Observational Properties ofStellar ContinuaBalmer Jump, Continuum Slope Filters and Colors Balmer Line Wings Correcting for Interstellar Reddening

Cool, faintHot, brightTopTo centerHigh opac.Low opac.

Balmer Jump Measures change in opacity due to onset of H b-f absorption from n=2 levelAt 3647 lower opacity, see deeper into atmosphere (hotter, more flux)

Change in the Balmer Jump Along the Main SequenceF, G stars: also consider H- opacity Saha equation: Depends on both T (n=2 to n=1 levels of H) and pressure (log g or ne)

Change in the Balmer Jump Along the Main SequenceHot F stars: H- opacity small for

Change in the Balmer Jump Along the Main SequenceHot stars: scattering contributes more to opacity compared to the declining role of neutral H b-f (since more H ionized) decline in Balmer Jump with increasing T

Sampling Spectra with FiltersBJ

Johnson Colors(Buser & Kurucz 1978, A&A, 70, 555)

Johnson UBV ColorsU 3650 , B 4400 , V 5500 Balmer Jump: U measures flux from both sides but most influenced by drop in flux for

Trends in Color-Color Diagram

Balmer Line Wings in Early-Type Stars: T dependence

Balmer Line Wings in Early-Type Stars: log g dependence

Balmer Line WingsLine depth line / continuumHot stars: line NH(n=2) (H b-b) and continuum NH(n=3) (H b-f, Paschen contin.) Line wings formed by collisional Stark broadening ne (density of perturbers) Line depth NH(n=2) ne / NH(n=3) depends on both T and P (log g)

Balmer Line WingsCool stars: Continuum opacity mainly due to H- Line depth NH(n=2) ne / N(H-) NH(n=2) ne / [NH(n=1) ne f(T)] = NH(n=2) / [NH(n=1) f(T)] depends only on T

Balmer Jump and Balmer Line Wings in Strmgren Filter System u 3500 , v 4110 , b 4670 , y 5470 , H wide and narrow

Balmer JumpBalmer WingsHot starsTne, TCool starsne, TT

Strmgren Indicesb-yslope of Paschen continuumc1 = (u-v) (v-b) or actual grad. across BJ long gradient measure of Balmer Jump = m(30 ) m(150 ) measures strength of Hm1 = (v-b) (b-y) measure metallic line absorptions (stronger in blue-green than yellow-red)

Trends in Strmgren Indices(Lester, Gray, & Kurucz 1986, ApJS, 61, 509)c1 : T hot; T, ne cool : T, ne hot; T cool

ISM Reddening Important for Distant Stars (hot, luminous ones)Q = reddening free parameter

Reddening Free ParametersJohnson Q Strmgren reddening free indices

Reddening, Extinction in other Bands (Fitzpatrick 1999, PASP, 111, 63)