The York Extinction Solver (YES) is a facility for evaluating the Galactic or localized extinction of any celestial body in any filter from any index of reddening. Its development was motivated by a need to free extinction estimates from errors arising from shifts in effective wavelengths, which permeate astronomy. For example, it has been common practice to estimate the extinction of a disk galaxy from an estimate of reddening derived from elliptical galaxies and a reddening law derived from broad-band photometry of O and B stars without any accommodation whatsoever for the differences between the spectral energy distributions! YES facilitates the determination of the ratio of total to selective extinction for a target without concern as to the shape of the spectrum of the reddening probe. Further background is given by McCall (2004, A.J., 128, 2144).
The amount of dust along a sight-line toward a target is quantified by the optical depth at 1 micrometer. Not only is this quantity monochromatic, but it also is relatively insensitive to grain properties and is roughly equal numerically to E(B–V). YES is set up to allow a user to determine the optical depth at 1 micrometer of the target from an estimate of the colour excess of a probe of reddening, and then to determine the extinction of the target from the optical depth.
Normally, two steps are required to estimate the extinction of a target from an index of reddening. YES facilitates both.
This function determines the optical depth at 1 micrometer in the direction of the target from the colour excess of a probe of reddening (the nature of which is usually not the same as that of the target) by utilizing a spectral energy distribution (SED) characteristic of the probe. For example, if E(B–V) is taken from Schlegel et al. (1998, Ap.J., 500, 525), then the SED of the probe should be that of an elliptical galaxy at rest, because the colour excess is defined by K-corrected B–V colours of elliptical galaxies.
From a colour excess derived for any pair of filters, YES will iteratively compute:
OR
This function determines the extinction of the target from optical depths at 1 micrometer by utilizing a SED characteristic of the target.
From estimates for the optical depths for Galactic and localized dust in the foreground of the target, YES will compute for any set of filters 1, 2, and 3:
Besides enabling the computation of optical depth and extinction, YES facilitates inspection of reddening laws, SEDs, and filter response curves. Also, list input is possible for each of the main functions.
This function evaluates "monochromatic" extinction coefficients for a specified reddening law. The user may experiment with any reddening law in YES without affecting the laws used in computations of optical depth or extinction. For a specified range of wavelengths, YES will provide
To ease the job of computing optical depths or extinctions for a large number of objects or extinction coefficients for a large number of wavelength intervals, there is the option of uploading a list of tasks. To aid with the creation of lists, sample list input files can be downloaded.
To permit inspection of the data employed by YES, the following files can be downloaded:
Integral to calculations are laws of reddening. YES permits a wide range of choices, even allowing different laws for Galactic and localized dust (i.e., foreground dust in the frame of reference of the reddening probe or target). Note, however, that the reddening law which is the basis of determining how the SED of a disk galaxy changes with tilt cannot be altered.
Reddening laws for Galactic and localized dust which are employed in computing optical depth from reddening and computing extinction from optical depth can be checked and/or modified by clicking on a button on the input form for each function. Normally, the defaults would be accepted, though (see below). The reddening law used to compute extinction coefficients is specified directly in the input form for this function. The selection has no effect on calculations done by the other functions.
It is important to realize that many laws of reddening are constructed, at least in part, from observations acquired through broad-band filters. Often, in assigning derived coefficients to wavelengths, shifts in effective wavelengths are not accommodated. Any such law will lead to an errant estimate of extinction for any target whose apparent SED does not match the apparent SED of the sources used to derive the law.
Although YES allows different choices for reddening laws, it defaults to using a truly monochromatic reddening law (that of Fitzpatrick 1999 re-normalized to the optical depth at 1 micrometer) whose shape has been tuned to give A(V)/E(B–V)=3.07 for Vega. To within +/– 0.05, this is the mean value of the ratio of total to selective extinction for the diffuse component of the interstellar medium of the Milky Way after independent measurements are adjusted to a common SED (McCall and Armour 2000, in Mapping the Hidden Universe, eds. Kraan-Korteweg, R. C., Henning, P. A., and Andernach, H. 2000, ASP Conf. Series Vol. 218, p. 1). It is the appropriate value to adopt for light which traverses a long path through the Milky Way, such as is the case for extragalactic probes or targets.
