Fragmentation Of Molecular Cloud Cores Research Articles

DISCOVERY OF A PLANETARY-MASS COMPANION TO A BROWN DWARF IN TAURUS

We have performed a survey for substellar companions to young brown dwarfs in the Taurus star-forming region using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. In these data, we have discovered a candidate companion at a projected separation of 0.105" from one of the brown dwarfs, corresponding to 15 AU at the distance of Taurus. To determine if this object is a companion, we have obtained images of the pair at a second epoch with the adaptive optics system at Gemini Observatory. The astrometry from the Hubble and Gemini data indicates that the two objects share similar proper motions and thus are likely companions. We estimate a mass of 5-10 Mjup for the secondary based on a comparison of its bolometric luminosity to the predictions of theoretical evolutionary models. This object demonstrates that planetary-mass companions to brown dwarfs can form on a timescale of <=1 Myr. Companion formation on such a rapid timescale is more likely to occur via gravitational instability in a disk or fragmentation of a cloud core than through core accretion. The Gemini images also reveal a possible substellar companion (rho=0.23") to a young low-mass star that is 12.4" from the brown dwarf targeted by Hubble. If these four objects comprise a quadruple system, then its hierarchical configuration would suggest that the fragmentation of molecular cloud cores can produce companions below 10 Mjup.

Gravitational Collapse and Fragmentation of Molecular Cloud Cores with GADGET‐2

The collapse and fragmentation of molecular cloud cores is examined numerically with unprecedentedly high spatial resolutions, using the publicly released code GADGET-2. As templates for the model clouds we use the standard isothermal test case in the variant calculated by Burkert & Bodenheimer in 1993 and the centrally condensed, Gaussian cloud advanced by Boss in 1991. A barotropic equation of state is used to mimic the nonisothermal collapse. We investigate both the sensitivity of fragmentation to thermal retardation and the level of resolution needed by smoothed particle hydrodynamics (SPH) to achieve convergence to existing Jeans-resolved, finite-difference (FD) calculations. We find that working with 0.6-1.2 million particles, acceptably good convergence is achieved for the standard test model. In contrast, convergent results for the Gaussian-cloud model are achieved using from 5 to 10 million particles. If the isothermal collapse is prolonged to unrealistically high densities, the outcome of collapse for the Gaussian cloud is a central adiabatic core surrounded by dense trailing spiral arms, which in turn may fragment in the late evolution. If, on the other hand, the barotropic equation of state is adjusted to mimic the rise of temperature predicted by radiative transfer calculations, the outcome of collapse is a protostellar binary core. At least, during the early phases of collapse leading to formation of the first protostellar core, thermal retardation not only favors fragmentation but also results in an increased number of fragments, for the Gaussian cloud.

The impact of magnetic fields on single and binary star formation

We have performed magnetohydrodynamics (MHD) simulations of the collapse and fragmentation of molecular cloud cores using a new algorithm for MHD within the smoothed particle hydrodynamics (SPH) method that enforces the zero magnetic divergence constraint. We find that the support provided by magnetic fields over thermal pressure alone has several important effects on fragmentation and the formation of binary and multiple systems, and on the properties of massive circumstellar discs. The extra support suppresses the tendency of molecular cloud cores to fragment due to either initial density perturbations or disc fragmentation. Furthermore, unlike most previous studies, we find that magnetic pressure plays the dominant role in inhibiting fragmentation rather than magnetic tension or magnetic braking. In particular, we find that if the magnetic field is aligned with the rotation axis of the molecular cloud core, the effects of the magnetic field on fragmentation and disc structure are almost entirely due to magnetic pressure, while if the rotation axis is initially perpendicular to the magnetic field, magnetic tension plays a greater role and can actually aid fragmentation. Despite these effects, and contrary to several past studies, we find that strongly perturbed molecular cloud cores are able to fragment to form wide binary systems even in the presence of quite strong magnetic fields. For massive circumstellar discs, we find that slowing of the collapse caused by the magnetic support decreases the mass infall rate on to the disc and, thus, weakens gravitational instabilities in young massive circumstellar discs. This not only reduces the likelihood that they will fragment, but also decreases the importance of spiral density waves in providing angular momentum transport and in promoting planet formation.

