A Search for a Sub-Earth Sized Companion to GJ 436 and a Novel Method to Calibrate Warm Spitzer IRAC Observations

We discovered evidence for a possible additional 0.75 R_Earth transiting planet in the NASA EPOXI observations of the known M dwarf exoplanetary system GJ 436. Based on an ephemeris determined from the EPOXI data, we predicted a transit event in an extant Spitzer Space Telescope 8 micron data set of this star. Our subsequent analysis of those Spitzer data confirmed the signal of the predicted depth and at the predicted time, but we found that the transit depth was dependent on the aperture used to perform the photometry. Based on these suggestive findings, we gathered new Warm Spitzer Observations of GJ 436 at 4.5 microns spanning a time of transit predicted from the EPOXI and Spitzer 8 micron candidate events. The 4.5 micron data permit us to rule out a transit at high confidence, and we conclude that the earlier candidate transit signals resulted from correlated noise in the EPOXI and Spitzer 8 micron observations. In the course of this investigation, we developed a novel method for correcting the intrapixel sensitivity variations of the 3.6 and 4.5 micron channels of the Infrared Array Camera (IRAC) instrument. We demonstrate the sensitivity of Warm Spitzer observations of M dwarfs to confirm sub-Earth sized planets. Our analysis will inform similar work that will be undertaken to use Warm Spitzer observations to confirm rocky planets discovered by the Kepler mission.Comment: 22 pages, 8 figures, accepted for publication in PAS

Large Uncertainties in the Thermodynamics of Phosphorus (III) Oxide (P4_4O6_6) Have Significant Implications for Phosphorus Species in Planetary Atmospheres

Phosphorus (III) oxide (P$_4$O$_6$) has been suggested to be a major component of the gas phase phosphorus chemistry in the atmospheres of gas giant planets and of Venus. However, P$_4$O$_6$'s proposed role is based on thermodynamic modeling, itself based on values for the free energy of formation of P$_4$O$_6$ estimated from limited experimental data. Values of the standard Gibbs free energy of formation ($\Delta$Go(g)) of P$_4$O$_6$ in the literature differ by up to ~656 kJ/mol, a huge range. Depending on which value is assumed, P$_4$O$_6$ may either be the majority phosphorus species present or be completely absent from modeled atmospheres. Here, we critically review the literature thermodynamic values and compare their predictions to observed constraints on P$_4$O$_6$ geochemistry. We conclude that the widely used values from the NIST/JANAF database are almost certainly too low (predicting that P$_4$O$_6$ is more stable than is plausible). We show that, regardless of the value of $\Delta$Go(g) for P$_4$O$_6$ assumed, the formation of phosphine from P$_4$O$_6$ in the Venusian atmosphere is thermodynamically unfavorable. We conclude that there is a need for more robust data on both the thermodynamics of phosphorus chemistry for astronomical and geological modeling in general and for understanding the atmosphere of Venus and the gas giant planets in particular.Comment: Article published in ACS Earth Space Chem. https://pubs.acs.org/doi/full/10.1021/acsearthspacechem.3c0001

Near-infrared transit photometry of the exoplanet HD 149026b

The transiting exoplanet HD 149026b is an important case for theories of planet formation and planetary structure, because the planet's relatively small size has been interpreted as evidence for a highly metal-enriched composition. We present observations of 4 transits with the Near Infrared Camera and Multi-Object Spectrometer on the Hubble Space Telescope, within a wavelength range of 1.1--2.0 $\mu$m. Analysis of the light curve gives the most precise estimate yet of the stellar mean density, $\rho_\star = 0.497^{+0.042}_{-0.057}$ g cm$^{-3}$. By requiring agreement between the observed stellar properties (including $\rho_\star$) and stellar evolutionary models, we refine the estimate of the stellar radius: $R_\star = 1.541^{+0.046}_{-0.042}$ R_\sun. We also find a deeper transit than has been measured at optical and mid-infrared wavelengths. Taken together, these findings imply a planetary radius of $R_p = 0.813^{+0.027}_{-0.025}$ $R_{\rm Jup}$, which is larger than earlier estimates. Models of the planetary interior still require a metal-enriched composition, although the required degree of metal enrichment is reduced. It is also possible that the deeper NICMOS transit is caused by wavelength-dependent absorption by constituents in the planet's atmosphere, although simple model atmospheres do not predict this effect to be strong enough to account for the discrepancy. We use the 4 newly-measured transit times to compute a refined transit ephemeris.Comment: 18 pages, 13 figures, accepted for publication in Ap

Combination of ablation and embolization for intermediate-sized liver metastases from colorectal cancer: what can we learn from treating primary liver cancer?

