202 research outputs found
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
Solenoidal versus compressive turbulence forcing
We analyze the statistics and star formation rate obtained in high-resolution
numerical experiments of forced supersonic turbulence, and compare with
observations. We concentrate on a systematic comparison of solenoidal
(divergence-free) and compressive (curl-free) forcing, which are two limiting
cases of turbulence driving. Our results show that for the same RMS Mach
number, compressive forcing produces a three times larger standard deviation of
the density probability distribution. When self-gravity is included in the
models, the star formation rate is more than one order of magnitude higher for
compressive forcing than for solenoidal forcing.Comment: 1 page, to appear in the proceedings of the IAU General Assembly
Joint Discussion 14 "FIR2009: The ISM of Galaxies in the Far-Infrared and
Sub-Millimetre", ed. M. Cunningha
An Unbiased 1.3 mm Emission Line Survey of the Protoplanetary Disk Orbiting LkCa 15
The outer (>30 AU) regions of the dusty circumstellar disk orbiting the ~2-5
Myr-old, actively accreting solar analog LkCa 15 are known to be chemically
rich, and the inner disk may host a young protoplanet within its central
cavity. To obtain a complete census of the brightest molecular line emission
emanating from the LkCa 15 disk over the 210-270 GHz (1.4 - 1.1 mm) range, we
have conducted an unbiased radio spectroscopic survey with the Institute de
Radioastronomie Millimetrique (IRAM) 30 meter telescope. The survey
demonstrates that, in this spectral region, the most readily detectable lines
are those of CO and its isotopologues 13CO and C18O, as well as HCO+, HCN, CN,
C2H, CS, and H2CO. All of these species had been previously detected in the
LkCa 15 disk; however, the present survey includes the first complete coverage
of the CN (2-1) and C2H (3-2) hyperfine complexes. Modeling of these emission
complexes indicates that the CN and C2H either reside in the coldest regions of
the disk or are subthermally excited, and that their abundances are enhanced
relative to molecular clouds and young stellar object environments. These
results highlight the value of unbiased single-dish line surveys in guiding
future high resolution interferometric imaging of disks.Comment: 35 pages, 9 figures, accepted for publication in The Astrophysical
Journa
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
Collisional excitation of doubly and triply deuterated ammonia NDH and ND by H
The availability of collisional rate coefficients is a prerequisite for an
accurate interpretation of astrophysical observations, since the observed media
often harbour densities where molecules are populated under non--LTE
conditions. In the current study, we present calculations of rate coefficients
suitable to describe the various spin isomers of multiply deuterated ammonia,
namely the NDH and ND isotopologues. These calculations are based on
the most accurate NH--H potential energy surface available, which has
been modified to describe the geometrical changes induced by the nuclear
substitutions. The dynamical calculations are performed within the
close--coupling formalism and are carried out in order to provide rate
coefficients up to a temperature of = 50K. For the various
isotopologues/symmetries, we provide rate coefficients for the energy levels
below 100 cm. Subsequently, these new rate coefficients are used
in astrophysical models aimed at reproducing the NHD, NDH and ND
observations previously reported towards the prestellar cores B1b and 16293E.
We thus update the estimates of the corresponding column densities and find a
reasonable agreement with the previous models. In particular, the
ortho--to--para ratios of NHD and NHD are found to be consistent with
the statistical ratios
Nitrogen chemistry and depletion in starless cores
We investigated the chemistry of nitrogen--containing species, principally
isotopomers of CN, HCN, and HNC, in a sample of pre-protostellar cores. We used
the IRAM 30 m telescope to measure the emission in rotational and hyperfine
transitions of CN, HCN, 13CN, H13CN, HN13C, and HC15N, in L 1544, L 183, Oph D,
L 1517B, L 310. The observations were made along axial cuts through the dust
emission peak, at a number of regularly--spaced offset positions. The
observations were reduced and analyzed to obtain the column densities, using
the measurements of the less abundant isotopic variants in order to minimize
the consequences of finite optical depths in the lines. The observations were
compared with the predictions of a free--fall gravitational collapse model,
which incorporates a non-equilibrium treatment of the relevant chemistry. We
found that CN, HCN, and HNC remain present in the gas phase at densities well
above that at which CO depletes on to grains. The CN:HCN and the HNC:HCN
abundance ratios are larger than unity in all the objects of our sample.
