ABSTRACT
By combining ground-based spectrographic observations of variability in the chromospheric emission from Sun-like stars with the variability seen in their eigenmode frequencies, it is ...possible to relate the changes observed at the surfaces of these stars to the changes taking place in the interior. By further comparing this variability to changes in the relative flux from the stars, one can obtain an expression for how these activity indicators relate to the energy output from the stars. Such studies become very pertinent when the variability can be related to stellar cycles as they can then be used to improve our understanding of the solar cycle and its effect on the energy output from the Sun. Here, we present observations of chromospheric emission in 20 Sun-like stars obtained over the course of the nominal 4 yr Kepler mission. Even though 4 yr is too short to detect stellar equivalents of the 11 yr solar cycle, observations from the Kepler mission can still be used to analyse the variability of the different activity indicators thereby obtaining information of the physical mechanism generating the variability. The analysis reveals no strong correlation between the different activity indicators, except in very few cases. We suggest that this is due to the sparse sampling of our ground-based observations on the one hand and that we are likely not tracing cyclic variability on the other hand. We also discuss how to improve the situation.
Superflares on solar-like stars Vasilyev, V.; Reinhold, T.; Shapiro, A. I. ...
Astronomy and astrophysics (Berlin),
12/2022, Letnik:
668
Journal Article
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Context.
Over the past years, thousands of stellar flares have been detected by harvesting data from large photometric surveys. These detections, however, do not account for potential sources of ...contamination such as background stars or small Solar System objects appearing in the same aperture as the primary target.
Aims.
We present a new method for identifying the true flare sources in large photometric surveys using data from the
Kepler
mission as an illustrative example. The new method considers not only the brightness excess in the stellar light curves, but also the location of this excess in the pixel-level data.
Methods.
Potential flares are identified in two steps. First, we search the light curves for at least two subsequent data points exceeding a 5
σ
threshold above the running mean. For these two cadences, we subtract the “quiet” stellar flux from the
Kepler
pixel data to obtain new images where the potential flare is the main light source. In the second step, we use a Bayesian approach to fit the point spread function of the instrument to determine the most likely location of the flux excess on the detector. We match this location with the position of the primary target and other stars from the
Gaia
DR2 catalog within a radius of 10 arcsec around the primary
Kepler
target. When the location of the flux excess and the target star coincide, we associate the event with a flare on the target star.
Results.
We applied our method to 5862 main-sequence stars with near-solar effective temperatures. From the first step we found 2274 events exceeding the 5σ level in at least two consecutive points in the light curves. Applying the second step reduced this number to 342 superflares. Of these, 283 flares occurred on 178 target stars and 47 events are associated with fainter background stars; in 10 cases the flare location could not be distinguished between the target and a background star. We also present cases where flares were reported previously but our technique could not attribute them to the target star.
Conclusions.
We conclude that identifying outliers in the light curves alone is insufficient to attribute them to stellar flares and that flares can only be uniquely attributed to a certain star when the instrument pixel-level data together with the point spread function are taken into account. As a consequence, previous flare statistics are likely contaminated by instrumental effects and unresolved astrophysical sources.
Spectroscopy of transiting exoplanets can be used to investigate their atmospheric properties
and habitability. Combining radial velocity (RV) and transit data provides additional information
on ...exoplanet physical properties. We detect a transiting rocky planet with an orbital period
of 1.467 days around the nearby red dwarf star Gliese 486. The planet Gliese 486 b is
2.81 Earth masses and 1.31 Earth radii, with uncertainties of 5%, as determined from RV data
and photometric light curves. The host star is at a distance of ~8.1 parsecs, has a J-band magnitude of ~7.2, and is observable from both hemispheres of Earth. On the basis of these properties and the planet’s short orbital period and high equilibrium temperature, we show that this terrestrial planet is suitable for emission and transit spectroscopy.
This is our reply to the comment of T. Metcalfe and J. van Saders on the Science report "The Sun is less active than other solar-like stars" by T. Reinhold, A. I. Shapiro, S. K. Solanki, B. T. ...Montet, N. A. Krivova, R. H. Cameron, E. M. Amazo-Gomez. We hope that both the comment and our reply lead to fruitful discussions which of the two presented scenarios is more likely.
