Searches for Stars with High Proper Motion

Stars with high proper motion (PM) are popular targets to observe for a variety of astrophysical reasons, including their propensity to appear in catalogs of stars thought to host systems of planets. Finding prior observations of such sources, or planning for future observations, can present special challenges when searching MAST. Here are points to keep in mind: 

• Most stars with high PM are known only by a catalog designation, which may not be recognized by standard tools that resolve names to coordinates.
  • You must pay attention to the reference system and epoch of the source catalog. 
• Searches by object name (in MAST APIs or a Web application like the Portal) will be resolved to positions in the ICRS reference frame at epoch 2000.0. Searches by position are not re-interpreted to a particular epoch.
• MAST search tools assume ICRS coordinates (epoch 2000.0)

In MAST web applications source positions can be visualized on a sky background, but different elements of the visualization correspond to different epochs: 

• For Portal searches, the displayed sky background in AstroView is in the ICRS frame, with an epoch that depends on the background survey. The background survey images were obtained over a span of years, so any given high-PM source may be blurry and appear at an unexpected location relative to background stars.
  
  • For DSS (the default) the epoch is roughly 1990; for Pan-STARRS the epoch is about 2012.5. 
• The displayed footprints of the observation apertures are in the ICRS frame at the epoch of each observation. 
• Cone searches (of a circular region of sky about a position) match an observation if the cone (at epoch 2000.0) matches any part of the footprint (defined at the epoch of the observation).

For sources with low proper motion, these various epochs typically do not matter. However, to obtain accurate matches of observations of targets with high proper motion, users must either:

• correct the source position to the current epoch, and then re-execute a search, this time by position
• use a cone search radius that is large enough to include the source position at all epochs of MAST mission observation footprints

In either case, note that the Observation footprints will have to be interpreted in the current epoch to know if they apply to your target.

Example

To understand the implications of high proper-motion targets on MAST searches, consider the example of Trappist-1. This source is an M-dwarf and a well known exoplanet host star; it is designated in GAIA-DR3 as 2635476908753563008. It has a proper motion in excess of 1 arcsec/yr. 

Coordinates

The coordinates for this source are fundamentally drawn from GAIA (see e.g. the Single Object Search page), and are listed in the table below. The SANTA query (the MAST resolver engine) to SIMBAD returns the coordinates in the ICRS reference frame, and epoch=2000.0. GAIA-DR3 coordinates are in the ICRS reference frame at epoch 2016.0.

The GAIA DR3 ID is: 2635476908753563008. Note that other catalog identifiers, notably 2MASS (J23062928-0502285) and WISE (J230630.01-050233.9), include coordinates which presumably apply to the reference epoch of those catalogs.

The difference in epoch from SIMBAD vs. GAIA shows the star has moved >17 arcsec. 

To compute the coordinates for some future epoch, say 2027-Feb-5 at noon UTC, use utilities in astropy: 

import astropy.units as u
from astropy.time import Time
from astropy.coordinates import SkyCoord

# ICRS coordinates for Trappist-1
ra = 346.62652162764
dec = -5.04352831943
pm_ra = 930.788
pm_dec = -479.038
# Create a SkyCoord object for the epoch of GAIA
c_2016 = SkyCoord(ra=ra*u.deg, 
         dec=dec*u.deg,
         pm_ra_cosdec=pm_ra*u.mas/u.yr,
         pm_dec=pm_dec*u.mas/u.yr,
         obstime=Time(2016.0, format='jyear', scale='tcb')
         )
# Compute coordinates for new epoch
c_new = c_2016.apply_space_motion(Time('2027-02-05T12:00:00'))
print(f'RA: {c_new.ra.deg}, Dec: {c_new.dec.deg}')

'RA: 346.62888293361175, Dec: -5.044738873941171'  

Visualization with AstroView

AstroView displays Observations in MAST in the ICRS reference frame that match a query. The most common query is either by position on the sky (in celestial coordinates, ICRS reference frame), or by common source name; source names are resolved to coordinates in the ICRS frame at epoch 2000.0. AstroView displays other elements (some optionally, like the coordinate grid) to set the astronomical context of the observations. These include: 

Background Survey

Astroview displays images from one of a selection of wide-area sky surveys. The epoch of AstroView all-sky images depends upon the selected background survey, and even the position on the sky, since the contributing images were obtained over time during the course of each survey. The epoch for any given region of sky is effectively the mean epoch of the image stack from the survey used to create the sky background. For two popular surveys: 

DSS

The Digitized Sky Survey is the default background survey because of its all-sky coverage, and average resolution and depth. Photographic plates, the source medium for the DSS, were taken between 1970 and 2000, with a mid-point of about 1990. Note that the red-blue dispersion of point-sources is in part an artifact of a systematic difference in time between the mean epochs of the red vs. the blue plates. 

Pan-STARRS

CCD images from the Pan-STARRS-1 survey were were obtained from 2010 through 2014; the mid-point of the survey is about 2012.5. The color background images used in MAST were created from stacked images in multiple passbands, which of course spanned some range in time. 

Observation Footprints

Observations in MAST include a spatial extent (a footprint) of an instrument aperture, which is characterized by a polygon or a simple geometric shape. The coordinates of the footprint for each Observation are defined in the ICRS reference frame at the epoch of that observation. This is of course different than the epoch of the background survey image. 

Cone Search for Trappist-1

An AstroView rendering is shown in the figure below, which is the result of performing an Object cone search for this star ("Trappist-1, r=1m") and filtering for the JWST mission. The footprints are from matching JWST Observations, which were obtained from 2022 July to 2023 Dec. The position in DSS indicates that the epoch of that background images is about 1990.9. The source position at different epochs is consistent with the GAIA proper motion measurement. 

Implications for Understanding Prior Observations

There are >3 M stars brighter than m=12 in the GAIA catalog, which are easily bright enough to obtain light curves from TESS. Of those, >185,000 have PM >50 mas/yr, which would result in displacements of 1.7 arcsec during the lifetime of the HST mission. Such displacements would potentially complicate searches for prior Observations in MAST, and in planning future, small-aperture observations with our Flagship missions. Some key steps can help future Observation planning for high PM stars:

• Be sure to obtain good coordinates and proper-motions for your targets, ideally from GAIA 
• If you search for your target by name (using the name-resolver in the Porta)l, note that the returned coordinates will be at the 2000.0 epoch
• Re-compute the coordinates of your target at the anticipated epoch of the Observation
• Execute a MAST search at the anticipated coordinates, with a search radius large enough to include the target position at the earliest epoch of interest
• Recognize that the displayed footprints in the MAST Portal reflect the epochs of those prior observations. 

For Further Reading...

• Portal Guide
• MAST Mission Search Guide
• astropy.coordinates
• SIMBAD