J/AJ/155/257 Photometry & RV follow-up observations of K2-18 (Sarkis+, 2018)
The CARMENES search for exoplanets around M dwarfs: a low-mass planet
in the temperate zone of the nearby K2-18.
Sarkis P., Henning T., Kurster M., Trifonov T., Zechmeister M., Tal-Or L.,
Anglada-Escude G., Hatzes A.P., Lafarga M., Dreizler S., Ribas I.,
Caballero J.A., Reiners A., Mallonn M., Morales J.C., Kaminski A.,
Aceituno J., Amado P.J., Bejar V.J.S., Hagen H.-J., Jeffers S.,
Quirrenbach A., Launhardt R., Marvin C., Montes D.
<Astron. J., 155, 257-257 (2018)>
=2018AJ....155..257S 2018AJ....155..257S (SIMBAD/NED BibCode)
ADC_Keywords: Stars, nearby ; Stars, dwarfs ; Stars, M-type ;
Photometry, UBVRI ; Radial velocities ; Spectroscopy ; Exoplanets
Keywords: stars: activity - stars: individual: K2-18 - stars: low-mass
Abstract:
K2-18 is a nearby M2.5 dwarf, located at 34 pc and hosting a transiting
planet that was first discovered by the K2 mission and later confirmed
with Spitzer Space Telescope observations. With a radius of ∼2 R⊕
and an orbital period of ∼33 days, the planet lies in the temperate zone
of its host star and receives stellar irradiation similar to that of Earth.
Here we perform radial velocity follow-up observations with the visual
channel of CARMENES with the goal of determining the mass and density
of the planet. We measure a planetary semi-amplitude of Kb∼3.5 m/s and
a mass of Mb∼9 M⊕, yielding a bulk density around
ρb∼4 g/cm3. This indicates a low-mass planet with a composition
consistent with a solid core and a volatile-rich envelope. A signal at
9 days was recently reported using radial velocity measurements taken
with the HARPS spectrograph. This was interpreted as being due to a second
planet. We see a weaker, time- and wavelength-dependent signal in the
CARMENES data set and thus favor stellar activity for its origin. K2-18 b
joins the growing group of low-mass planets detected in the temperate zone
of M dwarfs. The brightness of the host star in the near-infrared makes
the system a good target for detailed atmospheric studies with the
James Webb Space Telescope.
Description:
We monitored the host star K2-18 for photometric variability with the
robotic 1.2 m twin-telescope STELLA on Tenerife (Strassmeier et al.
2004AN....325..527S 2004AN....325..527S) and its wide-field imager WiFSIP. From 2017 February
until 2017 June, we obtained blocks of four exposures in Johnson B and
four exposures in Cousins R over 33 nights. The exposure time was 120 s
in B and 60 s in R.
We monitored K2-18 between 2016 December and 2017 June with CARMENES.
CARMENES (Calar Alto search for M dwarfs with Exo-earths with Near-infrared
and optical Echelle Spectrographs) is a pair of high-resolution echelle
spectrographs (Quirrenbach et al. 2014SPIE.9147E..1FQ) mounted on the
3.5 m telescope of the Calar Alto Observatory (CAHA) in Spain. The VIS
channel covers the wavelength range from 0.52 to 0.96 µm and has a
spectral resolution R=94600 (Quirrenbach et al. 2016SPIE.9908E..12Q 2016SPIE.9908E..12Q),
with a demonstrated precision similar to HARPS and better than Keck/HIRES
(Trifonov et al. 2018, J/A+A/609/A117).
