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Total 9   
9
  • Chang Won Lee
  • Shinyoung Kim
  • Tie Liu, Kee-Tae Kim, Mika Juvela, Ke Wang, Ken'ichi Tatematsu, James Di Francesco, Sheng-Yuan Liu, Yuefang Wu, Mark Thompson, Gary Fuller, David Eden, Di Li, I. Ristorcelli, Sung-ju Kang, Yuxin Lin, D. Johnstone, J. H. He, P. M. Koch, Patricio Sanhueza, Sheng-Li Qin, Q. Zhang, N. Hirano, Paul F. Goldsmith, Neal J. Evans II, Glenn J. White, Minho Choi, Chang Won Lee, L. V. Toth, Steve Mairs, H.-W. Yi, Mengyao Tang, Archana Soam, N. Peretto, Manash R. Samal, Michel Fich, Harriet Parsons, Jinghua Yuan, Chuan-Peng Zhang, Johanna Malinen, George J. Bendo, A. Rivera-Ingraham, Hong-Li Liu, Jan Wouterloot, Pak Shing Li, Lei Qian, Jonathan Rawlings, Mark G. Rawlings, Siyi Feng, Yuri Aikawa, S. Akhter, Dana Alina, Graham Bel, J.-P. Bernard, Andrew Blain, Rebeka B?gner, L. Bronfman, D.-Y. Byun, Scott Chapman, Huei-Ru Chen, M. Chen, Wen-Ping Chen, X. Chen, Xuepeng Chen, A. Chrysostomou, Giuliana Cosentino, M. R. Cunningham, K. Demyk, Emily Drabek-Maunder, Yasuo Doi, C. Eswaraiah, Edith Falgarone, O. Feher, Helen Fraser, Per Friberg, G. Garay, J. X. Ge, W. K. Gear, Jane Greaves, X. Guan, Lisa Harvey-Smith, Tetsuo HASEGAWA, J. Hatchell, Yuxin He, C. Henkel, T. Hirota, W. Holland, A. Hughes, E. Jarken, Tae-Geun Ji, Izaskun Jimenez-Serra, Miju Kang, Koji S. Kawabata, Gwanjeong Kim, Jungha Kim, Jongsoo Kim, Shinyoung Kim, B.-C. Koo, Woojin Kwon, Yi-Jehng Kuan, K. M. Lacaille, Shih-Ping Lai, C. F. Lee, J.-E. Lee, Y.-U. Lee1, Dalei Li, Hua-bai Li, N. Lo, John A. P. Lopez, Xing Lu, A-Ran Lyo, D. Mardones, A. Marston, P. McGehee, F. Meng, L. Montier, Julien Montillaud, T. Moore, O. Morata, Gerald H. Moriarty-Schieven, S. Ohashi, Soojong Pak, Geumsook Park, R. Paladini, Kate M Pattle, Gerardo Pech, V.-M. Pelkonen, K. Qiu, Zhi-Yuan Ren, John Richer, M. Saito, Takeshi Sakai, H. Shang, Hiroko Shinnaga, Dimitris Stamatellos, Y.-W. Tang, Alessio Traficante, Charlotte Vastel, S. Viti, Andrew Walsh, Bingru Wang, Hongchi Wang, Junzhi Wang, D. Ward-Thompson, Anthony Whitworth, Ye Xu, J. Yang, Yao-Lun Yang, Lixia Yuan, A. Zavagno, Guoyin Zhang, H.-W. Zhang, Chenlin Zhou, Jianjun Zhou, Lei Zhu, Pei Zuo, and Chao Zhang
  • 2018
  • The Astrophysical Journal Supplement Series, Volume 234, Number 2
The low dust temperatures (<14 K) of Planck Galactic cold clumps (PGCCs) make them ideal targets to probe the initial conditions and very early phase of star formation. “TOP-SCOPE” is a joint survey program targeting ∼2000 PGCCs in J = 1-0 transitions of CO isotopologues and ∼1000 PGCCs in 850 μm continuum emission. The objective of the “TOP-SCOPE” survey and the joint surveys (SMT 10 m, KVN 21 m, and NRO 45 m) is to statistically study the initial conditions occurring during star formation and the evolution of molecular clouds, across a wide range of environments. The observations, data analysis, and example science cases for these surveys are introduced with an exemplar source, PGCC G26.53+0.17 (G26), which is a filamentary infrared dark cloud (IRDC). The total mass, length, and mean line mass (M/L) of the G26 filament are ∼6200 M⊙, ∼12 pc, and ∼500 M⊙ pc^-1, respectively. Ten massive clumps, including eight starless ones, are found along the filament. The most massive clump as a whole may still be in global collapse, while its denser part seems to be undergoing expansion owing to outflow feedback. The fragmentation in the G26 filament from cloud scale to clump scale is in agreement with gravitational fragmentation of an isothermal, nonmagnetized, and turbulent supported cylinder. A bimodal behavior in dust emissivity spectral index (β) distribution is found in G26, suggesting grain growth along the filament. The G26 filament may be formed owing to large-scale compression flows evidenced by the temperature and velocity gradients across its natal cloud.