Colorectal cancer liver metastases (CRLMs) are common. Treating CRLMs with thermal ablation can prolong survival, but compared to lesions smaller than 3 cm, local control rates and overall survival are relatively worse with larger, intermediate (3–5 cm) lesions. Local recurrence rates range between 1.7%–20.2% and 6.7%–68.9% for CRLMs less than 3 cm and greater than 3 cm, respectively. Worse outcomes are also present when ablating intermediate size hepatocellular carcinoma (HCC) and there are some pathological similarities with CRLMs, namely the presence of micrometastatic disease. Combining ablation with transarterial chemoembolization is more effective in treating intermediate-size HCC than ablation alone. A meta-analysis of robust randomized controlled trials demonstrated long-term improved survival with combination therapy compared to ablation alone (odds ratio at 1, 3 and 5 years of 2.74, 2.77 and 5.23, respectively). There is, however, minimal evidence for combination therapy in CRLMs, limited to a handful of studies that are predominantly retrospective and have heterogeneous inclusion criteria. Given the difficulty in successfully treating intermediate CRLMs, the strong evidence for combination therapy in intermediate HCC and potential pathological similarities, formal evaluation of combination treatment in CRLM is merited. This review highlights existing evidence for treatment of intermediate-size liver lesions and highlights where trials in CRLMs should focus

The Transit Light Curve Project. XII. Six Transits of the Exoplanet XO-2b

We present photometry of six transits of the exoplanet XO-2b. By combining the light-curve analysis with theoretical isochrones to determine the stellar properties, we find the planetary radius to be 0.996 +0.031/-0.018 rjup and the planetary mass to be 0.565 +/- 0.054 mjup. These results are consistent with those reported previously, and are also consistent with theoretical models for gas giant planets. The mid-transit times are accurate to within 1 min and are consistent with a constant period. However, the period we derive differs by 2.5 sigma from the previously published period. More data are needed to tell whether the period is actually variable (as it would be in the presence of an additional body) or if the timing errors have been underestimated.Comment: Accepted for publication in AJ. 20 pages, 3 tables, 4 figure

The NASA-UC Eta-Earth Program: I. A Super-Earth Orbiting HD 7924

We report the discovery of the first low-mass planet to emerge from the NASA-UC Eta-Earth Program, a super-Earth orbiting the K0 dwarf HD 7924. Keplerian modeling of precise Doppler radial velocities reveals a planet with minimum mass M_P sin i = 9.26 M_Earth in a P = 5.398 d orbit. Based on Keck-HIRES measurements from 2001 to 2008, the planet is robustly detected with an estimated false alarm probability of less than 0.001. Photometric observations using the Automated Photometric Telescopes at Fairborn Observatory show that HD 7924 is photometrically constant over the radial velocity period to 0.19 mmag, supporting the existence of the planetary companion. No transits were detected down to a photometric limit of ~0.5 mmag, eliminating transiting planets with a variety of compositions. HD 7924b is one of only eight planets known with M_P sin i < 10 M_Earth and as such is a member of an emerging family of low-mass planets that together constrain theories of planet formation.Comment: ApJ accepted, 10 pages, 10 figures, 4 table

The growth and hydrodynamic collapse of a protoplanet envelope

We have conducted three-dimensional self-gravitating radiation hydrodynamical models of gas accretion onto high mass cores (15-33 Earth masses) over hundreds of orbits. Of these models, one case accretes more than a third of a Jupiter mass of gas, before eventually undergoing a hydrodynamic collapse. This collapse causes the density near the core to increase by more than an order of magnitude, and the outer envelope to evolve into a circumplanetary disc. A small reduction in the mass within the Hill radius (R_H) accompanies this collapse as a shock propagates outwards. This collapse leads to a new hydrostatic equilibrium for the protoplanetary envelope, at which point 97 per cent of the mass contained within the Hill radius is within the inner 0.03 R_H which had previously contained less than 40 per cent. Following this collapse the protoplanet resumes accretion at its prior rate. The net flow of mass towards this dense protoplanet is predominantly from high latitudes, whilst at the outer edge of the circumplanetary disc there is net outflow of gas along the midplane. We also find a turnover of gas deep within the bound envelope that may be caused by the establishment of convection cells.Comment: 16 pages, 16 figures. Accepted for publication in MNRA

Preliminary Results on HAT-P-4, TrES-3, XO-2, and GJ 436 from the NASA EPOXI Mission

EPOXI (EPOCh + DIXI) is a NASA Discovery Program Mission of Opportunity using the Deep Impact flyby spacecraft. The EPOCh (Extrasolar Planet Observation and Characterization) Science Investigation will gather photometric time series of known transiting exoplanet systems from January through August 2008. Here we describe the steps in the photometric extraction of the time series and present preliminary results of the first four EPOCh targets.Comment: 4 pages, 2 figures. To appear in the Proceedings of the 253rd IAU Symposium: "Transiting Planets", May 2008, Cambridge, M

Five Kepler target stars that show multiple transiting exoplanet candidates

We present and discuss five candidate exoplanetary systems identified with the Kepler spacecraft. These five systems show transits from multiple exoplanet candidates. Should these objects prove to be planetary in nature, then these five systems open new opportunities for the field of exoplanets and provide new insights into the formation and dynamical evolution of planetary systems. We discuss the methods used to identify multiple transiting objects from the Kepler photometry as well as the false-positive rejection methods that have been applied to these data. One system shows transits from three distinct objects while the remaining four systems show transits from two objects. Three systems have planet candidates that are near mean motion commensurabilities---two near 2:1 and one just outside 5:2. We discuss the implications that multitransiting systems have on the distribution of orbital inclinations in planetary systems, and hence their dynamical histories; as well as their likely masses and chemical compositions. A Monte Carlo study indicates that, with additional data, most of these systems should exhibit detectable transit timing variations (TTV) due to gravitational interactions---though none are apparent in these data. We also discuss new challenges that arise in TTV analyses due to the presence of more than two planets in a system.Comment: Accepted to Ap