• Journal of The Korean Astronomical Society
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• v.37 no.4
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• pp.261-263
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• 2004

We present a progress report on HCN(1-0) line observations toward starless cores to probe inward motions. We have made a single pointing survey toward the central regions of 85 starless cores and performed mapping observations of 6 infall candidate starless cores. The distributions of the velocity difference between HCN(1-0) hyperfine lines and the optically thin tracer $N_2H^+$(1-0) are significantly skewed to the blue, meaning that HCN(1-0) frequently detects inward motions. Their skewness to the blue is even greater than that of CS(2-1) Lee et al., possibly implying more infall occurrence than CS(1-0). We identify 19 infall candidates by using several characteristics illustrating spectral infall asymmetry seen in HCN(1-0) hyperfine lines, CS(3-2), CS(2-1), $DCO^+(2-1)$ and $N_2H^+$ observations. The HCN(1-0) F(O-l) with the least optical depth usually shows a similar intensity distribution to that of $N_2H^+$ which closely traces the density distribution of the cores, indicating that HCN(1-0) is less chemically affected and so believed to reflect kinematics occurring in rather inner regions of the cores. Detailed radiative transfer model fits of the spectra are underway to analyze central infall kinematics in starless cores.

• Journal of The Korean Astronomical Society
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• v.34 no.4
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• pp.255-259
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• 2001

PMDSPH is a combined 3D particle-mesh and SPH code aimed to simulate the self-consistent dynamical evolution of spiral galaxies including live stellar and collisionless dark matter components, as well as an isothermal gas component. This paper describes some aspects of this code and shows how its application to the Milky Way helps to recover the gas flow within the Galactic bar region from the observed HI and CO longitude-velocity distributions.

• Journal of The Korean Astronomical Society
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• v.39 no.1
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• pp.9-17
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• 2006

In an attempt to investigate star formation activity and statistical properties of clumps of high Galactic latitude clouds (HLCs), we mapped the Polaris Flare region, PF121.3+25.5, in $^{12}CO\;and\;^{13}CO$ J = 1 - 0 using SRAO 6-m telescope and also observed its 12 $^{13}CO$ peak positions in CS J = 2 - 1 with TRAO 14-m telescope. $^{13}CO$ integrated intensity map shows clearly its clumpy structure and the locations of clumps well agree with $^{12}CO$morphology. CS line is not detected toward the 12 $^{13}CO$ peak positions, so we can conclude there are no dense $(\sim10^4\;cm^{-3})$ in this region. We decomposed 105 clumps from $^{13}CO$ map using GAUSSCLUMPS algorithm. The mass of clumps ranges from $7.8\;M_{\odot}\;to\;7.4{\times}10^{-2}\;M_{\odot}$ with a total mass of $66.4\;M_{\odot}$ The mass spectrum follows a power law, dN/dM ${\propto}\;M^{-\alpha}$ with a power index of ${\alpha}=1.91{\pm}0.13$. The virial masses of clumps are in the range of $10{\sim}100M_{LTE}$ and so these clumps are considered to be gravitationally unbound.

• Journal of The Korean Astronomical Society
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• v.55 no.5
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• pp.149-172
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• 2022

We present a new method which constructs an Hɪ super-profile of a galaxy which is based on profile decomposition analysis. The decomposed velocity profiles of an Hɪ data cube with an optimal number of Gaussian components are co-added after being aligned in velocity with respect to their centroid velocities. This is compared to the previous approach where no prior profile decomposition is made for the velocity profiles being stacked. The S/N improved super-profile is useful for deriving the galaxy's global Hɪ properties like velocity dispersion and mass from observations which do not provide sufficient surface brightness sensitivity for the galaxy. As a practical test, we apply our new method to 64 high-resolution Hɪ data cubes of nearby galaxies in the local Universe which are taken from THINGS and LITTLE THINGS. In addition, we also construct two additional Hɪ super-profiles of the sample galaxies using symmetric and all velocity profiles of the cubes whose centroid velocities are determined from Hermite h3 polynomial fitting, respectively. We find that the Hɪ super-profiles constructed using the new method have narrower cores and broader wings in shape than the other two super-profiles. This is mainly due to the effect of either asymmetric velocity profiles' central velocity bias or the removal of asymmetric velocity profiles in the previous methods on the resulting Hɪ super-profiles. We discuss how the shapes (𝜎_n/𝜎_b, A_n/A_b, and A_n/A_tot) of the new Hɪ super-profiles which are measured from a double Gaussian fit are correlated with star formation rates of the sample galaxies and are compared with those of the other two super-profiles.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.103-105
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• 2015

