Far-infrared study of K giants in the solar neighborhood: Connection between Li enrichment and mass-loss

Abstract

Context. A small group of red giant branch (RGB) stars are known to have anomalous Li enhancement whose origin is still not well understood. Some studies have proposed that the Li enhancement in RGB stars is correlated to their far-IR excess, a result of mass loss. Studies to confirm such a correlation have a significant bearing on our understanding of the Galactic Li enhancement. Aims. We searched for a correlation between the two anomalous properties of K giants: Li enhancement and IR excess from an unbiased survey of a large sample of RGB stars. Methods. A sample of 2000 low-mass K giants with accurate astrometry from the Hipparcos catalog was chosen for which Li abundances have been determined from low-resolution spectra. Far-IR data were collected from the WISE and IRAS catalogs. To probe the correlation between the two anomalies, we supplemented 15 Li-rich K giants discovered from this sample with 25 known Li-rich K giants from other studies. Dust shell evolutionary models and spectral energy distributions were constructed using the code DUSTY to estimate different dust shell properties, such as dust evolutionary time scales, dust temperatures, and mass-loss rates. Results. Among 2000 K giants, we found about two dozen K giants with detectable far-IR excess, and surprisingly, none of them are Li-rich. Similarly, the 15 new Li-rich K giants that were identified from the same sample show no evidence of IR excess. Of the total 40 Li-rich K giants, only 7 show IR excess. Important is that K giants with Li enhancement and/or IR excess begin to appear only at the bump on the RGB. Conclusions. Results show that K giants with IR excess are very rare, similar to K giants with Li enhancement. This may be due to the rapid differential evolution of dust shell and Li depletion compared to RGB evolutionary time scales. We also infer from the results that during the bump evolution, giants probably undergo some internal changes, which are perhaps the cause of mass-loss and Li-enhancement events. However, the available observational results do not ascertain that these properties are correlated. That a few Li-rich giants have IR excess seems to be pure coincidence.

Study of Lithium Rich Giants with the GALAH Spectroscopic Survey

“…This is not the case as shown in Figure-4, as there is a factor of about four more Li-rich giants at the clump compared to those at the bump or post-bump evolution. One could make another argument against the bump as the site for Li origin based on very short Li depletion periods compared to evolutionary time of luminosity bump (see Palacios et al 2001;Bharat Kumar et al 2015). The enhanced Li may deplete within 60% of the evolutionary period of the bump, i.e Li-rich giants may become Li-normal within a few million years which is much shorter than the evolutionary periods between the bump and RC.…”

Spectroscopic study of two new super Li-rich red clump K giants

“…Though the numbers are too small to deduce firm conclusion, it would be interesting to comment on the large difference in number of known Li-rich giants among RC and RGB phases. Whether the difference in numbers is due to large difference in stellar evolutionary time scales between the two phases; red clump evolutionary time scales are relatively much longer compared to the bump time scales (Bharat Kumar et al 2015). A systematic survey of Li among the known RGB and RC giants is warranted to find clues for the origin of Li-excess.…”

“…Also, relatively much shorter Li depletion time scales compared to stellar evolutionary time scales from the bump to red clump (e.g. Bharat Kumar et al 2015) add to the argument in favour that the enhanced Li at the bump is very unlikely to survive all the way to red clump.…”

“…One of the key evidences for the addition of external Li rich material is the enhanced abundance of 6 Li which has not been observed either in RGB stars or in main sequence dwarfs with planets (Reddy et al 2002). Also, models based on external events predict infrared excess as well as enhanced rotation as a consequence of accretion process which do not seem to be evident in many of the Lirich stars (Bharat Kumar et al 2015;Rebull et al 2015). Of course, IR excess may not be a stringent criterion as Li and dust shells evolve on different time scales (Bharat Kumar et al 2015).…”

“…Also, models based on external events predict infrared excess as well as enhanced rotation as a consequence of accretion process which do not seem to be evident in many of the Lirich stars (Bharat Kumar et al 2015;Rebull et al 2015). Of course, IR excess may not be a stringent criterion as Li and dust shells evolve on different time scales (Bharat Kumar et al 2015). As per vsini values, as shown in Table 4 among 9 L-rich giants at least 4 stars do show projected vsini which are two to three times of the average observed values for K giants (e.g.…”