PHOENIX Model Atmosphere Research Articles

Estimating fundamental parameters of nearby M dwarfs from SPIRou spectra

ABSTRACT We present the results of a study aiming at retrieving the fundamental parameters of M dwarfs from spectra secured with SPIRou, the near-infrared high-resolution spectropolarimeter installed at the Canada–France–Hawaii Telescope (CFHT), in the framework of the SPIRou Legacy Survey (SLS). Our study relies on comparing observed spectra with two grids of synthetic spectra, respectively, computed from PHOENIX and MARCS model atmospheres, with the ultimate goal of optimizing the precision at which fundamental parameters can be determined. In this first step, we applied our technique to 12 inactive M dwarfs with effective temperatures (Teff) ranging from 3000 to 4000 K. We implemented a benchmark to carry out a comparison of the two models used in this study. We report that the choice of model has a significant impact on the results and may lead to discrepancies in the derived parameters of 30 K in Teff and 0.05 dex to 0.10 dex in surface gravity (log g) and metallicity ($\rm {[M/H]}$), as well as systematic shifts of up to 50 K in Teff and 0.4 dex log g and $\rm {[M/H]}$. The analysis is performed on high signal-to-noise ratio template SPIRou spectra, averaged over multiple observations corrected from telluric absorption features and sky lines, using both a synthetic telluric transmission model and principal component analysis. With both models, we retrieve Teff, log g, and $\rm {[M/H]}$ estimates in good agreement with reference literature studies, with internal error bars of about 30 K, 0.05 dex, and 0.1 dex, respectively.

Long Term Evolution of Surface Features on the Red Supergiant AZ Cyg

Abstract We present H-band interferometric observations of the red supergiant (RSG) AZ Cyg that were made with the Michigan Infra-Red Combiner (MIRC) at the six-telescope Center for High Angular Resolution Astronomy (CHARA) Array. The observations span 5 yr (2011–2016), which offers insight into the short and long-term evolution of surface features on RSGs. Using a spectrum of AZ Cyg obtained with SpeX on the NASA InfraRed Telescope Facility (IRTF) and synthetic spectra calculated from spherical MARCS, spherical PHOENIX, and SAtlas model atmospheres, we derive T eff is between 3972 K and 4000 K and log g between −0.50 and 0.00, depending on the stellar model used. Using fits to the squared visibility and GAIA parallaxes, we measure its average radius R = 911 − 50 + 57 R ⊙ . Reconstructions of the stellar surface using our model-independent imaging codes SQUEEZE and OITOOLS.jl show a complex surface with small bright features that appear to vary on a timescale of less than one year and larger features that persist for more than one year. The 1D power spectra of these images suggest a characteristic size of 0.52–0.69 R ⋆ for the larger, long lived features. This is close to the values of 0.51–0.53 R ⋆ that are derived from 3D RHD models of stellar surfaces. We conclude that interferometric imaging of this star is in line with predictions of 3D RHD models but that short-term imaging is needed to more stringently test predictions of convection in RSGs.

The First Radio Spectrum of a Rapidly Rotating A-type Star

Abstract The radio spectra of main-sequence stars remain largely unconstrained due to the lack of observational data to inform stellar atmosphere models. As such, the dominant emission mechanisms at long wavelengths, how they vary with spectral type, and how much they contribute to the expected brightness at a given radio wavelength are still relatively unknown for most spectral types. We present radio continuum observations of Altair, a rapidly rotating A-type star. We observed Altair with NOEMA in 2018 and 2019 at 1.34, 2.09, and 3.22 mm and with the Very Large Array in 2019 at 6.7 and 9.1 mm. In the radio spectra, we see a brightness temperature minimum at millimeter wavelengths followed by a steep rise to temperatures larger than the optical photosphere, behavior that is unexpected for A-type stars. We use these data to produce the first submillimeter to centimeter spectrum of a rapidly rotating A-type star informed by observations. We generated both PHOENIX and KINICH-PAKAL model atmospheres and determine the KINICH-PAKAL model better reproduces Altair’s radio spectrum. The synthetic spectrum shows a millimeter brightness temperature minimum followed by significant emission over that of the photosphere at centimeter wavelengths. Together, these data and models show how the radio spectrum of an A-type star can reveal the presence of a chromosphere, likely induced by rapid rotation, and that a Rayleigh Jean’s extrapolation of the stellar photosphere is not an adequate representation of a star’s radio spectrum.

