Guillermo Gonzalez

“Sculptor-ing” the Galaxy? The Chemical Compositions of Red Giants in the Sculptor Dwarf Spheroidal Galaxy

We used high-resolution, high signal-to-noise ratio spectra obtained with the Very Large Telescope and the UV-Visual Echelle Spectrograph to determine abundances of 17 elements in four red giants in the Sculptor (Scl) dwarf spheroidal galaxy. Our [Fe/H]-values range from -2.10 to -0.97, confirming previous findings of a large metallicity spread. We combined our data with similar data for five Scl giants studied recently by Shetrone et al. to form one of the largest samples of high-resolution abundances yet obtained for a dwarf spheroidal galaxy, covering essentially the full known metallicity range in this galaxy. These properties allow us to establish trends of [X/Fe] with [Fe/H] for many elements X. The trends are significantly different from the trends seen in Galactic halo and globular cluster stars. This conclusion is evident for most of the elements from oxygen to manganese. We compare our Scl sample with the most similar Galactic counterparts and find substantial differences remain even with these stars. The many discrepancies in the relationships between [X/Fe] as seen in Scl compared with Galactic field stars indicate that our halo cannot be made up in bulk of stars similar to those presently seen in dwarf spheroidal galaxies like Scl, corroborating similar conclusions reached by Shetrone et al., Fulbright, and Tolstoy et al. These results have serious implications for the Searle-Zinn and hierarchical galaxy formation scenarios. We also find that the most metal-rich star in our sample is a heavy element–rich star. This star and the [Ba/Eu] trend we see indicate that asymptotic giant branch stars must have played an important role in the evolution of the s-process elements in Scl. A very high percentage of such heavy-element stars are now known in dwarf spheroidals compared with the halo, further mitigating against the formation of the halo from such objects.

Parent Stars of Extrasolar Planets. VII. New Abundance Analyses of 30 Systems

The results of new spectroscopic analyses of 30 stars with giant planet and/or brown dwarf companions are presented. Values for Teff and [Fe/H] are used in conjunction with Hipparcos data and Padua isochrones to derive masses, ages, and theoretical surface gravities. These new data are combined with spectroscopic and photometric metallicity estimates of other stars harboring planets and published samples of F, G, and K dwarfs to compare several subsets of planet bearing stars with similarly well-constrained control groups. The distribution of [Fe/H] values continues the trend uncovered in previous studies in that stars hosting planetary companions have a higher mean value than otherwise similar nearby stars. We also investigate the relationship between stellar mass and the presence of giant planets, and we find statistically marginal but suggestive evidence of a decrease in the incidence of radial velocity companions orbiting relatively less massive stars. If confirmed with larger samples, this would represent a critical constraint to both planetary formation models, as well as to estimates of the distribution of planetary systems in our Galaxy.

Rummaging through Earth's Attic for Remains of Ancient Life

Abstract We explore the likelihood that early remains of Earth, Mars, and Venus have been preserved on the Moon in high enough concentrations to motivate a search mission. During the Late Heavy Bombardment, the inner planets experienced frequent large impacts. Material ejected by these impacts near the escape velocity would have had the potential to land and be preserved on the surface of the Moon. Such ejecta could yield information on the geochemical and biological state of early Earth, Mars, and Venus. To determine whether the Moon has preserved enough ejecta to justify a search mission, we calculate the amount of terran material incident on the Moon over its history by considering the distribution of ejecta launched from the Earth by large impacts. In addition, we make analogous estimates for Mars and Venus. We find, for a well-mixed regolith, that the median surface abundance of terran material is roughly 7 ppm, corresponding to a mass of approximately 20,000 kg of terran material over a 10×10-square-km area. Over the same area, the amount of material transferred from Venus is 1–30 kg and material from Mars as much as 180 kg. Given that the amount of terran material is substantial, we estimate the fraction of this material surviving impact with intact geochemical and biological tracers.

A search for 6Li in stars with planets

Using very high-resolution (R ∼ 125 000) and high-quality (signal-to-noise ratio ≥ 350) spectra, we have searched for 6 Li in stars hosting extrasolar planets. From detailed profile-fitting of the Li I resonance line at 6707.7 A, we find no significant amount of 6 Li relative to 7 Li for any of eight planet-bearing stars ( 6 Lι/ 7 Li ≤ 0.0-4.03) with strong Li I lines. In particular, we do not confirm the presence of 6 Li with 6 Lι/ 7 Li = 0.13 reported by Israelian et al. for HD 82943, a star with two known planets. Several of the eight stars plus HD 219542 A, the planetless primary of a binary, have been identified in the literature as possible recipients of accreted terrestrial material. For all of the planet-hosting stars and an additional five planetless stars, we find no 6 Li.

