Stellar Surface Activity Research Articles

Measuring the Spot Variability of T Tauri Stars Using Near-infrared Atomic Fe and Molecular OH Lines

As part of the Young Exoplanets Spectroscopic Survey, this study explores the spot variability of 13 T Tauri Stars (TTSs) in the near-infrared H band, using spectra from the Immersion GRating INfrared Spectrometer. By analyzing effective temperature (T eff) sensitive lines of atomic Fe i at ∼1.56259 μm and ∼1.56362 μm, and molecular OH at ∼1.56310 and ∼1.56317 μm, we develop an empirical equivalent width ratio (EWR) relationship for T eff in the range of 3400–5000 K. This relationship allows for precise relative T eff estimates to within tens of Kelvin and demonstrates compatibility with solar metallicity target models. However, discrepancies between observational data and model predictions limit the extension of the T eff–EWR relationship to a broader parameter space. Our study reveals that both classical and weak-line TTSs can exhibit T eff variations exceeding 150 K over a span of 2 yr. The detection of a quarter-phase delay between the EWR and radial velocity phase curves in TTSs indicates spot-driven signals. A phase delay of 0.06 ± 0.13 for CI Tau, however, suggests additional dynamics, potentially caused by planetary interaction, inferred from a posited 1:1 commensurability between the rotation period and orbital period. Moreover, a positive correlation between T eff variation amplitude and stellar inclination angle supports the existence of high-latitude spots on TTSs, further enriching our understanding of stellar surface activity in young stars.

Measuring stellar surface rotation and activity with the PLATO mission

The Planetary Transits and Oscillations of stars mission (PLATO) will allow us to measure surface rotation and monitor photometric activity of tens of thousands of main sequence solar-type and subgiant stars. This paper is the first of a series dedicated to the preparation of the analysis of stellar surface rotation and photospheric activity with the near-future PLATO data. We describe in this work the strategy that will be implemented in the PLATO pipeline to measure stellar surface rotation, photometric activity, and long-term modulations. The algorithms are applied on both noise-free and noisy simulations of solar-type stars, which include activity cycles, latitudinal differential rotation, and spot evolution. PLATO simulated systematics are included in the noisy light curves. We show that surface rotation periods can be recovered with confidence for most of the stars with only six months of observations and that the recovery rate of the analysis significantly improves as additional observations are collected. This means that the first PLATO data release will already provide a substantial set of measurements for this quantity, with a significant refinement on their quality as the instrument obtains longer light curves. Measuring the Schwabe-like magnetic activity cycle during the mission will require that the same field be observed over a significant timescale (more than four years). Nevertheless, PLATO will provide a vast and robust sample of solar-type stars with constraints on the activity-cycle length. Such a sample is lacking from previous missions dedicated to space photometry.

Surface magnetic activity of the fast-rotating G5 giant IN Comae, central star of the faint planetary nebula LoTr 5

Context. On the asymptotic giant branch, low to intermediate mass stars blow away their outer envelopes, forming planetary nebulae. Dynamic interaction between the planetary nebula and its central progenitor is poorly understood. The interaction is even more complex when the central object is a binary star with a magnetically active component, as is the case for the target in this paper.Aims. We aim to quantify the stellar surface activity of the cool binary component of IN Com and aim to explain its origin. In general, we need a better understanding of how central binary stars in planetary nebulae evolve and how this evolution could develop such magnetically active stars as IN Com.Methods. We present a time series of 13 consecutive Doppler images covering six months in 2017 that we used to measure the surface differential rotation with a cross-correlation method. Hitherto unpublished high-precision photometric data from 1989 to 2017 are presented. We applied Fourier-transformation-based frequency analysis to both photometry and spectra. Very high resolution (R ≈ 200 000) spectra were used to update IN Com’s astrophysical parameters by means of spectral synthesis.Results. Our time-series Doppler images show cool and warm spots coexisting with an average surface temperature contrast of −1000 K and +300 K with respect to the effective temperature. Approximately 8% of the stellar surface is covered with cool spots and ∼3% with warm spots. A consistent cool polar spot is seen in all images. The average lifetime of the cool spots is not much more than a few stellar rotations (one month), while the warm spots appear to live longer (three months) and are mostly confined to high latitudes. We found anti-solar surface differential rotation with a shear coefficient ofα = −0.026 ± 0.005 suggesting an equatorial rotation period of 5.973 ± 0.008 d. We reconfirm the 5.9 day rotation period of the cool star from photometry, radial velocities, and Hαline-profile variations. A long-termV-brightness variation with a likely period of 7.2 yr is also found. It appears in phase with the orbital radial velocity of the binary system in the sense that it is brightest at highest velocity and faintest at lowest velocity, that is, at the two phases of quadrature. We redetermine [Ba/Fe], [Y/Fe], and [Sr/Fe] ratios and confirm the overabundance of theses-process elements in the atmosphere of IN Com.

