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Planet Hunters X.

KIC 8462852 – Where’s the ﬂux?

†

arXiv:1509.03622v1 [astro-ph.SR] 11 Sep 2015

T. S. Boyajian

, D. M. LaCourse

, S. A. Rappaport

D. Fabrycky

, D. A. Fischer

, D. Gandolﬁ

5,6

, G. M. Kennedy

, M. C. Liu

, A. Moor

K. Olah

, K. Vida

, M. C. Wyatt

, W. M. J. Best

, F. Ciesla

, B. Cs´ k

, T. J. Dupuy

G. Handler

, K. Heng

, H. Korhonen

15,16

, J. Kov´ cs

, T. Kozakis

, L. Kriskovics

, J.

R. Schmitt

, Gy. Szabo

9,11,18

, R. Szabo

, J. Wang

1,19

, S. Goodman

, A. Hoekstra

, K. J.

Jek

Department

Amateur

of Astronomy, Yale University, New Haven, CT 06511, USA

Astronomer

Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA

Landessternwarte K¨ nigstuhl, Zentrum f¨ r Astronomie der Universit¨ t Heidelberg, K¨ nigstuhl 12, D-69117 Heidelberg, Germany

Dipartimento di Fisica, Universit´ di Torino, via P. Giuria 1, I-10125, Torino, Italy

Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu HI 96822, USA

Konkoly Observatory, Research Centre of Astronomy and Earth Sciences, Hungarian Academy of Sciences, H-1121 Budapest, Konkoly Th. M. ut 15 –17, Hungary

Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637

ELTE Gothard Astrophysical Observatory, H-9704 Szombathely, Szent Imre herceg ut 112, Hungary

The University of Texas at Austin, Department of Astronomy, 2515 Speedway C1400, Austin, TX 78712, USA

Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland

University of Bern, Center for Space and Habitability, Sidlerstrasse 5, CH-3012, Bern, Switzerland

Finnish Centre for Astronomy with ESO (FINCA), University of Turku, V¨ is¨ l¨ ntie 20, FI-21500 Piikki¨ , Finland

a aa

Centre for Star and Planet Formation, Niels Bohr Institute, University of Copenhagen, Øster Voldgade 5-7, DK-1350, København K, Denmark

Carl Sagan Institute, Cornell University, Ithaca, NY 14853, USA

Gothard-Lend¨ let Research Team, H-9704 Szombathely, Szent Imre herceg ut 112, Hungary

California Institute of Technology, Pasadena, CA 91109, USA

14 September 2015

ABSTRACT

Over the duration of the

Kepler

mission, KIC 8462852 was observed to undergo irregularly

shaped, aperiodic dips in ﬂux down to below the 20% level. The dipping activity can last for

between 5 and 80 days. We characterize the object with high-resolution spectroscopy, spectral

energy distribution ﬁtting, and Fourier analyses of the

Kepler

light curve. We determine that

KIC 8462852 is a main-sequence F3 V/IV star, with a rotation period

∼

0.88

d, that exhibits

no signiﬁcant IR excess. In this paper, we describe various scenarios to explain the mysterious

events in the

Kepler

light curve, most of which have problems explaining the data in hand.

By considering the observational constraints on dust clumps orbiting a normal main-sequence

star, we conclude that the scenario most consistent with the data in hand is the passage of

a family of exocomet fragments, all of which are associated with a single previous breakup

event. We discuss the necessity of future observations to help interpret the system.

Key words:

stars: individual (KIC 8462852), chaos, stars: peculiar, stars: activity, comets:

general, planets and satellites: dynamical evolution and stability

Based on observations obtained with the Nordic Optical Telescope, oper-

ated on the island of La Palma jointly by Denmark, Finland, Iceland, Nor-

way, and Sweden, in the Spanish Observatorio del Roque de los Muchachos

of the Instituto de Astroﬁsica de Canarias.

