1,542 research outputs found
Weather on Other Worlds. IV. H emission and photometric variability are not correlated in L0T8 dwarfs
Recent photometric studies have revealed that surface spots that produce flux
variations are present on virtually all L and T dwarfs. Their likely magnetic
or dusty nature has been a much-debated problem, the resolution to which has
been hindered by paucity of diagnostic multi-wavelength observations. To test
for a correlation between magnetic activity and photometric variability, we
searched for H emission among eight L3T2 ultra-cool dwarfs with
extensive previous photometric monitoring, some of which are known to be
variable at 3.6 m or 4.5 m. We detected H only in the
non-variable T2 dwarf 2MASS J125453930122474. The remaining seven objects do
not show H emission, even though six of them are known to vary
photometrically. Combining our results with those for 86 other L and T dwarfs
from the literature show that the detection rate of H emission is very
high (94) for spectral types between L0 and L3.5 and much smaller (20)
for spectral types L4, while the detection rate of photometric variability
is approximately constant (3055) from L0 to T8 dwarfs. We conclude
that chromospheric activity, as evidenced by H emission, and
large-amplitude photometric variability are not correlated. Consequently, dust
clouds are the dominant driver of the observed variability of ultra-cool dwarfs
at spectral types at least as early as L0.Comment: 12 pages, 4 figures, accepted for publication in Ap
Rotational modulation of the linear polarimetric variability of the cool dwarf TVLM 51346546
Aims: We aimed to monitor the optical linear polarimetric signal of the
magnetized, rapidly rotating M8.5 dwarf TVLM 51346546.
Methods: - and -band linear polarimetry images were collected with the
ALFOSC instrument of the 2.56-m Nordic Optical Telescope on two consecutive
nights covering about 0.5 and 4 rotation cycles in the and filters,
respectively. We also obtained simultaneous intensity curves by means of
differential photometry. The typical precision of the data is 0.46\%
(), 0.35\% () in the linear polarization degree and 9 mmag
(), 1.6 mmag () in the differential intensity curves.
Results: Strong and variable linear polarization is detected in the and
filters, with values of maximum polarization ( = 1.300.35 \%)
similar for both bands. The intensity and the polarimetric curves present a
sinusoid-like pattern with a periodicity of 1.98 h, which we ascribe to
structures in TVLM 51346's surface synchronized with rotation. We found that
the peaks of the intensity and polarimetric curves occur with a phase
difference of 0.180.01, and that the maximum of the linear polarization
happens nearly half a period (0.590.03) after the radio pulse. We
discussed different scenarios to account for the observed properties of the
light curves.Comment: Accepted for publication in Astronomy and Astrophysic
Simultaneous optical and near-infrared linear spectropolarimetry of the earthshine
Aims: We aim to extend our current observational understanding of the
integrated planet Earth spectropolarimetry from the optical to the
near-infrared wavelengths. Major biomarkers like O and water vapor
are strong flux absorbents in the Earth atmosphere and some linear polarization
of the reflected stellar light is expected to occur at these wavelengths.
Methods: Simultaneous optical ( m) and near-infrared (
m) linear spectropolarimetric data of the earthshine were acquired by
observing the nightside of the waxing Moon. The data have sufficient spectral
resolution (2.51 nm in the optical, and 1.83 and 2.91 nm in the near-infrared)
to resolve major molecular species present in the Earth atmosphere.
