Potential Drivers of Mid-Infrared Variability in Young Stars: testing physical models with multi-epoch near-infrared spectra of YSOs in ρ Oph

Abstract

Recent studies have identified several young stellar objects (YSOs) which exhibit significant mid-infrared (mid-IR) variability. A wide range of physical mechanisms may be responsible for these variations, including changes in a YSO's accretion rate or in the extinction or emission from the inner disk. We have obtained and analyzed multi-epoch near-infrared (NIR) spectra for five actively accreting YSOs in the $\rho$ Oph star-forming region along with contemporaneous mid-IR light curves obtained as part of the YSOVAR Spitzer/IRAC survey. Four of the five YSOs exhibit mid-IR light curves with modest ($\sim 0.2$--0.4 mag) but statistically significant variations over our 40-day observation window. Measuring the strengths of prominent photospheric absorption lines and accretion sensitive \ion{H}{1} and \ion{He}{1} lines in each NIR spectrum, we derive estimates of each YSO's spectral type, effective temperature ({\Teff}), and $H$ band extinction ($A_H$), and analyze the time evolution of their NIR veiling ($r_H$ and $r_K$) and mass accretion rates ({\Macc}). Defining a YSO's evolutionary stage such that heavily veiled, high accretion rate objects are less evolved than those with lower levels of veiling and ongoing accretion, we infer that GY 314 is the most evolved YSO in our sample, with GY 308 and GY 292 at progressively earlier evolutionary stages. Leveraging our multi-epoch, multi-wavelength dataset, we detect significant variations in mass accretion rates over timescales of days to weeks, but find that extinction levels in these YSOs remain relatively constant. We find no correlation between these YSO mid-IR light curves and time-resolved veiling or mass accretion rates, such that we are unable to link their mid-IR variability with physical processes localized near the inner edge of the circumstellar disk or within regions which are directly responsive to mass accretion...

21 pages, 11 figures

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