E. Roueff - Academia.edu (original) (raw)
Papers by E. Roueff
Astronomy & Astrophysics, 2008
Context. Transfer in lines controls the gas cooling of photon dominated regions (PDR) and provide... more Context. Transfer in lines controls the gas cooling of photon dominated regions (PDR) and provides many of the observational constraints that are available for their modelling. Aims. The interpretation of infrared and radio observations by the new generation of instruments, such as Herschel, requires sophisticated line radiative-transfer methods. The effect of dust emission on the excitation of molecular species in molecular regions is investigated in detail to explicitly show the origin of various approximations used in the literature. Application to H 2 O is emphasised. Methods. The standard 1D radiative transfer equation is written as a function of the space variable (as opposed to the usual optical depth). This permits to simultaneously consider all pumping contributions to a multi-level species in a non-uniform slab of dust and gas. This treatment is included in the Meudon PDR Code (available at http://aristote.obspm.fr/MIS/). Results. Infrared emission from hot grains at the edge of the PDR may penetrate deep inside the cloud, providing an efficient radiation source to excite some species at a location where cold grains no longer emit. This leads to non-negligible differences with classical escape probability methods for some lines, e.g. water. Cooling efficiency does not follow directly from line emissivities. The infrared pumping contribution leads to a higher excitation that enhances collisional de-excitation and reduces cooling efficiency.
VizieR On-line Data Catalog: J/ApJ/783/36. Originally published in: 2014ApJ...783...36MA catalog ... more VizieR On-line Data Catalog: J/ApJ/783/36. Originally published in: 2014ApJ...783...36MA catalog for anionic C3H- molecule to 2THz from t of programs is provided in Table2
Astronomy & Astrophysics, 2018
The isocyanate radical (NCO) is the simplest molecule containing the backbone of the peptide bond... more The isocyanate radical (NCO) is the simplest molecule containing the backbone of the peptide bond, C(=O)–N. This bond has a prebiotic interest since it links two amino acids to form large chains of proteins. It is also present in some organic molecules observed in space such as HNCO, NH2CHO, and CH3NCO. In this letter, we report the first detection in space of NCO towards the dense core L483. We also report the identification of the ion H2NCO+, which definitively confirms its presence in space, and observations of HNCO, HOCN, and HCNO in the same source. For NCO, we derive a column density of 2.2 × 1012 cm−2, which means that it is only ~5 times less abundant than HNCO. We find that H2NCO+, HOCN, and HCNO have abundances relative to HNCO of 1/400, 1/80, and 1/160, respectively. Both NCO and H2NCO+ are involved in the production of HNCO and several of its isomers. We have updated our previous chemical models involving NCO and the production of the CHNO isomers. Taking into account th...
Journal of Physics B: Atomic and Molecular Physics, 1977
Cross sections are computed for the low-energy fine-structure excitation of singly ionized carbon... more Cross sections are computed for the low-energy fine-structure excitation of singly ionized carbon (2P electronic ground state) by atomic hydrogen (2S electronic ground state) in a close-coupling formulation; recent accurate potential energy curves are employed. The effects of rotational coupling and of the fine-structure energy defect are investigated. The excitation cross section is found to be larger than previously computed. The cooling function is calculated for temperatures of astrophysical interest.
Astronomy & Astrophysics, 2017
Aims. The formation epoch of protostellar disks is debated because of the competing roles of rota... more Aims. The formation epoch of protostellar disks is debated because of the competing roles of rotation, turbulence, and magnetic fields in the early stages of low-mass star formation. Magnetohydrodynamics simulations of collapsing cores predict that rotationally supported disks may form in strongly magnetized cores through ambipolar diffusion or misalignment between the rotation axis and the magnetic field orientation. Detailed studies of individual sources are needed to cross check the theoretical predictions. Methods. We present 0.06–0.1′′ resolution images at 350 GHz toward B1b-N and B1b-S, which are young class 0 protostars, possibly first hydrostatic cores. The images have been obtained with ALMA, and we compare these data with magnetohydrodynamics simulations of a collapsing turbulent and magnetized core. Results. The submillimeter continuum emission is spatially resolved by ALMA. Compact structures with optically thick 350 GHz emission are detected toward both B1b-N and B1b-S,...
