8.4 The atmospheric absorption spectrum at millimeter wavelengths, ATM

Calculations of zenith atmosphere opacity at 2.5 and 2.9 km, the altitude of the IRAM sites, have been made by [Cernicharo 1988] and [Cernicharo & Pardo 1999]. A computer programme, ATM, repeating these calculations has been installed on-line on the IRAM telescopes of Pico Veleta and Plateau de Bure; it is activated at each calibration or skydip and allows to interpret the observed sky emissivity in terms of water and oxygen contributions and of upper and lower sideband opacities. (During skydips, the antenna is pointed successively at different elevations and the emission of the sky measured at each step; the sky emission variation is fitted by an exponential function of the air mass, and the atmosphere opacity and average temperature readily derived). Note that the opacities derived from sky emissivity observations do not always agree with those calculated from the measurement of p, T, and RH on the site, as water vapor is not at hydrostatic equilibrium.

Some of the results for the band 20-500 GHz are shown on Fig.8.4. One recognizes from left to right, the (blended) forest of fine structure transitions from O₂, near 60 GHz, the $1,1\leftarrow 0,1$ fine structure line of O₂ at 118.75 GHz, the third lowest lines of para water (still 200 K above the ground level), at 183.31 GHz, and the fourth ortho water line (420 K above ortho ground level), at 325.15 GHz. The fundamental line of ortho water $(1_{10}\leftarrow 1_{01})$, at 556.94 GHz is visible. The water and oxygen lines delineate the 4 atmospheric ``windows'' of the millimetre spectrum (called the 3 mm, 2 mm, 1.3 mm and 0.8 mm windows). Water is seen to dominate completely atmospheric absorption above 150 GHz.

  

Figure: Zenith opacity of the standard winter atmosphere at an altitude of 2.5 km for 1 mm of precipitable water vapor, as a function of frequency (GHz). The contributions of H₂O, O₂ and O₃ are shown in the three upper panels. The red line shows the same zenith opacity at a scale of 1/20. One may note the importance of the water line wings above 150 GHz, compared to those of O₂ (a consequence of the absence of electric dipole moment in the latter molecule) and O₃. Courtesy [Cernicharo & Pardo 1999]

Less than one millimeter of precipitable water vapor corresponds to exceptionally good winter weather conditions on the sites of Pico Veleta and Plateau de Bure. Such conditions seldom happen even on Mauna Kea. Two to three millimeter of water are standard by clear winter nights at these observatories; six to ten millimeter of water are typical of clear summer nights.

The typical zenith atmosphere opacities, in the dips of the 1.3 mm and 0.8 mm windows (e.g. at the frequencies of the J=2-1 and 3-2 rotational transitions of CO, 230.54 and 345.80 GHz ), are respectively 0.15-0.2 and 0.5-0.7 in winter. The astronomical signals at these frequencies are attenuated by factors of respectively $\simeq 1.2$ and 2 at zenith, 1.3 and 2.8 at 45 degree elevation, and 1.7 and 6 at 20 degree elevation. Larger attenuations are the rule in summer and in winter by less favorable conditions.

The J=1-0 line of CO, at 115.27 GHz, is close to the 118.75 GHz oxygen line. Although this latter is relatively narrow, it raises by $\simeq$0.3 the atmosphere opacity (which is 0.35-0.4). The atmosphere attenuation is then intermediate between those at 230 and 345 GHz (by dry weather, however, it is more stable than the latter, since the water contribution is small). The measurement of accurate CO line intensity ratios (even not considering the problems linked to differences in beam size and receiver sideband gain ratios) requires therefore good weather, a high source elevation, and a careful monitoring of the atmosphere.

A catalogue of lines intensities in several standard astronomical sources, measured with the IRAM 30 m telescope has been published [Mauersberger et al 1989]. The lines intensities were calibrated by the chopper-wheel method, following the above recipes. The reader is referred to this report for details.