arXiv subjects

Giampaolo Piotto

Publications and source records attributed to Giampaolo Piotto.

At least 55 records · Page 3Linked to original sources

arXiv2018

Combining low- to high-resolution transit spectroscopy of HD 189733b. Linking the troposphere and the thermosphere of a hot gas giant

Space-borne low-to medium-resolution (R~10^2-10^3) transmission spectroscopy of atmospheres detect the broadest spectral features (alkali doublets, molecular bands, scattering), while high-resolution (R~10^5), ground-based observations probe the sharpest features (cores of the alkali lines, molecular lines).The two techniques differ by:(1) The LSF of ground-based observations is 10^3 times narrower than for space-borne observations;(2)Space-borne transmission spectra probe up to the base of thermosphere, while ground-based observations can reach pressures down to 10^(-11);(3)Space-borne observations directly yield the transit depth of the planet, while ground-based observations measure differences in the radius of the planet at different wavelengths.It is challenging to combine both techniques.We develop a method to compare theoretical models with observations at different resolutions.We introduce PyETA, a line-by-line 1D radiative transfer code to compute transmission spectra at R~10^6 (0.01 A) over a broad wavelength range.An hybrid forward modeling/retrieval optimization scheme is devised to deal with the large computational resources required by modeling a broad wavelength range (0.3-2 $μ$m) at high resolution.We apply our technique to HD189733b.Here, HST observations reveal a flattened spectrum due to scattering by aerosols, while high-resolution ground-based HARPS observations reveal the sharp cores of sodium lines.We reconcile these results by building models that reproduce simultaneously both data sets, from the troposphere to the thermosphere. We confirm:(1)the presence of scattering by tropospheric aerosols;(2)that the sodium core feature is of thermospheric origin.Accounting for aerosols, the sodium cores indicate T up to 10000K in the thermosphere.The precise value of the thermospheric temperature is degenerate with the abundance of sodium and altitude of the aerosol deck.

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arXiv2018

An ultra-short period rocky super-Earth with a secondary eclipse and a Neptune-like companion around K2-141

Ultra-short period (USP) planets are a class of low mass planets with periods shorter than one day. Their origin is still unknown, with photo-evaporation of mini-Neptunes and in-situ formation being the most credited hypotheses. Formation scenarios differ radically in the predicted composition of USP planets, it is therefore extremely important to increase the still limited sample of USP planets with precise and accurate mass and density measurements. We report here the characterization of an USP planet with a period of 0.28 days around K2-141 (EPIC 246393474), and the validation of an outer planet with a period of 7.7 days in a grazing transit configuration. We derived the radii of the planets from the K2 light curve and used high-precision radial velocities gathered with the HARPS-N spectrograph for mass measurements. For K2-141b we thus inferred a radius of $1.51\pm0.05~R_\oplus$ and a mass of $5.08\pm0.41~M_\oplus$, consistent with a rocky composition and lack of a thick atmosphere. K2-141c is likely a Neptune-like planet, although due to the grazing transits and the non-detection in the RV dataset, we were not able to put a strong constraint on its density. We also report the detection of secondary eclipses and phase curve variations for K2-141b. The phase variation can be modeled either by a planet with a geometric albedo of $0.30 \pm 0.06$ in the Kepler bandpass, or by thermal emission from the surface of the planet at $\sim$3000K. Only follow-up observations at longer wavelengths will allow us to distinguish between these two scenarios.

