2,480 research outputs found
Characterization of Secondary Organic Aerosol (SOA) formed by the Reaction of β-caryophyllene, Soot and Ozone: Climate Impact
Diesel soot (black carbon, BC) is an important light absorbing aerosol component in atmosphere that can cause tropospheric heating. Laboratory studies have found it to be unreactive to ozone at ambient temperature. The low uptake coefficient i.e., γ 300 K = 2× 10-7 , of the soot-O3 reaction indicates a low probability of irreversible O3 loss from gas phase to surface-adsorbed product (Particle phase). This shows clearly that at low temperature soot is not reactive with atmospheric oxidants. In contrast, sesquiterpenes (SQT) such as β-caryophyllene (C15H24), which are produced primarily by plants, are extremely reactive with ozone. For example, the residence time of β- caryophyllene in the atmosphere is only 2 min in the presence of 60 ppb ozone. Thus, ozonolysis reaction of β-caryophyllene is expected to be a significant source of biogenic secondary organic aerosols. These oxidized products may condense onto soot particles, and a question arises as to how they will partition between the soot surface, vapor phase, and aqueous aerosol phases. Liquid chromatography- mass spectrometry (LC/MS), Fourier transform infrared (FTIR) and UVVis spectroscopies are being used to study the β- caryophyllene-dark ozonolysis reaction at low ozone levels (40-60 ppb). Products identified include low molecular weight highly volatile and water soluble products such as formaldehyde, acetaldehyde, acetone, and acetic acid. Also identified are high molecular weight components (~350 Dalton) with lower water solubility and vapor pressures. The SOA coatings of these SQTs on soot are being evaluated to determine their hygroscopicity. As these compounds absorb in the IR and UV-Vis they can add to radiative forcing by submicron aerosols and need to be better understood for climate modeling
The Evolution of L and T Dwarfs in Color-Magnitude Diagrams
We present new evolution sequences for very low mass stars, brown dwarfs and
giant planets and use them to explore a variety of influences on the evolution
of these objects. We compare our results with previous work and discuss the
causes of the differences and argue for the importance of the surface boundary
condition provided by atmosphere models including clouds.
The L- to T-type ultracool dwarf transition can be accommodated within the
Ackerman & Marley (2001) cloud model by varying the cloud sedimentation
parameter. We develop a simple model for the evolution across the L/T
transition. By combining the evolution calculation and our atmosphere models,
we generate colors and magnitudes of synthetic populations of ultracool dwarfs
in the field and in galactic clusters. We focus on near infrared color-
magnitude diagrams (CMDs) and on the nature of the ``second parameter'' that is
responsible for the scatter of colors along the Teff sequence. Variations in
metallicity and cloud parameters, unresolved binaries and possibly a relatively
young population all play a role in defining the spread of brown dwarfs along
the cooling sequence. We find that the transition from cloudy L dwarfs to
cloudless T dwarfs slows down the evolution and causes a pile up of substellar
objects in the transition region, in contradiction with previous studies. We
apply the same model to the Pleiades brown dwarf sequence. Taken at face value,
the Pleiades data suggest that the L/T transition occurs at lower Teff for
lower gravity objects. The simulated populations of brown dwarfs also reveal
that the phase of deuterium burning produces a distinctive feature in CMDs that
should be detectable in ~50-100 Myr old clusters.Comment: Accepted for publication in the ApJ. 52 pages including 20 figure
Microlensing Events from Measurements of the Deflection Angle
Microlensing events are now regularly being detected by monitoring the flux
of a large number of potential sources and measuring the combined magnification
of the images. This phenomenon could also be detected directly from the
gravitational deflection, by means of high precision astrometry using
interferometry. Relative astrometry at the level of 10\muas may become
possible in the near future. The gravitational deflection can be measured by
astrometric monitoring of a bright star having a background star within a small
angular separation. This type of monitoring program will be carried out for the
independent reasons of discovering planets from the angular motion they induce
on the nearby star around which they are orbiting, and for measuring
parallaxes, proper motions and orbits of binary stars. We discuss three
applications of the measurement of gravitational deflections by astrometric
monitoring: measuring the mass of the bright stars that are monitored,
measuring the mass of brown dwarfs or giant planets around the bright stars,
and detecting microlensing events by unrelated objects near the line of sight
to the two stars. We discuss the number of stars whose mass could be measured
by this procedure. We also give expressions for the number of expected
microlensing events by unrelated objects, which could be stars, brown dwarfs,
or other compact objects accounting for dark matter in the halo or in the disk.Comment: submitted to ApJ Letter
Atmospheric, Evolutionary, and Spectral Models of the Brown Dwarf Gliese 229 B
Theoretical spectra and evolutionary models that span the giant planet--brown
dwarf continuum have been computed based on the recent discovery of the brown
dwarf, Gliese 229 B. A flux enhancement in the 4--5 micron window is a
universal feature from Jovian planets to brown dwarfs. We confirm the existence
of methane and water in Gl 229 B's spectrum and find its mass to be 30 to 55
Jovian masses. Although these calculations focus on Gliese 229 B, they are also
meant to guide future searches for extra-solar giant planets and brown dwarfs.Comment: 8 pages, plain TeX, plus four postscript figures, gzipped and
uuencoded, accepted for Scienc
On the Radii of Close-in Giant Planets
The recent discovery that the close-in extrasolar giant planet, HD209458b,
transits its star has provided a first-of-its-kind measurement of the planet's
radius and mass. In addition, there is a provocative detection of the light
reflected off of the giant planet, Boo b. Including the effects of
stellar irradiation, we estimate the general behavior of radius/age
trajectories for such planets and interpret the large measured radii of
HD209458b and Boo b in that context. We find that HD209458b must be a
hydrogen-rich gas giant. Furthermore, the large radius of close-in gas giant is
not due to the thermal expansion of its atmosphere, but to the high residual
entropy that remains throughout its bulk by dint of its early proximity to a
luminous primary. The large stellar flux does not inflate the planet, but
retards its otherwise inexorable contraction from a more extended configuration
at birth. This implies either that such a planet was formed near its current
orbital distance or that it migrated in from larger distances (0.5 A.U.),
no later than a few times years of birth.Comment: aasms4 LaTeX, 1 figure, accepted to Ap.J. Letter
- …