Additional Keplerian Signals in the HARPS data for Gliese 667C: Further Analysis

AbstractA re-analysis of Gliese 667C HARPS precision radial velocity data was carried out with a Bayesian multi-planet Kepler periodogram (from 0 to 7 planets) based on a fusion Markov chain Monte Carlo algorithm. The most probable number of signals detected is six with a Bayesian false alarm probability of 0.012. The residuals were shown to be consistent with white noise. The six signals detected include two previously reported with periods of 7.198 (b) and 28.14 (c) days, plus additional periods of 30.82, 38.82, 53.22, and 91.3 days. The existence of these Keplerian-like signals suggest the possibility of additional planets in the habitable zone of Gl 667C although some of the signals could be artifacts arising from the sampling or stellar surface activity. N-body orbital integrations are being undertaken to determine which of these signals are consistent with a stable planetary system. Preliminary results demonstrate that four of the signals, with periods of 7.2, 28.1, 38.8, & 91 d, are consistent with a stable 4 planet system on time scales of 107 yr. The M sin i values are ~5.5, 4.4, 1.9, and 4.7 M⊕, respectively.

Signals embedded in the radial velocity noise

The abilities of radial velocity exoplanet surveys to detect the lowest-mass extra-solar planets are currently limited by a combination of instrument precision, lack of data, and "jitter". Jitter is a general term for any unknown features in the noise, and reflects a lack of detailed knowledge of stellar physics (asteroseismology, starspots, magnetic cycles, granulation, and other stellar surface phenomena), as well as the possible underestimation of instrument noise. We study an extensive set of radial velocities for the star HD 10700 ($\tau$ Ceti) to determine the properties of the jitter arising from stellar surface inhomogeneities, activity, and telescope-instrument systems, and perform a comprehensive search for planetary signals in the radial velocities. We perform Bayesian comparisons of statistical models describing the radial velocity data to quantify the number of significant signals and the magnitude and properties of the excess noise in the data. We reach our goal by adding artificial signals to the "flat" radial velocity data of HD 10700 and by seeing which one of our statistical noise models receives the greatest posterior probabilities while still being able to extract the artificial signals correctly from the data. We utilise various noise components to assess properties of the noise in the data and analyse the HARPS, AAPS, and HIRES data for HD 10700 to quantify these properties and search for previously unknown low-amplitude Keplerian signals. ...

EVIDENCE FOR THE IMPACT OF STELLAR ACTIVITY ON THE DETECTABILITY OF SOLAR-LIKE OSCILLATIONS OBSERVED BY KEPLER

We use photometric observations of solar-type stars, made by the NASA Kepler Mission, to conduct a statistical study of the impact of stellar surface activity on the detectability of solar-like oscillations. We find that the number of stars with detected oscillations falls significantly with increasing levels of activity. The results present strong evidence for the impact of magnetic activity on the properties of near-surface convection in the stars, which appears to inhibit the amplitudes of the stochastically excited, intrinsically damped solar-like oscillations.

