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.
IS THE GALACTIC BULGE DEVOID OF PLANETS? Penny, Matthew T.; Henderson, Calen B.; Clanton, Christian
The Astrophysical journal,
10/2016, Letnik:
830, Številka:
2
Journal Article
Recenzirano
Odprti dostop
ABSTRACT We consider a sample of 31 exoplanetary systems detected by gravitational microlensing and investigate whether or not the estimated distances to these systems conform to the Galactic ...distribution of planets expected from models. We derive the expected distribution of distances and relative proper motions from a simulated microlensing survey, correcting for the dominant selection effects that affect the sensitivity of planet detection as a function of distance, and compare it to the observed distribution using Anderson-Darling (AD) hypothesis testing. Taking the relative abundance of planets in the bulge to that in the disk, , as a model parameter, we find that our model is consistent with the observed distribution only for (for a p-value threshold of 0.01) implying that the bulge may be devoid of planets relative to the disk. Allowing for a dependence of planet abundance on metallicity and host mass, or an additional dependence of planet sensitivity on event timescale, does not restore consistency for . We examine the distance estimates of some events in detail, and conclude that some parallax-based estimates could be significantly in error. Only by combining the removal of one problematic event from our sample and the inclusion of strong dependences of planet abundance or detection sensitivity on host mass, metallicity, and event timescale are we able to find consistency with the hypothesis that the bulge and disk have equal planet abundance.
ABSTRACT Simultaneous space- and ground-based microlensing surveys, such as K2's Campaign 9 (K2C9) and WFIRST, facilitate measuring the masses and distances of free-floating planet (FFP) candidates, ...which are identified as single-lens events with timescales that are of the order of 1 day. Measuring the mass and distance of an FFP lens requires determining the size of the source star , measuring the microlens parallax , and using high-resolution imaging to search for the lens flux from a possible host star. Here we investigate the accessible parameter space for each of these components considering different satellites for a range of FFP masses, Galactic distances, and source star properties. We find that at the beginning of K2C9, when its projected separation from the Earth is 0.2 au, it will be able to measure for Jupiter-mass FFP candidates at distances larger than ∼2 kpc and to Earth-mass lenses at ∼8 kpc. At the end of K2C9, when = 0.81 au, it is sensitive to planetary-mass lenses for distances 3.5 kpc, and even then only to those with mass MJup. From lens flux constraints we find that it will be possible to exclude hosts down to the deuterium-burning limit for events within ∼2 kpc. This indicates that the ability to characterize FFPs detected during K2C9 is optimized for events occurring toward the beginning of the campaign. WFIRST, on the other hand, will be able to detect and characterize FFP masses down to or below super-Earths throughout the Galaxy during its entire microlensing survey.
We present adaptive optics imaging from the NIRC2 instrument on the Keck II telescope that resolves the exoplanet host (and lens) star as it separates from the brighter source star. These ...observations yield the K-band brightness of the lens and planetary host star, as well as the lens-source relative proper motion, , in the heliocentric reference frame. The measurement allows for the determination of the microlensing parallax vector, , which had only a single component determined by the microlensing light curve. The combined measurements of and KL provide the masses of the host star, , and planet, mp = 3.27 0.32MJupiter with a projected separation of 3.4 0.5 au. This confirms the tentative conclusion of a previous paper that this super-Jupiter mass planet, OGLE-2005-BLG-071Lb, orbits an M dwarf. Such planets are predicted to be rare by the core accretion theory and have been difficult to find with other methods, but there are two such planets with firm mass measurements from microlensing, and an additional 11 planetary microlens events with host mass estimates and planet mass estimates >2 Jupiter masses that could be confirmed by high angular follow-up observations. We also point out that OGLE-2005-BLG-071L has separated far enough from its host star that it should be possible to measure the host-star metallicity with spectra from a high angular resolution telescope such as Keck, the Very Large Telescope, the Hubble Space Telescope, or the James Webb Space Telescope.
