2,717 research outputs found
The Schizophrenic Spectrum of LSR 1610-0040: a Peculiar M Dwarf/Subdwarf
We present a moderate resolution (R=2000), 0.8-4.1 micron spectrum of LSR
1610-0040, a high proper motion star classified as an early-type L subdwarf by
Lepine and collaborators based on its red-optical spectrum. The near-infrared
spectrum of LSR 1610-0040 does not fit into the (tentative) M/L subdwarf
sequence but rather exhibits a mix of characteristics found in the spectra of
both M dwarfs and M subdwarfs. In particular, the near-infrared spectrum
exhibits a Na I doublet and CO overtone bandheads in the K band, and Al I and K
I lines and an FeH bandhead in the H band, all of which have strengths more
typical of field M dwarfs. Furthermore the spectrum of Gl 406 (M6 V) provides a
reasonably good match to the 0.6-4.1 micron spectral energy distribution of LSR
1610. Nevertheless the near-infrared spectrum of LSR 1610 also exhibits
features common to the spectra of M subdwarfs including a strong Ti I multiplet
centered at ~0.97 microns, a weak VO band at ~1.06 microns, and possible
collision-induced H_2 absorption in the H and K bands. We discuss a number of
possible explanations for the appearance of the red-optical and near-infrared
spectrum of LSR 1610-0040. Although we are unable to definitively classify LSR
1610-0040, the preponderance of evidence suggests that it is a mildly
metal-poor M dwarf. Finally, we tentatively identify a new band of TiO at ~0.93
microns in the spectra of M dwarfs.Comment: Accepted for publication in the Astronomical Journa
A Unified Theory for the Atmospheres of the Hot and Very Hot Jupiters: Two Classes of Irradiated Atmospheres
We highlight the importance of gaseous TiO and VO opacity on the highly
irradiated close-in giant planets. The atmospheres of these planets naturally
fall into two classes that are somewhat analogous to the M- and L-type dwarfs.
Those that are warm enough to have appreciable opacity due to TiO and VO gases
we term the ``pM Class'' planets, and those that are cooler we term ``pL
Class'' planets. We calculate model atmospheres for these planets, including
pressure-temperature profiles, spectra, and characteristic radiative time
constants. We show that pM Class planets have hot stratospheres 2000 K
and appear ``anomalously'' bright in the mid infrared secondary eclipse, as was
recently found for planets HD 149026b and HD 209458b. This class of planets
absorbs incident flux and emits thermal flux from high in their atmospheres.
Consequently, they will have large day/night temperature contrasts and
negligible phase shifts between orbital phase and thermal emission light
curves, because radiative timescales are much shorter than possible dynamical
timescales. The pL Class planets absorb incident flux deeper in the atmosphere
where atmospheric dynamics will more readily redistribute absorbed energy. This
will lead to cooler day sides, warmer night sides, and larger phase shifts in
thermal emission light curves. Around a Sun-like primary this boundary occurs
at 0.04-0.05 AU. The eccentric transiting planets HD 147506b and HD
17156b alternate between the classes. Thermal emission in the optical from pM
Class planets is significant red-ward of 400 nm, making these planets
attractive targets for optical detection. The difference in the observed
day/night contrast between ups Andromeda b (pM Class) and HD 189733b (pL Class)
is naturally explained in this scenario. (Abridged.)Comment: Accepted to the Astrophysical Journa
Magnetic and Metal-Insulator Transitions in beta-Na0.5CoO2 and gamma-K0.5CoO2 -NMR and Neutron Diffraction Studies-
Co-oxides beta-Na0.5CoO2 and gamma-K0.5CoO2 have been prepared by the Na
de-intercalation from alpha-NaCoO2 and by the floating-zone method,
respectively. It has been found that successive phase transitions take place at
temperatures Tc1 and Tc2 in both systems. The appearance of the internal
magnetic field at Tc1 with decreasing temperature T indicates that the
antiferromagnetic order exists at T < Tc1, as in gamma-Na0.5CoO2. For
beta-Na0.5CoO2, the transition temperatures and the NMR parameters determined
from the data taken for magnetically ordered state are similar to those of
gamma-Na0.5CoO2, indicating that the difference of the stacking ways of the
CoO2 layers between these systems do not significantly affect their physical
properties. For gamma-K0.5CoO2, the quantitative difference of the physical
quantities are found from those of beta- and gamma-Na0.5CoO2. The difference
between the values of Tci (i = 1 and 2) of these systems might be explained by
considering the distance between CoO2 layers.Comment: 8 pages, 14 figures, 1 Tabl
Non-Linearity Corrections and Statistical Uncertainties Associated with Near-Infrared Arrays
We derive general equations for non-linearity corrections and statistical
uncertainty (variance) estimates for data acquired with near-infrared detectors
employing correlated double sampling, multiple correlated double sampling
(Fowler sampling) and uniformly-spaced continuous readout techniques. We
compare our equation for the variance on each pixel associated with Fowler
