Search

We report the discovery of a resolved (0.9 arcsec) substellar companion to a member of the 1–5 Myr Taurus star-forming region. The host star (2M0437) is a single mid-M type (T eff ≈ 3100 K) dwarf with a position, space motion, and colour–magnitude that support Taurus membership, and possible affiliation with a ∼2.5-Myr-old subgroup. A comparison with stellar models suggests a 2–5 Myr age and a mass of 0.15–0.18M⊙. Although K2 detected quasi-periodic dimming from close-in circumstellar dust, the star lacks detectable excess infrared emission from a circumstellar disc and its H α emission is not commensurate with accretion. Astrometry based on 3 yr of AO imaging shows that the companion (2M0437b) is comoving, while photometry of two other sources at larger separation indicates that they are likely heavily reddened background stars. A comparison of the luminosity of 2M0437b with models suggests a mass of 3–5 MJUP, well below the deuterium burning limit, and an effective temperature of 1400–1500 K, characteristic of a late L spectral type. The H − K colour is redder than the typical L dwarf, but comparable to other directly detected young planets, e.g. those around HR 8799. The discovery of a super-Jupiter around a very young, very low-mass star challenges models of planet formation by either core accretion (which requires time) or disc instability (which requires mass). We also detected a second, comoving, widely separated (75 arcsec) object that appears to be a heavily extincted star. This is certainly a fellow member of this Taurus subgroup and statistically likely to be a bound companion. [ABSTRACT FROM AUTHOR]

M dwarf stars are high-priority targets for searches for Earth-size and potentially Earth-like planets, but their planetary systems may form and evolve in very different circumstellar environments than those of solar-type stars. To explore the evolution of these systems, we obtained transit spectroscopy and photometry of the Neptune-size planet orbiting the ≈650-Myr-old Hyades M dwarf K2-25. An analysis of the variation in spectral line shape induced by the Doppler 'shadow' of the planet indicates that the planet's orbit is closely aligned with the stellar equator (⁠|$\lambda =-1.7_{-3.7}^{+5.8}$| deg), and that an eccentric orbit found by previous work could arise from perturbations by another planet on a coplanar orbit. We detect no significant variation in the depth of the He i line at 1083 nm during transit. A model of atmospheric escape as an isothermal Parker wind with a solar composition shows that this non-detection is not constraining compared to escape rate predictions of ∼0.1 M⊕  Gyr−1; at such rates, at least several Gyr are required for a Neptune-like planet to evolve into a rocky super-Earth. [ABSTRACT FROM AUTHOR]

We obtained high-resolution infrared spectroscopy and short-cadence photometry of the 600–800 Myr Praesepe star K2-100 during transits of its 1.67-d planet. This Neptune-size object, discovered by the NASA K2 mission, is an interloper in the 'desert' of planets with similar radii on short-period orbits. Our observations can be used to understand its origin and evolution by constraining the orbital eccentricity by transit fitting, measuring the spin-orbit obliquity by the Rossiter–McLaughlin effect, and detecting any extended, escaping the hydrogen–helium envelope with the 10 830 -Å line of neutral helium in the 2s3S triplet state. Transit photometry with 1-min cadence was obtained by the K2 satellite during Campaign 18 and transit spectra were obtained with the IRD spectrograph on the Subaru telescope. While the elevated activity of K2-100 prevented us from detecting the Rossiter–McLaughlin effect, the new photometry combined with revised stellar parameters allowed us to constrain the eccentricity to e < 0.15/0.28 with 90/99 per cent confidence. We modelled atmospheric escape as an isothermal, spherically symmetric Parker wind, with photochemistry driven by ultraviolet radiation, which we estimate by combining the observed spectrum of the active Sun with calibrations from observations of K2-100 and similar young stars in the nearby Hyades cluster. Our non-detection (<5.7 m Å) of a transit-associated He  i line limits mass-loss of a solar-composition atmosphere through a T ≤ 10000 K wind to <0.3 M⊕ Gyr−1. Either K2-100b is an exceptional desert-dwelling planet, or its mass-loss is occurring at a lower rate over a longer interval, consistent with a core accretion-powered scenario for escape. [ABSTRACT FROM AUTHOR]

