The Surface Magnetic Activity of the Weak-Line T Tauri Stars TWA 7 and TWA 25

We present an analysis of spectropolarimetric observations of the low-mass weak-line T Tauri stars TWA 25 and TWA 7. The large-scale surface magnetic fields have been reconstructed for both stars using the technique of Zeeman Doppler imaging. Our surface maps reveal predominantly toroidal and non-axisymmetric fields for both stars. These maps reinforce the wide range of surface magnetic fields that have been recovered, particularly in pre-main sequence stars that have stopped accreting from the (now depleted) central regions of their discs. We reconstruct the large scale surface brightness distributions for both stars, and use these reconstructions to filter out the activity-induced radial velocity jitter, reducing the RMS of the radial velocity variations from 495 m s −1 to 32 m s −1 for TWA 25, and from 127 m s −1 to 36 m s −1 for TWA 7, ruling out the presence of close-in giant planets for both stars. The TWA 7 radial velocities provide an example of a case where the activity-induced radial velocity variations mimic a Keplerian signal that is uncorrelated with the spectral activity indices. This shows the usefulness of longitudinal magnetic field measurements in identifying activity-induced radial velocity variations.

X-ray emission in the enigmatic CVSO 30 system

CVSO 30 is a young, active, weak-line T Tauri star; it possibly hosts the only known planetary system with both a transiting hot-Jupiter and a cold-Jupiter candidate (CVSO 30 b and CVSO 30 c). We analyzed archival ROSAT, Chandra, and XMM-Newton data to study the coronal emission in the system. According to our modeling, CVSO 30 shows a quiescent X-ray luminosity of ≈8 × 1029 erg s−1. The X-ray absorbing column is consistent with interstellar absorption. XMM-Newton observed a flare, during which a transit of the candidate CVSO 30 b was expected, but no significant transit-induced variation in the X-ray flux is detectable. While the hot-Jupiter candidate CVSO 30 b has continuously been undergoing mass loss powered by the high-energy irradiation, we conclude that its evaporation lifetime is considerably longer than the estimated stellar age of 2.6 Myr.

Magnetic topologies of young suns: the weak-line T Tauri stars TWA 6 and TWA 8A

ABSTRACT We present a spectropolarimetric study of two weak-line T Tauri stars (wTTSs), TWA 6, and TWA 8A, as part of the MaTYSSE (Magnetic Topologies of Young Stars and the Survival of close-in giant Exoplanets) programme. Both stars display significant Zeeman signatures that we have modelled using Zeeman Doppler Imaging (ZDI). The magnetic field of TWA 6 is split equally between poloidal and toroidal components, with the largest fraction of energy in higher order modes, with a total unsigned flux of 840 G, and a poloidal component tilted 35° from the rotation axis. TWA 8A has a 70 per cent poloidal field, with most of the energy in higher order modes, with an unsigned flux of 1.4 kG (with a magnetic filling factor of 0.2), and a poloidal field tilted 20° from the rotation axis. Spectral fitting of the very strong field in TWA 8A (in individual lines, simultaneously for Stokes I and V) yielded a mean magnetic field strength of 5.9 ± 0.2 kG. The higher field strengths recovered from spectral fitting suggests that a significant proportion of magnetic energy lies in small-scale fields that are unresolved by ZDI. So far, wTTSs in MaTYSSE appear to show that the poloidal-field axisymmetry correlates with the magnetic field strength. Moreover, it appears that classical T Tauri stars (cTTSs) and wTTSs are mostly poloidal and axisymmetric when mostly convective and cooler than ∼4300 K, with hotter stars being less axisymmetric and poloidal, regardless of internal structure.

Aplikasi Algoritma Differential Evolution untuk Desain Optimal Load Frequency Control pada Pembangkit Listrik Tenaga Hibrid Angin dan Diesel

<p class="IndexTerms"><span lang="IN">Perubahan fluktuasi frekuensi sangat mempengaruhi kualitas daya pada sumber energi terbarukan turbin angin yang dihibrid dengan diesel. Sistem hibrid adalah jaringan terkontrol dari beberapa pembangkit tenaga energi terbaharukan seperti : turbin angin, sel surya, mikrohidro dan sebagainya. Ada beberapa permasalahan yang dapat meningkatkan osilasi frekuensi rendah, seperti tidak optimalnya setting gain dan kecilnya waktu konstan pada Automatic Voltage Regulator, Terlalu banyak jaringan transmisi yang panjang sehingga kemampuan lemah (weak line). Dalam penerapannya sistem wind-diesel dikontrol dengan kontroler PID, namun dalam penyetelan nilai gain dari PID masih dalam metode trial-error saja, sehingga sulit untuk mendapatkan nilai optimum dari PID. Dalam penelitian ini diterapkan desain kontrol dengan menggunakan Metode Cerdas dalam mencari nilai optimum Proporsional Intergral Derivatif (PID) untuk mengatur frekuensi beban dengan program Matlab/ Simulink. Pemodelan wind-diesel menggunakan fungsi transfer dari diagram turbin angin dan diesel. Respon sistem dengan Simulink/ Matlab dengan membandingkan dengan sistem tak terkontrol dan dengan metode PID-Trial Error, menunjukkan bahwa besar overshoot dan respon keadaan mantap (Settling Time) pada sistem terkontrol <em>Differential Evolution (DEVO) </em>menjadi berkurang dan lebih cepat.</span></p><p class="IndexTerms"><span lang="IN"><br /></span></p><p class="IndexTerms"><em><strong><span lang="IN">Abstract</span></strong></em></p><p class="IndexTerms"><em>Changes in load frequency greatly affect the power quality of renewable wind turbine energy sources with diesel. Hybrid power system is a network consisting of several renewable energy plants such as wind power, solar power, hydro power, and others. Some problems can increase the low frequency oscillations in the system, such as gain settings and small time constants of the non-optimal Automatic Voltage Regulator, long transmission lines so that their capabilities are weak (weak line). In some previous studies, the wind-diesel system is controlled by conventional Proportional, Integral, Derivative (PID) controllers, but the PID gain setting is still in the trial-error method, making it difficult to obtain optimal PID values. In this research, we proposed a method of optimizing PID parameters in wind-diesel by using intelligent method based on Differential Evolution (DEVO). The objective function of this research is to minimize Time Absolute Error (ITAE), so that the overshoot will be muted properly. Wind-diesel modeling uses the diagram of the transfer function of wind-diesel. From the analysis results obtained optimal PID parameters respectively, Kp = 79.9999, Ki = 59.9998, Kd = 9.9006. The smallest resulting overshoot is -7.932e-05 to 6.792e-10 pu where the smallest overshoot value is compared to other controller models. The frequency response obtained by optimal PID tuning will dampen the frequency oscillation due to load changes, indicated by small overshoot and fast settling time to steady state conditions.</em><em></em></p><p class="IndexTerms"><span lang="IN"><br /></span></p>