A Galactic Latitude (Paul Green)
To remind ourselves about extinction...
It's characterized by optical astronomers via a color excess
E(B-V)= (B-V) - (B-V)0which translates to visual extinction AV via
AV= R* E(B-V)where R is 3.1±0.1 Fortunately, this constant holds for almost any optical wavelength of interest, not just V band (except where large dust particles dominate, like in young star forming regions).
Derive the absolute extinction at any wavelength (Al) via
Al = E(B-V) * [a*Rv + b]Values of a and b are functions of wavelength x in units of inverse microns. I put the formulae from Cardelli, J. A., Clayton, G. C., & Mathis, J. S. 1989, ApJ, 345, 245
For efficient selection and classification of objects from color-color plots, we should not tolerate any more reddening than E(B-V)= 0.1
Crudely, E(B-V) can be related to Galactic latitude via
E(B-V)= A + B*cosec(|b|)where nobody agrees on A and B. (There's no REAL correlation because the data are not very conclusive - see Sparks et al. 1987, MNRAS, 225, 769). However, A=0 and B=0.03 is the best guess for a smooth function, so that on average, we want to include only
sin|b| > 0.3or
|b| > 17.5°, call it 20°The equation
E(B-V)= -0.055 + 1.987e-22*NhGalis eq.7 of Burstein & Heiles 1978, ApJ, 225, 40 and assumes a constant dust-to-gas ratio.
This means reddening is discounted for Galactic columns Nh>2.77e20. This is certainly not correct, esp. in application to the far UV or soft X-rays, but the errors in the determination of the reddening law are about 0.04mag in E(B-V) due to scatter in the data. To exclude negative reddenings then, it is reasonable to use
E(B-V)=max[0,-0.055 + 1.987e-22*NhGal)]Requiring E(B-V)< 0.1 thus means Nh>8e20, which is justifiable from the point of view of facilitating comparison with previous soft X-ray (esp. ROSAT) observations.
B Planetary Observations:
The observing will be complicated and may involve nudge mode. They are extremely bright in the visual. Coordinates are not yet known. For Jupiter, auoral emission extends out to Io.
C Galaxies and Galaxy clusters (Alexey Vikhlinin):
Criteria still being formalized and quantified. Several tests are made:
Date Last Modified: 22 April 2002