Monthly Notices of the Royal Astronomical Society, 2014
The availability of collisional rate coefficients with H 2 is a prerequisite for interpretation o... more The availability of collisional rate coefficients with H 2 is a prerequisite 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 NH 2 D/para-H 2 (J 2 = 0, 2) collisional system, for energy levels up to J τ = 7 7 (E u ∼ 735 K) and for gas temperatures in the range T = 5-300 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 J τ = 4 2 (E u ∼ 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 per cent for the levels below J τ = 4 2 , while we estimate an accuracy of 30 per cent for higher levels.
Journal de Physique, 1988
Journal de Physique, 1977
Proceedings of the International Astronomical Union
ABSTRACT
Journal de Physique, 1989
The Astrophysical Journal, 2015
Small hydrocarbons, such as C 2 H, C 3 H and C 3 H 2 are more abundant in photo-dissociation regi... more Small hydrocarbons, such as C 2 H, C 3 H and C 3 H 2 are more abundant in photo-dissociation regions (PDRs) than expected based on gas-phase chemical models. To explore the hydrocarbon chemistry further, we observed a key intermediate species, the hydrocarbon ion l-C 3 H + , in the Horsehead PDR with the Plateau de Bure Interferometer at high-angular resolution (6 ′′). We compare with previous observations of C 2 H and cC 3 H 2 at similar angular resolution and new gas-phase chemical model predictions to constrain the dominant formation mechanisms of small hydrocarbons in low-UV flux PDRs. We find that, at the peak of the HCO emission (PDR position), the measured l-C 3 H + , C 2 H and cC 3 H 2 abundances are consistent with current gas-phase model predictions. However, in the first PDR layers, at the 7.7 µm PAH band emission peak, which are more exposed to the radiation field and where the density is lower, the C 2 H and cC 3 H 2 abundances are underestimated by an order of magnitude. At this position, the l-C 3 H + abundance is also underpredicted by the model but only by a factor of a few. In addition, contrary to the model predictions, l-C 3 H + peaks further out in the PDR than the other hydrocarbons, C 2 H and cC 3 H 2. This cannot be explained by an excitation effect. Current gas-phase photochemical models thus cannot explain the observed abundances of hydrocarbons, in particular in the first PDR layers. Our observations are consistent with a topdown hydrocarbon chemistry, in which large polyatomic molecules or small carbonaceous grains are photo-destroyed into smaller hydrocarbon molecules/precursors. Subject headings: astrochemistry molecular data molecular processes ISM: abundances ISM: molecules photon-dominated region (PDR)
Monthly Notices of the Royal Astronomical Society, 2015
Recently an accurate analysis of absorption spectra of molecular hydrogen, observed with the Cosm... more Recently an accurate analysis of absorption spectra of molecular hydrogen, observed with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope, in the photosphere of white dwarf stars GD133 and GD29-38 was published in a Letter [Phys. Rev. Lett. 113, 123002 (2014)], yielding a constraint on a possible dependence of the proton-electron mass ratio on a gravitational field of strength 10,000 times that at the Earth's surface. In the present paper further details of that study are presented, in particular a re-evaluation of the spectrum of the B 1 Σ + u − X 1 Σ + g (v ′ , v ′′) Lyman bands relevant for the prevailing temperatures (12,000-14,000 K) of the photospheres. An emphasis is on the calculation of so-called K i-coefficients, that represent the sensitivity of each individual line to a possible change in the proton-electron mass ratio. Such calculations were performed by semi-empirical methods and by ab initio methods providing accurate and consistent values. A full listing is provided for the molecular physics data on the Lyman bands (wavelengths λ i , line oscillator strengths f i , radiative damping rates Γ i , and sensitivity coefficients K i) as required for the analyses of H 2-spectra in hot dwarf stars. A similar listing of the molecular physics parameters for the C 1 Π u − X 1 Σ + g (v ′ , v ′′) Werner bands is provided for future use in the analysis of white dwarf spectra.
