In this (provocative!) review I will discuss a few recent ideas, invoking particular physical mechanisms, which lead to significant revisions of our conventional understanding of low-mass star, brown dwarf and gaseous planet mechanical and thermal properties. If time allows, I will also briefly address issues about these object formation mechanisms. Finally I will mention how on-going and future observational projects will help us constrain the theoretical scenarios
Despite their widespread use, evolutionary models of low-mass pre-main sequence (PMS) stars are poorly constrained by observations. Fewer than 20 PMS M dwarfs in binary systems have measured dynamical masses to a precision of 25% or better through astrometric monitoring. The vast majority of these systems are extremely young (<10 Myr). We have identified a new sample of more than 50 stars in binary systems with ages 10-100 Myr where dynamical masses can be measured through high-resolution AO imaging of young moving group members. Through astrometric orbit monitoring with the Differential Speckle Survey Instrument at the Discovery Channel Telescope and Gemini Observatory as well as Keck/NIRC2, combined with radial velocity observations from TRES and CHIRON, we have measured dynamical masses for more than a dozen systems. We then combine relative and absolute broadband photometry with medium-resolution spectroscopy to compare the measured masses and atmospheric parameters to those predicted by stellar evolutionary models. I will present initial results of this survey, which provides the first opportunity to test evolutionary models for many stars with masses between 0.1 and 0.5 solar masses and ages between 10 and 100 Myr.
I will present new results for pre-Main sequence and early brown dwarf evolutionary models accounting for the effect of early accretion history. First, I will present self-consistent numerical simulations fully coupling numerical hydrodynamics models of collapsing prestellar cores and evolutionary models of the central protostar or proto-brown dwarf. I will in particular analyse the main impact of consistent accretion history on Li depletion. I will also present the first attempt to describe the multi-dimensional structure of accreting young stars based on fully compressible time implicit multi-dimensional hydrodynamics simulations. I will discuss the relevance of assumptions and treatments of accretion used in 1D stellar evolution codes.
High-energy photons (X-ray to NUV; 5 – 3200 Å) from exoplanet host stars regulate the atmospheric temperature profiles and photochemistry on orbiting planets, influencing the production of potential “biomarker” gases. However, few observational and theoretical constraints exist on the high-energy irradiance from typical (i.e., weakly active) M and K dwarf exoplanet host stars. Towards the development of an empirical database of stellar spectra to support exoplanet atmosphere modeling, we present results from a panchromatic survey (Hubble/Chandra/XMM/optical) of M and K dwarf exoplanet hosts. The MUSCLES Treasury Survey (Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems) combines UV and X-ray observations with reconstructed Lyman-alpha and EUV (100-900 Å) radiation to create 5 Ångström to 5 micron stellar irradiance spectra. These data are now publically available as a High-Level Science Product on MAST. We find that all low-mass exoplanet host stars exhibit significant chromospheric/transition region/coronal emission -- no “UV/X-ray inactive” M or K dwarfs are observed. The F(FUV)/F(NUV) flux ratio, a driver for possible abiotic production of the suggested biomarkers O2 and O3, increases by ~3 orders of magnitude as the star’s habitable zone moves inward from 1 to 0.1 AU, while the incident FUV (912 – 1700 Å) and XUV (5 – 900 Å) radiation field strengths are approximately constant across this range. Far-UV flare activity is common in ‘optically inactive’ M dwarfs, and we present tentative evidence for the interaction of massive planets with the upper atmospheres of their cool host stars.
The Sun has a steady 11-year cycle in magnetic activity most well-known by the rising and falling in the occurrence of dark sunspots on the solar disk in visible bandpasses. The 11-year cycle is also manifest in the variations of emission in the Ca II H & K line cores, due to non-thermal (i.e. magnetic) heating in the lower chromosphere. The large variation in Ca II H & K emission allows for study of the patterns of long-term variability in other stars thanks to synoptic monitoring with the Mount Wilson Observatory HK photometers (1966-2003) and Lowell Observatory Solar-Stellar Spectrograph (1994-present). Overlapping measurements for a set of 27 nearby solar-analog (spectral types G0-G5) stars were used to calibrate the two instruments and construct time series of magnetic activity up to 50 years in length. Precise properties of fundamental importance to the dynamo are available from Hipparcos, the Geneva-Copenhagen Survey, and CHARA interferometry. Using these long time series and measurements of fundamental properties, we do a comparative study of stellar "twins" to explore the sensitivity of the stellar dynamo to small changes to structure, rotation, and composition. We also compare this sample to the Sun and find hints that the regular periodic variability of the solar cycle may be rare among its nearest neighbors in parameter space.
