Chromospheric Activity Indicators Research Articles

Long-term magnetic activity of a sample of M-dwarf stars from the HARPS program

Aims. Other stars are known to have magnetic cycles similar to that of the Sun. The relationship between these activity variations and the observed radial-velocity is still not satisfactorily understood. In this first paper our aim is to assess the long-term activity variations in the low end of the main sequence, having in mind a planetary search perspective. Methods. We used a sample of 30 M0-M5.5 stars from the HARPS M-dwarf planet search program with a median timespan of observations of 5.2 years. We computed chromospheric activity indicators based on the Ca II H and K, H{\alpha}, He I D3, and Na I D1 and D2 lines. All data were binned in to average out undesired effects such as rotationally modulated atmospheric inhomogeneities. We searched for long-term variability of each index and determined the correlations between them. Results. While the S_CaII, H{\alpha}, and Na I indices showed significant variability for a fraction of our stellar sample (39%, 33%, and 37%, respectively), only 10% of our stars presented significant variability in the He i index. We therefore conclude that this index is a poor activity indicator at least for this type of stars. Although the H{\alpha} shows good correlation with S_Ca II for the most active stars, the correlation is lost when the activity level decreases. This result appears to indicate that the Ca II-H{\alpha} correlation is dependent on the activity level of the star. The Na I lines correlate very well with the S_Ca II index for the stars with low activity levels we used, and are thus a good chromospheric activity proxy for early-M dwarfs. We therefore strongly recommend the use of the Na I activity index because the signal-to-noise ratio in the sodium lines spectral region is always higher than for the calcium lines.

Chromospheric activities and kinematics for solar type dwarfs and subgiants: analysis of the activity distribution and the AVR

Aims. In this work we present chromospheric activity indices, kinematics, radial-velocities, and rotational velocities for more than 850 FGK-type dwarfs and subgiant stars in the southern hemisphere and test how best to calibrate and measure S-indices from echelle spectra.

Chromospheric activity of stars with planets

Context. Signatures of chromospheric activity enhancement have been found for a dozen stars, pointing to a possible star-planet interaction. Nevertheless in the coronal activity regime, there is no conclusive observational evidence for such an interaction. Does star-planet interaction manifest itself only for a few particular cases, without having a major effect on stars with planets in general? Aims. We aim to add additional observational constraints to support or reject the major effects of star-planet interactions in stellar activity, based on CaII chromospheric emission flux. Methods. We performed a statistical analysis of CaII emission flux of stars with planets, as well as a comparison between CaII and X-ray emission fluxes, searching for dependencies on planetary parameters. Results. In the present sample of stars with planets, there are no significant correlations between chromospheric activity indicator log(R'HK) and planetary parameters. Further, the distribution of the chromospheric activity indicator for stars without planets is not distinguishable from the one with planets.

Effect of magnetic activity saturation in chromospheric flux-flux relationships

We present a homogeneous study of chromospheric and coronal flux-flux relationships using a sample of 298 late-type dwarf active stars with spectral types F to M. The chromospheric lines were observed simultaneously in each star to avoid spread due to long term variability. Unlike other works, we subtract the basal chromospheric contribution in all the spectral lines studied. For the first time, we quantify the departure of dMe stars from the general relations. We show that dK and dKe stars also deviate from the general trend. Studying the flux-colour diagrams we demonstrate that the stars deviating from the general relations are those with saturated X-ray emission and that those stars also present saturation in the H$\alpha$ line. Using several age spectral indicators, we show that they are younger stars than those following the general relationships. The non-universality of flux-flux relationships found in this work should be taken into account when converting between fluxes in different chromospheric activity indicators.

A likely exoplanet orbiting the oscillating K-giantαArietis

Aims. To search for extrasolar planets around intermediate-mass stars, we are conducting a precise RV survey around K-giants. Methods. We present high-accuracy RV measurements of {\alpha} Ari made from November 2003 to February 2010. This star belongs to our sample of 55 K-giants studied for extrasolar planet and pulsation searches using the fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) attached to the 1.8-m telescope at Bohyunsan Optical Astronomy Observatory (BOAO) in Korea. Results. We find a planetary companion with long-period and low-amplitude radial velocity (RV) variations in oscillating K2 III star {\alpha} Ari (HD 12929). We do not find the correlation between RV variations and chromospheric activity indicators (Ca II H & K region, H{\alpha} line). The bisector analysis also shows that the bisector velocity span (BVS) does not show any obvious correlation with RV variation but has periodic component that may be attributed to the rotation of the star. If the RV variation is indeed caused by a planetary companion, an orbital solution with a period of P = 381 days, a semi-amplitude of K = 41 m/s, and an eccentricity of e = 0.25 fits the data best. Assuming a possible stellar mass of M_star = 1.5 M_sun, we estimate the minimum mass for the planetary companion of m2 sin i = 1.8 M_Jupiter with an orbital semi-major axis of 1.2 AU. Conclusions. Our finding of a likely exoplanet in {\alpha} Ari supports searching for extrasolar planets around giant stars with multiperiodic oscillations.