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arXiv2017

Precise Masses in the WASP-47 System

We present precise radial velocity observations of WASP-47, a star known to host a hot Jupiter, a distant Jovian companion, and, uniquely, two additional transiting planets in short-period orbits: a super-Earth in a ~19 hour orbit, and a Neptune in a ~9 day orbit. We analyze our observations from the HARPS-N spectrograph along with previously published data to measure the most precise planet masses yet for this system. When combined with new stellar parameters and reanalyzed transit photometry, our mass measurements place strong constraints on the compositions of the two small planets. We find unlike most other ultra-short-period planets, the inner planet, WASP-47 e, has a mass (6.83 +/- 0.66 Me) and radius (1.810 +/- 0.027 Re) inconsistent with an Earth-like composition. Instead, WASP-47 e likely has a volatile-rich envelope surrounding an Earth-like core and mantle. We also perform a dynamical analysis to constrain the orbital inclination of WASP-47 c, the outer Jovian planet. This planet likely orbits close to the plane of the inner three planets, suggesting a quiet dynamical history for the system. Our dynamical constraints also imply that WASP-47 c is much more likely to transit than a geometric calculation would suggest. We calculate a transit probability for WASP-47 c of about 10%, more than an order of magnitude larger than the geometric transit probability of 0.6%.

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arXiv2017

Three's Company: An additional non-transiting super-Earth in the bright HD 3167 system, and masses for all three planets

HD 3167 is a bright (V = 8.9), nearby K0 star observed by the NASA K2 mission (EPIC 220383386), hosting two small, short-period transiting planets. Here we present the results of a multi-site, multi-instrument radial velocity campaign to characterize the HD 3167 system. The masses of the transiting planets are 5.02+/-0.38 MEarth for HD 3167 b, a hot super-Earth with a likely rocky composition (rho_b = 5.60+2.15-1.43 g/cm^3), and 9.80+1.30-1.24 MEarth for HD 3167 c, a warm sub-Neptune with a likely substantial volatile complement (rho_c = 1.97+0.94-0.59 g/cm^3). We explore the possibility of atmospheric composition analysis and determine that planet c is amenable to transmission spectroscopy measurements, and planet b is a potential thermal emission target. We detect a third, non-transiting planet, HD 3167 d, with a period of 8.509+/-0.045 d (between planets b and c) and a minimum mass of 6.90+/-0.71 MEarth. We are able to constrain the mutual inclination of planet d with planets b and c: we rule out mutual inclinations below 1.3 degrees as we do not observe transits of planet d. From 1.3-40 degrees, there are viewing geometries invoking special nodal configurations which result in planet d not transiting some fraction of the time. From 40-60 degrees, Kozai-Lidov oscillations increase the system's instability, but it can remain stable for up to 100Myr. Above 60 degrees, the system is unstable. HD 3167 promises to be a fruitful system for further study and a preview of the many exciting systems expected from the upcoming NASA TESS mission.

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arXiv2017

The Kepler-19 system: a thick-envelope super-Earth with two Neptune-mass companions characterized using Radial Velocities and Transit Timing Variations

We report a detailed characterization of the Kepler-19 system. This star was previously known to host a transiting planet with a period of 9.29 days, a radius of 2.2 R$_\oplus$ and an upper limit on the mass of 20 M$_\oplus$. The presence of a second, non-transiting planet was inferred from the transit time variations (TTVs) of Kepler-19b, over 8 quarters of Kepler photometry, although neither mass nor period could be determined. By combining new TTVs measurements from all the Kepler quarters and 91 high-precision radial velocities obtained with the HARPS-N spectrograph, we measured through dynamical simulations a mass of $8.4 \pm 1.6$ M$_\oplus$ for Kepler-19b. From the same data, assuming system coplanarity, we determined an orbital period of 28.7 days and a mass of $13.1 \pm 2.7$ M$_\oplus$ for Kepler-19c and discovered a Neptune-like planet with a mass of $20.3 \pm 3.4$ M$_\oplus$ on a 63 days orbit. By comparing dynamical simulations with non-interacting Keplerian orbits, we concluded that neglecting interactions between planets may lead to systematic errors that could hamper the precision in the orbital parameters when the dataset spans several years. With a density of $4.32 \pm 0.87$ g cm$^{-3}$ ($0.78 \pm 0.16$ $ρ_\oplus$) Kepler-19b belongs to the group of planets with a rocky core and a significant fraction of volatiles, in opposition to low-density planets characterized by transit-time variations only and the increasing number of rocky planets with Earth-like density. Kepler-19 joins the small number of systems that reconcile transit timing variation and radial velocity measurements.

