9P/Tempel 1

The Deep Impact Comet

 

Amateur Monitoring by the Observadores_Cometas Group

 

This is the only official source of homologised Spanish amateur CCD monitoring data of 9P/Tempel 1.

 

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1. Light curve, photometry and production rate of dust (curva de luz, fotometría y producción de polvo)

2. Contributing observers and equipment list (Relación de Observadores)

2. Image library (Galería de imágenes)

3. Texto en castellano (Últimas observaciones recibidas a 24 de septiembre)

4. Comparativa del 9P con otros cometas.

5. NASA: página del 9P/Tempel 1. Imágenes y animaciones.

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The light curve of 9P/Tempel 1 from the data obtained by members of the “Observadores_cometas” group in different standard apertures. The data is obtained using the FOCAS routine developed by Julio Castellano. The magnitudes have been converted to standard Johnson-Kron-Cousins R magnitudes using calculated colour transformations.

 

In some early data the comet’s low altitude and in general the bright sky background – most contributing observers suffer from moderate to severe light pollution – have affected the data quality, particularly in larger apertures. The following figures are the sky brightness in magnitudes per square arcsecond for observations in the data set and give an idea of the difficulty in observing faint objects. Note that the sky brightness may include a significant component of moonlight.

 

 

Despite the careful background calculation it must be warned that an 18^th magnitude point source comet is 10.2 magnitudes fainter (a factor of 12 000) than the integrated sky in a 60” aperture for the median sky brightness, thus the light curve in larger apertures should be treated with great caution at its fainter limit.

 

A fairer idea of the comet’s activity can be obtained from its light curve in the 10” aperture for which there is more photometry available and which is less affected by the sky background. As the comet was effectively a point source at early epochs there is no great loss of information involved.

 

 

CCD photometry of 9P/Tempel 1 in a constant aperture of 10” from the Observadores_Cometas Group. The magnitudes have been transformed to Johnson-Kron-Cousins R. Note that the physical aperture size at the comet has reduced from    25 400km to 7 700km over the period covered by the light curve.

 

The light curve shows how rapidly the comet has brightened over the 126 days covered so far. However, as the physical aperture size has reduced to less than half the original aperture size, having decreased from 25 400km to 7700km, the true increase in activity is even greater than suggested by the light curve. However, countering this is the fact that as the geocentric distance has reduced to a third from 3.49 to 1.06AU, a large part of the comet’s brightening is also down to its reduced distance from the Earth.

 

A least squares fit to the light curve shows that the rate of brightening in the 10” aperture has increased significantly since mid-December 2004.

 

From 2004 October 2nd to December 11th

276 to 206 days before perihelion,

2.80 < r < 2.39AU

40 measurements

Mean rate of brightening 0.018 mags./day

 

From 2004 December 18^th to 2005 March 1st

199 to 125 days before perihelion,

2.36 < r < 1.93AU

214 measurements

Mean rate of brightening 0.049 mags./day

 

To measure the true activity of the comet and separate out the effects of decreasing physical aperture and the comet’s approach to the Earth we convert the CCD photometry to Afr (Afrho), the equivalent column of dust through the coma, as proposed by Mike A’Hearn and collaborators (A’Hearn et al.: 1984, AJ, 89, 579). The advantage of this parameter is that it is not model dependent.

 

 

Conversion of the CCD photometry presented in the light curves above to the equivalent column of dust through the coma (Afrho).

 

Note the considerable dispersion, particularly of the early data. Part of the reason for this is that the coma did not fill the aperture and part is due to the difficulties in obtaining reliable photometry with a large aperture when the comet is four orders of magnitude fainter than the integrated sky background.

 

If we take just the data in the 10” aperture which is more reliable for faint comets, we see the trend more clearly.

 

 

The same graphic as the previous one with only data for the 10” aperture displayed.

 

We see that the value of Afrho has increased from 4.2-cm at r=2.79AU in early October to the current value of 265+/-12cm at a heliocentric distance of r=1.93AU. In other words, an increase in the true rate of emission of dust of approximately a factor of 40.

 

What is interesting is that the rate of increase of brightness in the 10” aperture is not an artefact of the changing geometry, but is seen clearly in the results for Afr too. In other words, there was a major change in the behaviour of the comet in mid-December at r=2.35AU with a considerable increase in its activity.

 

A cluster of points can be seen in late February that are clearly brighter than the trend. These are observations on two nights by two independent teams (MPC 213 and MPC J76), which were separated by some hundreds of kilometres and which agree to 0.1 magnitudes. There is a suggestion that there may have been a small outburst. When the data are plotted on a linear Afr scale (below) the evidence is at least suggestive.

 

                       

 

Recent values of Afr from the data presented above on a linear scale.

 

                       

 

Recent photometric data in 10”, 20” and 30”. Different symbols are used for the photometry in the 10” aperture obtained with the multiaperture photometry routine and photometry taken as part of routine astrometry. Note that most of the points taken in February 26^th and 27^th superimpose; what appears to be 3 points is actually 12 individual measures.

 

When we look at the multiaperture data the results in the 20” aperture are also suggestive that there may have been an event in the light curve; the data in 30” show too much scatter to permit a conclusion.

 

The rate of dust production is estimated from the values of Afrho by assuming the conversion factor calculated by Osip and collaborators (Osip et al.: 1992, Icarus, 98, 115).

 

                     

 

The estimated production rate of dust from the nucleus of Comet 9P/Tempel 1 from the photometry reported above.

 

A least squares fit to the dust production shows it increasing as a r^-10.6. However, it is obvious that a simple power law is not a good fit to the data as the dust production was stable or even decreased slightly between r=2.7 and r=2.35AU and is even faster than the overall fit at r<2.35AU. A fit to just the data at r<2.35AU gives an r^-15.7 power law. Even if we assume some gas contamination of the measurements (although this is likely to be very small in a low activity comet at this heliocentric distance), the rate of increase of dust production is still exceptionally high.

 

There are simultaneous CCD photometry and total visual magnitude estimates each by two observers on 2005 March 1^st. The total visual magnitude estimates (which differ by only 0.1 mags.) convert to a water vapour production rate of 125kg/s, while the value of Afr of 265+/-12 cm converts to an approximate dust production rate of 47.7+/-2.2 kg/s. This would imply a very low gas to dust ratio of »2.5 suggesting that either the inner coma is extremely dusty at present or, more likely, the assumed conversion factor from Afr to rate of dust emission is too small – other calibrations have suggested that the conversion factor may be about 60% larger, although there is no reason why it should be the same value for all comets. However, even if the conversion factor is as much as double the vale assumed here the inner coma would still be very dusty.

 

                       

 

The least squares power law fit to dust production in 9P/Tempel 1 at r<2.35AU from the data shown above. From r=2.35AU to r=1.9AU the dust production rate has increased as r^-15.7.