Showing papers by "Anthony G. A. Brown published in 1997"

Improved Methods for Identifying Moving Groups

Abstract: We present a new procedure for the identification of moving groups. It is a combination of two independent member selection methods. One is a modern implementation of a classical convergent point method for proper motion data. The other objectively identifies moving groups in velocity space using proper motions and parallaxes. We briefly describe both methods, and illustrate their power by applying them to Hipparcos measurements of a field containing the Upper Scorpius subgroup of the Scorpio-Centaurus association (Sco OB2). We show how our membership selection procedure not only improves the list of previously known B and A-type members, but also identifies many new members, including a significant number of F stars. We apply our procedure to other nearby OB associations elsewhere in these proceedings (Hoogerwerf et al.: de Zeeuw et al.).

High s/n echelle spectroscopy in young stellar groups. i. observations and data reduction

Abstract: This is the first paper in a series in which we study the kinematical structure and dynamical evolution of OB associations and young stellar clusters. By means of high-resolution high-S/N spectroscopic observations, we aim at obtaining accurate stellar radial and rotational velocities and information on binarity in a selected number of such systems. In the present paper, we discuss the observations and data reduction for a sample of 156 early-type, established or probable, members of the Sco OB2 association. These stars form a subset of a larger sample that was observed by HIPPARCOS satellite. The observations presented here were preformed at the ESO 1.5 m telescope using the ECHELEC echelle CCD spectrograph. The formal S/N obtained is typically 70 at 3800 A and 300 at 4050 A. A new data reduction package was developed. For the sake of qualilty control over the final spectra, we critically evaluated the performance of each procedure in the reduction. Emphasis is on techniques for detecting systematic errors in an empirical way and for maximally eliminating them, at least on a differential level. The accuracy of our reduced spectra is limited by intrinsic instrumental imperfections. Local random errors are given and the amplitude of systematic residuals that (may) occur in certain spectral regions is estimated in considerable detail. In anticipation of next papers in this series, we finally show that the accuracy of differential radial velocities (for spectra of the same star) is essentially limited by centering and zero-point uncertainties amounting to greater or equal to 1km s1 (rms).

The Hyades: distance, structure, dynamics, and age

Abstract: We use absolute trigonometric parallaxes from the Hipparcos Catalogue to determine individual distances to members of the Hyades cluster, from which the 3-dimensional structure of the cluster can be derived. Inertially-referenced proper motions are used to rediscuss distance determinations based on convergent-point analyses. A combination of parallaxes and proper motions from Hipparcos, and radial velocities from ground-based observations, are used to determine the position and velocity components of candidate members with respect to the cluster centre, providing new information on cluster membership: 13 new candidate members within 20 pc of the cluster centre have been identified. Farther from the cluster centre there is a gradual merging between certain cluster members and field stars, both spatially and kinematically. Within the cluster, the kinematical structure is fully consistent with parallel space motion of the component stars with an internal velocity dispersion of about 0.3 km/s. The spatial structure and mass segregation are consistent with N-body simulation results, without the need to invoke expansion, contraction, rotation, or other significant perturbations of the cluster. The quality of the individual distance determinations permits the cluster zero-age main sequence to be accurately modelled. The helium abundance for the cluster is determined to be Y=0.26+/-0.02 which, combined with isochrone modelling including convective overshooting, yields a cluster age of 625+/-50 Myr. The distance to the observed centre of mass is 46.34+/-0.27 pc, corresponding to a distance modulus m-M=3.33+/-0.01 mag for the objects within 10 pc of the cluster centre (roughly corresponding to the tidal radius). Discrepancies with previous distance estimates are investigated and explained.

High s/n echelle spectroscopy in young stellar groups : ii. rotational velocities of early-type stars in sco ob2

Abstract: 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.

Some Considerations in Making Full Use of the HIPPARCOS Catalogue

Abstract: 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.

Some Considerations in making full Use of the Hipparcos Catalogue

Abstract: 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.

Structure and Evolution of Nearby OB Associations

Abstract: We present the first 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 fields on the sky. We use a new method to identify moving groups in these fields (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 firm basis for studies of galactic and extragalactic star forming regions.

Improved Methods for Identifying Moving Groups

Abstract: We present a new procedure for the identification of moving groups. It is a combination of two independent member selection methods. One is a modern implementation of a classical convergent point method for proper motion data. The other objectively identifies moving groups in velocity space using proper motions and parallaxes. We briefly describe both methods, and illustrate their power by applying them to Hipparcos measurements of a field containing the Upper Scorpius subgroup of the Scorpio-Centaurus association (Sco OB2). We show how our membership selection procedure not only improves the list of previously known B and A-type members, but also identifies many new members, including a significant number of F stars. We apply our procedure to other nearby OB associations elsewhere in these proceedings (Hoogerwerf et al.; de Zeeuw et al.).

Structure and Evolution of Nearby OB Associations

Abstract: We present the first 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 fields on the sky. We use a new method to identify moving groups in these fields (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 firm basis for studies of galactic and extragalactic star forming regions.

New Results for Three Nearby OB Associations

Abstract: We have developed a new procedure to identify moving groups, and have applied it to carry out a census of the nearby OB associations based on Hipparcos parallaxes and proper motions (see de Bruijne et al. and de Zeeuw et al. elsewhere in this volume). Here we present three illustrative cases. For $\alpha$ Persei our method allows us to refine the bright end of the membership list, while for Collinder 121 the Hipparcos data change the whole appearance of the association. Finally, we report the discovery of a new association in the field of Cepheus OB2.

New Results for Three Nearby OB Associations

Abstract: We have developed a new procedure to identify moving groups, and have applied it to carry out a census of the nearby OB associations based on Hipparcos parallaxes and proper motions (see de Bruijne et al. and de Zeeuw et al. elsewhere in this volume). Here we present three illustrative cases. For $\alpha$ Persei our method allows us to refine the bright end of the membership list, while for Collinder 121 the Hipparcos data change the whole appearance of the association. Finally, we report the discovery of a new association in the field of Cepheus OB2.