arXiv subjects

Anita L. Cochran

Publications and source records attributed to Anita L. Cochran.

28 records · Page 2Linked to original sources

arXiv2006

Visible and Near-Infrared Spectrophotometry of the Deep Impact Ejecta of Comet 9P/Tempel 1

We have obtained optical spectrophotometry of the evolution of comet 9P/Tempel 1 after the impact of the Deep Impact probe, using the Supernova Integral Field Spectrograph (SNIFS) at the UH 2.2m telescope, as well as simultaneous optical and infrared spectra using the Lick Visible-to-Near-Infrared Imaging Spectrograph (VNIRIS) spectrograph. The spatial distribution and temporal evolution of the "violet band" CN (0-0) emission and of the 630 nm [OI] emission was studied. We found that CN emission centered on the nucleus increased in the two hours after impact, but that this CN emission was delayed compared to the light curve of dust-scattered sunlight. The CN emission also expanded faster than the cloud of scattering dust. The emission of [OI] at 630 nm rose similarly to the scattered light, but then remained nearly constant for several hours after impact. On the day following the impact, both CN and [OI] emission concentrated on the comet nucleus had returned nearly to pre-impact levels. We have also searched for differences in the scattering properties of the dust ejected by the impact compared to the dust released under normal conditions. Compared to the pre-impact state of the comet, we find evidence that the color of the comet was slightly bluer during the post-impact rise in brightness. Long after the impact, in the following nights, the comet colors returned to their pre-impact values. This can be explained by postulating a change to a smaller particle size distribution in the ejecta cloud, in agreement with the findings from mid-infrared observatons, or by postulating a large fraction of clean ice particles, or by a combination of these two.

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arXiv2006

Observations of Comet 9P/Tempel 1 with the Keck 1 HIRES Instrument During Deep Impact

We report high-spectral resolution observations of comet 9P/Tempel 1 before, during and after the impact on 4 July 2005 UT of the Deep Impact spacecraft with the comet. These observations were obtained with the HIRES instrument on Keck 1. We observed brightening of both the dust and gas, but at different rates. We report the behavior of OH, NH, CN, C$_{3}$, CH, NH$_{2}$ and C$_{2}$ gas. From our observations, we determined a CN outflow velocity of at least 0.51 km sec$^{-1}$. The dust color did not change substantially. To date, we see no new species in our spectra, nor do we see any evidence of prompt emission. From our observations, the interior material released by the impact looks the same as the material released from the surface by ambient cometary activity. However, further processing of the data may uncover subtle differences in the material that is released as well as the time evolution of this material.

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arXiv2006

The impact and rotational lightcurves of Comet 9P/Tempel 1

UVES and HIRES high-resolution spectra of comet 9P/Tempel 1 are used to investigate the impact and rotational light curves of various species with a view toward building a simple model of the distribution and activity of the sources. The emission by OH, NH, CN, C3, CH, C2, NH2 and OI, are analyzed, as well as the light scattered by the dust. It is found that a simple model reproduces fairly well the impact lightcurves of all species combining the production of the observed molecules and the expansion of the material throughout the slit. The rotational light curve for each species is explained in terms of a single model with three sources.

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arXiv2002

A McDonald Observatory Study of Comet 19P/Borrelly: Placing the Deep Space 1 Observations into a Broader Context

We present imaging and spectroscopic data on comet 19P/Borrelly that were obtained around the time of the Deep Space 1 encounter and in subsequent months. In the four months after perihelion, the comet showed a strong primary (sunward) jet that is aligned with the nucleus' spin axis. A weaker secondary jet on the opposite hemisphere appeared to became active around the end of 2001, when the primary jet was shutting down. We investigated the gas and dust distributions in the coma, which exhibited strong asymmetries in the sunward/anti-sunward direction. A comparison of the CN and C2 distributions from 2001 and 1994 (during times when the viewing geometry was almost identical) shows a remarkable similarity, indicating that the comet's activity is essentially repeatable from one apparition to the next. We also measured the dust reflectivities as a function of wavelength and position in the coma, and, though the dust was very red overall, we again found variations with respect to the solar direction. We used the primary jet's appearance on several dates to determine the orientation of the rotation pole to be alpha = 214^{circ}, delta = -5^{circ}. We compared this result to published images from 1994 to conclude that the nucleus is near a state of simple rotation. However, data from the 1911, 1918 and 1925 apparitions indicate that the pole might have shifted by 5-10^{circ} since the comet was discovered. Using our pole position and the published nongravitational acceleration terms, we computed a mass of the nucleus of 3.3 times10^{16} g and a bulk density of 0.49 g cm^{-3} (with a range of 0.29 < rho < 0.83 g cm^{-3}). This result is the least model-dependent comet density known to date.