†

The data presented herein were obtained at the W.M. Keck Observatory,

INTRODUCTION

tion analysis. We discover a wide M-dwarf companion to the sys-

tem and argue that with the data sets we have in-hand, we can

exclude the presence of an additional gravitationally bound com-

panion nearby. In Section

we visit possible explanations for the

peculiar observations of KIC 8462852, including instrumental arti-

facts, intrinsic/extrinsic variability, and a variety of scenarios invok-

ing light-blocking events. In Section

we conclude by discussing

future observations needed to constrain the nature of the object.

For over four years, NASA’s

Kepler

mission measured the bright-

ness of objects within a

∼

100

square-degree patch of sky in the

direction of the constellations Cygnus and Lyrae. The program’s

targets were primarily selected to address the

Kepler

mission goals

of discovering Earth-like planets orbiting other stars.

Kepler

tar-

geted over

150, 000

stars, primarily with a 30-minute observing

cadence, leading to over 2.5-billion data points per year (>

bil-

lion data points over the nominal mission lifetime).

The

Kepler

mission’s data processing and identiﬁcation of

transiting planet candidates was done in an automated manner

through sophisticated computer algorithms (e.g.,

Jenkins et al.

2010).

Complementary to this analysis, the Zooniverse citizen sci-

ence network provided the means to crowd source the review of

light curves with the Planet Hunters project

(e.g.,

Fischer et al.

2012).

In this framework, Planet Hunter volunteers view 30 day

segments of light curves in the ‘Classify’ web interface. A vol-

unteer’s main task is to identify signals of transiting planets by

harnessing the human eye’s unique ability for pattern recognition.

This process has shown to have a detection efﬁciency to identify

planetary transits

85%

using the ﬁrst Quarter of

Kepler

data

(Schwamb

et al. 2012).

The Planet Hunters project has now discov-

ered almost a hundred exoplanet candidates, including several con-

ﬁrmed systems (Fischer

et al. 2012; Lintott et al. 2013; Schwamb

et al. 2013; Wang et al. 2013; Schmitt et al. 2014).

Because Planet Hunter volunteers look at every light curve by

eye, serendipitous discoveries are inevitable, especially in rich data

sets such as that which

Kepler

has provided. As such, a key aspect

of the Planet Hunters project is the ‘Talk’ interface. ‘Talk’ is a back-

end site where volunteers can discuss light curves and present fur-

ther analysis on objects viewed in the main ‘Classify’ interface. In

a handful of cases, such as the discovery of the unusual cataclysmic

variable, KIC 9406652 (Gies

et al. 2013),

the default aperture mask

used to generate the

Kepler

light curve was not perfectly centered

on the object of interest. Because of this, interesting events in the

Kepler

light curve would appear to come and go as a result of the

shifting orientation of the aperture mask when the spacecraft un-

derwent a quarterly rotation. Events such as these are tagged and

discussed on ‘Talk’, making it possible to return to the raw data tar-

get pixel ﬁles (TPF) to extract improved light curves with modiﬁed

aperture masks, for example.

This paper presents the discovery of a mysterious dipping

source, KIC 8462852, from the Planet Hunters project. In just the

ﬁrst quarter of

Kepler

data, Planet Hunter volunteers identiﬁed

KIC 8462852’s light curve as a “bizarre”, “interesting”, “giant tran-

sit” (Q1 event depth was 0.5% with a duration of

days). As new

Kepler

data were released in subsequent quarters, discussions con-

tinued on ‘Talk’ about KIC 8462852’s light curve peculiarities, par-

ticularly ramping up pace in the ﬁnal observations quarters of the

Kepler mission.

In this work we examine the full 4 years of

Kepler

observa-

tions of KIC 8462852 as well as supplemental information pro-

vided by additional ground- and space-based observations. In Sec-

tion

we characterize KIC 8462852 using

Kepler

photometry,

spectroscopic analysis, AO imaging, and spectral energy distribu-

DATA

KIC 8462852, also known as TYC 3162-665-1 and 2MASS

J20061546+4427248, is a

∼

mag star in the Kepler ﬁeld

of view. As mentioned above in the previous section, it was identi-

ﬁed serendipitously by the Planet Hunters project, and was deemed

an interesting object that was worthy of further investigation. In the

following sections, we characterize the system with data from

Ke-

pler

as well as additional data from various targeted and archived

programs.