In solar type stars, the attenuation of convective blueshift due to magnetic activity dominates the radial velocity variations over small mass planets. Models of stars different from the Sun will request a good knowledge of these properties to be extrapolated. It is therefore crucial to precisely determine not only the amplitude of the convective blueshift for different types of stars, but also the dependence of this convective blueshift on magnetic activity, as they are key factors in models producing the radial velocity variations. We study a large sample of G and K stars and focus on their temporally averaged properties, i.e. the activity level and a criterion allowing to characterize the amplitude of the convective blueshift using the variation of the velocity versus the intensity at the bottom of spectral lines. We find this criteria to depend on spectral type, on wavelength (this dependence is correlated with the temperature and the activity level as well) and on the activity level. We find that the convective blueshift decreases when the effective temperature decreses, in good agreement with models. The relative dependence of the convective blueshift on activity (with respect to the average convective blueshift for a given stellar type) seems to be constant over the considered range of spectral types. We finally compare the observed RV variation amplitudes with those derived from our convective blueshift estimations using a simple law and find a general agreement on the amplitude and trend, and show that inclination plays a major role.
Measuring fundamental properties of substellar and planetary mass objects is at the heart of much of "Cool Star" research. Measurements of effective temperature, gravity, mass, radius, and metallicity hold the key to our understanding of substellar objects, including their formation, atmospheres and evolution. Parallax measurements along with broad band photometry and spectral coverage from the optical through the mid infrared have led to breakthroughs in our understanding of atmospheric physics of brown dwarfs and directly imaged exoplanets. The small subpopulations of age-calibrated sources such as low gravity moving group members as well as subdwarfs (the extremes) provide critical benchmarks, allowing us to decouple the effects of age, metallicity, and gravity on the population as a whole. The substellar population spans hot, young M dwarfs to extremely cold and low-mass Y dwarfs allowing us to study an extreme range of temperatures. In this review talk, I will present our current understanding of the fundamental properties of substellar and planetary mass objects. I will show trends in luminosity, absolute magnitude, and temperature that are correlated with age, mass, and atmosphere properties. While many of the measurements made rely partially on models to fill in missing information, I will also comment on consistency and discrepancies between observations and current model predictions.
Our understanding of brown dwarfs and directly imaged exoplanets relies on theoretical models of their evolution and emergent spectra. Hence testing such models is paramount. Such tests are substantially more powerful when the distances are directly known for the objects, but the necessary parallaxes have been lacking for key portions of parameter space due to the distance and faintness of relevant targets. We present here parallaxes for 65 young (~10-100 Myr) brown dwarfs and planetary-mass objects from the Hawaii Infrared Parallax Program, about a factor of 5 increase over previous studies and with substantially higher precision. Combined with results for older field objects, we construct an empirical map of the influence of temperature and gravity on ultracool objects from ~100 Mjup to ~5 Mjup. Comparison with state-of-the-art theoretical reveals that the models are generally too blue and/or faint compared to the data. We demonstrate that the effect of reduced gravity (i.e. lower mass) can be distinctly charted on infrared color-magnitude diagrams and show that the gravity dependence of the L/T transition differs from the current predictions. Finally, we find that the low-gravity sequence and the locus of directly imaged substellar companions largely coincide but not entirely, raising the question of whether the two populations span a common range of physical properties.
Modeling of substellar atmospheres is crucial for understanding their properties from observations, including determining their temperatures, masses, compositions, and atmospheric circulation. I will review what we (think we) understand about brown dwarf and exoplanet atmospheres from the past two decades of atmosphere studies. Model atmospheres have provided templates at different effective temperatures, gravities, and metallicities to compare to observations, allowing us to connect their spectra to these physical properties. We have learned about mixing in substellar atmospheres by comparing to models that include disequilibrium chemistry. Clouds play a key role in shaping the spectra of brown dwarfs and planets, from refractory iron and silicate clouds in hot objects to volatile clouds in colder objects. l will discuss the current frontiers of substellar atmosphere modeling, including studies of the coldest objects and the youngest objects, studies of variable objects, and the development of data-driven retrieval algorithms. Models for cold objects have been developed, which include the effects of water ice clouds; these models have been compared to spectral observations of the coldest known brown dwarf. The youngest objects include planet-mass brown dwarfs and directly-imaged planets; gravity may strongly effect the chemistry and cloud formation on these objects. Brown dwarfs of all spectral types have now been observed to be variable, and atmospheric circulation models will be important to understand this variability. Lastly, new data-driven modeling techniques have been pioneered in the past three years that allow us to study new properties of atmospheres that grid models cannot probe.
Recent advances in atmospheric retrieval techniques applied to brown dwarf spectra have allowed us to extract detailed information about their thermal structures and molecular abundances beyond the classic “logg-Teff” grid model fits. With these new diagnostic modeling tools we can gain a deeper understanding of the physical and chemical processes operating in substellar atmospheres. In Line et al. (2015) we applied powerful Bayesian atmospheric retrieval tools to the SpeX low-res spectra of two benchmark brown dwarf systems. In that investigation we were able to demonstrate the validity of the approach by obtaining similar metallicities, C-to-O ratios, and ages for the brown dwarf companions to their host stars. Here, we more widely apply this approach to the low-res SpeX spectra of 11 late T-dwarf objects (T7-T8) in order to identify diagnostic trends indicative of particular physical processes. Given the retrieved thermal profiles, we determine which regions of the atmosphere are in or out of radiative equilibrium and where the radiative convective boundary occurs. We also show trends in the retrieved composition with temperature. One exciting find is an increasing trend in the alkali metal abundance with increasing temperature--an additional line of evidence supporting the “rainout” processes predicted to occur in substellar atmospheres. We also show that these late T-dwarfs are definitively cloud free within a simple gray cloud framework. This investigation demonstrates the power of atmospheric retrieval tools applied to quality spectra of small sample of objects and lay’s the groundwork for future studies involving brown dwarfs,self-luminous directly imaged planets, and upcoming hi-quality transiting planet data from JWST and beyond.
