These are the latest articles published in The Astrophysical Journal Letters.

• Open/close Ubiquitous Torsional Motions in Type II Spicules

  B. De Pontieu et al. 2012 ApJ 752 L12 Tag this article

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  Spicules are long, thin, highly dynamic features that jut out ubiquitously from the solar limb. They dominate the interface between the chromosphere and corona and may provide significant mass and energy to the corona. We use high-quality observations with the Swedish 1 m Solar Telescope to establish that so-called type II spicules are characterized by the simultaneous action of three different types of motion: (1) field-aligned flows of order 50-100 km s ^–1, (2) swaying motions of order 15-20 km s ^–1, and (3) torsional motions of order 25-30 km s ^–1. The first two modes have been studied in detail before, but not the torsional motions. Our analysis of many near-limb and off-limb spectra and narrowband images using multiple spectral lines yields strong evidence that most, if not all, type II spicules undergo large torsional modulation and that these motions, like spicule swaying, represent Alfvénic waves propagating outward at several hundred km s ^–1. The combined action of the different motions explains the similar morphology of spicule bushes in the outer red and blue wings of chromospheric lines, and needs to be taken into account when interpreting Doppler motions to derive estimates for field-aligned flows in spicules and determining the Alfvénic wave energy in the solar atmosphere. Our results also suggest that large torsional motion is an ingredient in the production of type II spicules and that spicules play an important role in the transport of helicity through the solar atmosphere.

• Open/close Origin of the Cosmic-Ray Spectral Hardening

  Nicola Tomassetti 2012 ApJ 752 L13 Tag this article

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  Recent data from ATIC, CREAM, and PAMELA indicate that the cosmic-ray energy spectra of protons and nuclei exhibit a remarkable hardening at energies above 100 GeV nucleon ^–1. We propose that the hardening is an interstellar propagation effect that originates from a spatial change of the cosmic-ray transport properties in different regions of the Galaxy. The key hypothesis is that the diffusion coefficient is not separable into energy and space variables as usually assumed. Under this scenario, we can reproduce the observational data well. Our model has several implications for cosmic-ray acceleration/propagation physics and can be tested by ongoing experiments such as the Alpha Magnetic Spectrometer or Fermi-LAT.

• Open/close Discovery of Three Distant, Cold Brown Dwarfs in the WFC3 Infrared Spectroscopic Parallels Survey

  D. Masters et al. 2012 ApJ 752 L14 Tag this article

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  We present the discovery of three late-type (≥T4.5) brown dwarfs, including a probable Y dwarf, in the WFC3 Infrared Spectroscopic Parallels (WISP) survey. We use the G141 grism spectra to determine the spectral types of the dwarfs and derive distance estimates based on a comparison with nearby T dwarfs with known parallaxes. These are the most distant spectroscopically confirmed T/Y dwarfs, with the farthest at an estimated distance of ~400 pc. We compare the number of cold dwarfs found in the WISP survey with simulations of the brown dwarf mass function. The number found is generally consistent with an initial stellar mass function dN/ dM M ^–α with α = 0.0-0.5, although the identification of a Y dwarf is somewhat surprising and may be indicative of either a flatter absolute magnitude/spectral-type relation than previously reported or an upturn in the number of very-late-type brown dwarfs in the observed volume.

• Open/close Discovery of Super-Li-rich Red Giants in Dwarf Spheroidal Galaxies

  Evan N. Kirby et al. 2012 ApJ 752 L16 Tag this article

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  Stars destroy lithium (Li) in their normal evolution. The convective envelopes of evolved red giants reach temperatures of millions of kelvin, hot enough for the ⁷Li( p, α) ⁴He reaction to burn Li efficiently. Only about 1% of first-ascent red giants more luminous than the luminosity function bump in the red giant branch exhibit A(Li) > 1.5. Nonetheless, Li-rich red giants do exist. We present 15 Li-rich red giants—14 of which are new discoveries—among a sample of 2054 red giants in Milky Way dwarf satellite galaxies. Our sample more than doubles the number of low-mass, metal-poor ([Fe/H] –0.7) Li-rich red giants, and it includes the most-metal-poor Li-enhanced star known ([Fe/H] = –2.82, A(Li) _NLTE = 3.15). Because most of the stars have Li abundances larger than the universe's primordial value, the Li in these stars must have been created rather than saved from destruction. These Li-rich stars appear like other stars in the same galaxies in every measurable regard other than Li abundance. We consider the possibility that Li enrichment is a universal phase of evolution that affects all stars, and it seems rare only because it is brief.

• Open/close General Relativistic Simulations of Magnetized Plasmas around Merging Supermassive Black Holes

  Bruno Giacomazzo et al. 2012 ApJ 752 L15 Tag this article

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  Coalescing supermassive black hole binaries are produced by the mergers of galaxies and are the most powerful sources of gravitational waves accessible to space-based gravitational observatories. Some such mergers may occur in the presence of matter and magnetic fields and hence generate an electromagnetic counterpart. In this Letter, we present the first general relativistic simulations of magnetized plasma around merging supermassive black holes using the general relativistic magnetohydrodynamic code Whisky. By considering different magnetic field strengths, going from non-magnetically dominated to magnetically dominated regimes, we explore how magnetic fields affect the dynamics of the plasma and the possible emission of electromagnetic signals. In particular, we observe a total amplification of the magnetic field of ~2 orders of magnitude, which is driven by the accretion onto the binary and that leads to much stronger electromagnetic signals, more than a factor of 10 ⁴ larger than comparable calculations done in the force-free regime where such amplifications are not possible.