A search for star-planet interactions in theυAndromedae system at X-ray and optical wavelengths

Close-in, giant planets are expected to influence their host stars via tidal or magnetic interaction. But are these effects strong enough in suitable targets known so far to be observed with today's instrumentation? The upsilon And system, an F8V star with a Hot Jupiter, was claimed to undergo cyclic changes in chromospheric activity indicators with its innermost planet's period. We want to investigate the stellar chromospheric and coronal activity over several months. We therefore monitored the star in X-rays as well as at optical wavelengths to test coronal and chromospheric activity indicators for planet-induced variability, making use of the Chandra X-ray Observatory as well as the echelle spectrographs FOCES and HRS at Calar Alto (Spain) and the Hobby-Eberly Telescope (Texas, US). The stellar activity level is low, as seen both in X-rays as in Ca II line fluxes; the chromospheric data show variability with the stellar rotation period. We do not find activity variations in X-rays or in the optical which can be traced back to the planet. Gaining observational evidence for star-planet interactions in X-rays remains challenging.

The Big Bear Solar Observatory Ca II K‐line index for solar cycle 23

AbstractWe present an analysis of 2634 Ca II K‐line full‐disk filtergrams obtained with the 15‐cm aperture photometric full‐disk telescope at Big Bear Solar Observatory during the period from 1996 January 1 to 2005 October 24. Using limb darkening corrected and contrast enhanced filtergrams, solar activity indices were derived, which are sensitive to the 11‐year solar activity cycle and 27‐day rotational period of plages around active regions and the bright chromospheric network. The present work extends an earlier study (solar cycle 22), which was based on video data. The current digital data are of much improved quality with higher spatial resolution and a narrower passband ameliorating photometric accuracy. The time series of chromospheric activity indices cover most of solar cycle 23. One of the most conspicuous features of the Ca II K indices is the secondary maximum in late 2001/early 2002 after an initial decline of chromospheric activity during the first half of 2001. We conclude that a secular trend exists in the Ca II K indices, which has its origin in the bright chromospheric network and brightenings related to decaying active regions. Superposed on this secular trend are the signatures of recurring, long‐lived active regions, which are clusters of persistent and continuously emerging magnetic flux. Such features are less visible, when the activity belts on both side of the equator are devoid of the brightenings related to decaying active regions as was the case in October/November 2003 at a time when a superactivity complex including several naked‐eye sunspots emerged (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Chromospheric activity among fast-rotating M dwarfs in the open cluster NGC 2516

We report radial velocities (RVs), projected equatorial velocities (v sin i) and CaII triplet (CaT) chromospheric activity indices for 237 late-K to mid-M candidate members of the young open cluster NGC 2516. These stars have rotation periods between 0.1 and 15 days. Intermediate resolution spectra were obtained using the Giraffe spectrograph at the Very Large Telescope. Membership was confirmed on the basis of RVs for 210 targets. We see a marked increase in the fraction of rapidly rotators as we move to cooler spectral types. About 20 per cent of M0-M1 stars have v sin i >15km/s, increasing to 90 per cent of M4 stars. Activity indices derived from the first two lines of the CaT (8498A and 8542A) show differing dependencies on rotation period and mass for stars lying above and below the fully convective boundary. Higher mass stars, of spectral type K3-M2.5, show chromospheric activity which increases with decreasing Rossby number (the ratio of period to convective turnover time), saturating for Rossby numbers <0.1. For cooler stars, which are probably fully convective and almost all of which have Rossby numbers <0.1, there is a clear decrease in chromospheric activity as (V-I)_0 increases, amounting to a fall of about a factor of 2-3 between spectral types M2.5 and M4. This decrease in activity levels at low Rossby numbers is not seen in X-ray observations of the coronae of cluster M-dwarfs or of active field M-dwarfs. There is no evidence for supersaturation of chromospheric activity for stars of any spectral type at Rossby numbers <0.01. We suggest that the fall in the limiting level of chromospheric emission beyond spectral type M3 in NGC~2516 is, like the simultaneous increase in rotation rates in field stars, associated with a change in the global magnetic topology as stars approach the fully convective boundary and not due to any decrease in dynamo-generated magnetic flux.

