Sylvio Klose, Eike Guenther, and Jens Woitas (Thüringer Landessternwarte Tautenburg)

report:

High-resolution spectra of SN 2002ap were aquired using the Echelle-spectrograph at the Tautenburg 2-m telescope on February 2.8 UT and 3.8 UT. The detector is a 2k x 2k CCD chip with 15 micron pixels. The spectral resolution was R = 70 000 (0.04 Angstrom per pixel; slit 1.04 arcsec). At the time of the observations SN 2002ap was at an apparent magnitude of about V=12. The spectra were reduced in a standard fashion. Wavelength calibration was performed by means of ThAr lamps. The signal-to-noise ratio of the combined images is about 30. This is sufficiently high to check the spectrum for lines from the interstellar medium in the supernova host galaxy (M 74). In the combined spectra we detect an absorption feature at a heliocentric velocity of (660 +/- 2) km/s (see Fig. 1) what we attribute to redshifted NaI D1 at the velocity of the host [1, 2]. For the D1 component we measure an equivalent width of (30 +/- 5) milli-Angstrom, whereas the redshifted D2 component is blended with a telluric line. Assuming that the empirical relation between the equivalent width of Na I D1 and interstellar reddening in our Galaxy [3] is also representative for the interstellar medium in M 74 we arrive at a reddening in the supernova host of E(B-V) = (0.008 +/- 0.002) mag. Thus, assuming a ratio of total-to-selective extinction of about 3, our data are in agreement with A_V (host) = (0.025 +/- 0.005) magnitudes. In other words, at a projected distance of the supernova of about 13.5 kpc from the center of M 74 (assuming a distance of 10 Mpc, [4]), the supernova light is nearly unaffected by any dust in M 74.

References:

[1] Tormen, G. & Burstein, D.: Astrophys. J. Suppl. Ser. 96 (1995) 123
[2] Smartt, S. & Meikle, P.: IAUC 7822 (2002)
[3] Munari, F. & Zwitter, T.: Astron. Astrophys. 318 (1997) 269
[4] Chen, P. C. et al.: Astrophys. J. 395 (1992) L 41

Fig. 1: Echelle-spectrum of Supernova 2002ap in the ScI galaxy Messier 74 (red color) aquired with the Tautenburg Echelle-spectrograph. For comparison we have overplotted the Echelle-spectrum of Alpha Lyrae (in white color). The redshifted Na D1 component is indicated. Note that both spectra were not taken during the same night. Therefore, the heliocentric velocity of the Galactic NaI D1,2 lines is slightly shifted from each other. Emission lines are due to light pollution.