Doppler imaging of LQ Hydrae for 1998–2002

Cole, Elizabeth; Hackman, T.; Käpylä, Maarit J.; Ilyin, I.; Kochukhov, O.; Piskunov, Nikolai

doi:10.1051/0004-6361/201425440

Cited by 17 publications

(42 citation statements)

References 42 publications

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“…We also note that the A120, page 10 of 13 observing seasons for spectrometry and photometry mostly do not overlap. In addition a low signal-to-noise ratio as well as less than ideal phase coverage for several observing seasons was reported, which increases the uncertainties even further (Cole et al 2014a). …”

Section: Comparison With Doppler Imagingmentioning

confidence: 99%

“…In Cole et al (2014a) the DI technique was applied to spectrometry of seven observing seasons. The resulting surface temperature maps were used to determine the epochs of the temperature minima, interpreted as starspots, for every season.…”

Section: Comparison With Doppler Imagingmentioning

confidence: 99%

“…The DI and ZDI maps (e.g., Strassmeier et al 1993;Saar et al 1994;Rice & Strassmeier 1998;Donati 1999;Donati et al 2003b;Cole et al 2014a) show both high-and low-latitude spot activity on the object to the extent that Cole et al (2014a) define the distribution over latitude to be bimodal. The relative strength of the two latitudinal spot regions has been reported to be highly variable over time so that during some epochs the nearequator spots dominate, while during others the nearly polar features are the strongest.…”

Section: Introductionmentioning

confidence: 99%

“…The spot distribution from DI and ZDI has been postulated to be concentrated onto active longitudes during some epochs. Therefore, the spot distribution has occasionally been highly non-axisymmetric (e.g., Saar et al 1994), but during different epochs no clear signs of active longitudes have been found (e.g., Cole et al 2014a). In addition, Cao & Gu (2014) found rotational modulation of chromospheric emission during 2006−2012, which indicates the presence and change of chromospheric active regions over the surface of LQ Hya over a considerable timespan.…”

Section: Introductionmentioning

confidence: 99%

“…Finally, we perform local CFs to study local segments in an attempt to identify interesting phase behavior. Here we also make a comparison to the continuous period search (CPS) method results by Lehtinen et al (2012) and the recent DI results of Cole et al (2014a). In Sects.…”

Section: Introductionmentioning

confidence: 99%

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Multiperiodicity, modulations, and flip-flops in variable star light curves

Olspert

Käpylä

Pelt

et al. 2015

A&A

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Aims. We study LQ Hya photometry for 1982-2014 with the carrier fit (CF) method and compare our results to earlier photometric analysis and recent Doppler imaging maps. Methods. As the rotation period of the object is not known a priori, we utilize different types of statistical methods first (least-squares fit of harmonics, phase dispersion statistics) to estimate various candidates for the carrier period for the CF method. Secondly, a global fit to the whole data set and local fits to shorter segments are computed with the period that is found to be optimal. Results. The harmonic least-squares analysis of all the available data reveals a short period, of close to 1.6 days, as a limiting value for a set of significant frequencies. We interpret this as the rotation period of the spots near the equatorial region. In addition, the distribution of the significant periods is found to be bimodal, hinting of a longer-term modulating period, which we set out to study with a two-harmonic CF model. A weak modulation signal is, indeed retrieved, with a period of roughly 6.9 yr. The phase dispersion analysis gives a clear symmetric minimum for coherence times lower than and around 100 days. We interpret this as the mean rotation pattern of the spots. Of these periods, the most significant and physically most plausible period statistically is the mean spot rotation period 1. d 60514, which is chosen to be used as the carrier period for the CF analysis. With the CF method, we seek any systematic trends in the spot distribution in the global time frame, and locally look for previously reported abrupt phase changes in rapidly rotating objects. During 2003During -2009, the global CF reveals a coherent structure rotating with a period of 1. d 6037, while during most other times the spot distribution appears somewhat random in phase. Conclusions. The evolution of the spot distribution of the object is found to be very chaotic, with no clear signs of an azimuthal dynamo wave that would persist over longer timescales, although the short-lived coherent structures occasionally observed do not rotate with the same speed as the mean spot distribution. The most likely explanation of the bimodal period distribution is attributed to the high-and low-latitude spot formation regions confirmed from Doppler imaging and Zeeman Doppler imaging.

