We announce the public release of the 'dark' stellar evolution code DarkStars. The code simultaneously solves the equations of WIMP capture and annihilation in a star with those of stellar evolution assuming approximate hydrostatic equilibrium. DarkStars includes the most extensive WIMP microphysics of any dark evolution code to date. The code employs detailed treatments of the capture process from a range of WIMP velocity distributions, as well as composite WIMP distribution and conductive energy transport schemes based on the WIMP mean-free path in the star. We give a brief description of the input physics and practical usage of the code, as well as examples of its application to dark stars at the Galactic centre.
Deep Dive into The DarkStars code: a publicly available dark stellar evolution package.
We announce the public release of the ‘dark’ stellar evolution code DarkStars. The code simultaneously solves the equations of WIMP capture and annihilation in a star with those of stellar evolution assuming approximate hydrostatic equilibrium. DarkStars includes the most extensive WIMP microphysics of any dark evolution code to date. The code employs detailed treatments of the capture process from a range of WIMP velocity distributions, as well as composite WIMP distribution and conductive energy transport schemes based on the WIMP mean-free path in the star. We give a brief description of the input physics and practical usage of the code, as well as examples of its application to dark stars at the Galactic centre.
The last two years have seen strong interest in the impacts of dark matter upon stellar structure and evolution. The predominant focus has been on self-annihilating WIMP (weakly-interacting massive particle) dark matter, because it has the ability to affect stellar structure by annihilating in stellar cores [1][2][3][4][5][6][7] and collapsing protostellar halos. [8][9][10][11][12] Interest has been driven by the prospect of providing constraints upon the nature of dark matter, 2,7 by intrinsic curiosity in the resultant 'dark stars' themselves, 4,12,13 and by their possible impacts upon early-universe processes like reionisation. 14,15 We have previously discussed the possibility that main-sequence dark stars could exist at the centre of our own Galaxy. 3,7,16,17 In those papers we utilised a form of the standard stellar evolution code stars [18][19][20] modified to include the effects of dark matter capture and annihilation. This modified code is DarkStars, and in these proceedings we announce its public release. DarkStars is written in Fortran95, and can be freely downloaded from http://www.fysik.su.se/ ~pat/darkstars. Below we give outlines of the code's input physics and practical usage, along with some simple examples of stars evolved with it.
DarkStars includes gravitational capture of WIMPs from the galactic halo via the full equations of Gould, 21 including both spin-dependent and spin-independent scattering on the 22 most important atomic nuclei. The capture routines are adapted from the solar capture code in DarkSUSY. 22 Capture can be performed semi-analytically from either a standard isothermal WIMP halo or an isothermal halo where the WIMP velocity distribution has been truncated at the local escape velocity. Alternatively, numerical capture calculations can be performed on a velocity distribution derived 23 from the Via Lactea 24 simulation of a Milky Way-type galaxy, or any other arbitrary, user-supplied velocity distribution.
The distribution of WIMPs with height in a star is obtained by interpolating between two limiting distributions according to the value of the WIMP mean-free path in the star: one corresponding to WIMPs with very long mean-free paths, the other to WIMPs with very short meanfree paths. Conductive energy transport by weak-scattering events between atomic nuclei and WIMPs is taken into account in a manner consistent with this distribution: the conductive luminosity at each height is approximated by rescaling the known expression for the conductive luminosity at short mean-free paths, according to the actual value of the mean free path in the star. The annihilation luminosity at each height in the star is simply calculated as the product of the annihilation cross-section and square of the local WIMP density, and fed along with the conductive luminosity into the luminosity equation in the stellar solver. Full technical details of the input physics for DarkStars can be found in Ref. 7.
DarkStars operates with a simple text-file input, containing a series of switches and physical parameters with which to perform a particular evolutionary run. Switches allow choices between analytical and numerical capture, different halo velocity distributions, the inclusion or exclusion of annihilation and conductive energy transport effects, and the option to run in a special ‘reconvergence mode’ where the solution obtained at each timestep is converged twice (see Ref. 7 for details). Runs can be saved and restarted at will, and the input format provides the ability to make periodic saves during the course of a single evolutionary run.
The user can specify the WIMP mass, spin-dependent, spin-independent and annihilation cross-sections, as well as the stellar mass and metallicity, the initial population of WIMPs in the star and the ultimate percentage of energy lost to neutrinos in each annihilation. One may also opt to specify a constant stellar velocity through a WIMP halo with some particular local density and velocity dispersion, located at a position with a single well-defined Galactic escape velocity. Alternatively, runs can be performed along user-specified orbits, where these four parameters become dynamic quantities given in an additional text file. Orbits can also be looped if desired.
The code presently allows metallicities of Z = 0.03-0.0001 with full evolutionary functionality, and a Z = 0 mode valid only for protostellar evolution. The latter includes opacities taken from Ref. 25, but does not yet contain an implementation of the full opacity tables required to treat the case where a Pop III star passes from the Z = 0 regime into the nonmetal-free one by nuclear burning. DarkStars comes with ZAMS starting models for all non-zero metallicities; protostellar models must be supplied by the user.
In Fig. 1 we give some example evolutionary tracks computed with Dark-Stars, for a Z = 0.01, 1 M star. The three different paths result from immersing the star in different ambient ha
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