Mark W. Claire

Stellar SEDs from 0.3 to 2.5 μm: Tracing the Stellar Locus and Searching for Color Outliers in the SDSS and 2MASS

The Sloan Digital Sky Survey (SDSS) and Two Micron All Sky Survey (2MASS) are rich resources for studying stellar astrophysics and the structure and formation history of the Galaxy. As new surveys and instruments adopt similar filter sets, it is increasingly important to understand the properties of the ugrizJHKs stellar locus, both to inform studies of ‘normal’ main sequence stars as well as for robust searches for point sources with unusual colors. Using a sample of � 600,000 point sources detected by SDSS and 2MASS, we tabulate the position and width of the ugrizJHKs stellar locus as a function of g i color, and provide accurate polynomial fits. We map the Morgan-Keenan spectral type sequence to the median stellar locus by using synthetic photometry of spectral standards and by analyzing 3000 SDSS stellar spectra with a custom spectral typing pipeline, described in full in an attached Appendix. Having characterized the properties of ‘normal’ main sequence stars, we develop an algorithm for identifying point sources whose colors differ significantly from those of normal stars. This algorithm calculates a point source’s minimum separation from the stellar locus in a seven-dimensional color space, and robustly identifies objects with unusual colors, as well as spurious SDSS/2MASS matches. Analysis of a final catalog of 2117 color outliers identifies 370 white-dwarf/M dwarf (WDMD) pairs, 93 QSOs, and 90 M giant/carbon star candidates, and demonstrates that WDMD pairs and QSOs can be distinguished on the basis of their J Ks and r z colors. We also identify a group of objects with correlated offsets in the u g vs. g r and g r vs. r i color-color spaces, but subsequent follow-up is required to reveal the nature of these objects. Future applications of this algorithm to a matched SDSS-UKIDSS catalog may well identify additional classes of objects with unusual colors by probing new areas of color-magnitude space. Subject headings: surveys — stars:late-type — stars:early-type — Galaxy:stellar content — infrared:stars

The Evolution of Solar Flux from 0.1 nm to 160 μm: Quantitative Estimates for Planetary Studies

Understanding changes in the solar flux over geologic time is vital for understanding the evolution of planetary atmospheres because it affects atmospheric escape and chemistry, as well as climate. We describe a numerical parameterization for wavelength-dependent changes to the non-attenuated solar flux appropriate for most times and places in the solar system. We combine data from the Sun and solar analogs to estimate enhanced UV and X-ray fluxes for the young Sun and use standard solar models to estimate changing visible and infrared fluxes. The parameterization, a series of multipliers relative to the modern top of the atmosphere flux at Earth, is valid from 0.1 nm through the infrared, and from 0.6 Gyr through 6.7 Gyr, and is extended from the solar zero-age main sequence to 8.0 Gyr subject to additional uncertainties. The parameterization is applied to a representative modern day flux, providing quantitative estimates of the wavelength dependence of solar flux for paleodates relevant to the evolution of atmospheres in the solar system (or around other G-type stars). We validate the code by Monte Carlo analysis of uncertainties in stellar age and flux, and with comparisons to the solar proxies κ1 Cet and EK Dra. The model is applied to the computation of photolysis rates on the Archean Earth.

Pathways for Neoarchean pyrite formation constrained by mass-independent sulfur isotopes

It is generally thought that the sulfate reduction metabolism is ancient and would have been established well before the Neoarchean. It is puzzling, therefore, that the sulfur isotope record of the Neoarchean is characterized by a signal of atmospheric mass-independent chemistry rather than a strong overprint by sulfate reducers. Here, we present a study of the four sulfur isotopes obtained using secondary ion MS that seeks to reconcile a number of features seen in the Neoarchean sulfur isotope record. We suggest that Neoarchean ocean basins had two coexisting, significantly sized sulfur pools and that the pathways forming pyrite precursors played an important role in establishing how the isotopic characteristics of each of these pools was transferred to the sedimentary rock record. One of these pools is suggested to be a soluble (sulfate) pool, and the other pool (atmospherically derived elemental sulfur) is suggested to be largely insoluble and unreactive until it reacts with hydrogen sulfide. We suggest that the relative contributions of these pools to the formation of pyrite depend on both the accumulation of the insoluble pool and the rate of sulfide production in the pyrite-forming environments. We also suggest that the existence of a significant nonsulfate pool of reactive sulfur has masked isotopic evidence for the widespread activity of sulfate reducers in the rock record.

Using biogenic sulfur gases as remotely detectable biosignatures on anoxic planets.

