| Related sites for http://www.phys.lsu.edu/astro/nap98/bf.final.html |
| Scientific_American_Explorations__A_Parade_of_New_Planets Sightings of extrasolar planets are increasing. | | Solar_and_Stellar_Seismology Explains the study of a star's oscillations to determine the interior chemical composition. | | Space_Telescope_Science_Institute A small Hubble telescope image gallery of Quasars. | | SpaceWander_Virtual_Space_Trip! Multimedia experience of flying by spaceship through the solar system. Visit each planet. Real space pictures from NASA. In English, French, Italian, Spanish and Russian. Recommended for kids. | | Spectra_and_Spectra_Analysis__Stars Includes a section on spectral classification of stars. | | Star_Data_Pages Provides detailed information about stars: positions, proper motion, radial velocity, magnitudes, spectral class, color index, catalogue references, data about multiple star systems (i.e. orbital elem | | Star_Journey_@_nationalgeographic_com Star Journey. Journey through the stars with National Geographic Online. View the nighttime sky with popular National Geographic Star Chart... | | Star_Maps Three high resolution star maps in .tif format for download. Tycho, Hipparcos and Yale bright star maps available. | | Stars_and_Galaxies Behavior of stars, generation of energy, their origin, and life cycle. Also includes a wide range of interesting pictures of galaxies and related phenomena that can be observed in the universe through | | Summer_Sky_Tour A guide to give a tour of the summer night sky. | | White_Dwarfs,_Neutron_Stars,_And_Black_Holes All-text page on the various stages in the discovery and development of theories of white dwarfs, neutron stars and black holes. | | INTAS An international association formed by EU member states and others to promote East-West Scientific co-operation. | | Mepielan Mediterranean Programme for International Environmental Law and Negotiation. Aims to foster an integrated approach to environmental governance regimes in the Mediterranean area. | | NATO_Science_Program Offers support for international collaboration between scientists from countries of the Euro-Atlantic Partnership Council and the Mediterranean Dialogue. Information about scholarships, fellowships, | | The_Digit_Zero Some information about the number zero and links to other pages. | | Zero A history of Zero that describes the introduction of zero and the main persons that brought advantages to it. | | Zero_in_Four_Dimensions A paper discussing the historical, psychological, cultural, and logical perspectives of the number zero. | | Annual_International_Symposium_on_Macrocyclic_Chemistry Provides information on upcoming symposia and details organizing committee members. Includes archived information from past meetings. | | ChemCenter_Meeting_Locator Global database of contact info on meetings, workshops, short courses, and symposia related to chemistry. | | Chemistry_Conferences Searchable and browsable database of chemistry conferences worldwide; free event submission by users. | | Chemsoc_Conference_and_Events_Database A comprehensive, fully searchable database of events, large and small, around the world. A free online submission form allows organizers to publicise their events free of charge. | | Inorganic_Chemistry_Conference_2007 Club Mykonos, Western Cape, South Africa, 8 - 12 July 2007. Lists programme and gives details of abstract submission. | | PREP_Symposia_Series 3 - 6 June 2007, Baltimore, Maryland. International symposium, exhibit and workshops on preparative/process chromatography, ion exchange, adsorption/desorption processes, and related techniques. | | 1st_Georgian_Bay_International_Conference_on_Bioinorganic_Chemistry 22 - 26 May 2007, Parry Sound, Ontario, Canada. Bioinorganic chemistry of the porphyrinoids, metallothioneins, and of other systems of bioinorganic interest. | | Synthetic_Heterocyclic_Chemistry Barcelona, Spain, 12 - 13 April 2007. Includes online registration. | | 15th_European_Symposium_on_Organic_Chemistry University College Dublin, 8 - 13 July 2007. Describes provisional programme, includes call for abstracts. | | The_Waterborne_Symposium 14 - 16 February 2007, New Orleans, Louisiana. 