Stellar X-ray sources in the eROSITA early data release

The incidence (or space-density) of "eROSITA-stars"

I took the occasion of the first eROSITA data release (in the 150sqdeg full-depth field) to make a few plots to wrap my mind around 'which stars emit X-ray emission'.

I started with the primary optical identification catalog, then took the objects that have Gaia counter-parts, then took those that had an expected plxSN>3 (by means of mag and plx cut, see Rix,Hogg+21) and looked at them: a CAMD with the points color-coded  by L_X/L_G

Note that the L_G is simply the Gaia G-band flux multiplied by a (bandpass width) of 4000A (rough).

Clear pattern! But to which extent does that reflect a "selection effect"?

If we just look at the fluxes, we get:

And if we color-code the CAMD by just X-ray luminosity, we get:

Is this interesting?

• what's the role of the vast majority of X-ray undetected sources?
• calculate space density = f(M,color | L_x) or f(M,color | L_x/L_G)
• look-up the X-ray spectral diagnostics (in the eROSITA papers): accretion disk, vs chromospheres, etc.., as different physical processes play a role here. 

HWR's articulation of evolution in biology (after listening to NYT article about beauty)

There was a long article about the utility of beauty in evolution; and why "beauty" exists, even if its evolutionary "advantage" is not manifest. The podcast seemed a bit rambling; a more mathematical view point seems to reconcile all that. No idea which fraction of that is BS

1) Mutations in observable properties of organisms happen (those can be traced back to genetic mutations, or othe underlying mechanisms: 'epigenetics' etc..)

2) Such mutations are likely to persist unless they lead to, or significantly contribute to, extinction.  I.e. mutations are likely to persist if they are advantageous from a utalitarian perspective, but can persist also if they are neutral, or slightly (but non-traumatically) negative in that respect. 

Diversity exists when a) it can originate; b) it does not lead to extinction. 

3) The space of "organisms not bound for near-term extinction" is set by environmental boundary conditions that change with space and time. (Environment: both climate, location and "other organisms").  Evolution takes time, and hence the space of all survivable diversity is not fully filled.

The lowerMS luminosity function

 analogous to the WD LF paper, let's look at the lower/lowest MS; just brain dump; needs follow-up.

Question: what is the "bimodality"?

Query

SELECT top 16000 *

FROM gaiaedr3.gaia_source AS g

WHERE

g.phot_g_mean_mag < 19.5

and

g.parallax > 20.

and

g.phot_g_mean_mag +5*log10(g.parallax)-10 > 11.

and

g.bp_rp > 2.

Plot

at some point one could look at optical spectra, to understand the color-bimodality: