Alert Notice 469: Observations of TT Ari requested in support of MOST observations

August 25, 2012: Dr. Nikolaus Vogt (Universidad de Valparaiso, Chile) has requested photometry and spectroscopy of the novalike (VY Scl subtype) cataclysmic variable TT Ari in support of upcoming observations with the Canadian Microvariability and Oscillations of Stars (MOST) satellite.

The observations are being carried out to study superhump behavior, which is still not well understood despite the amount of research done in all classes of cataclysmic variables. The goals are discussed below.

Observing schedule
MOST will observe TT Ari between 2012 September 13 and October 20. Dr. Vogt writes: "For the first 22 days (until 5-6 Oct) it will share each MOST orbit with another Primary Science Target...We expect TT Ari will receive about 50% of each 101-minute satellite orbit (meaning intervals of about 50 minutes of continuous coverage with 50-minute gaps) during the first 22 days of the run. The final 14 days will be continuous coverage of TT Ari...The exact start and end dates of the run may change by 1 or 2 days depending on operational and logistical details, but the MOST Team will strive to preserve the total number of days scheduled."

Observing Instructions
Supporting photometric CCD observations of TT Ari are requested before, during, and after the MOST observations. From Dr. Vogt: "Serial observations, if possible of minimal duration one orbital period per night (3.3 hours), at any time between August and December 2012. Differential CCD photometry: Johnson UBV or white light (without filter) [s/n=50], time resolution 1 minute (in order to resolve the flickering and possible quasi-periodic oscillations with a ~20 minutes period)."

Simultaneous spectroscopic monitoring of TT Ari is also requested in order to confirm (and improve) the orbital period and to search for spectral variability in the superhump phase. "Spectroscopy: Wavelength range 4000–7000 Angstroms, including Halpha, time resolution required: less than or equal to 5 minutes, in order to resolve variations in radial velocity and line profiles with the orbital and/or the superhump period."

The Departamento de Fisica y Astronomia of the Valparaiso University will carry out photometry with small telescopes in central Chile but the assistance of other observers, particularly in other latitudes and longitudes, is requested.

Background and Goals
TT Ari, one of the brightest cataclysmic variables, exhibits occasional fadings of several magnitudes, from its usual high-state (maximum) magnitude of ~10.5V to a low-state magnitude as faint as 16V. These fadings occur every 20-25 years, and last between 500 and 1000 days. According to observations in the AAVSO International Database, TT Ari is currently magnitude 10.5V.

TT Ari exibits superhumps - both positive (the superhump period is longer than the orbital period) and negative (the superhump period is shorter than the orbital period). While positive superhumps are thought probably to be the result of an eccentric configuration in the accretion disk, the mechanism for negative superhumps is not yet understood except that it may be related to the disk's being warped out of the orbital plane, leading to complex torque phenomena.

MOST observed TT Ari in 2007 but not enough information about the superhumps was known at that time to obtain conclusive results. In January 2011 TT Ari recovered from a fading episode that began in 2009, so now is a good time to study the star again and compare the current behavior with that from 2007.

Goals include "to detect significant superhump period variations, as well as periodic signals in the range of several days (beat periods). We will analyse the new data, as well as the archival data from 2007, in the same way, in order to determine the accurate superhump ephemeris, its amplitude, light curve form and possible period changes in both data sets. We also will try to find a signal of the beat period between P(orbital) and P(superhump)...we will search for quasi-periodic oscillations (QPOs) which were reported by several authors, with periods mainly of the order of 20 minutes. Also the pronounced flickering will be analysed, and compared to the results of Kraicheva et al. (1999, A&A, 351, 607)...Occurrence, amplitudes and time scales of QPOs and of the flickering could vary with the phase of the beat period, the superhump period or other parameters. Only the uninterrupted MOST observations for weeks will be able to detect this kind of inter-correlations, which could contribute important facts for the correct understanding of superhumps and, more generally, of the rather complex time-dependent behaviour of accretion disks in cataclysmic variables."

Dr. Vogt also notes that observers who contribute reduced data used in future joint publications will be co-authors of them.

Coordinates: RA 02 06 53.09  Dec. +15 17 41.8 (2000.0)

Charts for TT Ari may be created using the AAVSO Variable Star Plotter (VSP) at

Please submit observations to the AAVSO International Database using the name TT ARI.

This campaign is being followed on the AAVSO Observing Campaigns page (

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen.


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