The Araucaria Project: First Cepheid Distance to the Sculptor Group Galaxy NGC 7793 From Variables Discovered in a Wide-Field Imaging Survey

Abstract

We have detected, for the first time, Cepheid variables in the Sculptor Group spiral galaxy NGC 7793. From wide-field images obtained in the optical V and I bands on 56 nights in 2003-2005, we have discovered 17 long-period (24-62 days) Cepheids whose periods and mean magnitudes define tight period-luminosity relations. We use the (V-I) Wesenheit index to determine a reddening-free true distance modulus to NGC 7793 of 27.68 ± 0.05 mag (internal error) ± 0.08 mag (systematic error). The comparison of the reddened distance moduli in V and I with the one derived from the Wesenheit magnitude indicates that the Cepheids in NGC 7793 are affected by an average total reddening of E(B-V)=0.08 mag, 0.06 of which is produced inside the host galaxy.As in the earlier Cepheid studies of the Araucaria Project, the reported distance is tied to an assumed LMC distance modulus of 18.50. The quoted systematic uncertainty takes into account effects like blending and possible inhomogeneous filling of the Cepheid instability strip on the derived distance. The reported distance value does not depend on the (unknown) metallicity of the Cepheids according to recent theoretical and empirical results. Our Cepheid distance is shorter, but within the errors consistent with the distance to NGC 7793 determined earlier with the TRGB and Tully-Fisher methods.-3 -The NGC 7793 distance of 3.4 Mpc is almost identical to the one our project had found from Cepheid variables for NGC 247, another spiral member of the Sculptor Group located close to NGC 7793 on the sky. Two other conspicuous spiral galaxies in the Sculptor Group, NGC 55 and NGC 300, are much nearer (1.9 Mpc), confirming the picture of a very elongated structure of the Sculptor Group in the line of sight put forward by Jerjen et al. and others.

A mass of less than 15 solar masses for the black hole in an ultraluminous X-ray source

“…We then retained orbital solutions predicting an intrinsic stellar flux consistent with that observed at minimum V light (V = 20.50 ± 0.03 mag). We assumed a distance of 3.7 ± 0.1 Mpc 14 , A V in the range of 0.07 (Galactic extinction 15 in the direction of NGC 7793) to 0.25 16 − 10×log(T eff ) and forced to be within the allowed range derived from M V values and T eff dependent bolometric corrections 60 (BC = -0.26, -0.49 and -0.71 for T eff = 10,000, 11,000 and 12,000 K, respectively).…”

Synthetic photometry of OB star clusters with stochastically sampled IMFs: analysis of models and HST observations

“…The clusters have different A V values due their slightly different observed magnitudes (see Table 5). (Pietrzyński et al 2010). It has a small bulge and a spiral filamentary morphology, and the following additional properties: an inclination of 47 • ; a colour excess due to the Galaxy of E(B − V) = 0.017 mag (Schlafly & Finkbeiner 2011)…”

First targeted search for gravitational-wave bursts from core-collapse supernovae in data of first-generation laser interferometer detectors

“…The progenitor of SN 2008bk was most likely a red supergiant with a radius of ∼500 R [112][113][114], which suggests an explosion time of ∼1 day after core collapse [8][9][10] [115]. Its host galaxy is NGC 7793, which is located at a Cepheiddistance D = (3.44 + 0.21 − 0.2) Mpc [116]. This distance estimate is consistent with D = (3.61 + 0.13 − 0.14) Mpc obtained by [117] based on the tip of the red giant branch method (e.g., [118]).…”