By combining ground-based spectrographic observations of variability in the chromospheric emission from Sun-like stars with the variability seen in their eigenmode frequencies, it is possible to ...relate the changes observed at the surfaces of these stars to the changes taking place in the interior. By further comparing this variability to changes in the relative flux from the stars, one can obtain an expression for how these activity indicators relate to the energy output from the stars. Such studies become very pertinent when the variability can be related to stellar cycles as they can then be used to improve our understanding of the solar cycle and its effect on the energy output from the Sun. Here we present observations of chromospheric emission in 20 Sun-like stars obtained over the course of the nominal 4-year Kepler mission. Even though 4 years is too short to detect stellar equivalents of the 11-year solar cycle, observations from the Kepler mission can still be used to analyse the variability of the different activity indicators thereby obtaining information of the physical mechanism generating the variability. The analysis reveals no strong correlation between the different activity indicators, except in very few cases. We suggest that this is due to the sparse sampling of our ground-based observations on the one hand and that we are likely not tracing cyclic variability on the other hand. We also discuss how to improve the situation.
ABSTRACT The mass and age of substellar objects are degenerate parameters leaving the evolutionary state of brown dwarfs ambiguous without additional information. Theoretical models are normally used ...to help distinguish between old, massive brown dwarfs and young, low-mass brown dwarfs but these models have yet to be properly calibrated. We have carried out an infrared high-contrast imaging program with the goal of detecting substellar objects as companions to nearby stars to help break degeneracies in inferred physical properties such as mass, age, and composition. Rather than using imaging observations alone, our targets are pre-selected based on the existence of dynamical accelerations informed from years of stellar radial velocity (RV) measurements. In this paper, we present the discovery of a rare benchmark brown dwarf orbiting the nearby (d = 18.69 0.19 pc), solar-type (G9V) star HD 4747 (Fe/H = −0.22 0.04) with a projected separation of only = 11.3 0.2 au (θ = 0 6). Precise Doppler measurements taken over 18 years reveal the companion's orbit and allow us to place strong constraints on its mass using dynamics ( ). Relative photometry (ΔKs = 9.05 0.14, , ) indicates that HD 4747 B is most likely a late-type L-dwarf and, if near the L/T transition, an intriguing source for studying cloud physics, variability, and polarization. We estimate a model-dependent mass of for an age of Gyr based on gyrochronology. Combining astrometric measurements with RV data, we calculate the companion dynamical mass ( ) and orbit (e = 0.740 0.002) directly. As a new mass, age, and metallicity benchmark, HD 4747 B will serve as a laboratory for precision astrophysics to test theoretical models that describe the emergent radiation of brown dwarfs.
The physical properties of faint stellar and substellar objects often rely on indirect, model-dependent estimates. For example, the masses of brown dwarfs are usually inferred using evolutionary ...models, which are age dependent and have yet to be properly calibrated. With the goal of identifying new benchmark objects to test low-mass stellar and substellar models, we have carried out a comprehensive adaptive optics survey as part of the TaRgetting bENchmark-objects with the Doppler Spectroscopy high-contrast imaging program. Using legacy radial velocity measurements from the High Resolution Echelle Spectrometer at Keck, we have identified several dozen stars that show long-term Doppler accelerations. We present follow-up high-contrast observations from the campaign and report the discovery of 31 comoving companions, as well as 11 strong candidate companions, to solar-type stars with well-determined parallax and metallicity values. Benchmark objects of this nature lend themselves to orbit determinations, dynamical mass estimates, and independent compositional assessment. This compendium of benchmark objects will serve as a convenient test group to substantiate theoretical evolutionary and atmospheric models near the hydrogen fusing limit.
The GALAH+ survey: Third data release Buder, Sven; Sharma, Sanjib; Kos, Janez ...
