Post-main-sequence Evolution of Icy Minor Planets. II. Water Retention and White Dwarf Pollution around Massive Progenitor Stars

Post-main-sequence Evolution of Icy Minor Planets. II. Water Retention and White Dwarf Pollution around Massive Progenitor Stars

Authors:

Malamud et al

Abstract:
Most studies suggest that the pollution of white dwarf (WD) atmospheres arises from the accretion of minor planets, but the exact properties of polluting material, and in particular the evidence for water in some cases, are not yet understood. Here we study the water retention of small icy bodies in exo-solar planetary systems, as their respective host stars evolve through and off the main sequence and eventually become WDs. We explore, for the first time, a wide range of star masses and metallicities. We find that the mass of the WD progenitor star is of crucial importance for the retention of water, while its metallicity is relatively unimportant. We predict that minor planets around lower-mass WD progenitors would generally retain more water and would do so at closer distances from the WD than compared with high-mass progenitors. The dependence of water retention on progenitor mass and other parameters has direct implications for the origin of observed WD pollution, and we discuss how our results and predictions might be tested in the future as more observations of WDs with long cooling ages become available.

DOES A DIFFERENTIATED, CARBONATE-RICH, ROCKY OBJECT POLLUTE THE WHITE DWARF SDSS J104341.53+085558.2?

DOES A DIFFERENTIATED, CARBONATE-RICH, ROCKY OBJECT POLLUTE THE WHITE DWARF SDSS J104341.53+085558.2?

Authors:

Melis et al

Abstract:

We present spectroscopic observations of the dust- and gas-enshrouded, polluted, single white dwarf star SDSS J104341.53+085558.2 (hereafter SDSS J1043+0855). Hubble Space Telescope Cosmic Origins Spectrograph far-ultraviolet spectra combined with deep Keck HIRES optical spectroscopy reveal the elements C, O, Mg, Al, Si, P, S, Ca, Fe, and Ni and enable useful limits for Sc, Ti, V, Cr, and Mn in the photosphere of SDSS J1043+0855. From this suite of elements we determine that the parent body being accreted by SDSS J1043+0855 is similar to the silicate Moon or the outer layers of Earth in that it is rocky and iron-poor. Combining this with comparison to other heavily polluted white dwarf stars, we are able to identify the material being accreted by SDSS J1043+0855 as likely to have come from the outermost layers of a differentiated object. Furthermore, we present evidence that some polluted white dwarfs (including SDSS J1043+0855) allow us to examine the structure of differentiated extrasolar rocky bodies. Enhanced levels of carbon in the body polluting SDSS J1043+0855 relative to the Earth–Moon system can be explained with a model where a significant amount of the accreted rocky minerals took the form of carbonates; specifically, through this model the accreted material could be up to 9% calcium-carbonate by mass.

The Fate of Icy Dwarf Exoplanets in White Dwarf Systems

POST-MAIN SEQUENCE EVOLUTION OF ICY MINOR PLANETS: IMPLICATIONS FOR WATER RETENTION AND WHITE DWARF POLLUTION

Authors:

Malamud et al

Abstract:

Most observations of polluted white dwarf atmospheres are consistent with accretion of water-depleted planetary material. Among tens of known cases, merely two involve accretion of objects that contain a considerable mass fraction of water. The purpose of this study is to investigate the relative scarcity of these detections. Based on a new and highly detailed model, we evaluate the retention of water inside icy minor planets during the high-luminosity stellar evolution that follows the main sequence. Our model fully considers the thermal, physical, and chemical evolution of icy bodies, following their internal differentiation as well as water depletion, from the moment of their birth and through all stellar evolution phases preceding the formation of the white dwarf. We also account for different initial compositions and formation times. Our results differ from previous studies, which have either underestimated or overestimated water retention. We show that water can survive in a variety of circumstances and in great quantities, and therefore other possibilities are discussed in order to explain the infrequency of water detection. We predict that the sequence of accretion is such that water accretes earlier, and more rapidly, than the rest of the silicate disk, considerably reducing the chance of its detection in H-dominated atmospheres. In He-dominated atmospheres, the scarcity of water detections could be observationally biased. It implies that the accreted material is typically intrinsically dry, which may be the result of the inside-out depopulation sequence of minor planets

Explorations of Dusty Debris Disk Geometry

Explorations of Dusty Debris Disk Geometry

Authors:

Dennihy et al

Abstract:

As the sample of white dwarfs with signatures of planetary systems has grown, statistical studies have begun to suggest our picture of compact debris disk formation from disrupted planetary bodies is incomplete. Here we present the results of an effort to extend the preferred dust disk model introduced by \citet{jur03} to include elliptical geometries. We apply this model the observed distribution of fractional infrared luminosities, and explore the difference in preferred parameter spaces for a circular and highly elliptical model on a well-studied dusty white dwarf.

