The⁸⁵Kr<i>s</i>‐Process Branching and the Mass of Carbon Stars

Abstract

We present new spectroscopic observations for a sample of C(N)-type red\ngiants. These objects belong to the class of Asymptotic Giant Branch stars,\nexperiencing thermal instabilities in the He-burning shell (thermal pulses).\nMixing episodes called third dredge-up enrich the photosphere with newly\nsynthesized C12 in the He-rich zone, and this is the source of the high\nobserved ratio between carbon and oxygen (C/O &gt; 1 by number). Our spectroscopic\nabundance estimates confirm that, in agreement with the general understanding\nof the late evolutionary stages of low and intermediate mass stars, carbon\nenrichment is accompanied by the appearance of s-process elements in the\nphotosphere. We discuss the details of the observations and of the derived\nabundances, focusing in particular on rubidium, a neutron-density sensitive\nelement, and on the s-elements Sr, Y and Zr belonging to the first s-peak. The\ncritical reaction branching at Kr85, which determines the relative enrichment\nof the studied species, is discussed. Subsequently, we compare our data with\nrecent models for s-processing in Thermally Pulsing Asymptotic Giant Branch\nstars, at metallicities relevant for our sample. A remarkable agreement between\nmodel predictions and observations is found. Thanks to the different neutron\ndensity prevailing in low and intermediate mass stars, comparison with the\nmodels allows us to conclude that most C(N) stars are of low mass (M &lt; 3Mo). We\nalso analyze the C12/C13 ratios measured, showing that most of them cannot be\nexplained by canonical stellar models. We discuss how this fact would require\nthe operation of an ad hoc additional mixing, currently called Cool Bottom\nProcess, operating only in low mass stars during the first ascent of the red\ngiant branch and, perhaps, also during the asymptotic giant branch.\n