The physical processes that control the thermal structure of lightly ionized winds from cool prot... more The physical processes that control the thermal structure of lightly ionized winds from cool protostars are discussed. Attention is concentrated on the hydrogen gas, and the heating, cooling, and chemical processes that affect the neutral and ionic species of atomic and molecular hydrogen are examined. Warm silicate dust may condense out of the cooling wind and may heat the gas through collisions. Singly ionized sodium atoms, which do not recombine for the mass-loss rates considered, set a lower limit to the ionization fraction in the wind. Magnetic fields, which are presumed to accelerate the wind, couple directly to the ionic component of the gas and transfer momentum and energy to the neutral component through collisions. This process of ambipolar diffusion is found to be the dominant source of heat input to the gas.
Proceedings of the International Astronomical Union, 1997
A magnetocentrifugally driven X-wind from the inner edge of the solar nebula around the young sun... more A magnetocentrifugally driven X-wind from the inner edge of the solar nebula around the young sun may process dustballs into the CAIs and chondrules found in chondritic meteorites (\cite{Shu-Shang-Lee). Time variability of the magnetic configuration could trigger flares, accelerating cosmic rays that spallogenically produce the short-lived radionuclides inferred for CAIs and chondrules.
The Journal of the Royal Astronomical Society of Canada Royal Astronomical Society of Canada, Apr 29, 1993
Newly formed stars produce sufficient Lyman continuum luminosity phi to significantly alter the s... more Newly formed stars produce sufficient Lyman continuum luminosity phi to significantly alter the structure and evolution of the accretion disk surrounding them. In the absence of a stellar wind, a nearly static, photoionized, 10(4) K, disk atmosphere, with a scale height that increases with r(3/2) , forms inside the gravitational radius r_g ~ 10(14) (M_* / M_⊙) cm where M_* is the mass of the central star. This ionized atmosphere is maintained by both the direct radiation from the central star and the diffuse field produced in the disk atmosphere by the significant fraction of hydrogen recombinations directly to the ground state. Beyond r_g the material evaporated from the disk is capable of escaping from the system and produces an ionized disk wind. The mass-loss due to this disk wind peaks at r_g. The inclusion of a stellar wind into the basic picture reduces the height of the inner disk atmosphere and introduces a new scale radius r_w where the thermal pressure of the material evaporated from the disk balances the ram pressure in the wind. In this case the mass-loss due to the disk wind peaks at r_w and is enhanced over the no-wind case. The photoevaporation of disks around newly formed stars has significance to both the UCHII problem and the dispersal of solar-type nebulae. High mass stars are intrinsically hot and thus yield sufficient Lyman luminosity to create disk mass-loss rates of order 2 times 10(-5) phi_ {49}(1/2) M_⊙ yr(-1) , where phi_ {49} = phi /(10(49) Lyman continuum photons s(-1) ) even without a stellar wind. This wind which will last for ~ 10(5) yrs if the disk mass is M_d ~ 0.3 M_*, yields sizes, emission measures and ages consistent with observations of UCHIIs. On the other end of the stellar scale, many newly formed low-mass stars are known to have enhanced extreme ultraviolet luminosity suggested to be due to boundary layer accretion. Assuming that the sun had such an enhanced Lyman luminosity phi ~ 10(41) s(-1) , for ~ 3times 10(7) yrs it is possible to remove all of the gas beyond the orbit of Saturn, r_g for the sun, associated with the minimum solar nebula. This process also has implications for the formation of the giant planets.
... For a typical massive star, the strong-wind case applies, and the observed physical size of t... more ... For a typical massive star, the strong-wind case applies, and the observed physical size of the UCHU region is of order 2r~ 1O'6lO1Tcm, assuming the disk extends to those kinds of distances. ... Beckwith, SVW and Sarget, AI 1992, ApJ, in press. ...
This paper presents a simple physical model for the bipolar molecular outflows that frequently ac... more This paper presents a simple physical model for the bipolar molecular outflows that frequently accompany star formation. The model forges an intrinsic link between the bipolar flow phenomenon and the process of star formation, and it helps to explain many of the systematics known for existing sources.
We use x-winds heated and ionized by the physical mechanisms discussed in earlier communications ... more We use x-winds heated and ionized by the physical mechanisms discussed in earlier communications by our group to model the free-free emission at centimeter wavelengths of the jets from young stellar objects (YSOs). With stellar parameters, mass-loss rates, and x-ray luminosities characteristic of Class 0 or Class I YSOs, we recover realistic spectral indices and radio maps. In particular, the 3.6 cm flux in models with mass loss rates ˜ 10-6 M⊙/yr or higher are comparable to existing observations of radio jets at 0".1 resolution. Lower mass-loss rates characteristic of Class II YSOs with revealed optical jets generally produce radio jets that are too weak to be detected at current sensitivity levels. We discuss the implications of these comparisons for theoretical models of the YSO outflow phenomenon.
