Microlensing Surveys for Exoplanets Gaudi, B. Scott
Annual review of astronomy and astrophysics,
01/2012, Letnik:
50, Številka:
1
Journal Article
Recenzirano
Unlike most other planet-detection techniques, gravitational microlensing does not rely on detection of photons from either the host or the planet. Rather, planets are discovered by their ...gravitational perturbation of light from a more distant source. I review the fundamental concepts of microlensing planet searches and discuss their practical application. I show how the strengths and peculiarities of the method flow from the basic manner in which planets are discovered. In particular, microlensing is sensitive to very low-mass planets on wide orbits and free-floating planets, and can be used to search for planets orbiting host stars with a broad range of masses and Galactocentric distances. However, microlensing events are rare and cannot be predicted in advance, the majority of the host stars are extremely faint, and the planetary signals typically last less than a day. These strengths motivate microlensing searches as powerful, complementary probes of unexplored parameter space that have already provided important constraints on the demographics of planets beyond the "snow line." However, the real-world challenges associated with the practical application of the method have driven the organization and evolution of the microlensing field and will continue to drive future developments from next-generation ground-based experiments through possible future space-based missions.
EXOFAST: A Fast Exoplanetary Fitting Suite in IDL Eastman, Jason; Gaudi, B. Scott; Agol, Eric
Publications of the Astronomical Society of the Pacific,
01/2013, Letnik:
125, Številka:
923
Journal Article
Recenzirano
Odprti dostop
ABSTRACT We present EXOFAST, a fast, robust suite of routines written in IDL that is designed to fit exoplanetary transits and radial velocity variations simultaneously or separately and characterize ...the parameter uncertainties and covariances with a differential evolution Markov chain Monte Carlo method. We describe how our code incorporates both data sets to derive simultaneously stellar parameters along with the transit and RV parameters, resulting in more self-consistent results on an example fit of the discovery data of HAT-P-3b that is well-mixed in under 5 minutes on a standard desktop computer. We describe in detail how our code works and outline ways in which the code can be extended to include additional effects or generalized for the characterization of other data sets-including non-planetary data sets. We discuss the pros and cons of several common ways to parameterize eccentricity, highlight a subtle mistake in the implementation of MCMC that could bias the inferred eccentricity of intrinsically circular orbits to significantly non-zero results, discuss a problem with IDL's built-in random number generator in its application to large MCMC fits, and derive a method to analytically fit the linear and quadratic limb darkening coefficients of a planetary transit. Finally, we explain how we achieved improved accuracy and over a factor of 100 improvement in the execution time of the transit model calculation. Our entire source code, along with an easy-to-use online interface for several basic features of our transit and radial velocity fitting, are available online at http://astroutils.astronomy.ohio-state.edu/exofast.
The Wide Field InfraRed Survey Telescope (WFIRST) is the next NASA astrophysics flagship mission, to follow the James Webb Space Telescope. The WFIRST mission was chosen as the top-priority large ...space mission of the 2010 astronomy and astrophysics decadal survey in order to achieve three primary goals: to study dark energy via a wide-field imaging survey, to study exoplanets via a microlensing survey, and to enable a guest observer program. Here we assess the ability of the several WFIRST designs to achieve the goal of the microlensing survey to discover a large sample of cold, low-mass exoplanets with semimajor axes beyond roughly one astronomical unit, which are largely impossible to detect with any other technique. We present the results of a suite of simulations that span the full range of the proposed WFIRST architectures, from the original design envisioned by the decadal survey, to the current design, which utilizes a 2.4 m telescope donated to NASA. By studying such a broad range of architectures, we are able to determine the impact of design trades on the expected yields of detected exoplanets. In estimating the yields we take particular care to ensure that our assumed Galactic model predicts microlensing event rates that match observations, consider the impact that inaccuracies in the Galactic model might have on the yields, and ensure that numerical errors in light-curve computations do not bias the yields for the smallest-mass exoplanets. For the nominal baseline WFIRST design and a fiducial planet mass function, we predict that a total of ∼1400 bound exoplanets with mass greater than ∼0.1 M⊕ should be detected, including ∼200 with mass 3 M⊕. WFIRST should have sensitivity to planets with mass down to ∼0.02 M⊕, or roughly the mass of Ganymede.