We used one-dimensional photochemical and radiative transfer models to study the potential of organic sulfur compounds (CS(2), OCS, CH(3)SH, CH(3)SCH(3), and CH(3)S(2)CH(3)) to act as remotely detectable biosignatures in anoxic exoplanetary atmospheres. Concentrations of organic sulfur gases were predicted for various biogenic sulfur fluxes into anoxic atmospheres and were found to increase with decreasing UV fluxes. Dimethyl sulfide (CH(3)SCH(3), or DMS) and dimethyl disulfide (CH(3)S(2)CH(3), or DMDS) concentrations could increase to remotely detectable levels, but only in cases of extremely low UV fluxes, which may occur in the habitable zone of an inactive M dwarf. The most detectable feature of organic sulfur gases is an indirect one that results from an increase in ethane (C(2)H(6)) over that which would be predicted based on the planets methane (CH(4)) concentration. Thus, a characterization mission could detect these organic sulfur gases-and therefore the life that produces them-if it could sufficiently quantify the ethane and methane in the exoplanets atmosphere.

ABIOTIC OZONE AND OXYGEN IN ATMOSPHERES SIMILAR TO PREBIOTIC EARTH

The search for life on planets outside our solar system will use spectroscopic identification of atmospheric biosignatures. The most robust remotely detectable potential biosignature is considered to be the detection of oxygen (O2) or ozone (O3) simultaneous to methane (CH4) at levels indicating fluxes from the planetary surface in excess of those that could be produced abiotically. Here we use an altitude-dependent photochemical model with the enhanced lower boundary conditions necessary to carefully explore abiotic O2 and O3 production on lifeless planets with a wide variety of volcanic gas fluxes and stellar energy distributions. On some of these worlds, we predict limited O2 and O3 buildup, caused by fast chemical production of these gases. This results in detectable abiotic O3 and CH4 features in the UV-visible, but no detectable abiotic O2 features. Thus, simultaneous detection of O3 and CH4 by a UV-visible mission is not a strong biosignature without proper contextual information. Discrimination between biological and abiotic sources of O2 and O3 is possible through analysis of the stellar and atmospheric context—particularly redox state and O atom inventory—of the planet in question. Specifically, understanding the spectral characteristics of the star and obtaining a broad wavelength range for planetary spectra should allow more robust identification of false positives for life. This highlights the importance of wide spectral coverage for future exoplanet characterization missions. Specifically, discrimination between true and false positives may require spectral observations that extend into infrared wavelengths and provide contextual information on the planet’s atmospheric chemistry.

High rates of anaerobic methanotrophy at low sulfate concentrations with implications for past and present methane levels.

Despite the importance of the anaerobic oxidation of methane (AOM) to global biogeochemical cycles, the relationship between sulfate concentration and the rate of AOM has not been previously experimentally constrained. Here, we present measurements showing substantial methane oxidation at low sulfate concentrations, with no significant decrease in the rate of AOM until sulfate levels are well below 1 mM. At sulfate levels below 1 mM, there appears to be a strong decoupling of AOM and sulfate reduction, with a (13)C-label transferred from methane to carbon dioxide occurring at a rate almost an order of magnitude faster than the observed rate of sulfate reduction. These results allow for the possibility that high rates of AOM occurred in the Archean oceans and that high rates of AOM may be found in freshwater environments (lakes, rivers, etc.) and deep ocean sediments today.

Using Dimers to Measure Biosignatures and Atmospheric Pressure for Terrestrial Exoplanets

Abstract We present a new method to probe atmospheric pressure on Earth-like planets using (O2-O2) dimers in the near-infrared. We also show that dimer features could be the most readily detectable biosignatures for Earth-like atmospheres and may even be detectable in transit transmission with the James Webb Space Telescope (JWST). The absorption by dimers changes more rapidly with pressure and density than that of monomers and can therefore provide additional information about atmospheric pressures. By comparing the absorption strengths of rotational and vibrational features to the absorption strengths of dimer features, we show that in some cases it may be possible to estimate the pressure at the reflecting surface of a planet. This method is demonstrated by using the O2 A band and the 1.06 μm dimer feature, either in transmission or reflected spectra. It works best for planets around M dwarfs with atmospheric pressures between 0.1 and 10 bar and for O2 volume mixing ratios above 50% of Earths present-...We present a new method to probe atmospheric pressure on Earth-like planets using (O2-O2) dimers in the near-infrared. We also show that dimer features could be the most readily detectable biosignatures for Earth-like atmospheres and may even be detectable in transit transmission with the James Webb Space Telescope (JWST). The absorption by dimers changes more rapidly with pressure and density than that of monomers and can therefore provide additional information about atmospheric pressures. By comparing the absorption strengths of rotational and vibrational features to the absorption strengths of dimer features, we show that in some cases it may be possible to estimate the pressure at the reflecting surface of a planet. This method is demonstrated by using the O2 A band and the 1.06 μm dimer feature, either in transmission or reflected spectra. It works best for planets around M dwarfs with atmospheric pressures between 0.1 and 10 bar and for O2 volume mixing ratios above 50% of Earths present-day level. Furthermore, unlike observations of Rayleigh scattering, this method can be used at wavelengths longer than 0.6 μm and is therefore potentially applicable, although challenging, to near-term planet characterization missions such as JWST. We also performed detectability studies for JWST transit transmission spectroscopy and found that the 1.06 and 1.27 μm dimer features could be detectable (SNR>3) for an Earth analogue orbiting an M5V star at a distance of 5 pc. The detection of these features could provide a constraint on the atmospheric pressure of an exoplanet and serve as biosignatures for oxygenic photosynthesis. We calculated the required signal-to-noise ratios to detect and characterize O2 monomer and dimer features in direct imaging-reflected spectra and found that signal-to-noise ratios greater than 10 at a spectral resolving power of R=100 would be required.

