Spectroscopy and photometry of the symbiotic star V4018 Sgr in outburst

ATel #4403; U. Munari (INAF Astr. Obs. Padua), P. Valisa, S. Dallaporta (ANS Collaboration), E. O. Waagen, A. Henden ( AAVSO Headquarters)
on 21 Sep 2012; 01:41 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

Tweet

Spectroscopy of the the symbiotic star V4018 Sgr (=AS 304), reported by AAVSO observers to be in outburst (cf. AAVSO Alert Notice 470), was obtained with the 0.6m telescope of the Schiaparelli Observatory in Varese (Italy). A low resolution, absolutely fluxed 4000-8650 Ang spectrum (4.24 Ang/pix) was recorded on Sept 13.90 UT. It shows the spectrum of the M giant overwhelmed by a blue continuum up to 6000 Ang, with only hydrogen Balmer and weak HeI lines visible in emission. The integrated absolute fluxes of representative emission lines are [in units of 10^(-13) erg cm^2 sec^(-1)] Hbeta = 34, HeI 5876 = 7.3, Halpha = 242, HeI 6678 = 5.0, and HeI 7065 = 6.0. Only feeble traces of [OIII] 5007 and OI 8446 are detectable. Compared with quiescence spectra (Allen (1984, Proc. ASA 5, 369; Munari and Zwitter 2002, A&A 383, 188; Gutierrez-Moreno et al. 1999, PASP 111, 571), all high ionization emission lines are gone, in particular HeII 4686 Ang and [FeVII] 6087, which were at times rivalling in intensity with Hbeta and HeI 5876, respectively. A medium resolution spectrum of V4018 Sgr was obtained on Sept 15.78 UT over the Halpha region (5312-6950 Ang, range 0.73 Ang/pix), and a low S/N Echelle spectrum (0.30 Ang/pix) of the same region was obtained on Sept 20.76 UT. No P-Cyg profiles affect the detected emission lines. HeI emission lines are very sharp, with a FWHM of 40 km/sec. The Halpha emission line displays a simple Gaussian-like emission core, characterized by a FWHM of 98 km/sec (corrected for instrumental resolution) and an heliocentric radial velocity of +105 km/sec, with the strong and extended symmetric wings typical of symbiotic stars (Sekeras and Skopal 2012, BaltA 21, 196). The heliocentric radial velocity of HeI emission lines is +80 km/sec. The 25 km/sec difference with Halpha, is close to the 22 km/sec difference found by van Winckel et al. (1993, A&AS 102, 401), who report the Halpha radial velocity to be +91 km/sec on 14 Sept 1989, while Munari and Buson (1993, MNRAS ) measured +74 km/sec on 31 July 1988. These Halpha profiles for 1988-89 did not present the absorption which is now apparent on the blue wing on Halpha on our Echelle spectrum for Sept 20.76, characterized by a FWHM of 60 km/sec and an heliocentric radial velocity of +49 km/sec. The outburst state of V4018 Sgr is confirmed by ANS Collaboration daily CCD photometry for Sept 13-20, which shows V4018 Sgr stable around B=11.66, V=11.00, Rc=10.35, Ic=9.68. These values are remarkably close to those reported by Munari et al. (1992, A&AS 93, 383) for V4018 Sgr in June 1990, during the previous bright outburst: B=11.69, V=11.19, Rc=10.64, Ic 9.87, J=8.59, H=7.71, K=7.40. Jurdana-Sepic and Munari (2010, PASP 122, 35) found V4018 Sgr even brighter, at B=11.2, on Asiago archive plates exposed in May 1961. For comparison, the median values for quiescence reported by Henden and Munari (2008, Baltic Astronomy 17, 293) are B=14.38, V=13.29, Rc=12.12, Ic=10.69. At the time of the previous outburst, Munari and Buson (1993, MNRAS 263, 267) obtained in 1992 low-res ultraviolet spectra with the IUE satellite that showed deep P-Cyg absorptions at large outflow velocity (2200 +/- 400 km/sec), a rarity among symbiotic stars.