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Mercury, September/October 1998 Table of Contents

Ronald H. Kaitchuck

It is a clear evening. I am standing outside the dome watching the western horizon, which is still glowing red. A few stars are beginning to become visible. It is unusually warm for November in Flagstaff, Arizona. My students - Carolyn Blakelock, Jeff Bryant, and Kim Weindorfer - and I are waiting for it to get dark enough to begin observations with the 31-inch telescope on Anderson Mesa. This telescope is jointly operated as the National Undergraduate Observatory by Lowell Observatory and a consortium of colleges and universities. This is a wonderful place for students to become involved in serious astronomical research: There are no "cookbook" labs here, and students work on real research projects using professional-grade instrumentation.

We had driven out to the Mesa early to open the dome, get the telescope ready, and begin the liquid nitrogen cooling of the CCD camera. Like many modern telescopes, the 31-inch telescope and its camera are operated remotely from a control room. The CCD images appear on computer monitors - no one looks through an eyepiece. Last night was only partially clear, but tonight looks promising.

We had traveled from Indiana for a five-night run on the telescope. This was only the start of our third night, but already we had developed a routine. The students would sit at the keyboards and operate the telescope and the camera. I would direct the operation from a chair in the corner and tell them stories of the "good-old days" when observers suffered in the dome in subzero weather. I suspected they knew that I didn't miss those days for a nanosecond. There is no question that modern observing is more comfortable, but it is also more efficient. Automation allows telescopes and instruments to operate in a consistent manner. There are few things for a sleepy astronomer to forget at 3:00 in the morning. The new generation of CCD cameras have roughly 20 times the quantum efficiency (light sensitivity) of photographic film. CCDs make it possible to do projects that once required much larger telescopes, and good quality images can be obtained with much shorter exposure times.

View from a hill. From his High Hill Observatory in Massachusetts and using his 17.5 inch reflector, amateur astronomer Gerald P. Dyck keeps vigil over a number of variable stars. A member of the American Association of Variable Star Observers, Dyck collaborates with the author in their monitoring of eruptive stars.

One of the goals for this trip to Arizona is to observe the early stages of a stellar eruption. The plan is to use images taken through different filters to record brightness and temperature changes as the star goes into outburst. The critical timing required by this project makes the Internet almost as important as the telescope. In the control room an email message arrives from Gerry Dyck suggesting possible target stars for tonight. Gerry routinely makes visual observations of these eruptive stars from his home in Massachusetts. The sky was darker now and at his suggestion we command the control computer to swing the telescope to the star AR Andromedae (or just "AR And" for short). It looked perfectly normal and faint last night, but Gerry suspects it's about "to blow."

AR And is one example of an eruptive class of variable stars called dwarf novae. These stars can brighten by a factor of 10 to 100 in a few hours and remain bright for a few days (or in some cases a few weeks). Outbursts occur at quasi-regular intervals of weeks to months. But they do not repeat exactly on time; hence, we depend on Gerry Dyck to make "predictions" for those few days a year we have access to a large telescope. His predictions are critical because we are interested in the earliest stages-the very onset of an outburst. Once the star has brightened enough for announcements to appear on the Internet, it is already too late for us.

A few years ago a former student of mine, Dr. Cathy Mansperger, caught the rise to outburst of another dwarf nova. She was observing with a spectrograph attached to the 72-inch telescope at Lowell Observatory. She recorded details of the earliest moments of the outburst. While she watched, the star changed temperature and began to increase in brightness. Her observations also indicated that the star had brightened slightly about 24 hours before the main outburst. These details would have been nearly impossible to catch without the collaboration of an amateur astronomer like Gerry Dyck. Gerry is not alone. He is one of many members of the American Association of Variable Star Observers (AAVSO). Since 1911 the AAVSO has collected variable star observations from dedicated amateur astronomers in 40 countries. For the most part, these observations are made at the eyepiece of a telescope by comparing the variable star to field stars of known brightness. A few of the Association's members now have their own CCD cameras and determine stars' brightnesses from the recorded images. The AAVSO has collected over 8.5 million observations. Their data bank is the reserve that professional astronomers draw on when trying to understand variable stars: The type of monitoring the AAVSO does is very difficult for professional astronomers to do because there are too few professional astronomers, far too few research-grade telescopes, and simply too many variable stars.

