16825830
The primordial binary population. I: A near-infrared adaptive optics search for close visual companions to A star members of Scorpius OB2
We present the results of a near-infrared adaptive optics survey with the aim to detect close companions to Hipparcos members in the three subgroups of the nearby OB association Sco OB2: Upper Scorpius (US), Upper Centaurus Lupus (UCL) and Lower Centaurus Crux (LCC). We have targeted 199 A-type and late B-type stars in the K S band, and a subset also in the J and H band. We find 151 stellar components other than the target stars. A brightness criterion is used to separate these components into 77 background stars and 74 candidate physical companion stars. Out of these 74 candidate companions, 41 have not been reported before (14 in US; 13 in UCL; 14 in LCC). The angular separation between primaries and observed companion stars ranges from 0.22 to 12.4. At the mean distance of Sco OB2 (130 pc) this corresponds to a projected separation of 28.6 AU to 1612 AU. Absolute magnitudes are derived for all primaries and observed companions using the parallax and interstellar extinction for each star individually. For each object we derive the mass from K S , assuming an age of 5 Myr for the US subgroup, and 20 Myr for the UCL and LCC subgroups. Companion star masses range from 0.10 M ○. to 3.0 M ○. . The mass ratio distribution follows f(q) = q -Γ with r = 0.33, which excludes random pairing. No close (ρ ≤ 3.75) companion stars or background stars are found in the magnitude range 12 mag 14 mag are observed. If these components are very low-mass companion stars, a gap in the companion mass distribution might be present. The small number of close low-mass companion stars could support the embryo-ejection formation scenario for brown dwarfs. Our findings are compared with and complementary to visual, spectroscopic, and astrometric data on binarity in Sco OB2. We find an overall companion star fraction of 0.52 in this association. This is a lower limit since the data from the observations and from literature are hampered by observational biases and selection effects. This paper is the first step toward our goal to derive the primordial binary population in Sco OB2.
118925453
High S/N Echelle spectroscopy in young stellar groups ? II. Rotational velocities of early-type stars in Sco OB2
We investigate the rotational velocities of early-type stars in the Sco OB2 association. We measure vsini for 156 established and probable members of the association. The mea- surements are performed with three different techniques, which are in increasing order of expected vsini: 1) converting the widths of spectral lines directly to vsini, 2) comparing arti- cially broadened spectra of low vsini stars to the target spec- trum, 3) comparing the Hei 4026 line prole to theoretical models. The sample is extended with literature data for 47 es- tablished members of Sco OB2. Analysis of the vsini distri- butions shows that there are no signicant differences between the subgroups of Sco OB2. We nd that members of the binary population of Sco OB2 on the whole rotate more slowly than the single stars. In addition, we nd that the B7{B9 single star members rotate signicantly faster than their B0{B6 counter- parts. We test various hypotheses for the distribution of vsini in the association. The results show that we cannot clearly ex- clude any form of random distribution of the direction and/or magnitude of the intrinsic rotational velocity vector. We also investigate the effects of rotation on colours in the Walraven photometric system. We show that positions of B7{B9 single dwarfs above the main sequence are a consequence of rota- tion. This establishes the influence of rotation on the Walraven colours, due primarily to surface gravity effects.
11339076
Learning about Galactic structure with Gaia astrometry
The Gaia mission is reviewed together with the expected contents of the final catalogue. It is then argued that the ultimate goal of Galactic structure studies with Gaia astrometry should be to build a dynamical model of our galaxy which is capable of explaining the contents of the Gaia catalogue. This will be possible only by comparing predicted catalogue data to Gaia’s actual measurements. To complement this approach the Gaia catalogue should be used to recalibrate photometric distance and abundance indicators across the HR‐diagram in order to overcome the lack of precise parallax data at the faint end of the astrometric survey. Using complementary photometric and spectroscopic data from other surveys will be essential in this respect.
7260362
1 STRUCTURE AND EVOLUTION OF NEARBY OB ASSOCIATIONS
We present the rst results of a comprehensive census of the stellar content of the nearby OB associations based on Hipparcos positions, proper motions and parallaxes for 12842 candidate member stars distributed over 21 elds on the sky. We use a new method to identify moving groups in these elds (see de Bruijne et al., these proceedings). Previously, astrometric membership in nearly all the nearby OB associations was known only for stars with spectral types earlier than B5. The Hipparcos measurements now allow us to identify members down to late F. This census provides a rm basis for studies of galactic and extragalactic star forming regions.
