Stefan C. Keller

The SkyMapper Telescope and The Southern Sky Survey

This paper presents the design and science goals for the SkyMapper telescope. SkyMapper is a 1.3-m telescope featuring a 5.7-square-degree field-of-view Cassegrain imager commissioned for the Australian National Universitys Research School of Astronomy and Astrophysics. It is located at Siding Spring Observatory, Coonabarabran, NSW, Australia and will see first light in late 2007. The imager possesses 16 384 × 16 384 0.5-arcsec pixels. The primary scientific goal of the facility is to perform the Southern Sky Survey, a six-colour and multi-epoch (four-hour, one-day, one-week, one-month and one-year sampling) photometric survey of the southerly 2π sr to g ∼23 mag. The survey will provide photometry to better than 3% global accuracy and astrometry to better than 50 milliarcsec. Data will be supplied to the community as part of the Virtual Observatory effort. The survey will take five years to complete.

Bump cepheids in the magellanic clouds: metallicities, the distances to the lmc and smc, and the pulsation-evolution mass discrepancy

We use nonlinear pulsation models to reproduce the observed light and color curves for two samples of bump Cepheid variables, 19 from the Large Magellanic Cloud and 9 from the Small Magellanic Cloud. This analysis determines the fundamental parameters mass, luminosity, effective temperature, metallicity, distance, and reddening for the sample of stars. The use of the light-curve shape alone to determine metallicity is a new modeling technique introduced here. The metallicity, distance, and reddening distributions for the two samples are in agreement with those of similar stellar populations in the literature. The distance modulus of the Large Magellanic Cloud is determined to be 18.54 ± 0.018, and the distance modulus of the Small Magellanic Cloud is determined to be 18.93 ± 0.024. The mean Cepheid metallicities are Z = 0.0091 ± 0.0007 and 0.0050 ± 0.0005 for the LMC and SMC, respectively. The masses derived from pulsation analysis are significantly less than those predicted by stellar evolutionary models with no or mild convective core overshoot. We show that this discrepancy cannot be accounted for by uncertainties in our input opacities or in mass-loss physics. We interpret the observed mass discrepancy in terms of enhanced internal mixing in the vicinity of the convective core during the main-sequence lifetime and find that the overshoot parameter Λc rises from 0.688 ± 0.009Hp at the mean LMC metallicity to 0.746 ± 0.009Hp in the SMC.

The GALAH survey: Scientific motivation

The Galactic Archaeology with HERMES (GALAH) survey is a large high-resolution spectroscopic survey using the newly commissioned High Efficiency and Resolution Multi-Element Spectrograph (HERMES) on the Anglo-Australian Telescope. The HERMES spectrograph provides high-resolution (R ~ 28 000) spectra in four passbands for 392 stars simultaneously over a 2 deg field of view. The goal of the survey is to unravel the formation and evolutionary history of the Milky Way, using fossil remnants of ancient star formation events which have been disrupted and are now dispersed throughout the Galaxy. Chemical tagging seeks to identify such dispersed remnants solely from their common and unique chemical signatures; these groups are unidentifiable from their spatial, photometric or kinematic properties. To carry out chemical tagging, the GALAH survey will acquire spectra for a million stars down to V ~ 14. The HERMES spectra of FGK stars contain absorption lines from 29 elements including light proton-capture elements, α-elements, odd-Z elements, iron-peak elements and n-capture elements from the light and heavy s-process and the r-process. This paper describes the motivation and planned execution of the GALAH survey, and presents some results on the first-light performance of HERMES.

A single low-energy, iron-poor supernova as the source of metals in the star SMSS J031300.36-670839.3

The element abundance ratios of four low-mass stars with extremely low metallicities (abundances of elements heavier than helium) indicate that the gas out of which the stars formed was enriched in each case by at most a few—and potentially only one—low-energy supernova. Such supernovae yield large quantities of light elements such as carbon but very little iron. The dominance of low-energy supernovae seems surprising, because it had been expected that the first stars were extremely massive, and that they disintegrated in pair-instability explosions that would rapidly enrich galaxies in iron. What has remained unclear is the yield of iron from the first supernovae, because hitherto no star has been unambiguously interpreted as encapsulating the yield of a single supernova. Here we report the optical spectrum of SMSS J031300.36−670839.3, which shows no evidence of iron (with an upper limit of 10−7.1 times solar abundance). Based on a comparison of its abundance pattern with those of models, we conclude that the star was seeded with material from a single supernova with an original mass about 60 times that of the Sun (and that the supernova left behind a black hole). Taken together with the four previously mentioned low-metallicity stars, we conclude that low-energy supernovae were common in the early Universe, and that such supernovae yielded light-element enrichment with insignificant iron. Reduced stellar feedback both chemically and mechanically from low-energy supernovae would have enabled first-generation stars to form over an extended period. We speculate that such stars may perhaps have had an important role in the epoch of cosmic reionization and the chemical evolution of early galaxies.

