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Register a new userMost young $alpha$-rich stars have high masses but are actually old
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Recent observations have revealed a population of $alpha$-element abundances enhanced giant stars with unexpected high masses ($gtrsim$1 $M_odot$) from asteroseismic analysis and spectroscopy. Assuming single-star evolution, their masses imply young ages ($tau<6$Gyr) incompatible with the canonical Galactic chemical evolution scenario. Here we study the chemistry and kinematics of a large sample of such $alpha$-rich, high-mass red giant branch (RGB) stars drawn from the LAMOST spectroscopic surveys. Using LAMOST and Gaia, we found these stars share the same kinematics as the canonical high-$alpha$ old stellar population in the Galactic thick disk. The stellar abundances show that these high-$alpha$ massive stars have $alpha$- and iron-peak element abundances similar to those of the high-$alpha$ old thick disk stars. However, a portion of them exhibit higher [(N+C)/Fe] and [Ba/Fe] ratios, which implies they have gained C- and Ba-rich materials from extra sources, presumably asymptotic giant branch (AGB) companions. The results support the previous suggestion that these RGB stars are products of binary evolution. Their high masses thus mimic young single stars, yet in fact they belong to an intrinsic old stellar population. To fully explain the stellar abundance patterns of our sample stars, a variety of binary evolution channels, such as, main-sequence (MS) + RGB, MS + AGB, RGB + RGB and RGB + AGB, are required, pointing to diverse formation mechanisms of these seemly rejuvenated cannibals. With this larger sample, our results confirm earlier findings that most, if not all, $alpha$-rich stars in the Galactic disk seem to be old.
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We derive age constraints for 1639 red giants in the APOKASC sample for which seismic parameters from Kepler, as well as effective temperatures, metallicities and [alpha/Fe] values from APOGEE DR12 are available. We investigate the relation between age and chemical abundances for these stars, using a simple and robust approach to obtain ages. We first derive stellar masses using standard seismic scaling relations, then determine the maximum possible age for each star as function of its mass and metallicity, independently of its evolutionary stage. While the overall trend between maximum age and chemical abundances is a declining fraction of young stars with increasing [alpha/Fe], at least 14 out of 241 stars with [alpha/Fe]>0.13 are younger than 6 Gyr. Five stars with [alpha/Fe]>0.2 have ages below 4 Gyr. We examine the effect of modifications in the standard seismic scaling relations, as well as the effect of very low helium fractions, but these changes are not enough to make these stars as old as usually expected for alpha-rich stars (i.e., ages greater than 8-9 Gyr). Such unusual alpha-rich young stars have also been detected by other surveys, but defy simple explanations in a galaxy evolution context.
The chemical abundances of a galaxys metal-poor stellar population can be used to investigate the earliest stages of its formation and chemical evolution. The Magellanic Clouds are the most massive of the Milky Ways satellite galaxies and are thought to have evolved in isolation until their recent accretion by the Milky Way. Unlike the Milky Ways less massive satellites, little is know about the Magellanic Clouds metal-poor stars. We have used the mid-infrared metal-poor star selection of Schlaufman & Casey (2014) and archival data to target nine LMC and four SMC giants for high-resolution Magellan/MIKE spectroscopy. These nine LMC giants with $-2.4lesssim[text{Fe/H}]lesssim-1.5$ and four SMC giants with $-2.6lesssim[text{Fe/H}]lesssim-2.0$ are the most metal-poor stars in the Magellanic Clouds yet subject to a comprehensive abundance analysis. While we find that at constant metallicity these stars are similar to Milky Way stars in their $alpha$, light, and iron-peak elemental abundances, both the LMC and SMC are enhanced relative to the Milky Way in the $r$-process element europium. These abundance offsets are highly significant, equivalent to $3.9sigma$ for the LMC, $2.7sigma$ for the SMC, and $5.0sigma$ for the complete Magellanic Cloud sample. We propose that the $r$-process enhancement of the Magellanic Clouds metal-poor stellar population is a result of the Magellanic Clouds isolated chemical evolution and long history of accretion from the cosmic web combined with $r$-process nucleosynthesis on a timescale longer than the core-collapse supernova timescale but shorter than or comparable to the thermonuclear (i.e., Type Ia) supernova timescale.
We report the detection of a large sample of high-$alpha$-metal-rich stars on the low giant branch with $2.6<logg<3.3$ dex in the LAMOST-MRS survey. This special group corresponds to an intermediate-age population of $5-9$ Gyr based on the $[Fe/H]$-$[C/N]$ diagram and age-$[C/N]$ calibration. A comparison group is selected to have solar $alpha$ ratio at super metallicity, which is young and has a narrow age range around 3 Gyr. Both groups have thin-disk like kinematics but the former shows slightly large velocity dispersions. The special group shows a larger extension in vertical distance toward 1.2 kpc, a second peak at smaller Galactic radius and a larger fraction of super metal rich stars with $[Fe/H]>0.2$ than the comparison group. These properties strongly indicate its connection with the outer bar/bulge region at $R=3-5$ kpc. A tentative interpretation of this special group is that its stars were formed in the X-shaped bar/bulge region, close to its corotation radius, where radial migration is the most intense, and brings them to present locations at 9 kpc and beyond. Low eccentricities and slightly outward radial excursions of its stars are consistent with this scenario. Its kinematics (cold) and chemistry ($[alpha/Fe]$ $sim 0.1$) further support the formation of the instability-driven X-shaped bar/bulge from the thin disk.
It has been known for a decade that hot stars with hot Jupiters tend to have high obliquities. Less is known about the degree of spin-orbit alignment for hot stars with other kinds of planets. Here, we re-assess the obliquities of hot Kepler stars with transiting planets smaller than Neptune, based on spectroscopic measurements of their projected rotation velocities (vsini). The basis of the method is that a lower obliquity -- all other things being equal -- causes sini to be closer to unity and increases the value of vsini. We sought evidence for this effect using a sample of 150 Kepler stars with effective temperatures between 5950 and 6550K and a control sample of 101 stars with matching spectroscopic properties and random orientations. The planet hosts have systematically higher values of vsini than the control stars, but not by enough to be compatible with perfect spin-orbit alignment. The mean value of sini is 0.856 +/- 0.036, which is 4-sigma away from unity (perfect alignment), and 2-sigma away from pi/4 (random orientations). There is also evidence that the hottest stars have a broader obliquity distribution: when modeled separately, the stars cooler than 6250K have <sini> = 0.928 +/- 0.042, while the hotter stars are consistent with random orientations. This is similar to the pattern previously noted for stars with hot Jupiters. Based on these results, obliquity excitation for early-G and late-F stars appears to be a general outcome of star and planet formation, rather than being exclusively linked to hot Jupiter formation.
We measure chemical abundances for over 20 elements of 15 N-rich field stars with high resolution ($R sim 30000$) optical spectra. We find that Na, Mg, Al, Si, and Ca abundances of our N-rich field stars are mostly consistent with those of stars from globular clusters (GCs). Seven stars are estimated to have [Al/Fe$]>0.5$, which is not found in most GC first generation stars. On the other hand, $alpha$ element abundances (especially Ti) could show distinguishable differences between in situ stars and accreted stars. We discover that one interesting star, with consistently low [Mg/Fe], [Si/Fe], [Ca/Fe], [Ti/Fe], [Sc/Fe], [V/Fe], and [Co/Fe], show similar kinematic and [Ba/Eu] as other stars from the dissolved dwarf galaxy $Gaia$-Sausage-Enceladus. The $alpha$-element abundances and the iron-peak element abundances of the N-rich field stars with metallicities $-1.25 le {rm [Fe/H]} le -0.95$ show consistent values with Milky Way field stars rather than stars from dwarf galaxies, indicating that they were formed in situ. In addition, the neutron capture elements of N-rich field stars show that most of them could be enriched by asymptotic giant branch (AGB) stars with masses around $3 - 5, M_{odot}$.
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