Models of Stars, Brown Dwarfs and Exoplanets

Within the next few years, GAIA and several instruments aiming at imag- ing extrasolar planets will see first light. In parallel, low mass planets are being searched around red dwarfs which offer more favourable conditions, both for radial velocity de- tection and transit studies, than solar-type stars. Authors of the model atmosphere code which has allowed the detection of water vapour in the atmosphere of Hot Jupiters re- view recent advancement in modelling the stellar to substellar transition. The revised solar oxygen abundances and cloud model allow for the first time to reproduce the pho- tometric and spectroscopic properties of this transition. Also presented are highlight results of a model atmosphere grid for stars, brown dwarfs and extrasolar planets.Comment: Refereed paper submitted to the british journal Philosophical Transactions A as an invited review to the Theo Murphy Meeting entitled "Water in the gas phase" held by the Kavli Royal Society in Chichely, GB, June 13-14th 201

High-precision stellar limb-darkening in exoplanetary transits

Characterization of the atmospheres of transiting exoplanets relies on accurate measurements of the extent of the optically thick area of the planet at multiple wavelengths with a precision $\lesssim$100 parts per million (ppm). Next-generation instruments onboard the James Webb Space Telescope (JWST) are expected to achieve $\sim$10 ppm precision for several tens of targets. A similar precision can be obtained in modelling only if other astrophysical effects, including the stellar limb-darkening, are accounted for properly. In this paper, we explore the limits on precision due to the mathematical formulas currently adopted to approximate the stellar limb-darkening, and to the use of limb-darkening coefficients obtained either from stellar-atmosphere models or empirically. We propose a new limb-darkening law with two coefficients, `power-2', which outperforms other two-coefficient laws adopted in the literature in most cases, and particularly for cool stars. Empirical limb-darkening based on two-coefficient formulas can be significantly biased, even if the light-curve residuals are nearly photon-noise limited. We demonstrate an optimal strategy to fitting for the four-coefficients limb-darkening in the visible, using prior information on the exoplanet orbital parameters to break some of the degeneracies that otherwise would prevent the convergence of the fit. Infrared observations taken with the James Webb Space Telescope (JWST) will provide accurate measurements of the exoplanet orbital parameters with unprecedented precision, which can be used as priors to improve the stellar limb-darkening characterization, and therefore the inferred exoplanet parameters, from observations in the visible, such as those taken with Kepler/K2, JWST, other past and future instruments

The role of convection, overshoot, and gravity waves for the transport of dust in M dwarf and brown dwarf atmospheres

Observationally, spectra of brown dwarfs indicate the presence of dust in their atmospheres while theoretically it is not clear what prevents the dust from settling and disappearing from the regions of spectrum formation. Consequently, standard models have to rely on ad hoc assumptions about the mechanism that keeps dust grains aloft in the atmosphere. We apply hydrodynamical simulations to develop an improved physical understanding of the mixing properties of macroscopic flows in M dwarf and brown dwarf atmospheres, in particular of the influence of the underlying convection zone. We performed 2D radiation hydrodynamics simulations including a description of dust grain formation and transport with the CO5BOLD code. The simulations cover the very top of the convection zone and the photosphere including the dust layers for effective temperatures between 900K and 2800K, all with logg=5 assuming solar chemical composition. Convective overshoot occurs in the form of exponentially declining velocities with small scale heights, so that it affects only the region immediately above the almost adiabatic convective layers. From there on, mixing is provided by gravity waves that are strong enough to maintain thin dust clouds in the hotter models. With decreasing effective temperature, the amplitudes of the waves become smaller but the clouds become thicker and develop internal convective flows that are more efficient in mixing material than gravity waves. The presence of clouds leads to a highly structured appearance of the stellar surface on short temporal and small spatial scales. We identify convectively excited gravity waves as an essential mixing process in M dwarf and brown dwarf atmospheres. Under conditions of strong cloud formation, dust convection is the dominant self-sustaining mixing component

