20 research outputs found
Detection of interstellar H_2D^+ emission
We report the detection of the 1_{10}-1_{11} ground state transition of
ortho-H_2D^+ at 372.421 GHz in emission from the young stellar object NGC 1333
IRAS 4A. Detailed excitation models with a power-law temperature and density
structure yield a beam-averaged H_2D^+ abundance of 3 x 10^{-12} with an
uncertainty of a factor of two. The line was not detected toward W 33A, GL
2591, and NGC 2264 IRS, in the latter source at a level which is 3-8 times
lower than previous observations. The H_2D^+ data provide direct evidence in
support of low-temperature chemical models in which H_2D^+ is enhanced by the
reaction of H_3^+ and HD. The H_2D^+ enhancement toward NGC 1333 IRAS 4A is
also reflected in the high DCO^+/HCO^+ abundance ratio. Simultaneous
observations of the N_2H^+ 4-3 line show that its abundance is about 50-100
times lower in NGC 1333 IRAS 4A than in the other sources, suggesting
significant depletion of N_2. The N_2H^+ data provide independent lower limits
on the H_3^+ abundance which are consistent with the abundances derived from
H_2D^+. The corresponding limits on the H_3^+$ column density agree with recent
near-infrared absorption measurements of H_3^+ toward W 33A and GL 2591.Comment: Standard AAS LaTeX format (15 pages + 2 figures
The Ionization Fraction in Dense Molecular Gas II: Massive Cores
We present an observational and theoretical study of the ionization fraction
in several massive cores located in regions that are currently forming stellar
clusters. Maps of the emission from the J = 1-> O transitions of C18O, DCO+,
N2H+, and H13CO+, as well as the J = 2 -> 1 and J = 3 -> 2 transitions of CS,
were obtained for each core. Core densities are determined via a large velocity
gradient analysis with values typically 10^5 cm^-3. With the use of
observations to constrain variables in the chemical calculations we derive
electron fractions for our overall sample of 5 cores directly associated with
star formation and 2 apparently starless cores. The electron abundances are
found to lie within a small range, -6.9 < log10(x_e) < -7.3, and are consistent
with previous work. We find no difference in the amount of ionization fraction
between cores with and without associated star formation activity, nor is any
difference found in electron abundances between the edge and center of the
emission region. Thus our models are in agreement with the standard picture of
cosmic rays as the primary source of ionization for molecular ions. With the
addition of previously determined electron abundances for low mass cores, and
even more massive cores associated with O and B clusters, we systematically
examine the ionization fraction as a function of star formation activity. This
analysis demonstrates that the most massive sources stand out as having the
lowest electron abundances (x_e < 10^-8).Comment: 35 pages (8 figures), using aaspp4.sty, to be published in
Astrophysical Journa
Tracing the Mass during Low-Mass Star Formation. II. Modelling the Submillimeter Emission from Pre-Protostellar Cores
We have modeled the emission from dust in pre-protostellar cores, including a
self-consistent calculation of the temperature distribution for each input
density distribution. Model density distributions include Bonnor-Ebert spheres
and power laws. The Bonnor-Ebert spheres fit the data well for all three cores
we have modeled. The dust temperatures decline to very low values (\Td \sim 7
K) in the centers of these cores, strongly affecting the dust emission.
Compared to earlier models that assume constant dust temperatures, our models
indicate higher central densities and smaller regions of relatively constant
density. Indeed, for L1544, a power-law density distribution, similar to that
of a singular, isothermal sphere, cannot be ruled out. For the three sources
modeled herein, there seems to be a sequence of increasing central
condensation, from L1512 to L1689B to L1544. The two denser cores, L1689B and
L1544, have spectroscopic evidence for contraction, suggesting an evolutionary
sequence for pre-protostellar cores.Comment: 22 pages, 9 figures, Ap. J. accepted, uses emulateapj5.st
Mechanism of Magnetic Flux Loss in Molecular Clouds
We investigate the detailed processes working in the drift of magnetic fields
in molecular clouds. To the frictional force, whereby the magnetic force is
transmitted to neutral molecules, ions contribute more than half only at cloud
densities , and charged grains contribute more
than 90% at . Thus grains play a decisive role
in the process of magnetic flux loss. Approximating the flux loss time by
a power law , where is the mean field strength in
the cloud, we find , characteristic to ambipolar diffusion,
only at . At higher densities,
decreases steeply with , and finally at , where magnetic fields
effectively decouple from the gas, is attained, reminiscent of
Ohmic dissipation, though flux loss occurs about 10 times faster than by Ohmic
dissipation. Ohmic dissipation is dominant only at . While ions and electrons drift in the direction of
magnetic force at all densities, grains of opposite charges drift in opposite
directions at high densities, where grains are major contributors to the
frictional force. Although magnetic flux loss occurs significantly faster than
by Ohmic dissipation even at very high densities as , the process going on at high densities is quite different from ambipolar
diffusion in which particles of opposite charges are supposed to drift as one
unit.Comment: 34 pages including 9 postscript figures, LaTex, accepted by
Astrophysical Journal (vol.573, No.1, July 1, 2002
CN and HCN in Dense Interstellar Clouds
We present a theoretical investigation of CN and HCN molecule formation in
dense interstellar clouds. We study the gas-phase CN and HCN production
efficiencies from the outer photon-dominated regions (PDRs) into the opaque
cosmic-ray dominated cores. We calculate the equilibrium densities of CN and
HCN, and of the associated species C+, C, and CO, as functions of the
far-ultraviolet (FUV) optical depth. We consider isothermal gas at 50 K, with
hydrogen particle densities from 10^2 to 10^6 cm^-3. We study clouds that are
exposed to FUV fields with intensities 20 to 2*10^5 times the mean interstellar
FUV intensity. We assume cosmic-ray H2 ionization rates ranging from 5*10^-17
s^-1, to an enhanced value of 5*10^-16 s^-1. We also examine the sensitivity of
the density profiles to the gas-phase sulfur abundance.Comment: Accepted for publication in ApJ, 33 pages, 8 figure
Deuterated Ammonia in Galactic Protostellar Cores
We report on a survey of \nh2d towards protostellar cores in low-mass star
formation and quiescent regions in the Galaxy. Twenty-three out of thirty-two
observed sources have significant (\gsim 5\sigma) \nh2d emission.
