Spectral Characterization of a Complete Equatorial Sample of 615 K Dwarfs

We present the results of a spectroscopic study to determine the stellar properties, activity levels, space motions, and ages of the nearest K dwarfs. The targeted stars are members of RKSTAR (RECONS K Star) Survey that includes ∼5000 K dwarf primaries within 50 pc. An initial study established a benchmark calibration set of 35 K dwarfs with known ages of 20 Myr to 5 Gyr and high resolution (R=80000) spectra acquired on the CHIRON echelle spectrometer on the SMARTS 1.5m telescope at CTIO. Two known spectral indicators of activity and youth — the Hα absorption line (6562.8 Å) and the Li I resonance line (6707.8 Å) — showed strong trends in the training set. A sample of 615 K dwarfs within 33 parsecs and found between declinations +30 and −30 degrees has now been observed with CHIRON and analyzed relative to the stars in the benchmark calibration set. Surprisingly, it appears that as many as ∼8% of these K dwarfs have spectroscopic features indicating that they are young and/or active. As expected, Galactic U V W space motions indicate that most of the stars fall into the thin (80%) and thick (20%) disk populations, with a single outlier, HD 134439, which is a known halo star. Overall, we find a set of metal poor K dwarfs with [Fe/H]values of −0.5 dex or less that account for 4% of the population in the solar neighborhood. Empirical SpecMatch has been used to measure stellar properties for this sample of K dwarfs: temperatures range from 3600–5500 K, metallicities range from −0.6 \u3c [Fe/H]\u3c +0.55, and rotational velocities (v sin i) range from less than 10 km/s to more than 50 km/s. Surface gravity values (log g values) have also been determined, although they are less reliable than the other quantities. Cross-matching with NASA’s Exoplanet Archive reveals that only 42 exoplanet host stars are among our sample, highlighting the limited focus on mid and late-type K dwarfs (Teff \u3c 4800 K) in large exoplanet surveys. As a result of this work, we have identified 500 K dwarfs that are calm host stars worthy of detailed efforts to detect terrestrial planets. This work will serve as a key resource for assessing host star suitability for exoplanet habitability

Mind the Gap I: Hα\alpha Activity of M Dwarfs Near the Partially/Fully Convective Boundary and a New Hα\alpha Emission Deficiency Zone on the Main Sequence

Since identifying the gap in the H-R Diagram (HRD) marking the transition between partially and fully convective interiors, a unique type of slowly pulsating M dwarf has been proposed. These unstable M dwarfs provide new laboratories in which to understand how changing interior structures result in potentially observable activity at the surface. In this work, we report the results of the largest high-resolution spectroscopic H$\alpha$ emission survey to date spanning this transition region, including 480 M dwarfs observed using the CHIRON spectrograph at CTIO/SMARTS 1.5-m. We find that M dwarfs with H$\alpha$ in emission are almost entirely found 0 to 0.5 magnitude above the top edge of the gap in the HRD, whereas effectively no stars in and below the gap show emission. Thus, the top edge of the gap marks a relatively sharp activity transition and there is no anomalous H$\alpha$ activity for stars in the gap. We also identify a new region at 10.3 $<M_{G}<$ 10.8 on the main sequence where fewer M dwarfs exhibit H$\alpha$ emission compared to M dwarfs above and below this magnitude range. Careful evaluation of literature results indicates that 1) rotation and H$\alpha$ activity distributions on the main sequence are closely related, and 2) fewer stars in this absolute magnitude range rotate in less than $\sim$13 days than populations surrounding this region. This result suggests that the most massive fully convective stars lose their angular momentum faster than both partially convective stars and less massive fully convective stars.Comment: 30 pages, 18 figures, and 6 table. Submitted to A

TESS Discovery of an ultra-short-period planet around the nearby M dwarf LHS 3844

Data from the newly-commissioned \textit{Transiting Exoplanet Survey Satellite} (TESS) has revealed a "hot Earth" around LHS 3844, an M dwarf located 15 pc away. The planet has a radius of $1.32\pm 0.02$ $R_\oplus$ and orbits the star every 11 hours. Although the existence of an atmosphere around such a strongly irradiated planet is questionable, the star is bright enough ($I=11.9$, $K=9.1$) for this possibility to be investigated with transit and occultation spectroscopy. The star's brightness and the planet's short period will also facilitate the measurement of the planet's mass through Doppler spectroscopy.Comment: 10 pages, 4 figures. Submitted to ApJ Letters. This letter makes use of the TESS Alert data, which is currently in a beta test phase, using data from the pipelines at the TESS Science Office and at the TESS Science Processing Operations Cente

TESS Discovery of an Ultra-short-period Planet around the Nearby M Dwarf LHS 3844

Data from the newly commissioned Transiting Exoplanet Survey Satellite has revealed a 'hot Earth' around LHS 3844, an M dwarf located 15 pc away. The planet has a radius of R ⊕ and orbits the star every 11 hr. Although the existence of an atmosphere around such a strongly irradiated planet is questionable, the star is bright enough (I = 11.9, K = 9.1) for this possibility to be investigated with transit and occultation spectroscopy. The star's brightness and the planet's short period will also facilitate the measurement of the planet's mass through Doppler spectroscopy

TOI 694b and TIC 220568520b: Two Low-mass Companions near the Hydrogen-burning Mass Limit Orbiting Sun-like Stars

