162 research outputs found
Building an Optimal Census of the Solar Neighborhood with Pan-STARRS Data
We estimate the fidelity of solar neighborhood (D < 100 pc) catalogs soon to
be derived from Pan-STARRS astrometric data. We explore two quantities used to
measure catalog quality: completeness, the fraction of desired sources included
in a catalog; and reliability, the fraction of entries corresponding to desired
sources. We show that the main challenge in identifying nearby objects with
Pan-STARRS will be reliably distinguishing these objects from distant stars,
which are vastly more numerous. We explore how joint cuts on proper motion and
parallax will impact catalog reliability and completeness. Using synthesized
astrometry catalogs, we derive optimum parallax and proper motion cuts to build
a census of the solar neighborhood with the Pan-STARRS 3 Pi Survey. Depending
on the Galactic latitude, a parallax cut pi / sigma pi > 5 combined with a
proper motion cut ranging from mu / sigma mu > 1-8 achieves 99% reliability and
60% completeness.Comment: 7 Pages, 4 Figures, 3 Tables. PASP in pres
On Estimating the QSO Transmission Power Spectrum
We present new methods to minimize the systematic and random errors for
measuring the transmission power spectrum from the Lyman-alpha forest. Sources
of systematic errors explored include metal line contamination and
continuum-fitting. We advocate the technique of trend-removal in place of
traditional continuum-fitting -- here, a spectrum is normalized by its
(smoothly varying) mean rather than its continuum -- this method is easily
automated and removes biases introduced by continuum-fitting. Trend- removal
can be easily applied to spectra where continuum-fitting is difficult, such as
when the resolution or signal-to-noise is low, or for spectra at high
redshifts. Furthermore, a measurement of the continuum power spectrum using
trend-removal, from either low redshift quasar spectra or the red-side of
Lyman-alpha, allows in principle the removal of spurious power introduced by
the continuum and thereby expanding scales probed to larger ones. We also
derive expressions for the shot-noise bias and variance of the power spectrum
estimate, taking into account the non-Poissonian nature of the shot-noise and
the non-Gaussianity of the cosmic fluctuations. An appropriate minimum variance
weighting of the data is given. Finally, we give practical suggestions on
observing strategy: the desired resolution and S/N for different purposes, and
how to distribute one's finite observing time among quasar targets. Also
discussed is the quasar spectroscopic study of the Sloan Digital Sky Survey,
which has the potential to measure the power spectrum at z ~ 2-4 accurate to
better than 1 % per mode -- the techniques presented here will be useful for
tackling the anticipated issues of shot-noise and continuum contamination.Comment: 35 pages, 14 figures, submitted to Ap
Mauna Kea Sky Transparency from CFHT SkyProbe Data
Nighttime sky transparency statistics on Mauna Kea are reported based on data
from the Canada-France-Hawaii Telescope SkyProbe monitor. We focus on the
period beginning with the start of MegaCam wide-field optical imager operations
in 2003, and continuing for almost three years. Skies were clear enough to
observe on 76% of those nights; attenuations were less than 0.2 magnitudes up
to 60% of the time. An empirical model of cloud attenuation and duration is
presented allowing us to further characterize the photometric conditions. This
is a good fit tothe SkyProbe data, and indicates that Mauna Kea skies are truly
photometric (without cloud) an average of 56% of the time, with moderate
seasonal variation. Continuous monitoring of transparency during the night is
necessary to overcome fluctuations in attenuation due to thin cloud.Comment: 10 pages, 6 figures, submitted to PAS
A Volume-Limited Sample of Ultracool Dwarfs. I. Construction, Space Density, and a Gap in the L/T Transition
We present a new volume-limited sample of L0-T8 dwarfs out to 25 pc defined
entirely by parallaxes, using our recent measurements from UKIRT/WFCAM along
with Gaia DR2 and literature parallaxes. With 369 members, our sample is the
largest parallax-defined volume-limited sample of L and T dwarfs to date,
yielding the most precise space densities for such objects. We find the local
