arXiv subjects

Hwankyung Sung

Publications and source records attributed to Hwankyung Sung.

At least 19 recordsLinked to original sources

arXiv2021

The Sejong Open Cluster Survey (SOS). VII. A Photometric Study of the Young Open Cluster IC 1590

Young open clusters are ideal laboratories to understand star formation process. We present deep UBV I and Halpha photometry for the young open cluster IC 1590 in the center of the H II region NGC 281. Early-type members are selected from UBV photometric diagrams, and low-mass pre-main sequence (PMS) members are identified by using Halpha photometry. In addition, the published X-ray source list and Gaia astrometric data are also used to isolate probable members. A total of 408 stars are selected as members. The mean reddening obtained from early-type members is 1 M_sun). The signature of mass segregation is detected from the difference in the slopes of the initial mass functions for the inner (r < 2.'5) and outer region of this cluster. We finally discuss the star formation history in NGC 281.

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arXiv2019

CCD UBV(RI)KC Photometry of the Open Clusters Juchert 9 and Berkeley 97

The CCD UBV(RI)KC photometry of the poorly studied open clusters Juchert 9 (Juc 9) and Berkeley 97 (Be 97), which are observed with the 0.84 m telescope at the San Pedro Mártir National Observatory, México has been analysed. For the likely cluster members, we determined the reddenings, E(B-V)=0.82 +- 0.04 (Juc 9) and E(B-V)=0.87 +- 0.05 (Be 97), from the early type stars. Our distance moduli/distances for only (B - V) colour are (Vo-Mv , d(kpc)) = (13.40 +- 0.10, 4.8 +- 0.2 kpc) (Juc 9) and (12.40 +- 0.12, 3.0 +- 0.2 kpc) (Be 97), respectively. The Gaia DR2 distances are d = 4.5 +- 1.2 kpc (Juc 9) and d = 3.1 +- 0.7 kpc (Be 97) from the median parallaxes with relative parallaxes < 0.20, which are in good agreement with the photometric distances within the uncertainties. The solar abundance PARSEC isochrones give us the intermediate ages, 30 +- 10 Myr for Juc 9 and 100 +- 30 Myr for Be 97.

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arXiv2019

The Sejong Open cluster Survey (SOS) VI. A small star-forming region in the high Galactic latitude molecular cloud MBM 110

We present optical photometric, spectroscopic data for the stars in the high Galactic latitude molecular cloud MBM 110. For the complete membership selection of MBM 110, we also analyze WISE mid-infrared data and Gaia astrometric data. Membership of individual stars is critically evaluated using the data mentioned above. The Gaia parallax of stars in MBM 110 is 2.667 +/- 0.095 mas (d = 375 +/- 13 pc), which confirms that MBM 110 is a small star-forming region in the Orion-Eridanus superbubble. The age of MBM 110 is between 1.9 Myr and 3.1 Myr depending on the adopted pre-main sequence evolution model. The total stellar mass of MBM 110 is between 16 M_sun (members only) and 23 M_sun (including probable members). The star formation efficiency is estimated to be about 1.4%. We discuss the importance of such small star formation regions in the context of the global star formation rate and suggest that a galaxy's star formation rate calculated from the Halpha luminosity may underestimate the actual star formation rate. We also confirm a young brown dwarf member based on photometry, spectroscopy, and astrometry.

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arXiv2018

Extended main sequence turn-off originated from a broad range of stellar rotational velocities