The evolution of stellar surface activity and possible effects on exoplanets

AbstractThe evolution of stellar activity involves a complex interplay between the interior dynamo mechanism, the emergent magnetic field configurations and their coupling with stellar winds, the subsequent angular momentum evolution, and fundamental stellar parameters. The discussion of the evolution of surface activity will emphasize the main sequence phase, from the ZAMS to stars of solar-age. We will focus particularly on the evolution of the fractional area coverages of spots on the surfaces of solar-type stars. We fit an empirical relation to the fractional mean spot area coverage as a function of age for ages greater than the Pleiades of the form log(MeanSpotCoverage) = 0.90(±0.26) − 1.03(±0.10)log(Age), where Age is in Myr. In addition, we summarize the relative evolution of radiative emissions in various short wavelength bands that are associated with stellar magnetic field-related activity. Possible effects on young planetary atmospheres also are appropriate to consider given that stellar surface activity is the origin of the high-energy component of the ambient radiation and particle fields in which planetary atmosphere evolution occurs.

Towards μ-arcsecond spatial resolution with Air Cherenkov Telescope arrays as optical intensity interferometers

In this poster contribution we highlight the equivalence between an Imaging Air Cherenkov Telescope (IACT) array and an Intensity Interferometer for a range of technical requirements. We touch on the differences between a Michelson and an Intensity Interferometer and give a brief overview of the current IACT arrays, their upgrades and next generation concepts (CTA, AGIS, completion 2015). The later are foreseen to include 30-90 telescopes that will provide 400-4000 different baselines that range in length between 50 m and a kilometre. Intensity interferometry with such arrays of telescopes attains 50μ-arc seconds resolution for a limiting mv ∼ 8.5. This technique opens the possibility of a wide range of studies, amongst others, probing the stellar surface activity and the dynamic AU scale circumstellar environment of stars in various crucial evolutionary stages. Here we discuss possibilities for using IACT arrays as optical Intensity Interferometers.

$\mathsf{\alpha}$ Eridani: rotational distortion, stellar and circumstellar activity

We explore the geometrical distortion and the stellar and circumstellar activity of a Eri (HD 10144), the brightest Be star in the sky. We present a thorough discussion of the fundamental parameters of the object for an independent determination of its rotational distortion. We used stellar atmosphere models and evolutionary tracks calculated for fast rotating early-type stars. If the star is a rigid rotator, its angular velocity rate is Ω/Ω c ≃ 0.8, so that its rotational distortion is smaller than the one inferred from recent interferometric measurements. We then discuss the stellar surface activity using high resolution and high S/N spectroscopic observations of He 1 and Mg II lines, which concern a period of Ha line emission decline. The variations in the He I lines are interpreted as due to non-radial pulsations. Time series analysis of variations was performed with the CLEANEST algorithm, which enabled us to detect the following frequencies: 0.49, 0.76, 1.27 and 1.72 c/d and pulsation degrees l ∼ (3-4) for v = 0.76 c/d; l ∼ (2-3) for v = 1.27 c/d and l ∼ (3-4) for v = 1.72 c/d. The study of the absolute deviation of the He I λ6678 A spectral line revealed mass ejection between 1997 and 1998. We conclude that the lowest frequency found, v = 0.49 c/d, is due to the circumstellar environment, which is present even at epochs of low emission in the wings of He I λ6678 A and Mg II λ4481 A line profiles, as well as during nearly normal aspects of the Hα line. This suggests that there may be matter around the star affecting some spectral regions, even though the object displays a B-normal like phase. The long-term changes of the Ha line emission in a Eri are studied. We pay much attention to the Ha line emission at the epoch of interferometric observations. The Ha line emission is modeled and interpreted in terms of varying structures of the circumstellar disc. We conclude that during the epoch of interferometric measurements there was enough circumstellar matter near the star to produce λ2.2 μm flux excess, which could account for the overestimated stellar equatorial angular diameter. From the study of the latest B?Be phase transition of a Eri we concluded that the Ha line emission formation regions underwent changes so that: a) the low Ha emission phases are characterized by extended emission zones in the circumstellar disc and a steep outward matter density decline; b) during the strong Hα emission phases the emitting regions are less extended and have a constant density distribution. The long-term variations of the Ha line in a Eri seem to have a 14-15 year cyclic B?Be phase transition. The disc formation time scales, interpreted as the periods during which the Ha line emission increases from zero to its maximum, agree with the viscous decretion model. On the other hand, the time required for the disc dissipation ranges from 6 to 12 years which questions the viscous disc model.

