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P. Rojo

Publications and source records attributed to P. Rojo.

35 records · Page 2Linked to original sources
arXiv2013

OTS44: Disk and accretion at the planetary border

We discover that the very low-mass brown dwarf OTS44 (M9.5, ~12 M_Jup) has significant accretion and a substantial disk, which demonstrates that the processes that accompany canonical star formation occur down to a central mass of a few Jupiter masses. We discover in VLT/SINFONI spectra that OTS44 has strong, broad, and variable Paschen beta emission that is evidence for active accretion at the planetary border. We also detect strong Halpha emission of OTS44 in a literature spectrum and determine an Halpha EW (-141 A) that is indicative of active accretion. Both the Pa beta and Halpha emission lines have broad profiles with wings extending to velocities of about +/-200 km/s. We determine the mass accretion rate of OTS44 based on Halpha to 7.6x10^{-12} Msun/yr, which shows that OTS44 has a relatively high mass-accretion rate considering its small central mass. This mass rate is nevertheless consistent with the general decreasing trend found for stars of several solar masses down to brown dwarfs. Furthermore, we determine the properties of the disk surrounding OTS44 through radiative transfer modeling of flux measurement from the optical to the far-IR (Herschel) by applying a Bayesian analysis. We find that OTS44 has a highly flared disk (beta >1.2) with a mass of 9.1x10^{-5} M_Sun, i.e. about 0.1 M_Jup or 30 M_Earth. We show that the ratio of disk-to-central-mass of about 10^{-2} found for objects between 0.03 Msun and 14 Msun is also valid for OTS44 at a mass of ~0.01 M_Sun. Our observations are in line with an isolated star-like mode of the formation of brown dwarfs down to 0.01 M_Sun.

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arXiv2013

Ground-based detection of Calcium and possibly Scandium and Hydrogen in the atmosphere of HD 209458b

Context: Since the first exoplanetary atmosphere detection using the Hubble Space Telescope, characterization of exoplanet atmospheres from the ground have been playing an increasingly important role in the analysis of such atmospheres thanks to the enhancement of telluric correction techniques. At present, several species have been discovered in the atmosphere of HD 209458b, all of them consistent with theoretical models. Aims: Data acquired using the High Dispersion Spectrograph on the Subaru telescope are re-analysed. We expect to discover new species in the atmosphere of the exoplanet HD 209458b. In addition to shed light on the atmospheric composition, we will derive the radial extension of the absorbents present in the atmosphere of the exoplanet. Methods: We present an alternative method to correct the telluric effects through the analysis of variations in spectral lines with the airmass. To search absorptions due to an exoplanetary atmosphere we implemented an algorithm to automatically search for all the features presenting an atmospheric signature in the transmission spectrum and through the wavelength range in the data. In order to estimate uncertainties we perform a bootstrapping analysis. Results: Absorption excess due to the transitions of Calcium at 6162.17A and 6493.78A, Scandium at 5526.79A, Hydrogen at 6562.8A and Sodium doublet are detected in the transmission spectrum at a level of -0.079+-0.012%, -0.138+-0.013%, -0.059+-0.012%, -0.123+-0.012%, -0.071+-0.016% using pass-bands of 0.5A, 0.4A, 0.5A, 1.1A and 0.6A, respectively. Conclusions: Models predict strong absorption in the Sodium resonance Doublet which was previously detected, also in this analysis. However, this is the first report of Calcium and possibly Scandium in HD 209458b, including the possible ground-based detection of Hydrogen. ... (continues)

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arXiv2013

Study of the impact of the post-MS evolution of the host star on the orbits of close-in planets. II. A giant planet in a close-in orbit around the RGB star HIP63242

Context: More than 40 planets have been found around giant stars, revealing a lack of systems orbiting interior to $\sim$ 0.6 AU. This observational fact contrasts with the planetary population around solar-type stars and has been interpreted as the result of the orbital evolution of planets due to the interaction with the host star and/or because of a different formation/migration scenario of planets around more massive stars. Aims: We are conducting a radial velocity study of a sample of 166 giant stars aimed at studying the population of close-in planets orbiting post-main sequence stars. METHODS: We have computed precision radial velocities from multi-epoch spectroscopic data, in order to search for planets around giant stars. Results: In this paper we present the discovery of a massive planet around the intermediate-mass giant star HIP\,63242. The best keplerian fit to the data lead to an orbital distance of 0.57 AU, an eccentricity of 0.23 and a projected mass of 9.2 \mjup. HIP\,63242\,b is the innermost planet detected around any intermediate-mass giant star and also the first planet detected in our survey.