The Fate of Exomoons in White Dwarf Planetary Systems

The Fate of Exomoons in White Dwarf Planetary Systems

Authors:

Payne et al

Abstract:

Roughly 1000 white dwarfs are known to be polluted with planetary material, and the progenitors of this material are typically assumed to be asteroids. The dynamical architectures which perturb asteroids into white dwarfs are still unknown, but may be crucially dependent on moons liberated from parent planets during post-main-sequence gravitational scattering. Here, we trace the fate of these exomoons, and show that they more easily achieve deep radial incursions towards the white dwarf than do scattered planets. Consequently, moons are likely to play a significant role in white dwarf pollution, and in some cases may be the progenitors of the pollution itself.

Merger of a White Dwarf-Neutron Star Binary to 1029 Carat Diamonds: Origin of the Pulsar Planets

Merger of a White Dwarf-Neutron Star Binary to 1029 Carat Diamonds: Origin of the Pulsar Planets

Authors:

Margalit et al

Abstract:

We show that the merger and tidal disruption of a C/O white dwarf (WD) by a neutron star (NS) binary companion provides a natural formation scenario for the PSR B1257+12 planetary system. Starting with initial conditions for the debris disk produced of the disrupted WD, we model its long term viscous evolution, including for the first time the effects of mass and angular momentum loss during the early radiatively inefficient accretion flow (RIAF) phase and accounting for the unusual C/O composition on the disk opacity. For plausible values of the disk viscosity α∼10−3−10−2 and the RIAF mass loss efficiency, we find that the disk mass remaining near the planet formation radius at the time of solid condensation is sufficient to explain the pulsar planets. Rapid rocky planet formation via gravitational instability of the solid carbon-dominated disk is facilitated by the suppression of vertical shear instabilities due to the high solid-to-gas ratio. Additional evidence supporting a WD-NS merger scenario includes (1) the low observed occurrence rate of pulsar planets (≲1% of NS birth), comparable to the expected WD-NS merger rate; (2) accretion by the NS during the RIAF phase is sufficient to spin PSR B1257+12 up to its observed 6 ms period; (3) similar models of `low angular momentum' disks, such as those produced from supernova fallback, find insufficient mass reaching the planet formation radius. The unusually high space velocity of PSR B1257+12 of ≳326kms−1 suggests a possible connection to the Calcium-rich transients, dim supernovae which occur in the outskirts of their host galaxies and were proposed to result from mergers of WD-NS binaries receiving SN kicks. The C/O disk composition implied by our model likely results in carbon-rich planets with diamond interiors.

Mass and eccentricity constraints on the planetary debris orbiting the white dwarf WD 1145+017

Mass and eccentricity constraints on the planetary debris orbiting the white dwarf WD 1145+017

Authors:

Gurri et al

Abstract:

Being the first of its kind, the white dwarf WD 1145+017 exhibits a complex system of disintegrating debris which offers a unique opportunity to study its disruption process in real time. Even with plenty of transit observations there are no clear constraints on the masses or eccentricities of such debris. Using N-body simulations we show that masses greater than approximately 1020 kg (a tenth of the mass of Ceres) or orbits that are not nearly circular (eccentricity>10−3) dramatically increase the chances of the system becoming unstable within two years, which would contrast with the observational data over this timespan. We also provide a direct comparison between transit phase shifts detected in the observations and by our numerical simulations.

Young White Dwarf WD 1536+520 has Solar-like Abudances of Rock Forming Elements

Solar Abundances of Rock Forming Elements, Extreme Oxygen and Hydrogen in a Young Polluted White Dwarf

Authors:

Farihi et al

Abstract:

The Teff = 20,800 K white dwarf WD 1536+520 is shown to have broadly solar abundances of the major rock forming elements O, Mg, Al, Si, Ca, and Fe, together with a strong relative depletion in the volatile elements C and S. In addition to the highest metal abundances observed to date, including log(O/He) = -3.4, the helium-dominated atmosphere has an exceptional hydrogen abundance at log(H/He) = -1.7. Within the uncertainties, the metal-to-metal ratios are consistent with the accretion of an H2O-rich and rocky parent body, an interpretation supported by the anomalously high trace hydrogen. The mixed atmosphere yields unusually short diffusion timescales for a helium atmosphere white dwarf, of no more than a few hundred yr, and equivalent to those in a much cooler, hydrogen-rich star. The overall heavy element abundances of the disrupted parent body deviate modestly from a bulk Earth pattern, and suggest the deposition of some core-like material. The total inferred accretion rate is 4.2e9 g/s, and at least 4 times higher than any white dwarf with a comparable diffusion timescale. Notably, when accretion is exhausted in this system, both metals and hydrogen will become undetectable within roughly 300 Myr, thus supporting a scenario where the trace hydrogen is related to the ongoing accretion of planetary debris.

The Composition of Fragmenting Asteroids Around White Dwarf WD 1145+017

Spectroscopic Evolution of Disintegrating Planetesimals: Minutes to Months Variability in the Circumstellar Gas Associated with WD 1145+017

Authors:

Redfield et al

Abstract:

With the recent discovery of transiting planetary material around WD 1145+017, a critical target has been identified that links the evolution of planetary systems with debris disks and their accretion onto the star. We present a series of observations, five epochs over a year, taken with Keck and the VLT, which for the first time show variability of circumstellar absorption in the gas disk surrounding WD 1145+017 on timescales of minutes to months. Circumstellar absorption is measured in more than 250 lines of 14 ions among ten different elements associated with planetary composition, e.g., O, Mg, Ca, Ti, Cr, Mn, Fe, Ni. Broad circumstellar gas absorption with a velocity spread of 225 km/s is detected, but over the course of a year blue shifted absorption disappears while redshifted absorption systematically increases. A correlation of equivalent width and oscillator strength indicates that the gas is not highly optically thick (median tau approximately 2). We discuss simple models of an eccentric disk coupled with magnetospheric accretion to explain the basic observed characteristics of these high resolution and high signal-to-noise observations. Variability is detected on timescales of minutes in the two most recent observations, showing a loss of redshifted absorption for tens of minutes, coincident with major transit events and consistent with gas hidden behind opaque transiting material. This system currently presents a unique opportunity to learn how the gas causing the spectroscopic, circumstellar absorption is associated with the ongoing accretion evidenced by photospheric contamination, as well as the transiting planetary material detected in photometric observations.

Observing Asteroid Fragmentation Around White Dwarf WD 1145+017

WD 1145+017 Photometric Observations During 8 Months of High Activity

Authors:

Gary et al

Abstract:

WD 1145+017 was observed from 2015 November to 2016 July for the purpose of characterizing transit behavior of the white dwarf by dust clouds thought to be produced by fragments of an asteroid in close orbit with the star. Fortuitously, most of these observations were carried out during a time when the overall "dip" activity was dramatically enhanced over that during its discovery with K2. By the end of our reported observations the dip activity had declined to a level close to its original state. Three notable events were observed. In 2016 January a large number of dust clouds appeared that had an orbital period of 4.4912 hours, and this event also marked the end of a 3-month interval of individual dip appearances that were dominated by short-lived activity producing an apparent 4.5004-hour periodicity that previous studies associated with the Kepler K2 "A" period. The second event was a 2016 April 21 appearance of four dip features with drift lines in a waterfall (date vs. phase) diagram that diverged from their origin date, and which lasted for two weeks. These dips appeared at a location in the orbit that cannot be explained as fragments that had just broken away from the "A" asteroid. The third event was the sudden appearance of a dip feature with a period of 4.6064 hours, which is essentially the same as the Kepler K2 "B" period. The evolution of dip shape, depth, and total fade amount provide constraints on dust production and loss mechanisms. Collisions can account for the sudden appearance of dust clouds, and the sudden increase in dust amount, but another mechanism for continual dust production is also required.

The gaseous debris disk of the white dwarf SDSS J1228+1040

The gaseous debris disk of the white dwarf SDSS J1228+1040. HST/COS search for far-ultraviolet signatures

Authors:

Hartmann et al

Abstract:

Gaseous and dust debris disks around white dwarfs (WDs) are formed from tidally disrupted planetary bodies. This offers an opportunity to determine the composition of exoplanetary material by measuring element abundances in the accreting WD's atmosphere. A more direct way to do this is through spectral analysis of the disks themselves. Currently, the number of chemical elements detected through disk emission-lines is smaller than that of species detected through lines in the WD atmospheres.