sampling with measurements obtained from data taken with the array installed in
the near-infrared cross-dispersed spectrograph (SpeX) at the NASA Infrared
Telescope Facility and find that it provides an accurate representation of the
empirical results. This comparison also reveals that the read noise associated
with a single readout of the SpeX array increases with the number of
non-destructive reads, n_r, as n_r^0.16. This implies that the {effective} read
noise of a stored image decreases as n_r^-0.34, shallower than the expected
rate of n_r^-0.5. The cause of this read noise behavior is uncertain, but may
be due to heating of the array as a result of the multiple read outs. Such
behavior may be generic to arrays that employ correlated or multiple correlated
double sampling readouts.Comment: 21 pages, accepted by PAS
Ammonia as a tracer of chemical equilibrium in the T7.5 dwarf Gliese 570D
We present the first analysis of an optical to mid-infrared spectrum of the
T7.5 dwarf Gliese 570D with model atmospheres, synthetic spectra, and brown
dwarf evolution sequences. We obtain precise values for the basic parameters of
Gl 570D: Teff=800 - 820K, log g (cm/s^2)=5.09 - 5.23, and log L/Lsun= -5.525 to
-5.551. The Spitzer IRS spectrum shows prominent features of ammonia (NH3) that
can only be fitted by reducing the abundance of NH3 by about one order of
magnitude from the value obtained with chemical equilibrium models. We model
departures from chemical equilibrium in the atmosphere of Gl 570D by
considering the kinetics of nitrogen and carbon chemistry in the presence of
vertical mixing. The resulting model spectrum reproduces the data very well.Comment: Accepted for publication in the ApJ. 10 pages, including 3 figure
2MASS J06164006-6407194: The First Outer Halo L Subdwarf
We present the serendipitous discovery of an L subdwarf, 2MASS
J06164006-6407194, in a search of the Two Micron All Sky Survey for T dwarfs.
Its spectrum exhibits features indicative of both a cool and metal poor
atmosphere including a heavily pressured-broadened K I resonant doublet, Cs I
and Rb I lines, molecular bands of CaH, TiO, CrH, FeH, and H2O, and enhanced
collision induced absorption of H2. We assign 2MASS 0616-6407 a spectral type
of sdL5 based on a comparison of its red optical spectrum to that of near
solar-metallicity L dwarfs. Its high proper motion (mu =1.405+-0.008 arcsec
yr-1), large radial velocity (Vrad = 454+-15 km s-1), estimated uvw velocities
(94, -573, 125) km s-1 and Galactic orbit with an apogalacticon at ~29 kpc are
indicative of membership in the outer halo making 2MASS 0616-6407 the first
ultracool member of this population.Comment: Accepted for publication in Ap
59Co-NMR Knight Shift of Superconducting Three-Layer NaxCoO2.yH2O
The superconducting state of NaxCoO2.yH2O with three CoO2 layers in a unit
cell has been studied by 59Co-NMR. The Knight shift measured for a peak of the
NMR spectra corresponding to the external magnetic field H along one of the
principal directions within the CoO2 plane, exhibits a rapid decrease with
decreasing temperature T below the superconducting transition temperature Tc,
indicating that the spin susceptibility is suppressed in the superconducting
phase, at least, for this field direction. Because differences of the
superconducting properties are rather small between this three-layer
NaxCoO2.yH2O and previously reported NaxCoO2.yH2O with two CoO2 layers within a
unit cell, the present result of the Knight shift studies indicates that the
Cooper pairs of the former system are in the singlet state as in the latter,
for which the spin susceptibility is suppressed for both directions of H
parallel and perpendicular to the CoO2 plane.Comment: 5 page
Socially Induced Synchronization of Every-other-day Egg Laying in a Seabird Colony
Spontaneous oscillator synchrony has been documented in a wide variety of electrical, mechanical, chemical, and biological systems, including the menstrual cycles of women and estrous cycles of Norway Rats (Rattus norvegicus). In temperate regions, many colonial birds breed seasonally in a time window set by photoperiod; some studies have suggested that heightened social stimulation in denser colonies can lead to a tightened annual reproductive pulse. It has been unknown, however, whether the analog of menstrual synchrony occurs in birdsthat is, whether avian ovulation cycles can synchronize on a daily timescale within the annual breeding pulse. We report every-other-day clutch-initiation and egg-laying synchrony in a breeding colony of Glaucous-winged Gulls (Larus glaucescens) and show that the level of synchrony declined with decreasing colony density. We also pose a mathematical model based on the hypothesis that preovulatory luteinizing hormone surges synchronize through social stimulation. Model predictions are consistent with observations. Finally, we suggest a procedure for identifying synchronous egg laying in other colonies and species. © 2010 by The American Ornithologists\u27 Union. All rights reserved
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
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