678 research outputs found
Herschel/PACS View Of Disks Around Low-Mass Stars And Brown Dwarfs In The TW Hydrae Association
We conducted Herschel/PACS observations of five very low-mass stars or brown dwarfs located in the TW Hya association with the goal of characterizing the properties of disks in the low stellar mass regime. We detected all five targets at 70 mu m and 100 mu m and three targets at 160 mu m. Our observations, combined with previous photometry from 2MASS, WISE, and SCUBA-2, enabled us to construct spectral energy distributions (SEDs) with extended wavelength coverage. Using sophisticated radiative transfer models, we analyzed the observed SEDs of the five detected objects with a hybrid fitting strategy that combines the model grids and the simulated annealing algorithm and evaluated the constraints on the disk properties via the Bayesian inference method. The modeling suggests that disks around low-mass stars and brown dwarfs are generally flatter than their higher mass counterparts, but the range of disk mass extends to well below the value found in T Tauri stars, and the disk scale heights are comparable in both groups. The inferred disk properties (i.e., disk mass, flaring, and scale height) in the low stellar mass regime are consistent with previous findings from large samples of brown dwarfs and very low-mass stars. We discuss the dependence of disk properties on their host stellar parameters and find a significant correlation between the Herschel far-IR fluxes and the stellar effective temperatures, probably indicating that the scaling between the stellar and disk masses (i.e., M-disk proportional to M-star) observed mainly in low-mass stars may extend down to the brown dwarf regime.Natural Science Foundation of Jiangsu Province of China BK20141046Youth Qianren Program of the National Science Foundation of ChinaNational Aeronautics and Space AdministrationStrategic Priority Research Program >The Emergence of Cosmological Structures> of the Chinese Academy of Sciences XDB09000000Astronom
Two Transiting Earth-size Planets Near Resonance Orbiting a Nearby Cool Star
Discoveries from the prime Kepler mission demonstrated that small planets (<
3 Earth-radii) are common outcomes of planet formation. While Kepler detected
many such planets, all but a handful orbit faint, distant stars and are not
amenable to precise follow up measurements. Here, we report the discovery of
two small planets transiting K2-21, a bright (K = 9.4) M0 dwarf located
656 pc from Earth. We detected the transiting planets in photometry
collected during Campaign 3 of NASA's K2 mission. Analysis of transit light
curves reveals that the planets have small radii compared to their host star,
2.60 0.14% and 3.15 0.20%, respectively. We obtained follow up NIR
spectroscopy of K2-21 to constrain host star properties, which imply planet
sizes of 1.59 0.43 Earth-radii and 1.92 0.53 Earth-radii,
respectively, straddling the boundary between high-density, rocky planets and
low-density planets with thick gaseous envelopes. The planets have orbital
periods of 9.32414 days and 15.50120 days, respectively, and have a period
ratio of 1.6624, very near to the 5:3 mean motion resonance, which may be a
record of the system's formation history. Transit timing variations (TTVs) due
to gravitational interactions between the planets may be detectable using
ground-based telescopes. Finally, this system offers a convenient laboratory
for studying the bulk composition and atmospheric properties of small planets
with low equilibrium temperatures.Comment: Updated to ApJ accepted version; photometry available alongside LaTeX
source; 10 pages, 7 figure
Investigating Atomic Details of the CaF(111) Surface with a qPlus Sensor
The (111) surface of CaF has been intensively studied with
large-amplitude frequency-modulation atomic force microscopy and atomic
contrast formation is now well understood. It has been shown that the apparent
contrast patterns obtained with a polar tip strongly depend on the tip
terminating ion and three sub-lattices of anions and cations can be imaged.
