327 research outputs found
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
Discovery of a Young L Dwarf Binary, SDSS J224953.47+004404.6AB
We report discovery of a young 0.32" L dwarf binary, SDSS J2249+0044AB, found
as the result of a Keck LGSAO imaging survey of young field brown dwarfs. Weak
K, Na, and FeH features as well as strong VO absorption in the integrated-light
J-band spectrum indicate a young age for the system. From spatially resolved
K-band spectra we determine spectral types of L3 and L5 for components A and B,
respectively. SDSS J2249+0044A is spectrally very similar to G196-3B, an L3
companion to a young M2.5 field dwarf. Thus, we adopt 100 Myr (the age estimate
of the G196-3 system) as the age of SDSS J2249+0044AB, but ages of 12-790 Myr
are possible. By comparison to G196-3B, we estimate a distance to SDSS
J2249+0044AB of 54 +- 16 pc and infer a projected separation of 17 +- 5 AU for
the binary. Comparison of the luminosities to evolutionary models at an age of
100 Myr yields masses of 0.029 and 0.022 Msun for SDSS J2249+0044A and B,
respectively. Over the possible ages of the system (12-790 Myr), the mass of
SDSS J2249+0044A could range from 0.011 to 0.070 Msun and the mass of SDSS
J2249+0044B could range from 0.009 to 0.065 Msun. Evolutionary models predict
that either component could be burning deuterium, which could result in a mass
ratio as low as 0.4, or alternatively, a reversal in the luminosities of the
binary. We find a likely proper motion companion, GSC 00568-01752, which lies
48.9" away (2600 AU) and has SDSS+2MASS colors consistent with an early M
dwarf. The photometric distance to GSC 00568-01752 is 53 +- 15 pc, in agreement
with our distance estimate for SDSS J2249+0044AB. The space motion of SDSS
J2249+0044AB shows no obvious coincidence with known young moving groups. The
unusually red near-IR colors, young age, and low masses of the binary make it
an important template for studying planetary-mass objects found by direct
imaging surveys.Comment: revised, accepted versio
Characterizing Young Brown Dwarfs using Low Resolution Near-IR Spectra
We present near-infrared (1.0-2.4 micron) spectra confirming the youth and
cool effective temperatures of 6 brown dwarfs and low mass stars with
circumstellar disks toward the Chamaeleon II and Ophiuchus star forming
regions. The spectrum of one of our objects indicates that it has a spectral
type of ~L1, making it one of the latest spectral type young brown dwarfs
identified to date. Comparing spectra of young brown dwarfs, field dwarfs, and
giant stars, we define a 1.49-1.56 micron H2O index capable of determining
spectral type to within 1 sub-type, independent of gravity. We have also
defined an index based on the 1.14 micron sodium feature that is sensitive to
gravity, but only weakly dependent on spectral type for field dwarfs. Our 1.14
micron Na index can be used to distinguish young cluster members (t <~ 5 Myr)
from young field dwarfs, both of which may have the triangular H-band continuum
shape which persists for at least tens of Myr. Using effective temperatures
determined from the spectral types of our objects along with luminosities
derived from near and mid-infrared photometry, we place our objects on the H-R
diagram and overlay evolutionary models to estimate the masses and ages of our
young sources. Three of our sources have inferred ages (t ~= 10-30 Myr)
significantly older than the median stellar age of their parent clouds (1-3
Myr). For these three objects, we derive masses ~3 times greater than expected
for 1-3 Myr old brown dwarfs with the bolometric luminosities of our sources.
The large discrepancies in the inferred masses and ages determined using two
separate, yet reasonable methods, emphasize the need for caution when deriving
or exploiting brown dwarf mass and age estimates.Comment: 11 pages, Accepted to Ap
Political institutions and debt crises
This paper shows that political institutions matter in explaining defaults on external and domestic debt obligations. We explore a large number of political and macroeconomic variables using a non-parametric technique to predict safety from default. The advantage of this technique is that it is able to identify patterns in the data that are not captured in standard probit analysis. We find that political factors matter, and do so in different ways for democratic and non-democratic regimes, and for domestic and external debt. In democracies, a parliamentary system or sufficient checks and balances almost guarantee the absence of default on external debt when economic fundamentals or liquidity are sufficiently strong. In dictatorships, high stability and tenure play a similar role for default on domestic debt
HN Peg B: A Test of Models of the L to T Dwarf Transition
Luhman and collaborators recently discovered an early-T dwarf companion to
the G0 dwarf star HN Peg, using Spitzer Infrared Array Camera (IRAC) images.
Companionship was established on the basis of the common proper motion inferred
from 1998 Two Micron All Sky Survey images and the 2004 IRAC images. In this
paper we present new near-infrared imaging data which confirms the common
proper motion of the system. We also present new 3 - 4 um spectroscopy of HN
Peg B, which provides tighter constraints on both the bolometric luminosity
determination and the comparison to synthetic spectra. New adaptive optics
imaging data are also presented, which shows the T dwarf to be unresolved,
providing limits on the multiplicity of the object. We use the age, distance
and luminosity of the solar-metallicity T dwarf to determine its effective
temperature and gravity, and compare synthetic spectra with these values, and a
range of grain properties and vertical mixing, to the observed 0.8 - 4.0 um
spectra and mid-infrared photometry. We find that models with temperature and
gravity appropriate for the older end of the age range of the system (0.5 Gyr)
can do a reasonable job of fitting the data, but only if the photospheric
condensate cloud deck is thin, and if there is significant vertical mixing in
the atmosphere. Dwarfs such as HN Peg B, with well-determined metallicity,
radius, gravity and temperature will allow development of dynamical atmosphere
models, leading to the solution of the puzzle of the L to T dwarf transition.Comment: 22 pages including 2 Tables and 5 Figures, accepted for publication
by the Astrophysical Journal, April 8 200
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