arXiv subjects

N. C. Santos

Publications and source records attributed to N. C. Santos.

At least 361 records · Page 20Linked to original sources

arXiv2013

The contribution of secondary eclipses as astrophysical false positives to exoplanet transit surveys

We investigate in this paper the astrophysical false-positive configuration in exoplanet-transit surveys that involves eclipsing binaries and giant planets which present only a secondary eclipse, as seen from the Earth. To test how an eclipsing binary configuration can mimic a planetary transit, we generate synthetic light curve of three examples of secondary-only eclipsing binary systems that we fit with a circular planetary model. Then, to evaluate its occurrence we model a population of binaries in double and triple system based on binary statistics and occurrence. We find that 0.061% +/- 0.017% of main-sequence binary stars are secondary-only eclipsing binaries mimicking a planetary transit candidate down to the size of the Earth. We then evaluate the occurrence that an occulting-only giant planet can mimic an Earth-like planet or even smaller planet. We find that 0.009% +/- 0.002% of stars harbor a giant planet that present only the secondary transit. Occulting-only giant planets mimic planets smaller than the Earth that are in the scope of space missions like Kepler and PLATO. We estimate that up to 43.1 +/- 5.6 Kepler Objects of Interest can be mimicked by this new configuration of false positives, re-evaluating the global false-positive rate of the Kepler mission from 9.4% +/- 0.9% to 11.3% +/- 1.1%. We note however that this new false-positive scenario occurs at relatively long orbital period compared with the median period of Kepler candidates.

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arXiv2013

New and updated stellar parameters for 71 evolved planet hosts. On the metallicity - giant planet connection

It is still being debated whether the well-known metallicity - giant planet correlation for dwarf stars is also valid for giant stars. For this reason, having precise metallicities is very important. Different methods can provide different results that lead to discrepancies in the analysis of planet hosts. To study the impact of different analyses on the metallicity scale for evolved stars, we compare different iron line lists to use in the atmospheric parameter derivation of evolved stars. Therefore, we use a sample of 71 evolved stars with planets. With these new homogeneous parameters, we revisit the metallicity - giant planet connection for evolved stars. A spectroscopic analysis based on Kurucz models in local thermodynamic equilibrium (LTE) was performed through the MOOG code to derive the atmospheric parameters. Two different iron line list sets were used, one built for cool FGK stars in general, and the other for giant FGK stars. Masses were calculated through isochrone fitting, using the Padova models. Kolmogorov-Smirnov tests (K-S tests) were then performed on the metallicity distributions of various different samples of evolved stars and red giants. All parameters compare well using a line list set, designed specifically for cool and solar-like stars to provide more accurate temperatures. All parameters derived with this line list set are preferred and are thus adopted for future analysis. We find that evolved planet hosts are more metal-poor than dwarf stars with giant planets. However, a bias in giant stellar samples that are searched for planets is present. Because of a colour cut-off, metal-rich low-gravity stars are left out of the samples, making it hard to compare dwarf stars with giant stars. Furthermore, no metallicity enhancement is found for red giants with planets ($\log g < 3.0$\,dex) with respect to red giants without planets.

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arXiv2013

Line-profile variations in radial-velocity measurements: Two alternative indicators for planetary searches

