The solirad (So) as a Convenient Unit for Quoting Astronomical Irradiances for Planetary Insolations and Exoplanetary Instellations

Measurements of physical parameters for stars and (exo)planets are often quoted in units normalized to the Sun and/or Earth. The nominal total solar irradiance, SN ⊙ , while based on a current best estimate with uncertainties, was adopted to be an exact reference value of 1361 W m−2 by IAU 2015 Resolution B3, corresponding to “the mean total electromagnetic energy from the Sun, integrated over all wavelengths, incident per unit area per unit time at distance 1 au”. In the planetary and exoplanetary science literature, the units employed for “flux”, “insolation”, “instellation”, etc., are often cumbersome or inconsistent. To simplify the quoting of irradiance units for astronomical applications, we introduce the portmanteau solirad, short for solar irradiance, as an abbreviated version of the longer IAU term “nominal total solar irradiance”. The solirad (So) is a unit of irradiance, where 1 solirad = 1 So = 1361 W m−2, equivalent to the IAU nominal total solar irradiance, and to an apparent bolometric magnitude of mbol ≃−26.832 mag (per IAU 2015 Resolution B2).

恒星与（系外）行星的物理参数测量结果，通常采用以太阳和/或地球为基准的归一化单位进行表述。标称总太阳辐照度$S_Nodot$虽基于当前带不确定度的最优估算值，但被国际天文学联合会（International Astronomical Union，IAU）2015年B3号决议采纳为精确参考值，即1361 W·m⁻²，其定义为“在1天文单位（astronomical unit，au）距离处，单位时间内入射到单位面积上的、太阳全波长范围的总电磁辐射平均能量”。在行星科学与系外行星科学的学术文献中，“flux（辐照度）”“insolation（日照入射通量）”“instellation（恒星光入射辐照度）”等术语对应的单位往往繁琐且不统一。为简化天文应用中辐照度单位的表述，我们将“solar irradiance（太阳辐照度）”拼合为混成词solirad，作为IAU定义的“标称总太阳辐照度”的简化术语。solirad（记作So）是辐照度单位，满足1 solirad = 1 So = 1361 W·m⁻²，与IAU定义的标称总太阳辐照度完全等效，同时对应视热星等$m_{bol} approx -26.832$星等（依据IAU 2015年B2号决议）。