arXiv subjects

Jacob L. Bean

Publications and source records attributed to Jacob L. Bean.

At least 145 records · Page 8Linked to original sources

arXiv2019

Constraining the evolution of stellar rotation using solar twins

The stellar Rotation $vs.$ Age relation is commonly considered as a useful tool to derive reliable ages for Sun-like stars. However, in the light of \kepler\ data, the presence of apparently old and fast rotators that do not obey the usual gyrochronology relations led to the hypothesis of weakened magnetic breaking in some stars. In this letter, we constrain the solar rotation evolutionary track using solar twins. Predicted rotational periods as a function of mass, age, [Fe/H] and given critical Rossby number ($Ro_{\rm crit}$) were estimated for the entire rotational sample. Our analysis favors the smooth rotational evolution scenario and suggests that, if the magnetic weakened breaking scenario takes place at all, it should arise after $Ro_{\rm crit}\gtrsim2.29$ or ages $\gtrsim$5.3 Gyr (at 95$\%$ confidence level).

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arXiv2019

Predicted Yield of Transits of Known Radial Velocity Exoplanets from the TESS Primary and Extended Missions

Radial velocity (RV) surveys have detected hundreds of exoplanets through their gravitational interactions with their host stars. Some will be transiting, but most lack sufficient follow-up observations to confidently detect (or rule out) transits. We use published stellar, orbital, and planetary parameters to estimate the transit probabilities for nearly all exoplanets that have been discovered via the RV method. From these probabilities, we predict that $25.5^{+0.7}_{-0.7}$ of the known RV exoplanets should transit their host stars. This prediction is more than double the amount of RV exoplanets that are currently known to transit. The Transiting Exoplanet Survey Satellite (TESS) presents a valuable opportunity to explore the transiting nature of many of the known RV exoplanet systems. Based on the anticipated pointing of TESS during its two-year primary mission, we identify the known RV exoplanets that it will observe and predict that $11.7^{+0.3}_{-0.3}$ of them will have transits detected by TESS. However, we only expect the discovery of transits for $\sim$3 of these exoplanets to be novel (i.e., not previously known). We predict that the TESS photometry will yield dispositive null results for the transits of $\sim$125 RV exoplanets. This will represent a substantial increase in the effort to refine ephemerides of known RV exoplanets. We demonstrate that these results are robust to changes in the ecliptic longitudes of future TESS observing sectors. Finally, we consider how several potential TESS extended mission scenarios affect the number of transiting RV exoplanets we expect TESS to observe.

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arXiv2018

Detection of Helium in the Atmosphere of the Exo-Neptune HAT-P-11b

The helium absorption triplet at a wavelength of 10,833 Å has been proposed as a way to probe the escaping atmospheres of exoplanets. Recently this feature was detected for the first time using Hubble Space Telescope (HST) WFC3 observations of the hot Jupiter WASP-107b. We use similar HST/WFC3 observations to detect helium in the atmosphere of the hot Neptune HAT-P-11b at the $4σ$ confidence level. We compare our observations to a grid of 1D models of hydrodynamic escape to constrain the thermospheric temperatures and mass loss rate. We find that our data are best fit by models with high mass loss rates of $\dot{M} \approx 10^{9}$ - $10^{11}$ g s$^{-1}$. Although we do not detect the planetary wind directly, our data are consistent with the prediction that HAT-P-11b is experiencing hydrodynamic atmospheric escape. Nevertheless, the mass loss rate is low enough that the planet has only lost up to a few percent of its mass over its history, leaving its bulk composition largely unaffected. This matches the expectation from population statistics, which indicate that close-in planets with radii greater than 2 R$_{\oplus}$ form and retain H/He-dominated atmospheres. We also confirm the independent detection of helium in HAT-P-11b obtained with the CARMENES instrument, making this the first exoplanet with the detection of the same signature of photoevaporation from both ground- and space-based facilities.