Aims. We introduce two methods to identify false-positive planetary signals in the context of radial-velocity exoplanet searches. The first is the bi-Gaussian cross-correlation function fitting, and the second is the measurement of asymmetry in radial-velocity spectral line information content, Vasy. Methods. We make a systematic analysis of the most used common line profile diagnosis, Bisector Inverse Slope and Velocity Span, along with the two proposed ones. We evaluate all these diagnosis methods following a set of well-defined common criteria and using both simulated and real data. We apply them to simulated cross-correlation functions created with the program SOAP and which are affected by the presence of stellar spots, and to real cross-correlation functions, calculated from HARPS spectra, for stars with a signal originating both in activity and created by a planet. Results. We demonstrate that the bi-Gaussian method allows a more precise characterization of the deformation of line profiles than the standard bisector inverse slope. The calculation of the deformation indicator is simpler and its interpretation more straightforward. More importantly, its amplitude can be up to 30% larger than that of the bisector span, allowing the detection of smaller-amplitude correlations with radial-velocity variations. However, a particular parametrization of the bisector inverse slope is shown to be more efficient on high-signal-to-noise data than both the standard bisector and the bi-Gaussian. The results of the Vasy method show that this indicator is more effective than any of the previous ones, being correlated with the radial-velocity with more significance for signals resulting from a line deformation. Moreover, it provides a qualitative advantage over the bisector, showing significant correlations with RV for active stars for which bisector analysis is inconclusive. (abridged)

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arXiv2013

Effect of stellar spots on high-precision transit light-curve

Stellar activity features such as spots can create complications in determining planetary parameters through spectroscopic and photometric observations. The overlap of a transiting planet and a stellar spot, for instance, can produce anomalies in the transit light curves that may lead to inaccurate estimation of the transit duration, depth and timing. For instance such inaccuracies can affect the precise derivation of the planet's radius. In this paper, we present the results of a quantitative study on the effects of stellar spots on high precision transit light curves. We show that spot anomalies can lead to the estimate of a planet radius that is 4% smaller than the real value. The effects on the transit duration can also be of the order of 4%, longer or shorter. Depending on the size and distribution of spots, anomalies can also produce transit timing variations with significant amplitudes. For instance, TTVs with signal amplitudes of 200 seconds can be produced when the spot is completely dark and as large as the largest Sun spot. Our study also indicates that the minimum size of a stellar spot with detectable affects on the high precision transit light curve is around 0.03 time the stellar radius for typical Kepler telescope precision. We also show that the strategy of including more free parameters (such as transit depth and duration) in the fitting procedure to measure the transit time of each individual transit will not produce accurate results in the case of active stars.

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arXiv2013

Is there plenty of metal-poor stars with planets in the Galactic thick disk?

We performed an uniform spectroscopic analysis of 1111 FGK dwarfs observed as part of the HARPS GTO planet search program. We applied a purely chemical approach, based on [alpha/Fe] ratio, to distinguish the various stellar components in the Galaxy. Apart from the well known Galactic thick and thin disks, we separated an alpha-enhanced stellar family at super-solar metallicities. The metal-rich high-alpha stars have orbits similar to the thin disk stars, but they are similar to thick disk stars in terms of age. Our data indicate that the incidence of stars with planets are greater among the chemically separated thick disk stars with [Fe/H] < -0.3 dex than they are among thin disk stars in the same [Fe/H] interval. Our results allow us to suppose that a certain chemical composition, and not the Galactic birth place of the stars, is the causative factor for that.

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arXiv2013

SWEET-Cat: A catalogue of parameters for Stars With ExoplanETs I. New atmospheric parameters and masses for 48 stars with planets

Due to the importance that the star-planet relation has to our understanding of the planet formation process, the precise determination of stellar parameters for the ever increasing number of discovered extra-solar planets is of great relevance. Furthermore, precise stellar parameters are needed to fully characterize the planet properties. It is thus important to continue the efforts to determine, in the most uniform way possible, the parameters for stars with planets as new discoveries are announced. In this paper we present new precise atmospheric parameters for a sample of 48 stars with planets. We then take the opportunity to present a new catalogue of stellar parameters for FGK and M stars with planets detected by radial velocity, transit, and astrometry programs. Stellar atmospheric parameters and masses for the 48 stars were derived assuming LTE and using high resolution and high signal-to-noise spectra. The methodology used is based on the measurement of equivalent widths for a list of iron lines and making use of iron ionization and excitation equilibrium principles. For the catalog, and whenever possible, we used parameters derived in previous works published by our team, using well defined methodologies for the derivation of stellar atmospheric parameters. This set of parameters amounts to over 65% of all planet host stars known, including more than 90% of all stars with planets discovered through radial velocity surveys. For the remaining targets, stellar parameters were collected from the literature.

