569 research outputs found
Strong CH^+ J = 1–0 emission and absorption in DR21
We report the first detection of the ground-state rotational transition of the methylidyne cation CH^+ towards the massive star-forming region DR 21 with the HIFI instrument onboard the Herschel satellite. The line profile exhibits a broad emission line, in addition to two deep and broad absorption features associated with the DR 21 molecular ridge and foreground gas. These observations allow us to determine a ^(12)CH^(+)J = 1–0 line frequency of ν = 835 137 ± 3 MHz, in good agreement with a recent experimental determination. We estimate the CH^+ column density to be a few 10^(13) cm^(-2) in the gas seen in emission, and >10^(14) cm^(-2) in the components responsible for the absorption, which is indicative of a high line of sight average abundance [CH^+] /[H] > 1.2 × 10^(-8). We show that the CH^+ column densities agree well with the predictions of state-of-the-art C-shock models in dense UV-illuminated gas for the emission line, and with those of turbulent dissipation models in diffuse gas for the absorption lines
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
Small scale variations of abundances of transiently heated grains in molecular clouds
IRAS images of a variety of fragments in nearby molecular clouds show that the energy distribution of their IR emission varies widely from cloud to cloud and from place to place within a given cloud. These variations at small scale are all the more unexpected since the colors of the IR emission of cold material differ very little at large scale: the colors of the cirrus emission above the 3kpc molecular ring are the same as those of the cirrus emission in the solar neighborhood. To quantitatively study these variations, 12, 60, and 100 microns brightnesses were obtained of small areas centered at different positions within the set of clouds and complexes. The range of observed 12/100 micron colors is given for each cloud. Variations by an order of magnitude are found in most clouds. Variations by a factor of 2 to 3 are observed within a cloud on scales as small as 0.5pc, the resolution of this study. It is concluded that large variations of the abundances of small particles with respect to those of the large grains responsible for the 100 micron emission are required to explain the observed color variations and that these abundances have to vary by large factors; an order of magnitude from cloud to cloud
CH^+(1–0) and ^(13)CH^+(1–0) absorption lines in the direction of massive star-forming regions
We report the detection of the ground-state rotational transition of the methylidyne cation CH^+ and its isotopologue ^(13)CH^+ toward the remote
massive star-forming regions W33A, W49N, and W51 with the HIFI instrument onboard the Herschel satellite. Both lines are seen only in
absorption against the dust continuum emission of the star-forming regions. The CH^+ absorption is saturated over almost the entire velocity
ranges sampled by the lines-of-sight that include gas associated with the star-forming regions (SFR) and Galactic foreground material. The CH^+
column densities are inferred from the optically thin components. A lower limit of the isotopic ratio [^(12)CH^+]/[^(13)CH^+] > 35.5 is derived from
the absorptions of foreground material toward W49N. The column density ratio, N(CH^+)/N(HCO^+), is found to vary by at least a factor 10,
between 4 and >40, in the Galactic foreground material. Line-of-sight ^(12)CH^+ average abundances relative to total hydrogen are estimated. Their
average value, N(CH^+)/N_H > 2.6 × 10^(−8), is higher than that observed in the solar neighborhood and confirms the high abundances of CH^+ in
the Galactic interstellar medium. We compare this result to the predictions of turbulent dissipation regions (TDR) models and find that these high
abundances can be reproduced for the inner Galaxy conditions. It is remarkable that the range of predicted N(CH^+)/N(HCO^+) ratios, from 1 to
~50, is comparable to that observed
Filamentary Structure and Helical Magnetic Fields in the Environment of a Starless Dense Core
