, hydrogen molecules are rapidly dissociated, rendering cooling inefficient. In both the cases we consider, the overall effect can be described by computing an equilibrium H2 abundance for the radiation flux and defining an effective shielding factor. Based on our numerical results, we develop a semianalytic model of the formation of the first stars and demonstrate how it can be coupled with large N-body simulations to predict the star formation rate in the early universe.

Early Structure Formation and Reionization in a Warm Dark Matter Cosmology

We study first structure formation in Lambda-dominated universes using large cosmological N-body/SPH simulations. We consider a standard LCDM model and a LWDM model in which the mass of the dark matter particles is taken to be m_X=10 keV. The linear power spectrum for the LWDM model has a characteristic cut-off at a wavenumber k=200 /Mpc, suppressing the formation of low mass (< 10^6 Msun) nonlinear objects early on. The absence of low mass halos in the WDM model makes the formation of primordial gas clouds with molecular hydrogen very inefficient at high redshifts. The first star-forming gas clouds form at z~21 in the WDM model, considerably later than in the CDM counterpart, and the abundance of these gas clouds differs by an order of magnitude between the two models. We carry out radiative transfer calculations by embedding massive Population III stars in the gas clouds. We show that the volume fraction of ionized gas rises up close to 100% by z=18 in the CDM case, whereas that of the WDM model remains extremely small at a level of a few percent. Thus the WDM model with m_X=10 keV is strongly inconsistent with the observed high optical depth by the WMAP satellite.

Formation of atoms, molecules and the first objects in the universe with a variable cosmological term

We investigate the thermal history in the universe with a variable cosmological term. In particular, we show how the formation of important molecules such as H 2 and HD is influenced during the epoch for the redshift from 10000 to 1. These molecules should dominate the cooling process of primordial gas clouds which triggers the collapse of them. It is found that the molecular formation is quickened by Δz ∼ 1000 compared with the case in the standard model due to the variable cosmological term. The qualitative estimate of the mass of the first objects has been performed with use of the time scales related to the thermal evolution. It is suggested that the formation scenarios of the first objects studied so far could be changed drastically if the variable cosmological term is included in the thermal history in the universe.

Effects of a Burst of Formation of First‐Generation Stars on the Evolution of Galaxies

First-generation (Population III) stars in the universe play an important role inearly enrichment of heavy elements in galaxies and intergalactic medium and thus affect the history of galaxies. The physical and chemical properties of primordial gas clouds are significantly different from those of present-day gas clouds observed in the nearby universe because the primordial gas clouds do not contain any heavy elements which are important coolants in the gas. Previous theoretical considerations have suggested that typical masses of the first-generation stars are between several $M_\odot$ and $\approx 10 M_\odot$ although it has been argued that the formation of very massive stars (e.g., $> 100 M_\odot$) is also likely. If stars with several $M_\odot$ are most popular ones at the epoch of galaxy formation, most stars will evolve to hot (e.g., $\gtrsim 10^5$ K), luminous ($\sim 10^4 L_\odot$) stars with gaseous and dusty envelope prior to going to die as white dwarf stars. Although the duration of this phase is short (e.g., $\sim 10^5$ yr), such evolved stars could contribute both to the ionization of gas in galaxies and to the production of a lot of dust grains if the formation of intermediate-mass stars is highly enhanced. We compare gaseous emission-line properties of such nebulae with some interesting high-redshift galaxies such asIRAS F10214+4724 and powerful radio galaxies.

On the Fragment Mass Scale of Primordial Gas Clouds

We discuss the evolution and the fragmentation of cylindrical gas clouds with primordial composition, using the virial equation. Comparing the timescale of collapse with that of fragmentation, we obtain the fragment mass scale for the various initial density. We estimate the minimum mass of a fragment that is formed from the primordial gas cloud. The mass scale is essentially determined by the Chandrasekhar mass. The effect of dark matter is also investigated. If the filamentary cloud consists not only of ordinary gas but also some dark matter, the fragmentation of the cylindrical cloud occurs at the lower density than the pure gas case. As a result, the fragment mass scale hardly becomes Chandrasekhar mass scale. In this case, massive star formation is strongly expected.