Star clusters have long been considered to comprise a simple stellar population, but this paradigm is being challenged, since apart from multiple populations in Galactic globular clusters^{1,2}, a number of intermediate-age star clusters exhibit a significant colour spread at the main sequence turn-off (MSTO)^{3,4,5,6,7,8,9,10,11}. A sequential evolution of multiple generations of stars formed over 100-200 million years is a natural explanation of this colour spread^{12}. Another approach to explain this feature is to introduce the effect of stellar rotation^{13}. However, its effectiveness has not yet been proven due to the lack of direct measurements of rotational velocities. Here we report the distribution of projected rotational velocities (V sin i) of stars in the Galactic open cluster M11, measured through a Fourier transform analysis. Cluster members display a broad V sin i distribution, and fast rotators including Be stars have redder colours than slow rotators. Monte Carlo simulations infer that cluster members have highly aligned spin axes and a broad distribution of equatorial velocities biased towards high velocities. Our findings demonstrate how stellar rotation affects the colours of cluster members, suggesting that the colour spread observed in populous clusters can be understood in the context of stellar evolution even without introducing multiple stellar populations.

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arXiv2018

Kinematic evidence for feedback-driven star formation in NGC 1893

OB associations are the prevailing star forming sites in the Galaxy. Up to now, the process of how OB associations were formed remained a mystery. A possible process is self-regulating star formation driven by feedback from massive stars. However, although a number of observational studies uncovered various signposts of feedback-driven star formation, the effectiveness of such feedback has been questioned. Stellar and gas kinematics is a promising tool to capture the relative motion of newborn stars and gas away from ionizing sources. We present high-resolution spectroscopy of stars and gas in the young open cluster NGC 1893. Our findings show that newborn stars and the tadpole nebula Sim 130 are moving away from the central cluster containing two O-type stars, and that the timescale of sequential star formation is about 1 Myr within a 9 parsec distance. The newborn stars formed by feedback from massive stars account for at least 18 per cent of the total stellar population in the cluster, suggesting that this process can play an important role in the formation of OB associations. These results support the self-regulating star formation model.

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arXiv2017

An Optical and Infrared Photometric Study of the Young Open Cluster IC 1805 in the Giant H II Region W4

We present deep wide-field optical CCD photometry and mid-infrared Spitzer/IRAC and MIPS 24micron data for about 100,000 stars in the young open cluster IC 1805. The members of IC 1805 were selected from their location in the various color-color and color-magnitude diagrams, and the presence of Halpha emission, mid-infrared excess emission, and X-ray emission. The reddening law toward IC 1805 is nearly normal (R_V = 3.05+/-0.06). However, the distance modulus of the cluster is estimated to be 11.9+/-0.2 mag (d = 2.4+/-0.2 kpc) from the reddening-free color-magnitude diagrams, which is larger than the distance to the nearby massive star-forming region W3(OH) measured from the radio VLBA astrometry. We also determined the age of IC 1805 (tau_MSTO = 3.5 Myr). In addition, we critically compared the age and mass scale from two pre-main-sequence evolution models. The initial mass function with a Salpeter-type slope of Gamma = -1.3+/-0.2 was obtained and the total mass of IC 1805 was estimated to be about 2700+/-200 M_sun. Finally, we found our distance determination to be statistically consistent with the Tycho-Gaia Astrometric Solution Data Release 1, within the errors. The proper motion of the B-type stars shows an elongated distribution along the Galactic plane, which could be explained by some of the B-type stars being formed in small clouds dispersed by previous episodes of star formation or supernova explosions.

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arXiv2016

A constraint on the formation timescale of the young open cluster NGC 2264: Lithium abundance of pre-main sequence stars

The timescale of cluster formation is an essential parameter in order to understand the formation process of star clusters. Pre-main sequence (PMS) stars in nearby young open clusters reveal a large spread in brightness. If the spread were considered as a result of a real spread in age, the corresponding cluster formation timescale would be about 5 -- 20 Myr. Hence it could be interpreted that star formation in an open cluster is prolonged for up to a few tens of Myr. However, difficulties in reddening correction, observational errors, and systematic uncertainties introduced by imperfect evolutionary models for PMS stars, can result in an artificial age spread. Alternatively, we can utilize Li abundance as a relative age indicator of PMS star to determine the cluster formation timescale. The optical spectra of 134 PMS stars in NGC 2264 have been obtained with MMT/Hectochelle. The equivalent widths have been measured for 86 PMS stars with a detectable Li line (3500 < T_eff [K] <= 6500). Li abundance under the condition of local thermodynamic equilibrium (LTE) was derived using the conventional curve of growth method. After correction for non-LTE effects, we find that the initial Li abundance of NGC 2264 is A(Li) = 3.2 +/- 0.2. From the distribution of the Li abundances, the underlying age spread of the visible PMS stars is estimated to be about 3 -- 4 Myr and this, together with the presence of embedded populations in NGC 2264, suggests that the cluster formed on a timescale shorter than 5 Myr.

