Origin of the early-type R stars: a binary-merger solution to a century-old problem?

Abstract

The early-R stars are carbon-rich K-type giants. They are enhanced in 12 C, 13 C and 14 N, have approximately solar oxygen, magnesium isotopes, s-process and iron abundances, have the luminosity of core-helium burning stars, are not rapid rotators, are members of the Galactic thick disk and, most peculiarly of all, are all single stars. Conventional single-star evolutionary models cannot explain such stars, but mergers in binary systems have been proposed to explain their origin. We have synthesized binary star populations to calculate the number of merged stars with helium cores which could be early-R stars. We find many possible evolutionary channels. The most common of which is the merger of a helium white dwarf with a hydrogen-burning red giant branch star during a common envelope phase followed by a helium flash in a rotating core which mixes carbon to the surface. All the channels together give ten times more early-R stars than we require to match recent Hipparcos observations -we discuss which channels are likely to be the true early-R stars and which are not. For the first time we have constructed a viable model of the early-R stars with which we can test some of our ideas regarding common envelope evolution in giants, stellar mergers, rotation, the helium flash and the origin of the early-R stars.

The chemical composition of carbon stars. The R-type stars

“…An extended discussion of these numerical simulations will be presented in an accompanying paper (Piersanti et al, in prep. ), but we can advance here that in the most favourable merging channels according to Izzard et al (2007), we do not obtain any carbon mixing (see also Zamora 2009).…”

Merging in the common envelope and the origin of early R-type stars

“…Following the statistical analysis by Izzard et al (2007), we focus attention on the merging of an He-WD with an RG star (their R3 channel). For this choice of parameters, the primary evolves toward the RGB after H exhausts at the center and overfills its own Roche lobe.…”

“…Later on, when the secondary exhausts H at the center, it ascends the Hayashi track and a common envelope (CE) episode occurs. Among all the possible systems representing this subtype in Izzard et al (2007), we select three systems that span the whole range of possible values for M WD and M RG in the R3a evolutionary channel by Izzard et al (2007, see their Fig. 2).…”

“…By assuming that such a merger could produce the physical conditions needed to give rise to a peculiar He-flash and a consequent dredge-up of carbon into the external layers, Izzard et al (2007) explore statistically different binary scenarios. They discuss different evolutionary channels and conclude that the most favorable one is the merger of an He white dwarf (He-WD) with an RG star.…”

“…The statistical analysis by Izzard et al (2007) produces ten times more stars than required to match the observed early-R to red clump star ratio. This discrepancy becomes more severe after the analysis by Zamora et al (2009), although this may not be a problem at all since only a few of these systems would finally reach the peculiar (and still unknown) conditions that lead to carbon mixing into the envelope.…”