2.1

Kepler

photometry

KIC 8462852 was observed throughout the main

Kepler

mission

(Quarters 0 – 17) under long-cadence (30-minute) observations

yielding an ultra-precise light curve spanning a time baseline

of four years. In this work, our analysis uses the normalized,

PDCSAP FLUX data. Note that we have thoroughly validated the

data to ensure that any ﬂux variations represent physical events in

or near the star (and they do); these processes are described in detail

within Section

4.1,

and we do not repeat them here.

In Figure

we present a montage of plots capturing much the

interesting ﬂux variations observed in the

Kepler

timeseries data.

The top two panels, ‘(a)’ and ‘(b)’, show the ﬂux time series for the

entire

Kepler

mission, but with different vertical ﬂux scales. These

show that the ﬂux is relatively constant for most of that time, but is

punctuated by a number of substantial dips in ﬂux, including a 15%

drop near day 800, and a whole sequence of dips (with one reach-

ing a depth of 22%) after day 1500. For convenience, we hereafter

refer to the two main dip structures between day 788 and 795 and

between day 1510 and 1570, as events ‘D800’ and ‘D1500’, re-

spectively. There are also other smaller dips, including two earlier

in the mission (around day 140 and day 260). Panel ‘(c)’ is a zoom

in on the dip D800. The remaining three panels are progressively

zoomed in around the exotic complex of dips at D1500. Virtually

all of the ﬂuctuations in intensity visible on these plots are real, i.e.,

not due to statistical or instrumental variations (Section

4.1).

There are modulations in the raw ﬂux data at the

∼

200

ppm

level which are visible by eye. To further explore whether any of

these modulations are periodic, or have a periodic component, we

generated a Fourier transform (FT) of the data with the dips excised

from the data train. Figure

shows the FT of the

Kepler

photometry

and one can see a clear periodicity of 0.88 day (1.14 cycles/day) and

its next two higher harmonics.

This 0.88-day signal is a broad feature that resembles typical

FTs of Kepler targets for early type stars (Balona

2013,

see their

ﬁgure 6). If this is a rotation period, then the projected rotational

velocity (from Section

2.2)

84±4

km s

−1

represents a minimum

stellar radius of

∼

1.46

R , consistent with the radius of an F-type

star (also see Section

2.2).

Also seen in Figure

just to the left

of the base frequency is a broad collection of smaller peaks. This

which is operated as a scientiﬁc partnership among the California Institute

of Technology, the University of California, and the National Aeronautics

and Space Administration. The Observatory was made possible by the gen-

erous ﬁnancial support of the W.M. Keck Foundation.

www.planethunters.org

KIC 8462852 – Where’s the ﬂux?

01 2

1.00

Normalized flux

0.95

0.90

0.85

0.80

0.75

500

1000

Time (BJD-2454833)

1500

1.01

1.00

Normalized flux

0.99

0.98

0.97

0.96

500

1000

Time (BJD-2454833)

1500

1.00

Normalized flux

0.90

Normalized flux

0.95

0.90

0.85

0.80

786

788

790 792 794 796

Time (BJD-2454833)

798

800

0.85

0.80

1500

1520

1540

1560

Time (BJD-2454833)

1580

1.01

1.00

Normalized flux

0.99

0.98

0.97

0.96

0.95

1500

1520

1540

1560

Time (BJD-2454833)

1580

Normalized flux

1.010

1.005

1.000

0.995

0.990

0.985

0.980

1500

1520

1540

1560

Time (BJD-2454833)

1580

Figure 1.

