We report the discovery of three very late T dwarfs in the UKIRT Infrared Deep Sky Survey (UKIDSS) Third Data Release: ULAS J101721.40+011817.9 (ULAS1017), ULAS J123828.51+095351.3 (ULAS1238) and ...ULAS J133553.45+113005.2 (ULAS1335). We detail optical and near-infrared (NIR) photometry for all three sources, and mid-IR photometry for ULAS1335. We use NIR spectra of each source to assign spectral types T8p (ULAS1017), T8.5 (ULAS1238) and T9 (ULAS1335) to these objects. ULAS1017 is classed as a peculiar T8 (T8p) due to appearing as a T8 dwarf in the J band, whilst exhibiting H- and K-band flux ratios consistent with a T6 classification. Through comparison to BT-Settl model spectra we estimate that ULAS1017 has 750K lapT sub(eff)lap 850K, and 5.0 lap logg(cms super(-2)) lap 5.5, assuming solar metallicity. This estimate for gravity is degenerate with varying metallicity. We estimate that ULAS1017 has an age of 1.6-15 Gyr, a mass of 33-70M sub(J) and lies at a distance of 31-54 pc. We do not estimate atmospheric parameters for ULAS1238 due to a lack of K-band photometry. We extend the unified scheme of Burgasser et al. to the type T9 and suggest the inclusion of the W sub(J) index to replace the now saturated J-band indices. We propose ULAS1335 as the T9 spectral type standard. ULAS1335 is the same spectral type as ULAS J003402.77-005206.7 and CFBDS J005910.90-011401.3. We argue that given the similarity of the currently known >T8 dwarfs to the rest of the T dwarf sequence, the suggestion of the Y0 spectral class for these objects is premature. Comparison of model spectra with that of ULAS1335 suggest a temperature below 600 K, possibly combined with low gravity and-or high metallicity. We find ULAS1335 to be extremely red in NIR to mid-IR colours, with H-4.49= 4.34 plus or minus 0.04. This is the reddest NIR to mid-IR colour yet observed for a T dwarf. The NIR to mid-IR spectral energy distribution of ULAS1335 further supports T sub(eff) < 600 K, and we estimate T sub(eff) similar to 550-600K for ULAS1335. We estimate that ULAS1335 has an age of 0.6-5.3 Gyr, a mass of 15-31M sub(J) and lies at a distance of 8-12 pc.
A recessed-gate AlGaN-GaN field-modulating plate (FP) field-effect transistor (FET) was successfully fabricated on an SiC substrate. By employing a recessed-gate structure on an FP FET, the ...transconductance was increased from 150 to 270 mS/mm, leading to an improvement in gain characteristics, and current collapse was minimized. At 2 GHz, a 48-mm-wide recessed FP FET exhibited a record output power of 230 W (4.8 W/mm) with 67% power-added efficiency and 9.5-dB linear gain with a drain bias of 53 V.
We report the discovery from K2 of two transiting hot Jupiter systems. K2-295 (observed in Campaign 8) is a K5 dwarf which hosts a planet slightly smaller than Jupiter, orbiting with a period of 4.0 ...d. We have made an independent discovery of K2-237 b (Campaign 11), which orbits an F9 dwarf every 2.2 d and has an inflated radius 60-70% larger than that of Jupiter. We use high-precision radial velocity measurements, obtained using the HARPS and FIES spectrographs, to measure the planetary masses. We find that K2-295 b has a similar mass to Saturn, while K2-237 b is a little more massive than Jupiter.
This paper reports the post‐growth annealing effects of low‐temperature grown Mg‐doped InGaN. By using MOVPE, 1 μm‐thick Mg‐doped InxGa1–xN (x ~ 0.36) films are grown at 570 °C. In order to activate ...the Mg acceptors, grown samples are treated by the conventional furnace annealing (FA) or the rapid thermal annealing (RTA). In the case of the FA at 650 °C for 20 min, the InGaN film is phase‐separated. On the other hand, the RTA at a temperature higher than 700 °C enables us to get p‐type samples. By using the RTA at 850 for 20 s, p‐type samples with a hole concentration 1018–1019 cm−3 are successfully obtained without phase separation.
