The Galactic bar, seen in stars of different [M/H]

 Which populations are bar-like in the Milky Way?

These are notes from a set of conversation between HWR and Madeline Lucey, during her visit to MPIA May 2023, augmented by a very few feasibility plots.  Worth exploring as a project?

The Milky Way has a prominent, presumably long-lived stellar bar, which is interesting in itself and whose dynamics affect the stellar dynamics well inside and outside of it. The exact length and pattern speed is still under debate, as is the question of how much it has slowed down.

Stellar populations of different age, abundances and kinematics (hot/cold) will contribute differently to forming and maintaining the bar, and will respond differently to the bar as an input perturbation.

Therefore it would be good to map, at first geometrically, the bar -- or the bar region in the MW -- in different stellar populations. Age might be the ideal population marker but is hard to determine consistently and precisely for stars that are 3-8kpc away from the Sun.  But metallicities, [M/H], are now available for vast samples in the inner Galaxy (say, RGC<5kpc). So, it appears feasible to make a metallicity-dependent map of the central galaxy.

However, naive maps n(X,Y | [M/H]) are likely to fail (== look confusing) for two reasons:

• (patchy) dust extinction and complex selection function
• imprecise distances that blur the image

A [M/H]-dependent bar map with RC stars

Does it make sense to try to construct a [M/H] dependent bar map?

Red clump (RC) stars of known metallicity are ~10% standard candles in their infrared fluxes (say, W1).  This implied distance precision of 5% is better than the expected parallax-based distance precision for heliocentric distances >3kpc. So, with good (perfect) identification of a set of stars as RCs of a given [M/H] a 5% distance precision map could conceivably be constructed.

The basic idea -- floated by HWR -- is to take advantage of the fact that the large majority of stars with luminosities within 0.5mag of M_W1(RC) ~ -1.7 and an appropriate temperature or color range are RCs anyway; once we know only that, we know their distance to 5-10%.  So one could consider: pick a broad RC candidate box in M_W1 (needs only poor parallax), Teff, logg space; identify likely RCs; then assume we know their photometric distance to 5-10%; and make a map.

Why might this work?  Rene Andrae's catalog contains 1.7M RC candidates beyond 4kpc (parallax<0.25), of which only 8000 have parallax S/N > 20.

Issues

• How to best identify RC stars, using XP (and Teff, logg,M/H), parallax, B,G,R,JHK,W1 etc..? There are simple ways (make cuts) and fancy ways (train and classify)
• How to deal with dust etc..