Ariel - Volume 11 Number 4

Executive Editor Ellen Feldman Leonardo S. Nasca, Jr. Business Managers Barbara L. Davies Martin B. Getzow News Editor Hugh A. Gelabert Features Editor Aaron D. Bleznak CAHS Editor Joan M. Greco Editorial Page Editor Samuel Markind Photography Editor Todd L. Demmy Sports Editor Paul F. Mansfiel

The Spectral Types of White Dwarfs in Messier 4

We present the spectra of 24 white dwarfs in the direction of the globular cluster Messier 4 obtained with the Keck/LRIS and Gemini/GMOS spectrographs. Determining the spectral types of the stars in this sample, we find 24 type DA and 0 type DB (i.e., atmospheres dominated by hydrogen and helium respectively). Assuming the ratio of DA/DB observed in the field with effective temperature between 15,000 - 25,000 K, i.e., 4.2:1, holds for the cluster environment, the chance of finding no DBs in our sample due simply to statistical fluctuations is only 6 X 10^(-3). The spectral types of the ~100 white dwarfs previously identified in open clusters indicate that DB formation is strongly suppressed in that environment. Furthermore, all the ~10 white dwarfs previously identified in other globular clusters are exclusively type DA. In the context of these two facts, this finding suggests that DB formation is suppressed in the cluster environment in general. Though no satisfactory explanation for this phenomenon exists, we discuss several possibilities.Comment: Accepted for Publication in Astrophys. J. 11 pages including 4 figures and 2 tables (journal format

The Space Motion of the Globular Cluster NGC 6397

As a by-product of high-precision, ultra-deep stellar photometry in the Galactic globular cluster NGC 6397 with the Hubble Space Telescope, we are able to measure a large population of background galaxies whose images are nearly point-like. These provide an extragalactic reference frame of unprecedented accuracy, relative to which we measure the most accurate absolute proper motion ever determined for a globular cluster. We find mu_alpha = 3.56 +/- 0.04 mas/yr and mu_delta = -17.34 +/- 0.04 mas/yr. We note that the formal statistical errors quoted for the proper motion of NGC 6397 do not include possible unavoidable sources of systematic errors, such as cluster rotation. These are very unlikely to exceed a few percent. We use this new proper motion to calculate NGC 6397's UVW space velocity and its orbit around the Milky Way, and find that the cluster has made frequent passages through the Galactic disk.Comment: 5 pages including 3 figures, accepted for publication in the Astrophysical Journal Letters. Very minor changes in V2. typos fixe

Deep ACS Imaging in the Globular Cluster NGC 6397: The Cluster Color Magnitude Diagram and Luminosity Function

We present the CMD from deep HST imaging in the globular cluster NGC 6397. The ACS was used for 126 orbits to image a single field in two colors (F814W, F606W) 5 arcmin SE of the cluster center. The field observed overlaps that of archival WFPC2 data from 1994 and 1997 which were used to proper motion (PM) clean the data. Applying the PM corrections produces a remarkably clean CMD which reveals a number of features never seen before in a globular cluster CMD. In our field, the main sequence stars appeared to terminate close to the location in the CMD of the hydrogen-burning limit predicted by two independent sets of stellar evolution models. The faintest observed main sequence stars are about a magnitude fainter than the least luminous metal-poor field halo stars known, suggesting that the lowest luminosity halo stars still await discovery. At the bright end the data extend beyond the main sequence turnoff to well up the giant branch. A populous white dwarf cooling sequence is also seen in the cluster CMD. The most dramatic features of the cooling sequence are its turn to the blue at faint magnitudes as well as an apparent truncation near F814W = 28. The cluster luminosity and mass functions were derived, stretching from the turn off down to the hydrogen-burning limit. It was well modeled with either a very flat power-law or a lognormal function. In order to interpret these fits more fully we compared them with similar functions in the cluster core and with a full N-body model of NGC 6397 finding satisfactory agreement between the model predictions and the data. This exercise demonstrates the important role and the effect that dynamics has played in altering the cluster IMF.Comment: 43 pages including 4 tables and 12 diagrams. Figures 2 and 3 have been bitmapped. Accepted for publication in the Astronomical Journa

Deep ACS Imaging in the Globular Cluster NGC6397: Dynamical Models

We present N-body models to complement deep imaging of the metal-poor core-collapsed cluster NGC6397 obtained with the Hubble Space Telescope. All simulations include stellar and binary evolution in-step with the stellar dynamics and account for the tidal field of the Galaxy. We focus on the results of a simulation that began with 100000 objects (stars and binaries), 5% primordial binaries and Population II metallicity. After 16 Gyr of evolution the model cluster has about 20% of the stars remaining and has reached core-collapse. We compare the color-magnitude diagrams of the model at this age for the central region and an outer region corresponding to the observed field of NGC6397 (about 2-3 half-light radii from the cluster centre). This demonstrates that the white dwarf population in the outer region has suffered little modification from dynamical processes - contamination of the luminosity function by binaries and white dwarfs with non-standard evolution histories is minimal and should not significantly affect measurement of the cluster age. We also show that the binary fraction of main-sequence stars observed in the NGC6397 field can be taken as representative of the primordial binary fraction of the cluster. For the mass function of the main-sequence stars we find that although this has been altered significantly by dynamics over the cluster lifetime, especially in the central and outer regions, that the position of the observed field is close to optimal for recovering the initial mass function of the cluster stars (below the current turn-off mass). More generally we look at how the mass function changes with radius in a dynamically evolved stellar cluster and suggest where the best radial position to observe the initial mass function is for clusters of any age.Comment: 34 pages, 11 figures, submitted to AJ, companion paper to 0708.403

