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Recently the Spitzer Space Telescope observed the transiting extrasolar planets, TrES-1 and HD209458b. These observations have provided the first estimates of the day side thermal flux from two extrasolar planets orbiting Sun-like stars. In this paper, synthetic spectra from atmospheric models are compared to these observations. The day-night temperature difference is explored and phase-dependent flux densities are predicted for both planets. For HD209458b and TrES-1, models with significant day-to-night energy redistribution are required to reproduce the observations. However, the observational error bars are large and a range of models remains viable., Comment: 8 pages, 7 figures, accepted for publication in the Astrophysical Journal

(abridged) We present an analysis of high resolution optical spectra for a sample of very young, mid- to late M, low-mass stellar and substellar objects in Upper Sco and Taurus. Effective temperatures (T_eff) and surface gravities are derived from a multi-feature spectral analysis using TiO, NaI and KI, through comparison with the latest synthetic spectra. In combination, they allow us to determine T_eff to within 50K and gravity to within 0.25 dex. Our high-resolution analysis does not require extinction estimates. Moroever, it yields T_eff and gravities independent of theoretical evolutionary models. We find that our gravities for most of the sample agree remarkably well with the isochrone predictions for the likely cluster ages. However, discrepancies appear in our coolest targets: these appear to have significantly lower gravity (by up to 0.75 dex) than our hotter objects, even though our entire sample covers a relatively narrow range in T_eff (about 300K). This drop in gravity is also implied by inter-comparisons of the data alone, without recourse to synthetic spectra. We consider, and argue against, dust opacity, cool stellar spots or metallicity differences leading to the observed spectral effects; a real decline in gravity is strongly indicated. Such gravity variations are contrary to the predictions of the evolutionary tracks, causing improbably low ages to be inferred from the tracks for our coolest targets. We venture that these results may arise from evolutionary model uncertainties related to accretion, deuterium-burning and/or convection effects. Finally, when combined with photometry and distances, our technique for deriving gravities and temperatures provides a way of obtaining masses and radii for substellar objects independent of evolutionary models, as presented in Paper II., 74 pages, incl. 11 figures, accepted ApJ

We present HST/WFPC2 observations of a sample of 134 ultra-cool objects (spectral types later than M7) coming from the DENIS, 2MASS and SDSS surveys, with distances estimated to range from 7 pc to 105 pc. Fifteen new ultra-cool binary candidates are reported here. Eleven known binaries are confirmed and orbital motion is detected in some of them. We estimate that the closest binary systems in this sample have periods between 5 and 20 years, and thus dynamical masses will be derived in the near future. For the calculation of binary frequency we restrict ourselves to systems with distances less than 20 pc. After correction of the binaries bias, we find a ratio of visual binaries (at the HST limit of detection) of around 10%, and that ~15% of the 26 objects within 20 parsecs are binary systems with separations between 1 and 8 A.U. The observed frequency of ultra-cool binaries is similar than that of binaries with G-type primaries in the separation range from 2.1 A.U. to 140 A.U. There is also a clear deficit of ultra-cool binaries with separations greater than 15 A.U., and a possible tendency for the binaries to have mass ratios near unity. Most systems have indeed visual and near-infrared brightness ratios between 1 and 0.3. We discuss our results in the framework of current scenarios for the formation and evolution of free-floating brown dwarfs.

The recent announcement that sodium absorption has been observed in the atmosphere of HD209458b, the only EGP observed to transit its parent star, is the first direct detection of an EGP atmosphere. We explore the possibility that neutral sodium is {\em not} in local thermodynamic equilibrium (LTE) in the outer atmosphere of irradiated EGPs and that the sodium concentration may be underestimated by models that make the LTE assumption. Our results indicate that it may not be necessary to invoke excessive photoionization, low metallicity, or even high altitude clouds to explain the observations.

Model atmospheres for cool giant stars are usually computed with detailed equation of state and opacity calculations that include many molecular species. We show that models of oxygen-rich stars with effective temperatures below 3000 K must also include dust grains. The effects of dust condensation in the equation of state and on the opacity must be included to obtain accurate temperature structures and spectra.

