Network


Latest external collaboration on country level. Dive into details by clicking on the dots.

Hotspot


Dive into the research topics where Youngjoo Yun is active.

Publication


Featured researches published by Youngjoo Yun.


Journal of the Korean Astronomical Society | 2014

FIRST DETECTION OF 22 GHZ H2O MASERS IN TX CAMELOPARDALIS

Se-Hyung Cho; Jaeheon Kim; Youngjoo Yun

Simultaneous time monitoring observations of H₂O 6 16 -5 23 , SiO J = 1-0, 2-1, 3-2, and 29 SiO v = 0, J = 1-0 lines are carried out in the direction of the Mira variable star TX Cam with the Korean VLBI Network single dish radio telescopes. For the first time, the H₂O maser emission from TX Cam is detected near the stellar velocity at five epochs from April 10, 2013 (o = 3.13) to June 4, 2014 (o = 3.89) including minimum optical phases. The intensities of H₂O masers are very weak compared to SiO masers. The variation of peak antenna temperature ratios among SiO v = 1, J = 1-0, J = 2-1, and J = 3-2 masers is investigated according to their phases. The shift of peak velocities of H₂O and SiO masers with respect to the stellar velocity is also investigated according to observed optical phases. The H₂O maser emission occurs around the stellar velocity during our monitoring interval. On the other hand, the peak velocities of SiO masers show a spread compared to the stellar velocity. The peak velocities of SiO J = 2-1, and J = 3-2 masers show a smaller spread with respect to the stellar velocity than those of SiO J = 1-0 masers. These simultaneous observations of multi-frequencies will provide a good constraint for maser pumping models and a good probe for investigating the stellar atmosphere and envelope according to their different excitation conditions.


Journal of the Korean Astronomical Society | 2016

THE AUTOMATIC CALIBRATION OF KOREAN VLBI NETWORK DATA

Jeffrey A. Hodgson; Sang-Sung Lee; G. Zhao; Juan-Carlos Algaba; Youngjoo Yun; Taehyun Jung; Do-Young Byun

The calibration of Very Long Baseline Interferometry (VLBI) data has long been a time consuming process. The Korean VLBI Network (KVN) is a simple array consisting of three identical antennas. Because four frequencies are observed simultaneously, phase solutions can be transferred from lower frequencies to higher frequencies in order to improve phase coherence and hence sensitivity at higher frequencies. Due to the homogeneous nature of the array, the KVN is also well suited for automatic calibration. In this paper we describe the automatic calibration of single-polarisation KVN data using the KVN Pipeline and comparing the results against VLBI data that has been manually reduced. We find that the pipelined data using phase transfer produces better results than a manually reduced dataset not using the phase transfer. Additionally we compared the pipeline results with a manually reduced phase-transferred dataset and found the results to be identical.


The Astrophysical Journal | 2018

Simultaneous VLBI Astrometry of H2O and SiO Masers toward the Semiregular Variable R Crateris

Dong-Jin Kim; Se-Hyung Cho; Youngjoo Yun; Yoon Kyung Choi; Dong-Hwan Yoon; Jaeheon Kim; Richard Dodson; Maria Rioja; Haneul Yang; Suk-Jin Yoon

We obtained, for the first time, astrometrically registered maps of the 22.2 GHz H2O and 42.8, 43.1, and 86.2 GHz SiO maser emission toward the semiregular b-type variable (SRb) R Crateris, at three epochs (2015 May 21, and 2016 January 7 and 26) using the Korean Very-long-baseline Interferometry Network. The SiO masers show a ring-like spatial structure, while the H2O maser shows a very asymmetric one-side outflow structure, which is located at the southern part of the ring-like SiO maser feature. We also found that the 86.2 GHz SiO maser spots are distributed in an inner region, compared to those of the 43.1 GHz SiO maser, which is different from all previously known distributions of the 86.2 GHz SiO masers in variable stars. The different distribution of the 86.2 GHz SiO maser seems to be related to the complex dynamics caused by the overtone pulsation mode of the SRb R Crateris. Furthermore, we estimated the position of the central star based on the ring fitting of the SiO masers, which is essential for interpreting the morphology and kinematics of a circumstellar envelope. The estimated stellar coordinate corresponds well to the position measured by Gaia.


