J. N. Yoon

Chungbuk National University, Chinsen, North Chungcheong, South Korea

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Publications (10)15.69 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: Table of R observations (HJD, magnitude) obtained at the 40cm telescope of the CBNU observatory, Korea. (1 data file).
    VizieR Online Data Catalog. 03/2009;
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    ABSTRACT: Aims: We study the variability of the nova-like cataclysmic variable TT Ari, on time-scales of between minutes and months. Methods: The observations in the filter R were obtained at the 40-cm telescope of the Chungbuk National University (Korea), 51 observational runs cover 226 h. The table of individual observations is available electronically. In our analysis, we applied several methods: periodogram, wavelet, and scalegram analysis. Results: TT Ari remained in a ``negative superhump'' state after its return from the ``positive superhump'' state, which lasted for 8 years. The ephemeris for 12 of the best pronounced minima is T_min=BJD 2 453 747.0700(47)+0.132322(53)E. The phases of minima may reach 0.2, which reflects the non-eclipse nature of these minima. The quasi-periodic oscillations (QPO) are present with a mean ``period'' of 21.6 min and mean semi-amplitude of 36 mmag. This value is consistent with the range 15-25 min reported for previous ``negative superhump'' states and does not support the hypothesis of secular decrease in the QPO period. Either the period, or the semi-amplitude show significant night-to-night variations. According to the position at the two-parameter diagrams (i.e. diagrams of pairs of parameters: time, mean brightness of the system, brightness of the source of QPO, amplitude, and timescale of the QPOs), the interval of observations was divided into 5 parts, showing different characteristics: 1) the ``pre-outburst'' stage; 2) the ``rise to outburst''; 3) ``top of the outbursts''; 4) ``post-outburst QPO'' state; and 5) ``slow brightening''. The the QPO source was significantly brighter during the 10-day outburst, than during the preceding interval. However, after the outburst, the large brightness of the QPO source still existed for about 30 days, producing the stage ``4''. The diagram for m_QPO(bar{m}) exhibits two groups in the brightness range 10fm6-10fm8, which correspond to larger and smaller amplitudes of the QPO. For the group ``5'' only, statistically significant correlations were found, for which, with increasing mean brightness, the period, amplitude, and brightness of the of QPO source also increase. The mean brightness at the ``negative superhump state'' varies within 10fm3-11fm2, so the system is brighter than at the ``positive superhump'' (11fm3), therefore the ``negative superhump'' phenomenon may be interpreted by a larger accretion rate. The system is an excellent laboratory for studying processes resulting in variations on timescales of between seconds and decades and needs further monitoring at various states of activity. Table 2 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/496/765
    Astronomy and Astrophysics 01/2009; 496:765-775. · 5.08 Impact Factor
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    ABSTRACT: We report the discovery of a multiple system with a quadruple-lined spectrum and a double eclipsing feature. Our photometric and high-resolution spectroscopic data show that V994 Herculis (V994 Her; ADS 11373 AB) is composed of two pairs of double-lined eclipsing binaries, which we designate as A and B. System A consists of a B8V+A0V binary with an orbital period of 2.083 264 d and system B of a A2V+A4V binary with 1.420 033 d. Our light curves show that both of them have a detached binary configuration. We derive masses and radii of four components (Aa, Ab, Ba and Bb) from the synthetic analyses of light curves and radial velocity curves. The masses of systems A and B are MAa= 2.83 ± 0.20 M⊙, MAb= 2.30 ± 0.16 M⊙, MBa= 1.87 ± 0.12 M⊙ and MBb= 1.86 ± 0.12 M⊙, with radii RAa= 2.15 ± 0.05 R⊙, RAb= 1.71 ± 0.04 R⊙, RBa= 1.59 ± 0.08 R⊙ and RBb= 1.50 ± 0.08 R⊙, respectively. These masses and radii are well consistent with the empirical relation for double-lined eclipsing binaries.
    Monthly Notices of the Royal Astronomical Society 08/2008; 389(4):1630 - 1636. · 5.52 Impact Factor
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    ABSTRACT: We study the variability of the cataclysmic variable DO Dra, on time-scales of between minutes and decades. The observations were obtained at the Korean 1m telescope at the Mt. Lemmon in 2006-2007, 14 observational runs cover 45h. Additionally, we have used 1509 patrol observations from the international AFOEV and VSOLJ databases. (1 data file).
