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Sana, H.,Rauw, G.,Sung, H.,Gosset, E.,Vreux, J.-M. Blackwell Publishing Ltd 2007 MONTHLY NOTICES- ROYAL ASTRONOMICAL SOCIETY Vol.377 No.3
<P>ABSTRACT</P><P>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<SUB>⊙</SUB>. 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 <I>L</I><SUB>X</SUB>/<I>L</I><SUB>bol</SUB> values that increase strongly with decreasing bolometric luminosity and can reach a saturation level (log <I>L</I><SUB>X</SUB>/<I>L</I><SUB>bol</SUB>∼−2.4) for non-flaring sources and even more extreme values during flares.</P>
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>