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1.
The fine structure of the quasar 3C 345 in polarized emission at 7 mm and 2 cm has been investigated. The kinematics is shown to correspond to an anticentrifuge: the thermal plasma of the surrounding space accretes onto the disk, flows to the center, and is ejected in the form of a rotating bipolar outflow that carries away the excess angular momentum as it accumulates. The bipolar outflow consists of a high-velocity central jet surrounded by a low-velocity component. The low-velocity flows are the rotating hollow tubes ejected from the peripheral part of the disk with a diameter ～Ø1 = 2.2 pc and from the region Ø2 = 1 pc. The high-velocity jet with a diameter Ø3 = 0.2 pc is ejected from the central part of the disk, while the remnant falls onto the forming central body. The ejection velocity of the high-velocity flow is v ? 0.06c. At a distance up to ～1 pc, the jet accelerates to an apparent velocity v ～ 8c. Further out, uniform motion is observed within ～2 pc following which deceleration occurs. The jet structure corresponding to a conical diverging helix with an increasing pitch is determined by gasdynamic instability. The counterjet structure is a mirror reflection of the nearby part of the jet. The brightness temperature of the fragment of the high-velocity flow at the exit from the counterjet nozzle is T b ≈ (1012?1013) K. The disk inclined at an angle of 60° to the plane of the sky shadows the jet ejector region. Ring currents observed in the tangential directions as parallel chains of components are excited in the rotating flows. The magnetic fields of the rotating bipolar outflow and the disk are aligned and oriented along the rotation axis. The translational motions of the jet and counterjet are parallel and antiparallel to the magnetic field, which determines their acceleration or deceleration. The quasar core is surrounded by a thermal plasma. The sizes of the HII region reach ～30 pc. The electron density decreases with increasing distance from the center from N e ≈ 108 to ≈105 cm?3. The observed emission from the jet fragments at the exit from the nozzle is partially absorbed by the thermal plasma, is refracted with increasing distance—moves with an apparent superluminal velocity, and decelerates as it goes outside the HII region. 相似文献
2.
The superfine structure of the jet formation region in the radio galaxy M87 has been investigated. An accretion disk and high-
and low-velocity jet and counterjet components have been identified. The high-velocity bipolar outflow is ejected from the
central disk region, a nozzle 4 mpc in diameter, while the low-velocity one is ejected from a ring 60 mpc in diameter and
14 mpc in width. The low-velocity plasma flow is a hollow tube with a built-in helix. The observed helical structure of the
high-velocity jet is determined by precession. The components of the structure, its disk and bipolar outflow, suggest solid-body
rotation. Ring currents and aligned magnetic fields are generated in them under the action of an external magnetic field.
The bipolar outflows are ejected coaxially but in opposite directions—along and opposite to the disk field. As a result, the
jet flow accelerates, while the counterjet one decelerates. This causes the extent of the region of radiative cooling of the
ejected relativistic electrons in the counterjet to decrease and maintains their “afterglow” at large distances in the jet.
The high collimation of the rotating flows is determined by their interaction with the environment. 相似文献
3.
We report the discovery of high-velocity dense gas from a bipolar outflow source near NGC 2068 in the L1630 giant molecular cloud. CO and HCO+ J =3→2 line wings have a bipolar distribution in the vicinity of LBS 17-H with the flow orientated roughly east–west and perpendicular to the elongation of the submillimetre dust continuum emission. The flow is compact (total extent ∼0.2 pc) and contains of the order of 0.1 M⊙ of swept-up gas. The high-velocity HCO+ emission is distributed over a somewhat smaller area <0.1 pc in extent.
A map of C18 O J =2→1 emission traces the LBS 17 core and follows the ambient HCO+ emission reasonably well, with the exception of the direction towards LBS 17-H where there is a significant anticorrelation between the C18 O and HCO+ . A comparison of beam-matched C18 O and dust-derived H2 column densities suggests that CO is depleted by up to a factor of ∼50 at this position if the temperature is as low as 9 K, although the difference is substantially reduced if the temperature is as high as 20 K. Chemical models of collapsing clouds can account for this discrepancy in terms of different rates of depletion on to dust grains for CO and HCO+ .
