Stellar and Planetary Parameters for K2's Late-type Dwarf Systems from C1 to C5

Abstract

The NASA K2 mission uses photometry to find planets transiting stars of various types. M dwarfs are of high interest since they host more short-period planets than any other type of main-sequence star and transiting planets around M dwarfs have deeper transits compared to other main-sequence stars. In this paper, we present stellar parameters from K and M dwarfs hosting transiting planet candidates discovered by our team. Using the SOFI spectrograph on the European Southern Observatory's New Technology Telescope, we obtained R≈1000J-, H-, and K-band (0.95-2.52 μm) spectra of 34 late-type K2 planet and candidate planet host systems and 12 bright K4-M5 dwarfs with interferometrically measured radii and effective temperatures. Out of our 34 late-type K2 targets, we identify 27 of these stars as M dwarfs. We measure equivalent widths of spectral features, derive calibration relations using stars with interferometric measurements, and estimate stellar radii, effective temperatures, masses, and luminosities for the K2 planet hosts. Our calibrations provide radii and temperatures with median uncertainties of 0.059 R e (16.09%) and 160 K (4.33%), respectively. We then reassess the radii and equilibrium temperatures of known and candidate planets based on our spectroscopically derived stellar parameters. Since a planet's radius and equilibrium temperature depend on the parameters of its host star, our study provides more precise planetary parameters for planets and candidates orbiting late-type stars observed with K2. We find a median planet radius and an equilibrium temperature of approximately 3R ⊕ and 500 K, respectively, with several systems (K2-18b and K2-72e) receiving near-Earth-like levels of incident irradiation.

Characterizing K2 Candidate Planetary Systems Orbiting Low-mass Stars. II. Planetary Systems Observed During Campaigns 1–7

“…We note that our radius estimates for all four planets are consistent with the revised values found by Martinez et al (2017), who used NTT/SOFI spectroscopy to characterize the host star and then scaled the R p /R å values from Crossfield et al (2016) accordingly.…”

“…As expected, Figure 2 also reveals that K2OIs with high FPPs tend to be larger than those with lower FPPs. Martinez et al (2017) noted a similar size difference between the radii of their planet candidates and validated planets (see their Figure 9) and remarked that the radius estimates for candidates tend to be more uncertain. The primary difference between our K2 planet candidates and the Kepler sample is that the K2 sample is biased toward brighter host stars.…”

Habitability Properties of Circumbinary Planets

“…Planets of M-dwarfs have on average smaller sizes than planets of Solarlike stars (Martinez et al, 2017). This observational fact may provide another answer to the problem of formation of planets of terrestrial type, again pointing out that abiogenesis can be concentrated on CBP of double Mdwarfs.…”

275 Candidates and 149 Validated Planets Orbiting Bright Stars in K2 Campaigns 0–10

“…We also found that inputting 2MASS photometry into isochrones along with the aforementioned stellar parameters had no noticeable effect on the resulting stellar radius and mass values (or their uncertainties). Although isochrones have been found to underestimate stellar radii before (Dressing et al 2017a(Dressing et al , 2017bMartinez et al 2017), this effect is confined to M dwarfs and late-K dwarfs, which are largely excluded from our host star sample since we do not consider stars with effective temperatures below 4250 K (see Section 4.2). The derived stellar masses and radii are reported in Table 2.…”

“…K2 has led to the discovery of numerous candidate and confirmed planets Foreman-Mackey et al 2015;Montet et al 2015;Vanderburg et al 2015bVanderburg et al , 2016bAdams et al 2016;Barros et al 2016;Crossfield et al 2016;Schlieder et al 2016;Sinukoff et al 2016;Pope et al 2016;Dressing et al 2017b;Hirano et al 2017;Martinez et al 2017), as well as to the identification of planets orbiting rare types of stars, including particularly bright nearby dwarf stars Vanderburg et al 2016a;Christiansen et al 2017;Crossfield et al 2017;Niraula et al 2017;Rodriguez et al 2017aRodriguez et al , 2017b, young, pre-main-sequence stars (Mann et al 2016;David et al 2016), and disintegrating planetary material transiting a white dwarf (Vanderburg et al 2015a).…”