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An activity indicator, which can provide a robust quantitative mapping between the stellar activity and the physical properties of its atmosphere, is important in exploring the physics of activity across spectral types. But the common activity indicators show large variability in their values which makes defining a robust quantitative scale difficult. Millimetre (mm) wavelengths probe the different atmospheric layers within the stellar chromosphere providing a tomographic view of the atmospheric dynamics. The project aims to define a robust mm-based activity indicator for the cool main-sequence stars ($\mathrm{T_{eff}} \sim$ 5000 - 7000 K). We derive the mm-brightness temperature ($\mathrm{T_B(\nu)}$) spectral indices ($\mathrm{\alpha_{mm}}$) for cool stars including the Sun using archival data in the 30 - 1000 GHz range. The derived values for $\mathrm{\alpha_{mm}}$ are explored as a function of various physical parameters and empirical power-law functions were derived. $\mathrm{\alpha_{mm}}$ estimates were also compared with other activity indicators. Despite the estimation errors, $\mathrm{\alpha_{mm}}$ values could well distinguish the cool stars, unlike common activity indicators. The low estimation errors on the derived trends of $\mathrm{\alpha_{mm}}$ versus physical parameters suggest that $\mathrm{\alpha_{mm}}$ could be a robust activity indicator. $\mathrm{\alpha_{mm}}$, which is linked to chromospheric thermal stratification and activity in cool stars can well distinguish and physically characterise the stars more robustly than common activity indicators. We emphasise the need for multi-frequency data across the mm-band for stars, with a range of physical parameters and gathered at multiple epochs during activity cycles. This will help explore $\mathrm{\alpha_{mm}}$ in a statistically robust manner and study the emergence of chromospheric heating on the main-sequence., Comment: Accepted in A&A Letters

The design and construction of CARMENES has been presented at previous SPIE conferences. It is a next-generation radial-velocity instrument at the 3.5m telescope of the Calar Alto Observatory, which was built by a consortium of eleven Spanish and German institutions. CARMENES consists of two separate échelle spectrographs covering the wavelength range from 0.52 to 1.71¿m at a spec-tral resolution of R < 80,000, fed by fibers from the Cassegrain focus of the telescope. CARMENES saw ¿First Light¿ on Nov 9, 2015. During the commissioning and initial operation phases, we established basic performance data such as throughput and spectral resolution. We found that our hollow-cathode lamps are suitable for precise wavelength calibration, but their spectra contain a number of lines of neon or argon that are so bright that the lamps cannot be used in simultaneous exposures with stars. We have therefore adopted a calibration procedure that uses simultaneous star / Fabry Pérot etalon exposures in combination with a cross-calibration between the etalons and hollow-cathode lamps during daytime. With this strategy it has been possible to achieve 1-2 m/s precision in the visible and 5-10 m/s precision in the near-IR; further improvements are expected from ongoing work on temperature control, calibration procedures and data reduction. Comparing the RV precision achieved in different wavelength bands, we find a ¿sweet spot¿ between 0.7 and 0.8¿m, where deep TiO bands provide rich RV information in mid-M dwarfs. This is in contrast to our pre-survey models, which predicted comparatively better performance in the near-IR around 1¿m, and explains in part why our near-IR RVs do not reach the same precision level as those taken with the visible spectrograph. We are now conducting a large survey of 340 nearby M dwarfs (with an average distance of only 12pc), with the goal of finding terrestrial planets in their habitable zones. We have detected the signatures of several previously known or suspected planets and also discovered several new planets. We find that the radial velocity periodograms of many M dwarfs show several significant peaks. The development of robust methods to distinguish planet signatures from activity-induced radial velocity jitter is therefore among our priorities. Due to its large wavelength coverage, the CARMENES survey is generating a unique data set for studies of M star atmospheres, rotation, and activity. The spectra cover important diagnostic lines for activity (H alpha, Na I D1 and D2, and the Ca II infrared triplet), as well as FeH lines, from which the magnetic field can be inferred. Correlating the time series of these features with each other, and with wavelength-dependent radial velocities, provides excellent handles for the discrimination between planetary companions and stellar radial velocity jitter. These data are also generating new insight into the physical properties of M dwarf atmospheres, and the impact of activity and flares on the habitability of M star planets. © 2018 SPIE., CARMENES is an instrument for the Centro Astronomico Hispano-Aleman de Calar Alto (CAHA, Almeria, Spain). CARMENES is funded by the German Max-Planck-Gesellschaft (MPG), the Spanish Consejo Superior de Investigaciones Cientificas (CSIC), the European Union through FEDER/ERF FICTS-2011-02 funds, and the members of the CARMENES Consortium (Max-Planck-Institut fur Astronomie, Instituto de Astrofisica de Andalucia, Landessternwarte Konigstuhl, Institut de Ciencies de l'Espai, Insitut fur Astrophysik Gottingen, Universidad Complutense de Madrid, Thuringer Landessternwarte Tautenburg, Instituto de Astrofisica de Canarias, Hamburger Sternwarte, Centro de Astrobiologia and Centro Astronomico Hispano-Aleman), with additional contributions by the Spanish Ministry of Science, the German Science Foundation through the Major Research Instrumentation Program and DFG Research Unit FOR2544 "Blue Planets around Red Stars", the Klaus Tschira Stiftung, the states of Baden-Wurttemberg and Niedersachsen, and by the Junta de Andalucia.