1110969
OB Associations, Open Clusters, and the Luminosity Calibration of the Nearer Stars
In the context of the luminosity calibration of the nearer stars I discuss the Hipparcos results on distances to nearby OB associations and open clusters. The shortcomings and assumptions in the analyses used to derive these results are pointed out and for the open clusters a comparison is made with results obtained from main sequence fitting. I conclude that given the considerable uncertainties in the latter technique there is no convincing evidence that the Hipparcos based distances to open clusters beyond the Hyades should not be trusted.
55115943
On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright O stars in the sky as predicted from their spectral type with the absolute magnitude calculated from their apparent magnitude and the Hipparcos parallax. We find that many stars appear to be much fainter than expected, up to five magnitudes. We find no evidence for a correlation between magnitude differences and the stellar rotational velocity as suggested for OB stars by Lamers et al. (1997, AA Howarth & Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) and find that they are consistent with the Hipparcos measurements. Although O stars conform nicely to the simulation, we notice that some B stars in the sample of Lamers et al. have a magnitude difference larger than expected.
53490118
Locating the γ-ray emission region of the flat spectrum radio quasar PKS 1510−089
I present a study of the high-energy γ-ray properties of the flat spectrum radio quasar, PKS 1510−089, based on 3.75 yr of observations with the Large Area Telescope detector on-board the Fermi γ-ray Space Telescope. Throughout the observing period, the 0.1 < Eγ < 300 GeV γ-ray flux was highly variable, undergoing several flaring events where the daily flux exceeded 10−5 photons cm−2 s−1 on three separate occasions. The increased photon statistics of these large flares allowed the observations to be re-analysed in 6 and 3 h intervals, revealing flux doubling time-scales as small as 1.3 ± 0.12 h during the flare rise time, and flux halving time-scales of 1.21 ± 0.15 h during the flare decay. These are the smallest variability time-scales measured to date at MeV–GeV energies for the flat spectrum quasar class of active galactic nuclei. 
 
The >10−5 photons cm−2 s−1 flare events were also studied in more detail in an attempt to uncover evidence for the location of PKS 1510−089's γ-ray emission region. In particular, two approaches were used: (i) searching for an energy dependence to the cooling time-scales, and (ii) searching for evidence of a spectral cut-off. The combined results of these two approaches, along with the confirmation of ≥20 GeV photon flux from PKS 1510−089, suggest the presence of multiple γ-ray emission regions being located in both the broad line region and molecular torus region of PKS 1510−089. 
 
An analysis of the highest photon events within the 3.75 yr data set finds PKS 1510−089 to be a source of ≥20 GeV γ-rays at the 13.5σ confidence level; a observational property which is difficult to explain in the traditional view that γ-ray emission from active galactic nuclei originates from the base of the relativistic jet. This gives further weight to the argument that there are multiple, simultaneously active γ-ray emission regions located along the relativistic jet of active galactic nuclei.
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.
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.
119096360
Downloading the nuclear structure shell model code OXBASH from INTERNET
This document explains how to download the OXBASH code for nuclear structure calculations from the NSCL anonymous server and get it running on VAX (VMS), ALPHA (OpenVMS) and IBM RS/6000 (aix) computers. The physics behind the code is explained in the OXBASH manual, which is part of the downloading package. Everybody is welcome to download the code and use it under the conditions described below.
122865130
Atmospheric sensing for the H.E.S.S. array
Several atmospheric monitoring instruments have been installed at the H.E.S.S. gamma‐ray observatory in Namibia. Firstly, Heitronics KT19 infrared radiometers, aligned paraxially with the H.E.S.S. telescopes, measure the infrared radiation of the water molecules. These allow us to detect clouds crossing the telescopes’ field of view and to estimate the humidity present in the atmosphere. For a general estimate of the atmosphere’s transmittance, i.e. the detection of any light‐attenuating aerosols, a ceilometer, which is a LIDAR with built‐in atmospheric data reduction code, is being used. It will be complemented soon by an instrument which will measure the transmissivity of the atmosphere at different wavelengths up to 500m above the ground. The overall status of the weather is monitored by a fully automated weatherstation. This paper describes the setup, the data analysis and how this will be used in order to improve the knowledge of the telescopes’ effective collection area.