Interstellar extinction

Extinction maps

By default, when a SampleObject instance is initialized, an estimate of the interstellar reddening E(B-V) in the direction of each target is automatically performed by the method SampleObject.interstellar_ext() and stored into the dedicated ebv attribute. Given its potential relevance, we decided to make this function a static method that could be easily operable in stand-alone mode.

The estimate of extinction (reddening) in a given band (color) is performed by integrating along the line of sight a suitable 3D extinction map. The integration algorithm – whose basic idea can be compared to Xiaolin Wu’s line algorithm – draws a line from the position of the Sun toward that of the star of interest; the value of each pixel crossed by the line is weighted according to the portion of the total distance spent by the line in the pixel itself. This method ensures a rapid and precise evaluation of the integral, allowing 10000 stars to be handled in ∼ 1 s under typical PC performances.

In stand-alone mode, the X-band extinction towards a list of stars would be computed through a command such as:

result = SampleObject.interstellar_ext(ra=ra_array,dec=dec_array,par=parallax_array,ext_map='leike',color='X')

where 'X' is any valid photometric filter or color (SEE HERE). Valid keywords for this method are the following:

  • ra: float or numpy array, optional. Right ascension of the star(s) [deg].

  • dec: float or numpy array, optional. Declination of the star(s) [deg].

  • l: float or numpy array, optional. Galactic longitude of the star(s) [deg].

  • b: float or numpy array, optional. Galactic latitude of the star(s) [deg].

  • par: float or numpy array, optional. Parallax of the star(s) [mas].

  • d: float or numpy array, optional. Distance of the star(s) [pc].

  • ext_map: string, optional. Extinction map to be used: must be ‘leike’ or ‘stilism’. Default: ‘leike’.

  • color: string, optional. Band in which the reddening/extinction is desired. Default: ‘B-V’.

  • error: bool, optional. Computes also the uncertainty on the estimate. Default: False.

No parameter is strictly required, but at one between 'ra' and 'l', one between 'dec' and 'b', one between 'par' and 'd' must be supplied. The outputs of the methods can be either:

  • a single float or numpy array, corresponding to the best estimate of reddening/extinction for the star(s), if ``error``==False`;

  • two floats or numpy arrays, corresponding to the same estimate(s) + associated uncertainties, if ``error``==True.

Two extinction maps can be currently selected:

  • the STILISM 3D extinction map by Lallement et al. (2019): a Sun-centered (6000x6000x800) pc grid, with step equal to 5 pc;

  • the Galactic extinction catalog by Leike et al. (2020): a Sun-centered (740x740x540) pc grid with step equal to 1 pc.

It is also possible to employ our scheme to generate integrated extinctions maps, at a fixed distance, for a given field of view SampleObject.plot_2D_ext(). See HERE for details.

Given the limitations of our approach for extinction determination (see our paper for details), and in order to allow for greater flexibility while retaining the general operating scheme, two alternatives have been implemented:

  • the possibility to set E(B-V) to zero:

file='1000stars.csv' #1000 random stars
example_object=madys.SampleObject(file,id_type='DR3',ext_map=None)

  • the possibility to provide, as an argument of the ebv keyword, a numpy array ebv_vector containing as many E(B-V) values as the amount of input stars:

file='1000stars.csv' #1000 random stars
example_object=madys.SampleObject(file,id_type='DR3',ext_map=None,ebv=ebv_vector)

Note

In default mode, no error on the derived estimates is returned since the download of additional heavy files would be required. The same is true in stand-alone mode. Nonetheless, it is possible to manually provide E(B-V) uncertainties at inizialization of a SampleObject instance through a the keyword ebv_err, provided that the corresponding ebv values have also been manually provided.

Extinction law

The conversion between extinction and reddening is mediated by a total-to-selective absorption ratio R = 3.16 (Wang & Chen 2019). We obtained a new extinction law (shown below) by combining the extinction law by Wang & Chen (2019) in the range [0.3; 2.0] µm and the diffuse average extinction by Gordon et al. (2021) in the range [6.5; 40.0] µm; a linear combination of the two is used in the intermediate range [2.0; 6.5] µm (see our paper for details).

_images/ext_law.png

The adopted extinction law goes farther in the mid-infrared than most commonly used parametrizations, delving into wavelength ranges of great scientific interest and now under the scrutiny of the James Webb Space Telescope.