Evidence for a moonlet belt in the region between Saturn's close-in moonrings Pandora and Pr... more Evidence for a moonlet belt in the region between Saturn's close-in moonrings Pandora and Prometheus is discussed. It is argued that little-known observations of magnetospheric electron density by Pioneer 11 imply substantial, ongoing injections of mass into the 2000 km region which surrounds the F ring. A hypothesis is presented that these events result naturally from interparticle collisions between the smaller members of an optically thin belt of moonlets. Also discussed is work on Uranus ring structure and photometry, image processing and analysis of the Jonian ring strucure, photometric and structural studies of the A ring of Saturn, and improvements to an image processing system for ring studies.
Proceedings of the International Astronomical Union
Models of molecular cloud cores have been proposed by Frank Shu and Zhi-Yun Li. These models have... more Models of molecular cloud cores have been proposed by Frank Shu and Zhi-Yun Li. These models have been used as initial states for magnetohydrodynamic collapse calculations to study the dynamics of the early stages of star formation. Implications about accretion rates outflows and angular momentum transport will be summarized. Synthetic maps and spectra generated from simulation data will be shown.
pp. 558–587. 11. Materials and methods are available as supporting material on Science Online. 12... more pp. 558–587. 11. Materials and methods are available as supporting material on Science Online. 12. Y. Amelin, A. N. Krot, I. D. Hutcheon, A. A. Ulyanov
We will review current ideas concerning the formation of sunlike stars and planetary systems. In ... more We will review current ideas concerning the formation of sunlike stars and planetary systems. In particular, we discuss the idea that the chondrules and CAIs found in chondritic meteorites might have been formed as solids entrained and melted in the bipolar wind that results from the interaction of the accreting protosolar nebula and the magnetosphere of the young protosun. Aerodynamic sorting and a mechanical selection for molten droplets that rain back onto the disk at planetary distances explain the size distributions and patterns of element segregation that we observe in carbonaceous and ordinary chondrties. Cosmic ray ions generated in the flares that accompany the general magnetic activity of the inner region may irradiate the precursor rocks before they are launched in the bipolar wind. Under reasonable scaling assumptions for the efficiency of the process in protostars, cosmic-ray bombardment suffices to generate the short-lived radionuclides (26) Al, (41) Ca, and (53) Mn at...
Ambipolar diffusion and X-ray interactions have the potential to heat the inner part (or jet) of ... more Ambipolar diffusion and X-ray interactions have the potential to heat the inner part (or jet) of the X-wind. We have recalculated the ambipolar diffusion coupling coefficient for atomic regions (where H+ is the dominant ion) using recently calculated cross sections by Krstíc and Schultz and by others. The coefficient for hot regions (T ≈ 104 K) is an order of magnitude larger than the values used in the past because the short-range part of the H-H+ interaction (including exchange effects) is larger than previously assumed. Consequently ambipolar diffusion plays only a minor role in heating the inner part of the X-wind and very likely in YSO jets in general. We also find that X-ray heating is enhanced in atomic regions through the significant probability for populating excited levels of the H atom in collisions with X-ray induced secondary electrons (e.g., Dalgarno, Yan, and Liu 1999). The standard Lyman-alpha cooling must then be reduced by an amount that can be greater than direct ...
We calculate free-free emission at centimeter wavelengths from jets from Young Stellar Objects (Y... more We calculate free-free emission at centimeter wavelengths from jets from Young Stellar Objects (YSOs). We use the x-wind model which is heated and ionized by several physical mechanisms, being of primary importance ionization by x-rays and heating by dissipation of mechanical energy (Shang et al. 2002). With stellar parameters, mass-loss rates, and x-ray luminosities characteristic of Class 0 or Class I YSOs, we recover realistic radio spectral indices and radio maps. In particular, the 3.6 cm flux in models with mass loss rates ˜ 10-6 Msun/yr are comparable to existing observations of radio jets at 0''.1 resolution. Lower mass-loss rates characteristic of Class II YSOs with revealed optical jets generally produce radio jets that are too weak to be detected at current sensitivity levels. We discuss the implications of these comparisons for theoretical models of the YSO outflow phenomenon.
