Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1978
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1978apj...226l..29m&link_type=abstract
Astrophysical Journal, Part 2 - Letters to the Editor, vol. 226, Nov. 15, 1978, p. L29-L33.
Astronomy and Astrophysics
Astronomy
43
Astronomical Photometry, Atmospheric Composition, Infrared Astronomy, Infrared Photometry, Stellar Atmospheres, White Dwarf Stars, Astronomical Catalogs, Atmospheric Models, Cool Stars, Helium, Hydrogen
Scientific paper
JHK photometry is presented for a number of cool white dwarfs, including five of the faintest stars with absolute magnitudes fainter than about 15. Comparison of the colors is made with the predictions of both hydrogen- and helium-rich model-atmosphere grids, primarily for the purpose of testing for the effects of the pressure-induced dipole opacity of molecular hydrogen. Three conclusions are offered. (1) The coolest observed stars lie near the temperature threshold at which IR photometry can provide a clear distinction between hydrogen- and helium-rich atmospheres. The two stars Wolf 489 and G107-70 are tentatively suggested as helium-rich, as their (V-K) colors are too red to match hydrogen models near log g = 8. (2) Hydrogen-atmosphere fits to the colors yield substantially higher effective temperatures for the cool stars than helium-atmosphere fits and yield higher temperatures than previous investigations fitting hydrogen-atmosphere models to optical/near-IR data. (3) There is a deficiency in the flux at blue wavelengths for all very cool stars relative to model predictions fitting V - K colors. Despite the absence or weakness of spectral features, it is argued that this is most likely due to blanketing by neutral metals.
Liebert James
Mould Jeremy
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