Furthermore, there is no observational evidence for large variations of these
ratios with increasing offset from the dust emission peak and hence with
density. Whilst the differential freeze--out of CN and CO can be understood in
terms of the current chemistry, the behaviour of the CN:HCN ratio is more
difficult to explain. Models suggest that most nitrogen is not in the gas phase
but may be locked in ices. Unambiguous conclusions require measurements of the
rate coefficients of the key neutral--neutral reactions at low temperatures
Deuterium fractionation in the Horsehead edge
Deuterium fractionation is known to enhance the [DCO+]/[HCO+] abundance ratio
over the D/H elemental ratio of about 1e-5 in the cold and dense gas typically
found in pre-stellar cores. We report the first detection and mapping of very
bright DCO+ J=3-2 and J=2-1 lines (3 and 4 K respectively) towards the
Horsehead photodissociation region (PDR) observed with the IRAM-30m telescope.
The DCO+ emission peaks close to the illuminated warm edge of the nebula (< 50"
or about 0.1 pc away). Detailed nonlocal, non-LTE excitation and radiative
transfer analyses have been used to determine the prevailing physical
conditions and to estimate the DCO+ and H13CO+ abundances from their line
intensities. A large [DCO+]/[HCO+] abundance ratio (>= 0.02) is inferred at the
DCO+ emission peak, a condensation shielded from the illuminating far-UV
radiation field where the gas must be cold (10-20 K) and dense (>= 2x10^5
cm-3). DCO+ is not detected in the warmer photodissociation front, implying a
lower [DCO+]/[HCO+] ratio (< 1e-3). According to our gas phase chemical
predictions, such a high deuterium fractionation of HCO+ can only be explained
if the gas temperature is below 20 K, in good agreement with DCO+ excitation
calculations.Comment: 4 pages, 3 PostScript figures. Accepted for publication in Astronomy
& Astrophysics in the letter section. Uses aa LaTeX macro
Selectivity of the photosensitiser Tookad® for photodynamic therapy evaluated in the Syrian golden hamster cheek pouch tumour model
The response to photodynamic therapy (PDT) with the photosensitiser (PS) Tookad was measured in the Syrian hamster cheek pouch model on normal mucosae and chemically induced squamous cell carcinoma. This PS is a palladium-bacteriopheophorbide presenting absorption peaks at 538 and 762 nm. The light dose, drug dose and drug injection-light irradiation times (DLI), ranging between 100 and 300 J cm(-2), 1-5 mg kg(-1) and 10-240 min respectively, were varied and the response to PDT was analysed by staging the macroscopic response and by the histological examination of the sections of the irradiated cheek pouch. A fast time decay of the tissular response with drug dose of 1-5 mg kg(-1) was observed for DLI ranging from 10 to 240 min and for light doses of 100-300 J cm(-2) delivered at a light dose rate of 150 mW cm(-2). A significantly higher level of tissular response was observed for squamous cell carcinoma compared to normal tissue. Nevertheless, the threshold level of the drug-light dose for a detectable response was not significantly different in the tumoral vs normal tissue. The highest response at the shortest DLIs and the absence of measurable response at DLI larger than 240 min at light dose of 300 J cm(-2) and drug dose of 5 mg kg(-1) reveals the predominantly vascular effect of Tookad. This observation suggests that Tookad could be effective in PDT of vascularised lesions
Wavelength-dependent effect of tetra(m-hydroxyphenyl)chlorin for photodynamic therapy in an ‘early' squamous cell carcinoma model
The purpose of the present study was to correlate the wavelength of the irradiation source with the phototoxic activity of tetra(m-hydroxyphenyl)chlorin (mTHPC) in healthy and neoplastic mucosae. The hamster tumour model for early squamous cell carcinoma was used in these experiments. In vitro and in vivo studies have shown that mTHPC absorbs significantly at 652 nm (1, 2). This wavelength is used currently in clinical mTHPC photodynamic therapy (PDT) trials. In order to study the wavelength dependence of the phototoxic effect on normal and tumour tissues, irradiation tests were performed 4 days after injection of 0.5mg kg-1 mTHPC. An argon-ion pumped dye laser was used as the light source. The light dose of 12 J cm-2 was delivered at a light dose rate of 150 mW cm-2. The wavelength was varied between 642.5 and 665 nm at 2.5-nm increments. The PDT damage was evaluated in serial Haematoxylin and Eosin stained sections using a tissue-damage scale. Light between 647.5 and 652.5 nm induced the highest damage to both the healthy and tumour mucosae. At wavelengths equal to or below 645 nm, and equal to or above 655 nm, tissue damage decreased. Wavelengths below 642 nm and above 660 nm did not induce any visible tissue damage. These results suggest that the in vivo optimal wavelength range for PDT with mTHPC is between 647 and 652 nm. This information is essential for selecting an appropriate light sourc
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