Nature Communications | 2018

Astrometrically registered maps of H2O and SiO masers toward VX Sagittarii

Dong-Hwan Yoon; Se-Hyung Cho; Youngjoo Yun; Yoon Kyung Choi; Richard Dodson; Maria Rioja; Jaeheon Kim; Hiroshi Imai; Dong-Jin Kim; Haneul Yang; Do-Young Byun

The supergiant VX Sagittarii is a strong emitter of both H2O and SiO masers. However, previous VLBI observations have been performed separately, which makes it difficult to spatially trace the outward transfer of the material consecutively. Here we present the astrometrically registered, simultaneous maps of 22.2 GHz H2O and 43.1/42.8/86.2/129.3 GHz SiO masers toward VX Sagittarii. The H2O masers detected above the dust-forming layers have an asymmetric distribution. The multi-transition SiO masers are nearly circular ring, suggesting spherically symmetric wind within a few stellar radii. These results provide the clear evidence that the asymmetry in the outflow is enhanced after the smaller molecular gas clump transform into the inhomogeneous dust layers. The 129.3 GHz maser arises from the outermost region compared to that of 43.1/42.8/86.2 GHz SiO masers. The ring size of the 129.3 GHz maser is maximized around the optical maximum, suggesting that radiative pumping is dominant.The red supergiant VX Sagittarii is a strong emitter of H2O and SiO masers, however its mass loss dynamics are still poorly understood. Here, the authors present astrometrically registered, simultaneous maps of SiO and H2O maser regions, and provide observational evidence for a break in spherical symmetry between the SiO and H2O maser zone.


11th Pacific Rim Conference on Stellar Astrophysics: Physics and Chemistry of the Late Stages of Stellar Evolution | 2016

KaVA ESTEMA project

Miyako Oyadomari; Hiroshi Imai; Se-Hyung Cho; Yoshiharu Asaki; Yoon-Kyong Choi; Jaeheon Kim; Youngjoo Yun; Naoko Matsumoto; Cheulhong Min; Tomoaki Oyama; Sung-Chul Yoon; Dong-Hwan Yoon; Dong-Jin Kim; Richard Dodson; Maria Rioja; Ross A. Burns; Gabor Orosz; Akiharu Nakagawa; James Chibueze O; Jun-ichi Nakashima; A. M. Sobolev

The ESTEMA (Expanded Study on Stellar Masers) project is one of three Large Programs of the KaVA (the combined array of the Korean VLBI Network and Japanese VLBI Exploration of Radio Astrometry), and conducted in 2015-2016. It aims to publish a database of the largest sample of VLBI images of circumstellar water (H2O) and silicon-monoxide (SiO) maser sources towards circumstellar envelopes (CSEs) of 80 evolved stars in late AGB to early post-AGB phase. Here we present the specifications of the ESTEMA observations and the planned scientific goals in order to share the basic information of the ESTEMA with astronomical community and encourage future collaborations with the ESTEMA and future follow-up observations for the targeted stars.


Journal of the Korean Astronomical Society | 2015

KVN MONITORING OBSERVATIONS TOWARD THE RECENT OUTBURST SYMBIOTIC STAR V407 CYGNI

Se-Hyung Cho; Jaeheon Kim; Youngjoo Yun

Simultaneous time monitoring observations of H₂O and SiO maser lines were performed toward the D-type symbiotic binary system V407 Cyg with the Korean VLBI Network single dish radio telescope. These monitoring observations were carried out from March 2, 2010 (optical phase ∅ = 0.0), 8 days before the nova outburst on March 10, 2010 to June 5, 2014 ( = 2.13). Eight days before the nova outburst, we detected the SiO v = 1, 2, J = 1–0 maser lines which exhibited values of 0.51 K (~ 6.70 Jy) and 0.71 K ( 9.30 Jy), respectively, while after the outburst we could not detect them on April 2 (∅ = 0.04), May 5 ( = 0.09), May 8 (∅ = 0.09), or on June 5, 2010 ( ∅ = 0.13) within the upper limits of our KVN observations. After restarting our monitoring observations, we detected SiO v = 2, J = 1–0 masers starting on October 20, 2011 (∅ = 0.83) and detected SiO v = 1, J = 1–0 masers starting on December 22, 2011 (∅ = 0.92). These results provide clear evidence of the interaction between the shock from the nova outburst and the SiO maser regions of the Mira envelope. The peak emission of SiO v = 1, 2, J = 1–0 masers always occurred at blueshifted velocities with respect to the stellar velocity except for that of SiO v = 1 at one epoch. These phenomena may be related to the redistribution of SiO maser regions after the outburst. The peak velocity variations of SiO masers associated with stellar pulsation phases show an increasing blueshifted trend during our monitoring interval after the outburst.


