David Field - Academia.edu (original) (raw)
Papers by David Field
Monthly Notices of the Royal Astronomical Society, 1979
Proceedings of the International Astronomical Union, 2006
Using radial velocities of vibrationally excited H2 emission in OMC1 we present the structure fun... more Using radial velocities of vibrationally excited H2 emission in OMC1 we present the structure functions and the scaling of the structure functions with their order at scales ranging from 70 AU to 30000 AU extending earlier related studies to scales lower by two orders of magnitude. The structure functions for OMC1 show clear deviations from power laws at 1500 AU. The scaling of the higher order structure functions with order deviates from predicted theoretical scalings. Observational results are compared with simulations of supersonic hydrodynamic turbulence. The unusual scaling is explained as a selection effect of preferentially observing the shocked part of the gas. The simulations are unable to reproduce the deviations from power laws of the structure functions.
Proceedings of the International Astronomical Union, 2006
Molecular clouds are usually thought to be dominated by turbulence where the structures are inher... more Molecular clouds are usually thought to be dominated by turbulence where the structures are inherently self-similar and lack characteristic scale. However self-similarity must break down at scales associated with star formation which imposes a characteristic scale. The turbulence may be driven by energy injection at some larger scale which also imposes characteristic scale. In order to understand the evolution of molecular clouds it is important to identify the departures from self-similarity associated with the scales of self-gravity and the driving of turbulence.We describe a method based on structure functions for determining whether a region of gas, such as a molecular cloud, is fractal or contains structure with characteristic scale sizes (Gustafsson, Lemaire & Field 2006). Using artificial data containing structure it is shown that derivatives of higher order structure functions provide a powerful way to detect the presence of characteristic scales should any be present and to...
Europhysics News, 2005
of British and Luxembourg origins, are involved in experimental tokamak research at the CRPP-EPFL... more of British and Luxembourg origins, are involved in experimental tokamak research at the CRPP-EPFL in Lausanne, Switzerland. Jo Lister is currently chairman of the EPS Plasma Physics Division. Both authors are engaged in extensive international collaborations and are fervent supporters of a positive decision to go ahead with ITER.
Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences, 1979
The theory of rotationally inelastic collisions between orbitally degenerate diatomic molecules a... more The theory of rotationally inelastic collisions between orbitally degenerate diatomic molecules and open-shell atoms is developed. Because of the orbital degeneracy two or more electronic potential energy surfaces are involved. Matrix elements of the interaction Hamiltonian are given, hyperfine coupling in the diatomic molecule also being included. From these it is apparent th at the parity of the initial Λ -doublet level will influence the inelastic scattering cross section for poles of interaction λ such that λ ≥ 2 Λ .An expression is developed for state-to-state cross sections using the restricted distorted wave Born approximation. A set of branching coefficients is defined which allows the representation of the parity dependence of the cross section in a simple parametric form. The theory is applied to collisional pumping as an excitation mechanism for interstellar maser action of OH and CH through the inversion of Λ -doublet populations. H atoms, H 2 , He, H + and H + 3 are con...
Physical Review Letters, 2009
Experimental data obtained using low energy electron beams are presented which show that films of... more Experimental data obtained using low energy electron beams are presented which show that films of N 2 O, of several hundred monolayers (ML), spontaneously acquire a positive potential of as high as 5 V. Films do not possess a dipole double layer but for >40 ML display a constant electric field within the material. This new phenomenon is attributed to dipole alignment. The phenomenon also shows a strong temperature dependence. This is revealed by the differing dependence of the surface potential on the film thickness at different temperatures and by electron transmission spectra which display marked structure at 62 K which is absent at 40 K.
