Early K-type Stars Research Articles

Confirming the 3:2 Resonance Chain of K2-138

Abstract The study of orbital resonances allows for the constraint of planetary properties of compact systems. K2-138 is an early K-type star with six planets, five of which have been proposed to be in the longest chain of 3:2 mean motion resonances. To observe and potentially verify the resonant behavior of K2-138's planets, we run N-body simulations using previously measured parameters. Through our analysis, we find that 99.2% of our simulations result in a chain of 3:2 resonances, although only 11% of them show a five-planet resonance chain. We find that we are able to use resonances to constrain the orbital periods and masses of the planets. We explore the possibility of this system forming in situ and through disk migration, and we investigate the potential compositions of each planet using a planet structure code.

A Pair of Warm Giant Planets near the 2:1 Mean Motion Resonance around the K-dwarf Star TOI-2202*

Abstract TOI-2202 b is a transiting warm Jovian-mass planet with an orbital period of P = 11.91 days identified from the Full Frame Images data of five different sectors of the TESS mission. Ten TESS transits of TOI-2202 b combined with three follow-up light curves obtained with the CHAT robotic telescope show strong transit timing variations (TTVs) with an amplitude of about 1.2 hr. Radial velocity follow-up with FEROS, HARPS, and PFS confirms the planetary nature of the transiting candidate (a b = 0.096 ± 0.001 au, m b = 0.98 ± 0.06 M Jup), and a dynamical analysis of RVs, transit data, and TTVs points to an outer Saturn-mass companion (a c = 0.155 ± 0.002 au, m c = 0.37 ± 0.10 M Jup) near the 2:1 mean motion resonance. Our stellar modeling indicates that TOI-2202 is an early K-type star with a mass of 0.82 M ⊙, a radius of 0.79 R ⊙, and solar-like metallicity. The TOI-2202 system is very interesting because of the two warm Jovian-mass planets near the 2:1 mean motion resonance, which is a rare configuration, and their formation and dynamical evolution are still not well understood.

Dynamical confirmation of a stellar mass black hole in the transient X-ray dipping binary MAXI J1305-704

ABSTRACT MAXI J1305-704 has been proposed as a high-inclination candidate black hole X-ray binary in view of its X-ray properties and dipping behaviour during outburst. We present photometric and spectroscopic observations of the source in quiescence that allow us to reveal the ellipsoidal modulation of the companion star and absorption features consistent with those of an early K-type star ($T_{\rm eff}=4610^{+130}_{-160}\, {\rm K}$). The central wavelengths of the absorption lines vary periodically at $P_{\rm orb}=0.394\pm 0.004\, {\rm d}$ with an amplitude of $K_2=554\pm 8\, {\rm km \, s^{-1}}$. They imply a mass function for the compact object of $f(M_1)=6.9\pm 0.3\, {\rm M}_\odot$, confirming its black hole nature. The simultaneous absence of X-ray eclipses and the presence of dips set a conservative range of allowed inclinations $60\, {\rm deg}\lt i\lt 82\, {\rm deg}$, while modelling of optical light curves further constrain it to $i=72^{+5}_{-8}\, {\rm deg}$. The above parameters together set a black hole mass of $M_1= 8.9_{-1.0}^{+1.6}\, {\rm M}_{\odot }$ and a companion mass of $M_2= 0.43\pm 0.16\, {\rm M}_{\odot }$, much lower than that of a dwarf star of the observed spectral type, implying it is evolved. Estimates of the distance to the system ($d=7.5^{+1.8}_{-1.4}\, {\rm kpc}$) and space velocity ($v_{\rm space}=270\pm 60 \, {\rm km\, s^{-1}}$) place it in the Galactic thick disc and favour a significant natal kick during the formation of the BH if the supernova occurred in the Galactic Plane.

Chromospheric activity of nearby Sun-like stars

Context.The chromospheric emission in the cores of the Ca IIH & K lines of late-type dwarfs is a well known indicator of magnetic activity that decreases with increasing stellar age.Aims.I use this indicator to investigate the formation history of nearby G- and early K-type stars with origins at galactocentric distances similar to that of the region where the Sun was born.Methods.A parent sample of single main-sequence stars with near-solar metallicity and known magnetic activity levels is built from catalogues of stellar atmospheric parameters and chromospheric activity indices. A kinematical approach usesGaiaastrometric data to differentiate thin disc stars from thick disc stars. Measured distributions ofR′HKchromospheric activity indices are compared with Monte Carlo simulations based on an empirical model of chromospheric activity evolution.Results.The thin disc includes a significant fraction of Sun-like stars with intermediate activity levels (2 × 10−5≤R′HK≤ 6 × 10−5), while most early K- and G-type stars from the thick disc are inactive (R′HK< 2 × 10−5). The chromospheric activity distribution among nearby Sun-like dwarfs from the thin disc can be explained by a combination of an old (>6–7 Gyr) star formation event (or events) and a more recent (<3 Gyr) burst of star formation. Such an event is not required to account for theR′HKindex distributions of nearby thick disc stars.Conclusions.The distribution of magnetic activity among local G- and early K-type stars with a near-solar metallicity bears the imprint of an important star formation event that occurred ~1.9–2.6 Gyr ago in the thin disc of the Milky Way.

