7,614 research outputs found
Doppler line profiles measurement of the Jovian Lyman Alpha emission with OAO-C
Observation of Jupiter made with the high resolution ultraviolet spectrometer of the Orbiting Astronomical Observatory copernicus in April and May, 1980, yield a Jovian Lyman alpha emission intensity of 7 + or 2.5 RR. This indicates a decrease by about a factor of two since the Voyager ultraviolet spectrometer measurements, nearly a year earlier. An unusually high column abundance of hydrogen atoms above the methane homopause at the Voyager epoch is indicated. Since the auroral charged particle bombardment of molecular hydrogen is expected to contribute significantly to the global population of the hydrogen atoms, it is suggested that at the time of the Voyager Jupiter encounter unusually high auroral activity existed, perhaps d to the high concentration of the Io plasma torus. The temporal variation of the Saturn lyman alpha emission, when contrasted with the Jovian data, reveals that the auroral processes are not nearly as important in determining the Saturn Lyman alpha intensity in the nonauroral region
Kepler-18b,c, and d: A System of Three Planets Confirmed by Transit Timing Variations, Light Curve Validation, Warm-Spitzer Photometry, and Radial Velocity Measurements
We report the detection of three transiting planets around a Sun-like star, which we designate Kepler-18. The transit signals were detected in photometric data from the Kepler satellite, and were confirmed to arise from planets using a combination of large transit-timing variations (TTVs), radial velocity variations, Warm-Spitzer observations, and statistical analysis of false-positive probabilities. The Kepler-18 star has a mass of 0.97 M_☉, a radius of 1.1 R_☉, an effective temperature of 5345 K, and an iron abundance of [Fe/H] = +0.19. The planets have orbital periods of approximately 3.5, 7.6, and 14.9 days. The innermost planet "b" is a "super-Earth" with a mass of 6.9 ± 3.4 M_⊕, a radius of 2.00 ± 0.10 R_⊕, and a mean density of 4.9 ± 2.4 g cm^3. The two outer planets "c" and "d" are both low-density Neptune-mass planets. Kepler-18c has a mass of 17.3 ± 1.9 M_⊕, a radius of 5.49 ± 0.26 R_⊕, and a mean density of 0.59 ± 0.07 g cm^3, while Kepler-18d has a mass of 16.4 ± 1.4 M_⊕, a radius of 6.98 ± 0.33 R_⊕ and a mean density of 0.27 ± 0.03 g cm^3. Kepler-18c and Kepler-18d have orbital periods near a 2:1 mean-motion resonance, leading to large and readily detected TTVs
Failure mechanisms of graphene under tension
Recent experiments established pure graphene as the strongest material known
to mankind, further invigorating the question of how graphene fails. Using
density functional theory, we reveal the mechanisms of mechanical failure of
pure graphene under a generic state of tension. One failure mechanism is a
novel soft-mode phonon instability of the -mode, whereby the graphene
sheet undergoes a phase transition and is driven towards isolated benzene rings
resulting in a reduction of strength. The other is the usual elastic
instability corresponding to a maximum in the stress-strain curve. Our results
indicate that finite wave vector soft modes can be the key factor in limiting
the strength of monolayer materials
Injectivity of satellite operators in knot concordance
Let P be a knot in a solid torus, K a knot in 3-space and P(K) the satellite
knot of K with pattern P. This defines an operator on the set of knot types and
induces a satellite operator P:C--> C on the set of smooth concordance classes
of knots. There has been considerable interest in whether certain such
functions are injective. For example, it is a famous open problem whether the
Whitehead double operator is weakly injective (an operator is called weakly
injective if P(K)=P(0) implies K=0 where 0 is the class of the trivial knot).
We prove that, modulo the smooth 4-dimensional Poincare Conjecture, any strong
winding number one satellite operator is injective on C. More precisely, if P
has strong winding number one and P(K)=P(J), then K is smoothly concordant to J
in S^3 x [0,1] equipped with a possibly exotic smooth structure. We also prove
that any strong winding number one operator is injective on the topological
knot concordance group. If P(0) is unknotted then strong winding number one is
the same as (ordinary) winding number one. More generally we show that any
satellite operator with non-zero winding number n induces an injective function
on the set of Z[1/n]-concordance classes of knots. We extend some of our
results to links.Comment: 16 pages; in second version we have added some results on operators
on links and string links and added some references to connections with Mazur
manifolds and corks; third version has only very minor changes and will
appear in Journal of Topolog
Confirmation of the Planet Hypothesis for the Long-period Radial Velocity Variations of Beta Geminorum
We present precise stellar radial velocity measurements for the K giant star
Beta Gem spanning over 25 years. These data show that the long period low
amplitude radial velocity variations found by Hatzes & Cochran (1993) are
long-lived and coherent. An examination of the Ca II K emission, spectral line
shapes from high resolution data (R = 210,000), and Hipparcos photometry show
no significant variations of these quantities with the RV period. These data
confirm the planetary companion hypothesis suggested by Hatzes & Cochran
(1993). An orbital solution assuming a stellar mass of 1.7 M_sun yields a
period, P = 589.6 days, a minimum mass of 2.3 M_Jupiter, and a semi-major axis,
and a = 1.6 AU. The orbit is nearly circular (e = 0.02). Beta Gem is the
seventh intermediate mass star shown to host a sub-stellar companion and
suggests that planet-formation around stars much more massive than the sun may
common.Comment: 10 pages, 9 figures, Astronomy and Astrophysics, in pres
The planet search programme at the ESO CES and HARPS. IV. The search for Jupiter analogues around solar-like stars
In 1992 we began a precision radial velocity (RV) survey for planets around
solar-like stars with the Coude Echelle Spectrograph and the Long Camera (CES
LC) at the 1.4 m telescope in La Silla (Chile). We have continued the survey
with the upgraded CES Very Long Camera (VLC) and HARPS, both at the 3.6 m
telescope, until 2007. The observations for 31 stars cover a time span of up to
15 years and the RV precision permit a search for Jupiter analogues. We perform
a joint analysis for variability, trends, periodicities, and Keplerian orbits
and compute detection limits. Moreover, the HARPS RVs are analysed for
correlations with activity indicators (CaII H&K and CCF shape). We achieve a
long-term RV precision of 15 m/s (CES+LC, 1992-1998), 9 m/s (CES+VLC,
1999-2006), and 2.8 m/s (HARPS, 2003-2009, including archive data), resp. This
enables us to confirm the known planets around Iota Hor, HR 506, and HR 3259. A
steady RV trend for Eps Ind A can be explained by a planetary companion. On the
other hand, we find previously reported trends to be smaller for Beta Hyi and
not present for Alp Men. The candidate planet Eps Eri b was not detected
despite our better precision. Also the planet announced for HR 4523 cannot be
confirmed. Long-term trends in several of our stars are compatible with known
stellar companions. We provide a spectroscopic orbital solution for the binary
HR 2400 and refined solutions for the planets around HR 506 and Iota Hor. For
some other stars the variations could be attributed to stellar activity. The
occurrence of two Jupiter-mass planets in our sample is in line with the
estimate of 10% for the frequency of giant planets with periods smaller than 10
yr around solar-like stars. We have not detected a Jupiter analogue, while the
detections limits for circular orbits indicate at 5 AU a sensitivity for
minimum mass of at least 1 M_Jup (2 M_Jup) for 13% (61%) of the stars.Comment: 63 pages, 24 figures (+33 online figures), 13 Tables, accepted for
publication in A&A (2012-11-13
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