Monthly Notices of the Royal Astronomical Society, 1996
A hydrodynamic pulsation model for long-period M-type Mira variables is used to estimate the phys... more A hydrodynamic pulsation model for long-period M-type Mira variables is used to estimate the physical conditions of number density, temperature and velocity field in the circumstellar envelope of a model star of 1 M 0 , 332-d period, surface temperature 3000 K and mass-loss rate 1.8 x 10-7 M0 yr-1 • These conditions have previously been used in a model of SiD masers to establish, for a single stellar phase, the nature and location of the zone in which SiD masers should form. Using Monte Carlo techniques, a synthetic image of 43-GHz emission is generated, showing that masers are predicted to form in an approximate ring 1 to 2 stellar radii from the photosphere, in agreement with the results of recent very long baseline interferometry (VLBI) observations. A new image of SiD masers around TX Cam, obtained using the Very Long Baseline Array (VLBA), is presented, which shows these characteristics. Models show that masers do not form in the region in which the stellar wind is fully developed. Amplification is dominated by tangential paths through the limbs of the circumstellar envelope. Synthetic lineshapes for a variety of maser transitions show structure and linewidths that are typical of observed lineshapes.
Astronomy and Astrophysics, 2006
We present near-infrared imaging and spectrocopic observations of the HII region N159A (∼10 pc) i... more We present near-infrared imaging and spectrocopic observations of the HII region N159A (∼10 pc) in the giant star-forming region N159 (50 pc) in the LMC. N159A was observed in the J and Ks bands at high spatial resolution ∼0.2 using the ESO Very Large Telescope UT4 (VLT), equipped with the NAOS adaptative optics system. Our data reveal the morphology of this region in unprecedented detail. The protostar P2, one of the first YSOs of Class I identified in the LMC is now resolved in two YSO candidates. The ultracompact HII region LI-LMC 1501W is found to be a tight cluster embedded in a compact HII region ionised by a late O source. A new multiple system composed of a tight star cluster and an YSO candidate, all embedded in a compact nebular region (0.4 pc) is also detected at the northeast edge of N159A. The stellar population of the whole N159A region appears composed of two main stellar populations, one with an age ≤3 Myr and the other one with a large range of age (300 Myr-10 Gyr). Using spectroscopy, one of the two exciting O stars in the HII region N159A is classified O5-O6.
Astronomy & Astrophysics, 2007
Aims. We present a new method for reproducing high spatial resolution observations of bow shocks ... more Aims. We present a new method for reproducing high spatial resolution observations of bow shocks by using 1D plane parallel shock models. As an example we analyse one bow shock located in the Orion Molecular Cloud (OMC1). Methods. We use high spatial resolution near-infrared observations of H 2 rovibrational emission to constrain shock models. These observations have been made at the ESO-VLT using a combination of the NACO adaptive optics system and infrared camera array and the Fabry-Perot interferometer. Three rovibrational H 2 lines have been observed: v = 1−0 S(1) at 2.12 µm, v = 1−0 S(0) at 2.23 µm and v = 2−1 S(1) at 2.25 µm. The spatial resolution is 0. 15 ∼ 70 AU. We analyse a single bow shock located in our field, featuring a very well defined morphology and high brightness. Results. One dimensional shock models are combined to estimate the physical properties of pre-shock density, shock velocity and transverse magnetic field strength along the bow shock. We find that the pre-shock density is constant at ∼5 × 10 5 cm −3 and shock velocities lie between ∼35 km s −1 in the wings of the shock and ∼50 km s −1 at the apex. We also find that the transverse magnetic field is stronger at the apex and weaker further down the wings varying between ∼2 and 4 mGauss. Predictions of shock velocity and magnetic field strength agree with previous independent observations.
Monthly Notices of the Royal Astronomical Society, 1997
Sputtering yields are calculated for the release of Si and ° from amorphous SiOz and of C from am... more Sputtering yields are calculated for the release of Si and ° from amorphous SiOz and of C from amorphous carbon, under impact of 4He +, 1ZC +, 160 +, 28Si + and 56Fe + ions, for energies in the range 40::;
Monthly Notices of the Royal Astronomical Society, 1984
Lecture Notes in Physics
OH masers are a potentially powerful probe of the physical conditions in massive star-forming reg... more OH masers are a potentially powerful probe of the physical conditions in massive star-forming regions, giving detailed information on the kinetic and dust temperatures, number densities of H2 and OH, bulk flows and magnetic fields in the vicinity of massive young stellar objects. Using current theories and data, we describe how bright maser spots, as seen in VLBI at 18
ESO Astrophysics Symposia, 2005
... Shocks and Star Formation in Orion 231 ... L. Salas, M. Rosado, I. Cruz-Gonzales et al.: ApJ ... more ... Shocks and Star Formation in Orion 231 ... L. Salas, M. Rosado, I. Cruz-Gonzales et al.: ApJ 511, 822 (1999) 5. S. Beckwith, SE Persson, G. Neugebauer & EE Becklin: ApJ 223, 464 (1978) 6. T. Doi, CR O'Dell & P. Hartigan: AJ 124, 445 (2002) 7. JS Richer, DS Shepherd, S. Cabrit ...