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arXiv2013

The HARPS search for southern extra-solar planets. XXXIV. A planetary system around the nearby M dwarf GJ163, with a super-Earth possibly in the habitable zone

The meter-per-second precision achieved by today velocimeters enables the search for 1-10 M_Earth planets in the habitable zone of cool stars. This paper reports on the detection of 3 planets orbiting GJ163 (HIP19394), a M3 dwarf monitored by our ESO/HARPS search for planets. We made use of the HARPS spectrograph to collect 150 radial velocities of GJ163 over a period of 8 years. We searched the RV time series for coherent signals and found 5 distinct periodic variabilities. We investigated the stellar activity and casted doubts on the planetary interpretation for 2 signals. Before more data can be acquired we concluded that at least 3 planets are orbiting GJ163. They have orbital periods of P_b=8.632+-0.002, P_c=25.63+-0.03 and P_d=604+-8 days and minimum masses msini = 10.6+-0.6, 6.8+-0.9, and 29+-3 M_Earth, respectively. We hold our interpretations for the 2 additional signals with periods P_(e)=19.4 and P_(f)=108 days. The inner pair presents an orbital period ratio of 2.97, but a dynamical analysis of the system shows that it lays outside the 3:1 mean motion resonance. GJ163c, in particular, is a super-Earth with an equilibrium temperature of T_eq = (302+-10) (1-A)^(1/4) K and may lie in the so called habitable zone for albedo values (A=0.34-0.89) moderately higher than that of Earth (A_Earth=0.2-0.3).

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arXiv2013

Deriving precise parameters for cool solar-type stars. Optimizing the iron line list

Temperature, surface gravity, and metallicitity are basic stellar atmospheric parameters necessary to characterize a star. We aim to improve the description of the spectroscopic temperatures especially for the cooler stars where the differences with the Infrared Flux Method are higher, as presented in previous work. Our spectroscopic analysis is based on the iron excitation and ionization balance, assuming Kurucz model atmospheres in LTE. The abundance analysis is determined using the code MOOG. We optimize the line list using a cool star with high resolution and high signal-to-noise spectrum, as a reference in order to check for weak, isolated lines. We test the quality of the new line list by re-deriving stellar parameters for 451 stars with high resolution and signal-to-noise HARPS spectra, that were analyzed in a previous work with a larger line list. The comparison in temperatures between this work and the latest IRFM shows that the differences for the cooler stars are significantly smaller and more homogeneously distributed than in previous studies for stars with temperatures below 5000 K. We use the new line list to re-derive parameters for some of the cooler stars that host planets. Finally, we present the impact of the new temperatures on the [Cr I/Cr II] and [Ti I/Ti II] abundance ratios that previously showed systematic trends with temperature.

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arXiv2013

Kinematics and chemical properties of the Galactic stellar populations. The HARPS FGK dwarfs sample

(Abridged) We analyze chemical and kinematical properties of about 850 FGK solar neighborhood long-lived dwarfs observed with the HARPS high-resolution spectrograph. The stars in the sample have logg > 4 dex, 5000 < Teff < 6500 K, and -1.39 < [Fe/H] < 0.55 dex. We apply a purely chemical analysis approach based on the [alpha/Fe] vs. [Fe/H] plot to separate Galactic stellar populations into the thin disk, thick disk and high-alpha metal-rich (hamr). Our analysis shows a negative gradient of the rotational velocity of the thin disk stars with [Fe/H] (-17 km s^-1 dex^-1), and a steep positive gradient for both the thick disk and hamr stars with the same magnitude of about +42 km s^-1 dex^-1. For the thin disk stars we observed no correlation between orbital eccentricities and metallicity, but observed a steep negative gradient for the thick disk and hamr stars with practically the same magnitude (about -0.18 dex^-1). Our results suggest that radial migration played an important role in the formation and evolution of the thin disk. For the thick disk stars it is not possible to reach a firm conclusion about their origin. Based on the eccentricity distribution of the thick disk stars only their accretion origin can be ruled out, and the heating and migration scenario could explain the positive steep gradient of V_phi with [Fe/H]. Analyzing the hamr stellar population we found that they share properties of both the thin and thick disk population. A comparison of the properties of the hamr stars with that of the subsample of stars from the N-body/SPH simulation using radial migration suggest that they may have originated from the inner Galaxy. Further detailed investigations would help to clarify their exact nature and origin.