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arXiv2018

Thorium in solar twins: implications for habitability in rocky planets

We have investigated the thorium (Th) abundance in a sample of 53 thin disc solar twins covering a wide range of ages. These data provide constrains on the mantle energy budget of terrestrial planets that can be formed over the evolution of the Galaxy's thin disc. We have estimated Th abundances with an average precision of 0.025\,dex (in both [Th/H] and [Th/Fe]) through comprehensive spectral synthesis of a Th\,II line present at 4019.1290\,Å, using very high resolution (R\,=\,115,000) high quality HARPS spectra obtained at the ESO La Silla Observatory. We have confirmed that there is a large energy budget from Th decay for maintaining mantle convection inside potential rocky planets around solar twins, from the Galactic thin disc formation until now, because the pristine [Th/H]$_{\rm ZAMS}$ is super-solar on average under a uniform dispersion of 0.056\,dex (varying from +0.037 up to +0.138\,dex based on linear fits against isochrone stellar age). Comparing to neodymium (Nd) and europium (Eu), two others neutron-capture elements, the stellar pristine abundance of Th follows Eu along the Galactic thin disc evolution, but it does not follow Nd, probably because neodymium has a significant contribution from the $s$-process (about 60\,per\,cent).

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arXiv2018

A Framework for Prioritizing the TESS Planetary Candidates Most Amenable to Atmospheric Characterization

A key legacy of the recently launched TESS mission will be to provide the astronomical community with many of the best transiting exoplanet targets for atmospheric characterization. However, time is of the essence to take full advantage of this opportunity. JWST, although delayed, will still complete its nominal five year mission on a timeline that motivates rapid identification, confirmation, and mass measurement of the top atmospheric characterization targets from TESS. Beyond JWST, future dedicated missions for atmospheric studies such as ARIEL require the discovery and confirmation of several hundred additional sub-Jovian size planets (R_p < 10 R_Earth) orbiting bright stars, beyond those known today, to ensure a successful statistical census of exoplanet atmospheres. Ground-based ELTs will also contribute to surveying the atmospheres of the transiting planets discovered by TESS. Here we present a set of two straightforward analytic metrics, quantifying the expected signal-to-noise in transmission and thermal emission spectroscopy for a given planet, that will allow the top atmospheric characterization targets to be readily identified among the TESS planet candidates. Targets that meet our proposed threshold values for these metrics would be encouraged for rapid follow-up and confirmation via radial velocity mass measurements. Based on the catalog of simulated TESS detections by Sullivan et al. (2015), we determine appropriate cutoff values of the metrics, such that the TESS mission will ultimately yield a sample of $\sim300$ high-quality atmospheric characterization targets across a range of planet size bins, extending down to Earth-size, potentially habitable worlds.

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arXiv2018

The Transiting Exoplanet Community Early Release Science Program for JWST

The James Webb Space Telescope (JWST) presents the opportunity to transform our understanding of planets and the origins of life by revealing the atmospheric compositions, structures, and dynamics of transiting exoplanets in unprecedented detail. However, the high-precision, time-series observations required for such investigations have unique technical challenges, and prior experience with other facilities indicates that there will be a steep learning curve when JWST becomes operational. In this paper we describe the science objectives and detailed plans of the Transiting Exoplanet Community Early Release Science (ERS) Program, which is a recently approved program for JWST observations early in Cycle 1. The goal of this project, for which the obtained data will have no exclusive access period, is to accelerate the acquisition and diffusion of technical expertise for transiting exoplanet observations with JWST, while also providing a compelling set of representative datasets that will enable immediate scientific breakthroughs. The Transiting Exoplanet Community ERS Program will exercise the time-series modes of all four JWST instruments that have been identified as the consensus highest priorities, observe the full suite of transiting planet characterization geometries (transits, eclipses, and phase curves), and target planets with host stars that span an illustrative range of brightnesses. The observations in this program were defined through an inclusive and transparent process that had participation from JWST instrument experts and international leaders in transiting exoplanet studies. Community engagement in the project will be centered on a two-phase Data Challenge that culminates with the delivery of planetary spectra, time-series instrument performance reports, and open-source data analysis toolkits in time to inform the agenda for Cycle 2 of the JWST mission.

