COMMISSIONS 27 AND 42 OF THE IAU INFORMATION BULLETIN ON VARIABLE STARS Number 4048 Konkoly Observatory Budapest 12 July 1994 HU ISSN 0374 - 0676 OPTICAL SPECTROSCOPY OF A FLARE ON PROXIMA CENTAURI On 9 March 1994, a large flare event on Proxima Centauri (Gliese 551 = LHS 49 = V645 Cen) was serendipitously observed with the CTIO 1.0-m telescope Cassegrain Image Tube Spectrograph, the "2D-Frutti". Presented here is a spectrum taken during the flare event and, for comparison, spectra taken during quiescence on the two following nights. Proxima Cen was observed as part of a spectral-typing program being carried out during non-photometric nights on the CTIO 1.0-m telescope. The 2D-Frutti spectrometer was configured with grating #26 (600 lines/mm, 5000A blaze) in first order to provide coverage in the spectral range of 4200-7600A with a resolution of 3-4A around Halpha (6563A). Because the observing conditions were non-photometric, these spectra are not flux calibrated. Instead, an instrumental spectral response function was derived by comparing observations of A-type field stars to the flux distribution of A-type spectrophotometric standard stars. Table 1 contains details of the individual observations and lists equivalent widths (EW), measured with respect to the local continuum, for the hydrogen Balmer lines during the flare and on subsequent nights during quiescent periods. Figure 1 shows the spectrum of Proxima Cen while in flare and, for comparison, one of the quiescent period spectra. Table 1 Spectroscopic Observations of Proxima Centauri UT Date Exposure Time EW(A) (at midpoint) (sec) Halpha Hbeta Hgamma Notes 1994 March 09 7:33 480 27 92 173 1 1994 March 10 7:27 600 4 6 20 1994 March 10 8:59 600 3 6 19 1994 March 10 9:09 600 3 3 14 1994 March 11 6:27 600 3 <2 10 2 Notes.- Equivalent widths are reported to the nearest Angstrom; (1) Also seen in emission are He I(4471A) EW = 10A, He I(5876A) EW = 7A, Na I D lines (5890A and 5896A), and He I(6678A) present but weak; (2) Hbeta present in emission but weak. Despite being the most nearby example of a dMe flare star, only a few optical spectra of Proxima Cen are found in the literature (e.g. Thackeray 1950; Mathioudakis & Doyle 1991). The flare spectrum in Figure 1 shows strong enhancement of Balmer line emission along with emission lines of He I (4471A, 5876A, and 6678A) and the Na I D lines (5890A and 5896A). The Halpha equivalent width in flare is enhanced by factor of about 10 over that in the observed quiescent period observations and by a factor of 15 over the quiescent Halpha equivalent width reported by Mathioudakis & Doyle (1991). The Hbeta and Hgamma lines show much larger enhancement factors during the flare. The continuum during the flare appears to be enhanced blueward of 4500A. [FIGURE 1] Figure 1: top, Proxima Centauri in flare on 9 March 1994. The hydrogen Balmer lines are the most prominent emission lines in this spectrum. Bottom, Proxima Centauri during a quiescent period on 10 March 1994. These spectra are not flux calibrated. Although Proxima has a lower rate of flare production than other flare stars (Walker 1981), it apparently does experience large flares from time to time. For comparison, a Delta U = 5.0 magnitude flare event on UV Ceti, a classic prototypical flare star, produced EW(Halpha) = 67.5A and EW(He I (5875A)) = 15.5A (Eason et al. 1992). Because no earlier observations were made of Proxima Cen on 9 March, the serendipitous flare spectrum presented here may not represent flare maximum for this particular event. The author made these observations as a visiting astronomer to Cerro Tololo Inter- American Observatory. The author would like to thank Dr. Chris Smith (CTIO) for deriving the instrumental spectral response curve for these data. Brian M. PATTEN Institute for Astronomy University of Hawaii at Manoa 2680 Woodlawn Drive Honolulu, Hawaii 96822 USA References: Eason, E. L. E., Giampapa, M. S., Radick, R. R., Worden, S. P., & Hege, E. K., 1992, AJ, 104, 1161 [BIBCODE 1992AJ....104.1161E ] Mathioudakis, M. & Doyle, J. G., 1991, A&A, 244, 409 [BIBCODE 1991A&A...244..409M ] Thackeray, A. D., 1950, MNRAS, 110, 45 [BIBCODE 1950MNRAS.110...45T ] Walker, A. R., 1981, MNRAS, 195, 1029 [BIBCODE 1981MNRAS.195.1029W ]