--------------------------------------------- PLANET TEAM ANOMALY ALERT FOR EROS BLG-2000-5 --------------------------------------------- In this anomaly alert, the PLANET collaboration announces the passage of the first fold-caustic crossing the microlensing event EROS BLG-2000-5 and characterizes the caustic parameters below. Due to the early alert of this event and the excellent characterization of its first caustic crossing, we are in an exceptionally favorable situation to predict and monitor the second caustic crossing. If observed spectroscopically, this second crossing would permit the first detailed, spatially-resolved spectrum of any normal star except the Sun. We strongly urge spectral observations of this second crossing, preferably with a large aperture telescope. A summary of this event and the PLANET team characterization of the first fold caustic follow. Event: EROS-2000-BUL-005 RA (J2000): 17:53:11.5 Dec (J2000): -30:55:35 On approximately 29 April 2000, the EROS collaboration issued an alert that EROS BLG-2000-5 was a probable microlensing event. On 8 June 2000, the MPS collaboration issued an anomaly alert, saying that source had brightened by 0.5 mag from the previous night and had continued to brighten by 0.1 mag in 40 minutes. PLANET began more intensive observations immediately, obtaining data over most of the caustic crossing, which lasted more than 1 day. We fit the flux to the form (e.g. Afonso et al. 2000 ApJ, 534, 894) flux = alpha * G[-(t-t_cc)/Delta t] + F_cc + omega(t-t_cc) where t_cc is the time that the center of the source crosses the caustic, Delta t is the time from the limb-crossing to the center-crossing, F_cc is the source flux just before the crossing, G is a specific function, and alpha and omega are parameters. We find t_cc(HJD) = 2,451,705.000 +/- 0.002 Delta t = 0.510 +/- 0.003 days alpha/F_cc = 2.09 +/- 0.06 By comparing the PLANET data with the data presented by EROS on their web page (http://www-dapnia.cea.fr/Spp/Experiences/EROS/alertes.html), we estimate F_cc/F_base = 3.3 where F_base is the baseline flux. These parameters should be useful for constraining fits to the microlensing data and so predicting the time of the second crossing. A discussion of how spectroscopic observations of caustics resolve the stellar atmosphere of the source can be found in recent literature (see, for example, Lennon et al. 1996, ApJ, 471, L23; Gaudi & Gould 1999 ApJ, 513, 619; Heyrovsky, Sasselov, & Loeb 2000, ApJ, astro-ph/9902273; and Sackett 2000, astro-ph/0004237, and references therein.) According to the EROS alert, the source at baseline has R_Cousins = 17.2 V_Johnson = 18.4 The peak brightness is given by flux_peak = 1.39*alpha + F_cc, hence the calibrated peak magnitude was approximately R_peak = 14.4 V_peak = 15.6 The second peak can be expected to be of comparable brightness. Because the first crossing was well characterized, it should be possible to predict the time of the second crossing very accurately. The exceptional brightness, long duration, and likely high accuracy of the prediction of the time of the second crossing are what make this event especially favorable for spectroscopic observations. PLANET continues to observe this event. Finding charts and future updated information can be found on the PLANET web site at http://www.astro.rug.nl/~planet. EROS information on this event can be found at the EROS alert web page at: http://www-dapnia.cea.fr/Spp/Experiences/EROS/alertes.html. ---------------------------------------------