Search

The Neptunian desert is a region in period-radius parameter space with very few Neptune-sized planets at short orbital periods. Amongst these, LTT 9779 b is the only known Neptune with a period shorter than 1 d to retain a significant H–He atmosphere. If the Neptune desert is the result of X-ray/EUV-driven photoevaporation, it is surprising that the atmosphere of LTT 9779 b survived the intense bombardment of high-energy photons from its young host star. However, the star has low measured rotational broadening, which points to the possibility of an anomalously slow spin period and hence a faint X-ray emission history that may have failed to evaporate the planet's atmosphere. We observed LTT 9779 with XMM-Newton and measured an upper limit for its X-ray luminosity that is a factor of 15 lower than expected for its age. We also simulated the evaporation past of LTT 9779 b and found that the survival of its atmosphere to the present day is consistent with an unusually faint XUV irradiation history that matches both the X-ray and rotation velocity measurements. We conclude that the anomalously low X-ray irradiation of the one Neptune seen to survive in Neptunian desert supports the interpretation of the desert as primarily a result of photoevaporation. [ABSTRACT FROM AUTHOR]

Very little is known about the young planet population because the detection of small planets orbiting young stars is obscured by the effects of stellar activity and fast rotation, which mask planets within radial velocity and transit data sets. The few planets that have been discovered in young clusters generally orbit stars too faint for any detailed follow-up analysis. Here, we present the characterization of a new mini-Neptune planet orbiting the bright (V = 9) and nearby K2 dwarf star, HD 18599. The planet candidate was originally detected in TESS  light curves from sectors 2, 3, 29, and 30, with an orbital period of 4.138 d. We then used HARPS and FEROS radial velocities, to find the companion mass to be 25.5 |$\pm$| 4.6  |$M_{\oplus }$|⁠. When we combine this with the measured radius from TESS  of 2.70 |$\pm$| 0.05  |$R_{\oplus }$|⁠ , we find a high planetary density of 7.1 |$\pm$| 1.4 g cm |$^{-3}$|⁠. The planet exists on the edge of the Neptune Desert and is the first young planet (300 Myr) of its type to inhabit this region. Structure models argue for a bulk composition to consist of 23 per cent H |$_2$| O and 77 per cent Rock and Iron. Future follow-up with large ground- and space-based telescopes can enable us to begin to understand in detail the characteristics of young Neptunes in the galaxy. [ABSTRACT FROM AUTHOR]

We analyse 829 481 stars from the Next Generation Transit Survey (NGTS) to extract variability periods. We utilize a generalization of the autocorrelation function (the G-ACF), which applies to irregularly sampled time series data. We extract variability periods for 16 880 stars from late-A through to mid-M spectral types and periods between ∼0.1 and 130 d with no assumed variability model. We find variable signals associated with a number of astrophysical phenomena, including stellar rotation, pulsations, and multiple-star systems. The extracted variability periods are compared with stellar parameters taken from Gaia DR2, which allows us to identify distinct regions of variability in the Hertzsprung–Russell Diagram. We explore a sample of rotational main-sequence objects in period-colour space, in which we observe a dearth of rotation periods between 15 and 25 d. This 'bi-modality' was previously only seen in space-based data. We demonstrate that stars in sub-samples above and below the period gap appear to arise from a stellar population not significantly contaminated by excess multiple systems. We also observe a small population of long-period variable M-dwarfs, which highlight a departure from the predictions made by rotational evolution models fitted to solar-type main-sequence objects. The NGTS data spans a period and spectral type range that links previous rotation studies such as those using data from Kepler, K2 , and MEarth. [ABSTRACT FROM AUTHOR]

