Near Infrared Surface Properties of the Two Intrinsically Brightest Minor Planets (90377) Sedna and (90482) Orcus

We present low resolution K band spectra taken at the Gemini 8 meter telescope of (90377) Sedna and (90482) Orcus (provisional designations 2003 VB12 and 2004 DW, respectively), currently the two minor planets with the greatest absolute magnitudes (i.e. the two most reflective minor planets). We place crude limits on the surface composition of these two bodies using a Hapke model for a wide variety of assumed albedos. The unusual minor planet (90377) Sedna was discovered on November 14, 2003 UT at roughly 90 AU with 1.6 times the heliocentric distance and perihelion distance of any other bound minor planet. It is the first solar system object discovered between the Kuiper Belt and the Oort Cloud, and may represent a transition population between the two. The reflectance spectrum of (90377) Sedna appears largely featureless at the current signal-to-noise ratio, suggesting a surface likely to be highly processed by cosmic rays. For large grain models (100 micron to 1 cm) we find that (90377) Sedna must have less than 70% surface fraction of water ice and less than 60% surface fraction of methane ice to 3 sigma confidence. Minor planet (90482) Orcus shows strong water ice absorption corresponding to less than 50% surface fraction for grain models 25 micron and larger. Orcus cannot have more than 30% of its surface covered by large (100 mm to 1 cm) methane grains to 3 sigma confidence.Comment: Accepted for publication in the Astrophysical Journa

NEW BACKGROUND INFRARED SOURCES FOR STUDYING THE GALACTIC CENTER'S INTERSTELLAR GAS

We are nearing completion of a low-resolution 2.0-2.5~$mu$m (4000-5000~cm$^{-1}$) survey of $sim$500 very red point-like objects in the Central Molecular Zone (CMZ) of the Milky Way Galaxy. The goal is to find bright objects with intrinsically featureless or nearly featureless spectra that are suitable as background light sources for high-resolution infrared absorption spectroscopy of H$_3^+$ and CO in the Galactic center's interstellar gas, on sightlines spread across the CMZ. Until recently very few such objects had been known outside of two clusters of hot and luminous stars close to the very center. We have used Spitzer Space Telescope 3.6--8.0-$mu$m photometry and 2-Micron All Sky Survey 1.0--2.5-$mu$m photometry to identify candidates with a significant probability of being stars embedded in circumstellar dust, and over the last several years have been acquiring low resolution spectra of them to determine their natures. The low resolution spectra, which encompass the wavelengths of the first overtone band heads of CO, which are prominent in cool stellar photospheres, show that by far the majority of candidates are very cool and/or highly reddened red giants, which are unsuitable as background sources because of their complex spectra . However, approximately ten percent of the candidates have featureless or nearly featureless spectra and are useful for investigations of the interstellar gas. Most of these have continua rising steeply to longer wavelengths and are luminous, dust embedded stars

Search for H₃⁺ isotopologues toward CRL 2136 IRS 1

Context. Deuterated interstellar molecules frequently have abundances relative to their main isotopologues much higher than the overall elemental D-to-H ratio in the cold dense interstellar medium. H₃⁺ and its isotopologues play a key role in the deuterium fractionation; however, the abundances of these isotopologues have not been measured empirically with respect to H₃⁺ to date. Aims. Our aim was to constrain the relative abundances of H₂D⁺ and D₃⁺ in the cold outer envelope of the hot core CRL 2136 IRS 1. Methods. We carried out three observations targeting H₃⁺ and its isotopologues using the spectrographs CRIRES at the VLT, iSHELL at IRTF, and EXES on board SOFIA. In addition, the CO overtone band at 2.3 μm was observed by iSHELL to characterize the gas on the line of sight. Results. The H₃⁺ ion was detected toward CRL 2136 IRS 1 as in previous observations. Spectroscopy of lines of H₂D⁺ and D₃⁺ resulted in non-detections. The 3σ upper limits of N(H₂D⁺)/N(H₃⁺) and N(D₃⁺)/N(H₃⁺) are 0.24 and 0.13, respectively. The population diagram for CO is reproduced by two components of warm gas with the temperatures 58 and 530 K, assuming a local thermodynamic equilibrium (LTE) distribution of the rotational levels. Cold gas (<20 K) makes only a minor contribution to the CO molecular column toward CRL 2136 IRS 1. Conclusions. The critical conditions for deuterium fractionation in a dense cloud are low temperature and CO depletion. Given the revised cloud properties, it is no surprise that H₃⁺ isotopologues are not detected toward CRL 2136 IRS 1. The result is consistent with our current understanding of how deuterium fractionation proceeds