We aim to understand the correlation between cloud formation and alkali line formation in substellar atmospheres.We perform line profile calculations for Na I and K I based on the coupling of our kinetic model for the formation and composition of dust grains with 1D radiative transfer calculations in atmosphere models for brown dwarfs and giant gas planets. The Na I and K I line profiles sensibly depend on the way clouds are treated in substellar atmosphere simulations. The kinetic dust formation model results in the highest pseudo-continuum compared to the limiting cases., Comment: 5 pages, Accepted for publication in MNRAS

We present plane-parallel and spherical LTE and NLTE atmospheric models of a variety of stellar parameters of the red giant star Arcturus (alpha Boo, HD124897, HR5340) and study their ability to fit the measured absolute flux distribution. Our NLTE models include tens of thousands of the strongest lines in NLTE, and we investigate separately the effects of treating the light metals and the Fe group elements Fe and Ti in NLTE. We find that the NLTE effects of Fe group elements on the model structure and flux distribution are much more important than the NLTE effects of all the light metals combined, and serve to substantially increases the violet and near UV flux level as a result of NLTE Fe over-ionization. Both the LTE and NLTE models predict significantly more flux in the blue and UV bands than is observed. We find that within the moderately metal-poor metallicity range, the effect of NLTE on the overall UV flux level decreases with decreasing metallicity. These results suggest that there may still be important UV opacity missing from the models. We find that models of solar metallicity giants of similar spectral type to Arcturus fit well the observed flux distributions of those stars from the red to the near UV band. This suggests that the blue and near UV flux discrepancy is metallicity dependent, increasing with decreasing metallicity.

We present VLT FORS1 spectroscopy that temporally resolves the second caustic crossing of the Bulge K giant source of microlensing event EROS 2000-BLG-5, the first time this has been accomplished for several phases of a caustic transit. The ~1 angstrom H-alpha equivalent width of the source star increases slightly as the center of the star egresses the caustic and then plummets by 30% during the final limb crossing. These changes are not seen in contemporaneous spectra of control stars in the FORS1 slit, but are qualitatively consistent with expectations from stellar atmosphere models as the caustic differentially magnifies different portions of the stellar face of the target. Observations such as these in a variety of stellar lines are equivalent to atmospheric tomography and are expected to provide a direct test of stellar models., 15 pages, including 1 table and 4 figures. As accepted by ApJ Letters, vol 550

We present comparisons which show good agreement between observed and synthetic spectra for water vapour transitions in a range of M dwarfs, The observations were made from 2.85 to 3.40 mu m where water vapour transitions are strong in cool stars but relatively weak in the Earth's atmosphere, allowing reliable observations to be made, The synthetic spectra were computed using a stellar atmosphere code and include preliminary ab initio calculations for re-vibrational bands up to J = 30, Synthetic spectra indicate that changes in metallicity and gravity have a small effect on the strength of the observed water bands whereas temperature changes produce large differences in strength, Formally, we find similar effective temperatures to those found in previous work. However, since the molecular opacity at the peak of the flux distribution is not well determined, uncertainties in the model atmosphere structure and the effective temperature scale remain.Detailed line profiles can be modelled for atomic lines because their damping constants are known, but they are not known for molecular transitions, Atomic lines computed with Voigt profiles and Van der Waals pressure broadening give an averaged full width half maximum of around 50 km s(-1), For the observed water vapour transitions to match this generation of synthetic spectra we use Gaussian profiles with a full width half maximum of 2 km s(-1) to model the pressure broadening of water vapour transitions, Examination of the model structure indicates that water vapour lines are formed relatively high in the photosphere at pressures about an order of magnitude lower than those of atomic lines, These results strongly suggest that water vapour transitions are not pressure broadened sufficiently to overlap; as previously assumed when modelling molecular transitions in cool dwarfs using the Just Overlapping Line Approximation, The inferred lack of pressure broadening allows flux to escape between water lines, even within a region of strong water vapour absorption, and leads to weaker water band strengths. We demonstrate that this result is likely to explain much of the past discrepancy between observed and theoretical spectral energy distributions for M dwarfs

We present our latest DRIFT‐PHOENIX model results for dust cloud formation in cool atmospheres and investigate the influence of the metallicity [M/H] on the dust cloud structure on the example of a typical mid‐L dwarf (Teff = 2000 K, log (g) = 5.0). Our new dust model simulates the formation of TiO2 seed particles (nucleation) and their subsequent gravitational settling, accompanied by kinetic growth and evaporation of seven solid species on top of these seeds. The gas phase depletion by each of the considered surface reactions and the element replenishment by convective up‐mixing are consistently coupled into this mechanism. The result is a stationary cloud structure in phase‐non‐equilibrium. We observe a comparably low decrease of the dust particle number density and a weak increase of the mean grain size in the lower dust cloud for decreasing metallicities down to [M/H] = −4.0. For even lower metallicities to [M/H] = −6.0, these trends are reversed.

