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fbtwitterlinkedinvimeoflicker grey 14rssslideshare1
Attwood, Rhianne Elizabeth
Languages: English
Types: Doctoral thesis
Subjects: QB

Classified by OpenAIRE into

arxiv: Astrophysics::Galaxy Astrophysics, Astrophysics::Earth and Planetary Astrophysics, Astrophysics::Solar and Stellar Astrophysics
In this thesis we investigate the influence of certain physical effects on the collapse and fragmentation of isolated, low-mass, low-turbulence cores, in particular on the mass distribution, binary statistics and kinematics of the resulting stars. We perform numerical simulations using a Smoothed Particle Hydrodynamics code to model this mode of star formation. Firstly we model acoustic oscillations of a self-gravitating isentropic monatomic gas sphere using our SPH code and find that if the smoothing lengths are adjusted so as to keep the number of neighbours in the range AAu, NNE1B should be set to zero, to reduce the level of numerical dissipation and diffusion. We suggest that this should become a standard test for codes simulating star formation, since pressure waves generated by the switch from approximate isothermality to approximate adiabaticity play a crucial role in the fragmentation of collapsing cores. We perform a large ensemble of SPH simulations of cores having different levels of turbulence, using a new, more realistic treatment of thermodynamics, developed by Stamatellos et al. (2007), which takes into account the thermal history of protostellar gas and captures the thermal inertia effects. We compare the results with simulations using a standard barotropic equation of state. We find that increasing the level of turbulence generally tends to reduce the fraction of the core mass which is converted into stars, and increase the number of stars formed by a single core. Using the new treatment results in more protostellar objects being formed, and a higher proportion of brown dwarfs. Of the multiple systems that form, they tend to have shorter periods, higher eccentricities and higher mass ratios. We also note that in our simulations the process of fragmentation is often bimodal, in the following sense. The first protostar to form is usually, at the end, the most massive, i.e. the primary. However, frequently a disc-like structure subsequently forms round this primary, and then, once it has accumulated sufficient mass, quickly fragments to produce several secondaries. We believe that this delayed fragmentation of a disc-like structure is likely to be an important source of very low-mass stars in nature (both low-mass hydrogen-burning stars and brown dwarf stars). We also model the evolution of an ensemble of prestellar cores in the Ophiuchus Main Cloud using initial conditions for the sizes and levels of turbulence constrained by the observations of Motte et al. (1998) and Andre' et al. (2007), and the recently revised core masses of Stamatellos et al. (2007). We find that star formation in these core is extremely efficient with typically the formation of a single star, but we also see the formation of multiple systems in a number of cores. We find that the number of stars formed by a core is highest if the core has high mass, and/or if it has a high initial level of turbulence, and/or if it starts from a low initial density. We explain why. Finally we explore the effect metallicity has on the mass distribution and binary statistics of stars formed from low-mass low-turbulence cores. We find that reducing the metallicity decreases the number of stars formed from a single core and reduces the number of brown dwarfs formed. It also reduces the binary frequency.
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    • 2 Smoothed Particle Hydrodynamics 2.1 Self-Gravitating Compressible F l o w ........................................................... 2.2 The concept of S P H ......................................................................................... 2.3 K e r n e l s ........................................................................................................... 2.4 SPH equations................................................................................................... 2.5 Smoothing L e n g th s ......................................................................................... 2.6 Artificial V is c o s ity ......................................................................................... 2.6.1 Time-dependent viscosity ..............................................................
    • 7 Summary 163 7.1 Numerical diffusion and numerical dissipation instar formation codes . 163 7.2 Treatment of the thermodynamics in collapsing c o r e s ................................ 164 7.3 Prestellar cores in the Ophiuchus Main C l o u d ............................................. 165 7.4 The effect of metallicity on the core co llap se.................................................166 7.5 Future w o r k ........................................................................................................ 167
    • 1.1 Taurus molecular cloud seen in extinction, taken from Dobashi et al. (2005) and modified by Nutter (private communication). The contour levels are Av= l, 2 ,4 .........................................................................................
    • 2.1 The structure of a 2-dimensional tree constructed for a simple distribution. Depending on the value of 9, we can determine whether to calculate the gravitational accelerations directly or approximate the particles as a clu ster................................................................................................................
    • 2.2 Graphical representation of MPT for when n = 5. Arrows indicate the steps that are allowed. By enforcing this, all particles will remain synchronised at the end of A t ...........................................................................
    • 3.1 Solution to the Lane Emden equation for n = 3 / 2 ....................................
    • 3.2 The two-dimensional lattice with inter-particle separation s......................