Automatisierte Klassifikation Stellarer Spektren im Hamburger Quasar-Survey

Der Hamburger Quasar-Survey (HQS) ist eine großflächige Objektivprismendurchmusterung, für die am Schmidtteleskop des Calar~Alto-Observatoriums in Spanien 14 000 deg² des Nordhimmels auf galaktischen Breiten |b|>20° mit dem 1.°7-Prisma beobachtet wurden. Die photographischen Daten von 1 046 000 blauen Objekten, mit 1 889 000 Schwärzungsspektren von ~40 Å Auflösung und einem S/N von 4-20 wurden in Hamburg im Zuge des Programms zur Suche nach leuchtkräftigen Quasaren extrahiert und digitalisiert. Diese Datenbasis ist als ergiebige Quelle lichtschwacher blauer Sterne erkannt worden und wurde im Rahmen einer Kollaboration zwischen der Hamburger Sternwarte, der Christian-Albrechts-Universität und der Dr. Remeis-Sternwarte Bamberg (Univ. Erlangen) für die Suche nach Weißen Zwergen und anderen Spätphasen der Sternentwicklung zur Verfügung gestellt. Bei spektroskopischen Nachbeobachtungen heißer Sterne, die visuell aus den Prismenspektren ausgewählt wurden, wurden etwa 80 Weiße Zwerge des Spektraltyps DA entdeckt. In einer quantitativen Analyse dieser Spektren sind durch den Fit von Modellatmosphären die atmosphärischen Parameter T_eff und log g bestimmt, und aus diesen unter Verwendung theoretischer Abkühlsequenzen, die eine Masse-Radius-Beziehung herstellen, die Massen und Leuchtkräfte abgeleitet worden. Für eine umfassendere Suche in der vollständigen Datenbasis digitalisierter Spektren sind Programme zur Umwandlung der Daten aus dem an der Hamburger Sternwarte verwendeten Binärformat in das standardisierte FITS-Format entwickelt worden. Basierend auf einer astrometrischen Identifikation sind bekannte Sterne aus Literaturkatalogen Weißer Zwerge und heißer Subdwarfs mit den Spektren abgeglichen worden, um die Vollständigkeit der Datenbasis zu prüfen und ein automatisiertes Suchverfahren für Weiße Zwerge zu entwickeln und zu testen. Diese Suche ist im Rahmen eines Systems zur Spektralanalyse und Klassifikation der stellaren Spektren realisiert worden, das auf einem chi²-Minimierungsverfahren auf einem Modellgitter theoretischer Spektren basiert. Die theoretischen Spektren sind durch Entwicklung eines Verfahrens zur Transformation von flusskalibrierten Spektren in photographische Schwärzungen erzeugt worden, welches unter Verwendung der bekannten heißen Sterne als Eichstandards kalibriert und getestet wurde. Das Programm erlaubt eine schnelle und voll automatisierte Klassifizierung aller stellaren Spektren mit einer ungefähren Bestimmung von log g und T_eff und einer groben Abschätzung der Metallizität von Hauptreihensternen. Weiße Zwerge können anhand der log g-Bestimmung mit ~85% Vollständigkeit ausgewählt werden. Für den Typ DA ist eine Temperaturbestimmung von etwa 20-25% Genauigkeit möglich. Mit diesem Verfahren ist ein Katalog von knapp 3000 Kandidaten für kühle DA mit Temperaturen von etwa 9000-30 000 K erstellt worden. Aus der gleichförmigen Verteilung der Sterne bezüglich der Milchstraße kann eine Kontamination mit helleren Sternen der galaktischen Scheibe zu mehr als etwa 30% ausgeschlossen werden. Die Temperaturen von 1100 der als DA ausgewählten Sterne würden sie in die Nähe des sog. ZZ Ceti-Instabilitätsstreifens, in dem pulsierende Weiße Zwerge auftreten, rücken

Alkali Line Profiles in Degenerate Dwarfs

Ultracool stellar atmospheres show absorption by alkali resonance lines severely broadened by collisions with neutral perturbers. In the coolest and densest atmospheres, such as those of T dwarfs, Na I and K I broadened by molecular hydrogen and helium can come to dominate the entire optical spectrum. Their profiles have been successfully modelled with accurate interaction potentials in the adiabatic theory, computing line profiles from the first few orders of a density expansion of the autocorrelation function. The line shapes in the emergent spectrum also depend on the distribution of absorbers as a function of depth, which can be modelled with improved accuracy by new models of dust condensation and settling. The far red K I wings of the latest T dwarfs still show missing opacity in these models, a phenomenon similar to what has been found for the Na I line profiles observed in extremely cool, metal-rich white dwarfs. We show that the line profile in both cases is strongly determined by multiple-perturber interactions at short distances and can no longer be reproduced by a density expansion, but requires calculation of the full profile in a unified theory. Including such line profiles in stellar atmosphere codes will further improve models for the coolest and densest dwarfs as well as for the deeper atmosphere layers of substellar objects in general.Comment: VI Serbian Conference on Spectral Line Shapes in Astrophysics; to be published by the American Institute of Physics, eds. Milan S. Dimitrijevic and Luka C. Popovic; 6 pages, 6 figure