34th annual symposium on polymer coatings topics. Gives registration information. | | The_Astronomer\'s_Telegrams For reporting and commenting on new astronomical observations of transient sources. | | About_Astronomy_&_Space_-_Two_Architectures_Chosen_for_Terrestrial_Planet_Finder Discover NASA's plan for advanced telescope searches for Earth-like planets around other stars. Learn about 2 separate missions with distinct designs to achieve the goal. | | Anglo-Australian_Planet_Search Long-term southern hemisphere program being carried with the 3.9 m Anglo-Australian Telescope. | | Arizona_Search_for_Planets_(ASP) Full scale survey project for extra-solar planets. Overview and publications. | | The_Big_Occulting_Steerable_Satellite_(project) Mission designed to provide improved resolution of closely spaced objects with comparable brightnesses (binary stars, microlensing events) and to facilitate separation of dim objects from nearby brig | | California_and_Carnegie_Planet_Search The site of California and Carnegie program for extrasolar planet search. | | Darwin_Space_Inferometer_Project_(ESA_SCIENCE) Overview and updates on the ESA's Earth-like planet hunting space-telescope project. | | Eddington_(ESA_SCIENCE) ESA space-telescope designed to seek out extrasolar planets and investigate the internal structure of stars. (Mission cancelled Nov. 2003) | | European_Extra-Solar_Planet_Consortium EXPORT is a consortium of European astronomers using the telescopes on La Palma and Tenerife to study extra-solar planets, as well as the formation and evolution of protoplanetary systems. | | ExNPS_Road_Map_-_August_1996 NASA Origins program report on the progress and future plans for the discovery of extrasolar planetary systems. | | Extrasolar_Group_Study Planet-finding assessment by the School of Physics and Astronomy, University of Birmingham 2004. | | Extrasolar_Planet_Detection_with_the_AFOE Extrasolar Planet Detection with the AFOE The Advanced Fiber-Optic Echelle (AFOE) spectrometer is a fiber-fed, bench-mounted echelle spectrograph, located at the 1.5m telescope of the Whipple Observa | | The_Extrasolar_Planets_Encyclopedia The reference site for astronomer involved in the search for extrasolar planets. |
|
THE FORMATION OF BINARY STARS THE FORMATION OF COMMON-ENVELOPE, PRE-MAIN-SEQUENCE BINARY STARSJ.E. TOHLINE, J.E. CAZES, AND H.S. COHLLouisiana State UniversityDepartment of Physics & Astronomy,202 Nicholson Hall,Baton Rouge, LA 70803-4001 U.S.A.1. IntroductionRecent observational investigations of the frequency of occurrenceof pre-main-sequence binary stars have reinforced earlier suspicions that ''binary formation is the primary branch of the star-formation process'' (Mathieu 1994). As Bodenheimer et al. (1993) have reviewed, a number of different theories have been proposed to explain the preponderance of binary stars. Klein et al. (1998) show how the directfragmentation of protostellar gas clouds may occur in early phases of collapse (at cloud densities n = 103 - 1010 cm-3). But at higher densities, clouds are unable to cool efficiently upon contraction. Consequently, direct fragmentation becomes problematical. Because higher mean densities are associated with systems having shorter dynamical times, one is led to consider mechanisms other than direct cloud fragmentation for forming binary systems with orbital periods less than a few hundred years. Here we investigate whether such binaries can formby spontaneous fission of rapidly rotating protostars.2. The Classical Fission HypothesisAs Chandrasekhar (1969) has reviewed (see also Durisen & Tohline 1985), if protostellar objects are assumed to be self-gravitating, incompressible fluids with uniform vorticity, one can show analytically that their allowed equilibrium configurations are defined by spheroids or ellipsoids. Classically, models describing the slow contraction of rotating protostellar gas clouds have been formulated around such analyticallyprescribable equilibrium configurations. For example, a large, slowly rotating gas cloud with a relatively small ratio of rotational to gravitational potential energy T/|W| will resemble a Maclaurin spheroid. As it contracts conserving angular momentum and mass,its evolution will proceed along the Maclaurin sequence through progressively flatter configurations of higher T/|W|. At a sufficiently high T/|W|, one finds that the axisymmetric