Astrophysics and Space Science Library, 2003
Coupling maser models to dynamical stellar envelope models provides a new step towards the full u... more Coupling maser models to dynamical stellar envelope models provides a new step towards the full understanding of the physics of mass-losing stars. Here, we describe how such numerical simulations of H2O and SiO maser emission yield maser variability and the proper motions of maser clumps during a stellar pulsation cycle. These calculations place important constraints on stellar models and provide new insights on the conditions leading to, and predominantly affecting, stellar maser emission. This work also constitutes the foundations towards our final goal: the development of dynamical models of AGB stars which calculate accurately and self-consistently the time-dependent intensity and location of SiO, H2O and OH maser emisson, in addition to the inclusion of time-dependent chemistry and dust formation. Such models will provide a "big picture" view of mass-losing stellar envelopes throughout their pulsation cycles.
The Astrophysical Journal, 2014
A fundamental question about the early evolution of low-mass protostars is when circumstellar dis... more A fundamental question about the early evolution of low-mass protostars is when circumstellar disks may form. High angular resolution observations of molecular transitions in the (sub)millimeter wavelength windows make it possible to investigate the kinematics of the gas around newly-formed stars, for example to identify the presence of rotation and infall. IRAS16293-2422 was observed with the extended Submillimeter Array (eSMA) resulting in subarcsecond resolution (0.46 ′′ × 0.29 ′′ , i.e. ∼ 55 × 35 AU) images of compact emission from the C 17 O (3-2) and C 34 S (7-6) transitions at 337 GHz (0.89 mm). To recover the more extended emission we have combined the eSMA data with SMA observations of the same molecules. The emission of C 17 O (3-2) and C 34 S (7-6) both show a velocity gradient oriented along a northeastsouthwest direction with respect to the continuum marking the location of one of the components of the binary, IRAS16293A. Our combined eSMA and SMA observations show that the velocity field on the 50-400 AU scales is consistent with a rotating structure. It cannot be explained by simple Keplerian rotation around a single point mass but rather needs to take into account the enclosed envelope mass at the radii where the observed lines are excited. We suggest that IRAS 16293-2422 could be among the best candidates to observe a pseudo-disk with future high angular resolution observations.
The Astrophysical Journal Supplement Series, 2014
We have surveyed a sample of massive star-forming regions located over a range of distances from ... more We have surveyed a sample of massive star-forming regions located over a range of distances from the Galactic centre for methyl formate, HCOOCH3, and its isotopologues H 13 COOCH3 and HCOO 13 CH3. The observations were carried out with the APEX telescope in the frequency range 283.4-287.4 GHz. Based on the APEX observations, we report tentative detections of the 13 C-methyl formate isotopologue HCOO 13 CH3 towards the following four massive star-forming regions: Sgr B2(N-LMH), NGC 6334 IRS 1, W51 e2 and G19.61-0.23. In addition, we have used the 1 mm ALMA science verification observations of Orion-KL and confirm the detection of the 13 C-methyl formate species in Orion-KL and image its spatial distribution. Our analysis shows that the 12 C/ 13 C isotope ratio in methyl formate toward Orion-KL Compact Ridge and Hot Core-SW components (68.4±10.1 and 71.4±7.8, respectively) are, for both the 13 C-methyl formate isotopologues, commensurate with the average 12 C/ 13 C ratio of CO derived toward Orion-KL. Likewise, regarding the other sources, our results are consistent with the 12 C/ 13 C in CO. We also report the spectroscopic characterization, which includes a complete partition function, of the complex H 13 COOCH3 and HCOO 13 CH3 species. New spectroscopic data for both isotopomers H 13 COOCH3 and HCOO 13 CH3, presented in this study, has made it possible to measure this fundamentally important isotope ratio in a large organic molecule for the first time.
