E(B-V) and the Interstellar Extinction CurvesΒΆ

The ETC supports eight different extinction relations. The Milky Way extinction curves are taken from Cardelli, Clayton, & Mathis [CCM]. In addition to curves appropriate for the diffuse and dense ISM cases, two additional curves have been added to provide more choices for Rv:

  • Milky Way Diffuse: An average Galactic extinction curve for diffuse ISM (Rv=3.1)
  • Milky Way Dense: A Galactic extinction curve for dense/molecular ISM (Rv=5.0)
  • Milky Way CCM1: Rv=2.1
  • Milky Way CCM2: Rv=4.0

The Large and Small Magellanic Cloud extinction curves are taken from [Gordon] et al:

  • LMC Average: Large Magellanic Cloud extinction (Rv=3.41) away from 30Dor
  • LMC Supershell: Large Magellanic Cloud extinction for the Supershell/in the 30Dor region but it does not apply to the 30 Dor Nebula (Rv=2.76)
  • SMC Bar: Small Magellanic Cloud extinction (Rv=2.74)

A general extra-galactic extinction curve is taken from [Calzetti] et al:

  • Starburst (attenuation law): Appropriate for stellar continuum

The following table should aid users in selecting the most appropriate extinction relation, it contains the ETC exposure times for a S/N=10 observation with a Kurucz O5 with V=15 and E(B-V)=0.4, in COS/FUV G130M/1309A through the Primary Science Aperture (PSA).

Model Extinction Exptime
Milky Way Diffuse Rv=3.1 308 sec
Milky Way Dense Rv=5.0 84 sec
Milky Way CCM Rv=2.1 612 sec
Milky Way CCM Rv=4.0 166 sec
LMC Average Rv=3.41 464 sec
LMC2 Supershell Rv=2.76 493 sec
SMC Bar Rv=2.74 3808 sec
Starburst ... 92 sec

Normally, the extinction factor is applied by default before the flux is normalized to the specified value in Sec.4. That is, in this case the normalized flux will correspond to the actual observed flux.

One can, however, specify an alternate computation order, in which the extinction is applied after the normalization takes place. This is useful when planning observations of targets where the “observed magnitude” is being calculated from the absolute magnitude and distance for a region of space in which there is a known, measured extinction.

Graph comparing extinction laws
[CCM]Cardelli, Clayton & Mathis (ApJ, 345, 245, 1989)
[Gordon]Gordon et al. (2003, ApJ, 594, 279)
[Calzetti]Calzetti et al. (2000. ApJ, 533, 682)

Top


Previous topic

User-Supplied Spectra

Next topic

Redshift


Return to the ETC