    VizieR Online Data Catalog. 07/2008;
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    ABSTRACT: Aims: We study the variability of the cataclysmic variable DO Dra, on time-scales of between minutes and decades. Methods: The observations were obtained at the Korean 1 m telescope at the Mt. Lemmon in 2006-2007, 14 observational runs cover 45 h. The table of individual observations is available electronically. Additionally, we have used 1509 patrol observations from the international AFOEV and VSOLJ databases. Results: The characteristic decay time dt/dm = 0.902(3) days/mag was estimated from our 3 nights of CCD R observations, which cover the descending branch of the outburst in 2006. The range of the outburst cycle is from 311d to 422^d, contrary to a previous estimate of 870^d. The ``quiescent'' data show a photometric wave with a cycle ~ 303(15)^d. We analyzed the profile of the ``composite'' (or ``mean'') outburst. We discovered however, that a variety of different outburst heights and durations had occurred, contrary to theoretical predictions. The analysis of the historical data has shown a correlation between the decay time dt/dm and the outburst maximum brightness with a slope d(dt/dm)/dm = 0.37(9). With increasing maximum brightness, we find that the decay time also increases; this is in contrast to the model predictions, which indicate that outbursts should have a constant shape. This is interpreted as representing the presence of outburst-to-outburst variability of the magnetospheric radius. A presence of a number of missed weak narrow outbursts is predicted from this statistical relationship. We tabulate characteristics of the ``quasi-orbital'' variations, which indicate that an amplitude maximum occurs between quiescence and the outburst peak. The semi-amplitude of the spin variability does not exceeded 0.02 mag. A new type of variability is detected, during 3 subsequent nights in 2007: periodic (during one nightly run) oscillations with rapidly-decreasing frequency from 86 to 47 cycles/day and a semi-amplitude increasing from 0.06m to 0.10m, during a monotonic brightness increase from 14.27m to 14.13m. This phenomenon was observed only during an unusually prolonged event of ~1 mag brightening in 2007 (lasting till autumn), during which no (expected) outburst was detected. We refer to this behaviour as to the ``transient periodic oscillations'' (TPO). We attribute the frequency decrease to ``beat''-type of the variability, probably caused by irradiation of a cloud that is spiralling down to the white dwarf. Its frequency would then increase and coverge towards the spin frequency. To study this new and interesting phenomenon, new regular photometric and spectral (in a ``target of opportunity'' mode) observations are required. Table 3 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/486/855
    Astronomy and Astrophysics 01/2008; 486(3):855-865. · 5.08 Impact Factor
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    [show abstract] [hide abstract]
    ABSTRACT: Aims. We study the variability of the cataclysmic variable DO Dra, on time-scales of between minutes and decades.Methods. The observations were obtained at the Korean 1 m telescope at the Mt. Lemmon in 2006-2007, 14 observational runs cover 45 h. The table of individual observations is available electronically. Additionally, we have used 1509 patrol observations from the international AFOEV and VSOLJ databases.Results. The characteristic decay time d$t$/d$m$ = 0.902(3) days/mag was estimated from our 3 nights of CCD $R$ observations, which cover the descending branch of the outburst in 2006. The range of the outburst cycle is from $311^{\rm d}$ to $422^{\rm d},$ contrary to a previous estimate of 870$^{\rm d}$. The “quiescent” data show a photometric wave with a cycle ~ $303(15)^{\rm d}$. We analyzed the profile of the “composite” (or “mean”) outburst. We discovered however, that a variety of different outburst heights and durations had occurred, contrary to theoretical predictions. The analysis of the historical data has shown a correlation between the decay time d$t$/d$m$ and the outburst maximum brightness with a slope d(d$t$/d$m$)/d$m$ = 0.37(9). With increasing maximum brightness, we find that the decay time also increases; this is in contrast to the model predictions, which indicate that outbursts should have a constant shape. This is interpreted as representing the presence of outburst-to-outburst variability of the magnetospheric radius. A presence of a number of missed weak narrow outbursts is predicted from this statistical relationship. We tabulate characteristics of the “quasi-orbital” variations, which indicate that an amplitude maximum occurs between quiescence and the outburst peak. The semi-amplitude of the spin variability does not exceeded 0.02 mag. A new type of variability is detected, during 3 subsequent nights in 2007: periodic (during one nightly run) oscillations with rapidly-decreasing frequency from 86 to 47 cycles/day and a semi-amplitude increasing from $0\fm06$ to $0\fm10,$ during a monotonic brightness increase from $14\fm27$ to $14\fm13.$ This phenomenon was observed only during an unusually prolonged event of ~$1$ mag brightening in 2007 (lasting till autumn), during which no (expected) outburst was detected. We refer to this behaviour as to the “transient periodic oscillations” (TPO). 
