• Journal of The Korean Astronomical Society
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• v.22 no.2
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• pp.127-140
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• 1989

Problem of the diffuse radiation (DFR) transfer is solved exactly for pure hydrogen nebulae of uniform density, and accuracies of the on-the-spot (OTS) approximation are critically examined. For different values of density and spectral types of the central star, we have calculated the degree of ionization and the kinetic temperature of electrons as functions of distance from the central star, and compared them with the corresponding results of the OTS approximation. At most locations inside an HII region. the DFR ionizes considerable amount of hydrogen; therefore, the OTS approximation under-estimates the size of ionized regions. The exact treatment of the DFR transfer results in an about 10 to 20 percent increase in the classical $Str{\ddot{o}}mgren$ radius. The OTS approximation overestimates the local heating rate by raising the density of neutral hydogens. Consequently, it predicts higher values for the local electron temperature. The OTS approximation also exaggerates the dependence of electron temperature on density. When the hydrogen density is changed from $10/cm^3$ to $10^3/cm^3$ with an 06.5V star, the OTS approximation shows an about 3,000 K difference in the electron temperature, while the exact treatment of the DFR-transfer reduces the difference to about 1,000 K. The OTS approximation fails to demonstrate the brightening of the electron temperature close to the ionization boundary.

• Journal of The Korean Astronomical Society
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• v.40 no.4
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• pp.137-140
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• 2007

Triggered star formation is not a new idea - it's been around for at least the last 30 years. Although it has never disappeared from the scene, it seems to be enjoying something of a renaissance in recent years, in both observational and theoretical studies. Here we give a brief discussion of the background of triggered star formation, describe some of our own recent observational efforts in this regard, and briefly mention some initial conclusions that may be drawn.

• Journal of The Korean Astronomical Society
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• v.37 no.4
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• pp.265-268
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• 2004

Molecular clouds present many levels of structure, including clumps and cores of varying size and density. We present a brief summary of these cores, describing their observed physical properties and their place in the star formation process. We conclude with some speculation about pre-proto-stellar stages of molecular cores and the observational challenges in their observation.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.54.1-54.1
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• 2017

The UKIRT Wide-field Infrared survey for Fe+(UWIFE) is an unbiased survey of the first Galactic quadrant, with narrow-band filter centered on $1.644{\mu}m$. This survey covers $7^{\circ}$ < l < $62^{\circ}$ and |b| < $1.5^{\circ}$, where active interaction of stars and interstellar medium is expected. With median seeing of 0.8 arcsec, 5 - sigma detection limit of 18.7 mag and surface brightness limit of $8.1{\times}10^{-20}W\;m^{-2}arcsec^{-2}$, this survey gives an opportunity to statistically study Galactic [Fe II] - emitting sources for the first time. In order to identify Ionized Fe Objects (IFOs) in survey area systematically, we conducted visual inspection and automatic detection simultaneously. Total of ~300 extended IFOs are identified, most of them are found out to be part of supernova remnants (SNRs), young stellar objects, HII regions and planetary nebulae. The majority of IFOs are new discoveries which reveal shocked structures in high-extinction region. Spatial distribution of IFOs suggest that they trace Galactic structure. As a part of spectroscopic follow-up, we observed SNR candidate IFO J183740.829-061452.41 with IGRINS (Immersion Grating Infrared Spectrograph, Yuk+2010), mounted on 2.7m Harlan Smith telescope. This unknown arc-like, 6'-long IFO is coincident with inner part of radio continuum loop G25.8+0.2, which has been known as HII region. However, interior of this radio shell is filled with diffuse soft X-ray emission, and possible association of hard X-ray pulsar / pulsar wind nebula makes the nature of the IFO unclear. The H and K-band 2D spectrum shows shock-ionized [Fe II] filaments, which is apart from photoionized HII filaments. In this presentation we present basic statistics of newly identified IFOs, as well as the follow-up study of IFO J183740.829-061452.41.

• Journal of The Korean Astronomical Society
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• v.27 no.1
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• pp.31-44
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• 1994

The 4.8GHz formaldehyde absorption line in the dark clouds in M17 and NGC 2024 regions has been mapped. In both nebulae, we detected two $H_2CO$ line components. In M17, the 24km $S^{-1}$ cloud is closely associated with the HII region located in front of the radio continuum source, and the 19km $S^{-1}$ cloud is associated with the visual dark clouds with a larger extent which are closer to us. The 19km $S^{-1}$ cloud has a mass motion approaching to the HII region. In both clouds, a velocity gradient from the north-east to the south-west directions is observed. The linewidth has no variation indicating no collapsing motion. In NGC 2024, the 9km $S^{-1}$ feature is extended along the dark bar in front of the bright nebula and a weak second component at 13km $S^{-1}$ is confined to the immediate vicinity of the radio source. Indications are that the 9km $S^{-1}$ cloud is physically associated with the dark bar and the 13km $S^{-1}$ cloud is located behind the radio source. The angular extent, the column density, and the total mass of the clouds are derived. The radial velocities of other molecular lines observed in these clouds are compared.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.44.2-45
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• 2016

