755 research outputs found
Gravoturbulent Star Formation: Effects of the Equation of State on Stellar Masses
Stars form by gravoturbulent fragmentation of interstellar gas clouds. The
supersonic turbulence ubiquitously observed in Galactic molecular gas generates
strong density fluctuations with gravity taking over in the densest and most
massive regions. Collapse sets in to build up stars and star clusters.
Turbulence plays a dual role. On global scales it provides support, while at
the same time it can promote local collapse. Stellar birth is thus intimately
linked to the dynamic behavior of parental gas clouds, which governs when and
where protostellar cores form, and how they contract and grow in mass via
accretion from the surrounding cloud material to build up stars. The equation
of state plays a pivotal role in the fragmentation process. Under typical cloud
conditions, massive stars form as part of dense clusters following the "normal"
mass function observed, e.g. in the solar neighborhood. However, for gas with
an effective polytropic index greater than unity star formation becomes biased
towards isolated massive stars. This is relevant for understanding the
properties of zero-metallicity stars (Population III) or stars that form under
extreme environmental conditions like in the Galactic center or in luminous
starbursts.Comment: 9 pages, 4 figure, to be published in the Proceedings of the IAU
Colloquium No. 227, 2005, "Massive Star Birth: A Crossroads of Astrophysics
Evolution of the ISM in Luminous IR Galaxies
Molecules that trace the high-density regions of the interstellar medium may
be used to evaluate the changing physical and chemical environment during the
ongoing nuclear activity in (Ultra-)Luminous Infrared Galaxies. The changing
ratios of the HCN(1-0), HNC(1-0), HCO+(1-0), CN(1-0) and CN(2-1), and CS(3-2)
transitions were compared with the HCN(1-0)/CO(1-0) ratio, which is proposed to
represent the progression time scale of the starburst. These diagnostic
diagrams were interpreted using the results of theoretical modeling using a
large physical and chemical network to describe the state of the nuclear ISM in
the evolving galaxies. Systematic changes are seen in the line ratios as the
sources evolve from early stage for the nuclear starburst (ULIRGs) to later
stages. These changes result from changing environmental conditions and
particularly from the lowering of the average density of the medium. A
temperature rise due to mechanical heating of the medium by feedback explains
the lowering of the ratios at later evolutionary stages. Infrared pumping may
affect the CN and HNC line ratios during early evolutionary stages. Molecular
transitions display a behavior that relates to changes of the environment
during an evolving nuclear starburst. Molecular properties may be used to
designate the evolutionary stage of the nuclear starburst. The HCN(1-0)/CO(1-0)
and HCO+(1-0)/HCN(1-0) ratios serve as indicators of the time evolution of the
outburst.Comment: To be published in Astronomy and Astrophysics - 11 pages, 9 figures,
1 tabl
Molecular Hydrogen in Diffuse Interstellar Clouds of Arbitrary Three-Dimensional Geometry
We have constructed three-dimensional models for the equilibrium abundance of
molecular hydrogen within diffuse interstellar clouds of arbitrary geometry
that are illuminated by ultraviolet radiation. The position-dependent photo-
dissociation rate of H within such clouds was computed using a 26-ray
approximation to model the attenuation of the incident ultraviolet radiation
field by dust and by H line absorption. We have applied our modeling
technique to the isolated diffuse cloud G236+39, assuming that the cloud has a
constant density and that the thickness of the cloud along the line of sight is
at every point proportional to the 100 um continuum intensity measured by IRAS.