8
  • Sang-Sung Lee
  • Sincheol Kang
  • Jae-Young Kim, Sang-Sung Lee, Jeffrey A. Hodgson, Juan-Carlos Algaba, Guang-Yao Zhao, Motoki Kino, Do-Young Byun, Sincheol Kang
  • 2018
  • A&A Volume 610
We study the centimeter- to millimeter-wavelength synchrotron spectrum of the core of the radio galaxy M 87 at ≤0.8 mas ~ 110Rs spatial scales using four years of fully simultaneous, multi-frequency VLBI data obtained by the Korean VLBI Network (KVN). We find a core spectral index α of ≥-0.37 (S ∝ ν+α) between 22 and 129 GHz. By combining resolution-matched flux measurements from the Very Long Baseline Array (VLBA) at 15 GHz and taking the Event Horizon Telescope (EHT) 230 GHz core flux measurements in epochs 2009 and 2012 as lower limits, we find evidence of a nearly flat core spectrum across 15 and 129 GHz, which could naturally connect the 230 GHz VLBI core flux. The extremely flat spectrum is a strong indication that the jet base does not consist of a simple homogeneous plasma, but of inhomogeneous multi-energy components, with at least one component with the turn-over frequency ≥100 GHz. The spectral shape can be qualitatively explained if both the strongly (compact, optically thick at <100 GHz) and the relatively weakly magnetized (more extended, optically thin at <100 GHz) plasma components are colocated in the footprint of the relativistic jet.
7
  • Sang-Sung Lee
  • Sincheol Kang
  • Jeffrey A. Hodgson, Bindu Rani, Sang-Sung Lee, Juan-Carlos Algaba, Motoki Kino, Sascha Trippe, Jong-Ho Park, Guang-Yao Zhao, Do-Young Byun, Sincheol Kang, Jae-Young Kim, Jeong-Sook Kim, Soon-Wook Kim, Atsushi Miyazaki, Kiyoaki Wajima, Junghwan Oh, Dae-won Kim, Mark Gurwell
  • 2018
  • Monthly Notices of the Royal Astronomical Society, Volume 475, Issue 1
3C 84 (NGC 1275) is a well-studied misaligned sctive galactic nucleus (AGN), which hasbeen active in γ-rays since at least 2008. We have monitored the source at four wavelengths(14, 7, 3, and 2 mm) using the Korean VLBI network (KVN) since 2013 as part of theinterferometric monitoring of γ-ray bright AGN (iMOGABA) programme, and it exhibitsbright radio emission both near the central SMBH and in a slowly moving feature located tothe south known as C3. Other facilities have also detected these short-term variations abovea slowly rising trend at shorter wavelengths, such as in γ-ray and 1-mm total intensity lightcurves. We find that the variations in the γ -rays and 1-mm total intensity light curves arecorrelated, with the γ-ray leading and lagging the radio emission. Analysis of the 2-mmKVN data shows that both the γ-rays and 1-mm total intensity short-term variations are bettercorrelated with the SMBH region than C3, likely placing the short-term variations in C1. Weinterpret the emission as being due to the random alignment of spatially separated emissionregions. We place the slowly rising trend in C3, consistent with previous results. Spectralanalysis of the γ-ray data shows that the γ-ray flaring is inconsistent with blazar-like γ -rayemission. Additionally, we report that since mid-2015, a large mm-wave radio flare has beenoccurring in C3, with a large γ-ray flare coincident with the onset of this flare at all radiowavelengths.