The Role of Shock Heating in the Evolution of Massive Primordial Clouds

The role of waves in the evolution of massive primordial gas clouds is investigated. The increase of the ionization degree in the cloud due to the heating, and subsequent non-equilibrium cooling causes the formation of hydrogen molecules, which are the most important coolant in the metal-free gas In this chapter we review the effect of waves with various Mach numbers and pre-shock densities in detail and discuss how they affect the following evolution of the parent cloud. By comparison of various timcscales, we show that the chemical evolution and the thermal evolution of the primordial gas are quite simple, in spite of the fact that the system as a whole undergoes non-equilibrium evolution. This result, which is demonstrated in a simple diagram (the . shock diagram), greatly helps us to interpret and predict the thermal evolution of the primordial gas clouds. Finally, we consider the fragmentation process itself, and we investigate the possibility that the cooling instability can create much smaller cloudlets than can be formed through the Jeans instability.

H2Absorption Measurement of Primordial Clouds

The observability of hydrogen molecules in absorption was investigated for infrared lines against bright infrared sources. The absorption efficiency of the hydrogen molecules becomes comparable with or larger than that of the dust grains in the metal-poor condition expected in the early Universe. If we can use bright infrared continuum sources behind the molecular gas clouds, the absorption measurement of the hydrogen molecules will be an important technique to explore the primordial gas clouds that are contracting into first-generation objects.

Star Formation in the Primordial Gas: Overview

Recently, the study for the star formation process in the primordial gas progresses very much. We show the outline of the thermal and dynamical evolution of primordial gas clouds in the universe after decoupling. The thermal process of the primordial gas is much different from the thermal process of the present-day interstellar gas. Especially, hydrogen molecule is the most important coolant at the low temperature gas (∼ 10 3 K). Considering the formation of H 2 and the cooling by H 2 line emission, we can study the thermal evolution of the primordial gas and investigate the star formation process. We propose the following scenario to form primordial stars efficiently. First, by pancake collapse of the overdensity regions in the expanding universe. disk like clouds form A disk like cloud has a tendency to fragment into filamentary clouds. Filamentary clouds are formed and they collapse dynamically. After collapse is almost halted by pressure force, the filament fragments into cloud cores. Finally, a primordial star forms in a cloud core.

Absorption Measurement of Hydrogen Molecules in the Early Universe

Abstract We investigate the observability of hydrogen molecules in absorption. The absorption efficiency of hydrogen molecules becomes comparable with, or larger than, that of the dust grains in the metal-poor condition expected in the early Universe. If we can use bright infrared continuum sources behind the molecular-gas clouds, the absorption measurement of the hydrogen molecules could be an important technique to explore the primordial gas clouds that are contracting into first-generation objects.

AAS Acknowledges Achievements in Astronomy

The American Astronomical Society recently announced the winners of several of its awards for 2001. These awards recognize contributions to astronomy and astrophysics. The Henry Norris Russell Lectureship, the society’s highest honor, will be presented to Wallace L. W. Sargent, Ira S. Bowen Professor of Astronomy at Caltech. AAS praises Sargent for being an “intellectual leader and educator” throughout his career and recognizes his contributions to astronomical spectroscopy, including his work in stellar spectroscopy of A-type stars that “led to the discovery of the helium-3 isotope in star 3 Centauri A.” He involved many of his students in his work in extragalactic spectroscopy, “which produced the first evidence for a black hole in galaxy M87 and culminated with his studies of quasar absorption lines.” His demonstration that the Lymanalpha forest absorption arises from intergalactic primordial gas clouds “provided a fundamental new probe of primeval gas in the early universe.”The society is presenting a new award this year. The inaugural AAS Education Prize, given in recognition of outstanding contributions to the education of the public, of students, and the next generation of professional astronomers, will go to Frank D. Drake for his “inspiration and leadership in many areas of education and public outreach in astronomy.” Drake’s “wide-ranging popularizations, his tireless help for journalists, and his championship of education and public information through the SETI Institute have helped scientists, educators, and the world at large to think rationally about life in the cosmos.” The citation also noted that his course work for and mentorship of students, and his development of the science of SETI (the search for extraterrestrial intelligence) through Project Ozma, the Drake Equation, and Project Phoenix, have “brought the excitement of the cosmic quest to several generations.” Drake is a professor emeritus of astronomy and astrophysics at the University of California, Santa Cruz, and distinguished scientist and chairman of the board of trustees of the SETI Institute. Bruce Elmgreen, a research staff member at IBM’s T. J. Watson Research Center in Yorktown Heights, New York, will receive the Dannie Heineman Prize for Astrophysics, given jointly by AAS and the American Institute of Physics. Elmgreen is being recognized for contributions that “span a remarkable range from theoretical studies of key processes in the interstellar medium to the physics of galaxy-wide starbursts, to investigations of dynamical features, including spiral arms and bars in galaxies.” The Newton Lacy Pierce Prize will be given this year to Kenneth Sembach, a research scientist at Johns Hopkins University. Sembach is being acknowledged for work “that has been important in increasing our understanding of the structure and elemental abundances of the gaseous component of the Galaxy, especially of the galactic halo, as well as in discovering new facets of the high-velocity cloud phenomenon in the galactic periphery.” Uros Seljak will receive the Helen B. Warner Prize for Astronomy for his “contributions to the theoretical understanding of the cosmic microwave background anisotropies and to the development of numerical and analytical tools that have been widely adopted for the comparison of observational data and cosmological models in that area.” Seljak is an assistant professor of physics at Princeton University. Michael J. Kurtz, an astronomer and computer scientist at the Harvard–Smithsonian Center for Astrophysics in Cambridge, Massachusetts, will be awarded the George Van Biesbroeck Prize. AAS acknowledges Kurtz’s contributions as “the visionary designer of the Astrophysics Data System, which clearly has revolutionized for over a decade the speed and thoroughness in which astronomers now can search and access the vast and still growing technical literature.”The Bruno Rossi Prize, given by AAS’s high-energy astrophysics division, will be shared by Andrew Fabian and Yasuo Tanaka for “their discovery, with the ASCA satellite [Advanced Satellite for Cosmology and Astrophysics], of broad iron K lines in active galactic nuclei, which demonstrate the effects of the strong gravitational field characteristic of black holes.” Fabian is a Royal Society Research Professor with the Institute of Astronomy at the University of Cambridge, UK. Tanaka is a professor emeritus of astronomy and astrophysics at the Institute of Space and Astronautical Science in Sagamihara, Japan. Sargent PPT|High resolution© 2001 American Institute of Physics.

