onboard SOHO and the SECCHI instrument onboard STEREO. The detected
objects usually belong to one of a series of “families” that have similar orbital
characteristics. By far the best known of these families is the Kreutz family which
is a group of sun-grazers that includes C/1965 S1 Ikeya-Seki and C/2011 W3
Lovejoy. Other sun-skirting groups include the Meyer, Marsden, and Kracht families. Backward orbital integrations indicate that the Marsden and Kracht families
may be related (Ohtsuka et al. 2003).
Families are assumed to arise from the result of the break-up of a parent object
with tidal forces at a previous perihelion passage being a logical assumption. Further
splitting at subsequent perihelion passages of the fragments is considered likely.
Kreutz family comets appear to arrive in clusters that may point to this process. The
observed morphologies of Kreutz family objects are diverse which may indicate
inhomogeneity of the progenitor.
The absolute brightness of Kreutz family comets appears to peak before they
reach perihelion (around 0.005 AU from the Sun) and then declines as they get even
closer. This would not be consistent with insolation-driven water ice sublimation but
may be caused by sublimation of what are normally considered to be refractories
such as olivine and pyroxene as the surface temperature rises (Kimura et al. 2002).
Use of Eq. (2.100) results in temperatures above 4000 K at 0.01 AU but of course
sublimation will be a strong coolant, the magnitude of which is difficult to quantify,
and the point source approximation for the solar input no longer holds at these
distances.
The loss of more refractory material can provide additional information on the
composition as illustrated by the detection of emission lines from of O, Na, K, Ca, V,
Cr, Mn, Fe, Co, Ni, and Cu in C/1965 S1 Ikeya-Seki (Preston 1967; Slaughter 1969).
C/2003 K7 was observed at 0.016 AU when grains were rapidly sublimated. The
relative abundances of H: C: Si were approximately 1: 0.0035: 0.045 and therefore
carbon-poor compared to what we have seen at 67P (cf. Fig. 4.70). On the other
hand, analysis of observations of C/2011 W3 Lovejoy has provided abundance ratios
by number for H: C: N: O: Si: Fe as 1.0: 0.035: 0.004: 0.5: 0.015: 0.025 respectively
which seems to be more in keeping with the 67P results but with a very high iron
content. Strength estimates from the break-ups of sun-grazers are of the order of
0.5–10 Pa (e.g. C/2012 S1 ISON; Steckloff et al. 2015).
While sun-grazers are spectacular, it remains a challenge to establish their
relationship to other comets and whether we can use the information obtained to
constrain the properties of other cometary objects.
432
6 Comet-Like Activity in Related Objects
objects usually belong to one of a series of “families” that have similar orbital
characteristics. By far the best known of these families is the Kreutz family which
is a group of sun-grazers that includes C/1965 S1 Ikeya-Seki and C/2011 W3
Lovejoy. Other sun-skirting groups include the Meyer, Marsden, and Kracht families. Backward orbital integrations indicate that the Marsden and Kracht families
may be related (Ohtsuka et al. 2003).
Families are assumed to arise from the result of the break-up of a parent object
with tidal forces at a previous perihelion passage being a logical assumption. Further
splitting at subsequent perihelion passages of the fragments is considered likely.
Kreutz family comets appear to arrive in clusters that may point to this process. The
observed morphologies of Kreutz family objects are diverse which may indicate
inhomogeneity of the progenitor.
The absolute brightness of Kreutz family comets appears to peak before they
reach perihelion (around 0.005 AU from the Sun) and then declines as they get even
closer. This would not be consistent with insolation-driven water ice sublimation but
may be caused by sublimation of what are normally considered to be refractories
such as olivine and pyroxene as the surface temperature rises (Kimura et al. 2002).
Use of Eq. (2.100) results in temperatures above 4000 K at 0.01 AU but of course
sublimation will be a strong coolant, the magnitude of which is difficult to quantify,
and the point source approximation for the solar input no longer holds at these
distances.
The loss of more refractory material can provide additional information on the
composition as illustrated by the detection of emission lines from of O, Na, K, Ca, V,
Cr, Mn, Fe, Co, Ni, and Cu in C/1965 S1 Ikeya-Seki (Preston 1967; Slaughter 1969).
C/2003 K7 was observed at 0.016 AU when grains were rapidly sublimated. The
relative abundances of H: C: Si were approximately 1: 0.0035: 0.045 and therefore
carbon-poor compared to what we have seen at 67P (cf. Fig. 4.70). On the other
hand, analysis of observations of C/2011 W3 Lovejoy has provided abundance ratios
by number for H: C: N: O: Si: Fe as 1.0: 0.035: 0.004: 0.5: 0.015: 0.025 respectively
which seems to be more in keeping with the 67P results but with a very high iron
content. Strength estimates from the break-ups of sun-grazers are of the order of
0.5–10 Pa (e.g. C/2012 S1 ISON; Steckloff et al. 2015).
While sun-grazers are spectacular, it remains a challenge to establish their
relationship to other comets and whether we can use the information obtained to
constrain the properties of other cometary objects.
432
6 Comet-Like Activity in Related Objects