한국천문학회지

221

LONG-SLIT SPECTROSCOPY OF PARSEC-SCALE JETS FROM DG TAURI

Oh, Heeyoung
Journal of the Korean astronomical society = 천문학회지
48, n.2
pp.113-123
2015
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We present observational results from optical long-slit spectroscopy of parsec-scale jets of DG Tau. From HH 158 and HH 702, the optical emission lines of Hα, [O i] λλ6300, 6363, [N ii] λλ6548, 6584, and [S ii] λλ6716, 6731 are obtained. The kinematics and physical properties (i.e., electron density, electron temperature, ionization fraction, and mass-loss rate) are investigated along the blueshifted jet up to 650′′ distance from the source. For HH 158, the radial velocity ranges from −50 to −250 km s−1. The proper motion of the knots is 0.′′196 − 0.′′272 yr−1. The electron density is ∼104 cm−3 close to the star, and decreases to ∼102 cm−3 at 14′′ away from the star. Ionization fraction indicates that the gas is almost neutral in the vicinity of the source. It increases up to over 0.4 along the distance. HH 702 is located at 650′′ from the source. It shows ∼ −80 km s−1 in the radial velocity. Its line ratios are similar to those at knot C of HH 158. The mass-loss rate is estimated to be about ∼ 10−7 M⊙ yr−1, which is similar to values obtained from previous studies.

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222

MEASURING THE CORE SHIFT EFFECT IN AGN JETS WITH THE EXTENDED KOREAN VLBI NETWORK

JUNG, TAEHYUN
Journal of the Korean astronomical society = 천문학회지
48, n.5
pp.277-284
2015
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We present our efforts for extending the simultaneous multi-frequency receiver system of the Korean Very Long Baseline Interferometry (VLBI) Network (KVN) to global baselines in order to measure the frequency-dependent position shifts in Active Galactic Nuclei (AGN) jets, the so called core shift effect, with an unprecedented accuracy (a few micro-arcseconds). Millimeter VLBI observations with simultaneous multi-frequency receiver systems, like those of the KVN, enable us to explore the innermost regions of AGN and high precision astrometry. Such a system is capable of locating the frequency dependent opacity changes accurately. We have conducted the feasibility test-observations with the interested partners by implementing the KVN-compatible systems. Here we describe the science case for measuring the core shift effect in the AGN jet and report progress and future plans on extending the simultaneous multi-frequency system to global baselines.

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223

POLARIZATION OF LYMAN α EMERGENT FROM A THICK SLAB OF NEUTRAL HYDROGEN

AHN, SANG-HTEON
Journal of the Korean astronomical society = 천문학회지
48, n.3
pp.195-202
2015
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Star forming galaxies found in the early universe exhibit asymmetric Lyα emission line that results from multiple scattering in a neutral thick medium surrounding the Lyα emission source. It is expected that emergent Lyα will be significantly polarized through a large number of resonance scattering events followed by a number of successive wing scatterings. In this study we adopt a Monte Carlo method to calculate the polarization of Lyα transferred in a very thick static slab of HI. Resonantly scattered radiation associated with transitions between <graphic></graphic> is only weakly polarized and therefore linear polarization of the emergent Lyα is mainly dependent on the number of off-resonant wing scattering events. The number of wing scattering events just before escape from the slab is determined by the product of the Doppler parameter a and the line center optical depth τ0, which, in turn, determines the behavior of the linear polarization of Lyα. This result is analogous to the study of polarized radiative transfer of Thomson scattered photons in an electron slab, where the emergent photons are polarized in the direction perpendicular to the slab when the scattering optical depth is small and polarized in the parallel direction when the slab is optically thick. Our simulated spectropolarimetry of Lyα shows that the line center is negligibly polarized, the near wing parts polarized in the direction parallel to the slab and the far wing parts are polarized in the direction perpendicular to the slab. We emphasize that the flip of polarization direction in the wing parts of Lyα naturally reflects the diffusive nature of the Lyα transfer process in thick neutral media.

