arXiv subjects

Mercedes Lopez-Morales

Publications and source records attributed to Mercedes Lopez-Morales.

At least 73 records · Page 4Linked to original sources

arXiv2014

The Kepler-10 planetary system revisited by HARPS-N: A hot rocky world and a solid Neptune-mass planet

Kepler-10b was the first rocky planet detected by the Kepler satellite and con- firmed with radial velocity follow-up observations from Keck-HIRES. The mass of the planet was measured with a precision of around 30%, which was insufficient to constrain models of its internal structure and composition in detail. In addition to Kepler-10b, a second planet transiting the same star with a period of 45 days was sta- tistically validated, but the radial velocities were only good enough to set an upper limit of 20 Mearth for the mass of Kepler-10c. To improve the precision on the mass for planet b, the HARPS-N Collaboration decided to observe Kepler-10 intensively with the HARPS-N spectrograph on the Telescopio Nazionale Galileo on La Palma. In to- tal, 148 high-quality radial-velocity measurements were obtained over two observing seasons. These new data allow us to improve the precision of the mass determina- tion for Kepler-10b to 15%. With a mass of 3.33 +/- 0.49 Mearth and an updated radius of 1.47 +0.03 -0.02 Rearth, Kepler-10b has a density of 5.8 +/- 0.8 g cm-3, very close to the value -0.02 predicted by models with the same internal structure and composition as the Earth. We were also able to determine a mass for the 45-day period planet Kepler-10c, with an even better precision of 11%. With a mass of 17.2 +/- 1.9 Mearth and radius of 2.35 +0.09 -0.04 Rearth, -0.04 Kepler-10c has a density of 7.1 +/- 1.0 g cm-3. Kepler-10c appears to be the first strong evidence of a class of more massive solid planets with longer orbital periods.

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arXiv2014

The contribution of the major planet search surveys to EChO target selection

The EChO core science will be based on a three tier survey, each with increasing sensitivity, in order to study the population of exo-planets from super-Earths to Jupiter-like planets, in the very hot to temperate zones (temperatures of 300 K - 3000 K) of F to M-type host stars. To achieve a meaningful outcome an accurate selection of the target sample is needed. In this paper we analyse the targets, suitable for EChO observations, expected to result from a sample of present and forthcoming detection surveys. Exoplanets currently known are already sufficient to provide a large and diverse sample. However we expect the results from these surveys to increase the sample of smaller planets that will allow us to optimize the EChO sample selection.

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arXiv2013

Stellar Diameters and Temperatures V. Eleven Newly Characterized Exoplanet Host Stars

We use near-infrared interferometric data coupled with trigonometric parallax values and spectral energy distribution fitting to directly determine stellar radii, effective temperatures, and luminosities for the exoplanet host stars 61 Vir, $ρ$ CrB, GJ 176, GJ 614, GJ 649, GJ 876, HD 1461, HD 7924, HD 33564, HD 107383, and HD 210702. Three of these targets are M dwarfs. Statistical uncertainties in the stellar radii and effective temperatures range from 0.5% -- 5% and from 0.2% -- 2%, respectively. For eight of these targets, this work presents the first directly determined values of radius and temperature; for the other three, we provide updates to their properties. The stellar fundamental parameters are used to estimate stellar mass and calculate the location and extent of each system's circumstellar habitable zone. Two of these systems have planets that spend at least parts of their respective orbits in the system habitable zone: two of GJ 876's four planets and the planet that orbits HD 33564. We find that our value for GJ 876's stellar radius is more than 20% larger than previous estimates and frequently used values in the astronomical literature.