Dayside thermal inversion in the atmosphere of WASP-19b

Context. Observations of ultra-hot Jupiters indicate the existence of thermal inversion in their atmospheres, with dayside temperatures greater than 2200 K. Various physical mechanisms such as non-local thermal equilibrium, cloud formation, disequilibrium chemistry, ionisation, hydrodynamic waves, and associated energy have been omitted in previous spectral retrievals, while they play an important role in the thermal structure of their upper atmospheres. Aims. We aim to explore the atmospheric properties of WASP-19b to understand its largely featureless thermal spectra using a state-of-the-art atmosphere code that includes a detailed treatment of the most important physical and chemical processes at play in such atmospheres. Methods. We used the one-dimensional line-by-line radiative transfer code PHOENIX in its spherical symmetry configuration including the BT-Settl cloud model and C/O disequilibrium chemistry to analyse the observed thermal spectrum of WASP-19b. Results. We find evidence for a thermal inversion in the dayside atmosphere of the highly irradiated ultra-hot Jupiter WASP-19b, with Teq ~ 2700 K. At these high temperatures we find that H2O dissociates thermally at pressures below 10−2 bar. The inverted temperature-pressure profiles of WASP-19b show evidence of CO emission features at 4.5 μm in its secondary eclipse spectra. Conclusions. We find that the atmosphere of WASP-19b is thermally inverted. We infer that the thermal inversion is due to the strong impinging radiation. We show that H2O is partially dissociated in the upper atmosphere above about τ = 10−2, but is still a significant contributor to the infrared opacity, dominated by CO. The high-temperature and low-density conditions cause H2O to have a flatter opacity profile than in non-irradiated brown dwarfs. Altogether these factors make H2O more difficult to identify in WASP-19b. We suggest that the state-of-the-art PHOENIX model atmosphere code is well suited to the study of this new class of extrasolar planets, ultra-hot Jupiters.

Physics of Eclipsing Binaries. IV. The Impact of Interstellar Extinction on the Light Curves of Eclipsing Binaries

Traditionally, the effects of interstellar extinction on binary star light curves have been treated as a uniform reduction in the observed brightness of the system that is independent of the orbital phase. However, unless the orbital plane of the system coincides with the plane of the sky, or if the two stars are completely identical and present with minimal mutual irradiation and tidal/rotational distortions, then this is unlikely to be an accurate representation of the effect of interstellar extinction. Here, we present an updated treatment of interstellar extinction as incorporated in the PHOEBE 2.2 release (publicly available from http://phoebe-project.org) and assess the importance of using such an approach in the modeling of different types of binary systems. We also present the incorporation of PHOENIX model atmospheres into the PHOEBE 2.2 release, providing increased fidelity on computed observables down to lower temperatures than previously available. The importance of these new code developments is then highlighted via an extincted toy model of the eclipsing white-dwarf–subdwarf binary SDSS J235524.29+044855.7, demonstrating that, in the age of the Rubin Observatory Legacy Survey of Space and Time as well as complementary space-based photometric missions, a proper accounting for extinction and as well as the use of realistic model atmospheres will be essential in deriving accurate binary parameters.