The very high resolution of the HIFI instrument (134 kHz-1MHz) on board of Herschel needs very ac... more The very high resolution of the HIFI instrument (134 kHz-1MHz) on board of Herschel needs very accurate laboratory measurements to detect unambiguously the signature of stable and unstable molecular species. Concerning the pure rotation spectra of new species, and particularly of open shell molecules, the first prediction could be far away and up to few hundred MHz. The 15N/14N ratio is not well measured in the ISM. However, the 15N/14N in the isotopomers is a potential tracer of the formation processes and the possible link with cometary molecules. Recent measurements include the detection of 15NH_2D N15NH+ and 15NH_3. The NH and NH_2 species are the simplest nitrogen radicals and are intermediate products in the NH_3 synthesis. They have been easily detected by Herschel and it therefore is interesting to now search for 15NH and 15NH_2. No spectrocopic data have been reported for these two radicals up to now. We present here the studies with high resolution spectroscopy in the THz ...
Sulfur is an abundant element which remains undepleted in diffuse interstellar gas (Av<1) but ... more Sulfur is an abundant element which remains undepleted in diffuse interstellar gas (Av<1) but it is traditionally assumed to deplete on dust grains at higher densities and larger Av. Photodissociation regions (PDRs) are an interesting intermediate medium between translucent and dark clouds where the chemistry and energetics are dominated by the illuminating FUV radiation field. Thus they can provide new insights about the sulfur depletion problem. However, physical and chemical gradients in PDRs take place at small angular scales (~1 to 10''). Aperture synthesis observations are therefore required to resolve such gradients. Besides, a complete understanding of molecular excitation is needed to correctly determine molecular abundances but also the preavailing physical conditions. Hence, multi-J observations at increasing frequencies are also required. Such high angular resolution and broad frequency coverage observations will be provided by ALMA in the near future. In th...
We report here our latest developments concerning the access to Atomic and Molecular Linelists Da... more We report here our latest developments concerning the access to Atomic and Molecular Linelists Databases within the Virtual Observatories, addressing the definition of standards through a proposed Data Model (AMLDM) , an access protocol to linelists (SLAP), and their implementation on customized spectroscopic data from the CDMS/JPL databases. Currently a sub-set of CDMS data can be retrieved from BASECOL database using the SLAP protocol.
The Astrophysical Journal, 2011
Young stars surrounded by disks with very low mass accretion rates are likely in the final stages... more Young stars surrounded by disks with very low mass accretion rates are likely in the final stages of inner disk evolution and therefore particularly interesting to study. We present ultraviolet (UV) observations of the ∼5-9 Myr old stars RECX-1 and RECX-11, obtained with the Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph on the Hubble Space Telescope, as well as optical and near-infrared spectroscopic observations. The two stars have similar levels of near-UV emission, although spectroscopic evidence indicates that RECX-11 is accreting and RECX-1 is not. The line profiles of Hα and He i λ10830 in RECX-11 show both broad and narrow redshifted absorption components that vary with time, revealing the complexity of the accretion flows. We show that accretion indicators commonly used to measure mass accretion rates, e.g., U-band excess luminosity or the Ca ii triplet line luminosity, are unreliable for low accretors, at least in the middle K spectral range. Using RECX-1 as a template for the intrinsic level of photospheric and chromospheric emission, we determine an upper limit of 3 × 10 −10 M yr −1 for RECX-11. At this low accretion rate, recent photoevaporation models predict that an inner hole should have developed in the disk. However, the spectral energy distribution of RECX-11 shows fluxes comparable to the median of Taurus in the near-infrared, indicating that substantial dust remains. Fluorescent H 2 emission lines formed in the innermost disk are observed in RECX-11, showing that gas is present in the inner disk, along with the dust.