LBS 17-H has a previously known water maser coincident with it but there are no known near-infrared, IRAS or radio continuum sources associated with this object, leading to the conclusion that it is probably very young. A greybody fit to the continuum data gives a luminosity of only 1.7 L⊙ and a submillimetre-to-bolometric luminosity ratio of 0.1, comfortably satisfying the criteria for classification as a class 0 protostar candidate. 相似文献
A map of C
LBS 17-H has a previously known water maser coincident with it but there are no known near-infrared, IRAS or radio continuum sources associated with this object, leading to the conclusion that it is probably very young. A greybody fit to the continuum data gives a luminosity of only 1.7 L
4.
The fine structure of the nucleus of the Seyfert galaxy NGC 1275 was investigated in 2005–2010 at a wavelength of 2 cm with a resolution as high as 50 μas. The structure consists of two parallel identical systems, eastern and western, spaced 0.5 pc apart in the plane of the sky. Each of them contains an ejector and a bipolar outflow. There are extended regions, lobes, at the extension of the bipolar outflows in the ?10° and 170° directions at distances of 5 pc northward and 6.5 pc southward of the active zone. The observed difference between the jet and counterjet sizes by a factor of ~3 and between the distances to the lobes by a factor of 0.8 is determined by the difference between their velocities and by the change of sign of the outflow acceleration in the period of silence. The high-velocity bipolar outflows are surrounded by three pairs of low-velocity components. The diameters of the low-velocity coaxial outflows and the third component are Ø1 ≈ 0.3 pc, Ø2 ≈ 0.8 pc, and Ø3 ≈ 1.4 pc at the detection limit. The outer low-velocity components of the outflows encompass both high-velocity outflows. The velocities of the outflows and their brightness temperatures increase exponentially as the center of the high-velocity outflows is approached. The brightness temperatures of the high-velocity outflows at the ejector exit are T b > 1012 K. The spectral line velocities in the nuclear region differ by ~600 km s?1 due to the velocity difference between the two systems. In the case of Keplerian motion, the revolution period is ~5 × 103 yr, and the mass of the central massive bodies, black holes, is M ≈ 107M⊙. The fine structure suggests a vortical nature of the formation. In the case under consideration, two parallel vortices spaced ~0.5 pc apart and shifted by ~0.5 pc relative to each other were formed. The surrounding material inflows onto the disk of each system, is transferred in a spiral to the center, and is ejected in the ?10° and 170° directions as an excess angular momentum is accumulated. The interaction with the surrounding medium accelerates and collimates the rotating outflows. The residual material falls to the forming central massive body, a black hole, whose gravitational field stabilizes and accelerates the system formation process. 相似文献
5.
The fine core-jet structure of the radio galaxy M87 has been investigated in the millimeter-decimeter wave band. A counterjet
whose extent is ρ(λ) ≈ 0.036λ pc, where the wavelength λ is expressed in centimeters, has been identified. At a wavelength of 2 cm, the brightness of the jet and counterjet decreases
exponentially to the minimally detectable level. However, the decline for the jet slows down from a level of several percent
of the peak value. The bipolar jet consists of a highly collimated relativistic plasma flow surrounded by a nonrelativistic
low-velocity component. The low-velocity jet flow includes a helical component observable up to a distance of 20 mas or 1.6
pc. The reaction of the flow produces a multimode precession responsible for the helical shape of the relativistic jet with
a variable pitch and a curved axis. The helical structures of the jet and counterjet are mirror reflections of each other
relative to the ejector. The apparent size of the accretion disk seen edge-on reaches 1.5 mas or 0.12 pc. 相似文献
6.
