对类太阳恒星周围小型系外行星的搜寻受到恒星物理变化的限制。虽然色球层变化已被充分研究，但观测、建模和理解光球谱线强度、形状和位移中远更微小的波动仍具挑战性。极精密径向速度光谱仪如今使得极精密恒星光谱成为可能，而将太阳视为一颗恒星的时间序列则允许监测其光球层变化。谱线强度的波动很可能与径向速度偏移相关，并可识别出用于监测的可观测代理指标。从HARPS-N对太阳作为恒星的三年观测中，选取了一千条低噪声光谱，并测量了Fe I、Fe II、Mg I、Mn I、H-alpha、H-beta、H-gamma、Na I和G-band中的谱线吸收。研究了它们的变化及其可能的大气起源，同时也考虑了同步测量的色球发射和视向速度。观测到系统性的谱线强度变化，大体上跟随Ca II H & K发射的太阳周演化，但幅度更小，通常处于亚百分比水平。在铁线中，蓝端Fe II的振幅最大，而不同强度的绿光Mg I三重线之间以及不同巴耳末线之间的变化趋势会改变符号。G-band核心的变化大于整个G-band的变化，与理论预测一致。在半禁戒Mg I 457.1 nm线中未检测到变化。超精细分裂的Mn I行为与Fe I大体相似。对于更长波长的谱线，地球大气吸收限制了可达到的精度。因此，太阳光球光谱中的微变化在各种特征之间显示出系统性特征。这些特征所测量的内容不同于经典的Ca II H & K指数，同时仍反映出磁区域的强烈影响。
英文摘要：
Searches for small exoplanets around solar-type stars are limited by stellar physical variability. While chromospheric variability is well studied, observing, modeling. and understanding the much smaller fluctuations in photospheric spectral line strengths, shapes, and shifts is challenging. Extreme precision radial-velocity spectrometers now enable extreme precision stellar spectroscopy and time series of the Sun seen as a star permit monitoring of its photospheric variability. Fluctuations in their line strengths may well correlate with radial-velocity excursions and identify observable proxies for their monitoring. From three years of HARPS-N observations of the Sun-as-a-star, one thousand low-noise spectra are selected, and line absorption measured in Fe I, Fe II, Mg I, Mn I, H-alpha, H-beta, H-gamma, Na I, and the G-band. Their variations and likely atmospheric origins are examined, also with respect to simultaneously measured chromospheric emission and apparent radial velocity. Systematic line-strength variability is seen, largely shadowing the solar-cycle evolution of Ca II H & K emission, but with smaller amounts, typically on a sub-percent level. Among iron lines, greatest amplitudes are for Fe II in the blue, while the trends change sign among differently strong lines in the green Mg I triplet and between Balmer lines. Variations in the G-band core are greater than of the full G-band, in line with theoretical predictions. No variation is detected in the semi-forbidden Mg I 457.1 nm. Hyperfine split Mn I behaves largely similar to Fe I. For lines at longer wavelengths, telluric absorption limits the achievable precision. Microvariability in the solar photospheric spectrum thus displays systematic signatures among various features. These measure something different than the classical Ca II H & K index while still reflecting a strong influence from magnetic regions.
