The Reddening, Distance, and Star Formation Rate of IC10

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Session 76 -- Spirals I
Display presentation, Wednesday, 11, 1995, 9:20am - 6:30pm

[76.18] The Reddening, Distance, and Star Formation Rate of IC10

P. Massey, T. E. Armandroff (NOAO)

IC10 has typically been assumed to be an outlying member of the Local Group, with a distance of 1-3 Mpc based upon HII region diameters, degree of resolution into stars, planetary nebula, and the Tully-Fisher relation. (de Vaucouleurs \& Ables 1965; de Vaucouleurs 1978; Sandage \& Tammann 1974; Yahil et al 1977; Jacoby \& Lesser 1981; Bottinelli et al 1984). Recently Wilson (1994) has used IR observations of Cepheids to suggest that it is actually quite a bit closer: 0.24 Mpc. Knowledge of the distance of IC10 is important as it allows us to better characterize our local neighborhood, as well as bearing directly on the interpretation of the high star-formation rate suggested by the Hunter \& Gallagher (1985) study, as well as our own discovery of a large number of Wolf-Rayet candidates (Massey, Armandroff \& Conti 1992).

Our new data on IC10 consists of spectroscopy of the Wolf-Rayet candidates plus $BV$ CCD photometry of the galaxy. These data constrain the reddening and distance modulus, as well as confirming that IC10 is currently producing massive stars at a prodigious rate. We have spectroscopically confirmed 15 Wolf-Rayet stars, a factor of 4 more than is known in NGC 6822. $BV$ photometry of the resolved stellar content of IC10 may be compared to that of the LMC to derive the reddening; we obtain from this a value of $E(B-V)=0.75 \pm 0.05$ using the ``blue plume" seen in the CMD. From the $BV$ photometry of the individual WR stars we derive a similar number: $E(B-V)=0.80\pm0.05$. This is similar to the 0.87 value inferred from integrated light studies, but is inconsistent with the Wilson (1994) determination of 1.4-1.9. We can also estimate the distance to IC10 by assuming that the brightest WC stars in IC10 are similar to the brightest found in the LMC; this leads to a distance of 0.87 Mpc, assuming a distance modulus to the LMC of 18.3. With our distance the surface density of W-R stars is then $\ge11.7$ per kpc$^2$, considerably higher than that of any other galaxy in the Local Group. (For comparison, the densest OB associations in M33 contain 7.9 W-R stars/kpc$^2$, while the overall average in the LMC is 2.0 W-R stars/kpc$^2$.)

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