Planetary tidal interactions and the rotational evolution of low-mass stars

Context. The surface angular velocity evolution of low-mass stars is now globally understood and the main physical mechanisms involved in it are observationally quite constrained. However, while the general behaviour of these mechanisms is grasped, their theoretical description is still under ongoing work. This is the case, for instance, about the description of the physical process that extracts angular momentum from the radiative core, which could be described by several theoretical candidates. Additionally, recent observations showed anomalies in the rotation period distribution of open cluster, main sequence, early K-type stars that cannot be reproduced by current angular momentum evolution models. Aims. In this work, we study the parameter space of star-planet system’s configurations to investigate if including the tidal star-planet interaction in angular momentum evolution models could reproduce the anomalies of this rotation period distribution. Methods. To study this effect, we use a parametric angular momentum evolution model that allows for core-envelope decoupling and angular momentum extraction by magnetized stellar wind that we coupled to an orbital evolution code where we take into account the torque due to the tides raised on the star by the planet. We explore different stellar and planetary configurations (stellar mass from 0.5 to 1.0 M⊙ and planetary mass from 10 M⊕ to 13 Mjup) to study their effect on the planetary orbital and stellar rotational evolution. Results. The stellar angular momentum is the most impacted by the star-planet interaction when the planet is engulfed during the early main sequence phase. Thus, if a close-in Jupiter-mass planet is initially located at around 50% of the stellar corotation radius, a kink in the rotational period distribution opens around late and early K-type stars during the early main sequence phase. Conclusions. Tidal star-planet interactions can create a kink in the rotation period distribution of low-mass stars, which could possibly account for unexpected scatter seen in the rotational period distribution of young stellar clusters.

The Belgian Repository of Fundamental Atomic Data and Stellar Spectra (BRASS) Identifying Fruitful Methods for Producing Atomic Data

The Belgian repository of fundamental atomic data and stellar spectra (BRASS) aims to provide the largest systematic and homogeneous quality assessment to-date of input atomic data required for stellar spectral synthesis. In addition to quality-assessed atomic data, BRASS shall also provide of a range of extremely high-quality benchmark stellar spectra spanning late B-type stars to early K-type stars. In this paper, we summarise the project’s progress and available results to-date. We provide a brief comparison between our results and the BRASS project’s compiled and cross-matched atomic literature, with the goal of providing useful feedback to the atomic community on which methods may produce more reliable and accurate atomic data. We hope that the examples presented here stimulate further investigation by the atomic physics community.

LIMIT CYCLES CAN REDUCE THE WIDTH OF THE HABITABLE ZONE.

The liquid water habitable zone (HZ) describes the orbital distance at which a terrestrial planet can maintain above-freezing conditions through regulation by the carbonate-silicate cycle. Recent calculations have suggested that planets in the outer regions of the HZ cannot maintain stable, warm climates, but rather should oscillate between long, globally glaciated states and shorter periods of climatic warmth. Such conditions, similar to "Snowball Earth" episodes experienced on Earth, would be inimical to the development of complex land life, including intelligent life. Here, we build on previous studies with an updated energy balance climate model to calculate this "limit cycle" region of the HZ where such cycling would occur. We argue that an abiotic Earth would have a greater CO2 partial pressure than today because plants and other biota help to enhance the storage of CO2 in soil. When we tune our abiotic model accordingly, we find that limit cycles can occur but that previous calculations have overestimated their importance. For G stars like the Sun, limit cycles occur only for planets with CO2 outgassing rates less than that on modern Earth. For K- and M-star planets, limit cycles should not occur; however, M-star planets may be inhospitable to life for other reasons. Planets orbiting late G-type and early K-type stars retain the greatest potential for maintaining warm, stable conditions. Our results suggest that host star type, planetary volcanic activity, and seafloor weathering are all important factors in determining whether planets will be prone to limit cycling.

LONG-ORBITAL-PERIOD PREPOLARS CONTAINING EARLY K-TYPE DONOR STARS. BOTTLENECK ACCRETION MECHANISM IN ACTION

ABSTRACT We studied two objects identified as cataclysmic variables (CVs) with periods exceeding the natural boundary for Roche-lobe-filling zero-age main sequence (ZAMS) secondary stars. We present observational results for V1082 Sgr with a 20.82 hr orbital period, an object that shows a low luminosity state when its flux is totally dominated by a chromospherically active K star with no signs of ongoing accretion. Frequent accretion shutoffs, together with characteristics of emission lines in a high state, indicate that this binary system is probably detached, and the accretion of matter on the magnetic white dwarf takes place through stellar wind from the active donor star via coupled magnetic fields. Its observational characteristics are surprisingly similar to V479 And, a 14.5 hr binary system. They both have early K-type stars as donor stars. We argue that, similar to the shorter-period prepolars containing M dwarfs, these are detached binaries with strong magnetic components. Their magnetic fields are coupled, allowing enhanced stellar wind from the K star to be captured and channeled through the bottleneck connecting the two stars onto the white dwarf’s magnetic pole, mimicking a magnetic CV. Hence, they become interactive binaries before they reach contact. This will help to explain an unexpected lack of systems possessing white dwarfs with strong magnetic fields among detached white+red dwarf systems.