What a difference a couple of nights can make. On 4 November 1997, an image of the dwarf nova AR Andromadae and surrounding sky (left) was obtained at the National Undergraduate Observatory outside Flagstaff, AZ. Two nights later, AR And erupted (right).

Our collaboration with an amateur astronomer is not unique. Bob Fried is an amateur with a very sophisticated observatory in Flagstaff, Arizona. He collaborates with professional astronomers around the world, making critical observations they can't. For many years Doug Hall, a professional astronomer at Vanderbilt University, directed the photoelectric photometry work of a large group of amateurs. And astronomer Joe Patterson, at Columbia University, directs a group of amateurs through an organization called The Center for Backyard Astrophysics. The contributions to science made by such collaborations are very significant but not widely known to the public.

Will it or won't it? Dwarf novae erupt at quasi-regular intervals of weeks to months, and AAVSO member Gerald P. Dyck's numerous observations of selected stars enable him to make predictions about when a dwarf nova may next erupt.

Here at the observatory in Flagstaff, the first image appears on the computer screen. We quickly compare it to last night's images and confirm our first impressions-AR And is brighter. We shoot another 30-second exposure just to be sure. There is no doubt; an outburst has started. We immediately begin an automated sequence of exposures through blue, yellow, red, and infrared filters. I telephone the headquarters of AAVSO and leave a message on their answering machine announcing the outburst. Back East, through spotty clouds, Gerry Dyck also notes the outburst of AR And. Over the next few hours the star becomes 16 times brighter than normal, and my Internet connection comes alive with email traffic from the AAVSO and VSNET (Variable Star Net out of Japan) as observers exchange comments about AR And and other stars in outburst. With each image, AR And is brighter. We are all pretty excited-this is just what we hoped to see! The students become involved in the excitement of doing science while seeing explosive phenomena in real time. For me it's a reminder of why I came to love astronomy, a love that goes all the way back to when I was a 12-year-old making observations from my backyard for the AAVSO.

Over the last few decades the work of amateur and professional astronomers has led to a general understanding of the dwarf novae and their outbursts. Armed with this knowledge, let's take an imaginary journey to AR And. This journey is purely imaginary because, traveling at light speed, the voyage would take many decades. (This also means that any outburst that my students and I see tonight actually happened decades earlier; the light from the outburst takes that long to reach the Earth.) As we approach, the star appears to grow ever brighter due to the inverse-square-law behavior of light, e.g. if we halve our distance to the star it appears four times brighter. Just hours before arriving at AR And we begin to see some structure that was not visible from the Earth. Most of the light from AR And does not come from a star but rather from a gaseous disk, looking somewhat like Saturn's rings, surrounding a tiny white-hot star. The inner portions of the disk near the star are exceptionally bright and blue-white in color. The outer edges are dimmer and redder in color. The color indicates the temperature. The blue-white regions are 60,000 K or more, while the outer portions of the disk are only 3000 K.

Almost lost in the glare of the disk is a larger but much fainter and cooler red companion star. The two stars orbit about the common center of mass, much like the weights on the ends of a baton spin about a point between them on the connecting rod. These stars are so small and close together that it takes less than four hours for them to complete an orbit about each other. The white star is only about the size of the Earth but is as massive as our Sun. The companion is a lower-main sequence dwarf with only a fraction of the Sun's mass. The gravity of the white dwarf distorts the red dwarf into a teardrop shape with the pointed end (called the inner Lagrangian point) directed toward the white dwarf. And to put it all in perspective, the pair forms a compact binary system that could actually fit inside our Sun.

