2-D Models of Protostars: III. Effects of Stellar Temperature

We model how the mid-infrared colors of Young Stellar Objects (YSOs) vary with stellar temperature. The spectral energy distribution (SED) of each object has contributions from thermal emission of circumstellar dust, from direct stellar photospheric emission, and from scattered stellar emission. We first isolate the effects of stellar contributions (direct+scattered) to the SED using homologous “Class I” models: the distribution of circumstellar matter is chosen to scale with stellar temperature T⋆ such that the shape of the thermal contribution to the SED remains constant. The relative contribution of stellar direct and scattered light varies with T⋆, changing the 1 − 10μm (mid-infrared; MIR) colors. Stellar light contributes more to the MIR emission of YSOs with lower temperature stars (T⋆ ∼ 4000 K) because the emission peak wavelength of the star is closer to that of the thermal radiation. In YSOs with hotter central stars, since the peak of the stellar and thermal spectra are more separated in wavelength, the 1− 10μm spectrum is closer to a pure thermal spectrum and the objects are redder. Next we consider realistic Class 0, I, and II source models and find that the other dominant effect of varying stellar temperature on YSO SEDs is that of the inner disk wall: In high-T⋆ models, the dust destruction radius is much further out with a consequently larger inner disk wall that contributes relatively more to the 2− 10μm flux. This effect partially offsets that of the stellar contribution leading to varying behaviors of the 2−10μm flux: In Class 0 sources, the trend is for higher T⋆ models to have redder colors. In Class I sources, the trend applies Space Science Institute, 4750 Walnut St. Suite 205, Boulder, CO 80301; [email protected] University of Wisconsin-Madison, Dept. of Astronomy, 475 N. Charter St., Madison, WI 53706; [email protected] Ritter Observatory, MS 113, Department of Physics & Astronomy, University of Toledo, Toledo, OH 43606-3390; [email protected] School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9AD, Scotland the Brave; [email protected]