K2 DISCOVERS A BUSY BEE: AN UNUSUAL TRANSITING
NEPTUNE FOUND IN THE BEEHIVE CLUSTER
Item Type Article
Authors Obermeier, Christian; Henning, Thomas; Schlieder, Joshua E.;
Crossfield, Ian J. M.; Petigura, Erik; Howard, Andrew W.; Sinukoff,
Evan; Isaacson, Howard T.; Ciardi, David R.; David, Trevor J.;
Hillenbrand, Lynne A.; Beichman, Charles A.; Howell, Steve
B.; Horch, Elliott; Everett, Mark; Hirsch, Lea; Vogt, Steven S.;
Christiansen, Jessie L.; Lépine, Sébastien; Aller, Kimberly M.; Liu,
Michael C.; Saglia, R. P.; Livingston, John; Kluge, Matthias
Citation K2 DISCOVERS A BUSY BEE: AN UNUSUAL TRANSITING
NEPTUNE FOUND IN THE BEEHIVE CLUSTER 2016, 152 (6):223
The Astronomical Journal
DOI 10.3847/1538-3881/152/6/223
Publisher IOP PUBLISHING LTD
Journal The Astronomical Journal
Rights © 2016. The American Astronomical Society. All rights reserved.
Download date 10/08/2022 06:54:21
Item License http://rightsstatements.org/vocab/InC/1.0/
Version Final published version
Link to Item http://hdl.handle.net/10150/622698
K2 DISCOVERS A BUSY BEE: AN UNUSUAL TRANSITING NEPTUNE
FOUND IN THE BEEHIVE CLUSTER
Christian Obermeier
1,2,3
, Thomas Henning
1
, Joshua E. Schlieder
4,5,16
, Ian J. M. Crossfield
6,17
, Erik A. Petigura
7,18
,
Andrew W. Howard
8
, Evan Sinukoff
8,19
, Howard Isaacson
9
, David R. Ciardi
5
, Trevor J. David
10
,
Lynne A. Hillenbrand
10
, Charles A. Beichman
9
, Steve B. Howell
4
, Elliott Horch
11
, Mark Everett
12
, Lea Hirsch
9
,
Johanna Teske
13,16
, Jessie L. Christiansen
5
, Sébastien Lépine
14
, Kimberly M. Aller
8
, Michael C. Liu
8
,
Roberto P. Saglia
2
, John Livingston
15
, and Matthias Kluge
2,3
1
Max Planck Institut für Astronomie, Heidelberg, Germany
2
Max-Planck-Institute for Extraterrestrial Physics, Garching, Germany
3
University Observatory Munich (USM), Ludwig-Maximilians-Universität, Munich, Germany
4
NASA Ames Research Center, Moffett Field, CA, 91125, USA
5
NASA Exoplanet Science Institute, California Institute of Technology, Pasadena, CA, 91125, USA
6
Lunar & Planetary Laboratory, University of Arizona, 1629 E. University Blvd., Tucson, AZ, 85721, USA
7
Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, 91125, USA
8
Institute for Astronomy, University of Hawai‘iatMānoa, Honolulu, HI, 96822, USA
9
Astronomy Department, University of California, Berkeley, CA, 94720, USA
10
Department of Astronomy, California Institute of Technology, Pasadena, CA, 91125, USA
11
Department of Physics, Southern Connecticut State University, New Haven, CT, 06515, USA
12
National Optical Astronomy Observatory, Tucson, AZ, 85719, USA
13
Carnegie Department of Terrestrial Magnetism, Washington, DC, 20015, USA
14
Department of Physics and Astronomy, Georgia State University, GA, USA
15
Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Bunkyo-ku, Tokyo 113-0033, Japan
Received 2016 May 16; revised 2016 July 20; accepted 2016 August 3; published 2016 December 6
ABSTRACT
Open clusters have been the focus of several exoplanet surveys, but only a few planets have so far been discovered.