A high-resolution spectroscopic survey of late-type stars: chromospheric activity, rotation, kinematics, and age

[NASA ADS]

Sounding stellar cycles with Kepler - I. Strategy for selecting targets

The long-term monitoring and high photometric precision of the Kepler satellite will provide a unique opportunity to sound the stellar cycles of many solar-type stars using asteroseismology. This can be achieved by studying periodic changes in the amplitudes and frequencies of the oscillation modes observed in these stars. By comparing these measurements with conventional ground-based chromospheric activity indices, we can improve our understanding of the relationship between chromospheric changes and those taking place deep in the interior throughout the stellar activity cycle. In addition, asteroseismic measurements of the convection zone depth and differential rotation may help us determine whether stellar cycles are driven at the top or at the base of the convection zone. In this paper, we analyze the precision that will be possible using Kepler to measure stellar cycles, convection zone depths, and differential rotation. Based on this analysis, we describe a strategy for selecting specific targets to be observed by the Kepler Asteroseismic Investigation for the full length of the mission, to optimize their suitability for probing stellar cycles in a wide variety of solar-type stars.

Chromospheric activity on the RS Canum Venaticorum binary SZ Piscium

Aims. We present the new high-resolution echelle spectra of SZ Psc, obtained in Nov. 2004 and Sep.- Dec. 2006, and study its chromospheric activity. Methods. By means of the spectral subtraction technique, we analyze our spectroscopic observations including several optical chromospheric activity indicators (the He I D(3), Na I D(1), D(2), H alpha, and Ca II infrared triplet lines). Results. All indicators show that the chromospheric activity of the system is associated with the cooler component. We find that the values of EW(8542)/EW(8498) are in the range 1-3, which indicates optically thick emission in plage-like regions. The 2006 data suggest the presence of active longitude phenomena. For the Ca II 8542 and 8662 and the Ha lines, it seems that the excess emission is stronger near the two quadratures of system. This may be anti-correlated with the behavior of the Na I D(1) line. The absorption features are detected in the subtracted Ha lines, which could be explained by prominence-like extended material seen on the stellar disk or by mass transfer from the cooler component to the hotter one.

The NStED Stellar and Exoplanet Hosting Star Service

AbstractThe NASA Star and Exoplanet Database (NStED) is a general purpose stellar archive with the aim of providing support for NASA's planet finding and characterization goals, stellar astrophysics, and the planning of NASA and other space missions. There are two principal components of NStED: a database of (currently) 140,000 nearby stars and exoplanet-hosting stars, and an archive dedicated to high precision photometric surveys for transiting exoplanets. We present a summary of the NStED stellar database, functionality, tools, and user interface. NStED currently serves the following kinds of data for 140,000 stars (where available): coordinates, multiplicity, proper motion, parallax, spectral type, multiband photometry, radial velocity, metallicity, chromospheric and coronal activity index, and rotation velocity/period. Furthermore, the following derived quantities are given wherever possible: distance, effective temperature, mass, radius, luminosity, space motions, and physical/angular dimensions of habitable zone. Queries to NStED can be made using constraints on any combination of the above parameters. In addition, NStED provides tools to derive specific inferred quantities for the stars in the database, cross-referenced with available extra-solar planetary data for those host stars. NStED can be accessed at http://nsted.ipac.caltech.edu.

The Na i D resonance lines in main-sequence late-type stars

We study the sodium D lines (D1: 5895.92 \AA; D2: 5889.95 \AA) in late-type dwarf stars. The stars have spectral types between F6 and M5.5 (B-V between 0.457 and 1.807) and metallicity between [Fe/H] = -0.82 and 0.6. We obtained medium resolution echelle spectra using the 2.15-m telescope at the argentinian observatory CASLEO. The observations have been performed periodically since 1999. The spectra were calibrated in wavelength and in flux. A definition of the pseudo-continuum level is found for all our observations. We also define a continuum level for calibration purposes. The equivalent width of the D lines is computed in detail for all our spectra and related to the colour index (B-V) of the stars. When possible, we perform a careful comparison with previous studies. Finally, we construct a spectral index (R_D') as the ratio between the flux in the D lines, and the bolometric flux. We find that, once corrected for the photospheric contribution, this index can be used as a chromospheric activity indicator in stars with a high level of activity. Additionally, we find that combining some of our results, we obtain a method to calibrate in flux stars of unknown colour.