configuration is no longer the lowest energy state available. Instead, there is an ellipsoidal configuration to which the gas cloud will prefer to evolve.Furthermore, if one follows evolution alonga more and more distorted ellipsoidal sequence (such as the Jacobisequence or any one of the Riemann sequences), one finds that eventuallyother configurations with even higher order surface distortions becomeenergetically favorable. For example (see Fig. 3 of Durisen &Tohline 1985), there is a ''dumbbell-binary sequence'' that branchessmoothly off of the Jacobi ellipsoid sequence. One might imagine, therefore,that binary stars form from the slow contraction of a rapidly rotatinggas cloud along the Maclaurin, then Jacobi (or Riemann), then dumbbell-binary sequences. In reality, the picture is not this clear. Most significantly, detailed work on ellipsoidal figures of equilibrium has only been completed for incompressible fluid systems. It is not at all clear to what extent the results carry over to more realistic structures having compressible equations of state.3. Problems Promoting the Fission Hypothesis While examining the structure of rotatinggas clouds that form the compressible analogues of Maclaurin spheroids, Ostriker & Bodenheimer (1968) showed thatmodels with reasonable degrees of compressibility must incorporate a significant degree of differential rotationif they are to possess reasonably high values of T/|W| and, therefore, be physically interesting in the context of the fission hypothesis. Employing 3D numerical hydrodynamics techniques, Durisen et al. (1986), Williams & Tohline (1988), and Houser et al. (1994) have examined the relative stability of rapidly rotating, compressible gas clouds that are initially in axisymmetric equilibrium but which reside just past a critical bifurcation point along the axisymmetric sequence according to Ostriker & Bodenheimer (1968). Invariably these simulations have shown that models with T/|W| > 0.27 are dynamically unstable toward the growth of a nonaxisymmetric deformationbut, unlike their uniformly rotating, incompressible counterparts, the eigenmode to which these structures appear to be unstable has a spiral character. Movie1  Quicktime (5,907K) Employing a significantly improved finite-difference simulation code and improved spatial resolution (1283 grid zones), we recently have repeated the simulation that was first reported in Durisen et al. (1986). Movie1 shows the nonlinear development of the two-armed, spiral-mode instability.The evolution is shown in the inertial reference frame and covers 20 central initial rotation periods. Each frame of Movie1 displays four nested isodensity contours at r/rmax = 0.8, 0.4, 0.04, and 0.004. Via the trailing spiral structure, gravitational torques are able to effectively redistribute angular momentum on a dynamical time scale; a relatively small amount of material is shed into an equatorial disk (this disk material is not visible in Movie1 because rdisk < 0.004 rmax);and the central object (containing most of the initial object's mass) settles down into a new equilibrium configuration. Clearly, evolution to a binary star system as suggested by the classical fission hypothesis does not occur.It is primarily because simulations of this type have not produced a binary star system that the classical fission hypothesis haslost favor within the star formation community over the past decade(Bodenheimer et al. 1993). 4. Fission Hypothesis RevivedInterestingly, the instability illustrated by Movie1 produces a final steady-state object (hereafter referred to as the ''final bar'') that is dynamically stable, has a T/|W| = 0.25, and has a decidedly nonaxisymmetric structure. In many respects this final bar appearsto be a compressible analog of a Riemann ellipsoid but, as Movie2 illustrates, the configuration possesses nontrivial internal motions. In the first frame of Movie2, 108 test particles have been lined up along the major axis of the final bar. Thereafter the particles are followed as they move along equatorial-plane streamlines of the flow, as viewed in a frame of reference that is rotating with the overall pattern speed of the final bar. The illustrated flow is entirely prograde and largely differential, but there is a smallvolume near the center of the