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[en] We estimate the long-duration gamma-ray burst (LGRB) progenitor rate using our recent work on the effects of environmental metallically on LGRB formation in concert with supernovae (SNe) statistics via an approach patterned loosely off the Drake equation. Beginning with the cosmic star formation history, we consider the expected number of broad-line Type Ic events (the SNe type associated with LGRBs) that are in low-metallicity host environments adjusted by the contribution of high-metallicity host environments at a much reduced rate. We then compare this estimate to the observed LGRB rate corrected for instrumental selection effects to provide a combined estimate of the efficiency fraction of these progenitors to produce LGRBs and the fraction of which are beamed in our direction. From this we estimate that an aligned LGRB occurs for approximately every 4000 ± 2000 low-metallically broad-lined SNe Ic. Therefore, if one assumes a semi-nominal beaming factor of 100, then only about one such supernova out of 40 produce an LGRB. Finally, we propose an off-axis LGRB search strategy of targeting only broad-line Type Ic events that occur in low-metallicity hosts for radio observation.
[en] Recently, it has been suggested that the metallicity aversion of Long-duration Gamma Ray Bursts (LGRBs) is not intrinsic to their formation, but rather a consequence of the anti-correlation between star formation and metallicity seen in the general galaxy population. To investigate this proposal, we compare the metallicity of the hosts of LGRBs, broad-lined Type Ic (Ic-bl) supernovae (SNe), and Type II SNe to each other and to the metallicity distribution of star-forming galaxies using the Sloan Digital Sky Survey (SDSS) to represent galaxies in the local universe and the Team Keck Redshift Survey (TKRS) for galaxies at intermediate redshifts. The differing metallicity distributions of LGRB hosts and the star formation in local galaxies forces us to conclude that the low-metallicity preference of LGRBs is not primarily driven by the anti-correlation between star formation and metallicity, but rather must be overwhelmingly due to the astrophysics of the LGRBs themselves. Three quarters of our LGRB sample are found at metallicities below 12+log(O/H) < 8.6, while less than a one-tenth of local star formation is at similarly low metallicities. However, our SN samples are statistically consistent with the metallicity distribution of the general galaxy population. Additionally, we show that the star formation rate distribution of the LGRB and SNe host populations are consistent with the star formation rate distribution of the SDSS galaxy sample. This provides further evidence that the low-metallicity distribution of LGRBs is not caused by the general properties of star-forming galaxies. Using the TKRS population of galaxies, we can exclude the possibility that the LGRB host metallicity aversion is caused by the decrease in galaxy metallicity with redshift, as this effect is clearly much smaller than the observed LGRB host metallicity bias over the redshift span of our sample. The presence of the strong metallicity difference between LGRBs and Type Ic-bl SNe largely eliminates the possibility that the observed LGRB metallicity bias is a byproduct of a difference in the initial mass functions of the galaxy populations. Rather, metallicity below half-solar must be a fundamental component of the evolutionary process that separates LGRBs from the vast majority of Type Ic-bl SNe and from the bulk of local star formation
[en] There is now strong evidence that long-duration gamma-ray bursts (LGRBs) are preferentially formed in low-metallicity environments. However, the magnitude of this effect and its functional dependence on metallicity have not been well characterized. In our previous paper, we compared the metallicity distribution of LGRB host galaxies to that of star-forming galaxies in the local universe. Here we build upon this work by in effect dividing one distribution by the other, and thus directly determine the relative rate of LGRB formation as a function of metallicity in the low-redshift universe. We find a dramatic cutoff in LGRB formation above a metallicity of in the KK04 scale, with LGRBs forming between 10 and 50 times more frequently per unit star formation below this cutoff than above. Furthermore, our data suggest that the rate of LGRB formation per unit star formation continues to fall above this break. We estimate that the LGRB formation rate per unit star formation may drop by as much as a factor of 100 between one-third solar and solar metallicity.
