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Total 9   
  • Chang Won Lee
  • Gwanjeong Kim
  • Gwanjeong Kim; Chang Won Lee; Gopinathan Maheswar; Mi-Ryang Kim; Archana Soam; Masao Saito; Kazuhiro Kiyokane; Sungeun Kim
  • 2019-01-24
  • The Astropysical Journal Supplement Series
We present the results of a systematic search for molecular outflows in 68 Very Low Luminosity Objects (VeLLOs) from single-dish observations in CO isotopologues, finding 16 VeLLOs that show clear outflow signatures in the CO maps. Together with an additional three VeLLOs from the literature, we analyzed the outflow properties for these 19 VeLLOs, identifying 15 VeLLOs as proto-brown-dwarf (proto-BD) candidates and 4 VeLLOs as likely faint protostar candidates. The proto-BD candidates are found to have a mass accretion rate (~10−8–10−7 M ⊙ yr−1) lower than that of the protostar candidates (gsim10−6 M ⊙ yr−1). Their accretion luminosities are similar to or smaller than their internal luminosities, implying that many proto-BD candidates might have had either small accretion activity in a quiescent manner throughout their lifetimes, or be currently exhibiting relatively higher (or episodic) mass accretion than in the past. Outflows of many proto-BDs show strong trends of being less active if they are fainter or have less massive envelopes. The outflow forces and internal luminosities for more than half of the proto-BD candidates seem to follow the evolutionary track of a protostar with an initial envelope mass of ~0.08 M ⊙, indicating that some BDs may form in less massive dense cores in a similar way to normal stars. But, because there also exists a significant fraction (about 40%) of proto-BDs with a much weaker outflow force than expected from the relations for protostars, we should not rule out the possibility of other formation mechanism for BDs.
  • Jeong Yoo Hong
  • Syeun Kim
  • Syeun Kim; Eun-Jung Choi; Sunki Cho; Jeong Yoo Hong
  • 2019-02-14
  • Space Policy
This article reviews South Korea's response model for space object impacts, re-entries, and collisions. These are countermeasure models that are prepared for natural or artificial space objects falling in South Korea or colliding in space, and they employ the Ministry of Science and ICT (MSIT) to perform 4 stages that include prevention, preparation, response, and restoration. However, the article suggests that this process is inefficient given that the MSIT specializes in research and development but is expected to perform disaster response tasks in the event of natural or manmade space object impacts and re-entries. The research presents 2 alternative models to the current ones: the first model specifies that the MSIT and the Ministry of the Interior and Safety (MOIS) work together, and the second model designates the MOIS as the only agency responsible for the disaster response. These alternative models would more effectively handle disaster management than would the current models.
  • Jungha Hwang
  • Hyangpyo Kim
  • Peter H. Yoon; Jungha Hwang; Hyangpyo Kim; Jungjoon Seough
  • 2019-04-14
  • JGR Space Physics
Quasi thermal fluctuations in the Langmuir/upper?hybrid frequency range are pervasively observed in space plasmas including the radiation belt and the ring current region of inner magnetosphere as well as the solar wind. The quasi thermal noise spectroscopy may be employed in order to determine the electron density and temperature as well as to diagnose the properties of energetic electrons when direct measurements are not available. However, when employing the technique, one must carefully take the spacecraft orientation into account. The present paper takes the upper?hybrid and multiple harmonic—or (n + 1/2)fce—emissions measured by the Van Allen Probes as an example in order to illustrate how the spacecraft antenna geometrical factor can be incorporated into the theoretical interpretation. This method can in principle be applied to other spacecraft, including the Parker Solar Probe.
  • Lee, Joon Hyeop
  • Pak, Mina
  • Lee, Joon Hyeop; Pak, Mina; Song, Hyunmi; Lee, Hye-Ran; Kim, Suk; Jeong, Hyunjin
  • 2019-10-18
  • ApJ, 884, 104
In our recent report, observational evidence supports that the rotational direction of a galaxy tends to be coherent with the average motion of its nearby neighbors within 1 Mpc. We extend the investigation to neighbors at farther distances in order to examine if such dynamical coherence is found even in large scales. The Calar Alto Legacy Integral Field Area (CALIFA) survey data and the NASA-Sloan Atlas (NSA) catalog are used. From the composite map of velocity distribution of “neighbor” galaxies within 15 Mpc from the CALIFA galaxies, the composite radial profiles of the luminosity-weighted mean velocity of neighbors are derived. These profiles show unexpectedly strong evidence of the dynamical coherence between the rotation of the CALIFA galaxies and the average line-of- sight motion of their neighbors within several-megaparsec distances. Such a signal is particularly strong when the neighbors are limited to red ones: the luminosity-weighted mean velocity at 1?
