Photoelectric Photometry Newsletter

Hipparcos

Hipparcos was a European Space Agency satellite whose prime purpose was to measure parallaxes (and therefore distances) of over 100,000 stars with a high degree of accuracy. Another million stars were measured with a lower accuracy. These results have already provided new insights into many areas of astronomy. AAVSO visual observers played a crucial role in this mission, by providing information on the brightness of large amplitude variables such as Miras, so that the exposure times of the measurements would be appropriate. Our director Janet Mattei was the coordinator of this effort. Her excellent work, and that of the AAVSO visual observers, was deeply appreciated and acknowledged by the Hipparcos project team.

As a by-product of these missions, the satellite monitored the brightness of these stars over a 3.5-year period. Up to 300 observations of each star were obtained of each star. These measurements are a "gold mine" of information about stellar variability, which can be used by astronomers and students for years to come.

The Hipparcos data were first released to the scientists who had proposed the measurements, or who had been otherwise involved in the planning and execution of the mission. The first results from the data were presented in May at "Hipparcos Venice 197", and the proceedings of this symposium have just been distributed as ESA Special Publication 402. It is a massive document - 862 pages of frontier science. The Hipparcos data were publicly released in the summer - on CD-ROM, in hard copy, and on the Internet.

All of the AAVSO photoelectric program stars were observed, both photometrically and astrometrically, by Hipparcos. I was interested to see how the Hipparcos results compared with our ground-based results which, in the case of the small-amplitude red variables, were published last year in the Publications of the Astronomical Society of the Pacific.

The Hipparcos "epoch photometry" (as the individual photometric measurements are called) has already been extensively analyzed. For the stars which were periodic - i.e. for which a period could be found - the phase curves were published as part of the Hipparcos database. Stars which were variable, but not obviously periodic, were classified as 'unsolved".

Almost all of the AAVSO SARV's were classified as "unsolved". This was undoubtedly because the variability is complex. For many of the stars, there are both short-term (weeks) and long-term (months to years) variations. Even the most periodic of the SARV's - EU Del with a period of 62.74 days - varies irregularly in amplitude and light curve shape.

This suggests to me that the AAVSO photometry and the Hipparcos photometry are complementary - the AAVSO photometry extends for ten years or more; the Hipparcos photometry was more concentrated, and does not have the unfortunate seasonal gaps that plague ground-based data. This is one more example of the fact that grou-nd-based and space-based astronomy are complementary. Even visual observations have a major role to play in astronomy in the space age - as every AAVSO observer knows!