Hwankyung Sung

[ITAL]UBVRI[/ITAL] and Hα Photometry of the Young Open Cluster NGC 6231

New UBVI and Hα photometry has been performed for the young open cluster NGC 2244. We classified 30 OB stars as being members of the cluster using proper-motion data and spectral types from previous investigators, along with photometric diagrams obtained in this study. We measured Hα emission strength of the stars by Hα photometry and set up a selection criterion to select pre–main-sequence (PMS) stars with Hα emission. Fourteen PMS stars and seven PMS candidates were found using the criterion. In addition, six stars found near the positions of ROSAT HRI X-ray sources were assumed to be PMS stars and the optical counterparts of these X-ray sources. We determined a reddening of E(B-V) = 0.47 ± 0.04 for the cluster and a total-to-selective extinction ratio of RV = 3.1 ± 0.2. The distance modulus derived was V0 - MV = 11.1. By comparing our photometric results with theoretical evolution models, we derived a main-sequence turnoff age of 1.9 Myr and a PMS age spread of about 6 Myr. The slope of the initial mass function, Γ, calculated in the mass range 0.5 ≤ log m ≤ 2.0 could be flat (Γ = -0.7 ± 0.1).

UBVRI and Hα Photometry of the Young Open Cluster NGC 6231

New UBVI and Hα photometry has been performed for the young open cluster NGC 2244. We classified 30 OB stars as being members of the cluster using proper-motion data and spectral types from previous investigators, along with photometric diagrams obtained in this study. We measured Hα emission strength of the stars by Hα photometry and set up a selection criterion to select pre–main-sequence (PMS) stars with Hα emission. Fourteen PMS stars and seven PMS candidates were found using the criterion. In addition, six stars found near the positions of ROSAT HRI X-ray sources were assumed to be PMS stars and the optical counterparts of these X-ray sources. We determined a reddening of E(B-V) = 0.47 ± 0.04 for the cluster and a total-to-selective extinction ratio of RV = 3.1 ± 0.2. The distance modulus derived was V0 - MV = 11.1. By comparing our photometric results with theoretical evolution models, we derived a main-sequence turnoff age of 1.9 Myr and a PMS age spread of about 6 Myr. The slope of the initial mass function, Γ, calculated in the mass range 0.5 ≤ log m ≤ 2.0 could be flat (Γ = -0.7 ± 0.1).

The Pre-Main-Sequence Stars and Initial Mass Function of NGC 2264

UBVRI and Hα CCD photometry has been performed in the southern region of NGC 2264 around the Cone Nebula. A nearly complete list of pre–main-sequence (PMS) members in the cluster has been made from the compilation of Hα emission stars identified in this study plus those selected in previous investigations, together with stars with X-ray emission. Using the H-R diagram, we tested a set of four PMS evolution models based on the PMS age and age spread estimate for the cluster and the mass-age relationship among individual stars. The resultant initial mass function (IMF) is in good agreement with the field star IMF of the solar neighborhood, with an IMF slope Γ = -1.7 in the mass range 0.3 ≤ log m ≤ 0.8.

The Open Cluster NGC 2516. I. Optical Photometry

We present UBVI CCD photometry of the intermediate-age open cluster NGC 2516. From this new photometry we derive the cluster parameters [V0 - MV = 7.77 ± 0.11 mag, E(B-V) = 0.112 ± 0.024 mag], age [log (age) = 8.2 ± 0.1], and a photometric abundance ([Fe/H] = -0.10 ± 0.04) from a comparison with theoretical isochrones of the Padua group. We find that the X-ray bright stars [log LX(ergs s-1)29.0] are systematically bluer in B-V and U-B. It was necessary to modify the photometric membership criterion used by Sung & Bessell due to the UV excess of X-ray bright stars and due to the abrupt increase in the slope of the (MV, B-V) ZAMS relation at B-V1.4. A minimum binary frequency 40% (±5%) was estimated from the distribution of distance moduli of unevolved photometric members. The photometric abundance obtained in this study is similar to that of the Pleiades and therefore the lower metallicity is not the cause of strong X-ray emission of F-type stars in NGC 2516. We believe that this results from them being close binaries, as suggested by their different distribution of distance moduli compared to other cluster members and surmise that the tidal forces between the two stars must increase their X-ray activity.

