Neutrinos from stars in the Milky Way

TL;DR

This work predicts the cumulative Galactic stellar neutrino flux (GSνF) at Earth by combining Gaia-based spatial distributions with a diverse set of MESA stellar models spanning $0.08\,M_\odot$ to $100\,M_\odot$, evolved from the pre-main sequence to final fates. The GSνF, spanning from keV to MeV energies, is shaped by thermal processes at low energies and thermonuclear processes at higher energies, with the thin disk and distances of $5-10$ kpc driving the dominant contribution. The study carefully models the Milky Way with a two-infall SFH, three Galactic components, and a consistent IMF, and computes the energy spectra by merging pp-chain, CNO-cycle, and $^{18}$F-related channels with several thermal neutrino processes, while noting the absence of flavor conversion in the source. The resulting GSνF is several orders of magnitude below solar and DSNB backgrounds, presenting substantial observational challenges but offering a potential new avenue for low-energy neutrino astronomy and tests of beyond-Standard-Model physics through neutrino propagation and interactions. Directional information and solar flux modulation could help distinguish the GSνF from solar and DSNB backgrounds, enabling insights into the Galactic stellar population and contributing to neutrino astrophysics and New Physics explorations.

Abstract

Neutrinos are produced during stellar evolution by means of thermal and thermonuclear processes. We model the cumulative neutrino flux expected at Earth from all stars in the Milky Way: the Galactic stellar neutrino flux (GS$ν$F). We account for the star formation history of our Galaxy and reconstruct the spatial distribution of Galactic stars by means of a random sampling procedure based on Gaia Data Release 2. We use the stellar evolution code $\texttt{MESA}$ to compute the neutrino emission for a suite of stellar models with solar metallicity and zero-age-main-sequence mass between $0.08M_\odot$ and $100\ M_\odot$, from their pre-main sequence phase to their final fates. We then reconstruct the evolution of the neutrino spectral energy distribution for each stellar model in our suite. The GS$ν$F lies between $\mathcal{O}(1)$ keV and $\mathcal{O}(10)$ MeV, with thermal (thermonuclear) processes responsible for shaping neutrino emission at energies smaller (larger) than $0.1$ MeV. Stars with mass larger than $\mathcal{O}(1\ M_\odot)$, located in the thin disk of the Galaxy, provide the largest contribution to the GS$ν$F. Moreover, most of the GS$ν$F originates from stars distant from Earth about $5-10$ kpc, implying that a large fraction of stellar neutrinos can reach us from the Galactic Center. Solar neutrinos and the diffuse supernova neutrino background have energies comparable to those of the GS$ν$F, challenging the detection of the latter. However, directional information of solar neutrino and GS$ν$F events, together with the annual modulation of the solar neutrino flux, could facilitate the GS$ν$F detection; this will kick off a new era for low-energy neutrino astronomy, also providing a novel probe to discover New Physics.

Figures (6)

• Figure 1: Galactic stellar neutrino and antineutrino flux at Earth for all flavors as a function of neutrino energy (dashed line), including solar neutrinos. The contributions from low-mass ($0.08 M_\odot \lesssim M < 0.9 M_\odot)$, intermediate-mass ($0.9 M_\odot \leq M \lesssim 8 M_\odot$), and high-mass ($M \geq 8 M_\odot$) stars are displayed as solid lines in orange, red, and blue, respectively. The light and dark gray shaded areas indicate the flux expected at Earth from solar neutrinos and the DSNB, respectively, as from Ref. Vitagliano:2019yzm.
• Figure 2: Top left: Sketch of the Milky Way, highlighting its three components: the bulge, the thin disk, and thick disk. To guide the eye, we also mark the distance of the Solar System from the Galactic Center. Top right: SFH of the Milky Way ($\zeta$, Eq. \ref{['eq:sfh']}), normalized to unity and in in arbitrary units (a.u.), as a function of the lookback time, assuming the Universe is $13.5$ Gyr old. We assume the two-infall formation model. The red line corresponds to the first infall episode, responsible for the formation of the bulge and thick disk; the orange line represents the second infall episode, leading to the formation of the thin disk. Bottom: IMF ($\xi$, in black, Eq. \ref{['eq:imf']}) normalized to unity, in arbitrary units, and as a function of the ZAMS mass. The vertical dashed lines indicate the ZAMS mass of the progenitors evolved in MESA in this work; the shaded bands indicate the range of ZAMS masses for which each model, marked by a dashed line, is assumed to be representative. The orange, red, and blue bands correspond to our low- ($88.9\%$), intermediate- ($10.5\%$), and high-mass ($0.6\%$) stellar populations.
• Figure 3: Galactic stellar neutrino and antineutrino flux at Earth for all flavors and from all low-mass (top), intermediate-mass (middle), and high-mass (bottom) stars in our Galaxy. The (anti)neutrino emission from the bulge, thin disk, and thick disk is displayed from left to right, respectively. The total emission for each mass range is marked by a black dashed line; the solid (dotted) lines correspond to thermonuclear (thermal) neutrinos. The (anti)neutrino emission from low- and high-mass stars is dominated by thermonuclear processes, while the low-energy tail of intermediate-mass stars is due to thermal processes. Overall, the largest (anti)neutrino emission comes from the thin disk, independent of the ZAMS mass range.
• Figure 4: Galactic stellar flux of neutrinos and antineutrinos (black dashed line) as a function of the neutrino energy and for all flavors. The solid and dotted lines distinguish between the thermonuclear or thermal origin of neutrino emission, respectively. The flux from solar neutrinos and the DSNB, modeled as in Ref. Vitagliano:2019yzm, are shown as light and dark gray bands, respectively. Left: Contributions to the GS$\nu$F for different distances from Earth. We distinguish among $5$ distance bins in the range between $0$ and $20$ kpc from Earth ($0$--$6$, $6$--$8$, $8$--$10$, $10$--$15$, and $15$--$20$ kpc plotted in orange, red, blue, and green, respectively). The GS$\nu$F is dominated from stars between $6$ and $10$ kpc from Earth. Right: Contributions to the GS$\nu$F from the bulge, the thin disk, and the thick disk are plotted in purple, magenta, and light blue, respectively. Stars in the thin disk of the Milky Way provide the largest contribution to the GS$\nu$F.
• Figure 5: Differential neutrino emission rate from the thermonuclear reactions contributing to the GS$\nu$F as a function of the distance from Earth from the bulge, thin disk, and thick disk in the left, middle, and right panels, respectively. The orange, red, and blue lines correspond to the flux from the pp chain, the CNO cycle, and $^{18}$F neutrinos from He burning flashes (see main text for details). The distance of the Galactic Center from Earth is marked by a gray dashed vertical line. The flux from the CNO cycle dominates, with the largest contribution coming from the thin disk.