We find that our model can successfully account for observed variations in the
ratio of 100 umu continuum intensity to HI column density, with larger values
of that ratio occurring along lines of sight in which the molecular hydrogen
fraction is expected to be largest. Using a standard chi^2 analysis to assess
the goodness of fit of our models, we find (at the 60sigma level) that a
three-dimensional model is more successful in matching the observational data
than a one-dimensional model in which the geometrical extent of the cloud along
the line of sight is assumed to be very much smaller than its extent in the
plane-of-the-sky. If D is the distance to G236+39, and given standard
assumptions about the rate of grain-catalysed H_2 formation, we find that the
cloud has an extent along the line of sight that is 0.9+-0.1 times its mean
extent projected onto the plane of the sky; a gas density of (53+-8)(100 pc/D)
cm^-3; and is illuminated by a radiation field of (1.1+-0.2) (100 pc/D) times
the mean interstellar radiation field estimated by Draine (1978). The derived
100 um emissivity per nucleon is (1.13+-0.06)x10^-20 MJy sr^-1 cm^2.Comment: 27 pages LaTex, uses aaspp4.sty, ApJ August
The complexity that the first stars brought to the Universe: Fragility of metal enriched gas in a radiation field
The initial mass function (IMF) of the first (Population III) stars and
Population II (Pop II) stars is poorly known due to a lack of observations of
the period between recombination and reionization. In simulations of the
formation of the first stars, it has been shown that, due to the limited
ability of metal-free primordial gas to cool, the IMF of the first stars is a
few orders of magnitude more massive than the current IMF. The transition from
a high-mass IMF of the first stars to a lower-mass current IMF is thus
important to understand. To study the underlying physics of this transition, we
performed several simulations using the cosmological hydrodynamic adaptive mesh
refinement code Enzo for metallicities of 10^{-4}, 10^{-3}, 10^{-2}, and
10^{-1} Z_{\odot}. In our simulations we include a star formation prescription
that is derived from a metallicity dependent multi-phase ISM structure, an
external UV radiation field, and a mechanical feedback algorithm. We also
implement cosmic ray heating, photoelectric heating and gas-dust
heating/cooling, and follow the metal enrichment of the ISM. It is found that
the interplay between metallicity and UV radiation leads to the co-existence of
Pop III and Pop II star formation in non-zero metallicity (Z/Z_{\odot}
\geq10^{-2}) gas. A cold (T10^{-22} g cm^{-3}) gas
phase is fragile to ambient UV radiation. In a metal-poor (Z/Z_{\odot}
\leq10^{-3}) gas, the cold and dense gas phase does not form in the presence of
a radiation field of F_{0}\sim10^{-5}-10^{-4} erg cm^{-2} s^{-1}. Therefore,
metallicity by itself is not a good indicator of the Pop III-Pop II transition.
Metal-rich (Z/Z_{\odot}\geq10^{-2}) gas dynamically evolves two to three orders
of magnitude faster than metal poor gas (Z/Z_{\odot}\leq10^{-3}). The
simulations including SNe show that pre-enrichment of the halo does not affect
the mixing of metals.Comment: Published in Ap
Molecular properties of (U)LIRGs: CO, HCN, HNC and HCO+
The observed molecular properties of a sample of FIR-luminous and OH
megamaser (OH-MM) galaxies have been investigated. The ratio of high and
low-density tracer lines is found to be determined by the progression of the
star formation in the system. The HCO+/HCN and HCO+/HNC line ratios are good
proxies for the density of the gas, and PDR and XDR sources can be
distinguished using the HNC/HCN line ratio. The properties of the OH-MM sources
in the sample can be explained by PDR chemistry in gas with densities higher
than 10^5.5 cm^-3, confirming the classical OH-MM model of IR pumped
amplification with (variable) low gains.Comment: 5 pages, 2 figures, to appear in: IAU Symposium 242 Astrophysical
Masers and their Environment
Lyman Alpha Radiation From Collapsing Protogalaxies II: Observational Evidence for Gas Infall
We model the spectra and surface brightness distributions for the Lyman alpha
(Lya) radiation expected from protogalaxies that are caught in the early stages
of their assembly. We use the results of a companion paper to characterize the
radiation emerging from spherically collapsing gas clouds. We then modify the
intrinsic spectra to incorporate the effect of subsequent resonant scattering
in the intergalactic medium (IGM). Using these models, we interpret a number of
recent observations of extended Lya blobs (LABs) at high redshift. We suggest,