As we travel closer, we can see a narrow stream of gas flowing from the pointed end of the red dwarf toward the white dwarf. Because both stars are moving, the stream does not fall straight toward the white dwarf but instead misses the star and passes to one side, merging with the gas in the disk; the stream's impact creates a bright flickering spot at the disk's outer edge. The disk owes its existence to mass transferred from the red dwarf via the stream. Mass transfer is common in close binary star systems and is found in all dwarf novae. The disk is a very energetic place. Matter swirls around in tight orbits traveling at speeds of hundreds of kilometers per second. In just the same way that Mercury travels faster than Pluto, orbital motion requires that gas in the inner disk travel faster than gas in the outer disk, and this leads the disk to resemble something like a circular, multi-lane highway. A driver will pass cars in the outer lanes while being passed by cars in the inner lanes. These speed differences in a disk lead to frictional heating as faster gas parcels on the inner orbits rub past parcels on the outer orbits. This also causes the gas in the disk to lose orbital energy and angular momentum. As a result, the disk is made to glow and the gas slowly spirals inward with orbit. The inner disk, where gas is merging with the white dwarf, is especially brilliant, radiating mostly in the ultraviolet portion of the electromagnetic spectrum. The gas that accretes onto the white dwarf is replaced by fresh stream material from the companion star.

Anatomy of a cataclysmic variable. In this illustration of the dwarf nova Z Chamaeleontis, a low-mass, main-sequence companion star loses gas to the higher-mass white dwarf (white spot at center of the accretion disk). The gas siphoned from the companion star has a great deal of angular momentum and cannot fall directly onto the white dwarf. Instead, the gas falls into orbit around the white dwarf and slowly spirals through the accretion disk before ultimately reaching the white dwarf's surface.

Dropping an object can cause a tremendous energy release. For example, if a book is dropped onto the floor there is an energy release-most obviously in the form of sound. The impact becomes more energetic if the book is dropped from a greater height or if a heavier book is used. The impact of matter onto a white dwarf is very energetic because the star is small yet massive, making the gravitation force at its surface very strong. An astronomy textbook with a mass of one kilogram that is dropped onto a white dwarf will release almost as much energy as that released by the atomic bombs used in World War II! Mass accretion is a very energetic process, and it is the energy source of the dwarf novae outbursts.

Astronomers believe that when a dwarf nova system is in quiescence, or quiet state, the matter arriving from the stream accumulates somewhere in the outer disk and is not efficiently transferred inward. Friction is required to transfer matter through the disk. The friction at any point in the disk increases with temperature. The temperature depends on the opacity of the gas-that is, how easily light can escape from the gas into space and cool the disk. If the opacity is low, for example, light easily escapes and cools the disk. For inner regions of the disk where the temperature is above 10,000 K, hydrogen is completely ionized: This makes the gas fairly opaque, which makes friction in the gas high, which makes for efficient transport of matter inward through the disk.

For regions of the disk further out, hydrogen is not ionized and the disk is fairly transparent and cooler. Thus, gas friction is low and inward matter transport inefficient. Meanwhile, "fresh" matter will continue to arrive through the stream from the secondary star. Gas will accumulate until compression due to all that material forces the temperature high enough to ionize hydrogen. Then friction suddenly increases and the matter spirals inward, falling toward the white dwarf. The passage of this matter through the disk raises the temperature of the inner disk and the outburst begins. Once the matter in the disk thins out, the temperature in the outer disk drops, the friction decreases, and the mass build-up begins again.

At least, that is how we think it works. Observations during the beginning stages of an outburst will enable astronomers to test this theory by actually determining how the temperature changes. But these observations are not easy to get. The outburst cycle is not perfectly periodic and even for the same dwarf nova the strength and duration of each outburst is not the same. This is why the observations by amateur astronomers are so important. The AAVSO database allows one to determine the average time between outbursts for each dwarf nova system and to make an intelligent guess as to when the next one will occur.

Ready for students. The 31-inch NURO telescope is prepared for another night of observations.

RONALD H. KAITCHUCK is a professor in the Department of Physics and Astronomy at Ball State University in Muncie, Indiana. His research interest in interacting binary stars betrays his passion for things variable and eruptive. His email address is 00rhkaitchuc@bsuvc.bsu.edu.