Stellar model atmospheres

An //Extract Stellar// request computes line strengths in a model atmosphere, and the request has to name one. The form offers two grids and an upload:

• ATLAS9 (Castelli & Kurucz) - the default

• MARCS

• Upload own model atmosphere... - your own .krz file

This page describes which models each choice can actually reach, since neither grid fills the Teff x log g box its endpoints suggest.

What each choice resolves to

Both grids live in $VALD_HOME/MODELS/STELLAR, which holds far more than the two families below: seven Castelli/ATLAS9 metallicity families (am20 am15 am10 am05 ap00 ap02 ap05) and some 2400 MARCS models over ten metallicities and four alpha/CNO patterns. A grid choice maps to exactly one metallicity family. Were a choice to span several, the metallicity of the atmosphere you got would depend on filename sort order.

Note the two different conventions for log g in the filename: ATLAS9 writes 10 x log g as two digits (G45 = 4.5), MARCS writes a signed decimal (g+4.5).

The MARCS models offered here are the plane-parallel ones (the p prefix). The line selection cannot use a spherical model: an uploaded MARCS model with MODEL TYPE= 3 in its header is rejected, and must be converted to plane-parallel geometry first.

You always get a model, even outside the grid

A request whose Teff and log g fall outside the grid - or between its nodes - is not an error. The nearest node is used instead, by the distance

    dist = |dTeff| + 100 x |d(10 log g)|

so 0.1 dex in log g weighs the same as 100 K in Teff, and Teff is followed in preference to gravity. Nothing in the result file announces this. The model that was actually used is shown as Model atmosphere on the request's own status page - worth a glance whenever the parameters sit near an edge of the coverage below.

The nodes of the other grid are never considered: they are named by a pattern this grid does not recognise, so a MARCS request can never be answered with an ATLAS9 atmosphere merely because it lies nearer in Teff.

Coverage

● a model is present, ○ the combination does not exist.

ATLAS9 (Castelli & Kurucz), [M/H] = 0.0

476 models: Teff 3500-50000 K at 76 values (250 K apart up to 13000 K, 1000 K apart above it) x log g 0.0-5.0 in steps of 0.5. That is 57% of the 76 x 11 box: the grid is a staircase, because low gravities are not computed for hot models. Above 40000 K only log g >= 4.5 exists, and at 50000 K only log g = 5.0.

MARCS, plane-parallel, z+0.00

173 models: Teff 2500-8000 K (100 K steps to 4000 K, 250 K above it) x log g 3.0-5.5. 90% filled - much closer to a full box than ATLAS9, but note that it starts at log g 3.0, so MARCS has no giants here, and that log g 5.5 exists only for the coolest dwarfs.

Choosing between them

• Teff above 8000 K: ATLAS9 is the only choice. MARCS stops at 8000 K.

• Giants and supergiants (log g < 3.0): ATLAS9 only.

• Cool dwarfs below 3500 K: MARCS only, which reaches down to 2500 K.

• Where both apply (roughly 3500-8000 K, log g 3.0-5.0), the grids differ in

  • physics, not just in sampling: MARCS uses a more complete treatment of molecular opacity, which matters most for the coolest stars. The two also differ in microturbulence, 2 km/s for this ATLAS9 family against 1 km/s for MARCS.
• Anything else - a different metallicity, a different microturbulence, a

  • spherical or interpolated model - has to be uploaded as a .krz file.