Study of a red clump giant, KIC 11087027, with high rotation and strong infrared excess - Evidence of tidal interaction for high lithium abundance

A small group of red giants show very high Li abundances contrary to the general understanding that the Li gets destroyed in stars. This has been an anomaly ever since the discovery of the Li-rich giant by Wallerstein & Sneden (1982). Recent studies demonstrated that high lithium abundance among red clump (RC) giants is common (Kumar et al., 2020) and showed that the He-flash event, immediately preceding the RC, holds the key for Li enhancement (Kumar et al., 2011; Casey et al., 2019; Deepak & Reddy, 2019; Singh et al., 2019; Yan et al., 2021). The average Li abundance of RC giants, post He-flash, is about a factor of 40 more than their counterparts on the upper red giant branch (RGB), closer to the RGB tip (Kumar et al., 2020). In addition, the study by Singh et al. (2021) showed that A(Li) evolves with the g-mode period spacing ($\Delta\Pi_{1}$) evolution. During transition phase from the degenerate core to the convective core He burning, the ($\Delta\Pi_{1}$) value increases. As a result, very high A(Li) values are mostly seen at relatively lower ($\Delta\Pi_{1}$) values, i.e., immediately after the He-flash. The normal Li RC giants are seen at high $\Delta\Pi_{1}$ values. From this, they hypothesised that the super Li-rich (SLR) giant phase (A(Li)¹¹1A(Li)=12+$\rm log(\frac{N(Li)}{(N(H)})$$>$3.2 dex) is a short-lived phenomenon, and giants with very high Li abundance have undergone Li enhancement very recently, suggesting the He-flash as the source of Li enhancement. Further, the study by Mallick et al. (2023) showed occurrence of Li-rich giants only among low-mass (M$\leq$ 2M_⊙) giants and none among intermediate (M$>$2M_⊙) mass RC giants providing an indirect evidence that Li enhancement occurs during the He-flash as the He-flash expected to happen in only low-mass giants (Miller Bertolami et al., 2020). We also note that there are Li abundance studies among cluster giants. Some of the super Li-rich giants in the clusters found to be on the RGB (Sanna et al., 2020; Nagarajan et al., 2023; Tsantaki et al., 2023). If these giants are indeed ascending the RGB first time then there may be multiple ways for Li-enrichment in giants.

However, it is not understood what physical mechanism is responsible for the production and transportation of Li to the photosphere. Recent attempts to explain the physical process include internal gravity waves generated during He-flash (e.g. Schwab, 2020; Jermyn & Fuller, 2022) and thermohaline mixing (Gao et al., 2022). There are proposals in literature to drive the convection and mixing of A(Li) with the upper atmospheres, like the differential rotation due to faster core rotation (Fekel & Balachandran, 1993; Simon & Drake, 1989). Stars may get faster rotations due to tidal interaction in binary systems driving the desired mixing process(Denissenkov & Herwig, 2004; Casey et al., 2019). Any external events like mergers or tidal interactions with companions may spin-up the star, generating a strong magnetic field and creating circumstellar envelopes. One would expect all or most of these features if the Li enhancement occurred recently due to external events like mergers.

In this paper, we analysed high-resolution spectra of KIC11087027 for the first time. The star is a super Li-rich giant with features such as high rotation, infrared excess (IR-excess) and high chromospheric activity, indicating some recent binary interaction.

KIC11087027 was found in the sample of 12500 stars common in LAMOST (Zhao et al., 2006; Cui et al., 2012; Zhao et al., 2012) and Kepler (Borucki et al., 2010) field studied by Singh et al. (2019). This star was excluded in Singh et al. (2019) because of the absence of oscillation modes in Kepler Power Density Spectra (PDS). The star has peculiar features like the presence of rotational modulation, flares, and IR-excess. To supplement the photometric data, we obtained high-resolution (R=60,000) optical spectra using the 2.0-m Hanle Chandra Telescope (HCT) equipped with Hanle Echelle Spectrograph (HESP)²²2https://www.iiap.res.in/hesp/ for abundance analysis. We took three frames, each with a 40-minute exposure. The spectra cover the wavelength range from 3700 to 9300 Å. We obtained calibration image frames - bias, flat, and Th-Ar lamp for wavelength calibration. We also obtained high-rotation hot star spectra for removing telluric lines. We followed the standard data reduction procedure for Echelle spectral reduction using IRAF ³³3https://iraf-community.github.io/. The resultant spectral image has a signal-to-noise ratio (SNR) of 86. The extracted spectrum was wavelength calibrated and continuum fitted for further analysis and derivation of stellar atmospheric parameters and elemental abundances.

Red giants with very high Li and strong IR-excess are rare because both properties seem transient and evolve differently. Though we do not have much information about IR-excess, the large surveys showed no evidence of IR-excess among giants (Bharat Kumar et al., 2015), and only a couple of Li-rich giants and SLRs are known to have IR-excess (Mallick et al., 2022). The general lack of IR excess among Li-rich giants indicates either no ejection of mass during the event that caused Li enrichment or the mass loss occurred, but the resultant IR-excess probably diluted faster than the Li depletion. In that sense, the star KIC11087027 is unusual, with very high Li and large IR-excess. This suggests Li-enrichment occurred recently, assuming the high Li and IR-excess result from a single event that is responsible for high Li among RC giants. Further, it is demonstrated that $\Delta\Pi_{1}$ is a proxy to the time evolution of a degenerate core into a convective core i.e., older RC giants with normal Li have relatively much higher $\Delta\Pi_{1}$ values (Singh et al., 2021). The estimated value of $\Delta\Pi_{1}$=187 s from spectra Wang et al. (2023) indicates that KIC 11087027 is a young RC and Li-enrichment occurred very recently. The star also shows very high stellar rotation (V_rot=19.5 km s^-1), which causes the star to have high chromospheric activity with flares. What process might have created all the above signatures in a single star is unclear. If these properties are a consequence of a single event like He-flash and evolve at different time scales, one would expect stars with all these properties to be uncommon. Here, we also note, the study by Sneden et al. (2022) which shows a subtle correlation between the strength of chromospheric line at He I 10830Å  and the high A(Li) in giants. They also show higher binary fraction among Li-rich giants compared to Li-poor implying some kind of binary interaction for Li production and chromospheric activity.

