436 research outputs found
A Candidate Young Massive Planet in Orbit around the Classical T Tauri Star CI Tau
The ~2 Myr old classical T Tauri star CI Tau shows periodic variability in
its radial velocity (RV) variations measured at infrared (IR) and optical
wavelengths. We find that these observations are consistent with a massive
planet in a ~9-day period orbit. These results are based on 71 IR RV
measurements of this system obtained over 5 years, and on 26 optical RV
measurements obtained over 9 years. CI Tau was also observed photometrically in
the optical on 34 nights over ~one month in 2012. The optical RV data alone are
inadequate to identify an orbital period, likely the result of star spot and
activity induced noise for this relatively small dataset. The infrared RV
measurements reveal significant periodicity at ~9 days. In addition, the full
set of optical and IR RV measurements taken together phase coherently and with
equal amplitudes to the ~9 day period. Periodic radial velocity signals can in
principle be produced by cool spots, hot spots, and reflection of the stellar
spectrum off the inner disk, in addition to resulting from a planetary
companion. We have considered each of these and find the planet hypothesis most
consistent with the data. The radial velocity amplitude yields an Msin(i) of
~8.1 M_Jup; in conjunction with a 1.3 mm continuum emission measurement of the
circumstellar disk inclination from the literature, we find a planet mass of
~11.3 M_Jup, assuming alignment of the planetary orbit with the disk.Comment: 61 pages, 13 figures, accepted for publication in The Astrophysical
Journa
Deuteron photo-disintegration with polarised photons in the energy range 30 - 50 MeV
The reaction d(\vec\gamma,np) has been studied using the tagged and polarised
LADON gamma ray beam at an energy 30 - 50 MeV to investigate the existence of
narrow dibaryonic resonances recently suggested from the experimental
measurements in a different laboratory. The beam was obtained by Compton
back-scattering of laser light on the electrons of the storage ring ADONE.
Photo-neutron yields were measured at five neutron angle \vartheta_n = 22,
55.5, 90, 125 and 157 degrees in the center of mass system.Our results do not
support the existence of such resonances.Comment: 16 pages, Latex, 22 figures, 1 table. Nucl. Phys. A to appea
Follow-Up Observations of PTFO 8-8695: A 3 MYr Old T-Tauri Star Hosting a Jupiter-mass Planetary Candidate
We present Spitzer 4.5\micron\ light curve observations, Keck NIRSPEC radial
velocity observations, and LCOGT optical light curve observations of
PTFO~8-8695, which may host a Jupiter-sized planet in a very short orbital
period (0.45 days). Previous work by \citet{vaneyken12} and \citet{barnes13}
predicts that the stellar rotation axis and the planetary orbital plane should
precess with a period of days. As a consequence, the observed
transits should change shape and depth, disappear, and reappear with the
precession. Our observations indicate the long-term presence of the transit
events ( years), and that the transits indeed do change depth, disappear
and reappear. The Spitzer observations and the NIRSPEC radial velocity
observations (with contemporaneous LCOGT optical light curve data) are
consistent with the predicted transit times and depths for the $M_\star = 0.34\
M_\odot$ precession model and demonstrate the disappearance of the transits. An
LCOGT optical light curve shows that the transits do reappear approximately 1
year later. The observed transits occur at the times predicted by a
straight-forward propagation of the transit ephemeris. The precession model
correctly predicts the depth and time of the Spitzer transit and the lack of a
transit at the time of the NIRSPEC radial velocity observations. However, the
precession model predicts the return of the transits approximately 1 month
later than observed by LCOGT. Overall, the data are suggestive that the
planetary interpretation of the observed transit events may indeed be correct,
but the precession model and data are currently insufficient to confirm firmly
the planetary status of PTFO~8-8695b.Comment: Accepted for publication in The Astrophysical Journa
Reactive ion etching of Ta–Si–N diffusion barriers in CF_(4)+O_(2)
Ta_(36)Si_(14)N_(50) amorphous layers were reactive ion etched in CF_(4)+O_(2) plasmas. The etch depth was determined as a function of gas composition, pressure, and cathode power. Adding small amounts of O_2 to CF_4 increased the etch rates up to approximately 15% O_2 concentration, with etch rates then decreasing with further addition of O_2. Etch rates increased with both pressure and power. Etching proceeded only after an initial delay time which depended upon gas composition and power. The delay is probably caused by a surface native oxide which must be removed before etching can commence. The presence of a surface oxide was observed from Auger electron spectroscopy intensity depth profile measurements and is estimated to be 2 nm thick. Under optimal conditions, the etch rate of Ta_(36)Si_(14)N_(50) is about seven times higher than for SiO_2, thus providing a high degree of selectivity for integrated circuit processing
Optical-Model Description of Time-Reversal Violation
A time-reversal-violating spin-correlation coefficient in the total cross
section for polarized neutrons incident on a tensor rank-2 polarized target is
calculated by assuming a time-reversal-noninvariant, parity-conserving
``five-fold" interaction in the neutron-nucleus optical potential. Results are
presented for the system for neutron incident energies
covering the range 1--20 MeV. From existing experimental bounds, a strength of
keV is deduced for the real and imaginary parts of the five-fold
term, which implies an upper bound of order on the relative -odd
strength when compared to the central real optical potential.Comment: 11 pages (Revtex
Epidemics on contact networks: a general stochastic approach
Dynamics on networks is considered from the perspective of Markov stochastic
processes. We partially describe the state of the system through network motifs
and infer any missing data using the available information. This versatile
approach is especially well adapted for modelling spreading processes and/or
population dynamics. In particular, the generality of our systematic framework
and the fact that its assumptions are explicitly stated suggests that it could
be used as a common ground for comparing existing epidemics models too complex
for direct comparison, such as agent-based computer simulations. We provide
many examples for the special cases of susceptible-infectious-susceptible (SIS)
and susceptible-infectious-removed (SIR) dynamics (e.g., epidemics propagation)
and we observe multiple situations where accurate results may be obtained at
low computational cost. Our perspective reveals a subtle balance between the
complex requirements of a realistic model and its basic assumptions.Comment: Main document: 16 pages, 7 figures. Electronic Supplementary Material
(included): 6 pages, 1 tabl
The angular distribution of the reaction
The reaction is very important for low-energy
( MeV) antineutrino experiments. In this paper we calculate
the positron angular distribution, which at low energies is slightly backward.