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224

INTERFEROMETRIC MONITORING OF GAMMA–RAY BRIGHT ACTIVE GALACTIC NUCLEI II: FREQUENCY PHASE TRANSFER

ALGABA, JUAN-CARLOS
Journal of the Korean astronomical society = 천문학회지
48, n.5
pp.237-255
2015
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The Interferometric Monitoring of Gamma–ray Bright Active galactic nuclei (iMOGABA) program provides not only simultaneous multifrequency observations of bright gamma–ray detected active galactic nuclei (AGN), but also covers the highest Very Large Baseline Interferometry (VLBI) frequencies ever being systematically monitored, up to 129 GHz. However, observation and imaging of weak sources at the highest observed frequencies is very challenging. In the second paper in this series, we evaluate the viability of the frequency phase transfer technique to iMOGABA in order to obtain larger coherence time at the higher frequencies of this program (86 and 129 GHz) and image additional sources that were not detected using standard techniques. We find that this method is applicable to the iMOGABA program even under non–optimal weather conditions.

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225

RADIO EMISSION FROM WEAK SPHERICAL SHOCKS IN THE OUTSKIRTS OF GALAXY CLUSTERS

Kang, Hyesung
Journal of the Korean astronomical society = 천문학회지
48, n.2
pp.155-164
2015
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In <xref>Kang (2015)</xref> we calculated the acceleration of cosmic-ray electrons at weak spherical shocks that are expected to form in the cluster outskirts, and estimated the diffuse synchrotron radiation emitted by those electrons. There we demonstrated that, at decelerating spherical shocks, the volume integrated spectra of both electrons and radiation deviate significantly from the test-particle power-laws predicted for constant planar shocks, because the shock compression ratio and the flux of inject electrons decrease in time. In this study, we consider spherical blast waves propagating through a constant density core surrounded by an isothermal halo with ρ ∝ r−n in order to explore how the deceleration of the shock affects the radio emission from accelerated electrons. The surface brightness profile and the volumeintegrated radio spectrum of the model shocks are calculated by assuming a ribbon-like shock surface on a spherical shell and the associated downstream region of relativistic electrons. If the postshock magnetic field strength is about 0.7 or 7 µG, at the shock age of ∼ 50 Myr, the volume-integrated radio spectrum steepens gradually with the spectral index from αinj to αinj + 0.5 over 0.1–10 GHz, where αinj is the injection index at the shock position expected from the diffusive shock acceleration theory. Such gradual steepening could explain the curved radio spectrum of the radio relic in cluster A2266, which was interpreted as a broken power-law by <xref>Trasatti et al. (2015)</xref>, if the relic shock is young enough so that the break frequency is around 1 GHz.

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226

MASS TRANSFER AND LIGHT TIME EFFECT STUDIES FOR AU SERPENTIS

Amin, S.M.
Journal of the Korean astronomical society = 천문학회지
48, n.1
pp.1-7
2015
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The orbital period changes of the W UMa eclipsing binary AU Ser are studied using the (O-C) method. We conclude that the period variation is due to mass transfer from the primary star to the secondary one at a very low and decreasing rate <TEX>$dP/dt=-8.872{\times}10^{-8}$</TEX>, superimposed on the sinusoidal variation due to a third body orbiting the binary with period <TEX>$42.87{\pm}3.16$</TEX> years, orbital eccentricity <TEX>$e=0.52{\pm}0.12$</TEX> and a longitude of periastron passage <TEX>${\omega}=133^{\circ}.7{\pm}15$</TEX>. On studying the magnetic activity, we have concluded that the Applegate mechanism failed to describe the cycling variation of the (O-C) diagram of AU Ser.

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227

NONTHERMAL RADIATION FROM RELATIVISTIC ELECTRONS ACCELERATED AT SPHERICALLY EXPANDING SHOCKS

Kang, Hyesung
Journal of the Korean astronomical society = 천문학회지
48, n.1
pp.9-20
2015
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We study the evolution of the energy spectrum of cosmic-ray electrons accelerated at spherically expanding shocks with low Mach numbers and the ensuing spectral signatures imprinted in radio synchrotron emission. Time-dependent simulations of diffusive shock acceleration (DSA) of electrons in the test-particle limit have been performed for spherical shocks with parameters relevant for typical shocks in the intracluster medium. The electron and radiation spectra at the shock location can be described properly by the test-particle DSA predictions with instantaneous shock parameters. However, the volume integrated spectra of both electrons and radiation deviate significantly from the test-particle power-laws, because the shock compression ratio and the flux of injected electrons at the shock gradually decrease as the shock slows down in time. So one needs to be cautious about interpreting observed radio spectra of evolving shocks based on simple DSA models in the test-particle regime.

