634 research outputs found
The Polytropic Equation of State of Interstellar Gas Clouds
Models are presented for the polytropic equation of state of
self-gravitating, quiescent interstellar gas clouds. A detailed analysis,
including chemistry, thermal balance, and radiative transfer, is performed for
the physical state of the gas as a function of density, metallicity, velocity
field, and background radiation field. It is found that the stiffness of the
equation of state strongly depends on all these physical parameters, and the
adiabatic index varies between 0.2-1.4. The implications for star formation, in
particular at high redshift and in starburst galaxies, and the initial stellar
mass function are discussed.Comment: Accepted by Ap
Molecular Lines as Diagnostics of High Redshift Objects
Models are presented for CO rotational line emission by high redshift
starburst galaxies. The influence of the cosmic microwave background on the
thermal balance and the level populations of atomic and molecular species is
explicitly included. Predictions are made for the observability of starburst
galaxies through line and continuum emission between z=5 and z=30. It is found
that the Millimeter Array could detect a starburst galaxy with ~10^5 Orion
regions, corresponding to a star formation rate of about 30 Mo yr^{-1}, equally
well at z=5 or z=30 due to the increasing cosmic microwave background
temperature with redshift. Line emission is a potentially more powerful probe
than dust continuum emission of very high redshift objects.Comment: 15 pages LaTex, uses aasms4.sty, Accepted by ApJ
CO+ in M 82: A Consequence of Irradiation by X-rays
Based on its strong CO+ emission it is argued that the M 82 star-burst galaxy
is exposed to a combination of FUV and X-ray radiation. The latter is likely to
be the result of the star-burst superwind, which leads to diffuse thermal
emission at ~0.7 keV, and a compact hard, 2-10 keV, source (but not an AGN).
Although a photon-dominated region (FUV) component is clearly present in the
nucleus of M 82, and capable of forming CO+, only X-ray irradiated gas of
density 10^3-10^5 cm^-3 can reproduce the large, ~(1-4)x10^13 cm^-2, columns of
CO+ that are observed toward the proto-typical star-burst M 82. The total X-ray
luminosity produced by M 82 is weak, ~10^41 erg s^-1, but this is sufficient to
drive the formation of CO+.Comment: added discussion on more recent X-ray observation
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
Probing high-redshift quasars with ALMA. I. Expected observables and potential number of sources
(abridged) We explore how ALMA observations can probe high-redshift galaxies
in unprecedented detail. We discuss the main observables that are excited by
the large-scale starburst, and formulate expectations for the chemistry and the
fluxes in the center of active galaxies, where chemistry may be driven by the
absorption of X-ray photons. We show that such X-ray dominated regions (XDRs)
should be large enough to be resolved with ALMA, and predict the expected
amount of emission in CO and various fine-structure lines. We discuss how such
XDRs can be distinguished from a strong starburst on the same spatial scales
based on the CO line SED. Our models are compared to known sources like NGC
1068 and APM 08279. We also analyze the properties of the z=6.42 quasar SDSS
J114816.64+525150.3, and find that the observed emission in CO, [CII] and [CI]
requires a dense warm and a low-density cold gas component. We estimate the
expected number of sources at redshifts higher than 6, finding that one could
expect one black hole with solar masses per arcmin.Comment: 15 pages, 17 figures, accepted by A&
Far-Infrared and Sub-Millimeter Observations and Physical Models of the Reflection Nebula Ced 201
ISO [C II] 158 micron, [O I] 63 micron, and H_2 9 and 17 micron observations
are presented of the reflection nebula Ced 201, which is a photon-dominated
region illuminated by a B9.5 star with a color temperature of 10,000 K (a cool
PDR). In combination with ground based [C I] 609 micron, CO, 13CO, CS and HCO+
data, the carbon budget and physical structure of the reflection nebula are
constrained. The obtained data set is the first one to contain all important
cooling lines of a cool PDR, and allows a comparison to be made with classical
PDRs. To this effect one- and three-dimensional PDR models are presented which
incorporate the physical characteristics of the source, and are aimed at
understanding the dominant heating processes of the cloud. The contribution of
very small grains to the photo-electric heating rate is estimated from these
models and used to constrain the total abundance of PAHs and small grains.
