Add version of FM90 using B3 instead of C3

docs/dust_extinction/choose_model.rst

@@ -183,8 +183,11 @@ used to fit measured extinction curves.  These models allow features in the
 extinction curve to be measured (e.g., 2175 A bump or 10 micron silicate
 feature).  The :class:`~dust_extinction.shapes.P92` is the most
 general as it covers the a very broad wavelength range.  The
-:class:`~dust_extinction.shapes.FM90` model has been extensively used,
-but only covers the UV wavelength range. The
+:class:`~dust_extinction.shapes.FM90` model covers the UV wavelength range
+and has been extensively shown to fit all known UV extinction curves. 
+The :class:`~dust_extinction.shapes.FM90_B3` model provides a variant
+of the FM90 model that uses B3 instead of C3 as B3 = explicit 2175 A 
+bump height = C3/gamma^2.
 :class:`~dust_extinction.shapes.G21` model focuses on the NIR/MIR
 wavelength range from 1-40 micron.
 
@@ -194,6 +197,8 @@ wavelength range from 1-40 micron.
 +============+==============+==================+===================+
 | FM90       | 3.13 - 11.0  |    0.0912 - 0.32 |  6                |
 +------------+--------------+------------------+-------------------+
+| FM90_B3    | 3.13 - 11.0  |    0.0912 - 0.32 |  6                |
++------------+--------------+------------------+-------------------+
 | P92        | 0.001 - 1000 |     0.001 - 1000 |  19 (24 possible) |
 +------------+--------------+------------------+-------------------+
 | G21        | 0.025 - 1    |           1 - 40 |  10               |

docs/dust_extinction/model_flavors.rst

@@ -492,6 +492,9 @@ Shape fitting models
    These models are used to fit the detailed shape of dust extinction curves.
    The FM90 (Fitzpatrick & Mass 1990) model uses 6 parameters to fit the
    shape of the ultraviolet extinction.
+   Note there are two forms of the FM90 model, FM90 that implements the model
+   as published and FM90_B3 that B3 = C3/gamma^2 as the explicit amplitude of
+   the 2175 A bump (easier to interpret).
    The P92 (Pei 1992) uses 19 parameters to fit the shape of the X-ray to
    far-infrared extinction.
    The G21 (Gordon et al. 2021) models uses 10 parameters to fit the shape

dust_extinction/__init__.py

@@ -2,5 +2,5 @@
 
 try:
     __version__ = _version(__name__)
-except PackageNotFoundError:
+except PackageNotFoundError:  # pragma: no cover
     pass

dust_extinction/shapes.py

@@ -6,7 +6,7 @@
 
 from .helpers import _get_x_in_wavenumbers, _test_valid_x_range
 
-__all__ = ["FM90", "P92", "G21"]
+__all__ = ["FM90", "FM90_B3", "P92", "G21"]
 
 x_range_FM90 = [1.0 / 0.35, 1.0 / 0.09]
 x_range_P92 = [1.0 / 1e3, 1.0 / 1e-3]
@@ -179,7 +179,7 @@ class FM90(Fittable1DModel):
        slope of liner term
 
     C3: float
-       amplitude of "2175 A" bump
+       strength of "2175 A" bump (true amplitude is C3/gamma^2)
 
     C4: float
        amplitude of FUV rise
@@ -332,6 +332,128 @@ def fit_deriv(in_x, C1, C2, C3, C4, xo, gamma):
     #             'C4': outputs_unit[self.outputs[0]]}
 