In the case of KIC 11087027, we discuss two plausible scenarios. One is an in-situ process in single-star evolution, and the other is the stars’ binary evolution, where orbital synchronisation causes high rotation aiding high Li abundance. It is highly likely that He-flash holds the key to high Li anomaly among red clump giants (Casey et al., 2019; Deepak & Reddy, 2019; Singh et al., 2019, 2021; Mallick et al., 2023). Of course, we need to understand the nature of some of the very high Li-rich giants reported in clusters. These giants appear to be on the RGB and well before the RGB tip (Sanna et al., 2020; Nagarajan et al., 2023) i.e before the He-flash event. It is important to analyse the cluster Li-rich giants using asteroseismology to find whether these are ascending the RGB. We will address this in the later study.

As per the RC Li-rich giants, the question remains about the physical process that mixes internally produced Li with the upper layers. Is He-flash the sole process? And does external aid, such as tidal locking, also play a role? Supporting the high levels of Li among red clump giants Schwab (2020) constructed models in which the powerful gravity waves generated during the He-flash provide a conducive environment for large-scale convection. For this, they required diffusion coefficient D_mix=10¹¹cm²s^-1, which is a few orders more than expected in stars ascending the RGB.

Let’s assume Li gets produced inside the star through the Cameron-Fowler mechanism (Cameron & Fowler, 1971), and the He-flash aids large-scale mixing. Though the models predict no observable feature like IR-excess due to the He-flash at the centre, it is not unreasonable to suggest mass loss occurs due to the flash, and the severity of mass loss depends on the intensity of the He-flash. The resultant IR-excess is diluted faster since it is a one-time event. This may be why red clump giants with IR-excess are rare. Importantly, all the SLRs or Li-rich giants don’t need to have IR-excess as they may evolve at different time-scales. The red clump giants with IR-excess are found more likely to be super Li-rich or Li-rich (Mallick et al., 2022) suggesting that IR-excess dilutes faster than Li depletion.

Concerning the high rotational velocity of KIC11087027, it is expected that the He-flash and the consequent reduction in star size by a factor of $\sim$10 will spin-up the star. Given the V_rot $\sim$0.7 km s^-1 for the giants at the RGB tip one would expect V_rot $\sim$ 7 km s^-1 for a young RC giant. This expected value due to a sudden drop in the size of the giant from the RGB tip to the RC is much smaller than the observed value of 19.5 km s^-1. This implies high rotation in KIC11087027 and other Li-rich RC stars found in the literature probably arose due to external factors like mergers, or tidal locking. A primary giant at or near the RGB tip, which is tidally locked with its binary star, may have a larger rotational velocity than a single star. In the case of KIC11087027, observed V_rot=19.5 km s^-1 may be the combination of tidal locking and the after-effect of the He-flash.

If we assume the star in tidal locking with close binary has slightly more V_rot say 2 km s^-1 rather the expected value of 0.7 km s^-1, the resulting post He-flash V_rot value matches with the observed high velocity. The large V_rot will increase the diffusion coefficient to 10¹¹ cm² s^-1 (Casey et al., 2019) as D${}_{\rm mix}\propto\Omega^{2}$. Models show that a significant increase in the value of D_mix may result in the observed lithium abundance in the photosphere(Denissenkov & VandenBerg, 2003).

In this study, we reported results from Kepler photometric light curve and high-resolution spectral data. Based on variation in radial velocity and Gaia astrometric data, we suggest that KIC 11087027 has an unresolved companion. The observed high V_rot=19.5 km s^-1 may be a result of a combination of tidal synchronization in a binary system and the effect of He-flash. The high rotation probably results in a high diffusion coefficient hence, the high Li abundance. The presence of several transient features like very high Li abundance, multiple dust shells, high rotation and strong flares provide evidence that either the star has undergone a merger-induced He-flash (Mallick et al., 2022) or the tidally locked primary star has undergone the He-flash very recently. Also, to have a comprehensive understanding of origin of Li enhanced giants it is imperative to understand the nature of Li-rich giants in clusters which are reported to be ascending the RGB.

We thank anonymous referees for useful comments which has improved our manuscript. This study is supported by the National Natural Science Foundation of China under grant No. 11988101, and the National Key R$\&$D Program of China No.2019YFA0405500. Guoshoujing Telescope (the Large Sky Area Multi-Object Fiber Spectroscopic Telescope LAMOST) is a National Major Scientific Project built by the Chinese Academy of Sciences. LAMOST is operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences. This work presents results from the European Space Agency (ESA) space mission Gaia. Gaia data are being processed by the Gaia Data Processing and Analysis Consortium (DPAC). Funding for the DPAC is provided by national institutions, in particular the institutions participating in the Gaia MultiLateral Agreement (MLA). The Gaia mission website is https://www.cosmos.esa.int/gaia. We gratefully acknowledge the entire team of Kepler space telescope which is funded by NASA’s Science Mission.