Evidence for Evolution Among Primordial Disks in the 5 Myr Old Upper Scorpius OB Association

TL;DR

This study investigates the evolution of primordial disks in the 5 million-year-old Upper Scorpius OB association by analyzing near-infrared spectra, accretion indicators, and spectral energy distributions to understand disk structure and accretion activity.

Contribution

It provides new observational evidence of disk evolution at 5 Myr, including accretion rates and inner disk radii, using combined near-infrared and mid-infrared data with radiative transfer models.

Findings

Some disks show inner radii larger than dust sublimation radius.

Accretion activity is detected in nearly half of the sources.

Disk structures vary significantly among the observed stars.

Abstract

Moderate-resolution, near-infrared spectra between 0.8 and 5.2 microns were obtained for 12 late-type (K0-M3) disk-bearing members of the ~5 Myr old Upper Scorpius OB association using SpeX on the NASA Infrared Telescope Facility. For most sources, continuum excess emission first becomes apparent between ~2.2 and 4.5 microns and is consistent with that produced by single-temperature blackbodies having characteristic temperatures ranging from ~500 to 1300 K. The near-infrared spectra for 5 of 12 Upper Scorpius sources exhibit Pa-gamma, Pa-beta and Br-gamma emission, indicators of disk accretion. Using a correlation between Pa-beta and Br-gamma emission line luminosity and accretion luminosity, mass accretion rates (Mdot) are derived for these sources that range from Mdot = 3.5 X 10^{-10} to 1.5 X 10^{-8} MSun per yr. Merging the SpeX observations with Spitzer Space Telescope mid-infrared (5.4-37.0 micron) spectroscopy and 24 and 70 micron broadband photometry, the observed spectral energy distributions are compared with those predicted by two-dimensional, radiative transfer accretion disk models. Of the 9 Upper Scorpius sources examined in this analysis, 3 exhibit spectral energy distributions that are most consistent with models having inner disk radii that substantially exceed their respective dust sublimation radii. The remaining Upper Scorpius members possess spectral energy distributions that either show significant dispersion among predicted inner disk radii or are best described by models having inner disk rims coincident with the dust sublimation radius.