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RISS 인기검색어
- 검색키워드
- 저자 : Michael S. Bessell (검색결과 22 건)
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American Institute of Physics 2010 The Astronomical journal Vol.140 No.6
<P>We have studied the star formation history and the initial mass function (IMF) using the age and mass derived from spectral energy distribution (SED) fitting and from color-magnitude diagrams (CMDs). We also examined the physical and structural parameters of more than 1000 pre-main-sequence stars in NGC 2264 using the online SED fitting tool (SED fitter) of Robitaille et al. We have compared the physical parameters of central stars from SED fitter and other methods. The temperature of the central star is, in many cases, much higher than that expected from its spectral type. The mass and age from SED fitter are not well matched with those from CMDs. We have made some suggestions to improve the accuracy of temperature estimates in the SED fitter. In most cases, these parameters of individual stars from the SED fitter in a star-forming region (SFR) or in the whole cluster showed nearly no systematic variation with age or with any other relevant parameter. On the other hand, the median properties of stars in NGC 2264 SFRs showed an evident evolutionary effect and were interrelated to each other. Such differences are caused by a larger age spread within an SFR than between them. The cumulative distribution of stellar ages showed a distinct difference among SFRs. A Kolmogorov-Smirnov test gave a very low probability of them being from the same population. The results indicate that star formation in NGC 2264 started at the surface region (Halo and Field regions) about 6-7 Myr ago, propagated into the molecular cloud and finally triggered the recent star formation in the Spokes cluster. The kind of sequential star formation that started in the low-density surface region (Halo and Field regions) implies that star formation in NGC 2264 was triggered by an external source. The IMF of NGC 2264 was determined in two different ways. One method used the stellar mass from the SED fitting tool, the other used the stellar mass from CMDs. The first IMF showed a distinct peak at m <img entity='thkap' SRC='http://ej.iop.org/icons/Entities/ap.gif' ALT='≈' ALIGN='ABSMIDDLE' /> 2 M<SUB>☉</SUB>, but the second did not. We attribute the peak as an artifact of the SED fitting tool because there is no such IMF with a peak at m <img entity='thkap' SRC='http://ej.iop.org/icons/Entities/ap.gif' ALT='≈' ALIGN='ABSMIDDLE' /> 2 M<SUB>☉</SUB>. The slope of the IMF of NGC 2264 for massive stars (log m ≥ 0.5) is –1.7 ± 0.1, which is somewhat steeper than the so-called standard Salpeter-Kroupa IMF. We also present data for 79 young brown dwarf candidates.</P>
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Sung, Hwankyung,Bessell, Michael S.,Chun, Moo-Young,Yi, Jonghyuk,Nazé,, Y.,Lim, Beomdu,Karimov, R.,Rauw, G.,Park, Byeong-Gon,Hur, Hyeonoh American Astronomical Society 2017 The Astrophysical journal Supplement series Vol.230 No.1
<P>We present deep wide-field optical CCD photometry and mid-infrared Spitzer/IRAC and MIPS 24 mu m 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 H alpha 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-circle dot. 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.</P>
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THE INITIAL MASS FUNCTION AND YOUNG BROWN DWARF CANDIDATES IN NGC 2264. III. PHOTOMETRIC DATA
Sung, Hwankyung,Bessell, Michael S.,Chun, Moo-Young,Karimov, Rivkat,Ibrahimov, Mansur American Institute of Physics 2008 The Astronomical journal Vol.135 No.2
