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Post Info TOPIC: XO-2b
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XO-2 binary system
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Title: The dissimilar chemical composition of the planet-hosting stars of the XO-2 binary system
Author: I. Ramirez, S. Khanal, P.Aleo, A. Sobotka, F. Liu, L. Casagrande, J. Melendez, D. Yong, D. L. Lambert, M. Asplund

Using high-quality spectra of the twin stars in the XO-2 binary system, we have detected significant differences in the chemical composition of their photospheres. The differences correlate strongly with the elements' dust condensation temperature. In XO-2N, volatiles are enhanced by about 0.015 dex and refractories are overabundant by up to 0.090 dex. On average, our error bar in relative abundance is 0.012 dex. We present an early metal-depletion scenario in which the formation of the gas giant planets known to exist around these stars is responsible for a 0.015 dex offset in the abundances of all elements while 20 M_Earth of non-detected rocky objects that formed around XO-2S explain the additional refractory-element difference. An alternative explanation involves the late accretion of at least 20 M_Earth of planet-like material by XO-2N, allegedly as a result of the migration of the hot Jupiter detected around that star. Dust cleansing by a nearby hot star as well as age or Galactic birthplace effects can be ruled out as valid explanations for this phenomenon.

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Blobrana


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RE: XO-2b
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Title: XO-2b: a hot Jupiter with a variable host star that potentially affects its measured transit depth
Author: Robert T. Zellem, Caitlin A. Griffith, Kyle A. Pearson, Jake D. Turner, Gregory W. Henry, Michael W. Williamson, M. Ryleigh Fitzpatrick, Johanna K. Teske, Lauren I. Biddle

The transiting hot Jupiter XO-2b is an ideal target for multi-object photometry and spectroscopy as it has a relatively bright (V-mag = 11.25) K0V host star (XO-2N) and a large planet-to-star contrast ratio (Rp/Rs~0.015). It also has a nearby (31.21") binary stellar companion (XO-2S) of nearly the same brightness (V-mag = 11.20) and spectral type (G9V), allowing for the characterization and removal of shared systematic errors (e.g., airmass brightness variations). We have therefore conducted a multiyear (2012--2015) study of XO-2b with the University of Arizona's 61" (1.55~m) Kuiper Telescope and Mont4k CCD in the Bessel U and Harris B photometric passbands to measure its Rayleigh scattering slope to place upper limits on the pressure-dependent radius at, e.g., 10~bar. Such measurements are needed to constrain its derived molecular abundances from primary transit observations. We have also been monitoring XO-2N since the 2013--2014 winter season with Tennessee State University's Celestron-14 (0.36~m) automated imaging telescope to investigate stellar variability, which could affect XO-2b's transit depth. Our observations indicate that XO-2N is variable, potentially due to spots, with a peak-to-peak amplitude of 0.00455±0.00090~R-mag and period of 29.87±0.19 days. Due to the likely influence of XO-2N's variability on the derivation of XO-2b's transit depth, we cannot bin multiple nights of data to decrease our uncertainties, preventing us from constraining its gas abundances. This study demonstrates that long-term monitoring programs of exoplanet host stars are crucial for understanding host star variability.

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Blobrana


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Title: APOSTLE: Longterm Transit Monitoring and Stability Analysis of XO-2b
Authors: Praveen Kundurthy, Rory Barnes, Andrew C. Becker, Eric Agol, Benjamin F. Williams, Noel Gorelick, Amy Rose

The Apache Point Survey of Transit Lightcurves of Exoplanets (APOSTLE) observed 10 transits of XO-2b over a period of three years. We present measurements which confirm previous estimates of system parameters like the normalised semi-major axis (a/R_{*}), stellar density (rho_{*}), impact parameter (b) and orbital inclination (i_{orb}). Our errors on system parameters like a/R_{*} and rho_{*} have improved by ~40% compared to previous best ground-based measurements. Our study of the transit times show no evidence for transit timing variations and we are able to rule out co-planar companions with masses \ge 0.20 Earth masses in low order mean motion resonance with XO-2b. We also explored the stability of the XO-2 system given various orbital configurations of a hypothetical planet near the 2:1 mean motion resonance. We find that a wide range of orbits (including Earth-mass perturbers) are both dynamically stable and produce observable TTVs. We find that up to 51% of our stable simulations show TTVs that are smaller than the typical transit timing errors (~20 sec) measured for XO-2b, and hence remain undetectable.

