GRACES observations of young [α/Fe]-rich stars

Abstract

We measure chemical abundance ratios and radial velocities in four massive (i.e., young) [α/Fe]-rich red giant stars using high-resolution high-S/N spectra from ES-PaDOnS fed by Gemini-GRACES. Our differential analysis ensures that our chemical abundances are on the same scale as the Alves-Brito et al. (2010) study of bulge, thin and thick disk red giants. We confirm that the program stars have enhanced [α/Fe] ratios and are slightly metal poor. Aside from lithium enrichment in one object, the program stars exhibit no chemical abundance anomalies when compared to giant stars of similar metallicity throughout the Galaxy. This includes the elements Li, O, Si, Ca, Ti, Cr, Ni, Cu, Ba, La, and Eu. Therefore, there are no obvious chemical signatures that can help to reveal the origin of these unusual stars. While our new observations show that only one star (not the Li-rich object) exhibits a radial velocity variation, simulations indicate that we cannot exclude the possibility that all four could be binaries. In addition, we find that two (possibly three) stars show evidence for an infrared excess, indicative of a debris disk. This is consistent with these young [α/Fe]-rich stars being evolved blue stragglers, suggesting their apparent young age is a consequence of a merger or mass transfer. We would expect a binary fraction of ∼50% or greater for the entire sample of these stars, but the signs of the circumbinary disk may have been lost since these features can have short timescales. Radial velocity monitoring is needed to confirm the blue straggler origin.

Constraining the Milky Way assembly history with Galactic Archaeology

“…This has recently become evident with the unexpected results of Chiappini et al (2015) and Martig et al (2015), who used CoRoT (Baglin et al 2006) and Kepler (Gilliland et al 2010) asteroseismic ages, respectively, combined with APOGEE chemical information, to show the existence of significantly young high-[α/Fe] stars (but see also Yong et al 2016). This is a largely unexpected result for chemical evolution modeling, because [α/Fe] has been thought to always be a good proxy for age.…”

KELT-21b: A Hot Jupiter Transiting the Rapidly Rotating Metal-poor Late-A Primary of a Likely Hierarchical Triple System

“…Such stars found by Chiappini et al (2015) were primarily in the inner Galactic disk, and they invoked formation at the end of the Galactic bar; although this is inconsistent with KELT-21ʼs current circular orbit and location near the solar circle, as mentioned earlier it could still have formed in the inner Galaxy and experienced radial mixing to move it to its current location, although this would have needed to be rapid in order to move the star several kiloparsecs within the 1.6 Gyr since it formed. The planet orbiting KELT-21 also seems to be at odds with the other proposed explanation for young α-rich stars, namely, that they are blue stragglers formed from stellar mergers (Jofré et al 2016;Yong et al 2016). Such a collision would have destroyed any short-period planet already around one of the stars; KELT-21b would have needed to either form from material thrown off in the collision, or have survived the collision at a larger semimajor axis and only migrated after the collision.…”

Confirming chemical clocks: asteroseismic age dissection of the Milky Way disk(s)

“…9). Understanding the origin of these stars has been the subject of a number of recent studies and they have been attributed to migrators from the Galactic bar (Chiappini et al 2015) as well as evolved blue stragglers (Martig et al 2015;Chiappini et al 2015;Yong et al 2016;Jofre et al 2016). In the former case it is believed that these stars formed in reservoirs of al- most inert gas close to the end of the Galactic bar, while the latter scenario proposes that the young α-rich stars are the product of mass transfer or stellar merger events.…”