Galaxies contain not only stars, but also interstellar matter, mainly gas which is found under different phases, and at different temperatures: ionised gas at several millions of degrees Kelvin, atomic gas around 100 Kelvin and molecular gas at 10 Kelvin. Also gas may be found around stars in outflowing winds, as in e.g. the Solar Wind. One may suspect the existence of gas which is still cooler, but it would be extremely difficult to detect in emission. Depending on its amount this gas could play an important rôle in the physics of stars and galaxies. Two astronomers from the Observatory of Paris-Meudon, Thibaut Le Bertre et Eric Gérard, discovered atomic hydrogen at a temperature close to the cosmic background in the direction of the Asymptotic Giant Branch star IRC+10216. The observations have been done with the recently refurbished Nançay radio-telescope.

An image obtained in near-infrared (K' band) of the circumstellar dust shell around IRC+10216. It has been obtained with 6-m SAO russian telescope (Weigelt et al. 1998, A and A 333, L51)

Stars of mass comparable to the Sun (Mo) and up to 5-6 Mo, after having spent several billions of years burning quietly hydrogen into helium inside their cores, go through a more active phase in which hydrogen and helium are burnt into carbon. During this phase the stellar atmosphere increases in size considerably, by 2 to 3 orders of magnitude. The star is now a red giant on the Asymptotic Giant Branch (AGB). The atmosphere becomes unstable, so that the luminosity and the radius vary in time. It is also during this phase, that these stars undergo mass loss at a large rate, which may reach 10^-5 Mo per year, and with an expansion velocity in the range 5 to 20 km/s. This phase will last for typically one million years and therefore the stars get progressively enshrouded into a shell of gas and dust which may reach a size of about one light year before getting disolved into the interstellar medium.

The matter at the surface of a giant star is at a temperature between 2000 and 3000 degrees Kelvin. However when it is ejected it expands into a larger and larger volume, and therefore cools down. On the other hand, the matter is heated by the interstellar radiation and by cosmic rays. The expectation was that matter in expanding shells could be found at temperatures down to 10 to 15 Kelvin.

In fact, observations done recently with the Nançay radio-telescope in the atomic hydrogen (HI) line at 21 cm have shown that the matter around IRC+10216 could be still colder and be found at a temperature of 4 Kelvin, very close to the cosmic background one (3 Kelvin).

In the interstellar medium, the observations show that atomic hydrogen is in general at a temperature around 50 Kelvin, whereas in the molecular clouds which are more dense, the matter may be at temperatures around 10 to 15 Kelvin. However, observations of HI on the line of sight of extra-galactic radio-sources reveal the existence of interstellar regions where the atomic hydrogen could at a lower temperature, around 20 Kelvin. Recently the canadian astronomers, Knee and Brunt (Nature volume 412, page 308), have discovered a large atomic interstellar cloud which is very cold, with a temperature around 10 Kelvin.

The Nançay radio-telescope with which the observations have been obtained

(credit: Vincent Meens, CNES-Toulouse)

The new focal system of the Nançay radio-telescope

(credit: Vincent Meens, CNES-Toulouse)

Reference

Le Bertre T., Gérard E.: 2001, "Cold HI in IRC+10216", Astron. and Astrophys., 378, L29

Contact
Thibaut Le Bertre (DEMIRM, Paris Observatory)
Eric Gérard (ARPEGES, Paris Observatory)