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Abstract The chemical abundance measurements of host stars and their substellar companions provide a powerful tool to trace the formation mechanism of the planetary systems. We present a detailed high-resolution spectroscopic analysis of a young M-type star, DH Tau A, which is located in the Taurus molecular cloud belonging to the Taurus-Auriga star-forming region. This star is host to a low-mass companion, DH Tau b, and both the star and the companion are still in their accreting phase. We apply our technique to measure the abundances of carbon and oxygen using carbon- and oxygen-bearing molecules, such as CO and OH, respectively. We determine a near-solar carbon-to-oxygen abundance ratio of C/O = 0.555 ± 0.063 for the host star DH Tau A. We compare this stellar abundance ratio with that of the companion from our previous study ( $\mathrm{C}/\mathrm{O}={0.54}_{-0.05}^{+0.06}$), which also has a near-solar value. This confirms the chemical homogeneity in the DH Tau system, which suggests a formation scenario for the companion consistent with a direct and relatively fast gravitational collapse rather than a slow core accretion process. 

Abstract Stellar spin down is a critical yet poorly understood component of stellar evolution. In particular, results from the Kepler Mission imply that mature age, solar-type stars have inefficient magnetic braking, resulting in a stalled spin-down rate. However, a large number of precise asteroseismic ages are needed for mature (≥3 Gyr) stars in order to probe the regime where traditional and stalled spin-down models differ. In this paper, we present a new asteroseismic benchmark star for gyrochronology discovered using reprocessed Kepler short cadence data. KIC 11029516 (Papayu) is a bright (Kp= 9.6 mag) solar-type star with a well-measured rotation period (21.1 ± 0.8 days) from spot modulation using 4 yr of Kepler long-cadence data. We combine asteroseismology and spectroscopy to obtainT_eff= 5888 ± 100 K, [Fe/H] = 0.30 ± 0.06 dex,M= 1.24 ± 0.05M_⊙,R= 1.34 ± 0.02R_⊙, and age of 4.0 ± 0.4 Gyr, making Papayu one of the most similar stars to the Sun in terms of temperature and radius with an asteroseismic age and a rotation period measured from spot modulation. We find that Papayu sits at the transition of where traditional and weakened spin-down models diverge. A comparison with stars of similar zero-age main-sequence temperatures supports previous findings that weakened spin-down models are required to explain the ages and rotation periods of old solar-type stars. 

Astronomers have found more than a dozen planets transiting stars that are 10–40 million years old1, but younger transiting planets have remained elusive. The lack of such discoveries may be because planets have not fully formed at this age or because our view is blocked by the protoplanetary disk. However, we now know that many outer disks are warped or broken2; provided the inner disk is depleted, transiting planets may thus be visible. Here we report observations of the transiting planet IRAS 04125+2902 b orbiting a 3-million-year-old, 0.7-solar-mass, pre-main-sequence star in the Taurus Molecular Cloud. The host star harbours a nearly face-on (30 degrees inclination) transitional disk3 and a wide binary companion. The planet has a period of 8.83 days, a radius of 10.7 Earth radii (0.96 Jupiter radii) and a 95%-confidence upper limit on its mass of 90 Earth masses (0.3 Jupiter masses) from radial-velocity measurements, making it a possible precursor of the super-Earths and sub-Neptunes frequently found around main-sequence stars. The rotational broadening of the star and the orbit of the wide (4 arcseconds, 635 astronomical units) companion are both consistent with edge-on orientations. Thus, all components of the system are consistent with alignment except the outer disk; the origin of this misalignment is unclear. 

Abstract We used the Immersion GRating Infrared Spectrometer (IGRINS) to determine fundamental parameters for 61 K- and M-type young stellar objects (YSOs) located in the Ophiuchus and Upper Scorpius star-forming regions. We employed synthetic spectra and a Markov chain Monte Carlo approach to fit specificK-band spectral regions and determine the photospheric temperature (T), surface gravity ( $\log g$), magnetic field strength (B), projected rotational velocity ( $v\sin i$), andK-band veiling (r_K). We determinedBfor ∼46% of our sample. Stellar parameters were compared to the results from Taurus-Auriga and the TW Hydrae association presented in Paper I of this series. We classified all the YSOs in the IGRINS survey with infrared spectral indices from Two Micron All Sky Survey and Wide-field Infrared Survey Explorer photometry between 2 and 24μm. We found that Class II YSOs typically have lower $\log g$and $v\sin i$, similarB, and higherK-band veiling than their Class III counterparts. Additionally, we determined the stellar parameters for a sample of K and M field stars also observed with IGRINS. We have identified intrinsic similarities and differences at different evolutionary stages with our homogeneous determination of stellar parameters in the IGRINS YSO survey. Considering $\log g$as a proxy for age, we found that the Ophiuchus and Taurus samples have a similar age. We also find that Upper Scorpius and TWA YSOs have similar ages, and are more evolved than Ophiuchus/Taurus YSOs.