Axions and the white dwarf luminosity function (original) (raw)
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Axions and the Cooling of White Dwarf Stars
Astrophysical Journal, 2008
White dwarfs are the end product of the lifes of intermediate-and low-mass stars and their evolution is described as a simple cooling process. Recently, it has been possible to determine with an unprecedented precision their luminosity function, that is, the number of stars per unit volume and luminosity interval. We show here that the shape of the bright branch of this function is only sensitive to the averaged cooling rate of white dwarfs and we propose to use this property to check the possible existence of axions, a proposed but not yet detected weakly interacting particle. Our results indicate that the inclusion of the emission of axions in the evolutionary models of white dwarfs noticeably improves the agreement between the theoretical calculations and the observational white dwarf luminosity function. The best fit is obtained for m a cos 2 β ≈ 5 meV, where m a is the mass of the axion and cos 2 β is a free parameter. We also show that values larger than 10 meV are clearly excluded. The existing theoretical and observational uncertainties do not yet allow the confirmation of the existence of axions, but our results clearly show that if their mass is of the order of few meV, the white dwarf luminosity function is sensitive enough to detect their existence.
White dwarfs as physics laboratories: the case of axions
White dwarfs are almost completely degenerate objects that cannot obtain energy from thermonuclear sources, so their evolution is just a gravothermal cooling process. Recent improvements in the accuracy and precision of the luminosity function and in pulsational data of variable white dwarfs suggest that they are cooling faster than expected from conventional theory. In this contribution we show that the inclusion of an additional cooling term due to axions able to interact with electrons with a coupling constant g_ae ~(2-7)x10^{-13} allows to fit better the observations.
White dwarfs are almost completely degenerate objects that cannot obtain energy from the thermonuclear sources and their evolution is just a gravothermal process of cooling. The simplicity of these objects, the fact that the physical inputs necessary to understand them are well identified, although not always well understood, and the impressive observational background about white dwarfs make them the most well studied Galactic population. These characteristics allow to use them as laboratories to test new ideas of physics. In this contribution we discuss the robustness of the method and its application to the axion case. Comment: 4 pages, 1 figure, to appear in the Proceedings for the 6th Patras meeting on Axions, WIMPs and WISPs
An independent limit on the axion mass from the variable white dwarf star R548
Journal of Cosmology and Astroparticle Physics, 2012
Pulsating white dwarfs with hydrogen-rich atmospheres, also known as DAV stars, can be used as astrophysical laboratories to constrain the properties of fundamental particles like axions. Comparing the measured cooling rates of these stars with the expected values from theoretical models allows us to search for sources of additional cooling due to the emission of weakly interacting particles. In this paper, we present an independent inference of the mass of the axion using the recent determination of the evolutionary cooling rate of R548, the DAV class prototype. We employ a state-of-the-art code which allows us to perform a detailed asteroseismological fit based on fully evolutionary sequences. Stellar cooling is the solely responsible of the rates of change of period with time (Π) for the DAV class. Thus, the inclusion of axion emission in these sequences notably influences the evolutionary timescales, and also the expected pulsational properties of the DAV stars. This allows us to compare the theoreticalΠ values to the corresponding empirical rate of change of period with time of R548 to discern the presence of axion cooling. We found that if the dominant period at 213.13 s in R548 is associated with a pulsation mode trapped in the hydrogen envelope, our models indicate the existence of additional cooling in this pulsating white dwarf, consistent with axions of mass m a cos 2 β ∼ 17.1 meV at a 2σ confidence level. This determination is in agreement with the value inferred from another well-studied DAV, G117−B15A. We now have two independent and consistent estimates of the mass of the axion obtained from DAVs, although additional studies of other pulsating white dwarfs are needed to confirm this value of the axion mass.
Axions and the pulsation periods of variable white dwarfs revisited
Astronomy and Astrophysics, 2010
Context. Axions are the natural consequence of the introduction of the Peccei-Quinn symmetry to solve the strong CP problem. All the efforts to detect such elusive particles have failed up to now. Nevertheless, it has been recently shown that the luminosity function of white dwarfs is best fitted if axions with a mass of a few meV are included in the evolutionary calculations. Aims. Our aim is to show that variable white dwarfs can provide additional and independent evidence about the existence of axions. Methods. The evolution of a white dwarf is a slow cooling process that translates into a secular increase of the pulsation periods of some variable white dwarfs, the so-called DAV and DBV types. Since axions can freely escape from such stars, their existence would increase the cooling rate and, consequently, the rate of change of the periods as compared with the standard ones. Results. The present values of the rate of change of the pulsation period of G117-B15A are compatible with the existence of axions with the masses suggested by the luminosity function of white dwarfs, in contrast with previous estimations. Furthermore, it is shown that if such axions indeed exist, the drift of the periods of pulsation of DBV stars would be noticeably perturbed.