We report the discovery of two transiting planets orbiting K2-290 (EPIC 249624646), a bright (V = 11.11) late F-type star residing in a triple-star system. It was observed during Campaign 15 of the K2 mission, and in order to confirm and characterize the system, follow-up spectroscopy and AO imaging were carried out using the FIES, HARPS, HARPS-N, and IRCS instruments. From AO imaging and Gaia data we identify two M-dwarf companions at a separation of 113 ± 2 and |$2467_{-155}^{+177}$|  au. From radial velocities, K2 photometry, and stellar characterization of the host star, we find the inner planet to be a mini-Neptune with a radius of 3.06 ± 0.16 R⊕ and an orbital period of P = 9.2 d. The radius of the mini-Neptune suggests that the planet is located above the radius valley, and with an incident flux of F ∼ 400 F⊕, it lies safely outside the super-Earth desert. The outer warm Jupiter has a mass of 0.774 ± 0.047 MJ and a radius of 1.006 ± 0.050 RJ, and orbits the host star every 48.4 d on an orbit with an eccentricity e < 0.241. Its mild eccentricity and mini-Neptune sibling suggest that the warm Jupiter originates from in situ formation or disc migration. [ABSTRACT FROM AUTHOR]

We report the discovery of 47 new T dwarfs in the Fourth Data Release (DR4) from the Large Area Survey (LAS) of the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey with spectral types ranging from T0 to T8.5. These bring the total sample of LAS T dwarfs to 80 as of DR4. In assigning spectral types to our objects we have identified eight new spectrally peculiar objects, and divide seven of them into two classes. H2O- H-early have a H2O- H index that differs with the H2O- J index by at least two subtypes. CH4- J-early have a CH4- J index that disagrees with the H2 index by at least two subtypes. We have ruled out binarity as a sole explanation for both types of peculiarity, and suggest that they may represent hitherto unrecognized tracers of composition and/or gravity. Clear trends in and are apparent for our sample, consistent with weakening absorption in the red wing of the K i line at 0.77 μm with decreasing effective temperature. We have used our sample to estimate space densities for T6–T9 dwarfs. By comparing our sample to Monte Carlo simulations of field T dwarfs for various mass functions of the form , we have placed weak constraints on the form of the field mass function. Our analysis suggests that the substellar mass function is declining at lower masses, with negative values of α preferred. This is at odds with results for young clusters that have been generally found to have . [ABSTRACT FROM AUTHOR]

We report the discovery of three very late T dwarfs in the UKIRT Infrared Deep Sky Survey (UKIDSS) Third Data Release: ULAS J101721.40+011817.9 (ULAS1017), ULAS J123828.51+095351.3 (ULAS1238) and ULAS J133553.45+113005.2 (ULAS1335). We detail optical and near-infrared (NIR) photometry for all three sources, and mid-IR photometry for ULAS1335. We use NIR spectra of each source to assign spectral types T8p (ULAS1017), T8.5 (ULAS1238) and T9 (ULAS1335) to these objects. ULAS1017 is classed as a peculiar T8 (T8p) due to appearing as a T8 dwarf in the J band, whilst exhibiting H- and K-band flux ratios consistent with a T6 classification. Through comparison to BT-Settl model spectra we estimate that ULAS1017 has , and , assuming solar metallicity. This estimate for gravity is degenerate with varying metallicity. We estimate that ULAS1017 has an age of 1.6–15 Gyr, a mass of 33– and lies at a distance of 31–54 pc. We do not estimate atmospheric parameters for ULAS1238 due to a lack of K-band photometry. We extend the unified scheme of Burgasser et al. to the type T9 and suggest the inclusion of the index to replace the now saturated J-band indices. We propose ULAS1335 as the T9 spectral type standard. ULAS1335 is the same spectral type as ULAS J003402.77−005206.7 and CFBDS J005910.90−011401.3. We argue that given the similarity of the currently known >T8 dwarfs to the rest of the T dwarf sequence, the suggestion of the Y0 spectral class for these objects is premature. Comparison of model spectra with that of ULAS1335 suggest a temperature below 600 K, possibly combined with low gravity and/or high metallicity. We find ULAS1335 to be extremely red in NIR to mid-IR colours, with . This is the reddest NIR to mid-IR colour yet observed for a T dwarf. The NIR to mid-IR spectral energy distribution of ULAS1335 further supports , and we estimate for ULAS1335. We estimate that ULAS1335 has an age of 0.6–5.3 Gyr, a mass of 15– and lies at a distance of 8–12 pc. [ABSTRACT FROM AUTHOR]