18,226 research outputs found
LGS AO Science Impact: Present and Future Perspectives
The recent advent of laser guide star adaptive optics (LGS AO) on the largest
ground-based telescopes has enabled a wide range of high angular resolution
science, previously infeasible from ground-based and/or space-based
observatories. As a result, scientific productivity with LGS has seen enormous
growth in the last few years, with a factor of ~10 leap in publication rate
compared to the first decade of operation. Of the 54 refereed science papers to
date from LGS AO, half have been published in the last ~2 years, and these LGS
results have already made a significant impact in a number of areas. At the
same time, science with LGS AO can be considered in its infancy, as astronomers
and instrumentalists are only beginning to understand its efficacy for
measurements such as photometry, astrometry, companion detection, and
quantitative morphology. We examine the science impact of LGS AO in the last
few years of operations, largely due to the new system on the Keck II 10-meter
telescope. We review currently achieved data quality, including results from
our own ongoing brown dwarf survey with Keck LGS. We assess current and
near-future performance with a critical eye to LGS AO's capabilities and
deficiencies. From both qualitative and quantitative considerations, it is
clear that the era of regular and important science from LGS AO has arrived.Comment: Invited review for Proc of the SPIE, "Astronomical Telescopes and
Instrumentation" (Marseilles, France, June 2008). Minor typos fixed, upgrade
to Figure 3, and addition of new Gemini LGS wor
A Near-Infrared Spectroscopic Study of Young Field Ultracool Dwarfs
We present a near-infrared (0.9-2.4 microns) spectroscopic study of 73 field
ultracool dwarfs having spectroscopic and/or kinematic evidence of youth
(~10-300 Myr). Our sample is composed of 48 low-resolution (R~100) spectra and
41 moderate-resolution spectra (R>~750-2000). First, we establish a method for
spectral typing M5-L7 dwarfs at near-IR wavelengths that is independent of
gravity. We find that both visual and index-based classification in the near-IR
provide consistent spectral types with optical spectral types, though with a
small systematic offset in the case of visual classification at J and K band.
Second, we examine features in the spectra of ~10 Myr ultracool dwarfs to
define a set of gravity-sensitive indices based on FeH, VO, K, Na and H-band
continuum shape. We then create an index-based method for classifying the
gravities of M6-L5 dwarfs that provides consistent results with gravity
classifications from optical spectroscopy. Our index-based classification can
distinguish between young and dusty objects. Guided by the resulting
classifications, we propose a set of low-gravity spectral standards for the
near-IR. Finally, we estimate the ages corresponding to our gravity
classifications.Comment: Published in ApJ. IDL program for calculating indices
(allers13_index.pro) included in the source gzipped ta
Kelu-1 is a Binary L Dwarf: First Brown Dwarf Science from Laser Guide Star Adaptive Optics
(Abridged) We present near-IR imaging of the nearby L dwarf Kelu-1 obtained
with the Keck sodium laser guide star adaptive optics (LGS AO) system as part
of a high angular resolution survey for substellar binaries. Kelu-1 was one of
the first free-floating L dwarfs identified, and the origin of its
overluminosity compared to other similar objects has been a long-standing
question. Our images clearly resolve Kelu-1 into a 0.29'' (5.4 AU) binary, and
a previous non-detection by HST demonstrates that the system is a true physical
pair. Binarity explains the properties of Kelu-1 that were previously noted to
be anomalous compared to other early-L dwarfs. We estimate spectral types of
L1.5-L3 and L3-L4.5 for the two components, giving model-derived masses of
0.05-0.07 Msun and 0.045-0.065 Msun for an estimated age of 0.3-0.8 Gyr. More
distant companions are not detected to a limit of 5-9 Mjup. The presence of
lithium absorption indicates that both components are substellar, but the
weakness of this feature relative to other L dwarfs can be explained if only
Kelu-1B is Li-bearing. Determining whether both or only one of the components
possesses lithium could constrain the age of Kelu-1 (and other Li-bearing L
binaries) with higher precision than is possible for most ultracool field
objects. These results are the first LGS AO observations of brown dwarfs and
demonstrate the potential of this new instrumental capability for substellar
astronomy.Comment: 24 pages, Astrophysical Journal, in press (Nov 20, 2005 issue). Note
that Figure 1 of the PDF version is degraded by arxiv.org, but the Postscript
version is fine. Version 2 includes very minor changes to match the published
versio
Theoretical Predictions for Surface Brightness Fluctuations and Implications for Stellar Populations of Elliptical Galaxies
(Abridged) We present new theoretical predictions for surface brightness
fluctuations (SBFs) using models optimized for this purpose. Our predictions
agree well with SBF data for globular clusters and elliptical galaxies. We
provide refined theoretical calibrations and k-corrections needed to use SBFs
as standard candles. We suggest that SBF distance measurements can be improved
by using a filter around 1 micron and calibrating I-band SBFs with the
integrated V-K galaxy color. We also show that current SBF data provide useful
constraints on population synthesis models, and we suggest SBF-based tests for
future models. The data favor specific choices of evolutionary tracks and
spectra in the models among the several choices allowed by comparisons based on
only integrated light. In addition, the tightness of the empirical I-band SBF
calibration suggests that model uncertainties in post-main sequence lifetimes
are less than +/-50% and that the IMF in ellipticals is not much steeper than
that in the solar neighborhood. Finally, we analyze the potential of SBFs for
probing unresolved stellar populations. We find that optical/near-IR SBFs are
much more sensitive to metallicity than to age. Therefore, SBF magnitudes and
colors are a valuable tool to break the age/metallicity degeneracy. Our initial
results suggest that the most luminous stellar populations of bright cluster
galaxies have roughly solar metallicities and about a factor of three spread in
age.Comment: Astrophysical Journal, in press (uses Apr 20, 2000 version of
emulateapj5.sty). Reposted version has a minor cosmetic change to Table
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