70483395
The Primordial Binary Population in OB Associations
We describe our method to find the primordial binary population in the Sco~OB2 association. We present the results of our ADONIS and VLT/NACO near-infrared adaptive optics binarity surveys of A and late-B stars in Sco OB2. We combine these results with literature data on visual, spectroscopic, and astrometric binary stars. With our observations we remove part of the selection effects present in the combined dataset. Using simulated observations the remaining biases can be removed in order to derive the true binary population. Detailed N-body simulations, including stellar and binary evolution are required to derive constraints on the binary population which was present just after the natal gas has been removed from Sco~OB2, i.e., the primordial binary population
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.
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.
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.
116941208
Brane tunneling and virtual brane-antibrane pairs
We survey barrier penetration by quantum tunneling for four cases: nonrelativistic point particles, scalar fields, relativistic point particles, and DBI branes. We examine two novel features that arise for DBI brane tunneling: the rate can sometimes increase as the barrier gets higher; and the instanton "wrinkles". We show that these features can be understood as the effect of the quantum sea of virtual brane-antibrane pairs. This sea exponentially augments the decay rate, with possible cosmological consequences.
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.
119299813
Small Steps and Giant Leaps in the Landscape
For landscapes of field-theory vacua, we identify an effect that can greatly enhance the decay rates to wildly distant minima - so much so that such transitions may dominate over transitions to near neighbors. We exhibit these 'giant leaps' in both a toy two-field model and, in the thin-wall limit, amongst the four-dimensional vacua of six-dimensional Einstein-Maxwell theory, and it is argued that they are generic to landscapes arising from flux compactifications. We discuss the implications for the cosmological constant and the stability of stringy de Sitter.
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.
117895476
Stellar Imager (SI): developing and testing a predictive dynamo model for the Sun by imaging other stars
The Stellar Imager mission concept is a space-based UV/Optical interferometer designed to resolve surface magnetic activity and subsurface structure and flows of a population of Sun-like stars, in order to accelerate the development and validation of a predictive dynamo model for the Sun and enable accurate long-term forecasting of solar/stellar magnetic activity.