We present near-infrared photometric properties of red supergiant stars (RSGs) in three galaxies NGC 4449, NGC 5055 and NGC 5457. The near-infrared imaging data of WFCAM UKIRT were used and combined with optical archive data to identify the RSGs in the galaxies. We found that the RSGs can be identified from the foreground Galactic stars in (i-K, ri) colour-colour diagram. The effective temperatures and luminosities of the identified RSGs are estimated from JHK photometry using MARCS model. In the H-R diagram, the majority of RSGs in the galaxies are distributed between $logL/L{\odot}=4.8$ and 5.7, and their effective temperature and luminosities agree with the current evolutionary tracks with masses in the range $9-30M{\odot}$. We also compared the spatial distribution of RSGs with the HII regions. A tight spatial correlation between RSGs and HII region was found in NGC 4449 and NGC 5457. We do not find a clear metallicity dependance on the RSG effective temperature in the three galaxies, but the maximum luminosity of the three galaxies is constant at $logL/L{\odot}{\sim}5.6$. Additional spectroscopy data, including photometry are essential to examine whether the physical properties of RSGs change with metallicity.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.1
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• pp.83.2-83.2
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• 2012

We have carried out simultaneous 22GHz $H_2O$ and 44GHz Class I $CH_3OH$ maser survey of 112 ultra-compact HII regions (UCHIIs) twice in 2010 and 2011. We detected $H_2O$ maser and $CH_3OH$ maser emission from 76(68%) and 49(44%) UCHIIs, respectively. Among them 15 $H_2O$ masers and 32 $CH_3OH$ masers are new detections. These high detection rates suggest that the occurrence periods of both masers are significantly overlapped with the UCHII phase. $CH_3OH$ masers always have small (<10 km s-1) relative velocities with respect to the natal molecular cores, while $H_2O$ masers often show larger velocities. We find 20 UCHIIs with $H_2O$ maser lines at relative velocities >30 km s-1. The formation and disappearance of $H_2O$ masers is frequent over one-year time interval. In contrast, $CH_3OH$ masers usually do not show substantial variation in intensity, velocity, or shape. The isotropic luminosities of both masers well correlate with the bolometric luminosities of the central stars when data points of lowand intermediate-mass protostars are added: $L_{H_2O}=5.89{\times}10^{-9}{(L_{bol})^{0.69}}$ and $L_{CH_3OH}=4.27{\times}10^{-9}{(L_{bol})^{0.62}}$. They also tend to increase with the 2cm radio continuum luminosity of UCHIIs and the 850 um continuum luminosity of the associated molecular cores. We discuss some individual sources.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.1
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• pp.58.3-59
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• 2015

Dynamical expansion of H II regions plays a key role in dispersing surrounding gas and therefore in limiting the efficiency of star formation in molecular clouds. We use analytic methods and numerical simulations to explore expansions of spherical dusty H II regions, taking into account the effects of direct radiation pressure, gas pressure, and total gravity of the gas and stars. Simulations show that the structure of the ionized zone closely follows Draine (2011)'s static equilibrium model in which radiation pressure acting on gas and dust grains balances the gas pressure gradient. Strong radiation pressure creates a central cavity and a compressed shell at the ionized boundary. We analytically solve for the temporal evolution of a thin shell, finding a good agreement with the numerical experiments. We estimate the minimum star formation efficiency required for a cloud of given mass and size to be destroyed by an HII region expansion. We find that typical giant molecular clouds in the Milky Way can be destroyed by the gas-pressure driven expansion of an H II region, requiring an efficiency of less than a few percent. On the other hand, more dense cluster-forming clouds in starburst environments can be destroyed by the radiation pressure driven expansion, with an efficiency of more than ~30 percent that increases with the mean surface density, independent of the total (gas+stars) mass. The time scale of the expansion is always smaller than the dynamical time scale of the cloud, suggesting that H II regions are likely to be a dominant feedback process in protoclusters before supernova explosions occurs.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.67.1-67.1
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• 2013

It has been known that HCN is one of ubiquitous high-density gas tracer, and the abundance ratio between HCN and its Isomer, HNC sensitively depends on kinetic temperature in star-forming regions. Here we investigate the molecular abundance ratio toward three different evolutionary phases of massive star formation: Infrared Dark Clouds, High-mass Protostellar Objects and Ultracompact HII Regions. We obtained the abundances of HCN and HNC using optically thin H13CN and HN13C lines observed with the KVN single-dish telescopes and MAMBO 1.2mm and SCUBA $850{\mu}m$ continuum data. According to our results, the ratio of [HCN]/[HNC] increases statistically with the evolutionary stage, indicative of the effect of temperature. We also found a strong anti-correlation between the column density of molecular hydrogen and the HNC abundance.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.2
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• pp.53.3-53.3
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• 2015

We analyzed both HCN J=1-0 and HNC J=1-0 line profiles to study the inflow motions in different evolutionary stages of massive star formation; infrared dark clouds (IRDCs), high-mass protostellar object (HMPOs), and ultra-compact HII regions (UCHIIs). The infall asymmetry in HCN spectra seems to be prevalent throughout all the three evolutionary phases, with IRDCs showing the largest excess in blue profile. In the case of HNC spectra, the prevalence of blue sources does not appear, excepting for IRDCs. We suggest that this line is not appropriate to trace infall motion in evolved stages of massive star formation because of an astrochemical effect. This result spotlights the importance of considering chemistry in dynamical study in star-forming regions. The fact that the IRDCs show the highest blue excess in both infall tracers indicates that the most active infall occurs in the early phase of star formation, i.e., the IRDC phase rather than in the later phases. However, the UCHIIs is likely still accreting matters. We also found that the absorption dips of the HNC spectra in all blue sources are red--shifted relative to their central velocities. These red-shifted absorption dips may indicate the observational signature of overall collapse although observations with better resolutions are needed to examine this feature more in detail.