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Extinction towards the cluster R136 in the Large Magellanic Cloud: An extinction law from the near-infrared to the ultraviolet

Sarah A. BrandsAlex de KoterJoachim M. Bestenlehner ...+3 G\"otz Gr\"afener
Mar 2023
The cluster R136 in the giant star-forming region 30 Doradus in the LargeMagellanic Cloud (LMC) offers a unique opportunity to resolve a stellarpopulation in a starburst-like environment. We obtain the near-infrared toultraviolet extinction towards 50 stars in the core of R136, employing the`extinction without standards' method. To assure good fits over the fullwavelength range, we combine and modify existing extinction laws. We detect astrong spatial gradient in the extinction properties across the core of R136,coinciding with a gradient in density of cold gas that is part of a molecularcloud lying northeast of the cluster. In line with previous measurements ofR136 and the 30 Doradus region, we obtain a high total-to-relative extinction($R_V = 4.38 \pm 0.87$). However, the high values of $R_V$ are accompanied byrelatively strong extinction in the ultraviolet, contrary to what is observedfor Galactic sightlines. The relatively strong ultraviolet extinction suggeststhat the properties of the dust towards R136 differ from those in the MilkyWay. For $R_{V} \sim 4.4$, about three times fewer ultraviolet photons canescape from the ambient dust environment relative to the canonical Galacticvalue of $R_{V} \sim 3.1$ at the same $A_{V}$. Therefore, if dust in the R136star-bursting environment is characteristic for cosmologically distantstar-bursting regions, the escape fraction of ultraviolet photons from suchregions is overestimated by a factor of three relative to the standard MilkyWay assumption for the total-to-selective extinction. Furthermore, a comparisonwith average curves tailored to other regions of the LMC shows that largedifferences in ultraviolet extinction exist within this galaxy. Furtherinvestigation is required in order to decipher whether or not there is arelation between $R_V$ and ultraviolet extinction in the LMC.