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arXiv2013

The GAPS programme with HARPS-N at TNG. I: Observations of the Rossiter-McLaughlin effect and characterisation of the transiting system Qatar-1

A long-term multi-purpose observational programme has started with HARPS-N@TNG aimed to characterise the global architectural properties of exoplanetary systems. In this first paper we fully characterise the transiting system Qatar-1. We exploit HARPS-N high-precision radial velocity measurements obtained during a transit to measure the Rossiter-McLaughlin effect in the Qatar-1 system, and out-of-transit measurements to redetermine the spectroscopic orbit. New photometric transit light-curves are analysed and a spectroscopic characterisation of the host star atmospheric parameters is performed based on various methods (line equivalent width ratios, spectral synthesis, spectral energy distribution). We achieved a significant improvement in the accuracy of the orbital parameters and derived the spin-orbit alignment of the system; this information, combined with the spectroscopic determination of the host star properties, allows us to derive the fundamental physical parameters for star and planet (masses and radii). The orbital solution for the Qatar-1 system is consistent with a circular orbit and the system presents a sky-projected obliquity of lambda = -8.4+-7.1 deg. The planet, with a mass of 1.33+-0.05 M_J, is found to be significantly more massive than previously reported. The host star is confirmed to be metal-rich ([Fe/H]= 0.20+-0.10) and slowly rotating (vsinI = 1.7+-0.3 km/s), though moderately active, as indicated by strong chromospheric emission in the Ca II H&K line cores (logR'_HK about -4.60). The system is well aligned and fits well within the general lambda vs Teff trend. We definitely rule out any significant orbital eccentricity. The evolutionary status of the system is inferred based on gyrochronology, and the present orbital configuration and timescale for orbital decay are discussed in terms of star-planet tidal interactions.

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arXiv2013

Searching for the signatures of terrestrial planets in F-, G-type main-sequence stars

We have studied the volatile-to-refractory abundance ratios to investigate their possible relation with the low-mass planetary formation. We present a fully differential chemical abundance analysis using high-quality HARPS and UVES spectra of 61 late F- and early G-type main-sequence stars, 29 are planet hosts and 32 are stars without detected planets. As the previous sample of solar analogs, these stars slightly hotter than the Sun also provide very accurate Galactic chemical abundance trends in the metallicity range $-0.3<{\rm [Fe/H]}<0.4$. Stars with and without planets show similar mean abundance ratios. Moreover, when removing the Galactic chemical evolution effects, these mean abundance ratios, $Δ{\rm [X/Fe]_{SUN-STARS}}$, versus condensation temperature tend to exhibit less steep trends with nearly null or slightly negative slopes. We have also analyzed a sub-sample of 26 metal-rich stars, 13 with and 13 without known planets and find the similar, although not equal, abundance pattern with negative slopes for both samples of stars with and without planets. Using stars at S/N $\ge 550$ provides equally steep abundance trends with negative slopes for both stars with and without planets. We revisit the sample of solar analogs to study the abundance patterns of these stars, in particular, 8 stars hosting super-Earth-like planets. Among these stars having very low-mass planets, only four of them reveal clear increasing abundance trends versus condensation temperature. Finally, we have compared these observed slopes with those predicted using a simple model which enables us to compute the mass of rocks which have formed terrestrial planets in each planetary system. We do not find any evidence supporting the conclusion that the volatile-to-refractory abundance ratio is related to the presence of rocky planets.

