arXiv subjects

A. Sozzetti

Publications and source records attributed to A. Sozzetti.

At least 271 records · Page 15Linked to original sources

arXiv2014

New Variable Stars Discovered by the APACHE Survey. I. Results After the First Observing Season

We present more than 80 new variable stars discovered during the first observing season of the APACHE survey. APACHE is a project aimed at detecting extrasolar planets transiting nearby, bright M dwarfs by using an array of small-aperture telescopes. Despite the fact that the survey is targeted to a well-defined sample of cool stars, we also reduce and analyze data for all the detected field stars. Since July 2012 dozens of different stellar fields have been monitored, leading to the detection of several variables for which we propose a classification and estimate a period, when a periodicity is evident in the data. Thanks to the SuperWASP public archive, we have also retrieved and analyzed photometric data collected by the SWASP survey, which helped us to refine the classification and the period estimation of many variables found in the APACHE database. Some of the variables present peculiarities and thus are discussed separately.

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arXiv2014

Gaia, Non-Single Stars, Brown Dwarfs, and Exoplanets

In its all-sky survey, Gaia will monitor astrometrically and photometrically millions of main-sequence stars with sufficient sensitivity to brown dwarf companions within a few AUs from their host stars and to transiting brown dwarfs on very short periods, respectively. Furthermore, thousands of detected ultra-cool dwarfs in the backyard of the Sun will have direct (absolute) distance estimates from Gaia, and for these Gaia astrometry will be of sufficient precision to reveal any orbiting companions with masses as low as that of Jupiter. Gaia observations thus bear the potential for critical contributions to many important questions in brown dwarfs astrophysics (how do they form in isolation and as companions to stars? Can planets form around them? What are their fundamental parameters such as ages, masses, and radii? What is their atmospheric physics?), and their connection to stars and planets. The full legacy potential of Gaia in the realm of brown dwarf science will be realized when combined with other detection and characterization programs, both from the ground and in space.

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arXiv2014

The HARPS search for southern extra-solar planets XXXV. The interesting case of HD41248: stellar activity, no planets?

The search for planets orbiting metal-poor stars is of uttermost importance for our understanding of the planet formation models. However, no dedicated searches have been conducted so far for very low mass planets orbiting such objects. Only a few cases of low mass planets orbiting metal-poor stars are thus known. Amongst these, HD41248 is a metal-poor, solar-type star on which a resonant pair of super-Earth like planets has In the present paper we present a new planet search program that is using the HARPS spectrograph to search for Neptunes and Super-Earths orbiting a sample of metal-poor FGK dwarfs. We then present a detailed analysis of an additional 162 radial velocity measurements of HD41248, obtained within this program, with the goal of confirming the existence of the proposed planetary system. We analyzed the precise radial velocities, obtained with the HARPS spectrograph, together with several stellar activity diagnostics and line profile indicators. A careful analysis shows no evidence for the planetary system previously announced. One of the signals, with a period of about 25 days, is shown to be related to the rotational period of the star, and is clearly seen in some of the activity proxies. The remaining signal (P~18 days) could not be convincingly retrieved in the new data set. We discuss possible causes for the complex (evolving) signals observed in the data of HD41248, proposing that they may be explained by the appearance and disappearance of active regions on the surface of a star with strong differential rotation, or by a combination of the sparse data sampling and active region evolution.

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arXiv2014

The Gaia Survey Contribution to EChO Target Selection and Characterization

The scientific output of the proposed EChO mission (in terms of spectroscopic characterization of the atmospheres of transiting extrasolar planets) will be maximized by a careful selection of targets and by a detailed characterization of the main physical parameters (such as masses and radii) of both the planets and their stellar hosts. To achieve this aim, the availability of high-quality data from other space-borne and ground-based programs will play a crucial role. Here we identify and discuss the elements of the Gaia catalogue that will be of utmost relevance for the selection and characterization of transiting planet systems to be observed by the proposed EChO mission.