6
  • Sang-Sung Lee
  • Sin-Cheol Kang
  • Juan-Carlos Algaba, Sang-Sung Lee, Dae-Won Kim, Bindu Rani, Jeffrey Hodgson, Motoki Kino, Sascha Trippe, Jong-Ho Park, Guang-Yao Zhao, Do-Young Byun, Mark Gurwell, Sin-Cheol Kang, Jae-Young Kim, Jeong-Sook Kim, Soon-Wook Kim, Benoit Lott, Atsushi Miyazaki, Kiyoaki Wajima
  • 2018
  • The Astrophysical Journal, Volume 852, Number 1
We present multi-frequency simultaneous VLBI radio observations of the flat spectrum radio quasar 1633+382 (4C 38.41) as part of the interferometric monitoring of γ-ray-bright active galactic nuclei (iMOGABA) program combined with additional observations in the radio, optical, X-rays, and γ-rays carried out during the period 2012 March-2015 August. The monitoring of this source reveals a significant long-lived increase in its activity for approximately two years in the radio bands, which correlates with a similar increase in all other bands from submillimeter to γ-rays. A significant correlation is also found between radio fluxes and simultaneous spectral indices during this period. The study of the discrete correlation function indicates time lags smaller than the uncertainties of ~40 days among both radio bands and high-energy bands, and a time lag of ~70 days, with γ-rays leading radio emission. We interpret this as showing that the high-energy and radio fluxes arise from different emitting regions, located at 1 ± 13 and 40 ± 13 pc from the central engine respectively.
5
  • Sang-Sung Lee
  • Sincheol Kang
  • Guang-Yao Zhao, Juan Carlos Algaba, Sang-Sung Lee, Taehyun Jung, Richard Dodson, Maria Rioja, Do-Young Byun, Jeffrey Hodgson, Sincheol Kang, Dae-Won Kim, Jae-Young Kim, Jeong-Sook Kim, Soon-Wook Kim, Motoki Kino, Atsushi Miyazaki, Jong-Ho Park,Sascha Trippe, Kiyoaki Wajima
  • 2018
  • The Astronomical Journal, Volume 155, Number 1
Atmospheric propagation effects at millimeter wavelengths can significantly alter the phases of radio signals and reduce the coherence time, putting tight constraints on high-frequency Very Long Baseline Interferometry (VLBI) observations. In previous works, it has been shown that non-dispersive (e.g., tropospheric) effects can be calibrated with the frequency phase transfer (FPT) technique. The coherence time can thus be significantly extended. Ionospheric effects, which can still be significant, remain however uncalibrated after FPT as well as the instrumental effects. In this work, we implement a further phase transfer between two FPT residuals (i.e., so-called FPT-square) to calibrate the ionospheric effects based on their frequency dependence. We show that after FPT-square, the coherence time at 3 mm can be further extended beyond 8 hr and the residual phase errors can be sufficiently canceled by applying the calibration of another source, which can have a large angular separation from the target (>20°) and significant temporal gaps. Calibrations for all-sky distributed sources with a few calibrators are also possible after FPT-square. One of the strengths and uniqueness of this calibration strategy is the suitability for high-frequency all-sky survey observations including very weak sources. We discuss the introduction of a pulse calibration system in the future to calibrate the remaining instrumental effects, allowing the possibility of imaging the source structure at high frequencies with FPT-square, where all phases are fully calibrated without involving any additional sources.
4
  • Junga Hwang
  • Hyangpyo Kim
  • Hyangpyo Kim, Junga Hwang, Jaeheung Park, Jacob Bortnik, Jaejin Lee
  • 2018
  • Journal of Geophysical Research: Space Physics, Volume 123, Issue 2, Pages: 1325-1336
It is well known that electromagnetic ion cyclotron (EMIC) waves play an important role in controlling particle dynamics inside the Earth's magnetosphere, especially in the outer radiation belt. In order to understand the results of wave-particle interactions due to EMIC waves, it is important to know how the waves are distributed and what features they have. In this paper, we present some statistical analyses on the spatial distribution of EMIC waves in the low Earth orbit by using Swarm satellites from December 2013 to June 2017 (~3.5 years) as a function of magnetic local time, magnetic latitude, and magnetic longitude. We also study the wave characteristics such as ellipticity, wave normal angle, peak frequency, and wave power using our automatic wave detection algorithm based on the method of Bortnik et al. (2007, https://doi.org/10.1029/2006JA011900). We also investigate the geomagnetic control of the EMIC waves by comparing with geomagnetic activity represented by Kp and Dst indices. We find that EMIC waves are detected with a peak occurrence rate at midlatitude including subauroral region, dawn sector (3-7 magnetic local time), and linear polarization dominated with an oblique propagating direction to the background magnetic field. In addition, our result shows that the waves have some relation with geomagnetic activity; that is, they occur preferably during the geomagnetic storm's late recovery phase at low Earth orbit.
