CEN34 -- High-Mass YSO in M17 or Background Post-AGB Star?

TL;DR

This study analyzes the nature of CEN34, a candidate high-mass YSO in M17, and finds it more consistent with being a background post-AGB star rather than a young stellar object, based on spectral and kinematic evidence.

Contribution

The paper provides a detailed spectral and kinematic analysis to distinguish between a high-mass YSO and a background evolved star, favoring the post-AGB star interpretation.

Findings

CEN34's spectral features match a supergiant star with T_eff ~ 5250 K.

Radial velocity of CEN34 differs significantly from M17's stars.

Spectral energy distribution suggests a background evolved star rather than a YSO.

Abstract

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.