The inner gaseous regions of protoplanetary disks are of special interest in the formation and evolution of planets and stars because they are the likely birthplaces of planets and serve as the accretion reservoir for young stars. The study of inner disks may give rise to a better understanding of the dynamics, physical and chemical structure, and gas content of the region. As a first step, we have developed a 1+1D disk radiative transfer package as an extension to the well established multipurpose stellar atmosphere program PHOENIX. The solution of the equations of momentum and energy conservation as well as the radiative transfer equation is adopted for the physical conditions in and the geometry of disks. Irradiation by the central star is treated in detail. Comparison of our models with high‐resolution infrared spectra will enable us to constrain the structure, dynamics, and gas content of disks, and thus give new insights on the physical processes governing star and planet formation. Additionally, we...

The physical conditions in a variety of objects (e.g. hot exoplanets and close binaries) are fundamentally influenced by external irradiation. In static cases this leads to the development of zones of different temperatures on the ‘day‐side’. In order to combine spectra from all zones to a full visible circular disk and to obtain a 1.5D spectrum we need to calculate the weight of each patch. In the following we present the geometrical considerations and calculate the observed projected area of constant temperature in an irradiated object for specific re‐radiation angles. This allows non‐isotropic models to be used. We supply an IDL code to calculate the observed projected area for any patch given the phase and angle between surface and line of sight as well as a proper weighting for each by numerical integration. We end up with a simple approach to upgrade a 1D irradiation model to a quasi 1.5D one. This method can be applied e.g. to irradiated secondaries in close binaries.

We present our code for the calculation of the 1D structure and synthetic spectra of accretion disks. The code is an extension of the well‐tested stellar atmosphere code PHOENIX and is therefore capable of including large lists of atomic and molecular lines as well as a large set of dust species. We assume the standard accretion disk model for geometrically thin disks and solve the radiative transfer equation in the vertical direction for a number of disk rings with different radii. The combination of these rings yields the total disk spectrum. Comparison to observations of the T Tauri star GQ Lup shows the capability of our code. Additionally, we will show first results of 3D radiative transfer calculations. We plan to investigate the effect of rotating disks on the line profile by means of a two‐level atom.

Continuum and line blanketing models for the photospheres of novae in the early stages of their outbursts are presented. The expanding envelopes are characterized by a very slow increase of density with decreasing radius which leads to very large geometrical extensions and large temperature differences between the inner and outer parts. The spectra show a large IR excess and a small Balmer jump which may be either in absorption or in emission. For the parameters considered (Teff = 104, 1.5 × 104, 2 × 104K, R = 1011 cm, solar composition), most lines are in absorption. The effects of both modifications in the temperature structure (e.g. by heating from shock fronts) and changes in the abundances of the heavy elements on the emergent spectra are briefly discussed.

We have analyzed the early optically thick ultraviolet spectra of Nova OS And 1986 using a grid of spherically symmetric, non-LTE, line-blanketed, expanding model atmospheres and synthetic spectra with the following set of parameters: $5,000\le$ T$_{model}$ $\le 60,000$K, solar abundances, $\rho \propto r^{-3}$, $\v_{max} = 2000\kms$, $L=6 \times 10^{4}\Lsun$, and a statistical or microturbulent velocity of 50 $\kms$. We used the synthetic spectra to estimate the model parameters corresponding to the observed {\it IUE} spectra. The fits to the observations were then iteratively improved by changing the parameters of the model atmospheres, in particular T$_{model}$ and the abundances, to arrive at the best fits to the optically thick pseudo-continuum and the features found in the {\it IUE} spectra. The {\it IUE} spectra show two different optically thick subphases. The earliest spectra, taken a few days after maximum optical light, show a pseudo-continuum created by overlapping absorption lines. The later observations, taken approximately 3 weeks after maximum light, show the simultaneous presence of allowed, semi-forbidden, and forbidden lines in the observed spectra. Analysis of these phases indicate that OS And 86 had solar metallicities except for Mg which showed evidence of being underabundant by as much as a factor of 10. We determine a distance of 5.1 kpc to OS And 86 and derive a peak bolometric luminosity of $\sim$ 5 $\times$ 10$^4$ L$_{\odot}$. The computed nova parameters provide insights into the physics of the early outburst and explain the spectra seen by {\it IUE}. Lastly, we find evidence in the later observations for large non-LTE effects of Fe{\sc ii} which, when included, lead to much better agreement with the observations., Comment: 10 pages, latex, MNRASTEX, figures not included, full text plus figures available at http://brian.la.asu.edu or at ftp://brian.la.asu.edu/pub/preprints/NLTE_OSAnd_86.ps, MNRAS, in press