Monthly Notices of the Royal Astronomical Society, 2012
We employ a state-of-the-art asteroseismological model of G117−B15A, the archetype of the H-rich atmosphere (DA) white dwarf pulsators (also known as DAV or ZZ Ceti variables), and use the most recently measured value of the rate of period change for the dominant mode of this pulsating star to derive a new constraint on the mass of axion, the still conjectural non-barionic particle considered as candidate for dark matter of the Universe. Assuming that G117−B15A is truly represented by our asteroseismological model, and in particular, that the period of the dominant mode is associated to a pulsation g-mode trapped in the H envelope, we find strong indications of the existence of extra cooling in this star, compatible with emission of axions of mass m a cos 2 β = 17.4 +2.3 −2.7 meV.
Journal of Cosmology and Astroparticle Physics, 2016
We employ an asteroseismic model of L19−2, a relatively massive (M ⋆ ∼ 0.75M ⊙) and hot (T eff ∼ 12 100 K) pulsating DA (H-rich atmosphere) white dwarf star (DAV or ZZ Ceti variable), and use the observed values of the temporal rates of period change of its dominant pulsation modes (Π ∼ 113 s and Π ∼ 192 s), to derive a new constraint on the mass of the axion, the hypothetical non-barionic particle considered as a possible component of the dark matter of the Universe. If the asteroseismic model employed is an accurate representation of L19−2, then our results indicate hints of extra cooling in this star, compatible with emission of axions of mass m a cos 2 β 25 meV or an axion-electron coupling constant of g ae 7 × 10 −13 .
Axion stars in the infrared limit
Journal of High Energy Physics, 2015
Following Ruffini and Bonazzola, we use a quantized boson field to describe condensates of axions forming compact objects. Without substantial modifications, the method can only be applied to axions with decay constant, f a , satisfying δ = (f a / M P ) 2 1, where M P is the Planck mass. Similarly, the applicability of the Ruffini-Bonazzola method to axion stars also requires that the relative binding energy of axions satisfies ∆ = 1 − (E a / m a ) 2 1, where E a and m a are the energy and mass of the axion. The simultaneous expansion of the equations of motion in δ and ∆ leads to a simplified set of equations, depending only on the parameter, λ = √ δ / ∆ in leading order of the expansions. Keeping leading order in ∆ is equivalent to the infrared limit, in which only relevant and marginal terms contribute to the equations of motion. The number of axions in the star is uniquely determined by λ. Numerical solutions are found in a wide range of λ. At small λ the mass and radius of the axion star rise linearly with λ. While at larger λ the radius of the star continues to rise, the mass of the star, M , attains a maximum at λ max 0.58. All stars are unstable for λ > λ max .
Astrophysical methods to constrain axions and other novel particle phenomena
Physics Reports, 1990
Prologue 3 7.1. Energy loss and energy transfer in the Sun; first 12. 1 he stellar energy loss argument 4 constraints 60 1.3. Other methods of stellar particle physics 8 7.2. Results from a germanium spectrometer 62 2. Axion phenomenology 10 7.3. A magnetic conversion experiment 62 2.1. Generic features of the Peeeei-Quinn mechanism 10 7.4. Radiative particle decays and solar-y-rays 63 2.2. The most common axion models 15 8. Red giants and horizontal branch stars 63 2.3. Fine points of axion properties (7 8.1. The general agenda 64 3. Axion cosmology 22 8.2. The evolution of low-mass stars 65 3.1. Inflationary scenario 23 8.3. Suppression of the helium flash by particle emission 68 3.2. Topological structures 24 8.4. Reduction of the helium burning phase 71 3.3. Thermally produced axions 26 8.5. Core mass at the helium flash 74 3.4. Decaying axions and a glow of the night sky 26 9. The white dwarf luminosity function 77 3.5. Experimental search for galactic axions 27 9.1. White dwarfs: theoretical and observed properties 77 4. Emission rates from stellar plasmas 28 9.2. Cooling theory for white dwarfs 79 4.1. General discussion of the emission rates 28 9.3. Neutrino losses included 81 4.2. Absorption rates 29 9.4. Axion bounds 81 4.3. Many-body effects in stellar plasmas 30 10. Cooling of nascent and young neutron stars 82 4.4. Compton process 39 10.1. Birth and cooling of neutron stars 82 4.5. Electron-positron annihilation 41 10.2. Supernova explosions and new particle physics 87 4.6. Bremsstrahlung by electrons 42 1(1.3. SN 1987A bounds on novel cooling phenomena 89 4.7. Axio-recombination and the axio-electric effect 44 10.4. Non-detection of new particles from SN 1987A 93 4.8. Bremsstrahlung by nucleons 45 10.5. SN 1987A axion bounds from numerical investiga-4.9. Primakoff effect and axion-photon mixing 49 tions 93 4.10. Plasmon decay rate 54 10.6. Axion trapping 97 .5. Energy transfer 55 10.7. Axion bounds from Einstein observations 97 5.1. Radiative transfer by massive bosons 56 10.8. What if neutron stars are strange quark stars? 98 5.2. Opacity contribution of massive pseudoscalars 56 11. Summary of axion and neutrino hounds 98 6. Exotic energy loss of low-mass stars; analytic treatment 57 11.1. Neutrinos 98 6.1. The equations of stellar structure 57 11.2. Axions 100 6.2. Homologous changes 58 References 104