We have conducted a long-term, wide-field, high-cadence photometric monitoring survey of ~50,000 stars in the Lagoon Nebula H II region. This first paper presents rotation periods for 290 low-mass ...stars in NGC 6530, the young cluster illuminating the nebula, and for which we assemble a catalog of infrared and spectroscopic disk indicators, estimated masses and ages, and X-ray luminosities. The distribution of rotation periods we measure is broadly uniform for 0.5 days < P < 10 days; the short-period cutoff corresponds to breakup. We observe no obvious bimodality in the period distribution, but we do find that stars with disk signatures rotate more slowly on average. The stars' X-ray luminosities are roughly flat with rotation period, at the saturation level (log L sub(X)/L sub(bol) approx = -3.3). However, we find a significant positive correlation between L sub(X)/L sub(bol) and corotation radius, suggesting that the observed X-ray luminosities are regulated by centrifugal stripping of the stellar coronae. The period-mass relationship in NGC 6530 is broadly similar to that of the Orion Nebula Cluster (ONC), but the slope of the relationship among the slowest rotators differs from that in the ONC and other young clusters. We show that the slope of the period-mass relationship for the slowest rotators can be used as a proxy for the age of a young cluster, and we argue that NGC 6530 may be slightly younger than the ONC, making it a particularly important touchstone for models of angular momentum evolution in young, low-mass stars.
We present Keck/NIRC2 adaptive optics imaging of planetary microlensing event MOA-2007-BLG-400 that resolves the lens star system from the source. We find that the MOA-2007-BLG-400L planetary system ...consists of a 1.71 ± 0.27M(sub Jup) planet orbiting a 0.69 ± 0.04M⨀ K-dwarf host star at a distance of 6.89 ± 0.77 kpc from the Sun. So, this planetary system probably resides in the Galactic bulge. The planet–host star projected separation is only weakly constrained due to the close-wide light-curve degeneracy; the 2σ projected separation ranges are 0.6–1.0 au and 4.7–7.7 au for close and wide solutions, respectively. This host mass is at the top end of the range of masses predicted by a standard Bayesian analysis. Our Keck follow-up program has now measured lens-source separations for six planetary microlensing events, and five of these six events have host star masses above the median prediction under the assumption that assumes that all stars have an equal chance of hosting planets detectable by microlensing. This suggests that more massive stars may be more likely to host planets of a fixed mass ratio that orbit near or beyond the snow line. These results also indicate the importance of host star mass measurements for exoplanets found by microlensing. The microlensing survey imaging data from NASA’s Nancy Grace Roman Space Telescope (formerly WFIRST) mission will be doing mass measurements like this for a huge number of planetary events.
Abstract
We present an adaptive optics (AO) analysis of images from the Keck II telescope NIRC2 instrument of the planetary microlensing event MOA-2009-BLG-319. The ∼10 yr baseline between the event ...and the Keck observations allows the planetary host star to be detected at a separation of 66.5 ± 1.7 mas from the source star, consistent with the light-curve model prediction. The combination of the host star brightness and light-curve parameters yields host star and planet masses of
M
host
= 0.524 ± 0.048
M
⊙
and
m
p
= 67.3 ± 6.2
M
⊕
at a distance of
D
L
= 7.1 ± 0.7 kpc. The star−planet projected separation is 2.03 ± 0.21 au. The planet-to-star mass ratio of this system,
q
= (3.857 ± 0.029) × 10
−4
, places it in the predicted “planet desert” at 10
−4
<
q
< 4 × 10
−4
according to the runaway gas accretion scenario of the core accretion theory. Seven of the 30 planets in the Suzuki et al. sample fall in this mass ratio range, and this is the third with a measured host mass. All three of these host stars have masses of 0.5 ≤
M
host
/
M
⊙
≤ 0.7, which implies that this predicted mass ratio gap is filled with planets that have host stars within a factor of two of 1
M
⊙
. This suggests that runaway gas accretion does not play a major role in determining giant planet masses for stars somewhat less massive than the Sun. Our analysis has been accomplished with a modified DAOPHOT code that has been designed to measure the brightness and positions of closely blended stars. This will aid in the development of the primary method that the Nancy Grace Roman Space Telescope mission will use to determine the masses of microlens planets and their hosts.