Journal of the Korean Astronomical Society | 2014

DEVELOPMENT OF 2.8-GHZ SOLAR FLUX RECEIVERS

Youngjoo Yun; Yong-Sun Park; Chang-Hee Kim; Bangwon Lee; Jung-Hoon Kim; Saeho Yoo; Chul-Hwan Lee; Jinwook Han; Young Yun Kim

We report the development of solar flux receivers operating at 2.8 GHz to monitor solar radio activity. Radio waves from the sun are amplified, filtered, and then transmitted to a power meter sensor without frequency down-conversion. To measure solar flux, a calibration scheme is designed with a noise source, an ambient load, and a hot load at 100 ℃. The receiver is attached to a 1.8 m parabolic antenna in Icheon, owned by National Radio Research Agency, and observation is being conducted during day time on a daily basis. We compare the solar fluxes measured for last seven months with solar fluxes obtained by DRAO in Penticton, Canada, and by the Hiraiso solar observatory in Japan, and finally establish equations to convert observed flux to the so-called Penticton flux with an accuracy better than 3.2 sfu.


The Astrophysical Journal | 2016

SiO MASERS AROUND WX PSC MAPPED with the KVN and VERA ARRAY (KaVA)

Youngjoo Yun; Se-Hyung Cho; Hiroshi Imai; Jaeheon Kim; Yoshiharu Asaki; James O. Chibueze; Yoon Kyung Choi; Richard Dodson; Dong-Jin Kim; Kozue Kusuno; Naoko Matsumoto; Cheulhong Min; Miyako Oyadomari; Maria Rioja; Dong-Hwan Yoon; Do-Young Byun; Hyunsoo Chung; Moon-Hee Chung; Yoshiaki Hagiwara; Myoung-Hee Han; Seog-Tae Han; Tomoya Hirota; Mareki Honma; Jung-Wook Hwang; Do-Heung Je; Takaaki Jike; Dong-Kyu Jung; Taehyun Jung; Ji-hyun Kang; Jiman Kang


The Astrophysical Journal | 2016

TWO NEW SiO MASER SOURCES IN HIGH-MASS STAR-FORMING REGIONS

Se-Hyung Cho; Youngjoo Yun; Jaeheon Kim; Tie Liu; Kee-Tae Kim; Minho Choi


Proceedings of the International Astronomical Union | 2017

A study on evolved stars by simultaneous observations of H2O and SiO masers using KVN

Se-Hyung Cho; Youngjoo Yun; Jaeheon Kim; Dong-Hwan Yoon; Dong-Jin Kim; Yoon Kyung Choi; Richard Dodson; Maria Rioja; Hiroshi Imai

Collaboration


Dive into the Youngjoo Yun's collaboration.

Top Co-Authors

Avatar

Se-Hyung Cho

Korea Astronomy and Space Science Institute

View shared research outputs
Top Co-Authors

Avatar

Jaeheon Kim

Korea Astronomy and Space Science Institute

View shared research outputs
Top Co-Authors

Avatar

Dong-Hwan Yoon

Seoul National University

View shared research outputs
Top Co-Authors

Avatar
Top Co-Authors

Avatar
Top Co-Authors

Avatar

Do-Young Byun

Korea Astronomy and Space Science Institute

View shared research outputs
Top Co-Authors

Avatar

Maria Rioja

University of Western Australia

View shared research outputs
Top Co-Authors

Avatar

Richard Dodson

University of Western Australia

View shared research outputs
Top Co-Authors

Avatar
Top Co-Authors

Avatar
Researchain Logo
Decentralizing Knowledge