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A Cold, Nearby Cloud in the Local Bubble
Verschuur (1969) Knapp & Verschuur (1972) - identify cloud complex with very narrow H I 21 cm emission indicative of T < 25 K in high Galactic latitude region Crovisier & Kazes (1980) Crovisier et al. (1985) - H I 21 cm absorption toward 3C 225 & 3C 237 indicative of cloud H I spin T of 20 K Heiles & Troland (2003) - map cloud using Leiden-Dwingeloo survey data (above); measure cloud H I spin T of 22 & 17 K and log N(H I) of & from 3C 225a,b (6 arcmin sep.) 21 cm absorption Distance unknown…
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Interstellar Na I Absorption in 3C 225 Region
Recent high-resolution (1.3 km/s) observations with the KPNO Coude Feed have revealed interstellar Na I toward a number of nearby stars in the 3C 225 region. The Na I lines are strong & narrow - easily detected toward even late-type stars!
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The Na I lines are detected at the H I cloud velocity (vLSR=3
The Na I lines are detected at the H I cloud velocity (vLSR=3.5 km/s) toward 23 stars The nearest of these stars are at well-measured distances of 41.5, 42.1, and 42.8 pc Nine of the stars (such as HD above) exhibit additional interstellar Na I absorption components - the nearest of these stars are at distances of about 160 pc
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Conclusions 1. Assuming similar turbulent H I & Na I motions, a comparison of the H I & Na I absorption line-widths in the area of 3C 225 yields a cloud temperature of 20 K and a turbulent velocity of 0.4 km/s 2. The distance of the cloud must be less than 45 pc 3. Since the “edge” of the Local Bubble has been measured to be about 100 pc in the 3C 225 direction (Lallement et al. 2003), this cold cloud must be located well within the tenuous local ISM Thus, the 3C 225 cloud may be the coldest, nearest ISM cloud Continuing work includes an Na I/H I study of the cloud’s small-scale structure as well as additional observations to better constrain its distance
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