8726146
A maximum stellar surface density in dense stellar systems
We compile observations of the surface mass density profiles of dense stellar systems, including globular clusters in the Milky Way and nearby galaxies, massive star clusters in nearby starbursts, nuclear star clusters in dwarf spheroidals and late-type discs, ultra-compact dwarfs, and galaxy spheroids spanning the range from low-mass 'cusp' bulges and ellipticals to massive 'core' ellipticals. We show that in all cases the maximum stellar surface density attained in the central regions of these systems is similar, � max ∼ 10 11 Mkpc −2 (∼20 g cm −2 ), despite the fact that the systems span ∼7 orders of magnitude in total stellar mass M ∗ and ∼ 5i n effective radius Re, and have a wide range in effective surface density M ∗/R 2. The surface density limit is reached on a wide variety of physical scales in different systems and is thus not a limit on three-dimensional stellar density. Given the very different formation mechanisms involved in these different classes of objects, we argue that a single piece of physics likely determinesmax. The radiation fields and winds produced by massive stars can have a significant influence on the formation of both star clusters and galaxies, while neither supernovae nor black hole accretion is important in star cluster formation. We thus conclude that feedback from massive stars likely accounts for the observedmax, plausibly because star formation reaches an Eddington-like flux that regulates the growth of these diverse systems. This suggests that current models of galaxy formation, which focus on feedback from supernovae and active galactic nuclei, are missing a crucial ingredient.
118571843
Effect of collective neutrino oscillations on the neutrino mechanism of core-collapse supernovae
In the seconds after collapse of a massive star, the newborn protoneutron star (PNS) radiates neutrinos of all flavours. The absorption of electron-type neutrinos below the radius of the stalled shockwave may drive explosions (the ‘neutrino mechanism’). Because the heating rate is proportional to the square of neutrino energy, flavour conversion of μ and τ neutrinos to electron-type neutrinos via collective neutrino oscillations (CνO) may in principle increase the heating rate and drive explosions. In order to assess the potential importance of CνO for the shock revival, we solve the steady-state boundary value problem of spherically symmetric accretion between the PNS surface (rν) and the shock (rS), including a scheme for flavour conversion via CνO. For a given rν, PNS mass (M), accretion rate () and assumed values of the neutrino energies from the PNS, we calculate the critical neutrino luminosity above which accretion is impossible and explosion results. We show that CνO can decrease the critical luminosity by a factor of at most ∼1.5, but only if the flavour conversion is fully completed inside rS and if there is no matter suppression. The magnitude of the effect depends on the model parameters (M, and rν) through the shock radius and the physical scale for flavour conversion. We quantify these dependencies and find that CνO could lower the critical luminosity only for small M and , and large rν. However, for these parameter values CνO are suppressed due to matter effects. By quantifying the importance of CνO and matter suppression at the critical neutrino luminosity for explosion, we show in agreement with previous studies that CνO are unlikely to affect the neutrino mechanism of core-collapse supernovae significantly.
16516458
The Ionization State of Sodium in Galactic Winds
Roughly 80% of ultraluminous infrared galaxies (ULIRGs) show blueshifted absorption in the resonance lines of neutral sodium, indicating that cool winds are common in such objects, as shown by Rupke et al. and by Martin. The neutral sodium (Na I) columns indicated by these absorption lines are ~1013-(3 × 1014) cm-2, a surprising result given that the bolometric luminosities range over a factor of only 4. We show that the gas in ULIRG outflows is likely to be in photoionization equilibrium. The very small ULIRG sample of Goldader et al. demonstrates that the ratio of UV flux to FIR flux varies by a factor ~100 from object to object. While the Goldader et al. sample does not overlap with those of Rupke et al. and Martin, we show that such a large variation in UV flux will produce a similar variation in the column of neutral sodium for a fixed mass flux and density. However, if the cold gas is in pressure equilibrium with a hot outflow with a mass-loss rate similar to the star formation rate, the range of ionization state is significantly smaller. Measurements of the UV flux for objects in the Martin and Rupke et al. catalogs will definitively determine whether photoionization effects are responsible for the wide variation seen in the sodium columns. If they are, a determination of the gas density and mass-loss rate in the cool winds will follow, with attendant improvements in our understanding of wind driving mechanisms and of the feedback effects of galaxies on their surroundings.
17438989
Magnetic Protoneutron Star Winds and r-Process Nucleosynthesis
Because of their neutron richness and association with supernovae, postexplosion protoneutron star winds are thought to be a likely astrophysical site for rapid neutron-capture nucleosynthesis (the r-process). However, the most recent models of spherical neutrino-driven protoneutron star winds do not produce robust r-process nucleosynthesis for "canonical" neutron stars with a gravitational mass of 1.4 M☉ and coordinate radius of 10 km. These models fail variously; the flow entropy is too low, the electron fraction is too high, or the dynamical expansion timescale is too long. To date, no models have included the effects of an ordered dipole magnetic field. We show that a strong magnetic field can trap the outflow in the neutrino heating region, thus leading to much higher matter entropy. We estimate both the trapping timescale and the resulting entropy amplification. For sufficiently large energy deposition rates, the trapped matter emerges dynamically from the region of closed magnetic field lines and escapes to infinity. We find that ordered dipoles with surface fields of 6 × 1014 G increase the asymptotic entropy sufficiently for robust r-process nucleosynthesis.