Solar photospheric spectrum microvariability III. Radial velocities and line profiles in magnetic active-region granulation
Lund University（隆德大学）；Zentrum für Astronomie der Universität Heidelberg, Landessternwarte（海德堡大学天文中心，州立天文台）；Leibniz-Institut für Astrophysik Potsdam（波茨坦阿诺德·爱因斯坦天体物理研究所）；Instituto de Astrofísica de Canarias（加那利群岛天体物理学研究所）；Texas Advanced Computing Center, The University of Texas at Austin（德克萨斯大学奥斯汀分校高级计算中心）
Spectroscopy across spatially resolved stellar surfaces reveals spectral line profiles free from rotational broadening, whose gradual changes from disk center toward the stellar limb reflect an atmospheric fine structure that is possible to model by 3-D hydrodynamics. Previous studies of photospheric spectral lines across stellar disks exist for the Sun and HD209458 (G0 V) and are now extended to the planet-hosting HD189733A to sample a cooler K-type star and explore the future potential of the method. During exoplanet transit, stellar surface portions successively become hidden and differenti…
The CoRoT satellite has made available high precision photometric observations of a large number of stars of different spectral types. Continuous photometric time series allow the characterization of stellar microvariability in a systematic way. We determine an index indicating the level of activity, derived from photometric data, for a large sample of stars with different color temperatures. We also assess to what extent this index can be related to an estimated Rossby number for stars whose rotation period can be estimated. We also estimate a characteristic lifetime of the surface heterogene…
We present an 80-d long uninterrupted high-cadence K2 light curve of the B1Iab supergiant rho Leo (HD 91316), deduced with the method of halo photometry. This light curve reveals a dominant frequency of $f_{\rmrot}=0.0373$d$^{-1}$ and its harmonics. This dominant frequency corresponds with a rotation period of 26.8d and is subject to amplitude and phase modulation. The K2 photometry additionally reveals multiperiodic low-frequency variability ($<1.5 $d$^{-1}$) and is in full agreement with low-cadence high-resolution spectroscopy assembled during 1800 days. The spectroscopy reveals rotational …
I briefly review the current state of the modelling of photospheric activity based on the high-precision optical light curves obtained with MOST, CoRoT, and Kepler. These models can be used to search for active longitudes where activity is preferentially concentrated, estimate the amplitude of stellar differential rotation, and look for short-term activity cycles as, e.g., in the case of CoRoT-2. In the case of a late-type star accompanied by a transiting hot Jupiter, the small light modulations observed during transits when a dark spot is occulted by the disc of the planet are also briefly co…
We present the results of contemporaneous spectroscopic and photometric monitoring of the young solar-type star HD171488 (Prot~1.337 d) aimed at studying surface inhomogeneities at photospheric/chromospheric levels. Echelle FOCES spectra (R~40000) and Johnson photometry have been performed in 2006. Spectral type, rotational velocity, metallicity, and gravity were determined using a code developed by us. The metallicity was measured from the analysis of iron lines. The spectral subtraction technique was applied to the most relevant chromospheric diagnostics included in the FOCES spectral range …
The CoRoT satellite has recently discovered a hot Jupiter that transits across the disc of a F9V star called CoRoT-6 with a period of 8.886 days. We model the photospheric activity of the star and use the maps of the active regions to study stellar differential rotation and the star-planet interaction. We apply a maximum entropy spot model to fit the optical modulation as observed by CoRoT during a uninterrupted interval of about 140 days. Photospheric active regions are assumed to consist of spots and faculae in a fixed proportion with solar-like contrasts. Individual active regions have life…
Recent Kepler photometry has revealed that about half of white dwarfs (WDs) have periodic, low-level (~ 1e-4 - 1e-3), optical variations. Hubble Space Telescope (HST) ultraviolet spectroscopy has shown that up to about one half of WDs are actively accreting rocky planetary debris, as evidenced by the presence of photospheric metal absorption lines. We have obtained HST ultraviolet spectra of seven WDs that have been monitored for periodic variations, to test the hypothesis that these two phenomena are causally connected, i.e. that the optical periodic modulation is caused by WD rotation couple…
