Browsing by Subject "late-type"

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An Ancient Extrasolar System With Five Sub-Earth-Size Planets
(2015-02) Campante, T. L.; Barclay, Thomas; Swift, Jonathan J.; Huber, Daniel; Adibekyan, V. Z.; Cochran, William; Burke, C. J.; Isaacson, Howard; Quintana, Elisa V.; Davies, G. R.; Aguirre, V. S.; Ragozzine, D.; Riddle, R.; Baranec, C.; Basu, S.; Chaplin, W. J.; Christensen-Dalsgaard, J.; Metcalfe, T. S.; Bedding, T. R.; Handberg, R.; Stello, D.; Brewer, J. M.; Hekker, S.; Karoff, C.; Kolbl, R.; Law, N. M.; Lundkvist, M.; Miglio, A.; Rowe, Jason F.; Santos, N. C.; Van Laerhoven, C.; Arentoft, T.; Elsworth, Y. P.; Fischer, Debra A.; Kawaler, Steven D.; Kjeldsen, H.; Lund, M. N.; Marcy, Geoffrey W.; Sousa, S. G.; Sozzetti, A.; White, T. R.; Cochran, William
The chemical composition of stars hosting small exoplanets (with radii less than four Earth radii) appears to be more diverse than that of gas-giant hosts, which tend to be metal-rich. This implies that small, including Earth-size, planets may have readily formed at earlier epochs in the universe's history when metals were more scarce. We report Kepler spacecraft observations of Kepler-444, a metal-poor Sun-like star from the old population of the Galactic thick disk and the host to a compact system of five transiting planets with sizes between those of Mercury and Venus. We validate this system as a true five-planet system orbiting the target star and provide a detailed characterization of its planetary and orbital parameters based on an analysis of the transit photometry. Kepler-444 is the densest star with detected solar-like oscillations. We use asteroseismology to directly measure a precise age of 11.2 +/- 1.0Gyr for the host star, indicating that Kepler-444 formed when the universe was less than 20% of its current age and making it the oldest known system of terrestrial-size planets. We thus show that Earth-size planets have formed throughout most of the universe's 13.8 billion year history, leaving open the possibility for the existence of ancient life in the Galaxy. The age of Kepler-444 not only suggests that thick-disk stars were among the hosts to the first Galactic planets, but may also help to pinpoint the beginning of the era of planet formation.
Hot Bottom Burning And S-Process Nucleosynthesis In Massive Agb Stars At The Beginning Of The Thermally-Pulsing Phase
(2013-07) Garcia-Hernandez, D. A.; Zamora, O.; Yague, A.; Uttenthaler, S.; Karakas, A. I.; Lugaro, M.; Ventura, P.; Lambert, D. L.; D. L. Lambert
We report the first spectroscopic identification of massive Galactic asymptotic giant branch (AGB) stars at the beginning of the thermal pulse (TP) phase. These stars are the most Li-rich massive AGBs found to date, super Li-rich AGBs with log epsilon (Li) similar to 3-4. The high Li overabundances are accompanied by weak or no s-process element (i.e. Rb and Zr) enhancements. A comparison of our observations with the most recent hot bottom burning (HBB) and s-process nucleosynthesis models confirms that HBB is strongly activated during the first TPs but the Ne-22 neutron source needs many more TP and third dredge-up episodes to produce enough Rb at the stellar surface. We also show that the short-lived element Tc, usually used as an indicator of AGB genuineness, is not detected in massive AGBs, which is in agreement with the theoretical predictions when the Ne-22 neutron source dominates the s-process nucleosynthesis.
Origin of Lithium Enrichment in K Giants
(2011-03) Kumar, Yerra B.; Reddy, Bacham E.; Lambert, David L.; Lambert, David L.
In this Letter, we report on a low-resolution spectroscopic survey for Li-rich K giants among 2000 low-mass (M <= 3 M-circle dot) giants spanning the luminosity range from below to above the luminosity of the clump. Fifteen new Li-rich giants including four super Li-rich K giants (log epsilon(Li) >= 3.2) were discovered. A significant finding is that there is a concentration of Li-rich K giants at the luminosity of the clump or red horizontal branch. This new finding is partly a consequence of the fact that our low-resolution survey is the first large survey to include giants well below and above the red giant branch (RGB) bump and clump locations in the H-R diagram. Origin of the lithium enrichment may be plausibly attributed to the conversion of He-3 via Be-7 to Li-7 by the Cameron-Fowler mechanism but the location for the onset of the conversion is uncertain. Two possible opportunities to effect this conversion are discussed: the bump in the first ascent of the RGB and the He-core flash at the tip of the RGB. The finite luminosity spread of the Li-rich giants serves to reject the idea that Li enhancement is, in general, a consequence of a giant swallowing a large planet.