Montage of ﬂux time series for KIC 8462852 showing different portions of the 4-year

Kepler

observations with different vertical scalings. The top

two panels show the entire

Kepler

observation time interval. The starting time of each

Kepler

quarter is marked and labeled with a red vertical line in the top

panel ‘(a)’. Panel ‘(c)’ is a blowup of the dip near day 793, (D800). The remaining three panels, ‘(d)’, ‘(e)’, and ‘(f)’, explore the dips which occur during the

90-day interval from day 1490 to day 1580 (D1500). Refer to Section

2.1

for details. See Section

2.1

for details.

1.4

Frequency (cycles/day)

1.2

1.0

0.8

300.0

600.0

900.0

1200.0

Kepler time (days)

Figure 2.

Fourier transform for KIC 8462852. The peaks are labeled with

the harmonic numbers starting with 1 for the base frequency. Refer to Sec-

tion

2.1

for details.

Figure 3.

The STFT for the

Kepler

ﬂux time series. The main base period of

∼

0.88

days is present throughout the span of observations. We identify (at

least) two additional frequencies appearing around day 400 and 1400, cor-

responding to periods of 0.96 to 0.90 days, which we attribute to differential

rotation. Refer to Section

2.1

for details.

suggests that something more complicated than a single rotating

surface inhomogeneity is producing the observed signal.

We investigate the stability of the frequencies observed in the

FT by performing a Short-Term Fourier Transform (STFT), again

clipping the data in the dipping regions. In the STFT method, the

data are broken up into “short” segments of

∼

d, the FT is com-

puted and displayed vertically on the plot, and this is repeated as a

function of time, with overlap in time segments to gain back some

temporal resolution.

The STFT is presented in Figure

This shows that the

0.88 day signal is present in most of the

Kepler

time series, with

the strongest presence occurring around day 1200. Interestingly

however, around day 400 and day 1400, we see major contribu-

tions at different frequencies, corresponding to

∼

0.96

days and

∼

0.90

days, respectively. We conclude that these are the source of

the broad collection of peaks to the left of the base frequency noted

above. These low-frequency side-bands could possibly be due to

regions contrasted in ﬂux (e.g., starspots, chemically peculiar re-

gions) appearing at higher latitudes coupled with differential ro-

tation. This is consistent with the differential rotation (or inferred

fractional frequency difference of

∼

10%)

for F-type stars (Rein-

hold et al. 2013).

We would like to note however, that we can-

not completely discount the possibility that these periods are due

to pulsations. The position of KIC 8462852 is within the Gamma

Doradus (γ Dor) region of the instability strip, where pulsations

are observed at

cycles d

−1

(e.g.,

Uytterhoeven et al. 2011).

Our interpretation of starspots relies on comparing the STFT of

KIC 8462852 to the STFT of known

Dor pulsators: we ﬁnd that

the dominant frequencies for

Dor stars do not evolve with time in

the STFT.

We also report on the presence of a possible 10 – 20 day pe-

riod (Figure

2),

which, when present, is visible by eye in the light

curve

. We illustrate this in Figure

showing zoomed in regions

of the

Kepler

light curve. The star’s 0.88 d period is evident in

each section as the high-frequency ﬂux variations. The panel sec-

ond from the bottom ‘(c)’) shows no low-frequency (10 – 20 day)

variations, but the rest do. We have no current hypothesis to explain

this signal.

2.2

Spectroscopy

Also present in the raw SAP data.

We obtained two high resolution (R

= 47000)

spectra of

KIC 8462852 with the FIES spectrograph (Frandsen

& Lindberg

1999; Telting et al. 2014)

mounted at the 2.56-m Nordic Optical

Telescope (NOT) of Roque de los Muchachos Observatory in La

Palma, Spain. The observations were performed on 11 August and

5 November 2014. The data were reduced using standard proce-

dures, which include bias subtraction, ﬂat ﬁelding, order tracing

and extraction, and wavelength calibration. The extracted spectra

have a S/N ratio of 45–55 per pixel at 5500 A.

Following the same spectral analysis procedure described in

Rappaport et al.

(2015), we used the co-added FIES spectrum

to determine the stellar effective temperature

eﬀ

, surface grav-

ity

log

projected rotational velocity

sin

metal abundance

[M/H], and spectral type of KIC 8462852 (Table

2).