The fine structure of the active region, the bulge, of the blazar OJ 287 has been investigated with a resolution of 20 μas (0.1 pc) at a wavelength of 7 mm, the epochs of 2007–2017. The structure and kinematics correspond to a vortex nature. The surrounding matter, the plasma, is transferred to the center along two arms from opposite directions. The emerging excess angular momentum is carried away along the rotation axis by bipolar outflows, rotating coaxial tubes, in a direction X ≈ ?120? in the plane of the sky as it is accumulated. The central high-velocity bipolar outflow has a helical shape. The diameters of the low-velocity flows are ø1 ≈ 0.3 and ø2 ≈ 0.65 mas, or 1.4 and 3 pc, respectively. Ring currents whose tangential directions are observed as parallel chains of components are excited in the flow walls. The peak brightness temperature of the nozzle reaches Tb ≈ 1012?1013 K. A “disk” with a diameter ø ≈ 0.5 mas (≈2.2 pc) is observed by the absorption of synchrotron radiation. The disk is inclined to the plane of the sky at an angle of 60? in the jet direction. The fragments are seen from a distance of ～0.2 mas outside the absorption zone. The jet sizes exceed considerably the counterjet ones. An enhanced supply of plasma from the northern arm gives rise to an independent vortex 0.2 mas away from the central one in the NW direction. As in the first case, the helical central bipolar outflow is surrounded by a low-velocity component ø ≈ 0.28 mas in diameter with built-in ring currents. The jet is ejected in the direction X = ?50? in the plane of the sky. The jet orientation changes, X = ?130? at a distance of 1 mas. A high activity of the central and two side nozzles spaced 0.22 mas apart in the direction X = ?40? is occasionally observed simultaneously. The active region of the blazar is observed through an ionized medium, a screen, whose influence is significant even at a wavelength of 7 mm. The absorption and refraction of the transmitted emission in the screen affect the apparent brightness relative to the positions of the fragments. 相似文献
7.
The superfine structure of the quasar 3C 273 has been investigated at wavelengths λ = 2 and 6 cm with angular resolutions up to φ = 20 μas for epochs 2005–2014. We have identified a nozzle and a bipolar outflow: a jet and a counterjet consisting of coaxial high- and low-velocity components. The separation between the nozzles in the plane of the sky is Δρ = 0.84 ± 0.16 pc; the flow ejection velocity is v ≤ 0.1c. The nozzle brightness temperature reaches T b ≈ 45 × 1012 K, φ = 20 μas, λ = 2 cm. The ejected electrons radiatively cool at a distance up to ≤4 pc. However, the jet afterglow is observed at a 8% level at a distance up to ρ ≈ 16 pc; the acceleration compensates for the radiative losses. The reduction in the emission level of the central flow at large distances determines the jet bifurcation. The counterjet shape is a mirror reflection of the initial part of the jet, suggesting a symmetry and identity of the ejected flows. The counterjet and jet nozzles are in the near and remote parts of the active region, respectively. The emission from the nozzles is absorbed by a factor of 2 and 15, respectively. The absorption decreases with increasing distance and the brightness of the jet fragments rises to its maximum at 0.5 pc from the nozzle. Arclike structures, arm fragments, are observed in the region of the nozzles. The relativistic plasma comes to the nozzles and is ejected. The brightness temperature of the arclike structures reaches 10% of the peak value, which is determined by the a smaller optical depth, the visibility in the transverse direction. The central high-velocity flow is surrounded by low-velocity components, hollow tubes being ejected as an excess angular momentum is accumulated. The remainder of the material flows along the arms toward the disk center until the next accumulation of an excess angular momentum and the process is repeated. The diameter of the outer nozzle is Ø = 25 pc and, further out, decreases exponentially; Ø n ≈ 80 exp(?1.15n) pc. The flow kinematics, collimation, and acceleration have a vortical nature. Ring currents producing magnetic fields, which accelerate and stabilize the processes, are generated in the rotating flows (tubes). The tangential directions of the currents are observed as parallel chains of components. 相似文献
8.
The structure of the AGN object 1803+784 has been investigated at a wavelength of 7 mm with a limiting angular resolution
reaching 20 μas. The ejector nozzle surrounded by a ring structure, an accretion disk, has been identified. The nozzle size
is ∼0.1 pc, the diameter of the ring structure is ∼1.4 pc, and its width is ∼0.25 pc. The reaction of the plasma flow produces
a multimode precession responsible for the conical helical structure of the jet with a variable step and a curved axis. The
viewing angle of the flow ejection is ∼40°. The central part of the ejected flow moving along the axis accelerates to a relativistic
velocity. The apparent velocity reaches 12 s at a distance of ∼1 mas or ∼6 pc from the ejector. The outer part of the flow
moves along a helix around a high-velocity component whose step is a factor of 4 smaller, because the longitudinal velocity
is relatively low. The plasma is ejected almost toward the observer, as confirmed by its high brightness temperature T
b ≈ 8 × 1013 K and highly beamed emission. The polarized emission from the nozzle is axisymmetric. The orientation of the polarization
of the flow along the whole length is aligned with the direction of its motion, suggesting the excitation of a ring magnetic
field around it and self-focusing. 相似文献
9.
