Find some outliers

[dfm]

Maybe these will be things like black holes! @katiebreivik

One open question is how much can we learn from just a measurement of the semi-amplitude? Do we have some expectations for where on the CMD would be interesting to look and what the semi-amplitude range we would expect with a baseline of ~650 days? Is there anything we should do in advance or just wait until we have a large sample?

[adrn]

Woot! (copied this plot from #6 to discuss in this context)

OK so the reason I brought up expanding the axis limits on your plot is that I think we'll have better luck looking for compact objects around more massive primary stars - especially if we can get to 1–5 Msun. Obviously there are competing selection effects here: more massive stars live less time and are rarer, but they are brighter so in a mag-limited sample we should see them over a larger volume. Do you think we can push your CMD grid cells down to BP-RP ~ -0.25?

[dfm]

@adrn: there are ~5 sources with BP-RP<0 across all magnitudes in your sample of plx-cut Gaia targets with measured RV-errors:

so given that sample, I don't think that we could reliably do much of anything. Do you think it would be possible to expand some of the cuts to get a larger sample?

[adrn]

Ah you are plotting observed BP-RP right, not reddening-corrected?

[dfm]

That sounds like an astronomy question :D

It's just the bp_rp column.

[adrn]

Ah, ok. The massive/young stars will also be biased toward the galactic midplane, so extinction will be more important on the blue end. So, as with any project 😆, I'm confused about whether we should work in observed BP-RP or dust-corrected colors...On one hand, the dust correction is imperfect so there could be spurious features in the CMD in extinction-corrected magnitudes. OTOH, the dust-corrected colors will map more directly to physical parameters (effective temperature / stellar mass / whatever)...hmmm

[dfm]

haha - yeah! It's definitely not obvious.

[katiebreivik]

Here are some hacky figures from the results of 100 simulated populations of BH + bright stars in the Galaxy. These have already fulfilled a set of cuts:

• orbital period less than 10 yr (so maybe a bit long for the baseline)
• orbital motion of bright star on the sky is at least as large as 3 times the Gaia single pointing position errors
• 5 < V_ext < 12 (I pulled this from the Gaia RV literature for good RV precision)

I only have G and B-V, both corrected or non-corrected for extinction. Any of the funky striping you see (especially in the non-extinction-corrected plots) is because we have the same binary at different distances, so the brightness changes but the color stays the same. First, G vs B-V with a colorbar showing the semi amplitude:

As @adrn predicted -- many of the systems are SUPER affected by the extinction, this is even more pronounced with more massive stars being younger/closer to the disk:

For the histogram you showed, this is out of the gold sample right? Wouldn't APOGEE just miss most of the massive stars while Gaia might not?

Maybe a good idea is to target systems for which we can guess a mass based on photometry but have semi amplitudes such that the companion is likely to be more massive based on assumptions for orbital period, inclination, and eccentricity?

[dfm]

Awesome - thanks @katiebreivik! Which histogram were you asking about? The ones that I made above are all for all Gaia sources with reported RV errors, >= 3 RVS transits, and some cut on parallax S/N (@adrn would need to tell us the details about that one) so APOGEE doesn't enter.

Maybe a good idea is to target systems for which we can guess a mass based on photometry but have semi amplitudes such that the companion is likely to be more massive based on assumptions for orbital period, inclination, and eccentricity?

I think that this sounds like an excellent plan. Hopefully I'll have some initial test cases next week that we can look at in more detail.

For my purposes, the main question that I need to answer right now is whether it makes more sense to estimate the measurement noise as a function dereddened color or not, but I'm going to open a separate issue about that.

[katiebreivik]

Thanks for clarifying! I was thinking that we were still considering the cross-matched Gaia+gold sample catalog!

[katiebreivik]

Following up on the idea of limiting the parameter space --here is a figure for the minimum semi amplitude of a dark stellar candidate, modulo a few assumptions, as a function of mass and as a function of color for a bright ZAMS star. (The color would definitely have to be corrected so #8 will be important!)

The assumptions are:

• min K means max orbital period, so I've taken the 650 day baseline
• the requirement for BH candidate is that the dark companion to the bright star has a mass that is the maximum of (5 Msun, the bright star mass), where 5 Msun is a slam dunk minimum BH mass
• average value of sin(i) gives i=0.7 rad
• it is easier to observe circular orbits with RVs than eccentric orbits so I chose e=0

With this approach, we are totally insensitive to any BHs that are less massive than their bright star companion, so this will probably be most successful for ~1 Msun stars with large semi amplitudes.