AAVSO: 2005: A Stellar Year for the AAVSO

Several papers made use of the AAVSO extensive data archives on Mira variables and other AGB stars. Two large studies used AAVSO light curves directly to study the pulsation behavior and physics of these highly evolved stars. First, Smith (East Tennessee State University) and collaborators compared the AAVSO optical light curves of a sample of Mira and semiregular variables to the near-infrared light curves from the COBE satellite's DIRBE instrument, and found that while the infrared peak lags the optical peak in Mira stars, there is no lag in the semiregular star light curves. Second, the AAVSO's own Matthew Templeton, along with coauthors Janet Mattei and Lee Anne Willson (Iowa State University) completed an extensive search for long-term period changes among a sample of over 500 Mira stars. All of the previous candidates for period changes were confirmed, and they also put limits on the numbers of AGB stars expected to undergo evolutionary events like thermal pulses at any given time. They also discussed several interesting cases of shorter-term period changes with (as-yet) unknown causes.

Other work on Mira and AGB stars used AAVSO data to correlate optical data with multiwavelength observations, or to schedule new observations. Tenenbaum (Pomona College & Maria Mitchell Observatory) et al correlated AAVSO optical observations of R Coronae Borealis stars with ground-based infrared spectroscopy obtained from Kitt Peak and Lick Observatories. They used this information to study how the abundance of carbon monoxide (CO) changes as a function of the temperatures of these highly-evolved stars. Ireland (Univ. of Sydney) et al used the Sydney University Stellar Interferometer to resolve dust shells around the Mira stars R Carinae and RR Scorpii, and used AAVSO visual data to determine the phases at which the observations were taken. Karovska (Harvard-Smithsonian CfA) et al used time-of-minimum estimates to schedule observations of the Mira AB system with the Chandra X-ray Observatory, and made the first confirmed detection of X-ray emission from Mira itself, rather than the interaction of Mira's stellar wind with its white dwarf companion (Mira B, or VZ Ceti).

Cataclysmic Variables

The AAVSO and its observing community have always made important contributions to the study of Cataclysmic Variables of all kinds, and this past year continued that tradition. AAVSO data were used directly to study the optical variability, and were also correlated with data taken at other wavelengths with ground- and space-based observatories. As always, the AAVSO also participated in several monitoring efforts during the year, to provide alerts for professional astronomers wanting to schedule observations at critical times, and to provide the optical component of multiwavelength campaigns.

Click on the above image to see the long-term light curve for Z Cam.

In one large study, Shafter (San Diego State U. & NASA GSFC), Cannizzo (NASA GSFC) and Waagen (AAVSO) used AAVSO light curves for 16 Z Cam-type dwarf novae to better understand why these stars exhibit the curious pattern of outbursts and standstills. They found that there is a relation between their orbital periods and their outburst intervals. Sokoloski (Smithsonian Astrophysical Observatory) et al. used a combination of KAIT photometry and AAVSO visual data to study and describe a new type of activity in the symbiotic star Z Andromedae; this phenomenon, which they describe as a "combination nova" involves both a disk instability in the white dwarf accretion disk, and thermonuclear burning on the white dwarf, combining elements of both classical and dwarf nova outbursts.

Novae were also represented among publications in 2005. Hachisu (Univ. of Tokyo) and Kato (Keio University), studying V1974 Cyg (Nova Cyg 1992), combined AAVSO visual data with archival IUE and ROSAT data to develop a new theoretical model of nova light curves. Their work showed that much of the optical light we observe in novae comes from a specific type of emission process ("free-free emission") in the ejecta shell, rather than from the white dwarf primary. Ak (Istanbul Univ.) et al. combined their own time-series photometry of V2540 Ophiuchi (Nova Oph 2002) with AAVSO and AFOEV long-term light curves. From the long-term data, they characterized the decay rate and measured the "period" of the transition-phase oscillations. Mason (ESO, Santiago) et al. used AAVSO archival data (among others) for Nova LMC 1995 to measure this nova's decay rate, and provide context for their own observations of Novae SMC 2001 and LMC 2002.

AAVSO data were frequently used to trigger satellite observations, or to check the activity state of archival data. Gaensicke (Univ. of Warwick) et al. used the long-term AAVSO light curve of AM Herculis to determine the activity state of several observations made with the FUSE and Hubble Space Telescope satellites. The activity state was used to interpret the spectral data in the context of models for this system. Pandel (UCSB) et al. used the activity state information for ten dwarf novae, along with data from the XMM-Newton satellite to study the boundary layer -- the interface between the accretion disk and the white dwarf surface -- in these systems. Similar use of AAVSO data was made for data of HT Camelopardalis (Evans and Hellier, Keele Univ.), IP Pegasi (Saito et al., Univ. Federal de Santa Catarina), SS Cygni and WX Hydri (Long et al., STScI), GK Persei (Vrielmann et al., Univ. Hamburg), GY Cancri, IR Comae, and HT Cas (Feline et al., Univ. Sheffield).

M80: A Dense Globular Cluster
Credit: F. R. Ferraro (ESO /Bologna Obs.), M. Shara (STSci /AMNH) et al., & the Hubble Heritage Team (AURA/ STScI/ NASA)

Shara (American Museum of Natural History) et al. used the AAVSO data archives for SS Cygni and U Geminorum, not to analyze these stars individually, but to assess how well infrequent observations of the globular cluster M80 could detect and characterize dwarf novae and their outbursts. They chose a random set of observations of each star, to see how often the stars were caught in outburst and quiescence, and used this to determine statistically how well the activity of hypothetical systems in M80 could be characterized with different observing cadences.

AAVSO observers participated in several new discoveries in 2005. Templeton (AAVSO) et al. studied the newly-discovered CV ASAS J002511+1217.2, combining the extensive visual and time-series CCD data collected by AAVSO observers with spectroscopic data by co-authors Ryan Leaman and Paula Szkody (Univ. of Washington). They found that this new system is likely a member of the rare WZ Sagittae class of short-period, infrequent outbursters. Likewise, AAVSO observers Berto Monard, Donn Ray Starkey, and Arto Oksanen participated in a campaign by Imada (Kyoto Univ.) et al. on SDSS J013701.06-091234.9, highlighting the importance of amateur follow-ups of discoveries from new surveys like the Sloan Digital Sky Survey.

Other stars

The Mira and cataclysmic variables are most well-represented in literature, but data for other types of stars are often requested by astronomers. One example is the data for RZ Ari, a low-amplitude semiregular, which was used by Richichi (ESO, Munich) and the Calar Alto lunar occultation program to determine that the average brightness of this star is slowly increasing with time. Another is the interesting case of FH Leo, described by Dall (ESO, Santiago) et al. Originally classified as a novalike variable, analysis of the AAVSO long-term light curve reveals this star is likely an ordinary main sequence star undergoing occasional bursts of unknown activity. They suggest it could be any of planetary transits, planetary accretion, or magnetic activity.

Your observations matter!

Variable stars are a fascinating pursuit for amateur and professional astronomers alike, and there are probably as many reasons for the excitement people have for them as there are variable star observers! Since its founding, the AAVSO has provided a link between amateurs with a love for variable stars, and professional scientists seeking to learn about the underlying physics of stars and stellar variability. The AAVSO is now a caretaker for a century-old archive of variable star data, amounting to more than 12.5 million individual observations. These observations, whether they be visual or CCD, positive or fainter-than, are precious snapshots of these stars' histories, and are helping scientists to uncover more about the universe we live in with ever-finer detail. They also stand as testament to the excitement you, the observing community, have for this wonderful pursuit.

Keep up the great work in 2006 and beyond!