We have modeled irradiated planets located near a dM5 and a G2 primary star. The impinging radiation field was explicitly included in the solution of the radiative transfer equation and in the computation of the atmospheric structure. We find that large errors in both the thermal and reflected flux will result from models which do not include the impinging radiation in a self-consistent manner. A cool (teff = 500K) and a hot (teff = 1000K) planet were modeled at various orbital separations from both the dM5 and the G2 primary. In all scenarios, we compared the effects of the irradiation in two extreme cases: one where dust clouds form and remain suspended in the atmosphere, and another where dust clouds form but completely settle out of the atmosphere. The atmospheric structure and emergent spectrum strongly depend on the presence or absence of dust clouds. We find that, in the absence of dust opacity, the impinging radiation significantly alters the innermost layers of an EGP atmosphere and that they are actually brighter in the optical than dusty planets. Our models also indicate that the planet-to-star brightness ratio in the optical will be less that 1 x 10^-5 for objects like tau bootis which is consistent with recently reported upper limit values., 19 pages, 15 figures

We present analyses of high resolution and medium resolution spectra of early L dwarfs. We used our latest set of model atmospheres to reproduce and analyze the observed features. We can model the optical flux and the atomic line profiles with the best accuracy to date. The models used to reproduce the observations include dust condensation and dust opacities. Compared to previous studies using older models we find that our dust treatment is much improved. The derived parameters for the objects are well in the expected range for old very low mass objects. This is also supported by the absence of Li in most of the objects. For the objects showing Li we can be almost certain that those are brown dwarfs. However, a spectral analysis in general, and this one in particular can only very roughly determine mass and age., AASTeX5.0. 26 pages, including all figures, Accepted for ApJ

We present the extension of our NextGen model atmosphere grid to the regime of giant stars. The input physics of the models presented here is nearly identical to the NextGen dwarf atmosphere models, however spherical geometry is used self-consistently in the model calculations (including the radiative transfer). We re-visit the discussion of the effects of spherical geometry on the structure of the atmospheres and the emitted spectra and discuss the results of NLTE calculations for a few selected models., Comment: ApJ, in press (November 1999), 13 pages, also available at http://dilbert.physast.uga.edu/~yeti/PAPERS and at ftp://calvin.physast.uga.edu/pub/preprints/NG-giants.ps.gz

Long thought by some researchers to be an oddity, GD 165B has instead proven to be the first example of a class of very cool objects (the L dwarfs) which, due to dust formation in their photosphere, lack the dominant bands of TiO seen in warmer M dwarfs.

We present our NextGen Model Atmosphere grid for low mass stars for effective temperatures larger than $3000\K$. These LTE models are calculated with the same basic model assumptions and input physics as the VLMS part of the NextGen grid so that the complete grid can be used, e.g., for consistent stellar evolution calculations and for internally consistent analysis of cool star spectra. This grid is also the starting point for a large grid of detailed NLTE model atmospheres for dwarfs and giants (Hauschildt et al, in preparation). The models were calculated from $3000\K$ to $10000\K$ (in steps of $200\K$) for $3.5 \le \logg \le 5.5$ (in steps of 0.5) and metallicities of $-4.0 \le \mh \le 0.0$. We discuss the results of the model calculations and compare our results to the Kurucz 1994 grid. Some comparisons to standard stars like Vega and the Sun are presented and compared with detailed NLTE calculations.

We present phase-resolved near-infrared broadband photometry of four short-period cataclysmic variables (HU Aqr, WZ Sge, TY Psc, and V592 Cas). Coupled with ultraviolet and optical data obtained from the literature, we have modeled the spectral energy distributions of these four cataclysmic variables, as well as that of the twin of WZ Sge, AL Com. The secondary stars contribute no more than 20%-50% of the near-infrared flux except for the polar HU Aqr, where the secondary contributes ~75% of the near-infrared flux. For the systems located above the orbital period minimum, the temperatures of the secondary stars match those for the expected main-sequence secondary stars. However, our modeling places WZ Sge below the orbital period minimum and shows it containing a secondary star of less than 1700 K—the coldest "star" yet identified.