We show that weak magnetism and recoil corrections have a large effect on the
angular distribution, making it isotropic at about 15 MeV and slightly forward
at higher energies. We also show that the behavior of the cross section and the
angular distribution can be well-understood analytically for MeV by calculating to , where is the nucleon mass. The
correct angular distribution is useful for separating events from other reactions and detector backgrounds, as well as for
possible localization of the source (e.g., a supernova) direction. We comment
on how similar corrections appear for the lepton angular distributions in the
deuteron breakup reactions and . Finally, in the reaction , the
angular distribution of the outgoing neutrons is strongly forward-peaked,
leading to a measurable separation in positron and neutron detection points,
also potentially useful for rejecting backgrounds or locating the source
direction.Comment: 10 pages, including 5 figure
Can sacrificial feeding areas protect aquatic plants from herbivore grazing? Using behavioural ecology to inform wildlife management
Effective wildlife management is needed for conservation, economic and human well-being objectives. However, traditional population control methods are frequently ineffective, unpopular with stakeholders, may affect non-target species, and can be both expensive and impractical to implement. New methods which address these issues and offer effective wildlife management are required. We used an individual-based model to predict the efficacy of a sacrificial feeding area in preventing grazing damage by mute swans (Cygnus olor) to adjacent river vegetation of high conservation and economic value. The accuracy of model predictions was assessed by a comparison with observed field data, whilst prediction robustness was evaluated using a sensitivity analysis. We used repeated simulations to evaluate how the efficacy of the sacrificial feeding area was regulated by (i) food quantity, (ii) food quality, and (iii) the functional response of the forager. Our model gave accurate predictions of aquatic plant biomass, carrying capacity, swan mortality, swan foraging effort, and river use. Our model predicted that increased sacrificial feeding area food quantity and quality would prevent the depletion of aquatic plant biomass by swans. When the functional response for vegetation in the sacrificial feeding area was increased, the food quantity and quality in the sacrificial feeding area required to protect adjacent aquatic plants were reduced. Our study demonstrates how the insights of behavioural ecology can be used to inform wildlife management. The principles that underpin our model predictions are likely to be valid across a range of different resource-consumer interactions, emphasising the generality of our approach to the evaluation of strategies for resolving wildlife management problems
Gemini Near Infrared Spectrograph -- Distant Quasar Survey: Augmented Spectroscopic Catalog and a Prescription for Correcting UV-Based Quasar Redshifts
Quasars at most often have redshifts measured from rest-frame
ultraviolet emission lines. One of the most common such lines, C IV
, shows blueshifts up to , and in
rare cases even higher. This blueshifting results in highly uncertain redshifts
when compared to redshift determinations from rest-frame optical emission
lines, e.g., from the narrow [O III] feature. We present
spectroscopic measurements for 260 sources at
having
mag from the Gemini Near Infrared
Spectrograph - Distant Quasar Survey (GNIRS-DQS) catalog, augmenting the
previous iteration which contained 226 of the 260 sources whose measurements
are improved upon in this work. We obtain reliable systemic redshifts based on
[O III] for a subset of 121 sources which we use to calibrate
prescriptions for correcting UV-based redshifts. These prescriptions are based
on a regression analysis involving C IV full-width-at-half-maximum intensity
and equivalent width, along with the UV continuum luminosity at a rest-frame
wavelength of 1350 A. Applying these corrections can improve the accuracy and
the precision in the C IV-based redshift by up to
and , respectively, which correspond to
Mpc and Mpc in comoving distance at . Our prescriptions
also improve the accuracy of the best available multi-feature redshift
determination algorithm by , indicating that the
spectroscopic properties of the C IV emission line can provide robust redshift
estimates for high-redshift quasars. We discuss the prospects of our
prescriptions for cosmological and quasar studies utilizing upcoming large
spectroscopic surveys.Comment: 20 pages (AASTeX 6.3.1), 8 figures, accepted for publication in Ap
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