The first three years of the outburst and light-echo evolution of V838 Mon and the nature of its progenitor

V838 Mon has undergone one of the most mysterious stellar outbursts on record, with (a) a large amplitude (Delta B ~ 10 mag) and multi-maxima photometric pattern, (b) a cool spectral type at maximum becoming cooler and cooler with time during the descent, until it reached the never-seen-before realm of L-type supergiants, never passing through optically thin or nebular stages, (c) the development of a spectacular, monotonically expanding light-echo in the circumstellar material, and (d) the identification of a massive and young B3V companion, unaffected by the outburst. In this talk we review the photometric and spectroscopic evolution during the first three full years of outburst, the light-echo development and infer the nature of the progenitor, which was brighter and hotter in quiescence than the B3V companion and with an inferred ZAMS mass of about 65 Msun.Comment: to appear in the Proceedings of the Colloquium "Interacting Binaries: Accretion, Evolution and Outcome", held in Cefalu' (Sicily) July 4-10, 2004, L.A. Antonelli et al. eds., American Institute of Physics Conf. Proc. series, in press. 6 pages, 4 figure

On the accuracy of GAIA radial velocities

We have obtained 782 real spectra and used them as inputs for 6700 automatic cross-correlation runs to the aim of investigating the radial velocity accuracy that GAIA could potentially achieve as function of spectral resolution and signal-to-noise ratio. We have explored the dispersions 0.25, 0.5, 1 and 2 Ang/pix (bracketing the 0.75 Ang/pix currently baselined for the 8490--8740 Ang GAIA range centered on the near-infrared CaII triplet) over S/N ranging from 10 to 110. We have carefully maintained the condition FWHM (PSF) = 2 pixels during the acquisition of the 782 input spectra, and therefore the resolutions that we have explored are 0.5, 1, 2 and 4 Ang corresponding to resolving powers R=17200, 8600, 4300 and 2150. We have investigated late-F to early-M stars (constituting the vast majority of GAIA targets), slowly rotating (V_{rot} sin i = 4 km/sec, as for field stars at these spectral types), of solar metallicity ( = -0.07) and not binary. The results are accurately described by the simple law: lg sigma = 0.6(lg S/N)^2 - 2.4(lg S/N) + 1.75(lg D) + 3, where sigma is the cross-correlation standard error (in km/sec) and D is the spectral dispersion (in Ang/pix). The spectral dispersion has turned out to be the dominant factor governing the accuracy of radial velocities, with S/N being less important and the spectral mis-match being a weak player. These results are relevant not only within the GAIA context but also to ground-based observers because the absence of telluric absorptions and proximity to the wavelengths of peak emission make the explored 8490--8740 Ang interval an interesting option for studies of cool stars with conventional telescopes.Comment: 12 pages, 2 figures, 5 tables, in press in Baltic Astronom

The narrow and moving HeII lines in nova KT Eri

We present outburst and quiescence spectra of the classical nova KT Eri and discuss the appearance of a sharp HeII 4686 Ang emission line, whose origin is a matter of discussion for those novae that showed a similar component. We suggest that the sharp HeII line, when it first appeared toward the end of the outburst optically thick phase, comes from the wrist of the dumbbell structure characterizing the ejecta as modeled by Ribeiro et al. (2013). When the ejecta turned optically thin, the already sharp HeII line became two times narrower and originated from the exposed central binary. During the optically thin phase, the HeII line displayed a large change in radial velocity that had no counterpart in the Balmer lines (both their narrow cores and the broad pedestals). The large variability in radial velocity of the HeII line continued well into quiescence, and it remains the strongest emission line observed over the whole optical range.Comment: in press in A&

BVRcIc photometric evolution and flickering during the 2010 outburst of the recurrent nova U Scorpii

CCD BVRcIc photometric observations of the 2010 outburst of the recurrent nova U Scorpii are presented. The light-curve has a smooth development characterized by t2(V)=1.8 and t3(V)=4.1 days, close to the t2(V)=2.2 and t3(V)=4.3 days of 1999 outburst. The plateau phase in 2010 has been brighter, lasting shorter and beginning earlier than in the 1999 outburst. Flickering, with an amplitude twice larger in $I_{\rm C}$ than in $B$ band, was absent on day +4.8 and +15.7, and present on day +11.8, with a time scale of about half an hour.Comment: published March 1