45 research outputs found
Relating jet structure to photometric variability: the Herbig Ae star HD 163296
Herbig Ae/Be stars are intermediate-mass pre-main sequence stars surrounded
by circumstellar dust disks. Some are observed to produce jets, whose
appearance as a sequence of shock fronts (knots) suggests a past episodic
outflow variability. This "jet fossil record" can be used to reconstruct the
outflow history. We present the first optical to near-infrared (NIR)
VLT/X-shooter spectra of the jet from the Herbig Ae star HD 163296. We
determine physical conditions in the knots, as well as their kinematic "launch
epochs". Knots are formed simultaneously on either side of the disk, with a
regular interval of ~16 yr. The velocity dispersion versus jet velocity and the
energy input are comparable in both lobes. However, the mass loss rate,
velocity, and shock conditions are asymmetric. We find Mjet/Macc ~ 0.01-0.1,
consistent with magneto-centrifugal jet launching models. No evidence for dust
is found in the high-velocity jet, suggesting it is launched within the
sublimation radius (<0.5 au). The jet inclination measured from proper motions
and radial velocities confirms it is perpendicular to the disk. A tentative
relation is found between the structure of the jet and the photometric
variability of the source. Episodes of NIR brightening were previously detected
and attributed to a dusty disk wind. We report for the first time significant
optical fadings lasting from a few days up to a year, coinciding with the NIR
brightenings. These are likely caused by dust lifted high above the disk plane;
this supports the disk wind scenario. The disk wind is launched at a larger
radius than the high-velocity atomic jet, although their outflow variability
may have a common origin. No significant relation between outflow and accretion
variability could be established. Our findings confirm that this source
undergoes periodic ejection events, which may be coupled with dust ejections
above the disk plane.Comment: 20 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
Lowâtemperature growth of high resistivity GaAs by photoassisted metalorganic chemical vapor deposition
We report the photoassisted lowâtemperature (LT) metalorganic chemical vapor deposition (MOCVD) of high resistivity GaAs. The undoped asâgrown GaAs exhibits a resistivity of âŒ106 Ωâcm, which is the highest reported for undoped material grown in the MOCVD environment. Photoassisted growth of doped and undoped device quality GaAs has been achieved at a substrate temperature of 400â°C in a modified atmospheric pressure MOCVD reactor. By using silane as a dopant gas, the LT photoassisted doped films have high levels of doping and electron mobilities comparable to those achieved by MOCVD for growth temperatures, Tgâł600â°C
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Relating jet structure to photometric variability: the Herbig Ae star HD 163296
Herbig Ae/Be stars are intermediate-mass pre-main sequence stars surrounded by circumstellar dust disks. Some are observed to produce jets, whose appearance as a sequence of shock fronts (knots) suggests a past episodic outflow variability. This âjet fossil recordâ can be used to reconstruct the outflow history. We present the first optical to near-infrared (NIR) spectra of the jet from the Herbig Ae star HD 163296, obtained with VLT/X-shooter. We determine the physical conditions in the knots and also their kinematic âlaunch epochsâ. Knots are formed simultaneously on either side of the disk, with a regular interval of ~16 yr. The velocity dispersion versus jet velocity and the energy input are comparable between both lobes. However, the mass-loss rate, velocity,and shock conditions are asymmetric. We find áčjet/áčacc ~ 0.01â0.1, which is consistent with magneto-centrifugal jet launching models. No evidence of any dust is found in the high-velocity jet, suggesting a launch region within the sublimation radius (<0.5 au). The jet inclination measured from proper motions and radial velocities confirms that it is perpendicular to the disk. A tentative relation is found between the structure of the jet and the photometric variability of the central source. Episodes of NIR brightening were previously detected and attributed to a dusty disk wind. We report for the first time significant optical fadings lasting from a few days up to a year, coinciding with the NIR brightenings. These are very likely caused by dust lifted high above the disk plane, and this supports the disk wind scenario. The disk wind is launched at a larger radius than the high-velocity atomic jet, although their outflow variability may have a common origin. No significant relation between outflow and accretion variability could be established. Our findings confirm that this source undergoes periodic ejection events, which may be coupled with dust ejections above the disk plane.Astronom
Superior Inhibitory Control and Resistance to Mental Fatigue in Professional Road Cyclists
Purpose: Given the important role of the brain in regulating endurance performance, this comparative study sought to determine whether professional road cyclists have superior inhibitory control and resistance to mental fatigue compared to recreational road cyclists. Methods: After preliminary testing and familiarization, eleven professional and nine recreational road cyclists visited the lab on two occasions to complete a modified incongruent colour-word Stroop task (a cognitive task requiring inhibitory control) for 30 min (mental exertion condition), or an easy cognitive task for 10 min (control condition) in a randomized, counterbalanced cross-over order. After each cognitive task, participants completed a 20-min time trial on a cycle ergometer. During the time trial, heart rate, blood lactate concentration, and rating of perceived exertion (RPE) were recorded. Results: The professional cyclists completed more correct responses during the Stroop task than the recreational cyclists (705±68 vs 576±74, p = 0.001). During the time trial, the recreational cyclists produced a lower mean power output in the mental exertion condition compared to the control condition (216±33 vs 226±25 W, p = 0.014). There was no difference between conditions for the professional cyclists (323±42 vs 326±35 W, p = 0.502). Heart rate, blood lactate concentration, and RPE were not significantly different between the mental exertion and control conditions in both groups. Conclusion: The professional cyclists exhibited superior performance during the Stroop task which is indicative of stronger inhibitory control than the recreational cyclists. The professional cyclists also displayed a greater resistance to the negative effects of mental fatigue as demonstrated by no significant differences in perception of effort and time trial performance between the mental exertion and control conditions. These findings suggest that inhibitory control and resistance to mental fatigue may contribute to successful road cycling performance. These psychobiological characteristics may be either genetic and/or developed through the training and lifestyle of professional road cyclists