Variations in the brightness of brown dwarfs and extrasolar planets offer a highly effective probe of ultra-cool atmospheres. Phase mapping of rotating objects reveals longitudinal variations, while wavelength-dependent monitoring enables a vertical probe of cloud layers and of temperature-pressure profiles. Such differential approaches are effective because they measure perturbations around an otherwise constant set of conditions, and so carry higher precision in determining atmospheric relations than is attainable by forward or inverse atmospheric modelling efforts. A most dramatic demonstration of the power of brown dwarf variability observations has been the resolved surface flux map of the secondary component in the nearest substellar binary, Luhman 16B. The past four years have seen a resurgence of brown dwarf variability science, often with superior precision using Spitzer and Hubble. Over 80 L, T, and Y dwarfs have now been monitored extensively and precisely, in sequences as long as 20 hours per object, and over time scales spanning minutes to years. With unbiased variability detection rates of approximately 50%, it is now possible to consider the role of surface inhomogeneities---hot spots, clouds, bands, and large-scale storms---in a statistical sample that spans a range of atmospheric conditions. Tantalizing evidence has already emerged for dependencies on effective temperature, surface gravity, rotation period, and viewing geometry. I will present an overview of the results from brown dwarf variability studies, and will highlight the key implications for the astrophysics of substellar atmospheres.
Wide-field surveys such as Kepler have produced reams of data suitable for investigating stellar activity of cooler stars. Starspot activity on these stars produces quasi-sinusoidal light curves whose phase and amplitude vary as active regions grow and decay over time. Here we investigate, firstly, whether there is a correlation between the size of starspots - assumed to be related to the amplitude of the sinusoid - and their decay timescale and, secondly, whether any correlation varies depending on the stellar effective temperature. To determine this, an autocorrelation function (ACF) was produced for samples of stars from Kepler and fitted with an apodised periodic function, using a Monte Carlo Markov Chain (MCMC) to measure the periods and decay timescales of the light curves. The light curve amplitudes, representing spot size were measured from the root-mean-squared scatter of the normalised light curves. Additionally we measured the amplitude of the shorter-timescale “flicker” caused by granular convection. The results show a correlation between the decay time of star spots and their inferred size, and that it heavily depends on the temperature of the star. Cooler stars have spots that last much longer, in particular for stars with longer rotational periods. This is consistent with current theories of diffusive mechanisms causing star spot decay. We also find that the Sun is not unusually quiet for its spectral type - stars with similar rotation periods and temperatures tend to have (comparatively) smaller star spots. I will go through the motivation of the project, the Kepler target selection, the methods used to fit the autocorrelation functions and discuss the key points of interest that emerge from the results.
The Gaia benchmark stars are a set of 40 typical Milky Way stars whose stellar parameters are currently used as calibrator pillars for the automatic characterisation of large spectroscopic surveys such as Gaia-ESO. Since these stars are very different from each other, this small sample is ideal for performing highly detailed analyses for a large variety of type of stars, allowing us to learn about deep aspects of spectroscopy beyond the Sun. In this talk, I will present current activities related to the analysis of Benchmark stars, and discuss the applications of our work in the context of future large spectroscopic surveys such as 4MOST and weave.
Recent work has shown a strong correlation between the [C/N] ratios measured by the the APOGEE survey and the masses, metallicities, and evolutionary states of red giant and red clump stars. The existence of these correlations is predicted by theory, but the absolute values of [C/N] as a function of mass, metallicity, and position on the giant branch are sensitive to numerous systematic effects, both observational and theoretical. We will present a comparison between the measured [C/N] ratios and theoretical predictions for the APOKASC sample, where the masses, compositions, and evolutionary states of the stars are known as a result of a joint analysis of seismic and spectroscopic data. Discrepancies between the predicted and observed values point to incorrect physics in terms of diffusion and mixing in stellar models and to incorrect absolute abundances in C and N for stellar abundance measurements. Improvements in both areas is critical in this age of precision stellar astrophysics.
