arXiv subjects

Saul Rappaport

Publications and source records attributed to Saul Rappaport.

At least 37 records · Page 2Linked to original sources

arXiv2021

Recurring Planetary Debris Transits and Circumstellar Gas around White Dwarf ZTF J0328$-$1219

We present follow-up photometry and spectroscopy of ZTF J0328$-$1219 strengthening its status as a white dwarf exhibiting transiting planetary debris. Using TESS and Zwicky Transient Facility photometry, along with follow-up high speed photometry from various observatories, we find evidence for two significant periods of variability at 9.937 and 11.2 hr. We interpret these as most likely the orbital periods of different debris clumps. Changes in the detailed dip structures within the light curves are observed on nightly, weekly, and monthly timescales, reminiscent of the dynamic behavior observed in the first white dwarf discovered to harbor a disintegrating asteroid, WD 1145+017. We fit previously published spectroscopy along with broadband photometry to obtain new atmospheric parameters for the white dwarf, with $M_{\star} = 0.731 \pm 0.023\,M_{\odot}$, $T_{\mathrm{eff}} = 7630 \pm 140\,$K, and $\mathrm{[Ca/He]}=-9.55\pm0.12$. With new high-resolution spectroscopy, we detect prominent and narrow Na D absorption features likely of circumstellar origin, with velocities $21.4\pm1.0$ km s$^{-1}$ blue-shifted relative to atmospheric lines. We attribute the periodically modulated photometric signal to dusty effluents from small orbiting bodies such as asteroids or comets, but are unable to identify the most likely material that is being sublimated, or otherwise ejected, as the environmental temperatures range from roughly 400K to 600K.

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arXiv2020

Stellar Flares from the First Tess Data Release: Exploring a New Sample of M-dwarfs

We perform a study of stellar flares for the 24,809 stars observed with 2 minute cadence during the first two months of the TESS mission. Flares may erode exoplanets' atmospheres and impact their habitability, but might also trigger the genesis of life around small stars. TESS provides a new sample of bright dwarf stars in our galactic neighborhood, collecting data for thousands of M-dwarfs that might host habitable exoplanets. Here, we use an automated search for flares accompanied by visual inspection. Then, our public allesfitter code robustly selects the appropriate model for potentially complex flares via Bayesian evidence. We identify 1228 flaring stars, 673 of which are M-dwarfs. Among 8695 flares in total, the largest superflare increased the stellar brightness by a factor of 16.1. Bolometric flare energies range from 10^31.0 to 10^36.9 erg, with a median of 10^33.1 erg. Furthermore, we study the flare rate and energy as a function of stellar type and rotation period. We solidify past findings that fast rotating M-dwarfs are the most likely to flare, and that their flare amplitude is independent of the rotation period. Finally, we link our results to criteria for prebiotic chemistry, atmospheric loss through coronal mass ejections, and ozone sterilization. Four of our flaring M dwarfs host exoplanet candidates alerted on by TESS, for which we discuss how these effects can impact life. With upcoming TESS data releases, our flare analysis can be expanded to almost all bright small stars, aiding in defining criteria for exoplanet habitability.

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arXiv2020

Kepler-78 and the Ultra-Short-Period Planets

Compared to the Earth, the exoplanet Kepler-78b has a similar size (1.2 $R_\oplus$) and an orbital period a thousand times shorter (8.5 hours). It is currently the smallest planet for which the mass, radius, and dayside brightness have all been measured. Kepler-78b is an exemplar of the ultra-short-period (USP) planets, a category defined by the simple criterion $P_{\rm orb} < 1$ day. We describe our Fourier-based search of the Kepler data that led to the discovery of Kepler-78b, and review what has since been learned about the population of USP planets. They are about as common as hot Jupiters, and they are almost always smaller than 2 $R_\oplus$. They are often members of compact multi-planet systems, although they tend to have relatively large period ratios and mutual inclinations. They might be the exposed rocky cores of "gas dwarfs," the planets between 2-4 $R_\oplus$ in size that are commonly found in somewhat wider orbits.