A 20'' × 20'' field located ~7' from the center of the massive galactic globular cluster ω Centauri (NGC 5139) was observed by the NIC2 camera of the Near-Infrared Camera and Multiobject Spectrometer on board the Hubble Space Telescope (HST) through the F110W and F160W broadband filters centered at 1.1 and 1.6 μm for a total of 3000 and 4000 s for the two filters, respectively. Standard photometric analysis of the resulting images yields 340 stars with a signal above a 10 σ threshold in both filters, covering the range of HST m160 magnitudes between 20 and 26, the deepest probe yet of a globular cluster in this wavelength region. These objects form a well-defined sequence in the m160 versus m110-m160 plane that is consistent with the theoretical near-IR color-magnitude diagram expected from recent low-mass stellar model calculations. The resulting stellar luminosity function increases steadily with increasing magnitude up to a peak at m16025, where it turns over and drops slowly down to the detection limit set by the incompleteness limit of 60% at m16026. With the theoretical mass-luminosity relationship that provides the best fit to the IR color-magnitude diagram, we obtain an excellent fit to the observed luminosity function down to a mass of ~0.2 M with a power-law mass function having a slope of α=-1.

We present detailed NLTE Ti I calculations in model atmospheres of cool dwarf and giant stars. A fully self-consistent NLTE treatment for a Ti I model atom with 395 levels and 5279 primary bound-bound transitions is included, and we discuss the implication of departures from LTE in this atom for the strengths of Ti I lines and TiO molecular bands in cool star spectra. We show that in the atmospheric parameter range investigated, LTE is a poor approximation to Ti I line formation, as expected from the low collisional rates in cool stars. The secondary effects of Ti I overionization on the TiO number density and the TiO molecular opacities, however, are found to be negligible in the molecular line-forming region for the relatively small parameter range studied in this paper.

We present preliminary nongray model atmospheres and interiors for cool brown dwarfs. The resulting synthetic spectra are compared to available spectroscopic and photometric observations of the coolest brown dwarf yet discovered, Gl 229B (Nakajima et al.). Despite the grainless nature of the present models, we find that the resulting synthetic spectra provide an excellent fit to most of the spectral features of the brown dwarf. We confirm the presence of methane absorption and the substellar nature of Gl 229B. These preliminary models set an upper limit for the effective temperature of 1000 K. We also compute the evolution of brown dwarfs with solar composition and masses from 0.02 to 0.065 M☉. While uncertainties in the age of the system yield some indeterminateness for the mass of Gl 229B, the most likely solution is m ≈ 0.04-0.055 M☉. In any case, we can set an upper limit m = 0.065 M☉ for a very unlikely age, t = 10 Gyr.

We present the discovery of a co-moving planetary-mass companion ~42" (~2000 AU) from a young M3 star, GU Psc, likely member of the young AB Doradus Moving Group (ABDMG). The companion was first identified via its distinctively red i - z color (> 3.5) through a survey made with Gemini-S/GMOS. Follow-up Canada-France-Hawaii Telescope/WIRCam near-infrared (NIR) imaging, Gemini-N/GNIRS NIR spectroscopy and Wide-field Infrared Survey Explorer photometry indicate a spectral type of T3.5+-1 and reveal signs of low gravity which we attribute to youth. Keck/Adaptive Optics NIR observations did not resolve the companion as a binary. A comparison with atmosphere models indicates Teff = 1000-1100 K and logg = 4.5-5.0. Based on evolution models, this temperature corresponds to a mass of 9-13 MJup for the age of ABDMG (70-130 Myr). The relatively well-constrained age of this companion and its very large angular separation to its host star will allow its thorough characterization and will make it a valuable comparison for planetary-mass companions that will be uncovered by forthcoming planet-finder instruments such as Gemini Planet Imager and SPHERE., 18 p., 10 fig., published in ApJ. May 20th, 2014: Small corrections compared to the journal version: 1) addition of the an important reference to 1RSX 1609-2105b from Lafreni\`ere et al. 2008, 2010, 2) update of the url for Andrew Mann metallicity calibration, 3) correction of a few factual mistakes in section 2.2.4), 4) aesthetic reformatting of urls

