color-selection of OB stars

How to get a simple query criterion to select massive stars in SDSS-V?

Approach:

Find a locus in color-color space that contains hot stars (reddened by different amounts);

Then use Gaia parallax information (NOT: good parallaxes) to weed out all stars in this
color-color-locus that are inconsistent with being luminous (absK<0), or too faint to
be SDSS-V observable mH<12.

At the end, we need to see how complete and contaminated such a sample is.

Starting point: 

take a sample of W1mag-luminous stars from GDR2 x ALLWISE (courtesy Alcione Mora)

(all stars absW1 < -2, or so) and look at them in some color-color space. Below, the Y-axis is chosen to be constant under reddening.

There is a distinct 'plume' in the bottom left, which is (conjecture) reddened blue (massive, luminous) stars, while the mass of objects around (4,0.4) is luminous giants, AGB stars etc.. This is verified by comparison with the literature catalog of Galactic OB stars https://heasarc.gsfc.nasa.gov/w3browse/all/galobstars.html (green points).

Possible Implementation:

We must not rely on "good parallaxes" in any query (as they are usually not available); instead we query on sensible initial 2MASS colors, and only existence in the Gaia catalog (G<21), nothing else

SELECT        g.source_id,g.ra,g.dec,g.l,g.b,g.parallax,g.parallax_error,g.pmra,g.pmra_error,g.pmdec,

              g.pmdec_error,g.astrometric_n_good_obs_al,g.astrometric_excess_noise,g.astrometric_chi2_al

              ,g.phot_g_mean_mag,g.phot_bp_mean_mag,g.phot_rp_mean_mag,g.phot_variable_flag,g.teff_val,g.a_g_val,

         tm.j_m,tm.j_msigcom,tm.h_m,tm.h_msigcom,tm.ks_m,tm.ks_msigcom,g.phot_g_mean_mag - tm.h_m AS g_min_h 

FROM gaiadr2.gaia_source AS g

INNER JOIN gaiadr1.tmass_best_neighbour AS xmatch

ON g.source_id = xmatch.source_id

INNER JOIN gaiadr1.tmass_original_valid AS tm

ON tm.tmass_oid = xmatch.tmass_oid

WHERE  b between -15 and 15

and

tm.h_m - tm.ks_m -0.48*(tm.j_m - tm.h_m) between -0.12 and 0.5

and 

tm.j_m - tm.h_m < 0.8

and

tm.h_m < 12.5

This yields (in an analogous color-color) plot a hint of bimodality (with hot stars bottom left plume, again) ...  but these distributions (1.7M stars) are mushy.

We now excise all stars where Gaia information tells us that they are inconsistent (at say 1 sigma) with being luminous, absK< 0, (i.e. OB stars) (After 7Myears, H<0 corresponds to M>5M_sun; Padova isochrones, is that correct?). Effectively this does a ``forground clean".

kmag + 5log10(parallax+parallax_error)-10 < 0. This leaves 0.6M stars (i.e. 1.1M stars excised).

Note again that Gaia only comes in in two ways:

-- there must be some detection (G<21-ish)

-- and Gaia parallax measurement (however 'lousy') must not be inconsistent with this being a luminous star.

This yields a nice hot-stars r-- red giant/AGB bimodality; showing that the color-color locus of reddened hot stars is distinct from that of RGB, RC, AGB etc. stars. This is illustrated again by comparison with literature OB stars (reference above)

Taking the lower part of this bimodal distribution leaves ~75k candidates M>5M_sun and m_H<12.5; they are very much concentrated to the Galactic plane; the stripy pattern is Gaia scanning law....

Data at: https://www.dropbox.com/s/ogxmtxag05slkeg/initial_JHK_color_cut_consistent_with_K%3C0_J-H-W1_color_cut.fits?dl=0

Clearly all this needs verification, clean up etc.. but that's what I mean by algorithmic selection.

A few notes:

-- need to check how good or bad this initial strawman selection is

-- this could be done in a single query

-- we should understand why the bluest stars from the literature are being missed (Rigel too bright etc..)

-- the bimodality-based sample-selection should not be done by hand as here

-- when challenged, I could do the same for low-mass young stars; but I leave this to experts.