Monthly notices of the Royal Astronomical Society,
2021, Letnik:
506, Številka:
1
Journal Article
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ABSTRACT
The ensemble of chemical element abundance measurements for stars, along with precision distances and orbit properties, provides high-dimensional data to study the evolution of the Milky ...Way. With this third data release of the Galactic Archaeology with HERMES (GALAH) survey, we publish 678 423 spectra for 588 571 mostly nearby stars (81.2 per cent of stars are within <2 kpc), observed with the HERMES spectrograph at the Anglo-Australian Telescope. This release (hereafter GALAH+ DR3) includes all observations from GALAH Phase 1 (bright, main, and faint survey, 70 per cent), K2-HERMES (17 per cent), TESS-HERMES (5 per cent), and a subset of ancillary observations (8 per cent) including the bulge and >75 stellar clusters. We derive stellar parameters Teff, log g, Fe/H, vmic, vbroad, and vrad using our modified version of the spectrum synthesis code Spectroscopy Made Easy (sme) and 1D marcs model atmospheres. We break spectroscopic degeneracies in our spectrum analysis with astrometry from Gaia DR2 and photometry from 2MASS. We report abundance ratios X/Fe for 30 different elements (11 of which are based on non-LTE computations) covering five nucleosynthetic pathways. We describe validations for accuracy and precision, flagging of peculiar stars/measurements and recommendations for using our results. Our catalogue comprises 65 per cent dwarfs, 34 per cent giants, and 1 per cent other/unclassified stars. Based on unflagged chemical composition and age, we find 62 per cent young low-$\alpha$, 9 per cent young high-$\alpha$, 27 per cent old high-$\alpha$, and 2 per cent stars with Fe/H ≤ −1. Based on kinematics, 4 per cent are halo stars. Several Value-Added-Catalogues, including stellar ages and dynamics, updated after Gaia eDR3, accompany this release and allow chrono-chemodynamic analyses, as we showcase.
We report the first planet discovery from the two-wheeled Kepler (K2) mission: HIP 116454 b. The host star HIP 116454 is a bright (V = 10.1, K = 8.0) K1 dwarf with high proper motion and a ...parallax-based distance of 55.2 + or - 5.4 pc. Based on high-resolution optical spectroscopy, we find that the host star is metal-poor with Fe/H = -0.16 + or - 0.08 and has a radius Rlow * = 0.716 + or - 0.024 R sub(middot in circle) and mass Mlow * = 0.775 + or - 0.027 M sub(middot in circle). The star was observed by the Kepler spacecraft during its Two-Wheeled Concept Engineering Test in 2014 February. During the 9 days of observations, K2 observed a single transit event. Using a new K2 photometric analysis technique, we are able to correct small telescope drifts and recover the observed transit at high confidence, corresponding to a planetary radius of Rp = 2.53 + or - 0.18 R sub(+ in circle). Radial velocity observations with the HARPS-N spectrograph reveal a 11.82 + or - 1.33 M sub(+ in circle) planet in a 9.1 day orbit, consistent with the transit depth, duration, and ephemeris. Follow-up photometric measurements from the MOST satellite confirm the transit observed in the K2 photometry and provide a refined ephemeris, making HIP 116454 b amenable for future follow-up observations of this latest addition to the growing population of transiting super-Earths around nearby, bright stars.
We present high spatial resolution images that demonstrate that WASP-12b and HAT-P-8b orbit the primary stars of hierarchical triple star systems. In each case, two distant companions with colors and ...brightnesses consistent with Mdwarfs co-orbit the hot Jupiter planet host as well as one another. Our adaptive optics images spatially resolve the secondary around WASP-12, previously identified by Bergfors et al. and Crossfield et al. into two distinct sources separated by 84.3+ or -0.6 mas (21+ or -3 AU).We find that the secondary to HAT-P-8, also identified by Bergfors et al., is in fact composed of two stars separated by 65.3+ or -0.5 mas (15+ or -1 AU). Our follow-up observations demonstrate physical association through common proper motion. HAT-P-8 C has a particularly low mass, which we estimate to be 0.18 + or - 0.02 M sub(middot in circle) using photometry. Due to their hierarchy, WASP-12 BC and HAT-P-8 BC will enable the first dynamical mass determination for hot Jupiter stellar companions. These previously well studied planet hosts now represent higher-order multi-star systems with potentially complex dynamics, underscoring the importance of diffraction-limited imaging and providing additional context for understanding the migrant population of transiting hot Jupiters.
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