International audienceThe environment of L1512, a starless dense core, has been mapped at high angular resolution in the 12CO (J=2-1) line over more than 1 pc, with a few positions observed in the 12CO (J=3-2) and (J=4-3) lines. The gas outside the dense core is structured in several filaments, roughly 1 pc long and ~0.1 pc thick, converging at the dense core position. Small longitudinal (~1 km s-1 pc-1) but large transverse (up to 8 km s-1 pc-1) velocity gradients are observed. Remarkably, the transverse gradients can be seen to change sign periodically, along at least one of the filaments. Thus, there are oscillations in the toroidal velocity within the filaments, which may be a signature of a magnetohydrodynamic instability developing in filaments permeated by a helical magnetic field. In the case of L1512, according to the analysis of Fiege & Pudritz, the growth rate of the instability is low, corresponding to a timescale of the order of 1 Myr. We deduce from the wavelength of the oscillations that the toroidal component of the magnetic field dominates the poloidal component. The toroidal component helps confine the filaments, which are not otherwise confined by self-gravity (m/mvir~0.2), by the pressure of the galactic H I layer, or by external turbulent pressure. We find that the velocity gradients in the vicinity of the dense core provide an estimate for an upper limit to the accretion rate onto the dense core of M=4×10-6 Msolar yr-1. For the gas characteristics in the filaments, we find that a broad range of density and temperature is allowed for the gas, from nH2=2×103 cm-3 for the coldest case (Tk=20 K) down to nH2=180 cm-3 for the warmest (Tk=250 K)
Boyle's law and gravitational instability
We have re-examined the classical problem of the macroscopic equation of
state for a hydrostatic isothermal self-gravitating gas cloud bounded by an
external medium at constant pressure. We have obtained analytical conditions
for its equilibrium and stability without imposing any specific shape and
symmetry to the cloud density distribution. The equilibrium condition can be
stated in the form of an upper limit to the cloud mass; this is found to be
inversely proportional to the power 3/2 of a form factor \mu characterizing the
shape of the cloud. In this respect, the spherical solution, associated with
the maximum value of the form factor, \mu = 1, turns out to correspond to the
shape that is most difficult to realize. Surprisingly, the condition that
defines the onset of the Bonnor instability (or gravothermal catastrophe) can
be cast in the form of an upper limit to the density contrast within the cloud
that is independent of the cloud shape. We have then carried out a similar
analysis in the two-dimensional case of infinite cylinders, without assuming
axisymmetry. The results obtained in this paper generalize well-known results
available for spherical or axisymmetric cylindrical isothermal clouds that have
had wide astrophysical applications, especially in the study of the
interstellar medium.Comment: 9 pages, 2 figures, to appear in A&
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
Heating of the molecular gas in the massive outflow of the local ultraluminous-infrared and radio-loud galaxy 4C12.50
We present a comparison of the molecular gas properties in the outflow vs. in
the ambient medium of the local prototype radio-loud and ultraluminous-infrared
galaxy 4C12.50 (IRAS13451+1232), using new data from the IRAM Plateau de Bure
interferometer and 30m telescope, and the Herschel space telescope. Previous
H_2 (0-0) S(1) and S(2) observations with the Spitzer space telescope had
indicated that the warm (~400K) molecular gas in 4C12.50 is made up of a
1.4(+-0.2)x10^8 M_sun ambient reservoir and a 5.2(+-1.7)x10^7 M_sun outflow.
The new CO(1-0) data cube indicates that the corresponding cold (25K) H_2 gas
mass is 1.0(+-0.1)x10^10 M_sun for the ambient medium and <1.3x10^8 M_sun for
the outflow, when using a CO-intensity-to-H_2-mass conversion factor alpha of
0.8 M_sun /(K km/s pc^2). The combined mass outflow rate is high, 230-800
M_sun/yr, but the amount of gas that could escape the galaxy is low. A
potential inflow of gas from a 3.3(+-0.3)x10^8 M_sun tidal tail could moderate
any mass loss. The mass ratio of warm-to-cold molecular gas is >= 30 times
higher in the outflow than in the ambient medium, indicating that a
non-negligible fraction of the accelerated gas is heated to temperatures at
which star formation is inefficient. This conclusion is robust against the use
of different alpha factor values, and/or different warm gas tracers (H_2 vs.
H_2 plus CO): with the CO-probed gas mass being at least 40 times lower at 400K
than at 25K, the total warm-to-cold mass ratio is always lower in the ambient
gas than in the entrained gas. Heating of the molecular gas could facilitate
the detection of new outflows in distant galaxies by enhancing their emission
in intermediate rotational number CO lines.Comment: A&A, in pres
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