1. 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…

2. 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!

3. 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

5. 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