based on the angular size, energetics, as well as the relatively shallow
surface brightness profiles, and double-peaked spectra, that several of these
LABs may be associated with collapsing protogalaxies. We suggest two follow-up
observations to diagnose the presence of gas infall. High S/N spectra of LABs
should reveal a preferential flattening of the surface brightness profile at
the red side of the line. Complementary imaging of the blobs at redshifted
Balmer alpha wavelengths should reveal the intrinsic Lya emissivity and allow
its separation from radiative transfer effects. We show that Lya scattering by
infalling gas can reproduce the observed spectrum of Steidel et al's LAB2 as
accurately as a recently proposed outflow model. Finally, we find similar
evidence for infall in the spectra of point-like Lyman alpha emitters. The
presence of scattering by the infalling gas implies that the intrinsic Lya
luminosities, and derived quantities, such as the star-formation rate, in these
objects may have been underestimated by about an order of magnitude.Comment: Accepted for Publication in ApJ, 11 emulateapj pages with 6 figures,
together with a companion pape
The formation of massive primordial stars in the presence of moderate UV backgrounds
Radiative feedback from populations II stars played a vital role in early
structure formation. Particularly, photons below the Lyman limit can escape the
star forming regions and produce a background ultraviolet (UV) flux which
consequently may influence the pristine halos far away from the radiation
sources. These photons can quench the formation of molecular hydrogen by
photo-detachment of . In this study, we explore the impact of such
UV radiation on fragmentation in massive primordial halos of a few times ~M. To accomplish this goal, we perform high resolution
cosmological simulations for two distinct halos and vary the strength of the
impinging background UV field in units of . We further make use of
sink particles to follow the evolution for 10,000 years after reaching the
maximum refinement level. No vigorous fragmentation is observed in UV
illuminated halos while the accretion rate changes according to the thermal
properties. Our findings show that a few 100-10, 000 solar mass protostars are
formed when halos are irradiated by at and
suggest a strong relation between the strength of UV flux and mass of a
protostar. This mode of star formation is quite different from minihalos, as
higher accretion rates of about M/yr are observed by
the end of our simulations. The resulting massive stars are the potential
cradles for the formation of intermediate mass black holes at earlier cosmic
times and contribute to the formation of a global X-ray background.Comment: Submitted to APJ, comments are welcome. High resolution copy is
available at http://www.astro.physik.uni-goettingen.de/~mlatif/IMBHs_apj.pd
How realistic UV spectra and X-rays suppress the abundance of direct collapse black holes
Observations of high redshift quasars at indicate that they harbor
supermassive black holes (SMBHs) of a billion solar masses. The direct collapse
scenario has emerged as the most plausible way to assemble SMBHs. The nurseries
for the direct collapse black holes are massive primordial halos illuminated
with an intense UV flux emitted by population II (Pop II) stars. In this study,
we compute the critical value of such a flux () for
realistic spectra of Pop II stars through three-dimensional cosmological
simulations. We derive the dependence of on the radiation
spectra, on variations from halo to halo, and on the impact of X-ray
ionization. Our findings show that the value of is a few
times and only weakly depends on the adopted radiation spectra in
the range between K. For three simulated halos
of a few times ~M, varies from . The impact of X-ray ionization is almost
negligible and within the expected scatter of for
background fluxes of . The computed estimates of
have profound implications for the quasar abundance at
as it lowers the number density of black holes forming through an
isothermal direct collapse by a few orders of magnitude below the observed
black holes density. However, the sites with moderate amounts of
cooling may still form massive objects sufficient to be compatible with
observations.Comment: Accepted for publication in MNRAS, comments are welcom
L-band ATS 5/Orion/S. S. Manhattan marine navigation and communication experiment Final report
L-band signals relayed by synchronous satellite for navigation and data communicatio
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