Prospects for C/2002 T7 (LINEAR)
Abstract
There is considerable anticipation of the prospects
of two bright naked eye comets being visible in Spring
2003. For observers in northern hemisphere mid-latitudes there is even the
unusual prospect of seeing two bright comets in the same visual field in the
evening sky. Although some aspects of C/2002 T7 (LINEAR) advise considerable
caution, its activity at this time is unusual and it has the potential to be
very bright.
Introduction
C/2002 T7 (LINEAR) is one of an increasing number
of LINEAR “asteroids” that are found to be cometary.
Its cometary nature was first noticed by Peter Birtwhistle at Great Shefford and
it was announced as a comet on 2002 Oct. 29 (1). The object was first reported
on 2002 Oct. 14 at r=6.92AU, but five pre-discovery images were also identified
in LINEAR data from 2002 Oct. 12 (2). The initial orbit solution, later
confirmed to be substantially correct showed that the comet would have a
relatively close approach to the Earth in May 2003 (Dmin=0.266AU, 40 million kilometres on 2003 May
18th) at T+27 days with the comet still well inside the Earth’s
orbit after perihelion at 0.615AU. This led to the early suggestion that the
comet could reach naked-eye visibility in 2003 April-June (3).
The comet’s orbit is found to be slightly
hyperbolic (e=1.000499 at perihelion) and opening further while in the inner
solar system (4), in other words it is dynamically new and thus an Oort Cloud object. Its absolute magnitude m0=+5.0[1][1]
at discovery (assuming a 4th power brightening law) suggested that
it is a Halley-class object that could reach m1=+1.3 at
minimum geocentric distance. However, given that dynamically new objects often
follow a 3rd power law it is reasonable to expect the comet to be
fainter than this. However, even a 3rd power law from discovery
would lead to m1=+3.7 at minimum geocentric distance and probable
naked-eye visibility.
The light curve and
previsions
When analysing the light curve it is necessary to take
care to separate CCD magnitudes, often of only the innermost coma, and total
visual magnitude estimates. For example, the estimate from a light curve fit by
Yoshida (5) groups both visual and CCD data and fails to make the significant
correction from R to V. Although the comet remains very strongly condensed and
has a small coma, total visual magnitude estimates already appear to be as much
as 2 magnitudes brighter than CCD data. Similarly, CCD data is taken with a
wide variety of methods and apertures and should be standardised if it is to be
employed usefully. Increasingly, CCD observers are using a method involving
standard aperture sizes that allows meaningful magnitudes to be calculated for
any physical aperture size (6). This allows far greater significance to be made
of amateur CCD photometry.
Photometry calculated using USNO A2.0 “R”
magnitudes may be converted to the standard Kron-Cousins
“R” magnitude by the relation:
RK-C = 0.993 RUSNO A2.0 +
0.066
We can then use this normalised magnitude to
calculate the light curve parameters. One of the most useful values is Afr, measured in
centimetres. This is the product of the dust albedo
and filling factor and can be visualised as the equivalent column of dust
summed through the coma.
Afr = e(2 ln(D+r) - ln r + ln r) - (MQ+m)/1.086 + 50.546)
Afr is measure of the
level of dust emission and thus the level of activity of a comet. To see it in
context, a fairly low activity object such as 67P/Churyumov-Gerasimenko had a
value of approximately 130-cm at perihelion and some 300-cm at the peak of its
outburst in 2002 October. In contrast, 1P/Halley gave peak values of »25000-cm in 1986.
For C/2002 T7 (LINEAR) the value of »80-cm at discovery has risen to »800-cm in mid-August
2003 although still at r=3.8AU.
Extrapolation to perihelion is extremely dangerous,
especially for a dynamically new object, but 218 determinations of Afr over 6.75 < r
< 3.81AU (below) would give a predicted value of Afr=151000-cm at
perihelion, some six times higher than 1P/Halley. At present there is no
evidence that the activity is flattening out as an almost identical result is
obtained using only data to conjunction in 2003 April.
This is an illustration of the potential of
the comet, although not necessarily how bright it will really get. The light
curve of the comet in an aperture of 10 arcseconds is
shown below. The data is identified by observing station (observations from http://groups.yahoo.com/group/Observadores_cometas/).
Despite the fact that the equivalent physical aperture size reduces from
44400km at discovery, to 33400km in mid-August 2003, the rate of increase in
brightness is rapid. Note that almost all of the data agrees to within ±0.2 magnitudes,
showing that consistent photometry can be obtained even with an unfiltered CCD.
To get a realistic idea of the brightness of the
comet at perihelion from the CCD data we must make an assumption about the coma
brightness profile and calculate the light curve for a fixed physical aperture.
We assume that there is a relation of the type
R = log a + b log r
Where “R” is the measured magnitude in R and “r” is
the aperture diameter in arcseconds, with “a” and “b”
constants. For a “typical” comet with a 1/r coma profile b » -2.5.
Given that the coma is so strongly condensed at present it is not feasible to
measure “b” accurately from the data, thus we have to assume a value of b = -2.5 and use a
physical aperture close to the current physical size of the 10 arcsecond aperture used in CCD photometry so that any error
in this assumption will not have a disproportionate effect on the results.
For the purposes of this extrapolation an aperture
of 25000km is used. This is equivalent to a coma diameter of 2.2 arcminutes at closest approach to the Earth. The light
curve below once again illustrates the potential of C/2002 T7 (LINEAR) and the
danger of making extrapolations to perihelion. For a 25000km aperture the best
fit to the light curve, assuming solar colours, is
m1 = 4.64 + 5 log D + 12.83 log r
This would give a predicted total visual magnitude
at perihelion (the vertical line to the left of the y-axis) of +2.2 within a
2.2 arcminute diameter of coma. Since the visual coma
diameter will undoubtedly be much larger than this readers can come to their
own conclusions about the possible total visual magnitude for the full coma in
late May 2004. However, the extrapolation to perihelion is still very large and
more cautious readers will allow for a substantial flattening of the slope as
the comet gets closer to the Sun and passes from activity dominated by
low-temperature volatiles such as CO to water-dominated sublimation.
Conclusions
As of mid-August 2003 C/2002 T7 (LINEAR) is still
250 days from perihelion and at r=3.9AU in the low-temperature volatile
dominated light curve regime. Although the activity at this point is high and
the extrapolation to perihelion suggests that the comet may be extremely
bright, it is likely that the change to water vapour sublimation dominated
activity at around 3AU will lead to a substantial reduction in the rate of
increase of activity, thus it is still premature to assume that the comet will
be a bright naked-eye object.
1. 1. Birtwhistle, P.: 2002, IAUC 8003 (http://cfa-www.harvard.edu/iauc/08000/08003.html)
2.
2. Marsden,
B.G.: 2002, MPEC 2002-U43 (http://cfa-www.harvard.edu/mpec/K02/K02U43.html)
3. 3. Green, D.: 2002, IAUC 8003 (http://cfa-www.harvard.edu/iauc/08000/08003.html)
4.
4. Marsden,
B.G.: 2003, MPC 48096
5. 5. Yoshida, S.: 2003, http://www.aerith.net/comet/catalog/2002T7/2002T7.html
6. 6. Kidger, M.R.: 2003, Astronomy & Astrophysics,
In Press