We assess the far-ultraviolet (FUV) spectrum of one well-studied object (SDSS J122859.93+104032.9) to search for disk signatures at wavelengths greater than 1050 angstrom, where the broad absorption lines of the Lyman series effectively block the WD photospheric flux. We performed FUV observations (950-1240 angstrom) with the Hubble Space Telescope/Cosmic Origins Spectrograph and used archival optical spectra. We compared them with non-local thermodynamic equilibrium model spectra. In addition, we investigate the Ca II infrared triplet (IRT) line profiles to constrain disk geometry and composition. 

<1050 absorption="" addition="" and="" angstrom="" archival="" block="" br="" broad="" ca="" compared="" composition.="" constrain="" disk="" effectively="" equilibrium="" flux.="" fuv="" geometry="" hubble="" ii="" in="" infrared="" investigate="" line="" lines="" lyman="" model="" non-local="" observations="" of="" optical="" origins="" osmic="" performed="" photospheric="" profiles="" series="" space="" spectra.="" spectrograph="" telescope="" the="" them="" thermodynamic="" to="" triplet="" used="" wd="" we="" where="" with=""> No disk emission-lines were detected in the FUV spectrum, indicating that the disk effective temperature is about 5000K. The long-time variability of the Ca II IRT was reproduced with a precessing disk model of bulk Earth-like composition, having a surface mass density of 0.3g/cm2 and an extension from 55 to 90 WD radii. The disk has a spiral shape that precesses with a period of approximately 37 years, confirming previous results.

HD 107148B: the Second White Dwarf Found Orbiting an Exoplanet Host Star at 1,000 AU

Follow-up spectroscopic observations of HD 107148 B: A new white dwarf companion of an exoplanet host star

Authors:

Mugrauer et al

Abstract:

We report on our follow-up spectroscopy of HD 1071478 B, a recently detected faint co-moving companion of the exoplanet host star HD 107148 A. The companion is separated from its primary star by about 35″ (or 1790 AU of projected separation) and its optical and near infrared photometry is consistent with a white dwarf, located at the distance of HD 107148 A. In order to confirm the white dwarf nature of the co-moving companion, we obtained follow-up spectroscopic observations of HD 107148 B with CAFOS at the CAHA 2.2 m telescope. According to our CAFOS spectroscopy HD 107148 B is a DA white dwarf with an effective temperature in the range between 5900 and 6400K. The properties of HD 107148 B can further be constrained with the derived effective temperature and the known visual and infrared photometry of the companion, using evolutionary models of DA white dwarfs. We obtain for HD 107148 B a mass of 0.56 ± 0.05 M⊙, a luminosity of (2.0 ± 0.2) × 10–4 L⊙, log g [cm s–2]) = 7.95 ± 0.09, and a cooling age of 2100 ± 270 Myr. With its white dwarf companion the exoplanet host star HD 107148 A forms an evolved stellar system, which hosts at least one exoplanet. So far, only few of these evolved systems are known, which represent only about 5 % of all known exoplanet host multiple stellar systems. HD 107148 B is the second confirmed white dwarf companion of an exoplanet host star with a projected separation to its primary star of more than 1000 AU.

Gray transits of WD 1145+017 over the visible band⋆

Gray transits of WD 1145+017 over the visible band⋆

Authors:

Alonso et al

Abstract:

We have observed several relatively deep transits of the white dwarf WD 1145+017 with the Gran Telescopio Canarias (GTC) in the wavelength range 480 to 920 nm. The observations covered approximately one hour on 2016 January 18 and two hours on 2016 January 20. There was variable extinction of the white dwarf during much of that time, but this extinction was punctuated by four sharp transits with depths ranging from 25% to 40%. The spectrum was dispersed with a grism and the flux data were ultimately summed into four bands centered at 0.53, 0.62, 0.71, and 0.84 μm. After careful normalization, we find that the flux light curves in all four bands are consistently the same, including through the deepest dips. We use these results to compute Ångström exponents, α, for the particles responsible for the extinction and find |⟨ α ⟩| ≲ 0.06, assuming that the extinction is relatively optically thin. We use the complex indices of refraction for common minerals to set constraints on the median sizes of possible dust grains and find that particle sizes ≲0.5 μm can be excluded for most common minerals.

