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Sanchez C Contreras - One of the best experts on this subject based on the ideXlab platform.

  • mass linear Momentum and kinetic energy of bipolar flows in protoplanetary nebulae
    Astronomy and Astrophysics, 2001
    Co-Authors: V Bujarrabal, A Castrocarrizo, J Alcolea, Sanchez C Contreras
    Abstract:

    We have studied the CO emission from protoplanetary nebulae (PPNe). Our sample is composed of 37 objects and includes, we think, all well identied PPNe detected in CO, together with the two yellow hypergiants emitting in CO and one young PN. We present a summary of the existing CO data, including accurate new observations of the 12 CO and 13 CO J = 1{0 and J = 2{1 lines in 16 objects. We identify in the nebulae a slowly expanding shell (represented in the spectra by a central core) and a fast outflow (corresponding to the line wings), that in the well studied PPNe is known to be bipolar. Excluding poor data, we end up with a sample of 32 Sources (including the 16 observed by us); fast flows are detected in 28 of these nebulae, being absent in only 4. We present a method to estimate from these data the mass, \scalar" Momentum and kinetic energy of the dierent components of the molecular outflows. We argue that the uncertainties of our method can hardly lead to signicant overestimates of these parameters, although underestimates may be present in not well studied objects. The total nebular mass is often as high as 1 M, and the mass-loss rate, that (presumably during the last stages of the AGB phase) originated the nebula, had typical values 10 4 M yr 1 . The Momentum corresponding to this mass ejection process in most studied nebulae is accurately coincident with the maximum Momentum that radiation pressure, acting through absorption by dust grains, is able to supply (under expected conditions). We estimate that this high-eciency process lasts about 1000{10 000 yr, after which the star has ejected a good fraction of its mass and the AGB phase ends. On the other hand, the fast molecular outflows, that have probably been accelerated by shock interaction with axial post-AGB jets, carry a signicant fraction of the nebular mass, with a very high Momentum (in most cases between 10 37 and 10 40 gc m s 1 ) and very high kinetic energy (usually between 10 44 and 10 47 erg). In general, yellow hypergiants and post-AGB objects with low initial mass show nebular masses and momenta that are, respectively, higher and lower than these values. We compare the momenta of the fast outflows with those that can be supplied by radiation pressure, taking into account the expected short acceleration times and some eects that can increase the Momentum transfer. We nd that in about 80% of PPNe, the fast molecular flows have too high momenta to be powered by radiation pressure. In some cases the Momentum of the outflow is1000 larger than that carried by radiation pressure; such high factors are dicult to explain even under exceptional conditions. Wind interaction is the basic phenomenon in the PN shaping from the former AGB envelopes; we conclude that this interaction systematically takes place along a dominant direction and that this process is not powered by radiation pressure. Due to the lack of theoretical studies, the possible Momentum Source remains a matter of speculation.

  • mass linear Momentum and kinetic energy of bipolar flows in protoplanetary nebulae
    Astronomy and Astrophysics, 2001
    Co-Authors: V Bujarrabal, A Castrocarrizo, J Alcolea, Sanchez C Contreras
    Abstract:

