Search

The Geostationary Carbon Cycle Observatory (GeoCarb), to be launched in 2023, will be capable of measuring solar-induced chlorophyll fluorescence (SIF) and add to the current space-based record which started in 1995. GeoCarb will be unique as it will be the first geostationary satellite capable of sensing SIF over the American continents. Consequently, SIF measurements from GeoCarb can be performed much more flexibly compared to polar-orbiting platforms. With its scan mirror assembly, the instrument can point to any location on the Earth disc. This will allow measurements to be collected at various times of day, and measurement locations can be re-visited several times within a day. In expectation of the launch, we conduct an extensive, SIF-focused simulation study and explore the capability and limitations of the instrument and its particular sampling approach. Using cloud information from real measurements, as well as other observation- and model-based data, we produce over four million atmospheric simulations of GeoCarb measurements that the instrument would see throughout a full day. We then apply dedicated SIF retrieval algorithms on the simulated spectra and investigate the results along with cloud-screening performance and emergent retrieval biases and subsequent bias correction. Finally, we make comparisons with currently operating instruments where appropriate and show future science users of GeoCarb SIF what a typical day of measurements will yield.

The direct study of molecular gas in inner protoplanetary disks is complicated by uncertainties in the spatial distribution of the gas, the time-variability of the source, and the comparison of observations across a wide range of wavelengths. Some of these challenges can be mitigated with far-ultraviolet spectroscopy. Using new observations obtained with the HST-Cosmic Origins Spectrograph, we measure column densities and rovibrational temperatures for CO and H2 observed on the line-of-sight through the AA Tauri circumstellar disk. CO A-X absorption bands are observed against the far-UV continuum. The CO absorption is characterized by log(N(^{12}CO)) = 17.5 +/- 0.5 cm^{-2} and T_rot(CO) = 500$^{+500}_{-200} K, although this rotational temperature may underestimate the local kinetic temperature of the CO-bearing gas. We also detect ^{13}CO in absorption with an isotopic ratio of ~20. We do not observe H2 absorption against the continuum; however, hot H2 (v > 0) is detected in absorption against the LyA emission line. We measure the column densities in eight individual rovibrational states, determining a total log(N(H2)) = 17.9^{+0.6}_{-0.3} cm^{-2} with a thermal temperature of T(H2) = 2500^{+800}_{-700} K. The high-temperature of the molecules, the relatively small H2 column density, and the high-inclination of the AA Tauri disk suggest that the absorbing gas resides in an inner disk atmosphere. If the H2 and CO are co-spatial within a molecular layer ~ 0.6 AU thick, this region is characterized by ~ 10^{5} cm^{-3} with an observed ratio of ~ 0.4. We also find evidence for a departure from a purely thermal H2 distribution, suggesting that excitation by continuum photons and H2 formation may be altering the level populations in the molecular gas., Comment: 12 pages, 4 figures. ApJ - accepted

We have used archival far-ultraviolet spectra from observations made by HST/STIS and FUSE to determine the column densities and rotational excitation temperatures for CO and H2, respectively, along the lines of sight to 23 Galactic O and B stars. The sightlines have reddening values in the range E(B-V)= 0.07-0.62, sampling the diffuse to translucent interstellar medium. We find that the H2 column densities range from 5x10^18-8x10^20 cm^-2 and the CO from upper limits around 2x10^12 cm^-2 to detections as high as 1.4x10^16 cm^-2. CO increases with increasing H2, roughly following a power law of factor \~2. The CO/H2 column density ratio is thus not constant, and ranges from 10^-7 - 10^-5, with a mean value of 3x10^-6. The sample segregates into "diffuse" and "translucent" regimes, the former having a molecular fraction less than ~0.25 and A_V/d, emualateapj style, 6 figures, 3 tables, accepted on 21 Nov 2006 for publication in The Astrophysical Journal

The reflection nebula NGC 2023 was observed by a rocket-borne long-slit imaging spectrograph in the 900 -- 1400 Angstrom bandpass on 2000 February 11. A spectrum of the star, as well as that of the nebular scattered light, was recorded. Through the use of a Monte Carlo modeling process, the scattering properties of the dust were derived. The albedo is low, 0.2 -- 0.4, and decreasing toward shorter wavelengths, while the phase function asymmetry parameter is consistent with highly forward-scattering grains, g~0.85. The decrease in albedo, while the optical depth increases to shorter wavelengths, implies that the far-UV rise in the extinction curve is due to an increase in absorption efficiency., 16 pages, 11 figures, accepted for publication in the Astrophysical Journal