Collapse and Fragmentation of Molecular Cloud Cores. IX. Magnetic Braking of Initially Filamentary Clouds

The collapse and fragmentation of initially filamentary, magnetic molecular clouds is calculated in three dimensions with a gravitational, radiative hydrodynamics code. The code includes magnetic field effects in an approximate manner: magnetic pressure, tension, braking, and ambipolar diffusion are all modelled. Three types of outcomes are observed: direct collapse and fragmentation into a multiple protostar system, periodic contraction and expansion without collapse, or periodic contraction and expansion leading eventually to collapse. While the models begin their evolution at rest except for the assumed solid-body rotation, they develop weakly supersonic velocity fields as a result of the rebounding prior to collapse. The models show that magnetically-supported clouds subject to magnetic braking can undergo dynamic collapse leading to protostellar fragmentation on scales of 10 AU to 100 AU, consistent with typical binary star separations.

Fragmentation of a Molecular Cloud Core versus Fragmentation of the Massive Protoplanetary Disk in the Main Accretion Phase

The fragmentation of molecular cloud cores a factor of 1.1 denser than the critical Bonnor-Ebert sphere is examined though three-dimensional numerical simulations. A nested grid is employed to resolve fine structure down to 1 AU while following the entire structure of the molecular cloud core of radius 0.14 pc. A total of 225 models are shown to survey the effects of initial rotation speed, rotation law, and amplitude of bar mode perturbation. The simulations show that the cloud fragments whenever the cloud rotates sufficiently slowly to allow collapse but fast enough to form a disk before first-core formation. The latter condition is equivalent to $\Omega_0 t_{\rm ff} \gtrsim 0.05$, where $\Omega_0$ and $t_{\rm ff}$ denote the initial central angular velocity and the freefall time measured from the central density, respectively. Fragmentation is classified into six types: disk-bar, ring-bar, satellite, bar, ring, and dumbbell types according to the morphology of collapse and fragmentation. When the outward decrease in initial angular velocity is more steep, the cloud deforms from spherical at an early stage. The cloud deforms into a ring only when the bar mode m = 2 perturbation is very minor. The ring fragments into two or three fragments via ring-bar type fragmentation and into at least three fragments via ring type fragmentation. When the bar mode is significant, the cloud fragments into two fragments via either bar or dumbbell type fragmentation. These fragments eventually merge due to their low angular momenta, after which several new fragments form around the merged fragment via satellite type fragmentation.

The Jeans Condition and Collapsing Molecular Cloud Cores: Filaments or Binaries?

The 1997 and 1998 studies by Truelove and colleagues introduced the Jeans condition as a necessary condition for avoiding artificial fragmentation during protostellar collapse calculations. They found that when the Jeans condition was properly satisfied with their adaptive mesh refinement (AMR) code, an isothermal cloud with an initial Gaussian density profile collapsed to form a thin filament rather than the binary or quadruple protostar systems found in previous calculations. Using a completely different self-gravitational hydrodynamics code introduced by Boss & Myhill in 1992 (B&M), we present here calculations that reproduce the filamentary solution first obtained by Truelove et al. in 1997. The filamentary solution only emerged with very high spatial resolution with the B&M code, with effectively 12,500 radial grid points (R12500). Reproducing the filamentary collapse solution appears to be an excellent means for testing the reliability of self-gravitational hydrodynamics codes, whether grid-based or particle-based. We then show that in the more physically realistic case of an identical initial cloud with nonisothermal heating (calculated in the Eddington approximation with code B&M), thermal retardation of the collapse permits the Gaussian cloud to fragment into a binary protostar system at the same maximum density where the isothermal collapse yields a thin filament. However, the binary clumps soon thereafter evolve into a central clump surrounded by spiral arms containing two more clumps. A roughly similar evolution is obtained using the AMR code with a barotropic equation of state—formation of a transient binary, followed by decay of the binary to form a central object surrounded by spiral arms, though in this case the spiral arms do not form clumps. When the same barotropic equation of state is used with the B&M code, the agreement with the initial phases of the AMR calculation is quite good, showing that these two codes yield mutually consistent results. However, the B&M barotropic result differs significantly from the B&M Eddington result at the same maximum density, demonstrating the importance of detailed radiative transfer effects. Finally, we confirm that even in the case of isothermal collapse, an initially uniform density sphere can collapse and fragment into a binary system, in agreement with the 1998 results of Truelove et al. Fragmentation of molecular cloud cores thus appears to remain as a likely explanation of the formation of binary stars, but the sensitivity of these calculations to the numerical resolution and to the thermodynamical treatment demonstrates the need for considerable caution in computing and interpreting three-dimensional protostellar collapse calculations.