The Astrophysical Journal, 2010
The internal energy available in vibrationally excited H 2 molecules can be used to overcome or d... more The internal energy available in vibrationally excited H 2 molecules can be used to overcome or diminish the activation barrier of various chemical reactions of interest for molecular astrophysics. In this article we investigate in detail the impact on the chemical composition of interstellar clouds of the reactions of vibrationally excited H 2 with C + , He + , O, OH, and CN, based on the available chemical kinetics data. It is found that the reaction of H 2 (v > 0) and C + has a profound impact on the abundances of some molecules, especially CH + , which is a direct product and is readily formed in astronomical regions with fractional abundances of vibrationally excited H 2 , relative to ground state H 2 , in excess of ∼ 10 −6 , independently of whether the gas is hot or not. The effects of these reactions on the chemical composition of the diffuse clouds ζ Oph and HD 34078, the dense PDR Orion Bar, the planetary nebula NGC 7027, and the circumstellar disk around the B9 star HD 176386 are investigated through PDR models. We find that formation of CH + is especially favored in dense and highly FUV illuminated regions such as the Orion Bar and the planetary nebula NGC 7027, where column densities in excess of 10 13 cm −2 are predicted. In diffuse clouds, however, this mechanism is found to be not efficient enough to form CH + with a column density close to the values derived from astronomical observations.
The Astrophysical Journal, 2007
Measurements using the Far Ultraviolet Spectroscopic Explorer (FUSE) show that the gas-phase D/H ... more Measurements using the Far Ultraviolet Spectroscopic Explorer (FUSE) show that the gas-phase D/H ratio in the solar neighborhood is highly variable and suggest that the elemental abundance of deuterium in the Galactic disk may be higher than previously thought. We investigate in this Letter the possible consequences of such an enhanced D/H ratio on the deuterium fractionation in singly and multiply deuterated molecules in cold, dense gas. We show that a modest increase in the elemental abundance of deuterium can lead to significant enhancements in abundances of multiply deuterated species.
The Astrophysical Journal, 2007
We present observations of the J=1-0, 2-1, and 3-2 rotational transitions of N2H+ and N2D+ toward... more We present observations of the J=1-0, 2-1, and 3-2 rotational transitions of N2H+ and N2D+ toward a sample of prototypical dark clouds. The data have been interpreted using nonlocal radiative transfer models. For all sources previously studied through millimeter-continuum observations, we find a good agreement between the volume density estimated from our N2H+ data and that estimated from the dust
Astronomy & Astrophysics, 2008
Context. Transfer in lines controls the gas cooling of photon dominated regions (PDR) and provide... more Context. Transfer in lines controls the gas cooling of photon dominated regions (PDR) and provides many of the observational constraints that are available for their modelling. Aims. The interpretation of infrared and radio observations by the new generation of instruments, such as Herschel, requires sophisticated line radiative-transfer methods. The effect of dust emission on the excitation of molecular species in molecular regions is investigated in detail to explicitly show the origin of various approximations used in the literature. Application to H 2 O is emphasised. Methods. The standard 1D radiative transfer equation is written as a function of the space variable (as opposed to the usual optical depth). This permits to simultaneously consider all pumping contributions to a multi-level species in a non-uniform slab of dust and gas. This treatment is included in the Meudon PDR Code (available at http://aristote.obspm.fr/MIS/). Results. Infrared emission from hot grains at the edge of the PDR may penetrate deep inside the cloud, providing an efficient radiation source to excite some species at a location where cold grains no longer emit. This leads to non-negligible differences with classical escape probability methods for some lines, e.g. water. Cooling efficiency does not follow directly from line emissivities. The infrared pumping contribution leads to a higher excitation that enhances collisional de-excitation and reduces cooling efficiency.
VizieR On-line Data Catalog: J/ApJ/783/36. Originally published in: 2014ApJ...783...36MA catalog ... more VizieR On-line Data Catalog: J/ApJ/783/36. Originally published in: 2014ApJ...783...36MA catalog for anionic C3H- molecule to 2THz from t of programs is provided in Table2
Astronomy & Astrophysics, 2018
The isocyanate radical (NCO) is the simplest molecule containing the backbone of the peptide bond... more The isocyanate radical (NCO) is the simplest molecule containing the backbone of the peptide bond, C(=O)–N. This bond has a prebiotic interest since it links two amino acids to form large chains of proteins. It is also present in some organic molecules observed in space such as HNCO, NH2CHO, and CH3NCO. In this letter, we report the first detection in space of NCO towards the dense core L483. We also report the identification of the ion H2NCO+, which definitively confirms its presence in space, and observations of HNCO, HOCN, and HCNO in the same source. For NCO, we derive a column density of 2.2 × 1012 cm−2, which means that it is only ~5 times less abundant than HNCO. We find that H2NCO+, HOCN, and HCNO have abundances relative to HNCO of 1/400, 1/80, and 1/160, respectively. Both NCO and H2NCO+ are involved in the production of HNCO and several of its isomers. We have updated our previous chemical models involving NCO and the production of the CHNO isomers. Taking into account th...
