From YST: can you remind me how do you infer the distance modulus is phot_g_mean_mag+1.27-1.5*bp_rp ? I always thought distance modulus is apparent - absolute magnitude. In other words, why 1.27-1.5*bp_rp is a good approximation for g band absolute magnitude?
Qualitatively, this is based on the fact that RC stars are remarkably good standard candles
in the near IR, say W1. Then you can ask, can I predict the W1 apparent magnitude from
phot_g_mean_mag and bp_rp, combining the effects of effective temperature spread
and reddening: and phot_g_mean_mag -1.5*bp_rp+ const. is the answer.
Then operationally you can take the RCs in APOGEE (which have a range of temperatures
and reddenings, and [Fe/H]..) and ask
what constant (and what bp_rp slope) to I need to predict the parallax (not the distance)
for the whole sample:
varpi_predicted = 10^ ( -(phot_g_mean_mag -a * bp_rp + b - 10)/5)
and solve for a and b in a chi2 sense (varpi_obs - varpi_predicted)
Then you can go to clusters etc.. and ask whether there are any Teff,[Fe/H] residuals,
which there seem to be none at the 1% level...
in the near IR, say W1. Then you can ask, can I predict the W1 apparent magnitude from
phot_g_mean_mag and bp_rp, combining the effects of effective temperature spread
and reddening: and phot_g_mean_mag -1.5*bp_rp+ const. is the answer.
Then operationally you can take the RCs in APOGEE (which have a range of temperatures
and reddenings, and [Fe/H]..) and ask
what constant (and what bp_rp slope) to I need to predict the parallax (not the distance)
for the whole sample:
varpi_predicted = 10^ ( -(phot_g_mean_mag -a * bp_rp + b - 10)/5)
and solve for a and b in a chi2 sense (varpi_obs - varpi_predicted)
Then you can go to clusters etc.. and ask whether there are any Teff,[Fe/H] residuals,
which there seem to be none at the 1% level...