Parent stars of extrasolar planets – IX. Lithium abundances

We compare the Li abundances of a sample of stars with planets discovered with the Doppler method to a sample of stars without detected planets. We prepared the samples by combining the Li abundances reported in several recent studies in a consistent way. Our results confirm recent claims that the Li abundances of stars with planets are smaller than those of stars without planets near the solar temperature. We also find that the vsini and R HK anomalies correlate with the Li abundance anomalies. These results suggest that planet formation processes have altered the rotation and Li abundances of stars that host Doppler-detected planets. We encourage others to test these findings with additional observations of Li in stars with temperatures between 5600 and 6200 K.

Abundance Analyses of Field RV Tauri Stars. VI. An Extended Sample

An abundance analysis is presented and discussed for a sample of 14 RV Tauri stars. The present abundance data and those from our previous papers and by other workers are combined in an attempt to further understanding of the dust-gas separation process that afflicts many RV Tauri variables. We propose that a stars intrinsic (i.e., initial) metallicity is given by the photospheric zinc abundance. Variables warmer than about 5000 K and with an initial metallicity [Fe/H] ≥ -1 are affected by dust-gas separation. Variables of all metallicities and cooler than about Teff 5000 K are unaffected by dust-gas separation. The RV Tauri variables show a spread in their C abundances, with the lower boundary of the points in the C versus Zn plane falling close to the predicted trend for giants after the first dredge-up. The upper boundary is inhabited by a few stars that are carbon-rich. The O abundances in the mean follow the predicted trend from unevolved stars, in line with the expectation that photospheric O abundance is unaffected by the first dredge-up. An evolutionary scenario involving mass loss by a first-ascent or early-AGB red giant, the primary star of a binary, is sketched.

Reseeding of early earth by impacts of returning ejecta during the late heavy bombardment

Mounting attention has focused on interplanetary transfer of microorganisms (panspermia), particularly in reference to exchange between Mars and Earth. In most cases, however, such exchange requires millions of years, over which time the transported microorganisms must remain viable. During a large impact on Earth, however, previous work (J.C. Armstrong et al., 2002, Icarus 160, 183–196) has shown that substantial amounts of material return to the planet of origin over a much shorter period of time (< 5000 years), considerably mitigating the challenges to the survival of a living organism. Conservatively evaluating experiments performed [by others] on Bacillus subtilis and Deinococcus radiodurans to constrain biological survival under impact conditions, we estimate that if the Earth were hit by a sterilizing impactor ∼ 300 km in diameter, with a relative velocity of 30 km s−1 (such as may have occurred during the Late Heavy Bombardment), an initial cell population in the ejecta of order 103–105 cells kg−1 would in most cases be sufficient for a single modern organism to survive and return to an again-clement planet 3000–5000 years later. Although little can be said about the characteristics or distribution of ancient life, our calculations suggest that impact reseeding is a possible means by which life, if present, could have survived the Late Heavy Bombardment.

Parent Stars of Extrasolar Planets. VIII. Chemical Abundances for 18 Elements in 31 Stars

We present the results of detailed spectroscopic abundance analyses for 18 elements in 31 nearby stars with planets (SWPs). The resulting abundances are combined with other similar studies of nearby SWPs and compared to a sample of nearby stars without detected planets. We find some evidence for abundance differences between these two samples for Al, Si and Ti. Some of our results are in conflict with a recent study of SWPs in the SPOCS data base. We encourage continued study of the abundance patterns of SWPs to resolve these discrepancies.

The Chemical Compositions of Stars with Planets: A Review

A number of trends among the properties of exoplanets have become evident in the years since the first one was announced in 1995. One particularly interesting trend began to emerge in 1997: the incidence of giant planets correlates with the metallicity of the host star. This has since been established with a high degree of statistical significance by several research groups. Other, more subtle trends are beginning to appear as the sample size continues to grow and the statistics improve. I review the state of our knowledge concerning the observed compositional trends and their possible causes and suggest several research directions.

ELEMENTAL ABUNDANCES OF THREE RED GIANTS IN TERZAN 7, A GLOBULAR CLUSTER ASSOCIATED WITH THE SAGITTARIUS GALAXY

Chemical abundances in three giants in Terzan 7, a globular cluster associated with the Sagittarius dwarf galaxy, have been determined using high-resolution spectra obtained with the UVES spectrograph on the ESO 8.2 m Kueyen telescope. We find the overall metallicity of the stars to be [Fe/H]= -0.61 ± 0.07, which is slightly higher than that previously evaluated from photometry and used for the age determination of this cluster. This metallicity yields an age of about 6 Gyr, which is slightly lower than derived from previous estimates. The relative abundance ratios of various chemical elements to iron lie between those of its host galaxys metal-poor and metal-rich stars and reveal an intriguing similarity to the pattern seen in the Large Magellanic Cloud.