93 research outputs found
Spectral Classification; Old and Contemporary
Beginning with a historical account of the spectral classification, its
refinement through additional criteria is presented. The line strengths and
ratios used in two dimensional classifications of each spectral class are
described. A parallel classification scheme for metal-poor stars and the
standards used for classification are presented. The extension of spectral
classification beyond M to L and T and spectroscopic classification criteria
relevant to these classes are described. Contemporary methods of
classifications based upon different automated approaches are introduced.Comment: To be published in "Principles and Perspectives in Cosmochemistry"
Lecture Notes on Kodai School on Synthesis of Elements in Stars: Ed Aruna
Goswami & Eswar Reddy, Springer Verlag, 2009, 17 pages, 10 figure
The Age of the Milky Way Inner Halo
The Milky Way galaxy is observed to have multiple components with distinct
properties, such as the bulge, disk, and halo. Unraveling the assembly history
of these populations provides a powerful test to the theory of galaxy formation
and evolution, but is often restricted due to difficulties in measuring
accurate stellar ages for low mass, hydrogen-burning stars. Unlike these
progenitors, the "cinders" of stellar evolution, white dwarf stars, are
remarkably simple objects and their fundamental properties can be measured with
little ambiguity from spectroscopy. Here I report observations and analysis of
newly formed white dwarf stars in the halo of the Milky Way, and a comparison
to published analysis of white dwarfs in the well-studied 12.5 billion-year-old
globular cluster Messier 4. From this, I measure the mass distribution of the
remnants and invert the stellar evolution process to develop a new relation
that links this final stellar mass to the mass of their immediate progenitors,
and therefore to the age of the parent population. By applying this technique
to a small sample of four nearby and kinematically-confirmed halo white dwarfs,
I measure the age of local field halo stars to be 11.4 +/- 0.7 billion years.
This age is directly tied to the globular cluster age scale, on which the
oldest clusters formed 13.5 billion years ago. Future (spectroscopic)
observations of newly formed white dwarfs in the Milky Way halo can be used to
reduce the present uncertainty, and to probe relative differences between the
formation time of the last clusters and the inner halo.Comment: Published in Nature, 2012, 486, 90. Second version corrects a missing
reference (#10) in the third paragraph and Figure 1 captio
The Classification of T Dwarfs
We discuss methods for classifying T dwarfs based on spectral morphological
features and indices. T dwarfs are brown dwarfs which exhibit methane
absorption bands at 1.6 and 2.2 . Spectra at red optical (6300--10100
{\AA}) and near-infrared (1--2.5 ) wavelengths are presented, and
differences between objects are noted and discussed. Spectral indices useful
for classification schemes are presented. We conclude that near-infrared
spectral classification is generally preferable for these cool objects, with
data sufficient to resolve the 1.17 and 1.25 K I doublets lines being
most valuable. Spectral features sensitive to gravity are discussed, with the
strength of the K-band peak used as an example. Such features may be used to
derive a two-dimensional scheme based on temperature and mass, in analogy to
the MK temperature and luminosity classes.Comment: 15 pages, 6 figures, conference proceedings for IAU Ultracool Dwarf
Stars session, ed. I. Steele & H. Jone
Gemini planet imager observational calibrations V: Astrometry and distortion
This is the final version of the article. Available from SPIE via the DOI in this record.From Conference Volume 9147: Ground-based and Airborne Instrumentation for Astronomy V, Suzanne K. Ramsay; Ian S. McLean; Hideki Takami, Montréal, Quebec, Canada, June 22, 2014We present the results of both laboratory and on sky astrometric characterization of the Gemini Planet Imager (GPI). This characterization includes measurement of the pixel scale∗ of the integral field spectrograph (IFS), the position of the detector with respect to north, and optical distortion. Two of these three quantities (pixel scale and distortion) were measured in the laboratory using two transparent grids of spots, one with a square pattern and the other with a random pattern. The pixel scale in the laboratory was also estimate using small movements of the artificial star unit (ASU) in the GPI adaptive optics system. On sky, the pixel scale and the north angle are determined using a number of known binary or multiple systems and Solar System objects, a subsample of which had concurrent measurements at Keck Observatory. Our current estimate of the GPI pixel scale is 14.14 ± 0.01 millarcseconds/pixel, and the north angle is -1.00 ± 0.03°. Distortion is shown to be small, with an average positional residual of 0.26 pixels over the field of view, and is corrected using a 5th order polynomial. We also present results from Monte Carlo simulations of the GPI Exoplanet Survey (GPIES) assuming GPI achieves ∼1 milliarcsecond relative astrometric precision. We find that with this precision, we will be able to constrain the eccentricities of all detected planets, and possibly determine the underlying eccentricity distribution of widely separated Jovians.The Gemini Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil), and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina). This publication makes use of data obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. P.K. and J.R.G. thank support from NASA NNX11AD21G, NSF AST-0909188, and the University of California LFRP-118057. Q.M.K is a Dunlap Fellow at the Dunlap Institute for Astronomy & Astrophysics, University of Toronto. The Dunlap Institute is funded through an endowment established by the David Dunlap family and the University of Toronto
Multiplicity of young brown dwarfs in Cha I
How frequent are brown dwarf binaries? Do brown dwarfs have planets? Are
current theoretical pre-main-sequence evolutionary tracks valid down to the
substellar regime? - Any detection of a companion to a brown dwarf takes us one
step forward towards answering these basic questions of star formation.
We report here on a search for spectroscopic and visual companions to young
brown dwarfs in the Cha I star forming cloud.