Here, we study the details of atomic contrast formation on CaF(111) with
small-amplitude force microscopy utilizing the qPlus sensor that has been shown
to provide utmost resolution at high scanning stability. Step edges resulting
from cleaving crystals in-situ in the ultra-high vacuum appear as very sharp
structures and on flat terraces, the atomic corrugation is seen in high clarity
even for large area scans. The atomic structure is also not lost when scanning
across triple layer step edges. High resolution scans of small surface areas
yield contrast features of anion- and cation sub-lattices with unprecedented
resolution. These contrast patterns are related to previously reported
theoretical results.Comment: 18 pages, 9 Figures, presented at 7th Int Conf Noncontact AFM
Seattle, USA Sep 12-15 2004, accepted for publication in Nanotechnology,
http://www.iop.or
THE HAWAII INFRARED PARALLAX PROGRAM. II. YOUNG ULTRACOOL FIELD DWARFS
(Abridged) We present a large, uniform analysis of young (~10-150 Myr)
ultracool dwarfs, based on new high-precision IR parallaxes for 68 objects. We
find that low-gravity (VL-G) late-M and L dwarfs form a continuous sequence in
IR color-magnitude diagrams, separate from field objects and current
theoretical models. VL-G objects also appear distinct from young substellar
(brown dwarf and exoplanet) companions, suggesting the two populations have a
different range of physical properties. In contrast, at the L/T transition,
young, old, and peculiar objects all span a narrow range in near-IR absolute
magnitudes. At a given spectral type, the IR absolute magnitudes of young
objects can be offset from ordinary field dwarfs, with the largest offsets
occurring in the Y and J bands for late-M dwarfs (brighter than the field) and
mid/late-L dwarfs (fainter than the field). Overall, low-gravity (VL-G) objects
have the most uniform photometric behavior while intermediate-gravity (INT-G)
objects are more diverse, suggesting a third governing parameter beyond
spectral type and gravity class. We examine the moving group memberships for
all young ultracool dwarfs with parallaxes, changing/refuting the status of 23
objects and fortifying the status of another 28 objects. We use our resulting
age-calibrated sample to establish empirical young isochrones and find a
declining frequency of VL-G objects relative to INT-G objects with increasing
age. Notable objects in our sample include high-velocity INT-G objects; very
red, late-L dwarfs with high surface gravities; candidate disk-bearing members
of the MBM20 cloud and beta Pic moving group; and very young distant
interlopers. Finally, we provide a comprehensive summary of the absolute
magnitudes and spectral classifications of 102 young ultracool dwarfs, found in
the field and as substellar companions to young stars.Comment: ApJ, in press, 138 pages including 33 figures and 15 tables.
Compilation of young ultracool dwarfs and young substellar (brown dwarf and
exoplanet) companions available at the Database of Ultracool Parallaxes (see
http://www.as.utexas.edu/~tdupuy/plx
Ion Mobility Shift of Isotopologues in a High Kinetic Energy Ion Mobility Spectrometer (HiKE-IMS) at Elevated Effective Temperatures
Ion mobility spectrometers (IMS) separate ions mainly by ionâneutral collision cross section and to a lesser extent by ion mass and effective temperature. When investigating isotopologues, the difference in collision cross section can be assumed negligible. Since the mobility shift of isotopologues is thus mainly caused by their difference in mass and effective temperature, the investigation of isotopologues can provide important insights into the theory of ion mobility. However, in classical IMS operated at ambient pressure, cluster formation with neutral molecules occurs, which significantly influences the mobility shift of isotopologues and thus makes a sound investigation of the effect of ion mass and effective temperature on the ion mobility difficult. In this work, the relative ion mobility of several organic compounds and their 13C-labeled isotopologues is studied in a High Kinetic Energy Ion Mobility Spectrometer (HiKE-IMS) at high reduced electric fields up to 120 Td, which allows the investigation of nonclustered ion species and thus enables a sound investigation of the mobility shift of isotopologues. The results show that the measured relative ion mobilities of isotopologues having the same effective temperature and, thus, their ion mass dominating the relative ion mobility agree well with theoretical relative ion mobilities predicted by the theory of ion mobility
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
Near-infrared Linear Polarization of Ultracool Dwarfs
We report on near-infrared J- and H-band linear polarimetric photometry of
eight ultracool dwarfs (two late-M, five L0-L7.5, and one T2.5) with known
evidence for photometric variability due to dust clouds, anomalous red infrared
colors, or low-gravity atmospheres. The polarimetric data were acquired with
the LIRIS instrument on the William Herschel Telescope. We also provide
mid-infrared photometry in the interval 3.4-24 micron for some targets obtained
with Spitzer and WISE, which has allowed us to confirm the peculiar red colors
of five sources in the sample. We can impose modest upper limits of 0.9% and
1.8% on the linear polarization degree for seven targets with a confidence of
99%. Only one source, 2MAS, J02411151-0326587 (L0), appears to be strongly
polarized (P ~ 3%) in the J-band with a significance level of P/sigma_P ~ 10.
The likely origin of its linearly polarized light and rather red infrared
colors may reside in a surrounding disk with an asymmetric distribution of
grains. Given its proximity (66 +/- 8 pc), this object becomes an excellent
target for the direct detection of the disk.Comment: Accepted for publication in Ap
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