  • Hyung-Chul Lim
  • Simon Kim
  • Simon Kim, Hyung-Chul Lim, Byoungsoo Kim
  • 2019-06
  • Journal of Astronomy and Space Sciences 36.2, 97-103
An automated signal-acquisition method for the NASA’s space geodesy satellite laser ranging (SGSLR) system is described as a selection of two system parameters with specified probabilities. These parameters are the correlation parameter: the minimum received pulse number for a signal-acquisition and the frame time: the minimum time for the correlation parameter. The probabilities specified are the signal-detection and false-acquisition probabilities to distinguish signals from background noise. The steps of parameter selection are finding the minimum set of values by fitting a curve and performing a graphdomain approximation. However, this selection method is inefficient, not only because of repetition of the entire process if any performance values change, such as the signal and noise count rate, but also because this method is dependent upon system specifications and environmental conditions. Moreover, computation is complicated and graph-domain approximation can introduce inaccuracy. In this study, a new method is proposed to select the parameters via a conditional equation derived from characteristics of the signal-detection and false-acquisition probabilities. The results show that this method yields better efficiency and robustness against changing performance values with simplicity and accuracy and can be easily applied to other satellite laser ranging (SLR) systems.
  • Joon Hyeop Lee
  • Mina Pak, Hye-Ran Lee
  • Mina Pak, Joon Hyeop Lee, Hyunjin Jeong, Suk Kim, Rory Smith, Hye-Ran Lee
  • 2019-08-06
  • ApJ, 880, 149
We investigate the stellar population properties of passive spiral galaxies in the Calar Alto Legacy Integral Field Area survey. Nine spiral galaxies that have (NUV−r > 5 (NUV is near-UV filter) and no/weak nebular emission lines in their spectra are selected as passive spirals. Our passive spirals lie in the redshift range of 0.001 < z < 0.021 and have a stellar mass range of 10.2 < log(M_star/M_solar) < 10.8. They clearly lie in the domain of early-type galaxies in the Wide-field Infrared Survey Explorer infrared color–color diagram. We analyze the stellar populations out to two effective radii, using the best-fitting model to the measured absorption line-strength indices in the Lick Observatory Image Dissector Scanner system. We find that stellar populations of the passive spirals span a wide range, even in their centers, and hardly show any common trend among themselves either. We compare the passive spirals with Lenticular (S0-type) galaxies (S0s) selected in the same mass range. S0s cover a wide range in age, metallicity, and [α/Fe], and stellar populations of the passive spirals are encompassed in the spread of the S0 properties. However, the distribution of passive spirals are skewed toward higher values of metallicity, lower [α/Fe], and younger ages at all radii. These results show that passive spirals are possibly related to S0s in their stellar populations. We infer that the diversity in the stellar populations of S0s may result from different evolutionary pathways of S0 formation, and passive spirals may be one of the possible channels.
  • Joon Hyeop Lee
  • Mina Pak, Hye-Ran Lee
  • Joon Hyeop Lee, Mina Pak, Hye-Ran Lee, Hyunmi Song
  • 2019-02-10
  • ApJ, 872, 78
We present our discovery of observational evidence for the coherence between galaxy rotation and the average line-of-sight motion of neighbors. We use the Calar Alto Legacy Integral Field Area (CALIFA) survey data analyzed with the Python CALIFA STARLIGHT Synthesis Organizer platform, and the NASA-Sloan Atlas catalog. After estimating the projected angular momentum vectors of 445 CALIFA galaxies, we build composite maps of their neighbor galaxies on the parameter space of line-of-sight velocity versus projected distance. The composite radial profiles of the luminosity-weighted mean velocity of neighbors show striking evidence for dynamical coherence between the rotational direction of the CALIFA galaxies and the average moving direction of their neighbor galaxies. The signal of such dynamical coherence is significant for the neighbors within 800 kpc from the CALIFA galaxies, for which the luminosity-weighted mean velocity is as large as 61.7 ± 17.6 km s-1 (3.5σ significance to the bootstrap uncertainty) when the angular momentum is measured at R e < R ≤ 2R e of each CALIFA galaxy. In the comparison of the subsamples, we find that faint, blue, or kinematically misaligned galaxies show stronger coherence with neighbor motions than bright, red, or kinematically well-aligned galaxies do. Our results indicate that (1) the rotation of a galaxy (particularly at its outskirt) is significantly influenced by interactions with its neighbors up to 800 kpc, (2) the coherence is particularly strong between faint galaxies and bright neighbors, and (3) galaxy interactions often cause internal kinematic misalignment or possibly even kinematically distinct cores.