The physical processes that control the thermal structure of lightly ionized winds from cool prot... more The physical processes that control the thermal structure of lightly ionized winds from cool protostars are discussed. Attention is concentrated on the hydrogen gas, and the heating, cooling, and chemical processes that affect the neutral and ionic species of atomic and molecular hydrogen are examined. Warm silicate dust may condense out of the cooling wind and may heat the gas through collisions. Singly ionized sodium atoms, which do not recombine for the mass-loss rates considered, set a lower limit to the ionization fraction in the wind. Magnetic fields, which are presumed to accelerate the wind, couple directly to the ionic component of the gas and transfer momentum and energy to the neutral component through collisions. This process of ambipolar diffusion is found to be the dominant source of heat input to the gas.
Proceedings of the International Astronomical Union, 1997
A magnetocentrifugally driven X-wind from the inner edge of the solar nebula around the young sun... more A magnetocentrifugally driven X-wind from the inner edge of the solar nebula around the young sun may process dustballs into the CAIs and chondrules found in chondritic meteorites (\cite{Shu-Shang-Lee). Time variability of the magnetic configuration could trigger flares, accelerating cosmic rays that spallogenically produce the short-lived radionuclides inferred for CAIs and chondrules.
The Journal of the Royal Astronomical Society of Canada Royal Astronomical Society of Canada, Apr 29, 1993
Newly formed stars produce sufficient Lyman continuum luminosity phi to significantly alter the s... more Newly formed stars produce sufficient Lyman continuum luminosity phi to significantly alter the structure and evolution of the accretion disk surrounding them. In the absence of a stellar wind, a nearly static, photoionized, 10(4) K, disk atmosphere, with a scale height that increases with r(3/2) , forms inside the gravitational radius r_g ~ 10(14) (M_* / M_⊙) cm where M_* is the mass of the central star. This ionized atmosphere is maintained by both the direct radiation from the central star and the diffuse field produced in the disk atmosphere by the significant fraction of hydrogen recombinations directly to the ground state. Beyond r_g the material evaporated from the disk is capable of escaping from the system and produces an ionized disk wind. The mass-loss due to this disk wind peaks at r_g. The inclusion of a stellar wind into the basic picture reduces the height of the inner disk atmosphere and introduces a new scale radius r_w where the thermal pressure of the material evaporated from the disk balances the ram pressure in the wind. In this case the mass-loss due to the disk wind peaks at r_w and is enhanced over the no-wind case. The photoevaporation of disks around newly formed stars has significance to both the UCHII problem and the dispersal of solar-type nebulae. High mass stars are intrinsically hot and thus yield sufficient Lyman luminosity to create disk mass-loss rates of order 2 times 10(-5) phi_ {49}(1/2) M_⊙ yr(-1) , where phi_ {49} = phi /(10(49) Lyman continuum photons s(-1) ) even without a stellar wind. This wind which will last for ~ 10(5) yrs if the disk mass is M_d ~ 0.3 M_*, yields sizes, emission measures and ages consistent with observations of UCHIIs. On the other end of the stellar scale, many newly formed low-mass stars are known to have enhanced extreme ultraviolet luminosity suggested to be due to boundary layer accretion. Assuming that the sun had such an enhanced Lyman luminosity phi ~ 10(41) s(-1) , for ~ 3times 10(7) yrs it is possible to remove all of the gas beyond the orbit of Saturn, r_g for the sun, associated with the minimum solar nebula. This process also has implications for the formation of the giant planets.
... For a typical massive star, the strong-wind case applies, and the observed physical size of t... more ... For a typical massive star, the strong-wind case applies, and the observed physical size of the UCHU region is of order 2r~ 1O'6lO1Tcm, assuming the disk extends to those kinds of distances. ... Beckwith, SVW and Sarget, AI 1992, ApJ, in press. ...
This paper presents a simple physical model for the bipolar molecular outflows that frequently ac... more This paper presents a simple physical model for the bipolar molecular outflows that frequently accompany star formation. The model forges an intrinsic link between the bipolar flow phenomenon and the process of star formation, and it helps to explain many of the systematics known for existing sources.
We use x-winds heated and ionized by the physical mechanisms discussed in earlier communications ... more We use x-winds heated and ionized by the physical mechanisms discussed in earlier communications by our group to model the free-free emission at centimeter wavelengths of the jets from young stellar objects (YSOs). With stellar parameters, mass-loss rates, and x-ray luminosities characteristic of Class 0 or Class I YSOs, we recover realistic spectral indices and radio maps. In particular, the 3.6 cm flux in models with mass loss rates ˜ 10-6 M⊙/yr or higher are comparable to existing observations of radio jets at 0".1 resolution. Lower mass-loss rates characteristic of Class II YSOs with revealed optical jets generally produce radio jets that are too weak to be detected at current sensitivity levels. We discuss the implications of these comparisons for theoretical models of the YSO outflow phenomenon.