Long-term stellar activity variations can affect the detectability of long-period and Earth-analogue extrasolar planets. We have, for 54 stars, analysed the long-term trend of five activity indicators: log  |$R^{\prime }_\mathrm{{HK}}$|⁠ , the cross-correlation function (CCF) bisector span, CCF full-width-at-half-maximum, CCF contrast, and the area of the Gaussian fit to the CCF; and studied their correlation with the RVs. The sign of the correlations appears to vary as a function of stellar spectral type, and the transition in sign signals a noteworthy change in the stellar activity properties where earlier type stars appear more plage dominated. These transitions become more clearly defined when considered as a function of the convective zone depth. Therefore, it is the convective zone depth (which can be altered by stellar metallicity) that appears to be the underlying fundamental parameter driving the observed activity correlations. In addition, for most of the stars, we find that the RVs become increasingly redshifted as activity levels increase, which can be explained by the increase in the suppression of convective blueshift. However, we also find a minority of stars where the RVs become increasingly blueshifted as activity levels increase. Finally, using the correlation found between activity indicators and RVs, we removed RV signals generated by long-term changes in stellar activity. We find that performing simple cleaning of such long-term signals enables improved planet detection at longer orbital periods. [ABSTRACT FROM AUTHOR]

We report the detection of a new planetary system orbiting the nearby M2.5V star GJ 357, using precision radial velocities from three separate echelle spectrographs, High Accuracy Radial velocity Planet Searcher (HARPS), High Resolution Echelle Spectrograph (HiRES), and Ultraviolet and Visible Echelle Spectrograph (UVES). Three small planets have been confirmed in the system, with periods of 9.125 ± 0.001, 3.9306 ± 0.0003, and 55.70 ± 0.05 d, and minimum masses of 3.33 ± 0.48, 2.09 ± 0.32, and 6.72 ± 0.94 M⊕, respectively. The second planet in our system, GJ 357 c, was recently shown to transit by the Transiting Exoplanet Survey Satellite (TESS), but we could find no transit signatures for the other two planets. Dynamical analysis reveals the system is likely to be close to coplanar, is stable on Myr time-scales, and places strong upper limits on the masses of the two non-transiting planets GJ 357 b and GJ 357 d of 4.25 and 11.20 M⊕, respectively. Therefore, we confirm the system contains at least two super-Earths, and either a third super-Earth or mini-Neptune planet. GJ 357 b  and  GJ 357 c are found to be close to a 7:3 mean motion resonance, however no libration of the orbital parameters was found in our simulations. Analysis of the photometric light curve of the star from the TESS , when combined with our radial velocities, reveals GJ 357 c has an absolute mass, radius, and density of |$2.248^{+0.117}_{-0.120}$|  M⊕, |$1.167^{+0.037}_{-0.036}$|  R⊕, and |$7.757^{+0.889}_{-0.789}$|  g cm−3, respectively. Comparison to super-Earth structure models reveals the planet is likely an iron-dominated world. The GJ 357 system adds to the small sample of low-mass planetary systems with well constrained masses, and further observational and dynamical follow-up is warranted to better understand the overall population of small multiplanet systems in the solar neighbourhood. [ABSTRACT FROM AUTHOR]

We present high cadence detections of two superflares from a bright G8 star (V = 11.56) with the Next Generation Transit Survey (NGTS).We improve upon previous super are detections by resolving the flare rise and peak, allowing us to fit a solar flare inspired model without the need for arbitrary break points between rise and decay. Our data also enables us to identify substructure in the flares. From changing star-spot modulation in the NGTS data, we detect a stellar rotation period of 59 h, along with evidence for differential rotation. We combine this rotation period with the observed ROSATX-ray flux to determine that the star's X-ray activity is saturated. We calculate the flare bolometric energies as 5.4+0.8-0.7 x 1034 and 2.6+0.4-0.3 x 1034 erg and compare our detections with G star superflares detected in the Kepler survey. We find our main flare to be one of the largest amplitude superflares detected from a bright G star. With energies more than 100 times greater than the Carrington event, our flare detections demonstrate the role that ground-based instruments such as NGTS can have in assessing the habitability of Earth-like exoplanets, particularly in the era of PLATO. [ABSTRACT FROM AUTHOR]