Rb-Rich Asymptotic Giant Branch Stars in the Magellanic Clouds
(2009-11) Garcia-Hernandez, D. A.; Manchado, A.; Lambert, David L.; Plez, B.; Garcia-Lario, P.; D'Antona, F.; Lugaro, M.; Karakas, A. I.; van Raai, M. A.; Lambert, David L.
We present high-resolution (R similar to 60,000) optical spectra of a carefully selected sample of heavily obscured and presumably massive O-rich asymptotic giant branch (AGB) stars in the Magellanic Clouds. We report the discovery of strong Rb I lines at 7800 angstrom in four Rb-rich LMC stars at luminosities equal to or greater than the standard adopted luminosity limit for AGB stars (M(bol) similar to -7.1), confirming that "hot bottom burning" may produce a flux excess in the more massive AGB stars. In the SMC sample, just one of the five stars with M(bol) < -7.1 was detected in Rb; the other stars may be massive red supergiants. The Rb-rich LMC AGB stars might have stellar masses of at least similar to 6-7 M(circle dot). Our abundance analyses show that these Rb-rich stars are extremely enriched in Rb by up to 10(3)-10(5) times solar but seem to have only mild Zr enhancements. The high Rb/Zr ratios, if real, represent a severe problem for the s-process, even if the (22)Ne source is operational as expected for massive AGB stars; it is not possible to synthesize copious amounts of Rb without also overproducing Zr. The solution to the problem may lie with an incomplete present understanding of the atmospheres of luminous AGB stars.
Secretly Eccentric: The Giant Planet And Activity Cycle Of GJ 328
(2013-09) Robertson, Paul; Endl, Michael; Cochran, William D.; MacQueen, Phillip J.; Boss, Alan P.; Robertson, Paul; Endl, Michael; Cochran, William D.; MacQueen, Phillip J.
We announce the discovery of a similar to 2 Jupiter-mass planet in an eccentric 11 yr orbit around the K7/M0 dwarf GJ 328. Our result is based on 10 years of radial velocity (RV) data from the Hobby-Eberly and Harlan J. Smith telescopes at McDonald Observatory, and from the Keck Telescope at Mauna Kea. Our analysis of GJ 328's magnetic activity via the Na I D features reveals a long-period stellar activity cycle, which creates an additional signal in the star's RV curve with amplitude 6-10 m s(-1). After correcting for this stellar RV contribution, we see that the orbit of the planet is more eccentric than suggested by the raw RV data. GJ 328b is currently the most massive, longest-period planet discovered around a low-mass dwarf.
Spectro-Thermometry Of M Dwarfs And Their Candidate Planets: Too Hot, Too Cool, Or Just Right?
(2013-12) Mann, Andrew W.; Gaidos, Eric; Ansdell, Megan; Mann, Andrew W.
We use moderate-resolution spectra of nearby late K and M dwarf stars with parallaxes and interferometrically determined radii to refine their effective temperatures, luminosities, and metallicities. We use these revised values to calibrate spectroscopic techniques to infer the fundamental parameters of more distant late-type dwarf stars. We demonstrate that, after masking out poorly modeled regions, the newest version of the PHOENIX atmosphere models accurately reproduce temperatures derived bolometrically. We apply methods to late-type hosts of transiting planet candidates in the Kepler field, and calculate effective temperature, radius, mass, and luminosity with typical errors of 57 K, 7%, 11%, and 13%, respectively. We find systematic offsets between our values and those from previous analyses of the same stars, which we attribute to differences in atmospheric models utilized for each study. We investigate which of the planets in this sample are likely to orbit in the circumstellar habitable zone. We determine that four candidate planets (KOI 854.01, 1298.02, 1686.01, and 2992.01) are inside of or within 1 sigma of a conservative definition of the habitable zone, but that several planets identified by previous analyses are not (e.g., KOI 1422.02 and KOI 2626.01). Only one of the four habitable-zone planets is Earth sized, suggesting a downward revision in the occurrence of such planets around M dwarfs. These findings highlight the importance of measuring accurate stellar parameters when deriving parameters of their orbiting planets.