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arXiv2017

Two massive rocky planets transiting a K-dwarf 6.5 parsecs away

HD 219134 is a K-dwarf star at a distance of 6.5 parsecs around which several low-mass planets were recently discovered. The Spitzer space telescope detected a transit of the innermost of these planets, HD 219134 b, whose mass and radius (4.5 MEarth and 1.6 REarth respectively) are consistent with a rocky composition. Here, we report new highprecision time-series photometry of the star acquired with Spitzer revealing that the second innermost planet of the system, HD 219134 c, is also transiting. A global analysis of the Spitzer transit light curves and the most up-to-date HARPS-N velocity data set yields mass and radius estimations of 4.74+-0.19 MEarth and 1.602+-0.055 REarth for HD 219134 b, and of 4.36+-0.22 MEarth and 1.511+-0.047 REarth for HD 219134 c. These values suggest rocky compositions for both planets. Thanks to the proximity and the small size of their host star (0.778+-0.005 Rsun), these two transiting exoplanets - the nearest to the Earth to date - are well-suited for a detailed characterization (precision of a few percent on mass and radius, constraints on the atmospheric properties...) that could give important constraints on the nature and formation mechanism of the ubiquitous short-period planets of a few Earth masses.

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arXiv2016

Kepler-21b: A rocky planet around a V = 8.25 magnitude star

HD 179070, aka Kepler-21, is a V = 8.25 F6IV star and the brightest exoplanet host discovered by Kepler. An early detailed analysis by Howell et al. (2012) of the first thirteen months (Q0 - Q5) of Kepler light curves revealed transits of a planetary companion, Kepler-21b, with a radius of about 1.60 +/- 0.04 R_earth and an orbital period of about 2.7857 days. However, they could not determine the mass of the planet from the initial radial velocity observations with Keck-HIRES, and were only able to impose a 2-sigma upper limit of 10 M_earth. Here we present results from the analysis of 82 new radial velocity observations of this system obtained with HARPS-N, together with the existing 14 HIRES data points. We detect the Doppler signal of Kepler-21b with a radial velocity semi-amplitude K = 2.00 +/- 0.65 m/s, which corresponds to a planetary mass of 5.1 +/- 1.7 M_earth. We also measure an improved radius for the planet of 1.639 (+0.019, -0.015) R_earth, in agreement with the radius reported by Howell et al. (2012). We conclude that Kepler-21b, with a density of 6.4 +/- 2.1 g/cm^3, belongs to the population of terrestrial planets with iron, magnesium silicate interiors, which have lost the majority of their envelope volatiles via stellar winds or gravitational escape. The radial velocity analysis presented in this paper serves as example of the type of analysis that will be necessary to confirm the masses of TESS small planet candidates.

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arXiv2016

The Orbit and Mass of the Third Planet in the Kepler-56 System

While the vast majority of multiple-planet systems have their orbital angular momentum axes aligned with the spin axis of their host star, Kepler-56 is an exception: its two transiting planets are coplanar yet misaligned by at least 40 degrees with respect to their host star. Additional follow-up observations of Kepler-56 suggest the presence of a massive, non-transiting companion that may help explain this misalignment. We model the transit data along with Keck/HIRES and HARPS-N radial velocity data to update the masses of the two transiting planets and infer the physical properties of the third, non-transiting planet. We employ a Markov Chain Monte Carlo sampler to calculate the best-fitting orbital parameters and their uncertainties for each planet. We find the outer planet has a period of 1002 $\pm$ 5 days and minimum mass of 5.61 $\pm$ 0.38 Jupiter masses. We also place a 95% upper limit of 0.80 m/s/yr on long-term trends caused by additional, more distant companions.