The 22 m diameter Mopra telescope in Australia is being used to undertake an improved survey of the CO J = 1-0 line at 3mm along the 4th quadrant of the Galaxy, achieving an order of magnitude better spatial and spectral resolution (i.e. 0.6 and 0.1 km/s) than the Dame et al. (2001) survey that is publically available for the Southern Galactic plane. Furthermore, the Mopra CO survey includes the four principal isotopologues of the CO molecule (i.e. $^{12}CO$, $^{13}CO$, $C^{18}O$ and $C^{17}O$). The survey makes use of an 8 GHz-wide spectrometer and a fast mode of on-the-fly mapping developed for the Mopra telescope, where the cycle time has been reduced to just 1/4 of a second. 38 square degrees of the Galaxy, from $l=306-344^{\circ}$, $b=0{\pm}5^{\circ}$ have currently been surveyed, together with additional 9 sq. deg. regions around the Carina complex and the Central Molecular Zone. We present new results from the survey (see also Burton et al., 2013, 2014). The Mopra CO data are being made publically available as they are published; for the latest release see the project website at www.phys.unsw.edu.au/mopraco.

• Journal of The Korean Astronomical Society
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• v.37 no.4
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• pp.257-259
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• 2004

We compare the results of the surveys of starless cores performed with CS (2-1) and (3-2) lines to study inward motions in the cores. The velocity shifts of the CS(3-2) and (2-1) lines with respect to $N_2H^+$ are found to correlate well with each other and to have similar number distributions, implying that, in many cores, systematic inward motions of gaseous material may occur over a range of density of at least a factor ${\~}$4. Fits of the CS spectra to a 2-layer radiative transfer model in ten infall candidates suggest that the median effective line-of-sight speed of the inward-moving gas is ${\~}0.07 km\;s^{-l}$ for CS (3-2) and ${\~} 0.04 km\;s^{-l}$ for CS(2-1). Considering that the optical depth obtained from the fits is usually smaller in CS(3-2) than in (2-1) line, this may indicate that CS(3-2) usually traces inner, denser gas with greater inward motions than CS(2-1) implying that many of the infall candidates have faster infall toward the center. However, this conclusion may not be representative of all starless core infall candidates, due to the statistically small number analyzed here. Further line observations will be useful to test this conclusion.

• Publications of The Korean Astronomical Society
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• v.33 no.3
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• pp.45-57
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• 2018

Low-mass star-formation studies deal with the birth of individual solar-type stars as it occurs in nearby molecular clouds. While this isolated mode of star formation may not represent the most common form of stellar birth, its study often provides first evidence for the general ingredients of star formation, such as gravitational infall, disk formation, or outflow acceleration. Here I briefly review the current status and the main challenges in our understanding of low-mass star formation, with emphasis in the still mysterious pre-stellar phase. In addition to presenting by-now classical work, I also show how ALMA is starting to play a decisive role driving progress in this field.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.133-137
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• 2015

The Galactic center uniquely provides opportunities to resolve how star clusters form in neutral gas overdensities engulfed in a large-scale accretion flow. We have performed sensitive Green Bank 100m Telescope (GBT), Karl G. Jansky Very Large Array (JVLA), and Submillimeter Array (SMA) mapping observations of molecular gas and thermal dust emission surrounding the Galaxy's supermassive black hole (SMBH) Sgr $A^{\ast}$. We resolved several molecular gas streams orbiting the center on ${\gtrsim}10$ pc scales. Some of these gas streams appear connected to the well-known 2-4 pc scale molecular circumnuclear disk (CND). The CND may be the tidally trapped inner part of the large-scale accretion flow, which incorporates inflow via exterior gas filaments/arms, and ultimately feeds gas toward Sgr $A^{\ast}$. Our high resolution GBT+JVLA $NH_3$ images and SMA+JCMT 0.86 mm dust continuum image consistently reveal abundant dense molecular clumps in this region. These gas clumps are characterized by ${\gtrsim}100$ times higher virial masses than the derived molecular gas masses based on 0.86 mm dust continuum emission. In addition, Class I $CH_3OH$ masers and some $H_2O$ masers are observed to be well associated with the dense clumps. We propose that the resolved gas clumps may be pressurized gas reservoirs for feeding the formation of 1-10 solar-mass stars. These sources may be the most promising candidates for ALMA to probe the process of high-mass star-formation in the Galactic center.