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arXiv2013

A library of near-infrared integral field spectra of young M-L dwarfs

We present a library of near-infrared (1.1-2.45 microns) medium-resolution (R~1500-2000) integral field spectra of 15 young M6-L0 dwarfs, composed of companions with known ages and of isolated objects. We use it to (re)derive the NIR spectral types, luminosities and physical parameters of the targets, and to test (BT-SETTL, DRIFT-PHOENIX) atmospheric models. We derive infrared spectral types L0+-1, L0+-1, M9.5+-0.5, M9.5+-0.5, M9.25+-0.25, M8+0.5-0.75, and M8.5+-0.5 for AB Pic b, Cha J110913-773444, USco CTIO 108B, GSC 08047-00232 B, DH Tau B, CT Cha b, and HR7329B, respectively. BT-SETTL and DRIFT-PHOENIX models yield close Teff and log g estimates for each sources. The models seem to evidence a 600-300+600 K drop of the effective temperature at the M-L transition. Assuming the former temperatures are correct, we derive new mass estimates which confirm that DH Tau B, USco CTIO 108B, AB Pic b, KPNO Tau 4, OTS 44, and Cha1109 lay inside or at the boundary of the planetary mass range. We combine the empirical luminosities of the M9.5-L0 sources to the Teff to derive semi-empirical radii estimates that do not match "hot-start" evolutionary models predictions at 1-3 Myr. We use complementary data to demonstrate that atmospheric models are able to reproduce the combined optical and infrared spectral energy distribution, together with the near-infrared spectra of these sources simultaneously. But the models still fail to represent the dominant features in the optical. This issue casts doubts on the ability of these models to predict correct effective temperatures from near-infrared spectra alone. We advocate the use of photometric and spectroscopic data covering a broad range of wavelengths to study the properties of very low mass young companions to be detected with the planet imagers (Subaru/SCExAO, LBT/LMIRCam, Gemini/GPI, VLT/SPHERE).

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arXiv2013

TraMoS project III: Improved physical parameters, timing analysis, and star-spot modelling of the WASP-4b exoplanet system from 38 transit observations

We report twelve new transit observations of the exoplanet WASP-4b from the Transit Monitoring in the South Project (TraMoS) project. These transits are combined with all previously published transit data for this planet to provide an improved radius measurement of Rp = 1.395 +- 0.022 Rjup and improved transit ephemerides. In a new homogeneous analysis in search for Transit Timing Variations (TTVs) we find no evidence of those with RMS amplitudes larger than 20 seconds over a 4-year time span. This lack of TTVs rules out the presence of additional planets in the system with masses larger than about 2.5 M_earth, 2.0 M_earth, and 1.0 M_earth around the 1:2, 5:3 and 2:1 orbital resonances. Our search for the variation of other parameters, such as orbital inclination and transit depth also yields negative results over the total time span of the transit observations. Finally we perform a simple study of stellar spots configurations of the system and conclude that the star rotational period is about 34 days.

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arXiv2013

Stellar activity as a tracer of moving groups

We present the results of a study of the stellar activity in the solar neighborhood using complete kinematics (galactocentric velocities U,V,W) and the chromospheric activity index $\log R'_{\rm{HK}}$. We analyzed the average activity level near the centers of known moving groups using a sample of 2529 stars and found that the stars near these associations tend to be more active than field stars. This supports the hypothesis that these structures, or at least a significant part of them, are composed of kinematically bound, young stars. We confirmed our results by using Galaxy Evolution Explorer (GALEX) UV data and kinematics taken from the Geneva-Copenhagen Survey for the stars in the sample. Finally, we present a compiled catalog with kinematics and activities for 2529 stars and a list of potential moving group members selected based on their stellar activity level.