Two Candidate Short Period Rocky Planets Around White Dwarfs Found

A Search for short-period Rocky Planets around WDs with the Cosmic Origins Spectrograph (COS)

Authors:

Sandhaus et al

Abstract:

The search for transiting habitable exoplanets has broadened to include several types of stars that are smaller than the Sun in an attempt to increase the observed transit depth and hence the atmospheric signal of the planet. Of all spectral types, white dwarfs are the most favorable for this type of investigation. The fraction of white dwarfs that possess close-in rocky planets is unknown, but several large angle stellar surveys have the photometric precision and cadence to discover at least one if they are common. Ultraviolet observations of white dwarfs may allow for detection of molecular oxygen or ozone in the atmosphere of a terrestrial planet. We use archival Hubble Space Telescope data from the Cosmic Origins Spectrograph to search for transiting rocky planets around UV-bright white dwarfs. In the process, we discovered unusual variability in the pulsating white dwarf GD 133, which shows slow sinusoidal variations in the UV. While we detect no planets around our small sample of targets, we do place stringent limits on the possibility of transiting planets, down to sub-lunar radii. We also point out that non-transiting small planets in thermal equilibrium are detectable around hotter white dwarfs through infrared excesses, and identify two candidates.

Circumstellar Debris and Pollution at White Dwarf Stars

Circumstellar Debris and Pollution at White Dwarf Stars

Authors:

Farihi et al

Abstract:

Circumstellar disks of planetary debris are now known or suspected to closely orbit hundreds of white dwarf stars. To date, both data and theory support disks that are entirely contained within the preceding giant stellar radii, and hence must have been produced during the white dwarf phase. This picture is strengthened by the signature of material falling onto the pristine stellar surfaces; disks are always detected together with atmospheric heavy elements. The physical link between this debris and the white dwarf host abundances enables unique insight into the bulk chemistry of extrasolar planetary systems via their remnants. This review summarizes the body of evidence supporting dynamically active planetary systems at a large fraction of all white dwarfs, the remnants of first generation, main-sequence planetary systems, and hence provide insight into initial conditions as well as long-term dynamics and evolution.

Liberating exomoons in white dwarf planetary systems

Liberating exomoons in white dwarf planetary systems

Authors:

Payne et al

Abstract:

Previous studies indicate that more than a quarter of all white dwarf (WD) atmospheres are polluted by remnant planetary material, with some WDs being observed to accrete the mass of Pluto in 10^6 years. The short sinking timescale for the pollutants indicate that the material must be frequently replenished. Moons may contribute decisively to this pollution process if they are liberated from their parent planets during the post-main-sequence evolution of the planetary systems. Here, we demonstrate that gravitational scattering events among planets in WD systems easily triggers moon ejection. Repeated close encounters within tenths of a planetary Hill radii are highly destructive to even the most massive, close-in moons. Consequently, scattering increases both the frequency of perturbing agents in WD systems, as well as the available mass of polluting material in those systems, thereby enhancing opportunities for collision and fragmentation and providing more dynamical pathways for smaller bodies to reach the WD. Moreover, during intense scattering, planets themselves have pericenters with respect to the WD of only a fraction of an AU, causing extreme Hill-sphere contraction, and the liberation of moons into WD-grazing orbits. Many of our results are directly applicable to exomoons orbiting planets around main sequence stars.

Remnant Planetary Systems Around Bright White Dwarfs

Remnant Planetary Systems Around Bright White Dwarfs

Authors:

Barber et al

Abstract:

We cross-correlate several sources of archival photometry for 1265 bright (V∼16 mag) white dwarfs (WDs) with available high signal-to-noise spectroscopy. We find 381 WDs with archival Spitzer+IRAC data and investigate this subsample for infrared excesses due to circumstellar dust. This large data set reveals 15 dusty WDs, including three new debris disks and the hottest WD known to host dust (WD 0010+280). We study the frequency of debris disks at WDs as function of mass. The frequency peaks at 12.5% for 0.7-0.75 M⊙ WDs (with 3 M⊙ main-sequence star progenitors) and falls off for stars more massive than this, which mirrors predicted planet occurrence rates for stars of different masses.