During the last decade astronomers have been trying to search for chemical signatures of terrestrial planet formation in the atmospheres of the hosting stars. Several studies suggested that the chemical abundance trend with the condensation temperature, Tc, is a signature of rocky planet formation. In particular, it was suggested that the Sun shows 'peculiar' chemical abundances due to the presence of the terrestrial planets in our solar-system. However, the rocky material accretion or the trap of rocky materials in terrestrial planets is not the only explanation for the chemical 'peculiarity' of the Sun, or other Sun-like stars with planets. In this talk I will make a very brief review of this topic, and present our last results for the particular case of Zeta Reticuli binary system: A very interesting and well-known system (known in science fiction and ufology as the world of Grey Aliens, or Reticulans) where one of the components hosts an exo-Kuiper belt, and the other component is a 'single', 'lonely' star.
The determination of atmospheric parameters and chemical abundances depends on the use of radiative transfer codes to compute synthetic spectra and/or derive abundances from equivalent widths (SPECTRUM, Gray & Corbally 1994; WIDTH, Kurucz 1993 & Sbordone et al. 2004; SME, Valenti & Piskunov 1996; Turbospectrum, Alvarez et al. 1998 & Plez 2012; MOOG, Sneden et al. 2012). However, to extract scientific conclusions about stellar aggregates or the Galaxy (for instance), it is common to mix results from different surveys/studies where different setups were used to derive parameters and abundances. These inhomogeneities can lead us to inaccurate conclusions. We studied one aspect of the problem: What differences originate from the use of different radiative transfer c odes? Using exactly the same spectroscopic pipeline (based on iSpec, Blanco-Cuaresma et al. 2014), we executed a homogeneous analysis (based on iSpec, Blanco-Cuaresma et al. 2014) of the Gaia FGK Benchmark Stars and studied the level of agreement between the most popular radiative transfer codes.
Over the last decade, precision space-based photometry has provided high signal-to-noise detections of oscillations in thousands of red giant stars. Combined with asteroseismic modeling and interpretation, this wealth of data has allowed for unprecedented characterization of red giant interiors, whose core properties can now be measured more precisely than the core of our own Sun. I will review recent progress in this field, focusing on four asteroseismic breakthroughs: 1. The ability to determine the evolutionary stage of red giants and to distinguish hydrogen and helium-burning stars. 2. Measurements of the rotation rates of the cores of red giants, which defy nearly all theoretical expectations. 3. The detection of strong magnetic fields in the cores of red giants, providing the first direct measurements of internal stellar magnetic fields. 4. New constraints on mixing in red giant cores, yielding evidence that convective overshoot and/or diffusive mixing processes are more efficient than expected.
Jennifer van Saders
Gyrochronology utilizes the spin-down of stars as a function of time to estimate stellar ages. Thanks to Kepler, we now have rotation periods numbering in the tens of thousands, making it a particularly enticing tool to obtain chronological information for studies of the Milky Way and extrasolar planets. However, gyrochronology is in its adolescence: it has been well-tested and validated in young and intermediate age open clusters, but old-star gyrochronology remains largely unexplored due to the technical challenges of obtaining an appropriate calibration sample. We present results on a sample of old, well-characterized asteroseismic target stars from the main Kepler mission to show that standard period-age relations fail in old field stars, and furthermore that they fail under predictable conditions. This unexpected behavior suggests a new class of magnetic braking models, where braking is weaker in stars with Rossby numbers of Ro > 2.0. This discovery both weakens the diagnostic power of gyrochronology in old stars and hints at an interesting transition occurring in our own Sun. We will discuss our progress to date in understanding this braking behavior, and place it in context of larger rotation samples.
Young and rapidly rotating stars are known for intense, dynamo generated magnetic fields. Spectropolarimetric observations of those stars among precisely aged clusters are key tools for gyrochronology and magnetochronology. We use ZDI maps of several young K-type stars of similar mass and radius but with various ages and rotational periods, to perform 3D numerical MHD simulations of their coronae and follow the evolution of their magnetic properties with age. Those simulations yield the coronal structure as well as the instant torque exerted by the rotating wind on the star. Coronal temperatures and density are set thanks to an evolutionary model that yields very different wind speeds. The speed spatial distribution is shaped by the three dimensional structure of the magnetic fields. For some stars of the sample, compression regions appear in the equatorial plane when the fast wind encounters the slow wind, within 30 stellar radii. We also find that the angular momentum loss follows the open flux formulation we derived in our recent work and that it is proportionnal to Ω3. The mass loss decreases with age, in agreement with observations of Lyman-alpha absorption at the astropause.
Observations of young open clusters have revealed a bimodal distribution of the rotation periods of solar-like stars that has proven difficult to explain under the existing rubric of magnetic braking. Recent studies suggest that magnetic complexity can play an important role in controlling stellar spin-down rates. In this talk I will discuss the missing term representing magnetic morphology in the context of stellar spin-down models. Using state-of-the-art magnetohydrodynamical magnetized wind simulations we have derived analytical expressions representing the magnetic field morphology dependence of mass and angular momentum loss rates. Magnetic field complexity provides a natural physical basis for stellar rotation evolution models requiring a rapid transition between weak and strong spin-down modes.
