Quantifying Interstellar Extinction at High Galactic Latitudes

Accurate knowledge of the interstellar medium (ISM) at high Galactic latitudes is crucial for future cosmic microwave background (CMB) polarization experiments due to extinction, albeit low, remaining a foreground larger than the anticipated signal in these regions. We develop a Bayesian model to identify a region of the Hertzsprung-Russell (HR) diagram suited to constrain the single-star extinction accurately at high Galactic latitudes. Using photometry from Gaia, 2MASS and ALLWISE together with parallax from Gaia, we employ nested sampling to fit the model to the data and analyse the posterior over stellar parameters for both synthetic and real data. Charting low variations in extinction is complex due to both systematic errors and degeneracies between extinction and other stellar parameters. The systematic errors can be minimised by restricting our data to a region of the HR diagram where the stellar models are most accurate. Moreover, the degeneracies can be significantly reduced by including astrophysical priors and spectroscopic constraints. We show accounting for the measurement error of the data and the assumed inaccuracies of the stellar models are critical in accurately recovering small variations in extinction. We compare our posterior to stellar parameters from the LAMOST and Gaia ESO spectroscopic surveys and demonstrate that a full posterior solution is necessary to understand both the extinction parameter and the effective temperature. We conclude by showing that under reasonable prior assumptions and using the posterior mean extinction for each star in a sample we can produce a dust map similar to other benchmark maps.
Comment: 27 pages, 19 figures