Journal of Physics B: Atomic and Molecular Physics, 1977
Cross sections are computed for the low-energy fine-structure excitation of singly ionized carbon... more Cross sections are computed for the low-energy fine-structure excitation of singly ionized carbon (2P electronic ground state) by atomic hydrogen (2S electronic ground state) in a close-coupling formulation; recent accurate potential energy curves are employed. The effects of rotational coupling and of the fine-structure energy defect are investigated. The excitation cross section is found to be larger than previously computed. The cooling function is calculated for temperatures of astrophysical interest.
Astronomy & Astrophysics, 2017
Aims. The formation epoch of protostellar disks is debated because of the competing roles of rota... more Aims. The formation epoch of protostellar disks is debated because of the competing roles of rotation, turbulence, and magnetic fields in the early stages of low-mass star formation. Magnetohydrodynamics simulations of collapsing cores predict that rotationally supported disks may form in strongly magnetized cores through ambipolar diffusion or misalignment between the rotation axis and the magnetic field orientation. Detailed studies of individual sources are needed to cross check the theoretical predictions. Methods. We present 0.06–0.1′′ resolution images at 350 GHz toward B1b-N and B1b-S, which are young class 0 protostars, possibly first hydrostatic cores. The images have been obtained with ALMA, and we compare these data with magnetohydrodynamics simulations of a collapsing turbulent and magnetized core. Results. The submillimeter continuum emission is spatially resolved by ALMA. Compact structures with optically thick 350 GHz emission are detected toward both B1b-N and B1b-S,...
Monthly Notices of the Royal Astronomical Society, 2014
The availability of collisional rate coefficients with H 2 is a prerequisite for interpretation o... more The availability of collisional rate coefficients with H 2 is a prerequisite 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 NH 2 D/para-H 2 (J 2 = 0, 2) collisional system, for energy levels up to J τ = 7 7 (E u ∼ 735 K) and for gas temperatures in the range T = 5-300 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 J τ = 4 2 (E u ∼ 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 per cent for the levels below J τ = 4 2 , while we estimate an accuracy of 30 per cent for higher levels.
Journal de Physique, 1988
Journal de Physique, 1977
Proceedings of the International Astronomical Union
ABSTRACT
Journal de Physique, 1989
The Astrophysical Journal, 2015
Small hydrocarbons, such as C 2 H, C 3 H and C 3 H 2 are more abundant in photo-dissociation regi... more Small hydrocarbons, such as C 2 H, C 3 H and C 3 H 2 are more abundant in photo-dissociation regions (PDRs) than expected based on gas-phase chemical models. To explore the hydrocarbon chemistry further, we observed a key intermediate species, the hydrocarbon ion l-C 3 H + , in the Horsehead PDR with the Plateau de Bure Interferometer at high-angular resolution (6 ′′). We compare with previous observations of C 2 H and cC 3 H 2 at similar angular resolution and new gas-phase chemical model predictions to constrain the dominant formation mechanisms of small hydrocarbons in low-UV flux PDRs. We find that, at the peak of the HCO emission (PDR position), the measured l-C 3 H + , C 2 H and cC 3 H 2 abundances are consistent with current gas-phase model predictions. However, in the first PDR layers, at the 7.7 µm PAH band emission peak, which are more exposed to the radiation field and where the density is lower, the C 2 H and cC 3 H 2 abundances are underestimated by an order of magnitude. At this position, the l-C 3 H + abundance is also underpredicted by the model but only by a factor of a few. In addition, contrary to the model predictions, l-C 3 H + peaks further out in the PDR than the other hydrocarbons, C 2 H and cC 3 H 2. This cannot be explained by an excitation effect. Current gas-phase photochemical models thus cannot explain the observed abundances of hydrocarbons, in particular in the first PDR layers. Our observations are consistent with a topdown hydrocarbon chemistry, in which large polyatomic molecules or small carbonaceous grains are photo-destroyed into smaller hydrocarbon molecules/precursors. Subject headings: astrochemistry molecular data molecular processes ISM: abundances ISM: molecules photon-dominated region (PDR)
