The amplitude and morphology of light curves of solar-like stars change substantially with increasing rotation rate: brightness variations get amplified and become more regular, which has so far not been explained. We develop a modelling approach for calculating brightness variations of stars with various rotation rates and use it to explain observed trends in stellar photometric variability. We combine numerical simulations of magnetic Flux Emergence And Transport (FEAT) with a model for stellar brightness variability to calculate synthetic light curves of stars as observed by the Kepler telescope. We compute the distribution of magnetic flux on the stellar surface for various rotation rates and degrees of active-region nesting (i.e., the tendency of active regions to emerge in the vicinity of recently emerged ones). Using the resulting maps of the magnetic flux, we compute the rotational variability of our simulated stellar light curves as a function of rotation rate and nesting of magnetic features and compare our calculations to Kepler observations. We show that both rotation rate and degree of nesting have a strong impact on the amplitude and morphology of stellar light curves. In order to explain the variability of the bulk of \K{} targets with known rotation rates, we need to increase the degree of nesting to values much larger than on the Sun. The suggested increase of nesting with the rotation rate can provide clues to the flux emergence process for high levels of stellar activity.
The solar brightness varies on timescales from minutes to decades. A clear identification of the physical processes behind such variations is needed for developing and improving physics-based models of solar brightness variability and reconstructing solar brightness in the past. This is, in turn, important for better understanding the solar-terrestrial and solar-stellar connections. We estimate the relative contributions of the continuum, molecular, and atomic lines to the solar brightness variations on different timescales. Our approach is based on the assumption that variability of the solar…
In addition to the `facular' brightening of active regions, the quiet Sun also contains a small scale magnetic field with associated brightenings in continuum radiation. We measure this contribution of quiet regions to the Sun's brightness from high spatial resolution (0"16-0"32) observations of the Swedish 1-m Solar Telescope (SST) and Hinode satellite. The line-of-sight magnetic field and continuum intensity near \ion{Fe}{i} 6302.5 Å are used to quantify the correlation between field strength and brightness. The data show that magnetic flux density contains a significant amount of intrinsica…
The unprecedented precision of broadband stellar photometry achieved with the planet-hunting missions CoRoT and \textit{Kepler} initiated a new era in examining the magnetically-driven brightness variations of hundreds of thousands of stars. Such brightness variations are well studied and understood for the Sun. The plethora of data allows to accurately compare solar and stellar brightness variations. An intriguing question is whether the observed trends in the stellar photometric variability (e.g. the dependence of the variability on the stellar rotation period) can be explained by utilising …
The net effect of the small scale magnetic field on the Sun's (bolometric) brightness is studied with realistic 3D MHD simulations. The direct effect of brightening within the magnetic field itself is consistent with measurements in high-resolution observations. The high 'photometric accuracy' of the simulations, however, reveal compensating brightness effects that are hard to detect observationally. The influence of magnetic concentrations on the surrounding nonmagnetic convective flows (a 'proximity effect') reduces the brightness by an amount exceeding the brightening by the magnetic concen…
Context. Comparison studies of Sun-like stars with the Sun suggest an anomalously low photometric variability of the Sun compared to Sun-like stars with similar magnetic activity. Comprehensive understanding of stellar variability is needed, to find a physical reasoning for this observation. Aims. We investigate the effect of metallicity and effective temperature on the photometric brightness change of Sun-like stars seen at different inclinations. The considered range of fundamental stellar parameters is sufficiently small so the stars, investigated here, still count as Sun-like or even as so…
Regular spaceborne measurements have revealed that solar brightness varies on multiple timescales, variations on timescales greater than a day being attributed to surface magnetic field. Independently, ground-based and spaceborne measurements suggest that Sun-like stars show a similar, but significantly broader pattern of photometric variability. To understand whether the broader pattern of stellar variations is consistent with the solar paradigm we assess relative contributions of faculae and spots to solar magnetically-driven brightness variability. We investigate how the solar brightness va…
