Abstracts

Overcoming Great Barriers in Galactic Archaeology 5-8 May 2009

In association with the Astrophysical Institute, Potsdam, the Australian National University, and the University of Sydney

Abstracts as of 1 May. (In no particular order!)

Stefan Keller

Talk: SkyMapper and Galactic Archaeology

Abstract: I will present the results of our characterization of substructure in the outer halo from SDSS and look forward to what we can achieve with SkyMapper and multi-object spectroscopy with instruments such as AAOmega and HERMES. Dominating the substructure of the outer halo is the Sagittarius (Sgr) Stream. I will discuss our studies to determine abundance ([Fe/H] and [alpha/fe]) trends along the southern trailing arm of Sgr, and their consequences for the nature of the progenitor dwarf and the chemodynamics of the halo. I will investigate our findings regarding the distances to and radial velocities of, putative Sgr debris that reveal marked discrepancies with current dynamical models and challenges for future modeling. I will similarly discuss our determinations of the distances and radial velocities of populations associated with other distinct halo substructures, some known such as the Virgo Overdensity, others recently discovered by us. I conclude with a demonstration of how SkyMapper will enable the construction of a metallicity map for the outer halo. When combined with radial velocities from multi-object spectroscopy, this will enable determination of the kinematic properties of Galactic components as a function of distance and metallicity.

Poster: Structure in the outer halo

Abstract: We present our detection of substructure in the outer halo (from 10-130 kpc) derived from the SDSS DR 7 dataset. In addition to recovery of the Sagittarius stream material and the Virgo Overdensity, a number of new tidal features are identified. The distances, luminosities, metallicities and radial velocities for these new features are presented and their origins discussed.

Mary williams

Talk: Chasing the dragon: hunting stellar streams in the solar neighbourhood

Abstract: Stellar streams in the solar neighbourhood can arise through various mechanisms: resonances, dissolved open clusters or via accreted satellite debris. Disentangling the origin of each stream however is difficult with some streams elusive and group membership potentially difficult to define. We will discuss our efforts to verify the existence (or non-existence) and origin of various solar neighbourhood streams, particularly the thick-disk Arcturus moving group. Understanding the origin of such a stream helps constrain the formation of the Galaxy’s thick disk.

john norris

Talk: Abundances in the BOOTES I and SEGUE 1 systems

Abstract: We shall report further abundances in putative low-luminosity dwarf spheroidal systems (dSph) obtained using the AAOmega multifibre facility on the Anglo-Australian Telescope. In particular we have determined carbon abundances for red giants in the chemically inhomogeneous Bootes I dSph to supplement our earlier work on its heavy element spread (Norris et al. 2008, ApJ, 689, L113). The very low luminosity (Mv ~ -1.5) Segue 1 system has been identified by Geha et al. (2008, astro-ph/0809.2781) as a dSph. We shall discuss the implications of the iron (based on the Ca II K line) and carbon abundances of stars in our sample that have radial velocities consistent with membership of the system.

Nicholas Martin

Talk: Faint dwarf galaxies in the Local Group

Abstract: I will present the latest in our understanding of the faint dwarf galaxies recently discovered in the Local Group over the last few years. From deep spectroscopic and photometric follow-up obtained with HST and 8m class telescopes, I will show what differentiates them from their brighter siblings or, on the contrary, which of their properties are but an extension to these of previously known satellites, Finally, these systems owing their discovery to wide-field surveys (SDSS and PAndAS), I will explain what the Pan-STARRS survey, that will start collecting data of the whole northern sky in mid-2009, should unravel on this topic.

Brad Gibson

Talk: The origin of the light distribution in disc galaxies

Abstract: To date, fully cosmological hydrodynamic disk simulations have only been undertaken with particle-based codes, such as GADGET, Gasoline, or CGD+. In light of the limitations of traditional implementations of smoothed particle hydrodynamics (SPH), it is important to explore complementary approaches to the SPH approach to galaxy formation. We present the first high-resolution cosmological disk simulations using an adaptive mesh refinement (AMR)-based hydrodynamical code, in this case, RAMSES. We analyse the temporal and spatial evolution of the simulated stellar disks’ vertical heating, velocity ellipsoid, stellar populations, and vertical and radial gradients, and compare and contrast them with a sample of cosmological SPH disks. Particular emphasis is placed upon the respective roles of radial migration warps, and star formation thresholds, in setting the breaks observed in the light profiles of disc galaxies. These results are the first in our long-term Galactic Archaeology program, the goals of which will be described.

