580 research outputs found
The Stellar Membership of the Taurus Star-forming Region
The high-precision astrometry from the second data release of the Gaia
mission has made it possible to greatly improve the census of members of nearby
clusters and associations. I have applied the Gaia data to the Taurus
star-forming region, refining the sample of known members and identifying
candidates for undiscovered members. The resulting samples of members and
candidates provide the best constraints to date on the distribution of ages and
the initial mass function (IMF) in Taurus. Several studies over the last 30
years have proposed the existence of a population of older stars (>=10 Myr)
that is associated with the Taurus clouds. The data from Gaia demonstrate that
such a population does not exist. Meanwhile, previous IMF estimates for small
fields surrounding the Taurus aggregates have exhibited a surplus of K7-M0
stars (0.7-0.8 Msun) relative to star-forming clusters like IC 348 and the
Orion Nebula Cluster. However, that difference disappears when the new census
of the entire region is considered, which should be complete for spectral types
earlier than M6-M7 at A_J<1. Thus, there is little variation in the stellar IMF
across the 3-4 orders of magnitude in stellar density that are present in
nearby star-forming regions. Finally, I note that the proper motions of two
previously known members, KPNO 15 and 2MASS J04355209+2255039, indicate that
they may have been ejected from the same location within the L1536 cloud ~7200
years ago.Comment: Astronomical Journal, in pres
Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun
Through a previous analysis of multi-epoch astrometry from the Wide-field
Infrared Survey Explorer (WISE), I identified WISE J085510.83-071442.5 as a new
high proper motion object. By combining astrometry from WISE and the Spitzer
Space Telescope, I have measured a proper motion of 8.1+/-0.1"/yr and a
parallax of 0.454+/-0.045" (2.20+0.24/-0.20 pc) for WISE J085510.83-071442.5,
giving it the third highest proper motion and the fourth largest parallax of
any known star or brown dwarf. It is also the coldest known brown dwarf based
on its absolute magnitude at 4.5um and its color in [3.6]-[4.5]. By comparing
M4.5 with the values predicted by theoretical evolutionary models, I estimate
an effective temperature of 225-260 K and a mass of 3-10 Mjup for the age range
of 1-10 Gyr that encompasses most nearby stars
Spectroscopy of Ultra-Cool Dwarfs
Ultra-cool dwarfs have been defined as dwarfs with spectral types of M7 or
later (Kirkpatrick, Henry, & Irwin 1997) and thus include the new L and T
spectral classes. The previous IAU report on ultra-cool dwarfs in 1999
described the optical and IR spectral features that characterize these spectral
types. In this report, I review the latest progress on the spectral
classification of ultra-cool dwarfs, which are divided into two categories: old
dwarfs in the field (>1 Gyr) and their young progenitors in the nearest
star-forming regions and open clusters (1-100 Myr). Because the former are
nearby and the latter are young, these two groups are relatively bright and
lend themselves to discovery and detailed study. Technically, the luminosity
class of young ultra-cool objects is closer to subgiant than dwarf;
nevertheless, I include them in this report on ultra-cool dwarfs.Comment: 5 pages, to appear in Reports on Astronomy, Vol. XXVA, Triennial
Report (2000-2002) of IAU Commission 45 (Stellar Classification), H. Rickman
(ed.
Discovery of a Binary Brown Dwarf at 2 Parsecs from the Sun
I am using multi-epoch astrometry from the Wide-field Infrared Survey
Explorer (WISE) to search for new members of the solar neighborhood via their
high proper motions. Through this work, I have identified WISE
J104915.57-531906.1 as a high proper motion object and have found additional
detections in images from the Digitized Sky Survey, the Two Micron All-Sky
Survey, and the Deep Near-Infrared Survey of the Southern Sky. I have measured
a parallax of 0.496+/-0.037" (2.0+/-0.15 pc) from the astrometry in these
surveys, making WISE J104915.57-531906.1 the third closest system to the Sun.
During spectroscopic observations with GMOS at Gemini Observatory, an i-band
acquisition image resolved it as a 1.5" (3 AU) binary. A spectrum was collected
for the primary, which I classify as L8+/-1. The secondary is probably near the
L/T transition as well given that it is only modestly fainter than the primary
(delta i=0.45 mag).Comment: Astrophysical Journal Letters, in pres
A New Parallax Measurement for the Coldest Known Brown Dwarf
WISE J085510.83-071442.5 was recently discovered as the coldest known brown
dwarf based on four epochs of images from the Wide-field Infrared Survey
Explorer and the Spitzer Space Telescope. We have improved the accuracy of its
parallax measurement by obtaining two additional epochs of Spitzer astrometry.
We derive a parallactic distance of 2.31+/-0.08 pc, which continues to support
its rank as the fourth closest known system to the Sun when compared to WISE
J104915.57-531906.1 AB (2.02+/-0.02 pc) and Wolf 359 (2.386+/-0.012 pc). The
new constraint on the absolute magnitude at 4.5um indicates an effective
temperature of 235-260 K based on four sets of theoretical models. We also show
the updated positions of WISE J085510.83-071442.5 in two color-magnitude
diagrams. Whereas Faherty and coworkers cited its location in MW2 versus J-W2
as evidence of water clouds, we find that those data can be explained instead
by cloudless models that employ non-equilibrium chemistry.Comment: Astrophysical Journal, in pres
The Spectral Energy Distribution of the Coldest Known Brown Dwarf
WISE J085510.83-071442.5 (hereafter WISE 0855-0714) is the coldest known
brown dwarf (~250 K) and the fourth closest known system to the Sun (2.2 pc).
