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Stellar Continua
• How do we measure stellar continua?• How precisely can we measure them?• What are the units?• What can we learn from the continuum?
– Temperature– Luminosity– Metallicity– Presence of binary companions
• Bolometric corrections
Measuring Stellar Flux Distributions
• Low resolution spectroscopy (R~600 or 50-100 Å)
• Wide spectral coverage• Access to fainter stars (why?)• Use a large (but not too large)
entrance aperture (why?)• Correct for sky brightness and telluric
extinction
Measuring Stellar Flux Distributions
• Four steps– Select a standard star (Vega)– Measure the shape of standard
star’s energy distribution (relative F vs. )
– Measure the standard star’s absolute flux at (at least) one wavelength
– Correct for line absorption
Primary Photometric Standards
• Vega (A0V)• For absolute flux, compare to standard
laboratory sources, usually black bodies• Flux measured in ergs cm-2 s-1 A-1 at the
top of the Earth’s atmosphere• Often plotted as
– F vs. A – F vs. wavenumber (cm-1 = 1/ in cm)– Log F + constant vs. A– Log F + constant vs. wavenumber
Stellar SEDs
Calculating F from V
• Best estimate for F at V=0 at 5556Å isF = 3.36 x 10-9 erg s-1 cm-2 Å-1
F = 996 photon s-1 cm-2 Å-1
F = 3.56 x 10-12 W m-2 Å-1
• We can convert V magnitude to F:Log F= -0.400V – 8.449 (erg s-1 cm-2 Å-1)Log F = -0.400V – 19.436 (erg s-1 cm-2 Å-1)
• To correct from 5556 to 5480 Å:
Log [F (5556)/F(5480)]=-0.006– 0.018(B-V)
What about the Sun?
• Absolute flux uncertain by about 2%
• Mv (~4.82) uncertain by about 0.02 mags
• B-V even more uncertain• values range from 0.619 to 0.686
Practice Problems
• Assuming an atmosphere + telescope + spectrograph+ detector efficiency of 10%, how many photons would be detected per Angstrom at 5480A using a 1.2-m telescope to observe a star with V=12 (and B-V=1.6) for one hour?
• Using the CTIO 4-m telescope, an astronomer obtained 100 photons per A at 5480 A in a one hour exposure. Again assuming an overall efficiency of 10%, what was the magnitude of the star if B-V=0?
Bound Free Continua
• Lyman– far UV
• Balmer– UV
• Paschen– optical
• Brackett– IR
• Pfund– more IR
Interpreting Stellar Flux DistributionsI. The Paschen Continuum
• The Paschen continuum slope (B-V) is a good temperature indicator
• Varies smoothly with changing temperature• Slope is negative (blue is brighter) for hot stars
and positive (visual is brighter) for cooler stars• B-V works as a temperature indicator from
3500K to 9000K (but depends on metallicity)• For hotter stars, neutral H and H- opacities
diminish, continuum slope dominated by Planck function, and the Rayleigh-Jeans approximation gives little temperature discrimination
The Paschen Continuum vs. Temperature
Flux Distributions
1.00E-07
1.00E-06
1.00E-05
1.00E-04
1.00E-03
1.00E-02
300 400 500 600 700 800 900 1000
Wavelength (nm)
Lo
g F
lux
4000 K
50,000 K
Interpreting Stellar Flux DistributionsII – The Balmer Jump
• The Balmer Jump is a measure of the change in the continuum height at 3647A due to hydrogen bound-free absorption
• Measured using U-B photometry• Sensitive to temperature BUT ALSO• Sensitive to pressure or luminosity (at
lower gravity, the Balmer jump is bigger – recall that bf depends on ionization, and hence on Pe)
• Works for 5000 < Teff < 10,000 (where Hbf opacity is significant)
Flux Distributions at T=8000
1.00E-06
1.00E-05
1.00E-04
200 300 400 500 600 700 800
Wavelength (nm)
Flux
Log g = 4.5
Log g = 1.5
Bolometric Flux
• Bolometric flux (Fbol) is the integral of F over all wavelengths
• Fbol is measured in erg cm-2 s-1 at the Earth• Luminosity includes the surface area
(where R is the distance from the source at which Fbol is measured):
• L is measured in units of erg s-1, R is distance, r is radius
dFFBol
0
422 44 TrFRL bol
Bolometric Corrections
• Can’t always measure Fbol
• Compute bolometric corrections (BC) to correct measured flux (usually in the V band) to the total flux
• BC is usually defined in magnitude units:
BC = mV – mbol = Mv - Mbol
constantlog5.2 V
bol
F
FBC
Bolometric Corrections from AQ-5
-4.5
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
-0.5 0 0.5 1 1.5 2B-V
BC
Main Sequence
Giants
Supergiants
Class Problem
• A binary system is comprised of an F0V star (B-V=0.30) and a G3IV star (B-V=0.72) of equal apparent V magnitude. – Which star has the larger bolometric
flux? – What is the difference between the
stars in Mbol?