We present the detection and follow-up observations of planetary candidates around low-mass stars observed by the K2 mission. Based on light-curve analysis, adaptive-optics imaging, and optical ...spectroscopy at low and high resolution (including radial velocity measurements), we validate 16 planets around 12 low-mass stars observed during K2 campaigns 5-10. Among the 16 planets, 12 are newly validated, with orbital periods ranging from 0.96 to 33 days. For one of the planets (K2-151b), we present ground-based transit photometry, allowing us to refine the ephemerides. Combining our K2 M-dwarf planets together with the validated or confirmed planets found previously, we investigate the dependence of planet radius Rp on stellar insolation and metallicity Fe/H. We confirm that for periods P 2 days, planets with a radius are less common than planets with a radius between 1-2 R⊕. We also see a hint of the "radius valley" between 1.5 and 2 R⊕, which has been seen for close-in planets around FGK stars. These features in the radius/period distribution could be attributed to photoevaporation of planetary envelopes by high-energy photons from the host star, as they have for FGK stars. For the M dwarfs, though, the features are not as well defined, and we cannot rule out other explanations such as atmospheric loss from internal planetary heat sources or truncation of the protoplanetary disk. There also appears to be a relation between planet size and metallicity: the few planets larger than about 3 R⊕ are found around the most metal-rich M dwarfs.
ABSTRACT We report the first ground-based transit observation of K2-3d, a 1.5 R⊕ planet supposedly within the habitable zone around a bright M-dwarf host star, using the Okayama 188 cm telescope and ...the multi(grz)-band imager MuSCAT. Although the depth of the transit (0.7 mmag) is smaller than the photometric precisions (1.2, 0.9, and 1.2 mmag per 60 s for the g, r, and z bands, respectively), we marginally but consistently identify the transit signal in all three bands, by taking advantage of the transit parameters from K2, and by introducing a novel technique that leverages multi-band information to reduce the systematics caused by second-order extinction. We also revisit previously analyzed Spitzer transit observations of K2-3d to investigate the possibility of systematic offsets in transit timing, and find that all the timing data can be explained well by a linear ephemeris. We revise the orbital period of K2-3d to be 44.55612 0.00021 days, which corrects the predicted transit times for 2019, i.e., the era of the James Webb Space Telescope, by ∼80 minutes. Our observation demonstrates that (1) even ground-based, 2 m class telescopes can play an important role in refining the transit ephemeris of small-sized, long-period planets, and (2) a multi-band imager is useful to reduce the systematics of atmospheric origin, in particular for bluer bands and for observations conducted at low-altitude observatories.
(ProQuest: ... denotes formulae and/or non-USASCII text omitted)We report on the discovery and characterization of the transiting planet K2-39b (EPIC 206247743b). With an orbital period of 4.6 days, ...it is the shortest-period planet orbiting a subgiant star known to date. Such planets are rare, with only a handful of known cases. The reason for this is poorly understood but may reflect differences in planet occurrence around the relatively high-mass stars that have been surveyed, or may be the result of tidal destruction of such planets. K2-39 (EPIC 206247743) is an evolved star with a spectroscopically derived stellar radius and mass of ... and ... , respectively, and a very close-in transiting planet, with a/Rlow * = 3.4. Radial velocity (RV) follow-up using the HARPS, FIES, and PFS instruments leads to a planetary mass of ... In combination with a radius measurement of 8.3 + or - 1.1 R+ in circle, this results in a mean planetary density of ... g cm super(-3). We furthermore discover a long-term RV trend, which may be caused by a long-period planet or stellar companion. Because K2-39b has a short orbital period, its existence makes it seem unlikely that tidal destruction is wholly responsible for the differences in planet populations around subgiant and main-sequence stars. Future monitoring of the transits of this system may enable the detection of period decay and constrain the tidal dissipation rates of subgiant stars.
Abstract
We report on the confirmation that the candidate transits observed for the star EPIC 211525389 are due to a short-period Neptune-sized planet. The host star, located in K2 campaign field 5, ...is a metal-rich (Fe/H = 0.26 ± 0.05) G-dwarf (T
eff = 5430 ± 70 K and log g = 4.48 ± 0.09), based on observations with the High Dispersion Spectrograph (HDS) on the Subaru 8.2 m telescope. High spatial resolution AO imaging with HiCIAO on the Subaru telescope excludes faint companions near the host star, and the false positive probability of this target is found to be <10−6 using the open source vespa code. A joint analysis of transit light curves from K2 and additional ground-based multi-color transit photometry with MuSCAT on the Okayama 1.88 m telescope gives an orbital period of P = 8.266902 ± 0.000070 d and consistent transit depths of R
p/R
⋆ ∼ 0.035 or (R
p/R
⋆)2 ∼ 0.0012. The transit depth corresponds to a planetary radius of
$R_{\rm p} = 3.59_{-0.39}^{+0.44}\,R_{\oplus }$
, indicating that EPIC 211525389 b is a short-period Neptune-sized planet. Radial velocities of the host star, obtained with the Subaru HDS, lead to a 3 σ upper limit of 90 M
⊕ (0.00027 M
⊙) on the mass of EPIC 211525389 b, confirming its planetary nature. We expect this planet, newly named K2-105 b, to be the subject of future studies to characterize its mass, atmosphere, and spin–orbit (mis)alignment, as well as investigate the possibility of additional planets in the system.