The Jeans Mass Constraint and the Fragmentation of Molecular Cloud Cores

The collapse and fragmentation of molecular cloud cores into binary and multiple protostar systems is a demanding computational problem, in part because of the large range of length scales involved. Truelove et al. proposed that the computational cell size Δx must be smaller than 1/4 of the Jeans length, λJ, if artificial (numerical) fragmentation is to be avoided. For a uniform Cartesian grid, Truelove et al.'s Jeans condition is equivalent to ensuring that the mass of a cell never exceeds ~1/64 of a Jeans mass. It is shown here that for a nonuniform spherical grid, artificial fragmentation can be avoided provided that the cell size of a cube with approximately the same volume as the spherical coordinate cell [Δx=(ΔxrΔxθΔx)1/3] is less than λJ/4 (i.e., that the mass inside each cell is much less than a Jeans mass), even if one of the three cell lengths (Δxr, Δxθ, or Δx) exceeds λJ/4. For a nonuniform grid, resolving a small fraction of a Jeans mass is less restrictive than resolving 1/4 of a Jeans length in each coordinate direction; resolving all three Jeans lengths is desirable but not necessary in order to avoid artificial fragmentation. The well-resolved collapse of an initially Gaussian-profile cloud is then followed with both isothermal and nonisothermal (Eddington approximation radiative transfer) thermodynamics and is shown to lead to fragmentation into a binary protostar system in both cases. The Jeans mass constraint appears to be a valuable indicator of physically realistic fragmentation.

Collapse and Fragmentation of Molecular Cloud Cores. V. Loss of Magnetic Field Support

The fragmentation mechanism has been quite successful at providing an explanation for the formation of binary stars during the collapse phase of dense cloud cores. However, nearly all fragmentation calculations to date have ignored the effects of magnetic fields, whereas magnetic fields are generally regarded as the dominant force in molecular clouds. Here, we present the first three-dimensional, radiative hydrodynamical models of the collapse and fragmentation of dense molecular cloud cores, including the effects of magnetic fields and ambipolar diffusion. Starting from a prolate, Gaussian cloud that would collapse and fragment in the absence of magnetic fields (a thermally supercritical cloud), we introduce sufficient magnetic field support [through the magnetic field pressure, B2/8π, with B = B0(ρ/ρ0)κ] to ensure a magnetically subcritical (stable) cloud. The effects of ambipolar diffusion are then simulated by reducing the magnetic pressure scaling factor (B0) over a specified time interval (=tAD), which leads to a magnetically supercritical cloud and collapse. The estimated timescale for ambipolar diffusion in these dense clouds is about 10 free-fall times. The numerical models show that when tAD is as long as 10 or even 20 free-fall times, fragmentation into a binary can still occur. The main effect of the magnetic field support is to delay somewhat the formation of the binary protostar. Once the dynamic collapse phase begins, a rapidly rotating, (βi = Erot/| Egrav | = 0.12) cloud fragments into a binary protostar. While it remains to be seen if magnetic fields can stifle fragmentation in slowly rotating clouds, rapidly rotating, magnetically supported clouds appear to be quite capable of forming binary stars.