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arXiv2013

Feasibility Studies for the Detection of $O_2$ in an Earth-like Exoplanet

We present the results of simulations on the detectability of $O_2$ in the atmosphere of Earth twins around nearby low mass stars using high resolution transmission spectroscopy. We explore such detectability with each of the three upcoming Extremely Large Telescopes (ELTs), i.e. GMT, TMT and E-ELT, and high resolution spectrographs, assuming such instruments will be available in all ELTs. With these simulations we extend previous studies by taking into account atmospheric refraction in the transmission spectrum of the exo-Earth and observational white and red noise contributions. Our studies reveal that the number of transits necessary to detect the $O_2$ in the atmosphere of an Earth twin around M-dwarfs is by far higher than the number of transits estimated by Snellen et al. (2013). In addition, our simulations show that, when accounting for typical noise levels associated to observations in the optical and near-infrared, the $O_2$ A-band at 760 nm is more favorable to detect the exoplanetary signal than the $O_2$ band at 1268 nm for all the spectral types, except M9V. We conclude that, unless unpredicted instrumental limitations arise, the implementation of pre-slit optics such as image slicers appear to be key to significantly improve the yield of this particular science case. However, even in the most optimistic cases, we conclude that the detection of $O_2$ in the atmosphere of an Earth twin will be only feasible with the ELTs if the planet is orbiting a bright close-by (d $\le$ 8 pc) M-dwarf with a spectral type later than M3.

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arXiv2013

Towards a better understanding of the distance scale from RR Lyrae variable stars: A case study for the inner halo globular cluster NGC 6723

We present BV photometry for 54 variables in the metal-rich inner halo globular cluster NGC6723. With the discovery of new RR Lyrae variables (RRLs), we obtain = 0.541 +/- 0.066 and = 0.292 +/- 0.030 day, n(c)/n(ab+c) = 0.167, and _int = 15.459 +/- 0.055. We carry out the Fourier decomposition analysis and obtain [Fe/H]_ZW = -1.23 +/- 0.11 and E(B-V) = 0.063 +/- 0.015 for NGC 6723. By calibrating the zero-point from the recent absolute trigonometric parallax measurements for RR Lyr, we derive the revised M_V(RR)-[Fe/H] relation, providing M_V(RR)= 0.52 at [Fe/H] = -1.50 and (m-M)_0 = 18.54 for Large Magellanic Cloud (LMC), in excellent agreement with others. We obtain (m-M)_0 = 14.65 +/- 0.05, equivalent to the distance from the Sun of 8.47 +/- 0.17 kpc, for NGC 6723 from various distance measurement methods using RRLs. We find that RRLs in NGC 6723 do not have magnitude dependency on the radial distance, due to a not severe degree of the apparent crowdedness. Finally, we show that there exists a relation between the degree of photometric contamination and the apparent crowdedness of the central region of globular cluster systems. The use of this relation can play a significantly role on mitigating the discrepancy to establish a cosmic distance scale using RRLs in resolved stellar populations in the near-field cosmology.

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arXiv2013

An Earth-sized planet with an Earth-like density

Kepler-78 (KIC 8435766) was identified by Sanchis-Ojeda et al. (2013) as harbouring a transiting planet of 1.16 times the size of the Earth and an orbital period of only 8.5 hours. While the exquisite Kepler photometry was able to determine its radius and period, the mass of the planet (and thus its mean density) remained unconstrained in the absence of precise radial-velocity measurements. Here we present an accurate mass measurement of Kepler-78b using the HARPS-N spectrograph, recently installed on the Telescopio Nazionale Galileo (INAF) at the Roque de los Muchachos Observatory, La Palma, Spain. These new data yield a mass of 1.86 Earth masses. The resulting mean density of the planet is 5.57 grams per cubic centimetre, which is similar to that of the Earth and implies a composition of iron and rock. Kepler-78b, which orbits a Sun-like star called Kepler 78 located in the Cygnus constellation at a distance of about 400 light years from us, is now the smallest exoplanet for which both the mass and radius are known accurately.

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arXiv2013

Benchmark Tests for Markov Chain Monte Carlo Fitting of Exoplanet Eclipse Observations

Ground-based observations of exoplanet eclipses provide important clues to the planets' atmospheric physics, yet systematics in light curve analyses are not fully understood. It is unknown if measurements suggesting near-infrared flux densities brighter than models predict are real, or artifacts of the analysis processes. We created a large suite of model light curves, using both synthetic and real noise, and tested the common process of light curve modeling and parameter optimization with a Markov Chain Monte Carlo (MCMC) algorithm. With synthetic white-noise models, we find that input eclipse signals are generally recovered within 10% accuracy for eclipse depths greater than the noise amplitude, and to smaller depths for higher sampling rates and longer baselines. Red-noise models see greater discrepancies between input and measured eclipse signals, often biased in one direction. Finally, we find that in real data, systematic biases result even with a complex model to account for trends, and significant false eclipse signals may appear in a non-Gaussian distribution. To quantify the bias and validate an eclipse measurement, we compare both the planet-hosting star and several of its neighbors to a separately-chosen control sample of field stars. Re-examining the Rogers et al. (2009) Ks-band measurement of CoRoT-1b finds an eclipse $3190^{+370}_{-440}$ ppm deep centered at $ϕ_{me}$=$0.50418^{+0.00197}_{-0.00203}$. Finally, we provide and recommend the use of selected datasets we generated as a benchmark test for eclipse modeling and analysis routines, and propose criteria to verify eclipse detections.

