43 Columbia University Departments of Astronomy and Physics New York, New York 10027 @S0002-7537~99!00801-X# This report covers the period September 1997 through During 1997-8 the activities of the Center for Backyard August 1998 and comprises an account of astronomical Astrophysics expanded. This is a network of astronomers, research carried out in the Department of Astronomy and the primarily amateur, who do stellar photometry with small Department of Physics. telescopes in their backyards. We typically observe a star Faculty and Research Associates were James Applegate, steadily for a few months, trying to amass the densest pos- Elena Aprile, Norman Baker, William Craig, Arlin Crotts, sible coverage by stressing long observation and distribution Karl-Ludwig Giboni, Charles Hailey, Jules Halpern, David of observers in longitude. This provides a time series well Helfand, Philip Kaaret, Stephen Kahn, Marc Kamionkowski, suited to the study of periodic signals, and immunized from Laura Kay ~Barnard!, Karen Leighly, Lloyd Motz ~Emeri- the ‘‘aliasing’’ problems inherent in data from a single site. tus!, Reshmi Mukherjee ~Barnard!, Robert Novick ~Emeri- The principal observers are in Belgium, Denmark, Maryland, tus!, Frederik Paerels, Joseph Patterson, Kevin Prendergast, Arizona, Illinois, New Zealand, and Australia. Occasional Andrew Rasmussen, R. Michael Rich, Malvin Ruderman, contributions also arrive from Russia, South Africa, Israel, Daniel Savin, Edward Spiegel, Marco Tavani, Wilhelmus Japan, and Chile. A grant from the Research Corporation van der Veen, and Jacqueline van Gorkom. enabled us to make this expansion in the scale of activity. Graduate students participating in research were Elizabeth Patterson and Kemp spent most of their time on this enter- Blanton, Ari Buchalter, Alex Casti, Tzu-Ching Chang, prise. Most programs involve the study of cataclysmic bina- Xuelei Chen, Xinzhong Chen, Jean Cottam, Catherine Cress, ries, justly famous for the many periods frequently present in Deepa Majmudar, Karl Forster, Akimi Fujita, Mario their light curves. Jimenez-Garate, Himel Ghosh, Ming Feng Gu, JaeSub Hong, Our study of the long-dormant dwarf nova EG Cancri Justin Howell, John Keck, Tomotake Kozu, Kaya Mori, Don revealed orbital and ‘‘superhump’’ periods of 86.36 and Neill, Masao Sako, Adrianne Slyz, Edgar Smith, Joshua 86.90 minutes, respectively. The offset by 0.6% is the small- Spodek, Ben Sugerman, John Tomsick, Robert Uglesich, est ever seen in a cataclysmic variable, and lends support to Matt Umurhan, Leven Wadley, Frank Wang, and Fang Xu. the idea that CVs of very short period lose thermal equilib- Undergraduates participating in research were Ian Adler, rium as their thermal timescales become longer than the ti- Scott Brown, Arindam Chatterjee, Elliott Eggleston, Justin mescale for gravitational radiation. In this theory, the sec- Detray, Irina Feygina Barnard , Susan Kassin, Ali Kinkha- ~ ! ondary star has a mass of 0.02 2 0.03M ( , but a radius of bwala, Miriam Krauss, Andreea Petric ~Barnard!, Yong 0.10R( . WZ Sagittae appears to be another star of this type. Moon, Scott Schnee, Charles Silver, Nigel Singh ~Cornell!, We studied the period excesses of all superhumping CVs, Lucianne Walkowicz ~Hopkins!, Beth Willman and Vanessa and found that the period excess appears to be a good signa- Yuille ~Barnard!. ture of the mass ratio. These CVs with a very low secondary- High school teachers participating in research were star mass exist and are even fairly numerous, possibly repre- Rachel Berger, Emily James and Britt Reichborn-Kjen- senting as much as 30% of all CVs. nerud. We also found two stars, AL Com and CP Eri, which Alex Casti, Catherine Cress, Eric Ford, Karl Forster, show large-amplitude superhumps at quiescence, contrary to Deepa Majmudar, David Schiminovich, Adrianne Slyz, Matt all known rules. These are likely to come from excitation of Umurhan, Frank Wang and Fang Xu received Ph.D. degrees. an instability at the 2:1 orbital resonance in the accretion Appointments during 1997–98 were held by Adjunct Pro- disk. fessor Michael Allison, Postdoctoral Research Scientists Large data sets, comprising typically ; 200 hr over ; 50 Valeri Egorov, Charles Liu, Uwe Oberlack, Francesco Pa- nights, have been collected on many other short-period CVs, parella, Frank Summers, Louis Tao, Ion Yadigaroglu, with the purpose of studying these precessional effects in Melinda Weil and Ming Zhao. disks. The most useful results come from the study of per- Marc Kamionkowski received the Helen B. Warner Prize manent superhumps in novalike variables. Essentially all for Astrophysics from the American Astronomical Society. novalike variables with Porb , 3.5 hr show superhumps. Their study and understanding will keep the group busy for years to come. 1. STARS & STELLAR EVOLUTION Rich, in collaboration with Jay Frogel ~OSU! and A. Ren- Crotts, along with S. Heathcote ~CTIO!, have measured zini ~ESO! has