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arXiv2019

A 9-Hr CV With One Outburst in 4 Years of Kepler Data

During a visual search through the Kepler main-field lightcurves, we have discovered a cataclysmic variable (CV) that experienced only a single 4-day long outburst over four years, rising to three times the quiescent flux. During the four years of non-outburst data the Kepler photometry of KIC 5608384 exhibits ellipsoidal light variations (`ELV') with a $\sim$12% amplitude and period of 8.7 hours. Follow-up ground-based spectral observations have yielded a high-quality radial velocity curve and the associated mass function. Additionally, H$α$ emission lines were present in the spectra even though these were taken while the source was presumably in quiescence. These emission lines are at least partially eclipsed by the companion K star. We utilize the available constraints of the mass function, the ELV amplitude, Roche-lobe filling condition, and inferred radius of the K star to derive the system masses and orbital inclination angle: $M_{\rm wd} \simeq 0.46 \pm 0.02 \, M_\odot$, $M_{\rm K} \simeq 0.41 \pm 0.03 \, M_\odot$, and $i \gtrsim 70^\circ$. The value of $M_{\rm wd}$ is the lowest reported for any accreting WD in a cataclysmic variable. We have also run binary evolution models using MESA to infer the most likely parameters of the pre-cataclysmic binary. Using the mass-transfer rates from the model evolution tracks we conclude that although the rates are close to the critical value for accretion disk stability, we expect KIC 5608384 to exhibit dwarf nova outbursts. We also conclude that the accreting white dwarf most likely descended from a hot subdwarf and, most notably, that this binary is one of the first bona fide examples of a progenitor of AM CVn binaries to have evolved through the CV channel.

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arXiv2018

A Compact Multi-Planet System With A Significantly Misaligned Ultra Short Period Planet

We report the discovery of a compact multi-planet system orbiting the relatively nearby (78pc) and bright ($K=8.9$) K-star, K2-266 (EPIC248435473). We identify up to six possible planets orbiting K2-266 with estimated periods of P$_b$ = 0.66, P$_{.02}$ = 6.1, P$_c$ = 7.8, P$_d$ = 14.7, P$_e$ = 19.5, and P$_{.06}$ = 56.7 days and radii of R$_P$ = 3.3 R$_{\oplus}$, 0.646 R$_{\oplus}$, 0.705 R$_{\oplus}$, 2.93 R$_{\oplus}$, 2.73 R$_{\oplus}$, and 0.90 R$_{\oplus}$, respectively. We are able to confirm the planetary nature of two of these planets (d & e) from analyzing their transit timing variations ($m_d= 8.9_{-3.8}^{+5.7} M_\oplus$ and $m_e=14.3_{-5.0}^{+6.4} M_\oplus$), confidently validate the planetary nature of two other planets (b & c), and classify the last two as planetary candidates (K2-266.02 & .06). From a simultaneous fit of all 6 possible planets, we find that K2-266 b's orbit has an inclination of 75.32$^{\circ}$ while the other five planets have inclinations of 87-90$^{\circ}$. This observed mutual misalignment may indicate that K2-266 b formed differently from the other planets in the system. The brightness of the host star and the relatively large size of the sub-Neptune sized planets d and e make them well-suited for atmospheric characterization efforts with facilities like the Hubble Space Telescope and upcoming James Webb Space Telescope. We also identify an 8.5-day transiting planet candidate orbiting EPIC248435395, a co-moving companion to K2-266.

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arXiv2017

Disintegrating Rocky Exoplanets

We discuss a new class of exoplanets that appear to be emitting a tail of dusty effluents. These disintegrating planets are found close to their host stars and have very hot, and likely molten, surfaces. The properties of the dust should provide a direct probe of the constituent material of these rocky bodies.

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arXiv2016

On the Detection of Non-Transiting Exoplanets with Dusty Tails

We present a way of searching for non-transiting exoplanets with dusty tails. In the transiting case, the extinction by dust during the transit removes more light from the beam than is scattered into it. Thus, the forward scattering component of the light is best seen either just prior to ingress, or just after egress, but with reduced amplitude over the larger peak that is obscured by the transit. This picture suggests that it should be equally productive to search for positive-going peaks in the flux from non-transiting exoplanets with dusty tails. We discuss what amplitudes are expected for different orbital inclination angles. The signature of such objects should be distinct from normal transits, starspots, and most - but not all - types of stellar pulsations.

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arXiv2016

Detection of binary and multiple systems among rapidly rotating K and M dwarf stars from Kepler data

From an examination of ~18,000 Kepler light curves of K- and M-stars we find some 500 which exhibit rotational periods of less than 2 days. Among such stars, approximately 50 show two or more incommensurate periodicities. We discuss the tools that allow us to differentiate between rotational modulation and other types of light variations, e.g., due to pulsations or binary modulations. We find that these multiple periodicities are independent of each other and likely belong to different, but physically bound, stars. This scenario was checked directly by UKIRT and adaptive optics imaging, time-resolved Fourier transforms, and pixel-level analysis of the data. Our result is potentially important for discovering young multiple stellar systems among rapidly rotating K- and M-dwarfs.