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arXiv2013

The Homogeneous Study of Transiting Systems (HoSTS) I. The Pilot Study of WASP-13

We present the fundamental stellar and planetary properties of the transiting planetary system WASP-13 within the framework of the Homogeneous Study of Transiting Systems (HoSTS). HoSTS aims to derive the fundamental stellar (Teff, [Fe/H], Mstar, Rstar), and planetary (Mpl, Rpl, Teq) physical properties of known transiting planets using a consistent methodology and homogeneous high-quality dataset. Four spectral analysis techniques are independently applied to a Keck+HIRES spectrum of WASP-13 considering two distinct cases: unconstrained parameters, and constrained log g from transit light curves. We check the derived stellar temperature against that from a different temperature diagnostic based on an INT+IDS Hα spectrum. The four unconstrained analyses render results that are in good agreement, and provide an improvement of 50% in the precision of Teff, and of 85% in [Fe/H] with respect to the WASP-13 discovery paper. The planetary parameters are then derived via the Monte-Carlo-Markov-Chain modeling of the radial velocity and light curves, in iteration with stellar evolutionary models to derive realistic uncertainties. WASP-13 (1.187 +- 0.065 Msun; 1.574 +- 0.048 Rsun) hosts a Saturn-mass, transiting planet (0.500 +- 0.037 MJup; 1.407 +- 0.052 RJup), and is at the end of its main-sequence lifetime (4-5.5 Gyr). Our analysis of WASP-13 showcases that both a detailed stellar characterization, and transit modeling are necessary to well determine the fundamental properties of planetary systems, which are paramount in identifying and determining empirical relationships between transiting planets and their hosts.

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arXiv2013

Volatile and refractory abundances of solar analogs with planets

We present a detailed abundance analysis of high-quality HARPS, UVES and UES spectra of 95 solar analogs, 33 with and 62 without detected planets. These spectra have S/N > 350. We investigate the possibility that the possible presence of terrestrial planets could affect the volatile-to-refratory abundance ratios. We do not see clear differences between stars with and without planets, either in the only seven solar twins or even when considering the whole sample of 95 solar analogs in the metallicity range -0.3< [Fe/H] < 0.5 . We demonstrate that after removing the Galactic chemical evolution effects the possible differences between stars with and without planets in these samples practically disappear and the volatile-to-refractory abundance ratios are very similar to solar values. We investigate the abundance ratios of volatile and refractory elements versus the condensation temperature of this sample of solar analogs, in particular, paying a special attention to those stars harbouring super-Earth-like planets.

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arXiv2013

On the functional form of the metallicity-giant planet correlation

It is generally accepted that the presence of a giant planet is strongly dependent on the stellar metallicity. A stellar mass dependence has also been investigated, but this dependence does not seem as strong as the metallicity dependence. Even for metallicity, however, the exact form of the correlation has not been established. In this paper, we test several scenarios for describing the frequency of giant planets as a function of its host parameters. We perform this test on two volume-limited samples (from CORALIE and HARPS). By using a Bayesian analysis, we quantitatively compared the different scenarios. We confirm that giant planet frequency is indeed a function of metallicity. However, there is no statistical difference between a constant or an exponential function for stars with subsolar metallicities contrary to what has been previously stated in the literature. The dependence on stellar mass could neither be confirmed nor be discarded.