We attribute the frequency decrease to “beat”-type of the variability, probably caused by irradiation of a cloud that is spiralling down to the white dwarf. Its frequency would then increase and coverge towards the spin frequency. To study this new and interesting phenomenon, new regular photometric and spectral (in a “target of opportunity” mode) observations are required.
    http://dx.doi.org/10.1051/0004-6361:20079056. 01/2008;
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    ABSTRACT: We present light curves of two short period binary systems, BL Eridani and GW Cephei. The BL Eridani was observed with VRI filters on 5 nights at Siding Spring Observatory (SSO) by the 50cm wide field robotic telescope equipped with a 2K CCD camera, which was developed by Korea Astronomy and Space Science Institute (KASI), and Yonsei University Observatory (YUO). Two nights were observed at SSO by automatic operation mode, and three nights were observed by automatic remote observation mode from KASI in Korea. With light curves covering full phase, five new times of minimum lights were determined. The magnitude difference of primary minimum and secondary minimum with V filter appeared as 0.25 magnitude, similar to the observation by Yamasaki (1998). The light curves were analyzed by mode 3 of WD binary code, and q-search method, which shows the best fit at q = 0.48 and i = 88.7 deg. Short period contact binary system GW Cephei was observed on 5 nights by the 61cm telescope with a 2K CCD camera at Sobaeksan Optical Astronomy Observatory (SOAO) of KASI with standard BVR filters. Full light curves were obtained, and five new times of minimum light were determined. We analyzed period variation of this system with 96 times of minimum light including our observations, which shows systematic variations that can be interpreted as the light time effect by a third body with a minimum mass of 0.17 solar mass. We assumed a cool spot on the surface of the massive secondary component responsible for the asymmetry of the light curves, and calculated photometric parameters of GW Cephei by WD code, which shows q = 2.59, and i = 85.4 deg with 16% fill-out factor.
    Proceedings of the International Astronomical Union 01/2007; 240.
  • J. N. Yoon, I. L. Andronov, S. M. Cha
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    ABSTRACT: TT Ari has been recently discovered to be at a relatively unstable phase by Andronov, Gazeas & Niarchos (2005, VSNET-Campaign 1630). Its prolonged "positive superhump" state from 1997 to 2004 has changed. In October-November, 2005, the star was fainter, and the superhumps are not pronounced. The dominating type of variability was quasi-periodic oscillations (QPO) with a time-scale of 10-35 minutes and amplitude 0.07 mag (B).
    The Astronomer's Telegram. 02/2006;
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    ABSTRACT: . We report on the presently continuing "outburst-like" event in TT Ari with an amplitude of 0.9m (R) and characteristic timescales dt/dm=15.87(11)days/mag (rise at JD 2453704-715) and dt/dm=4.95(7)days/mag (fall at HJD 2453721-724). TT Ari has been recently found to be out of the "positive superhump" state, which lasted from 1997 (Skillman et al. , 1998, ApJ, 503 L67; Andronov et al., 1999, AJ, 117, 574) to at least 2004 (Andronov, Ostrova & Burwitz 2004, VSNET-Campaign 1555, http://uavso.pochta.ru/TTAri), by Andronov, Gazeas & Niarchos (2005, VSNET-Campaign 1630, http://uavso.pochta.ru/ TTAri2005).
    The Astronomer's Telegram. 01/2006;
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    ABSTRACT: The nova-like variable TT Ari had shown a luminosity change with a drastic change of the light curve. This state is distincly different from both the bright "negative superhump" state and the "positive superhump" state known before.
    Information Bulletin on Variable Stars. 01/2005;