We have continued our studies of the fine structure of the active region in the blazar OJ 287 at wavelength λ = 2cm with a resolution of 20 μas, the epochs of 1995–2017. We have identified fragments of two arms along which the surrounding plasma comes to the nozzle. The brightness temperature of the flows rises as the nozzle is approached to Tb ? 1012 K. The high-velocity bipolar outflow surrounded by lowvelocity components carries away an excess angular momentum as it is accumulated. The high collimation and helicity of the flows are determined by rotation and precession, respectively. Ring currents responsible for the longitudinal magnetic fields are excited in the flows. The jet and counterjet are a mirror reflection of each other; the difference in sizes is determined by the acceleration/deceleration of the flows along/opposite to the magnetic field. The velocity of the high-velocity outflow is v ? 0.06 c. The brightness temperature of the nozzle reaches Tb ? 1014 K. The spectral index of the southern and northern nozzles is α ≈ 0.66 and ≈0.4, respectively; the difference is determined by absorption in the bulge. The separation between the nozzles is 12 μas or 0.05 pc. The central region of reduced brightness with a diameter ? ≈ 3.6 pc corresponds to the bulge inclined toward the jet at an angle of 65° to the plane of the sky. The counterjet is ejected toward the observer; the jet is ejected in the opposite direction and is visible outside the bulge from a distance of 1.5 pc. The structure and kinematics of the bulge correspond to a vortex nature. An enhanced supply of matter from the northern arm in the middle of 2000 increased the activity of the low-velocity nozzle. A secondary vortex located at a distance of 0.28 mas (1.3 pc) was formed. The high-velocity flow is ejected in a direction of ?110°. 相似文献
10.
L. I. Matveyenko V. A. Demichev S. S. Sivakon’ Ph. D. Diamond D. A. Graham 《Astronomy Letters》2005,31(12):816-823
We analyze the superfine structure of the supermaser H2O emission region in Orion KL over the period 1979–1999. The angular resolution reached 0.1 mas, which corresponds to 0.045 AU at a distance to Orion KL of 450 pc. We determined the velocity of the local standard of rest, VLSR = 7.65 km s?1. The formation of a protostar is accompanied by a structure that consists of an accretion disk, a bipolar outflow, and a surrounding envelope. The disk is at the stage of separation into protoplanetary rings. The disk plane is warped like the brim of a hat. The disk is 27 AU in diameter and ～0.3 AU in thickness. The rings contain ice granules. Radiation and stellar wind sublimate and blow away the water molecules to form halos around the rings, maser rings. The radiation from the rings is concentrated in the azimuthal plane, and its directivity reaches 10?3. The relative velocities of the rings located in the central part of the disk 15 AU in diameter correspond to rigid-body rotation, Vrot = ΩR. The rotation period is T ≈ 170 yr. The injector is surrounded by a toroidal structure 1.2 AU in diameter. The diameter of the injected flow does not exceed 0.05 AU. A highly collimated bipolar outflow with a diameter of ～0.1 AU is observed at a distance as large as 3 AU. Precession of the injector axis with a period of ～10 yr forms a spiral flow structure. The flow velocity is ～10 km s?1. The kinetic energy of the accreting matter and the disk is assumed to be transferred to the bipolar outflow, causing the rotation velocity distribution of the rings to deviate from the Keplerian velocity. The surrounding envelope amplifies the emission from the structure at a velocity of 7.65 km s?1 in a band of ～0.5 km s?1 by more than two orders of magnitude, which determines the supermaser emission. 相似文献
11.
Mao Xin-jie 《Astrophysics and Space Science》1998,260(3):397-404
The configuration of the magnetic field associated with a protostar surrounded by a circumstellar disk is assumed to be a
kind of magnetic mirror, which reflects the particles at its throat located nearby the disk midplane, and then extracts them
out of the star and the disk. Turbulent Alfven waves are excited due to anisotropic temperature distribution caused by the
existing magnetic field in the environment. Accelerated by turbulent Alfven waves, the particles coming out of the young stellar
object and the circumstellar disk can reach the expected velocities around 300 km s-1 at a typical distance 0.1 pc from the central star. The wave energy is converted from the thermal energy stored in the system
consisting of the early stage star associated with the disk and their environment, and a small fraction of which is enough.