Independent of the normal solar cycle, a decrease in the sunspot magnetic field strength has been observed using the Zeeman-split 1564.8nm Fe I spectral line at the NSO Kitt Peak McMath-Pierce telescope. Corresponding changes in sunspot brightness and the strength of molecular absorption lines were also seen. This trend was seen to continue in observations of the first sunspots of the new solar Cycle 24, and extrapolating a linear fit to this trend would lead to only half the number of spots in Cycle 24 compared to Cycle 23, and imply virtually no sunspots in Cycle 25. We examined synoptic obs…
This topical collection summarizes recent advances in observing and modeling irradiance variations of the Sun and Sun-like stars, emphasizing the links between surface magnetic fields and the resulting solar and stellar variability. In particular, the articles composing this collection summarize recent progress in i) solar-irradiance measurements; ii) modeling of solar- and stellar-irradiance variability; and iii) understanding of the effects of such variability on Earth's climate and exoplanet environments. This topical-collection overview article gives background and more details on these as…
How global faculae and network coverage relates to that of sunspots is relevant to the brightness variations of the Sun and Sun-like stars. We extend earlier studies that found the facular-to-sunspot-area ratio diminishes with total sunspot coverage. Chromospheric indices and the total magnetic flux enclosed in network and faculae, referred to here as 'facular indices', are modulated by the amount of facular and network present. We probed the relationship between various facular and sunspot indices through an empirical model that takes into account how active regions evolve. This model was inc…
Motivated by dark coronal lanes in SOHO / EIT 284 Å EUV observations we construct and optimize an atmosphere model of the AR 8535 sunspot by adding a cool and dense component in the volume of plasma along open field lines determined using the Potential Field Source Surface (PFSS) extrapolation. Our model qualitatively reproduces the observed reduced microwave brightness temperature in the northern part of the sunspot in the VLA observations from 13 May 1999 and provides a physical explanation for the coronal dark lanes. We propose application of this method to other sunspots with such observed…
We analyze the effect of solar features on the variability of the solar irradiance in three different spectral ranges. Our study is based on two solar-cycles' worth of full-disk photometric images from the San Fernando Observatory, obtained with red, blue and Ca II K-line filters. For each image we measure the photometric sum, Sigma, which is the relative contribution of solar features to the disk-integrated intensity of the image. The photometric sums in the red and blue continuum, Sigma_r and Sigma_b, exhibit similar temporal patterns: they are negatively correlated with solar activity, with…
Images of the Sun at millimeter wavelengths obtained by ALMA show a significant correspondence with the magnetograms. In this paper, we investigate this correspondence by comparing ALMA full-disk solar image taken at 1.2 mm with a SDO/HMI magnetogram and analyze their correlation. It is found that chromospheric network and active regions show a positive correlation where brightness temperature is increasing with the line-of-sight magnetic field strength, while sunspots have a negative correlation. Quiet Sun regions do not show any dependence of the brightness temperature with the magnetic fiel…
Context. The bipolar outflows associated with low-mass protostars create cavities in the infalling envelope. These cavities are illuminated by the central protostar and inner disk, creating a bipolar scattered light nebula at near-infrared and mid-infrared wavelengths. The variability of the scattered light nebula in both total intensity and intensity as a function of position in the scattered light nebula can provide important insights into the structure of the inner disk that cannot be spatially resolved. Aims. We aim to determine the likelihood that a warped inner disk is the origin of the …
Context. Monitoring of the photometric and chromospheric HK emission data series of stars similar to the Sun in age and average activity level showed that there is an empirical correlation between the average stellar chromospheric activity level and the photometric variability. In general, more active stars show larger photometric variability. Interestingly, the measurements and reconstructions of the solar irradiance show that the Sun is significantly less variable than indicated by the empirical relationship. Aims. We aim to identify possible reasons for the Sun to be currently outside of th…
Kepler observations revealed that hundreds of stars with near-solar fundamental parameters and rotation periods have much stronger and more regular brightness variations than the Sun. Here we identify one possible reason for the peculiar behaviour of these stars. Inspired by solar nests of activity, we assume that the degree of inhomogeneity of active-region (AR) emergence on such stars is higher than on the Sun. To test our hypothesis, we model stellar light curves by injecting ARs consisting of spots and faculae on stellar surfaces at various rates and nesting patterns, using solar AR proper…
