📝 Original Info
- Title: The temporal changes in the emission spectrum of Comet 9P/Tempel 1 after Deep Impact
- ArXiv ID: 0904.0592
- Date: 2011-02-11
- Authors: Researchers from original ArXiv paper
📝 Abstract
The time dependence of the changes in the emission spectra of Comet 9P/Tempel 1 after Deep Impact are derived and discussed. This was a unique event because for the first time it gave astronomers the opportunity to follow the time history of the formation and decay of O(1S), OH, CN, C2, C3, NH, and NH2. Least squares fits of a modified Haser model with constraints using known rate constants were fit to the observed data. In the case of OH a simple two-step Haser model provides a reasonable fit to the observations. Fitting the emissions from O(1S), CN, C2, C3, NH, and NH2 requires the addition of a delayed component to a regular two or three step Haser model. From this information a picture of the Deep Impact encounter emerges where there is an initial formation of gas and dust, which is responsible for the prompt emission that occurs right after impact. A secondary source of gas starts later after impact when the initial dust has dissipated enough so that solar radiation can reach the surface of freshly exposed material. The implications of this and other results are discussed in terms of the implications on the structure and composition of the comet's nucleus.
💡 Deep Analysis
Deep Dive into The temporal changes in the emission spectrum of Comet 9P/Tempel 1 after Deep Impact.
The time dependence of the changes in the emission spectra of Comet 9P/Tempel 1 after Deep Impact are derived and discussed. This was a unique event because for the first time it gave astronomers the opportunity to follow the time history of the formation and decay of O(1S), OH, CN, C2, C3, NH, and NH2. Least squares fits of a modified Haser model with constraints using known rate constants were fit to the observed data. In the case of OH a simple two-step Haser model provides a reasonable fit to the observations. Fitting the emissions from O(1S), CN, C2, C3, NH, and NH2 requires the addition of a delayed component to a regular two or three step Haser model. From this information a picture of the Deep Impact encounter emerges where there is an initial formation of gas and dust, which is responsible for the prompt emission that occurs right after impact. A secondary source of gas starts later after impact when the initial dust has dissipated enough so that solar radiation can reach the
📄 Full Content
The collision between the Deep Impact projectile and comet 9P/Tempel 1 on 4 July 2005 UT created unique conditions for studying the chemical processes responsible for the radicals that are observed in comets. It is the first time in the history of astronomy that an astronomical event initiated by man could be followed in real time. The energy expended in the collision was 19 x 10 9 J. The interaction of the material released in this collision with solar radiation was followed with the Keck I telescope on Mauna Kea using the high-resolution echelle spectrograph (HIRES). Excellent high-resolution spectra of the emissions from O( 1 S), OH, CH, CN, C 2 , C 3 , NH, and NH 2 radicals as a function of time after the collision were measured. All of these emissions were present before the encounter but we have devised a method to separate the emission due to the impact from those that were present before the event and thus as a result have been able to derive the temporal behavior of the emissions. The purpose is to directly determine the lifetime of a particular radical species from the variation of the emission intensity as a function of time. We are not determining scale lengths since we have an independent measurement of time. We employ the time after impact, which is a measured quantity, and only use the distance to separate the emission caused by the impact from the emission present before impact. No velocity is required to change scale length into time since time is measured directly at the telescope. Thus, the data that are extracted from the observations are similar to having a double beam spectrometer in the laboratory that measures the light intensity of the emissions present before impact at the same time as measuring the total light intensity after the impact at each wavelength. By subtracting the former from the later one obtains changes in the emission at each wavelength due to the impact. There have been no previous studies in the history of cometary science like this because there has been no previous time in recorded history where man has initiated an astronomical event and then had the tools to record the response as a function of time after the event.
Models have been developed to fit the temporal behavior of the emissions and thus provide new information about the chemical reactions as well as the properties of the cometary nucleus.
The Keck I telescope with the HIRES spectrograph was used to observe the aftermath of the impact. The spectrograph was equipped with the blue cross-disperser which means that it covered a spectral band pass between 3047-5894 Å at a resolution, R = λ/Δλ =47,000. This band pass and resolution allowed us to identify and assign the spectral features of OH, NH, CN, CH, C 3 , C 2 , NH 2 , and O ( 1 S). The extremely good image quality of the optics of the Keck telescope and the stable atmosphere resulted in seeing of 0.7 arcsec. Thus, by employing a slit size of 7.0 x 0.86 arcsec (or 4570 x 562 km at the comet) we were able to obtain excellent spatial resolution of ~ 457 km at the comet. This kind of spatial resolution allowed us to separate the temporal response of these emissions from the ambient emission already present in the coma of the comet.
The observations on 4 July 2005 started during nautical twilight, at 05:36:15, so that we could obtain a pre-impact spectrum. Eight degree twilight was at 05:29UT; 12 degree twilight (nautical) was at 05:58UT; 18 degree twilight (astronomical) was at 06:28UT. Complete details of the observations as well as of our preliminary reduction, where we extracted the integrated spectra over the entire slit length as well as only in the inner 0.7 arcsec, are detailed in Cochran et al. (2007). A log of the observations is given in that paper.
The impact caused the release of additional gas from the nucleus beyond the normal outflow of ambient gas. In order to understand this additional gas, it is necessary to remove the ambient gas signal from the post-impact observations. It was only after using the excellent spatial and spectral resolution of the Keck-HIRES that we were able to obtain the unique chemical signatures of the Deep Impact event. The impact caused an instantaneous release of gas followed by that gas flowing outwards from the impact site. In order to study the lifetimes of the molecules against photodissociation, we wanted to derive light curves that included only the gas produced by the impact with none of the ambient gas signature. Because the gas flowed outward at some finite speed (we assumed 0.55 km sec -1 but as we will show later in this paper, this is an upper limit to gas velocity.), it did not reach the end of our 7 arcsec slit until after our fifth observation post impact. Thus, the ends of our slit during these first observations would still be an accurate and concurrent measure of the ambient cometary spectrum. We therefore extracted the spectra from the ends of the slit, averaging over the spectra from both en
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