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arXiv2015

The initial mass function of young open clusters in the Galaxy: A preliminary result

The initial mass function (IMF) is an essential tool with which to study star formation processes. We have initiated the photometric survey of young open clusters in the Galaxy, from which the stellar IMFs are obtained in a homogeneous way. A total of 16 famous young open clusters have preferentially been studied up to now. These clusters have a wide range of surface densities (log sigma = -1 to 3 [stars pc^2] for stars with mass larger than 5M_sun) and cluster masses (M_cl = 165 to 50,000M_sun), and also are distributed in five different spiral arms in the Galaxy. It is possible to test the dependence of star formation processes on the global properties of individual clusters or environmental conditions. We present a preliminary result on the variation of the IMF in this paper.

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arXiv2015

Sejong open cluster survey (SOS) - V. The active star forming region Sh 2-255 -- 257

There is much observational evidence that active star formation is taking place in the HII regions Sh 2-255 -- 257. We present a photometric study of this star forming region (SFR) using imaging data obtained in passbands from the optical to the mid-infrared in order to study the star formation process. A total of 218 members were identified using various selection criteria based on their observational properties. The SFR is reddened by at least E(B-V) = 0.8 mag, and the reddening law toward the region is normal (R_V = 3.1). From the zero-age main sequence fitting method it is confirmed that the SFR is 2.1 +/- 0.3 kpc from the Sun. The median age of the identified members is estimated to be about 1.3 Myr from comparison of the Hertzsprung-Russell diagram (HRD) with stellar evolutionary models. The initial mass function (IMF) is derived from the HRD and the near-infrared (J, J-H) color-magnitude diagram. The slope of the IMF is about Gamma = -1.6 +/- 0.1, which is slightly steeper than that of the Salpeter/Kroupa IMF. It implies that low-mass star formation is dominant in the SFR. The sum of the masses of all the identified members provides the lower limit of the cluster mass (169M_sun). We also analyzed the spectral energy distribution (SED) of pre-main sequence stars using the SED fitting tool of Robitaille et al. and confirm that there is a significant discrepancy between stellar mass and age obtained from two different methods based on the SED fitting tool and the HRD.

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arXiv2015

Sejong Open Cluster Survey (SOS) - IV. The Young Open Clusters NGC 1624 and NGC 1931

Young open clusters located in the outer Galaxy provide us with an opportunity to study star formation activity in a different environment from the solar neighborhood. We present a UBVI and H alpha photometric study of the young open clusters NGC 1624 and NGC 1931 that are situated toward the Galactic anticenter. Various photometric diagrams are used to select the members of the clusters and to determine the fundamental parameters. NGC 1624 and NGC 1931 are, on average, reddened by = 0.92 +/- 0.05 and 0.74 +/- 0.17 mag, respectively. The properties of the reddening toward NGC 1931 indicate an abnormal reddening law (Rv,cl = 5.2 +/- 0.3). Using the zero-age main sequence fitting method we confirm that NGC 1624 is 6.0 +/- 0.6 kpc away from the Sun, whereas NGC 1931 is at a distance of 2.3 +/- 0.2 kpc. The results from isochrone fitting in the Hertzsprung-Russell diagram indicate the ages of NGC 1624 and NGC 1931 to be less than 4 Myr and 1.5 - 2.0 Myr, respectively. We derived the initial mass function (IMF) of the clusters. The slope of the IMF (Gamma_NGC 1624 = -2.0 +/- 0.2 and Gamma_NGC 1931 = -2.0 +/- 0.1) appears to be steeper than that of the Salpeter/Kroupa IMF. We discuss the implication of the derived IMF based on simple Monte-Carlo simulations and conclude that the property of star formation in the clusters seems not to be far different from that in the solar neighborhood.

