119272545
Bubbles of Nothing and the Fastest Decay in the Landscape
The rate and manner of vacuum decay are calculated in an explicit flux compactification, including all thick-wall and gravitational effects. For landscapes built of many units of a single flux, the fastest decay is usually to discharge just one unit. By contrast, for landscapes built of a single unit each of many different fluxes, the fastest decay is usually to discharge all the flux at once, which destabilizes the radion and begets a bubble of nothing. By constructing the bubble of nothing as the limit in which ever more flux is removed, we gain new insight into the bubble's appearance. Finally, we describe a new instanton that mediates simultaneous flux tunneling and decompactification. Our model is the thin-brane approximation to six-dimensional Einstein-Maxwell theory.
119317094
A Wrinkle in Coleman - De Luccia
The decay of a false vacuum in a quantum field theory with the conventional 1 2 (∂μφ) 2 kinetic term was first studied by Coleman[1] via the semiclassical approximation. The effects of gravitation were later included[2]. This work found application in the string landscape, where vacuum decay offers the mechanism by which the multitude of string vacua can be sampled [3]. However, the action appropriate in an open string context has a richer structure than that considered in [1, 2]. We will show that this structure can radically modify the lifetime of certain vacua. To exemplify these modifications, in this letter we consider the case where open string moduli explore the string landscape. Branes and the charges they carry are a crucial part of stable compactifications, and the dynamics of such objects under any process of vacuum selection warrants careful study. In the context of D3 branes probing a type IIB compactification[4] with warp factor 1/f(φ) and potential V (φ)[5], the relevant action is of the Dirac-Born-Infeld (DBI) type. Using this action we will discover two novel features: that the dominant instanton may have a “wrinkle” (see Fig. 2), and that raising the potential barrier can increase the tunneling rate. The net result is that as the barrier height reaches the string scale, the decay rate becomes orders of magnitude faster than the CDL prediction. This effect can be traced to the fact that nucleated bubbles of true vacuum can have parametrically lower domain wall tensions than those arising from the CDL theory. We assume the reader is familiar with the original work by Coleman and De Luccia, which we follow closely.
26270443
Boom and bust inflation: a graceful exit via compact extra dimensions.
A model of inflation is proposed in which compact extra dimensions allow a graceful exit without recourse to flat potentials or super-Planckian field values. Though bubbles of true vacuum are too sparse to uniformly reheat the Universe by colliding with each other, a compact dimension enables a single bubble to uniformly reheat by colliding with itself. This mechanism, which generates an approximately scale invariant perturbation spectrum, requires that inflation be driven by a bulk field, that vacuum decay be slow, and that the extra dimension be at least a hundred times larger than the false vacuum Hubble length.
17267013
Detection of satellite remnants in the Galactic Halo with Gaia– I. The effect of the Galactic background, observational errors and sampling
We address the problem of identifying remnants of satellite galaxies in the halo of our galaxy with Gaia data. The Gaia astrometric mission offers a unique opportunity to search for and study these remnants using full phase-space information for the halo of our Galaxy. However, the remnants have to be extracted from a very large data set (of the order of 109 stars) in the presence of observational errors and against a background population of Galactic stars. We address this issue through numerical simulations with a view towards timely preparations for the scientific exploitation of the Gaia data. 
 
 
 
We present a Monte Carlo simulation of the Gaia catalogue with a realistic number of entries. We use a model of the galaxy that includes separate light distributions and kinematics for the bulge, disc and stellar halo components. For practical reasons we exclude the region within Galactic coordinates: −90°≤l≤ 90° and −5°≤b≤ 5°, close to the Galactic plane and centre. Nevertheless, our catalogue contains 3.5 × 108 stars. No interstellar absorption has been modelled, as we limit our study to high Galactic latitudes. 
 
 
 
We perform tree code 106–body simulations of satellite dwarf galaxies in orbit around a rigid mass model of the Galaxy. We follow the simulations for 1010 yr. The resulting shrinking satellite cores and tidal tails are then added to the Monte Carlo simulation of the Gaia catalogue. To assign photometric properties to the particles we use a Hess diagram for the Solar neighbourhood for Galactic particles, while for the dwarf galaxy particles we use isochrones from the Padova group. When combining the Milky Way and dwarf galaxy models we include the complication that the luminosity function of the satellite is probed at various depths as a function of position along the tidal tails. The combined Galaxy and satellites model is converted to a synthetic Gaia catalogue using a detailed model for the expected astrometric and radial velocity errors, depending on magnitude, colour and sky position of the stars. 
 