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arXiv2014

The GAPS Programme with HARPS-N at TNG. III: The retrograde orbit of HAT-P-18b

The measurement of the Rossiter-McLaughlin effect for transiting exoplanets places constraints on the orientation of the orbital axis with respect to the stellar spin axis, which can shed light on the mechanisms shaping the orbital configuration of planetary systems. Here we present the interesting case of the Saturn-mass planet HAT-P-18b, which orbits one of the coolest stars for which the Rossiter-McLaughlin effect has been measured so far. We acquired a spectroscopic time-series, spanning a full transit, with the HARPS-N spectrograph mounted at the TNG telescope. The very precise radial velocity measurements delivered by the HARPS-N pipeline were used to measure the Rossiter-McLaughlin effect. Complementary new photometric observations of another full transit were also analysed to obtain an independent determination of the star and planet parameters. We find that HAT-P-18b lies on a counter-rotating orbit, the sky-projected angle between the stellar spin axis and the planet orbital axis being lambda=132 +/- 15 deg. By joint modelling of the radial velocity and photometric data we obtain new determinations of the star (M_star = 0.770 +/- 0.027 M_Sun; R_star= 0.717 +/- 0.026 R_Sun; Vsin(I_star) = 1.58 +/- 0.18 km/s) and planet (M_pl = 0.196 +/- 0.008 M_J; R_pl = 0.947 +/- 0.044 R_J) parameters. Our spectra provide for the host star an effective temperature T_eff = 4870 +/- 50 K, a surface gravity of log(g_star) = 4.57 +/- 0.07 cm/s, and an iron abundance of [Fe/H] = 0.10 +/- 0.06. HAT-P-18b is one of the few planets known to transit a star with T_eff < 6250 K on a retrograde orbit. Objects such as HAT-P-18b (low planet mass and/or relatively long orbital period) most likely have a weak tidal coupling with their parent stars, therefore their orbits preserve any original misalignment. As such, they are ideal targets to study the causes of orbital evolution in cool main-sequence stars.

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arXiv2014

The galactic habitable zone of the Milky Way and M31 from chemical evolution models with gas radial flows

The galactic habitable zone is defined as the region with sufficient abundance of heavy elements to form planetary systems in which Earth-like planets could be born and might be capable of sustaining life, after surviving to close supernova explosion events. Galactic chemical evolution models can be useful for studying the galactic habitable zones in different systems. We apply detailed chemical evolution models including radial gas flows to study the galactic habitable zones in our Galaxy and M31. We compare the results to the relative galactic habitable zones found with "classical" (independent ring) models, where no gas inflows were included. For both the Milky Way and Andromeda, the main effect of the gas radial inflows is to enhance the number of stars hosting a habitable planet with respect to the "classical" model results, in the region of maximum probability for this occurrence, relative to the classical model results. These results are obtained by taking into account the supernova destruction processes. In particular, we find that in the Milky Way the maximum number of stars hosting habitable planets is at 8 kpc from the Galactic center, and the model with radial flows predicts a number which is 38% larger than what predicted by the classical model. For Andromeda we find that the maximum number of stars with habitable planets is at 16 kpc from the center and that in the case of radial flows this number is larger by 10 % relative to the stars predicted by the classical model.

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arXiv2013

Parallaxes of five L Dwarfs with a Robotic Telescope

We report the parallax and proper motion of five L dwarfs obtained with observations from the robotic Liverpool Telescope. Our derived proper motions are consistent with published values and have considerably smaller errors. Based on our spectral type versus absolute magnitude diagram, we do not find any evidence for binaries among our sample, or, at least no comparable mass binaries. Their space velocities locate them within the thin disk and based on the model comparisons they have solar-like abundances. For all five objects, we derived effective temperature, luminosity, radius, gravity and mass from a evolutionary model(CBA00) and our measured parallax; moreover, we derived their effective temperature by integrating observed optical and near-infrared spectra and model spectra (BSH06 or BT-Dusty respectively) at longer wavelengths to obtain bolometric {\bf flux using} the classical Stefan-Boltzmann law: generally the three temperatures for one object derived using two different methods with three models are consistent, while at lower temperature(e.g. for L4) the differences among the three temperatures are slightly larger than that at higher temperature(e.g. for L1).