The Kepler spacecraft revealed an abundance of small planets around small cool stars, therefore, such cluster
members are prime targets for exoplanet transit searches. Keplerʼs new mission, K2, is targeting several open
clusters and star-forming regions around the ecliptic to search for transiting planets around their low-mass
constituents. Here, we report the discovery of the first transiting planet in the intermediate-age (800 Myr) Beehive
cluster (Praesepe). K2-95 is a faint (
=K p 15.5 mag
)
M
3.0 0.5
dwarf from K2ʼs Campaign 5 with an effective
temperature of
3
471 124 K
, approximately solar metallicity and a radius of
R
0
.402 0.050
. We detected a
transiting planet with a radius of
-
+
Å
R
3
.47
0.53
0.78
and an orbital period of 10.134 days. We combined photometry,
medium/high-resolution spectroscopy, adaptive optics/speckle imaging, and archival survey images to rule out
any false-positive detection scenarios, validate the planet, and further characterize the system. The planet’s radius
is very unusual as M-dwarf field stars rarely have Neptune-sized transiting planets. The comparatively large radius
of K2-95b is consistent with the other recently discovered cluster planets K2-25b (Hyades) and K2-33b (Upper
Scorpius), indicating systematic differences in their evolutionary states or formation. These discoveries from K2
provide a snapshot of planet formation and evolution in cluster environments and thus make excellent laboratories
to test differences between field-star and cluster planet populations.
Key words: eclipses – stars: individual (K2-95) – stars: low-mass – techniques: photometric – techniques:
spectroscopic
1. INTRODUCTION
Exoplanet science is still a young field, but what stands out is
the strong diversity in the properties of both detected planets
and their host stars. Already a short time after the first transiting
planet was detected by Charbonneau et al. ( 2000) andHenry
et al. (2000), surveys were started with a focus on open clusters
for a variety of reasons. The higher density of stars gives
surveys access to more stars for a given field of view. Age,
distance, and metallicity of the member stars are well
determined, yielding more precise estimates for the planetary
and stellar parameters. Furthermore, most observed field stars
are relatively old (1 Gyr) while many currently targeted
clusters present a younger sample (10–800 Myr). In addition,
planet formation in stellar clusters may well be very different
due to stronger and more frequent gravitational interactions
between the stars. Planets in younger clusters may also be
undergoing thermal evolution, radial contraction, or receiving
high irradiation from their active host stars. Therefore, open
clusters are an excellent laboratory to test planet formation and
evolution models. Initial transit surveys that focused on 47 Tuc
(Gilliland et al. 2000; Weldrake et al. 2005), NGC 2301
(Howell et al. 2005) and NGC 7789 (Bramich & Horne 2006
),
found no evidence for transiting planets. Since then, fourteen
planets have been discovered in open clusters, namely in NGC
6811 (Meibom et al. 2013), NGC 2423 (Lovis & Mayor 2007),
M67 (Brucalassi et al. 2014, 2016), the Beehive (Praesepe)
(Quinn et al. 2012; Malavolta et al. 2016), the Hyades (Sato
The Astronomical Journal, 152:223 (12pp), 2016 December doi:10.3847/1538-3881/152/6/223
© 2016. The American Astronomical Society. All rights reserved.
16
NASA Postdoctoral Program Fellow.
17
NASA Sagan Fellow.
18
Hubble Fellow.
19
NSERC Postgraduate Research Fellow.
20
Carnegie Origins Postdoctoral Fellow, jointly appointed by Carnegie DTM
and Carnegie Observations.
1
et al. 2007; Quinn et al. 2014; David et al. 2016a; Mann et al.
2016a) and Upper Scorpius (David et al. 2016b; Mann et al.
2016b). All planets in M67, the planet in NGC 2423, one
planet in the Hyades and the Praesepe planets were detected
with the radial velocity ( RV) method. All planets in NGC 6811,
one planet in the Hyades and the planet in Upper Scorpius were
discovered with the transit method. All detections were of
planets that likely harbor significant gaseous envelopes.
Additionally, a ∼2 Myr old hot Jupiter located in the Taurus-
Auriga star-forming region was detected via the RV method
(Donati et al. 2016).
All transiting planets were detected with the Kepler space
telescope. After the failure of two of its four reaction wheels,
the original mission of Kepler ended and was redirected for the
“second light” survey K2 (Howell et al. 2014). Instead of
continuously observing the same area over years, the K2
mission switches fields every three months, stabilized by the
two remaining reaction wheels and solar photon pressure for
the third axis (roll angle). However, the telescope still drifts
slowly and has to be corrected by firing the thrusters every 6 hr.