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arXiv2016

A 1.9 Earth radius rocky planet and the discovery of a non-transiting planet in the Kepler-20 system

Kepler-20 is a solar-type star (V = 12.5) hosting a compact system of five transiting planets, all packed within the orbital distance of Mercury in our own Solar System. A transition from rocky to gaseous planets with a planetary transition radius of ~1.6 REarth has recently been proposed by several publications in the literature (Rogers 2015;Weiss & Marcy 2014). Kepler-20b (Rp ~ 1.9 REarth) has a size beyond this transition radius, however previous mass measurements were not sufficiently precise to allow definite conclusions to be drawn regarding its composition. We present new mass measurements of three of the planets in the Kepler-20 system facilitated by 104 radial velocity measurements from the HARPS-N spectrograph and 30 archival Keck/HIRES observations, as well as an updated photometric analysis of the Kepler data and an asteroseismic analysis of the host star (MStar = 0.948+-0.051 Msun and Rstar = 0.964+-0.018 Rsun). Kepler-20b is a 1.868+0.066-0.034 REarth planet in a 3.7 day period with a mass of 9.70+1.41-1.44 MEarth resulting in a mean density of 8.2+1.5-1.3 g/cc indicating a rocky composition with an iron to silicate ratio consistent with that of the Earth. This makes Kepler-20b the most massive planet with a rocky composition found to date. Furthermore, we report the discovery of an additional non-transiting planet with a minimum mass of 19.96+3.08-3.61 MEarth and an orbital period of ~34 days in the gap between Kepler-20f (P ~ 11 days) and Kepler-20d (P ~ 78 days).

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arXiv2016

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. X. The radial distribution of stellar populations in NGC 2808

Due to their extreme helium abundance, the multiple stellar populations of the globular cluster NGC 2808 have been widely investigated from a photometric, spectroscopic, and kinematic perspective. The most striking feature of the color-magnitude diagram of NGC 2808 is the triple main sequence (MS), with the red MS corresponding to a stellar population with primordial helium, and the middle and the blue MS being enhanced in helium up to Y$\sim$0.32 and $\sim$0.38, respectively. A recent study has revealed that this massive cluster hosts at least five distinct stellar populations (A, B, C, D, and E). Among them populations A, B, and C correspond to the red MS, while populations C and D are connected to the middle and the blue MS. In this paper we exploit Hubble-Space-Telescope photometry to investigate the radial distribution of the red, the middle and the blue MS from the cluster center out to about 8.5 arcmin. Our analysis shows that the radial distribution of each of the three MSs is different. In particular, as predicted from multiple-population formation models, both the blue MS and the middle MS appears to be more concentrated than the red MS with a significance level for this result wich is above 3σ.

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arXiv2016

WISEP J060738.65+242953.4: A Nearby. Pole-On L8 Brown Dwarf with Radio Emission

We present a simultaneous, multi-wavelength campaign targeting the nearby (7.2 pc) L8/L9 (optical/near-infrared) dwarf WISEP J060738.65+242953.4 in the mid-infrared, radio, and optical. Spitzer Space Telescope observations show no variability at the 0.2% level over 10 hours each in the 3.6 and 4.5 micron bands. Kepler K2 monitoring over 36 days in Campaign 0 rules out stable periodic signals in the optical with amplitudes great than 1.5% and periods between 1.5 hours and 2 days. Non-simultaneous Gemini optical spectroscopy detects lithium, constraining this L dwarf to be less than ~2 Gyr old, but no Balmer emission is observed. The low measured projected rotation velocity (v sin i < 6 km/s) and lack of variability are very unusual compared to other brown dwarfs, and we argue that this substellar object is likely viewed pole-on. We detect quiescent (non-bursting) radio emission with the VLA. Amongst radio detected L and T dwarfs, it has the lowest observed L_nu and the lowest v sin i. We discuss the implications of a pole-on detection for various proposed radio emission scenarios.

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arXiv2016

The Canarias Einstein Ring: a Newly Discovered Optical Einstein Ring

We report the discovery of an optical Einstein Ring in the Sculptor constellation, IAC J010127-334319, in the vicinity of the Sculptor Dwarf Spheroidal Galaxy. It is an almost complete ring ($\sim 300^{\circ}$) with a diameter of $\sim 4.5\, {\rm arcsec}$. The discovery was made serendipitously from inspecting Dark Energy Camera (DECam) archive imaging data. Confirmation of the object nature has been obtained by deriving spectroscopic redshifts for both components, lens and source, from observations at the $10.4$ m Gran Telescopio CANARIAS (GTC) with the spectrograph OSIRIS. The lens, a massive early-type galaxy, has a redshift of ${\rm z}=0.581$ while the source is a starburst galaxy with redshift of ${\rm z}=1.165$. The total enclosed mass that produces the lensing effect has been estimated to be ${\rm M_{tot}=(1.86 \pm 0.23) \,\cdot 10^{12}\, {\rm M_{\odot}}}$.

