• 천문학논총
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• 제33권3호
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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.

• 천문학회지
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• 제53권5호
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• pp.103-115
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• 2020

We analyze the spatially resolved kinematics of gas and stars for a sample of ten hidden type 1 AGNs in order to investigate the nature of their central sources and the scaling relation with host galaxy stellar velocity dispersion. We select our sample from a large number of hidden type 1 AGNs, which are identified based on the presence of a broad (full width at half maximum ≳1000 km s^-1) component in the Hα line profile and which are frequently mis-classified as type 2 AGNs because AGN continuum and broad emission lines are weak or obscured in the optical spectral range. We used the Blue Channel Spectrograph at the 6.5-m Multiple Mirror Telescope to obtain long-slit data with a spatial scale of 0.3 arcsec pixel^-1. We detected broad Hβ lines for only two targets; however, the presence of strong broad Hα lines indicates that the AGNs we selected are all low-luminosity type 1 AGNs. We measured the velocity, velocity dispersion, and flux of stellar continuum and gas emission lines (i.e., Hβ and [O III]) as a function of distance from the center. The spatially resolved gas kinematics traced by Hβ or [O III] are generally similar to the stellar kinematics except for the inner center, where signatures of gas outflows are detected. We compare the luminosity-weighted effective stellar velocity dispersions with the black hole masses and find that our hidden type 1 AGNs, which have relatively low back hole masses, follow the same scaling relation as reverberation-mapped type 1 AGN and more massive inactive galaxies.

• 천문학회지
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• 제51권1호
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• pp.1-4
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• 2018

We study the pseudo-synchronous orbital motion of a binary system on the main sequence. The equations of the pseudo-synchronous orbit are derived up to $O(e^4)$ where e is the eccentricy of the orbit. We integrate the equations to present their solutions. The theoretical results are applied to the evolution of the orbit and spin of the binary star Y Cygni, which has a current eccentricity of $e_0\;=\;0.142$. We tabulate our numerical results for the evolution of the orbit and spin per century. The numerical results for the semi-major axes and rotational angular velocities in the evolutional time scales of three stages (synchronization, circularization, and collapse time scale) are also tabulated. Synchronization is achieved in about $5{\times}10^3\;years$ followed by circularization lasting about $1{\times}10^5\;years$ before decaying in $2{\times}10^5\;years$.

• 천문학회지
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• 제37권4호
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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.

• 천문학논총
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• 제30권2호
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• pp.583-584
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• 2015

In this paper, the numerical results concerning different orbits of a 3D axisymmetric non-rotating galactic potential are presented. We use $Paczy{\acute{n}}ski^{\prime}s$ gravitational potential with different birth velocity distributions for the isolated old Neutron Star (NS) population. We note some smooth non-constant segments corresponding to regular orbits as well as the characterization of their chaoticity. This is strongly related to the effect of different kick velocities due to supernovae mass-loss and natal kicks to the newly-formed NS. We further confirm that the dynamical motion of the isolated old NSs in the gravitational field becomes obvious, with some significant diffraction in the symmetry of their orbital characteristics.

• 천문학회지
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• 제37권4호
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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.

• 천문학회지
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• 제39권1호
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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.

• 천문학논총
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• 제30권2호
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• pp.119-120
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• 2015

We report results of the measurement of the trigonometric parallax of an $H_2O$ maser source in IRAS 22555+6213 with the VLBI Exploration of Radio Astrometry (VERA). The annual parallax was determined to be $0.278{\pm}0.019$ mas, corresponding to a distance of $3.66^{+0.30}_{-0.26}kpc$. Our results confirm that IRAS 22555+6213 is located in the Perseus arm. We computed the peculiar motion of IRAS 22555+6213 to be ($U_{src}$, $V_{src}$, $W_{src}$) = ($0{\pm}1$, $-32{\pm}1$, $9{\pm}1$) $km\;s^{-1}$, where $U_{src}$, $V_{src}$, and $W_{src}$ are directed toward the Galactic center, in the direction of Galactic rotation and toward the Galactic north pole, respectively. IRAS 22555+6213, NGC7538 and Cepheus A lie along the same line of sight, and are within $2^{\circ}$ on the sky. Their parallax distances, with which we derived their absolute position in the Milky Way, show that IRAS 22555+6213 and NGC7538 are associated with the Perseus arm, while Cepheus A is located in the Local arm. We compared the kinematic distances of IRAS 22555+6213 derived with flat and non-flat rotation curve with its parallax distance and found the kinematic distance derived from the non-flat rotation assumption ($-32km\;s^{-1}$ lag) to be consistent with the parallax distance.

• 천문학논총
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• 제30권2호
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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.

• 한국지구과학회지
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• 제26권7호
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• pp.725-731
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• 2005

우리은하 속의 행성상성운들의 운동자료를 분석하여, 은하 회전의 운동학적 특성과 진화에 대한 단서를 제공한다. 우리는 THE STRASBOURG-ESO CATALOGUE OF GALACTIC PLANETARY NEBULAE에 주어진 자료 502개를 이용하여 행성상성운의 운동학적 특성을 분석하였다. 은경($\iota$)이 $0^{\circ},\;90^{\circ},\;180^{\circ},\;270^{\circ}$인 4방향과 태양의 향점과 배점의 총 6방향에 대한 특성적인 평균시선속도 $(V_r)s$를 구하였다. 행성상성운들은 향점에서 69.0 km/s로 태양에 접근하고, 배점$(56{\circ},\;236{\circ})$에서 64.1km/s로 멀어지고 있는 경향을 보였다. z 방향으로 행성상성운의 분포를 보면 은하면 아래쪽에 더 많이 분포하지만, 큰 차이는 보이지 않았다. 우리은하에 속한 천체 중 제 3세대인 행성상성운들의 회전속도 성분이 태양에 비해 상대적으로 느림을 알 수 있는데 이는 4세대 별인 태양이 제 3세대 별들과 조우할 수 있음을 암시한다.