Photometric precision is therefore slightly lower than during
the Kepler mission but, as will be described in the following
section, can be corrected very well.
The Beehive cluster (M44), also called Praesepe, is an open
cluster targeted by K2 in Campaign 5. It is nearby
(
=
d
183 8 pc
, van Leeuwen 2009; Majaess et al. 2011)
and of intermediate age. Past estimates placed the age of
Praesepe at around 600 Myr (Fossati et al. 2008) but new
estimates that take into account the effects of rotation in its
high-mass members suggest an age as old 800 Myr (Brandt &
Huang 2015b). Furthermore, the kinematics (Madsen et al.
2002), metallicity (Dobbie et al. 2006) and age (Brandt &
Huang 2015b) of Praesepe are very similar to those of the
Hyades cluster. The age of Hyades was also redetermined to
800 Myr (Brandt & Huang 2015a; David & Hillenbrand 2015)
and it is now assumed that both clusters may share the same
origin.
Since the transit signal gets stronger with decreasing stellar
radius, M dwarfs are promising targets for the detection of
small planets in an open cluster. Dressing & Charbonneau
(2015) estimate an abundance of rocky and small sub-
Neptunian planets around those stars with periods shorter than
200 days with an average of 2.5±0.2 planets per star with
radii between
Å
R1 and 4
. Here, we present the discovery and
validation of a transiting Neptune-sized planet in the Praesepe
cluster detected in K2 Campaign 5 in orbit around the low-mass
star K2-95. In Section 2 we describe the layout of our
photometric and spectroscopic follow-up and detail the
subsequent results in Section 3. We validate the candidate as
a planet in Section 4, discuss the impact of our findings in the
context of exoplanets in clusters and the field in Section 5, and
provide concluding remarks in Section 6.
2. OBSERVATIONS
2.1. K2 Target Selection and Photometry
We identified the star K2-95 as a potential M dwarf target
and high probability member of the Praesepe cluster for our K2
Campaign 5 proposal (GO5006—PI Schlieder). Other groups
also proposed this star as a potential K2 target (GO5011—
PI Beichman, GO5048—PI Guzik, GO5095—PI Agueros,
GO5097—PI Johnson).
K2-95 was observed during K2 Campaign 5 with nearly
continuous photometry from 2015 April 27 to July 10. We
extracted the photometry from the pixel data which we
downloaded from the MAST.
21
Our photometric extraction pipeline is described in more
detail in Petigura et al. (2015) and Crossfield et al. (2015).
During K2 operations, the telescope is torqued by solar
radiation pressure which causes it to slowly roll around the
boresight. This motion causes stars to drift across the CCD by
about 1 pixel every 6 hr. As stars are sampled by different
pixels, intra-pixel sensitivity and flat-fielding variations cause
the apparent brightness of the star to change. Thruster fires to
correct for this drift affect the pointing and therefore pixel
position greatly, giving the overall photometry a saw-tooth
shape. We solve for the roll angle between each frame and an
arbitrary reference frame and model the time- and roll-
dependent brightness variations using a Gaussian process.
Further, we adjust the size of our square extraction aperture to
minimize the residual noise in the corrected light curve. This
balances two competing effects: larger apertures yield smaller
systematic errors while smaller apertures include less back-
ground noise. Our final square extraction aperture is r=1
pixel
»
4
. The resulting, de-trended light curve exhibits slow,
periodic, ∼1% modulations with a period of about 24 days. We
attribute this modulation to spots on the rotating stellar surface.
The timescale of this variation is long compared to other M
dwarfs in Praesepe and places K2-95 among the slowest
rotators in the cluster (see also Section 3.5). This variation is
fitted and removed to produce the final light curve which is
shown in the top panel of Figure 1.
We searched through the optimized light curve with the
TERRA algorithm which is described in more detail by
Petigura et al. (2013). In short, it searches for periodic box-
shaped photometric dimmings and fits them with a model from
Mandel & Agol (2002). Using TERRA, we detected a transit
signal in the K2-95 light curve with a period of
=P 10.132 days
and a signal-to-noise ratio (S/N) of 23.97.