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arXiv2016

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. VII. Implications from the Nearly Universal Nature of Horizontal Branch Discontinuities

The UV-initiative Hubble Space Telescope Treasury survey of Galactic globular clusters provides a new window into the phenomena that shape the morphological features of the horizontal branch (HB). Using this large and homogeneous catalog of UV and blue photometry, we demonstrate that the HB exhibits discontinuities that are remarkably consistent in color (effective temperature). This consistency is apparent even among some of the most massive clusters hosting multiple distinct sub-populations (such as NGC 2808, omega Cen, and NGC 6715), demonstrating that these phenomena are primarily driven by atmospheric physics that is independent of the underlying population properties. However, inconsistencies arise in the metal-rich clusters NGC 6388 and NGC 6441, where the discontinuity within the blue HB (BHB) distribution shifts ~1,000 K to 2,000 K hotter. We demonstrate that this shift is likely due to a large helium enhancement in the BHB stars of these clusters, which in turn affects the surface convection and evolution of such stars. Our survey also increases the number of Galactic globular clusters known to host blue-hook stars (also known as late hot flashers) from 6 to 23 clusters. These clusters are biased toward the bright end of the globular cluster luminosity function, confirming that blue-hook stars tend to form in the most massive clusters with significant self-enrichment.

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arXiv2015

The Kepler-454 System: A Small, Not-rocky Inner Planet, a Jovian World, and a Distant Companion

Kepler-454 (KOI-273) is a relatively bright (V = 11.69 mag), Sun-like starthat hosts a transiting planet candidate in a 10.6 d orbit. From spectroscopy, we estimate the stellar temperature to be 5687 +/- 50 K, its metallicity to be [m/H] = 0.32 +/- 0.08, and the projected rotational velocity to be v sin i <2.4 km s-1. We combine these values with a study of the asteroseismic frequencies from short cadence Kepler data to estimate the stellar mass to be 1.028+0:04-0:03 M_Sun, the radius to be 1.066 +/- 0.012 R_Sun and the age to be 5.25+1:41-1:39 Gyr. We estimate the radius of the 10.6 d planet as 2.37 +/- 0.13 R_Earth. Using 63 radial velocity observations obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and 36 observations made with the HIRES spectrograph at Keck Observatory, we measure the mass of this planet to be 6.8 +/- 1.4M_Earth. We also detect two additional non-transiting companions, a planet with a minimum mass of 4.46 +/- 0.12 M_J in a nearly circular 524 d orbit and a massive companion with a period >10 years and mass >12.1M_J . The twelve exoplanets with radii <2.7 R_Earth and precise mass measurements appear to fall into two populations, with those <1.6 R_Earth following an Earth-like composition curve and larger planets requiring a significant fraction of volatiles. With a density of 2.76 +/- 0.73 g cm-3, Kepler-454b lies near the mass transition between these two populations and requires the presence of volatiles and/or H/He gas.

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arXiv2015

Global Architecture of Planetary Systems (GAPS), a project for the whole Italian Community

The GAPS project is running since 2012 with the goal to optimize the science return of the HARPS-N instrument mounted at Telescopio Nazionale Galileo. A large number of astronomers is working together to allow the Italian community to gain an international position adequate to the HARPS-N capabilities in the exoplanetary researches. Relevant scientific results are being obtained on both the main guidelines of the collaboration, i.e., the discovery surveys and the characterization studies. The planetary system discovered around the southern component of the binary XO-2 and its characterization together with that of the system orbiting the northern component are a good example of the completeness of the topics matched by the GAPS project. The dynamics of some planetary systems are investigated by studying the Rossiter-McLaughlin effect, while host stars are characterized by means of asteroseismology and star-planet interaction.

