G. F. Porto de Mello

HR 6060: The Closest Ever Solar Twin?

The detailed analysis of the optical spectrum and evolutionary state of the G2 Va star HR 6060 shows this object to have atmospheric parameters, mass, chromospheric activity, and UBV colors indistinguishable from the solar ones within the observational uncertainties. Only its luminosity and age are slightly higher than solar. Its abundance pattern is solar, with the exception of a slight excess of Sc, V, and the elements heavier than Sr. HR 6060 surpasses all previously claimed solar twins in likeness to the Sun, and we recommend that it be considered for strong priority in the ongoing planet-searching programs as well as in SETI surveys.

Age and mass of solar twins constrained by lithium abundance

Aims. We analyze the non-standard mixing history of the solar twins HIP 55 459, HIP 79 672, HIP 56 948, HIP 73 815, and HIP 100 963, to determine as precisely as possible their mass and age. Methods. We computed a grid of evolutionary models with non-standard mixing at several metallicities with the Toulouse-Geneva code for a range of stellar masses assuming an error bar of ±50 K in Teff. We choose the evolutionary model that reproduces accurately the observed low lithium abundances observed in the solar twins. Results. Our best-fit model for each solar twin provides a mass and age solution constrained by their Li content and Teff determina

Abundance analysis of barium and mild barium stars

Aims. We compare and discuss abundances and trends in normal giants, mild barium, and barium stars, searching for differences and similarities between barium and mild barium stars that could help shed some light on the origin of these similar objects. Also, we search for nucleosynthetic effects possibly related to the s-process that were observed in the literature for elements like Cu in other types of s-process enriched stars. Methods. High signal to noise, high resolution spectra were obtained for a sample of normal, mild barium, and barium giants. Atmospheric parameters were determined from the Fe I and Fell lines. Abundances for Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, Eu, and Gd, were determined from equivalent widths and model atmospheres in a differential analysis, with the red giant ∈ Vir as the standard star. Results. The different levels of s-process overabundances of barium and mild barium stars were earlier suggested to be related to the stellar metallicity. Contrary to this suggestion, we found in this work no evidence of barium and mild barium having a different range in metallicity. However, comparing the ratio of abundances of heavy to light s-process elements, we found some evidence that they do not share the same neutron exposure parameter. The exact mechanism controlling this difference is still not clear. As a by-product of this analysis we identify two normal red giants misclassified as mild barium stars. The relevance of this finding is discussed. Concerning the suggested nucleosynthetic effects possibly related to the s-process, for elements like Cu, Mn, V and Sc, we found no evidence for an anomalous behavior in any of the s-process enriched stars analyzed here. However, further work is still needed since a clear [Cu/Fe] vs. [Ba/Fe] anticorrelation exists for other s-process enriched objects.

Copper and barium abundances in the Ursa Major Moving Group

We present Cu and Ba abundances for seven GK dwarf stars, members of the solar-metallicity, 0.3-Gyr-old Ursa Major Moving Group. All analysed member stars show [Ba/Fe] excesses of +0.3-plus, associated with [Cu/Fe] deficiencies of up to -0.23 dex. The present results suggest that there is an anticorrelation between the abundances of Cu and the heavy elements produced by the main component of the neutron-capture s-process. Other possible anomalies are Na and C deficiencies with respect to normal solar-metallicity stars. The new data do not confirm the recent claim that the group member HR 6094 is a Ba dwarf star.

MARVELS-1b: A Short-period, Brown Dwarf Desert Candidate from the SDSS-III Marvels Planet Search

We present a new short-period brown dwarf (BD) candidate around the star TYC 1240-00945-1. This candidate was discovered in the first year of the Multi-object APO Radial Velocity Exoplanets Large-area Survey (MARVELS), which is part of the Sloan Digital Sky Survey (SDSS) III, and we designate the BD as MARVELS-1b. MARVELS uses the technique of dispersed fixed-delay interferometery to simultaneously obtain radial velocity (RV) measurements for 60 objects per field using a single, custom-built instrument that is fiber fed from the SDSS 2.5 m telescope. From our 20 RV measurements spread over a ~370 day time baseline, we derive a Keplerian orbital fit with semi-amplitude K = 2.533 ± 0.025 km s^(–1), period P = 5.8953 ± 0.0004 days, and eccentricity consistent with circular. Independent follow-up RV data confirm the orbit. Adopting a mass of 1.37 ± 0.11 M_☉ for the slightly evolved F9 host star, we infer that the companion has a minimum mass of 28.0 ± 1.5 M_(Jup), a semimajor axis 0.071 ± 0.002 AU assuming an edge-on orbit, and is probably tidally synchronized. We find no evidence for coherent intrinsic variability of the host star at the period of the companion at levels greater than a few millimagnitudes. The companion has an a priori transit probability of ~14%. Although we find no evidence for transits, we cannot definitively rule them out for companion radii ≲ R_(Jup).