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arXiv2013

The CORALIE survey for southern extrasolar planets XVII. New and updated long period and massive planets

Since 1998, a planet-search program around main sequence stars within 50 pc in the southern hemisphere, is carried out with the CORALIE echelle spectrograph at La Silla Observatory. With an observing time span of more than 14 years, the CORALIE survey is now able to unveil Jovian planets on Jupiter's period domain. This growing period-interval coverage is important regarding to formation and migration models since observational constraints are still weak for periods beyond the ice line. Long-term precise Doppler measurements with the CORALIE echelle spectrograph, together with a few additional observations made with the HARPS spectrograph on the ESO 3.6m telescope, reveal radial velocity signatures of massive planetary companions in long period orbits. In this paper we present seven new planets orbiting HD27631, HD98649, HD106515A, HD166724, HD196067, HD219077, and HD220689 together with the CORALIE orbital parameters for three already known planets around HD10647, HD30562, and HD86226. The period range of the new planetary companions goes from 2200 to 5500 days and covers a mass domain between 1 and 10.5 MJup. Surprisingly, five of them present quite high eccentricities above e>0.57. A pumping scenario by Kozai mechanism may be invoked for HD106515Ab and HD196067b which are both orbiting stars in multiple systems. As the presence of a third massive body can't be inferred from the data of HD98649b, HD166724b, and HD219077b, the origin of the eccentricity of these systems remains unknown. Except for HD10647b, no constraint on the upper mass of the planets is provided by Hipparcos astrometric data. Finally it is interesting to note that the hosts of these long period planets show no metallicity excess.

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arXiv2013

The HARPS search for southern extrasolar planets: XXXIII. New multi-planet systems in the HARPS volume limited sample: a super-Earth and a Neptune in the habitable zone

The vast diversity of planetary systems detected to date is defying our capability of understanding their formation and evolution. Well-defined volume-limited surveys are the best tool at our disposal to tackle the problem, via the acquisition of robust statistics of the orbital elements. We are using the HARPS spectrograph to conduct our survey of ~850 nearby solar-type stars, and in the course of the past nine years we have monitored the radial velocity of HD103774, HD109271, and BD-061339. In this work we present the detection of five planets orbiting these stars, with m*sin(i) between 0.6 and 7 Neptune masses, four of which are in two multiple systems, comprising one super-Earth and one planet within the habitable zone of a late-type dwarf. Although for strategic reasons we chose efficiency over precision in this survey, we have the capability to detect planets down to the Neptune and super-Earth mass range, as well as multiple systems, provided that enough data points are made available.

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arXiv2013

Successful Asteroseismology for a Better Characterization of the Exoplanet HAT-P-7b

It is well known that asteroseismology is the unique technique permitting the study of the internal structure of pulsating stars using their pulsational frequencies, which is per se very important. It acquires an additional value when the star turns out to be a planet host. In this case, the asteroseismic study output may be a very important input for the study of the planetary system. With this in mind, we use the large time-span of the Kepler public data obtained for the star system HAT-P-7, first to perform an asteroseismic study of the pulsating star using Time-Dependent-Convection (TDC) models. Secondly, we make a revision of the planet properties in the light of the asteroseismic study.

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arXiv2012

Metallicity of M dwarfs III. Planet-metallicity and planet-stellar mass correlations of the HARPS GTO M dwarf sample

Aims. The aim of this work is the study of the planet-metallicity and the planet-stellar mass correlations for M dwarfs from the HARPS GTO M dwarf subsample Methods. We use a new method that takes advantage of the HARPS high-resolution spectra to increase the precision of metallicity, using previous photometric calibrations of [Fe/H] and effective temperature as starting values. Results. In this work we use our new calibration (rms = 0.08 dex) to study the planet-metallicity relation of our sample. The well-known correlation for Giant planet FGKM hosts with metallicity is present. Regarding Neptunians and smaller hosts no correlation is found but there is a hint that an anti-correlation with [Fe/H] may exist. We combined our sample with the California Planet Survey late-K and M-type dwarf sample to increase our statistics but found no new trends. We fitted a power law to the frequency histogram of the Jovian hosts for our sample and for the combined sample, f_p = C10^α[Fe/H], using two different approaches: a direct bin fitting and a bayesian fitting procedure. We obtained a value for C between 0.02 and 0.04 and for αbetween 1.26 and 2.94. Regarding stellar mass, an hypothetical correlation with planets was discovered, but was found to be the result of a detection bias.

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