    • 3.3 2-D plot of the isentropic monatomic sphere, constructed as described in section 3.2.3..................................................................................................
    • 3.4 Density profile of sphere before settling. Red line represents the analytical solution; green points represent the actual particle densities............... 46
    • 3.5 Density profile of the settled distribution......................................................
    • 3.6 Radial components of the gravitational accelerations (red open squares) . and hydrostatic accelerations (filled green circles) of the particles in a settled sphere, as a function of radius............................................................
    • 4.1 Schematic representation of the pseudo-cloud around an SPH particle.The location of the SPH particle inside its pseudo-cloud is not specified. Taken from Stamatellos et al. (2007a)...........................................................
    • 4.3 The variation with density and temperature of the pseudo-mean opacity. Isopycnic curves are plotted as in Fig. 4.2. For comparison the local opacity at density p = 10-6 g cm-3 is also plotted (dashed line). Taken from Stamatellos et al. (2007)......................................................................... 73
    • 4.4 Simulation of the collapse and fragmentation of a 5 .4 M© core, first evolved with the barotropic equation of state (top row) and then with the new treatment of the energy equation (bottom row), using identical initial conditions. Each snapshot shows the logarithm of the column density................................................................................................................
    • 4.5 Stellar masses as a function of time, for a selection of simulations. Note (i) the delay between the formation of the primary and the formation of a clutch of secondaries (this is the time during which the circumprimary disc accumulates, until it becomes Toomre unstable); and (ii) the rapid decline in the accretion rate onto the primary once the secondaries start to condense out.................................................................................................
    • 4.13 The distribution of eccentricities, e, for multiple protostars: (a) using the barotropic equation of state; (b) using the new treatment of the energy equation.............................................................................................................
    • 4.14 The distribution of mass ratios, q, for multiple protostars: (a) using the barotropic equation of state; (b) using the new treatment of the energy equation.............................................................................................................
    • 13CO map of Ophiuchus, taken from Loren (1989) and modified by Nut­
    • ter et al. (2006). The contour levels give antenna temperatures of 4, 5,
    • 6 ,7 ,8 ,1 0 , 12,14, 18, 20K.................................................................................. 109
    • Millimeter continuum mosaic of the 6 major clumps in the Ophiuchus
    • main cloud, from Motte et al. (1998)............................................................. 110
    • 5.3 Logarithm of the total mass of a core ( M ^ ) plotted against the number of stars formed, N *...............................................................................................121
    • 5.11 For each multiple system we plot the orbital eccentricity, e against the period, P .................................................................................................................129
    • 5.12 The distribution of mass ratios, q, for multiple protostars at the end of the simulations...................................................................................................... 130
    • 6.1 Stellar masses as a function of time, for simulationswith Z = Z0
    • 5.1 Estimated temperatures for each clump............................................................. 113
    • 5.3 Sample of recalculated values of 0 4 ^ /a THESM..........................................132
    • 6.1 Results of the simulations performed with metallicities Z = ZQ, Z = 0.1 ZQand Z = 0.01 ZQ, at time t = 0.3 Myr. See text for a description of each column.......................................................................................................... 145
    • Bromm V., Coppi P. S., Larson R. B., 2002, ApJ, 564, 23
    • Bromm V., Ferrara A., Coppi P. S., Larson R. B., 2001, MNRAS, 328, 969
    • Burgasser A. J., Reid I. N., Siegler N., Close L., Allen P., Lowrance P., Gizis J., 2007, in Reipurth B., Jewitt D., Keil K., ed, Protostars and Planets V, p. 427
    • Burkert A., Bodenheimer P., 1993, MNRAS, 264, 798
    • Burkert A., Bodenheimer P., 2000, ApJ, 543, 822
    • Caselli P., Walmsley C. M., Terzieva R., Herbst E., 1998, ApJ, 499, 234
    • Chapman S., Pongracic H., Disney M., Nelson A., Turner J., Whitworth A., 1992, Nature, 359, 207
    • Christlieb N., Wisotzki L., Reimers D., Homeier D., Koester D., Heber U., 2001, A&A, 366, 898
    • Hubber D. A., Goodwin S. P., Whitworth A. P., 2006, A&A, 450, 881
    • Hubber D. A., Whitworth A. P., 2005, A&A, 437, 113
    • Jappsen A.-K., Glover S. C. O., Klessen R. S., Mac Low M.-M., 2007, ApJ, 660, 1332
    • Jeans J. H., 1928, Astronomy and cosmogony. Cambridge [Eng.] The University press, 1928.
    • Jijina J., Myers P. C., Adams F. C., 1999, ApJS, 125, 161
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