Hot Jupiters (HJs), gas giant planets orbiting their host stars with periods on the order of days, commonly occur in the Galaxy, including relatively massive (1.6-2.4 M , i.e., A-type main-sequence ...stars) and evolved stars. The majority of A-type main-sequence stars have stellar binary companions, that can strongly affect the dynamical evolution of planets around either star. In this work, we investigate the effects of gravitational perturbations by a far away stellar companion on the orbital evolution of gas giant planets orbiting A-type stars, the so-called Eccentric Kozai-Lidov mechanism, including the effects of general relativity, post-main-sequence stellar evolution, and tides. We find that only 0.15% of A-type stars will host HJs during their main-sequence lifetimes. However, we also find a new class of planets, Temporary Hot Jupiters (THJs), that form during the post-main-sequence lifetime of about 3.7% of former A-type main-sequence stars. These THJs orbit on periods of tens to a hundred days and only exist for a few 100,000 years before they are engulfed, but they reach similar temperatures as "classical" HJs due to the increased stellar luminosities. THJs' spin-orbit angles will mostly be misaligned. THJ effects on the host stars' evolution could also be observable for longer than a few 100,000 years. Overall, we find that approximately 70% of all gas giant planets orbiting A-type stars will eventually be destroyed or engulfed by their star, about 25% during the main-sequence lifetime, about 45% during post-main-sequence evolution.
We present a semianalytic estimate of the expected yield of single-transit planets from the Transiting Exoplanet Survey Satellite (TESS). We use the TESS Candidate Target List-6 (CTL-6) as an input ...catalog of over four million sources. We predict that from the 200,000 stars selected to be observed with the high-cadence postage stamps (PSs) with the highest CTL-6 priority, there will be 241 single-transit events caused by planets detectable at a signal-to-noise ratio of S/N ≥ 7.3. We find a lower limit of an additional 977 events caused by single-transit planets in the full frame images (FFIs); this is a lower limit because the CTL-6 is incomplete below a TESS magnitude of . Of the single-transit events from the PSs and FFIs, 1091/1218 will have transit depths deeper than 0.1% and will thus be amenable for photometric follow-up from the ground, and 1195/1218 will have radial velocity signals greater than 1 m s−1. We estimate that the periods of 146 single transits will be constrained to better than 10% using the TESS photometry assuming circular orbits. We find that the number of planets detected by TESS in the PSs with periods days can be doubled by including single-transiting planets, while the number of planets with days can be increased by an order of magnitude. We predict 79 habitable zone planets in the TESS light curves from single transits, with 18 orbiting FGK stars.
ABSTRACT A microlensing survey by Sumi et al. exhibits an overabundance of short-timescale events (STEs; tE < 2 days) relative to what is expected from known stellar populations and a smooth ...power-law extrapolation down to the brown dwarf regime. This excess has been interpreted as a population of approximately Jupiter-mass objects that outnumber main-sequence stars nearly twofold; however the microlensing data alone cannot distinguish between events due to wide-separation (a 10 au) and free-floating planets. Assuming these STEs are indeed due to planetary-mass objects, we aim to constrain the fraction of these events that can be explained by bound but wide-separation planets. We fit the observed timescale distribution with a lens mass function comprised of brown dwarfs, main-sequence stars, and stellar remnants, finding and thus corroborating the initial identification of an excess of STEs. We then include a population of bound planets that are expected not to show signatures of the primary lens (host) in their microlensing light curves and that are also consistent with results from representative microlensing, radial velocity, and direct imaging surveys. We find that bound planets alone cannot explain the entire STE excess without violating the constraints from the surveys we consider and thus some fraction of these events must be due to free-floating planets, if our model for bound planets holds. We estimate a median fraction of STEs due to free-floating planets to be f = 0.67 (0.23 ≤ f ≤ 0.85 at 95% confidence) when assuming "hot-start" planet evolutionary models and f = 0.58 (0.14 ≤ f ≤ 0.83 at 95% confidence) for "cold-start" models. Assuming a delta-function distribution of free-floating planets of mass yields a number of free-floating planets per main-sequence star of N = 1.4 (0.48 ≤ N ≤ 1.8 at 95% confidence) in the "hot-start" case and N = 1.2 (0.29 ≤ N ≤ 1.8 at 95% confidence) in the "cold-start" case.