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arXiv2014

Reddening, Distance, and Stellar Content of the Young Open Cluster Westerlund 2

We present deep $UBVI_C$ photometric data of the young open cluster Westerlund 2. An abnormal reddening law of $R_{V,cl}=4.14\pm0.08$ was found for the highly reddened early-type members ($E(B-V)\geq 1.45$), whereas a fairly normal reddening law of $R_{V,fg}=3.33\pm0.03$ was confirmed for the foreground early-type stars ($E(B-V)_{fg}<1.05$). The distance modulus was determined from zero-age main-sequence (ZAMS) fitting to the reddening-corrected colour-magnitude diagram of the early-type members to be $V_0-M_V=13.9\pm0.14$ (random error) $_{-0.1}^{+0.4}$ (the upper limit of systematic error) mag ($d = 6.0 \pm 0.4 _{-0.3}^{+1.2}$ kpc). To obtain the initial mass function, pre-main-sequence (PMS) stars were selected by identifying the optical counterparts of Chandra X-ray sources and mid-infrared emission stars from the Spitzer GLIMPSE source catalog. The initial mass function shows a shallow slope of $Γ=-1.1 \pm 0.1$ down to $\log m = 0.7$. The total mass of Westerlund 2 is estimated to be at least 7,400 $M_{\odot}$. The age of Westerlund 2 from the main-sequence turn-on and PMS stars is estimated to be $\lesssim$ 1.5 Myr. We confirmed the existence of a clump of PMS stars located $\sim1$ arcmin north of the core of Westerlund 2, but we could not find any clear evidence for an age difference between the core and the northern clump.

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arXiv2014

Sejong Open Cluster Survey (SOS). III. The Young Open Cluster NGC 1893 in the H II Region W8

We present a UBV I and H alpha photometric study of the young open cluster NGC 1893 in the H II region W8 (IC 410 or Sh 2-236). A total of 65 early-type members are selected from photometric diagrams. A mean reddening of the stars is = 0.563 +/- 0.083 mag. The published photometric data in the near- and mid-infrared passbands are used to test the reddening law toward the cluster, and we confirm that the reddening law is normal (R_V = 3.1). Zero-age main sequence fitting gives a distance modulus of V_0 - M_V = 12.7 +/- 0.2 mag, equivalent to 3.5 +/- 0.3 kpc. From H alpha photometry 125 H alpha emission stars and candidates are identified as pre-main sequence (PMS). The lists of young stellar objects and X-ray sources published by previous studies allow us to select a large number of PMS members down to 1 M_sun. Isochrone fitting in the Hertzsprung-Russell diagram gives a turn-off age of 1.5 Myr and the median age of 1.9 Myr from the PMS members with a spread of 5 Myr. We derive the initial mass function (IMF) for stars with mass larger than 1 M_sun. The slope of the IMF (Gamma = -1.3 +/- 0.1) is well consistent with the Salpeter/Kroupa IMF. A total mass of the cluster appears to be in excess of 1,300 M_sun. Finally, we estimate the mass accretion rate of 82 PMS members in the mass range of 0.6 M_sun to 5 M_sun.

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arXiv2014

The Interstellar Reddening Law within 3kpc from the Sun

We have investigated the interstellar reddening law of young open clusters within 3kpc from the Sun using optical, near-IR 2MASS, and Spitzer IRAC data. The total-to-selective extinction ratio Rv of 162 young open clusters (log t{age} <= 7.3) listed in the open cluster database WEBDA is determined from the color excess ratios. The young open clusters in the Sgr-Car arm show a relatively higher $R_V$, those in the Per arm and in the Cygnus region of the local arm show a relatively smaller value, and those in the Mon-CMa region of the local arm show a normal value (Rv ~ 3.1).