Results: We find the highest values of linear polarization () at
the bluest wavelengths, which agrees with the literature. Linear polarization
intensity steadily decreases towards red wavelengths reaching a nearly flat
value beyond 0.8 m. In the near-infrared, we measured a polarization
degree of for the continuum. We report the detection of molecular
features due to O at m and HO at 0.6530.725
m, 0.7800.825 m, 0.93 and 1.12 m in the spectropolarimetric
data; most of them show high linear polarimetry degrees above the continuum. In
particular, the broad HO 1.12 m band displays a polarimetric
intensity as high as that of the blue optical. These features may become a
powerful tool to characterize Earth-like planets in polarized light.Comment: 4 pages, 3 figures. Accepted for publication as Letter in Astronomy
and Astrophysics on 23/01/201
Resonant tunneling through protected quantum dots at phosphorene edges
We theoretically investigate phosphorene zigzag nanorribons as a platform for
constriction engineering. In the presence of a constriction at the upper edge,
quantum confinement of edge protected states reveals resonant tunnelling
Breit-Wigner transmission peaks, if the upper edge is uncoupled to the lower
edge. Coupling between edges in thin constrictions gives rise to Fano-like and
anti-resonances in the transmission spectrum of the system.Comment: 8 pages,7 figure
A Statistical Survey of Peculiar L and T Dwarfs in SDSS, 2MASS, and WISE
We present the final results from a targeted search for brown dwarfs with
unusual near-infrared colors. From a positional cross-match of SDSS, 2MASS and
WISE, we have identified 144 candidate peculiar L and T dwarfs. Spectroscopy
confirms that 20 of the objects are peculiar or are candidate binaries. Nine of
the 420 objects in our sample are young (200 Myr; 2.1%) and another 8
(1.9%) are unusually red with no signatures of youth. With a spectroscopic
color of 2.58 0.11 mag, one of the new objects, the L6 dwarf
2MASS J03530419+0418193, is among the reddest field dwarfs currently known and
is one of the reddest objects with no signatures of youth known to date. We
have also discovered another potentially very low gravity object, the L1 dwarf
2MASS J00133470+1109403, and independently identified the young L7 dwarf 2MASS
J00440332+0228112, first reported by Schneider and collaborators. Our results
confirm that signatures of low gravity are no longer discernible in low to
moderate resolution spectra of objects older than 200 Myr. The 1.9% of
unusually red L dwarfs that do not show other signatures of youth could be
slightly older, up to 400 Myr. In this case a red color may be
more diagnostic of moderate youth than individual spectral features. However,
its is also possible that these objects are relatively metal-rich, and so have
an enhanced atmospheric dust content.Comment: 27 pages, 7 figures, 5 tables, accepted to A
The Prototypical Young L/T-Transition Dwarf HD 203030B Likely Has Planetary Mass
Upon its discovery in 2006, the young L7.5 companion to the solar analog HD
203030 was found to be unusual in being 200 K cooler than older late-L
dwarfs. HD 203030B offered the first clear indication that the effective
temperature at the L-to-T spectral type transition depends on surface gravity:
now a well-known characteristic of low-gravity ultra-cool dwarfs. An initial
age analysis of the G8V primary star indicated that the system was 130--400 Myr
old, and so the companion between 12--31 . Using moderate
resolution near-infrared spectra of HD 203030B, we now find features of very
low gravity comparable to those of 10--150 Myr-old L7--L8 dwarfs. We also
obtained more accurate near infrared and {\sl Spitzer}/IRAC photometry, and
find a color of mag---comparable to those
observed in other young planetary-mass objects---and a luminosity of
log() dex. We further reassess the
evidence for the young age of the host star, HD 203030, with a more
comprehensive analysis of the photometry and updated stellar activity
measurements and age calibrations. Summarizing the age diagnostics for both
components of the binary, we adopt an age of 100 Myr for HD 203030B and an age
range of 30--150 Myr. Using cloudy evolutionary models, the new companion age
range and luminosity result in a mass of 11 with a range of 8--15
, and an effective temperature of K.Comment: 12 pages, 7 figures, accepted for publication in A
Lucky Spectroscopy, an equivalent technique to Lucky Imaging. Spatially resolved spectroscopy of massive close visual binaries using the William Herschel Telescope
CONTEXT: Many massive stars have nearby companions whose presence hamper
their characterization through spectroscopy. AIMS: We want to obtain spatially
resolved spectroscopy of close massive visual binaries to derive their spectral
types. METHODS: We obtain a large number of short long-slit spectroscopic
exposures of five close binaries under good seeing conditions, select those
with the best characteristics, extract the spectra using multiple-profile
fitting, and combine the results to derive spatially separated spectra.
RESULTS: We demonstrate the usefulness of Lucky Spectroscopy by presenting the
spatially resolved spectra of the components of each system, in two cases with
separations of only ~0.3". Those are delta Ori Aa+Ab (resolved in the optical
for the first time) and sigma Ori AaAb+B (first time ever resolved). We also
spatially resolve 15 Mon AaAb+B, zeta Ori AaAb+B (both previously resolved with
GOSSS, the Galactic O-Star Spectroscopic Survey), and eta Ori AaAb+B, a system
with two spectroscopic B+B binaries and a fifth visual component. The systems
have in common that they are composed of an inner pair of slow rotators orbited
by one or more fast rotators, a characteristic that could have consequences for
the theories of massive star formation.Comment: Accepted for publication in A&A, 7 page
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