The Masses of Population II White Dwarfs

Globular star clusters are among the first stellar populations to have formed in the Milky Way, and thus only a small sliver of their initial spectrum of stellar types are still burning hydrogen on the main-sequence today. Almost all of the stars born with more mass than 0.8 M_sun have evolved to form the white dwarf cooling sequence of these systems, and the distribution and properties of these remnants uniquely holds clues related to the nature of the now evolved progenitor stars. With ultra-deep HST imaging observations, rich white dwarf populations of four nearby Milky Way globular clusters have recently been uncovered, and are found to extend an impressive 5 - 8 magnitudes in the faint-blue region of the H-R diagram. In this paper, we characterize the properties of these population II remnants by presenting the first direct mass measurements of individual white dwarfs near the tip of the cooling sequence in the nearest of the Milky Way globulars, M4. Based on Gemini/GMOS and Keck/LRIS multiobject spectroscopic observations, our results indicate that 0.8 M_sun population II main-sequence stars evolving today form 0.53 +/- 0.01 M_sun white dwarfs. We discuss the implications of this result as it relates to our understanding of stellar structure and evolution of population II stars and for the age of the Galactic halo, as measured with white dwarf cooling theory.Comment: Accepted for Publication in Astrophys. J. on Aug. 05th, 2009. 19 pages including 9 figures and 2 tables (journal format

Enlarging conference learning : at the crossroads of fat studies and conference pedagogies

This article stages an encounter between the field of fat studies and conference pedagogy scholarship. After laying the foundations for a reading of academic conferences as learning spaces, the authors present two examples—International Fat Studies Conferences held in Aotearoa, New Zealand, in 2012 and 2016—to unpack these ideas. The framing of fat studies conferences as pedagogical spaces sparks questions that travel in multiple directions. It calls us to consider possible modifications to the design of fat studies conferences, as well as how discussions about fat pedagogy may have a wider application to academic gatherings

Probing the Faintest Stars in a Globular Star Cluster

NGC 6397 is the second closest globular star cluster to the Sun. Using 5 days of time on the Hubble Space Telescope, we have constructed the deepest ever color-magnitude diagram for this cluster. We see a clear truncation in each of its two major stellar sequences. Faint red main sequence stars run out well above our observational limit and near to the theoretical prediction for the lowest mass stars capable of stable hydrogen-burning in their cores. We also see a truncation in the number counts of faint blue stars, namely white dwarfs. This reflects the limit to which the bulk of the white dwarfs can cool over the lifetime of the cluster. There is also a turn towards bluer colors in the least luminous of these objects. This was predicted for the very coolest white dwarfs with hydrogen-rich atmospheres as the formation of H2 causes their atmospheres to become largely opaque to infrared radiation due to collision-induced absorption.Comment: 12 pages, 4 figures. Full Resolution Figures in Science, 2006, 313, 93

The foot in forensic human identification - a review

The identification of human remains is a process which can be attempted irrespective of the stage of decomposition in which the remains are found or the anatomical regions recovered. In recent years, the discovery of fragmented human remains has garnered significant attention from the national and international media, particularly the recovery of multiple lower limbs and feet from coastlines in North America. While cases such as these stimulate public curiosity, they present unique challenges to forensic practitioners in relation to the identification of the individual from whom the body part originated. There is a paucity of literature pertaining to the foot in forensic human identification and in particular, in relation to the assessment of the parameters represented by the biological profile. This article presents a review of the literature relating to the role of the foot in forensic human identification and highlights the areas in which greater research is required. © 2013

Regulating terrorist content on social media: automation and the rule of law

Social media companies make extensive use of artificial intelligence in their efforts to remove and block terrorist content from their platforms. This article begins by arguing that, since such efforts amount to an attempt to channel human conduct, they should be regarded as a form of regulation that is subject to rule of law principles. The article then discusses three sets of rule of law issues. The first set concerns enforceability. Here the article highlights the displacement effects that have resulted from the automated removal and blocking of terrorist content and argues that regard must be had to the whole social media ecology, as well as to jihadist groups other than the so-called Islamic State and other forms of violent extremism. Since rule by law is only a necessary, and not a sufficient, condition for compliance with rule of law values, the article then goes on to examine two further sets of issues: the clarity with which social media companies define terrorist content and the adequacy of the processes by which a user may appeal against an account suspension or the blocking or removal of content. The article concludes by identifying a range of research questions that emerge from the discussion and that together form a promising and timely research agenda to which legal scholarship has much to contribute