Accurate tracers of the stellar magnetic field and rotation are cornerstones for the study of M dwarfs and for reliable detection and characterization of their exoplanetary companions. Such measurements are particularly challenging for old, slowly rotating, fully convective M dwarfs. To explore the use of new activity and rotation tracers, we examined multi-year near-infrared spectroscopic monitoring of two such stars -- GJ 699 (Barnard's Star) and Teegarden's Star -- carried out with Habitable Zone Planet Finder spectrograph. We detected periodic variations in absorption line widths across th…
It has been asserted that the primary star in the HD 189733 system steadily accretes evaporated exospheric gases from its ``hot Jupiter'' companion, rather like a T Tauri star accreting from a disk. We conduct statistical and periodogram analyses of the photometric time series of the primary, as acquired by the automated photoelectric telescope (APT), Microvariability and Oscillations of Stars (MOST), and Wise Observatory, to investigate this claim with the goal of revealing the presence of accretion shocks or photospheric accretion hotspots as are found in T Tauri systems such as AA Tau. None…
We study the connection between the chromospheric and photospheric behaviour of the active late-type star FK Comae. We use spot temperature modelling, light curve inversion based on narrow- and wide-band photometric measurements, Halpha observations from 1997-2010, and Doppler maps from 2004-2010 to compare the behaviour of chromospheric and photospheric features. Investigating low-resolution Halpha spectra we find that the changes in the chromosphere seem to happen mainly on a time scale longer than a few hours, but shorter variations were also observed. According to the Halpha measurements p…
From $5.5$ months of dual-band optical photometric monitoring at the $1$ mmag level, BRITE-Constellation has revealed two simultaneous types of variability in the O4I(n)fp star $ζ$ Puppis: one single periodic non-sinusoidal component superimposed on a stochastic component. The monoperiodic component is the $1.78$ d signal previously detected by Coriolis/SMEI, but this time along with a prominent first harmonic. The shape of this signal changes over time, a behaviour that is incompatible with stellar oscillations but consistent with rotational modulation arising from evolving bright surface inh…
In the present paper we study impact of the photospheric 5-minute oscillations on the ion cyclotron waves in the solar wind. We proceed from the assumption that the ion cyclotron waves in solar wind are experiencing modulation with a characteristic period of 5 minutes under the influence of Alfven waves driven by photospheric motions. The theory presented in our paper predicts a deep frequency modulation of the ion cyclotron waves. The frequency modulation is expected mainly from variations in orientation of the IMF lines. In turn, the variations in orientation are caused by the Alfven waves, …
We study the variability of solar activity using new photospheric proxies originally developed for the analysis of stellar magnetism with the CoRoT and Kepler photometric observations. These proxies are obtained by tracking the temporal modulations in the observations associated to the spots and magnetic features as the Sun rotates. We analyze here 21 years, spanning solar cycles 23 and 24, of the observations collected by the space-based photometric VIRGO and radial velocity GOLF instruments on board the SoHO satellite. The photospheric activity proxy Sph is then calculated for each of the th…
Context. The Kepler mission has recently discovered a brown dwarf companion transiting one member of the M4V+M5V visual binary system LHS 6343 AB with an orbital period of 12.71 days. Aims. The particular interest of this transiting system lies in the synchronicity between the transits of the brown dwarf C component and the main modulation observed in the light curve, which is assumed to be caused by rotating starspots on the A component. We model the activity of this star by deriving maps of the active regions that allow us to study stellar rotation and the possible interaction with the brown…
We analyze the photometric and spectroscopic variability of the classical T Tauri star V2129 Oph over several rotational cycles to test the dynamical predictions of magnetospheric accretion models. The photometric variability and the radial velocity variations in the photospheric lines can be explained by rotational modulation due to cold spots, while the radial velocity variations of the He I (5876 Å) line and the veiling variability are due to hot spot rotational modulation. The hot and cold spots are located at high latitudes and about the same phase, but the hot spot is expected to sit at …
