Observational Properties of Stellar Continua Balmer Jump, Continuum Slope Filters and Colors Balmer Line Wings Correcting for Interstellar Reddening

Top Cool, faint High opac. Hot, bright Low opac. To center

Balmer Jump Measures change in opacity due to onset of H b-f absorption from n=2 level At λ<3647Å high opacity, so radiation comes from upper atmosphere (cooler, less flux) At λ>3647Å lower opacity, see deeper into atmosphere (hotter, more flux)

Change in the Balmer Jump Along the Main Sequence F, 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 Sequence Hot F stars: H- opacity small for λ<3647 Late B, A stars: H- opacity negligible depends only on T

Change in the Balmer Jump Along the Main Sequence Hot 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 Filters BJ

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

Johnson UBV Colors U 3650 Å, B 4400 Å, V 5500 Å Balmer Jump: U measures flux from both sides but most influenced by drop in flux for λ<3647 Å Progressing up the MS, larger BJ → (U-B) looks redder (larger) (B-V) continues to get bluer (smaller) Lines important in UV and optical make stars appear redder (especially in U-B)

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 Wings Line depth α κline / κcontinuum Hot stars: κline α NH(n=2) (H b-b) and κcontinuum α NH(n=3) (H b-f, Paschen contin.) Line wings formed by collisional Stark broadening α ne (density of perturbers) → Line depth α NH(n=2) ne / NH(n=3) depends on both T and P (log g)

Balmer Line Wings Cool stars: Continuum opacity mainly due to H- → Line depth α NH(n=2) ne / N(H-) α 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 Strömgren Filter System u 3500 Å, v 4110 Å, b 4670 Å, y 5470 Å, Hβ wide and narrow Balmer Jump Balmer Wings Hot stars T ne, T Cool stars

Strömgren Indices b-y slope of Paschen continuum c1 = (u-v) – (v-b) or actual grad. across BJ – long λ gradient → measure of Balmer Jump β = m(30 Å) – m(150 Å) → measures strength of Hβ m1 = (v-b) – (b-y) → measure metallic line absorptions (stronger in blue-green than yellow-red)

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

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

Reddening Free Parameters Johnson Q Strömgren reddening free indices

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