Astrochemistry of Cosmic Phenomena, 1992
We combine a sophisticated model of maser propagation with a simple model of an accelerating mole... more We combine a sophisticated model of maser propagation with a simple model of an accelerating molecular outflow and show that the observation of different OH maser frequencies is consistent with emission from different parts of the outflow.
Science, 2003
Detailed laboratory experiments on the formation of HD from atom recombination on amorphous solid... more Detailed laboratory experiments on the formation of HD from atom recombination on amorphous solid water films show that this process is extremely efficient in a temperature range of 8 to 20 kelvin, temperatures relevant for H 2 formation on dust grain surfaces in the interstellar medium (ISM). The fate of the 4.5 electron volt recombination energy is highly dependent on film morphology. These results suggest that grain morphology, rather than the detailed chemical nature of the grain surface, is most important in determining the energy content of the H 2 as it is released from the grain into the ISM.
![Research paper thumbnail of High-Resolution Interstellar Spectroscopy and Star Formation [and Discussion]](https://attachments.academia-assets.com/89882737/thumbnails/1.jpg)
](Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 1981
During the past several years, high spatial and spectral resolution molecular spectroscopy has gr... more During the past several years, high spatial and spectral resolution molecular spectroscopy has greatly contributed to our knowledge of the physics, dynamics and chemistry of interstellar molecular clouds and thus has led to a better understanding of the conditions that lead to star formation. According to their physical properties, molecular clouds can be grouped into four different types: (i) the dark clouds, (ii) the molecular clouds associated with H+ regions, (iii) the ‘protostellar’ (or maser) environment, and (iv) the molecular envelopes of late-type stars. The first three types of cloud contain generally active regions of star formation. As typical examples the properties are discussed of individual clouds such as TMC 1 and L 183 for the cold clouds, S 140 and S 106 for the warm dark clouds with embedded infrared source, and Orion A for a region with associated H+ region. In S 140, NH 3 is clumped on a scale of not more than 20", whereas recent observations towards Orion...
Monthly Notices of the Royal Astronomical Society, 2005
We present Multi-Element Radio-Linked Interferometer Network (MERLIN) observations of OH maser an... more We present Multi-Element Radio-Linked Interferometer Network (MERLIN) observations of OH maser and radio continuum emission within a few hundred pc of the core of the ultraluminous infrared galaxy (ULIRG) Markarian 231. This is the only known OH megamaser galaxy classed as a Seyfert 1. Maser emission is identified with the 1665-and 1667-MHz transitions over a velocity extent of 720 km s −1. Both lines show a similar position-velocity structure including a gradient of 1.7 km s −1 pc −1 from NW to SE along the 420-pc major axis. The (unresolved) inner few tens of pc possess a much steeper velocity gradient. The maser distribution is modelled as a torus rotating about an axis inclined at ∼45 • to the plane of the sky. We estimate the enclosed mass density to be 320 ± 90 M pc −3 in a flattened distribution. This includes a central unresolved mass of 8 × 10 6 M. All the maser emission is projected against a region with a radio continuum brightness temperature 10 5 K, giving a maser gain of 2.2. The 1667:1665 MHz line ratio is close to 1.8 (the value predicted for thermal emission) consistent with radiatively pumped, unsaturated masers. This behaviour and the kinematics of the torus suggest that the size of individual masing regions is in the range 0.25-4 pc with a covering factor close to unity. There are no very bright compact masers, in contrast to galaxies such as the Seyfert 2 Markarian 273, where the masing torus is viewed nearer edge-on. The comparatively modest maser amplification seen from Markarian 231 is consistent with its classification in the unification scheme for Seyfert galaxies. Most of the radio continuum emission on 50-500 pc scales is probably of starburst origin but the compact peak is 0.4 per cent polarized by a magnetic field running north-south, similar to the jet direction on these scales. There is no close correlation between maser and continuum intensity, suggesting that much of the radio continuum must originate in the foreground and indeed the relative continuum brightness is slightly greater in the direction of the approaching jet. Comparisons with other data show that the jet changes direction close the nucleus and suggest that the sub-kpc disc hosting the masers and starburst activity is severely warped.