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arXiv2012

Weighing The Non-Transiting Hot Jupiter Tau BOO b

We report the detection of the orbital velocity of non-transiting hot Jupiter Tau Boo b. By employing high-resolution ground-based spectroscopy around 2.3 μm during one half night, we are able to detect carbon monoxide absorption lines produced in the planet atmosphere, which shift significantly in wavelength during the course of the observations due to the orbital motion of the planet. This detection of the planetary signal results in the determination of the orbital inclination being i = 47 (+7, -6) degrees and furthermore allow us to solve for the exact planetary mass being mp = 5.6 (0.7) MJup. This clearly confirms the planetary nature of the non-transiting companion to Tau Boo.

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arXiv2012

The GJ 436 System: Directly Determined Astrophysical Parameters of an M-Dwarf and Implications for the Transiting Hot Neptune

The late-type dwarf GJ 436 is known to host a transiting Neptune-mass planet in a 2.6-day orbit. We present results of our interferometric measurements to directly determine the stellar diameter ($R_{\star} = 0.455 \pm 0.018 R_{\odot}$) and effective temperature ($T_{\rm EFF} = 3416 \pm 54$ K). We combine our stellar parameters with literature time-series data, which allows us to calculate physical and orbital system parameters, including GJ 436's stellar mass ($M_{\star} = 0.507^{+ 0.071}_{- 0.062} M_{\odot}$) and density ($ρ_* = 5.37^{+ 0.30}_{- 0.27} ρ_\odot$), planetary radius ($R_{p} = 0.369^{+ 0.015}_{- 0.015} R_{Jupiter}$), planetary mass ($M_{p} = 0.078^{+ 0.007}_{- 0.008} M_{Jupiter}$), implying a mean planetary density of $ρ_{p} = 1.55^{+ 0.12}_{- 0.10} ρ_{Jupiter}$. These values are generally in good agreement with previous literature estimates based on assumed stellar mass and photometric light curve fitting. Finally, we examine the expected phase curves of the hot Neptune GJ 436b, based on various assumptions concerning the efficiency of energy redistribution in the planetary atmosphere, and find that it could be constrained with {\it Spitzer} monitoring observations.

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arXiv2012

Modeling Multi-Wavelength Stellar Astrometry. III. Determination of the Absolute Masses of Exoplanets and Their Host Stars

Astrometric measurements of stellar systems are becoming significantly more precise and common, with many ground and space-based instruments and missions approaching 1 microarcsecond precision. We examine the multi-wavelength astrometric orbits of exoplanetary systems via both analytical formulae and numerical modeling. Exoplanets have a combination of reflected and thermally emitted light that cause the photocenter of the system to shift increasingly farther away from the host star with increasing wavelength. We find that, if observed at long enough wavelengths, the planet can dominate the astrometric motion of the system, and thus it is possible to directly measure the orbits of both the planet and star, and thus directly determine the physical masses of the star and planet, using multi-wavelength astrometry. In general, this technique works best for, though is certainly not limited to, systems that have large, high-mass stars and large, low-mass planets, which is a unique parameter space not covered by other exoplanet characterization techniques. Exoplanets that happen to transit their host star present unique cases where the physical radii of the planet and star can be directly determined via astrometry alone. Planetary albedos and day-night contrast ratios may also be probed via this technique due to the unique signature they impart on the observed astrometric orbits. We develop a tool to examine the prospects for near-term detection of this effect, and give examples of some exoplanets that appear to be good targets for detection in the K to N infrared observing bands, if the required precision can be achieved.

