Testing modified Newtonian dynamics in the Milky Way

Modified Newtonian dynamics (MOND) is an empirical theory originally proposed to explain the rotation curves of spiral galaxies by modifying the gravitational acceleration, rather than by invoking dark matter. Here,we set constraints on MOND using an up-to-date compilation of kinematic tracers of the Milky Way and a comprehensive collection of morphologies of the baryonic component in the Galaxy. In particular, we find that the so-called standard interpolating function cannot explain at the same time the rotation curve of the Milky Way and that of external galaxies for any of the baryonic models studied, while the so-called simple interpolating function can for a subset of models. Upcoming astronomical observations will refine our knowledge on the morphology of baryons and will ultimately confirm or rule out the validity of MOND in the Milky Way. We also present constraints on MOND-like theories without making any assumptions on the interpolating function.

The lithium problem, a phenomenologists perspective

Thirty years after the first observation of the 7Li isotope in the atmosphere of metal-poor halo stars, the puzzle about its origin persists. Do current observations still support the existence of a plateau: a single value of lithium abundance, constant over several orders of magnitude in the metallicity of the target star? If this plateau exists, is it universal in terms of observational loci of target stars? Is it possible to explain such observations with known astrophysical processes? Can yet poorly explored astrophysical mechanisms explain the observations or do we need to invoke physics beyond the standard model of Cosmology and/or the standard model of Particle Physics to explain them? Is there a 6Li problem, and is it connected to the 7Li one? These questions have been discussed at the Paris workshop Lithium in the Cosmos, and I summarize here its contents, providing an overview from the perspective of a phenomenologist.

Main sequence stars with asymmetric dark matter

We study the effects of feebly or non-annihilating weakly interacting Dark Matter (DM) particles on stars that live in DM environments denser than that of our Sun. We find that the energy transport mechanism induced by DM particles can produce unusual conditions in the core of Main Sequence stars, with effects which can potentially be used to probe DM properties. We find that solar mass stars placed in DM densities of rhochi>= e2 GeV/cm3 are sensitive to Spin-Dependent scattering cross-section sigmsd >= e-37 cm2 and a DM particle mass as low as mchi=5 GeV, accessing a parameter range weakly constrained by current direct detection experiments.

WIMP Dark Matter and the First Stars: a critical overview

If Dark Matter (DM) is composed by Weakly Interacting Massive Particles, its annihilation in the halos harboring the earliest star formation episode may strongly influence the first generation of stars (Population III). Whereas DM annihilation at early stages of gas collapse does not dramatically affect the properties of the cloud, the formation of a hydrostatic object (protostar) and its evolution toward the main sequence may be delayed. This process involves DM concentrated in the center of the halo by gravitational drag, and no consensus is yet reached over whether this can push the initial mass of Population III to higher masses. DM can also be captured through scattering over the baryons in a dense object, onto or very close to the Main Sequence. This mechanism can affect formed stars and in principle prolonge their lifetimes. The strength of both mechanisms depends upon several environmental conditions and on DM parameters; such spread in the parameter space leads to very different scenarios for the observables in the Population. Here I summarize the state of the art in modelling and observational expectations, eventually highlighting the most critical assumptions and sources of uncertainty.

Self-annihilating dark matter and the CMB: reionizing the Universe and constraining cross sections

I summarize the recent advances in determining the effects of self-annihilating WIMP dark matter on the modification of the recombination history, at times earlier than the formation of astrophysical objects. Depending on mass and self-annihilation cross section, WIMP DM can reproduce sizable amounts of the total free electron abundance at z > 6; as known, this affects the CMB temperature and polarization correlation spectra, and can be used to place stringent bounds in the particle mass vs cross-section plane. WMAP5 data already strongly disfavor the region capable to explain the recent cosmic positron and electrons anomalies in terms of DM annihilation, whereas in principle the Planck mission has the potential to see a signal produced by a candidate laying in that region, or from WIMPs with thermal annihilation cross-sections <sv>=3e-26 cm3/s and masses below 50 GeV.

Effects of dark matter annihilation on the first stars

We study the evolution of the first stars in the universe (Population III) from the early pre-Main Sequence until the end of helium burning in the presence of WIMP dark matter annihilation inside the stellar structure. The two different mechanisms that can provide this energy source are the contemporary contraction of baryons and dark matter, and the capture of WIMPs by scattering off the gas with subsequent accumulation inside the star. We find that the first mechanism can generate an equilibrium phase, previously known as a dark star, which is transient and present in the very early stages of pre-MS evolution. The mechanism of scattering and capture acts later, and can support the star virtually forever, depending on environmental characteristic of the dark matter halo and on the specific WIMP model.

Dark matter annihilation effects on the first stars

We study the effects of WIMP dark matter (DM) on the collapse and evolution of the first stars in the Universe. Using a stellar evolution code, we follow the pre-Main Sequence (MS) phase of a grid of metal-free stars with masses in the range 5-600 solar mass forming in the centre of a 1e6 solar mass halo at redhisft z=20. DM particles of the parent halo are accreted in the proto-stellar interior by adiabatic contraction and scattering/capture processes, reaching central densities of order 1e12 GeV/cm3 at radii of the order of 10 AU. Energy release from annihilation reactions can effectively counteract the gravitational collapse, in agreement with results from other groups. We find this stalling phase (known as dark star) is transients and lasts from 2.1e3 yr (M=600 solar mass) to 1.8e4 yr (M=9 solar mass). Later in the evolution, DM scattering/capture rate becomes high enough that energy deposition from annihilations significantly alters the pre-MS evolution of the star in a way that depends on DM (i) velocity dispersion, (ii) density, (iii) elastic scattering cross section with baryons. For our fiducial set of parameters (10 km/s, 1e11 GeV/cm3, 1e-38 cm2) we find that the evolution of stars of mass lower than 40 solar masses freezes on the HR diagram before reaching the ZAMS. Stars with bigger masses manage to ignite nuclear reactions; however, DM burning prolonges their lifetimes by a factor 2 (5) for a 600 (40) solar mass star.

High Energy neutrino signals from the Epoch of Reionization

We perform a new estimate of the high energy neutrinos expected from GRBs associated with the first generation of stars in light of new models and constraints on the epoch of reionization and a more detailed evaluation of the neutrino emission yields. We also compare the diffuse high energy neutrino background from Population III stars with the one from ordinary stars (Population II), as estimated consistently within the same cosmological and astrophysical assumptions. In disagreement with previous literature, we find that high energy neutrinos from Population III stars will not be observable with current or near future neutrino telescopes, falling below both IceCube sensitivity and atmospheric neutrino background under the most extreme assumptions for the GRB rate. This rules them out as a viable diagnostic tool for these still elusive metal-free stars.