On the Initial Mass Function of Population III Stars

The collapse and fragmentation of filamentary primordial gas clouds are explored using one- and two-dimensional hydrodynamical simulations coupled with the nonequilibrium processes of hydrogen molecule formation. The cloud evolution is computed from the initial central density nc = 10-106 cm-3. The simulations show that depending on the initial density, there are two occasions for the fragmentation of primordial filaments. If a filament has relatively low initial density such as nc 105 cm-3, the radial contraction is slow as a result of less effective H2 cooling and appreciably decelerates at densities higher than a critical density, where LTE populations are achieved for the rotational levels of H2 molecules and the cooling timescale becomes accordingly longer than the free-fall timescale. This filament tends to fragment into dense clumps before the central density reaches 108-109 cm-3, where H2 cooling by three-body reactions is effective and the fragment mass is more massive than some tens of M?. In contrast, if a filament is initially as dense as nc 105 cm-3, the more effective H2 cooling with the help of three-body reactions allows the filament to contract up to n ~ 1012 cm-3. After the density reaches n ~ 1012 cm-3, the filament becomes optically thick to H2 lines and the radial contraction subsequently almost stops. At this final hydrostatic stage, the fragment mass is lowered down to ?1 M? because of the high density of the filament. The dependence of the fragment mass upon the initial density could be translated into the dependence on the local amplitude of random Gaussian density fields or the epoch of the collapse of a parent cloud. Hence, it is predicted that the initial mass function of Population III stars is likely to be bimodal with peaks of ?102 and ?1 M?, where the relative heights could be a function of the collapse epoch. Implications for the metal enrichment by Population III stars at high redshifts and baryonic dark matter are briefly discussed.

Primordial Star Formation under Far‐Ultraviolet Radiation

Thermal and chemical evolution of primordial gas clouds irradiated with far-ultraviolet (FUV; hν < 13.6 eV) radiation is investigated. In clouds irradiated by intense FUV radiation, sufficient hydrogen molecules to be important for cooling are never formed. However, even without molecular hydrogen, if the clouds are massive enough, they start collapsing via atomic hydrogen line cooling. Such clouds continue to collapse almost isothermally owing to successive cooling by H- free-bound emission up to the number density of 1016 cm-3. Inside the clouds, the Jeans mass eventually falls well below a solar mass. This indicates that hydrogen molecules are dispensable for low-mass primordial star formation, provided that fragmentation of the clouds occurs at sufficiently high density.

Collapse of low-mass clouds in the presence of a UV radiation field

The collapse of marginally Jeans-unstable primordial gas clouds in the presence of a UV radiation field is discussed. Assuming that the dynamical collapse proceeds approximately in an isothermal self-similar fashion, we investigate the thermal evolution of the collapsing central core until H2 cooling dominates photoheating and the temperature drops to below 104 K. Consequently, the mass of the cooled core is evaluated as Mcool=3.6×106 M⊙ (I21/1)−0.32. This scale depends only on the incident UV intensity, and provides a lower limit to the mass of collapsed objects in the UV radiation field.