2MASS wide field extinction maps. I. The Pipe nebula

Context: . Aims: . We present a 8°×6°, high resolution extinction map of the Pipe nebula using 4.5 million stars from the Two Micron All Sky Survey (2MASS) point source catalog. Methods: . The use of Nicer (Lombardi & Alves 2001, A&A, 377,1023), a robust and optimal technique to map the dust column density,allows us to detect a AV = 0.5 mag extinction at a 3-σlevel with a 1 arcmin resolution. Results: . (i ) We find forthe 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+24-58 pc. This,together with the total estimated mass, 10^4 {M}_⊙, 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 2.91 ×1020 cm-2 K-1 km-1 s in the range AV in [0.9, 5.4] mag. (iv ) We identify approximately1500 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. Conclusions: . 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 2 mag in A_V), and shows also an irreducible uncertainty in the dust-CO correlation of about 1 mag of visual extinction.