 
 
We explore the feasibility of detecting tidal streams in the halo using the energy versus angular momentum plane. We find that a straightforward search in this plane will be very challenging. The combination of the background population and the observational errors will make it difficult to detect tidal streams as discrete structures in the E–Lz plane. In addition the propagation of observational errors leads to apparent caustic structures in the integrals of motion space that may be mistaken for physical entities. Any practical search strategy will have to use a combination of pre-selection of high-quality data and complementary searches using the photometric data that will be provided by Gaia.
16900773
From Detailed Galaxy Simulations to a Realistic End-of-Mission Gaia Catalogue
We address the problem of identifying remnants of satellite galaxies in the halo of our galaxy with Gaia data. We make use of N-body simulations of dwarf galaxies being disrupted in the halo of our galaxy combined with a Monte Carlo model of the Milky Way galaxy. The models are converted to a simulated Gaia catalogue containing a realistic number ($\sim10^8$--$10^9$) of stars. The simulated catalogue can be used to study how to handle the large data set that Gaia will provide and to study issues such as how to best retrieve information on substructure in the Galactic halo. The techniques described are applicable to any set of N-body simulations of (parts of) the Galaxy.
16290019
The Orion OB1 Association II. The Orion-Eridanus Bubble
Observations of the interstellar medium in the vicinity of the Orion OB1 association show a cavity filled with hot ionized gas, surrounded by an expanding shell of neutral hydrogen (the Orion-Eridanus Bubble). In this paper we examine this cavity and the surrounding bubble with the aid of data from the Leiden/Dwingeloo HI survey. We present neutral-hydrogen maps for the Orion-Eridanus region which allow identification of the HI filaments and arcs delineating the Bubble and derivation of its expansion velocity. The HI data are compared to X-ray, CO and IRAS 100 micron data. Using models of wind blown bubbles that take the density stratification of the Galactic HI layer into account we show that the stellar winds and supernovae from stars in Orion OB1 can account for the size as well as the expansion velocity of the HI shell. However, density inhomogeneities in the ambient interstellar medium cause significant discrepancies between our model and the observed shell.
119251012
High energy gamma-ray properties of the FR I radio galaxy NGC 1275
We report on our study of the high-energy $\gamma-$ray emission from the FR I radio galaxy NGC 1275, based on two years of observations with the Fermi-LAT detector. Previous Fermi studies of NGC 1275 had found evidence for spectral and flux variability on monthly timescales during the first year of Fermi-LAT observations. This variability is also seen in the larger two year data set, during which we observe a large $\gamma-$ray flare (June-August 2010). The increased photon statistics from this large flare have allowed the discovery of flux variability from NGC 1275 on the timescales of days. The largest flux variation observed during this flare being a factor of $\sim 3$ from one day to the next and a resultant $e$-folding risetime of $1.51\pm0.2$ days. The two year averaged $E>$100 MeV $\gamma-$ray spectrum is adequately described by a power-law spectrum, with a photon index, $\Gamma$, of $2.09 \pm 0.02$, and a resultant integrated flux of $F_{\gamma}=(2.2\pm0.1) \times 10^{-7}$ ph cm$^{-2}$s$^{-1}$. While no hysteresis was observed in the photon index$-$flux ($\Gamma_{\gamma}$ vs F$_{\gamma}$) parameter space, there was obvious `harder-when-brighter' behaviour observed during the large $\gamma-$ray flare. Furthermore, during this large flare, NGC 1275 appeared to migrate from the FR I radio galaxy to the BL Lac object region of the photon index$-$luminosity ($\Gamma_{\gamma}$ vs L$_{\gamma}$) paramater space. In this paper we present details of our Fermi-LAT analysis of NGC 1275, including a brief discussion on its implications for $\gamma-$ray blazar sources.
6711134
The Large-Scale Distribution and Motions of Older Stars in Orion
Abstract: We review the current knowledge of the population of `older' stars in the Orion OB1 association, specifically those in subgroups 1a and 1b. We briefly outline the history of the subject and then continue with a summary of the present state of knowledge of the early-type stars in Orion OB1. New results from the Hipparcos parallaxes and proper motions will be presented. The main result is that subgroup 1a is located at about 330 pc from the Sun, much closer than the previously determined distance, and about 100 pc distant from the other subgroups of the association and the Orion molecular clouds. Unfortunately, due to the unfavorable kinematics of the association with respect to the Galactic background, Hipparcos proper motions do not allow a clear kinematic separation of the association from the field. For this purpose accurate and homogeneous radial velocities are needed. Traditionally, the massive O and B stars have received most of the attention in the studies of OB associations. However, we will present results showing that significant numbers of low-mass stars are associated with Orion OB1. Unbiased, optically complete, spectroscopic and photometric surveys of areas within subgroups 1a and 1b have the potential to determine the complete low-mass stellar population, down to the brown dwarf limit. This will provide much insight into the overall initial mass function and studies of the kinematics of the low-mass stars will yield insights into the dispersal of the association.