3
  • Inwoo Han
  • Gwanghui Jeong
  • TaeYang Bang, ByeongCheol Lee, GwangHui Jeong, Inwoo Han, MyeongGu Park
  • 2018
  • JKAS.2018.51.1.17
Detecting exoplanets around giant stars sheds light on the later-stage evolution of planetary systems. We observed the M giant HD 18438 and the K giant HD 158996 as part of a Search for Exoplanets around Northern circumpolar Stars (SENS) and obtained 38 and 24 spectra from 2010 to 2017 using the high-resolution Bohyunsan Observatory Echelle Spectrograph (BOES) at the 1.8m telescope of Bohyunsan Optical Astronomy Observatory in Korea. We obtained precise RV measurements from the spectra and found long-period radial velocity (RV) variations with period 719.0 days for HD 18438 and 820.2 days for HD 158996. We checked the chromospheric activities using Ca ii H and H α lines, HIPPARCOS photometry and line bisectors to identify the origin of the observed RV variations. In the case of HD 18438, we conclude that the observed RV variations with period 719.0 days are likely to be caused by the pulsations because the periods of HIPPARCOS photometric and H α EW variations for HD 18438 are similar to that of RV variations in Lomb-Scargle periodogram, and there are no correlations between bisectors and RV measurements. In the case of HD 158996, on the other hand, we did not find any similarity in the respective periodograms nor any correlation between RV variations and line bisector variations. In addition, the probability that the real rotational period can be as longer than the RV period for HD 158996 is only about 4.3%. Thus we conclude that observed RV variations with a period of 820.2 days of HD 158996 are caused by a planetary companion, which has the minimum mass of 14.0 M Jup , the semi-major axis of 2.1 AU, and eccentricity of 0.13 assuming the stellar mass of 1.8 M ⊙ . HD 158996 is so far one of the brightest and largest stars to harbor an exoplanet candidate.
2
  • Inwoo Han
  • Gwanghui Jeong
  • Byeong-Cheol Lee, D. Gadelshin, Inwoo Han, Dong-Il Kang, Kang-Min Kim, G. Valyavin, G. Galazutdinov, Gwanghui Jeong, N. Beskrovnaya, T. Burlakova, A. Grauzhanina, N.R. Ikhsanov, A.F. Kholtygin, A. Valeev, V. Bychkov, Myeong-Gu Park
  • 2018
  • Monthly Notices of the Royal Astronomical Society: Letters, Volume 473, Issue 1, Pages L41?L45
We present high-resolution spectropolarimetric observations of the spectroscopic binary χ Dra. Spectral lines in the spectrum of the main component χ Dra A show variable Zeeman displacement, which confirms earlier suggestions about the presence of a weak magnetic field on the surface of this star. Within about 2 yr of time base of our observations, the longitudinal component BL of the magnetic field exhibits variation from -11.5 ± 2.5 to +11.1 ± 2.1 G with a period of about 23?d. Considering the rotational velocity of χ Dra A in the literature and that newly measured in this work, this variability may be explained by the stellar rotation under the assumption that the magnetic field is globally stable. Our new measurements of the radial velocities (RV) in high-resolution I-spectra of χ Dra A refined the orbital parameters and reveal persistent deviations of RVs from the orbital curve. We suspect that these deviations may be due to the influence of local magnetically generated spots, pulsations, or a Jupiter-size planet orbiting the system.
1
  • Inwoo Han
  • Gwanghui Jeong
  • G. Jeong, B.-C. Lee, I. Han, M. Omiya, H. Izumiura, B. Sato, H. Harakawa, E. Kambe, D. Mkrtchian
  • 2018
  • A&A Volume 610
The purpose of this paper is to detect and investigate the nature of long-term radial velocity (RV) variations of K-type giants and to confirm planetary companions around the stars. We have conducted two planet search programs by precise RV measurement using the 1.8 m telescope at Bohyunsan Optical Astronomy Observatory (BOAO) and the 1.88 m telescope at Okayama Astrophysical Observatory (OAO). The BOAO program searches for planets around 55 early K giants. The OAO program is looking for 190 G-K type giants. In this paper, we report the detection of long period RV variations of three K giant stars, HD 40956, HD111591, and HD113996. We investigated the cause of the observed RV variations and conclude the substellar companions are most likely the cause of the RV variations. The orbital analyses yield P = 578.6 +/- 3.3 d, m sin i = 2.7 +/- 0.6 M_J, a = 1.4 +/- 0.1 AU for HD 40956; P = 1056.4 +/- 14.3 d, m sin i = 4.4 +/- 0.4 M_J, a = 2.5 +/- 0.1 AU for HD 111591; P = 610.2 +/- 3.8 d, m sin i = 6.3 +/- 1.0 M_J, a = 1.6 +/- 0.1 AU for HD113996.
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