Monthly Notices of the Royal Astronomical Society, 1979
Proceedings of the International Astronomical Union, 2006
Using radial velocities of vibrationally excited H2 emission in OMC1 we present the structure fun... more Using radial velocities of vibrationally excited H2 emission in OMC1 we present the structure functions and the scaling of the structure functions with their order at scales ranging from 70 AU to 30000 AU extending earlier related studies to scales lower by two orders of magnitude. The structure functions for OMC1 show clear deviations from power laws at 1500 AU. The scaling of the higher order structure functions with order deviates from predicted theoretical scalings. Observational results are compared with simulations of supersonic hydrodynamic turbulence. The unusual scaling is explained as a selection effect of preferentially observing the shocked part of the gas. The simulations are unable to reproduce the deviations from power laws of the structure functions.
Proceedings of the International Astronomical Union, 2006
Molecular clouds are usually thought to be dominated by turbulence where the structures are inher... more Molecular clouds are usually thought to be dominated by turbulence where the structures are inherently self-similar and lack characteristic scale. However self-similarity must break down at scales associated with star formation which imposes a characteristic scale. The turbulence may be driven by energy injection at some larger scale which also imposes characteristic scale. In order to understand the evolution of molecular clouds it is important to identify the departures from self-similarity associated with the scales of self-gravity and the driving of turbulence.We describe a method based on structure functions for determining whether a region of gas, such as a molecular cloud, is fractal or contains structure with characteristic scale sizes (Gustafsson, Lemaire & Field 2006). Using artificial data containing structure it is shown that derivatives of higher order structure functions provide a powerful way to detect the presence of characteristic scales should any be present and to...
Europhysics News, 2005
of British and Luxembourg origins, are involved in experimental tokamak research at the CRPP-EPFL... more of British and Luxembourg origins, are involved in experimental tokamak research at the CRPP-EPFL in Lausanne, Switzerland. Jo Lister is currently chairman of the EPS Plasma Physics Division. Both authors are engaged in extensive international collaborations and are fervent supporters of a positive decision to go ahead with ITER.
Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences, 1979
The theory of rotationally inelastic collisions between orbitally degenerate diatomic molecules a... more The theory of rotationally inelastic collisions between orbitally degenerate diatomic molecules and open-shell atoms is developed. Because of the orbital degeneracy two or more electronic potential energy surfaces are involved. Matrix elements of the interaction Hamiltonian are given, hyperfine coupling in the diatomic molecule also being included. From these it is apparent th at the parity of the initial Λ -doublet level will influence the inelastic scattering cross section for poles of interaction λ such that λ ≥ 2 Λ .An expression is developed for state-to-state cross sections using the restricted distorted wave Born approximation. A set of branching coefficients is defined which allows the representation of the parity dependence of the cross section in a simple parametric form. The theory is applied to collisional pumping as an excitation mechanism for interstellar maser action of OH and CH through the inversion of Λ -doublet populations. H atoms, H 2 , He, H + and H + 3 are con...
Physical Review Letters, 2009
Experimental data obtained using low energy electron beams are presented which show that films of... more Experimental data obtained using low energy electron beams are presented which show that films of N 2 O, of several hundred monolayers (ML), spontaneously acquire a positive potential of as high as 5 V. Films do not possess a dipole double layer but for >40 ML display a constant electric field within the material. This new phenomenon is attributed to dipole alignment. The phenomenon also shows a strong temperature dependence. This is revealed by the differing dependence of the surface potential on the film thickness at different temperatures and by electron transmission spectra which display marked structure at 62 K which is absent at 40 K.
