2013A&A...557A..51C

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 (550-2500nm) spectrum reveals several photospheric absorption features, such as Hα, the CaII triplet, and the CO bandhead, but lacks emission lines. The spectral features in the range 8375-8770Å are used to constrain an effective temperature T_eff=5250±250K (early-/mid-G) and a logg=2.0±0.3 (supergiant). The spectral energy distribution (SED) displays a faint infrared excess that resembles that of a high-mass YSO or an evolved star of intermediate mass. Moreover, the observed temperature and surface gravity are identical for high-mass YSOs and evolved stars. The radial velocity of CEN34 relative to the local standard of rest (V_LSR) as obtained from various photospheric lines is of the order of -60km/s and thus distinct from the +25km/s found for several OB stars in the cluster and for the associated molecular cloud. The SED modeling yields ∼10^–4M_☉ of circumstellar material, which contributes only a tiny fraction to the total visual extinction (11mag). The distance of CEN34 is between 2.0kpc and 4.5kpc. 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 an accretion rate >4x10^–3M_☉/yr over a very short time span (∼10³yrs), and it is currently undergoing a phase of gravitational contraction without any further mass gain. However, all the aforementioned characteristics of CEN34 are compatible with an evolved star of 5-7M_☉ and an age of 50-100Myr, so it is most likely a background post-AGB star with a distance between 2.0kpc and 4.5kpc. We consider the latter classification as the more likely interpretation. Further discrimination of the two possible scenarios should come from the stricter confinement of CEN34's distance.