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arXiv2015

A disintegrating minor planet transiting a white dwarf

White dwarfs are the end state of most stars, including the Sun, after they exhaust their nuclear fuel. Between 1/4 and 1/2 of white dwarfs have elements heavier than helium in their atmospheres, even though these elements should rapidly settle into the stellar interiors unless they are occasionally replenished. The abundance ratios of heavy elements in white dwarf atmospheres are similar to rocky bodies in the Solar system. This and the existence of warm dusty debris disks around about 4% of white dwarfs suggest that rocky debris from white dwarf progenitors' planetary systems occasionally pollute the stars' atmospheres. The total accreted mass can be comparable to that of large asteroids in the solar system. However, the process of disrupting planetary material has not yet been observed. Here, we report observations of a white dwarf being transited by at least one and likely multiple disintegrating planetesimals with periods ranging from 4.5 hours to 4.9 hours. The strongest transit signals occur every 4.5 hours and exhibit varying depths up to 40% and asymmetric profiles, indicative of a small object with a cometary tail of dusty effluent material. The star hosts a dusty debris disk and the star's spectrum shows prominent lines from heavy elements like magnesium, aluminium, silicon, calcium, iron, and nickel. This system provides evidence that heavy element pollution of white dwarfs can originate from disrupted rocky bodies such as asteroids and minor planets.

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arXiv2015

Multiwavelength Transit Observations of the Candidate Disintegrating Planetesimals Orbiting WD 1145+017

We present multiwavelength, multi-telescope, ground-based follow-up photometry of the white dwarf WD 1145+017, that has recently been suggested to be orbited by up to six or more, short-period, low-mass, disintegrating planetesimals. We detect 9 significant dips in flux of between 10% and 30% of the stellar flux from our ground-based photometry. We observe transits deeper than 10% on average every ~3.6 hr in our photometry. This suggests that WD 1145+017 is indeed being orbited by multiple, short-period objects. Through fits to the multiple asymmetric transits that we observe, we confirm that the transit egress timescale is usually longer than the ingress timescale, and that the transit duration is longer than expected for a solid body at these short periods, all suggesting that these objects have cometary tails streaming behind them. The precise orbital periods of the planetesimals in this system are unclear from the transit-times, but at least one object, and likely more, have orbital periods of ~4.5 hours. We are otherwise unable to confirm the specific periods that have been reported, bringing into question the long-term stability of these periods. Our high precision photometry also displays low amplitude variations suggesting that dusty material is consistently passing in front of the white dwarf, either from discarded material from these disintegrating planetesimals or from the detected dusty debris disk. For the significant transits we observe, we compare the transit depths in the V- and R-bands of our multiwavelength photometry, and find no significant difference; therefore, for likely compositions the radius of single-size particles in the cometary tails streaming behind the planetesimals in this system must be ~0.15 microns or larger, or ~0.06 microns or smaller, with 2-sigma confidence.

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arXiv2015

Tidally-driven Roche-Lobe Overflow of Hot Jupiters with MESA

Many exoplanets have now been detected in orbits with ultra-short periods, very close to the Roche limit. Building upon our previous work, we study the possibility that mass loss through Roche lobe overflow (RLO) may affect the evolution of these planets, and could possibly transform a hot Jupiter into a lower-mass planet (hot Neptune or super-Earth). We focus here on systems in which the mass loss occurs slowly ("stable mass transfer" in the language of binary star evolution) and we compute their evolution in detail with the binary evolution code MESA. We include the effects of tides, RLO, irradiation and photo-evaporation of the planet, as well as the stellar wind and magnetic braking. Our calculations all start with a hot Jupiter close to its Roche limit, in orbit around a sun-like star. The initial orbital decay and onset of RLO are driven by tidal dissipation in the star. We confirm that such a system can indeed evolve to produce lower-mass planets in orbits of a few days. The RLO phase eventually ends and, depending on the details of the mass transfer and on the planetary core mass, the orbital period can remain around a few days for several Gyr. The remnant planets have a rocky core and some amount of envelope material, which is slowly removed via photo-evaporation at nearly constant orbital period; these have properties resembling many of the observed super-Earths and sub-Neptunes. For these remnant planets we also predict an anti-correlation between mass and orbital period; very low-mass planets ($M_{\rm pl}\,\lesssim\,5\,M_{\oplus}$) in ultra-short periods ($P_{\rm orb}$<1d) cannot be produced through this type of evolution.

