174 research outputs found
Intermittency of interstellar turbulence: extreme velocity-shears and CO emission on milliparsec scale
The condensation of diffuse gas into molecular clouds occurs at a rate driven
largely by turbulent dissipation. This process still has to be caught in action
and characterized. A mosaic of 13 fields was observed in the CO(1-0) line with
the IRAM-PdB interferometer in the translucent environment of two low-mass
dense cores. The large size of the mosaic compared to the resolution (4 arcsec)
is unprecedented in the study of the small-scale structure of diffuse molecular
gas. Eight weak and elongated structures of thicknesses as small as 3 mpc (600
AU) and lengths up to 70mpc are found. These are not filaments because once
merged with short-spacing data, they appear as the sharp edges of larger-scale
structures. Six out of eight form quasi-parallel pairs at different velocities
and different position angles. This cannot be the result of chance alignment.
The velocity-shears estimated for the three pairs include the highest ever
measured far from star forming regions (780 km/s/pc). Because the large scale
structures have sharp edges, with little or no overlap, they have to be thin
CO-layers. Their edges mark a sharp transition between a CO-rich component and
a gas undetected in the CO line because of its low CO abundance, presumably the
cold neutral medium. We propose that these sharp edges are the first
directly-detected manifestations of the intermittency of interstellar
turbulence. The large velocity-shears reveal an intense straining field,
responsible for a local dissipation rate several orders of magnitude above
average, possibly at the origin of the thin CO-layers.Comment: 16 pages, 11 figures, Accepted for publication in Astronomy and
Astrophysic
Dissipative structures of diffuse molecular gas: I - Broad HCO(1-0) emission
Results: We report the detection of broad HCO+(1-0) lines (10 mK < T < 0.5
K). The interpretation of 10 of the HCO+ velocity components is conducted in
conjunction with that of the associated optically thin 13CO emission. The
derived HCO+ column densities span a broad range, , and the inferred HCO+ abundances, , are more than one order of magnitude above
those produced by steady-state chemistry in gas weakly shielded from UV
photons, even at large densities. We compare our results with the predictions
of non-equilibrium chemistry, swiftly triggered in bursts of turbulence
dissipation and followed by a slow thermal and chemical relaxation phase,
assumed isobaric. The set of values derived from the observations, i.e. large
HCO+ abundances, temperatures in the range of 100--200 K and densities in the
range 100--1000 cm3, unambiguously belongs to the relaxation phase. The
kinematic properties of the gas suggest in turn that the observed HCO+ line
emission results from a space-time average in the beam of the whole cycle
followed by the gas and that the chemical enrichment is made at the expense of
the non-thermal energy. Last, we show that the "warm chemistry" signature (i.e
large abundances of HCO+, CH+, H20 and OH) acquired by the gas within a few
hundred years, the duration of the impulsive chemical enrichment, is kept over
more than thousand years. During the relaxation phase, the \wat/OH abundance
ratio stays close to the value measured in diffuse gas by the SWAS satellite,
while the OH/HCO+ ratio increases by more than one order of magnitude.Comment: 14 page
Intermittency of interstellar turbulence: Parsec-scale coherent structure of intense velocity-shear
Guided by the duality of turbulence (random versus coherent we seek coherent
structures in the turbulent velocity field of molecular clouds, anticipating
their importance in cloud evolution. We analyse a large map (40' by 20')
obtained with the HERA multibeam receiver (IRAM-30m telescope) in a high
latitude cloud of the Polaris Flare at an unprecedented spatial (11") and
spectral (0.05 km/s) resolutions in the 12CO(2-1) line. We find that two
parsec-scale components of velocities differing by ~2 km/s, share a narrow
interface ( pc) that appears as an elongated structure of intense
velocity-shear, ~15 to 30 km/s/pc. The locus of the extrema of
line--centroid-velocity increments (E-CVI) in that field follows this
intense-shear structure as well as that of the 12CO(2-1) high-velocity line
wings. The tiny spatial overlap in projection of the two parsec-scale
components implies that they are sheets of CO emission and that discontinuities
in the gas properties (CO enrichment and/or increase of gas density) occur at
the position of the intense velocity shear. These results disclose spatial and
kinematic coherence between scales as small as 0.03 pc and parsec scales. They
confirm that the departure from Gaussianity of the probability density
functions of E-CVIs is a powerful statistical tracer of the intermittency of
turbulence. They disclose a link between large scale turbulence, its
intermittent dissipation rate and low-mass dense core formation
The ionization fraction gradient across the Horsehead edge: An archetype for molecular clouds
The ionization fraction plays a key role in the chemistry and dynamics of
molecular clouds. We study the H13CO+, DCO+ and HOC+ line emission towards the
Horsehead, from the shielded core to the UV irradiated cloud edge, i.e., the
Photodissociation Region (PDR), as a template to investigate the ionization
fraction gradient in molecular clouds. We analyze a PdBI map of the H13CO+
J=1-0 line, complemented with IRAM-30m H13CO+ and DCO+ higher-J line maps and
new HOC+ and CO+ observations. We compare self-consistently the observed
spatial distribution and line intensities with detailed depth-dependent
predictions of a PDR model coupled with a nonlocal radiative transfer
calculation. The chemical network includes deuterated species, 13C