ABSTRACT The Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets (MuSCAT) is an optical three-band ( -, - and -band) imager that was recently developed for the 188 cm ...telescope at Okayama Astrophysical Observatory with the aim of validating and characterizing transiting planets. In a pilot observation with MuSCAT we observed a primary transit of HAT-P-14b, a high-surface gravity (gp = 38 ms−2) hot Jupiter around a bright (V = 10) F-type star. From a 2.9 hr observation we achieved the five-minute binned photometric precisions of 0.028%, 0.022%, and 0.024% in the , , and bands, respectively, which provided the highest-quality photometric data for this planet. Combining these results with those of previous observations, we search for variations of transit timing and duration over five years as well as variations of planet-star radius ratio ( ) with wavelengths, but can find no considerable variation in any parameters. On the other hand, using the transit-subtracted light curves we simulate the achievable measurement error of with MuSCAT for various planetary sizes, assuming three types of host stars: HAT-P-14, the nearby K-dwarf HAT-P-11, and the nearby M-dwarf GJ1214. Comparing our results with the expected atmospheric scale heights, we find that MuSCAT is capable of probing the atmospheres of planets as small as a sub-Jupiter ( ) around HAT-P-14 in all bands, a Neptune ( ) around HAT-P-11 in all bands, and a super-Earth ( ) around GJ1214 in and bands. These results promise that MuSCAT will produce fruitful scientific outcomes in the K2 and TESS era.
ABSTRACT We validate a planet on a close-in orbit (P = 2.260455 0.000041 days) around K2-28 (EPIC 206318379), a metal-rich M4-type dwarf in the Campaign 3 field of the K2 mission. Our follow-up ...observations included multi-band transit observations from the optical to the near-infrared, low-resolution spectroscopy, and high-resolution adaptive optics (AO) imaging. We perform a global fit to all of the observed transits using a Gaussian process-based method and show that the transit depths in all of the passbands adopted for the ground-based transit follow-ups ( ) are within ∼2 of the K2 value. Based on a model of the background stellar population and the absence of nearby sources in our AO imaging, we estimate the probability that a background eclipsing binary could cause a false positive to be <2 × 10−5. We also show that K2-28 cannot have a physically associated companion of stellar type later than M4, based on the measurement of almost identical transit depths in multiple passbands. There is a low probability for an M4 dwarf companion ( ), but even if this were the case, the size of K2-28b falls within the planetary regime. K2-28b has the same radius (within 1 ) and experiences irradiation from its host star similar to the well-studied GJ 1214b. Given the relative brightness of K2-28 in the near-infrared ( mag and mH = 11.03 mag) and relatively deep transit (0.6%-0.7%), a comparison between the atmospheric properties of these two planets with future observations would be especially interesting.
Abstract
We present three-band simultaneous observations of a weak-line T-Tauri star, CVSO 30 (PTFO 8-8695), which is one of the youngest objects having a candidate transiting planet. The data were ...obtained with the Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets (MuSCAT) on the 188 cm telescope at Okayama Astrophysical Observatory in Japan. We observed the fading event in the
$g^{\prime }_2$
-,
$r^{\prime }_2$
-, and z
s, 2-bands simultaneously. As a result, we find a significant wavelength dependence of fading depths of about 3.1%, 1.7%, and 1.0% for the three bands. A cloudless H/He-dominant atmosphere of a hot Jupiter cannot explain this large wavelength dependence. Additionally, we rule out a scenario with the occultation of the gravity-darkened host star. Thus our result is in favor of the fading origin as a circumstellar dust clump or occultation of an accretion hotspot.
ABSTRACT We report on the detection and early characterization of a hot Jupiter in a three day orbit around K2-34 (EPIC 212110888), a metal-rich F-type star located in the K2 Cycle 5 field. Our ...follow-up campaign involves precise radial velocity (RV) measurements and high-contrast imaging using multiple facilities. The absence of a bright nearby source in our high-contrast data suggests that the transit-like signals are not due to light variations from such a companion star. Our intensive RV measurements show that K2-34b (EPIC 212110888b) has a mass of , confirming its status as a planet. We also detect the Rossiter-McLaughlin effect for K2-34b and show that the system has a good spin-orbit alignment ( degrees). High-contrast images obtained by the HiCIAO camera on the Subaru 8.2 m telescope reveal a faint companion candidate (CC) ( mag) at a separation of . Follow-up observations are needed to confirm that the CC is physically associated with K2-34. K2-34b appears to be an example of a typical "hot Jupiter," albeit one which can be precisely characterized using a combination of K2 photometry and ground-based follow-up.
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