Monthly Notices of the Royal Astronomical Society, 2015
Recently an accurate analysis of absorption spectra of molecular hydrogen, observed with the Cosm... more Recently an accurate analysis of absorption spectra of molecular hydrogen, observed with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope, in the photosphere of white dwarf stars GD133 and GD29-38 was published in a Letter [Phys. Rev. Lett. 113, 123002 (2014)], yielding a constraint on a possible dependence of the proton-electron mass ratio on a gravitational field of strength 10,000 times that at the Earth's surface. In the present paper further details of that study are presented, in particular a re-evaluation of the spectrum of the B 1 Σ + u − X 1 Σ + g (v ′ , v ′′) Lyman bands relevant for the prevailing temperatures (12,000-14,000 K) of the photospheres. An emphasis is on the calculation of so-called K i-coefficients, that represent the sensitivity of each individual line to a possible change in the proton-electron mass ratio. Such calculations were performed by semi-empirical methods and by ab initio methods providing accurate and consistent values. A full listing is provided for the molecular physics data on the Lyman bands (wavelengths λ i , line oscillator strengths f i , radiative damping rates Γ i , and sensitivity coefficients K i) as required for the analyses of H 2-spectra in hot dwarf stars. A similar listing of the molecular physics parameters for the C 1 Π u − X 1 Σ + g (v ′ , v ′′) Werner bands is provided for future use in the analysis of white dwarf spectra.
The very high resolution of the HIFI instrument (134 kHz-1MHz) on board of Herschel needs very ac... more The very high resolution of the HIFI instrument (134 kHz-1MHz) on board of Herschel needs very accurate laboratory measurements to detect unambiguously the signature of stable and unstable molecular species. Concerning the pure rotation spectra of new species, and particularly of open shell molecules, the first prediction could be far away and up to few hundred MHz. The 15N/14N ratio is not well measured in the ISM. However, the 15N/14N in the isotopomers is a potential tracer of the formation processes and the possible link with cometary molecules. Recent measurements include the detection of 15NH_2D N15NH+ and 15NH_3. The NH and NH_2 species are the simplest nitrogen radicals and are intermediate products in the NH_3 synthesis. They have been easily detected by Herschel and it therefore is interesting to now search for 15NH and 15NH_2. No spectrocopic data have been reported for these two radicals up to now. We present here the studies with high resolution spectroscopy in the THz ...
Sulfur is an abundant element which remains undepleted in diffuse interstellar gas (Av<1) but ... more Sulfur is an abundant element which remains undepleted in diffuse interstellar gas (Av<1) but it is traditionally assumed to deplete on dust grains at higher densities and larger Av. Photodissociation regions (PDRs) are an interesting intermediate medium between translucent and dark clouds where the chemistry and energetics are dominated by the illuminating FUV radiation field. Thus they can provide new insights about the sulfur depletion problem. However, physical and chemical gradients in PDRs take place at small angular scales (~1 to 10''). Aperture synthesis observations are therefore required to resolve such gradients. Besides, a complete understanding of molecular excitation is needed to correctly determine molecular abundances but also the preavailing physical conditions. Hence, multi-J observations at increasing frequencies are also required. Such high angular resolution and broad frequency coverage observations will be provided by ALMA in the near future. In th...
We report here our latest developments concerning the access to Atomic and Molecular Linelists Da... more We report here our latest developments concerning the access to Atomic and Molecular Linelists Databases within the Virtual Observatories, addressing the definition of standards through a proposed Data Model (AMLDM) , an access protocol to linelists (SLAP), and their implementation on customized spectroscopic data from the CDMS/JPL databases. Currently a sub-set of CDMS data can be retrieved from BASECOL database using the SLAP protocol.
