Getting Started with SIMBAD

If you’ve found something interesting in a flipbook and you want to know more about it, SIMBAD is the right place to start.  Just figure out the object’s coordinates using the numbers on the left and bottom of the images, type them in to SIMBAD’s coordinate query page.  And poof, you’ll have all the answers!  Right?

Well it’s not always that simple.  So let’s talk for a bit about what SIMBAD can tell you–and what it can’t tell you–about your favorite patch of sky. I’m going to try to explain here, from start to finish, how to use SIMBAD to reliably determine if if you have rediscovered a well known object with high proper motion, or if you have possibly found a new high-proper motion object, worth submitting on the Think-You’ve -Got-One form.

First of all, SIMBAD is an acronym. It stands for Set of Identifications, Measurements, and Bibliography for Astronomical Data.  SIMBAD began in 1972 as database of stars only (no galaxies, asteroids,planetary nebulae, clusters, novae, supernovae, etc.).  But other objects have since been added in. SIMBAD now contains about 4.5 million stars and 3.5 million nonstellar objects.

Note that SIMBAD’s inventory is still heavily biased towards stars.  Astronomers have cataloged far more galaxies than stars, for example, and a small fraction of these appear in SIMBAD.  The NASA/IPAC Extragalactic Database (NED), for example, contains more than 214 million distinct sources.  The VizieR database contains even more. But those extragalactic objects are not the high-proper-motion object we are seeking. And VizieR is much less user friendly than SIMBAD. So you’re better off sticking with SIMBAD to start.

When you do a SIMBAD search, start by typing in the coordindates of your object.  No comma needed.  SIMBAD offers you chance to select a coordinate system; go with the default, which is FK5. Leave the epoch and the equinox (i.e. for the coordinate system) both set to the year 2000. But where it says “define a radius”, change that number to 1 arc min.  Objects further than one arcminute from your search location are probably too far away to be the object you are looking for.

After you hit return or click, “Submit Query,” SIMBAD will take you to one of three places: a page saying “No astronomical object found”, a table of objects it found, or a page that’s all about one specific object.  Here’s what the “No astronomical object found” page looks like.  If you end up here, be sure to flag your object with the #notinsimbad tag in TALK and submit your object using the Think-You’ve-Got-One form!  If not, it’s still possible that your object is not in SIMBAD, and hence an interesting find.  So read on!

If SIMBAD finds more than one object within the radius you chose, it will take you to a table listing all the objects, in order of how far away they are in angular separation from the coordinates you typed in.  Here’s an example table.

The first place to look on the table is the column that says dist(asec). That’s because your fist task is to make sure the object SIMBAD is showing you corresponds to the object you care about!  If the distance in this column is more than about 60 asec (arcseconds), i.e. one arcminute, from the coordinates you entered, it may be too far away on the sky to match the object you care about, unless you were sloppy when you looked up its coordinates.  Note that the spacing between the tick marks on the left of the flipbooks is about 3 arcminutes (180 arcseconds).  I bet you were much more accurate than that with your measurement.

Next, since Backyard Worlds: Planet 9 is all about finding objects with high proper motions, you’ll probably want to check the Otype column and the Proper motions column.  The Otype column lists some codes that indicate what kind of object the object is, according to SIMBAD.

“PM*” means star with high proper motion. If you spot an object with this label, there’s a good chance you’re seeing it as a dipole or mover. But note that objects with high proper motion are also sometimes labeled “BD*”, “WD*”, “Fl*”, “LM*”, “SB*”, or even “*i*” or “Q?”!  Here is a guide to all of SIMBAD’s object type codes, and here are a few more, defined.

* = star

BD* = brown dwarf

WD* = white dwarf

Fl* = flare star

LM* = low mass star (i.e. less mass than the Sun)

SB*=spectroscopic binary

*i* = star in double system

Keep in mind that one star can sometimes have many known properties that would demand multiple classifications in SIMBAD’s system.  For example, you could have a low mass star that is also a flare stare and is in a spectroscopic binary system with a white dwarf.  That object should probably be labeled something like “LM*, Fl*,SB*, WD*”. But SIMBAD only gives it one of those labels.  Bummer.  So check that column for objects called “PM*”.  But just because you don’t see “PM*” on the table you get, doesn’t mean SIMBAD hasn’t located the dipole or mover you’re looking at.  It might just have given it a different Otype label.

