Anthony G. A. Brown

Exploring the Full Stellar Population of the Upper Scorpius OB Association

We investigate the stellar population and star formation history of the Upper Scorpius OB association, the most nearby region of recent massive star formation, over the full stellar mass range from 0.1 to 20 M?. The first part of this paper describes an extension of our large spectroscopic survey (Preibisch et al., published in 2001) for low-mass pre?main-sequence (PMS) stars in Upper Scorpius. Using the multiobject spectrograph 2dF at the Anglo-Australian Telescope, we obtained spectra of 469 stars with magnitudes R = 12.5?18.0 in a 6 deg2 area. Among these, we find 68 new PMS stars, nearly all of them M-type stars, by their strong lithium absorption lines. The total area covered by our 2dF survey is now 9 deg2 and contains 166 new PMS stars. Combining these results with our earlier investigation (Preibisch & Zinnecker) yields a sample of 250 PMS stars in the mass range ~0.1 to ~2 M?. The location of these stars in the HR diagram suggests a mean age of 5 Myr without a significant age spread. In the second part of this paper, we also consider the population of 114 high-mass members identified in detailed Hipparcos studies. We construct a combined HR diagram for the 364 high- and low-mass members and find that the whole stellar population is very well characterized by a very narrow age distribution around 5 Myr. We estimate individual masses for all members and construct an empirical mass function covering the mass range from 0.1 up to 20 M?. A power-law fit to the mass function gives a slope of ? ~ -2.6 above ~2 M? and a much flatter slope (? ~ -0.9) below ~0.6 M?. The initial mass function of Upper Sco is not identical, but within the errors consistent with recent determinations of the field initial mass function. There is certainly no deficit of low-mass stars in the Upper Sco OB association, but rather a small excess of low-mass stars. Our results on the stellar age distribution confirm earlier indications that the star formation process in Upper Sco was triggered and support previous conjectures that the triggering event was a supernova shock wave originating from the nearby Upper Centaurus?Lupus association.

Distance determination for RAVE stars using stellar models . II. Most likely values assuming a standard stellar evolution scenario

The RAdial Velocity Experiment (RAVE) is a spectroscopic survey of the Milky Way which already collected over 400000 spectra of ∼330000 different stars. We use the subsample of spectra with spectroscopically determined values of stellar parameters to determine the distances to these stars. The list currently contains 235064 high quality spectra which show no peculiarities and belong to 210872 different stars. The numbers will grow as the RAVE survey progresses. The public version of the catalog will be made available through the CDS services along with the ongoing RAVE public data releases. The distances are determined with a method based on the work by Breddels et al. (2010, A&A, 511, A16). Here we assume that the star undergoes a standard stellar evolution and that its spectrum shows no peculiarities. The refinements include: the use of either of the three isochrone sets, a better account of the stellar ages and masses, use of more realistic errors of stellar parameter values, and application to a larger dataset. The derived distances of both dwarfs and giants match within ∼21% to the astrometric distances of Hipparcos stars and to the distances of observed members of open and globular clusters. Multiple observations of a fraction of RAVE stars show that repeatability of the derived distances is even better, with half of the objects showing a distance scatter of <11%. RAVE dwarfs are ∼300 pc from the Sun, and giants are at distances of 1 to 2 kpc, and up to 10 kpc. This places the RAVE dataset between the more local Geneva-Copenhagen survey and the more distant and fainter SDSS sample. As such it is ideal to address some of the fundamental questions of Galactic structure and evolution in the pre-Gaia era. Individual applications are left to separate papers, here we show that the full 6-dimensional information on position and velocity is accurate enough to discuss the vertical structure and kinematic properties of the thin and thick disks.

The Gaia mission : Science, organization and present status

The ESA space astrometry mission Gaia will measure the positions, parallaxes and proper motions of the 1 billion brightest stars on the sky. Expected accuracies are in the 725 as range down to 15 mag and sub-mas accuracies at the faint limit (20 mag). The astrometric data are complemented by low-resolution spectrophotometric data in the 3301000 nm wavelength range and, for the brighter stars, radial velocity measurements. The scientific case covers an extremely wide range of topics in galactic and stellar astrophysics, solar system and exoplanet science, as well as the establishment of a very accurate, dense and faint optical reference frame. With a planned launch around 2012 and an (extended) operational lifetime of 6 years, final results are expected around 2021. We give a brief overview of the science goals of Gaia, the overall project organisation, expected performance, and some key technical features and challenges.

The design and performance of the Gaia photometric system

The European Gaia astrometry mission is due for launch in 2011. Gaia will rely on the proven principles of the ESA Hipparcos mission to create an all-sky survey of about one billion stars throughout our Galaxy and beyond, by observing all objects down to 20 mag. Through its massive measurement of stellar distances, motions and multicolour photometry, it will provide fundamental data necessary for unravelling the structure, formation and evolution of the Galaxy. This paper presents the design and performance of the broad- and medium-band set of photometric filters adopted as the baseline for Gaia. The 19 selected passbands (extending from the UV to the far-red), the criteria and the methodology on which this choice has been based are discussed in detail. We analyse the photometric capabilities for characterizing the luminosity, temperature, gravity and chemical composition of stars. We also discuss the automatic determination of these physical parameters for the large number of observations involved, for objects located throughout the entire Hertzsprung-Russell diagram. Finally, the capability of the photometric system (PS) to deal with the main Gaia science case is outlined.

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.

