No abstract

Starburst‐driven galactic winds are responsible for the transport of mass, in particular metal‐enriched gas, and energy out of galaxies and into the intergalactic medium. These outflows directly affect the chemical evolution of galaxies, and heat and enrich the intergalactic and intercluster medium. Currently, several basic problems preclude quantitative measurements of the impact of galactic winds: the unknown filling factors of, in particular, the soft X‐ray‐emitting gas prevent accurate measurements of densities, masses and energy content; multiphase temperature distributions of unknown complexity bias X‐ray‐determined abundances; unknown amounts of energy and mass may reside in hard to observe T∼105 K and T∼107.5 K phases; and the relative balance of thermal versus kinetic energy in galactic winds is not known. In an effort to address these problems, we perform an extensive hydrodynamical parameter study of starburst‐driven galactic winds, motivated by the latest observation data on the best‐studied starburst galaxy M82. We study how the wind dynamics, morphology and X‐ray emission depend on the ISM distribution of the host galaxy, the starburst star formation history and strength, and the presence and distribution of mass‐loading by dense clouds. We also investigate and discuss the influence of finite numerical resolution on the results of these simulations. We find that the soft X‐ray emission from galactic winds comes from low filling factor (η≲2 per cent) gas, which contains only a small fraction (≲10 per cent) of the mass and energy of the wind, irrespective of whether the wind models are strongly mass‐loaded or not. X‐ray observations of galactic winds do not directly probe the gas that contains the majority of the energy, mass or metal‐enriched gas in the outflow. X‐ray emission comes from a complex phase‐continuum of gas, covering a wide range of different temperatures and densities. No distinct phases, as are commonly assumed when fitting X‐ray spectra, are seen in our models. Estimates of the properties of the hot gas in starburst galaxies based on fitting simple spectral models to existing X‐ray spectra should be treated with extreme suspicion. The majority of the thermal and kinetic energy of these winds is in a volume‐filling hot, T∼107 K, component which is extremely difficult to probe observationally because of its low density and hence low emissivity. Most of the total energy is in the kinetic energy of this hot gas, a factor that must be taken into account when attempting to constrain wind energetics observationally. We also find that galactic winds are efficient at transporting large amounts of energy out of the host galaxy, in contrast to their inefficiency at transporting mass out of star‐forming galaxies.

Measurements of 500 Sun-like stars show that their properties differ from those predicted by stellar population models.

The X‐ray emission from the supermassive star η Car is simulated using a 3D model of the wind–wind collision. In the model the intrinsic X‐ray emission is spatially extended and energy dependent. Absorption due to the unshocked stellar winds and the cooled post‐shock material from the primary LBV star is calculated as the intrinsic emission is ray traced along multiple sightlines through the 3D spiral structure of the circumstellar environment. The observable emission is then compared to available X‐ray data, including the light curve observed by the Rossi X‐ray Timing Explorer (RXTE) and spectra observed by XMM–Newton. The orientation and eccentricity of the orbit are explored, as are the wind parameters of the stars and the nature and physics of their close approach. Our modelling supports a viewing angle with an inclination of ≃42°, consistent with the polar axis of the Homunculus nebula, and the projection of the observer's line of sight on to the orbital plane has an angle of ≃0°–30° in the prograde direction on the apastron side of the semimajor axis. However, there are significant discrepancies between the observed and model light curves and spectra through the X‐ray minimum. In particular, the hard flux in our synthetic spectra is an order of magnitude greater than observed. This suggests that the hard X‐ray emission near the apex of the wind–wind collision region (WCR) ‘switches off’ from periastron until two months afterwards. Further calculations reveal that radiative inhibition significantly reduces the pre‐shock velocity of the companion wind. As a consequence the hard X‐ray emission is quenched, but it is unclear whether the long duration of the minimum is due solely to this mechanism alone. For instance, it is possible that the collapse of the WCR on to the surface of the companion star, which would be aided by significant inhibition of the companion wind, could cause an extended minimum as the companion wind struggles to re‐establish itself as the stars recede. For orbital eccentricities, e≲ 0.95, radiative braking prevents a wind collision with the companion star's surface. Models incorporating a collapse/disruption of the WCR and/or reduced pre‐shock companion wind velocities bring the predicted emission and the observations into much better agreement.

We have measured solar-like oscillations in red giants using time-series photometry from the first 34 days of science operations of the Kepler Mission. The light curves, obtained with 30 minute sampling, reveal clear oscillations in a large sample of G and K giants, extending in luminosity from the red clump down to the bottom of the giant branch. We confirm a strong correlation between the large separation of the oscillations (Δν) and the frequency of maximum power (ν max ). We focus on a sample of 50 low-luminosity stars (ν max > 100 μHz, L 30 L ) having high signal-to-noise ratios and showing the unambiguous signature of solar-like oscillations. These are H-shellburning stars, whose oscillations should be valuable for testing models of stellar evolution and for constraining the star formation rate in the local disk. We use a new technique to compare stars on a singleéchelle diagram by scaling their frequencies and find well-defined ridges corresponding to radial and non-radial oscillations, including clear evidence for modes with angular degree l = 3. Measuring the small separation between l = 0 and l = 2 allows us to plot the so-called C-D diagram of δν 02 versus Δν. The small separation δν 01 of l = 1 from the midpoint of adjacent l = 0 modes is negative, contrary to the Sun and solar-type stars. The ridge for l = 1 is notably broadened, which we attribute to mixed modes, confirming theoretical predictions for low-luminosity giants. Overall, the results demonstrate the tremendous potential of Kepler data for asteroseismology of red giants.