[en] We quantify possible differences between turbulent dynamo action in the Sun and the dynamo action studied in idealized simulations. For this purpose, we compare Fourier-space shell-to-shell energy transfer rates of three incrementally more complex dynamo simulations: an incompressible, periodic simulation driven by random flow, a simulation of Boussinesq convection, and a simulation of fully compressible convection that includes physics relevant to the near-surface layers of the Sun. For each of the simulations studied, we find that the dynamo mechanism is universal in the kinematic regime because energy is transferred from the turbulent flow to the magnetic field from wavenumbers in the inertial range of the energy spectrum. The addition of physical effects relevant to the solar near-surface layers, including stratification, compressibility, partial ionization, and radiative energy transport, does not appear to affect the nature of the dynamo mechanism. The role of inertial-range shear stresses in magnetic field amplification is independent from outer-scale circumstances, including forcing and stratification. Although the shell-to-shell energy transfer functions have similar properties to those seen in mean-flow driven dynamos in each simulation studied, the saturated states of these simulations are not universal because the flow at the driving wavenumbers is a significant source of energy for the magnetic field.
[en] Following the optical imaging of exoplanet candidate Fomalhaut b (Fom b), we present a numerical model of how Fomalhaut's debris disk is gravitationally shaped by a single interior planet. The model is simple, adaptable to other debris disks, and can be extended to accommodate multiple planets. If Fom b is the dominant perturber of the belt, then to produce the observed disk morphology it must have a mass M pl < 3M J, an orbital semimajor axis a pl > 101.5 AU, and an orbital eccentricity e pl = 0.11-0.13. These conclusions are independent of Fom b's photometry. To not disrupt the disk, a greater mass for Fom b demands a smaller orbit farther removed from the disk; thus, future astrometric measurement of Fom b's orbit, combined with our model of planet-disk interaction, can be used to determine the mass more precisely. The inner edge of the debris disk at a ∼ 133 AU lies at the periphery of Fom b's chaotic zone, and the mean disk eccentricity of e ∼ 0.11 is secularly forced by the planet, supporting predictions made prior to the discovery of Fom b. However, previous mass constraints based on disk morphology rely on several oversimplifications. We explain why our constraint is more reliable. It is based on a global model of the disk that is not restricted to the planet's chaotic zone boundary. Moreover, we screen disk parent bodies for dynamical stability over the system age of ∼ 100 Myr, and model them separately from their dust grain progeny; the latter's orbits are strongly affected by radiation pressure and their lifetimes are limited to ∼ 0.1 Myr by destructive grain-grain collisions. The single planet model predicts that planet and disk orbits be apsidally aligned. Fomalhaut b's nominal space velocity does not bear this out, but the astrometric uncertainties may be large. If the apsidal misalignment proves real, our calculated upper mass limit of 3M J still holds. If the orbits are aligned, our model predicts M pl = 0.5M J, a pl = 115 AU, and e pl = 0.12. Parent bodies are evacuated from mean-motion resonances with Fom b; these empty resonances are akin to the Kirkwood gaps opened by Jupiter. The belt contains at least 3M + of solids that are grinding down to dust, their velocity dispersions stirred so strongly by Fom b that collisions are destructive. Such a large mass in solids is consistent with Fom b having formed in situ.
[en] We present spatially resolved near-IR spectroscopic observations of 15 young stars. Using a grism spectrometer behind the Keck interferometer, we obtained an angular resolution of a few milliarcseconds and a spectral resolution of 230, enabling probes of both gas and dust in the inner disks surrounding the target stars. We find that the angular size of the near-IR emission typically increases with wavelength, indicating hot, presumably gaseous material within the dust sublimation radius. Our data also clearly indicate Brγ emission arising from hot hydrogen gas, and suggest the presence of water vapor and carbon monoxide gas in the inner disks of several objects. This gaseous emission is more compact than the dust continuum emission in all cases. We construct simple physical models of the inner disk and fit them to our data to constrain the spatial distribution and temperature of dust and gas emission components.
[en] We present near-infrared observations of T Tauri and Herbig Ae/Be stars with a spatial resolution of a few milliarcseconds and a spectral resolution of ∼2000. Our observations spatially resolve gas and dust in the inner regions of protoplanetary disks, and spectrally resolve broad-linewidth emission from the Brγ transition of hydrogen gas. We use the technique of spectro-astrometry to determine centroids of different velocity components of this gaseous emission at a precision orders of magnitude better than the angular resolution. In all sources, we find the gaseous emission to be more compact than or distributed on similar spatial scales to the dust emission. We attempt to fit the data with models including both dust and Brγ-emitting gas, and we consider both disk and infall/outflow morphologies for the gaseous matter. In most cases where we can distinguish between these two models, the data show a preference for infall/outflow models. In all cases, our data appear consistent with the presence of some gas at stellocentric radii of ∼0.01 AU. Our findings support the hypothesis that Brγ emission generally traces magnetospherically driven accretion and/or outflows in young star/disk systems.
