Contributions of point extragalactic sources to the Cosmic Microwave Background bispectrum

All the analyses of Cosmic Microwave Background (CMB) temperature maps up--to--date show that CMB anisotropies follow a Gaussian distribution. On the other hand, astrophysical foregrounds which hamper the detection of the CMB angular power spectrum, are not Gaussian distributed on the sky. Therefore, they should give a sizeable contribution to the CMB bispectrum. In fact, the first year data of the Wilkinson Microwave Anisotropy Probe (WMAP) mission have allowed the {\it first} detection of the extragalactic source contribution to the CMB bispectrum at 41 GHz and, at the same time, much tighter limits than before to non--Gaussian primordial fluctuations. In view of the above and for achieving higher precision in current and future CMB measurements of non--Gaussianity, in this paper we discuss a comprehensive assessment of the bispectrum due to either uncorrelated and clustered extragalactic point sources in the whole frequency interval around the CMB intensity peak. Our calculations, based on current cosmological evolution models for sources, show that the reduced angular bispectrum due to point sources, $b_{ps}$, should be detectable in all WMAP and Planck frequency channels. We also find agreement with the results on $b_{ps}$ at 41 GHz coming from the analysis of the first year WMAP data. Moreover, by comparing $b_{ps}$ with the primordial reduced CMB bispectrum, we find that only the peak value of the primordial bispectrum (which appears at $l\simeq 200$) results greater than $b_{ps}$ in a frequency window around the intensity peak of the CMB. The amplitude of this window basically depends on the capability of the source detection algorithms (i.e., on the achievable flux detection limit, $S_{lim}$, for sources).Comment: 26 pages, 6 Figures, use AasTex5.0, ApJ, in press, Oct. 10, 2003 Issu

The (In)Stability of Planetary Systems

We present results of numerical simulations which examine the dynamical stability of known planetary systems, a star with two or more planets. First we vary the initial conditions of each system based on observational data. We then determine regions of phase space which produce stable planetary configurations. For each system we perform 1000 ~1 million year integrations. We examine upsilon And, HD83443, GJ876, HD82943, 47UMa, HD168443, and the solar system (SS). We find that the resonant systems, 2 planets in a first order mean motion resonance, (HD82943 and GJ876) have very narrow zones of stability. The interacting systems, not in first order resonance, but able to perturb each other (upsilon And, 47UMa, and SS) have broad regions of stability. The separated systems, 2 planets beyond 10:1 resonance, (we only examine HD83443 and HD168443) are fully stable. Furthermore we find that the best fits to the interacting and resonant systems place them very close to unstable regions. The boundary in phase space between stability and instability depends strongly on the eccentricities, and (if applicable) the proximity of the system to perfect resonance. In addition to million year integrations, we also examined stability on ~100 million year timescales. For each system we ran ~10 long term simulations, and find that the Keplerian fits to these systems all contain configurations which may be regular on this timescale.Comment: 37 pages, 49 figures, 13 tables, submitted to Ap

Overabundance of alpha-elements in exoplanet host stars

We present the results for a chemical abundance analysis between planet-hosting and stars without planets for 12 refractory elements for a total of 1111 nearby FGK dwarf stars observed within the context of the HARPS GTO programs. Of these stars, 109 are known to harbour high-mass planetary companions and 26 stars are hosting exclusively Neptunians and super-Earths. We found that the [X/Fe] ratios for Mg, Al, Si, Sc, and Ti both for giant and low-mass planet hosts are systematically higher than those of comparison stars at low metallicities ([Fe/H] < from -0.2 to 0.1 dex depending on the element). The most evident discrepancy between planet-hosting and stars without planets is observed for Mg. Our data suggest that the planet incidence is greater among the thick disk population than among the thin disk for mettallicities bellow -0.3 dex. After examining the [alpha/Fe] trends of the planet host and non-host samples we conclude that a certain chemical composition, and not the Galactic birth place of the stars, is the determinating factor for that. The inspection of the Galactic orbital parameters and kinematics of the planet-hosting stars shows that Neptunian hosts tend to belong to the "thicker" disk compared to their high-mass planet-hosting counterparts.We also found that Neptunian hosts follow the distribution of high-alpha stars in the UW vs V velocities space, but they are more enhanced in Mg than high-alpha stars without planetary companions. Our results indicate that some metals other than iron may also have an important contribution to planet formation if the amount of iron is low. These results may provide strong constraints for the models of planet formation, especially for planets with low mass.Comment: 10 pages, 8 figures, 3 tables, accepted for publication in Astronomy & Astrophysic

Are beryllium abundances anomalous in stars with giant planets?