The phase-folded light curve is shown in the bottom panel of
Figure 1, centered around the transit event. We subtracted the
best-fitting model transit and iterated the TERRA algorithm to
search for other transits but did not detect any secondary
signals. Visual inspection also did not reveal any additional
transit features.
2.2. Photometric Follow-up
We observed K2-95 with the 2.0 m Fraunhofer telescope
Wendelstein (Hopp et al. 2014), using the Wide Field Imager
(WFI)(Kosyra et al. 2014) on Mt.Wendelstein in the Bavarian
Alps. An independent transit detection from a ground-based
facility serves not only for period confirmation and estimation
of its uncertainty, but as evidence for the planetary nature of the
transit from a common eclipse depth at different wavelengths.
Multi-band transit photometry can be used to characterize the
planet’s atmosphere or rule out false-positive detections (Mislis
et al. 2010; Southworth et al. 2012; Mancini et al. 2013; Ciceri
et al. 2016). The limb darkening coefficients differ across
photometric bands and can be used to differentiate between
planetary signals and those of shallow-eclipse eclipsing
binaries (EBs). K2-95 was followed up in the i′-band on UT
2016 April 16 during suboptimal weather with seeing between
21
The Mikulski Archive for Space Telescopes.
2
The Astronomical Journal, 152:223 (12pp), 2016 December Obermeier et al.
1
and
3
and cirrus activity which led to aborting the
observations after about three hours, or around mid-transit.
However, due to the relative isolation of the target and
reference stars on the CCD, the data were still salvageable and
we could identify the transit after binning the data in
30 minutes intervals. The light curve seen in Figure 2 shows
the expected transit depth of 0.7% and agrees very well with
the overlaid best-fitting transit model from the K2 data,
adjusted for the respective i′-band limb darkening coefficients.
This light curve is already time-corrected and indicates a slight
shift in phase. This implies that our initial period estimate may
have been off by a few seconds per cycle, an effect seen in the
follow-up of previous K2 planet discoveries (see Beichman
et al. 2016), but it is still inside the period uncertainty (see also
Section 4.2) of ≈60 s. Following up transiting planets over
larger baselines and therefore improving period accuracy is a
valuable step in preserving the ephemeris for future studies.
2.3. IRTF/SpeX
We observed our target with the near-infrared cross-
dispersed spectrograph (SpeX, Rayner et al. 2003) on the
3.0 m NASA Infrared Telescope Facility on Mauna Kea. While
K2 targets are already pre-characterized with broadband
photometry, spectral typing is essential for more accurate
stellar properties. K2-95 was observed on UT 2015 December
09 under excellent conditions with a clear sky and an average
seeing of
0
.5
. We used the instrument’s short cross-dispersed
mode (SXD) with the 0.3 ×
15
slit which provides a
wavelength range of 0.68–2.5
mm
and a resolution of R ≈
2000. The target was placed at two locations along the slit and
was observed in an ABBA pattern with 16×185 s integrations
for a total integration time of 2960 s. For telluric correction and
wavelength calibration, we observed an A0 standard star plus
arc and flat lamp exposures right after the target. We reduced
the data with the SpeXTool package (Vacca et al. 2003;
Cushing et al. 2004) which performs flat fielding, sky
subtraction, bad pixel removal and subsequently spectral
extraction and combination, telluric correction, wavelength
+flux calibration and order merging. We achieved a median
S/N of 70 per resolution element in the J- (
m1.25 m
), 80 in the
H- (
m1.6 m
) and 60 in the K-band (
m
2
.2 m
). We compare the
JHK-band spectra to late-type standards from the IRTF Spectral
Library (Rayner et al. 2009), seen in Figure 3. The best visual
match for K2-95 lies between M2 and M3 standards across all
infrared bands.