<P>We have performed deep wide-field CCD photometry of the young open cluster NGC 2264 to study the extent of star-forming regions (SFRs) and the shape of the initial mass function. In this paper, we present VRI and Hα photometry for more than 67,000 stars. From the spatial distribution of the selected Hα emission stars, we identify two active SFRs and a less active halo region surrounding these two SFRs. There are several Hα emission stars in the field region outside the halo region, and these may be newly formed stars in the Mon OB1 association surrounding the cluster. The locus of pre-main-sequence (PMS) stars in the I<SUB>C</SUB> versus V − I<SUB>C</SUB> diagram is revised from the distribution of Hα and X-ray emission stars in the diagram. The mean reddening of late-type PMS stars is estimated to be E(B − V) <img entity='thkap' SRC='http://ej.iop.org/icons/Entities/ap.gif' ALT='≈' ALIGN='ABSMIDDLE' /> 0.2 mag using the distribution of X-ray emission stars in the 2MASS color-color diagram. We can confirm that the Hα emission stars below the PMS locus (so-called BMS stars) are bona-fide members of NGC 2264 from their spatial distribution as well as from their near-IR excess in the 2MASS color-color diagram. In addition, four objects around IRS-2 detected with the Spitzer IRAC are also classified as BMS stars.</P>
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THE INITIAL MASS FUNCTION AND THE SURFACE DENSITY PROFILE OF NGC 6231
Sung, Hwankyung,Sana, Hugues,Bessell, Michael S. American Institute of Physics 2013 The Astronomical journal Vol.145 No.2
<P>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 R<SUB>V</SUB> = 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 M<SUB>☉</SUB>. The slope of the IMF of NGC 6231 (Γ = –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 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 <img entity='gsim' SRC='http://ej.iop.org/icons/Entities/gsim.gif' ALT='gsim' ALIGN='BASELINE' /> 8 M<SUB>☉</SUB>) 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) × 10<SUP>3</SUP> M<SUB>☉</SUB>. 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-rotating 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 timescale of ejection of the high-mass runaway star HD 153919 from NGC 6231, albeit the younger age cannot be entirely excluded.</P>
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SEJONG OPEN CLUSTER SURVEY (SOS). 0. TARGET SELECTION AND DATA ANALYSIS
Sung, Hwankyung,Lim, Beomdu,Bessell, Michael S.,Kim, Jinyoung S.,Hur, Hyeonoh,Chun, Moo-Young,Park, Byeong-Gon The Korean Astronomical Society 2013 Journal of The Korean Astronomical Society Vol.46 No.3
Star clusters are superb astrophysical laboratories containing cospatial and coeval samples of stars with similar chemical composition. We initiate 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' UBV I system. To achieve our main goal, we pay 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 - $T_{eff}$ relations, Sp - color relations, and $T_{eff}$ - 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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Hur, Hyeonoh,Sung, Hwankyung,Bessell, Michael S. American Institute of Physics 2012 The Astronomical journal Vol.143 No.2
<P>We present new UBVI<SUB>c</SUB> CCD photometry of the young open clusters Trumpler 14 (Tr 14) and Trumpler 16 (Tr 16) in the η Carina Nebula. We also identify the optical counterpart of Chandra X-ray sources and Two Micron All Sky Survey point sources. The members of the clusters were selected from the proper-motion study, spectral types, reddening characteristics, and X-ray or near-IR excess emission. An abnormal reddening law R<SUB>V, cl</SUB> = 4.4 ± 0.2 was obtained for the stars in the η Carina Nebula using the 141 early-type stars with high proper-motion membership probability (P<SUB>μ</SUB> <img entity='gE' SRC='http://ej.iop.org/icons/Entities/gE.gif' ALT='gE' ALIGN='BASELINE' /> 70%). We determined the distance to each cluster and conclude that Tr 14 and Tr 16 have practically the same distance modulus of V<SUB>0</SUB> – M<SUB>V</SUB> = 12.3 ± 0.2 mag (d = 2.9 ± 0.3 kpc). The slope of the initial mass function was determined to be Γ = –1.3 ± 0.1 for Tr 14, Γ = –1.3 ± 0.1 for Tr 16, and Γ = –1.4 ± 0.1 for all members in the observed region for the stars with log m<img entity='gE' SRC='http://ej.iop.org/icons/Entities/gE.gif' ALT='gE' ALIGN='BASELINE' />0.2. We also estimated the age of the clusters to be about 1-3 Myr from the evolutionary stage of evolved stars and low-mass pre-main-sequence stars.</P>
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