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Section: Comparison With Doppler Imagingmentioning

confidence: 99%

Section: Comparison With Doppler Imagingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Multiperiodicity, modulations, and flip-flops in variable star light curves

Olspert

Käpylä

Pelt

et al. 2015

A&A

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Activity trends in young solar-type stars

Lehtinen

Jetsu

Hackman

et al. 2016

A&A

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Aims. We study a sample of 21 young and active solar-type stars with spectral types ranging from late F to mid K and characterize the behaviour of their activity. Methods. We apply the continuous period search (CPS) time series analysis method on Johnson B-and V-band photometry of the sample stars, collected over a period of 16 to 27 years. Using the CPS method, we estimate the surface differential rotation and determine the existence and behaviour of active longitudes and activity cycles on the stars. We supplement the time series results by calculating new log R HK = log F HK /σT 4 eff emission indices for the stars from high resolution spectroscopy.Results. The measurements of the photometric rotation period variations reveal a positive correlation between the relative differential rotation coefficient and the rotation period as k ∝ P 1.36 rot , but do not reveal any dependence of the differential rotation on the effective temperature of the stars. Secondary period searches reveal activity cycles in 18 of the stars and temporary or persistent active longitudes in 11 of them. The activity cycles fall into specific activity branches when examined in the log P rot /P cyc vs. log Ro −1 , where Ro −1 = 2Ωτ c , or log P rot /P cyc vs. log R HK diagram. We find a new split into sub-branches within this diagram, indicating multiple simultaneously present cycle modes. Active longitudes appear to be present only on the more active stars. There is a sharp break at approximately log R HK = −4.46 separating the less active stars with long-term axisymmetric spot distributions from the more active ones with non-axisymmetric configurations. In seven out of eleven of our stars with clearly detected long-term non-axisymmetric spot activity the estimated active longitude periods are significantly shorter than the mean photometric rotation periods. This systematic trend can be interpreted either as a sign of the active longitudes being sustained from a deeper level in the stellar interior than the individual spots or as azimuthal dynamo waves exhibiting prograde propagation.

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Short-term evolution and coexistence of spots, plages and flare activity on LQ Hydrae

Soriano

Strassmeier

2017

A&A

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Aims. We aim to study the short-term evolution of the chromospheric and photospheric activity of the young, single K2 dwarf LQ Hya. Methods. Four months of quasi-simultaneous spectroscopic and photometric observations were used to study the variations of the photometric light curve, the evolution of the chromospheric activity from the Hα and Hβ lines, and the distribution of cool spots from Doppler maps. Results. During our observations one side of the star was more active than the other. The equivalent width of the Hα line from the least active hemisphere increased from ≈0.7 Å at the beginning of the observation to 1.0 Å at the end. The basal emission of the most active hemisphere remained roughly constant at EW Hα ≈ 1.0 Å. Intense flare activity was observed during the first twenty days, where at least four different events were detected. The line asymmetries of the Hα line suggest that one of the flares could have produced a mass ejection with a maximum projected speed of 70 km s −1 . The rotational modulation of the V-band photometry showed clear anti-correlation with the chromospheric activity. The difference in brightness between the opposite hemispheres decreased from 0 m . 16 to 0 m . 09 in two months. Three spots gradually moving apart from each other are dominating the photospheric Doppler maps. The comparison between the maps and the Hα line as the star rotates reveals the spatial coexistence of chromospheric Hα emission and photospheric spots. Conclusions. Our results indicate that the active regions of LQ Hya can live for at least four months. The detected changes in the photometric light curve and the spectroscopic Doppler images seem to be more a consequence of the spatial redistribution of the active regions rather than due to changes in their strength. Only one of the active regions shows significant changes in its chromospheric emission.

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Doppler imaging of LQ Hydrae for 1998–2002

Cited by 17 publications

References 42 publications

Multiperiodicity, modulations, and flip-flops in variable star light curves

Multiperiodicity, modulations, and flip-flops in variable star light curves

Activity trends in young solar-type stars

Short-term evolution and coexistence of spots, plages and flare activity on LQ Hydrae

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