Elizabeth Wylie de Boer

Talk: Evidence of tidal debris from Omega Centauri in the Kapteyn moving group

Abstract: This study presents results of a detailed kinematic and chemical analysis of 17 members of the Kapteyn moving group. It does not appear chemically homogenous. However, both the kinematics and the chemical abundance pattern seen in 15 of the stars in this group are very similar to those observed in the well-studied cluster, Omega Centauri. It is suggested that some members of this moving group are perhaps a remnant of the tidal debris of Omega Cen, left in the Galactic disk during the merger event which deposited Omega Cen into the Milky Way.

Amanda Karakas & John Lattanzio

Talk: Abundance anomalies in globular clusters and the role of AGB stars

Abstract: One of the more popular theories to account for the abundance anomalies in globular cluster stars in the “self-pollution scenario” where the polluters were a previous generation of massive asymptotic giant branch (AGB) stars. This idea has proven attractive because: 1) the hot bottom burning experienced by these objects qualitatively provides an ideal proton-capture environment to produce helium and convert C and O to N, Ne to Na and Mg to Al; and 2) the slow winds from these stars allow their retention by the cluster’s gravitational potential. In this talk we critically examine the AGB self-pollution hypothesis, and discuss some of the problems encountered by all pollution models. Ways around some of these problems are discussed (eg, no third dredge-up, the “external pollution” model).

Tomaz ZWitter

Talk: Present state and promises of the RAVE survey

Abstract: RAdial Velocity Experiment (RAVE) is an ambitious ongoing spectroscopic survey which already secured over a third million stellar spectra. For many of these spectra not only the radial velocity but also stellar parameters have been determined from spectral analysis. This requires automated reduction, analysis and classification techniques which have to be checked against other spectroscopic and photometric datasets. The adopted solutions are important for planning of future large scale spectroscopic surveys and also for any RAVE user who wants to take full advantage of the claimed rather impressive typical errors: less than 2 km/s error in radial velocity, 200 K in temperature, 0.3 dex in gravity and 0.2 dex in metallicity. The talk will present the first two data releases which include results on 51,829 spectra and outline the improvements planned for the next approximately yearly data releases. Possibilities for RAVE's extension using a modern multiple-object spectrograph will be discussed.

Peter Cottrell

Talk: Surveying the s- and r-process elements in globular clusters using SALT, VLT and AAT

Abstract: We are exploring various techniques to extract s- and r-process abundances for large numbers of stars in globular clusters in order to investigate the abundance distribution of these species within and between clusters.

Our research is based around instruments on the Southern African Large Telescope (SALT), data from UVES on the VLT and AAOmega on the AAT, so spanning a large range of spectral resolution.

We have and will use the Robert Stobie Spectrograph (RSS) on SALT in its multi-object spectroscopy (MOS) and Fabry-Pérot (FP) modes. RSS has a field of view of 8 arcminutes and is therefore ideally suited for imaging galactic globular clusters.

In MOS mode we will simultaneously obtain spectra of several hundred stars in each cluster at a resolving power of about 8,000. We have demonstrated that this is sufficient to derive s-and r-process element abundances (Y, Zr, La, Nd and Eu) with a precision of better than 0.3 dex using synthetic spectra and some performance verification phase long slit spectra from RSS.

In FP mode at its highest resolution (R~8,000) we will scan across the line profile of some key s-process elements, in particular Ba, to obtain the abundances of many hundreds of stars in each cluster. This requires photometric quality observations to ensure that the flux in the line is well calibrated. We will use spectrum synthesis techniques to extract abundances with a precision of about 0.5 dex.

The UVES data was acquired by two of the authors (IM & JvL) for a different purpose but its resolution will enable r- and s-process elemental abundances to be extracted for several globular clusters – 47 Tuc, NGC362 and NGC6388. The AAOmega data for 47 Tuc is being explored to investigate these heavy element abundances at medium resolution.

Gary da Costa

Talk: M22: A [Fe/H] abundance range revealed

Abstract: Intermediate resolution spectra at the Ca II triplet have been obtained for 56 candidate red giants in the field of the globular cluster M22 with the VLT/FORS instrument. Spectra were also obtained for a number of red giants in standard globular clusters to provide a calibration of the observed line strengths with overall abundance [Fe/H]. For the 41 M22 member stars that lie within the V − VHB bounds of the calibration, we find an abundance distribution that is substantially broader than that expected from the observed errors alone. We argue that this broad distribution cannot solely be the result of differential reddening. Instead we conclude that, as has long been suspected, M22 is similar to Omega Cen in having an intrinsic dispersion in heavy element abundance. The observed M22 abundance distribution rises sharply to a peak at [Fe/H] " –1.9 with a broad tail to higher abundances: the highest abundance star in our sample has [Fe/H] " –1.4 dex. If the unusual properties of Omega Cen have their origin in a scenario in which the cluster is the remnant nucleus of a disrupted dwarf galaxy, then such a scenario likely also applies to M22.