Past ultraviolet and optical observations of stars hosting close-in Jupiter-mass planets have shown that some of these stars present an anomalously low chromospheric activity, significantly below the basal level. For the hot Jupiter planet host WASP-13, observations have shown that the apparent lack of activity is possibly caused by absorption from the intervening interstellar medium (ISM). Inspired by this result, we study the effect of ISM absorption on activity measurements (S and log R 'HK indices) for main-sequence late-type stars. To this end, we employ synthetic stellar photospheric spectra combined with varying amounts of chromospheric emission and ISM absorption. We present the effect of ISM absorption on activity measurements by varying several instrumental (spectral resolution), stellar (projected rotational velocity, effective temperature, and chromospheric emission flux), and ISM parameters (relative velocity between stellar and ISM Ca ii lines, broadening b-parameter, and Ca ii column density). We find that for relative velocities between the stellar and ISM lines smaller than 30-40 km s-1 and for ISM Ca ii column densities log NCaII = 12, the ISM absorption has a significant influence on activity measurements. Direct measurements and three dimensional maps of the Galactic ISM absorption indicate that an ISM Ca ii column density of log NCaII = 12 is typically reached by a distance of about 100 pc along most sight lines. In particular, for a Sun-like star lying at a distance greater than 100 pc, we expect a depression (bias) in the log R 'HK value larger than 0.05-0.1 dex, about the same size as the typical measurement and calibration uncertainties on this parameter. This work shows that the bias introduced by ISM absorption must always be considered when measuring activity for stars lying beyond 100 pc. We also consider the effect of multiple ISM absorption components. We discuss the relevance of this result for exoplanet studies and revise the latest results on stellar activity versus planet surface gravity correlation. We finally describe methods with which it would be possible to account for ISM absorption in activity measurements and provide a code to roughly estimate the magnitude of the bias. Correcting for the ISM absorption bias may allow one to identify the origin of the anomaly in the activity measured for some planet-hosting stars. [ABSTRACT FROM AUTHOR]

The hunt for Earth analogue planets orbiting Sun-like stars has forced the introduction of novel methods to detect signals at, or below, the level of the intrinsic noise of the observations. We present a new global periodogram method that returns more information than the classic Lomb–Scargle periodogram method for radial velocity signal detection. Our method uses the minimum mean-squared error as a framework to determine the optimal number of genuine signals present in a radial velocity timeseries using a global search algorithm, meaning that we can discard noise spikes from the data before a follow-up analysis. This method also allows us to determine the phase and amplitude of the signals we detect, meaning that we can track these quantities as a function of time to test if the signals are stationary or non-stationary. We apply our method to the radial velocity data for GJ876 as a test system to highlight how the phase information can be used to select against the non-stationary sources of detected signals in radial velocity data, such as rotational modulation of star spots. Analysis of this system yields two new statistically significant signals in the combined Keck and HARPS velocities with periods of 10 and 15 d. Although a planet with a period of 15 d would relate to a Laplace resonant chain configuration with three of the other planets (8:4:2:1), we stress that the follow-up dynamical analyses are needed to test the reliability of such a six-planet system. [ABSTRACT FROM AUTHOR]

We present the results of a study of the stellar activity in the solar neighborhood using complete kinematics (galactocentric velocities U, V, W) and the chromospheric activity index log R0 HK. We analyzed the average activity level near the centers of known moving groups using a sample of 2529 stars and found that the stars near these associations tend to be more active than field stars. This supports the hypothesis that these structures, or at least a significant part of them, are composed of kinematically bound, young stars. We confirmed our results by using GALaxy Evolution EXplorer (GALEX) UV data and kinematics taken from the Geneva- Copenhagen Survey for the stars in the sample. Finally, we present a compiled catalog with kinematics and activities for 2529 stars and a list of potential moving group members selected based on their stellar activity level. [ABSTRACT FROM AUTHOR]