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arXiv2018

Community targets for JWST's early release science program: evaluation of WASP-63b

We present observations of WASP-63b by the Hubble Space Telescope (HST) as part of "A Preparatory Program to Identify the Single Best Transiting Exoplanet for JWST Early Release Science". WASP-63b is one of the community targets under consideration for the James Webb Space Telescope (JWST) Early Release Science (ERS) program. We present a spectrum derived from a single observation by HST Wide Field Camera 3 in the near infrared. We engaged groups across the transiting exoplanet community to participate in the analysis of the data and present results from each. There is general agreement amongst all results that we find an H2O absorption feature with 3.5-4.0 sigma significance. However, the feature is muted in comparison to a clear atmosphere at solar composition. Although the detection of the water feature is robust, the reasons for the muting of this feature are ambiguous due to a degeneracy between clouds and composition. The data does not yield robust detections of any molecular species other than H2O. The group was motivated to perform an additional set of retrieval exercises to investigate an apparent bump in the spectrum at ~ 1.55 um. We explore possible disequilibrium chemistry and find this feature is consistent with super-solar HCN abundance but it is questionable if the required mixing ratio of HCN is chemically and physically plausible. The ultimate goal of this study is to vet WASP-63b as a potential community target to best demonstrate the capabilities and systematics of JWST instruments for transiting exoplanet science. In the case of WASP-63b, the presence of a detectable water feature indicates that WASP-63b remains a plausible target for ERS observations.

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arXiv2018

The Chemical Homogeneity of Sun-like Stars in the Solar Neighborhood

The compositions of stars are a critical diagnostic tool for many topics in astronomy such as the evolution of our Galaxy, the formation of planets, and the uniqueness of the Sun. Previous spectroscopic measurements indicate a large intrinsic variation in the elemental abundance patterns of stars with similar overall metal content. However, systematic errors arising from inaccuracies in stellar models are known to be a limiting factor in such studies, and thus it is uncertain to what extent the observed diversity of stellar abundance patterns is real. Here we report the abundances of 30 elements with precisions of 2% for 79 Sun-like stars within 100 parsecs. Systematic errors are minimized in this study by focusing on solar twin stars and performing a line-by-line differential analysis using high-resolution, high-signal-to-noise spectra. We resolve [X/Fe] abundance trends in galactic chemical evolution at precisions of $10^{-3}$ dex Gyr$^{-1}$ and reveal that stars with similar ages and metallicities have nearly identical abundance patterns. Contrary to previous results, we find that the ratios of carbon-to-oxygen and magnesium-to-silicon in solar metallicity stars are homogeneous to within 10% throughout the solar neighborhood, implying that exoplanets may exhibit much less compositional diversity than previously thought. Finally, we demonstrate that the Sun has a subtle deficiency in refractory material relative to >80% of solar twins (at 2$σ$ confidence), suggesting a possible signpost for planetary systems like our own.

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arXiv2018

From thermal dissociation to condensation in the atmospheres of ultra hot Jupiters: WASP-121b in context

A new class of exoplanets has emerged: the ultra hot Jupiters, the hottest close-in gas giants. Most of them have weaker than expected spectral features in the $1.1-1.7μm$ bandpass probed by HST/WFC3 but stronger spectral features at longer wavelengths probed by Spitzer. This led previous authors to puzzling conclusions about the thermal structures and chemical abundances of these planets. Using the SPARC/MITgcm, we investigate how thermal dissociation, ionization, H$^-$ opacity and clouds shape the thermal structures and spectral properties of ultra hot Jupiters with a special focus on WASP-121b. We expand our findings to the whole population of ultra hot Jupiters through analytical quantification of the thermal dissociation and its influence on the strength of spectral features. We predict that most molecules are thermally dissociated and alkalies are ionized in the dayside photospheres of ultra hot Jupiters. This includes H$_{\rm 2}$O, TiO, VO, and H$_{\rm 2}$ but not CO, which has a stronger molecular bond. The vertical molecular gradient created by the dissociation significantly weakens the spectral features from water while the $4.5μm$ CO feature remain unchanged. The water band in the HST/WFC3 bandpass is further weakened by H$^-$ continuum opacity. Molecules are expected to recombine before reaching the limb, leading to order of magnitude variations of the chemical composition and cloud coverage between the limb and the dayside. Overall, molecular dissociation provides a qualitative understanding of the lack of strong spectral feature of water in the $1-2μm$ bandpass observed in most ultra hot Jupiters. Quantitatively, however, our model does not provide a satisfactory match to the WASP-121b emission spectrum. Together with WASP-33b and Kepler-33Ab, they seem the outliers among the population of ultra hot Jupiters in need of a more thorough understanding.

