Long-term observations of Uranus and Neptune at 90 GHz with the IRAM 30 m telescope

(1985–2005)

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany e-mail: kramer@ph1.uni-koeln.de
² IRAM, Nucleo Central, Avda. Divina Pastora 7, 18012 Granada, Spain
³ LESIA (LAM – bât. 18), 5 Place Jules Janssen, 92195 Meudon, France
⁴ IRAM, 300 Rue de la Piscine, Domaine Universitaire, 38406 St. Martin d`Hères, France

Received: 24 April 2007
Accepted: 23 January 2008

Abstract

Context. The planets Uranus and Neptune with small apparent diameters are primary calibration standards.

Aims. We investigate their variability at ~90 GHz using archived data taken with the IRAM 30 m telescope during the 20 year period 1985 to 2005.

Methods. We calibrate the planetary observations against non-variable secondary standards (NGC 7027, NGC 7538, W3OH, K3-50A) observed almost simultaneously.

Results. Between 1985 and 2005, the viewing angle of Uranus changed from south-pole to equatorial. We find that the disk brightness temperature declines by almost 10% (~) over this time span indicating that the south-pole region is significantly brighter than average. Our finding is consistent with recent long-term radio observations at 8.6 GHz. Both data sets show a rapid decrease of the Uranus brightness temperature during 1993, indicating a temporal, planetary scale change. We do not find indications for a variation of Neptune's brightness temperature at the 8% level.

Conclusions. If Uranus is to be used as a calibration source, and if accuracies better than 10% are required, the Uranus sub-earth point latitude needs to be taken into account.