A featureless transmission spectrum for the Neptune-mass exoplanet GJ 436b

GJ 436b is a warm—approximately 800 kelvin—exoplanet that periodically eclipses its low-mass (half the mass of the Sun) host star, and is one of the few Neptune-mass planets that is amenable to detailed characterization. Previous observations have indicated that its atmosphere has a ratio of methane to carbon monoxide that is 10^5 times smaller than predicted by models for hydrogen-dominated atmospheres at these temperatures. A recent study proposed that this unusual chemistry could be explained if the planet’s atmosphere is significantly enhanced in elements heavier than hydrogen and helium. Here we report observations of GJ 436b’s atmosphere obtained during transit. The data indicate that the planet’s transmission spectrum is featureless, ruling out cloud-free, hydrogen-dominated atmosphere models with an extremely high significance of 48σ. The measured spectrum is consistent with either a layer of high cloud located at a pressure level of approximately one millibar or with a relatively hydrogen-poor (three per cent hydrogen and helium mass fraction) atmospheric composition

A physically motivated and empirically calibrated method to measure effective temperature, metallicity, and Ti abundance of M dwarfs

The ability to perform detailed chemical analysis of Sun-like F-, G-, and K-type stars is a powerful tool with many applications including studying the chemical evolution of the Galaxy and constraining planet formation theories. Unfortunately, complications in modeling cooler stellar atmospheres hinders similar analysis of M-dwarf stars. Empirically-calibrated methods to measure M dwarf metallicity from moderate-resolution spectra are currently limited to measuring overall metallicity and rely on astrophysical abundance correlations in stellar populations. We present a new, empirical calibration of synthetic M dwarf spectra that can be used to infer effective temperature, Fe abundance, and Ti abundance. We obtained high-resolution (R~25,000), Y-band (~1 micron) spectra of 29 M dwarfs with NIRSPEC on Keck II. Using the PHOENIX stellar atmosphere modeling code (version 15.5), we generated a grid of synthetic spectra covering a range of temperatures, metallicities, and alpha-enhancements. From our observed and synthetic spectra, we measured the equivalent widths of multiple Fe I and Ti I lines and a temperature-sensitive index based on the FeH bandhead. We used abundances measured from widely-separated solar-type companions to empirically calibrate transformations to the observed indices and equivalent widths that force agreement with the models. Our calibration achieves precisions in Teff, [Fe/H], and [Ti/Fe] of 60 K, 0.1 dex, and 0.05 dex, respectively and is calibrated for 3200 K < Teff < 4100 K, -0.7 < [Fe/H] < +0.3, and -0.05 < [Ti/Fe] < +0.3. This work is a step toward detailed chemical analysis of M dwarfs at a similar precision achieved for FGK stars.Comment: accepted for publication in ApJ, all synthetic spectra available at http://people.bu.edu/mveyette/phoenix

Epsilon Indi Ba, Bb: a spectroscopic study of the nearest known brown dwarfs

The discovery of Epsilon Indi Ba and Bb, a nearby binary brown dwarf system with a main-sequence companion, allows a concerted campaign to characterise the physical parameters of two T dwarfs providing benchmarks against which atmospheric and evolutionary models can be tested. Some recent observations suggest the models at low mass and intermediate age may not reflect reality with, however, few conclusive tests. We are carrying out a comprehensive characterisation of these, the nearest known brown dwarfs, to allow constraints to be placed upon models of cool field dwarfs. We present broadband photometry from the V- to M-band and the individual spectrum of both components from 0.6-5.1 microns at a resolution of up to R=5000. A custom analytic profile fitting routine was implemented to extract the blended spectra and photometry of both components separated by 0.7 arcsec. We confirm the spectral types to be T1 and T6, and notably, we do not detect lithium at 6708A in the more massive object which may be indicative both of the age of the system and the mass of the components.Comment: 4 pages, 2 figures, to appear in proceedings of Cool Stars 1