1. Discovery of an Extraordinarily Massive Young Stellar Cluster Donald F. Figer Rochester Institute of Technology/STScI J. MacKenty, M. Robberto, K. Smith (STScI), F. Najarro (CSIC), R. Kudritzki (UH), A. Herrero (IAC)

2. Main Points Our team is announcing that we have found the richest cluster of red supergiants in the Galaxy. We made this finding using 2MASS, IRMOS, and the Spitzer Space Telescope. We identified 14 red supergiants using spectra. The cluster is one of the most massive in Galaxy. The cluster is near rare high energy objects. The cluster is a hotbed of supernovae activity. The research was sponsored by NASA LTSA.

3. Logic Flow of Discovery 1.Selected target from list of possible clusters. 2.Used 2MASS image to target stars for spectroscopic observations. 3.Obtained infrared spectra with IRMOS. 4.Obtained mid-infrared fluxes from Spitzer. 5.Compared data to well known red supergiants. 6.Discovered 14 red supergiants in cluster. 7.Associated high energy objects with cluster. 8.Inferred age, mass, and status of cluster.

5. Location in Galaxy

6. Need for Infrared Observations Optically visible stars in blue.

7. Suspected Red Supergiants Infrared Brightness Infrared Color

8. Spectra Obtained with IRMOS Intensity Conclusion: Stars are Red Supergiants

10. Red supergiants are one thousand times larger than the Sun and one hundred times larger than a red giant. (a quarter mile across on this scale!)

11. Cluster Age Conclusion: Age~10 Myr Theory Observation Brightness

12. Cluster Mass

13. Supernovae Rate

14. 5.8  m+3.6  m+20cm

15. HESS TeV Source Near Cluster All High Energy TeV Sources in Sky

16. High Energy Objects

17. Most Massive Stars Project Goal is to find the most massive stars in the Galaxy. The sample includes hundreds of candidate stellar clusters. The red supergiant cluster is the first target in the sample. We will use IRMOS, Hubble, Spitzer, Chandra, VLA, and GTC to observe more candidates.

18. Conclusions We found a new Galactic stellar cluster. It contains three times as many red supergiants as any other Galactic cluster. It is associated with rare high energy objects. It likely had an initial mass of >20,000 M . There are likely more similar clusters.