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arXiv2002

A Search for N2+ in Spectra of Comet C/2002C1 (Ikeya-Zhang)

We report low- and high-resolution spectra of comet C/2002 C1 (Ikeya-Zhang) from McDonald Observatory. The comet had a well-developed ion tail including CO+, CO2+, CH+, and H2O+. We used our high-resolution spectra to search for N2+. None was detected and we placed upper limits on N2+/CO+ of 5.4 times 10^{-4}. N2+ was detected in the low-resolution spectra but we show that this emission was probably telluric in origin (if cometary, we derive N2+/CO+ = 5.5 times 10^{-3}, still very low). We discuss the implications for the conditions in the early solar nebula of the non-detection of N2+. These depend on whether the H2O ice was deposited in the amorphous or crystalline form. If H2O was deposited in its crystalline form, the detection of CO+ but not N2+ has implications for H2O/H2 in the early solar nebula.

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arXiv2002

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

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 from 3830A--10192A. The spectra from 3830--5776A are continuous; from 5777--10192A there are increasing interorder gaps. The comet was at a heliocentric distance of 0.66au and a geocentric distance of 1.0au. 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 in order 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 community. This atlas should prove a valuable tool for future studies of comets.

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arXiv2001

Observations of O (1S) and O (1D) in Spectra of C/1999 S4 (LINEAR)

We report on high spectral resolution observations of comet C/1999 S4 (LINEAR) obtained at McDonald Observatory in June and July 2000. We report unequivocal detections of the O (1S) and O (1D) metastable lines in emission in the cometary spectrum. These lines are well separated from any telluric or cometary emission features. We have derived the ratio of the two red doublet lines and show they are consistent with the predictions of the branching ratio. We also derived a ratio of 0.06+/-0.01 for the green line flux to the sum of the red line fluxes. This ratio is consistent with H2O as the dominant parent for atomic oxygen. We have measured the widths of the lines and show that the widths imply that there must be some parent of atomic oxygen in addition to the H2O.

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arXiv2000

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 N$_2$ in comets is important to our understanding of the condensation of ices in the solar nebula. In addition, N$_2$ is a tracer of Ar so study of N$_2$ 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 N$_2$ 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.

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arXiv1998

The Calibration of the HST Kuiper Belt Object Search: Setting the Record Straight

The limiting magnitude of the HST data set used by Cochran et al. (1995) to detect small objects in the Kuiper belt is reevaluated, and the methods used are described in detail. It is shown, by implanting artificial objects in the original HST images, and re-reducing the images using our original algorithm, that the limiting magnitude of our images (as defined by the 50% detectability limit) is $V=28.4$. This value is statistically the same as the value found in the original analysis. We find that $\sim50%$ of the moving Kuiper belt objects with $V=27.9$ are detected when trailing losses are included. In the same data in which these faint objects are detected, we find that the number of false detections brighter than $V=28.8$ is less than one per WFPC2 image. We show that, primarily due to a zero-point calibration error, but partly due to inadequacies in modeling the HST'S data noise characteristics and Cochran et al.'s reduction techniques, Brown et al. 1997 underestimate the SNR of objects in the HST dataset by over a factor of 2, and their conclusions are therefore invalid.

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arXiv1995

The Discovery of Halley-sized Kuiper Belt Objects Using HST

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<28.6. This must be viewed as a lower limit until our search of parameter space is complete. Our observations imply that there are >2x10^8 objects of this size in the Kuiper belt with inclinations <12 degrees and within ~40 AU of the Sun.

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