avacuum wavelength.

b critical density for collisions with electrons at Te=104 K.

spectrum widely used to study the molecular jets usually associated with young Class 0 and I protostars. The far-IR, on the other hand, contains the rotational transitions of the most abundant molecules in non-dissociative shocked gas, such as CO and H2O.

Observations in the IR, however, suffer several limitations. At ground, atmospheric transmission is above 50% only in few windows below 20 |m. Longer wavelengths can be observed only with telescopes mounted on aircraft or satellite missions. In addition to the absorption, the atmosphere emits in the infrared, introducing a strong background that needs to be removed from the scientific observations. Below 2.2 |m, the main contribution to the sky emission comes from the forest of strong OH roto-vibrational lines. Above 2.5 | m there is thermal emission both by the atmosphere and by the telescope with its warm optics that dominates the background. Chopping and beam-switching techniques are required to limit the small scale spatial and temporal fluctuations of this background. Because of that, observations above 2.5 |m from ground remain limited in sensitivity and are much more efficiently done from space.

The main limitation on the spatial resolution that can be reached in the IR comes from diffraction. For a 8 m telescope at a wavelength of 2 |m, the diffraction disk corresponds to an angular diameter of ~ 0" = 0.5A|m/Dm = 0.12", while at 10|m the angular diameter is 0.9". The diffraction limit at a given wavelength can be compared to the seeing disk, whose size is described as A/r0, where r0, the Fried parameter, is the length over which the incoming wave-front is not significantly disturbed by motions in the atmosphere. r0 depends on the wavelength as r0 x A6/5, with the seeing disk varying as 0 x A-0 2. At 2 |m, in good atmospheric conditions, the seeing is of the order of 0.4-0.5", which means that the resolution of an 8-m diffraction-limited IR instrument is about a factor of 4 better than a seeing-limited instrument. Adaptive Optics (AO) devices can, therefore, improve the spatial resolution of a near-IR device on a large telescope, but this improvement becomes less and less important at longer wavelengths or with telescopes 4 m or less. For example, the diffraction-limited NICS camera on HST (2.4 m) has a resolution similar to an 8-m seeing-limited telescope on ground (Fig. 1).

Furthermore, the big limitation of the so-far operational AO systems is the need of optical guiding stars in a relatively small field of view of a dozen arcsec. In practice, this implies that the target source should be relatively bright in the R band, making it impossible to use these devices to observe embedded YSOs. In the study of jets, this is a significant limitation, since bright IR lines are usually observed more frequently in jets from very young and optically invisible protostars. At present, the only instrument making use of a guiding star IR sensor is NACO working at VLT; it, however, requires stars with K magnitude greater than 8, making limiting its use to only few targets. In the near future, the use of artificial laser guide stars will alleviate this problem, although long exposures will still need a natural star for tip-tilt motions not corrected with laser (see the lecture by Esposito in this volume).

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