Abstract
The Nancy Grace Roman Space Telescope (Roman) will perform a Galactic Exoplanet Survey (RGES) to discover bound exoplanets with semimajor axes greater than 1 au using gravitational ...microlensing. Roman will even be sensitive to planetary-mass objects that are not gravitationally bound to any host star. Such free-floating planetary-mass objects (FFPs) will be detected as isolated microlensing events with timescales shorter than a few days. A measurement of the abundance and mass function of FFPs is a powerful diagnostic of the formation and evolution of planetary systems, as well as the physics of the formation of isolated objects via direct collapse. We show that Roman will be sensitive to FFP lenses that have masses from that of Mars (0.1
M
⊕
) to gas giants (
M
≳ 100
M
⊕
) as isolated lensing events with timescales from a few hours to several tens of days, respectively. We investigate the impact of the detection criteria on the survey, especially in the presence of finite-source effects for low-mass lenses. The number of detections will depend on the abundance of such FFPs as a function of mass, which is at present poorly constrained. Assuming that FFPs follow the fiducial mass function of cold, bound planets adapted from Cassan et al., we estimate that Roman will detect ∼250 FFPs with masses down to that of Mars (including ∼60 with masses ≤
M
⊕
). We also predict that Roman will improve the upper limits on FFP populations by at least an order of magnitude compared to currently existing constraints.
We observed two full orbital phase curves of the transiting brown dwarf KELT-1b, at 3.6 and 4.5 m, using the Spitzer Space Telescope. Combined with previous eclipse data from Beatty et al., we ...strongly detect KELT-1b's phase variation as a single sinusoid in both bands, with amplitudes of 964 36 ppm at 3.6 m and 979 54 ppm at 4.5 m, and confirm the secondary eclipse depths measured by Beatty et al. We also measure noticeable eastward hotspot offsets of 28 4 3 5 at 3.6 m and 18 6 5 2 at 4.5 m. Both the day-night temperature contrasts and the hotspot offsets we measure are in line with the trends seen in hot Jupiters, though we disagree with the recent suggestion of an offset trend by Zhang et al. Using an ensemble analysis of Spitzer phase curves, we argue that nightside clouds are playing a noticeable role in modulating the thermal emission from these objects, based on: (1) the lack of a clear trend in phase offsets with equilibrium temperature, (2) the sharp day-night transitions required to have non-negative intensity maps, which also resolves the inversion issues raised by Keating & Cowan, (3) the fact that all the nightsides of these objects appear to be at roughly the same temperature of 1000 K, while the dayside temperatures increase linearly with equilibrium temperature, and (4) the trajectories of these objects on a Spitzer color-magnitude diagram, which suggest colors only explainable via nightside clouds.
Exoplanets have been observed around stars at all stages of stellar evolution, in many cases orbiting in configurations that will eventually lead to the planets being engulfed or consumed by their ...host stars, such as hot Jupiters or ultrashort period planets. Furthermore, objects such as polluted white dwarfs provide strong evidence that the consumption of planets by stars is a common phenomenon. This consumption causes several significant changes in the stellar properties, such as changes to the stellar spin, luminosity, chemical composition, or mass-loss processes. Here, we explore this wide variety of effects for a comprehensive range of stellar and planetary masses and stages of stellar evolution, from the main sequence over red giants to white dwarfs. We determine that planet consumption can cause transient luminosity features that last on the order of centuries to millennia, and that the post-consumption stellar spins can often reach breakup speeds. Furthermore, stellar mass loss can be caused by this spin-up, as well as through surface grazing interactions, leading to to the formation of unusual planetary nebula shapes or collimated stellar gas ejections. Our results highlight several observable stellar features by which the presence or previous existence of a planet around a given star can be deduced. This will provide future observational campaigns with the tools to better constrain exoplanet demographics, as well as planetary formation and evolution histories.
Abstract
For the majority of short-period exoplanets transiting massive stars with radiative envelopes, the spin angular momentum of the host star is greater than the planetary orbital angular ...momentum. In this case, the orbits of the planets will undergo nodal precession, which can significantly impact the probability that the planets transit their parent star. In particular, for some combinations of the spin–orbit angle
ψ
and the inclination of the stellar spin
i
*
, all such planets will eventually transit at some point over the duration of their precession period. Thus, as the time over which the sky has been monitored for transiting planets increases, the frequency of planets with detectable transits will increase, potentially leading to biased estimates of exoplanet occurrence rates, especially orbiting more-massive stars. Furthermore, due to the dependence of the precession period on orbital parameters such as spin–orbit misalignment, the observed distributions of such parameters may also be biased. We derive the transit probability of a given exoplanet in the presence of nodal precession induced by a rapidly spinning host star. We find that the effect of nodal precession has already started to become relevant for some short-period planets, i.e., hot Jupiters, orbiting massive stars, by increasing transit probabilities by order of a few percent for such systems within the original Kepler field. We additionally derive simple expressions to describe the time evolution of the impact parameter
b
for applicable systems, which should aid in future investigations of exoplanet nodal precession and spin–orbit alignment.
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