Anita L. Cochran

Large Excess of Heavy Nitrogen in Both Hydrogen Cyanide and Cyanogen from Comet 17P/Holmes

From millimeter and optical observations of the Jupiter-family comet 17P/Holmes performed soon after its huge outburst of 2007 October 24, we derive 14N/15N = 139 ± 26 in HCN and 14N/15N = 165 ± 40 in CN, establishing that HCN has the same nonterrestrial isotopic composition as CN. The same conclusion is obtained for the long-period comet C/1995 O1 (Hale-Bopp) after a reanalysis of previously published measurements. These results are compatible with HCN being the prime parent of CN in cometary atmospheres. The15N excess relative to the Earths atmospheric value indicates that N-bearing volatiles in the solar nebula underwent important N isotopic fractionation at some stage of solar system formation. HCN molecules never isotopically equilibrated with the main nitrogen reservoir in the solar nebula before being incorporated in Oort Cloud and Kuiper Belt comets. The 12C/13C ratios in HCN and CN are measured to be consistent with the terrestrial value.

CHARA Array K'-Band Measurements of the Angular Dimensions of Be Star Disks

We present the firstK 0 -band,long-baseline interferometric observations of the northern Be starsCas,� Per,� Tau, andDra. The measurements were made with multiple telescope pairs of the CHARA Array interferometer and in every case the observations indicate that the circumstellar disks of the targets are resolved. We fit the interferometric visibilities with predictions from a simple disk model that assumes an isothermal gas in Keplerian rotation. We derive fitsof thefourmodelparameters(diskbasedensity,radialdensityexponent,disknormalinclination,andpositionangle) for each of the targets. The resulting densities are in broad agreement with prior studies of the IR excess flux, and the resultingorientationsgenerallyagreewiththosefrominterferometricHandcontinuumpolarimetricobservations.We find that the angular size of the K 0 diskemissionis smaller thanthatdeterminedfor the Hemission, and weargue that thedifferenceisthe resultof a larger Hopacityandtherelativelylarger neutral hydrogenfractionwithincreasingdisk radius. All the targets are known binaries with faint companions, and we find that companions appear to influence the interferometric visibilities in the cases ofPer andDra. We also present contemporaneous observations of the H� , H� ,andBremissionlines.Syntheticmodelprofilesoftheselinesthatarebasedonthesamediskinclinationandradial densityexponentasderivedfromtheCHARA Arrayobservationsmatchtheobservedemissionlinestrengthif thedisk base density is reduced by � 1.7 dex.

The McDonald Observatory faint comet survey: gas production in 17 comets

Abstract We present spectral data from 152 observations of 17 comets obtained with an Intensified Dissector Scanner spectrograph at McDonald Observatory. We present the details of the observations and reduction of these data to production rates. These production rates are then analyzed to show that comets seem to be reasonably homogeneous as a group. The ratios of production rates remain constant with activity level and with heliocentric distance, with the exception of Q(NH2)/Q(CN), which shows a heliocentric distance dependence.

Absolute spectrophotometry of Titan, Uranus, and Neptune: 30,500–10,500 Å

Absolute measurements of the geometric albedo spectra of Titan, Uranus, and Neptune from 3500 to 10,500 A are reported. The measurements have spectral resolution of about 7 A and high signal-to-noise ratio owing to the superb efficiency of the spectrograph and Reticon detector used to acquire them. Agreement is excellent with albedo measurements of Uranus G. W. Lockwood, B. L. Lutz, D. T. Thompson, and A. Warnock (1983, Astrophys. J., 266, 402–414). The high precision and spectral resolution of the data make possible quantitative measurements of the effects of Raman scattering by H2 in the atmospheres of Uranus and Neptune.

Spectrophotometry of the continuum in 18 comets

Abstract We have studied the continuum emission in spectra of 18 comets. We find that the gas-to-dust ratio (GDR) increases with increasing activity (as measured by the production of CN). This may be explained by having dust production depend on the gas production raised to the 0.55 ± 0.17 power (1σ error). We interpret this to indicate that cometary gas production in the range of heliocentric distances (and gas production rates) for which we have data, is primarily through a porous mantle. Approximately equal numbers of comets show changes in continuum brightness with distance from the optocenter equal to, or significantly slower than, the canonical (distance)−1 falloff. All continua were red compared to the solar spectrum, with an average reddening of 22% per 1000 A (between 4400 and 5700 A). There is some evidence for a change in the slope of the continuum with distance from the optocenter in some comets. Trends in these observables throughout the apparition of each comet are discussed, and comparisons among comets are made.

