MRT Unit Standards

As more scientific analyses and observations are interlinked via the web it becomes more important to adopt standardized units so that information can be easily compared and used outside of specialized fields. In minimize potential confusion, the AAS machine readable (MR) tables use standard SI units in most cases. For example, the string 0.1nm to express Ångströms (Å=10^-10m), since the Ångström is a non-standard unit. Another example is mW/m2, the milliwatt per m², which is identical to the CGS erg/cm²/s unit that is not used outside astronomy.

Unit syntax should abide by the following rules:

• Rule 1: any unit is described by a single word — no space is allowed. For instance, the Ångström is coded 0.1nm, and never 0.1 nm; the kilometer-per-second is coded km/s, or km.s-1, but never km / s or km s-1.
• Rule 2: the only allowed numerical factor is at the very beginning of the Unit string. The structure of the unit is therefore the concatenation of factor and unit_expression and we will write the “number of pixels per Å” as 10pix/nm, and neither pix/0.1nm nor pix/(0.1nm). The numerical factor may include the letter x for the multiplication, as 1.5×10+11 to express the number 1.5×10¹¹
• Rule 3: The operators to express a compound unit are
  • / for the division — as in km/s
  • . for the multiplication (the dot is however understood as a decimal point in the leading numerical factor) — as in kW.h
  • nothing for a power — as m2 for m² or 10+21 for 10²¹.

  Note that + or – signs are not operators, but represent the leading sign of numeric values.

• Rule 4: a simple (non-compound) unit is made of a basic unit symbol, eventually preceded by a multiple prefix.

When possible choose the shortest syntax. For example, km/s to km.s-1.

Only simple power functions of physical units are accepted, which means that e.g. Msun^3/2 (solar mass at a 3/2 power) is not valid. However for units expressed on a base 10 logarithmic scale, use bracketed units such as [Msun] to indicate logM_{sun}.

Note that a more extensive list of allowed units is provided by VizieR.

The basic symbols listed include basic standard SI units (b), the extensions listed by the IAU style book marked (a), other frequent physical extensions (e), and a few further astronmical extensions (c).