Characterizing stellar parameters from high-resolution spectra of main sequence cool stars. I. The G2V–K2V stars

Abstract The goal of the present study is to construct, test, and validate a high-resolution synthetic spectral library using PHOENIX model atmospheres and develop a reliable tool to estimate stellar parameters from high-resolution optical and/or near-infrared spectra of M dwarfs. We report here the preliminary results of tests characterizing main sequence G–K stars from high-resolution spectra. We anchored the atomic line-list using the stellar standards Sun, ξ Boo A, and ϵ Eri to ensure the synthetic spectra computed with PHOENIX reproduce their observed counterparts. These stars were chosen because their parameters are very well characterized, and on which the absolute accuracy of our method depends on. We successfully estimated the stellar parameters with associated error bars for 17 stars. Using a pseudo Monte Carlo statistical analysis, we present overall improved uncertainties on the stellar parameters compared to those in the literature (on average 9 K, 0.014 dex, and 0.008 dex for the effective temperature, the surface gravity, and the metallicity, respectively). Our estimated stellar parameters are also in good agreement with values found in the literature.

Extremely Irradiated Hot Jupiters: Non-oxide Inversions, H− Opacity, and Thermal Dissociation of Molecules

Abstract Extremely irradiated hot Jupiters, exoplanets reaching dayside temperatures >2000 K, stretch our understanding of planetary atmospheres and the models we use to interpret observations. While these objects are planets in every other sense, their atmospheres reach temperatures at low pressures comparable only to stellar atmospheres. In order to understand our a priori theoretical expectations for the nature of these objects, we self-consistently model a number of extreme hot Jupiter scenarios with the PHOENIX model atmosphere code. PHOENIX is well-tested on objects from cool brown dwarfs to expanding supernovae shells, and its expansive opacity database from the UV to far-IR make PHOENIX well-suited to understanding extremely irradiated hot Jupiters. We find several fundamental differences between hot Jupiters at temperatures >2500 K and their cooler counterparts. First, absorption by atomic metals like Fe and Mg, molecules including SiO and metal hydrides, and continuous opacity sources like H−, all combined with the short-wavelength output of early-type host stars, result in strong thermal inversions, without the need for TiO or VO. Second, many molecular species, including H2O, TiO, and VO are thermally dissociated at pressures probed by transit and eclipse observations, potentially biasing retrieval algorithms that assume uniform vertical abundances. We discuss other interesting properties of these objects, as well as future prospects and predictions for observing and characterizing this unique class of astrophysical object, including the first self-consistent model of the hottest known Jovian planet, KELT-9b.

Stellar and Planetary Characterization of the Ross 128 Exoplanetary System from APOGEE Spectra

Abstract The first detailed chemical abundance analysis of the M-dwarf (M4.0) exoplanet-hosting star Ross 128 is presented here, based upon near-infrared (1.5–1.7 μm), high-resolution (R ∼ 22,500) spectra from the SDSS Apache Point Galactic Evolution Experiment survey. We determined precise atmospheric parameters T eff = 3231 ± 100 K, log g = 4.96 ± 0.11 dex and chemical abundances of eight elements (C, O, Mg, Al, K, Ca, Ti, and Fe), finding Ross 128 to have near solar metallicity ([Fe/H] = +0.03 ± 0.09 dex). The derived results were obtained via spectral synthesis (1D LTE) adopting both MARCS and PHOENIX model atmospheres; stellar parameters and chemical abundances derived from the different adopted models do not show significant offsets. Mass–radius modeling of Ross 128b indicates that it lies below the pure-rock composition curve, suggesting that it contains a mixture of rock and iron, with the relative amounts of each set by the ratio of Fe/Mg. If Ross 128b formed with a subsolar Si abundance, and assuming the planet’s composition matches that of the host star, it likely has a larger core size relative to the Earth despite this producing a planet with a Si/Mg abundance ratio ∼34% greater than the Sun. The derived planetary parameters—insolation flux (S Earth = 1.79 ± 0.26) and equilibrium temperature (T eq = 294 ± 10 K)—support previous findings that Ross 128b is a temperate exoplanet in the inner edge of the habitable zone.