Ion-molecule chemistry, which preferentially enhances deuterium in molecules
above its cosmological value of \scnot{1.6}{-5} sufficiently explains these
abundances. NH2D/NH3 ratios towards Class 0 sources yields information about
the ``fossil remnants'' from the era prior to the onset of core collapse and
star formation. We compare our observations with predictions of gas-phase
chemical networks.Comment: 16 Pages, 7 Figures, Accepted to Ap.J., to appear in the June 20,
2001 editio
Gas and Dark Matter Spherical Dynamics
We investigate the formation of spherical cosmological structures following
both dark matter and gas components. We focus on the dynamical aspect of the
collapse assuming an adiabatic, , fully ionized primordial
plasma. We use for that purpose a fully Lagrangian hydrodynamical code designed
to describe highly compressible flows in spherical geometry. We investigate
also a "fluid approach" to describe the mean physical quantities of the dark
matter flow. We test its validity for a wide range of initial density contrast.
We show that an homogeneous isentropic core forms in the gas distribution,
surrounded by a self-similar hydrostatic halo, with much higher entropy
generated by shock dissipation. We derive analytical expressions for the size,
density and temperature of the core, as well as for the surrounding halo. We
show that, unless very efficient heating processes occur in the intergalactic
medium, we are unable to reproduce within adiabatic models the typical core
sizes in X-ray clusters. We also show that, for dynamical reasons only, the gas
distribution is naturally antibiased relative to the total mass distribution,
without invoking any reheating processes. This could explain why the gas
fraction increases with radius in very large X-ray clusters. As a preparation
for the next study devoted to the thermodynamical aspect of the collapse, we
investigate the initial entropy level required to solve the core problem in
X-ray clusters.Comment: 26 pages, 5 figures, accepted for publication in The Astrophysical
Journa
Cosmic-ray propagation in molecular clouds
Cosmic-rays constitute the main ionising and heating agent in dense,
starless, molecular cloud cores. We reexamine the physical quantities necessary
to determine the cosmic-ray ionisation rate (especially the cosmic ray spectrum
at E < 1 GeV and the ionisation cross sections), and calculate the ionisation
rate as a function of the column density of molecular hydrogen. Available data
support the existence of a low-energy component (below about 100 MeV) of
cosmic-ray electrons or protons responsible for the ionisation of diffuse and
dense clouds. We also compute the attenuation of the cosmic-ray flux rate in a
cloud core taking into account magnetic focusing and magnetic mirroring,
following the propagation of cosmic rays along flux tubes enclosing different
amount of mass and mass-to-flux ratios. We find that mirroring always dominates
over focusing, implying a reduction of the cosmic-ray ionisation rate by a
factor of 3-4 depending on the position inside the core and the magnetisation
of the core.Comment: To appear in "Cosmic Rays in Star-Forming Environments", Proceedings
of the 2nd Session of the Sant Cugat Forum on Astrophysics. D. F. Torres and
O. Reimer (Editors), 2013, Springer, 25 pages, 11 figure
APEX mapping of H3O+ in the Sgr B2 region
The cosmic-ray ionization rate (zeta) of dense molecular clouds is a key
parameter for their dynamics and chemistry. Variations of zeta are well
established, but it is unclear if these are related to source column density or
to Galactic location. Using the APEX telescope, we have mapped the 364 GHz line
of H3O+ in the Sgr B2 region and observed the 307 GHz line at selected
positions. With the IRAM 30-m telescope we have observed the 203 GHz line of
H2O-18 at the same positions. Strong H3O+ emission is detected over a ~3x2 pc
region, indicating H3O+ column densities of 10^15 - 10^16 cm^-2 in an 18" beam.
The H3O+ abundance of ~3 x 10^-9 and H3O+/H2O ratio of ~1/50 in the Sgr B2
envelope are consistent with models with zeta ~4 x 10^-16 s^-1, 3x lower than
derived from H3+ observations toward Sgr A, but 10x that of local dense clouds.
The ionization rates of interstellar clouds thus seem to be to first order
determined by the ambient cosmic-ray flux, while propagation effects cause a
factor of ~3 decrease from diffuse to dense clouds.Comment: Accepted by A&A Letters (APEX special issue); four A4 pages, two
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