CEN34 -- High-Mass YSO in M17 or Background Post-AGB Star?
  We investigate the proposed high-mass young stellar object (YSO) candidate
CEN34, thought to be associated with the star forming region M17. Its optical
to near-infrared (550-2500 nm) spectrum reveals several photospheric absorption
features, such as H{\alpha}, Ca triplet and CO bandheads but lacks any emission
lines. The spectral features in the range 8375-8770{\AA} are used to constrain
an effective temperature of 5250\pm250 (early-/mid-G) and a surface gravity of
2.0\pm0.3 (supergiant). The spectral energy distribution of CEN34 resembles the
SED of a high-mass YSO or an evolved star. Moreover, the observed temperature
and surface gravity are identical for high-mass YSOs and evolved stars. The
radial velocity relative to LSR (V_LSR) of CEN34 as obtained from various
photospheric lines is of the order of -60 km/s and thus distinct from the +25
km/s found for several OB stars in the cluster and for the associated molecular
cloud. The SED modeling yields ~ 10^{-4} M_sun of circumstellar material which
contributes only a tiny fraction to the total visual extinction (11 mag). In
the case of a YSO, a dynamical ejection process is proposed to explain the
V_LSR difference between CEN34 and M17. Additionally, to match the temperature
and luminosity, we speculate that CEN34 had accumulated the bulk of its mass
with accretion rate > 4x10^{-3} M_sun/yr in a very short time span (~ 10^3
yrs), and currently undergoes a phase of gravitational contraction without any
further mass gain. However, all the aforementioned characteristics of CEN34 are
compatible with an evolved star of 5-7 M_sun and an age of 50-100 Myrs, most
likely a background post-AGB star with a distance between 2.0 kpc and 4.5 kpc.
We consider the latter classification as the more likely interpretation.
Further discrimination between the two possible scenarios should come from the
more strict confinement of CEN34's distance.
