1,052 research outputs found
Determination of jet fuel thermal deposit rate using a modified JFTOT
Three fuels having different breakpoint temperatures were studied in the modified jet fuel thermal oxidation tester. The lower stability fuel with a breakpoint of 240 C was first stressed at a constant temperature. After repeating this procedure at several different temperatures, an Arrehenius plot was drawn from the data. The correlation coefficient and the energy of activation were calculated to be 0.97 and 8 kcal/mole respectively. Two other fuels having breakpoint temperatures of 271 C and 285 C were also studied in a similar manner. A straight line was drawn through the data at a slope equivalent to the slope of the lower stability fuel. The deposit formation rates for the three fuels were determined at 260 C, and a relative deposit formation rate at this temperature was calculated and plotted as a function of the individual fuel's breakpoint temperatures
The Galactic disk mass function: reconciliation of the HST and nearby determinations
We derive and parametrize the Galactic mass function (MF) below 1 \msol
characteristic of both single objects and binary systems. We resolve the long
standing discrepancy between the MFs derived from the HST and from the nearby
luminosity functions, respectively. We show that this discrepancy stemmed from
{\it two} cumulative effects, namely (i) incorrect color-magnitude determined
distances, due a substantial fraction of M dwarfs in the HST sample belonging
to the metal-depleted, thick-disk population, as corrected recently by Zheng et
al. and (ii) unresolved binaries. We show that both the nearby and HST MF for
unresolved systems are consistent with a fraction 50% of M-dwarf
binaries, with the mass of both the primaries and the companions originating
from the same underlying single MF. This implies that 30% of M dwarfs
should have an M dwarf companion and 20% should have a brown dwarf
companion, in agreement with recent determinations. The present calculations
show that the so-called "brown-dwarf desert" should be reinterpreted as a lack
of high mass-ratio (m_2/m_1\la 0.1) systems, and does not preclude a
substantial fraction of brown dwarfs as companions of M dwarfs or for other
brown dwarfs.Comment: 16 pages, Latex file, uses aasms4.sty, to appear in ApJ Letter
On the Correlation between the Magnetic Activity Levels, the Metallicities and the Radii of Low-Mass Stars
The recent burst in the number of radii measurements of very low-mass stars
from eclipsing binaries and interferometry of single stars has opened more
questions about what can be causing the discrepancy between the observed radii
and the ones predicted by the models. The two main explanations being proposed
are a correlation between the radius of the stars and their activity levels or
their metallicities. This paper presents a study of such correlations using all
the data published to date. The study also investigates correlations between
the radii deviation from the models and the masses of the stars. There is no
clear correlation between activity level and radii for the single stars in the
sample. Those single stars are slow rotators with typical velocities v_rot sini
< 3.0 km s^-1. A clear correlation however exists in the case of the faster
rotating members of binaries. This result is based on the of X-ray emission
levels of the stars. There also appears to be an increase in the deviation of
the radii of single stars from the models as a function of metallicity, as
previously indicated by Berger et al. (2006). The stars in binaries do not seem
to follow the same trend. Finally, the Baraffe et al. (1998) models reproduce
well the radius observations below 0.30-0.35Msun, where the stars become fully
convective, although this result is preliminary since almost all the sample
stars in that mass range are slow rotators and metallicities have not been
measured for most of them. The results in this paper indicate that stellar
activity and metallicity play an important role on the determination of the
radius of very low-mass stars, at least above 0.35Msun.Comment: 22 pages, 4 figures. Accepted for publication on Ap
Multiepoch Radial Velocity Observations of L Dwarfs
We report on the development of a technique for precise radial-velocity
measurements of cool stars and brown dwarfs in the near infrared. Our technique
is analogous to the Iodine (I2) absorption cell method that has proven so
successful in the optical regime. We rely on telluric CH4 absorption features
to serve as a wavelength reference, relative to which we measure Doppler shifts
of the CO and H2O features in the spectra of our targets. We apply this
technique to high-resolution (R~50,000) spectra near 2.3 micron of nine L
dwarfs taken with the Phoenix instrument on Gemini-South and demonstrate a
typical precision of 300 m/s. We conduct simulations to estimate our expected
precision and show our performance is currently limited by the signal-to-noise
of our data. We present estimates of the rotational velocities and systemic
velocities of our targets. With our current data, we are sensitive to
companions with M sin i > 2MJ in orbits with periods less than three days. We
identify no companions in our current data set. Future observations with
improved signal-to-noise should result in radial-velocity precision of 100 m/s
for L dwarfs.Comment: Accepted for publication in ApJ, 24 pages, 7 figure
The Big Occulting Steerable Satellite (BOSS)
Natural (such as lunar) occultations have long been used to study sources on
small angular scales, while coronographs have been used to study high contrast
sources. We propose launching the Big Occulting Steerable Satellite (BOSS), a
large steerable occulting satellite to combine both of these techniques. BOSS
will have several advantages over standard occulting bodies. BOSS would block
all but about 4e-5 of the light at 1 micron in the region of interest around
the star for planet detections. Because the occultation occurs outside the
telescope, scattering inside the telescope does not degrade this performance.