Pre-main-sequence (PMS) stars more massive than about 0.35 solar masses transition from hosting fully convective interiors to configurations with a radiative core and outer convective envelope during their gravitational contraction. Observational evidence has emerged that PMS stars (at least those above ~0.5 solar masses) are born with simple, axisymmetric, magnetic fields, with tilted kilo-Gauss dipole and/or octupole components. As they evolve across the HR diagram, and in particular once they progress onto Henyey tracks, their large-scale magnetic fields become complex, multipolar, and non-axisymmetric, with weak (~0.1 kG) dipole components. I will demonstrate, by comparing Hayashi to Henyey track PMS stars from a sample of ~1000 in 5 star forming regions, that this observed magnetic topology transition has corresponding signatures at X-ray wavelengths, and in the rotation period distributions of accreting systems. X-ray emission decays faster with age for higher mass PMS stars. The increase in magnetic complexity and the decay of coronal X-ray emission from young early K to late G-type PMS stars, the progenitors of main-sequence A-type stars, is consistent with the dearth of X-ray detections of the latter, as well as the lack of magnetic field detections in most Herbig stars.
Despite the prevalence of fully-convective stars, very few members of the field population have measured rotation periods. The lack of observational constraints at field ages has hampered studies of rotational evolution. We present rotation periods for 387 nearby mid-to-late M dwarfs in the Northern hemisphere, and new detections for M dwarfs in the Southern hemisphere. These measurements are derived from photometry from the MEarth North and South transit surveys, and include detections from 0.1 to 140 days. The period distribution is mass dependent: as the mass decreases, the slowest rotators at a given mass have longer periods, and the fastest rotators have shorter periods. We find a dearth of stars with intermediate rotation periods, which suggests that fully-convective stars undergo rapid angular momentum evolution. The typical detected rotator has stable, sinusoidal photometric modulations at a semi-amplitude of 0.5 to 1%. We find no correlation between period and amplitude for stars below 0.25 Msun, and discuss the rotation-magnetic activity relation. We use Galactic kinematics and established age-velocity relations to estimate the M dwarf spin-down timescale. We find that stars with P<10 days are on average <2 Gyrs, and that those with P>70 days are about 5 Gyrs.
The recent progress in high spatial resolution techniques, spanning wavelengths from the visual to the radio regime, is leading to valuable insights into the complex dynamical atmospheres of AGB stars and their wind forming regions. Striking examples are detections of asymmetries and inhomogeneities in the photospheric and dust-forming layers which are probably related to large-scale convective flows predicted by 3D "star-in-a-box" models. Furthermore, high-resolution observations allow to measure dust condensation distances and they give information about the chemical composition and sizes of dust grains close to the star. These are essential constraints for understanding wind acceleration and developing a predictive theory of mass loss on the AGB, which is a crucial ingredient of stellar and galactic chemical evolution models.
I will present our ALMA observations of the CO emission around the carbon AGB star R Sculptoris. The data reveal the known detached shell and a previously unknown, binary induced, spiral shape. The observations confirm a formation of the shell during a thermal pulse about 2300 years ago. The full analysis of the ALMA data shows that the shell around R Scl in fact is entirely filled with molecular gas, and hence not as detached as previously thought. This has implications for the mass-loss rate evolution immediately after the pulse, indicating a much higher mass-loss rate than previously assumed. Comparing the ALMA images to our optical observations of polarised, dust scattered light, we further show that the distributions of the dust and gas coincide almost perfectly, implying a common evolution of the dust and gas, and constraining the wind-driving mechanism. The mass-loss process and amount of mass lost during the thermal pulse cycle affect the chemical evolution of the star, its lifetime on the AGB, and the return of heavy elements to the ISM. New high-resolution ALMA observations constrain the parameters of the binary system and the inner spiral, and will allow for a detailed hydrodynamical modelling of the gas and dust during and after the last thermal pulse. Our results present the only direct measurements of the thermal pulse evolution currently available. They greatly increase our understanding of this fundamental period of stellar evolution, and the implications it has for the chemical evolution of evolved stars, the ISM, and galaxies.
Flares result from the sudden reconnection and relaxation of magnetic fields in the coronae of stellar atmospheres. The highly dynamic atmospheric response produces radiation across the electromagnetic spectrum, from the radio to X-rays, on a range of timescales, from seconds to days. New high resolution data of solar flares have revealed the intrinsic spatial properties of the flaring chromosphere, which is thought to be where the majority of the flare energy is released as radiation in the optical and near-UV continua and emission lines. New data of stellar flares have revealed the detailed properties of the broadband (white-light) continuum emission, which provides straightforward constraints for models of the transformation of stored magnetic energy in the corona into thermal energy of the lower atmosphere. In this talk, we discuss the physical processes that produce several important spectral phenomena in the near-ultraviolet and optical as revealed from new radiative-hydrodynamic models of flares on the Sun and low mass stars. We present recent progress with high-flux nonthermal electron beams in reproducing the observed optical continuum color temperature of T~10,000 K and the Balmer jump properties in the near-ultraviolet. These beams produce dense, heated chromospheric condensations, which can explain the shape and strength of the continuum emission in M dwarf flares and the red-wing asymmetries in the chromospheric emission lines in recent observations of solar flares from the Interface Region Imaging Spectrograph. Current theoretical challenges and future modeling directions will be discussed, as well as observational synergies between solar and stellar flares.