Fine Structure of the Chromospheric Activity in Solar Type Stars : The H-alpha Line

A calibration of Hα as both a chromospheric diagnostic and an age indicator is presented, complementing the works previously done on this subject (Herbig 1985; Pasquini & Pallavicini 1991). The chromospheric diagnostic was built with a statistically significant sample, covering nine years of observations, and including 175 solar neighborhood stars. Regarding the age indicator, the presence of stars for which very accurate ages are determined, such as those belonging to clusters and kinematic groups, lends confidence to our analysis. We also investigate the possibility that stars of the same age might have gone through different tracks of chromospheric decay, identifying - within the same age range - effects of metallicity and mass. These parameters, however, as well as age, seem to be significant only for dwarf stars, losing their meaning when we analyze stars in the subgiant branch. This result suggests that, in these evolved stars, the emission mechanism cannot be magnetohydrodynamical in nature, in agreement with recent models (Fawzy et al. 2002c, and references therein). The Sun is found to be a typical star in its Hα chromospheric flux, for its age, mass and metallicity. As a byproduct of this work, we developed an automatic method to determine temperatures from the wings of Hα, which means the suppression of the error inherent to the visual procedure used in the literature.

Accurate and homogeneous abundance patterns in solar-type stars of the solar neighbourhood: a chemo-chronological analysis

Aims. We report the derivation of abundances of C, Na, Mg, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, Zr, Ba, Ce, Nd, and Sm in a sample of 25 solar-type stars of the solar neighbourhood, correlating the abundances with the stellar ages, kinematics, and orbital parameters. Methods. The spectroscopic analysis, based on data of high resolution and high signal-to-noise ratio, was differential to the Sun and applied to atomic line equivalent widths supplemented by the spectral synthesis of C and C2 features. We also performed a statistical study by using the method of tree clustering analysis, searching for groups of stars sharing similar elemental abundance patterns. We derived the stellar parameters from various criteria, with average errors of 30 K, 0.13 dex, and 0.05 dex, respectively, for Teff, log g ,a nd [Fe/H]. The average error of the [X/Fe] abundance ratios is 0.06 dex. Ages were derived from theoretical HR diagrams and membership of the stars in known kinematical moving groups. Results. We identified four stellar groups: one having, on average, over-solar abundances (� [X/H]� =+ 0.26 dex), another with undersolar abundances (� [X/H]� = −0.24 dex), and two with intermediate values (� [X/H]� = −0.06 and +0.06 dex) but with distinct chemical patterns. Stars sharing solar metallicity, age, and Galactic orbit possibly have non-solar abundance ratios, a possible effect either of chemical heterogeneity in their natal clouds or migration. A trend of [Cu/Fe] with [Ba/Fe] seems to exist, in agreement with previous claims in the literature, and maybe also of [Sm/Fe] with [Ba/Fe]. No such correlation involving C, Na, Mn, and Zn is observed. The [X/Fe] ratios of various elements show significant correlations with age. [Mg/Fe], [Sc/Fe], and [Ti/Fe] increase with age. [Mn/Fe] and [Cu/Fe] display a more complex behaviour, first increasing towards younger stars up to the solar age, and then decreasing, a result we interpret as possibly related to time-varying yields of SN Ia and the weak s-process in massive stars. The steepest negative age relation is due to [Ba/Fe], but only for stars younger than the Sun, and a similar though less significant behaviour is seen for Zr, Ce, and Nd. [Sr/Fe] and [Y/Fe] show a linearly increasing trend towards younger stars. The [Cu/Ba] and [Sm/Ba] therefore decrease for younger stars. We found that [Ba/Mg], [Ba/Zn], and [Sr,Y,Ba/Sm] increase but only for stars younger than the Sun, whereas the [Sr/Mg], [Y/Mg], [Sr/Zn], and [Y/Zn] ratios increase linearly towards younger stars over the whole age range.

On the link between rotation, chromospheric activity and Li abundance in subgiant stars

