The origin of metallic atoms and ions in the cometary comae is investigated theoretically. Two effects are revealed in the comas of bright comets: (1) the Na anomalous type effect is possible within the gas-dust jets of comet P/Halley 1986 3 due to cooling cometary dust by cryogenic gas flow from the nucleus; and (2) the production of ions of refractory elements (Fe(+), Si(+), etc.) at large heliocentric distances is possible in the comas of the Halley type dusty comets due to high-velocity impacts between cometary and zodiacal dust particles. Spectral observations of comets with high sensitivity and spatial resolution are important for studying both comets and interplanetary dust

Ibadov, S.

English

NASA Technical Reports Server

Asteroids, Comets, Meteors 1991, pp. 241-244
Lunar and Planetary Institute, Houston, 1992 5S,,e" 9 0 241
Z,. z 0
NET/_LI.IC AT_IS AND ION8 IN i_NETSm 1_3NET HALLEY 1986 Ill
S. Ibad_
Institute of _trophysics, Du_hanbe 734670, USSR
Abltract
The origin of metallic atoms and ions in the cometary comae is
investigated theoretically. Two effects are revealed in the comas of
bright comets_ namely Na anomalous type effect is possible within the
gas-dust jets of comet P/Halley 1986 Ill due to cooling cometary dust by
cryogenic gas flo_ from the nucleus, and the production of ions of
refractory elements (Fe+, Si + , etc.) at large heliocentric distances is
possible in the comas of the Halley type dusty comets due to high-velocity
impacts between cometary and zodiacal dust particles. Spectral
observations of comets with high sensitivity and spatial resolution are
important for studying both comets and interplanetary dust.
Introduction
The investigation of a metallic component in comets is important in
different aspects. The emissions of the refractory metallic atoms (Fe, Niv
Sitetc.) have usually been detected in Sun-grazing comets at heliocentric
distances R _ 0.01AU (see_ e.g., Preston, 1967; Oppenheimer_ 1980, and
references therein). Neanwhile_ in situ measurements carried out Nithin
the coma of comet P/Halley 1986 III by VEGA and GIOTTO spacecraft near R =
1AU led to the discovery of metal ions of the Fe + type (6ringauz et al.t
1986; _rankowsky et al._ 1986; Balsiger etal., 1986).
Anomalous distribution (i.e. intensity maxiu displaced frum the
cometary nucleus towards the Sun) of the Na D-line emissions in the head
of the bright comet Nrkos was detected by 200-inch Palomar telescope in
1957, the origin of which is the puzzle in the physics of comets (of.
6reenstein and Arpigny_ 1962; Wurm, 1963; Dobrovolsky, 1966; Ibadov,
1985).
Netallic atoms have the Io_ ionization potentials and in this
connection the short photoionizatLon time scale - of the order of 10 4 sec
at R = 1AU (5hk!ovsky_ 1960; Wurmw 1963), so that generation mechanisms
of metal at_ may also be important in the problem of the ionization of
the cometary matter, particularly_ of the inner coma, giving the onset of
the ray structure of the ion tails of comets (see Narochnik v 1964; Jackson
and Donn, 1%8; _unm, 1968).
In the present report the problem of the origin of metal atoms and ions
in cometary comae related to the data from VEGA and GIOTTO spacecraft is
considered taking into account both modern vie_ on the physical conditions
in the near-nuclear region of bright comets and high-velocity interaction
between cometary coma and Zodiacal dust cloud.
Possible analalous distribution of sodium atom elission in the
comet H_lley
c_ of
Sodiu_ atolmissionsare observed at heliocentric distances R _ 1AU not
only in the cometary heads but also in the type II cometary tails {see
Levin, 1964), i.e. at cometocentric distances which are essentially more
https://ntrs.nasa.gov/search.jsp?R=19930009981 2020-03-17T07:31:15+00:00Z
242 Asteroids, Comets, Meteors 1991
than the photoionization scale length for sodium atoms. This indicates
that the source of metallic atoms are dust particles emitted from the
cometary nucleus and irradiated by solar radiation.- -
The rate of evaporation of dust particles depends on their temperature