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arXiv2013

Sejong Open Cluster Survey (SOS). II. IC 1848 Cluster in the H II Region W5 West

IC 1848 is one of the young open clusters in the giant star forming Cas OB6 association. Several interesting aspects relating to star formation processes in giant star forming regions attracted us to study the initial mass function (IMF), star formation mode, and properties of pre-main sequence stars (PMS). A UBVI and H alpha photometric study of the young open cluster IC 1848 was conducted as part of the "Sejong Open cluster Survey" (SOS). We have selected 105 early-type members from photometric diagrams. Their mean reddening is = 0.660 +/- 0.054 mag. Using the published photometric data with near- and mid-infrared archival data we confirmed the normal reddening law (R_V = 3.1) toward the cluster (IC 1848). A careful zero-age main sequence fitting gives a distance modulus of V_0 - M_V = 11.7 +/- 0.2 mag, equivalent to 2.2 +/- 0.2 kpc. H alpha photometry and the list of young stellar objects identified by Koenig et al. permitted us to select a large number of PMS stars comprising 196 H alpha emission stars, 35 H alpha emission candidates, 5 Class I, 368 Class II, and 24 transition disk candidates. From the Hertzsprung-Russell diagram using stellar evolution models, we estimate an age of 5 Myr from several evolved stars and 3 Myr from the PMS stars. The IMF was derived from stars with mass larger than 3 M_sun, and the slope is slightly steeper (Gamma = -1.6 +/- 0.2) than the Salpeter/Kroupa IMF. Finally, we estimated the mass accretion rate of PMS stars with a UV excess. The mean mass accretion rate is about 1.4 x 10^-8 M_sun yr^-1 in the mass range of 0.5 M_sun to 2 M_sun, whereas intermediate-mass stars (>= 2.5 M_sun) exhibit a much higher accretion rate of dM/dt > 10^-6 M_sun yr^-1.

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arXiv2013

[Fe II] 1.64 um Features of Jets and Outflows from Young Stellar Objects in the Carina Nebula

We present [Fe II] 1.64 μm imaging observations for jets and outflows from young stellar objects (YSOs) over the northern part (~ 24'x45') of the Carina Nebula, a massive star forming region. The observations were performed with IRIS2 of Anglo-Australian Telescope and the seeing was ~1.5"+-0.5". Eleven jet and outflow features are detected at eight different regions, and are named as Ionized Fe Objects (IFOs). One Herbig-Haro object candidate missed in Hubble Space Telescope Hα observations is newly identified as HHc-16, referring our [Fe II] images. IFOs have knotty or longish shapes, and the detection rate of IFOs against previously identified YSOs is 1.4 %, which should be treated as a lower limit. Four IFOs show an anti-correlated peak intensities in [Fe II] and Hα, where the ratio I([Fe II])/I(Hα) is higher for longish IFOs than for knotty IFOs. We estimate the outflow mass loss rate from the [Fe II] flux, using two different methods. The jet-driving objects are identified for three IFOs (IFO-2, -4, and -7), for which we study the relations between the outflow mass loss rate and the YSO physical parameters from the radiative transfer model fitting. The ratios of the outflow mass loss rate over the disk accretion rate are consistent for IFO-4 and -7 with the previously reported values (10^-2-10^+1), while it is higher for IFO-2. This excess may be from the underestimation of the disk accretion rate. The jet-driving objects are likely to be low- or intermediate-mass stars. Other YSO physical parameters, such as luminosity and age, show reasonable relations or trends.

