Long-term Observations of Uranus and Neptune at 90 GHz with the IRAM 30 m Telescope. Astron
received date; accepted date: 23/01/2008 Context. The planets Uranus and Neptune with small apparent diameters are primary calibration standards. Aims. We investigate their variability at ∼ 90GHz using archived data taken at the IRAM 30m telescope during the 20 years period 1985 to 2005. Methods. We...
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| Format: | Text |
| Language: | English |
| Published: |
2008
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.244.7575 http://arxiv.org/pdf/0801.4452v1.pdf |
| Summary: | received date; accepted date: 23/01/2008 Context. The planets Uranus and Neptune with small apparent diameters are primary calibration standards. Aims. We investigate their variability at ∼ 90GHz using archived data taken at the IRAM 30m telescope during the 20 years period 1985 to 2005. Methods. We calibrate the planetary observations against non-variable secondary standards (NGC7027, 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 % ( ∼ 2σ) 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 8.6GHz by Klein & Hofstadter (2006). Both data sets do moreover show a rapid decrease of the Uranus brightness temperature during the year 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 calibration source, and if accuracies better than 10 % are required, the Uranus sub-earth point latitude needs to be taken into account. |
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