10704036
Exploring the consequences of pairing algorithms for binary stars
Knowledge of the binary population in stellar groupings provides important information about the outcome of the star forming process in different environments. Binarity is also a key ingredient in stellar population studies, and is a prerequisite for calibrating the binary evolution channels. In this paper we present an overview of several commonly used methods of pairing individual stars into binary systems, which we refer to as pairing functions. These pairing functions are frequently used by observers and computational astronomers, either for their mathematical convenience or because they roughly describe the expected outcome of the star forming process. We discuss the consequences of each pairing function for interpreting observations and numerical simulations. The binary fraction and mass ratio distribution generally depend strongly on the selection of the range in primary spectral type in a sample. The mass ratio distribution and binary fraction derived from a binarity survey among a mass-limited sample of targets is thus not representative of the population as a whole. Neither theory nor observations indicate that random pairing of binary components from the mass distribution, the simplest pairing function, is realistic. It is more likely that companion stars are formed in a disc around a star or that a pre-binary core fragments into two binary components. The results of our analysis are important for (i) the interpretation of the observed mass ratio distribution and binary fraction for a sample of stars; (ii) a range of possible initial condition algorithms for star cluster simulations; and (iii) how to distinguish between the different star formation scenarios.
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.
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.
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.
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.
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.
188687701
Atmospheric monitoring at the H.E.S.S. site
35625352
Status of the nuclear shell model
In this review we concentrate on recent results of shell model calculations and related experimental data obtained for the sd-shell nuclei. Model spaces and effective interactions, Gamow-Teller, magnetic dipole, electric quadrupole, and higher multipole matrix elements, and electron scattering form factors are discussed. (AIP)
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.
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.
117836000
A search for close companions in Sco OB2
Using adaptive optics we study the binary population in the nearby OB association Scorpius OB2. We present the first results of our near-infrared adaptive optics survey among 199 (mainly) A- and B-type stars in Sco OB2. In total 151 components other than the target stars are found, out of which 77 are probably background stars. Our findings are compared with data collected from literature. Out of the remaining 74 candidate physical companions 42 are new, demonstrating that many stars A/B stars have faint, close companions.