We present our latest results on the solar-stellar connection by studying 18 solar analogs that we identified among the Kepler seismic sample (Salabert et al., 2016a). We measured their magnetic activity properties using observations collected by the Kepler satellite and the ground-based, high-resolution Hermes spectrograph. The photospheric (Sph) and chromospheric (S) magnetic activity proxies of these seismic solar analogs are compared in relation to solar activity. We show that the activity of the Sun is actually comparable to the activity of the seismic solar analogs. Furthermore, we repor…
Despite the important advances in space asteroseismology during the last decade, the early phases of evolution of stars with masses above $\sim$15 M$_{\odot}$ have been only vaguely explored up to now. Our goal is to detect, analyze and interpret variability in the early-B type supergiant HD\,2905 using long-term, ground based, high resolution spectroscopy. We gather a total of 1141 high-resolution spectra covering some 2900 days. We complement these observations with the $Hipparcos$ light curve, which includes 160 data points obtained during a time span of $\sim$1200 days. We investigate spec…
The structure of electric current and magnetic helicity in the solar corona is closely linked to solar activity over the 11-year cycle, yet is poorly understood. As an alternative to traditional current-free "potential field" extrapolations, we investigate a model for the global coronal magnetic field which is non-potential and time-dependent, following the build-up and transport of magnetic helicity due to flux emergence and large-scale photospheric motions. This helicity concentrates into twisted magnetic flux ropes, which may lose equilibrium and be ejected. Here, we consider how the magnet…
The Oef category gathers rapidly rotating and evolved O-stars displaying a centrally reversed He II 4686 emission line. The origin of the variability of their photospheric and wind spectral lines is debated, with rotational modulation or pulsations as the main contenders. To shed new light on this question, we analysed high-quality and high-cadence TESS photometric time series for five Oef stars. We also collected a new time series of spectra for one target (lambda Cep) which had been the subject of specific debates in the last years. These observations reveal the variety of Oef behaviours. Wh…
The initial success of the Rigidly Rotating Magnetosphere (RRM) model application to the B2Vp star sigma OriE by Townsend, Owocki & Groote (2005) triggered a renewed era of observational monitoring of this archetypal object. We utilize high-resolution spectropolarimetry and the magnetic Doppler imaging (MDI) technique to simultaneously determine the magnetic configuration, which is predominately dipolar, with a polar strength Bd = 7.3-7.8 kG and a smaller non-axisymmetric quadrupolar contribution, as well as the surface distribution of abundance of He, Fe, C, and Si. We describe a revised RRM …
Massive stars show a variety of spectral variability: presence of discrete absorption components in UV P-Cygni profiles, optical line profile variability, X-ray variability, radial velocity modulations. Our goal is to study the spectral variability of single OB stars to better understand the relation between photospheric and wind variability. For that, we rely on high spectral resolution, high signal-to-noise ratio optical spectra collected with the spectrograph NARVAL on the Telescope Bernard Lyot at Pic du Midi. We investigate the variability of twelve spectral lines by means of the Temporal…
Solar photospheric magnetic field plays a dominant role in the variability of total solar irradiance (TSI). The modulation of magnetic flux at six specific ranges on TSI is characterized for the first time. The daily flux values of magnetic field at four ranges are extracted from MDI/{\sl SOHO}, together with daily flux of active regions (MF$_{\text{ar}}$) and quiet regions (MF$_{\text{qr}}$); the first four ranges (MF$_{1-4}$) are: 1.5--2.9, 2.9--32.0, 32.0--42.7, and 42.7--380.1 ($\times 10^{18}$ Mx per element), respectively. Cross-correlograms show that MF$_4$, MF$_{\text{qr}}$, and MF$_{\…