The multitude of activity phenomena on the Sun's surface are related to magnetic fields believed to be driven by a dynamo mechanism acting in the tachocline. This global dynamo involves the generation and evolution of the largest features of the sun, such as sunspots, the overall magnetic polarity of the sun, and its short and long-term changes over the solar activity cycle. Recent findings have rapidly changed our picture of the principles driving solar dynamo and at the origin of the observed amplitude fluctuations in cycle strength. Stellar data have achieved a richness comparable to that of solar data thanks to a wealth of spectro-polarimetric information and statistical studies of unprecedentedly huge samples of stars observed by space-borne telescopes (KEPLER, COROT). It is essential, for the understanding of the effect of stellar mass on the resulting magnetic activity, to fully exploit the wealth of these observations. Solar like stars are known to show chromospheric activity similar to that on the Sun in the Ca II H and K emission. However the tachocline moves towards increasing depths with later spectral types, disappearing around M4. What is the role of the tachocline? How does its presence differentiate magnetic activity with respect to fully convective stars? How does stellar internal structure affect the principles driving stellar dynamo? I will review the latest developments in solar dynamo which need to be brought to the knowledge of the stellar community to open a new window on stellar observables holding clues on the above mentioned questions.
Jose A. Caballero
CARMENES, the brand-new, Spanish-German, two-channel, ultra-stabilised, high-resolution spectrograph at the 3.5 m Calar Alto telescope, started its science survey on 01 Jan 2016. In one shot, it covers from 0.52 to 1.71 μm with resolution R = 94,600 (λ < 0.96 μm) and 80,400 (λ > 0.96 μm). During guaranteed time observations, CARMENES carries out the programme for which the instrument was designed: radial-velocity monitoring of bright, nearby, low-mass dwarfs with spectral types between M0.0V and M9.5V. Carmencita is the 'CARMEN(ES) Cool dwarf Information and daTa Archive', our input catalogue, from which we select the circa 300 targets being observed during guaranteed time. Besides that, Carmencita is perhaps the most comprehensive database of bright, nearby M dwarfs ever built, as well as a useful tool for forthcoming exoplanet hunters: ESPRESSO, HPF, IRD, SPIRou, TESS or even PLATO. Carmencita contains dozens of parameters measured by us or compiled from the literature for about 2200 M dwarfs in the solar neighbourhood brighter than J = 11.5 mag: accurate coordinates, spectral types, photometry from ultraviolet to mid-infrared, parallaxes and spectro-photometric distances, rotational and radial velocities, Hα equivalent widths, X-ray count rates and hardness ratios, close and wide multiplicity data, proper motions, Galactocentric space velocities, metallicities, full references, homogeneously derived astrophysical parameters, and much more. In my talk, I will explain how we build Carmencita standing on the shoulders of giants and observing with 2-m class telescopes, and produce a dozen MSc theses and several PhD theses in the process (http://carmenes.caha.es).
Active M dwarf stars host powerful and distinctly non-solar magnetic dynamos, which manifest themselves by ubiquitous surface activity phenomena such as flares and X-ray emission. Direct evidence of strong magnetic fields IS detected in the intensity and polarisation line profiles, yet analyses of these observables lead to contradictory conclusions about magnetic field strengths and topologies. Here we present a set of complementary projects aimed at developing self-consistent, physically-sound empirical models of M-dwarf magnetism. On the theoretical and modelling side, we discuss improvements of the diagnosis of field strengths using Zeeman broadening of atomic and molecular lines. We also present results of an in-depth study of polarised radiative transfer in M-dwarf atmospheres, taking into account both global and local magnetic field components, and assess the resulting impact on interpretation of the Zeeman Doppler maps of M dwarfs. On the observational side, we report the first ever spectropolarimetric observations of M dwarfs in all four Stokes parameters and present the serendipitous discovery of a sudden evolution of the magnetic field topology in one of the brightest active M dwarfs.
In this talk I will review the recent works on magnetism of cool, main-sequence stars, their winds and potential impact on surrounding exoplanets. The winds of these stars are very tenuous and persist during their lifetime. Although carrying just a small fraction of the stellar mass, these magnetic winds regulate the rotation of the star. Since cool stars are likely to be surrounded by planets, understanding the host star winds and magnetism is a key step towards characterisation of exoplanetary environments. Although these environments may be potentially dangerous for a planet's atmosphere, the interaction between exoplanets and the host star winds can provide other avenues for planet detection and maybe even assess planetary properties, which would otherwise remain unknown.