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arXiv2013

Astrometric Detection of Giant Planets Around Nearby M Dwarfs: The Gaia Potential

[abridged] We carry out numerical simulations to gauge the Gaia potential for precision astrometry of exoplanets orbiting a sample of known dM stars within 30 pc from the Sun. (1) It will be possible to accurately determine orbits and masses for Jupiter-mass planets with orbital periods in the range 0.2 10. Given present-day estimates of the planet fraction f_p around M dwarfs, 100 giant planets could be found by Gaia around the sample. Comprehensive screening by Gaia of the reservoir of 4x10^5 M dwarfs within 100 pc could result in 2600 detections and as many as 500 accurate orbit determinations. The value of f_p could then be determined with an accuracy of 2%, an improvement by over an order of magnitude with respect to the most precise values available to-date; (2) in the same period range, inclination angles corresponding to quasi-edge-on configurations will be determined with enough precision (a few percent) so that it will be possible to identify intermediate-separation planets which are potentially transiting within the errors. Gaia could alert us of the existence of 10 such systems. More than 250 candidates could be identified assuming solutions compatible with transit configurations within 10% accuracy, although a large fraction of these (85%) could be false positives; (3) for well-sampled orbits, the uncertainties on planetary ephemerides, separation and position angle, will degrade at typical rates of < 1 mas/yr and < 2 deg/yr, respectively; (4) planetary phases will be measured with typical uncertainties of several degrees, resulting (under the assumption of purely scattering atmospheres) in phase-averaged errors on the phase function of 0.05, and expected uncertainties in the determination of the emergent flux of intermediate-separation (0.3<a<2.0 AU) giant planets of 20%. [abridged]

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arXiv2013

Parallaxes of Southern Extremely Cool objects (PARSEC) II: Spectroscopic Follow-up and Parallaxes of 52 Targets

We present near-infrared spectroscopy for 52 ultracool dwarfs, including two newly discovered late-M dwarfs, one new late-M subdwarf candidate, three new L and four new T dwarfs. We also present parallaxes and proper motions for 21 of them. Four of the targets presented here have previous parallax measurements, while all the others are new values. This allow us to populate further the spectral sequence at early types (L0-L4). Combining the astrometric parameters with the new near-infrared spectroscopy presented here, we are able to investigate further the nature of some of the objects. In particular, we find that the peculiar blue L1 dwarf SDSS J133148.92-011651.4 is a metal-poor object, likely a member of the galactic thick disk. We discover a new M subdwarf candidate, 2MASS J20115649-6201127. We confirm the low-gravity nature of EROS-MP J0032-4405, DENIS-P J035726.9-441730, and 2MASS J22134491-2136079. We present two new metal-poor dwarfs: the L4pec 2MASS J19285196-4356256 and the M7pec SIPS2346-5928. We also determine the effective temperature and bolometric luminosity of the 21 targets with astrometric measurements, and we obtain a new polynomial relation between effective temperature and near-infrared spectral type. The new fit suggests a flattening of the sequence at the transition between M and L spectral types. This could be an effect of dust formation, that causes a more rapid evolution of the spectral features as a function of the effective temperature.

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arXiv2013

NPARSEC: NTT Parallaxes of Southern Extremely Cool objects. Goals, targets, procedures and first results

The discovery and subsequent detailed study of T dwarfs has provided many surprises and pushed the physics and modeling of cool atmospheres in unpredicted directions. Distance is a critical parameter for studies of these objects to determine intrinsic luminosities, test binarity and measure their motion in the Galaxy. We describe a new observational program to determine distances across the full range of T dwarf sub-types using the NTT/SOFI telescope/instrument combination. We present preliminary results for ten objects, five of which represent new distances.

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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

The APACHE Project

First, we summarize the four-year long efforts undertaken to build the final setup of the APACHE Project, a photometric transit search for small-size planets orbiting bright, low-mass M dwarfs. Next, we describe the present status of the APACHE survey, officially started in July 2012 at the site of the Astronomical Observatory of the Autonomous Region of the Aosta Valley, in the Western Italian Alps. Finally, we briefly discuss the potentially far-reaching consequences of a multi-technique characterization program of the (potentially planet-bearing) APACHE targets.

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arXiv2013

The GAPS Programme with HARPS-N@TNG II: No giant planets around the metal-poor star HIP 11952

In the context of the program Global Architecture of Planetary Systems (GAPS), we have performed radial velocity monitoring of the metal-poor star HIP 11952 on 35 nights over about 150 days using the newly installed high resolution spectrograph HARPS-N at the TNG and HARPS at ESO 3.6m telescope. The radial velocities show a scatter of 7 m/s, compatible with the measurement errors for such a moderately warm metal-poor star (Teff = 6040+-120K; [Fe/H] =-1.9+-0.1). We then exclude the presence of the two giant planets with periods of 6.95+-0.01 d and 290.0+-16.2 d and radial velocity semi-amplitudes of 100.3+-19.4 m/s and 105.2+-14.7 m/s, respectively, which had recently been announced. This result is important considering that HIP 11952 was thought to be the most metal-poor star hosting a planetary system with giant planets, thus challenging some models of planet formation.

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arXiv2012

A long-period massive planet around HD106515A

We have performed RV monitoring of the components of the binary system HD 106515 over about 11 years using the high resolution spectrograph SARG at TNG. The primary shows long-period radial velocity variations that indicate the presence of a low mass companion whose projected mass is in the planetary regime (m sin i = 9.33 Mjup). The 9.8 years orbit results quite eccentric (e=0.57), as typical for massive giant planets. Our results confirm the preliminary announcement of the planet included in Mayor et al. (2011). The secondary instead does not show significant RV variations. The two components do not differ significantly in chemical composition, as found for other pairs for which one component hosts giant planets. Adaptive optics images obtained with AdOpt@TNG do not reveal additional stellar companions. From the analysis of the relative astrometry of the components of the wide pair we put an upper limit on the mass of the newly detected companion of about 0.25 Msun. State of art or near future instrumentation can provide true mass determination, thanks to the availability of the wide companion HD106515B as reference. Therefore, HD106515Ab will allow deeper insight in the transition region between planets and brown dwarfs.

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arXiv2012

Photometric transit search for planets around cool stars from the western Italian Alps: A pilot study

[ABRIDGED] In this study, we set out to a) demonstrate the sensitivity to <4 R_E transiting planets with periods of a few days around our program stars, and b) improve our knowledge of some astrophysical properties(e.g., activity, rotation) of our targets by combining spectroscopic information and our differential photometric measurements. We achieve a typical nightly RMS photometric precision of ~5 mmag, with little or no dependence on the instrumentation used or on the details of the adopted methods for differential photometry. The presence of correlated (red) noise in our data degrades the precision by a factor ~1.3 with respect to a pure white noise regime. Based on a detailed stellar variability analysis, a) we detected no transit-like events; b) we determined photometric rotation periods of ~0.47 days and ~0.22 days for LHS 3445 and GJ 1167A, respectively; c) these values agree with the large projected rotational velocities (~25 km/s and ~33 km/s, respectively) inferred for both stars based on the analysis of archival spectra; d) the estimated inclinations of the stellar rotation axes for LHS 3445 and GJ 1167A are consistent with those derived using a simple spot model; e) short-term, low-amplitude flaring events were recorded for LHS 3445 and LHS 2686. Finally, based on simulations of transit signals of given period and amplitude injected in the actual (nightly reduced) photometric data for our sample, we derive a relationship between transit detection probability and phase coverage. We find that, using the BLS search algorithm, even when phase coverage approaches 100%, there is a limit to the detection probability of ~90%. Around program stars with phase coverage >50% we would have had >80% chances of detecting planets with P<1 day inducing fractional transit depths >0.5%, corresponding to minimum detectable radii in the range 1.0-2.2 R_E. [ABRIDGED]