Based on spectra taken with UVES at the VLT, we found significant radial
velocity (RV) variations for five bona-fide and candidate brown dwarfs in Cha
I. They can be caused by either a (substellar or planetary) companion or
stellar activity. A companion causing the detected RV variations would have
about a few Jupiter masses. We are planning further UVES observations in order
to explore the nature of the detected RV variations. We also found that the RV
dispersion is only ~ 2km/s indicating that there is probably no run-away brown
dwarf among them.
Additionally a search for companions by direct imaging with the HST and SOFI
(NTT) has yielded to the detection of a few companion candidates in larger
orbits.Comment: Conference proceeding "Origins of stars and planets: The VLT view",
ESO, Garching, April 24-27 200
Stationary Black Holes: Uniqueness and Beyond
The spectrum of known black-hole solutions to the stationary Einstein
equations has been steadily increasing, sometimes in unexpected ways. In
particular, it has turned out that not all black-hole-equilibrium
configurations are characterized by their mass, angular momentum and global
charges. Moreover, the high degree of symmetry displayed by vacuum and
electro-vacuum black-hole spacetimes ceases to exist in self-gravitating
non-linear field theories. This text aims to review some developments in the
subject and to discuss them in light of the uniqueness theorem for the
Einstein-Maxwell system.Comment: Major update of the original version by Markus Heusler from 1998.
Piotr T. Chru\'sciel and Jo\~ao Lopes Costa succeeded to this review's
authorship. Significantly restructured and updated all sections; changes are
too numerous to be usefully described here. The number of references
increased from 186 to 32
The changing landscape of disaster volunteering: opportunities, responses and gaps in Australia
There is a growing expectation that volunteers will have a greater role in disaster management in the future compared to the past. This is driven largely by a growing focus on building resilience to disasters. At the same time, the wider landscape of volunteering is fundamentally changing in the twenty-first century. This paper considers implications of this changing landscape for the resilience agenda in disaster management, with a focus on Australia. It first reviews major forces and trends impacting on disaster volunteering, highlighting four key developments: the growth of more diverse and episodic volunteering styles, the impact of new communications technology, greater private sector involvement and growing government expectations of and intervention in the voluntary sector. It then examines opportunities in this changing landscape for the Australian emergency management sector across five key strategic areas and provides examples of Australian responses to these opportunities to date. The five areas of focus are: developing more flexible volunteering strategies, harnessing spontaneous volunteering, building capacity to engage digital (and digitally enabled) volunteers, tapping into the growth of employee and skills-based volunteering and co-producing community-based disaster risk reduction. Although there have been considerable steps taken in Australia in some of these areas, overall there is still a long way to go before the sector can take full advantage of emerging opportunities. The paper thus concludes by identifying important research and practice gaps in this area
Gemini-LIGHTS: Herbig Ae/Be and massive T-Tauri protoplanetary disks imaged with Gemini Planet Imager
This is the final version. Available on open access from the American Astronomical Society via the DOI in this recordWe present the complete sample of protoplanetary disks from the Gemini- Large Imaging with GPI
Herbig/T-tauri Survey (Gemini-LIGHTS) which observed bright Herbig Ae/Be stars and T-Tauri stars
in near-infrared polarized light to search for signatures of disk evolution and ongoing planet formation.
The 44 targets were chosen based on their near- and mid-infrared colors, with roughly equal numbers
of transitional, pre-transitional, and full disks. Our approach explicitly did not favor well-known,
“famous” disks or those observed by ALMA, resulting in a less-biased sample suitable to probe the
major stages of disk evolution during planet formation. Our optimized data reduction allowed polarized
flux as low as 0.002% of the stellar light to be detected, and we report polarized scattered light around
80% of our targets. We detected point-like companions for 47% of the targets, including 3 brown
dwarfs (2 confirmed, 1 new), and a new super-Jupiter mass candidate around V1295 Aql. We searched
for correlations between the polarized flux and system parameters, finding a few clear trends: presence
of a companion drastically reduces the polarized flux levels, far-IR excess correlates with polarized flux
for non-binary systems, and systems hosting disks with ring structures have stellar masses < 3 M .
Our sample also included four hot, dusty “FS CMa” systems and we detected large-scale (> 100 au)
scattered light around each, signs of extreme youth for these enigmatic systems. Science-ready images
are publicly available through multiple distribution channels using a new FITS file standard jointly
developed with members of the VLT/SPHERE team.NSFEuropean Research Council (ERC)ANI
Exploring new physics frontiers through numerical relativity
The demand to obtain answers to highly complex problems within strong-field gravity has been met with significant progress in the numerical solution of Einstein's equations - along with some spectacular results - in various setups. We review techniques for solving Einstein's equations in generic spacetimes, focusing on fully nonlinear evolutions but also on how to benchmark those results with perturbative approaches. The results address problems in high-energy physics, holography, mathematical physics, fundamental physics, astrophysics and cosmology
QCD and strongly coupled gauge theories : challenges and perspectives
We highlight the progress, current status, and open challenges of QCD-driven physics, in theory and in experiment. We discuss how the strong interaction is intimately connected to a broad sweep of physical problems, in settings ranging from astrophysics and cosmology to strongly coupled, complex systems in particle and condensed-matter physics, as well as to searches for physics beyond the Standard Model. We also discuss how success in describing the strong interaction impacts other fields, and, in turn, how such subjects can impact studies of the strong interaction. In the course of the work we offer a perspective on the many research streams which flow into and out of QCD, as well as a vision for future developments.Peer reviewe
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