We report here results of spectropolarimetric observations of the classical T Tauri star DN Tau carried out (at 2 epochs) with ESPaDOnS at the Canada-France-Hawaii Telescope within the `Magnetic Protostars and Planets' programme. We infer that DN Tau, with a photospheric temperature of 3,950+-50 K, a luminosity of 0.8+-0.2 Lsun and a rotation period of 6.32 d, is a ~2Myr-old fully-convective 0.65+-0.05 Msun star with a radius of 1.9+-0.2 Dsun, viewed at an inclination of 35+-10degr. Clear circularly-polarized Zeeman signatures are detected in both photospheric and accretion-powered emission li…
We report here results of spectropolarimetric observations of the ~8Myr classical TTauri star (cTTS) TWHya carried out with ESPaDOnS at the Canada-France-Hawaii Telescope (CFHT) in the framework of the `Magnetic Protostars and Planets' (MaPP) programme, and obtained at 2 different epochs (2008 March and 2010 March). Obvious Zeeman signatures are detected at all times, both in photospheric lines and in accretion-powered emission lines. Significant intrinsic variability and moderate rotational modulation is observed in both photospheric and accretion proxies. Using tomographic imaging, we recons…
The photospheric spot activity of some of the stars with transiting planets discovered by the CoRoT space experiment is reviewed. Their out-of-transit light modulations are fitted by a spot model previously tested with the total solar irradiance variations. This approach allows us to study the longitude distribution of the spotted area and its variations versus time during the five months of a typical CoRoT time series. The migration of the spots in longitude provides a lower limit for the surface differential rotation, while the variation of the total spotted area can be used to search for sh…
We report on high-resolution optical time-series spectroscopy of the central star of the `Eskimo' planetary nebula NGC~2392. Datasets were secured with the ESO 2.3m in 2006 March and CFHT 3.6m in 2010 March to diagnose the fast wind and photospheric properties of the central star. The HeI and HeII recombination lines reveal evidence for clumping and temporal structures in the fast wind that are erratically variable on timescales down to ~ 30 min. (i.e. comparable to the characteristic wind flow time). We highlight changes in the overall morphology of the wind lines that cannot plausibly be exp…
We address the problem where the X-ray emission lines are formed and investigate orbital dynamics using Chandra HETG observations, photoionizing calculations and numerical wind-particle simulations.The observed Si XIV (6.185 A) and S XVI (4.733 A) line profiles at four orbital phases were fitted with P Cygni-type profiles consisting of an emission and a blue-shifted absorption component. In the models, the emission originates in the photoionized wind of the WR companion illuminated by a hybrid source: the X-ray radiation of the compact star and the photospheric EUV-radiation from the WR star. …
We have carried out numerical simulation based on the equations of radiation magnetohydrodynamics to study the interaction of solar granules and small-scale magnetic fields in photospheric regions with various magnetic fluxes. Four sequences of 2D time-dependent models were calculated for photospheric regions with average vertical magnetic fluxes of 0, 10, 20, and 30 mT. The models exhibit no substantial variations in their temperature structure with varying average field strength, while the density and gas pressure profiles display gross changes. The solar granulation brightness field also va…
The CARMENES instrument is searching for periodic radial-velocity (RV) variations of M dwarfs, which are induced by orbiting planets. However, there are other potential sources of such variations, including rotational modulation caused by stellar activity. We aim to investigate four M dwarfs (Ross 318, YZ CMi, TYC 3529-1437-1, and EV Lac) with different activity levels and spectral sub-types. Our goal is to compare the periodicities seen in 22 activity indicators and the stellar RVs, and to examine their stability over time. For each star, we calculated GLS periodograms of pseudo-equivalent wi…
Phase-resolved observations of the solar-type star Ksi Bootis A were obtained using the NARVAL spectropolarimeter at the Telescope Bernard Lyot (Pic du Midi, France) during years 2007, 2008, 2009 and 2010. The data sets enable us to study both the rotational modulation and the long-term evolution of various magnetic and activity tracers. Here, we focus on the large-scale photospheric magnetic field (reconstructed by Zeeman-Doppler Imaging), the Zeeman broadening of the FeI 846.84 nm magnetic line, and the chromospheric CaII H and H alpha emission.