The Sun and stars with low magnetic activity levels, become photometrically brighter when their activity increases. Magnetically more active stars display the opposite behaviour and get fainter when their activity increases. We reproduce the observed photometric trends in stellar variations with a model that treats stars as hypothetical Suns with coverage by magnetic features different from that of the Sun. The presented model attributes the variability of stellar spectra to the imbalance between the contributions from different components of the solar atmosphere, such as dark starspots and br…
The irradiance of the Sun is modulated on all time scales. Small-scale solar magnetic elements composed of quiet-Sun network and active region plages contribute to this modulation on solar cycle time scales. The evaluation of their contrast as a function of their magnetic field strength is an important constraint for models of solar irradiance variation. In this thesis, we improve and extend the results of earlier contrast studies taking advantage of the high resolution data delivered by the balloon-borne solar observatory Sunrise during its science flights in 2009 and 2013. In addition to the…
We investigate the variability of the brightness distribution and the changing density structure of the protoplanetary disk around DR Tau, a classical T Tauri star. DR Tau is known for its peculiar variations from the ultraviolet (UV) to the mid-infrared (MIR). Our goal is to constrain the temporal variation of the disk structure based on photometric and MIR interferometric data. We observed DR Tau with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) at three epochs separated by about nine years, two months, respectively. We fit the spectral…
Magnetic activity of the Sun and other stars causes their brightness to vary. We investigate how typical the Sun's variability is compared to other solar-like stars, i.e. those with near-solar effective temperatures and rotation periods. By combining four years of photometric observations from the Kepler space telescope with astrometric data from the Gaia spacecraft, we measure photometric variabilities of 369 solar-like stars. Most of the solar-like stars with well-determined rotation periods show higher variability than the Sun and are therefore considerably more active. These stars appear n…
Some close binaries of the beta Lyrae type show photometric cycles longer than the orbital one, which are possibly related to changes in their accretion disks. We aim to understand the short- and long-scale changes observed in the light curve of the eclipsing system OGLE-BLG-ECL-157529. In particular, we want to shed light on the contribution of the disk to these changes, especially those related to the long cycle, occurring on timescales of hundreds of days. We studied I-band OGLE photometric times series spanning 18.5 years, constructing disk models by analyzing the orbital light curve at 52…
We utilize Kepler data to study the precision differential photometric variability of solar-type and cooler stars at different timescales, ranging from half an hour to 3 months. We define a diagnostic that characterizes the median differential intensity change between data bins of a given timescale. We apply the same diagnostics to SOHO data that has been rendered comparable to Kepler. The Sun exhibits similar photometric variability on all timescales as comparable solar-type stars in the Kepler field (it is not unusually quiet). The previously-defined photometric "range" serves as our activit…
Results from BVRI photometric observations of the young stellar object V2492 Cyg collected during the period from August 2010 to December 2017 are presented. The star is located in the field of the Pelican Nebula and it was discovered in 2010 due to its remarkable increase in the brightness by more than 5 mag in R-band. According to the first hypothesis of the variability V2492 Cyg is an FUor candidate. During subsequent observations it was reported that the star shows the characteristics inherent to EXor- and UXor-type variables. The optical data show that during the whole time of observation…
Photometry from the Kepler mission is optimized to detect small, short duration signals like planet transits at the expense of long-term trends. This long-term variability can be recovered in photometry from the Full Frame Images (FFIs), a set of calibration data collected approximately monthly during the Kepler mission. Here, we present f3, an open-source package to perform photometry on the Kepler FFIs in order to detect changes in the brightness of stars in the Kepler field of view over long time baselines. We apply this package to a sample of 4,000 Sun-like stars with measured rotation per…