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arXiv2012

ROPS: A New Search for Habitable Earths in the Southern Sky

We present the first results from our Red Optical Planet Survey (ROPS) to search for low mass planets orbiting late type dwarfs (M5.5V - M9V) in their habitable zones (HZ). Our observations, with the red arm of the MIKE spectrograph (0.5 - 0.9 microns) at the 6.5 m Magellan Clay telescope at Las Campanas Observatory indicate that >= 92 per cent of the flux lies beyond 0.7 microns. We use a novel approach that is essentially a hybrid of the simultaneous iodine and ThAr methods for determining precision radial velocities. We apply least squares deconvolution to obtain a single high S/N ratio stellar line for each spectrum and cross correlate against the simultaneously observed telluric line profile, which we derive in the same way. Utilising the 0.62 - 0.90 micron region, we have achieved an r.m.s. precision of 10 m/s for an M5.5V spectral type star with spectral S/N ~160 on 5 minute timescales. By M8V spectral type, a precision of ~30 m/s at S/N = 25 is suggested, although more observations are needed. An assessment of our errors and scatter in the radial velocity points hints at the presence of stellar radial velocity variations. Of our sample of 7 stars, 2 show radial velocity signals at 6-sigma and 10-sigma of the cross correlation uncertainties. If the signals are planetary in origin, our findings are consistent with estimates of Neptune mass planets that predict a frequency of 13 - 27 per cent for early M dwarfs.Our current analysis indicates the we can achieve a sensitivity that is equivalent to the amplitude induced by a 6 M_Earth planet orbiting in the habitable zone. Based on simulations, we estimate that <10 M_Earth habitable zone planets will be detected in a new stellar mass regime, with <=20 epochs of observations.

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arXiv2012

Transit Monitoring in the South (TraMoS) project: Discarding Transit Timing Variations in WASP-5b

We report nine new transit epochs of the extrasolar planet, observed in the Bessell-I band with SOAR at the Cerro Pachon Observatory and with the SMARTS 1-m Telescope at CTIO, between August 2008 and October 2009. The new transits have been combined with all previously published transit data for this planet to provide a new Transit Timing Variations (TTVs) analysis of its orbit. We find no evidence of TTVs RMS variations larger than 1 min over a 3 year time span. This result discards the presence of planets more massive than about 5 M_earth, 1 M_earth and 2 M_earth around the 1:2, 5:3 and 2:1 orbital resonances. These new detection limits exceed by ~5-30 times the limits imposed by current radial velocity observations in the Mean Motion Resonances of this system. Our search for the variation of other parameters, such as orbital inclination and transit depth also yields negative results over the total time span of the transit observations. This result supports formation theories that predict a paucity of planetary companions to Hot Jupiters.

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arXiv2011

Study of the impact of the post-MS evolution of the host star on the orbits of close-in planets. I. Sample definition and physical properties

Context: To date, more than 30 planets have been discovered around giant stars, but only one of them has been found to be orbiting within 0.6 AU from the host star, in direct contrast to what is observed for FGK dwarfs. This result suggests that evolved stars destroy/engulf close-in planets during the red giant phase. Aims: We are conducting a radial velocity survey of 164 bright G and K giant stars in the southern hemisphere with the aim of studying the effect of the host star evolution on the inner structure of planetary systems. In this paper we present the spectroscopic atmospheric parameters (\Teff, \logg, $ξ$, [Fe/H]) and the physical properties (mass, radius, evolutionary status) of the program stars. In addition, rotational velocities for all of our targets were derived. Methods: We used high resolution and high S/N spectra to measure the equivalent widths of many Fe{\sc\,i} and Fe{\sc\,ii} lines, which were used to derive the atmospheric parameters by imposing local thermodynamic and ionization equilibrium. The effective temperatures and metallicities were used, along with stellar evolutionary tracks to determine the physical properties and evolutionary status of each star. Results: We found that our targets are on average metal rich and they have masses between $\sim$\,1.0\,M$_\odot$ and 3.5\,M$_\odot$. In addition, we found that 122 of our targets are ascending the RGB, while 42 of them are on the HB phase.

