WR 38/38a and the ratio of total-to-selective extinction in Carina

TL;DR

This study reanalyzes the open cluster Shorlin 1 associated with WR 38/38a, revealing that dust extinction in the Carina region is often 'anomalous' with a higher R value, impacting distance estimates and stellar parameters.

Contribution

It provides a revised analysis of cluster reddening, distance, and dust extinction ratios, highlighting widespread anomalous dust properties in the Carina region and refining parameters for associated Wolf-Rayet stars.

Findings

Reddening E(B-V)=1.45±0.07 for Shorlin 1

Distance to cluster approximately 2.94±0.12 kpc

Total-to-selective extinction ratio R≈4.0±0.1 in the region

Abstract

A reanalysis of the (seemingly very distant) open cluster Shorlin 1, the group of stars associated with WR 38 and WR 38a, is made on the basis of existing UBV and JHKs observations for cluster members. The 2MASS observations, in particular, imply a mean cluster reddening of E(B-V)=1.45+-0.07 and a distance of 2.94+-0.12 kpc. The reddening agrees with the UBV results provided that the local reddening slope is described by E(U-B)/E(B-V)=0.64+-0.01, but the distance estimates in the 2MASS and UBV systems agree only if the ratio of total-to-selective extinction for the associated dust is R=Av/E(B-V)=4.0+-0.1. Both results are similar to what has been obtained for adjacent clusters in the Eta Carinae region by similar analyses, which suggests that `anomalous' dust extinction is widespread through the region, particularly for groups reddened by relatively nearby dust. Dust associated with the Eta Carinae complex itself appears to exhibit more `normal' qualities. The results have direct implications for the interpretation of distances to optical spiral arm indicators for the Galaxy at l=287-291 degrees, in particular the Carina arm here is probably little more than ~2 kpc distant, rather than 2.5-3 kpc distant as implied in previous studies. Newly-derived intrinsic parameters for the two cluster Wolf-Rayet stars WR 38 (WC4) and WR 38a (WN5) are in good agreement with what is found for other WR stars in Galactic open clusters, which was not the case previously.