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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

•

• 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)