1. 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

2. Measuring Stellar Flux Distributions
Low resolution spectroscopy
Wide spectra coverage
Access to fainter stars
Typically R~600
Use a large (but not too large) entrance aperture
Correct for sky brightness and telluric extinction

3. Primary Photometric Standards
Vega (A0V)
Compared 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
Fl vs. A
Fn vs. wavenumber (cm-1 = 1/l in cm)
Log Fl + constant vs. A
Log Fn + constant vs. wavenumber

4. Calculating Fl from V Best estimate for Fl at V=0 at 5556A is
Fl = 3.54 x 10-9 erg s-1 cm-2 A-1
Fl = 990 photon s-1 cm-2 A-1
Fl = 3.54 x W m-2 A-1
We can convert V magnitude to Fl:
Log Fl = V – (erg s-1 cm-2 A-1)
Log Fn = V – (erg s-1 cm-2 A-1)
With color correction for 5556 > 5480 A:
Log Fl =-0.400V –8.451 – 0.018(B-V) (erg s-1 cm-2 A-1)

5. Class 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?

6. Interpreting Stellar Flux Distributions I. 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

7. The Paschen Continuum vs. Temperature
50,000 K
4000 K

8. Interpreting Stellar Flux Distributions II – 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 kbf depends on ionization, and hence on Pe)
Works for 5000 < Teff < 10,000 (where Hbf opacity is significant)

9. Flux Distributions at T=8000
Log g = 4.5
Log g = 1.5

10. Bolometric Flux
Bolometric flux (Fbol) is the integral of Fn 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

11. 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

12. Bolometric Corrections

13. 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 magnitude.
Which star has the larger bolometric flux?
What is the difference between the stars in Mbol?