Lithium Abundances in α Persei Stars

We revisit the scatter in lithium abundances among stars in the α Persei cluster. Among the factors that we examine, inhomogeneous reddening, stellar spots, and stellar surface activity are the main contributors to this scatter. Thus there is no solid evidence requiring the observed dispersion in lithium abundances to be intrinsic to the cluster member stars. Instead, the scatter is probably induced by observational effects.

Why solar astronomers should be interested in stars

I discuss the scientific reasons why solar astronomers should be interested in other stars and why Solar Physics should be put in the broader context of Stellar Astrophysics. Selected topics include stellar interiors and helioseismology, dynamo action and activity cycles, stellar surface activity, stellar chromospheres, coronae and winds, and the study of the Sun in time.

A False Planet around HD 192263

We present new high-precision Stromgren photometry and Ca II H and K spectrophotometry of HD 192263. Based on radial velocity variations detected previously by two groups, this K2 V star was thought to host a 0.75 MJup (minimum mass) planetary companion in a 24 day orbit. Our photometric observations reveal periodic variations that match the purported planetary orbital period, while the Ca II H and K emission fluxes are modulated on half the planetary period. This suggests that rotational modulation of the visibility of stellar surface activity is the source of the observed radial velocity variations. Therefore, HD 192263 should be removed from lists of stars with well-established planetary companions unless further observations and analysis can support the existence of the planet in spite of the star's intrinsic variations.

Star clusters: Optimal targets for a photometric planetary search program

By targeting stars in some of the most populous open star clusters, the stringent requirements of a photometric search for planets as small as the Earth, i.e., that large numbers of stars must be monitored frequently and with extremely high precision, can be achieved in a ground‐based observing program. Data for the cluster NGC 188 confirms previous studies that even with a telescope of modest aperture (0.9 m), high‐precision relative photometry of moderately bright stars is possible; even under marginal conditions of thin clouds and variable seeing, high‐frequency atmospheric fluctuations (scintillation) can be largely corrected for. By continuously monitoring the many hundreds of stars in the richest old clusters simultaneously in two colors, it should also be possible to model explicitly the slower variations in atmospheric extinction that could mask the small light variations caused by transit events. Furthermore, the stars selected will automatically be main sequence stars of known age and distance; there are several old open clusters with ages of billions of years that together contain several thousand stars of solar mass or less and which are virtually identical to the Sun in chemical composition. An effective search program will require one or more telescopes of at least 4 m in aperture dedicated to the project for a period of several years. Even very large ground‐based telescopes can be built for a small fraction of the cost of a space mission and would be usable for other projects at the end of the search period. By targeting four of the “best” of the old open clusters, the project could uncover as many as 25 planetary systems in a few years of observing, or at least set meaningful upper limits on the frequency of planetary systems. Furthermore, the same data set will also be useful to investigate stellar surface activity as a function of stellar age, to derive precise parallaxes of the target clusters (which will help to set the cosmic distance scale) and possibly to conduct asteroseismological studies. The data will also likely lead to the discovery of serendipitous events such as cosmologically distant supernova explosions along the line of sight.

