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

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

3. 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)

4. 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)

5. 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

6. 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

7. Sampling Spectra with FiltersBJ

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

9. 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)

10. Trends in Color-Color Diagram

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

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

13. 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)

14. 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

15. 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

17. 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)

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

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

20. Reddening Free Parameters
Johnson Q
Strömgren reddening free indices

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