In this paper we present beryllium (Be) abundances in a large sample of 41 extra-solar planet host stars, and for 29 stars without any known planetary-mass companion, spanning a large range of effective temperatures. The Be abundances were derived through spectral synthesis done in standard Local Thermodynamic Equilibrium, using spectra obtained with various instruments. The results seem to confirm that overall, planet-host stars have ``normal'' Be abundances, although a small, but not significant, difference might be present. This result is discussed, and we show that this difference is probably not due to any stellar ``pollution'' events. In other words, our results support the idea that the high-metal content of planet-host stars has, overall, a ``primordial'' origin. However, we also find a small subset of planet-host late-F and early-G dwarfs that might have higher than average Be abundances. The reason for the offset is not clear, and might be related either to the engulfment of planetary material, to galactic chemical evolution effects, or to stellar-mass differences for stars of similar temperature.Comment: 15 pages, 9 figures, accepted for publication in Astronomy & Astrophysic

Nitrogen abundances in Planet-harbouring stars

We present a detailed spectroscopic analysis of nitrogen abundances in 91 solar-type stars, 66 with and 25 without known planetary mass companions. All comparison sample stars and 28 planet hosts were analysed by spectral synthesis of the near-UV NH band at 3360 \AA observed at high resolution with the VLT/UVES,while the near-IR NI 7468 \AA was measured in 31 objects. These two abundance indicators are in good agreement. We found that nitrogen abundance scales with that of iron in the metallicity range -0.6 <[Fe/H]< +0.4 with the slope 1.08 \pm 0.05. Our results show that the bulk of nitrogen production at high metallicities was coupled with iron. We found that the nitrogen abundance distribution in stars with exoplanets is the high [Fe/H] extension of the curve traced by the comparison sample of stars with no known planets. A comparison of our nitrogen abundances with those available in the literature shows a good agreement.Comment: 15 pages, 7 figures, Accepted for publication in A&

Evolved stars hint to an external origin of enhanced metallicity in planet-hosting stars

Exo-planets are preferentially found around high metallicity main sequence stars. We aim at investigating whether evolved stars share this property, and what this tells about planet formation. Statistical tools and the basic concepts of stellar evolution theory are applied to published results as well as our own radial velocity and chemical analyses of evolved stars. We show that the metal distributions of planet-hosting (P-H) dwarfs and giants are different, and that the latter do not favor metal-rich systems. Rather, these stars follow the same age-metallicity relation as the giants without planets in our sample. The straightforward explanation is to attribute the difference between dwarfs and giants to the much larger masses of giants' convective envelopes. If the metal excess on the main sequence is due to pollution, the effects of dilution naturally explains why it is not observed among evolved stars. Although we cannot exclude other explanations, the lack of any preference for metal-rich systems among P-H giants could be a strong indication of the accretion of metal-rich material. We discuss further tests, as well as some predictions and consequences of this hypothesis.Comment: A&A, in pres

Searching for the signatures of terrestial planets in solar analogs

We present a fully differential chemical abundance analysis using very high-resolution (R >~ 85,000) and very high signal-to-noise (S/N~800 on average) HARPS and UVES spectra of 7 solar twins and 95 solar analogs, 24 are planet hosts and 71 are stars without detected planets. The whole sample of solar analogs provide very accurate Galactic chemical evolution trends in the metalliciy range -0.3<[Fe/H]<0.5. Solar twins with and without planets show similar mean abundance ratios. We have also analysed a sub-sample of 28 solar analogs, 14 planet hosts and 14 stars without known planets, with spectra at S/N~850 on average, in the metallicity range 0.14<[Fe/H]<0.36 and find the same abundance pattern for both samples of stars with and without planets. This result does not depend on either the planet mass, from 7 Earth masses to 17.4 Jupiter masses, or the orbital period of the planets, from 3 to 4300 days. In addition, we have derived the slope of the abundance ratios as a function of the condensation temperature for each star and again find similar distributions of the slopes for both stars with and without planets. In particular, the peaks of these two distributions are placed at a similar value but with opposite sign as that expected from a possible signature of terrestial planets. In particular, two of the planetary systems in this sample, containing each of them a Super-Earth like planet, show slope values very close to these peaks which may suggest that these abundance patterns are not related to the presence of terrestial planets.Comment: Accepted for publication in The Astrophysical Journa

Light elements in stars with exoplanets

It is well known that stars orbited by giant planets have higher abundances of heavy elements when compared with average field dwarfs. A number of studies have also addressed the possibility that light element abundances are different in these stars. In this paper we will review the present status of these studies. The most significant trends will be discussed.Comment: 10 pages, 6 figures. Submitted to the proceedings of IAU symposium 268: Light elements in the universe

Higher depletion of lithium in planet host stars: no age and mass effect

Recent observational work by Israelian et al. has shown that sun-like planet host stars in the temperature range 5700K < Teff < 5850K have lithium abundances that are significantly lower than those observed for "single" field stars. In this letter we use stellar evolutionary models to show that differences in stellar mass and age are not responsible for the observed correlation. This result, along with the finding of Israelian et al., strongly suggest that the observed lithium difference is likely linked to some process related to the formation and evolution of planetary systems.Comment: 4 pages, 4 figures, letter accepted for publication in Astronomy & Astrophysic