Radial velocity perturbations induced by stellar surface inhomogeneities including spots, plages and granules currently limit the detection of Earth-twins using Doppler spectroscopy. Indeed, the effects of stellar surface inhomogeneities on observed stellar radial velocities are extremely difficult to characterize because except for the Sun, stellar surfaces cannot be resolved, and thus developing optimal correction techniques to extract true stellar radial velocities is extremely challenging. To learn more about possible ways to correct for radial velocity stellar signals we have built a solar telescope to feed full-disk sunlight into the HARPS-N spectrograph. This setup enables long-term observation of the Sun as a star with state-of-the-art sensitivity to radial velocity changes. We can then use Solar Dynamic Observatory (SDO) images to link any radial velocity perturbation with physical changes on the solar surface. During this talk, I will present the first year of data. With observations of the Sun every possible day for a few hours, this data set represents our best chance of understanding deeply stellar signals, to test the best observational strategies to look for exoplanets, and to find correction techniques to mitigate the impact of stellar signals. I will show the first detection of Jupiter as an exoplanet, and first attempts to model stellar signals using SDO images, Gaussian-process regression, and spectroscopic observables. If successful, these new methods to mitigate stellar signals should be directly applicable to others stars, as the same spectrograph is used, and should enable the detection of Venus over the next two years, thus demonstrating the possibility of detecting Earth-twins around other solar-type stars using the radial velocity technique.
We present an updated view of the rotation-activity-age relation for (old) field M dwarfs based on K2 rotation periods and flares, archival X-ray and UV data, and dedicated Chandra and XMM-Newton observations. The rotation-activity-age relation can be used as a proxy for magnetic fields -- which are difficult to measure in M dwarfs -- and is, therefore, key to studies of (i) the predicted dynamo transition at the fully convective boundary (SpT~M3), (ii) differences in angular momentum loss through magnetized winds with respect to solar-type stars, and (iii) the evaporation of planet atmospheres, especially relevant for M dwarfs because of the small separation of their planets' habitable zones where they are strongly exposed to the stellar high-energy emission. We use a two-fold approach to obtain tight observational constraints: (1) In our study of the activity-rotation relation of nearby, proper-motion selected M dwarfs using photometric time-series from the K2 mission combined with X-ray and UV data we find strong evidence for an abrupt change of optical photometric activity (flares, rotation cycle amplitude and residual variability) at Prot ~ 10d, and an unexpectedly steep decline of X-ray activity in the unsaturated regime of slow rotators. (2) The age-activity relation is investigated through deep X-ray observations of a sample of M dwarfs in wide binaries with white dwarfs. Here the white dwarf serves as a chronometer for the age of the M dwarf. Our results provide important input for accurate angular momentum evolution models and planet atmosphere escape calculations for M dwarfs.
Discovering Earth-like exoplanets using the radial velocity (RV) technique is mainly being challenged by stellar activity. Only by understanding the variable signals imposed by the star itself will we be able to find the underlying planetary signals. Activity indicators, such as the FWHM of the spectral lines and the Ca II H&K emission, are already being used to understand the stellar variations. However, the high-resolution spectra that we have from the RV searches contain much more information. In this talk I will explain about a new technique of extracting information on the stellar magnetic field directly from the intensity spectrum. By combining thousands of lines, we can extract the Zeeman broadening effect. We apply this to the high-resolution spectra time series taken with HARPS and HARPS-N in order to disentangle the stellar magnetic signal from the signals from Earth-like planets.
The chemical evolution of the Universe is governed by the nucleosynthesis contribution from stars, which in turn is determined primarily by the initial stellar mass. I will review the status of theoretical stellar evolutionary models and yields of single stars between about 1 to 10 solar mass. Stars in this mass range evolve to become cool red giants after the main sequence. It is during the giant branches that these stars experience mixing events that can significantly change the surface composition of the envelope. Observed enrichments include carbon, lithium, nitrogen, fluorine, and heavy elements synthesized by the slow neutron capture process (the s-process). Cool evolved stars release their stellar yields through strong outflows or winds, in contrast to massive stars that explode as core-collapse supernovae. I will discuss the main uncertainties affecting theoretical calculations and highlight areas of recent progress. Finally, I will discuss the role that cool evolved stars play in the broader picture of Galactic chemical evolution.
Miras are useful tracers of stellar populations in the Milky Way because their distances can be determined accurately based on period-luminoisty relation. We have been conducting a large-scale survey of the northern disk using Kiso Wide-Field Camera attached to Schmidt telescope at Kiso observatory. The KISOGP (KWFC Intensive Survey of the Galactic Plane) project have made 40-70 epoch observations in I-band of about 320 sq. degrees for over 3 years starting in 2012. In the data analysis so far, we detected more than 700 Miras with periods between 100 and 600 days, approximately 90% of which were not previously reported as variable stars. Preliminary estimates of distance locate these Miras in heavily-reddened regions in the disk with few Miras previously known in the distance range of 2 to 8 kpc but some being further than 10 kpc. We are also carrying out follow-up observations: (1) low-resolution optical/IR spectroscopy for classification of carbon-rich and oxygen-rich Miras, and (2) search for SiO maser emissions to study their mass-loss features and radial velocities. In this presentation, we'll present some characteristics of these Miras based on the various data mentioned above and discuss what they tell us about the structure and evolution of the Galactic disk.
