2006A&A...454..781L

We present a 8°x6°, high resolution extinction map of the Pipe nebula using 4.5 million stars from the Two Micron All Sky Survey (2MASS) point source catalog. The use of Nicer (Lombardi & Alves, 2001A&A...377.1023L), a robust and optimal technique to map the dust column density, allows us to detect a A_V=0.5mag extinction at a 3-σ level with a 1-arcmin resolution. (i) We find for the Pipe nebula a normal reddening law, E(J-H)=(1.85/-0.15)E(H-K). (ii) We measure the cloud distance using Hipparchos and Tycho parallaxes, and obtain 130⁺²⁴_–58pc. This, together with the total estimated mass, 10⁴M_☉, makes the Pipe the closest massive cloud complex to Earth. (iii) We compare the Nicer extinction map to the NANTEN ¹²CO observations and derive with unprecedented accuracy the relationship between the near-infrared extinction and the ¹²CO column density and hence (indirectly) the ¹²CO X-factor, that we estimate to be 2.91x10²⁰/cm2/K/km/s in the range A_V ∈[0.9, 5.4]mag. (iv) We identify approximately 1500 OH/IR stars located within the Galactic bulge in the direction of the Pipe field. This represents a significant increase of the known numbers of such stars in the Galaxy. Our analysis confirms the power and simplicity of the color excess technique to study molecular clouds. The comparison with the NANTEN ¹²CO data corroborates the insensitivity of CO observations to low column densities (up to approximately 2mag in A_V), and shows also an irreducible uncertainty in the dust-CO correlation of about 1mag of visual extinction.