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arXiv2015

Tests of the planetary hypothesis for PTFO 8-8695b

The T Tauri star PTFO 8-8695 exhibits periodic fading events that have been interpreted as the transits of a giant planet on a precessing orbit. Here we present three tests of the planet hypothesis. First, we sought evidence for the secular changes in light-curve morphology that are predicted to be a consequence of orbital precession. We observed 28 fading events spread over several years, and did not see the expected changes. Instead we found that the fading events are not strictly periodic. Second, we attempted to detect the planet's radiation, based on infrared observations spanning the predicted times of occultations. We ruled out a signal of the expected amplitude. Third, we attempted to detect the Rossiter-McLaughlin effect by performing high-resolution spectroscopy throughout a fading event. No effect was seen at the expected level, ruling out most (but not all) possible orientations for the hypothetical planetary orbit. Our spectroscopy also revealed strong, time-variable, high-velocity Hα and Ca H & K emission features. All these observations cast doubt on the planetary hypothesis, and suggest instead that the fading events represent starspots, eclipses by circumstellar dust, or occultations of an accretion hotspot.

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arXiv2014

Tracking the Stellar Longitudes of Starspots in Short-Period Kepler Binaries

We report on a new method for tracking the phases of the orbital modulations in very short-period, near-contact, and contact binary systems systems in order to follow starspots. We apply this technique to Kepler light curves for 414 binary systems that were identified as having anticorrelated O-C curves for the midtimes of the primary and secondary eclipses, or in the case of non-eclipsing systems, their light-curve minima. This phase tracking approach extracts more information about starspot and binary system behavior than may be easily obtained from the O-C curves. We confirm the hypothesis of Tran et al. (2013) that we can successfully follow the rotational motions of spots on the surfaces of the stars in these binaries. In ~34% of the systems, the spot rotation is retrograde as viewed in the frame rotating with the orbital motion, while ~13% show significant prograde spot rotation. The remaining systems show either little spot rotation or erratic behavior, or sometimes include intervals of both types of behavior. We discuss the possibility that the relative motions of spots are related to differential rotation of the stars. It is clear from this study that the motions of the starspots in at least 50% of these short-period binaries are not exactly synchronized with the orbits.

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arXiv2014

Eclipse Timing Variation Analyses of Eccentric Binaries with Close Tertiaries in the Kepler field

We report eclipse timing variation analyses of 26 compact hierarchical triple stars comprised of an eccentric eclipsing ('inner') binary and a relatively close tertiary component found in the {\em Kepler} field. We simultaneously fit the primary and secondary $O-C$ curves of each system for the light-travel time effect (LTTE), as well as dynamical perturbations caused by the tertiary on different timescales. For the first time, we include those contributions of three-body interactions which originate from the eccentric nature of the inner binary. These effects manifest themselves both on the period of the triple system, $P_2$, and on the longer "apse-node" timescale. We demonstrate that consideration of the dynamically forced rapid apsidal motion yields an efficient and independent tool for the determination of the binary orbit's eccentricity and orientation, as well as the 3D configuration of the triple. Modeling the forced apsidal motion also helps to resolve the degeneracy between the shapes of the LTTE and the dynamical delay terms on the $P_2$ timescale, due to the strong dependence of the apsidal motion period on the triple's mass ratio. This can lead to the independent determination of the binary and tertiary masses without the need for independent radial velocity measurements. Through the use of our analytic method for fitting $O-C$ curves we have obtained robust solutions for system parameters for the ten most ideal triples of our sample, and only somewhat less robust, but yet acceptable, fits for the remaining systems. Finally we study the results of our 26 system parameter fits via a set of distributions of various physically important parameters, including mutual inclination angle, and mass and period ratios.

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arXiv2014

The Relation between the Transit Depths of KIC 12557548b & the Stellar Rotation Period

Kawahara and collaborators analyzed the transits of the candidate disintegrating Mercury-mass planet KIC 12557548b and suggested that the transit depths were correlated with the phase of the stellar rotation. We analyze the transit depths of KIC 12557548b and confirm that there is indeed a robust, statistically significant signal in the transit depths at the rotation period of the spotted host star. This signal is more prominent in the first-half of the Kepler data, and is not due to leakage of the rotating spot signal into our measurement of the transit depths, or due to unocculted starspots. We investigate the suggestion that this signal could be due to an active region on the star, emitting enhanced ultraviolet or X-ray radiation leading to an increased mass loss rate of the planet; we confirm that such a scenario could cause both modulation of the transit depths of KIC 12557548b, and small enough transit-timing variations that they might not be detected in the Kepler data. Our preferred explanation for the fact that the transit depths of KIC 12557548b are modulated with the stellar rotation phase is that the candidate transiting planet is occulting starspots on this highly spotted star; such a scenario could cause transit depth variations as large as have been observed, and cause transit-timing variations small enough that they are arguably consistent with the Kepler data.