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arXiv2011

A Uniform Search for Secondary Eclipses of Hot Jupiters in Kepler Q2 Lightcurves

We present the results of searching the Kepler Q2 public dataset for the secondary eclipses of 76 hot Jupiter planet candidates from the list of 1,235 candidates published by Borucki et al. (2011). This search has been performed by modeling both the Kepler PDC light curves and new light curves produced via our own photometric pipeline. We derive new stellar and planetary parameters for each system with robust errors. We find 16 systems with 1-2 sigma, 14 systems with 2-3 sigma, and 6 systems with >3 sigma confidence level secondary eclipse detections in at least one light curve. We find that the majority of detected planet candidates emit more light than expected due to thermal blackbody emission in the optical Kepler bandpass, and present a trend of increasing excess emission with decreasing maximum effective planetary temperature. We explore modeling biases, significant planetary albedos, non-LTE or other thermal emission, significant internal energy generation, and exoplanet mis-identification as possible causes of both the excess emission and its correlation with expected planetary temperature. Although we find no single cause is able to explain all of the planet candidates, significant planetary albedos, with a general trend of increasing planetary albedos with decreasing atmospheric temperatures, is able to explain most of the systems. We estimate an 11% false positive rate in the current Kepler planet candidate sample of hot Jupiters. We also establish robust upper limits on the eclipse depth for all systems, and find that a significant fraction of these systems have very low albedos, significantly augmenting and extending the sample of albedo determinations to planets as cool as 1200 K. Finally, we note that continued observations with Kepler, and improved techniques for the removal of systematic noise in the Kepler data, are needed to better characterize these systems.

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arXiv2011

55 Cancri: Stellar Astrophysical Parameters, a Planet in the Habitable Zone, and Implications for the Radius of a Transiting Super-Earth

The bright star 55 Cancri is known to host five planets, including a transiting super-Earth. The study presented here yields directly determined values for 55 Cnc's stellar astrophysical parameters based on improved interferometry: $R=0.943 \pm 0.010 R_{\odot}$, $T_{\rm EFF} = 5196 \pm 24$ K. We use isochrone fitting to determine 55 Cnc's age to be 10.2 $\pm$ 2.5 Gyr, implying a stellar mass of $0.905 \pm 0.015 M_{\odot}$. Our analysis of the location and extent of the system's habitable zone (0.67--1.32 AU) shows that planet f, with period $\sim$ 260 days and $M \sin i = 0.155 M_{Jupiter}$, spends the majority of the duration of its elliptical orbit in the circumstellar habitable zone. Though planet f is too massive to harbor liquid water on any planetary surface, we elaborate on the potential of alternative low-mass objects in planet f's vicinity: a large moon, and a low-mass planet on a dynamically stable orbit within the habitable zone. Finally, our direct value for 55 Cancri's stellar radius allows for a model-independent calculation of the physical diameter of the transiting super-Earth 55 Cnc e ($\sim 2.05 \pm 0.15 R_{\earth}$), which, depending on the planetary mass assumed, implies a bulk density of 0.76 $ρ_{\earth}$ or 1.07 $ρ_{\earth}$.

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arXiv2011

Magnetic Fields in Earth-like Exoplanets and Implications for Habitability around M-dwarfs

We present estimations of dipolar magnetic moments for terrestrial exoplanets using the Olson & Christiansen (2006) scaling law and assuming their interior structure is similar to Earth. We find that the dipolar moment of fast rotating planets (where the Coriolis force dominates convection in the core), may amount up to ~80 times the magnetic moment of Earth, M_Earth, for at least part of the planets' lifetime. For very slow rotating planets (where the force of inertia dominates), the dipolar magnetic moment only reaches up to ~1.5 M_Earth. Applying our calculations to currently confirmed rocky exoplanets, we find that CoRoT-7b, Kepler-10b and 55 Cnc e can sustain dynamos up to ~ 18, 15 and 13 M_Earth, respectively. Our results also indicate that the magnetic moment of rocky exoplanets not only depends on their rotation rate, but also on their formation history, thermal state, age and composition, as well as the geometry of the field. These results apply to all rocky planets, but have important implications for the particular case of exoplanets in the Habitable Zone of M-dwarfs.