As a next step, you’ll want to look at the column labeled “Proper Motion”.  This column is really sort of two columns smushed into one; it lists a pair of numbers.  The first number is its motion west to east due to its proper motion. The second number is its motion from south to north) due to its proper motion.  That first number is motion in the direction of increasing Right Ascension, so it’s sometimes called the Right Ascension component. The second number is motion in the direction of increasing declination, so it’s sometimes called the declination component.  Just note that there are some additional weird subtleties about actually figuring out how the Right Ascension of a star actually varies in time because of the nature of the equatorial coordinate system.

Proper motion, as you may recall, is how an object moves on the sky as a result of having a different orbit around the Galactic center than the Sun does.  Objects near the Sun (Planet nine is an extreme example) can also shift around on the sky due to parallax, i.e. the effect of the Earth’s orbit.  Parallactic motion alone would make a star move round and round in an ellipse. Proper motion goes in a straight line.  Together, parallactic motion and proper motion combine to make stars move in a squiggly path like this (below). Experts in astrometry (the craft of measuring stellar positions) look at plots like this one and disentangle the two kinds of motion from it.

Now, what really matters to us is figuring out if the object might be a dipole or mover that we could spot.  And that depends on a different number than what’s listed in SIMBAD: the total proper motion. To get the total proper motion, you need to add together the two proper motion numbers you read in SIMBAD in a kind of funny way: you square them both, sum the squares and take the square root.  Anyway, if a object’s total proper motion is bigger than about 100 milliarcseconds per year, it will probably show up as a dipole.  If it’s bigger than about 1000 milliarcseconds per year, it will probably show up as a mover.  If you don’t see any objects with total proper motion greater than 100 milliarcseconds per year on this table, flag your object as #notinsimbad and submit your object on the Think-You’ve -Got-One form!

A third possibility is that SIMBAD will find only one single object within the search radius you entered. In that case, SIMBAD will jump right to a page on that specific object, like the one below for the star Vega (its proper motion and parallactic motion are shown above). You’ll notice that this page also lists the object’s proper motion, if it is known.  I’ve circled it in red on the screen shot below. The units here are milliarcseconds per year again, which is what we want.

Let’s pretend your SIMBAD search pulled up this page. I’m super lazy so I’m just going to do it using Google as a calculator. Type sqrt(200.94^2 +286.23^2) into a Google search bar and you get 349.720597763.  That’s clearly higher than 100 but less than 1000.  So this star should show up as a dipole.  Too bad.  It’s already a well known high-proper-motion object. You can flag it with the #known tag on talk.  But don’t submit it to the Think-You’ve-Got-One form.

Of course, sometimes, SIMBAD only finds one object, and it takes you directly that object’s page, and the object is clearly not the one you’re seeing in the flipbook!  What if SIMBAD sends you directly to a page like the one below, for an active galaxy called M 81? There’s no proper motion listed because object outside the Milky Way don’t have proper motions.

If this happens, it means SIMBAD couldn’t find any high-proper-motion objects near the one you found.  So go ahead and  flag your object as #notinsimbad and submit your object on the Think-You’ve -Got-One form.

To summarize:  SIMBAD can send you to one of three different pages.  But–no matter where it sends you–if you can’t find an object on that page with TOTAL proper motion > 100 milliarcseconds per year, then you have an object worth flagging with #notinsimbad and submitting on the Think-You’ve-Got-One form.

One last comment.  Good scientists take lots of notes.   So I strongly encourage you to write notes in a subject’s TALK page about what you learned from SIMBAD.  If you did find a high-proper-motion object in SIMBAD within a 1 arcsec search radius, type in some information about it, like its name, proper motion, otype, and distance to the coordiantes you used.  That way people can check your work, and maybe they won’t need to repeat your search.  We can also use your notes to develop a better understanding for our ability to recover known objects at Backyard Worlds: Planet 9.  That’s an important way to measure the power of our search.

OK I think that’s enough for now.  There’s lots more you can learn from SIMBAD of course. But maybe we’ll come back to that another day.

See you on TALK!

Best,

Marc