Spectroscopic studies of yellow supergiants in the open cluster NGC 129

Abstract

Spectroscopic studies of three yellow supergiants in the open cluster NGC 129, the classical Cepheid DL Cas, SAO 21450, and SAO 21482, have been performed on the basis of high-resolution spectra. For the two nonvariable cluster supergiants, the atmospheric parameters and chemical composition have been determined for the first time. SAO 21450 (T _eff = 6541 ± 16 K, log g = 2.00, V _t = 4.20 km s⁻¹) has nearly solar abundances of the key elements in the evolution of yellow supergiants (CNO, Na, Mg, and Al), while SAO 21482 (T _eff = 4506 ± 50 K, log g = 1.10, V _t = 9.90 km s⁻¹) exhibits an overabundance of carbon ([C/H] = +0.34 dex) and aluminum and nearly solar N, O, Na, and Mg abundances. The abundances of the key elements in the Cepheid DL Cas are typical for an object that has passed the first dredge-up: a C underabundance, N and Na overabundances, and nearly solar O, Mg, and Al abundances. In all objects, the abundances of iron [Fe/H] = −0.01 dex, α-elements, Fe-peak elements, and r- and s-process elements are virtually identical and nearly solar. The radial velocities of SAO 21482 measured from metal absorption lines have confirmed its membership in NGC 129. The knifelike shape of the Hα and Hβ line profiles in SAO 21482 and the asymmetry of the Mg Ib 5183.618 Å line in SAO 21482 and DL Cas as well as the absorption lines of neutral atoms and ions of metals in the Cepheid suggest the existence of extended gaseous envelopes around them. The positions of the objects on the T _eff−L diagram among the tracks of evolutionary masses for the objects show the following: (1) the primary component of SAO 21450 has a mass of 6.6 M _⊙ and approaches the blue edge of the Cepheid instability strip (CIS) for the first time, while its companion of possible spectral type B5 V has a mass of 4.8 M _⊙; (2) DL Cas is on the path of its CIS with a mass of 5.8 M _⊙ and has lost ~1.5 M _⊙ after the first dredge-up; (3) SAO 21482 with a mass of no more than 7.3 M _⊙ has passed the red edge of the CIS and probably enters the asymptotic giant branch. The theoretical CNO abundance estimates based on evolutionary tracks approximately coincide with the observed ones, while the age estimates for the supergiants are close to the mean cluster age, (7.6 ± 0.4) × 10⁷ yr.