19 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
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 photometric data of the young open cluster
Westerlund 2. An abnormal reddening law of was found for
the highly reddened early-type members (), whereas a fairly
normal reddening law of was confirmed for the foreground
early-type stars (). 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
(random error) (the upper limit of systematic error) mag ( 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 down to . The total mass of Westerlund 2
is estimated to be at least 7,400 . 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 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
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.Comment: 13 pages, 7 figures, 3 tables, accepted for publication in MNRA