M subdwarfs are low-metallicity M dwarfs that typically inhabit the halo population of the Galaxy. Metallicity controls the opacity of stellar atmospheres; in metal-poor stars, hydrostatic ...equilibrium is reached at a smaller radius, leading to smaller radii for a given effective temperature. We compile a sample of 88 stars that span spectral classes K7 to M6 and include stars with metallicity classes from solar-metallicity dwarf stars to the lowest metallicity ultra subdwarfs to test how metallicity changes the stellar radius. We fit models to Palomar Double Spectrograph (DBSP) optical spectra to derive effective temperatures (Teff) and we measure bolometric luminosities (Lbol) by combining broad wavelength-coverage photometry with Gaia parallaxes. Radii are then computed by combining the Teff and Lbol using the Stefan-Boltzman law. We find that for a given temperature, ultra subdwarfs can be as much as five times smaller than their solar-metallicity counterparts. We present color-radius and color-surface brightness relations that extend down to Fe/H of −2.0 dex, in order to aid the radius determination of M subdwarfs, which will be especially important for the WFIRST exoplanetary microlensing survey.
I investigate the possibility of constraining the flux of the lens (i.e., host star) for the types of planetary systems the Korean Micro-lensing Telescope Network is predicted to find. I examine the ...potential to obtain lens flux measurements by (1) imaging the lens once it is spatially resolved from the source, (2) measuring the elongation of the point-spread function of the microlensing target (lens+source) when the lens and source are still unresolved, and (3) taking prompt follow-up photometry. In each case I simulate the observing programs for a representative example of current ground-based adaptive optics (AO) facilities, future ground-based AO facilities, and future space telescopes. I find that undetected blend stars would cause catastrophic failures for (16 bullet fsubbin)% of planet detections, where fsubbin is the binary fraction, with the majority of these failures occurring for host stars with mass 0.3 M.
The Korean Microlensing Telescope Network (KMTNet) will consist of three 1.6 m telescopes each with a 4 deg super(2) field of view (FoV) and will be dedicated to monitoring the Galactic Bulge to ...detect exoplanets via gravitational microlensing. KMTNet's combination of aperture size, FoV, cadence, and longitudinal coverage will provide a unique opportunity to probe exoplanet demographics in an unbiased way. Here we present simulations that optimize the observing strategy for and predict the planetary yields of KMTNet. We find preferences for four target fields located in the central Bulge and an exposure time of t sub(exp) = 120 s, leading to the detection of ~2200 microlensing events per year. We estimate the planet detection rates for planets with mass and separation across the ranges 0.1 < or =, slant M sub(p)/M sub(+ in circle) < or =, slant 1000 and 0.4 < or =, slant a/AU < or =, slant 16, respectively. Normalizing these rates to the cool-planet mass function of Cassan et al, we predict KMTNet will be approximately uniformly sensitive to planets with mass 5 < or =, slant M sub(p)/M sub(+ in circle) < or =, slant 1000 and will detect ~20 planets per year per dex in mass across that range. For lower-mass planets with mass 0.1 < or =, slant M sub(p)/M sub(+ in circle) < 5, we predict KMTNet will detect ~10 planets per year. We also compute the yields KMTNet will obtain for free-floating planets (FFPs) and predict KMTNet will detect ~1 Earth-mass FFP per year, assuming an underlying population of one such planet per star in the Galaxy. Lastly, we investigate the dependence of these detection rates on the number of observatories, the photometric precision limit, and optimistic assumptions regarding seeing, throughput, and flux measurement uncertainties.
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