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Abstract:
We present two new techniques for constraining the evolutionary ages of intermediate-mass (2-8 M‚òâ), pre-main-sequence stars (IMPS) in obscured, massive Galactic star-forming regions using combined infrared (IR) and X-ray point-source photometry catalogs containing thousands of objects. High-spatial-resolution X-ray images identify IMPS with or without IR excess emission from circumstellar dusk disks. IMPS complete their evolution across the Henyey tracks to reach the ZAMS as AB stars in <10 Myr, hence placing them on the HR diagram by modeling IR SEDs gives a more robust constraint on (model-dependent) evolutionary age than is possible for lower-mass stars that slowly descend the Hayashi tracks. Very young IMPS with GK spectral types produce intrinsic, strong coronal X-ray emission that rapidly decays with time following the development of a radiative zone. We hence observe an age-dependent dearth of intermediate-mass stars in an X-ray-selected stellar mass function compared to a standard stellar initial mass function. In the process we identify candidate unresolved binary star systems in which the IR-detected primaries are A or late B-type stars (including Herbig Ae/Be stars), but the observed X-ray emission must originate from lower- mass, T Tauri companions. These techniques will be applied to calibrate star formation rates in a sample of lumious Galactic H II regions. This work has been supported by the NSF via awards CAREER-1454224 and DUE-1356133 (Cal-Bridge) and by NASA through Chandra Award G07-18003B.