Objects:
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RA (ICRS) DE Designation(s)
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11 30 14.52 +07 35 18.3 K2-18 = 2MASS J11301450+0735180
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table4.dat 25 27 WiFSIP photometry in the Johnson B filter
table5.dat 25 27 WiFSIP photometry in the Cousins R filter
table6.dat 106 58 Radial velocities obtained with CARMENES and
the spectroscopic activity indicators
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See also:
J/A+A/608/A35 : K2-18 HARPS time-series (Cloutier+, 2017)
J/A+A/609/A117 : CARMENES radial velocity curves of 7 M-dwarf (Trifonov+, 2018)
J/A+A/612/A49 : 324 CARMENES M dwarfs velocities (Reiners+, 2018)
Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
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1- 11 F11.6 d BJD [7812.628906/7921.402344] Barycentric Julian Date
(BJD-2450000)
13- 18 F6.4 mag DBmag [0.9855/1.0153] Differential Johnson B band
magnitude
20- 25 F6.4 mag e_DBmag [0.0019/0.0062] Uncertainty in DBmag
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Byte-by-byte Description of file: table5.dat
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Bytes Format Units Label Explanations
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1- 11 F11.6 d BJD [7812.628906/7921.41] Barycentric Julian Date
(BJD-2450000)
13- 18 F6.4 mag DRmag [0.9904/1.0115] Differential Cousins R band
magnitude
20- 25 F6.4 mag e_DRmag [0.0018/0.0057] Uncertainty in DRmag
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Byte-by-byte Description of file: table6.dat
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Bytes Format Units Label Explanations
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1- 11 F11.6 d BJD [7735.61786/7924.3808] Barycentric Julian
Date (BJD-2450000)
13- 18 F6.2 m/s RV [-13.49/9.82] Radial velocity
20- 24 F5.2 m/s e_RV [1.61/11.91] Uncertainty in RV
26- 31 F6.2 m/s RV(blue) [-30.21/16.88] The blue order radial velocity
(1)
33- 37 F5.2 m/s e_RV(blue) [2.08/15.66] Uncertainty in RV(blue) (1)
39- 44 F6.2 m/s RV(red) [-14.18/12.35] The red order radial velocity
(1)
46- 50 F5.2 m/s e_RV(red) [1.69/10.86] Uncertainty in RV(red) (1)
52- 57 F6.4 --- CaII(IRT1) [0.567/0.6074] The Ca II H and K, calcium
infrared triplet activity indicator
(8498.02Å) (dex)
59- 64 F6.4 --- e_CaII(IRT1) [0.0021/0.0105] Uncertainty in CaII(IRT1)
(dex)
66- 71 F6.4 --- CaII(IRT2) [0.4228/0.474] The Ca II H and K, calcium
infrared triplet activity indicator
(8542.09Å) (dex)
73- 78 F6.4 --- e_CaII(IRT2) [0.0022/0.0132] Uncertainty in CaII(IRT2)
(dex)
80- 85 F6.4 --- CaII(IRT3) [0.4004/0.4449] The Ca II H and K, calcium
infrared triplet activity indicator
(8662.14Å) (dex)
87- 92 F6.4 --- e_CaII(IRT3) [0.002/0.0123] Uncertainty in CaII(IRT3)
(dex)
94- 99 F6.4 --- Halpha [0.93/0.9715] Hα activity index
(6562.81Å) (dex)
101-106 F6.4 --- e_Halpha [0.0021/0.0121] Uncertainty in Halpha (dex)
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Note (1): As a final test, we looked at the RV measurements in the red and blue
orders of CARMENES-VIS. If the RVs are dominated by activity due to active
regions on the stellar surface, then the planetary semi-amplitude in the blue
part of the spectrum should be more affected by activity, whereas the red part
should be less affected. As a result, if the star is active, a single
Keplerian fit to the data should yield different planetary semi-amplitudes for
different orders. The RV measurements for K2-18 are available at 42 orders. We
calculated an RV weighted mean average for the first and second half of the
orders, which we will refer to as the blue RVs and as the red RVs,
respectively. The blue orders cover the wavelength range from 561 to 689 nm,
whereas the red orders cover the range from 697 to 905 nm.
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History:
From electronic version of the journal
(End) Tiphaine Pouvreau [CDS] 08-Jan-2019