Planetary host stars: evaluating uncertainties in cool model atmospheres

M-dwarfs are emerging in the literature as promising targets for detecting low-mass, Earth-like planets. An important step in this process is to determine the stellar parameters of the M-dwarf host star as accurately as possible. Different well-tested stellar model atmosphere simulations from different groups are widely applied to undertake this task. This paper provides a comparison of different model atmosphere families to allow a better estimate of systematic errors on host-star stellar parameter introduced by the use of one specific model atmosphere family only. We present a comparison of the ATLAS9, MARCS, Phoenix and Drift-Phoenix model atmosphere families including the M-dwarf parameter space (T$_{\rm eff}=2500$K$\,...\,$4000K, log(g)=3.0$\,...\,$5.0, [M/H]=$-2.5\,...\,0.5$). We examine the differences in the (T$_{\rm gas}$, p$_{\rm gas}$)-structures, in synthetic photometric fluxes and in colour indices. Model atmospheres results for higher log(g) deviate considerably less between different models families than those for lower log(g) for all T$_{\rm eff}=2500$K$\,...\,$4000K examined. We compiled the broad-band synthetic photometric fluxes for all available model atmospheres (incl. M-dwarfs and brown dwarfs) for the UKIRT WFCAM ZYJHK, 2MASS JHKs and Johnson UBVRI filters, and calculated related colour indices. Synthetic colours in the IR wavelengths diverge by no more than 0.15 dex amongst all model families. For all spectral bands considered, model discrepancies in colour diminish for higher T$_{\rm eff}$ atmosphere simulations. We notice differences in synthetic colours between all model families and observed example data (incl. Kepler 42 and GJ1214).

THE κ ANDROMEDAE SYSTEM: NEW CONSTRAINTS ON THE COMPANION MASS, SYSTEM AGE, AND FURTHER MULTIPLICITY

Kappa Andromedae is a B9IVn star at 52 pc for which a faint substellar companion separated by 55 AU was recently announced. In this work, we present the first spectrum of the companion, "kappa And B," using the Project 1640 high-contrast imaging platform. Comparison of our low-resolution YJH-band spectra to empirical brown dwarf spectra suggests an early-L spectral type. Fitting synthetic spectra from PHOENIX model atmospheres to our observed spectrum allows us to constrain the effective temperature to ~2000K, as well as place constraints on the companion surface gravity. Further, we use previously reported log(g) and effective temperature measurements of the host star to argue that the kappa And system has an isochronal age of 220 +/- 100 Myr, older than the 30 Myr age reported previously. This interpretation of an older age is corroborated by the photometric properties of kappa And B, which appear to be marginally inconsistent with other 10-100 Myr low-gravity L-dwarfs for the spectral type range we derive. In addition, we use Keck aperture masking interferometry combined with published radial velocity measurements to rule out the existence of any tight stellar companions to kappa And A that might be responsible for the system's overluminosity. Further, we show that luminosity enhancements due to a nearly "pole-on" viewing angle coupled with extremely rapid rotation is unlikely. Kappa And A is thus consistent with its slightly evolved luminosity class (IV) and we propose here that kappa And, with a revised age of 220 +/- 100 Myr, is an interloper to the 30 Myr Columba association with which it was previously associated. The photometric and spectroscopic evidence for kappa And B combined with our re-assesment of the system age implies a substellar companion mass of 50^{+16}_{-13} Jupiter Masses, consistent with a brown dwarf rather than a planetary mass companion.

The atmospheric structure and fundamental parameters of the red supergiants AH Scorpii, UY Scuti, and KW Sagittarii

We present the atmospheric structure and the fundamental properties of the red supergiants (RSGs) AH Sco, UY Sct, and KW Sgr based on VLTI/AMBER observations. We carried out spectro-interferometric observations of AH Sco, UY Sct, and KW Sgr in the near-infrared K band with the VLTI/AMBER instrument, and compared the data to a new grid of hydrostatic PHOENIX model atmospheres. In our visibility data, we observe molecular layers of water and CO in extended atmospheres. For a uniform disk modeling, we observe size increases at the water band of 10% to 25% and at the CO bandheads of 20%-35% with respect to the near continuum bandpass. The PHOENIX atmosphere models predict the spectra and the continuum visibility values, but cannot reproduce the large extensions of the molecular layers. This indicates that the opacities of the molecular bands are included, but that the model atmospheres are too compact compared to the observations. The observed extended layers may be levitated by processes such as pulsation or convection, which are not included in the hydrostatic atmospheric models. Comparing the continuum visibility values to PHOENIX models, we estimate the Rosseland-mean photospheric angular diameters. Together with the distance and the spectro-photometry, we calculate radii and effective temperatures. The location of the targets in the HR-diagram is confirmed to be close to, and possibly slightly to the right of, the Hayashi limit of recent evolutionary tracks corresponding to masses between about 20 Msun and 40 Msun