2.4. Keck/HIRES
We obtained a high-resolution optical spectrum of K2-95
using the HIRES echelle spectrometer on the 10 m Keck I
telescope (Vogt et al. 1994) on UT 2015 December 23. High-
resolution spectroscopy can be used to rule out false-positive
detection scenarios such as EBs by searching for secondary line
features that are created by a possible companion star. Our
observation followed the procedures of the California Planet
Search (CPS, Howard et al. 2010). We used the “C2” decker,
providing a spectral resolution of R=55,000, and subtracted
the sky from the stellar spectrum. We utilized the HIRES
exposure meter to automatically terminate the exposure when
Figure 1. Top: calibrated and normalized K2 photometry for K2-95. The upper red lines indicate the detected transits with the corresponding points also marked in
red. Bottom: period-folded light curve with the best-fitting transit model overlaid as a red line.
Figure 2. Normalized photometry in the i′-band for K2-95, recorded with the Wendelstein WFI. We overlaid the best-fitting transit model from the K2 data, adapted
with appropriate quadratic limb darkening parameters for the i′-band. The binned points (black) agree very well with the model (red line ); however, the transit was
shifted by about 27 minutes (new center indicated by the blue line) which indicates an error in the initial period estimate within the fitting uncertainties. The original
points (light gray) are shown in the background.
3
The Astronomical Journal, 152:223 (12pp), 2016 December Obermeier et al.
S/N=32 per pixel was achieved. The HIRES spectrum
was reduced using standard CPS procedures and cover
∼3600–8000Å. Two additional spectra were obtained on UT
December 24 and 29 using a redder setting of HIRES at
R=48,000; these data are described in J. Pepper et al. (2016,
in preparation).
2.5. Keck/NIRC2
We obtained high-resolution NIR images of K2-95 using
NIRC2 on the 10 m Keck II telescope using the target as a
natural guide star to drive the AO system. High-resolution
imaging is a useful tool for constraining the probability of a
blended background star. We observed the target on UT 2016
January 16 in the K-band, following a multi-point dither pattern
with integration times short enough to avoid saturation. We
used the dithered images to subtract the sky background and
remove dark current, then aligned, flat-fielded, and stacked the
individual images. The star appears single and has no close
companions within several arcseconds. To estimate the
sensitivity of the NIRC2 observations, we injected fake sources
with S/N=5 into the combined image at separations that are
integral multiples of the star’s FWHM. We show our final
image and the 5σ sensitivity curve in the left panel of Figure 4.
2.6. Gemini-N/DSSI
We also obtained speckle imaging of K2-95 in two narrow
band filters centered at 880 and 692 nm using the DSSI camera
(Horch et al. 2009) on the 8 m Gemini North telescope on UT
2016 January 16. We followed a standard observing procedure
where the star was centered in the field, guiding was
established, and many images were taken using 60 ms
exposures. The data were reduced and combined into a final
reconstructed image using the techniques described in Horch
et al. (2011) and Howell et al. (2012). These procedures
perform automatic model fits (single, double, triple) and
provide estimates of the magnitude difference and separation
for multiple systems. K2-95 was found to be a single star. We
measured the background sensitivity of the reconstructed DSSI
image, using a series of concentric annuli centered on the
target. The innermost annulus is at the telescope diffraction
limit where our sensitivity is zero. The sensitivities in the
subsequent annuli are interpolated using a cubic spline to
produce a smooth sensitivity curve. The 880 nm reconstructed
DSSI image and sensitivity curve are shown in the right panel
of Figure 4.
2.7. Archival Imaging
Data taken from photographic plates, now digitally scanned
and available online
22
, cover several decades of astrometry.
Our target was first observed in 1954 by the Digital Sky
Survey (DSS) in the red and blue channels with an additional
epoch from 1989 and 1990, respectively. We show the
DSS-red plates from 1954 and 1989 in Figure 5. The images
Figure 3. JHK-band IRTF/SpeX spectra of K2-95, compared to K4V–M6V standard spectra from the IRTF spectral library. Every spectrum is normalized to the
continuum. The target is a best visual match for types M2V and M3V in all three bands, which is very clear in the K-band. This is consistent with both our SED fitting
results and the spectral typing using spectroscopic indices.
22
http://irsa.ipac.caltech.edu/applications/finderchart/
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The Astronomical Journal, 152:223 (12pp), 2016 December Obermeier et al.