BOSS could be combined with a space telescope at the Earth-Sun L2 point to
yield very long integration times, in excess of 3000 seconds. If placed in
Earth orbit, integration times of 160--1600 seconds can be achieved from most
major telescope sites for objects in over 90% of the sky. Applications for BOSS
include direct imaging of planets around nearby stars. Planets separated by as
little as 0.1--0.25 arcseconds from the star they orbit could be seen down to a
relative intensity as little as 1e-9 around a magnitude 8 (or brighter) star.
Other applications include ultra-high resolution imaging of compound sources,
such as microlensed stars and quasars, down to a resolution as little as 0.1
milliarcseconds.Comment: 25pages, 4 figures, uses aaspp4, rotate, and epsfig. Submitted to the
Astrophysical Journal. For more details see
http://erebus.phys.cwru.edu/~boss
A Dedicated M-Dwarf Planet Search Using The Hobby-Eberly Telescope
We present first results of our planet search program using the 9.2 meter
Hobby-Eberly Telescope (HET) at McDonald Observatory to detect planets around
M-type dwarf stars via high-precision radial velocity (RV) measurements.
Although more than 100 extrasolar planets have been found around solar-type
stars of spectral type F to K, there is only a single M-dwarf (GJ 876, Delfosse
et al. 1998; Marcy et al. 1998; Marcy et al. 2001) known to harbor a planetary
system. With the current incompleteness of Doppler surveys with respect to
M-dwarfs, it is not yet possible to decide whether this is due to a fundamental
difference in the formation history and overall frequency of planetary systems
in the low-mass regime of the Hertzsprung-Russell diagram, or simply an
observational bias. Our HET M-dwarf survey plans to survey 100 M-dwarfs in the
next 3 to 4 years with the primary goal to answer this question. Here we
present the results from the first year of the survey which show that our
routine RV-precision for M-dwarfs is 6 m/s. We found that GJ 864 and GJ 913 are
binary systems with yet undetermined periods, while 5 out of 39 M-dwarfs reveal
a high RV-scatter and represent candidates for having short-periodic planetary
companions. For one of them, GJ 436 (rms = 20.6 m/s), we have already obtained
follow-up observations but no periodic signal is present in the RV-data.Comment: 12 pages, 14 figures, accepted for publication in the Astronomical
Journa
Design Considerations for a Ground-based Transit Search for Habitable Planets Orbiting M dwarfs
By targeting nearby M dwarfs, a transit search using modest equipment is
capable of discovering planets as small as 2 Earth radii in the habitable zones
of their host stars. The MEarth Project, a future transit search, aims to
employ a network of ground-based robotic telescopes to monitor M dwarfs in the
northern hemisphere with sufficient precision and cadence to detect such
planets. Here we investigate the design requirements for the MEarth Project. We
evaluate the optimal bandpass, and the necessary field of view, telescope
aperture, and telescope time allocation on a star-by-star basis, as is possible
for the well-characterized nearby M dwarfs. Through these considerations, 1,976
late M dwarfs (R < 0.33 Rsun) emerge as favorable targets for transit
monitoring. Based on an observational cadence and on total telescope time
allocation tailored to recover 90% of transit signals from planets in habitable
zone orbits, we find that a network of ten 30 cm telescopes could survey these
1,976 M dwarfs in less than 3 years. A null result from this survey would set
an upper limit (at 99% confidence) of 17% for the rate of occurrence of planets
larger than 2 Earth radii in the habitable zones of late M dwarfs, and even
stronger constraints for planets lying closer than the habitable zone. If the
true occurrence rate of habitable planets is 10%, the expected yield would be
2.6 planets.Comment: accepted to PAS
Discovery of three nearby L dwarfs in the Southern Sky
We report the discovery of three L dwarfs in the solar vicinity within 30
parsecs. These objects were originally found as proper motion objects from a
combination of R and I photographic plates measured as part of the SuperCOSMOS
Sky Surveys. We subsequently identified these objects as bona fide brown dwarf
candidates on the basis of their R-I colour, as first criterion, and
subsequently their J-K colours when the infrared data were available from the
2MASS database. Spectroscopic observations in the optical with the ESO
3.6m/EFOSC2 and in the near-infrared with the NTT/SOFI led to the
classification of their spectral types as early L dwarfs.Comment: 4 pages including 2 figures, accepted for publication in Astronomy
and Astrophysics Letter
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