Monthly Notices of the Royal Astronomical Society, 1996
A hydrodynamic pulsation model for long-period M-type Mira variables is used to estimate the phys... more A hydrodynamic pulsation model for long-period M-type Mira variables is used to estimate the physical conditions of number density, temperature and velocity field in the circumstellar envelope of a model star of 1 M 0 , 332-d period, surface temperature 3000 K and mass-loss rate 1.8 x 10-7 M0 yr-1 • These conditions have previously been used in a model of SiD masers to establish, for a single stellar phase, the nature and location of the zone in which SiD masers should form. Using Monte Carlo techniques, a synthetic image of 43-GHz emission is generated, showing that masers are predicted to form in an approximate ring 1 to 2 stellar radii from the photosphere, in agreement with the results of recent very long baseline interferometry (VLBI) observations. A new image of SiD masers around TX Cam, obtained using the Very Long Baseline Array (VLBA), is presented, which shows these characteristics. Models show that masers do not form in the region in which the stellar wind is fully developed. Amplification is dominated by tangential paths through the limbs of the circumstellar envelope. Synthetic lineshapes for a variety of maser transitions show structure and linewidths that are typical of observed lineshapes.
Astronomy and Astrophysics, 2006
We present near-infrared imaging and spectrocopic observations of the HII region N159A (∼10 pc) i... more We present near-infrared imaging and spectrocopic observations of the HII region N159A (∼10 pc) in the giant star-forming region N159 (50 pc) in the LMC. N159A was observed in the J and Ks bands at high spatial resolution ∼0.2 using the ESO Very Large Telescope UT4 (VLT), equipped with the NAOS adaptative optics system. Our data reveal the morphology of this region in unprecedented detail. The protostar P2, one of the first YSOs of Class I identified in the LMC is now resolved in two YSO candidates. The ultracompact HII region LI-LMC 1501W is found to be a tight cluster embedded in a compact HII region ionised by a late O source. A new multiple system composed of a tight star cluster and an YSO candidate, all embedded in a compact nebular region (0.4 pc) is also detected at the northeast edge of N159A. The stellar population of the whole N159A region appears composed of two main stellar populations, one with an age ≤3 Myr and the other one with a large range of age (300 Myr-10 Gyr). Using spectroscopy, one of the two exciting O stars in the HII region N159A is classified O5-O6.
Astronomy & Astrophysics, 2007
Aims. We present a new method for reproducing high spatial resolution observations of bow shocks ... more Aims. We present a new method for reproducing high spatial resolution observations of bow shocks by using 1D plane parallel shock models. As an example we analyse one bow shock located in the Orion Molecular Cloud (OMC1). Methods. We use high spatial resolution near-infrared observations of H 2 rovibrational emission to constrain shock models. These observations have been made at the ESO-VLT using a combination of the NACO adaptive optics system and infrared camera array and the Fabry-Perot interferometer. Three rovibrational H 2 lines have been observed: v = 1−0 S(1) at 2.12 µm, v = 1−0 S(0) at 2.23 µm and v = 2−1 S(1) at 2.25 µm. The spatial resolution is 0. 15 ∼ 70 AU. We analyse a single bow shock located in our field, featuring a very well defined morphology and high brightness. Results. One dimensional shock models are combined to estimate the physical properties of pre-shock density, shock velocity and transverse magnetic field strength along the bow shock. We find that the pre-shock density is constant at ∼5 × 10 5 cm −3 and shock velocities lie between ∼35 km s −1 in the wings of the shock and ∼50 km s −1 at the apex. We also find that the transverse magnetic field is stronger at the apex and weaker further down the wings varying between ∼2 and 4 mGauss. Predictions of shock velocity and magnetic field strength agree with previous independent observations.
Monthly Notices of the Royal Astronomical Society, 1997
Sputtering yields are calculated for the release of Si and ° from amorphous SiOz and of C from am... more Sputtering yields are calculated for the release of Si and ° from amorphous SiOz and of C from amorphous carbon, under impact of 4He +, 1ZC +, 160 +, 28Si + and 56Fe + ions, for energies in the range 40::;
Monthly Notices of the Royal Astronomical Society, 1984
Lecture Notes in Physics
OH masers are a potentially powerful probe of the physical conditions in massive star-forming reg... more OH masers are a potentially powerful probe of the physical conditions in massive star-forming regions, giving detailed information on the kinetic and dust temperatures, number densities of H2 and OH, bulk flows and magnetic fields in the vicinity of massive young stellar objects. Using current theories and data, we describe how bright maser spots, as seen in VLBI at 18
ESO Astrophysics Symposia, 2005
... Shocks and Star Formation in Orion 231 ... L. Salas, M. Rosado, I. Cruz-Gonzales et al.: ApJ ... more ... Shocks and Star Formation in Orion 231 ... L. Salas, M. Rosado, I. Cruz-Gonzales et al.: ApJ 511, 822 (1999) 5. S. Beckwith, SE Persson, G. Neugebauer & EE Becklin: ApJ 223, 464 (1978) 6. T. Doi, CR O'Dell & P. Hartigan: AJ 124, 445 (2002) 7. JS Richer, DS Shepherd, S. Cabrit ...