The simultaneous photometric and spectroscopic observations of the RR Lyrae variables in the globular cluster, M3, published in Jurcsik et al. (2017, Paper I) made it possible to perform Baade-Wesselink (BW) analysis of a large sample of Blazhko stars for the first time. The BW distances of Blazhko stars turned out to be unreliable, as significantly different distances were obtained for the stars of the Blazhko sample and also for the same star in different modulation phases. Even the results of small modulation-amplitude Blazhko stars may be doubtful. This result warns that the application of…
We present optical ESO time-series and UV archival (FUSE, HST, IUE) spectroscopy of the H-rich central star of He 2-138. Our study targets the central star wind in a very young planetary nebula, and explores physical conditions that may provide clues to the nature of the preceding post-AGB super-wind phases of the star. We provide evidence for a dense, slowly accelerating outflow that is variable on time-scales of hours. Line-synthesis modelling (SEI and CMFGEN) of low and high ionization UV and optical lines is interpreted in terms of an asymmetric, two-component outflow, where high-speed hig…
Chemically peculiar (CP) stars of the upper main sequence are characterised by specific anomalies in the photospheric abundances of some chemical elements. The group of CP2 stars, which encompasses classical Ap and Bp stars, exhibits strictly periodic light, spectral, and spectropolarimetric variations that can be adequately explained by the model of a rigidly rotating star with persistent surface structures and a stable global magnetic field. Using observations from the Kepler K2 mission, we find that the B9pSi star HD 174356 displays a light curve both variable in amplitude and shape, which …
We present simultaneous multiwavelength observations of the ultra-active star AB~Doradus obtained in the X-ray range with the eROSITA instrument on board the Russian--German Spectrum-Roentgen-Gamma mission (SRG), and in the optical range obtained with the Transiting Exoplanet Survey Satellite (TESS). Thanks to its fortuitous location in the vicinity of the southern ecliptic pole, AB~Dor was observed by these missions simultaneously for almost 20 days. With the hitherto obtained data we study the long-term evolution of the X-ray flux from AB~Dor and the relation between this observable and the …
Finding solar-analog stars with fundamental properties as close as possible to the Sun and studying the characteristics of their surface magnetic activity is a very promising way to understand the solar variability and its associated dynamo process. However, the identification of solar-analog stars depends on the accuracy of the estimated stellar parameters. Thanks to the photometric CoROT and Kepler space missions, the addition of asteroseismic data was proven to provide the most accurate fundamental properties that can be derived from stellar modeling today. Here, we present our latest resul…
Context. Kepler-17 is a G2V sun-like star accompanied by a transiting planet with a mass of ~2.5 Jupiter masses and an orbital period of 1.486 d, recently discovered by the Kepler space telescope. This star is highly interesting as a young solar analogue. Aims. We used about 500 days of high-precision, high-duty-cycle optical photometry collected by Kepler to study the rotation of the star and the evolution of its photospheric active regions. Methods. We applied a maximum-entropy light curve inversion technique to model the flux rotational modulation induced by active regions that consist of d…
Pre-main sequence stars are variable sources. In diskless stars this variability is mainly due to the rotational modulation of dark photospheric spots and active regions, as in main sequence stars even if associated with a stronger magnetic activity. Aims. We aim at analyzing the simultaneous optical and X-ray variability in these stars to unveil how the activity in the photosphere is connected with that in the corona, to identify the dominant surface magnetic activity, and to correlate our results with stellar properties, such as rotation and mass. Methods. We analyzed the simultaneous optica…
We report here results of spectropolarimetric observations of the classical T Tauri star (cTTS) GQ Lup carried out with ESPaDOnS at the Canada-France-Hawaii Telescope (CFHT) in the framework of the "Magnetic Protostars and Planets" (MaPP) programme, and obtained at 2 different epochs (2009 July & 2011 June). From these observations, we first infer that GQ Lup has a photospheric temperature of 4,300+-50Â K and a rotation period of 8.4+-0.3 d; it implies that it is a 1.05+-0.07 Msun star viewed at an inclination of ~30deg, with an age of 2-5 Myr, a radius of 1.7+-0.2 Rsun, and has just started t…
FU Orionis systems are young stars undergoing outbursts of disc accretion and where the optical spectrum contains lines associated with both the disc photosphere and a wind component. Previous observations of the prototype FU Orionis have suggested that the wind lines and the photospheric lines are modulated with periods of 14.54 and 3.54 days respectively (Herbig et al. 2003). We have re-observed the system at higher spectral resolution, by monitoring variations of optical line profiles over 21 nights in 2007 and have found periods of 13.48 and 3.6 days in the wind and disc components consist…