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arXiv2014

The Mass of Kepler-93b and The Composition of Terrestrial Planets

Kepler-93b is a 1.478 +/- 0.019 Earth radius planet with a 4.7 day period around a bright (V=10.2), astroseismically-characterized host star with a mass of 0.911+/-0.033 solar masses and a radius of 0.919+/-0.011 solar radii. Based on 86 radial velocity observations obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and 32 archival Keck/HIRES observations, we present a precise mass estimate of 4.02+/-0.68 Earth masses. The corresponding high density of 6.88+/-1.18 g/cc is consistent with a rocky composition of primarily iron and magnesium silicate. We compare Kepler-93b to other dense planets with well-constrained parameters and find that between 1-6 Earth masses, all dense planets including the Earth and Venus are well-described by the same fixed ratio of iron to magnesium silicate. There are as of yet no examples of such planets with masses > 6 Earth masses: All known planets in this mass regime have lower densities requiring significant fractions of volatiles or H/He gas. We also constrain the mass and period of the outer companion in the Kepler-93 system from the long-term radial velocity trend and archival adaptive optics images. As the sample of dense planets with well-constrained masses and radii continues to grow, we will be able to test whether the fixed compositional model found for the seven dense planets considered in this paper extends to the full population of 1-6 Earth mass planets.

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arXiv2014

Characterizing K2 Planet Discoveries: A super-Earth transiting the bright K-dwarf HIP 116454

We report the first planet discovery from the two-wheeled Kepler (K2) mission: HIP 116454 b. The host star HIP 116454 is a bright (V = 10.1, K = 8.0) K1-dwarf with high proper motion, and a parallax-based distance of 55.2 +/- 5.4 pc. Based on high-resolution optical spectroscopy, we find that the host star is metal-poor with [Fe/H] = -.16 +/- .18, and has a radius R = 0.716 +/- .0024 R_sun and mass M = .775 +/- .027 Msun. The star was observed by the Kepler spacecraft during its Two-Wheeled Concept Engineering Test in February 2014. During the 9 days of observations, K2 observed a single transit event. Using a new K2 photometric analysis technique we are able to correct small telescope drifts and recover the observed transit at high confidence, corresponding to a planetary radius of Rp = 2.53 +/- 0.18 Rearth. Radial velocity observations with the HARPS-N spectrograph reveal a 11.82 +/- 1.33 Mearth planet in a 9.1 day orbit, consistent with the transit depth, duration, and ephemeris. Follow-up photometric measurements from the MOST satellite confirm the transit observed in the K2 photometry and provide a refined ephemeris, making HIP 116454 b amenable for future follow-up observations of this latest addition to the growing population of transiting super-Earths around nearby, bright stars.

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arXiv2014

Revising the ages of planet-hosting stars

This article aims to measure the age of planet-hosting stars (SWP) through stellar tracks and isochrones computed with the \textsl{PA}dova \& T\textsl{R}ieste \textsl{S}tellar \textsl{E}volutionary \textsl{C}ode (PARSEC). We developed algorithms based on two different techniques for determining the ages of field stars: \emph{isochrone placement} and \emph{Bayesian estimation}. Their application to a synthetic sample of coeval stars shows the intrinsic limits of each method. For instance, the Bayesian computation of the modal age tends to select the extreme age values in the isochrones grid. Therefore, we used the isochrone placement technique to measure the ages of 317 SWP. We found that $\sim6\%$ of SWP have ages lower than 0.5 Gyr. The age distribution peaks in the interval [1.5, 2) Gyr, then it decreases. However, $\sim7\%$ of the stars are older than 11 Gyr. The Sun turns out to be a common star that hosts planets, when considering its evolutionary stage. Our SWP age distribution is less peaked and slightly shifted towards lower ages if compared with ages in the literature and based on the isochrone fit. In particular, there are no ages below 0.5 Gyr in the literature.

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