Rotation periods of 12 000 main-sequenceKeplerstars: Dependence on stellar spectral type and comparison withvsiniobservations

Aims: We aim to measure the starspot rotation periods of active stars in the Kepler field as a function of spectral type and to extend reliable rotation measurements from F-, G-, and K-type to M-type stars. Methods: Using the Lomb-Scargle periodogram we searched more than 150 000 stellar light curves for periodic brightness variations. We analyzed periods between 1 and 30 days in eight consecutive Kepler quarters, where 30 days is an estimated maximum for the validity of the PDC_MAP data correction pipeline. We selected stable rotation periods, i.e., periods that do not vary from the median by more than one day in at least six of the eight quarters. We averaged the periods for each stellar spectral class according to B - V color and compared the results to archival vsini data, using stellar radii estimates from the Kepler Input Catalog. Results: We report on the stable starspot rotation periods of 12 151 Kepler stars. We find good agreement between starspot velocities and vsini data for all F-, G- and early K-type stars. The 795 M-type stars in our sample have a median rotation period of 15.4 days. We find an excess of M-type stars with periods less than 7.5 days that are potentially fast-rotating and fully convective. Measuring photometric variability in multiple Kepler quarters appears to be a straightforward and reliable way to determine the rotation periods of a large sample of active stars, including late-type stars.

Coronal abundances of X-ray bright pre-main sequence stars in the Taurus molecular cloud

Aims. We studied the thermal properties and chemical composition of the X-ray emitting plasma of a sample of bright members of the Taurus Molecular Cloud to investigate possible differences among classical and weak-lined T Tauri stars, and possible dependences of the abundances on the stellar activity level and/or on the presence of accretion/circumstellar material. Methods. We used medium-resolution X-ray spectra obtained with the sensitive EPIC/PN camera in order to analyze the possible sample. The PN spectra of 20 bright (LX ∼ 10 30 –10 31 erg s −1 ) Taurus members, with at least ∼4500 counts, were fitted using thermal models of optically thin plasma with two components and variable abundances of O, Ne, Mg, Si, S, Ar, Ca, and Fe. Extensive preliminary investigations were employed to study the performances of the PN detectors regarding abundance determinations, and finally to check the results of the fittings. Results. We found that the observed X-ray emission of the studied stars can be attributed to coronal plasma having similar thermal properties and chemical composition both in the classical and in the weak-lined T Tauri stars. The results of the fittings did not reveal a correlation between the abundance patterns and activity or accretion/disk presence. The iron abundance of these active stars is significantly lower than the solar photospheric value (∼0.2 solar). An indication of slightly different coronal properties in stars with different spectral type is found in this study. G-type and early K-type stars have, on average, slightly higher Fe abundances (Fe ∼ 0.24 solar) than stars with later spectral type (Fe ∼ 0.15 solar), confirming previous findings from high-resolution X-ray spectroscopy. Stars of the former group are also found to have, on average, hotter coronae.

The dependence of macroturbulence on luminosity in early K-type stars

view Abstract Citations (58) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The dependence of macroturbulence on luminosity in early K-type stars. Smith, M. A. ; Dominy, J. F. Abstract The velocity-dissection technique and Fourier techniques are applied to high-quality Reticon data in order to determine rotational and macroturbulent velocities for 17 early K stars and three late G stars of luminosity classes Ib, II, III, and IV. Several tests are made which indicate that the effects of various random and systematic errors are minor. The results suggest that rotational velocities cannot be discerned among early K subgiants and supergiants but are marginally detectable in some giants. This is explained in terms of present ideas concerning rotational deceleration due to evolutionary expansion. No evidence is found for large differences in the macroturbulence velocity distribution among late-type stars of luminosity classes Ib through V. The results obtained for the K1 giant Beta Cep confirm previous reports of a correlation between photospheric and chromospheric turbulence. Publication: The Astrophysical Journal Pub Date: July 1979 DOI: 10.1086/157209 Bibcode: 1979ApJ...231..477S Keywords: Early Stars; K Stars; Late Stars; Stellar Atmospheres; Stellar Luminosity; Stellar Rotation; Turbulent Flow; Chromosphere; G Stars; Photosphere; Spectral Resolution; Velocity Distribution; Astrophysics; K Stars:Luminosities; Late-Type Stars:Turbulence full text sources ADS | data products SIMBAD (21)

INFRARED OBJECT IN THE REGION OF IC 1895

nor any other with as short a period as 0^33. To what class of variables AT Herculis belongs cannot be ascertained on the basis of present observations. If the small amplitude of 0^5 is correct, AT Herculis may be an eclipsing variable with an early K-type star as the brighter component. The present observations also show that the star is about half a magnitude fainter than given in the General Catalogue of Variable Stars.