We search for persistent and quasi-periodic release events of streamer blobs during 2007 with the Large Angle Spectrometric Coronagraph on the \textit{Solar and Heliospheric Observatory} and assess the velocity of the slow solar wind along the plasma sheet above the corresponding streamer by measuring the dynamic parameters of blobs. We find 10 quasi-periodic release events of streamer blobs lasting for three to four days. In each day of these events, we observe three-five blobs. The results are in line with previous studies using data observed near the last solar minimum. Using the measured b…
The variability of the spectral solar irradiance (SSI) over the course of the 11-year solar cycle is one of the manifestations of solar magnetic activity. There is a strong evidence that the SSI variability has an effect on the Earth's atmosphere. The faster rotation of the Sun in the past lead to a more vigorous action of solar dynamo and thus potentially to larger amplitude of the SSI variability on the timescale of the solar activity cycle. This could led to a stronger response of the Earth's atmosphere as well as other solar system planets' atmospheres to the solar activity cycle. We calcu…
The Kepler mission provides an exciting opportunity to study the lightcurves of stars with unprecedented precision and continuity of coverage. This is the first look at a large sample of stars with photometric data of a quality that has heretofore been only available for our Sun. It provides the first opportunity to compare the irradiance variations of our Sun to a large cohort of stars ranging from vary similar to rather different stellar properties, at a wide variety of ages. Although Kepler data is in an early phase of maturity, and we only analyze the first month of coverage, it is suffici…
The published Mount Wilson Doppler-shift measurements of the solar velocity field taken in 1967--1982 are revisited with a more accurate model, which includes two terms representing the meridional flow and three terms corresponding to the convective limb shift. Integration of the recomputed data over the visible hemisphere reveals significant variability of the net radial velocity at characteristic time scales of 0.1--10 years, with a standard deviation of 1.4 \ms. This result is supported by independent published observations. The implications for exoplanet detection include reduced sensitivi…
Magnetic features on the surfaces of cool stars cause variations of their brightness. Such variations have been extensively studied for the Sun. Recent planet-hunting space telescopes allowed measuring brightness variations in hundred thousands of other stars. The new data posed the question of how typical is the Sun as a variable star. Putting solar variability into the stellar context suffers, however, from the bias of solar observations being made from its near-equatorial plane, whereas stars are observed at all possible inclinations. We model solar brightness variations at timescales from …
We analyze space-based time series photometry of Sun-like stars, mostly in the Pleiades, but also field stars and the Sun itself. We focus on timescales between roughly 1 hour and 1 day. In the corresponding frequency band these stars display brightness fluctuations with a decreasing power-law continuous spectrum. K2 and Kepler observations show that the RMS flicker due to this Mid-Frequency Continuum (MFC) can reach almost 1%, approaching the modulation amplitude from active regions. The MFC amplitude varies by a factor up to 40 among Pleiades members with similar Teff, depending mainly on th…
Young and active stars generally have regular, almost sinusoidal, patterns of variability attributed to their rotation, while the majority of older and less active stars, including the Sun, have more complex and non-regular light-curves which do not have clear rotational-modulation signals. Consequently, the rotation periods have been successfully determined only for a small fraction of the Sun-like stars observed by transit-based planet-hunting missions, such as CoRoT, Kepler, and TESS. This suggests that only a small fraction of such systems have been properly identified as solar-like analog…
Bright points (BPs) are manifestations of small magnetic elements in the solar photosphere. Their brightness contrast not only gives insight into the thermal state of the photosphere (and chromosphere) in magnetic elements, but also plays an important role in modulating the solar total and spectral irradiance. Here we report on simultaneous high-resolution imaging and spectropolarimetric observations of BPs using Sunrise balloon-borne observatory data of the quiet Sun at disk center. BP contrasts have been measured between 214 nm and 525 nm, including the first measurements at wavelengths belo…
The latitudinal distribution of starspots deviates from the solar pattern with increasing rotation rate. Numerical simulations of magnetic flux emergence and transport can help model the observed stellar activity patterns and the associated brightness variations. We set up a composite model for the processes of flux emergence and transport on Sun-like stars, to simulate stellar brightness variations for various levels of magnetic activity and rotation rates. Assuming that the distribution of magnetic flux at the base of the convection zone follows solar scaling relations, we calculate the emer…
