Lick Indices and Spectral Energy Distribution Analysis based on an M31
  Star Cluster Sample: Comparisons of Methods and Models
  Application of fitting techniques to obtain physical parameters---such as
ages, metallicities, and $\alpha$-element to iron ratios---of stellar
populations is an important approach to understand the nature of both galaxies
and globular clusters (GCs). In fact, fitting methods based on different
underlying models may yield different results, and with varying precision. In
this paper, we have selected 22 confirmed M31 GCs for which we do not have
access to previously known spectroscopic metallicities. Most are located at
approximately one degree (in projection) from the galactic center. We performed
spectroscopic observations with the 6.5 m MMT telescope, equipped with its Red
Channel Spectrograph. Lick/IDS absorption-line indices, radial velocities,
ages, and metallicities were derived based on the $\rm EZ\_Ages$ stellar
population parameter calculator. We also applied full spectral fitting with the
ULySS code to constrain the parameters of our sample star clusters. In
addition, we performed $\chi^2_{\rm min}$ fitting of the clusters' Lick/IDS
indices with different models, including the Bruzual & Charlot models (adopting
Chabrier or Salpeter stellar initial mass functions and 1994 or 2000 Padova
stellar evolutionary tracks), the GALEV, and the Thomas et al. models. For
comparison, we collected their $UVBRIJK$ photometry from the Revised Bologna
Catalogue (v.5) to obtain and fit the GCs' spectral energy distributions
(SEDs). Finally, we performed fits using a combination of Lick/IDS indices and
SEDs. The latter results are more reliable and the associated error bars become
significantly smaller than those resulting from either our Lick/IDS
indices-only or our SED-only fits.