    We have studied the CO emission from protoplanetary nebulae (PPNe). Our sample is composed of 37 objects and includes, we think, all well identied PPNe detected in CO, together with the two yellow hypergiants emitting in CO and one young PN. We present a summary of the existing CO data, including accurate new observations of the 12 CO and 13 CO J = 1{0 and J = 2{1 lines in 16 objects. We identify in the nebulae a slowly expanding shell (represented in the spectra by a central core) and a fast outflow (corresponding to the line wings), that in the well studied PPNe is known to be bipolar. Excluding poor data, we end up with a sample of 32 Sources (including the 16 observed by us); fast flows are detected in 28 of these nebulae, being absent in only 4. We present a method to estimate from these data the mass, \scalar" Momentum and kinetic energy of the dierent components of the molecular outflows. We argue that the uncertainties of our method can hardly lead to signicant overestimates of these parameters, although underestimates may be present in not well studied objects. The total nebular mass is often as high as 1 M, and the mass-loss rate, that (presumably during the last stages of the AGB phase) originated the nebula, had typical values 10 4 M yr 1 . The Momentum corresponding to this mass ejection process in most studied nebulae is accurately coincident with the maximum Momentum that radiation pressure, acting through absorption by dust grains, is able to supply (under expected conditions). We estimate that this high-eciency process lasts about 1000{10 000 yr, after which the star has ejected a good fraction of its mass and the AGB phase ends. On the other hand, the fast molecular outflows, that have probably been accelerated by shock interaction with axial post-AGB jets, carry a signicant fraction of the nebular mass, with a very high Momentum (in most cases between 10 37 and 10 40 gc m s 1 ) and very high kinetic energy (usually between 10 44 and 10 47 erg). In general, yellow hypergiants and post-AGB objects with low initial mass show nebular masses and momenta that are, respectively, higher and lower than these values. We compare the momenta of the fast outflows with those that can be supplied by radiation pressure, taking into account the expected short acceleration times and some eects that can increase the Momentum transfer. We nd that in about 80% of PPNe, the fast molecular flows have too high momenta to be powered by radiation pressure. In some cases the Momentum of the outflow is1000 larger than that carried by radiation pressure; such high factors are dicult to explain even under exceptional conditions. Wind interaction is the basic phenomenon in the PN shaping from the former AGB envelopes; we conclude that this interaction systematically takes place along a dominant direction and that this process is not powered by radiation pressure. Due to the lack of theoretical studies, the possible Momentum Source remains a matter of speculation.

Martine Baelmans - One of the best experts on this subject based on the ideXlab platform.

  • hybrid fluid kinetic model for neutral particles in the plasma edge
    Nuclear materials and energy, 2019
    Co-Authors: Niels Horsten, Giovanni Samaey, Martine Baelmans
    Abstract:

    Abstract We elaborate a hybrid fluid-kinetic model for neutral particles in the plasma edge. A macro/fluid model with kinetic corrections in the closure terms of the fluid moment equations is solved in the entire domain. The kinetic corrections follow from a Monte Carlo simulation of the micro/kinetic part. We compare two hybrid models with different underlying fluid approximations: (i) a pure pressure-diffusion equation; and (ii) a continuity and parallel Momentum equation with pressure-diffusion transport only retained in the directions perpendicular to the magnetic field lines. We asses their performance for a high recycling slab case with fixed background plasma. Compared to a Monte Carlo simulation of the full kinetic equation, the statistical error on the particle Source is reduced with 40% and 47% for respectively the hybrid model without and with the parallel Momentum equation for the same computational time. For the parallel Momentum Source there is only a reduction for the model with parallel Momentum equation (about 69%). Unfortunately, the statistical error on the ion energy Source slightly increases for both hybrid models.

V Bujarrabal - One of the best experts on this subject based on the ideXlab platform.

  • mass linear Momentum and kinetic energy of bipolar flows in protoplanetary nebulae
    Astronomy and Astrophysics, 2001
    Co-Authors: V Bujarrabal, A Castrocarrizo, J Alcolea, Sanchez C Contreras
    Abstract:

    We have studied the CO emission from protoplanetary nebulae (PPNe). Our sample is composed of 37 objects and includes, we think, all well identied PPNe detected in CO, together with the two yellow hypergiants emitting in CO and one young PN. We present a summary of the existing CO data, including accurate new observations of the 12 CO and 13 CO J = 1{0 and J = 2{1 lines in 16 objects. We identify in the nebulae a slowly expanding shell (represented in the spectra by a central core) and a fast outflow (corresponding to the line wings), that in the well studied PPNe is known to be bipolar. Excluding poor data, we end up with a sample of 32 Sources (including the 16 observed by us); fast flows are detected in 28 of these nebulae, being absent in only 4. We present a method to estimate from these data the mass, \scalar" Momentum and kinetic energy of the dierent components of the molecular outflows. We argue that the uncertainties of our method can hardly lead to signicant overestimates of these parameters, although underestimates may be present in not well studied objects. The total nebular mass is often as high as 1 M, and the mass-loss rate, that (presumably during the last stages of the AGB phase) originated the nebula, had typical values 10 4 M yr 1 . The Momentum corresponding to this mass ejection process in most studied nebulae is accurately coincident with the maximum Momentum that radiation pressure, acting through absorption by dust grains, is able to supply (under expected conditions). We estimate that this high-eciency process lasts about 1000{10 000 yr, after which the star has ejected a good fraction of its mass and the AGB phase ends. On the other hand, the fast molecular outflows, that have probably been accelerated by shock interaction with axial post-AGB jets, carry a signicant fraction of the nebular mass, with a very high Momentum (in most cases between 10 37 and 10 40 gc m s 1 ) and very high kinetic energy (usually between 10 44 and 10 47 erg). In general, yellow hypergiants and post-AGB objects with low initial mass show nebular masses and momenta that are, respectively, higher and lower than these values. We compare the momenta of the fast outflows with those that can be supplied by radiation pressure, taking into account the expected short acceleration times and some eects that can increase the Momentum transfer. We nd that in about 80% of PPNe, the fast molecular flows have too high momenta to be powered by radiation pressure. In some cases the Momentum of the outflow is1000 larger than that carried by radiation pressure; such high factors are dicult to explain even under exceptional conditions. Wind interaction is the basic phenomenon in the PN shaping from the former AGB envelopes; we conclude that this interaction systematically takes place along a dominant direction and that this process is not powered by radiation pressure. Due to the lack of theoretical studies, the possible Momentum Source remains a matter of speculation.

  • mass linear Momentum and kinetic energy of bipolar flows in protoplanetary nebulae
    Astronomy and Astrophysics, 2001
    Co-Authors: V Bujarrabal, A Castrocarrizo, J Alcolea, Sanchez C Contreras
    Abstract:

    We have studied the CO emission from protoplanetary nebulae (PPNe). Our sample is composed of 37 objects and includes, we think, all well identied PPNe detected in CO, together with the two yellow hypergiants emitting in CO and one young PN. We present a summary of the existing CO data, including accurate new observations of the 12 CO and 13 CO J = 1{0 and J = 2{1 lines in 16 objects. We identify in the nebulae a slowly expanding shell (represented in the spectra by a central core) and a fast outflow (corresponding to the line wings), that in the well studied PPNe is known to be bipolar. Excluding poor data, we end up with a sample of 32 Sources (including the 16 observed by us); fast flows are detected in 28 of these nebulae, being absent in only 4. We present a method to estimate from these data the mass, \scalar" Momentum and kinetic energy of the dierent components of the molecular outflows. We argue that the uncertainties of our method can hardly lead to signicant overestimates of these parameters, although underestimates may be present in not well studied objects. The total nebular mass is often as high as 1 M, and the mass-loss rate, that (presumably during the last stages of the AGB phase) originated the nebula, had typical values 10 4 M yr 1 . The Momentum corresponding to this mass ejection process in most studied nebulae is accurately coincident with the maximum Momentum that radiation pressure, acting through absorption by dust grains, is able to supply (under expected conditions). We estimate that this high-eciency process lasts about 1000{10 000 yr, after which the star has ejected a good fraction of its mass and the AGB phase ends. On the other hand, the fast molecular outflows, that have probably been accelerated by shock interaction with axial post-AGB jets, carry a signicant fraction of the nebular mass, with a very high Momentum (in most cases between 10 37 and 10 40 gc m s 1 ) and very high kinetic energy (usually between 10 44 and 10 47 erg). In general, yellow hypergiants and post-AGB objects with low initial mass show nebular masses and momenta that are, respectively, higher and lower than these values. We compare the momenta of the fast outflows with those that can be supplied by radiation pressure, taking into account the expected short acceleration times and some eects that can increase the Momentum transfer. We nd that in about 80% of PPNe, the fast molecular flows have too high momenta to be powered by radiation pressure. In some cases the Momentum of the outflow is1000 larger than that carried by radiation pressure; such high factors are dicult to explain even under exceptional conditions. Wind interaction is the basic phenomenon in the PN shaping from the former AGB envelopes; we conclude that this interaction systematically takes place along a dominant direction and that this process is not powered by radiation pressure. Due to the lack of theoretical studies, the possible Momentum Source remains a matter of speculation.