  • Taehyun Jung
  • Il je Cho
  • S. Issaoun, M. D. Johnson, L. Blackburn, C. D. Brinkerink, M. Moscibrodzka, A. Chael, C. Goddi, I. Marti-Vidal, J. Wagner, S. S. Doeleman, H. Falcke, T. P. Krichbaum, K. Akiyama, U. Bach, K. L. Bouman, G. C. Bower, A. Broderick, I. Cho, G. Crew, J. Dexter, V. Fish, R. Gold, J. L. Gomez, K. Hada, A. Hernandez-Gomez, M. Janßen, M. Kino, M. Karmer, L. Loinard, R.-S. Lu, S. Markoff, D. P. Marrone, L. D. Matthews, J. M. Moran, C. Muller, E. Ros, H. Rottmann, S. Sanchez, R. P. J. Tilanus, P. de Vicente, M. Wielgus, J. A. Zensus, G.-Y. Zhao
  • 2019
  • ApJ
The Galactic center supermassive black hole Sagittarius A* (Sgr A*) is one of the most promising targets to study the dynamics of black hole accretion and outflow via direct imaging with very long baseline interferometry (VLBI). At 3.5 mm (86 GHz), the emission from Sgr A* is resolvable with the Global Millimeter VLBI Array (GMVA). We present the first observations of Sgr A* with the phased Atacama Large Millimeter/submillimeter Array (ALMA) joining the GMVA. Our observations achieve an angular resolution of ∼87 μas, improving upon previous experiments by a factor of two. We reconstruct a first image of the unscattered source structure of Sgr A* at 3.5 mm, mitigating the effects of interstellar scattering. The unscattered source has a major-axis size of 120 ± 34 μas (12 ± 3.4 Schwarzschild radii) and a symmetrical morphology (axial ratio of {1.2}-0.2+0.3), which is further supported by closure phases consistent with zero within 3σ. We show that multiple disk-dominated models of Sgr A* match our observational constraints, while the two jet-dominated models considered are constrained to small viewing angles. Our long-baseline detections to ALMA also provide new constraints on the scattering of Sgr A*, and we show that refractive scattering effects are likely to be weak for images of Sgr A* at 1.3 mm with the Event Horizon Telescope. Our results provide the most stringent constraints to date for the intrinsic morphology and refractive scattering of Sgr A*, demonstrating the exceptional contribution of ALMA to millimeter VLBI.
  • Jaewon Yoo
  • M. T. Soumagnac, C.G. Sabiu, R. Barkana, J. Yoo
  • 2019
Baryon Acoustic Oscillations (BAOs) in the early Universe are predicted to leave an as yet undetected signature on the relative clustering of total mass versus luminous matter. This signature, a modulation of the relative large-scale clustering of baryons and dark matter, offers a new angle to compare the large scale distribution of light versus mass. A detection of this effect would provide an important confirmation of the standard cosmological paradigm and constrain alternatives to dark matter as well as non-standard fluctuations such as Compensated Isocurvature Perturbations (CIPs). The first attempt to measure this effect in the SDSS-III BOSS Data Release 10 CMASS sample remained inconclusive but allowed to develop a method, which we detail here and use to conduct the second observational search. When using the same model as in our previous study and including CIPs in the model, the DR12 data are consistent with a null-detection, a result in tension with the strong evidence previously measured with the DR10 data. This tension remains when we use a more realistic model taking into account our knowledge of the survey flux limit, as the data then privilege a zero effect. In the absence of CIPs, we obtain a null detection consistent with both the absence of the effect and the amplitude predicted in previous theoretical studies. This shows the necessity of more accurate data in order to prove or disprove the theoretical predictions.
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