Astrophysics and Space Science Library, 2003
Coupling maser models to dynamical stellar envelope models provides a new step towards the full u... more Coupling maser models to dynamical stellar envelope models provides a new step towards the full understanding of the physics of mass-losing stars. Here, we describe how such numerical simulations of H2O and SiO maser emission yield maser variability and the proper motions of maser clumps during a stellar pulsation cycle. These calculations place important constraints on stellar models and provide new insights on the conditions leading to, and predominantly affecting, stellar maser emission. This work also constitutes the foundations towards our final goal: the development of dynamical models of AGB stars which calculate accurately and self-consistently the time-dependent intensity and location of SiO, H2O and OH maser emisson, in addition to the inclusion of time-dependent chemistry and dust formation. Such models will provide a "big picture" view of mass-losing stellar envelopes throughout their pulsation cycles.
The Astrophysical Journal, 2014
A fundamental question about the early evolution of low-mass protostars is when circumstellar dis... more A fundamental question about the early evolution of low-mass protostars is when circumstellar disks may form. High angular resolution observations of molecular transitions in the (sub)millimeter wavelength windows make it possible to investigate the kinematics of the gas around newly-formed stars, for example to identify the presence of rotation and infall. IRAS16293-2422 was observed with the extended Submillimeter Array (eSMA) resulting in subarcsecond resolution (0.46 ′′ × 0.29 ′′ , i.e. ∼ 55 × 35 AU) images of compact emission from the C 17 O (3-2) and C 34 S (7-6) transitions at 337 GHz (0.89 mm). To recover the more extended emission we have combined the eSMA data with SMA observations of the same molecules. The emission of C 17 O (3-2) and C 34 S (7-6) both show a velocity gradient oriented along a northeastsouthwest direction with respect to the continuum marking the location of one of the components of the binary, IRAS16293A. Our combined eSMA and SMA observations show that the velocity field on the 50-400 AU scales is consistent with a rotating structure. It cannot be explained by simple Keplerian rotation around a single point mass but rather needs to take into account the enclosed envelope mass at the radii where the observed lines are excited. We suggest that IRAS 16293-2422 could be among the best candidates to observe a pseudo-disk with future high angular resolution observations.
The Astrophysical Journal Supplement Series, 2014
We have surveyed a sample of massive star-forming regions located over a range of distances from ... more We have surveyed a sample of massive star-forming regions located over a range of distances from the Galactic centre for methyl formate, HCOOCH3, and its isotopologues H 13 COOCH3 and HCOO 13 CH3. The observations were carried out with the APEX telescope in the frequency range 283.4-287.4 GHz. Based on the APEX observations, we report tentative detections of the 13 C-methyl formate isotopologue HCOO 13 CH3 towards the following four massive star-forming regions: Sgr B2(N-LMH), NGC 6334 IRS 1, W51 e2 and G19.61-0.23. In addition, we have used the 1 mm ALMA science verification observations of Orion-KL and confirm the detection of the 13 C-methyl formate species in Orion-KL and image its spatial distribution. Our analysis shows that the 12 C/ 13 C isotope ratio in methyl formate toward Orion-KL Compact Ridge and Hot Core-SW components (68.4±10.1 and 71.4±7.8, respectively) are, for both the 13 C-methyl formate isotopologues, commensurate with the average 12 C/ 13 C ratio of CO derived toward Orion-KL. Likewise, regarding the other sources, our results are consistent with the 12 C/ 13 C in CO. We also report the spectroscopic characterization, which includes a complete partition function, of the complex H 13 COOCH3 and HCOO 13 CH3 species. New spectroscopic data for both isotopomers H 13 COOCH3 and HCOO 13 CH3, presented in this study, has made it possible to measure this fundamentally important isotope ratio in a large organic molecule for the first time.
