CEN 34 – high-mass YSO in M 17 or background post-AGB star?^⋆

¹ Purple Mountain Observatory, Chinese Academy of Sciences, 2 West Beijing Road, 210008 Nanjing, PR China
e-mail: zwchen@pmo.ac.cn
² Astronomisches Institut, Ruhr-Universität Bochum, Universitätsstrasse 150, 44801 Bochum, Germany
³ University of Chinese Academy of Sciences, 100039 Beijing, PR China
⁴ Instituto de Astronomía, Universidad Católica del Norte, Avenida Angamos 0610, Casilla 1280 Antofagasta, Chile

Received: 14 April 2013
Accepted: 10 July 2013

Abstract

We investigate the proposed high-mass young stellar object (YSO) candidate CEN 34, thought to be associated with the star-forming region M 17. Its optical to near-infrared (550−2500 nm) spectrum reveals several photospheric absorption features, such as Hα, the Ca ii triplet, and the CO bandhead, but lacks emission lines. The spectral features in the range 8375−8770 Å are used to constrain an effective temperature T_eff = 5250 ± 250 K (early-/mid-G) and a log g = 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 CEN 34 relative to the local standard of rest (V_LSR) as obtained from various photospheric lines is of the order of −60 km s^-1 and thus distinct from the +25 km s^-1 found for several OB stars in the cluster and for the associated molecular cloud. The SED modeling yields 10^-4 M_⊙ of circumstellar material, which contributes only a tiny fraction to the total visual extinction (11 mag). The distance of CEN 34 is between 2.0 kpc and 4.5 kpc. In the case of a YSO, a dynamical ejection process is proposed to explain the V_LSR difference between CEN 34 and M 17. Additionally, to match the temperature and luminosity, we speculate that CEN 34 had accumulated the bulk of its mass with an accretion rate >4 × 10^-3M_⊙/yr over a very short time span (~10³ yrs), and it is currently undergoing a phase of gravitational contraction without any further mass gain. However, all the aforementioned characteristics of CEN 34 are compatible with an evolved star of 5−7 M_⊙ and an age of 50−100 Myr, so it is 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 of the two possible scenarios should come from the stricter confinement of CEN 34’s distance.