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arXiv2014

A Study of the Shortest-Period Planets Found With Kepler

We present the results of a survey aimed at discovering and studying transiting planets with orbital periods shorter than one day (ultra--short-period, or USP, planets), using data from the {\em Kepler} spacecraft. We computed Fourier transforms of the photometric time series for all 200,000 target stars, and detected transit signals based on the presence of regularly spaced sharp peaks in the Fourier spectrum. We present a list of 106 USP candidates, of which 18 have not previously been described in the literature. In addition, among the objects we studied, there are 26 USP candidates that had been previously reported in the literature which do not pass our various tests. All 106 of our candidates have passed several standard tests to rule out false positives due to eclipsing stellar systems. A low false positive rate is also implied by the relatively high fraction of candidates for which more than one transiting planet signal was detected. By assuming these multi-transit candidates represent coplanar multi-planet systems, we are able to infer that the USP planets are typically accompanied by other planets with periods in the range 1-50 days, in contrast with hot Jupiters which very rarely have companions in that same period range. Another clear pattern is that almost all USP planets are smaller than 2 $R_\oplus$, possibly because gas giants in very tight orbits would lose their atmospheres by photoevaporation when subject to extremely strong stellar irradiation. Based on our survey statistics, USP planets exist around approximately $(0.51\pm 0.07)\%$ of G-dwarf stars, and $(0.83\pm 0.18)\%$ of K-dwarf stars.

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arXiv2014

Multiwavelength Observations of the Candidate Disintegrating sub-Mercury KIC 12557548b

We present multiwavelength photometry, high angular resolution imaging, and radial velocities, of the unique and confounding disintegrating low-mass planet candidate KIC 12557548b. Our high angular resolution imaging, which includes spacebased HST/WFC3 observations in the optical, and groundbased Keck/NIRC2 observations in K'-band, allow us to rule-out background and foreground candidates at angular separations greater than 0.2 arcsec that are bright enough to be responsible for the transits we associate with KIC 12557548. Our radial velocity limit from Keck/HIRES allows us to rule-out bound, low-mass stellar companions to KIC 12557548 on orbits less than 10 years, as well as placing an upper-limit on the mass of the candidate planet of 1.2 Jupiter masses; therefore, the combination of our radial velocities, high angular-resolution imaging, and photometry are able to rule-out most false positive interpretations of the transits. Our precise multiwavelength photometry includes two simultaneous detections of the transit of KIC 12557548b using CFHT/WIRCam at 2.15 microns and the Kepler space telescope at 0.6 microns, as well as simultaneous null-detections of the transit by Kepler and HST/WFC3 at 1.4 microns. Our simultaneous HST/WFC3 and Kepler null-detections, provide no evidence for radically different transit depths at these wavelengths. Our simultaneous CFHT/WIRCam detections in the near-infrared and with Kepler in the optical reveal very similar transit depths (the average ratio of the transit depths at ~2.15 microns compared to ~0.6 microns is: 1.02 +/- 0.20). This suggests that if the transits we observe are due to scattering from single-size particles streaming from the planet in a comet-like tail, then the particles must be ~0.5 microns in radius or larger, which would favor that KIC 12557548b is a sub-Mercury, rather than super-Mercury, mass planet.

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arXiv2013

The Roche limit for close-orbiting planets: Minimum density, composition constraints, and application to the 4.2-hour planet KOI 1843.03

The requirement that a planet must orbit outside of its Roche limit gives a lower limit on the planet's mean density. The minimum density depends almost entirely on the orbital period and is immune to systematic errors in the stellar properties. We consider the implications of this density constraint for the newly-identified class of small planets with periods shorter than half a day. When the planet's radius is known accurately, this lower limit to the density can be used to restrict the possible combinations of iron and rock within the planet. Applied to KOI 1843.03, with a radius of 0.6 Earth radii and the shortest known orbital period of 4.245 hr, the planet's mean density must be greater than approximately 7 g/cm^3. By modeling the planetary interior subject to this constraint, we find the composition of the planet must be mostly iron, with at most a modest fraction of silicates (less than approximately 30% by mass).

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