Mike Rich

Talk: The bulge radial velocity assay

Abstract: We have been using the Hydra spectrograph at CTIO to undertake a survey of radial velocity of 2MASS-selected red giants, from -10<l<+10, and -3<b<-8.

The survey has reached 10,000 stars, and has produced a rotation curve and velocity dispersion profile at l=-4 and l=-8. We find no evidence for cold stream structures in our fields, and coaddition of the data largely rules out either a very hot (spheroid) component, or very cold (disky) subpopulation. We have fit our velocities to an N-body bar by Juntai Shen, and the fit is excellent; the bar angle is constrained at 20 deg.

The bulge has an abundance gradient, a feature seen in other edge on classical and boxy bulges. However, the radial velocities are clearly consistent with an N-body bar that would conceivably have formed via a buckling instability (that would presumably not leave an abundance gradient as its signature). We consider that this is a problem in the formation of bars.

We discuss strategies for future surveys, including HERMES and APOGEE. The preliminary indications are that the bulge's kinematic uniformity leaves little room for multiple populations contributing to its formation.

fred Watson

Talk: RAVEing mad - the agony and the ecstasy of a million-star survey

Abstract: When RAVE was first proposed, it was anticipated that a modest pilot survey with the existing 6dF instrument on the UK Schmidt Telescope would be followed by a main survey using a new, purpose-built multi-fibre system. When funding for this ambitious instrument did not materialise, the decision was taken to 'RAVE on' towards a million stars, endowing the project with the legacy of an instrument not ideally suited to the task in hand. Nevertheless, the observing team has succeeded in gathering data on a significant fraction of that target, and recent developments promise to speed progress during the survey's remaining years. In this talk, I will describe some of the instrumental and observational aspects of the survey, and present an analysis of the economics of RAVEing.

Paul HARDING

Talk: Recent results from SEGUE

Abstract: I will present some recent results from SEGUE (the Sloan Extension for Galactic Understanding and Exploration). These include discovery of new ultra-cool subdwarfs, accurate globular cluster CMDs in ugriz for the first time, and abundance patterns in the outer halo.

SEAN RYAN ET AL.

Talk: Lithium abundances and isotope ratios in halo dwarfs

S. G. Ryan, A. E. Garcia Perez, A. Hosford & J. E. Norris

The primordial 7Li abundance inferred from halo dwarfs does not agree with the value expected from big bang nucleosynthesis calculations when the WMAP-inferred baryon-to-photon ratio is used. We are investigating some of the possible solutions to this problem and will report on findings to date. Although 6Li is not expected to be produced primordially, it too is amenable to study in the spectra of metal-poor dwarfs. We present the results of our analyses of this isotope, in the context of published detections of 6Li in such stars.

Poster: Temperature Scales and the "Lithium Problem"

A. Hosford, S. G. Ryan, A. E. Garcia Perez, J. E. Norris, K. A. Olive & R. Collet

Abstract: In calculating the abundances of Li, A(Li), from spectral measurements, the effective temperature is the most important atmospheric parameter, as Li has a high sensitivity to temperature: Delta A / Delta Teff ~ 0.065 dex per 100 K for halo main-sequence turnoff stars. It is the aim of this work to derive a Teff scale using the excitation energy method. In this work we report on the results of an LTE analysis and the continuing analysis of the effects of NLTE on the Teff scale.

Torgny Karlsson

Talk: Stellar relics, dwarf galaxies, and dark matter

Abstract: The oldest metal-poor stars are relics of a fiery cosmic era. These lingering remains hold important clues to the very first stellar objects and to the sequence of events that ultimately led to the formation of the Milky Way and its galactic companions in the local group. In particular, I will discuss a possible formation scenario for the population of carbon-enhanced metal-poor stars with no observed enhancements of s-process elements. Furthermore, I will argue that the apparent absence of the chemical signature of so-called pair-instability supernovae (PISNe), which are a natural consequence of current theoretical models for primordial star formation at the highest masses, may arise from a subtle observational selection effect. Finally, I will briefly discuss a new model of chemical evolution, specifically aimed to map out the metal enrichment in Dwarf Galaxies as a function of baryon and dark matter content.