An Introduction to VizieR

So you’ve been playing with Backyard Worlds: Planet 9 for a day or two now, and finding some interesting critters.  You read the F.A.Q. and learned how to read an object’s R.A. and declination using the numbers on the bottom and left edge of the images.  You’ve even looked up some of your favorite objects in SIMBAD using those coordinates. And most of them are listed in SIMBAD, maybe as high proper motion stars.  But some of them are #notinsimbad!!

If you’re starting to find objects that are not in SIMBAD, it’s time to take a peek a VizieR.  VizieR is harder to use than SIMBAD, but it also has many many more sources in it.  So if you find something that’s #notinvizier, well that’s a big deal.  And even if the object you found is in VizieR, it’s entirely possible that nobody realized that it’s a moving object! That’s also a big deal.

So how do you use VizieR?  First, you’ll want to type in your object’s coordinates. but don’t type them into the search bar on the top of the page.  Type them into the search bar that is labeled “Search by Position across 16780 tables”  (the number of tables might be different when you get there).

Next click “Go”.   (You can’t just hit return.)

Now you should see a huge page full of tables like the one below.  Each one contains the results from searching a different catalog.  Every year, VizieR adds more tables to its repository, so every year the pages get longer and longer.

This one below lists sources from the USNO-A2.0 Catalog.  USNO is the U.S. Naval Observatory.  They have a long history of carefully and accurately measuring the positions of stars. Can you guess why the Naval Observatory got interested in positions of stars?

Anyway, turn your attention now to the column labeled “r” on the left of the table. Every table has one.  This column shows the angular distance between the source in the table and the coordinates you entered.  That’s important!  The sources are listed in order by distance.  So when you look through the tables, start by looking at the source in the first row; it’s the closest one to where you thought your source was located. Then work your way down.  It’s easy to make a mistake of 1 arcminute (roughly 0.017 degrees) when you are are reading the coordinates of your favorite object off of the flipbook axes.  But the tickmarks on the flipbooks are about 2.4 arcminutes apart; you probably didn’t get the coordinates wrong by that much.

Now, since Backyard Worlds: Planet 9 is all about finding moving objects, the next thing you might want to do is search the page for known moving objects!  There are two phenomena that make objects in our project move across the sky: proper motion and parallax.  Parallax is caused by the Earth’s orbital motion, which changes our point of view. Proper motion is caused by the object’s own motion through space. Proper motion is generally easier to measure; many more objects have measured proper motion than measured parallax.

Proper motions are measured in milliarcseconds per year (mas/yr).   A milliarcsecond is 1/1296000000 of a circle, i.e. a very very small angle.   A typical pizza in a New York pizzeria is about one milliarcsecond across…if you are looking at it while standing in California.

Now on to the business of deciding whether you have made a big discovery. Let’s search the VizieR page for measurements of proper motion. On my Mac, I can search within a webpage using the COMMAND F buttons on my keyboard.  There is probably a similar keystroke on a PC.  There are two directions of proper motion: proper motion in  Right Ascension and proper motion in declination.  The corresponding search terms to use are “pmra” and “pmdec”.  Go ahead and pick one of those search terms and do a search, and see what you get.

If someone else has already found your dipole/mover there will be a source somewhere on the page with a proper motion–ra or dec–that’s greater than about 100 milliarcseconds per year (mas/yr).  If you can’t find such an object in the whole page, then your object is #notinvizier.  

On Backyard Worlds: Planet 9, objects that we see as movers tend to move at least 900 milliarcseconds per yer.  Dipoles can range down to about 100 milliarcseconds per year if they are bright.  So the idea here is to see if anyone else has previously published a dipole or mover near your search coordinates.

Here’s an example of a catalog on VizieR that lists proper motions.   One of the measured proper motions is circled in red because it’s more than 100 milliarcseconds. That’s what you have to keep an eye out for.  But note that this particular object, with pmRA=106.0 milliarcseconds per year, is r=1.6012 arcminutes away from the search coordinates.   So that might be a different object after all…

Now, there are many catalogs of stellar proper motions.  You may find they contain contradictory information!  I want to call your attention to one called “Gaia DR 1”.   That’s a catalog containing the first data release from ESA’s GAIA mission.   It just came out this summer.  It is very deep and very accurate–probably the most reliable proper motion catalog.   If you find that a particular source located, say, r=0.394 arcseconds from your search coordinates appears in multiple astrometric catalogs, the Gaia DR1 measurements are the ones I would trust.  If you can’t find the object in Gaia DR1, a decent rule of thumb is to go with the catalog with the most recent publication date.

Note: do not trust the proper motions from the AllWISE Catalog!  They are not really proper motions! They are mislabeled in VizieR.

After you finish examining the data on proper motion, the next thing to do is to search the page for “type” to lean what other researchers think the object’s spectral type is. You may find nothing, or you may find that different catalogs have differing opinions.  This article by Alan MacRobert provides a good introduction to spectral types and stellar classification. If you do not see a spectral type listed, we may want to follow it up to get a spectral type, even if someone else previously recognized the object’s high proper motion. If you do not spot a spectral type on VizieR, flag the object with the #nospectraltype flag on TALK.

Conflicting measurements are one potential pitfall with using VizieR.  There are more potential pitfalls.  For example, while you are using VizieR, that just because a table comes up where the name of the table mentions “quasar” for example, that doesn’t mean your object is a quasar.  It may be a table from a paper that is mostly about quasars–but this particular table is a list of the rejects, or the calibrators.

All this can get confusing, I know! VizieR is a powerful tool meant for professional astronomical research, and it is not very user-friendly. Don’t worry if it doesn’t make sense at first glance; congratulate yourself on making the effort to use it!  Don’t be afraid to ignore this whole blog post and just focus on doing classifications or using SIMBAD. And don’t be afraid to ask us for help, e.g. on TALK.

And remember, if you find a mover or dipole that is not in VizieR, or which is in VizieR but has no spectral type, be sure to make a note of it in TALK using the #notinvizier or the #nospectraltype tags.  And be sure to submit it using the Think You’ve Got One form. 

Thanks to Lucero Lopez, Cara_na and Sehajroop Bath for requesting this article!  Good luck to everyone!

Marc Kuchner