Ca II H+K fluxes from S-indices of large samples: a reliable and consistent conversion based on PHOENIX model atmospheres

Context. Historic stellar activity data based on chromospheric line emission using O.C. Wilson’s S-index reach back to the 1960ies and represent a very valuable data resource both in terms of quantity and time-coverage. However, these data are not flux-calibrated and are therefore difficult to compare with modern spectroscopy and to relate to quantitative physics. Aims. In order to make use of the rich archives of Mount Wilson and many other S-index measurements of thousands of main sequence stars, subgiants and giants in terms of physical Ca ii H+K line chromospheric surface fluxes and the related R-index, we seek a new, simple but reliable conversion method of the S-indices. A first application aims to obtain the (empirical) basal chromospheric surface flux to better characterise stars with minimal activity levels. Methods. We collect 6024 S-indices from six large catalogues from a total of 2530 stars with well-defined parallaxes (as given by the Hipparcos catalogue) in order to distinguish between main sequence stars (2133), subgiants (252) and giants (145), based on their positions in the Hertzsprung-Russell diagram. We use the spectra of a grid of PHOENIX model atmospheres to obtain the photospheric contributions to the S-index. To convert the latter into absolute Ca ii H+K chromospheric line flux, we first derive new, colour-dependent photospheric flux relations for, each, main sequence, subgiant and giant stars, and then obtain the chromospheric flux component. In this process, the PHOENIX models also provide a very reliable scale for the physical surface flux. Results. For very large samples of main sequence stars, giants and subgiants, we obtain the chromospheric Ca ii H+K line surface fluxes in the colour range of 0.44 < B −V < 1.6 and the related R-indices. We determine and parametrize the lower envelopes, which we find to well coincide with historic work on the basal chromospheric flux. There is good agreement in the apparently simpler cases of inactive giants and subgiants, and distinguishing different luminosity classes proves important. Main sequence stars, surprisingly, show a remarkable lack of inactive chromospheres in the B − V range of 1.1 to 1.5. Finally, we intoduce a new, “pure” and universal activity indicator: a derivative of the R-index based on the non-basal, purely activity-related Ca ii H+K line surface flux, which puts different luminosity classes on the same scale. Conclusions. The here presented conversion method can be used to directly compare historical S-indices with modern chromospheric Ca ii H+K line flux measurements, in order to derive activity records over long periods of time or to establish the long-term variability of marginally active stars, for example. The numerical simplicity of this conversion allows for its application to very large stellar samples.

SPECTRAL TYPING OF LATE-TYPE STELLAR COMPANIONS TO YOUNG STARS FROM LOW-DISPERSION NEAR-INFRARED INTEGRAL FIELD UNIT DATA

We used the Project 1640 near-infrared coronagraph and integral field spectrograph to observe 19 young solar type stars. Five of these stars are known binary stars and we detected the late-type secondaries and were able to measure their JH spectra with a resolution of R\sim30. The reduced, extracted, and calibrated spectra were compared to template spectra from the IRTF spectral library. With this comparison we test the accuracy and consistency of spectral type determination with the low-resolution near-infrared spectra from P1640. Additionally, we determine effective temperature and surface gravity of the companions by fitting synthetic spectra calculated with the PHOENIX model atmosphere code. We also present several new epochs of astrometry of each of the systems. Together these data increase our knowledge and understanding of the stellar make up of these systems. In addition to the astronomical results, the analysis presented helps validate the Project 1640 data reduction and spectral extraction processes and the utility of low-resolution, near-infrared spectra for characterizing late-type companions in multiple systems.