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228

MOLECULAR OUTFLOWS FROM NEWLY FORMED MASSIVE STARS

KIM, KEE-TAE
Journal of the Korean astronomical society = 천문학회지
48, n.6
pp.365-380
2015
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We map 6 massive young stellar objects (YSOs) in the CO J=2-1 line and survey 18 massive YSOs, including the six, in the HCO+ J=1−0, SiO J=2−1, H2O 616 − 523 maser, and CH3OH 70 − 61 A+ maser lines. We detect CO bipolar outflows in all the six mapped sources. Four of them are newly discovered (07299−1651, 21306+5540, 22308+5812, 23133+6050), while 05490+2658 is mapped in the CO J=2-1 line for the first time. The detected outflows are much more massive and energetic than outflows from low-mass YSOs with masses >20 M⊙ and momenta >300 M⊙ km s−1. They have mass outflow rates (3−6)×10−4 M⊙ yr−1, which are at least one order of magnitude greater than those observed in low-mass YSOs. We detect HCO+ and SiO line emission in 18 (100%) and 4 (22%) sources, respectively. The HCO+ spectra show high-velocity wings in 11 (61%) sources. We detect H2O maser emission in 13 (72%) sources and 44 GHz CH3OH maser emission in 8 (44%) sources. Of the detected sources, 5 H2O and 6 CH3OH maser sources are new discoveries. 20081+3122 shows high-velocity (>30 km s−1) H2O maser lines. We find good correlations of the bolometric luminosity of the central (proto)star with the mechanical force, mechanical luminosity, and mass outflow rate of molecular outflow in the bolometric luminosity range of 10−1−106 L⊙, and identified 3 intermediate- or high-mass counterparts of Class O objects.

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229

STUDY OF MILLI-JANSKY SEYFERT GALAXIES WITH STRONG FORBIDDEN HIGH-IONIZATION LINES USING THE VERY LARGE ARRAY SURVEY IMAGES

LAL, DHARAM V.
Journal of the Korean astronomical society = 천문학회지
48, n.6
pp.399-412
2015
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We study the radio properties at 1.4 GHz of Seyfert galaxies with strong forbidden highionization lines (FHILs), selected from the Sloan Digital Sky Survey - a large-sized sample containing nearly equal proportion of diverse range of Seyfert galaxies showing similar redshift distributions compiled by using the Very Large Array survey images. The radio detection rate is low, 49%, which is lower than the detection rate of several other known Seyfert galaxy samples. These galaxies show low star formation rates and the radio emission is dominated by the active nucleus with ≤10% contribution from thermal emission, and possibly, none show evidence for relativistic beaming. The radio detection rate, distributions of radio power, and correlations between radio power and line luminosities or X-ray luminosity for narrow-line Seyfert 1 (NLS1), Seyfert 1 and Seyfert 2 galaxies are consistent with the predictions of the unified scheme hypothesis. Using correlation between radio and [O III] λ 5007 Å luminosities, we show that ∼8% sample sources are radio-intermediate and the remaining are radio-quiet. There is possibly an ionization stratification associated with clouds on scales of 0.1-1.0 kpc, which have large optical depths at 1.4GHz, and it seems these clouds are responsible for free-free absorption of radio emission from the core; hence, leading to low radio detection rate for these FHIL-emitting Seyfert galaxies

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230

MILGROM’S LAW AND Λ’S SHADOW: HOW MASSIVE GRAVITY CONNECTS GALACTIC AND COSMIC DYNAMICS

Trippe, Sascha
Journal of the Korean astronomical society = 천문학회지
48, n.3
pp.191-194
2015
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Massive gravity provides a natural solution for the dark energy problem of cosmology and is also a candidate for resolving the dark matter problem. I demonstrate that, assuming reasonable scaling relations, massive gravity can provide for Milgrom’s law of gravity (or “modified Newtonian dynamics”) which is known to remove the need for particle dark matter from galactic dynamics. Milgrom’s law comes with a characteristic acceleration, Milgrom’s constant, which is observationally constrained to a0 ≈ 1.1 × 10−10 ms−2 . In the derivation presented here, this constant arises naturally from the cosmologically required mass of gravitons like <graphic></graphic> , with Λ, H0, and ΩΛ being the cosmological constant, the Hubble constant, and the third cosmological parameter, respectively. My derivation suggests that massive gravity could be the mechanism behind both, dark matter and dark energy.

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