GeminiFocus October 2017 - Page 7

spheric lithium content, being about 0.3 dex more abundant than its B companion (Figure 2). As lithium is most likely destroyed when it reaches the high temperatures of the stellar interior by means of stellar convection, this great difference between two similar coeval stars is surprising and unexpected. In addition, we also found that star A is en- hanced in refractory elements when com- pared to its stellar companion; here we identify as “refractory” and “volatile” those species with condensation temperatures (T C ) > 900 K and < 900 K, respectively. To obtain this result, we plotted the abundance differ- ence (B – A) between both stars versus T C for each chemical species. Figure 3 also shows weighted linear fits (long dashed lines) to all species and only to the refractory compo- nents. A similar refractory to volatile content between both stars would correspond to null slopes in this plot. However, the nega- tive slopes in Figure 3 point notably toward a higher content of refractory species in star A than in its stellar companion. tis stars, δ Scuti stars, and blue strag- glers. The so-called λ Bootis stars are a small group (only 2%) of population I stars, that show moderate to extreme surface underabundances of iron (Fe)- peak elements, but with solar abun- dances of carbon, nitrogen, and oxy- gen. In contrast, both stars A and B in the HAT-P-4 system are metal rich, and their effective temperatures are lower than those of the λ Bootis group; this allowed us to discard them as belong- ing to the λ Bootis class. δ Scuti stars are regularly pulsating variables with ~A6–F6 spectral types, located in the instability strip of the Hertzsprung-Russell diagram. While most of these stars present overabundances of heavy elements (be- tween 0.5 – 1.0 dex), both the effective tem- perature and mass of stars A and B are not only lower than those of the δ Scuti group, but they also lie out of the instability strip boundaries. Furthermore, no stellar pulsa- tions have been reported in either compo- nent A or B; so we also abandoned a possible δ Scuti classification. Finally, blue stragglers are stars significantly bluer than those placed on the main-sequence turnoff of the cluster (or population) to which they belong. Figure 2. GRACES spectra near the 6,707.8 Å lithium line for star A in HAT- P-4 (blue dotted line) and its B companion (black continuous line). The lithium content is notably enhanced in star A. Figure 3. Differential abundances (B - A) vs. condensation temperature T C . Long dashed lines are weighted linear fits to all species and to refractory species. The solar-twins trend of Meléndez et al. (2009) is shown with a continuous line for comparison. Chemically Peculiar Stars? Our attempts at classifying the HAT-P-4 components to other stars with peculiar chemical patterns also brought surprises. We looked at three cases in particular: λ Boo- October 2017 GeminiFocus 5