J/AJ/168/161 K2-2 b: light curves and radial velocities (Thygesen+, 2024)
The K2 and TESS synergy.
III. Search and rescue of the lost ephemeris for K2's first planet.
Thygesen E., Rodriguez J.E., de Beurs Z.L., Vanderburg A.,
Livingston J.H., Irwin J., Venner A., Cretignier M., Collins K.A.,
Bieryla A., Charbonneau D., Crossfield I.J.M., Dumusque X., Kielkopf J.,
Latham D.W., Werner M.
<Astron. J., 168, 161 (2024)>
=2024AJ....168..161T 2024AJ....168..161T
ADC_Keywords: Exoplanets ; Photometry, UBVRI; Radial velocities;
Spectra, optical
Keywords: Exoplanet astronomy ; Exoplanet systems ; Exoplanet catalogs ;
Exoplanets ; Ephemerides ; Transits
Abstract:
K2-2 b/HIP 116454 b, the first exoplanet discovery by K2 during its
Two-Wheeled Concept Engineering Test, is a sub-Neptune
(2.5±0.1R⊕, 9.7±1.2M⊕) orbiting a relatively
bright (KS=8.03) K-dwarf star on a 9.1d period. Unfortunately, due to
a spurious follow-up transit detection and ephemeris degradation, the
transit ephemeris for this planet was lost. In this work, we recover
and refine the transit ephemeris for K2-2 b, showing a ∼40σ
discrepancy from the discovery results. To accurately measure the
transit ephemeris and update the parameters of the system, we jointly
fit space-based photometric observations from NASA's K2, Transiting
Exoplanet Survey Satellite, and Spitzer missions with new photometric
observations from the ground, as well as radial velocities from
HARPS-N that are corrected for stellar activity using a new modeling
technique. Ephemerides becoming lost or significantly degraded, as is
the case for most transiting planets, highlights the importance of
systematically updating transit ephemerides with upcoming large
efforts expected to characterize hundreds of exoplanet atmospheres.
K2-2 b sits at the high-mass peak of the known radius valley for
sub-Neptunes, and is now well-suited for transmission spectroscopy
with current and future facilities. Our updated transit ephemeris will
ensure no more than a 13min uncertainty through 2030.
Description:
The discovery analysis for K2-2 b included a 47d long light curve from
the Microvariablity and Oscillations of Stars (MOST) Space Telescope,
but future follow-up attempts to reobserve the transit with Spitzer
and HST showed no transit during or near the predicted window. This
ultimately led to the idea that the MOST observations were not
reliably constraining the transit ephemeris. Our new observations from
MEarth, ULMT, Spitzer, and TESS confirm this hypothesis.
A single transit of K2-2 b was observed at 30min cadence during the
Kepler Two-Wheel Concept Engineering Test during 2014 February. See
Section 2.2.
MEarth was used to initially recover the transit of K2-2 b and
constrain the ephemeris, observing multiple partial and full transits.
MEarth consists of 16 separate 0.4m telescopes using custom 715nm
longpass filters. Telescopes 1-8 are a part of the MEarth-North
Observatory at Fred Lawrence Whipple Observatory (FLWO) on Mount
Hopkins, Arizona, while the other eight telescopes (numbered as 11-18)
are part of the MEarth-South Observatory located at Cerro Tololo
Inter-American Observatory (CTIO) on Cerro Tololo, Chile. K2-2 was
observed using a subset of four telescopes from each observatory with
1min cadence on UT 2016 September 21 and 30, and UT 2016 October 9.
See Section 2.3.
Once the ephemeris was refined from the MEarth observations, an
ingress of K2-2 b was observed using the University of Louisville
Manner Telescope (ULMT; formerly MVRC) at the Mt. Lemmon summit of
Steward Observatory, Arizona. The observation was made in the r' band
with 50s exposure time on UT 2016 October 10. See Section 2.4.
With the ephemeris more precisely constrained, Spitzer was used to
observe a single transit of K2-2 b on UT 2017 April 1. The observation
was 10.5hr long, and was taken with the InfraRed Array Camera channel
2 (4.5um) with a 2s exposure time. See Section 2.5.
A single transit was observed by the Transiting Exoplanet Survey
Satellite (TESS) in each of Sectors 42 and 70 (on 2021-Aug-21 and
2023-Sp-21). We used the 120s cadence light curves in our global fits.
