137 research outputs found
Hawaii quasar and T dwarf survey. I. Method and discovery of faint field ultracool dwarfs
The Hawaii Quasar and T dwarf survey (HQT Survey) is a wide-field, red optical survey carried out with the
Suprime-Cam mosaic CCD camera on the 8.2 m Subaru telescope. The HQT survey is designed to search for
low-luminosity (M_(AB1450) 5.7) as well as T dwarfs, both of which are selected
by their very red I − z' colors. We use an optical narrowband filter NB816 to break a well-known I − z' color degeneracy between high-z quasars and foreground M and L dwarfs, which are more numerous than quasars.
This paper is the first in a series of papers from the HQT survey and we report on the discovery of six faint
(19 ≤ J ≤ 20) ultracool dwarfs found over a ~9.3 deg^2 area with a limiting magnitude of z'_(AB) ≤ 23.3. These
dwarfs were confirmed by near-IR imaging and/or spectroscopy conducted at various facilities on Mauna Kea.
With estimated distances of 60–170 pc, these are among the most distant spectroscopically confirmed field brown
dwarfs to date. Limits on the proper motions of these ultracool dwarfs suggest that they are old members of the
Galactic disk, though future follow-up observations are necessary to minimize errors. Our finding rate of ultracool
dwarfs is within model predictions of Liu et al. However, the large brightening amplitude (~1 mag) previously
reported for the L/T transition objects appears to overpredict the numbers. We also examine how the survey field latitude affects the survey sensitivity to the vertical scale height of ultracool dwarfs
Fragmentation Kinematics in Comet 332P/Ikeya-Murakami
We present initial time-resolved observations of the split comet
332P/Ikeya-Murakami taken using the Hubble Space Telescope. Our images reveal a
dust-bathed cluster of fragments receding from their parent nucleus at
projected speeds in the range 0.06 to 3.5 m s from which we estimate
ejection times from October to December 2015. The number of fragments with
effective radii 20 m follows a differential power law with index
= -3.60.6, while smaller fragments are less abundant than
expected from an extrapolation of this power-law. We argue that, in addition to
losses due to observational selection, torques from anisotropic outgassing are
capable of destroying the small fragments by driving them quickly to rotational
instability. Specifically, the spin-up times of fragments 20 m in
radius are shorter than the time elapsed since ejection from the parent
nucleus. The effective radius of the parent nucleus is 275 m
(geometric albedo 0.04 assumed). This is about seven times smaller than
previous estimates and results in a nucleus mass at least 300 times smaller
than previously thought. The mass in solid pieces, kg, is about
4% of the mass of the parent nucleus. As a result of its small size, the parent
nucleus also has a short spin-up time. Brightness variations in time-resolved
nucleus photometry are consistent with rotational instability playing a role in
the release of fragments.Comment: 19 pages, 1 table, 4 figures, To be published on ApJ
Detection of Earth-impacting asteroids with the next generation all-sky surveys
We have performed a simulation of a next generation sky survey's (Pan-STARRS
1) efficiency for detecting Earth-impacting asteroids. The steady-state
sky-plane distribution of the impactors long before impact is concentrated
towards small solar elongations (Chesley and Spahr, 2004) but we find that
there is interesting and potentially exploitable behavior in the sky-plane
distribution in the months leading up to impact. The next generation surveys
will find most of the dangerous impactors (>140m diameter) during their
decade-long survey missions though there is the potential to miss difficult
objects with long synodic periods appearing in the direction of the Sun, as
well as objects with long orbital periods that spend much of their time far
from the Sun and Earth. A space-based platform that can observe close to the
Sun may be needed to identify many of the potential impactors that spend much
of their time interior to the Earth's orbit. The next generation surveys have a
good chance of imaging a bolide like 2008TC3 before it enters the atmosphere
but the difficulty will lie in obtaining enough images in advance of impact to
allow an accurate pre-impact orbit to be computed.Comment: 47 pages, 16 figures, 2 table
The Ursa Major Cluster of Galaxies. I. Cluster Definition and Photometric Data
The Ursa Major Cluster has received remarkably little attention, although it
is as near as the Virgo Cluster and contains a comparable number of HI-rich
galaxies. In this paper, criteria for group membership are discussed and data
are presented for 79 galaxies identified with the group. Of these, all 79 have
been imaged at B,R,I bands with CCDs, 70 have been imaged at K' with a HgCdTe
array detector, and 70 have been detected in the HI 21cm line. A complete
sample of 62 galaxies brighter than M(B)=-16.5 is identified. Images and
gradients in surface brightness and color are presented at a common linear
scale. As has been seen previously, the galaxies with the reddest global colors
are reddest at the centers and get bluer at large radii. However, curiously,
among the galaxies with the bluest global colors there are systems with very
blue cores that get redder at large radii.Comment: A LATEX file without figures. The postscript version (7.1Mb in
gzipped format) including all the tables, figures and scanned versions of the
plates can be retrieved as preprint no.208 from
http://www.astro.rug.nl:80/~secr/ Accepted for publication in The
Astronomical Journa
Discovery of a Methane Dwarf from the IfA-Deep Survey
We present the discovery of a distant methane dwarf, the first from the
Institute for Astronomy (IfA) Deep Survey. The object ("IfA 0230-Z1") was
identified from deep optical I and z'-band imaging, being conducted as an
IfA-wide collaboration using the prime-focus imager Suprime-Cam on the Subaru
8.2-m Telescope. IfA 0230-Z1 is extremely red in the Iz'J (0.8--1.2 micron)
bands but relatively blue in J-H; such colors are uniquely characteristic of T
dwarfs. A near-IR spectrum taken with the Keck Telescope shows strong H2O
absorption and a continuum break indicative of CH4, confirming the object has a
very cool atmosphere. Comparison with nearby T dwarfs gives a spectral type of
T3-T4 and a distance of ~45 pc. Simple estimates based on previous T dwarf
discoveries suggest that the IfA survey will find a comparable number of T
dwarfs as the 2MASS survey, albeit at a much larger average distance. We also
discuss the survey's ability to probe the galactic scale height of ultracool (L
and T) dwarfs.Comment: Astrophysical Journal Letters, in pres
- …