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Solar analogs, broadly defined as stars similar to the Sun in mass or spectral type, provide a useful laboratory for exploring the range of Sun-like behaviors and exploring the physical mechanisms underlying some of the Suns most elusive processes like coronal heating and the dynamo. We describe a series of heliophysics-motivated, but astrophysics-like studies of solar analogs. We argue for a range of stellar observations, including (a) the identification and fundamental parameter determination of new solar analogs, and (b) characterizing emergent properties like activity, magnetism, and granulation. These parameters should be considered in the framework of statistical studies of the dependences of these observables on fundamental stellar parameters like mass, metallicity, and rotation.
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The investigation of star forming regions have enormously benefited from the recent advent of the ALMA interferometer. More specifically, the unprecedented combination of high-sensitivity and high-angular resolution provided by ALMA allows one to shed light on the jet/disk systems associated with a Sun-like mass protostar. Also astrochemistry enjoyed the possibility to analyze complex spectra obtained using large bandwidths: several interstellar Complex Organic Molecules (iCOMs; C-bearing species with at least 6 atoms) have been imaged around protostars. This in turn boosted the study of the astrochemistry at work during the earliest phases of star formation paving the way to the chemical complexity in planetary systems where Life could emerge. There is mounting evidence that the observations of iCOMs can be used as unique tool to shed light, on Solar System scales (< 50 au), on the molecular content of protostellar disk. The increase of iCOMs abundances occur only under very selective physical conditions, such as those associated low-velocity shocks found where the infalling envelope is impacting the rotating accretion disk. The imaging of these regions with simpler molecules such as CO or CS is indeed paradoxically hampered by their high abundances and consequently high line opacities which do not allow the observers to disentangle all the emitting components at these small scales. In this respect, we review the state-of-the art of the ALMA analysis about the standard Sun-like star forming region in Orion named HH 212. We show (i) how all the physical components involved in the formation of a Sun-like star can be revealed only by observing different molecular tracers, and (ii) how the observation of iCOMs emission, observed to infer the chemical composition of star forming regions, can be used also as unique tracer to image protostellar disks on Solar System scales.
An accurate forecast of flare and CME initiation requires precise measurements of the magnetic energy build up and release in the active regions of the solar atmosphere. We designed a new space weather mission that performs such measurements using new optical instruments based on the Hanle and Zeeman effects. The mission consists of two satellites, one orbiting the L1 Lagrangian point (Spacecraft Earth, SCE) and the second in heliocentric orbit at 1AU trailing the Earth by 80$^circ$ (Spacecraft 80, SC80). Optical instruments measure the vector magnetic field in multiple layers of the solar atmosphere. The orbits of the spacecraft allow for a continuous imaging of nearly 73% of the total solar surface. In-situ plasma instruments detect solar wind conditions at 1AU and ahead of our planet. Earth directed CMEs can be tracked using the stereoscopic view of the spacecraft and the strategic placement of the SC80 satellite. Forecasting of geoeffective space weather events is possible thanks to an accurate surveillance of the magnetic energy build up in the Sun, an optical tracking through the interplanetary space, and in-situ measurements of the near-Earth environment.
Near- to mid-infrared period-magnitude relations and also the period-bolometric luminosity relation of OGLE-III Mira-like variables in the LMC are derived. The relations have a kink, and the period at which the break occurs is quantitatively obtained. There are many Mira-like variables whose fluxes at the optical and the near-infrared wavebands are fainter than the ones predicted by the period-magnitude relations. The deviation is due to the circumstellar extinction, and the amount of the deviation is found to be strongly correlated with near-infrared colors. The empirical formulae relating the amount of the deviation and the near-infrared colors are derived. These relations are useful to accurately calculate the distances to the dusty Mira-like variables, because the dimmed fluxes due to the circumstellar extinction can be estimated. In a manner analogous to the interstellar extinction law, the ratios of deviations at any two different wavebands are calculated. The ratios are found to change with the pulsation period, indicating that the dust properties are subject to change as Mira-like variables evolve.
The time-variable velocity fields of solar-type stars limit the precision of radial-velocity determinations of their planets masses, obstructing detection of Earth twins. Since 2015 July we have been monitoring disc-integrated sunlight in daytime using a purpose-built solar telescope and fibre feed to the HARPS-N stellar radial-velocity spectrometer. We present and analyse the solar radial-velocity measurements and cross-correlation function (CCF) parameters obtained in the first 3 years of observation, interpreting them in the context of spatially-resolved solar observations. We describe a Bayesian mixture-model approach to automated data-quality monitoring. We provide dynamical and daily differential-extinction corrections to place the radial velocities in the heliocentric reference frame, and the CCF shape parameters in the sidereal frame. We achieve a photon-noise limited radial-velocity precision better than 0.43 m s$^{-1}$ per 5-minute observation. The day-to-day precision is limited by zero-point calibration uncertainty with an RMS scatter of about 0.4 m s$^{-1}$. We find significant signals from granulation and solar activity. Within a day, granulation noise dominates, with an amplitude of about 0.4 m s$^{-1}$ and an autocorrelation half-life of 15 minutes. On longer timescales, activity dominates. Sunspot groups broaden the CCF as they cross the solar disc. Facular regions temporarily reduce the intrinsic asymmetry of the CCF. The radial-velocity increase that accompanies an active-region passage has a typical amplitude of 5 m s$^{-1}$ and is correlated with the line asymmetry, but leads it by 3 days. Spectral line-shape variability thus shows promise as a proxy for recovering the true radial velocity.
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 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 the 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 the solar activity. We show that the activity of the Sun is comparable to the activity of the seismic solar analogs, within the maximum-to-minimum temporal variations of the 11-year solar activity cycle. Furthermore, we report on the discovery of temporal variability in the acoustic frequencies of the young (1 Gyr-old) solar analog KIC10644253 with a modulation of about 1.5 years, which agrees with the derived photospheric activity Sph (Salabert et al, 2016b). It could be the signature of the short-period modulation, or quasi-biennal oscillation, of its magnetic activity as observed in the Sun and in the 1-Gyr-old solar analog HD30495. In addition, the lithium abundance and the chromospheric activity estimated from HERMES confirms that KIC10644253 is a young and more active star than the Sun.
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