• 천문학회보
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• 제38권1호
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• pp.31.2-31.2
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• 2013

We use grid-based hydrodynamic simulations to study star formation in nuclear rings in barred galaxies. The gaseous medium is assumed to be infinitesimally thin, isothermal, and unmagnetized. To investigate various situations, we vary the total gas content in the bar regions and the bar growth time. We find that star formation rate (SFR) in a nuclear ring is determined by the mass inflow rate to the ring rather than the total gas mass in the ring. The SFR shows a strong primary burst and weak secondary bursts at early time, and declines to small values at late time. The primary burst is caused by the rapid gas infall to the ring due to the bar growth, with its duration and peak depending on the bar growth time. The secondary bursts result from re-infall of the ejected gas by star formation feedback of the primary burst. When the SFR is low, ages of young star clusters exhibit an azimuthal gradient along the ring since star formation takes place mostly near the contact points between the dust lanes and the nuclear ring. When the SFR is large, on the other hand, star formation is widely distributed throughout the whole length of the ring, with no apparent age gradient of star clusters. Regardless of SFR, star clusters have a positive radial age gradient, with younger clusters located closer to the ring, since the ring shrinks in size over time.

• 한국지구과학회지
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• 제35권5호
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• pp.362-374
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• 2014

This study investigated the initial mass function (IMF) and star formation history of high-mass stars in the Small Magellanic Cloud (SMC) using a population synthesis technique. We used the photometric survey catalog of Lee (2013) as the observable quantities and compare them with those of synthetic populations based on Bayesian inference. For the IMF slope (${\Gamma}$) range of -1.1 to -3.5 with steps of 0.1, five types of star formation models were tested: 1) continuous; 2) single burst at 10 Myr; 3) single burst at 60 Myr; 4) double bursts at those epochs; and 5) a complex hybrid model. In this study, a total of 125 models were tested. Based on the model calculations, it was found that the continuous model could simulate the high-mass stars of the SMC and that its IMF slope was -1.6 which is slightly steeper than Salpeter's IMF, i.e., ${\Gamma}=-1.35$.

• 천문학회지
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• 제29권spc1호
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• pp.77-78
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• 1996

We perfom a large project for complex study of Blue Compact Galaxies (BCGs) with strong star formation, which includes optical spectroscopy, BVR CCD photometry and HI 21 cm radio survey. The most interesting galaxies are studied also with HST and VLA. In the frame of this project we began the study of H$\alpha$ morphology of BCGs with 6-m telescope. We present and discuss here the results for the first 6 galaxies. We found the noticeable variety of forms for H$\alpha$ morphology comparing to broad band images: from very compact HII region in very center of stellar body (Mark 996, possible dwarf post-merger, old galaxy experiencing strong star formation burst), to very extended gas emission encompassing the whole area traced by stars (SBS 0335-052, the most probable young galaxy in formation).

• 대한후두음성언어의학회지
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• 제7권1호
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• pp.50-55
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• 1996

Acoustic analysis study was performed on 20 normal subjects by speaking nonsense syllables composed of Korean bilabial stops$(/P, P^{\star}, P^{h}/)$ and their preceding and/or following vowel /a/ (that is, $[pa, p^{\star}a, p^{h}a, apa, ap^{\star}a, ap^{h}a]$) with an ultraminiature pressure, sensor. in their mouths. Speech materials were phonated twice, once with a moderate voice, another time with a loud voice. The acoustic signal and intraoral pressure were recorded simultaneously on computer. By these procedures, we were to measure the intraoral pressure, closure duration and VOT of Korean bilabial stops, and to compare the values one another according to the intensity of phonation and the position of the target consonants. Intraoral pressure was measured by the peak intraoral pressure value of Its wave closure duration by the time interval between the onset of intraoral pressure build-up and the burst meaning the release of closure ; Voice onset time(VOT) on by the time interval between the burst and the onset or glottal vibration. Heavily aspirated bilabial stop consonant /$p^h$/ showed the highest intraoral pressure value, unaspirated /$p^{\star}$/, the second, slightly aspirated /P/, the lowest. The syllable initial bilabial stops showed higher intraoral pressure than word initial stops, and the value of loudly phonated consonants were higher than moderate consonants. The longest closure duration period was that of /$p^{\star}$/ and the shortest, /P/, and the duration was longer in word initial position and in the moderate voice. In VOT, the order of the longest to shortest was $/{p^h}/, /p/, /{p^\star}/$, and the value was shorer when the consonant was in intervocalic position and when it was phonated with a loud voice.

• 천문학회보
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• 제39권1호
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• pp.44.1-44.1
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• 2014

Blue Compact Dwarf galaxies (BCDs) are systems that recently have experienced the burst of star formation. As one of the causes for active star formation in BCDs, tidal interaction (fly-by or merger) has been suggested. A pair of BCDs, ESO 435-IG20 and ESO 435-IG16 are separated by only ~80 kpc in projection at a similar redshift (at a ~9 Mpc distance), and hence suspected to be a good example of such case. Intergalactic atomic hydrogen gas found in HIPASS survey is also suggestive of this hypothesis. In this study, we probe the HI morphology and kinematics of this BCD pair using ATCA HI data to study detailed interaction history. We investigate various star formation tracers of the pair to study how responsible tidal interaction is for triggering star formation in these galaxies.

