Follow-Up Observing Begins!
In our last blog post, we invited you to submit interesting targets to follow up with the Tillinghast 1.5m telescope at Mt. Hopkins this spring. Thank you to jessicamh, Gez Quiruga, arvintan, kmasterdo, silviug, wtaskew, cpitney, Pini2013, Ted91, Vinokurov, michiharu and everyone else who submitted targets! Thanks to your help, we picked out 102 objects to follow up this spring. The observing starts tomorrow night.
And guess what? We’ve got more follow-up observing planned for the fall semester, and also for the Southern hemisphere, with help from our new collaborators, Luciano Garcia and Mercedes Gomez from Observatorio Astronómico de Córdoba and Christoph Baranec from the University of Hawaii.
So we’re keeping that target submission form open. From now on, whenever you find an interesting target, anywhere in the sky, feel free to submit it.
And now that we’ve been through this process, I can better explain how we decided what to follow up this time. This part of the blog post is going to be a bit technical–so feel free to skip it, or ask us for more info if you get tripped up by the jargon.
Main sequence stars (aka dwarfs)
Luminosity Class IV stars: A IV, F IV, G IV, K IV, M IV.
A III, F III, G III and K III stars
T Tauri stars and Herbig Ae stars
objects with distance < 200 parsecs
objects with proper motion > 30 milliarcsec/year
We generally don’t want to follow up:
galaxies, Active Galactic Nuclei
blends (i.e. two objects so close together that we can’t analyze them separately)
Also mixed in the lists of possible targets were:
and plenty of objects where we can’t tell what it is
These objects went onto a “Maybe” list, to be followed-up as second priorities.
We could read some of this information from the SIMBAD spectral type. The quality of this information varies, and the SIMBAD spectral type includes a data quality letter (A,B,C,D, or E) where A is the best. Since the purpose of this observing run is to weed out blends and to get more accurate spectral types, we figured it was OK to look at objects where the spectral type quality was poor. But we threw out objects classified in SIMBAD or VizieR as M giants, Cepheids, Be stars, galaxies, Active Galactic Nuclei, eclipsing binaries, O stars or supergiants.
The most common contaminants are M giants and supergiants. We want to avoid those. But some M stars are main sequence stars (dwarfs). Like this one: AWI00003dm Disks around these M dwarfs are rare and interesting and worth extra points! So we must be careful weeding out the M giants and supergiants.
M giants are sneaky! They come with many different labels in SIMBAD and VizieR: Long Period Variables (LPVs), SR+L, Slow Irregular Variables, Miras, Semi-regular Variables, Semiregular pulsating Variables, Carbon stars. All those are kinds of M giants/supergiants and they tend to make their own dust, so we can’t use dust around them as an indicator of a planetary system. We’re not following them up.
Sometimes you can spot an M giant even when there’s no known spectral type. For example, subtract the V magnitude from the K magnitude. If V – K > 3.29, you’re looking at an M star. Then, if a star has a measured distance of thousands of parsecs, you can bet it’s a giant or supergiant. So we declared some objects to be M giants based on color and distance. A real M dwarf is so faint we can only see it if is much closer than 100 parsecs.
Here’s more information about how to guess a star’s spectral type based on its color: http://www.stsci.edu/~inr/intrins.html
If you know you’re looking at an M star, another good clue that it’s a giant/supergiant is if it is highly variable (e.g. amplitude more than one magnitude). So we looked up the variability amplitude for our targets in VizieR as well.
For an M star with no parallax measurement and no variability measurement, it can be hard to tell if you’re looking at a dwarf of giant or supergiant. So I put objects like that on the “maybe” list.
And finally–all the subjects on Disk Detective are preselected to have a certain degree of redness (we require the WISE 4 magnitude to be at most the WISE 1 magnitude – 0.25). But that’s not sufficient to find M star debris disks, since M stars are so cold, and therefore intrinsically red colored. We had to additionally weed out M stars with WISE 4 magnitude > WISE 1 magnitude + 1.0. (I know that sounds terribly confusing–it’s confusing because in the astronomical magnitude system, brighter objects have lower magnitudes. But adding this second criterion says that we are being more demanding when it comes to M stars in terms of how much brighter they need to be in the WISE 4 band than the WISE 1 band.)
Whew—that’s a lot of detail, I know. But now you can see why we try to weed out all those blends and multiples etc. using the handy animated flipbooks on the DiskDetective site before we start all the detailed research on each one.
Here are all the objects on our current version of the follow-up list for the Tillinghast 1.5 m for this spring, below (this list includes the maybes). Thanks again for all your hard work. And keep our fingers crossed for good weather at Mt. Hopkins!