Aims. We aim to investigate the long-term temporal evolution of the magnetic field of the solar-type star ξBootis A, both from direct magnetic field measurements and from the simultaneous estimate of indirect activity indicators. Methods. We obtained seven epochs of high-resolution, circularly-polarized spectra from the NARVAL spectropolarimeter between 2007 and 2011, for a total of 76 spectra. Using approximately 6,100 photospheric spectral lines covering the visible domain, we employed a cross-correlation procedure to compute a mean polarized line profile from each spectrum. The large-scale …
EX Lupi is a young star, prototype of EXor variables. Its spectrum is very rich in emission lines, including many metallic lines. It has been also proposed to have a close companion. We use the metallic emission lines to study the accretion structures and to test the companion hypothesis. We analyse 54 spectra taken in 5 years of quiescence time. We study the line profile variability and the radial velocity of the metallic emission lines. We use the velocity signatures of different species with various excitation conditions and their time dependency to track the dynamics associated to accretio…
The total solar irradiance (TSI) varies on timescales of minute to centuries. On short timescales it varies due to the superposition of intensity fluctuations produced by turbulent convection and acoustic oscillations. On longer scale times, it changes due to photospheric magnetic activity, mainly because of the facular brightenings and dimmings caused by sunspots. While modern TSI variations have been monitored from space since 1970s, TSI variations over much longer periods can only be estimated using either historical observations of magnetic features, possibly supported by flux transport mo…
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 H$α$ and H$β$ lines, and the distribution of cool spots from Doppler maps. During our observations one side of the star was more active than the other. The equivalent width of the H$α$ 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 a H$α$ EW of ~1.0 Å. Int…
Total and spectral solar irradiance are key parameters in the assessment of solar influence on changes in the Earth's climate. We present a reconstruction of daily solar irradiance obtained using the SATIRE-S model spanning 1974 to 2013 based on full-disc observations from the KPVT, SoHO/MDI and SDO/HMI. SATIRE-S ascribes variation in solar irradiance on timescales greater than a day to photospheric magnetism. The solar spectrum is reconstructed from the apparent surface coverage of bright magnetic features and sunspots in the daily data using the modelled intensity spectra of these magnetic s…
Light curves of solar-type stars often show gradual fluctuations due to rotational modulation by magnetic features (starspots and faculae) on stellar surfaces. Two quantitative measures of modulated light curves are employed as the proxies of magnetic activity for solar-type stars observed with Kepler telescope. The first is named autocorrelation index $i_{AC}$, which describes the degree of periodicity of the light curve, the second is the effective fluctuation range of the light curve $R_{eff}$, which reflects the depth of rotational modulation. The two measures are complementary and depict …
Massive stars play a significant role in the chemical and dynamical evolution of galaxies. However, much of their variability, particularly during their evolved supergiant stage, is poorly understood. To understand the variability of evolved massive stars in more detail, we present a study of the O9.2Ib supergiant $ζ$ Ori Aa, the only currently confirmed supergiant to host a magnetic field. We have obtained two-color space-based BRIght Target Explorer photometry (BRITE) for $ζ$ Ori Aa during two observing campaigns, as well as simultaneous ground-based, high-resolution optical CHIRON spectrosc…
The hard X-ray source IGR J11215-5952 is a peculiar transient, displaying very short X-ray outbursts every 165 days. We obtained high-resolution spectra of the optical counterpart, HD 306414, at different epochs, spanning a total of three months, before and around the 2007 February outburst with the combined aims of deriving its astrophysical parameters and searching for orbital modulation. We fit model atmospheres generated with the fastwind code to the spectrum. We also cross-correlated each individual spectrum to the best-fit model to derive radial velocities. From its spectral features, we…
Our goal is to determine the rotational and magnetic-related activity properties of stars at different stages of evolution. We have focussed our attention on 6 young loose stellar associations within 100 pc and ages in the range 8-70 Myr: TW Hydrae (~8 Myr), beta Pictoris (~10 Myr), Tucana/Horologium, Columba, Carina (~30 Myr), and AB Doradus (~70 Myr). Additional data on alpha Persei and the Pleiades from the literature is also considered. Rotational periods of stars showing rotational modulation due to photospheric magnetic activity (i.e. starspots) have been determined applying the Lomb-Sca…
We introduce a method of measuring a lower limit to the amplitude of surface differential rotation from high-precision, evenly sampled photometric time series. It is applied to main-sequence late-type stars whose optical flux modulation is dominated by starspots. An autocorrelation of the time series was used to select stars that allow an accurate determination of starspot rotation periods. A simple two-spot model was applied together with a Bayesian information criterion to preliminarily select intervals of the time series showing evidence of differential rotation with starspots of almost con…
