124 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
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 4 Gyr M-dwarf Gyrochrone from CFHT/MegaPrime Monitoring of the Open Cluster M67
We present stellar rotation periods for late K- and early M-dwarf members of
the 4 Gyr old open cluster M67 as calibrators for gyrochronology and tests of
stellar spin-down models. Using Gaia EDR3 astrometry for cluster membership and
Pan-STARRS (PS1) photometry for binary identification, we build this set of
rotation periods from a campaign of monitoring M67 with the
Canada-France-Hawaii Telescope's MegaPrime wide field imager. We identify 1807
members of M67, of which 294 are candidate single members with significant
rotation period detections. Moreover, we fit a polynomial to the period versus
color-derived effective temperature sequence observed in our data. We find that
the rotation of very cool dwarfs can be explained by a simple solid-body
spin-down between 2.7 and 4 Gyr. We compare this rotational sequence to the
predictions of gyrochronological models and find that the best match is
Skumanich-like spin-down, P_rot \propto t^0.62, applied to the sequence of
Ruprecht 147. This suggests that, for spectral types K7-M0 with near-solar
metallicity, once a star resumes spinning down, a simple Skumanich-like is
sufficient to describe their rotation evolution, at least through the age of
M67. Additionally, for stars in the range M1-M3, our data show that spin-down
must have resumed prior to the age of M67, in conflict with predictions of the
latest spin-down models.Comment: 21 pages, 16 figures, Accepted for publication by Ap
Evidence for Distinct Components of the Galactic Stellar Halo from 838 RR Lyrae Stars Discovered in the LONEOS-I Survey
We present 838 ab-type RR Lyrae stars from the Lowell Observatory Near Earth
Objects Survey Phase I (LONEOS-I). These objects cover 1430 deg^2 and span
distances ranging from 3-30 kpc from the Galactic Center. Object selection is
based on phased, photometric data with 28-50 epochs. We use this large sample
to explore the bulk properties of the stellar halo, including the spatial
distribution. The period-amplitude distribution of this sample shows that the
majority of these RR Lyrae stars resemble Oosterhoff type I, but there is a
significant fraction (26 %) which have longer periods and appear to be
Oosterhoff type II. We find that the radial distributions of these two
populations have significantly different profiles (rho_{OoI} ~ R^(-2.26 +-
0.07) and rho_{OoII} ~ R^(-2.88 +- 0.11). This suggests that the stellar halo
was formed by at least two distinct accretion processes and supports dual-halo
models.Comment: 18 pages, 28 figures, apjemulated, minor corrections and
clarifications. Accepted to ApJ on Jan 21, 200
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