Observations of the pure rotational H_2 lines with ISO, in particular the S(3)
line, indicate the presence of a small amount of very warm, approximately 330
K, molecular gas. This gas cannot be accommodated by the presented models.Comment: 32 pages, 7 figures, in LaTeX. To be published in Ap
Diagnostics of the molecular component of PDRs with mechanical heating. II: line intensities and ratios
CO observations in active galactic nuclei and star-bursts reveal high kinetic
temperatures. Those environments are thought to be very turbulent due to
dynamic phenomena such as outflows and high supernova rates. We investigate the
effect of mechanical heating (MH) on atomic fine-structure and molecular lines,
and their ratios. We use those ratios as a diagnostic to constrain the amount
of MH in an object and also study its significance on estimating the H2 mass.
Equilibrium PDRs models were used to compute the thermal and chemical balance
for the clouds. The equilibria were solved for numerically using the optimized
version of the Leiden PDR-XDR code. Large velocity gradient calculations were
done as post-processing on the output of the PDR models using RADEX. High-J CO
line ratios are very sensitive to MH. Emission becomes at least one order of
magnitude brighter in clouds with n~10^5~cm^-3 and a star formation rate of 1
Solar Mass per year (corresponding to a MH rate of 2 * 10^-19 erg cm^-3 s^-1).
Emission of low-J CO lines is not as sensitive to MH, but they do become
brighter in response to MH. Generally, for all of the lines we considered, MH
increases excitation temperatures and decreases the optical depth at the line
centre. Hence line ratios are also affected, strongly in some cases. Ratios
involving HCN are a good diagnostic for MH, such as HCN(1-0)/CO(1-0) and
HCN(1-0)/HCO^+(1-0). Both ratios increase by a factor 3 or more for a MH
equivalent to > 5 percent of the surface heating, as opposed to pure PDRs. The
first major conclusion is that low-J to high-J intensity ratios will yield a
good estimate of the MH rate (as opposed to only low-J ratios). The second one
is that the MH rate should be taken into account when determining A_V or
equivalently N_H, and consequently the cloud mass. Ignoring MH will also lead
to large errors in density and radiation field estimates.Comment: 38 pages, to appear in A&
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
FUV and X-ray irradiated protoplanetary disks: a grid of models I. The disk structure
Context. Planets are thought to eventually form from the mostly gaseous (~99%
of the mass) disks around young stars. The density structure and chemical
composition of protoplanetary disks are affected by the incident radiation
field at optical, FUV, and X-ray wavelengths, as well as by the dust
properties.
Aims. The effect of FUV and X-rays on the disk structure and the gas chemical
composition are investigated. This work forms the basis of a second paper,
which discusses the impact on diagnostic lines of, e.g., C+, O, H2O, and Ne+
observed with facilities such as Spitzer and Herschel.
Methods. A grid of 240 models is computed in which the X-ray and FUV
luminosity, minimum grain size, dust size distribution, and surface density
distribution are varied in a systematic way. The hydrostatic structure and the
thermo-chemical structure are calculated using ProDiMo.
Results. The abundance structure of neutral oxygen is stable to changes in
the X-ray and FUV luminosity, and the emission lines will thus be useful
tracers of the disk mass and temperature. The C+ abundance distribution is
sensitive to both X-rays and FUV. The radial column density profile shows two
peaks, one at the inner rim and a second one at a radius r=5-10 AU. Ne+ and
other heavy elements have a very strong response to X-rays, and the column
density in the inner disk increases by two orders of magnitude from the lowest
(LX = 1e29 erg/s) to the highest considered X-ray flux (LX = 1e32 erg/s). FUV
confines the Ne+ ionized region to areas closer to the star at low X-ray
luminosities (LX = 1e29 erg/s). H2O abundances are enhanced by X-rays due to
higher temperatures in the inner disk and higher ionization fractions in the
outer disk. The line fluxes and profiles are affected by the effects on these
species, thus providing diagnostic value in the study of FUV and X-ray
irradiated disks around T Tauri stars. (abridged)Comment: 47 pages, accepted by Astronomy and Astrophysics, a high resolution
version of the paper is located at
http://www.astro.rug.nl/~meijerink/disk_paperI_xrays.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
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