Anaerobic methanotrophy and the rise of atmospheric oxygen

In modern marine sediments, the anoxic decomposition of organic matter generates a significant flux of methane that is oxidized microbially with sulphate under the seafloor and never reaches the atmosphere. In contrast, prior to ca 2.4 Gyr ago, the ocean had little sulphate to support anaerobic oxidation of methane (AOM) and the ocean should have been an important methane source. As atmospheric O2 and seawater sulphate levels rose on the early Earth, AOM would have increasingly throttled the release of methane. We use a biogeochemical model to simulate the response of early atmospheric O2 and CH4 to changes in marine AOM as sulphate levels increased. Semi-empirical relationships are used to parameterize global AOM rates and the evolution of sulphate levels. Despite broad uncertainties in these relationships, atmospheric O2 concentrations generally rise more rapidly and to higher levels (of order approx. 10−3 bar versus approx. 10−4 bar) as a result of including AOM in the model. Methane levels collapse prior to any significant rise in O2, but counter-intuitively, methane re-rises after O2 rises to higher levels when AOM is included. As O2 concentrations increase, shielding of the troposphere by stratospheric ozone slows the effective reaction rate between oxygen and methane. This effect dominates over the decrease in the methane source associated with AOM. Thus, even with the inclusion of AOM, the simulated Late Palaeoproterozoic atmosphere has a climatologically significant level of methane of approximately 50 ppmv.

Ensemble properties of comets in the Sloan Digital Sky Survey

We present the ensemble properties of 31 comets (27 resolved and 4 unresolved) observed by the Sloan Digital Sky Survey (SDSS). This sample of comets represents about 1 comet per 10 million SDSS photometric objects. Five-band (u, g, r, i, z) photometry is used to determine the comets colors, sizes, surface brightness profiles, and rates of dust production in terms of the Afp formalism. We find that the cumulative luminosity function for the Jupiter Family Comets in our sample is well fit by a power law of the form N(<H) {proportional_to} 10{sup (0.49{+-}0.05)H} for H < 18, with evidence of a much shallower fit N(<H) {proportional_to} 10{sup (0.19{+-}0.03)H} for the faint (14.5 < H < 18) comets. The resolved comets show an extremely narrow distribution of colors (0.57 {+-} 0.05 in g - r for example), which are statistically indistinguishable from that of the Jupiter Trojans. Further, there is no evidence of correlation between color and physical, dynamical, or observational parameters for the observed comets.

The ultraviolet, optical, and infrared properties of Sloan Digital Sky Survey sources detected by GALEX

We discuss the ultraviolet, optical, and infrared properties of the Sloan Digital Sky Survey (SDSS) sources detected by the Galaxy Evolution Explorer (GALEX) as part of its All–sky Imaging Survey (AIS) Early Release Observations (ERO). Virtually all (> 99%) of the GALEX sources in the overlap region are detected by SDSS; those without an SDSS counterpart within our 6 search radius are mostly unflagged GALEX artifacts. GALEX sources represent ∼2.5% of all SDSS sources within these fields and about half are optically unresolved. Most unresolved GALEX/SDSS sources are bright (r < 18) blue turn–off thick disk stars and are typically detected only in the GALEX near–UV band. The remaining unresolved sources include low–redshift quasars (z < 2.2), white dwarfs, and white dwarf/M dwarf pairs, and these dominate the optically unresolved sources detected in both GALEX bands. Almost all the resolved SDSS sources detected by GALEX are fainter than the SDSS “main” spectroscopic limit (conversely, of the SDSS galaxies in the “main” spectroscopic sample, about 40% are detected in at least one GALEX band). These sources have colors consistent with those of blue (spiral) galaxies (u − r < 2.2), and most are detected in both GALEX bands. Measurements of Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195 NASA Harriett G. Jenkins Predoctoral Fellow; agueros@astro.washington.edu Kapteyn Institute, Postbus 800, Groningen 9700 AV, The Netherlands Department of Astronomy, 610 Space Sciences Building, Cornell University, Ithaca, NY 14853 Department of Physics & Astronomy, University of Pittsburgh, 3941 O’Hara Street, Pittsburgh, PA 15260 Princeton University Observatory, Princeton, NJ 08544