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Johannesburg Centre,
Astronomical Society of Southern Africa
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VARIABLE OF THE MONTH - OY CARINAE
This star featured in one of the first VOTM articles, as far as I remember. Just to show how much our amateur observations are prized, I suggested to our Editor that he print the AAVSO announcement below. For the uninitiated, it should be pointed out that the observations by myself on February 21 and by Rod Stubbings on February 22 are crucial in that they define the time of the superoutburst. THE AMERICAN ASSOCIATION OF VARIABLE STAR OBSERVERS SPECIAL AAVSO ALERT NOTICE (February 23, 2000) 1004-69 OY CAR IN OUTBURST - EUVE SATELLITE SCHEDULED TO OBSERVE IT FOR AAVSO OBSERVERS! As reported in AAVSO News Flash 580 on February 22, the dwarf nova type, SU UMa subtype, cataclysmic variable OY Car is in superoutburst. The following observations have been reported to the AAVSO: Feb 17.7340 UT, <15.0, R.
Stubbings, Drouin, Victoria; Australia; AAVSO observers around the world have been awarded 100,000 seconds of EUVE satellite time to observe the current superoutburst of OY Car with the satellite! This is the very first time that amateur astronomers have been awarded time on the EUVE satellite. The EUVE time has been awarded to the observers by the Director of EUVE, from his own discretionary time, because of the vital contributions of variable star observers in the scheduling of EUVE observations of cataclysmic variables and the important results that have been obtained in correlating the EUV and x-ray data with optical observations. In choosing the target of opportunity for EUVE observations, we checked the good EUV candidates, and of these, four cataclysmic variables stand out, namely SS Cyg, U Gem, VW Hyi, and OY Car. These stars are bright, close, and have low hydrogen column densities. For this first-ever Target-of-Opportunity observation by AAVSO observers we have chosen the dwarf nova (SU UMa subtype) cataclysmic variable OY Car, because it is a very interesting eclipsing system, it has been observed only once with EUVE, its superoutburst was expected during the EUVE observing window, and observations of it can answer some very important questions about these systems, and also because we want to give a unique opportunity to our Southern-Hemisphere observers. Later this year we will be collaborating in the observations of SS Cyg with EUVE and Chandra x-ray satellite. For our EUVE observing run on OY Car, we are collaborating with our colleague Dr. Christopher Mauche at Lawrence Livermore National Laboratory in California. The purpose of the EUVE observations is: - First, to measure the delay between the turn-on in the optical and EUV, and thereby better understand the heating waves which cause the accretion disks of dwarf novae to go from quiescence (low accretion rate) to outburst (high accretion rate); - Second, to better understand the interaction between the boundary layer and the stellar wind of OY Car; - Third, to better understand the cause of superoutbursts. Existing FUV and EUV data on OY Car and VW Hyi hint that the short-wavelength light curves of superoutbursts are double-humped. This same phenomenon is quite apparent in the optical in some SU UMa systems, but in OY Car and VW Hyi it appears that the short-wavelength-flux light curves are double-humped, whereas the optical light curves are smooth (optical light falls only slowly throughout the light curve). The EUV flux tell us about the accretion rate at the very center of the accretion disk, whereas the optical light comes primarily from the outer disk. Simultaneous optical and EUV light curves therefore provide a spatial dissection of the accretion disk and provide information to constrain the cause of superoutbursts. The period of eclipses of OY Car is 01h 30m 54s. The eclipse elements are given in the General Catalogue of Variable Stars (4th edition, Kholopov et al., 1985) as: Min (I) Heliocentric = 2443993.553241 + 0.0631209247 E (days) The first predicted eclipse for Feb 23 is at 01 hr 00m 06s (vsnet-alert 4253). As OY Car is an SU UMa-type dwarf nova, during a superoutburst it exhibits superhumps, small-amplitude variations slightly longer in period than the orbital period of the system. For OY Car, the superhump period is 01hr 32m 57s. A plea to our Southern-Hemisphere observers in particular -- please monitor the superoutburst of OY Car as closely as you can, particularly the eclipses and superhumps, with observations every 3 to 5 minutes. We highly recommend CCD observations. Good optical coverage is essential in correlating and analyzing the EUVE data. Accompanying is an AAVSO preliminary 'f' scale chart for OY Car prepared by C. Scovil and incorporating the comparison star sequence from Publ. RASNZ #8, p. 10, 1980. Please use this chart to observe OY Car, and report your observations to AAVSO Headquarters. Please be sure to indicate which comparison stars you used to make your estimates. We very much appreciate the EUVE Director's allocation of 100,000 seconds to AAVSO observers worldwide. We sincerely thank the observers above for their valuable observations which alerted us to the superoutburst and the superb EUVE team for their efforts. CHARTS AVAILABLE ON AAVSO WEB AND FTP SITES Electronic copies of the chart for OY Car mentioned in this Alert Notice are available through our web site at the following address: The charts may also be obtained directly from our FTP site: ftp.aavso.org (198.116.78.5) The answering machine at AAVSO Headquarters is on nights and weekends for your convenience. Please call our charge-free number (888-802-STAR = 888-802-7827) to report your observations. We also encourage observers to send observations by fax to 617-354-0665 or by e-mail through the Internet to observations@aavso.org. Please note that this Special Alert Notice is being distributed electronically only and not on paper. Many thanks for your valuable astronomical contributions and your efforts. Good observing! Janet A. Mattei Elizabeth O. Waagen Danie Overbeek |
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