September 2008

Linda French in Pisco Elqui

Professor of Physics Linda French

Another so-so night! Right now we are waiting and hoping that the clouds will clear. I just made a quick trip down to the kitchen–my night lunch got packed without milk to add to that essential of the astronomer’s life, coffee. Even on a cloudy night, this is a beautiful spot to be. To the west is the Pacific, to the east, the higher Andes. We are at about 7,000 feet altitude, and we look over to peaks of 12,000 and 14,000 feet. Right now they are still covered with snow at high altitudes.

In the valleys lie small towns–Ovalle, Andacollo, and right below us, Vicuna, home of the beloved poet Gabriela Mistral. I will say more about “La Gabriela” to close this blog, but Vicuna itself is worthy of a mention. The clear air and dark skies that astronomers sought on the peaks is still to be found in the valleys as well. About ten years ago, the people of Vicuna, with the assistance of Cerro Tololo astronomers and technical staff, planned and constructed a municipal observatory, El Observatorio de Mamalluca. Its purpose is solely to allow the public to view the beauty of the southern skies. Five years ago my husband and I visited Mamalluca, and I was thrilled to see for the first time the Southern Cross and some of the glorious star clusters that those of us who live in the Northern Hemisphere never see. After all, when I am on Tololo, if it is clear I am observing–and if it’s cloudy, I can’t see the sky! Another town, Andacollo, has followed with its own observatory, and the people of Chile are seeing what a valuable natural resource their dark skies are.

As one drives up the Elqui Valley it narrows; the mountains rise sharply on either side. Because we are on the edge of a desert, the higher elevations are arid, while the regions on either side of the river are fertile. This region produces most of the grapes used for pisco, a delicious grape brandy that packs a considerable wallop.

Someone asked about my suitcase–it did arrive just in time for the trip up the mountain, but not before I went to the mall. While I’d prefer to be haggling in the marketplace for one-of-a-kind crafts, there’s something to be said for having enough socks and underwear! We stopped at the airport on the way up the mountain, where the flight with my suitcase had just arrived from Santiago.

So what are we trying to find out here? Those who have been to one of my talks on asteroids can skip this paragraph. Asteroids shine (in visible light) by reflecting sunlight. Most of them are somewhat irregularly shaped, like potatoes, and they all rotate. If they are the same color on all sides, then we on Earth see more light reflected when the broad side of the potato faces us than when it is end-on. Thus, we see a graph with two peaks and two valleys when we plot the asteroid’s brightness with time. The time it takes to repeat that pattern is its rotation period, and the difference between maximum and minimum light gives a clue about its shape.

There are good reasons for thinking that asteroids made of dense materials like iron should rotate faster, on average, than those made of rock. We believe that some asteroids are “rubble piles” of debris from earlier collisions, only loosely held together by gravity, and we think that those asteroids would fly apart if they rotated very fast. (Think of those carnival rides which use “centrifugal force” to hold you against a rotating wheel as the floor falls away from your feet.) So, some astronomers predict that it might be possible to measure the average density of a group of asteroids by determining their rotation rates. This is important for the Trojan asteroids because we do not know their composition; even knowing an average density would be a help. And it turns out that the Trojans which have been studied seem to rotate more slowly than main belt asteroids, consistent with a low density. But the data set is small, and far more work needs to be done. Since the brightness changes quite a lot with rotation for most Trojans, we should be able to see the effects in somewhat cloudy skies.

One of the things I like about actually going to an observatory is the chance to talk with astronomers from other disciplines, other institutions, and other parts of the world. So far we’ve met two Dutch astronomers, an Australian living and working in Chile, and two Americans working for the U.S. National Observatories. Most people are doing galactic and extragalactic research. It’s enlivening to hear about their work and to tell them about ours. Sometimes lasting relationships spring up; when I was here in 2003, Gautham Narayan ’05 and I met Chris Stubbs who was then just moving to Harvard University. Gautham is now doing his Ph. D. research under Chris’s direction.

About Gabriela Mistral: Born in Vicuna, she began her career as a schoolteacher in the tiny hamlet of Monte Grande, further up the Elqui Valley. Almost entirely self-taught, she became a diplomat and, eventually, the first Latin American to win the Nobel Prize for Literature. The spirit of the Valley permeates her poems. When I first came to Chile, while being driven up to the observatory, some of her lines came into my head. I asked the driver, “Wasn’t Gabriela Mistral from Chile?” He said, “She was from that town right over there.” So much to do and learn, so little time!