See Section 2.6.
We included archival spectroscopy to determine the host-star
properties and to refine the mass measurement of K2-2 b. In
particular, to better characterize the host star in the global fit, we
used metallicity measurements of K2-2 from the Tillinghast Reflector
Echelle Spectrograph (TRES) on the 1.5m Tillinghast Reflector at the
FLWO and the Stellar Parameter Classification from Buchhave+
(2012Natur.486..375B 2012Natur.486..375B).
We used a total of 105 spectra of K2-2, including those used in
Vanderburg+ (2015ApJ...800...59V 2015ApJ...800...59V) and Bonomo+ (2023A&A...677A..33B 2023A&A...677A..33B),
acquired using the High Accuracy Radial velocity Planet Searcher for
the Northern hemisphere (HARPS-N) on the 3.6m TNG at the Roque de los
Muchachos Observatory, in order to better characterize the mass of
K2-2 b. Each observation had either 15 or 30min of exposure time, with
a resolving power of R=115,000. The observations occurred in two main
blocks, separated by ∼2.5yr; the first run was from UT 2014-Jul-07 to
2017-Dec-06, and the second from UT 2020-Jun-25 to 2023-Nov-27. See
Section 2.7.
Objects:
---------------------------------------------------------------
RA (IRCS) DE Designation(s) (Period)
---------------------------------------------------------------
23 35 49.28 +00 26 43.8 K2-2b = HIP 116454b (P=9.1004157)
---------------------------------------------------------------
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
k2.dat 32 284 K2 light curve of K2-2 b
(Data behind Figure 1)
mearth.dat 47 5176 MEarth (custom 715nm) light curve of K2-2 b
(Data behind Figure 1)
spitzer.dat 32 315 Spitzer 4.5um light curve of K2-2 b
(Data behind Figure 1)
tess.dat 36 25454 TESS T band light curve of K2-2 b
(Data behind Figure 1)
ulmt.dat 41 711 ULMT r' band light curve of K2-2 b
(Data behind Figure 1)
fig2.dat 83 105 HARPS-N RVs and cross-correlation functions
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See also:
II/246 : 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)
I/311 : Hipparcos, the New Reduction (van Leeuwen, 2007)
II/294 : The SDSS Photometric Cat. R7 (Adelman-McCarthy+, 2009)
IV/34 : K2 Ecliptic Plane Input Catalog (EPIC) (Huber+, 2017)
IV/39 : TESS Input Catalog version 8.2 (TIC v8.2) (Paegert+, 2021)
I/355 : Gaia DR3 Part 1. Main source (Gaia Collaboration, 2022)
J/ApJ/683/1076 : Transits of exoplanet XO-3b (Winn+, 2008)
J/ApJ/736/12 : Transit light curves of GJ1214 (Berta+, 2011)
J/A+A/529/A75 : Limb-darkening coefficients (Claret+, 2011)
J/other/Nat/486.375 : Stellar parameters of KOI stars (Buchhave+, 2012)
J/ApJ/791/35 : Detect: 715 Kepler planet candid. host stars (Law+, 2014)
J/ApJS/226/7 : Planet candid. discov.: K2's 1st yr (Crossfield+, 2016)
J/ApJS/222/14 : Planetary candid.: 1st yr K2 mission (Vanderburg+, 2016)
J/A+A/600/A30 : Limb-darkening for TESS satellite (Claret, 2017)
J/AJ/154/109 : CKS. III. Planet radii (Fulton+, 2017)
J/AJ/156/277 : 60 valid planets: K2 campaigns 5-8 (Livingston+, 2018)
J/AJ/156/102 : TESS Input Cat. & Candidate Target List (Stassun+, 2018)
J/A+A/610/A20 : HITEP. II. Transiting exoplanets imaging (Evans+, 2018)
J/AJ/157/169 : Identify exoplanets: deep learning in K2 (Dattilo+, 2019)
J/AJ/157/174 : Transiting planets in Kepler-47 system (Orosz+, 2019)
J/ApJS/244/11 : Planet candidates & EBs: K2 campaigns 0-8 (Kruse+, 2019)
J/AJ/160/209 : K2 & TESS Syn. I. Param & LC, 4 stars (Ikwut-Ukwa+, 2020)
J/ApJS/254/42 : Hipparcos-Gaia (EDR3) Cat. of Accelerat. (Brandt, 2021)