The connection rotation-CaII emission flux-lithium abundance is analyzed for a sample of bona fide subgiant stars, with evolutionary status determined from HIPPARCOS trigonometric parallax measurements and from the Toulouse-Geneva code. The distribution of rotation and CaII emission flux as a function of eective temperature shows a discontinuity located around the same spectral type, F8IV. Blueward of this spectral type, subgiants have a large spread of values of rotation and CaII flux, whereas stars redward of F8IV show essentially low rotation and low CaII flux. The strength of these declines depends on stellar mass. The abundance of lithium also shows a sudden decrease. For subgiants with mass lower than about 1.2 M the decrease is located later than that in rotation and CaII flux, whereas for masses higher than 1.2 M the decrease in lithium abundance is located around the spectral type F8IV. The discrepancy between the location of the discontinuities of rotation and CaII emission flux and log n(Li) for stars with masses lower than 1.2 M seems to reflect the sensitivity of these phenomena to the mass of the convective envelope. The drop in rotation, which results mostly from a magnetic braking, requires an increase in the mass of the convective envelope less than that required for the decrease in log n(Li). The location of the discontinuity in log n(Li) for stars with masses higher than 1.2 M, in the same region of the discontinuities in rotation and CaII emission flux, may also be explained by the behavior of the deepening of the convective envelope. The more massive the star is, the earlier is the increase of the convective envelope. In contrast to the relationship between rotation and CaII flux, which is fairly linear, the relationship between lithium abundance and rotation shows no clear tendency toward linear behavior. Similarly, no clear linear trend is observed in the relationship between lithium abundance and CaII flux. In spite of these facts, subgiants with high lithium content also have high rotation and high CaII emission flux.

Abundances of Mn, Co and Eu in a sample of 20 F-G disk stars : the influence of hyperfine structure splitting

We present Mn, Co and Eu abundances for a sample of 20 disk F and G dwarfs and subgiants with metallicities in the range −0.8 ≤ [Fe/H] ≤ +0.3. We investigate the influence of hyperfine structure (HFS) on the derived abundances of Mn and Co by using HFS data from different sources in the literature, as well as calculated HFS from interaction factors A and B. Eu abundances were obtained from spectral synthesis of one Eu  line that takes into account HFS from a series of recent laboratory measurements. For the lines analysed in this study, we find that for manganese, the differences between abundances obtained with different HFSs are no greater than 0.10 dex. Our cobalt abundances are even less sensitive to the choice of HFS than Mn, presenting a 0.07 dex maximum difference between determinations with different HFSs. However, the cobalt HFS data from different sources are significantly different. Our abundance results for Mn offer an independent confirmation of literature results, favouring type Ia supernovae as the main nucleosynthesis site of Mn production, in contrast to trends of Mn versus metallicity previously reported in the literature. For Co, we obtain [Co/Fe] ∼ 0.0 in the range −0.3 < [Fe/H] < +0. 3a nd [Co/Fe] rising to a level of +0.2 when [Fe/H] decreases from −0. 3t o−0.8, in disagreement with recent results in the literature. The observed discrepancies may be attributed to the lack of HFS in the works we used for comparison. Our results for Eu are in accordance with low-mass type II supernovae being the main site of the r-process nucleosynthesis.

A photometric and spectroscopic survey of solar twin stars within 50 parsecs of the Sun - I. Atmospheric parameters and color similarity to the Sun

Context. Solar twins and analogs are fundamental in the characterization of the Sun’s place in the context of stellar measurements, as they are in understanding how typical the solar properties are in its neighborhood. They are also important for representing sunlight observable in the night sky for diverse photometric and spectroscopic tasks, besides being natural candidates for harboring planetary systems similar to ours and possibly even life-bearing environments. Aims. We report a photometric and spectroscopic survey of solar twin stars within 50 parsecs of the Sun. Hipparcos absolute magnitudes and (B − V) Tycho colors were used to define a 2σ box around the solar values, where 133 stars were considered. Additional stars resembling the solar UBV colors in a broad sense, plus stars present in the lists of Hardorp, were also selected. All objects were ranked by a color-similarity index with respect to the Sun, defined by uvby and BV photometry. Methods. Moderately high-resolution, high signal-to-noise ratio spectra were used for a subsample of equatorial-southern stars to derive Teff ,l ogg (both ionization and evolutionary), and spectroscopic [Fe/H] with average internal errors better than 50 K, 0.20 dex, and 0.08 dex, respectively. Ages and masses were estimated from theoretical HR diagrams. Results. The color-similarity index proved very successful, since none of the best solar-analog and twin candidates that were photometrically and spectroscopically found to be good solar matches differed from the Sun by more than 3σ in their colors. We identify and quantitatively rank many new excellent solar analogs, which are fit to represent the Sun in the night sky to varying degrees of accuracy and in a wide range of contexts. Some of them are faint enough (V Tycho ∼ 8.5) to be of interest for moderately large telescopes. We also identify two stars with near-UV spectra indistinguishable from the Sun’s, although only HD 140690 also has atmospheric parameters matching the Sun’s, besides a high degree of photometric fidelity. This object is proposed as a prime solar analog in both the UV and visible ranges, a rare object. We present five new “probable” solar twin stars, besides five new “possible” twins, the best candidates being HD 98649, HD 118598, HD 150248, and HD 164595. Masses and isochronal ages for the best solar twin candidates lie very close to the solar values within uncertainties, but chromospheric activity levels range somewhat. We propose that the solar twins be emphasized in the ongoing searches for extra-solar planets and SETI searches.