Aims. Our understanding of magnetic fields in late-type stars is strongly driven by what we know of the solar magnetic field. For this reason, it is crucial to understand how typical the solar dynamo is. To do this we need to compare the solar magnetic field with that of other stars as similar to the Sun as possible, both in stellar parameters and age, hence activity. We present here the detection of a magnetic field in three planet-hosting solar-like stars having a mass, age, and activity level comparable to that of the Sun. Methods. We used the HARPSpol spectropolarimeter to obtain high-resolution high-quality circularly polarised spectra of HD70642, HD117207, and HD154088, using the least-squares deconvolution technique to detect the magnetic field. From the Stokes I spectra, we calculated the log R'HK activity index for each star. We compared the position of the stars in the Hertzsprung-Russell diagram to evolutionary tracks, to estimate their mass and age. We used the lithium abundance, derived from the Stokes I spectra, to further constrain the ages. Results. We obtained a definite magnetic field detection for both HD70642 and HD154088, while for HD117207 we obtained a marginal detection. Due to the lower signal-to-noise ratio of the observations, we were unable to detect the magnetic field in the second set of observations available for HD117207 and HD154088. On the basis of effective temperature, mass, age, and activity level the three stars can be considered solar analogs. Conclusions. HD70642, HD117207, and HD154088 are ideal targets for a comparative study between the solar magnetic field and that of solar analogs. [ABSTRACT FROM AUTHOR]

ABSTRACT We present the first results from our Red Optical Planet Survey to search for low-mass planets orbiting late-type dwarfs (M5.5V-M9V) in their habitable zones. Our observations with the red arm of the Magellan Inamori Kyocera Echelle spectrograph (0.5-0.9 m) at the 6.5-m Magellan Clay telescope at Las Campanas Observatory indicate that ≥92 per cent of the flux lies beyond 0.7 m. We use a novel approach that is essentially a hybrid of the simultaneous iodine and ThAr methods for determining precision radial velocities. We apply least squares deconvolution to obtain a single high signal-to-noise ratio (S/N) stellar line for each spectrum and cross-correlate against the simultaneously observed telluric line profile, which we derive in the same way. Utilizing the 0.62-0.90 m region, we have achieved an rms precision of 10 ms−1 for an M5.5V spectral type star with spectral S/N ∼ 160 on 5-min time-scales. By M8V spectral type, a precision of ∼30 ms−1 at S/N = 25 is suggested, although more observations are needed. An assessment of our errors and scatter in the radial velocity points hints at the presence of stellar radial velocity variations. Of our sample of seven stars, two show radial velocity signals at 6σ and 10σ of the cross-correlation uncertainties. We find that chromospheric activity (via Hα variation) does not have an impact on our measurements and are unable to determine a relationship between the derived photospheric line profile morphology and radial velocity variations without further observations. If the signals are planetary in origin, our findings are consistent with estimates of Neptune mass planets that predict a frequency of 13-27 per cent for early M dwarfs. Our current analysis indicates the we can achieve a sensitivity that is equivalent to the amplitude induced by a 6 M⊕ planet orbiting in the habitable zone. Based on simulations, we estimate that <10 M⊕ habitable zone planets will be detected in a new stellar mass regime, with ≤20 epochs of observations. Higher resolution and greater instrument stability indicate that photon-limited precisions of 2 ms−1 are attainable on moderately rotating M dwarfs (with vsin i≤ 5 km s−1) using our technique. [ABSTRACT FROM AUTHOR]

We have acquired high-resolution echelle spectra of 225 F6-M5 type stars in the Southern hemisphere. The stars are targets or candidates to be targets for the Anglo-Australian Planet Search. Ca ii H& K line cores were used to derive activity indices for all of these objects. The indices were converted to the Mt. Wilson system of measurements and values determined. A number of these stars had no previously derived activity indices. In addition, we have also included the stars from Tinney et al. using our Mt. Wilson calibration. The radial-velocity instability (also known as jitter) level was determined for all 21 planet-host stars in our data set. We find the jitter to be at a level considerably below the radial-velocity signatures in all but one of these systems. 19 stars from our sample were found to be active and thus have high levels of jitter. Radial-velocity analysis for planetary companions to these stars should proceed with caution. [ABSTRACT FROM AUTHOR]