X-ray Study of Low-mass Young Stellar Objects in the ρ Ophiuchi ...

Chapter 3Review of the ρ Ophiuchi Cloud3.1 The ρ Ophiuchi Molecular CloudThe ρ Ophiuchi molecular cloud (hereafter, ρ Oph) is one of the nearest and most well-knownstar-forming regions in our Galaxy, which is located about 15 ◦ above the Galactic center (Figure2.3).The distance to ρ Oph is somewhat uncertain. Chini (1981) made optical–infrared photometricobservations and estimated the distance to be 165±20 pc from the mean distance modulus of threebright and heavily reddened stars. Alternatively, Knude & Høg (1998) calculated optical reddening(E B−V ) of MS stars and giants using the Hipparcos and T ycho catalogues, then found an abruptincrease of E B−V for stars with the distance larger than 120 pc. Recently, however, de Zeeuw etal. (1999) proposed the distance of 145±2 pc from the Hipparcos positions, proper motions, andparallaxes. In this thesis, we adopt the distance of 145 pc.The total size of the ρ Oph complex is ∼10 ◦ × 20 ◦ ( ∼ = 25 pc × 50 pc). The ongoing starformation is less active in the north-east region but is concentrated in the south-west region withthe size of ∼1 pc × 2 pc called “ρ Oph main body”. The radio observations of the ρ Oph main bodyregion have been frequently made since 1980’s. The C 18 O J=1–0 (Wilking & Lada, 1983; Tachihara,Mizuno, & Fukui, 2000) and 13 CO J=1–0 (Loren, 1989) mappings revealed the distribution of lowdensity molecular gas and were used to estimate the total mass and average density of the mainbody to be 500–1000 M ⊙ and 10 4 –10 5 cm −3 , respectively. Loren, Wootten, & Wilking (1990)observed higher density gas with DCO + emission lines and divided the main body into sevensmaller scale cores called ρ Oph A, B1, B2, C, D, E, and F. The scale, mass, and density of thesecores are ∼0.1 pc, 10–100 M ⊙ , and 10 5 –10 6 cm −3 , respectively. The millimeter-submillimeter radiocontinuum emission (850 µm: Johnstone et al. 2000; 1.3 mm: Motte et al. 1998; see also Figures15