1,259 research outputs found
How Sound Are Our Ultralight Axion Approximations?
Ultralight axions (ULAs) are a promising dark-matter candidate. ULAs may have implications for small-scale challenges to the ΛCDM model and arise in string scenarios. ULAs are already constrained by cosmic microwave background (CMB) experiments and large-scale structure surveys, and will be probed with much greater sensitivity by future efforts. It is challenging to compute observables in ULA scenarios with sufficient speed and accuracy for cosmological data analysis because the ULA field oscillates rapidly. In past work, an effective fluid approximation has been used to make these computations feasible. Here this approximation is tested against an exact solution of the ULA equations, comparing the induced error of CMB observables with the sensitivity of current and future experiments. In the most constrained mass range for a ULA dark-matter component (10−27  eV≤max≤10−25  eV), the induced bias on the allowed ULA fraction of dark matter from Planck data is less than 1σ. In the cosmic-variance limit (including temperature and polarization data), the bias is ≲2σ for primary CMB anisotropies, with more severe biases (as high as ∼4σ) resulting for less reliable versions of the effective fluid approximation. If all of the standard cosmological parameters are fixed by other measurements, the expected bias rises to 4−20σ (well beyond the validity of the Fisher approximation), though the required level of degeneracy breaking will not be achieved by any planned surveys
Constraining mixing in massive stars in the Small Magellanic Cloud
Context. The evolution of massive stars is strongly influenced by internal
mixing processes such as semiconvection, convective core overshooting, and
rotationally induced mixing. None of these is currently well constrained. Aims.
We investigate models for massive stars in the Small Magellanic Cloud (SMC). We
aim to constrain the various mixing efficiencies by comparing model results to
observations. Methods. We use the stellar evolution code MESA to compute more
than 60 grids of detailed evolutionary models for stars with initial masses of
9 to 100 Msol, in each grid assuming different combinations of mixing
efficiencies. Results. We find that for most of the combinations of the mixing
efficiencies, models in a wide mass range spend core-helium burning either only
as blue supergiants, or only as red supergiants. The latter case corresponds to
models that maintain a shallow slope of the hydrogen/helium (H/He) gradient
separating the core and the envelope of the models. Only a small part of the
mixing parameter space leads to models that produce a significant number of
blue and red supergiants, which both exist abundantly in the SMC.
Interestingly, these models contain steep H/He gradients, as is required to
understand the hot, hydrogen-rich Wolf-Rayet stars in the SMC. We find that
unless it is very fast, rotation has a limited effect on the H/He profiles in
our models. Conclusions. While we use specific implementations of the
considered mixing processes, they comprehensively probe the two first order
structural parameters: the core mass and the H/He gradient in the core-envelope
interface. Our results imply that, in massive stars, the H/He gradients above
the helium cores become very steep. Our model grids can be tested with future
observational surveys of the massive stars in the SMC, and thereby help to
considerably reduce the uncertainties in models of massive star evolution.Comment: Accepted for publication in Astronomy and Astrophysics. 16 pages, 14
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Structural evaluation of candidate designs for the large space telescope primary mirror
Structural performance analyses were conducted on two candidate designs (Itek and Perkin-Elmer designs) for the large space telescope three-meter mirror. The mirror designs and the finite-element models used in the analyses evaluation are described. The results of the structural analyses for several different types of loading are presented in tabular and graphic forms. Several additional analyses are also reported: the evaluation of a mirror design concept proposed by the Boeing Co., a study of the global effects of local cell plate deflections, and an investigation of the fracture mechanics problems likely to occur with Cervit and ULE. Flexibility matrices were obtained for the Itek and Perkin-Elmer mirrors to be used in active figure control studies. Summary, conclusions, and recommendations are included
Tensor Detection Severely Constrains Axion Dark Matter