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arXiv2012

The frequency of giant planets around metal-poor stars

Context. The discovery of about 700 extrasolar planets, so far, has lead to the first statistics concerning extrasolar planets. The presence of giant planets seems to depend on stellar metallicity and mass. For example, they are more frequent around metal-rich stars,with an exponential increase in planet occurrence rates with metallicity. Aims. We analyzed two samples of metal-poor stars (-2.0 \leq [Fe/H] \leq 0.0) to see if giant planets are indeed rare around these objects. Radial velocity datasets were obtained with two different spectrographs (HARPS and HIRES). Detection limits for these data,expressed in minimum planetary mass and period, are calculated. These produce trustworthy numbers for the planet frequency. Methods. A general Lomb Scargle (GLS) periodogram analysis was used together with a bootstrapping method to produce the detection limits. Planet frequencies were calculated based on a binomial distribution function within metallicity bins. Results. Almost all hot Jupiters and most giant planets should have been found in these data. Hot Jupiters around metal-poor stars have a frequency lower than 1.0% at one sigma. Giant planets with periods up to 1800 days, however, have a higher frequency of $f_p = 2.63^{+2.5}_{-0.8}%$. Taking into account the different metallicities of the stars, we show that giant planets appear to be very frequent $(f_p = 4.48^{+4.04}_{-1.38}%)$ around stars with [Fe/H] > -0.7, while they are rare around stars with [Fe/H] \leq -0.7 (\leq 2.36% at one sigma). Conclusions. Giant planet frequency is indeed a strong function of metallicity, even in the low-metallicity tail. However, the frequencies are most likely higher than previously thought.

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arXiv2012

Gaia Universe Model Snapshot : A statistical analysis of the expected contents of the Gaia catalogue

Context. This study has been developed in the framework of the computational simulations executed for the preparation of the ESA Gaia astrometric mission. Aims. We focus on describing the objects and characteristics that Gaia will potentially observe without taking into consideration instrumental effects (detection efficiency, observing errors). Methods. The theoretical Universe Model prepared for the Gaia simulation has been statistically analyzed at a given time. Ingredients of the model are described, giving most attention to the stellar content, the double and multiple stars, and variability. Results. In this simulation the errors have not been included yet. Hence we estimate the number of objects and their theoretical photometric, astrometric and spectroscopic characteristics in the case that they are perfectly detected. We show that Gaia will be able to potentially observe 1.1 billion of stars (single or part of multiple star systems) of which about 2% are variable stars, 3% have one or two exoplanets. At the extragalactic level, observations will be potentially composed by several millions of galaxies, half million to 1 million of quasars and about 50,000 supernovas that will occur during the 5 years of mission. The simulated catalogue will be made publicly available by the DPAC on the Gaia portal of the ESA web site http://www.rssd.esa.int/gaia/.

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arXiv2012

Astrometry and Exoplanet Characterization: Gaia and Its Pandora's Box

In its all-sky survey, Gaia will monitor astrometrically hundreds of thousands of main-sequence stars within $\approx200$ pc, looking for the presence of giant planetary companions within a few AUs from their host stars. Indeed, Gaia observations will have great impact is the astrophysics of planetary systems (e.g., Casertano et al. 2008), in particular when seen as a complement to other techniques for planet detection and characterization (e.g., Sozzetti 2011). In this paper, I briefly address some of the relevant technical issues associated with the precise and accurate determination of astrometric orbits of planetary systems using Gaia data. I then highlight some of the important synergies between Gaia high-precision astrometry and other ongoing and planned, indirect and direct planet-finding and characterization programs, both from the ground and in space, and over a broad range of wavelengths, providing preliminary results related to one specific example of such synergies.

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