Collapse and Fragmentation of Molecular Cloud Cores. IV. Oblate Clouds and Small Cluster Formation

view Abstract Citations (40) References (53) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Collapse and Fragmentation of Molecular Cloud Cores. IV. Oblate Clouds and Small Cluster Formation Boss, Alan P. Abstract Binary and multiple stars are believed to form primarily by the gravitational collapse of dense molecular cloud cores and by the fragmentation of the clouds into systems of protostars. Previous numerical calculations have shown that initially prolate (or cylindrical) cloud cores may collapse and fragment into binary or higher order protostellar systems. Observations of precollapse clouds are consistent with a significant fraction being oblate as well as prolate, yet the collapse of oblate clouds has not been investigated to date with a three-dimensional hydrodynamics code. We present here three-dimensional calculations of the isothermal collapse of oblate cloud cores, with initial axial ratios of 2:1 and uniform or centrally condensed (Gaussian) radial density profiles. The calculations are performed with a spatially and temporally second-order accurate hydrodynamics code with relatively high resolution in the azimuthal direction (Nφ = 128). Oblate clouds with moderate solid-body rotation rates collapse in two steps. First, the clouds collapse along their short axes to form thin pancakes, which are unsupported by centrifugal effects. Second, the pancakes collapse radially inward along their midplanes until centrifugal support is achieved in their inner regions. During the second phase, the surface density of the pancakes becomes so high [σ(r) > σζ(r) ≍(c2sπ/(Gr)] that the inner regions of the pancakes fragment into multiple protostars. Fragmentation occurs when the initial ratio of thermal to gravitational energy (αι) is about 0.4 or less. Varying the ratio of rotational to gravitational energy (βι) from 0.14 to 0.018 has relatively little effect, because the highly fragmentable pancakes are formed primarily as a result of the oblate geometry of the initial clouds rather than by rotational flattening. The most dramatic result is that oblate clouds appear to fragment into a larger number of protostars than do prolate clouds with similar values of αι and βι. The models imply that centrally condensed, oblate cloud cores close to virial equilibrium (α ≍ ½) should collapse and fragment into small clusters containing on the order of 10 protostars. Publication: The Astrophysical Journal Pub Date: September 1996 DOI: 10.1086/177686 Bibcode: 1996ApJ...468..231B Keywords: HYDRODYNAMICS; ISM: CLOUDS; ISM: MOLECULES; STARS: FORMATION; GALAXY: OPEN CLUSTERS AND ASSOCIATIONS: GENERAL full text sources ADS | data products SIMBAD (10) Related Materials (9) Part 1: 1993ApJ...410..157B Part 2: 1995ApJ...439..224B Part 2: 1997ApJ...483..309B Part 3: 1995ApJ...451..218B Part 3: 1999ApJ...520..744B Part 7: 2002ApJ...568..743B Part 8: 2005ApJ...622..393B Part 9: 2007ApJ...658.1136B Part 10: 2009ApJ...697.1940B

Collapse and Fragmentation of Molecular Cloud Cores. III. Initial Differential Rotation

