Merge pull request #328 from adrn/v21-frame

V21 frame

CHANGES.rst

@@ -12,6 +12,8 @@ New Features
 
 - Added the ability to use leapfrog integration within the ``DirectNBody`` integrator.
 
+- Added a new coordinate frame for the Vasiliev+2021 Sagittarius stream coordinate
+  system, ``SagittariusVasiliev21``.
 
 Bug fixes
 ---------

gala/coordinates/sgr.py

@@ -8,7 +8,7 @@
 import astropy.units as u
 from astropy.coordinates.matrix_utilities import rotation_matrix
 
-__all__ = ["SagittariusLaw10"]
+__all__ = ["SagittariusLaw10", "SagittariusVasiliev21"]
 
 
 class SagittariusLaw10(coord.BaseCoordinateFrame):
@@ -108,3 +108,91 @@ def sgr_to_galactic():
     spherical Galactic.
     """
     return galactic_to_sgr().T
+
+
+# -------------------------------------------------------------------------------------
+
+
+class SagittariusVasiliev21(coord.BaseCoordinateFrame):
+    """
+    A Heliocentric, right-handed spherical coordinate system defined by the orbit of the
+    Sagittarius dwarf galaxy, as described in Vasiliev et al. 2021,
+    https://ui.adsabs.harvard.edu/abs/2021MNRAS.501.2279V/abstract
+
+    Parameters
+    ----------
+    representation : `BaseRepresentation` or None
+        A representation object or None to have no data (or use the other
+        keywords).
+
+    Lambda : `Angle`, optional, must be keyword
+        The longitude-like angle corresponding to Sagittarius' orbit.
+    Beta : `Angle`, optional, must be keyword
+        The latitude-like angle corresponding to Sagittarius' orbit.
+    distance : `Quantity`, optional, must be keyword
+        The Distance for this object along the line-of-sight.
+
+    pm_Lambda_cosBeta : :class:`~astropy.units.Quantity`, optional, must be keyword
+        The proper motion along the stream in ``Lambda`` (including the
+        ``cos(Beta)`` factor) for this object (``pm_Beta`` must also be given).
+    pm_Beta : :class:`~astropy.units.Quantity`, optional, must be keyword
+        The proper motion in Declination for this object (``pm_ra_cosdec`` must
+        also be given).
+    radial_velocity : :class:`~astropy.units.Quantity`, optional, must be keyword
+        The radial velocity of this object.
+
+    """
+
+    default_representation = coord.SphericalRepresentation
+    default_differential = coord.SphericalCosLatDifferential
+
+    frame_specific_representation_info = {
+        coord.SphericalRepresentation: [
+            coord.RepresentationMapping("lon", "Lambda"),
+            coord.RepresentationMapping("lat", "Beta"),
+            coord.RepresentationMapping("distance", "distance"),
+        ]
+    }
+
+    _default_wrap_angle = 180 * u.deg
+
+    def __init__(self, *args, **kwargs):
+        wrap = kwargs.pop("wrap_longitude", True)
+        super().__init__(*args, **kwargs)
+        if wrap and isinstance(
+            self._data,
+            (coord.UnitSphericalRepresentation, coord.SphericalRepresentation),
+        ):
+            self._data.lon.wrap_angle = self._default_wrap_angle
+
+    # TODO: remove this. This is a hack required as of astropy v3.1 in order
+    # to have the longitude components wrap at the desired angle
+    def represent_as(self, base, s="base", in_frame_units=False):
+        r = super().represent_as(base, s=s, in_frame_units=in_frame_units)
+        if hasattr(r, "lon"):
+            r.lon.wrap_angle = self._default_wrap_angle
+        return r
+
+    represent_as.__doc__ = coord.BaseCoordinateFrame.represent_as.__doc__
+
+
+# Galactic to Sgr coordinates
+@frame_transform_graph.transform(
+    coord.StaticMatrixTransform, coord.Galactic, SagittariusVasiliev21
+)
+def galactic_to_sgr_v21():
+    """Compute the transformation from ICRS to Sagittarius coordinates"""
+    if not hasattr(SagittariusVasiliev21, "_R"):
+        R = np.diag([1.0, -1.0, -1.0]) @ B @ C @ D
+        SagittariusVasiliev21._R = R
+
+    return SagittariusVasiliev21._R
+
+
+# Sgr to Galactic coordinates
+@frame_transform_graph.transform(
+    coord.StaticMatrixTransform, SagittariusVasiliev21, coord.Galactic
+)
+def sgr_to_galactic_v21():
+    """Compute the inverse transformation from Sagittarius coordinates to ICRS"""
+    return galactic_to_sgr_v21().T