[en] We present the discovery of the optical afterglow and early-type host galaxy of the short-duration GRB 100117A. The faint afterglow is detected 8.3 hr after the burst with rAB = 25.46 ± 0.20 mag. Follow-up optical and near-infrared observations uncover a coincident compact red galaxy, identified as an early-type galaxy at a spectroscopic redshift of z ∼ 0.915 with a mass of ∼3 x 1010 Msun, an age of ∼1 Gyr, and a luminosity of LB ≅ 0.5 L*. From a possible weak detection of [O II]λ3727 emission at z = 0.915 we infer an upper bound on the star formation rate of ∼0.1 Msun yr-1, leading to a specific star formation rate of ∼<0.004 Gyr-1. Thus, GRB 100117A is only the second short burst to date with a secure early-type host (the other being GRB 050724 at z = 0.257) and it has one of the highest short gamma-ray burst (GRB) redshifts. The offset between the host center and the burst position, 470 ± 310 pc, is the smallest to date. Combined with the old stellar population age, this indicates that the burst likely originated from a progenitor with no significant kick velocity. However, from the brightness of the optical afterglow we infer a relatively low density of n ∼ 3 x 10-4 ε-3e,-1ε-1.75B,-1 cm-3. The combination of an optically faint afterglow and host suggests that previous such events may have been missed, thereby potentially biasing the known short GRB host population against z ∼> 1 early-type hosts.
[en] We detect the optical afterglow and host galaxy of GRB 070714B. Our observations of the afterglow show an initial plateau in the light curve for approximately the first 5-25 minutes, and then steepening to a power-law decay with index α = 0.86 ± 0.10 for the period between 1 and 24 hr postburst. This is consistent with the X-ray light curve which shows an initial plateau followed by a similar subsequent decay. At late time, we detect a host galaxy at the location of the optical transient. Gemini Nod and Shuffle spectroscopic observations of the host show a single emission line at 7167 A which, based on a griz JHK photometric redshift, we conclude is the 3727 A [O II] line. We therefore find a redshift of z = 0.923. This redshift, as well as a subsequent probable spectroscopic redshift determination of GRB 070429B at z = 0.904 by two other groups significantly exceeds the previous highest spectroscopically confirmed short burst redshift of z = 0.546 for GRB 051221. This dramatically moves back the time at which we know short bursts were being formed and suggests that the present evidence for an old progenitor population may be observationally biased.
[en] We report on the successful science verification phase of a new observing mode at the Keck Interferometer, which provides a line-spread function width and sampling of 150 km s-1 at the K'-band, at a current limiting magnitude of K' ∼ 7 mag with a spatial resolution of λ / 2 B ∼ 2.7 mas and a measured differential phase stability of unprecedented precision (3 mrad at K = 5 mag, which represents 3 μ as on the sky or a centroiding precision of 10-3). The scientific potential of this mode is demonstrated by the presented observations of the circumstellar disk of the evolved Be-star 48 Lib. In addition to indirect methods such as multi-wavelength spectroscopy and polarimetry, the spectro-interferometric astrometry described here provides a new tool to directly constrain the radial density structure in the disk. For the first time, we resolve several Pfund emission lines, in addition to Br γ, in a single interferometric spectrum, with adequate spatial and spectral resolution and precision to analyze the radial disk structure in 48 Lib. The data suggest that the continuum and Pf-emission originates in significantly more compact regions, inside the Br γ-emission zone. Thus, spectro-interferometric astrometry opens the opportunity to directly connect the different observed line profiles of Br γ and Pfund in the total and correlated flux to different disk radii. The gravitational potential of a rotationally flattened Be star is expected to induce a one-armed density perturbation in the circumstellar disk. Such a slowly rotating disk oscillation has been used to explain the well-known periodic V/R spectral profile variability in these stars, as well as the observed V/R cycle phase shifts between different disk emission lines. The differential line properties and linear constraints set by our data are consistent with theoretical models and lend direct support to the existence of a radius-dependent disk density perturbation. The data also show decreasing gas rotation velocities at increasing stellocentric radii as expected for Keplerian disk rotation, assumed by those models.