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Stellar, substellar, and planetary atmosphere models are all highly sensitive to the input opacities. Generational differences between various state-of-the-art stellar/planetary models are primarily because of incomplete and outdated atomic/molecular line-lists. Here we present a database of pre-computed absorption cross-sections for all isotopologues of key atmospheric molecules relevant to late-type stellar, brown dwarf, and planetary atmospheres: MgH, AlH, CaH, TiH, CrH, FeH, SiO, TiO, VO, and H2O. The pressure and temperature ranges of the computed opacities are between 10$^{-6}$--3000~bar and 75--4000~K, and their spectral ranges are 0.25--330~$\mu$m for many cases where possible. For cases with no pressure-broadening data, we use collision theory to bridge the gap. We also probe the effect of absorption cross-sections calculated from different line lists in the context of Ultra-Hot Jupiter and M-dwarf atmospheres. Using 1-D self-consistent radiative-convective thermochemical equilibrium models, we report significant variations in the theoretical spectra and thermal profiles of substellar atmospheres. With a 2000 K representative Ultra-Hot Jupiter, we report variations of up to 320 and 80 ppm in transmission and thermal emission spectra, respectively. For a 3000 K M-dwarf, we find differences of up to 125$\%$ in the spectra. We find that the most significant differences arise due to the choice of TiO line-lists, primarily below 1$\mu$m. In sum, we present (1) a database of pre-computed molecular absorption cross-sections, and (2) quantify biases that arise when characterizing substellar/exoplanet atmospheres due to line list differences, therefore highlighting the importance of correct and complete opacities for eventual applications to high precision spectroscopy and photometry., Comment: accepted in ApJS. 44 pages, 17 figures, 7 tables. Comments are welcome

Non-rocky sub-Jovian exoplanets in high-irradiation environments are rare. LTT 9779b, also known as Transiting Exoplanet Survey Satellite (TESS) object of interest (TOI) 193.01, is one of the few such planets discovered to date, and the first example of an ultrahot Neptune. The planet’s bulk density indicates that it has a substantial atmosphere, so to investigate its atmospheric composition and shed further light on its origin, we obtained Spitzer InfraRed Array Camera secondary eclipse observations of LTT 9779b at 3.6 and 4.5 μm. We combined the Spitzer observations with a measurement of the secondary eclipse in the TESS bandpass. The resulting secondary eclipse spectrum strongly prefers a model that includes CO absorption over a blackbody spectrum, incidentally making LTT 9779b the first TESS exoplanet (and the first ultrahot Neptune) with evidence of a spectral feature in its atmosphere. We did not find evidence of a thermal inversion, at odds with expectations based on the atmospheres of similarly irradiated hot Jupiters. We also report a nominal dayside brightness temperature of 2305 ± 141 K (based on the 3.6 μm secondary eclipse measurement), and we constrained the planet’s orbital eccentricity to e

V838 Mon erupted at the beginning of 2002 becoming an extremely luminous star. Among various scenarios proposed to explain the nature of the outburst the most promising is a stellar merger event. In this paper we investigate the observational properties of the star and its surroundings in the post outburst phase. We have obtained a high resolution optical spectrum of V838 Mon in October 2005 using the Keck I telescope.We have identified numerous atomic features and molecular bands present in the spectrum and provided an atlas of those features. In order to improve the spectrum interpretation we have performed simple modeling of the molecular bands. Our analysis indicates that the spectrum is dominated by molecular absorption features arising in photospheric regions with temperatures of ~2400 K and in colder outer layers, where the temperature decreases down to ~500 K. A number of resonance lines of neutral alkali metals are observed to show P-Cyg profiles. Particularly interesting are numerous prominent emission lines of [FeII]. All of them show practically the same profile, which can be well described by a Lorentzian profile. In the blue part of the spectrum photospheric signatures of the B-type companion are easily seen. We have fitted the observed spectrum with a synthetic one and the obtained parameters are consistent with the B3V type. We have also estimated radial and rotational velocities of the companion., Comment: accepted to ApJS, 21 pages, 15 figures; a paragraph about photometry has been added, typos corrected

High-resolution and high-S/N observations of the strong and relatively narrow λ5797 diffuse interstellar absorption band, recorded with the Ultra-High-Resolution Facility at the 3.9 m Anglo-Australian Telescope and the 0.9 m coude feed of the Kitt Peak National Observatory, are reported. For selected lines of sight and using resolving powers of up to 600,000, a remarkable degree of intrinsic ultrafine structure is found within the band. There exists an almost exact correspondence between the structure in the spectra recorded toward μ Sgr, ζ Per, and ζ Oph, including one particularly narrow component with a width that is comparable to the widths of atomic and molecular absorption lines along these lines of sight. The results provide a new and extremely exacting test against which theoretical or laboratory candidates can be assessed, and they point to a new approach to studies of diffuse band carrier distributions through their velocity signatures.