PHYSICAL PROPERTIES OF YOUNG BROWN DWARFS AND VERY LOW MASS STARS INFERRED FROM HIGH-RESOLUTION MODEL SPECTRA

By comparing near-infrared spectra with atmosphere models, we infer the effective temperature, surface gravity, projected rotational velocity, and radial velocity for 21 very-low-mass stars and brown dwarfs. The unique sample consists of two sequences in spectral type from M6-M9, one of 5-10 Myr objects and one of >1 Gyr field objects. A third sequence is comprised of only ~M6 objects with ages ranging from <1 Myr to >1 Gyr. Spectra were obtained in the J band at medium (R~2,000) and high (R~20,000) resolutions with NIRSPEC on the Keck II telescope. Synthetic spectra were generated from atmospheric structures calculated with the PHOENIX model atmosphere code. Using multi-dimensional least-squares fitting and Monte Carlo routines we determine the best-fit model parameters for each observed spectrum and note which spectral regions provide consistent results. We identify successes in the reproduction of observed features by atmospheric models, including pressure-broadened KI lines, and investigate deficiencies in the models, particularly missing FeH opacity, that will need to be addressed in order to extend our analysis to cooler objects. The precision that can be obtained for each parameter using medium- and high- resolution near-infrared spectra is estimated and the implications for future studies of very low mass stars and brown dwarfs are discussed.

6Li in the Atmosphere of GJ 117 Revisited

Detection of 6Li has been shown for energetic solar events, one chromospherically active binary, and several dwarf halo stars. We had previously found a 6Li/7Li = 0.03 ± 0.01 for active K dwarf GJ 117 using VLT UVES observations. Here we present high signal-to-noise (>1000) high spectral resolution observations taken with the McDonald Observatory's 2.7 m and echelle spectrometer of GJ 117. We have used the solar spectrum and template stars to eliminate possible blends, such as Ti I, in the 6Li spectral region. Our new analysis, using an updated PHOENIX model atmosphere, finds 6Li/7Li = 0.05 ± 0.02. In addition, bisector analysis showed no significant red asymmetries that would affect the lithium line profile. No changes above the statistical uncertainties are found between the VLT and McDonald data. The amount of 6Li derived for GJ 117 is consistent with creation in spallation reactions on the stellar surface, but we caution that uncertainties in the continuum level may cause additional uncertainty in the 6Li fraction.

Dense Spot Coverage and Polar Caps on SV Cam

We have used spectrophotometric data from nine Hubble Space Telescope orbits to eclipse-map the primary component of the RS CVn binary SV Cam. From these observations and its HIPPARCOS parallax we find that the surface flux in the eclipsed low-latitude region is about 30% lower than computed from the best fitting PHOENIX model atmosphere. This flux deficit can only be accounted for if about a third of the primary’s surface is covered with unresolved spots. Even when we extend the spottedness from the eclipsed region to the entire surface, there still remains an unaccounted flux deficit. This remaining flux deficit is explained by the presence of a large polar spot extending down to latitude 42 ± 6°.

Interferometric Constraints on Gravity Darkening with Application to the Modeling of Spica A &amp; B

AbstractIn 2005 we obtained very precise interferometric measurements of the pole-on rapid rotator Vega (A0 V) with the longest baselines of the Center for High Angular Angular Resolution (CHARA) Array and the Fiber Linked Unit for Optical Recombination (FLUOR). For the analysis of these data, we developed a code for mapping sophisticated PHOENIX model atmospheres on to the surface of rotationally distorted stars described by a Roche-von Zeipel formalism. Given a set of input parameters for a star or binary pair, this code predicts the interferometric visibility, spectral energy distribution and high-resolution line spectrum expected for the system. For the gravity-darkened Vega, our model provides a very good match to the K-band interferometric data, a good match to the spectral energy distribution – except below 160 nm – and a rather poor match to weak lines in the high dispersion spectrum where the model appears overly gravity darkened. In 2006, we used the CHARA Array and FLUOR to obtain high precision measurements of the massive, non-eclipsing, double-line spectroscopic binary Spica, a 4-day period system where both components are gravity darkened rapid rotators. These data supplement recent data obtained with the Sydney University Stellar Interferometer (SUSI). Our study follows the classic 1971 study by Herbison-Evans et al. who resolved Spica as a binary with the Narrabri Stellar Intensity Interferometer (NSII). We will report on our progress modeling the new interferometric and archival spectroscopic data, with the goal towards better constraining the apsidal constant.

