14814611
Some Considerations in making full Use of the Hipparcos Catalogue
This contribution is intended as a `rough guide' to the Hipparcos Catalogue for the non-expert user. Some general aspects of the use of astrometric data are discussed as well as Hipparcos-specific applications. We discuss when and at what level one may expect systematic errors to occur in the Hipparcos Catalogue. Next we discuss the question of the interpretation of the measured parallaxes in terms of distances and luminosities of stars. What are the biases one should be aware of and how can these be corrected? When using the astrometric data to study the statistics of stars one should take the full covariance matrix of the errors on the astrometric parameters into account. We explain how to do this and discuss the specific case of a moving cluster. Finally, we address the question of the correlation of astrometric parameters over a given region of the sky. At present the Hipparcos Catalogue contains no identified systematic errors.
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.
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.
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.
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.
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.
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.
119263159
Evaluating the Gamma-Ray Evidence for Self-Annihilating Dark Matter from the Virgo Cluster
Based on three years of Fermi Large Area Telescope (LAT) gamma-ray data of the Virgo cluster, evidence for an extended emission associated with dark matter pair annihilation in the b b-bar channel has been reported by Han et al. [1]. After an in depth spatial and temporal analysis, we argue that the tentative evidence for a gamma-ray excess from the Virgo cluster is mainly due to the appearance of a population of previously unresolved gamma-ray point sources in the region of interest. These point sources are not part of the LAT second source catalogue (2FGL), but are found to be above the standard detection significance threshold when three or more years of LAT data is included.
188687701
Atmospheric monitoring at the H.E.S.S. site
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.