Astrochemistry of Cosmic Phenomena, 1992
We combine a sophisticated model of maser propagation with a simple model of an accelerating mole... more We combine a sophisticated model of maser propagation with a simple model of an accelerating molecular outflow and show that the observation of different OH maser frequencies is consistent with emission from different parts of the outflow.
Science, 2003
Detailed laboratory experiments on the formation of HD from atom recombination on amorphous solid... more Detailed laboratory experiments on the formation of HD from atom recombination on amorphous solid water films show that this process is extremely efficient in a temperature range of 8 to 20 kelvin, temperatures relevant for H 2 formation on dust grain surfaces in the interstellar medium (ISM). The fate of the 4.5 electron volt recombination energy is highly dependent on film morphology. These results suggest that grain morphology, rather than the detailed chemical nature of the grain surface, is most important in determining the energy content of the H 2 as it is released from the grain into the ISM.
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 1981
During the past several years, high spatial and spectral resolution molecular spectroscopy has gr... more During the past several years, high spatial and spectral resolution molecular spectroscopy has greatly contributed to our knowledge of the physics, dynamics and chemistry of interstellar molecular clouds and thus has led to a better understanding of the conditions that lead to star formation. According to their physical properties, molecular clouds can be grouped into four different types: (i) the dark clouds, (ii) the molecular clouds associated with H+ regions, (iii) the ‘protostellar’ (or maser) environment, and (iv) the molecular envelopes of late-type stars. The first three types of cloud contain generally active regions of star formation. As typical examples the properties are discussed of individual clouds such as TMC 1 and L 183 for the cold clouds, S 140 and S 106 for the warm dark clouds with embedded infrared source, and Orion A for a region with associated H+ region. In S 140, NH 3 is clumped on a scale of not more than 20", whereas recent observations towards Orion...
Monthly Notices of the Royal Astronomical Society, 2005
We present Multi-Element Radio-Linked Interferometer Network (MERLIN) observations of OH maser an... more We present Multi-Element Radio-Linked Interferometer Network (MERLIN) observations of OH maser and radio continuum emission within a few hundred pc of the core of the ultraluminous infrared galaxy (ULIRG) Markarian 231. This is the only known OH megamaser galaxy classed as a Seyfert 1. Maser emission is identified with the 1665-and 1667-MHz transitions over a velocity extent of 720 km s −1. Both lines show a similar position-velocity structure including a gradient of 1.7 km s −1 pc −1 from NW to SE along the 420-pc major axis. The (unresolved) inner few tens of pc possess a much steeper velocity gradient. The maser distribution is modelled as a torus rotating about an axis inclined at ∼45 • to the plane of the sky. We estimate the enclosed mass density to be 320 ± 90 M pc −3 in a flattened distribution. This includes a central unresolved mass of 8 × 10 6 M. All the maser emission is projected against a region with a radio continuum brightness temperature 10 5 K, giving a maser gain of 2.2. The 1667:1665 MHz line ratio is close to 1.8 (the value predicted for thermal emission) consistent with radiatively pumped, unsaturated masers. This behaviour and the kinematics of the torus suggest that the size of individual masing regions is in the range 0.25-4 pc with a covering factor close to unity. There are no very bright compact masers, in contrast to galaxies such as the Seyfert 2 Markarian 273, where the masing torus is viewed nearer edge-on. The comparatively modest maser amplification seen from Markarian 231 is consistent with its classification in the unification scheme for Seyfert galaxies. Most of the radio continuum emission on 50-500 pc scales is probably of starburst origin but the compact peak is 0.4 per cent polarized by a magnetic field running north-south, similar to the jet direction on these scales. There is no close correlation between maser and continuum intensity, suggesting that much of the radio continuum must originate in the foreground and indeed the relative continuum brightness is slightly greater in the direction of the approaching jet. Comparisons with other data show that the jet changes direction close the nucleus and suggest that the sub-kpc disc hosting the masers and starburst activity is severely warped.