J/A+A/648/A103 : Thorium line list (Dumusque+, 2021)
J/MNRAS/508/3877 : Catalogue of white dwarfs in Gaia EDR3 (Gentile+, 2021)
J/AJ/162/259 : Scal K2. IV. Camp 1-8 & 10-18 planets samp (Zink+, 2021)
J/ApJS/258/40 : ExoClock proj II. Exoplanet ephemerids (Kokori+, 2022)
J/A+A/677/A33 : 38 Kepler and K2 systems RVs (Bonomo+, 2023)
J/A+A/678/A2 : YARARA V2. 4 high-PM stars RVs (Cretignier+, 2023)
J/ApJS/265/4 : ExoClock proj III. 450 exoplanet ephemeri (Kokori+, 2023)
J/A+A/678/A90 : HD99492,HD147379,HD190007 HARPS-N data (Stalport+, 2023)
J/MNRAS/527/2940 : Sol activity components intensity prof (Cretignier+ 2024)
Byte-by-byte Description of file: k2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 14 F14.6 d BJD Barycentric Julian Date of Observation at TDB
16- 23 F8.6 --- Flux [0.998/1.01] Normalized Flux
25- 32 F8.6 --- e_Flux Uncertainty in Flux
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Byte-by-byte Description of file: mearth.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 5 A5 --- Tel MEarth Telescope
7- 20 F14.6 d BJD Barycentric Julian Date of Observation at TDB
22- 29 F8.6 --- Flux [0.77/1.06] Normalized Flux
31- 38 F8.6 --- e_Flux [1e-3/70e-3] Uncertainty in Flux
40- 47 F8.6 --- Airmass [1.16/2.93] Airmass of observation
--------------------------------------------------------------------------------
Byte-by-byte Description of file: spitzer.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 14 F14.6 d BJD Barycentric Julian Date of Observation at TDB
16- 23 F8.6 --- Flux [0.997/1.01] Normalized Flux
25- 32 F8.6 --- e_Flux [6e-4/6.5e-4] Uncertainty in Flux
--------------------------------------------------------------------------------
Byte-by-byte Description of file: tess.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 3 A3 --- Sect TESS Sector
5- 18 F14.6 d BJD Barycentric Julian Date of Observation at TDB
20- 27 F8.6 --- Flux [0.996/1.01] Normalized Flux
29- 36 F8.6 --- e_Flux [7.6e-4/7.7e-4] Uncertainty in Flux
--------------------------------------------------------------------------------
Byte-by-byte Description of file: ulmt.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 14 F14.6 d BJD Barycentric Julian Date of Observation at TDB
16- 23 F8.6 --- Flux [0.948/1.02] Normalized Flux
25- 32 F8.6 --- e_Flux [8e-4/2.5e-2] Uncertainty in Flux
34- 41 F8.6 --- Airmass [1.16/2.92] Airmass of observation
--------------------------------------------------------------------------------
Byte-by-byte Description of file: fig2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 14 F14.6 d BJD Barycentric Julian Date of Observation at TDB
16- 24 F9.6 m/s RVel [-9.8/11.4] HARPS-N Radial Velocity
26- 33 F8.6 m/s e_RVel [0.8/3.2] Uncertainty in RVel
35- 43 F9.6 --- ccf1 [-1.6e-3/2.7e-3] Cross-Correlation function,
location 1
45- 53 F9.6 --- ccf2 [-2.5e-3/4.2e-3] Cross-Correlation function,
location 2
55- 63 F9.6 --- ccf3 [-3.6e-3/4.6e-3] Cross-Correlation function,
location 3
65- 73 F9.6 --- ccf4 [-3.5e-3/3.9e-3] Cross-Correlation function,
location 4
75- 83 F9.6 --- ccf5 [-2.5e-3/1.8e-3] Cross-Correlation function,
location 5
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History:
From electronic version of the journal
References:
Ikwut-Ukwa et al. Paper I. 2020AJ....160..209I 2020AJ....160..209I Cat. J/AJ/160/209
Thygesen et al. Paper II. 2023AJ....165..155T 2023AJ....165..155T
Thygesen et al. Paper III. 2024AJ....168..161T 2024AJ....168..161T This catalog
(End) Prepared by [AAS], Robin Leichtnam [CDS] 23-May-2025