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Distance of nova Del 2013 from MMRD relations

ATel #5313; M. M.M. Santangelo, M. Pasquini, S. Gambogi, G. Cavalletti (OAC - Osservatorio Astronomico di Capannori and IRF - Istituto Ricerche Fotometriche, Italy)
on 23 Aug 2013; 15:56 UT
Credential Certification: Filippo Mannucci (filippo@arcetri.astro.it)

Subjects: Optical, Nova

Following ATEL 5295, in the course of the CATS (Capannori Astronomical Transients Survey) project, M.M.M. Santangelo and co-workers performed more UBVR photoelectric photometry and low resolution CCD long-slit spectrometry of nova Delphini 2013. The measurements were made with an Optec SSP-5A single channel photoelectric photometer (with a photomultiplier tube Hamamatsu R6358), and with a SBIG SGS spectrometer + CCD camera ST-7XME attached to the 0.30-m f/10 Schmidt-Cassegrain telescope at Osservatorio Astronomico di Capannori (OAC). The atmospheric extinction was measured each night with the method of Bouguer's lines. As comparison stars for the photometry we used HD 194113 (UBV measurements) and GSC 1644-1837 (BVR measurements). Pre-processing of the spectra included bias + dark + nightsky subtraction. The spectra were wavelength calibrated by means of Mercury and Neon lamps taken immediately after each nova spectrum, and they were flux calibrated by means of spectra of the spectrophotometric standard star Zeta Peg taken in the same night at a similar airmass of nova ones. These spectra of nova Delphini 2013 were taken in two spectral ranges from 385.4 to 715.2 nm and from 591.0 to 920.8 nm, at the dispersion of 0.43 nm/pixel (resolution R ~ 1000), through a 18 microns wide long-slit. Our photoelectric photometry shows that, after the end of a short post-maximum plateau, the V band light curve is fading linearly at a constant rate of 0.250 +/- 0.008 mag/day. If this slope will remain constant for the following two days we expect the nova should have reached the 2 magnitudes below level at maximum around Julian Day 2456529.5 . This would lead to a time t2 ~ 8.5 +/- 0.3 days (assuming as t-max the Julian Day 2456521.0 at V = 4.35 +/- 0.03, from AAVSO data). Using the nonlinear Mv-log(t2) relation of Downes & Duerbeck (2000, AJ 120, p.2007) this implies an absolute magnitude of Mv ~ -9.3 +/- 0.2 . The E(B-V) was estimated to be 0.17 from ATEL 5288 and 0.182 from ATEL 5297, leading to Av respectively of 0.53 and 0.56 mags; from the NED extinction calculator we get other two estimates for Av: 0.49 and 0.60 mags. We adopted a mean value of Av = 0.55 +/- 0.1 mags. So the distance of the nova is d ~ 4.2 +/- 0.4 kpc Using the linear Mv-log(t2) relation of Downes & Duerbeck (2000, AJ 120, p.2007) a t2 = 8.5 implies an absolute magnitude of Mv ~ -8.9 +/- 0.2. So, ceteris paribus, the distance changes to d ~ 3.5 +/- 0.4 kpc. As a final preliminary estimate, we can adopt a value around 4 kpc (or a bit less) for the distance of the nova DEL 2013. Our spectra of nova Delphini 2013, taken in the last few days, showed no more P Cyg effect but simple emission lines of H-alpha, H-beta, H-gamma, OI 777.4 nm, and various Fe II.