undertaken 3 large programs with the NIC- the expansion velocity of the outer circumstellar rings of SN MOS camera on board HST. The first is to image the Galac- 1987A, and the circumstellar gas just beyond. All three rings tic bulge at 2 6deg latitude as deeply as possible, with the in the SN 1987A nebula appear to have the same kinematic aim of reaching the hydrogen burning limit at M H 5 age. Crotts and Heathcote, along with J. Maza and M. Phil- 1 10. Preliminary reduction of the data shows that a very lips ~CTIO!, have also recorded changes in the spectrum of tight infrared lower main sequence has been measured, but SN 1987A in response to the impact of the ejecta upon this not down to the brown dwarfs as had been hoped. The other circumstellar nebula. two programs involve study of the stellar populations and 44 ANNUAL REPORT globular clusters in the central kpc of the M31 bulge. Prior the ISOPHOT instrument aboard the ISO satellite. After the ground-based studies find some evidence for extremely completion of the ISO mission in April 1998, twelve out of bright AGB stars, perhaps proving the presence of an inter- fourteen scheduled observations were actually done. For five mediate age population there. It has been argued that these sources we have data at 90 and 160 micron, for one source apparently bright stars are blends of stars, not intrinsically only at 90 micron and for another one only at 160 micron. luminous objects. The NICMOS imaging is intended to settle The images are roughly 15 x 35 arcmin in size and show the this debate, which has implications for the ages of spheroid distribution of dust as illuminated by the central mass losing populations in general. stars. This inner parts of the shells are clearly detected but it Rich, in collaboration with Beatriz Barbuy ~Universidade is not clear how far out we are able to detect the dust emis- de Sao Paulo! has studied the Galactic Center globular clus- sion. We are currently trying to better understand the instru- ter NGC 6558. This cluster appears to be similar to NGC ment and observing mode in order to determine how far out 6522 in having a heavily populated blue horizontal branch we are still detecting the extended dust shell. and a remarkably sparsely populated red giant branch. The Van der Veen in collaboration with Martin Groenewegen cluster is a core-collapse globular cluster that has experi- ~MPE, Munich!, Alain Omont ~IAP, Paris! and B. Lefloch enced much disk and bulge shocking in its lifetime. How- ~IRAM! observed an additional six evolved mass losing ever, it is virtually impossible to understand how any process stars. Combined with observations made in 1996 and 1997 could modify the population of the cluster by transforming we now have a total sample of 16 stars which are mapped at red giants into blue HB stars, since the latter must ignite 1300 micron using the bolometer array at the IRAM 30m helium in their cores. This would be very hard to do if the telescope at ~Pico Veleta, Spain!. The maps are about 1 arc- envelope of an RGB star were stripped in a collision with min square in size and are obtained with a 119 FWHM beam. another star. The similarity of NGC 6558 with NGC 6522 These data enable us to study the most recent mass loss and other Galactic center clusters is interesting, and better history ~last several thousand years! with a time resolution of color-magnitude diagrams are needed for this possible class a few hundred years. The sample includes a variety of late of globular clusters. type stars such as Carbon Stars, S-Stars, Miras and their Rich, in collaboration with M. Shara, M. Fall, and D. more massive equivalents M-supergiants. The data are now Zurek ~STScI! has used WFPC2 HST images to show that being reduced and analyzed. One of the results that was im- two sets of star clusters in the Small Magellanic Cloud were mediately obvious is that while many of the circumstellar formed in very short bursts. NGC 152, 411, and 419 have envelopes appear fairly round some show strong deviations identical CMDs with an age of 1.5 Gyr, while Kron 3, NGC from spherical symmetry. 339, 361, and 416 also have identical CMDs but with an age Van der Veen in collaboration with Martin Groenewegen of 8 Gyr. These clusters are widely separated in space but ~MPE, Munich! and Henry Matthews ~JAC, Hawaii! ob- appear to have formed nearly simultaneously. It has been tained maps at 450 and 850 micron of four evolved stars widely believed that the SMC has had a continuous period of using the SCUBA bolometer array at James Clark Maxwell cluster formation; these observations contradict that idea. Telescope ~JCMT, Mauna Kea, Hawaii!. This was a pilot Reduction of 30 LMC clusters is also underway, with the sample of a variety of evolved mass losing stars to see if finding that ages of most of the younger star clusters are their circumstellar dust shells are detectable at 450 and 850 younger than those derived from ground-based data.