Astrometry

Astrometry

Besides photometric information astrometrical knowledge of a source provides an important diagnostic tool. The importance has increased now Gaia's astrometric information is readily available.
Therefore Phosphorpy provides an interface to astrometry data from Gaia. This includes the coordinates, proper motion, parallax information and their associated uncertainties. An additional resource offered by Phosphorpy is the computed physical distance following the analysis of Bailer-Jones et al..

A DataSet object ds must exist and it should contain a set of coordinates. Then we get the astrometric information for the sources involved using

ds.astrometry

As before it is not required to make this explicit call, it can be part of a longer call chain.

Now we can take the total proper motion ():

proper_motion = ds.astrometry.total_proper_motion()

or investigate the proper motion in one of the two coordinates only:

proper_motion = ds.astrometry.total_proper_motion('RA')

Also, we can take the physical distance from the Bailer-Jones et al. catalogue or the distance computed by simply taking . In the example below, the first gives the distance using the Bailer-Jones et al. catalogue and the second uses the simple 1 over parallax distance:

bj_distance = ds.astrometry.distance()
parallax_distance = ds.astrometry.distance(kind='simple')

Valid keywords are:

keyword	type
simple
parallax
bailer-jones	[Bailer-Jones et al.](https://iopscience.iop.org/article/10.3847/1538-3881/aacb21)
bj	[Bailer-Jones et al.](https://iopscience.iop.org/article/10.3847/1538-3881/aacb21)

For the astrometry section one can select sources on the basis of different astrometric criteria such as proper motion in one of the coordinates, total proper motion, parallax, and distance. The selection criteria have the prefix 'set_' and suffix '_limit', implying that they can be called as follows:

# proper motion limit in RA direction (alternative Dec for the declination)
ds.astrometry.set_proper_motion_limit('RA', -20, -10)
# total proper motion limit
ds.astrometry.set_total_proper_motion_limit(10, 20)
# parallax limit in mas
ds.astrometry.set_parallax_limit(0.2, 1)
# distance limit in kpc
ds.astrometry.set_distance_limit(1, 5)

Plots

The astrometry section has the option to create some basic plots. For instance, to plot the distribution of the total proper motion

ds.astrometry.plot.proper_motion_hist()

or the parallax distribution

ds.astrometry.plot.parallax_hist()

In both cases the histograms range from the minimal value to the maximal value and the bin-size is set to auto (see hist auto bin for details). Other plots include a plot of parallax vs. the uncertainty in the parallax:

ds.astrometry.plot.parallax()

and a proper motion plot with the option to use the total proper motion (i.e. corrected for the 1/Declination effect)

ds.astrometry.plot.proper_motion(cos_correction=True)

All of these plots have the keyword path to pass a path to save the resulting figure on your harddisk.