The Initial Mass Function and Young Brown Dwarf Candidates in NGC 2264. I. The Initial Mass Function around S Monocerotis

The young open cluster NGC 2264 was observed on 2002 January 6?7 with the CFH12K mosaic CCD on the 3.6 m Canada-France-Hawaii Telescope to investigate the shape of the initial mass function (IMF). In order to select the pre?main-sequence (PMS) members more completely, Chandra X-Ray Observatory archival data were retrieved and used for the identification of X-ray?emitting weak-line T Tauri stars. In addition, the (R-I, V-I) diagram was used to exclude background late-type giants, and a statistical approach was applied to estimate the number of contaminating foreground main-sequence stars. The shape of the IMF of the Chandra field of view is flat between log m = -0.4 and ~-1.0 (m in solar mass units), with a weak signature of a dip near log m = -0.2. The overall shape of the IMF is very similar to those obtained for the Pleiades or the Trapezium. The fraction of disked PMS stars, as noted by Rebull and coworkers, does not show any signature of mass dependency. The age of PMS stars between log m = 0 and -1 is 3.1 ? 0.5 Myr.

The Initial Mass Function and Young Brown Dwarf Candidates in NGC 2264. III. Photometric Data

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 IC versus V − IC 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) 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.

An XMM-Newton view of the young open cluster NGC 6231 ⋆ III. Optically faint X-ray sources

We discuss the properties of the X-ray sources with faint optical counterparts in the very young open cluster NGC 6231. From their positions in the Hertzsprung‐Russell diagram, we find that the bulk of these objects probably consists of low-mass pre-main-sequence (PMS) stars with masses in the range 0.3‐3.0 M � . The age distribution of these objects indicates that low-mass star formation in NGC 6231 started more than 10 Myr ago and culminated in a starburst-like event about 1‐4 Myr ago when the bulk of the low-mass PMS stars as well as the massive cluster members formed. We find no evidence for a spatial age gradient that could point towards a sequential star formation process. Only a few X-ray sources have counterparts with a reddening exceeding the average value of the cluster or with infrared colours indicating the presence of a moderate near-IR excess. The X-ray spectra of the brightest PMS sources are best fitted by rather hard thermal plasma models and a significant fraction of these sources display flares in their light curve. The X-ray brightest flaring sources have decay times between 2 and 16 ks. The X-ray selected PMS stars in NGC 6231 have log LX/Lbol values that increase strongly with decreasing bolometric luminosity and can reach a saturation level (log LX/Lbol ∼ −2.4) for non-flaring sources and even more extreme values during flares.

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 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-Cousin’ UBVI 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.

Standard stars: CCD photometry, transformations and comparisons

We discuss variations of the atmospheric extinction coefficients and transformation equations to the standard UBVRI system based on observations of standard stars during 1996-1997 at Siding Spring Observatory using a thinned SITe CCD and coloured glass filters. In the transformation from the initial natural system to the Landolt version of the standard system, a large nonlinear term related to the Balmer discontinuity was required for the U transformation. We then modified the U filter, and the subsequent transformation to the SAAO version of the standard UBVRI system had only small nonlinear correction terms for U , B , and I. The correction terms relating to U and B are evidently due to the Balmer discontinuity, while that relating to I seems to be due to the Paschen discontinuity at λ ≈ 8200 A. We also compared the results with Landolts observations, and confirmed the difference between the two sets of standard stars (SAAO and Landolt).

CCD PHOTOMETRY OF STANDARD STARS AT MAIDANAK ASTRONOMICAL OBSERVATORY IN UZBEKSTAN: TRANSFORMATIONS AND COMPARISONS

Observation of standard stars is of crucial importance in stellar photometry. We have studied the standard transformation relations of the UBV RI CCD photometric system at the Maidanak Astronomical Observatory in Uzbekistan. All observations were made with the AZT-22 1.5m telescope, SITe 2k CCD or Fairchild 486 CCD, and standard Bessell UBV RI filters from 2003 August to 2007 September. We observed many standard stars around the celestial equator observed by SAAO astronomers. The atmospheric extinction coecients, photometric zero points, and time variation of photometric zero points of each night were determined. Secondary extinction coecients and photometric zero points were very stable, while primary extinction coecients showed a distinct seasonal variation. We also determined the transformation coecients for each filter. For B, V , R, and I filters, the transformation to the SAAO standard system could be achieved with a straight line or a combination of two straight lines. However, in the case of the U filter and Fairchild 486 CCD combination, a significant non-linear correction term - related to the size of Balmer jump or the strength of the Balmer lines - of up to 0:08 mags was required. We found that our data matched well the SAAO photometry in V , B i V , V i I, and R i I. But in U i B, the dierence in zero point was about 3 :6 mmag and the scatter was about 0:02 mag. We attribute the relatively large scatter in U iB to the larger error in U of the SAAO photometry. We confirm the mostly small dierences between the SAAO standard UBV RI system and the Landolt standard system. We also attempted to interpret the seasonal variation of the atmospheric extinction coecients in the context of scattering sources in the earth’s atmosphere.