We report the discovery of TOI 694 b and TIC 220568520 b, two low-mass stellar companions in eccentric orbits around metal-rich Sun-like stars, first detected by the Transiting Exoplanet Survey Satellite (TESS). TOI 694 b has an orbital period of 48.05131 ± 0.00019 days and eccentricity of 0.51946 ± 0.00081, and we derive a mass of 89.0 ± 5.3 MJup (0.0849 ± 0.0051 M⊙) and radius of 1.111 ± 0.017 RJup (0.1142 ± 0.0017 R⊙). TIC 220568520 b has an orbital period of 18.55769 ± 0.00039 days and eccentricity of 0.0964 ± 0.0032, and we derive a mass of 107.2 ± 5.2 MJup (0.1023 ± 0.0050 M⊙) and radius of 1.248 ± 0.018 RJup (0.1282 ± 0.0019 R⊙). Both binary companions lie close to and above the hydrogen-burning mass threshold that separates brown dwarfs and the lowest-mass stars, with TOI 694 b being 2σ above the canonical mass threshold of 0.075 M⊙. The relatively long periods of the systems mean that the magnetic fields of the low-mass companions are not expected to inhibit convection and inflate the radius, which according to one leading theory is common in similar objects residing in short-period tidally synchronized binary systems. Indeed we do not find radius inflation for these two objects when compared to theoretical isochrones. These two new objects add to the short but growing list of low-mass stars with well-measured masses and radii, and highlight the potential of the TESS mission for detecting such rare objects orbiting bright stars

HD 2685 b: a hot Jupiter orbiting an early F-type star detected by TESS

We report on the confirmation of a transiting giant planet around the relatively hot (Teff = 6801 ± 76 K) star HD 2685, whose transit signal was detected in Sector 1 data of NASA’s TESS mission. We confirmed the planetary nature of the transit signal using Doppler velocimetric measurements with CHIRON, CORALIE, and FEROS, as well as using photometric data obtained with the Chilean-Hungarian Automated Telescope and the Las Cumbres Observatory. From the joint analysis of photometry and radial velocities, we derived the following parameters for HD 2685 b: P = 4.12688^{+0.00005}_{-0.00004} days, e = 0.091^{+0.039}_{-0.047}, MP = 1.17 ± 0.12 MJ, and RP =1.44 ± 0.05 RJ. This system is a typical example of an inflated transiting hot Jupiter in a low-eccentricity orbit. Based on the apparent visual magnitude (V = 9.6 mag) of the host star, this is one of the brightest known stars hosting a transiting hot Jupiter, and it is a good example of the upcoming systems that will be detected by TESS during the two-year primary mission. This is also an excellent target for future ground- and space-based atmospheric characterization as well as a good candidate for measuring the projected spin-orbit misalignment angle through the Rossiter–McLaughlin effect

TESS Spots a Hot Jupiter with an Inner Transiting Neptune

Hot Jupiters are rarely accompanied by other planets within a factor of a few in orbital distance. Previously, only two such systems have been found. Here, we report the discovery of a third system using data from the Transiting Exoplanet Survey Satellite (TESS). The host star, TOI-1130, is an eleventh magnitude K-dwarf in Gaia G-band. It has two transiting planets: A Neptune-sized planet (3.65 ± 0.10) with a 4.1 days period, and a hot Jupiter with an 8.4 days period. Precise radial-velocity observations show that the mass of the hot Jupiter is For the inner Neptune, the data provide only an upper limit on the mass of 0.17 (3σ). Nevertheless, we are confident that the inner planet is real, based on follow-up ground-based photometry and adaptive-optics imaging that rule out other plausible sources of the TESS transit signal. The unusual planetary architecture of and the brightness of the host star make TOI-1130 a good test case for planet formation theories, and an attractive target for future spectroscopic observations

TOI 564 b and TOI 905 b: grazing and fully transiting hot Jupiters discovered by TESS

We report the discovery and confirmation of two new hot Jupiters discovered by the Transiting Exoplanet Survey Satellite (TESS): TOI 564 b and TOI 905 b. The transits of these two planets were initially observed by TESS with orbital periods of 1.651 and 3.739 days, respectively. We conducted follow-up observations of each system from the ground, including photometry in multiple filters, speckle interferometry, and radial velocity measurements. For TOI 564 b, our global fitting revealed a classical hot Jupiter with a mass of ${1.463}_{-0.096}^{+0.10}$ MJ and a radius of ${1.02}_{-0.29}^{+0.71}$ RJ. Also a classical hot Jupiter, TOI 905 b has a mass of ${0.667}_{-0.041}^{+0.042}$ MJ and radius of ${1.171}_{-0.051}^{+0.053}$ RJ. Both planets orbit Sun-like, moderately bright, mid-G dwarf stars with V ∼ 11. While TOI 905 b fully transits its star, we found that TOI 564 b has a very high transit impact parameter of ${0.994}_{-0.049}^{+0.083}$ , making it one of only ∼20 known systems to exhibit a grazing transit and one of the brightest host stars among them. Therefore, TOI 564 b is one of the most attractive systems to search for additional nontransiting, smaller planets by exploiting the sensitivity of grazing transits to small changes in inclination and transit duration over a timescale of several years

Water mist : one of the most common natural resources we know is also an environmentally friendly and economic substance in sophisticated fire suppression systems

Peer reviewed: NoNRC publication: Ye