Observational Properties of Stellar Continua Balmer Jump Continuum

Observational Properties of Stellar Continua Balmer Jump, Continuum Slope Filters and Colors Balmer Line Wings Correcting for Interstellar Reddening 1

2

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) 3

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) 4

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 5

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 6

Sampling Spectra with Filters BJ 7

Johnson Colors (Buser & Kurucz 1978, A&A, 70, 555) 8

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) 9

Trends in Color -Color Diagram 10

Balmer Line Wings in Early-Type Stars: T dependence 11

Balmer Line Wings in Early-Type Stars: log g dependence 12

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) 13

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 14

Balmer Jump and Balmer Line Wings in Strömgren Filter System • Balmer Jump Balmer Wings Hot stars T n e, T Cool stars n e, T T • u 3500 Å, v 4110 Å, b 4670 Å, y 5470 Å, Hβ wide and narrow 15

16

Strömgren Indices • b-y slope of Paschen continuum • c 1 = (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 • m 1 = (v-b) – (b-y) → measure metallic line absorptions (stronger in blue-green than yellow-red) 17