We present new photometric data and analysis of the long-duration UBV photoelectric observations for three candidates to protoplanetary objects - F-supergiants with IR-excesses located at large galactic latitudes, IRAS 18095+2704, IRAS 19386+0155, and IRAS 19500-1709. All three stars have revealed quasiperiodic low-amplitude variabilities caused by pulsations observed against the long-term trends of brightnesses. For IRAS 18095+2704=V887 Her we have found a pulsation period of 109 days and a linear trend of brightness under the constant colours if being averaged over the year timescale. The li…
While the quiet Sun magnetic field shows only little variation with the solar cycle, long-term variations cannot be completely ruled out from first principles. We investigate the potential effect of quiet Sun magnetism on spectral solar irradiance through a series of small-scale dynamo simulations with zero vertical flux imbalance ($\langle B_z\rangle=0$) and varying levels of small-scale magnetic field strength, and one weak network case with an additional flux imbalance corresponding to a flux density of $\langle B_z\rangle=100$ G. From these setups we compute the dependence of the outgoing …
We have measured the meridional motions of the magnetic elements in the Sun's surface layers since 1996 and find systematic and substantial variations. In general the meridional flow speed is fast at cycle minima and slow at cycle maxima. We find that these systematic variations are characterized by a weakening of the meridional flow on the poleward sides of the active (sunspot) latitudes. This can be interpreted as a inflow toward the sunspot zones superimposed on a more general poleward meridional flow profile. We also find variations in the meridional flow which vary from cycle-to-cycle. Th…
We carried out BVRI CCD photometric observations in the field of V582 Aur from 2009 August to 2013 February. We acquired high-, medium-, and low-resolution spectroscopy of V582 Aur during this period. To study the pre-outburst variability of the target and construct its historical light curve, we searched for archival observations in photographic plate collections. Both CCD and photographic observations were analyzed using a sequence of 14 stars in the field of V582 Aur calibrated in BVRI. The pre-outburst photographic observations of V582 Aur show low-amplitude light variations typical of T T…
The integrated brightness of the Sun shows variability on time-scales from minutes to decades. This variability is mainly caused by pressure mode oscillations, by granulation and by dark spots and bright faculae on the surface of the Sun. By analyzing the frequency spectrum of the integrated brightness we can obtain greater knowledge about these phenomena. It is shown how the frequency spectrum of the integrated brightness of the Sun in the frequency range from 0.1 to 3.2 mHz shows clear signs of both granulation, faculae and p-mode oscillations and that the measured characteristic time-scales…
Results from the BV(RI)c photometric observations of the young stellar object V2492 Cyg collected in the period from April 2018 to September 2020 are presented. These observations are a part of our monitoring of the star that began in 2010 and continuing to date. V2492 Cyg is located in the Pelican Nebula, and its variability was explained by both accretion and extinction events. The new photometric data show that the star continues to exhibit rapid irregular variability in all bands. In the period from March 2019 to May 2020, we registered a prolonged decrease event in the light curve of V249…
The Sun has long been considered a constant star, to the extent that its total irradiance was termed the solar constant. It required radiometers in space to detect the small variations in solar irradiance on timescales of the solar rotation and the solar cycle. A part of the difficulty is that there are no other constant natural daytime sources to which the Sun's brightness can be compared. The discovery of solar irradiance variability rekindled a long-running discussion on how strongly the Sun affects our climate. A non-negligible influence is suggested by correlation studies between solar va…
Multi-epoch observations have revealed the variability of pre-main sequence stars and/or their environment. Moreover, structures in orbital motion around the central star, resulting from planet-disk interaction, are predicted to cause temporal variations in the brightness distributions of protoplanetary disks. Through repeated observations with the Very Large Telescope Interferometer (VLTI) over nearly two decades, the ESO Archive has become a treasure chest containing high-resolution multi-epoch near- and mid-infrared observations of the potential planet-forming regions in protoplanetary disk…