The dusty type IIn Supernova 1998S

The type IIn SN 1998S is one of the most remarkable core-collapse supernovae ever observed. It underwent a complex interaction with a substantial circumstellar medium, resulting in radiation at wavelengths from radio to X-rays. IR and optical observations have revealed a wide variety of broad and narrow emission lines. Examination of the SN/CSM interaction and of the ejecta spectra has allowed us to deduce that the supernova probably arose from a massive, RSG progenitor having a large (>3200 AU radius), dusty circumstellar disk. SN 1998S also developed one of the strongest, most persistent infrared excesses ever seen in a supernova. IR/optical monitoring of SN 1998S has been carried out to nearly 1200 days post-explosion. This includes coverage to wavelengths as long as 4.7 microns, making SN 1998S only the second supernova (after SN 1987A) to be observed in this spectral region. Fading of the central and redshifted components of the late-time H I and He I line profiles suggests strongly that dust condensed in the ejecta. However, it is less clear whether the strong late-time IR emission arose from this dust, or from an IR echo in the dusty CSM. One interesting possibility is that dust condensed in the cool dense shell between the outer and reverse shocks, thus simultaneously producing both the line obscuration and the IR emission.Comment: 9 pages. {Invited talk given at ESO/MPA/MPE Workshop "From Twilight to Highlight: The Physics of Supernovae", Garching, Germany, 29-31 July 2002. To appear in the Proceedings, eds. W. Hillebrandt & B. Leibundgut (pub. Springer-Verlag series ``ESO Astrophysics Symposia'

Search for H₃⁺ isotopologues toward CRL 2136 IRS 1

Context. Deuterated interstellar molecules frequently have abundances relative to their main isotopologues much higher than the overall elemental D-to-H ratio in the cold dense interstellar medium. H₃⁺ and its isotopologues play a key role in the deuterium fractionation; however, the abundances of these isotopologues have not been measured empirically with respect to H₃⁺ to date. Aims. Our aim was to constrain the relative abundances of H₂D⁺ and D₃⁺ in the cold outer envelope of the hot core CRL 2136 IRS 1. Methods. We carried out three observations targeting H₃⁺ and its isotopologues using the spectrographs CRIRES at the VLT, iSHELL at IRTF, and EXES on board SOFIA. In addition, the CO overtone band at 2.3 μm was observed by iSHELL to characterize the gas on the line of sight. Results. The H₃⁺ ion was detected toward CRL 2136 IRS 1 as in previous observations. Spectroscopy of lines of H₂D⁺ and D₃⁺ resulted in non-detections. The 3σ upper limits of N(H₂D⁺)/N(H₃⁺) and N(D₃⁺)/N(H₃⁺) are 0.24 and 0.13, respectively. The population diagram for CO is reproduced by two components of warm gas with the temperatures 58 and 530 K, assuming a local thermodynamic equilibrium (LTE) distribution of the rotational levels. Cold gas (<20 K) makes only a minor contribution to the CO molecular column toward CRL 2136 IRS 1. Conclusions. The critical conditions for deuterium fractionation in a dense cloud are low temperature and CO depletion. Given the revised cloud properties, it is no surprise that H₃⁺ isotopologues are not detected toward CRL 2136 IRS 1. The result is consistent with our current understanding of how deuterium fractionation proceeds

Absolute calibration and atmospheric versus mineralogic origin of absorption features in 2.0 to 2.5 micron Mars spectra obtained during 1993