Hubble Space Telescope observations of SV Cam - II. First derivative light-curve modelling using phoenix and atlas model atmospheres

The variation of the specific intensity across the stellar disc is an essential input parameter in surface brightness reconstruction techniques such as Doppler imaging, where the relative intensity contributions of different surface elements are important in detecting star-spots. We use PHOENIX and ATLAS model atmospheres to model light curves derived from high precision (signal-to-noise ratio ≃ 5000) Hubble Space Telescope (HST) data of the eclipsing binary SV Cam (F9V+K4V), where the variation of specific intensity across the stellar disc will determine the contact points of the binary system light curve. For the first time, we use Χ 2 comparison fits to the first derivative profiles to determine the best-fitting model atmosphere. We show the wavelength dependence of the limb darkening and that the first derivative profile is sensitive to the limb-darkening profile very close to the limb of the primary star. It is concluded that there is only a marginal difference (<1σ) between the Χ 2 comparison fits of the two model atmospheres to the HST light curve at all wavelengths. The usefulness of the second derivative of the light curve for measuring the sharpness of the primary's limb is investigated, but we find that the data are too noisy to permit a quantitative analysis.

Direct Evidence for a Polar Spot on SV Camelopardalis

We have used spectrophotometric data from the Hubble Space Telescope (HST) to eclipse-map the primary component of the RS CVn binary SV Cam over nine HST orbits. We find from these observations and the Hipparcos parallax that the surface flux in the eclipsed low-latitude region of the primary is about 30% lower than that computed from a PHOENIX model atmosphere at the effective temperature that best fits the spectral energy distribution of the eclipsed flux. This can only be accounted for if about a third of the primary's surface is covered with unresolved dark starspots. Extending this to the full surface of the primary, we find that, even taking into account this spot-filling factor, there is an additional flux deficit on the primary star. This can only be explained if there is a large polar spot on the primary star extending from the pole to latitude 48° ± 6°.

Tests of stellar model atmospheres by optical interferometry

We present K-band interferometric measurements of the limb-darkened intensity profile of the M4 giant star psi Phoenicis obtained with VLTI/VINCI. High-precision squared visibility amplitudes in the second lobe of the visibility function were obtained employing two 8.2 m UTs. This succeeded one month after light from UTs was first combined for interferometric fringes. In addition, we sampled the visibility function at small spatial frequencies using the 40cm test siderostats. Our measurement constrains the diameter of the star as well as its CLV. We construct a spherical hydrostatic PHOENIX model atmosphere based on spectrophotometric data from the literature and confront its CLV prediction with our interferometric measurement. We compare as well CLV predictions by plane-parallel hydrostatic PHOENIX, ATLAS9, and ATLAS12 models. We find that the Rosseland angular diameter as predicted by comparison of the spherical PHOENIX model with spectrophotometry is in good agreement with our interferometric diameter measurement. The shape of our measured visibility function in the second lobe is consistent with all considered PHOENIX and ATLAS model predictions, and significantly different from UD and FDD models. We derive high-precision fundamental parameters for psi Phe, namely a Rosseland angular diameter of 8.13 +/- 0.2 mas, with the Hipparcos parallax corresponding to a Rosseland linear radius R of 86 +/- 3 Rsun and an effective temperature of 3550 +/- 50 K, with R corresponding to a luminosity of log (L/Lsun)=3.02 +/- 0.06. Together with evolutionary models, these values are consistent with a mass of 1.3 +/- 0.2 Msun, and a surface gravity of log g = 0.68 +/- 0.11.