The coefficient η, indicating the efficiency of converting thermal energy to wave energy, is equal to 10-11.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
12.
Kurt Liffman 《Astrophysics and Space Science》2007,311(1-3):69-74
We discuss the star-disk electric circuit for a young stellar object (YSO) and calculate the expected torques on the star
and the disk. We obtain the same disk magnetic field and star-disk torques as given by standard magnetohydrodynamic (MHD)
analysis. We show how a short circuit in the star-disk electric circuit may produce a magnetically-driven jet flow from the
inner edge of a disk surrounding a young star.
An unsteady bipolar jet flow is produced that flows perpendicular to the disk plane. Jet speeds of order hundreds of kilometers
per second are possible, while the outflow mass loss rate is proportional to the mass accretion rate and is a function of
the disk inner radius relative to the disk co-rotation radius. 相似文献
13.
Gamma rays have been observed from two blazars at TeV energies. One of these, Markarian 421, has been observed also at GeV energies and has roughly equal luminosity per decade at GeV and TeV energies. Photon-photon pair production on the infrared background radiation is expected to prevent observation above 1 TeV. However, the infrared background is not well known and it may be possible to observe the nearest blazars up to energies somewhat below 100 TeV where absorption on the cosmic microwave background will give a sharp cut-off. Blazars are commonly believed to correspond to low power radio galaxies, seen down along a relativistic jet; as such they are all expected to have the nuclear activity encircled by a dusty molecular torus, which subtends an angle of 90 degrees or more in width as seen from the central source. Photon-photon pair production can also take place on the infrared radiation produced at the AGN by this molecular torus and surrounding outer disk. We calculate the optical depth for escaping γ-rays produced near the central black hole and at various points along the jet axis for the case of blazars where the radiation is observed in a direction closely aligned with the jet. We find that the TeV emission site must be well above the top of the torus. For example, if the torus has an inner radius of 0.1 pc and an outer radius of 0.2 pc, then the emission site in Mrk 421 would have be at least 0.25 pc above the upper surface of the torus, and if Mrk 421 is observed above 50 TeV in the future, the emission site would have to be at least 0.5 pc above the upper surface. This has important implications for models of γ-ray emission in active galactic nuclei. 相似文献
14.
Light-echo measurements show that SN 1987A is 425 pc behind the LMC disk. It is continuing to move away from the disk at 18 km s-1. Thus, it has been suggested that SN 1987A was ejected from the LMC disk. However, SN 1987A is a member of a star cluster, so this entire cluster would have to have been ejected from the disk. We show that the cluster was formed in the LMC disk, with a velocity perpendicular to the disk of about 50 km s-1. Such high-velocity formation of a star cluster is unusual, having no known counterpart in the Milky Way. 相似文献
15.
F.P. Wilkin 《Astrophysics and Space Science》2003,287(1-4):175-178
I examine the question of purely accreting protostars, and set limits to the breakout time of a protostellar wind within the accretion flow forming the new star. Hypothesizing a wind launched from the protostellar surface, three temporal phases are derived: a crushed wind, a trapped wind, and an escaping wind. In the current model, evolution from one phase to the next is a consequence of the growing anisotropy of the infalling flow, a natural outcome of the collapse of a rotating cloud core. During the crushed wind phase, infall overcomes the wind at all solid angles, and the accretion directly strikes the protostellar surface. The trapped phase consists of a wind sufficiently strong to push material back from the stellar surface, but too weak to carry the heavy, shocked and swept-up infall out of the star's gravitational potential. Unless the wind turns on impulsively, a significant fraction of the pre-breakout life of the protostar may be spent in this trapped wind phase in which gas is launched from the protostar but is pulled back, crashing onto the protostellar and disk surfaces. It may be that some `starless cores' contain as-yet undetected, very young accreting protostars, and that episodic luminosity fluctuations associated with this trapped wind could be observed. 相似文献
16.
Neal J. Evans II Shudong Zhou Carsten Kömpe C. M. Walmsley 《Astrophysics and Space Science》1994,212(1-2):139-145
The globular molecular cloud B335 contains a single, deeply embedded, far-infrared source. Our recent observations of H2CO and CS lines toward this source provide direct kinematic evidence for collapse. Both the intensity and detailed shape of the line profiles match those expected from inside-out collapse inside a radius of 0.036 pc. The collapse began about 1.5 × 105 years ago, similar to the onset of the outflow. The mass accretion rate is about 10 times the outflow rate, and about 0.4M
should have now accumulated in the star and disk. Because B335 rotates only very slowly, any disk would still be very small (about 3 AU). The accretion luminosity should be adequate to power the observed luminosity. Consequently, we believe that B335 is indeed a collapsing protostar.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A. 相似文献
17.