Ka Tat Wong
We present the recent ALMA data of Mira (omi Cet) which show its radio photosphere, extended atmosphere, and inner wind at an unprecedented detail. Mira was observed in the 2014 ALMA Long Baseline Campaign with baselines up to 15 km. The data produce images of SiO and H2O emission/absorption at an angular resolution of ~30 mas at 220 GHz, which clearly resolve the wind of this prototypical Mira variable within the dust condensation radius. Very unique in the dataset is that molecular transition lines are seen in *absorption* towards the continuum source, even in lines which are dominated by maser emission, allowing detailed studies of physical conditions and chemistry along the line of sight. We have modelled the 28SiO J=5-4 v=0, 2 and H2O ν2=1 J(Ka,Kc)=5(5,0)-6(4,3) emission and absorption with the aim to understand the spatial structures of Mira’s extended atmosphere, dust condensation process, shock dissipation, and the kinematics. The results challenge previous hydrodynamic and dust-formation models of Mira and Mira variables.
The Milky Way is the one spiral galaxy that we can study in great detail. The advent of Gaia has over the past decades created a lot of activity to produce the necessary ground-based spectroscopic surveys that will complete Gaia. The role of cool stars in this context is of particular interest as they give the map of the properties of the Galaxy in terms of stellar ages and elemental abundances. I will discuss some of the past surveys, give example results from the on-going surveys and explore the potential of the future surveys such as those conducted by WEAVE, DESI, and 4MOST.
Over the past five years, the old assumption that metal-poor stars exist only in the Galactic halo has been shown to be false. Several observational campaigns have succeeded in finding very metal-poor giant stars within the confines of the Galactic bulge, and following the principle that the Milky Way formed “inside-out”, there is significant theoretical weight behind the idea that these stars are the oldest in the Galaxy. By studying the chemistry of these stars, we can gain insight into the earliest stages of the Milky Way’s formation, including what the very first stars of the Galaxy would have looked like. In this talk I will present the latest findings of the EMBLA survey, which has successfully identified more than 500 RGB stars in the bulge with [Fe/H]<-2. The survey has observed 50 with high-resolution spectrographs, and have found some peculiar chemical differences in them compared to the younger metal-poor stars found in the halo. This includes a lack of stars with the large carbon enhancement that is characteristic of the lowest metallicity objects in the halo. We have also been able to confirm – for a small subsample of our stars – that the majority are indeed on tightly-bound orbits, rather than passing through the bulge region on typical eccentric halo star orbits. This discovery confirms that these stars live permanently in the bulge, and that the bulge we see today has grown around them. We are following up some 200 of these stars with Kepler K2 Campaign 9, from which we will get highly accurate stellar parameters, and be able to derive ages. We hope the statistical age of all 200 stars will then confirm that we have truly found the remnants of the first stars of the Milky Way.
The Gemini Planet Imager (GPI) is a next-generation coronagraphic integral field unit with the sensitivity and resolution to detect planetary companions with separations of 0.2” to 1.0” around a large set of stars. An 890-hour GPI survey of 600 young, nearby stars commenced in late-2014, and approximately 200 stars have been observed thus far. The central aims of the program are: (1) the discovery of a population of giant planets with orbital radii of 5-50 AU comparable to Solar System gas giant orbits, (2) the characterization of the atmospheric properties of young planetary companions, and (3) spatially resolved imaging of debris disks. Initial results from GPI exoplanet observations include the discoveries of new cool companions including a planet and brown dwarf, and a number of resolved debris disks exhibiting a range of structures. An overview of the survey scope, current detection limits, and initial results will be presented.
Brown Dwarfs (BDs) present a link between stars and planets, and thus are important for our understanding of both star and planet formation and evolution. The formation of BDs is a matter of considerable dispute. They might have formed just like stars, which would imply a continuous extension of the IMF into the sub-stellar regime. Or they might form via other channels like disk fragmentation and photo-evaporation, which might imply an increase of BDs in denser star clusters like globular clusters. Although large surveys undertaken in the past decade have detected large numbers of BDs, we still do not know much about old, metal-poor BDs. This is where globular clusters come in: they are massive, and thus might have produced BDs in large numbers, and they are also the oldest and most metal-poor stellar aggregates in our Galaxy. In this talk, we present an analysis of deep HST/WFC3 near-IR (NIR) imaging data of the globular cluster M4. The best-photometry NIR colour-magnitude diagram (CMD) clearly shows the main sequence extending towards the expected end of the Hydrogen-burning limit and going beyond this point towards fainter sources. As such, this is the deepest NIR CMD of a globular cluster to date. Archival HST optical data were used for proper-motion cleaning of the CMD and for distinguishing the white dwarfs (WDs) from BD candidates. Comparing our observed CMDs with theoretical models, we conclude that we have reached beyond the H-burning limit in our NIR CMD and are probably just above or around this limit in our optical-NIR CMDs. We visually inspected the positions of all NIR sources which are fainter than the (NIR) H-burning limit and conclude that we found in total four good BD candidates.