657 research outputs found
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
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
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 ion–neutral 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
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