We present the first VLBI observations of the compact source S1 in the radio jet of NGC 1068. Roughly 1 pc in length and 0.2
pc wide, S1 resolves into clumps aligned perpendicular to the local radio jet axis. The radio continuum emission arises from
a hot (Te ~ 106 K), dense (ne ~ 106 cm-3) plasma, and the source of the radio emission is either thermal free-free emission
or Thomson-reflected synchrotron emission. The clouds comprising S1 are opaque to soft X-radiation, and we therefore propose
that S1 is the inner, ionized region, or ‘hot zone,’ of the obscuring medium surrounding the active nucleus. The covering
fraction of the hot zone is small, Cf ⋦ 10%. Since the covering fraction of the dusty, obscuring medium is probably much larger,
the obscuring disk must either flare or warp outside of the hot zone. That the radio jet and hot zone axes are perpendicular
also suggests that the hot zone may be considered to trace the outermost extent of the accretion disk.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
It is commonly accepted that stars form in molecular clouds by the gravitational collapse of dense gas. However, it is precisely not the infalling but the outflowing material that is primarily observed. Outflow motions prevail around both low and high mass young stellar objects. We present here results from a family of self-similar models that could possibly help to understand this paradox. The models take into account the heating of the central protostar for the deflection and acceleration of the gas. The models make room for all the ingredients observed around the central objects, i.e. molecular outflows, fast jets, accretion disks and infalling envelopes. We suggest that radiative heating and magnetic field may ultimately be the main energy sources driving outflows for both low and high mass stars. The models show that the ambient medium surrounding the jet is unhomogeneous in density, velocity, magnetic field. Consequently, we suggest that jets and outflows have a prehistory that is inprinted in their environment, and that this should have direct consequences on the setting of jet numerical simulations. 相似文献
19.
Jill M. Rathborne James M. Jackson Robert Simon Qizhou Zhang 《Astrophysics and Space Science》2009,324(2-4):155-162
Infrared dark clouds (IRDCs) are cold, dense molecular clouds identified as extinction features against the bright mid-infrared Galactic background. Our recent 1.2 mm continuum emission survey of IRDCs reveals many compact (<0.5 pc) and massive (10–2100 M⊙) cores within them. These prestellar cores hold the key to understanding IRDCs and their role in star formation. Here, we present high angular resolution spectral-line and mm/sub-mm continuum images obtained with the IRAM Plateau de Bure Interferometer and the Sub-Millimeter Array towards three high-mass IRDC cores. The high angular resolution images reveal that two of the cores are resolved into multiple, compact protostellar condensations, while the remaining core contains a single, compact protostellar condensation with a very rich molecular spectrum, indicating that it is a hot molecular core. The derived gas masses for these condensations suggest that each core is forming at least one high-mass protostar, while two of the cores are also forming lower-mass protostars. The close proximity of multiple protostars of disparate mass indicates that these IRDCs are in the earliest evolutionary states in the formation of stellar clusters. 相似文献
20.
Gravitational stability of gaseous protostellar disks is relevant to theories of planetary formation. Stable gas disks favor formation of planetesimals by the accumulation of solid material; unstable disks allow the possibility of direct condensation of gaseous protoplanets. We present the results of numerical experiments designed to test the stability of thin disks against large-scale, self-gravitational disruption. The disks are represented by a distribution of about 6 × 104 point masses on a two-dimensional (r, φ) grid. The motions of the particles in the self-consistent gravity field are calculated, and the evolving density distributions are examined for instabilities. Two parameters that have major influences on stability are varied: the initial temperature of the disk (represented by an imposed velocity dispersion), and the mass of the protostar relative to that of the disk. It is found that a disk as massive as 1M⊙, surrounding a 1M⊙ protostar, can be stable against long-wavelength gravitational disruption if its temperature is about 300°K or greater. Stability of a cooler disk requires that it be less massive, but even at 100°K a stable disk can have an appreciable fraction () of a solar mass. 相似文献