Late-Type Red Supergiants: Too Cool for the Magellanic Clouds - AstrophysicsReport as inadecuate




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Abstract: We have identified seven red supergiants (RSGs) in the Large Magellanic Cloud(LMC) and four RSGs in the Small Magellanic Cloud (SMC), all of which havespectral types that are considerably later than the average type observed intheir parent galaxy. Using moderate-resolution optical spectrophotometry andthe MARCS stellar atmosphere models, we determine their physical properties andplace them on the H-R diagram for comparison with the predictions of currentstellar evolutionary tracks. The radial velocities of these stars suggest thatthey are likely all members of the Clouds rather than foreground dwarfs or halogiants. Their locations in the H-R diagram also show us that those stars arecooler than the current evolutionary tracks allow, appearing to the right ofthe Hayashi limit, a region in which stars are no longer in hydrodynamicequilibrium. These stars exhibit considerable variability in their Vmagnitudes, and three of these stars also show changes in their effectivetemperatures (and spectral types) on the time-scales of months. One of thesestars, M2002 SMC 055188, was caught in an M4.5 I state, as late as that seenin HV 11423 at its recent extreme: considerable later, and cooler, than anyother supergiant in the SMC. In addition, we find evidence of variableextinction due to circumstellar dust and changes in the stars- luminosities,also consistent with our recent findings for HV 11423 - when these stars arehotter they are also dustier and more luminous. We suggest that these starshave unusual properties because they are in an unstable (and short-lived)evolutionary phase.



Author: Emily M. Levesque, Philip Massey, K. A. G. Olsen, Bertrand Plez

Source: https://arxiv.org/







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