Results from optical photometric observations of the pre-main sequence star GM Cep are reported in the paper. The star is located in the field of the young open cluster Trumpler 37 - a region of active star formation. GM Cep shows a large amplitude rapid variability interpreted as a possible outburst from EXor type in previous studies. Our data from BVRI CCD photometric observations of the star are collected from June 2008 to February 2011 in Rozhen observatory (Bulgaria) and Skinakas observatory (Crete, Greece). A sequence of sixteen comparison stars in the field of GM Cep was calibrated in t…
The Sun provides the energy required to sustain life on Earth and drive our planet's atmospheric circulation. However, establishing a solid physical connection between solar and tropospheric variability has posed a considerable challenge across the spectrum of Earth-system science. The canon of solar variability, the solar fiducial clock, lies almost exclusively with the 400 years of human telescopic observations that demonstrates the waxing and waning number of sunspots, over an 11(ish) year period. Recent research has demonstrated the critical importance of the underlying 22-year magnetic po…
The solar brightness varies on timescales from minutes to decades. Determining the sources of such variations, often referred to as solar noise, is of importance for multiple reasons: a) it is the background that limits the detection of solar oscillations, b) variability in solar brightness is one of the drivers of the Earth's climate system, c) it is a prototype of stellar variability which is an important limiting factor for the detection of extra-solar planets. Here we show that recent progress in simulations and observations of the Sun makes it finally possible to pinpoint the source of th…
Although the Sun is our closest star by many orders of magnitude and despite having sunspot records stretching back to ancient China, our knowledge of the Sun's magnetic field is far from complete. Indeed, even now, after decades of study, the most obvious manifestations of magnetic fields in the Sun (e.g. sunspots, flares and the corona) are scarcely understood at all. These failures in spite of intense effort suggest that to improve our grasp of magnetic fields in stars and of astrophysical dynamos in general, we must broaden our base of examples beyond the Sun; we must study stars with a va…
Results from UBVRI photometric observations of the pre-main sequence star GM Cep obtained in the period April 2011 - August 2014 are reported in the paper. Presented data are a continuation of our photometric monitoring of the star started in 2008. GM Cep is located in the field of the young open cluster Trumpler 37 and over the past years it has been an object of intense photometric and spectral studies. The star shows a strong photometric variability interpreted as a possible outburst from EXor type in previous studies. Our photometric data for a period of over six years show a large amplitu…
We investigate the long-period fluctuations in the brightness of the Sun as a star using the measurements of sunlight reflected from the planets (Jupiter, Mars) when the light hits the field of view of the LASCO C3 coronagraph (Large Angle and Spectrometric Coronagraph Experiment).
The Evershed flow is a systematic motion of gas that occurs in the penumbra of all sunspots. Discovered in 1909, it still lacks a satisfactory explanation. We know that the flow is magnetized, often supersonic, and that it shows conspicuous fine structure on spatial scales of 0.2"-0.3", but its origin remains unclear. The hope is that a good observational understanding of the relation between the flow and the penumbral magnetic field will help us determine its nature. Here I review advances in the characterization of the Evershed flow and sunspot magnetic fields from high-resolution spectrosco…
Context. Stellar spectral variability on timescales of a day and longer, arising from magnetic surface features such as dark spots and bright faculae, is an important noise source when characterising extra-solar planets. Current 1D models of faculae do not capture the geometric properties and fail to reproduce observed solar facular contrasts. Magnetoconvection simulations provide facular contrasts accounting for geometry. Aims. We calculate facular contrast spectra from magnetoconvection models of the solar photosphere with a view to improve (a) future parameter determinations for planets wit…
The rotational evolution of cool stars is governed by magnetised stellar winds which slow the stellar rotation during their main sequence lifetimes. Magnetic variability is commonly observed in Sun-like stars, and the changing strength and topology of the global field is expected to affect the torque exerted by the stellar wind. We present three different methods for computing the angular momentum loss in the solar wind. Two are based on MHD simulations from Finley & Matt (2018), with one using the open flux measured in the solar wind, and the other using remotely-observed surface magnetograms…