• 천문학회보
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• 제40권2호
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• pp.31.3-32
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• 2015

We present the physical properties of star forming regions in IC 10 obtained from Korea VLBI Network (KVN) 22GHz, the Submillimeter Array (SMA) CO, Very Large Array (VLA) HI 21cm, optical (U, B, V and H-alpha), and Spitzer infrared observations. IC 10 is a nearby (~0.7Mpc) irregular blue compact dwarf (BCD) galaxy which is likely to be experiencing an intense and recent burst of star formation. This nearby infant system showing high star formation rate but low metallicity (<20% of that of the Sun) provides critical environment of interstellar medium (ISM) under which current galactic star formation models are challenged. To make quantitative analysis of the ISM in the galaxy, we apply 2D cross-correlation technique to the multi-wavelength data for the first time. By cross-correlating different tracers of star formation, dust and gas phases in IC 10 in a two dimensional way, we discuss the gas properties and star formation history of the galaxy.

• 천문학회보
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• 제38권2호
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• pp.39.2-39.2
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• 2013

We use grid-based hydrodynamic simulations to study star formation history in nuclear rings of barred-spiral galaxies. In our previous study, we concentrated on bar-only galaxies without spirals, finding that the star formation rate (SFR) in a nuclear ring exhibits a strong primary burst at early time before decreasing to below 1 $M_{\odot}/yr$ at late time. The rapid decline is caused by the paucity of the gas in the bar region, due to early massive gas inflows to the nuclear ring. Since star formation in nuclear rings is observed to be sustained for about 1-2 Gyr, this requires mechanisms to supply the gas to the bar regions. In this work, we study the effect of spiral arms on the radial gas inflows and related star formation in the nuclear rings. We show that spiral arms are efficient to remove angular momentum of the gas to cause significant gas inflows to the bar region, provided the patten speed of the arms is much smaller than that of the bar. The inflowing gas is added to a nuclear ring, making the ring SFR episodic over a long period of time. The time interval of multiple bursts of star formation is a few tens to hundred million years, with the mean peak SFR of ${\sim}5M_{\odot}/yr$, consistent with observations of M100.

• 천문학회보
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• 제46권1호
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• pp.51.2-51.2
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• 2021

Nuclear rings are sites of intense star formation at the center of barred spiral galaxies. A straightforward but unanswered question is what controls star formation rate (SFR) in nuclear rings. To understand how the ring SFR is related to mass inflow rate, gas content, and background gravitational field, we run a series of semi-global hydrodynamic simulations of nuclear rings, adopting the TIGRESS framework to handle radiative heating and cooling as well as star formation and supernova feedback. We find: 1) when the mass inflow rate is constant, star formation proceeds in a remarkably steady fashion, without showing any burst-quench behavior suggested in the literature; 2) the steady state SFR has a simple linear relationship with the inflow rate rather than the ring gas mass; 3) the midplane pressure balances the weight of the overlying gas and the SFR surface density is linearly correlated with the midplane pressure, consistent with the self-regulated star formation theory. We suggest that the ring SFR is controlled by the mass inflow rate in the first place, while the gas mass adjusts to the resulting feedback in the course of achieving the vertical dynamical equilibrium.

• 천문학회보
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• 제43권1호
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• pp.45.3-46
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• 2018

Although mass accretion from the disk to the central protostar is a key process of low mass star formation, the accretion mechanism is still poorly understood. To investigate "episodic accretion", which has been suggested as an accretion mechanism in low mass star formation, we have carried out near-infrared spectroscopic observations of three very low-luminosity objects (VeLLOs) and one background source, using InfraRed Camera onboard the AKARI space telescope. The ice absorption features of $H_2O$, $CO_2$, and CO were detected around the wavelengths of 3.0, 4.26, and $4.67{\mu}m$, respectively. In addition, we revealed the XCN ice feature, which is attributed to high energy UV photons produced by the episodic burst accretion. The comparisons of the ice abundances of our targets with those of other YSOs observed previously with AKARI IRC imply that the three VeLLOs had experienced burst accretions although they are now in a very quiescent phase.

• 천문학회보
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• 제46권2호
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• pp.66.3-67
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• 2021

Low-mass stars form by the gravitational collapse of dense molecular cores. Observations and theories of low-mass protostars both suggest that accretion bursts happen in timescales of ~100 years with high accretion rates, so called episodic accretion. One mechanism that triggers accretion bursts is infalling fragments from the outer disk. Such fragmentation happens when the disk is massive enough, preferentially activated during the embedded phase of star formation (Class 0 and I). Most observations and models focus on the gas structure of the protostars undergoing episodic accretion. However, the dust and ice composition are poorly understood, but crucial to the chemical evolution through thermal and energetic processing via accretion burst. During the burst phase, the surrounding material is heated up, and the chemical compositions of gas and ice in the disk and envelope are altered by sublimation of icy molecules from grain surfaces. Such alterations leave imprints in the ice composition even when the temperature returns to the pre-burst level. Thus, chemical compositions of gas and ice retain the history of past bursts. Infrared spectral observations of the Spitzer and AKARI revealed a signature caused by substantial heating, toward many embedded protostars at the quiescent phase. We present the AKARI IRC 2.5-5.0 ㎛ spectra for embedded protostars to trace down the characteristics of accretion burst across the evolutionary stages. The ice compositions obtained from the absorption features therein are used as a clock to measure the timescale after the burst event, comparing the analyses of the gas component that traced the burst frequency using the different refreeze-out timescales. We discuss ice abundances, whose chemical change has been carved in the icy mantle, during the different timescales after the burst ends.