Our First Follow-Up Observing Proposal
Barely a week after the launch of DiskDetective, I looked at the calendar and saw an important deadline looming: the deadline to propose for time on the telescopes at the Fred Lawrence Whipple Observatory (FLWO). The FLWO is a cluster of telescopes nestled into the rocky terrain at the peak of Mount Hopkins, about an hour south of Tucson, Arizona. Among this group of telescopes is the 1.5m Tillinghast, which our science team members Thayne Currie and Scott Kenyon have used in the past for spectral typing large sample of stars—the perfect machine for some initial follow up of our Disk Detective candidates.
The telescopes are scheduled by trimester, so there are opportunities to propose once every four months. But filled with excitement from our launch, and all the interesting candidates you have been discussing up on Talk, I was suddenly eager to jump on this chance to take our disk search to the next level. So I emailed the science team and we all started writing.
Proposing for time on a telescope generally means preparing a four to six page document (the proposal) and submitting it to the Time Allocation Committee (TAC). The TAC, a panel of astronomers who have experience using the telescope at hand, compares all the proposals the receive and decides how many nights on the telescope to award each one. The telescopes are always oversubscribed, so the TAC generally has to disappoint many of the teams that propose, awarding them fewer nights than they ask for, or maybe none at all.
In any given proposal round, the Hubble Space Telescope is usually oversubscribed by a factor of four to one. The brand new ALMA telescope was recently oversubscribed by a factor of nine to one! But the 1.5m TiIllinghast is only oversubscribed by about 1.5 to one, so it seemed like there is a good chance we will get some observing time to test our ideas, if we could somehow get our act together in a flash.
The proposals generally consist of a list of targets, a description of the observing procedure and how the data will be analyzed, and a few pages explaining why the work is important (the “Science Justification”). Each section must be perfect if it is going to impress the TAC. With only three days to go, John Debes and I started passing around a draft of the science justification section. Thayne Currie and Scott Kenyon chimed in on the observing procedure and data analysis sections.
The concept of the proposal was simple. Many of the good candidates from Disk Detective (clean, uncontaminated point sources whose spectra energy distributions look like a star plus a disk) are stars that we know very little about. We may know how bright they are in one or two bands, or even have a crude spectrum. But for a disk candidate to be useful to help us understand how, where, and when planets form, we need to know more than that.
We need to know whether the star is on the main sequence like the Sun, whether it as a young star like HH 30, or whether it has evolved off the main sequence, like the red supergiant Betelgeuse. We need to know the star’s mass—stars range in mass down to less than one tenth of the Sun’s mass, and up to maybe 100 times the Sun’s mass. And of course, we need to be certain that the object we think is as star is not really a galaxy or AGN. The observations we had in mind can help fill in all this information by telling us the “spectral type” of the star.
The 1.5m Tillinghast telescope (the 1.5 meters refers to the diameter of the first mirror that the starlight hits) comes with your choice of two different spectrographs. We opted to propose for the FAST spectrograph, which provides spectra with a moderate level of resolution, but has a very high efficiency, so we will have time to take spectra of maybe 50 stars per night. If you’re used to looking at the SEDs of your favorite object, you can think of these spectra as filling in some of the fine details in the SEDs in the wavelength range around 0.4 to 0.7 microns. The fine details will show us absorption lines (dips in the spectrum) associated with various elements in the star near its surface. The widths of the lines tell us about the star’s mass. The relative strengths of the different lines will tell us the star’s temperature. With a bit of additional modeling, we can also learn about the amount of each element present in the star and even constrain the star’s age.
So the concept was straightforward, as far as these things go—business as usual for astronomers who study stars. What was scary about writing this proposal was the target list! We didn’t need to make the final selections just yet; if we win the telescope time, we will be allowed to submit the final list later on. But at this point we had to understand going in what kinds of targets were available if we were going to write something reasonable. Alissa Bans started digging through the data from the Disk Detective classifications and quickly got stuck; there was already just too much data from the more than 250,000 classifications we had already received to sort through and comprehend in one weekend. I thought for a bit that we were going to have to give up, and wait till next round.
Fortunately, at the same time, the Disk Detectives were submitting their favorite objects on the Talk pages, checking the RA and Dec in SIMBAD to make sure they were in the right part of the sky for this telescope to observe in the spring trimester, checking the V magnitude to make sure collecting a high quality spectrum wouldn’t take too long, and checking the literature to make sure they had not already been classified. In about 24 hours, we had lists of good targets from Pini2013, TED91, onetimegolfer, artman40, silviug, WizardHowl, Vinokurov and others. These lists gave us a notion of how our target options were distributed in terms of observability and what was known about them. And the mere fact that y’all were able to come up with these lists of good targets made us confident that the proposal was worth submitting, even in such a rush.
Tuesday morning, Scott Kenyon uploaded the finished proposal onto the proposal submission website. In a few weeks, the TAC will make its decision. With any luck we’ll be awarded a night or two of time to start following up our objects.
If that happens, none of us will actually go to Mount Hopkins. Instead a technician who is an expert at the ins and outs of this particular telescope and instrument will stay on the mountain and do the observing for us (that’s how they do it at this telescope). In fact, all our targets won’t even be observed in the same night. They will be mixed in with other targets all throughout the spring months, based on the weather and what’s convenient to point at. But we will start getting emails with lots of juicy new data for us to analyze on our favorite candidates.
So thank you again for all the hard work last weekend poring through the data and literature. The process gave us a chance to try out the next stage of the Disk Detective project and better understand what we’ll need to do to find the valuable new disks we’re looking for. And this first telescope proposal is just the beginning—so don’t worry if you missed it. The next opportunity for the FLWO 1.5m will be June 18. We are also looking into opportunities to proposal for time on telescopes in the Southern hemisphere so we can observe objects below the equator (declination <0). We’ll let you know about these as they come up.
There is nothing like first-hand evidence.