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Observational Properties ofStellar Continua
Balmer Jump, Continuum SlopeFilters and ColorsBalmer Line Wings
Correcting for Interstellar Reddening
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Cool, faint
Hot, bright
Top
To center
High opac.
Low opac.
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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)
D F FB 2 5 3647 3647. log ( ) / ( )
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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)
3647
3647 2
H N H
H N H H N nb f H
N H f T N n nH e 1
3647
3647
1
2
H N n n f T
H N H H N nH e
bf H
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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
3647
3647
1
2
N n n
N nH e
H
3647
3647
3
2
b f H
bf H
H N n
H N n
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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
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Sampling Spectra with FiltersBJ
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Johnson Colors(Buser & Kurucz 1978, A&A, 70, 555)
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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)
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Trends in Color-Color
Diagram
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Balmer Line Wings in Early-Type Stars: T dependence
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Balmer Line Wings in Early-Type Stars: log g dependence
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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)
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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
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Balmer Jump and Balmer Line Wings in Strömgren Filter System
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• u 3500 Å, v 4110 Å, b 4670 Å, y 5470 Å, Hβ wide and narrow
Balmer Jump
BalmerWings
Hot stars T ne, T
Cool stars ne, T T
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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)
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Trends in Strömgren Indices(Lester, Gray, & Kurucz 1986, ApJS, 61, 509)
c1 : T hot; T, ne cool β : T, ne hot; T cool
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ISM Reddening Important for Distant Stars (hot, luminous ones)
Q = reddening free parameter
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Reddening Free Parameters
• Johnson Q
• Strömgren reddening free indices
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Reddening, Extinction in other Bands (Fitzpatrick 1999, PASP, 111, 63)
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