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arXiv2018

The Solar Twin Planet Search: The age - chromospheric activity relation

It is well known that the magnetic activity of solar type stars decreases with age, but it is widely debated in the literature whether there is a smooth decline or if there is an early sharp drop until 1-2 Gyr followed by a relatively inactive constant phase. We revisited the activity-age relation using time-series observations of a large sample of solar twins whose precise isochronal ages and other important physical parameters have been determined. We measured the Ca II H and K activity indices using 9000 HARPS spectra of 82 solar twins. We measured an average solar activity of $S_{\rm MW}$ = 0.1712 $\pm$ 0.0017 during solar magnetic cycles 23$-$24 covered by HARPS observations and we also inferred an average of $S_{\rm MW}$ = 0.1694 $\pm$ 0.0025 for cycles 10$-$24, anchored on a S index vs. sunspot number correlation. Also, a simple relation between the average and dispersion of the activity levels of solar twins was found. This enabled us to predict the stellar variability effects on the age-activity diagram and, consequently, estimate the chromospheric age uncertainties due to the same phenomena. The age-activity relation is still statistically significant up to ages around 6$-$7 Gyr, in agreement with previous works using open clusters and field stars with precise ages. Our research confirms that Ca II H \& K lines remain a useful chromospheric evolution tracer until stars reach ages of at least 6$-$7 Gyr. We found an evidence that, for the most homogeneous set of old stars, the chromospheric activity indices seem to continue decreasing after the solar age towards the end of the main-sequence. Our results indicate that a significant part of the scatter observed in the age-activity relation of solar twins can be attributed to stellar cycle modulations effects. The Sun seems to have a normal activity level and variability for its age.

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arXiv2018

Global Climate and Atmospheric Composition of the Ultra-Hot Jupiter WASP-103b from HST and Spitzer Phase Curve Observations

We present thermal phase curve measurements for the hot Jupiter WASP-103b observed with Hubble/WFC3 and Spitzer/IRAC. The phase curves have large amplitudes and negligible hotspot offsets, indicative of poor heat redistribution to the nightside. We fit the phase variation with a range of climate maps and find that a spherical harmonics model generally provides the best fit. The phase-resolved spectra are consistent with blackbodies in the WFC3 bandpass, with brightness temperatures ranging from $1880\pm40$ K on the nightside to $2930 \pm 40$ K on the dayside. The dayside spectrum has a significantly higher brightness temperature in the Spitzer bands, likely due to CO emission and a thermal inversion. The inversion is not present on the nightside. We retrieved the atmospheric composition and found the composition is moderately metal-enriched ($\mathrm{[M/H]} = 23^{+29}_{-13}\times$ solar) and the carbon-to-oxygen ratio is below 0.9 at $3\,σ$ confidence. In contrast to cooler hot Jupiters, we do not detect spectral features from water, which we attribute to partial H$_2$O dissociation. We compare the phase curves to 3D general circulation models and find magnetic drag effects are needed to match the data. We also compare the WASP-103b spectra to brown dwarfs and young directly imaged companions and find these objects have significantly larger water features, indicating that surface gravity and irradiation environment play an important role in shaping the spectra of hot Jupiters. These results highlight the 3D structure of exoplanet atmospheres and illustrate the importance of phase curve observations for understanding their complex chemistry and physics.

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arXiv2018

MAROON-X: A Radial Velocity Spectrograph for the Gemini Observatory

MAROON-X is a red-optical, high precision radial velocity spectrograph currently nearing completion and undergoing extensive performance testing at the University of Chicago. The instrument is scheduled to be installed at Gemini North in the first quarter of 2019. MAROON-X will be the only RV spectrograph on a large telescope with full access by the entire US community. In these proceedings we discuss the latest addition of the red wavelength arm and the two science grade detector systems, as well as the design and construction of the telescope front end. We also present results from ongoing RV stability tests in the lab. First results indicate that MAROON-X can be calibrated at the sub-m/s level, and perhaps even much better than that using a simultaneous reference approach.