The Astrophysical Value of Continuous Photometric Monitoring from the Antarctic Plateau

AbstractAmong the astrophysical researches which will largely benefit from observation from the Antarctic Plateau, we consider those which require high-quality, continuous, daylight, uninterrupted observation in the optical–near-IR region. The systematic study of the stellar micro-oscillations and surface activity of individual objects, as well as of stellar associations extended for periods of weeks, will make a fundamental contribution to our knowledge of stellar structure and will ultimately provide the physical parameters of the stars. A similar consideration applies for the short-period (a few hours to a few days) binaries, which include the W UMa contact binaries. These objects are subject to continuously varying light curves associated with surface activity and secular effects, which cannot be adequately studied from the currently available ground telescopes. Finally, the exceptional seeing and low background will allow the systematic search for and study of supernovae in clusters of galaxies at cosmological distances, which will improve our knowledge of the geometry of the Universe at z ~ 0·7–1.

Search for dark companions of K and M giants: Preliminary results for α Boo

We are currently engaged in a long term search for substellar companions to K and M giant stars, and have obtained data from 1979 to the present. We present preliminary results for α Boo, encompassing selected measurements during the period from January 1979 to June 1984. We chose 4 groups of data, each group taken within an interval of up to 1.5 months, and examined the internal consistency of each set to look for short term trends. We found the radial velocity to be very consistent with a standard deviation of about ±50 m/s within time scales of hours, days, and weeks. We did not detect any significant radial velocity changes over these periods. This leads us to assume that the stellar surface activity is confined to the level of our uncertainty. Because of this consistency it was possible to take the means of the radial velocity measurements within each group and compare them to look for possible long term changes. We found variations of the radial velocity of about 200 m/s over the span of a few years, significant at the 6 to 8 σ level.

UBVRI and H-alpha photometry of FK COM

view Abstract Citations (28) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS UBVRI and H alpha photometry of FK Com. Holtzman, J. A. ; Nations, H. L. Abstract The authors have obtained UBVRI and Hα photometry of FK Com during February 1983. The distortion wave in the star's light curve is seen to have increased in amplitude and to have dramatically changed in shape since the previous observing season. In addition, a very energetic flare-like event was observed on the night of 21/22 February 1983. These data and the recent history of FK Com are discussed in terms of the starspot phenomenology for stellar surface activity. In particular, modeling of the light curves yields a best fit requiring two dark spots with a temperature difference between them and the surrounding photosphere of 600±150K and a combined area of 0.26±0.06 of the stellar hemisphere. Publication: The Astronomical Journal Pub Date: March 1984 DOI: 10.1086/113526 Bibcode: 1984AJ.....89..391H Keywords: H Alpha Line; Peculiar Stars; Stellar Spectrophotometry; Ubv Spectra; Giant Stars; Light Curve; Starspots; Stellar Color; Stellar Flares; Astrophysics full text sources ADS | data products SIMBAD (3) GCPD (1)

A remarkable H-alpha outburst in the RS CVn binary SZ PISCIUM - A transient mass-transfer event

The very unusual spectroscopic behavior of the RS CVn binary SZ Psc (K1IV + F8V; P = 3.97 days) is described. For much of the time the Ha line in the K star appears as a weak absorption feature, presumably partially filled by chromospheric emission. The Hα equivalent width ordinarily shows small variations that may be adequately explained by stellar surface activity. During a single month in 1978, however, the appearance and behavior of the Hα feature were bizarre. Over the course of a few nights the line profile changed from the expected weak absorption feature to a broad (∼300 km s<SUP>-1</SUP>) double-peaked emission. This emission profile persisted, weakening slowly, for several weeks. Velocity measures indicate the K star was the source of the emission. Such an Ha outburst, with its unusual profile and temporal behavior, has never been seen in other, more intensively observed RS CVn systems. An explanation in the strict context of surface phenomena does not appear feasible. Instead, we propose that the emission is circumstellar in origin, produced from a ring or shell of gas around the primary K component. We examine the possibility that this Hα outburst may have signaled a transient mass-transfer event in this barely detached system.