 
+class FM90_B3(Fittable1DModel):
+    r"""
+    Fitzpatrick & Massa (1990) 6 parameter ultraviolet shape model
+    Version with bump amplitude B3 = C3/gamma^2
+
+    Parameters
+    ----------
+    C1: float
+       y-intercept of linear term
+
+    C2: float
+       slope of liner term
+
+    B3: float
+       amplitude of "2175 A" bump
+
+    C4: float
+       amplitude of FUV rise
+
+    xo: float
+       centroid of "2175 A" bump
+
+    gamma: float
+       width of "2175 A" bump
+
+    Notes
+    -----
+    From Fitzpatrick & Massa (1990, ApJS, 72, 163)
+
+    Only applicable at UV wavelengths
+
+    Example showing a FM90 curve with components identified.
+
+    .. plot::
+        :include-source:
+
+        import numpy as np
+        import matplotlib.pyplot as plt
+        import astropy.units as u
+
+        from dust_extinction.shapes import FM90_B3
+
+        fig, ax = plt.subplots()
+
+        # generate the curves and plot them
+        x = np.arange(3.8,8.6,0.1)/u.micron
+
+        ext_model = FM90_B3()
+        ax.plot(x,ext_model(x),label='total')
+
+        ext_model = FM90_B3(B3=0.0, C4=0.0)
+        ax.plot(x,ext_model(x),label='linear term')
+
+        ext_model = FM90_B3(C1=0.0, C2=0.0, C4=0.0)
+        ax.plot(x,ext_model(x),label='bump term')
+
+        ext_model = FM90_B3(C1=0.0, C2=0.0, B3=0.0)
+        ax.plot(x,ext_model(x),label='FUV rise term')
+
+        ax.set_xlabel(r'$x$ [$\mu m^{-1}$]')
+        ax.set_ylabel(r'$E(\lambda - V)/E(B - V)$')
+
+        ax.legend(loc='best')
+        plt.show()
+    """
+    n_inputs = 1
+    n_outputs = 1
+
+    C1 = Parameter(description="linear term: y-intercept", default=0.10)
+    C2 = Parameter(description="linear term: slope", default=0.70)
+    B3 = Parameter(description="bump: amplitude", default=3.23)
+    C4 = Parameter(description="FUV rise: amplitude", default=0.41)
+    xo = Parameter(description="bump: centroid", default=4.60, min=0.0)
+    gamma = Parameter(description="bump: width", default=0.99, min=0.0)
+
+    x_range = x_range_FM90
+
+    @staticmethod
+    def evaluate(in_x, C1, C2, B3, C4, xo, gamma):
+        """
+        FM90 function
+
+        Parameters
+        ----------
+        in_x: float
+           expects either x in units of wavelengths or frequency
+           or assumes wavelengths in wavenumbers [1/micron]
+
+           internally wavenumbers are used
+
+        Returns
+        -------
+        exvebv: np array (float)
+            E(x-V)/E(B-V) extinction curve [mag]
+
+        Raises
+        ------
+        ValueError
+           Input x values outside of defined range
+        """
+        x = _get_x_in_wavenumbers(in_x)
+
+        # check that the wavenumbers are within the defined range
+        _test_valid_x_range(x, x_range_FM90, "FM90_B3")
+
+        # linear term
+        exvebv = C1 + C2 * x
+
+        # bump term
+        x2 = x ** 2
+        exvebv += (B3 * gamma ** 2) * (x2 / ((x2 - xo ** 2) ** 2 + x2 * (gamma ** 2)))
+
+        # FUV rise term
+        fnuv_indxs = np.where(x >= 5.9)
+        if len(fnuv_indxs) > 0:
+            y = x[fnuv_indxs] - 5.9
+            exvebv[fnuv_indxs] += C4 * (0.5392 * (y ** 2) + 0.05644 * (y ** 3))
+
+        # return E(x-V)/E(B-V)
+        return exvebv
+
+
 class P92(Fittable1DModel):
     r"""
     Pei (1992) 24 parameter shape model

dust_extinction/tests/helpers.py

@@ -13,7 +13,7 @@
     D22,
     G23,
 )
-from dust_extinction.shapes import FM90, P92, G21
+from dust_extinction.shapes import FM90, FM90_B3, P92, G21
 from dust_extinction.averages import (
     RL85_MWGC,
     RRP89_MWGC,
@@ -34,7 +34,7 @@
 param_ave_models_Rv = [CCM89, O94, F99, F04, VCG04, GCC09, M14, F19, D22, G23]
 param_ave_models_Rv_fA = [G16]
 param_ave_models = param_ave_models_Rv + param_ave_models_Rv_fA
-shape_models = [FM90, P92, G21]
+shape_models = [FM90, FM90_B3, P92, G21]
 ave_models = [
     RL85_MWGC,
     RRP89_MWGC,