974 research outputs found
Star Formation in Extreme Environments: The Effects of Cosmic Rays and Mechanical Heating
Context: Molecular data of extreme environments, such as Arp 220, but also
NGC 253, show evidence for extremely high cosmic ray (CR) rates (10^3-10^4 *
Milky Way) and mechanical heating from supernova driven turbulence.
Aims: The consequences of high CR rates and mechanical heating on the
chemistry in clouds are explored.
Methods: PDR model predictions are made for low, n=10^3, and high, n=10^5.5
cm^-3, density clouds using well-tested chemistry and radiation transfer codes.
Column densities of relevant species are discussed, and special attention is
given to water related species. Fluxes are shown for fine-structure lines of O,
C+, C, and N+, and molecular lines of CO, HCN, HNC, and HCO+. A comparison is
made to an X-ray dominated region model.
Results: Fine-structure lines of [CII], [CI], and [OI] are remarkably similar
for different mechanical heating and CR rates, when already exposed to large
amounts of UV. HCN and H2O abundances are boosted for very high mechanical
heating rates, while ionized species are relatively unaffected. OH+ and H2O+
are enhanced for very high CR rates zeta > 5 * 10^-14 s^-1. A combination of
OH+, OH, H2O+, H2O, and H3O+ trace the CR rates, and are able to distinguish
between enhanced cosmic rays and X-rays.Comment: 13 pages, 8 figures, A&A accepte
Extragalactic H3O+: Some Consequences
We discuss some implications of our recent detection of extragalactic H3O+:
the location of the gas in M82, the origin of energetic radiation in M82, and
the possible feedback effects of star formation on the cosmic ray flux in
galaxies.Comment: Five pages, one figure; contribution to proceedings of conference
"Far-infrared observations of the interstellar medium", December 2007, Bad
Honne
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&
Diagnostics of the Molecular Component of PDRs with Mechanical Heating
Context. Multitransition CO observations of galaxy centers have revealed that
significant fractions of the dense circumnuclear gas have high kinetic
temperatures, which are hard to explain by pure photon excitation, but may be
caused by dissipation of turbulent energy.
Aims. We aim to determine to what extent mechanical heating should be taken
into account while modelling PDRs. To this end, the effect of dissipated
turbulence on the thermal and chemical properties of PDRs is explored. Methods.
Clouds are modelled as 1D semi-infinite slabs whose thermal and chemical
equilibrium is solved for using the Leiden PDR-XDR code.
Results. In a steady-state treatment, mechanical heating seems to play an
important role in determining the kinetic temperature of the gas in molecular
clouds. Particularly in high-energy environments such as starburst galaxies and
galaxy centers, model gas temperatures are underestimated by at least a factor
of two if mechanical heating is ignored. The models also show that CO, HCN and
H2 O column densities increase as a function of mechanical heating. The HNC/HCN
integrated column density ratio shows a decrease by a factor of at least two in
high density regions with n \sim 105 cm-3, whereas that of HCN/HCO+ shows a
strong dependence on mechanical heating for this same density range, with
boosts of up to three orders of magnitude.
Conclusions. The effects of mechanical heating cannot be ignored in studies
of the molecular gas excitation whenever the ratio of the star formation rate
to the gas density is close to, or exceeds, 7 \times 10-6 M yr-1 cm4.5 . If
mechanical heating is not included, predicted column densities are
underestimated, sometimes even by a few orders of magnitude. As a lower bound
to its importance, we determined that it has non-negligible effects already
when mechanical heating is as little as 1% of the UV heating in a PDR.Comment: 26 pages, 14 figures in the text and 13 figures as supplementary
material. Accepted for publication in A&
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
Detection of extragalactic H3O+
The H3O+ molecule probes the oxygen chemistry and the ionization rate of
dense circumnuclear gas in galaxies. In particular, recent H3O+ observations
show variations in the cosmic-ray ionization rate by factors of 10 within
our Galaxy. Using the JCMT, we have observed the 364 GHz line of p-H3O+ in the
centers of M82 and Arp 220. In Arp 220, the line profile suggests that the
emission originates in the Western nucleus. In M82, both the eastern molecular
peak and the circumnuclear region contribute to the emission. The derived
column densities, abundances, and H3O+ / H2O ratios indicate ionization rates
similar to or even exceeding that in the Galactic Center. Model calculations of
the chemistry of irradiated molecular gas indicate a likely origin of this high
ionization rate in the extended, evolved starburst of M82. In contrast,
irradiation by X-rays from the AGN disk is the most likely model for Arp 220.Comment: Accepted by A&A Letters; 4-5 pages depending on paper format; two b/w
figure
Mechanical feedback in the molecular ISM of luminous IR galaxies
Aims: Molecular emission lines originating in the nuclei of luminous
infra-red galaxies are used to determine the physical properties of the nuclear
ISM in these systems.
Methods: A large observational database of molecular emission lines is
compared with model predictions that include heating by UV and X-ray radiation,
mechanical heating, and the effects of cosmic rays.
Results: The observed line ratios and model predictions imply a separation of
the observedsystems into three groups: XDRs, UV-dominated high-density (n>=10^5
cm-3) PDRs, and lower-density (n=10^4.5 cm-3) PDRs that are dominated by
mechanical feedback.
Conclusions: The division of the two types of PDRs follows naturally from the
evolution of the star formation cycle of these sources, which evolves from
deeply embedded young stars, resulting in high-density (n>=10^5 cm-3) PDRs, to
a stage where the gas density has decreased (n=10^4.5 cm-3) and mechanical
feedback from supernova shocks dominates the heating budget.Comment: 4 pages, 3 figures, published as Letter to the Editor in A&A (see
http://www.aanda.org/articles/aa/abs/2008/34/aa10327-08/aa10327-08.html
Irradiated ISM: Discriminating between Cosmic Rays and X-rays
The ISM of active galaxy centers is exposed to a combination of cosmic ray,
FUV and X-ray radiation. We apply PDR models to this ISM with both `normal' and
highly elevated (5\times 10^{-15}s^-1) cosmic-ray rates and compare the results
to those obtained for XDRs. Our existing PDR-XDR code is used to construct
models over a 10^3-10^5 cm^-3 density range and for 0.16-160 erg s^-1 cm^-2
impingent fluxes. We obtain larger high J (J>10) CO ratios in PDRs when we use
the highly elevated cosmic ray rate, but these are always exceeded by the
corresponding XDR ratios. The [CI] 609 mum/13CO(2-1) line ratio is boosted by a
factor of a few in PDRs with n~10^3 cm^-3 exposed to a high cosmic ray rate. At
higher densities ratios become identical irrespective of cosmic ray flux, while
XDRs always show elevated [CI] emission per CO column. The HCN/CO and HCN/HCO+
line ratios, combined with high J CO emission lines, are good diagnostics to
distinguish between PDRs under either low or high cosmic ray irradiation
conditions, and XDRs. Hence, the HIFI instrument on Herschel, which can detect
these CO lines, will be crucial in the study of active galaxies.Comment: accepted by Astrophysical Journal Letter
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