The quest for the Sun's siblings: an exploratory search in the Hipparcos Catalogue

We describe the results of a search for the remnants of the Sun’s birth cluster among stars in the Hipparcos Catalogue. This search is based on the predicted phase-space distribution of the Sun’s siblings from simple simulations of the orbits of the cluster stars in a smooth Galactic potential. For stars within 100 pc, the simulations show that it is interesting to examine those that have small space motions relative to the Sun. From amongst the candidate siblings thus selected, there are six stars with ages consistent with that of the Sun. Considering their radial velocities and abundances only one potential candidate, HIP 21158, remains, but essentially the result of the search is negative. This is consistent with predictions by Portegies Zwart on the number of siblings near the Sun. We discuss the steps that should be taken in anticipation of the data from the Gaia mission in order to conduct fruitful searches for the Sun’s siblings in the future.

Nested Shells Reveal the Rejuvenation of the Orion-Eridanus Superbubble

The Orion-Eridanus superbubble is the prototypical superbubble due to its proximity and evolutionary state. Here, we provide a synthesis of recent observational data from WISE and Planck with archival data, allowing to draw a new and more complete picture on the history and evolution of the Orion-Eridanus region. We discuss the general morphological structures and observational characteristics of the superbubble, and derive quantitative properties of the gas- and dust inside Barnards Loop. We reveal that Barnards Loop is a complete bubble structure which, together with the lambda Ori region and other smaller-scale bubbles, expands within the Orion-Eridanus superbubble. We argue that the Orion-Eridanus superbubble is larger and more complex than previously thought, and that it can be viewed as a series of nested shells, superimposed along the line of sight. During the lifetime of the superbubble, HII region champagne flows and thermal evaporation of embedded clouds continuously mass-load the superbubble interior, while winds or supernovae from the Orion OB association rejuvenate the superbubble by sweeping up the material from the interior cavities in an episodic fashion, possibly triggering the formation of new stars that form shells of their own. The steady supply of material into the superbubble cavity implies that dust processing from interior supernova remnants is more efficient than previously thought. The cycle of mass-loading, interior cleansing, and star formation repeats until the molecular reservoir is depleted or the clouds have been disrupted. While the nested shells come and go, the superbubble remains for tens of millions of years.

The impact of CCD radiation damage on Gaia astrometry - I. Image location estimation in the presence of radiation damage

The Gaia mission has been designed to perform absolute astrometric measurements with unprecedented accuracy; the end-of-mission parallax standard error is required to be of the order of 10 mu as for the brightest stars (V= 10) and 30 mu as for a G2V-type star of magnitude 15. These requirements set a stringent constraint on the accuracy of the estimation of the location of the stellar image on the charge-coupled device (CCD) for each observation: e.g. 0.3 mas or 0.005 pixel for the same V= 15 G2V star. However, the Gaia CCDs will suffer from charge transfer inefficiency (CTI) caused by radiation damage that will degrade the stellar image quality and may degrade the astrometric performance of Gaia if not properly addressed. For the first time at this level of detail, the potential impact of radiation damage on the performance of Gaia is investigated. In this paper (first of a series of papers), we focus on the evaluation of the CTI impact on the image location accuracy using a large set of CTI-free and damaged synthetic Gaia observations supported by experimental test results. We show that CTI decreases the stellar image signal-to-noise ratio and irreversibly degrades the image location estimation precision. As a consequence, the location estimation standard errors increase by up to 6 per cent in the Gaia operating conditions for a radiation damage level equivalent to the end-of-mission accumulated dose. We confirm that, in addition, the CTI-induced image distortion introduces a systematic bias in the image location estimation (up to 0.05 pixel or 3 mas in the Gaia operating conditions). Hence, a CTI-mitigation procedure is critical to achieve the Gaia requirements. We present a novel approach to CTI mitigation that enables, without correction of the raw data, unbiased estimation of the image location and flux from damaged observations. We show that its current implementation reduces the maximum measured location bias for the faintest magnitude to 0.005 pixel (similar to 4 x 10(-4) pixel at magnitude 15) and that the Gaia image location estimation accuracy is preserved. In the second paper, we will investigate how the CTI effects and CTI-mitigation scheme affect the final astrometric accuracy of Gaia by propagating the residual errors through the astrometric solution. (Less)

Detection of Satellite Remnants in the Galactic Halo with Gaia - II. A modified Great Circle Cell Method

We propose an extension of the GC3 streamer finding method of Johnston et al. (1996) that can be applied to the future Gaia database. The original method looks for streamers along great circles in the sky, our extension adds the kinematical restriction that velocity vectors should also be constrained to lie along these great circles, as seen by a Galactocentric observer. We show how to use these combined criteria starting from heliocentric observables. We test it by using the mock Gaia catalogue of Brown et al. (2005), which includes a realistic Galactic background and observational errors, but with the addition of detailed star formation histories for the simulated satellites. We investigate its success rate as a function of initial satellite luminosity, star formation history and orbit. We find that the inclusion of the kinematical restriction vastly enhances the contrast between a streamer and the background, even in the presence of observational errors, provided we use only data with good astrometric quality (fractional errors of 30 per cent or better). The global nature of the method diminishes the erasing effect of phase mixing and permits the recovery of merger events of reasonable dynamical age. Satellites with a star formation history different to that of the Galactic background are also better isolated. We find that satellites in the range of 10 8 10 9 ;

HIGH S/N ECHELLE SPECTROSCOPY IN YOUNG STELLAR GROUPS. I. OBSERVATIONS AND DATA REDUCTION

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).