Working group on nominal units for stellar and planetary astronomy

Parent Divisions: G and A
Support from Div. F, H, J

Chairman: Petr Harmanec
Deputy chairmen: Andrej Prsa, Guillermo Torres


Martin Asplund Div. H
Philip D. Bennett C36
Nicole Capitaine Div. A
Jorgen Christensen-Dalagaard Div. H
Eric Depagne C27
William Folkner Div. A, Div. F
Margit Haberreiter C36
Saskia Hekker C27
James Hilton Div. A
Donald Kurtz C25
Jaques Laskar Div. A
Eric E. Mamajek
Brian D. Mason C26
Eugene F. Milone C27
Michele M. Montgomery C27
Mercedes Richards C42
Susan Stewart Div. A

The basic properties of stars such as mass, radius, and luminosity have been traditionally expressed in the units of solar mass, solar radius and solar luminosity. With the rapidly increasing precision of astronomical observations and progress in theoretical modeling, this practice can lead to systematic errors because there is no consensus in the astronomical community on the values of those solar properties in SI (or cgs) units. Inconsistencies are already being demonstrated from different investigators using slightly different values for the conversion constants. A similar situation is presented in the field of extrasolar planets, where properties are typically expressed in terms of Jupiter or Earth's properties.

The task of this inter-divisional Working Group is to establish a set of nominal values (exact numbers) for conversion constants that are used to transform quantities in solar and planetary units to the SI or cgs system of units. To preserve practical comparison with the already published values, it is convenient to choose the nominal conversion constants as close as possible to the most accurately observed properties of the Sun, Jupiter and Earth to date.

Beyond this main topic, which will eventually converge to a proposal for the IAU resolution, the WG will also address the following topics where prolonged and continued discussion is foreseen:

1. We will make an effort to address the problem of the zero point of the scale of bolometric magnitudes and corrections, and discuss thoroughly whether some generally acceptable recommendations could be found.

2. The Working Group will also try to clarify potential ambiguities in the definition of orbital period and semi-major axes in more complicated situations in stellar and planetary astronomy.

This implies that the new WG will likely be active for more than one IAU triennial.