configuration that is moving harmonically.In Movie3 we illustrate the 3D flow-field of the final bar. In the first frame of Movie3, a vertical sheet of test particles has been alligned with the major axis of the final bar. The subsequentmotion of these particles illustrates that there is relatively little vertical fluid motion and, although it varies with R and q, the angular velocityw(x) is almost independent of z.It may be possible, therefore, to understand thisand similar systems in terms of the properties of simpler, 2D nonaxisymmetric structures. Movie2  Quicktime (5,747K) Movie3  Quicktime (3,376K) Andalib (1998) recently has developed a self-consistent-field technique that can be used to Movie4  Quicktime (6,927K) construct equilibrium models of infinitesimally thin, self-gravitating gaseous disks with (a) compressible equations of state, (b) nonaxisymmetric structures, and (c) nontrivial internal motions. By demandingthat the disks have uniform vortensity (defined as the ratio of vorticity to mass density),Andalib has successfully constructed equilibrium disks with polytropic indices 0 < n < 1.3 and minor-to-major axis ratiosin the range 0.06 < b/a < 0.80. Movie4 illustrates the internal flow of four of Andalib's compressible disks with nonaxisymmetric structures: one with fully retrograde internal motions (R); one with fully prograde internal motions (P); one with vortices sandwiched between separate regions of prograde and retrograde flow (V);and a common-envelope binary (dumbbell-shaped) configuration (D). The similarity between the flow illustrated in Movie2 and the flow in Andalib's model P (Movie4)is striking. Apparently Andalib's model provides a good 2D analog of the 3D ''final bar'' that formed as a result of our fully hydrodynamic simulation of the two-armed, spiral mode instability (Movie1). Furthermore, Andalib's work demonstrates that model P is just one among a series of compressible models with nontrivial internal flows that definesa smooth elliptical-dumbbell-binary sequence.We suspect, therefore, that the final bar sits on an analogous (3D) sequence and that, if it is cooled slowly,it will evolve along the sequence to a common-envelopebinary configuration such as the one illustrated by model D inMovie4. Additional support for this conjecture comes fromNew & Tohline (1997) who have demonstrated that stable, equal-masscommon-envelope binaries can be constructed for fully 3D fluid systems with a sufficiently compressible equation of state. In summary, it seems clear that a wide variety of rapidly rotating, nonaxisymmetric systems can be constructed with compressible equations of state. This work gives us renewed confidence that fission offers a viableroute to binary star formation. Future investigations designed to model the slow cooling and contraction of initially nonaxisymmetricconfigurations like the final bar described above should demonstrate whether or not this scenario is correct.5. AcknowledgmentsThis work has been supported, in part, by the U.S. National ScienceFoundation through grant AST-9528424and, in part, by grants ofhigh-performance-computing time at the San Diego Supercomputer Centerand through the PET programof the NAVOCEANO DoD Major Shared Resource Center in Stennis, MS.6. ReferencesAndalib, S.W. (1998), The Structure and Stability of Selected, 2-D Self-Gravitating Systems, Ph.D. Dissertation, Louisiana State UniversityBodenheimer, P., Ruzmaikina, T. and Mathieu, R.D. (1993), in Protostars and Planets, III, ed. E.H. Levy and J.I. Lunine. University of Arizona Press, Tucson, AZ, U.S.A. p. 367Chandrasekhar, S. (1969), Ellipsoidal Figures of Equilibrium. Yale University Press, New Haven, CT, U.S.A.Durisen, R.H., Gingold, R.A., Tohline, J.E. and Boss, A.P. (1986), Ap.J., 305 , p. 281Durisen, R.H. and Tohline, J.E. (1985), in Protostars and Planets, II, ed. D.C. Black and M. Mathews. University of Arizona Press, Tucson, AZ, U.S.A. p. 534Houser, J.L., Centrella, J.M. and Smith, S.C. (1994), Phys. Rev. Lett., 72, p. 1314Klein, R.I, McKee, C.F. and Fisher, R. (1998), These proceedingsMathieu, R.D. (1994), Ann. Rev. Astr. Ap., 32, p. 465New, K.B.C. and Tohline, J.E. (1997), Ap.J., 490, p. 311Ostriker, J.P. and Bodenheimer, P. (1968), Ap.J., 151, p. 1089Williams, H.A. and Tohline, J.E. (1988), Ap.J., 334, p. 449 |
|