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arXiv2011

Twenty-One New Light Curves of OGLE-TR-56b: New System Parameters and Limits on Timing Variations

Although OGLE-TR-56b was the second transiting exoplanet discovered, only one light curve, observed in 2006, has been published besides the discovery data. We present twenty-one light curves of nineteen different transits observed between July 2003 and July 2009 with the Magellan Telescopes and Gemini South. The combined analysis of the new light curves confirms a slightly inflated planetary radius relative to model predictions, with R_p = 1.378 +/- 0.090 R_J. However, the values found for the transit duration, semimajor axis, and inclination values differ significantly from the previous result, likely due to systematic errors. The new semimajor axis and inclination, a = 0.01942 +/- 0.00015 AU and i = 73.72 +/- 0.18 degrees, are smaller than previously reported, while the total duration, T_14 = 7931 +/- 38 s, is 18 minutes longer. The transit midtimes have errors from 23 s to several minutes, and no evidence is seen for transit midtime or duration variations. Similarly, no change is seen in the orbital period, implying a nominal stellar tidal decay factor of Q_* = 10^7, with a three-sigma lower limit of 10^5.7.

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arXiv2011

Chromospheric Activities and Kinematics for Solar Type Dwarfs and Subgiants: Analysis of the Activity Distribution and the AVR

In this work we present chromospheric activity indices, kinematics, radial-velocities and rotational velocities for more than 850 FGK-type dwarfs and subgiant stars in the southern hemisphere and test how best to calibrate and measure S-indices from echelle spectra. We confirm the bimodal distribution of chromospheric activities for such stars and highlight the role that the more active K-dwarfs play in biasing the number of active stars. We show that the age-activity relationship does appear to continue to ages older than the Sun if we simply compare main sequence stars and subgiant stars, with an offset of around 2.5 Gyrs between the peaks of both distributions. Also we show evidence for an increased spin-down timescale for cool K dwarfs compared with earlier F and G type stars. In addition, we show how kinematics can be used to preselect inactive stars for future planet search projects. We see the well known trend between projected rotational velocity and activity, however we also find a correlation between kinematic space velocity and chromospheric activity. It appears that after the Vaughan-Preston gap there is a quick step function in the kinematic space motion towards a significantly larger spread in velocities. We speculate on reasons for this correlation and provide some model scenarios to describe the bimodal activity distribution through magnetic saturation, residual low level gas accretion or accretion by the star of planets or planetesimals. Finally, we provide a new empirical measurement for the disk heating law, using the latest age-activity relationships to reconstruct the age-velocity distribution for local disk stars. We find a value of 0.337+/-0.045 for the exponent of this power law (i.e. sigma_tot proportional to t^0.337), in excellent agreement with those found using isochrone fitting methods and with theoretical disk heating models. [Abridged]

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arXiv2011

Study of Planetary Systems around Giant Stars

More than 450 exoplanets have currently been detected, most of them by the radial velocity (RV) technique. While the majority of exoplanets have been found around main-sequence (MS) FGK stars (M 1.5M*), only a small fraction (- 10%) have been discovered orbiting post- MS stars. However, such stars are known to host exoplanets and the detection fraction appears to be larger than for solar-type dwarfs. To date - 30 planets have been found orbiting giant stars which have revealed interesting properties that contrast with the results found for solar-type stars. We are carrying out a RV search for planets around giant stars in the southern hemisphere in order to study different formation scenarios for planets around intermediate-mass stars and the effect of the post-MS evolution of the host stars on the orbits of close-in planets (a 0.6 AU).