H Shirai - One of the best experts on this subject based on the ideXlab platform.

  • experimental determination of non diffusive toroidal Momentum flux in jt 60u
    Nuclear Fusion, 1994
    Co-Authors: K Nagashima, Y Koide, H Shirai
    Abstract:

    Toroidal Momentum transport is examined experimentally by using on- and off-axis tangential neutral beam injections on the JT-60U tokamak. From a steady state Momentum balance analysis-on the assumption that Momentum flux is diffusive-it is found that the profiles of the Momentum diffusivity are quite different in the two cases of on- and off-axis beam injections. In addition, transient toroidal Momentum transport was examined by using a Momentum Source modulation experiment. On the assumption that the toroidal Momentum flux consists of a diffusive and a convective flow term, it is found that there is non-diffusive inward flux of toroidal Momentum whose absolute value is comparable to that of the diffusive flux

Y Koide - One of the best experts on this subject based on the ideXlab platform.

  • Momentum transport and plasma rotation profile in toroidal direction in jt 60u l mode plasmas
    Nuclear Fusion, 2007
    Co-Authors: Minoru Yoshida, Y Koide, H Takenaga, H Urano, N Oyama, K Kamiya, Y Sakamoto, G Matsunaga, Y Kamada
    Abstract:

    The characteristics of Momentum transport and plasma rotation in the toroidal direction are studied, using near-perpendicular neutral beam injection (PERP-NBI), co tangential and counter (CTR) tangential NBI in JT-60U. Diffusive and non-diffusive terms of Momentum transport are evaluated from the transient analysis by using the Momentum Source modulation. Fast ion losses due to the toroidal field ripple, which locally induces the edge CTR rotation, are used as a novel Momentum Source. Parameter dependence of these transport coefficients i.e. the toroidal Momentum diffusivity (?) and the convection velocity (Vconv), and the relation between Momentum and heat diffusivities (?i) are investigated in L-mode plasmas systematically. The toroidal Momentum diffusivity increases with increasing heating power and decreases with increasing plasma current. The relation of ? and ?i to some non-dimensional parameters is investigated. A clear dependence of ?/?i on normalized plasma pressure (?N) is observed. It is also found that toroidal rotation velocity profiles in the case with and without external torque input can be almost reproduced by ? and Vconv estimated from the transient Momentum transport analysis at low ? (?N < 0.4).

  • experimental determination of non diffusive toroidal Momentum flux in jt 60u
    Nuclear Fusion, 1994
    Co-Authors: K Nagashima, Y Koide, H Shirai
    Abstract:

    Toroidal Momentum transport is examined experimentally by using on- and off-axis tangential neutral beam injections on the JT-60U tokamak. From a steady state Momentum balance analysis-on the assumption that Momentum flux is diffusive-it is found that the profiles of the Momentum diffusivity are quite different in the two cases of on- and off-axis beam injections. In addition, transient toroidal Momentum transport was examined by using a Momentum Source modulation experiment. On the assumption that the toroidal Momentum flux consists of a diffusive and a convective flow term, it is found that there is non-diffusive inward flux of toroidal Momentum whose absolute value is comparable to that of the diffusive flux