White Dwarf Star Steals Enough Material to Transform Star into Brown Dwarf

An irradiated brown-dwarf companion to an accreting white dwarf

Authors:

Hernández Santisteban et al

Abstract:

Interacting compact binary systems provide a natural laboratory in which to study irradiated substellar objects. As the mass-losing secondary (donor) in these systems makes a transition from the stellar to the substellar regime, it is also irradiated by the primary (compact accretor)1, 2. The internal and external energy fluxes are both expected to be comparable in these objects, providing access to an unexplored irradiation regime. The atmospheric properties of donors are largely unknown3, but could be modified by the irradiation. To constrain models of donor atmospheres, it is necessary to obtain accurate observational estimates of their physical properties (masses, radii, temperatures and albedos). Here we report the spectroscopic detection and characterization of an irradiated substellar donor in an accreting white-dwarf binary system. Our near-infrared observations allow us to determine a model-independent mass estimate for the donor of 0.055 ± 0.008 solar masses and an average spectral type of L1 ± 1, supporting both theoretical predictions and model-dependent observational constraints that suggest that the donor is a brown dwarf. Our time-resolved data also allow us to estimate the average irradiation-induced temperature difference between the dayside and nightside of the substellar donor (57 kelvin) and the maximum difference between the hottest and coolest parts of its surface (200 kelvin). The observations are well described by a simple geometric reprocessing model with a bolometric (Bond) albedo of less than 0.54 at the 2σ confidence level, consistent with high reprocessing efficiency, but poor lateral heat redistribution in the atmosphere of the brown-dwarf donor4, 5. These results add to our knowledge of binary evolution, in that the donor has survived the transition from the stellar to the substellar regime, and of substellar atmospheres, in that we have been able to test a regime in which the irradiation and the internal energy of a brown dwarf are comparable.

Did the Cool White Dwarf NLTT19868 eat a Terrestial Exoplanet?

Extreme abundance ratios in the polluted atmosphere of the cool white dwarf NLTT19868

Authors:

Kawka et al

Abstract:

We present an analysis of intermediate-dispersion spectra and photometric data of the newly identified cool, polluted white dwarf NLTT19868. The spectra obtained with X-shooter on the Very Large Telescope (VLT)-Melipal show strong lines of calcium, and several lines of magnesium, aluminium and iron. We use these spectra and the optical-to-near infrared spectral energy distribution to constrain the atmospheric parameters of NLTT19868. Our analysis shows that NLTT19868 is iron poor with respect to aluminium and calcium. A comparison with other cool, polluted white dwarfs shows that the Fe to Ca abundance ratio (Fe/Ca) varies by up to approximately two orders of magnitudes over a narrow temperature range with NLTT19868 at one extremum in the Fe/Ca ratio and, in contrast, NLTT888 at the other extremum. The sample shows evidence of extreme diversity in the composition of the accreted material: In the case of NLTT888, the inferred composition of the accreted matter is akin to iron-rich planetary core composition, while in the case of NLTT19868 it is close to mantle or bulk-Earth composition depleted by subsequent chemical separation at the bottom of the convection zone.

1/10th the Mass of Ceres' Worth of Drifting Asteroid Fragments Around WD 1145+017

Drifting Asteroid Fragments Around WD 1145+017

Authors:

Rappaport et al

Abstract:

We have obtained extensive photometric observations of the polluted white dwarf WD 1145+017 which has been reported to be transited by at least one, and perhaps several, large asteroids (or, planetesimals) with dust emission. We have carried out 53 observation sessions on 37 nights, totaling 192 hours, of this 17th magnitude star with small to modest size telescopes covering the interval 2015 November 1 to 2016 January 21. In all, we have detected some 237 significant dips in flux. Periodograms of the data reveal a significant periodicity of 4.5004 hours that is consistent with the dominant ("A") period detected with K2. The folded light curve at this period shows there is an hour-long depression in flux with a mean depth of nearly 10%. This depression is comprised of a series of shorter and sometimes deeper dips that do not always occur at exactly the same orbital phase, and which would be unresolvable with K2. In fact, we also find numerous dips in flux at other orbital phases. Nearly all of the dips associated with this activity appear to drift systematically in phase with respect to the "A" period by about 2.5 minutes per day with a dispersion of ~0.5 min/d, corresponding to a mean drift period of 4.4928 hours. In all, we can track approximately 15 of these drifting features. There is no detection of the "B"-"F" periods found with K2, but if they remain at the K2 levels we would not expect to have seen them. We explain the drifting motion as that of smaller bodies ('fragments') that break off from the asteroid and go into a slightly smaller orbit than that of the asteroid. If our interpretation is correct, we can use the drift rate to determine the mass of the asteroid. Under that scenario, we find that the mass of the asteroid is M_a ~= 10^{23} grams, or about 1/10th the mass of Ceres, with an uncertainty of about a factor of 2.