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.Comment: 21 pages, 16 figures, accepted for publication in J. of Korean Astronomical Society (JKAS

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.Comment: 45 pages, 32 figures, 9 tables, accepted for publication in ApJ

Reddening, distance, and stellar content of the young open cluster Westerlund 2

We present deep UBVIC photometric data of the young open cluster Westerlund 2. An abnormal reddening law of RV, cl = 4.14 ± 0.08 was found for the highly reddened early-type members (E(B−V)≧1.45E(B−V)≧1.45), whereas a fairly normal reddening law of RV, fg = 3.33 ± 0.03 was confirmed for the foreground early-type stars (E(B − V)fg < 1.05). The distance modulus was determined from zero-age main-sequence fitting to the reddening-corrected colour–magnitude diagram of the early-type members to be V0 − MV = 13.9 ± 0.14 (random error) +0.4−0.1−0.1+0.4 (the upper limit of systematic error) mag (d=6.0±0.4+1.2−0.3d=6.0±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 (Galactic Legacy Infrared Mid-Plane Survey Extraordinaire) source catalogue. The initial mass function shows a shallow slope of Γ = −1.1 ± 0.1 down to log m = 0.7. The total mass of Westerlund 2 is estimated to be at least 7 400 M⊙. The age of Westerlund 2 from the main-sequence turn-on and PMS stars is estimated to be ≲ 1.5 Myr. We confirmed the existence of a clump of PMS stars located ∼1 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

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 $\Gamma=-1.1 \pm 0.1$ 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.Comment: 25 pages, 22 figures, 6 tables, accepted for publication in MNRA

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 〈E(B-V)〉 = 4.19 ± 0.23 and 〈E(J-KS )〉 = 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 × 104 M⊙. 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

The Wide Integral Field Infrared Spectrograph: Commissioning Results and On-sky Performance

We have recently commissioned a novel infrared ($0.9-1.7$ $\mu$m) integral field spectrograph (IFS) called the Wide Integral Field Infrared Spectrograph (WIFIS). WIFIS is a unique instrument that offers a very large field-of-view (50$^{\prime\prime}$ x 20$^{\prime\prime}$) on the 2.3-meter Bok telescope at Kitt Peak, USA for seeing-limited observations at moderate spectral resolving power. The measured spatial sampling scale is $\sim1\times1^{\prime\prime}$ and its spectral resolving power is $R\sim2,500$ and $3,000$ in the $zJ$ ($0.9-1.35$ $\mu$m) and $H_{short}$ ($1.5-1.7$ $\mu$m) modes, respectively. WIFIS's corresponding etendue is larger than existing near-infrared (NIR) IFSes, which are mostly designed to work with adaptive optics systems and therefore have very narrow fields. For this reason, this instrument is specifically suited for studying very extended objects in the near-infrared such as supernovae remnants, galactic star forming regions, and nearby galaxies, which are not easily accessible by other NIR IFSes. This enables scientific programs that were not originally possible, such as detailed surveys of a large number of nearby galaxies or a full accounting of nucleosynthetic yields of Milky Way supernova remnants. WIFIS is also designed to be easily adaptable to be used with larger telescopes. In this paper, we report on the overall performance characteristics of the instrument, which were measured during our commissioning runs in the second half of 2017. We present measurements of spectral resolving power, image quality, instrumental background, and overall efficiency and sensitivity of WIFIS and compare them with our design expectations. Finally, we present a few example observations that demonstrate WIFIS's full capability to carry out infrared imaging spectroscopy of extended objects, which is enabled by our custom data reduction pipeline.Comment: Published in the Proceedings of SPIE Astronomical Telescopes and Instrumentation 2018. 17 pages, 13 figure