The Astrophysical Journal, 2011
Young stars surrounded by disks with very low mass accretion rates are likely in the final stages... more Young stars surrounded by disks with very low mass accretion rates are likely in the final stages of inner disk evolution and therefore particularly interesting to study. We present ultraviolet (UV) observations of the ∼5-9 Myr old stars RECX-1 and RECX-11, obtained with the Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph on the Hubble Space Telescope, as well as optical and near-infrared spectroscopic observations. The two stars have similar levels of near-UV emission, although spectroscopic evidence indicates that RECX-11 is accreting and RECX-1 is not. The line profiles of Hα and He i λ10830 in RECX-11 show both broad and narrow redshifted absorption components that vary with time, revealing the complexity of the accretion flows. We show that accretion indicators commonly used to measure mass accretion rates, e.g., U-band excess luminosity or the Ca ii triplet line luminosity, are unreliable for low accretors, at least in the middle K spectral range. Using RECX-1 as a template for the intrinsic level of photospheric and chromospheric emission, we determine an upper limit of 3 × 10 −10 M yr −1 for RECX-11. At this low accretion rate, recent photoevaporation models predict that an inner hole should have developed in the disk. However, the spectral energy distribution of RECX-11 shows fluxes comparable to the median of Taurus in the near-infrared, indicating that substantial dust remains. Fluorescent H 2 emission lines formed in the innermost disk are observed in RECX-11, showing that gas is present in the inner disk, along with the dust.
The Astrophysical Journal, 2010
The internal energy available in vibrationally excited H 2 molecules can be used to overcome or d... more The internal energy available in vibrationally excited H 2 molecules can be used to overcome or diminish the activation barrier of various chemical reactions of interest for molecular astrophysics. In this article we investigate in detail the impact on the chemical composition of interstellar clouds of the reactions of vibrationally excited H 2 with C + , He + , O, OH, and CN, based on the available chemical kinetics data. It is found that the reaction of H 2 (v > 0) and C + has a profound impact on the abundances of some molecules, especially CH + , which is a direct product and is readily formed in astronomical regions with fractional abundances of vibrationally excited H 2 , relative to ground state H 2 , in excess of ∼ 10 −6 , independently of whether the gas is hot or not. The effects of these reactions on the chemical composition of the diffuse clouds ζ Oph and HD 34078, the dense PDR Orion Bar, the planetary nebula NGC 7027, and the circumstellar disk around the B9 star HD 176386 are investigated through PDR models. We find that formation of CH + is especially favored in dense and highly FUV illuminated regions such as the Orion Bar and the planetary nebula NGC 7027, where column densities in excess of 10 13 cm −2 are predicted. In diffuse clouds, however, this mechanism is found to be not efficient enough to form CH + with a column density close to the values derived from astronomical observations.
The Astrophysical Journal, 2007
Measurements using the Far Ultraviolet Spectroscopic Explorer (FUSE) show that the gas-phase D/H ... more Measurements using the Far Ultraviolet Spectroscopic Explorer (FUSE) show that the gas-phase D/H ratio in the solar neighborhood is highly variable and suggest that the elemental abundance of deuterium in the Galactic disk may be higher than previously thought. We investigate in this Letter the possible consequences of such an enhanced D/H ratio on the deuterium fractionation in singly and multiply deuterated molecules in cold, dense gas. We show that a modest increase in the elemental abundance of deuterium can lead to significant enhancements in abundances of multiply deuterated species.
The Astrophysical Journal, 2007
We present observations of the J=1-0, 2-1, and 3-2 rotational transitions of N2H+ and N2D+ toward... more We present observations of the J=1-0, 2-1, and 3-2 rotational transitions of N2H+ and N2D+ toward a sample of prototypical dark clouds. The data have been interpreted using nonlocal radiative transfer models. For all sources previously studied through millimeter-continuum observations, we find a good agreement between the volume density estimated from our N2H+ data and that estimated from the dust