The atmospheres of novae at early times in their outbursts are very extended, expanding shells with low densities. Models of these atmospheres show that NLTE effects are very important and must be included in realistic calculations. We have, therefore, been improving our atmospheric studies by increasing the number of ions treated in NLTE. One of the most important ions is Fe II which has a complex structure and numerous lines in the observable spectrum. In this paper we investigate NLTE effects for Fe II for a wide variety of parameters. We use a detailed Fe II model atom with 617 level and 13675 primary lines, treated using a rate-operator formalism. We show that the radiative transfer equation in nova atmospheres {\em must} be treated with sophisticated numerical methods and that simple approximations, such as the Sobolev method, {\em cannot} be used because of the large number of overlapping lines in the co-moving frame. Our results show that the formation of the Fe II lines is strongly affected by NLTE effects. For low effective temperatures, $\Teff < 20,000\,$K, the optical Fe II lines are most influenced by NLTE effects, while for higher $\Teff$ the UV lines of Fe II are very strongly affected by NLTE. The departure coefficients are such that Fe II tends to be overionized in NLTE when compared to LTE. Therefore, Fe II-NLTE must be included with sophisticated radiative transfer in nova atmosphere models in order to reliably analyze observed nova spectra., 25 pages, latex, AASTEX, figures not included, full text plus figures available via http://brian.la.asu.edu/ or at ftp://brian.la.asu.edu/FeII-nova.tgz, to appear in ApJ

We have calculated a grid of more than 700 model atmospheres valid for a wide range of parameters encompassing the coolest known M~dwarfs, M~subdwarfs and brown dwarf candidates: $1500\le \teff \le 4000\,$K, $3.5\le \log(g)\le 5.5$, and $-4.0\le { [M/H]}\le +0.5$. Our equation of state includes 105 molecules and up to 27 ionization stages of 39 elements. In the calculations of the base grid of model atmospheres presented here, we include over 300 molecular bands of 4 molecules (TiO, VO, CaH, FeH) in the JOLA approximation, the water opacity of Ludwig (1971), collision induced opacities, b-f and f-f atomic processes, as well as about 2 million spectral lines selected from a list with more than 42 million atomic and 24 million molecular (H$_2$, CH, NH, OH, MgH, SiH, C$_2$, CN, CO, SiO) lines. High-resolution synthetic spectra are obtained using an opacity sampling method. The model atmospheres and spectra are calculated with the generalized stellar atmosphere code PHOENIX, assuming LTE, plane-parallel geometry, energy (radiative plus convective) conservation, and hydrostatic equilibrium. The model spectra give close agreement with observations of M~dwarfs across a wide spectral range from the blue to the near-IR, with one notable exception: the fit to the water bands. We discuss several practical applications of our model grid, e.g., broadband colors derived from the synthetic spectra. In light of current efforts to identify genuine brown dwarfs, we also show how low-resolution spectra of cool dwarfs vary with surface gravity, and how the high-resolution line profile of the Li~I resonance doublet depends on the Li abundance.

In this paper we describe the evolution of Nova Cas 1993 over the first two months of its outburst. We present an ultraviolet light curve that covers the period from announcement to just after dust began forming in the ejecta (1994 Feb. 15) and International Ultraviolet Explorer (IUE) spacecraft constraints forced us to halt our observations. We have used spherical, expanding, Non-local Thermodynamic Equilibrium (NLTE) stellar atmospheres to compute synthetic spectra and have compared the results to combined ultraviolet (low-resolution 1200-3400 A and high-resolution 2400-3300 A) spectra. Our fits show that the effective temperature of the ejecta increased from approx. 8000 to about approx. 16 000 K between 1993 Dec. 12 and 1993 Dec. 26. The temperature then increased more slowly to approx. 24 000 on 1994 Jan. 28. A preliminary abundance analysis shows evidence for hydrogen depletion, as we also found for Nova V1974 Cygni; however we find a larger enhancement of carbon, nitrogen, and oxygen. We also show that the principal mechanism for mass ejection in this nova is a radiation pressure driven wind and that mechanical driving is not necessary.