Modeling Grain Alignment by Radiative Torques and Hydrogen Formation Torques in Reflection Nebula

Reflection nebulae--dense cores--illuminated by surrounding stars offer a unique opportunity to directly test our quantitative model of grain alignment based on radiative torques (RATs) and to explore new effects arising from additional torques. In this paper, we first perform detailed modeling of grain alignment by RATs for the IC 63 reflection nebula illuminated both by a nearby $\gamma$ Cas star and the diffuse interstellar radiation field. We calculate linear polarization $p_{\lambda}$ of background stars by radiatively aligned grains and explore the variation of fractional polarization ($p_{\lambda}/A_V$) with visual extinction $A_{V}$ across the cloud. Our results show that the variation of $p_{V}/A_{V}$ versus $A_{V}$ from the dayside of IC 63 to its center can be represented by a power-law ($p_{V}/A_{V}\propto A_{V}^{\eta}$) with different slopes depending on $A_{V}$. We find a shallow slope $\eta \sim- 0.1$ for $A_{V}< 3$ and a very steep slope $\eta\sim -2$ for $A_{V}> 4$. We then consider the effects of additional torques due to H$_{2}$ formation and model grain alignment by joint action of RATs and H$_2$ torques. We find that $p_{V}/A_{V}$ tends to increase with an increasing magnitude of H$_{2}$ torques. In particular, the theoretical predictions obtained for $p_{V}/A_{V}$ and peak wavelength $\lambda_{\max}$ in this case show an improved agreement with the observational data. Our results reinforce the predictive power of the RAT alignment mechanism in a broad range of environmental conditions and show the effect of pinwheel torques in environments with efficient H$_2$ formation. Physical parameters involved in H$_2$ formation may be constrained using detailed modeling of grain alignment combined with observational data. In addition, we discuss implications of our modeling for interpreting latest observational data by {\it Planck} and other ground-based instruments.