119104483
Relativistic MHD winds from rotating neutron stars
Abstract We review here recent numerical results concerning the acceleration and collimation of relativistic outflows from neutron stars, and we discuss their implications for models of magnetars in their hypothesized initial high rotation states. New results with different injection conditions and extending to much larger distance from the central star are also presented and discussed, showing that, only in the case of weakly magnetized winds, collimated outflows are indeed possible. We finally comment on the possibility that newly born magnetars might trigger GRBs, and we show that numerical results do not support such hypothesis.
119250898
Diffuse Hard X-ray Emission in Starburst Galaxies as Synchrotron from Very High Energy Electrons
The origin of the diffuse hard X-ray (2-10?keV) emission from starburst galaxies is a long-standing problem. We suggest that synchrotron emission of 10-100?TeV electrons and positrons (e ?) can contribute to this emission, because starbursts have strong magnetic fields. We consider three sources of e ? at these energies: (1) primary electrons directly accelerated by supernova remnants, (2) pionic secondary e ? created by inelastic collisions between cosmic ray (CR) protons and gas nuclei in the dense interstellar medium of starbursts, and (3) pair e ? produced between the interactions between 10 and 100?TeV ?-rays and the intense far-infrared (FIR) radiation fields of starbursts. We create one-zone steady-state models of the CR population in the Galactic center (R ? 112 pc), NGC?253, M82, and Arp?220's nuclei, assuming a power-law injection spectrum for electrons and protons. We consider different injection spectral slopes, magnetic field strengths, CR acceleration efficiencies, and diffusive escape times, and include advective escape, radiative cooling processes, and secondary and pair e ?. We compare these models to extant radio and GeV and?TeV ?-ray data for these starbursts, and calculate the diffuse synchrotron X-ray and inverse Compton (IC) luminosities of these starbursts in the models which satisfy multiwavelength constraints. If the primary electron spectrum extends to ~PeV energies and has a proton/electron injection ratio similar to the Galactic value, we find that synchrotron emission contributes 2%-20% of their unresolved, diffuse hard X-ray emission. However, there is great uncertainty in this conclusion because of the limited information on the CR electron spectrum at these high energies. IC emission is likewise a minority of the unresolved X-ray emission in these starbursts, from 0.1% in the Galactic center to 10% in Arp?220's nuclei, with the main uncertainty being the starbursts' magnetic field. We also model generic starbursts, including submillimeter galaxies, in the context of the FIR-X-ray relation, finding that anywhere between 0% and 16% of the total hard X-ray emission is synchrotron for different parameters, and up to 2% in the densest starbursts assuming an E ?2.2 injection spectrum and a diffusive escape time of 10 Myr (E/3 GeV)?1/2 (h/100 pc). Neutrino observations by IceCube and?TeV ?-ray data from HESS, VERITAS, and CTA can further constrain the synchrotron X-ray emission of starbursts. Our models do not constrain the possibility of hard, second components of primary e ? from sources like pulsars in starbursts, which could enhance the synchrotron X-ray emission further.
119183491
SN 2008iy: an unusual Type IIn Supernova with an enduring 400-d rise time
We present spectroscopic and photometric observations of the Type IIn supernova (SN) 2008iy. SN 2008iy showed an unprecedentedly long rise time of ~400 d, making it the first known SN to take significantly longer than 100 d to reach peak optical luminosity. The peak absolute magnitude of SN 2008iy was M r ≈ -19.1 mag, and the total radiated energy over the first ~700 d was ~2 x 10 50 erg. Spectroscopically, SN 2008iy is very similar to the Type IIn SN 1988Z at late times and, like SN 1988Z, it is a luminous X-ray source (both SNe had an X-ray luminosity L X > 10 41 erg s -1 ). SN 2008iy has a growing near-infrared excess at late times similar to several other SNe IIn. The Hα emission-line profile of SN 2008iy shows a narrow P Cygni absorption component, implying a pre-SN wind speed of ~100 km s -1 . We argue that the luminosity of SN 2008iy is powered via the interaction of the SN ejecta with a dense, clumpy circumstellar medium. The ~400-d rise time can be understood if the number density of clumps increases with distance over a radius ~1.7 x 10 16 cm from the progenitor. This scenario is possible if the progenitor experienced an episodic phase of enhanced mass loss < 1 century prior to explosion or if the progenitor wind speed increased during the decades before core collapse. We favour the former scenario, which is reminiscent of the eruptive mass-loss episodes observed for luminous blue variable (LBV) stars. The progenitor wind speed and increased mass-loss rates serve as further evidence that at least some, and perhaps all, Type IIn SNe experience LBV-like eruptions shortly before core collapse. We also discuss the host galaxy of SN 2008iy, a subluminous dwarf galaxy, and offer a few reasons why the recent suggestion that unusual, luminous SNe preferentially occur in dwarf galaxies may be the result of observational biases.