A real-time fast radio burst: polarization detection and multiwavelength follow-up

Fast radio bursts (FRBs) are one of the most tantalizing mysteries of the radio sky; their progenitors and origins remain unknown and until now no rapid multiwavelength follow-up of an FRB has been possible. New instrumentation has decreased the time between observation and discovery from years to seconds, and enables polarimetry to be performed on FRBs for thefirst time. We have discovered an FRB (FRB 140514) in real-time on 2014 May 14 at 17:14:11.06 UTCattheParkesradiotelescopeandtriggeredfollow-upatotherwavelengthswithinhoursof theevent.FRB140514wasfoundwithadispersionmeasure(DM)of562.7(6)cm −3 pc,giving an upper limit on source redshift of z 0.5. FRB 140514 was found to be 21 ± 7 per cent (3σ) circularly polarized on the leading edge with a 1σ upper limit on linear polarization <10 per cent. We conclude that this polarization is intrinsic to the FRB. If there was any intrinsic linear polarization, as might be expected from coherent emission, then it may have been depolarized by Faraday rotation caused by passing through strong magnetic fields and/or high-density environments. FRB 140514 was discovered during a campaign to re-observe known FRB fields, and lies close to a previous discovery, FRB 110220; based on the difference in DMs of these bursts and time-on-sky arguments, we attribute the proximity to sampling bias and conclude that they are distinct objects. Follow-up conducted by 12 telescopes observing from X-ray to radio wavelengths was unable to identify a variable multiwavelength counterpart, allowing us to rule out models in which FRBs originate from nearby ( z< 0.3) supernovae and long duration gamma-ray bursts.

Testing LMC Microlensing Scenarios: The Discrimination Power of the SuperMACHO Microlensing Survey

Characterizing the nature and spatial distribution of the lensing objects that produce the previously measured microlensing optical depth toward the Large Magellanic Cloud (LMC) remains an open problem. We present an appraisal of the ability of the SuperMACHO Project, a next-generation microlensing survey directed toward the LMC, to discriminate between various proposed lensing populations. We consider two scenarios: lensing by a uniform foreground screen of objects and self-lensing by LMC stars. The optical depth for screen lensing is essentially constant across the face of the LMC, whereas the optical depth for self-lensing shows a strong spatial dependence. We have carried out extensive simulations, based on data obtained during the first year of the project, to assess the SuperMACHO surveys ability to discriminate between these two scenarios. In our simulations we predict the expected number of observed microlensing events for various LMC models for each of our fields by adding artificial stars to the images and estimating the spatial and temporal efficiency of detecting microlensing events using Monte Carlo methods. We find that the event rate itself shows significant sensitivity to the choice of the LMC luminosity function, limiting the conclusions that can be drawn from the absolute rate. If instead we determine the differential event rate across the LMC, we will decrease the impact of these systematic biases and render our conclusions more robust. With this approach the SuperMACHO Project should be able to distinguish between the two categories of lens populations. This will provide important constraints on the nature of the lensing objects and their contributions to the Galactic dark matter halo.

Revealing Substructure in the Galactic Halo: The SEKBO RR Lyrae Survey

We present a search for RR Lyrae variable stars from archival observations of the Southern Edgeworth-Kuiper Belt Object survey. The survey covers 1675 deg2 along the ecliptic to a mean depth of -->V = 19.5, i.e., a heliocentric distance of ~50 kpc for RR Lyrae stars. The survey reveals 2016 RR Lyrae candidates. Follow-up photometric monitoring of a subset of these candidates shows ~24% contamination by non-RR Lyrae variables. We derive a map of overdensity of RR Lyrae stars in the halo that reveals a series of structures coincident with the leading and trailing arms of debris from the Sagittarius dwarf galaxy. One of the regions of overdensity is found on the trailing arm, 200° from the main body of the Sagittarius dwarf at a distance of ~45 kpc. This distant detection of the stellar population of the outer trailing arm of Sagittarius offers a tight constraint on the motion of the dwarf galaxy. A distinctly separate region of overdensity is seen toward the Virgo overdensity.

Be stars in and around young clusters in the Magellanic Clouds

We present the results of a search for Be stars in six elds centered on the young clusters NGC 330 and NGC 346 in the SMC, and NGC 1818, NGC 1948, NGC 2004 and NGC 2100 in the LMC. Be stars were iden- tied by dierencing R band and narrow-band H CCD images. Our comparatively large images provide substan- tial Be star populations both within the clusters and in their surrounding elds. Magnitudes, positions and nding charts are given for the 224 Be stars found. The fraction of Be stars to normal B stars within each cluster is found to vary signicantly although the average ratio is similar to the average Be to B star ratio found in the Galaxy. In some clusters, the Be star population is weighted to mag- nitudes near the main sequence turn-o. The Be stars are redder in V I than normal main-sequence stars of similar magnitude and the redness increases with increasing H emission strength.

Jet-induced Star Formation in Centaurus A

The inner part of the northeast middle radio lobe of the radio galaxy Centaurus A is the site of complex interactions. This area contains a large H I cloud as well as filaments of ionized gas and associated blue knots, several of which exist along the northeastern edge of the radio-emitting zones. We observed the filaments and blue knots with the Hubble Space Telescope using WFPC2, and the ionized gas from the ground. Our sensitive, high angular resolution WFPC2 images reveal the presence of young stars, many concentrated in what appear to be OB associations, superimposed on a background sheet of older stars that is typical of the Cen A halo. The ages of the OB associations are estimated to be less than 15 Myr from a comparison of color-magnitude diagrams with those for the Large Magellanic Cloud star cluster NGC 2004, and younger stellar groups may be nearer regions of Hα emission. We discuss our data in the context of models for star formation stimulated by interactions between the radio jet and gas cloud.

Cepheid Mass Loss and the Pulsation-Evolutionary Mass Discrepancy

I investigate the discrepancy between the evolution and pulsation masses for Cepheid variables. A number of recent works have proposed that noncanonical mass loss can account for the mass discrepancy. This mass loss would be such that a 5 M☉ star loses approximately 20% of its mass by arriving at the Cepheid instability strip; a 14 M☉ star, none. Such findings would pose a serious challenge to our understanding of mass loss. I revisit these results in light of the Padova stellar evolutionary models and find evolutionary masses are (17 ± 5)% greater than pulsation masses for Cepheids between 5 < M/M☉ < 14. I find that mild internal mixing in the main-sequence progenitor of the Cepheid are able to account for this mass discrepancy.