The Discovery of Halley-sized Kuiper Belt Objects Using the Hubble Space Telescope

We report the statistical detection (at the >99% confidence level) of a population of 28th magnitude objects exhibiting proper motions of ~1 arcsecond per hour at quadrature in deep HST/WFPC2 images. The drift directions imply a preponderance of objects on prograde orbits concentrated near the ecliptic plane. We interpret this as the detection of objects which reside in the Kuiper belt near or beyond the orbit of Neptune, comparable in size to comet 1P/Halley (radii ~10 km for an albedo of 4%). Our observations imply a population of ~2.5x10^4 objects deg^-2 with V 2x10^8 objects of this size in the Kuiper belt with inclinations <12 degrees and within ~40 AU of the Sun.

N+2 and CO+ in Comets 122P/1995 S1 (deVico) and C/1995 O1 (Hale–Bopp)

We observed Comets 122P/1995 S1 (deVico) and C/1995 O1 (Hale–Bopp) with high spectral resolving power in order to determine the ratio of N+2/CO+ in their comae. While we clearly detected the CO+ in both of these comets, no N+2 was detected in either comet. From these spectra, we derive sensitive upper limits for N+2/CO+. These upper limits are substantially below other reported detections of N+2/CO+ in other comets. We discuss the prior N+2 detections and compare them with our observations. The abundance of N2 in comets is important to our understanding of the condensation of ices in the solar nebula. In addition, N2 is a tracer of Ar so the study of N2 allows an understanding of the role of comets for delivering volatiles to the terrestrial planets. It appears that many, if not most, comets are depleted in N2 and it will be necessary to search for a mechanism for depleting this molecule in order to be consistent with current models of the solar nebula.

A High Spectral Resolution Atlas of Comet 122P/de Vico

Abstract On 3 and 4 October 1995, we obtained high spectral resolving power (R=λ/Δλ=60,000) observations of Comet 122P/de Vico using the 2DCoude cross-dispersed echelle spectrograph on the 2.7-m telescope of McDonald Observatory. The spectra cover the wavelength range 3830–10,192 A. The spectra from 3830 to 5776 A are continuous, but from 5777 to 10,192 A there are increasing interorder gaps. The comet was at a heliocentric distance of 0.66 AU and a geocentric distance of 1.0 AU. Comet de Vico has a very high gas-to-dust ratio and the spectra have excellent signal/noise. These two factors combined to yield spectra with a large number of emission lines. We have collected laboratory molecular line lists and have used these line lists to identify as many of the detected lines as possible. We have identified 12,219 emission lines and have located another 4,055 lines which we cannot identify. We present representative spectra and identifications along with a description of our plans to make these spectra and identifications available to the scientific community. This atlas should prove to be a valuable tool for future studies of comets.

Spectrophotometric observations of P/Schwassmann-Wachmann 1 during outburst

Spectra of Comet Schwassmann-Wachmann 1 were obtained on November 26-27, 1978 and February 15, 1979 using the Cassegrain Digicon spectrograph, and on February 21-23, 1979 using the IDS spectrograph of McDonald Observatory. All spectra were obtained while the comet was in an extended series of outburst (12th-13th mag). The spectra were predominantly reflected solar light, indicating the outburst was very rich in dust. Weak emission between 4000 and 4050 A, and between 4240 and 4290 A was detected in November 1978 and is probably due to CO(+). These outbursts, and the detection of CO(+), indicate that comets are not inactive at distances as great as 6 AU.

THE VOLATILE COMPOSITION AND ACTIVITY OF COMET 103P/HARTLEY 2 DURING THE EPOXI CLOSEST APPROACH ∗

We report time-resolved measurements of the absolute and relative abundances of eight parent volatiles (H2O, CH3OH, C2H6, C2H2, NH3, HCN, H2CO, and HC3N) in the coma of 103P/Hartley 2 on UT 2010 November 4, the date the EPOXI spacecraft made its closest approach to the comet, using high-dispersion infrared spectroscopy with NIRSPEC at the W. M. Keck Observatory. Overall gas and dust production increased by roughly 60% between UT 10:49 and 15:54. Differences in the spatial distributions of species in the coma suggest icy sources of different composition in the nucleus of 103P/Hartley 2. However, differences in the relative abundances of species with time are minor, suggesting either internal compositional heterogeneity in 103P/Hartley 2 is small compared with the diversity of chemistry observed within the comet population, or more significant heterogeneity exists on scales smaller than our spatial resolution. Observations contemporaneous with the EPOXI encounter test how compositional heterogeneity over the surface and the inner coma of a comet manifests itself in remote-sensing observations of the bulk coma.