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arXiv2013

Sejong Open Cluster Survey (SOS). 0. Target Selection and Data Analysis

Star clusters are superb astrophysical laboratories containing cospatial and coeval samples of stars with similar chemical composition. We have initiated the Sejong Open cluster Survey (SOS) - a project dedicated to providing homogeneous photometry of a large number of open clusters in the SAAO Johnson-Cousins' $UBVI$ system. To achieve our main goal, we have paid much attention to the observation of standard stars in order to reproduce the SAAO standard system. Many of our targets are relatively small, sparse clusters that escaped previous observations. As clusters are considered building blocks of the Galactic disk, their physical properties such as the initial mass function, the pattern of mass segregation, etc. give valuable information on the formation and evolution of the Galactic disk. The spatial distribution of young open clusters will be used to revise the local spiral arm structure of the Galaxy. In addition, the homogeneous data can also be used to test stellar evolutionary theory, especially concerning rare massive stars. In this paper we present the target selection criteria, the observational strategy for accurate photometry, and the adopted calibrations for data analysis such as color-color relations, zero-age main sequence relations, Sp - Mv relations, Sp - Teff relations, Sp - color relations, and Teff - BC relations. Finally we provide some data analysis such as the determination of the reddening law, the membership selection criteria, and distance determination.

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arXiv2012

The Starburst Cluster Westerlund 1: The Initial Mass Function and Mass Segregation

Westerlund 1 is the most important starburst cluster in the Galaxy due to its massive star content. We have performed BVIc and JKs photometry to investigate the initial mass function (IMF). By comparing the observed color with the spectral type - intrinsic color relation, we obtain the mean interstellar reddening of =4.19+/-0.23 and =1.70+/-0.21. Due to the heavy extinction toward the cluster, the zero-age main sequence fitting method based on optical photometry proved to be inappropriate for the distance determination, while the near-infrared photometry gave a reliable distance to the cluster -- 3.8 kpc from the empirical relation. Using the recent theoretical stellar evolution models with rotation, the age of the cluster is estimated to be 5.0+/-1.0 Myr. We derived the IMF in the massive part and obtained a fairly shallow slope of Γ = -0.8 +/- 0.1. The integration of the IMF gave a total mass for the cluster in excess of 5.0 x 10^4 solar mass. The IMF shows a clear radial variation indicating the presence of mass segregation. We also discuss the possible star formation history of Westerlund 1 from the presence of red supergiants and relatively low-luminosity yellow hypergiants.

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arXiv2012

The Initial Mass Function and the Surface Density Profile of NGC 6231

We have performed new wide-field photometry of the young open cluster NGC 6231 to study the shape of the initial mass function (IMF) and mass segregation. We also investigated the reddening law toward NGC 6231 from optical to mid-infrared color excess ratios, and found that the total-to-selective extinction ratio is Rv = 3.2, which is very close to the normal value. But many early-type stars in the cluster center show large color excess ratios. We derived the surface density profiles of four member groups, and found that they reach the surface density of field stars at about 10', regardless of stellar mass. The IMF of NGC 6231 is derived for the mass range 0.8 -- 45 Msun. The slope of the IMF of NGC 6231 (Gamma = -1.1 +/- 0.1) is slightly shallower than the canonical value, but the difference is marginal. In addition, the mass function varies systematically, and is a strong function of radius - it is is very shallow at the center, and very steep at the outer ring suggesting the cluster is mass segregated. We confirm the mass segregation for the massive stars (m >~ 8 Msun) by a minimum spanning tree analysis. Using a Monte Carlo method, we estimate the total mass of NGC 6231 to be about 2.6 (+/- 0.6) x 10^3 Msun. We constrain the age of NGC 6231 by comparison with evolutionary isochrones. The age of the low-mass stars ranges from 1 to 7 Myr with a slight peak at 3 Myr. However the age of the high mass stars depends on the adopted models and is 3.5 +/- 0.5 Myr from the non- or moderately-rotating models of Brott et al. as well as the non-rotating models of Ekström et al. But the age is 4.0 -- 7.0 Myr if the rotating models of Ekström et al. are adopted. This latter age is in excellent agreement with the time scale of ejection of the high mass runaway star HD 153919 from NGC 6231, albeit the younger age cannot be entirely excluded.

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