We report the detection of the 21 cm line of neutral hydrogen in absorption at a redshift of 2.6365 toward the gravitationally lensed radio source MG J0414+0534. The absorption shows two velocity components, each with a width of ~150 km s-1, separated by about 200 km s-1. We determine a total H I column density of 7.5±1.3×1018 (Ts) cm-2 assuming an H I covering factor of unity. Assuming a Galactic dust-to-gas ratio and that all the reddening toward the source is due to dust associated with the observed H I 21 cm absorption line, we derive an upper limit to the H I spin temperature Ts of 1300 K. We do not detect absorption in the HCN (J=1-0) line toward MG J0414+0534 with a 3 σ optical depth limit of 0.047 at 19.2 km s-1 resolution. The implied upper limit on the HCN column density is 1×1013 cm-2. The redshift of the H I absorption centroid differs by 200 km s-1 from the centroid of the recently discovered CO J=3-2 emission from MG J0414+0534. We present two alternative scenarios that explain both the observed velocity offset between the CO emission and the H I absorption and the lack of molecular absorption toward MG J0414+0534. Either (1) the H I 21 cm line is absorbing the lensed active galactic nucleus and the regions giving rise to CO emission have a different spatial distribution than the radio continuum-emitting regions and are affected differently by gravitational lensing or (2) the H I 21 cm absorption is toward an extended, steep-spectrum radio component, which contributes substantially to the radio continuum emission at low frequencies but has a negligible flux density at higher frequencies. This latter model requires free-free absorption toward the steep-spectrum component in order to result in the observed Gigahertz-peaked spectrum of MG J0414+0534's integrated radio emission.

We present 4.5$��$m and 8$��$m photometric observations of 18 cool white dwarfs obtained with the Spitzer Space Telescope. Our observations demonstrate that four white dwarfs with T_eff< 6000 K show slightly depressed mid-infrared fluxes relative to white dwarf models. In addition, another white dwarf with a peculiar optical and near-infrared spectral energy distribution (LHS 1126) is found to display significant flux deficits in Spitzer observations. These mid-infrared flux deficits are not predicted by the current white dwarf models including collision induced absorption due to molecular hydrogen. We postulate that either the collision induced absorption calculations are incomplete or there are other unrecognized physical processes occuring in cool white dwarf atmospheres. The spectral energy distribution of LHS 1126 surprisingly fits a Rayleigh-Jeans spectrum in the infrared, mimicking a hot white dwarf with effective temperature well in excess of 10$^5$ K. This implies that the source of this flux deficit is probably not molecular absorption but some other process., 17 pages, 4 figures, ApJ in press, 10 May 2006

Les observations VLA des spectres d'absorption de OH, H 2 CO, et NH 3 vis-a-vis de la radiosource nucleaire dans la galaxie active Cen A sont presentees. Les observations permettent d'etudier les constituants et les conditions presents dans le gaz du couloir de poussiere, ainsi que la nature et la localisation du gaz decale vers le rouge qui a ete suppose proche, et tombant, dans le noyau.

Les observations spectroscopiques IR-proche de 320 geantes K et M de 6 champs a faible absorption le long de l'axe mineur du bulbe galactique et de 60 naines et geantes de type avance du voisinage solaire sont presentees. L'abondance en metal moyenne des geantes M du bulbe et les contraintes sur la contribution relative des naines et geantes de type avance sur la lumiere integree des galaxies E et S0 sont estimees a partir des indices de force des bandes d'absorption moleculaire et de donnees photometriques optiques et IR.

The results of an observational program aimed at setting up a network of faint near-infrared standards of sufficient accuracy are reported. The network covers both northern and southern hemispheres and includes standards red enough to provide at least a limited check on color transformations. The standards are set up at J (1.2 micron), H (1.6 micron), K (2.2 microns), and L (3.5 microns), and their H2O and CO molecular absorption indices are determined. The problem of color transformations between observatories is discussed briefly. All magnitudes presented are transformed to the natural system defined by the CIT observations.

Comparison of interferometric, photoelectric scans of interstellar absorption-line profiles of Na I, K I, Ca II, and CH(+) toward zeta Ophiuchi. Near a heliocentric velocity of -15 km/sec, there is a strong composite feature showing four components. The atomic and molecular absorption lines do not arise in a common volume of gas but rather in essentially different clouds whose atomic abundances and/or ionization balances vary substantially.