1,633 research outputs found
Three New Cool Brown Dwarfs Discovered with the Wide-field Infrared Survey Explorer (WISE) and an Improved Spectrum of the Y0 Dwarf WISE J041022.71+150248.4
As part of a larger search of Wide-field Infrared Survey Explorer (WISE) data
for cool brown dwarfs with effective temperatures less than 1000 K, we present
the discovery of three new cool brown dwarfs with spectral types later than T7.
Using low-resolution, near-infrared spectra obtained with the NASA Infrared
Telescope Facility and the Hubble Space Telescope we derive spectral types of
T9.5 for WISE J094305.98+360723.5, T8 for WISE J200050.19+362950.1, and Y0: for
WISE J220905.73+271143.9. The identification of WISE J220905.73+271143.9 as a Y
dwarf brings the total number of spectroscopically confirmed Y dwarfs to
seventeen. In addition, we present an improved spectrum (i.e. higher
signal-to-noise ratio) of the Y0 dwarf WISE J041022.71+150248.4 that confirms
the Cushing et al. classification of Y0. Spectrophotometric distance estimates
place all three new brown dwarfs at distances less than 12 pc, with WISE
J200050.19+362950.1 lying at a distance of only 3.9-8.0 pc. Finally, we note
that brown dwarfs like WISE J200050.19+362950.1 that lie in or near the
Galactic plane offer an exciting opportunity to measure their mass via
astrometric microlensing.Comment: Accepted for publication in the Astronomical Journa
Discovery of Four High Proper Motion L Dwarfs, Including a 10 pc L Dwarf at the L/T Transition
We discover four high proper motion L dwarfs by comparing the Wide-field
Infrared Survey Explorer (WISE) to the Two Micron All Sky Survey (2MASS). WISE
J140533.32+835030.5 is an L dwarf at the L/T transition with a proper motion of
0.85+/-0.02" yr^-1, previously overlooked due to its proximity to a bright star
(V=12 mag). From optical spectroscopy we find a spectral type of L8, and from
moderate-resolution J band spectroscopy we find a near-infrared spectral type
of L9. We find WISE J140533.32+835030.5 to have a distance of 9.7+/-1.7 pc,
bringing the number of L dwarfs at the L/T transition within 10 pc from six to
seven. WISE J040137.21+284951.7, WISE J040418.01+412735.6, and WISE
J062442.37+662625.6 are all early L dwarfs within 25 pc, and were classified
using optical and low-resolution near-infrared spectra. WISE
J040418.01+412735.6 is an L2 pec (red) dwarf, a member of the class of
unusually red L dwarfs. We use follow-up optical and low-resolution
near-infrared spectroscopy to classify a previously discovered (Castro & Gizis
2012) fifth object WISEP J060738.65+242953.4 as an (L8 Opt/L9 NIR), confirming
it as an L dwarf at the L/T transition within 10 pc. WISEP J060738.65+242953.4
shows tentative CH_4 in the H band, possibly the result of unresolved binarity
with an early T dwarf, a scenario not supported by binary spectral template
fitting. If WISEP J060738.65+242953.4 is a single object, it represents the
earliest onset of CH_4 in the H band of an L/T transition dwarf in the SpeX
Library. As very late L dwarfs within 10 pc, WISE J140533.32+835030.5 and WISEP
J060738.65+242953.4 will play a vital role in resolving outstanding issues at
the L/T transition.Comment: 45 pages, 12 figures, accepted for publication in Ap
Hubble Space Telescope Spectroscopy of Brown Dwarfs Discovered with the Wide-field Infrared Survey Explorer
We present a sample of brown dwarfs identified with the {\it Wide-field
Infrared Survey Explorer} (WISE) for which we have obtained {\it Hubble Space
Telescope} ({\it HST}) Wide Field Camera 3 (WFC3) near-infrared grism
spectroscopy. The sample (twenty-two in total) was observed with the G141 grism
covering 1.101.70 m, while fifteen were also observed with the G102
grism, which covers 0.901.10 m. The additional wavelength coverage
provided by the G102 grism allows us to 1) search for spectroscopic features
predicted to emerge at low effective temperatures (e.g.\ ammonia bands) and 2)
construct a smooth spectral sequence across the T/Y boundary. We find no
evidence of absorption due to ammonia in the G102 spectra. Six of these brown
dwarfs are new discoveries, three of which are found to have spectral types of
T8 or T9. The remaining three, WISE J082507.35280548.5 (Y0.5), WISE
J120604.38840110.6 (Y0), and WISE J235402.77024015.0 (Y1) are the
nineteenth, twentieth, and twenty-first spectroscopically confirmed Y dwarfs to
date. We also present {\it HST} grism spectroscopy and reevaluate the spectral
types of five brown dwarfs for which spectral types have been determined
previously using other instruments.Comment: Accepted for publication in the Astrophysical Journal. 20 pages, 18
figures, 7 table
Detection of Coulomb Charging around an Antidot in the Quantum Hall Regime
We have detected oscillations of the charge around a potential hill (antidot)
in a two-dimensional electron gas as a function of a large magnetic field B.