The recent detection of B-modes by BICEP2 has non-trivial implications for
axion dark matter implied by combining the tensor interpretation with
isocurvature constraints from Planck. In this paper the measurement is taken as
fact, and its implications considered, though further experimental verification
is required. In the simplest inflation models implies . If the axion decay constant constraints on
the dark matter (DM) abundance alone rule out the QCD axion as DM for (where accounts for theoretical
uncertainty). If then vacuum fluctuations of the axion field
place conflicting demands on axion DM: isocurvature constraints require a DM
abundance which is too small to be reached when the back reaction of
fluctuations is included. High QCD axions are thus ruled out. Constraints
on axion-like particles, as a function of their mass and DM fraction, are also
considered. For heavy axions with we find
, with stronger constraints on heavier
axions. Lighter axions, however, are allowed and (inflationary)
model-independent constraints from the CMB temperature power spectrum and large
scale structure are stronger than those implied by tensor modes.Comment: 6 pages, 1 figure. v2: Some discussion and references added. v3
Update on QCD discussion. Version accepted for publication in Physical Review
Letter
Baryons Still Trace Dark Matter: Probing CMB Lensing Maps For Hidden Isocurvature
Compensated isocurvature perturbations (CIPs) are primordial fluctuations that balance baryon and dark-matter isocurvature to leave the total matter density unperturbed. The effects of CIPs on the cosmic microwave background (CMB) anisotropies are similar to those produced by weak lensing of the CMB: smoothing of the power spectrum and generation of non-Gaussian features. Here, an entirely new CIP contribution to the standard estimator for the lensing-potential power spectrum is derived. Planck measurements of the temperature and polarization power spectrum, as well as estimates of CMB lensing, are used to place limits on the variance of the CIP fluctuations on CMB scales, Δ2rms(RCMB). The resulting constraint of Δ2rms(RCMB)\u3c4.3×10−3 at 95% confidence level (CL) using this new technique improves on past work by a factor of ∼3. We find that for Planck data our constraints almost reach the sensitivity of the optimal CIP estimator. The method presented here is currently the most sensitive probe of the amplitude of a scale-invariant CIP power spectrum, ACIP, placing an upper limit of ACIP\u3c0.017 at 95% CL. Future measurements of the large-scale CMB lensing-potential power spectrum could probe CIP amplitudes as low as Δ2rms(RCMB)=8×10−5 at 95% CL (corresponding to ACIP=3.2×10−4)
Subsonic structure and optically thick winds from Wolf--Rayet stars
Wolf-Rayet star's winds can be so dense and so optically thick that the
photosphere appears in the highly supersonic part of the outflow, veiling the
underlying subsonic part of the star, and leaving the initial acceleration of
the wind inaccessible to observations. We investigate the conditions and the
structure of the subsonic part of the outflow of Galactic WR stars, in
particular of the WNE subclass; our focus is on the conditions at the sonic
point. We compute 1D hydrodynamic stellar structure models for massive helium
stars adopting outer boundaries at the sonic point. We find that the outflows
of our models are accelerated to supersonic velocities by the radiative force
from opacity bumps either at temperatures of the order of 200kK by the Fe
opacity bump or of the order of 50kK by the HeII opacity bump. For a given
mass-loss rate, the conditions in the subsonic part of the outflow are
independent from the detailed physical conditions in the supersonic part. The
close proximity to the Eddington limit at the sonic point allows us to
construct a Sonic HR diagram, relating the sonic point temperature to the L/M
ratio and the stellar mass-loss rate, thereby constraining the sonic point
conditions, the subsonic structure, and the stellar wind mass-loss rates from
observations. The minimum mass-loss rate necessary to have the flow accelerated
to supersonic velocities by the Fe opacity bump is derived. A comparison of the
observed parameters of Galactic WNE stars to this minimum mass-loss rate
indicates that their winds are launched to supersonic velocities by the
radiation pressure arising from the Fe-bump. Conversely, models which do not
show transonic flows from the Fe opacity bump form inflated envelopes. We
derive an analytic criterion for the appearance of envelope inflation in the
subphotospheric layers.Comment: A&A, Forthcoming article. 13 pages+
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