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arXiv2010

A Cold Neptune-Mass Planet OGLE-2007-BLG-368Lb: Cold Neptunes Are Common

We present the discovery of a Neptune-mass planet OGLE-2007-BLG-368Lb with a planet-star mass ratio of q=[9.5 +/- 2.1] x 10^{-5} via gravitational microlensing. The planetary deviation was detected in real-time thanks to the high cadence of the MOA survey, real-time light curve monitoring and intensive follow-up observations. A Bayesian analysis returns the stellar mass and distance at M_l = 0.64_{-0.26}^{+0.21} M_\sun and D_l = 5.9_{-1.4}^{+0.9} kpc, respectively, so the mass and separation of the planet are M_p = 20_{-8}^{+7} M_\oplus and a = 3.3_{-0.8}^{+1.4} AU, respectively. This discovery adds another cold Neptune-mass planet to the planetary sample discovered by microlensing, which now comprise four cold Neptune/Super-Earths, five gas giant planets, and another sub-Saturn mass planet whose nature is unclear. The discovery of these ten cold exoplanets by the microlensing method implies that the mass ratio function of cold exoplanets scales as dN_{\rm pl}/d\log q \propto q^{-0.7 +/- 0.2} with a 95% confidence level upper limit of n < -0.35 (where dN_{\rm pl}/d\log q \propto q^n). As microlensing is most sensitive to planets beyond the snow-line, this implies that Neptune-mass planets are at least three times more common than Jupiters in this region at the 95% confidence level.

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arXiv2010

Near-infrared integral-field spectra of the planet/brown dwarf companion AB Pic b

Chauvin et al. 2005 imaged a co-moving companion at ~260 AU from the young star AB Pic A. Evolutionary models predictions based on J H K photometry of AB Pic b suggested a mass of ~13 - 14 MJup, placing the object at the deuterium-burning boundary. We used the adaptive-optics-fed integral field spectrograph SINFONI to obtain high quality medium-resolution spectra of AB Pic b (R = 1500-2000) over the 1.1 - 2.5 microns range. Our analysis relies on the comparison of our spectra to young standard templates and to the latest libraries of synthetic spectra developed by the Lyon's Group. AB Pic b is confirmed to be a young early-L dwarf companion. We derive a spectral type L0-L1 and find several features indicative of intermediate gravity atmosphere. A comparison to synthetic spectra yields Teff = 2000+100-300 K and log(g) = 4 +- 0.5 dex. The determination of the derived atmospheric parameters of AB Pic b is limited by a non-perfect match of current atmosphere spectra with our near-infrared observations of AB Pic b. The current treatment of dust settling and missing molecular opacity lines in the atmosphere models could be responsible. By combining the observed photometry, the surface fluxes from atmosphere models and the known distance of the system, we derive new mass, luminosity and radius estimates of AB Pic b. They confirm independently the evolutionary model predictions. We finally review the current methods used to characterize planetary mass companions and discuss them in the perspective of future planet deep imaging surveys.

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arXiv2009

The Calan-Hertfordshire Extrasolar Planet Search

The detailed study of the exoplanetary systems HD189733 and HD209458 has given rise to a wealth of exciting information on the physics of exoplanetary atmospheres. To further our understanding of the make-up and processes within these atmospheres we require a larger sample of bright transiting planets. We have began a project to detect more bright transiting planets in the southern hemisphere by utilising precision radial-velocity measurements. We have observed a constrained sample of bright, inactive and metal-rich stars using the HARPS instrument and here we present the current status of this project, along with our first discoveries which include a brown dwarf/extreme-Jovian exoplanet found in the brown dwarf desert region around the star HD191760 and improved orbits for three other exoplanetary systems HD48265, HD143361 and HD154672. Finally, we briefly discuss the future of this project and the current prospects we have for discovering more bright transiting planets.

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arXiv2008

A Metal-biased Planet Search

We have begun a metal-rich planet search project using the HARPS instrument in La Silla, Chile to target planets with a high potential to transit their host star and add to the number of bright benchmark transiting planets. The sample currently consists of 100, bright (7.5 </= V </= 9.5) solar-type stars (0.5 </= B-V </= 0.9) in the southern hemisphere which are both inactive (logR'HK </= -4.5) and metal-rich ([Fe/H] >/= 0.1 dex). We determined the chromospheric activity and metallicity status of our sample using high resolution FEROS spectra. We also introduce the first result from our HARPS planet search and show that the radial-velocity amplitude of this star is consistent with an orbiting planetary-mass companion (i.e. Msini < 0.5MJ) with a period of ~5 days. We are currently engaged in follow-up to confirm this signal as a bonafide orbiting planet.

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