L0-T8 dwarf population includes young objects (200
Myr) and subdwarfs, as expected from recent studies favoring
representative ages 4 Gyr for the ultracool field population. This is
also the first volume-limited sample to comprehensively map the transition from
L to T dwarfs (spectral types L8-T4). After removing binaries, we
identify a previously unrecognized, statistically significant (>4.4)
gap 0.5 mag wide in colors in the L/T transition,
i.e., a lack of such objects in our volume-limited sample, implying a rapid
phase of atmospheric evolution. In contrast, the most successful models of the
L/T transition to date the "hybrid" models of Saumon & Marley (2008)
predict a pile-up of objects at the same colors where we find a deficit,
demonstrating the challenge of modeling the atmospheres of cooling brown
dwarfs. Our sample illustrates the insights to come from even larger
parallax-selected samples from the upcoming Legacy Survey of Space and Time
(LSST) by the Vera Rubin Obsevatory.Comment: AJ, in press. 71 pages, 17 figures, 4 tables. Data for all members of
the volume-limited sample can be found in the UltracoolSheet at
http://bit.ly/UltracoolSheet , a compilation of 3000+ ultracool dwarfs and
imaged exoplanets, including photometry, J2000 positions, parallaxes, proper
motions, multiplicity, and spectroscopic classifications from multiple
surveys and numerous source
X-rays from Superbubbles in the Large Magellanic Cloud IV: The Blowout Structure of N44
We have used optical echelle spectra along with ROSAT and ASCA X-ray spectra
to test the hypothesis that the southern portion of the N44 X-ray bright region
is the result of a blowout structure. Three pieces of evidence now support this
conclusion. First, the filamentary optical morphology corresponding with the
location of the X-ray bright South Bar suggests the blowout description (Chu et
al 1993). Second, optical echelle spectra show evidence of high velocity (~90
km/sec) gas in the region of the blowout. Third, X-ray spectral fits show a
lower temperature for the South Bar than the main superbubble region of Shell
1. Such a blowout can affect the evolution of the superbubble and explain some
of the discrepancy discussed by Oey & Massey (1995) between the observed shell
diameter and the diameter predicted on the basis of the stellar content and
Weaver et al.'s (1977) pressure-driven bubble model.Comment: 15 pages, LaTeX + psfig, 1 tex file, 2 sty files, 7 PS files, also
available at: http://www.astro.washington.edu/gene/papers/papers.htm
A Volume-Limited Sample of Ultracool Dwarfs. II. The Substellar Age and Mass Functions in the Solar Neighborhood
We present the most precise constraints to date for the mass and age
distributions of single ultracool dwarfs in the solar neighborhood, based on an
updated volume-limited sample of 504 L, T, and Y dwarfs within 25 pc. We
develop a Monte Carlo approach using the
statistic to correct for incompleteness and obtain a space density of
pc for spectral types L0-Y2. We
calculate bolometric luminosities for our sample, using an updated
"super-magnitude" method for the faintest objects. We use our resulting
luminosity function and a likelihood-based population synthesis approach to
simultaneously constrain the mass and age distributions. We employ the fraction
of young L0-L7 dwarfs as a novel input for this analysis that is crucial for
constraining the age distribution. For a power-law mass function we find , indicating an
increase in numbers toward lower masses, consistent with measurements in nearby
star-forming regions. For an exponential age distribution we find , i.e., a population with fewer old
objects than often assumed, which may reflect dynamical heating of the Galactic
plane as much as the historical brown dwarf birthrate. We compare our analysis
to Kirkpatrick et al. (2021), who used a similar volume-limited sample.
Although our mass function measurements are numerically consistent, their
assumption of a flat age distribution is disfavored by our analysis, and we
identify several important methodological differences between our two studies.
Our calculation of the age distribution of solar neighborhood brown dwarfs is
the first based on a volume-limited sample.Comment: Accepted to ApJ. 49 pages, 14 figures, 6 table
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