Evidence for a moonlet belt in the region between Saturn's close-in moonrings Pandora and Pr... more Evidence for a moonlet belt in the region between Saturn's close-in moonrings Pandora and Prometheus is discussed. It is argued that little-known observations of magnetospheric electron density by Pioneer 11 imply substantial, ongoing injections of mass into the 2000 km region which surrounds the F ring. A hypothesis is presented that these events result naturally from interparticle collisions between the smaller members of an optically thin belt of moonlets. Also discussed is work on Uranus ring structure and photometry, image processing and analysis of the Jonian ring strucure, photometric and structural studies of the A ring of Saturn, and improvements to an image processing system for ring studies.
Proceedings of the International Astronomical Union
Models of molecular cloud cores have been proposed by Frank Shu and Zhi-Yun Li. These models have... more Models of molecular cloud cores have been proposed by Frank Shu and Zhi-Yun Li. These models have been used as initial states for magnetohydrodynamic collapse calculations to study the dynamics of the early stages of star formation. Implications about accretion rates outflows and angular momentum transport will be summarized. Synthetic maps and spectra generated from simulation data will be shown.
pp. 558–587. 11. Materials and methods are available as supporting material on Science Online. 12... more pp. 558–587. 11. Materials and methods are available as supporting material on Science Online. 12. Y. Amelin, A. N. Krot, I. D. Hutcheon, A. A. Ulyanov
We will review current ideas concerning the formation of sunlike stars and planetary systems. In ... more We will review current ideas concerning the formation of sunlike stars and planetary systems. In particular, we discuss the idea that the chondrules and CAIs found in chondritic meteorites might have been formed as solids entrained and melted in the bipolar wind that results from the interaction of the accreting protosolar nebula and the magnetosphere of the young protosun. Aerodynamic sorting and a mechanical selection for molten droplets that rain back onto the disk at planetary distances explain the size distributions and patterns of element segregation that we observe in carbonaceous and ordinary chondrties. Cosmic ray ions generated in the flares that accompany the general magnetic activity of the inner region may irradiate the precursor rocks before they are launched in the bipolar wind. Under reasonable scaling assumptions for the efficiency of the process in protostars, cosmic-ray bombardment suffices to generate the short-lived radionuclides (26) Al, (41) Ca, and (53) Mn at...
Ambipolar diffusion and X-ray interactions have the potential to heat the inner part (or jet) of ... more Ambipolar diffusion and X-ray interactions have the potential to heat the inner part (or jet) of the X-wind. We have recalculated the ambipolar diffusion coupling coefficient for atomic regions (where H+ is the dominant ion) using recently calculated cross sections by Krstíc and Schultz and by others. The coefficient for hot regions (T ≈ 104 K) is an order of magnitude larger than the values used in the past because the short-range part of the H-H+ interaction (including exchange effects) is larger than previously assumed. Consequently ambipolar diffusion plays only a minor role in heating the inner part of the X-wind and very likely in YSO jets in general. We also find that X-ray heating is enhanced in atomic regions through the significant probability for populating excited levels of the H atom in collisions with X-ray induced secondary electrons (e.g., Dalgarno, Yan, and Liu 1999). The standard Lyman-alpha cooling must then be reduced by an amount that can be greater than direct ...
We calculate free-free emission at centimeter wavelengths from jets from Young Stellar Objects (Y... more We calculate free-free emission at centimeter wavelengths from jets from Young Stellar Objects (YSOs). We use the x-wind model which is heated and ionized by several physical mechanisms, being of primary importance ionization by x-rays and heating by dissipation of mechanical energy (Shang et al. 2002). With stellar parameters, mass-loss rates, and x-ray luminosities characteristic of Class 0 or Class I YSOs, we recover realistic radio spectral indices and radio maps. In particular, the 3.6 cm flux in models with mass loss rates ˜ 10-6 Msun/yr are comparable to existing observations of radio jets at 0''.1 resolution. Lower mass-loss rates characteristic of Class II YSOs with revealed optical jets generally produce radio jets that are too weak to be detected at current sensitivity levels. We discuss the implications of these comparisons for theoretical models of the YSO outflow phenomenon.
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