41 research outputs found
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.Comment: 79 pages, 21 pages, 7 tables, Accepted for publication in the
Astronomical Journa
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.Comment: 53 pages, 12 figures, 4 tables, Accepted for publication in the
Astrophysical Journa
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' 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
Sejong open cluster survey (SOS) - III. The young open cluster NGC 1893 in the H II region W8
We present a UBVI and Hα photometric study of the young open cluster NGC 1893 in the HII 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 ± 0.08