Heavy sterile neutrinos in stellar core-collapse

We perform spherically symmetric simulations of the core collapse of a single progenitor star of zero age main sequence mass $M_{\mathrm{ZAMS}}=15M_{\odot }$ with two models of heavy sterile neutrinos in the mass range of $100\mathrm{MeV}/c^2$. According to both models, these hypothetical particles are copiously produced in the center, stream outwards and subsequently decay, releasing energy into final states (including neutrinos) of the Standard Model. We find that they can lead to a successful explosion in otherwise nonexploding progenitors. Depending on their unknown parameters (e.g., mass and coupling constants with matter), we obtain either no explosion or an explosion of one of two types, i.e., through heating of gas downstream of the stalled shock wave, similar to the standard scenario for supernova explosions, or through heating of gas at higher radii that ejects matter from the outer core or the envelope while the center continues to accrete matter. In both cases, the explosion energies can be very high. We presume that this new type of explosion would produce an electromagnetic signal that significantly differs from common events because of the relative absence of heavy elements in the ejecta. The combination of core-collapse simulations and astrophysical observations may further constrain the parameters of the sterile neutrinos.

Figure 1

One-loop diagram generating a magnetic dipole moment ${\mu }_{\mathrm{h}}$ for ${\nu }_{\mathrm{h}}=(N{\overline{N}}^c)$. Changing $N\to \tilde{N}$, $E\to \tilde{E}$ and $m_E\to m_{\tilde{E}E}$ we obtain an electromagnetic dipole transition ${\mu }_{\mathrm{tr}}$ between $\nu$ (the active neutrino mixed with $\tilde{N}$) and ${\nu }_{\mathrm{h}}$.

Figure 2

Time evolution of models R (top left), A4 (top right), F8 (bottom left), and A8 (bottom right) from Table 1. Upper subpanels: [logarithm of] entropy per baryon (color map), density (${\log }_{10}(\rho /1\mathrm{g}{\mathrm{cm}}^{-3})$; white isocontours), positive radial velocity (${\log }_{10}(v_r/1\mathrm{cm}{\mathrm{s}}^{-1})$; light blue isocontours). Lower subpanels: total (i.e., active and sterile) neutrino heating (shades of orange) and cooling (shades of blue), enclosed mass (in solar masses, white isocontour), explosion radius (yellow line). In both panels: shock radius (green line), neutrino sphere radius (proxy for the PNS radius; magenta), and gain radius (dashed salmon). Note that the color scales as well as the radial (vertical) and temporal (horizontal) scales may vary from panel to panel.

Figure 3

Maximum temperature (solid lines) and its location (dashed lines) in models: R (black), F8 (red), A23 (orange), A16 (green), and A4 (blue).

Figure 4

Total (volume integrated) cooling rate due to active (black) and sterile (blue dashed line) neutrinos, and total heating rate due to active (orange) and sterile (red) neutrinos in A4 (left) and F8 (right) models from Table 1. The cooling due to active neutrinos is equivalent to the luminosity they would have if only SM processes were included. The decays of sterile to active neutrinos add an additional contribution to their total luminosity, which is shown by the greed dash-dotted lines.

Figure 5

Explosion energy (Eq. (35); solid lines) and unbound mass (Eq. (36); dashed lines) in A4 (black), A4D (orange), A4T (green), and F8 (red) models from Table 1. Note the excellent agreement (convergence) between the models A4, A4D, and A4T for $t\lesssim 130\mathrm{ms}$.

Figure 6

Absorption (solid lines) and scattering (dashed lines) opacities of electron neutrinos with energies 75–101 MeV (i.e., the most populated energy bin; black) and of sterile neutrinos in simulation A4 (green) at $t=10\mathrm{ms}$ postbounce. With red solid line is marked the absorption opacity of sterile neutrinos in model F8. Density profile is marked with blue dotted line.

Figure 7

Explosion energies of all AMP (top) and FKP (bottom) models as a function of, respectively, the magnetic, ${\mu }_{\mathrm{h}}$, and transition moment, ${\mu }_{\mathrm{tr}}$ (AMP), and of mass, $m_{\mathrm{h}}$, and mixing angle ${\sin }^2\theta$ (FKP) of the sterile neutrinos. In all AMP models but A30 (with $m_{\mathrm{h}}=80\mathrm{MeV}/c^2$, $m_{\mathrm{h}}=3.4\times {10}^{-9}{\mu }_{\mathrm{B}}$, and ${\mu }_{\mathrm{tr}}=3.4\times {10}^{-11}{\mu }_{\mathrm{B}}$), sterile neutrino mass $m_{\mathrm{h}}=50\mathrm{MeV}/c^2$. Nonexploding models are marked with empty diamonds. Regions of the parameter space with an either too long (cosmological constraint) or too short (experimental contraints) lifetime of sterile neutrinos are marked with dark gray. Light gray denotes region not excluded by our simulations. Models in yellow region are too energetic ($E_{\mathrm{expl}}>2\times {10}^{52}\mathrm{erg}$). In the gray region of ${\mu }_{\mathrm{tr}}>{\mu }_{\mathrm{h}}$ (top), the production channel $\nu X->{\nu }_{h}X$ (not included in the AMP model nor in our code) is important relative to (or even dominant over) $e^{+}{e}^{-}->{\overline{\nu }}_h{\nu }_h$ and therefore exploring it numerically was beyond the scope of this paper.