fractionation reactions and HCO+/HOC+ isomerization reactions. The role of
neutral and charged PAHs in the cloud chemistry and ionization balance is
investigated. The detection of HOC+ reactive ion towards the Horsehead PDR
proves the high ionization fraction of the outer UV irradiated regions, where
we derive a low [HCO+]/[HOC+]~75-200 abundance ratio. In the absence of PAHs,
we reproduce the observations with gas-phase metal abundances, [Fe+Mg+...],
lower than 4x10(-9) (with respect to H) and a cosmic-rays ionization rate of
zeta=(5+/-3)x10(-17) s(-1). The inclusion of PAHs modifies the ionization
fraction gradient and increases the required metal abundance. The ionization
fraction in the Horsehead edge follows a steep gradient, with a scale length of
~0.05 pc (or ~25''), from [e-]~10(-4) (or n_e ~ 1-5 cm(-3)) in the PDR to a few
times ~10(-9) in the core. PAH^- anions play a role in the charge balance of
the cold and neutral gas if substantial amounts of free PAHs are present ([PAH]
>10(-8)).Comment: 13 pages, 7 figures, 6 tables. Accepted for publication in A&A
(english not edited
Collisional excitation of singly deuterated ammonia NHD by H
The availability of collisional rate coefficients with H is a
pre-requisite for interpretation of observations of molecules whose energy
levels are populated under non local thermodynamical equilibrium conditions. In
the current study, we present collisional rate coefficients for the NHD /
para--H() collisional system, for energy levels up to (735 K) and for gas temperatures in the range K. The
cross sections are obtained using the essentially exact close--coupling (CC)
formalism at low energy and at the highest energies, we used the
coupled--states (CS) approximation. For the energy levels up to
(215 K), the cross sections obtained through the CS formalism are
scaled according to a few CC reference points. These reference points are
subsequently used to estimate the accuracy of the rate coefficients for higher
levels, which is mainly limited by the use of the CS formalism. Considering the
current potential energy surface, the rate coefficients are thus expected to be
accurate to within 5\% for the levels below , while we estimate
an accuracy of 30\% for higher levels
Spitzer Infrared Spectrograph Detection of Molecular Hydrogen Rotational Emission towards Translucent Clouds
Using the Infrared Spectrograph on board the Spitzer Space Telescope, we have detected emission in the S(0), S(1), and S(2) pure-rotational (v = 0-0) transitions of molecular hydrogen (H_2) toward six positions in two translucent high Galactic latitude clouds, DCld 300.2–16.9 and LDN 1780. The detection of these lines raises important questions regarding the physical conditions inside low-extinction clouds that are far from ultraviolet radiation sources. The ratio between the S(2) flux and the flux from polycyclic aromatic hydrocarbons (PAHs) at 7.9 μm averages 0.007 for these six positions. This is a factor of about four higher than the same ratio measured toward the central regions of non-active Galaxies in the Spitzer Infrared Nearby Galaxies Survey. Thus, the environment of these translucent clouds is more efficient at producing rotationally excited H_2 per PAH-exciting photon than the disks of entire galaxies. Excitation analysis finds that the S(1) and S(2) emitting regions are warm (T ≳ 300 K), but comprise no more than 2% of the gas mass. We find that UV photons cannot be the sole source of excitation in these regions and suggest mechanical heating via shocks or turbulent dissipation as the dominant cause of the emission. The clouds are located on the outskirts of the Scorpius-Centaurus OB association and may be dissipating recent bursts of mechanical energy input from supernova explosions. We suggest that pockets of warm gas in diffuse or translucent clouds, integrated over the disks of galaxies, may represent a major source of all non-active galaxy H_2 emission
Molecules in the transition disk orbiting T Cha
We seek to establish the presence and properties of gas in the circumstellar
disk orbiting T Cha, a nearby (d~110 pc), relatively evolved (age ~5-7 Myr) yet
actively accreting 1.5 Msun T Tauri star. We used the APEX 12 m radiotelescope
to search for submillimeter molecular emission from the T Cha disk, and we
reanalyzed archival XMM-Newton spectroscopy of T Cha to ascertain the
intervening absorption due to disk gas along the line of sight to the star
(N_H). We detected submillimeter rotational transitions of 12CO, 13CO, HCN, CN
and HCO+ from the T Cha disk. The 12CO line appears to display a double-peaked
line profile indicative of Keplerian rotation. Analysis of the CO emission line
data indicates that the disk around T Cha has a mass (M_disk,H_2 = 80 M_earth)
similar to, but more compact (R_disk, CO~80 AU) than, other nearby, evolved
molecular disks (e.g. V4046 Sgr, TW Hya, MP Mus) in which cold molecular gas
has been previously detected. The HCO+/13CO and HCN/13CO, line ratios measured
for T Cha appear similar to those of other evolved circumstellar disks (i.e. TW
Hya and V4046 Sgr), while the CN/13CO ratio appears somewhat weaker. Analysis
of the XMM-Newton data shows that the atomic absorption toward T Cha is
1-2 orders of magnitude larger than toward the other nearby T Tauri with
evolved disks. Furthermore, the ratio between atomic absorption and optical
extinction N_H/A_V toward T Cha is higher than the typical value observed for
the interstellar medium and young stellar objects in the Orion Nebula Cluster.
This may suggest that the fraction of metals in the disk gas is higher than in
the interstellar medium. Our results confirm that pre-main sequence stars older
than ~5 Myr, when accreting, retain cold molecular disks, and that those
relatively evolved disks display similar physical and chemical properties.Comment: Accepted for publication on A&
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