YSO selection for SDSS-V

Towards defining YSOs targets for SDSS-V

Science goals:

Previous post, dealt with identifying samples of stars that are massive (>5M_sun). I'd like to explore what can be done interms of an "algorithmic, all-sky" sample definition for (lower-mass) YSOs.

YSO's can be discerned by (any combination of) the following observational properties:
1)  their SEDs (0.5-20mum) are not-just-a-simple-photosphere (..disks, accretion, etc..)
2)  they lie off the(ir) main sequence
3)  many (most?) of them show some flux variability
4)  they are clustered in position and velocity space.
5)  dominant selection contaminants are dust-reddened giants of various sorts.

To start, I took a simple stab at combining the aspects 1), 3) 5)  Basically, I tried to find the objects that have YSO-like WISE colors (i.e. W1-W2 significantly > 0), that vary on year-timescales by a > few percent, and are within 5kpc (to cut out background). 

What I do requires some detection in Gaia (for starters I took G<18);
this clearly will miss seriously embedded sources.

This approach uses two aspects of Gaia that are perhaps non-obvious:
-- already now, Gaia is a high-precision, all-sky variability survey: 

    at the moment one gets only the rmsvariability amplitude
    on timescales of a year, encoded in variability == sqrt(N_obs)*flux_error/flux 

    (needs correction at the faint end).
-- and while the parallax measurements for such WISE-color-selected samples 

   will be insignificant for much of the sample,  plx/plx_error < 1, this is still very   

 informative: e.g. at plx_error < 0.2mas, plx/plx_error < 1 means you are NOT nearby.

Having said that, I queried GaiaDR2 x WISE for  

 variability>0.05mag,W1-W2>0.2, Gaia_G<18  and no astrometry flags.
When plotted in WISE color space (excising bad W1 and W2 photometry, 

left-nad panel)  this selection yields 26.000 objects:

Stars selected by variability>0.05mag,W1-W2>0.2, Gaia_G<18

These stars have an interesting distribution in variability amplitude.

Then I sub-select the objects that lie within 5kpc from the Sun (at least at >1sigma confidence),
i.e. parallax - parallax_error > 0.2mas; this is eliminate background; indeed, this cleans the large majority of objects, leaving only slightly over 5000.  In WISE color space,  this leaves a distinct population (the blob around (1.7,0.6):

Subsample from above, selected to be within 5kpc

If I then plot the objects surviving this simple procedure on the sky they look pleasing to my eye
(color-coding is the distance ), when trying isolate a sample of YSO within 5kpc:

This is the on-sky distribution of the stars, selected as above, color-coded by distance.

with a zoom-in on the Orion region (different distance color-scheme)

What seems good to me is:
1) the prominent nearby SF regions are there;
2) there is no apparent bulge/disk contamination;
3) the parallaxes indicate little contamination (i.e. patches in (l,b) have very similar parallaxes); note
that except for requiring parallax - parallax_error > 0.2mas, parallaxes have not entered the selection.

The query that created the sample is:

SELECT * , sqrt(g.phot_g_n_obs)/g.phot_g_mean_flux_over_error as variability
FROM gaiadr2.gaia_source AS g
INNER JOIN gaiadr2.allwise_best_neighbour as xaw
    ON xaw.source_id = g.source_id
INNER JOIN gaiadr1.allwise_original_valid as allwise
    ON xaw.allwise_oid = allwise.allwise_oid
WHERE
g.phot_g_mean_mag < 18  /* magnitude cut, so that variability at 0.05mag can be established */
and
sqrt(g.phot_g_n_obs)/g.phot_g_mean_flux_over_error > 0.05/* variability selection rms > 0.05mag */
and
sqrt( g.astrometric_chi2_al / ( g.astrometric_n_good_obs_al - 5)) < 2. /* weed out bad astrometry */
and
allwise.w1mpro - allwise.w2mpro > 0.25  /* stay away from the boring-star-locus in WISE */
and
g.parallax - g.parallax_error > 0.2  /* stars that are within 5kpc; to weed out background */
Comments in blue must be removed before the query runs

[I did do poor-WISE-photometry-cleaning afterwards]
The resulting output file can be found at:
https://www.dropbox.com/s/1ylqx7c9f4kmkbd/YSO_subset_v2_of_var%3E0.05%3EW1-W2%3E0.25.fits?dl=0

Next steps and action items for the working group:

Obvious limitations:

By construction, the above procedure only selects YSOs with W1-W2>0.25 and G-band variability > 0.05mag, and G<18; i.e. YSOs that do not show significant "W2 excess" and lie near the normal stellar locus, and those that don't vary (by 0.05mag) will not be captured; and those
with Gaia-G > 18. The first aspect means that Class-III objects and more mature PMS phases (sorry about the term) will be missed; the second aspect means severely reddened Class 0 objects will also be absent. This may, will and can lead to an augmentation of the above query.

For such objects, their variability and their position in the CMD must play a larger role; I have started to explore this, and will post next.

Verification steps:

-- do known YSOs, whose established measurements should satisfy the above criteria, show up in the sample? Does the query pick what it purports to. I.e. what falls through the cracks because of crowding, but would still make a good fiber target?

-- what is the overlap/mismatch with established catalogs?

-- can we use existing spectral surveys (incl. SDSS-IV) to verify the purity of the selection?