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Observational Properties of Stellar Continua

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.

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Observational Properties of Stellar Continua

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  1. Observational Properties ofStellar Continua Balmer Jump, Continuum SlopeFilters and ColorsBalmer Line WingsCorrecting 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- opacitySaha 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 negligibledepends 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 Filters BJ

  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 ακline / κcontinuum • Hot stars: κlineαNH(n=2) (H b-b) and κcontinuum αNH(n=3) (H b-f, Paschen contin.)Line wings formed by collisional Stark broadening αne (density of perturbers)→ Line depth αNH(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 αNH(n=2) ne / N(H-)αNH(n=2) ne / [NH(n=1) nef(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

  16. Strömgren Indices • b-y slope of Paschen continuum • c1 = (u-v) – (v-b) oractual 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)

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

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

  19. Reddening Free Parameters • Johnson Q • Strömgren reddening free indices

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

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