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Model for dust extinction in the UV/opt/IR

NOTE: This model is now in the Xspec release and this page is superceded by the Xspec manual

Two models by Martin Still for extinction due to dust grains. The source code files required are msldst.f and mszdst.f with the parameter information in dust_lmodel.dat. Help information is given below.

zdust

Extinction by dust grains from Pei (1992, ApJ 395, 130), suitable for IR, optical and UV energy bands, including the full energy ranges of the Swift UVOT and XMM-Newton OM detectors. Three models are included which characterize the extinction curves of (1) the Milky Way, (2) the LMC and (3) the SMC. The models can be modified by redshift and can therefore be applied to extragalactic sources. The transmission is set to unity shortward of 912 Angstroms in the rest frame of the dust. This is incorrect physically but does allow the model to be used in combination with an X-ray photoelectric absorption model such as phabs. Parameter 1 (method) describes which extinction curve (MW, LMC or SMC) will be constructed and should never be allowed to float during a fit. The extinction at V, A(V) = E(B-V) x Rv. Rv should typically remain frozen for a fit. Standard values for Rv are MW = 3.08, LMC = 3.16 and SMC = 2.93 (from table 2 of Pei 1992), although these may not be applicable to more distant dusty sources. Performance reports, comments and suggestions to Martin Still (Martin.Still-at-gsfc.nasa.gov).

Model parameters are :

  • method: 1 = Milky Way, 2 = LMC, 3 = SMC
  • E(B-V), colour excess
  • Rv, ratio of total to selective extinction
  • Redshift (z)

zsmdust

Extinction by dust grains suited to starburst galaxies and the hosts of gamma ray bursts. The model can be applied over the IR, optical and UV energy bands, including the full energy ranges of the Swift UVOT and XMM-Newton OM detectors. The transmission is set to unity shortward of 912 Angstroms in the rest frame of the dust. This is incorrect physically but does allow the model to be used in combination with an X-ray photoelectric absorption model such as phabs. The extinction curve contains no spectral features and is characterized by a powerlaw slope over spectral wavelength. This model has been justified by e.g. Savaglio & Fall (2004, ApJ, 614, 293) because the apparent low metallicities within GRB hosts result in no significant spectral features within the extinction curve, unlike those found in local galaxies. The extinction at V, A(V) = E(B-V) x Rv. Standard values for Rv are Milky Way = 3.08, LMC = 3.16 and SMC = 2.93 (from table 2 of Pei 1992, ApJ, 395, 130), although these may not be applicable to more distant dusty sources. Performance reports, comments and suggestions to Martin Still (Martin.Still-at-gsfc.nasa.gov).

Model parameters are :

  • E(B-V), colour excess
  • ExtIndex, spectral index of the the extinction curve
  • Rv, ratio of total to selective extinction
  • Redshift (z)
Keith Arnaud, Lab. for High Energy Astrophysics, NASA/Goddard Space Flight Center

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Last modified: Tuesday, 14-Feb-2023 16:42:07 EST