We obtained new high resolution reflectance spectra of Mars during the 1993 opposition from Mauna Kea Observatory using the UKIRT CGS4 spectrometer. Fifty spectra of 1600-2000 km surface regions and a number of standard star spectra were obtained in the 2.04 to 2.44 micron wavelength region on 4 February 1993 UT. Near-simultaneous observations of bright standard stars were used to perform terrestrial atmospheric corrections and an absolute flux calibration. Using the known magnitude of the stars and assuming blackbody continuum behavior, the flux from Mars could be derived. A radiative transfer model and the HITRAN spectral line data base were used to compute atmospheric transmission spectra for Mars and the Earth in order to simulate the contributions of these atmospheres to our observed data. Also, we examined the ATMOS solar spectrum in the near-IR to try to identify absorption features in the spectrum of the Sun that could be misinterpreted as Mars features. Eleven absorption features were detected in our Mars spectra. Our data provide no conclusive identification of the mineralogy responsible for the absorption features we detected. However, examination of terrestrial spectral libraries and previous high spectral resolution mineral studies indicates that the most likely origin of these features is either CO3(sup 2-), HCO3(-), or HSO4(-) anions in framework silicates or possibly (Fe, Mg)-OH bonds in sheet silicates

H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate

Using high resolution infrared spectroscopy we have surveyed twenty sightlines for H3+ absorption. H3+ is detected in eight diffuse cloud sightlines with column densities varying from 0.6x10^14 cm^-2 to 3.9x10^14 cm^-2. This brings to fourteen the total number of diffuse cloud sightlines where H3+ has been detected. These detections are mostly along sightlines concentrated in the Galactic plane, but well dispersed in Galactic longitude. The results imply that abundant H3+ is common in the diffuse interstellar medium. Because of the simple chemistry associated with H3+ production and destruction, these column density measurements can be used in concert with various other data to infer the primary cosmic-ray ionization rate, zeta_p. Values range from 0.5x10^-16 s^-1 to 3x10^-16 s^-1 with an average of 2x10^-16 s^-1. Where H3+ is not detected the upper limits on the ionization rate are consistent with this range. The average value of zeta_p is about an order of magnitude larger than both the canonical rate and rates previously reported by other groups using measurements of OH and HD. The discrepancy is most likely due to inaccurate measurements of rate constants and the omission of effects which were unknown when those studies were performed. We believe that the observed column density of H3+ is the most direct tracer for the cosmic-ray ionization rate due to its simple chemistry. Recent models of diffuse cloud chemistry require cosmic-ray ionization rates on the order of 10^-16 s^-1 to reproduce observed abundances of various atomic and molecular species, in rough accord with our observational findings.Comment: Accepted to ApJ, 35 pages, 5 figures, 5 table

Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate

The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many interstellar molecules. In dense clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density, and temperature. On the other hand, observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models. However, diffuse cloud models have been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons, the electron fraction, and the cosmic-ray ionisation rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards zeta Persei) where the electron fraction is already known. Taken together, these results allow us to derive the value of the third uncertain model parameter: we find that the cosmic-ray ionisation rate in this sightline is forty times faster than previously assumed. If such a high cosmic-ray flux is indeed ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of H3+ can be resolved.Comment: 6 pages, Nature, in pres

Hot and Diffuse Clouds near the Galactic Center Probed by Metastable H3+

Using an absorption line from the metastable (J, K) = (3, 3) level of H3+ together with other lines of H3+ and CO observed along several sightlines, we have discovered a vast amount of high temperature (T ~ 250 K) and low density (n ~ 100 cm-3) gas with a large velocity dispersion in the Central Molecular Zone (CMZ) of the Galaxy, i.e., within 200 pc of the center. Approximately three fourths of the H3+ along the line of sight to the brightest source we observed, the Quintuplet object GCS 3-2, is inferred to be in the CMZ, with the remaining H3+ located in intervening spiral arms. About half of H3+ in the CMZ has velocities near ~ - 100 km s-1 indicating that it is associated with the 180 pc radius Expanding Molecular Ring which approximately forms outer boundary of the CMZ. The other half, with velocities of ~ - 50 km s-1 and ~ 0 km s-1, is probably closer to the center. CO is not very abundant in those clouds. Hot and diffuse gas in which the (3, 3) level is populated was not detected toward several dense clouds and diffuse clouds in the Galactic disk where large column densities of colder H3+ have been reported previously. Thus the newly discovered environment appears to be unique to the CMZ. The large observed H3+ column densities in the CMZ suggests an ionization rate much higher than in the diffuse interstellar medium in the Galactic disk. Our finding that the H3+ in the CMZ is almost entirely in diffuse clouds indicates that the reported volume filling factor (f &#8805; 0.1) for n &#8805; 104 cm-3 clouds in the CMZ is an overestimate by at least an order of magnitude.Comment: 33 pages, 5 figures, 3 table