High-Dispersion Optical Spectra of Nearby Stars Younger Than the Sun

We present high-dispersion (R ~ 16,000) optical (3900-8700 A) spectra of 390 stars obtained with the Palomar 60 inch telescope. majority of stars observed are part of the Spitzer Legacy Science Program The Formation and Evolution of Planetary Systems. Through detailed analysis we determine stellar properties for this sample, including radial and rotational velocities, Li I λ6708 and Hα equivalent widths, the chromospheric activity index R'_(HK), and temperature- and gravity-sensitive line ratios. Several spectroscopic binaries are also identified. From our tabulations, we illustrate basic age- and rotation-related correlations among measured indices. One novel result is that Ca II chromospheric emission appears to saturate at v sin i values above ~30 km s^(-1), similar to the well-established saturation of X-rays that originate in the spatially separate coronal region.

Activity and Kinematics of Ultracool Dwarfs, Including an Amazing Flare Observation

We present the activity and kinematics of a representative volume-limited (20 pc) sample of 152 late-M and L dwarfs (M7--L8) photometrically selected from the Two Micron All-Sky Survey (2MASS). Using new proper motion measurements and spectrophotometric distance estimates, we calculate tangential velocities. The sample has a mean tangential velocity of = 31.5 km/s, a velocity dispersion of sigma_tan = 20.7 km/s, and a maximum tangential velocity of V_tan = 138.8 km/s. These kinematic results are in excellent agreement with previous studies of ultracool dwarfs in the local solar neighborhood. H_alpha emission, an indicator of chromospheric activity, was detected in 63 of 81 late-M dwarfs and 16 of 69 L dwarfs examined. We find a lack of correlation between activity strength, measured by log(F_H_alpha/F_bol), and V_tan, though velocity distributions suggest that the active dwarfs in our sample are slightly younger than the inactive dwarfs. Consistent with previous studies of activity in ultracool dwarfs, we find that the fraction of H$\alpha$ emitting objects per spectral type peaks at spectral type M7 and declines through mid-L dwarfs. Activity strength is similarly correlated with spectral type for spectral types later than M7. Eleven dwarfs out of 150 show evidence of variability, ranging from small fluctuations to large flare events. We estimate a flare cycle of ~5% for late-M dwarfs and ~2% for L dwarfs. Observations of strong, variable activity on the L1 dwarf 2MASS J10224821+5825453 and an amazing flare event on the the M7 dwarf 2MASS J1028404$-$143843 are discussed.

On the age of stars harboring transiting planets

Results of photometric surveys have brought to light the existence of a population of giant planets orbiting their host stars even closer than the hot Jupiters (HJ), with orbital periods below 3 days. The reason why radial velocity surveys were not able to detect these very-hot Jupiters (VHJ) is under discussion. A possible explanation is that these close-in planets are short-lived, being evaporated on short time-scales due to UV flux of their host stars. In this case, stars hosting transiting VHJ planets would be systematically younger than those in the radial velocity sample. We have used the UVES spectrograph (VLT-UT2 telescope) to obtain high resolution spectra of 5 faint stars hosting transiting planets, namely, OGLE-TR-10, 56, 111, 113 and TrES-1. Previously obtained CORALIE spectra of HD189733, and published data on the other transiting planet-hosts were also used. The immediate objective is to estimate ages via Li abundances, using the Ca II activity-age relation, and from the analysis of the stellar rotational velocity. For the stars for which we have spectra, Li abundances were computed as in Israelian et al. (2004) using the stellar parameters derived in Santos et al. (2006). The chromospheric activity index $S_{US}$ was built as the ratio of the flux within the core of the Ca II H & K lines and the flux in two nearby continuum regions. The index $S_{US}$ was calibrated to Mount Wilson index $S_{MW}$ allowing the computation of the Ca II H & K corrected for the photospheric contribution. These values were then used to derive the ages by means of the Henry et al. (1996) activity-age relation. Bearing in mind the limitations of the ages derived by Li abundances, chromospheric activity, and stellar rotational velocities, none of the stars studied in this paper seem to be younger than 0.5 Gyr.

EUVE J0825-16.3 and EUVE J1501-43.6: Two dMe Double–Lined Spectroscopic Binaries

AbstractHigh-resolution echelle spectroscopic observations taken with the FEROS spectrograph at the ESO 2.2-m telescope confirm the binary nature of the dMe stars EUVE J0825−16.3 and EUVE J1501−43.6, previously reported by Christian &amp; Mathioudakis (2002). In these binary systems, emission of similar intensity from both components is detected in the Na i D1 &amp; D2, He i D3, Ca II H&amp;K, Ca II IRT and Balmer lines. We have determined precise radial velocities by cross-correlation with radial velocity standard stars, which have allowed us to obtain for the first time the orbital solution of these systems. Both binaries consist of two nearly equal M0V components with an orbital period shorter than 3.5 days. We have analyzed the behaviour of the chromospheric activity indicators (variability and possible flares). In addition, we have determined its rotational velocity and kinematics.