Radio emission of the quiet Sun is considered to be due to thermal bremsstrahlung emission of the hot solar atmosphere. The properties of the quiet Sun in the microwave band have been well studied, and they can be well described by the spectrum of bremsstrahlung emission. In the meter-wave and decameter-wave bands, properties of the quiet Sun have rarely been studied due to the instrumental limitations. In this work, we use the LOw Frequency ARray (LOFAR) telescope to perform high quality interferometric imaging spectroscopy observations of quiet Sun coronal emission at frequencies below 90~MH…
We examine the photometric variability of stars in the M 67 field using Kepler/K2-Campaign-5 light curves. Variabilities and periods were determined for 639 stars. The mean photometric period of 28 single Sun-like members stars in M 67 is 23.4 +/- 1.2 d. This corresponds to a gyro-age of 3.7 +/- 0.3 Gyr, assuming the periods can be associated with rotation. The intrinsic variabilities of the solar analogs are greater than the Sun's variability, as measure from VIRGO fluxes. We also find evidence that the single cluster members have a different distribution of variability than the binary member…
CCD photometric observations of the dwarf nova V1006 Cyg were carried out in 2015-2017 with 11 telescopes located at 7 observatories. They covered the 2015 superoutburst with rebrightening, five normal outbursts of ~4-day duration and one wide outburst that lasted at least seven days. The interval between normal outbursts was 16 and 22 days, and between superoutbursts is expected to be longer than 124 days. The positive superhumps with the mean period of 0^d.10544(10) and 0^d.10406(17) were detected during the 2015 superoutburst and during the short-term quiescence between rebrightening and th…
The absolute brightness temperature of the Sun at millimeter wavelengths is an important diagnostic of the solar chromosphere. Because the Sun is so bright, measurement of this property usually involves the operation of telescopes under extreme conditions and requires a rigorous performance assessment of the telescope. In this study, we establish solar observation and calibration techniques at 2.6-mm wavelength for the Nobeyama 45-m telescope and derive the absolute solar brightness temperature accurately. We tune the superconductor-insulator-superconductor (SIS) receiver by inducing different…
In previous work we identified six Sun-like stars observed by Kepler with exceptionally clear asteroseismic signatures of rotation. Here, we show that five of these stars exhibit surface variability suitable for measuring rotation. In order to further constrain differential rotation, we compare the rotation periods obtained from light-curve variability with those from asteroseismology. The two rotation measurement methods are found to agree within uncertainties, suggesting that radial differential rotation is weak, as is the case for the Sun. Furthermore, we find significant discrepancies betw…
HD 163296 is a Herbig Ae star that underwent a dramatic $\sim$0.8 magnitude drop in brightness in the V photometric band in 2001 and a brightening in the near-IR in 2002. Because the star possesses Herbig-Haro objects travelling in outflowing bipolar jets, it was suggested that the drop in brightness was due to a clump of dust entrained in a disk wind, blocking the line-on-sight toward the star. In order to quantify this hypothesis, we investigated the brightness drop at visible wavelengths and the brightening at near-IR wavelengths of HD 163296 using the Monte Carlo Radiative Transfer Code, H…
We present the analysis of several newly obtained and archived photometric and spectroscopic datasets of the intriguing and yet poorly understood 13.5-mag central star candidate of the bipolar planetary nebula Sh2-71. Photometric observations confirmed the previously determined quasi-sinusoidal lightcurve with a period of 68 days and also indicated periodic sharp brightness dips, possibly eclipses, with a period of 17.2 days. In addition, the comparison between U and V lightcurves revealed that the 68-day brightness variations are accompanied by a variable reddening effect of $ΔE(U-V)=0.38$. S…
High-precision time series have recently become available for many stars as a result of data from CoRoT, Kepler, and TESS and have been widely used to study stellar activity. They provide information integrated over the stellar disk, hence many degeneracies between spots and plages or sizes and contrasts. Our aim is to understand how to relate photometric variability to physical parameters in order to help the interpretation of these observations. We computed a large number of synthetic time series of brightness variations for old MS stars within the F6-K4 range, using consistent modeling for …
The magnetic fields of low-mass stars are observed to be variable on decadal timescales, ranging in behaviour from cyclic to stochastic. The changing strength and geometry of the magnetic field should modify the efficiency of angular momentum loss by stellar winds, but this has not been well quantified. In Finley et al. (2018) we investigated the variability of the Sun, and calculated the time-varying angular momentum loss rate in the solar wind. In this work, we focus on four low-mass stars that have all had their surface magnetic fields mapped for multiple epochs. Using mass loss rates deter…