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arXiv2018

A HST/WFC3 Thermal Emission Spectrum of the Hot Jupiter HAT-P-7b

Secondary eclipse observations of several of the hottest hot Jupiters show featureless, blackbody-like spectra or molecular emission features, which are consistent with thermal inversions being present in those atmospheres. Theory predicts a transition between warmer atmospheres with thermal inversions and cooler atmospheres without inversions, but the exact transition point is unknown. In order to further investigate this issue, we observed two secondary eclipses of the hot Jupiter HAT-P-7b with the Hubble Space Telescope (HST) WFC3 instrument and combined these data with previous Spitzer and Kepler secondary eclipse observations. The HST and Spitzer data can be well fit by a blackbody with $T=2692 \pm 14$ K, and the Kepler data point constrains the geometric albedo to $A_{g}=0.077 \pm 0.006$. We modeled these data with a 3D GCM and 1D self-consistent forward models. The 1D models indicate that the atmosphere has a thermal inversion, weak heat redistribution, and water dissociation that limits the range of pressures probed. This result suggests that WFC3 observations of HAT-P-7b and possibly some other ultra-hot Jupiters appear blackbody-like because they probe a region near the tropopause where the atmospheric temperature changes slowly with pressure. Additionally, the 1D models constrain the atmospheric metallicity ($[\text{M/H}]=-0.87^{+0.38}_{-0.34}$) and the carbon-to-oxygen ratio (C/O $<1$ at 99 % confidence). The solar composition 3D GCM matches the Spitzer data but generally underpredicts the flux in the WFC3 bandpass and cannot reproduce its featureless shape. This discrepancy could be explained by high atmospheric drag or nightside clouds, and may be better understood through further observation with the James Webb Space Telescope (JWST).

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arXiv2018

H- Opacity and Water Dissociation in the Dayside Atmosphere of the Very Hot Gas Giant WASP-18 b

We present one of the most precise emission spectra of an exoplanet observed so far. We combine five secondary eclipses of the hot Jupiter WASP-18 b (Tday=2900K) that we secured between 1.1 and 1.7 micron with the WFC3 instrument aboard the Hubble Space Telescope. Our extracted spectrum (S/N=50, R=40) does not exhibit clearly identifiable molecular features but is poorly matched by a blackbody spectrum. We complement this data with previously published Spitzer/IRAC observations of this target and interpret the combined spectrum by computing a grid of self-consistent, 1D forward models, varying the composition and energy budget. At these high temperatures, we find there are important contributions to the overall opacity from H- ions, as well as the removal of major molecules by thermal dissociation (including water), and thermal ionization of metals. These effects were omitted in previous spectral retrievals for very hot gas giants, and we argue that they must be included to properly interpret the spectra of these objects. We infer a new metallicity and C/O ratio for WASP-18 b, and find them well constrained to be solar ([M/H]=-0.01 (0.35), C/O<0.85 at 3 sigma confidence level), unlike previous work but in line with expectations for giant planets. The best fitting self-consistent temperature-pressure profiles are inverted, resulting in an emission feature at 4.5 micron seen in the Spitzer photometry. These results further strengthen the evidence that the family of very hot gas giant exoplanets commonly exhibit thermal inversions.

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arXiv2017

The temporal evolution of neutron-capture elements in the Galactic discs

Important insights into the formation and evolution of the Galactic disc(s) are contained in the chemical compositions of stars. We analysed high-resolution and high signal to noise HARPS spectra of 79 solar twin stars in order to obtain precise determinations of their atmospheric parameters, ages ($σ$$\sim$0.4 Gyr) and chemical abundances ($σ$$<$0.01~dex) of 12 neutron-capture elements (Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, and Dy). This valuable dataset allows us to study the [X/Fe]-age relations over a time interval of $\sim$10 Gyr and among stars belonging to the thin and thick discs. These relations show that i) the $s$-process has been the main channel of nucleosynthesis of $n$-capture elements during the evolution of the thin disc; ii) the thick disc is rich in $r$-process elements which suggests that its formation has been rapid and intensive. %; iii) a chemical continuity between the thin and thick discs is evident in the abundances of Ba. In addition, the heavy (Ba, La, Ce) and light (Sr, Y, Zr) $s$-process elements revealed details on the dependence between the yields of AGB stars and the stellar mass or metallicity. Finally, we confirmed that both [Y/Mg] and [Y/Al] ratios can be employed as stellar clocks, allowing ages of solar twin stars to be estimated with an average precision of $\sim$0.5~Gyr.