The field confines electrons around the antidot in closed orbits, the areas of
which are quantised through the Aharonov-Bohm effect. Increasing B reduces each
state's area, pushing electrons closer to the centre, until enough charge
builds up for an electron to tunnel out. This is a new form of the Coulomb
blockade seen in electrostatically confined dots. Addition and excitation
spectra in DC bias confirm the Coulomb blockade of tunnelling.Comment: 4 pages, 4 Postscript figure
Integrable, oblique travelling waves in quasi-charge-neutral two-fluid plasmas
International audienceA Hamiltonian description of oblique travelling waves in a two-fluid, charge-neutral, electron-proton plasma reveals that the transverse momentum equations for the electron and proton fluids are exactly integrable in cases where the total transverse momentum flux integrals, Py(d) and Pz(d), are both zero in the de Hoffman Teller (dHT) frame. In this frame, the transverse electric fields are zero, which simplifies the transverse momentum equations for the two fluids. The integrable travelling waves for the case Py(d)=Pz(d)=0, are investigated based on the Hamiltonian trajectories in phase space, and also on the longitudinal structure equation for the common longitudinal fluid velocity component ux of the electron and proton fluids. Numerical examples of a variety of travelling waves in a cold plasma, including oscillitons, are used to illustrate the physics. The transverse, electron and proton velocity components ujy and ujz (j=e, p) of the waves exhibit complex, rosette type patterns over several periods for ux. The role of separatrices in the phase space, the rotational integral and the longitudinal structure equation on the different wave forms are discussed
Strong Nebular Line Ratios in the Spectra of z~2-3 Star-forming Galaxies: First Results from KBSS-MOSFIRE
We present initial results of a deep near-IR spectroscopic survey covering
the 15 fields of the Keck Baryonic Structure Survey (KBSS) using MOSFIRE on the
Keck 1 telescope, focusing on a sample of 251 galaxies with redshifts 2.0< z <
2.6, star-formation rates 2 < SFR < 200 M_sun/yr, and stellar masses 8.6 <
log(M*/M_sun) < 11.4, with high-quality spectra in both H- and K-band
atmospheric windows. We show unambiguously that the locus of z~2.3 galaxies in
the "BPT" nebular diagnostic diagram exhibits a disjoint, yet similarly tight,
relationship between the ratios [NII]6585/Halpha and [OIII]/Hbeta as compared
to local galaxies. Using photoionization models, we argue that the offset of
the z~2.3 locus relative to z~ 0 is explained by a combination of harder
ionizing radiation field, higher ionization parameter, and higher N/O at a
given O/H than applies to most local galaxies, and that the position of a
galaxy along the z~2.3 star-forming BPT locus is surprisingly insensitive to
gas-phase oxygen abundance. The observed nebular emission line ratios are most
easily reproduced by models in which the net ionizing radiation field resembles
a blackbody with effective temperature T_eff = 50000-60000 K and N/O close to
the solar value at all O/H. We critically assess the applicability of
commonly-used strong line indices for estimating gas-phase metallicities, and
consider the implications of the small intrinsic scatter in the empirical
relationship between excitation-sensitive line indices and stellar mass (i.e.,
the "mass-metallicity" relation), at z~2.3.Comment: 41 pages, 25 figures, accepted for publication in the Astrophysical
Journal. Version with full-resolution figures available at
http://www.astro.caltech.edu/~ccs/mos_bpt_submit.pd
Magnetic moment non-conservation in magnetohydrodynamic turbulence models
The fundamental assumptions of the adiabatic theory do not apply in presence
of sharp field gradients as well as in presence of well developed
magnetohydrodynamic turbulence. For this reason in such conditions the magnetic
moment is no longer expected to be constant. This can influence particle
acceleration and have considerable implications in many astrophysical problems.
Starting with the resonant interaction between ions and a single parallel
propagating electromagnetic wave, we derive expressions for the magnetic moment
trapping width (defined as the half peak-to-peak difference in the
particle magnetic moment) and the bounce frequency . We perform
test-particle simulations to investigate magnetic moment behavior when
resonances overlapping occurs and during the interaction of a ring-beam
particle distribution with a broad-band slab spectrum.
We find that magnetic moment dynamics is strictly related to pitch angle
for a low level of magnetic fluctuation, , where is the constant and uniform background magnetic field.
Stochasticity arises for intermediate fluctuation values and its effect on
pitch angle is the isotropization of the distribution function .
This is a transient regime during which magnetic moment distribution
exhibits a characteristic one-sided long tail and starts to be influenced by
the onset of spatial parallel diffusion, i.e., the variance
grows linearly in time as in normal diffusion. With strong fluctuations
isotropizes completely, spatial diffusion sets in and
behavior is closely related to the sampling of the varying magnetic field
associated with that spatial diffusion.Comment: 13 pages, 10 figures, submitted to PR
The Exemplar T8 Subdwarf Companion of Wolf 1130
We have discovered a wide separation (188.5") T8 subdwarf companion to the
sdM1.5+WD binary Wolf 1130. Companionship of WISE J200520.38+542433.9 is
verified through common proper motion over a ~3 year baseline. Wolf 1130 is
located 15.83 +/- 0.96 parsecs from the Sun, placing the brown dwarf at a
projected separation of ~3000 AU. Near-infrared colors and medium resolution
(R~2000-4000) spectroscopy establish the uniqueness of this system as a
high-gravity, low-metallicity benchmark. Although there are a number of
low-metallicity T dwarfs in the literature, WISE J200520.38+542433.9 has the
most extreme inferred metallicity to date with [Fe/H] = -0.64 +/- 0.17 based on
Wolf 1130. Model comparisons to this exemplar late-type subdwarf support it
having an old age, a low metallicity, and a small radius. However, the
spectroscopic peculiarities of WISE J200520.38+542433.9 underscore the
importance of developing the low-metallicity parameter space of the most
current atmospheric models.Comment: Accepted to ApJ on 05 September 2013; 33 pages in preprint format, 8
figures, 3 table
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