It has been previously detected only in the J band and two mid-IR bands. To
better measure its spectral energy distribution (SED), we have performed deep
imaging of WISE 0855-0714 in six optical and near-IR bands with Gemini
Observatory, the Very Large Telescope, and the Hubble Space Telescope. Five of
the bands show detections, although one detection is marginal (S/N~3). We also
have obtained two epochs of images with the Spitzer Space Telescope for use in
refining the parallax of the brown dwarf. By combining astrometry from this
work and previous studies, we have derived a parallax of 0.449+/-0.008"
(2.23+/-0.04 pc). We have compared our photometry for WISE 0855-0714 to data
for known Y dwarfs and to the predictions of three suites of models by Saumon
et al. (2012) and Morley et al. (2012, 2014) that are defined by the presence
or absence of clouds and non-equilibrium chemistry. Our estimates of Y-J and
J-H for WISE 0855-0714 are redder than colors of other Y dwarfs, confirming a
predicted reversal of near-IR colors to redder values at temperatures below
300-400 K. In color-magnitude diagrams, no single suite of models provides a
clearly superior match to the sequence formed by WISE 0855-0714 and other Y
dwarfs. Instead, the best fitting model changes from one diagram to the next.
Similarly, all of the models have substantial differences from the SED of WISE
0855-0714. As a result, we are currently unable to constrain the presence of
clouds or non-equilibrium chemistry in its atmosphere.Comment: accepted for publication in the Astronomical Journa
Characterization of High Proper Motion Objects from the Wide-field Infrared Survey Explorer
We present an analysis of high proper motion objects that we have found in a
recent study and in this work with multi-epoch astrometry from the Wide-field
Infrared Survey Explorer (WISE). Using photometry and proper motions from 2MASS
and WISE, we have identified the members of this sample that are likely to be
late type, nearby, or metal poor. We have performed optical and near-infrared
spectroscopy on 41 objects, from which we measure spectral types that range
from M4-T2.5. This sample includes 11 blue L dwarfs and five subdwarfs; the
latter were also classified as such in the recent study by Kirkpatrick and
coworkers. Based on their spectral types and photometry, several of our
spectroscopic targets may have distances of <20 pc with the closest at ~12 pc.
The tangential velocities implied by the spectrophotometric distances and
proper motions indicate that four of the five subdwarfs are probably members of
the Galactic halo while several other objects, including the early-T dwarf WISE
J210529.08-623558.7, may belong to the thick disk.Comment: Astrophysical Journal, in pres
New Low-Mass Stars and Brown Dwarfs with Disks in the Chamaeleon I Star-Forming Region
We have used images obtained with the Infrared Array Camera and the Multiband
Imaging Photometer onboard the Spitzer Space Telescope to search for low-mass
stars and brown dwarfs with circumstellar disks in the Chamaeleon I
star-forming region. Through optical spectroscopy of sources with red colors in
these data, we have identified seven new disk-bearing members of the cluster.
Three of these objects are probably brown dwarfs according to their spectral
types (M8, M8.5, M8-L0). Three of the other new members may have edge-on disks
based on the shapes of their infrared spectral energy distributions. One of the
possible edge-on systems has a steeply rising slope from 4.5 to 24um,
indicating that it could be a class I source (star+disk+envelope) rather than a
class II source (star+disk). If so, then it would be one of the least massive
known class I protostars (M5.75, M~0.1 Msun).Comment: Astrophysical Journal, in pres
The Disk Population of the Upper Scorpius Association
We present photometry at 3-24um for all known members of the Upper Scorpius
association (~11 Myr) based on all images of these objects obtained with the
Spitzer Space Telescope and the Wide-field Infrared Survey Explorer. We have
used these data to identify the members that exhibit excess emission from
circumstellar disks and estimate the evolutionary stages of these disks.
Through this analysis, we have found ~50 new candidates for transitional,
evolved, and debris disks. The fraction of members harboring inner primordial
disks is 1.2 Msun) and increases with later types to a
value of ~25% at >=M5 (M<=0.2 Msun), in agreement with the results of previous
disk surveys of smaller samples of Upper Sco members. These data indicate that
the lifetimes of disks are longer at lower stellar masses, and that a
significant fraction of disks of low-mass stars survive for at least ~10 Myr.
Finally, we demonstrate that the distribution of excess sizes in Upper Sco and
the much younger Taurus star-forming region (~1 Myr) are consistent with the
same, brief timescale for clearing of inner disks.Comment: machine readable version of Table 1 available at
http://iopscience.iop.org/0004-637X/758/1/3
A Search for Substellar Companions to the Two Nearest Brown Dwarf Systems
WISE J104915.57-531906.1 A+B and WISE J085510.83-071442.5 were recently
discovered as the third and fourth closest known systems to the Sun,
respectively (2.0 and 2.3 pc). The former consists of a L8+T0.5 binary and the
latter is a probable Y dwarf and is the coldest known brown dwarf (~250 K). We
present a search for common proper motion companions to these brown dwarfs
using multi-epoch mid-infrared images from the Spitzer Space Telescope. We have
also obtained near-infrared adaptive optics images of WISE J104915.57-531906.1
A+B with the Very Large Telescope to search for companions at smaller
separations than reached by Spitzer. No new companions are detected in either
system. At projected separations of 25-420" (50-840 AU) for WISE
J104915.57-531906.1 A+B and 4-420" (9-970 AU) for WISE J085510.83-071442.5, the
Spitzer images are sensitive to companions with M_4.5<21.6 and 21.9,
respectively, which correspond to masses of >=1 M_Jup for ages of >=1 Gyr and
temperatures of >=150 K. The detection limit in the adaptive optics images of
WISE J104915.57-531906.1 A+B is dH~10 at 3-15" (6-30 AU), or >=7 M_Jup for >=1
Gyr.Comment: Astronomical Journal, in pres
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