Virtual columns

Virtual calculations are out-of-memory calculations and manipulations you can perform on the columnar data directly from the Explorer webapp interface.

The functionality mirrors how one could download and use the dataset locally, calculating new quantities from the columns within the dataset. All calculations are processed server-side, however, without any local downloads required.

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Example: parallax surface gravity

We can roughly calculate surface gravity from the Gaia parallaxes (plx) and magnitude (g_mag). The formula for a star of \(1\mathrm{M_\odot}\) is simply:

\[\log g_{\mathrm{plx}} = \log g_{\odot} + 4 \log \left( \frac{T_{\mathrm{eff}}}{T_{\mathrm{eff},\odot}} \right) +0.4 (G + 5 \log(\omega \times 10^{-3}) + 5 - M_{\mathrm{bol},\odot})\]

Note that \(G\) is the Gaia G magnitude, we convert parallax \(\omega\) from milliarcseconds to arcseconds. In this example, we do not add extinction effects, but the relevant column does exist in the dataset.

We can enter this as a new column by clicking on the Virtual Calculations menu and adding a column. This can be written in the virtual column syntax for an ASPCAP subset, with the columns mass, teff, and g_mag as:

4.44 + 4*log10(teff/5770) + 0.4*(5*log10(plx*1e-3) + 0.25 + g_mag)

where \(\log g_{\odot} = 4.44\) and \(M_{bol,\odot} = 4.75\) and \(T_{\rm eff} = 5770\).

The Virtual Columns add menu, which opens when you click "Add Virtual Column" in the sidebar menu.