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arXiv2017

The Science Case for an Extended Spitzer Mission

Although the final observations of the Spitzer Warm Mission are currently scheduled for March 2019, it can continue operations through the end of the decade with no loss of photometric precision. As we will show, there is a strong science case for extending the current Warm Mission to December 2020. Spitzer has already made major impacts in the fields of exoplanets (including microlensing events), characterizing near Earth objects, enhancing our knowledge of nearby stars and brown dwarfs, understanding the properties and structure of our Milky Way galaxy, and deep wide-field extragalactic surveys to study galaxy birth and evolution. By extending Spitzer through 2020, it can continue to make ground-breaking discoveries in those fields, and provide crucial support to the NASA flagship missions JWST and WFIRST, as well as the upcoming TESS mission, and it will complement ground-based observations by LSST and the new large telescopes of the next decade. This scientific program addresses NASA's Science Mission Directive's objectives in astrophysics, which include discovering how the universe works, exploring how it began and evolved, and searching for life on planets around other stars.

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arXiv2017

Spectroscopic binaries in the Solar Twin Planet Search program: from substellar-mass to M dwarf companions

Previous studies on the rotation of Sun-like stars revealed that the rotational rates of young stars converge towards a well-defined evolution that follows a power-law decay. It seems, however, that some binary stars do not obey this relation, often by displaying enhanced rotational rates and activity. In the Solar Twin Planet Search program we observed several solar twin binaries, and found a multiplicity fraction of $42\% \pm 6\%$ in the whole sample; moreover, at least three of these binaries (HIP 19911, HIP 67620 and HIP 103983) clearly exhibit the aforementioned anomalies. We investigated the configuration of the binaries in the program, and discovered new companions for HIP 6407, HIP 54582, HIP 62039 and HIP 30037, of which the latter is orbited by a $0.06$ M$_\odot$ brown dwarf in a 1-month long orbit. We report the orbital parameters of the systems with well-sampled orbits and, in addition, the lower limits of parameters for the companions that only display a curvature in their radial velocities. For the linear trend binaries, we report an estimate of the masses of their companions when their observed separation is available, and a minimum mass otherwise. We conclude that solar twin binaries with low-mass stellar companions at moderate orbital periods do not display signs of a distinct rotational evolution when compared to single stars. We confirm that the three peculiar stars are double-lined binaries, and that their companions are polluting their spectra, which explains the observed anomalies.

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arXiv2017

An Observational Diagnostic for Distinguishing Between Clouds and Haze in Hot Exoplanet Atmospheres

The nature of aerosols in hot exoplanet atmospheres is one of the primary vexing questions facing the exoplanet field. The complex chemistry, multiple formation pathways, and lack of easily identifiable spectral features associated with aerosols make it especially challenging to constrain their key properties. We propose a transmission spectroscopy technique to identify the primary aerosol formation mechanism for the most highly irradiated hot Jupiters (HIHJs). The technique is based on the expectation that the two key types of aerosols -- photochemically generated hazes and equilibrium condensate clouds -- are expected to form and persist in different regions of a highly irradiated planet's atmosphere. Haze can only be produced on the permanent daysides of tidally-locked hot Jupiters, and will be carried downwind by atmospheric dynamics to the evening terminator (seen as the trailing limb during transit). Clouds can only form in cooler regions on the night side and morning terminator of HIHJs (seen as the leading limb during transit). Because opposite limbs are expected to be impacted by different types of aerosols, ingress and egress spectra, which primarily probe opposing sides of the planet, will reveal the dominant aerosol formation mechanism. We show that the benchmark HIHJ, WASP-121b, has a transmission spectrum consistent with partial aerosol coverage and that ingress-egress spectroscopy would constrain the location and formation mechanism of those aerosols. In general, using this diagnostic we find that observations with JWST and potentially with HST should be able to distinguish between clouds and haze for currently known HIHJs.

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