Observing Satellites of the Milky Way at KPNOttasnimana | October 18, 2010
Hello, I am Tonima Tasnim Ananna, a sophomore from Bryn Mawr College, a declared Physics major (at Bryn Mawr), undeclared Astronomy major (at Haverford), and one of the members of the recent ASTR341 Observational Astronomy group which went to Kitt Peak National Observatory (KPNO) at Arizona to find some RR Lyrae stars in faint satellite galaxies of the Milky Way. I’m so appreciative that the Louis Green Fund was available to support this trip to an observatory.
The satellites that we observed are galaxies Segue 2 and Segue 3, and Ursa Major 2 as control. The telescope that we used at KPNO is a 36’’ telescope called WIYN 0.9m.
Before we start, I will give a little bit of background on RR Lyrae star. These starts are special because they are a type of Pulsating Variable star – a kind of stars which periodically change their radius and luminosity, and reside in the “instability strip” of a HR-Diagram. The pulsating variables have a shell with temperature around 40,000K – a temperature at which Helium is ionize from He+ to He++ – very close to its surface. This releases a lot of electrons into the shell, and as electrons scatter photons, this shell is very opaque to photons. The photons which random walk out of the hot core of the star cannot pass through this shell, and hence create a lot of pressure on the shell’s inner surface, forcing it to expand. As the hot gas expands, its temperature drops, and much like the surface of last scattering of the early Universe, the helium neutralizes and the shell’s opacity drops, and the photon flies free, suddenly increasing the luminosity of the star. But there is more to these stars than the cool Physics, these stars play a very useful part in gauging astronomical distances: their luminosity and period are related, so if we can find the period of such a star, we can find its absolute magnitude, and using the distance modulus, we can thus find its distance from us. This makes them excellent standard candles, and gives us the motivation to try to find them in faint galaxies like Segue 2 and 3, which have not yet been studied extensively for RR Lyrae.
To prepare for the observing run, at first we studied a handful of faint satellites of the Milky Way, prepared their hourly airmass table for the approximate time of our observation, and it turned out that the Segues make excellent objects for observation, with airmass smaller than 2 most of the time throughout the period of our observation. We approximated the expected background count based on lunar age at the time of our observation (around 7 days), and calculated exposure time that would give us a satisfactory signal to noise (100). The plan was to take images of the galaxies over a few hours (the RR Lyrae have periods of around half a day), and study the images for any changes in luminosity of stars over the period of observation. We used three different filters (B, V and I) to take the images.
The class split into three teams, and our team consisted of Assistant Professor Scott Engle, Andrew Sturner, and me. We were the earliest team there, and we observed the night of October 13th, 2010. We arrived at Tucson on the night of 10th, stayed at a Hyatt place for the night, went shopping for groceries the next day, had lunch at a restaurant called “Brushfire” which had excellent food and pictures of hell on its wall (to symbolize their food is spicy perhaps). Then we headed towards KPNO to observe. Tucson is surrounded by mountains, which was a treat for me because my native land, Bangladesh, is extremely two-dimensional. KPNO itself is on top of a mountain, elevated 7000ft above sea level, which, we found out, affords it exceptionally dark skies, far from city lights. We were inaugurated to our telescope’s system by another team working there on the night of the 12th. During the training, we had time to wander outside at the sight, and take in the amazing night sky above the mountain. The disk of the Milky Way was visible as a dense, thick disk of stars across the sky. Once Orion nebula rose, we saw the super-red Betelgeuse and whiter Rigel. Andrew and I even spotted a shooting meteor, which Scott missed because at that instant he was unfortunately looking through a pair of binoculars.
Prior to the KPNO trip, most of our experience with telescopes was limited to the 12 and 16’’ Cassegrain telescopes at Haverford. This semester, we also started using CCD cameras to take pictures of our objects of interest. The telescope which we used at KPNO was 36’’, and with all due respect to our beloved 16’’ telescope, the WIYN 0.9m had a much more sophisticated system, complete with a liquid Nitrogen Dewar to keep the pixels of the CCD camera (called S2KB) cool to avoid thermal electron emission, and a system to take “dome flats”, or images of a flat field to correct our object images for biases in the camera’s pixels’ outputs. Another notable difference between the WIYN telescope and the 16’’ was the use of the Guide cameras – the WIYN telescope uses two special guide cameras to keep track of a star, and constantly outputs the degree to which the star’s image deviates from its centre, to verify that the telescope is keeping track of the our object of interest. But the best part of all has to be how little time we actually spent in the dome: we only needed to go to the dome to take a peek to verify everything looks as it should, and to fill the liquid Nitrogen Dewar. This was very comforting for me because last year when we were working for hours in the dome with the 12’’ telescope, no matter how much clothes I wore I always seemed to under dress by two jackets, and am sure was losing enough heat to be luminous myself. (But it was still fun when we got to look at the craters of the moon, and blue-green Neptune and Uranus).
On the 12th, we woke up late because we have to stay awake throughout the coming night, had some food and went to meet Hillary, the sight supervisor, who gave us a second overview of the system. We filled the Dewar with liquid Nitrogen, and left to drive around the observatory. We were back a couple of hours later (around 4:30 pm) to take dome flats. We opened up the dome vents, turned on the exhaust fans and the dome flat lights (low intensity for our filters). We took 5 dome flats in each filter, 10 bias images (bias accounts for spontaneous reading by the camera for 0 second exposures), and by the time we were done with these, the sun has set, and it was time to start the Guider cameras to track a star. We looked through both guide cameras, and found an auspicious looking star (sorry about the bad pun) in the north camera. We started tracking it using a computer called MOSS, and entered the RA and DEC of Segue 3 in another computer called Olive. We moved the telescope to Segue 3, and using the focusing system of the computer Emerald, we analyzed a series of seven exposures of the galaxy for different focus values, and decided on the best focus value (in units particular to the system) for our images. The focus value changes with temperature, so we kept monitoring the temperature of the dome to see if we needed to make any adjustments. Luckily for us, temperature remained stable inside the dome and we only focused once per object. We started taking exposures (around 7 pm), and Professor Willman and her teams arrived. They were studying our activity the way we studied the other team the night before. We refilled the Dewar in the middle of observing Segue 3, because it needs refilling every 8 hours, and to show the other team how to do so. Then they left to rest, and we continued taking exposures of Segue 3, and completed 13 sets by midnight. Then we worked on a synoptic project which requested us to take pictures of M31 – Andromeda – in R band. We spent around one and a half hour taking flats, biases and images for this program, emailed them a notification, and moved to Segue 2. We refocused, took images of Segue 2 for a few hours – this time only in B and V band. We only managed to take 9 sets for Segue 2, and then had to move to Ursa Major 2. We took around 5 images of UM2 in V band, and then it was time for us to leave to catch an early flight back to Philly. We refilled the Dewar, and left pretty energized, even though we were up all night!
Now that we are back from the trip, we plan to analyze the images we took for a lab for ASTR341, and spot (fingers crossed) some standard stars that would help us estimate the distance to Segue 2 and 3. It is fun to solve problem sets, but it is much more satisfying to do something hands-on that produces a meaningful result, like this project. Even if we don’t get the result we are expecting, we still developed a better idea how real observers observe, and I personally got to observe the disk of Milky Way with naked eyes!