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Title: Transmission spectroscopy of exoplanet XO-2b observed with HST NICMOS
Authors: Nicolas Crouzet, Peter R. McCullough, Christopher J. Burke, Douglas Long

Spectroscopy during planetary transits is a powerful tool to probe exoplanet atmospheres. We present the near-infrared transit spectroscopy of XO-2b obtained with HST NICMOS. Uniquely for NICMOS transit spectroscopy, a companion star of similar properties to XO-2 is present in the field of view. We derive improved star and planet parameters through a photometric white-light analysis. We show a clear correlation of the spectrum noise with instrumental parameters, in particular the angle of the spectral trace on the detector. An MCMC method using a decorrelation from instrumental parameters is used to extract the planetary spectrum. Spectra derived independently from each of the 3 visits have a RMS of 430, 510, and 1000 ppm respectively. The same analysis is performed on the companion star after numerical injection of a transit with a depth constant at all wavelengths. The extracted spectra exhibit residuals of similar amplitude as for XO-2, which represent the level of remaining NICMOS systematics. This shows that extracting planetary spectra is at the limit of NICMOS' capability. We derive a spectrum for the planet XO-2b using the companion star as a reference. The derived spectrum can be represented by a theoretical model including atmospheric water vapour or by a flat spectrum model. We derive a 3-sigma upper limit of 1570 ppm on the presence of water vapour absorption in the atmosphere of XO-2b. In an appendix, we perform a similar analysis for the gas giant planet XO-1b.

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Title: GTC OSIRIS transiting exoplanet atmospheric survey: detection of sodium in XO-2b from differential long-slit spectroscopy
Authors: D. K. Sing, C. M. Huitson, M. Lopez-Morales, F. Pont, J.-M. Désert, D. Ehrenreich, P. A. Wilson, G. E. Ballester, J. J. Fortney, A. Lecavelier des Etangs, A. Vidal-Madjar

We present two transits of the hot-Jupiter exoplanet XO-2b using the Gran Telescopio Canarias (GTC). The time series observations were performed using long-slit spectroscopy of XO-2 and a nearby reference star with the OSIRIS instrument, enabling differential specrophotometric transit lightcurves capable of measuring the exoplanet's transmission spectrum. Two optical low-resolution grisms were used to cover the optical wavelength range from 3800 to 9300{\AA}. We find that sub-mmag level slit losses between the target and reference star prevent full optical transmission spectra from being constructed, limiting our analysis to differential absorption depths over ~1000{\AA} regions. Wider long slits or multi-object grism spectroscopy with wide masks will likely prove effective in minimising the observed slit-loss trends. During both transits, we detect significant absorption in the planetary atmosphere of XO-2b using a 50{\AA} bandpass centred on the Na I doublet, with absorption depths of Delta(R_pl/R_star)^2=0.049±0.017 % using the R500R grism and 0.047±0.011 % using the R500B grism (combined 5.2-sigma significance from both transits). The sodium feature is unresolved in our low-resolution spectra, with detailed modelling also likely ruling out significant line-wing absorption over an ~800{\AA} region surrounding the doublet. Combined with narrowband photometric measurements, XO-2b is the first hot Jupiter with evidence for both sodium and potassium present in the planet's atmosphere.

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Title: XO-2b: a Prograde Planet with a Negligible Eccentricity, and an Additional Radial Velocity Variation
Authors: Norio Narita, Teruyuki Hirano, Bun'ei Sato, Hiroki Harakawa, Akihiko Fukui, Wako Aoki, Motohide Tamura

We present precise radial velocities of XO-2 taken with the Subaru HDS, covering two transits of XO-2b with an interval of nearly two years. The data suggest that the orbital eccentricity of XO-2b is consistent with zero within 2 \sigma (e=0.045±0.024) and the orbit of XO-2b is prograde (the sky-projected spin-orbit alignment angle \lambda=10° ±72°). The poor constraint of \lambda is due to a small impact parameter (the orbital inclination of XO-2b is almost 90°. The data also provide an improved estimate of the mass of XO-2b as 0.62±0.02 Jupiter masses. We also find a long-term radial velocity variation in this system. Further radial velocity measurements are necessary to specify the cause of this additional variation.

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