view Abstract Citations (37) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Collapse and Fragmentation of Molecular Cloud Cores. III. Initial Differential Rotation Boss, Alan P. ; Myhill, Elizabeth A. Abstract Fragmentation during protostellar collapse is the leading mechanism for explaining the formation of binary and multiple stars. Most calculations of protostellar fragmentation have assumed the initial molecular cloud cores to be in solid body rotation, whereas differential rotation might characterize some molecular cloud cores. Here we use a second-order accurate, radiative hydrodynamics code to calculate the self-gravitational collapse of three-dimensional protostellar clouds, starting from centrally condensed (Gaussian), prolate (1.5:1 and 2:1 ratios) configurations, with varying degrees of differential rotation. The initial differential rotation is characterized by an exponent γι specifying the dependence of the initial angular velocity profile on the initial density (Ωι ∝ ργιι); γι = 0 produces initially solid body rotation, while γι = ⅔ results in the maximum degree of differential rotation consistent with spherical contraction at conserved mass and angular momentum. Depending on the value of γι, and the initial ratios of thermal (αι) and rotational (βι) to gravitational energy, three types of outcomes result: slow contraction, vigorous collapse without fragmentation, or collapse leading to fragmentation. Fragmentation occurs for γι = ⅔ clouds with initial 1.5:1 axis ratios when αι < 0.45 and βι ≍ 0.1, whereas γι = 0 clouds only fragment for αι < 0.3. For initial 2:1 axis ratios, fragmentation requires αι < 0.5 and βι ≍ 0.1 for γι = ⅔, compared to αι < 0.4 for clouds with γι = 0. These criteria quantify the extent to which initial differential rotation enhances rotationally driven fragmentation by concentrating the angular momentum per unit mass in the densest regions of the cloud. Gaussian clouds in rapid differential rotation can fragment even if they start collapse from a configuration close to virial equilibrium (αι + βι = ½). Publication: The Astrophysical Journal Pub Date: September 1995 DOI: 10.1086/176213 Bibcode: 1995ApJ...451..218B Keywords: HYDRODYNAMICS; ISM: CLOUDS; ISM: KINEMATICS AND DYNAMICS; ISM: MOLECULES; STARS: BINARIES: GENERAL; STARS: FORMATION full text sources ADS | Related Materials (9) Part 1: 1993ApJ...410..157B Part 2: 1995ApJ...439..224B Part 4: 1996ApJ...468..231B Part 5: 1997ApJ...483..309B Part 6: 1999ApJ...520..744B Part 7: 2002ApJ...568..743B Part 8: 2005ApJ...622..393B Part 9: 2007ApJ...658.1136B Part 10: 2009ApJ...697.1940B

Collapse and fragmentation of molecular cloud cores. 2: Collapse induced by stellar shock waves

The standard scenario for low-mass star formation involves 'inside-out' collapse of a dense molecular cloud core following loss of magnetic field support through ambipolar diffusion. However, isotopic anomalies in presolar grains and meteoritical inclusions imply that the collapse of the presolar cloud may have been triggered by a stellar shock wave. This paper explores 'outside-in' collapse, that is, protostellar collapse initiated directly by the compression of quiescent dense cloud cores impacted by relatively slow stellar shock waves. A second-order accurate, gravitational hydrodynamics code has been used to study both the spherically symmetrical and three-dimensional evolution of initially centrally condensed, isothermal, self-gravitating, solar-mass cloud cores that are struck by stellar shock waves with velocities up to 25 km/s and postshock temperatures of 10 to 10,000 K. The models show that such mild shock waves do not completely shred and destroy the cloud, and that the dynamical ram pressure can compress the cloud to the verge of self-gravitational collapse. However, compression caused by a high postshock temperature is a considerably more effective means of inducing collapse. Shock-induced collapse produces high initial mass accretion rates (greater than 10(exp -4) solar mass/yr in a solar-mass cloud) that decline rapidly to much lower values, depending on the presence (approximately 10(exp -6) solar mass/yr) or absence (approximately 10(exp -8) to 10(exp -7) solar mass/yr) of an infinite reservoir of mass. Stellar mass accretion rates approximately 10(exp -7) solar mass/yr have been previously inferred from the luminosities of T Tauri stars; balanced mass accretion (stellar rate = envelope rate) at approximately 10(exp -7) solar mass/yr could then be possible if accretion occurs from a finite mass reservoir. Fluid tracers are used to determine what fraction of the stellar shock material is incorporated into the resulting protostellar object and disk; roughly half the impinging material is injected into the collapsing cloud core when there is a high postshock temperature. The models are consistent with a scenario where an AGB star wind triggered the collapse of the presolar cloud while injecting about 0.01 solar mass of matter derived from the AGB star envelope, as has been separately inferred on the basis of nucleosynthesis calculations.

Collapse and fragmentation of molecular cloud cores. I - Moderately centrally condensed cores

3D calculations of the collapse of moderately centrally condensed molecular cloud cores with varied thermal and rotational energies are presented. The calculations are carried out using a newly developed and tested second-order accurate radiative hydrodynamics code. Because of the use of a second-order accurate numerical scheme and initial clouds that resemble both observed prolate molecular cloud cores and magnetically supported clouds at the initiation of the dynamic collapse phase, the new models provide a superior estimate of the likelihood of fragmentation as a mechanism for binary star formation.