LU Vel (GJ 375): A M3.5Ve Flare and Double-Lined Spectroscopic Binary

High resolution echelle spectroscopic observations taken with the FEROS spectrograph at the 2.2m telescope ESO confirm the binary nature of the flare M3.5V star LU Vel (GJ 375, RE J0958-462) previously reported by Christian and Mathioudakis (2002). Emission of similar intensity from both components is detected in the Balmer, Na I D1&D2, He I D3, Ca II H&K, and Ca II IRT lines. We have determined precise radial velocities by cross correlation with radial velocity standard stars, which have allowed us to obtain for the first time the orbital solution of the system. The binary consists of two near-equal M3.5V components with an orbital period shorter than 2 days. We have analyzed the behaviour of the chromospheric activity indicators (variability and possible flares). In addition, we have determined its rotational velocity and kinematics.

Oxygen from the λ7774 High‐Excitation Triplet in Open Cluster Dwarfs: Hyades

Oxygen abundances have been derived from the near-IR, high-excitation O I λ7774 triplet in high-resolution, high signal-to-noise ratio spectra of 45 Hyades dwarfs using standard one-dimensional, plane-parallel LTE models. Effective temperatures of the stellar sample range from 4319 to 6301 K, and the derived relative O abundances as a function of Teff evince a trichotomous morphology. At Teff > 6100 K, there is evidence of an increase in the O abundances with increasing Teff, consistent with non-LTE (NLTE) predictions. At intermediate Teff (5450 K ≤ Teff ≤ 6100 K), the O abundances are flat, and star-to-star values are in good agreement, having a mean value of [O/H] = +0.25 ± 0.02; however, systematic errors at the ≲0.10 dex level might exist. The O abundances for stars with Teff ≤ 5450 K show a striking increase with decreasing Teff, in stark contrast to expectations and canonical NLTE calculations. The cool Hyades triplet results are compared to those recently reported for dwarfs in the Pleiades cluster and the UMa moving group; qualitative differences between the trends observed in these stellar aggregates point to a possible age-related diminution of triplet abundance trends in cool open cluster dwarfs. Correlations with age-related phenomena, i.e., chromospheric activity and photospheric spots, faculae, and/or plages, are investigated. No correlation with Ca II H+K chromospheric activity indicators is observed. Multicomponent LTE "toy" models have been constructed in order to simulate photospheric temperature inhomogeneities that could arise from the presence of starspots, and we demonstrate that photospheric spots are a plausible source of the triplet trends among the cool dwarfs.

Optical and EUV observations of solar flare kernels

We present high-resolution spectral observations, covering the entire optical region (3800−9000 A), of a solar flare observed during a multiwavelength campaign. The flare, recorded on 2002 January 11, was a medium solar flare event (GOES class C7.5). The spectral observations were carried out using the Hamilton echelle spectrograph on the coude auxiliary telescope at Lick Observatory and with the Coronal Diagnostic Spectrometer (CDS) on board SoHO. The high signal-to-noise optical spectra are analysed using the same techniques as we applied to stellar flare data. Hα images obtained at Big Bear Solar Observatory (BBSO), plus magnetograms obtained with the Michelson Doppler Imager (MDI) on board SoHO and Transition Region And Coronal Explorer (TRACE) 1600 A were used in the flare analysis. We observe stellarlike behaviour in the main solar chromospheric activity indicators, which show either filling-in or emission during the flare. We find that the Balmer and Ca ii lines show asymmetric profiles, with red-shifted wings and blue-shifted cores. This behaviour could be explained by material expanding. During the flare, the Mg i and Fe i lines show a filling-in of the line profile indicating that the flare affected the lower atmosphere. There is some evidence for pre-flare heating as seen in Fe xix 592 A. Furthermore, O v 629 A shows an increase in flux some 10 min. before the coronal lines, perhaps indicating particle beam heating in the initial stages of the flare. We have also determined the main physical parameters at flare maximum. The electron densities and electron temperatures found for the flare imply that the Balmer emitting plasma originates in the chromosphere. The physical parameters obtained for the modelled flare are consistent with previously derived values for solar flares.