Strengthening the open cluster distance scale via VVV photometry

Abstract

Approximately 14% of known Galactic open clusters possess absolute errors ≤20% as evaluated from n ≥ 3 independent distance estimates, and the statistics for age estimates are markedly worse. That impedes such diverse efforts as calibrating standard candles and constraining masses for substellar companions. New data from the VVV survey may be employed to establish precise cluster distances with comparatively reduced uncertainties (≤10%). This is illustrated by deriving parameters for Pismis 19 and NGC 4349, two pertinent open clusters which hitherto feature sizable uncertainties (60%). Fundamental parameters determined for Pismis 19 from new VVV JHK_s photometry are d = 2.40 ± 0.15 kpc, ⟨E_J−H⟩= 0.34 ± 0.04, and log τ = 9.05 ± 0.10, whereas for NGC 4349 the analysis yielded d = 1.63 ± 0.13 kpc, E_J−H = 0.09 ± 0.02, log τ = 8.55 ± 0.10. The results exhibit a significant (≥5×) reduction in uncertainties, and indicate that: i) existing parameters for the substellar object NGC 4349 127b require revision, in part because the new cluster parameters imply that the host is 20% less-massive (M_∗/M_☉ ∼ 3.1); ii) R Cru is not a member of NGC 4349 and should be excluded from period-Wesenheit calibrations that anchor the distance scale; iii) and results for Pismis 19 underscore the advantages gleaned from employing deep VVV JHK_s data to examine obscured (A_V ∼ 4) and differentially reddened intermediate-age clusters.