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arXiv2011

Potential Biases in the Detection of Planetary Systems with Large Transit Timing Variations

The Transit Timing Variations (TTVs) technique provides a powerful tool to detect additional planets in transiting exoplanetary systems. In this paper we show how transiting planets with significant TTVs can be systematically missed, or cataloged as false positives, by current transit search algorithms, unless they are in multi-transit systems. If the period of the TTVs, P_TTV, is longer than the time baseline of the observations and its amplitude, A_TTV, is larger than the timing precision limit of the data, transiting planet candidates are still detected, but with incorrect ephemerides. Therefore, they will be discarded during follow-up. When P_TTV is shorter than the time baseline of the observations and A_TTV is sufficiently large, constant period search algorithms find an average period for the system, which results in altered transit durations and depths in the folded light curves. Those candidates can get subsequently discarded as eclipsing binaries, grazing eclipses, or blends. Also, for large enough A_TTVs, the transits can get fully occulted by the photometric dispersion of the light curves. These detection biases could explain the observed statistical differences between the frequency of multiple systems among planets detected via other techniques and those detected via transits. We suggest that new transit search algorithms allowing for non-constant period planets should be implemented.

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arXiv2011

Astrophysical Parameters and Habitable Zone of the Exoplanet Hosting Star GJ 581

GJ 581 is an M dwarf host of a multiplanet system. We use long-baseline interferometric measurements from the CHARA Array, coupled with trigonometric parallax information, to directly determine its physical radius to be $0.299 \pm 0.010 R_{\odot}$. Literature photometry data are used to perform spectral energy distribution fitting in order to determine GJ 581's effective surface temperature $T_{\rm EFF}=3498 \pm 56$ K and its luminosity $L=0.01205 \pm 0.00024 L_{\odot}$. From these measurements, we recompute the location and extent of the system's habitable zone and conclude that two of the planets orbiting GJ 581, planets d and g, spend all or part of their orbit within or just on the edge of the habitable zone.

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arXiv2010

Transit Timing Variation Analysis of OGLE-TR-132b with Seven New Transits

We report the results of the first transit timing variation (TTV) analysis of the very hot Jupiter OGLE-TR-132b, using ten transits collected over a seven-year period. Our analysis combines three previously published transit light curves with seven new transits, which were observed between February 2008 and May 2009 with the new MagIC-e2V instrument on the Magellan Telescopes in Chile. We provide a revised planetary radius of R_p = 1.23+/-0.07 R_J, which is slightly larger than, but consistent within the errors, of the previously published results. Analysis of the planet-to-star radius ratio, orbital separation, inclination and transit duration reveals no apparent variation in any of those parameters during the time span observed. We also find no sign of transit timing variations larger than -108+/-49 s, with most residuals very close to zero. This allows us to place an upper limit of 5-10 M_Earth for a coplanar, low-eccentricity perturber in either the 2:1 or 3:2 mean-motion resonance with OGLE-TR-132b. We similarly find that the data are entirely consistent with a constant orbital period and there is no evidence for orbital decay within the limits of precision of our data.

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arXiv2010

Exoplanet atmospheres: a brand-new and rapidly expanding research field

The field of exoplanets is quickly expanding from just the detection of new planets and the measurement of their most basic parameters, such as mass, radius and orbital configuration, to the first measurements of their atmospheric characteristics, such as temperature, chemical composition, albedo, dynamics and structure. Here I will overview some of the main findings on exoplanet atmospheres until September 2010, first from space and just in the past two years also from the ground.

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arXiv2010

Habitability of Planets Orbiting Cool Stars

Terrestrial planets are more likely to be detected if they orbit M dwarfs due to the favorable planet/star size and mass ratios. However, M dwarf habitable zones are significantly closer to the star than the one around our Sun, which leads to different requirements for planetary habitability and its detection. We review 1) the current limits to detection, 2) the role of M dwarf spectral energy distributions on atmospheric chemistry, 3) tidal effects, stressing that tidal locking is not synonymous with synchronous rotation, 4) the role of atmospheric mass loss and propose that some habitable worlds may be the volatile-rich, evaporated cores of giant planets, and 5) the role of planetary rotation and magnetic field generation, emphasizing that slow rotation does not preclude strong magnetic fields and their shielding of the surface from stellar activity. Finally we present preliminary findings of the NASA Astrobiology Institute's workshop "Revisiting the Habitable Zone." We assess the recently-announced planet Gl 581 g and find no obvious barriers to habitability. We conclude that no known phenomenon completely precludes the habitability of terrestrial planets orbiting cool stars.

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