This paper describes the small-signal characterization through delay-time analysis and high-power operation of the Ka-band of AlGaN/GaN heterojunction field-effect transistors (FETs). An FET with a ...gatewidth of 100 /spl mu/m and a gate length of 0.09 /spl mu/m has exhibited a current gain cutoff frequency (f/sub T/) of 81 GHz, a maximum frequency of oscillation (fmax) of 187 GHz, and a maximum stable gain of 10.5 dB at 30 GHz (8.3 dB at 60 GHz). Delay-time analysis has demonstrated channel electron velocities of 1.50/spl times/10/sup 7/ to 1.75/spl times/10/sup 7/ cm/s in a gate-length range of 0.09-0.25 /spl mu/m. State-of-the-art performance-saturated power of 5.8 W with a linear gain of 9.2 dB and a power-added efficiency of 43.2%-has been achieved at 30 GHz using a single chip having a gatewidth of 1.0 mm and a gate length of 0.25 /spl mu/m.
Recessed-gate AlGaN-GaN heterojunction field-effect transistors (FETs) with a field-modulating plate (FP) have been successfully fabricated for high-voltage and high-power microwave applications. The ...developed recessed-gate FP-FET with a gate length of 1 /spl mu/m exhibited an increased transconductance of 200 mS/mm. A series of current collapse measurements revealed that the recessed-gate FP-FET is highly desirable for collapse-free high-voltage power operation. Equivalent circuit analysis demonstrated that the gain loss due to the additional gate feedback capacitance associated with the FP electrode is considerably compensated by increasing the drain bias voltage to more than 30 V. A 48-mm-wide recessed-gate FP-FET biased at a drain voltage of 48 V exhibited a record saturated output power of 197 W with a linear gain of 10.1 dB and a power-added efficiency of 67% at 2 GHz.
We present the detection and follow-up observations of planetary candidates around low-mass stars observed by the K2 mission. Based on light-curve analysis, adaptive-optics imaging, and optical ...spectroscopy at low and high resolution (including radial velocity measurements), we validate 16 planets around 12 low-mass stars observed during K2 campaigns 5-10. Among the 16 planets, 12 are newly validated, with orbital periods ranging from 0.96 to 33 days. For one of the planets (K2-151b), we present ground-based transit photometry, allowing us to refine the ephemerides. Combining our K2 M-dwarf planets together with the validated or confirmed planets found previously, we investigate the dependence of planet radius Rp on stellar insolation and metallicity Fe/H. We confirm that for periods P 2 days, planets with a radius are less common than planets with a radius between 1-2 R⊕. We also see a hint of the "radius valley" between 1.5 and 2 R⊕, which has been seen for close-in planets around FGK stars. These features in the radius/period distribution could be attributed to photoevaporation of planetary envelopes by high-energy photons from the host star, as they have for FGK stars. For the M dwarfs, though, the features are not as well defined, and we cannot rule out other explanations such as atmospheric loss from internal planetary heat sources or truncation of the protoplanetary disk. There also appears to be a relation between planet size and metallicity: the few planets larger than about 3 R⊕ are found around the most metal-rich M dwarfs.
A novel GaAs power FET with a field-modulating plate (FP-FET) and operated at a drain bias voltage of 24 V was developed. Regarding the power performance of an FP-FET under wideband code division ...multiple access (WCDMA) operation, its peak output power significantly degraded in the saturated-output-power region when its FP length was short. However, by introducing a sufficiently large FP length, it was demonstrated that the peak-power degradation can be significantly suppressed. In particular, an FP-FET amplifier fabricated with an FP length of 1.5 /spl mu/m exhibited no WCDMA peak-power degradation and delivered an output power of 120 W with a linear gain of 14.2 dB at 2.14 GHz.
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