The plots in

Figure

show select regions of the observed spectrum (black)

along with the best ﬁt model (red). The temperature we derive

eﬀ

= 6750

140

K) is consistent with the photometric esti-

mate of

eﬀ

= 6584

+178

K from the revised Kepler Input Catalog

−279

properties (Huber

et al. 2014),

as well as with

eﬀ

= 6780

K de-

rived from the empirical

−

color-temperature relation from

Boyajian et al.

(2013). The projected rotational velocity we mea-

sure

sin

= 84

km s

−1

is also well in line with the one

predicted from rotation in Section

2.1,

if the 0.88 d signal is in

fact the rotation period. Overall, the star’s spectrum is unremark-

able, as it looks like an ordinary early F-star with no signs of any

emission lines or P-Cygni proﬁles. Finally, we use the stellar prop-

erties derived from our spectroscopic analysis to estimate a stellar

mass

= 1.43

M , luminosity

log

= 0.67

L , and radius

= 1.58

R , corresponding to a main-sequence F3 V star (Pecaut

KIC 8462852 – Where’s the ﬂux?

1.0010

1.0005

1.0000

0.9995

0.9990

0.9985

190

1.0010

1.0005

1.0000

200

210

220

230

240

Normalized flux

0.9995

0.9990

0.9985

360

370

380

390

1.0010

1.0005

1.0000

0.9995

0.9990

0.9985

1025

1.0010

1.0005

1.0000

0.9995

0.9990

0.9985

1030

1035

1040

1045

1050

1240

1250

1260

1270

1280

Time (BJD-2454833)

Figure 4.

Stacked plots showing a zoomed-in portion of the

Kepler

light

curve. The star’s rotation period of 0.88 d is seen in each panel as the high-

frequency modulation in ﬂux. With the exception of panel ‘c)’, a longer

term (10 –20 day) brightness variation is observed, also present in the FT

shown in Figure

Refer to Section

2.1

for details.

Figure 5.

NOT spectrum closeups for KIC 8462852, the best ﬁt stellar

model shown in red. Panels show region near H

α,

β,

Mg, and Na D (top

to bottom). The bottom panel shows both the stellar (broad) and interstellar

(narrow) counterparts of the Na D lines. Refer to Section

2.2

for details.

& Mamajek 2013)

. Combining the this radius, the projected rota-

tional velocity, and rotation period (Section

2.1),

we determine a

stellar rotation axis inclination of 68 degrees.

While interstellar medium features are not typically related

to indicators of astrophysically interesting happenings in stars, we

note the presence of stellar and interstellar Na D lines in our spec-

tra. In the bottom panel of Figure

we show a close up of the

region containing the Na D lines (λλ5890,

5896

A). Within the two

broad stellar features, there are two very deep and narrow Na D

lines with split line proﬁles, indicating the presence of two discrete

ISM clouds with different velocities of

∼

km s

−1

2.3

Imaging

http://www.pas.rochester.edu/˜emamajek/EEM_

dwarf_UBVIJHK_colors_Teff.txt

Figure

shows the UKIRT image of KIC 8462852 as well as a simi-

larly bright source

∼

away. The PSF of KIC 8462852 is asym-

metric by comparison, leading us to speculate that KIC 8462852

has a faint companion star about

1.5

−

away.

We observed KIC 8462852 on 2014 Oct 16 UT using the nat-

ural guide star adaptive optics (AO) system (Wizinowich

et al.

2000)

of the 10-meter Keck II Telescope on Mauna Kea, Hawaii.

We used the facility IR camera NIRC2 and the

(1.25

µm),

(1.64

µm),

and

(2.20

µm)

ﬁlters from the Mauna Kea Observato-

ries (MKO) ﬁlter consortium (Simons

& Tokunaga 2002; Tokunaga

et al. 2002).

We used NIRC2’s narrow camera, which produces a

0.00994 pixel

−1

scale and a 10.2 ﬁeld of view. Conditions were

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