exponentially, so that for the appearance of the anomalous distribution of
free sodium atoms in the cometary head it is essentially not only the
character of a distribution of dust particles concentration (for_
example, the presence of a dust envelope) in the Coma, but also of dust _
particles temperat_-_ T = T(r). This question is actual and requires the
special consideration in connection with the essential progress, achieved
during the last years in the understanding of the physical Conditions in
The temperature of gas in the inner comae of both telescopic and bright :
comets is extremely low: T_ _< _20_OK at c_etocentricfiistances r_< 10_ km
(see, e.g., Shimizu, !9"16; Bisikalo and Strelnitskij, 1985; _ov and
Shematovich, 1987). Along with that the density of molecules near the
nuclei of bright comets is sufficiently great (n >.10 f3 molecules/cm
see Dobrovolsky, 1966 p.204; Shulman, 1987 p.18),_ so that the application
of a vacuum approximation to the thermal regime of dust particles of the
cometary atmospheres become invalid: the depression of the temperature of
cometary dust due to cooling by intense gaseous flow from the subso!ar
zone of a nucleus is possible (Ibadov, 1987a) The displacement of the
zone of the onset of dust particles evaporation and, respectively, of the
zone of maximal concentration (and emission) of sodium atoms towards the
Sun is determined as
7x105 /k 1/2 ,
s
= 2.5 [0_ Vg Mg R3/2 /(s 1/4 3e3/4 _g ks)]1/2 = {rmax 2xi06 /k 1/2
Here the numeri_cal estimate is made for comet Hailey 1986 iiI, when the
formula parameters (in CE_) have the following values: accomodation
coefficient of the gas molecules (of the H: 0 type) on the surface of dust
grain M (Tg < 200 K) = I; the local mean thermal velocity of molecules_
of gas Y_ (_ 200 K) = 5xi0 4 cm/s; the gas production_rate of _the
nucleus _$ (_ = O.B AU) = 4xlO _ g/s, the molecular welght of subii_ting _
molecules _] = 18 (Gringauz et al., 1986); the integra! c_ffkc_ents Of an
absorption df the solar radiation and a heat emission from dust grain
E--_e=l for the upper line (corresponds to strongly absorbing particles
with the albedo A -_O) and _ = ge =#.Ifor the lower line (corresponds to
metallic particles); k_ = Sa /S_ is the anisotropy coefficient of matter
ejection from the cometary nucleus, S_ is the area of the effectively
emitting subsolar zone (i.e. the area occupied by gas-dust jets) of the
nucleus, S_ is the total surface of the nucleus, so that at k_ = 0.1
(Keller et al., 1986) we have rm¢x= 20 km and 70 km for the two types of
dust particles, .correspondingly. During the periods of cometary brightness
outbursts when M_ increases by several times r_ax= tO0 _. ?
Thus, the d£sp!acoment of the brightnes_ maximum o_= the sodium
D-lines (and emissions of other atoms and molecules ejected from grains)
towards the Sun to the cometocentric distance r_t_X = 20-70 km is possible_.....
within gas-dust jets of comet Halley due to depression of dust temperature
Asteroids, Comets, Meteors 1991 243
(and its evaporation rate) caused by expanding cryogenic relatively dense
gas in the nearnuclear region. As shows above formula, the gas production
rate of a comet may be determined, if the value of a displacement of
emission maximum of the sodium type volatile metal atoms is determined
from observations. So, it is important spectral observations of comets
with high spatial resolution.
Origin of refractory metal ions in the coma of comet Halley 19B6 III
r_e presence of the Fe+ type ions in the comet Halleyls coma was
discovered by the VEGA and GIOTTO in situ measurements on march 1986 (see
6ringauz et al., i987). At the same time calculations show that
at heliocentric distances under consideration (R >_0.8 AU) the sublimation
oi=refractory dust particles of the cometary coma in the field of
electromagnetic and corpuscular radiations of the Sun is negligibly small
for appearing the detectedions of the Fe type (Ibadov, 1980; Dobrovelsky
and Ibadov, 1981; Ip and Axford, 1986; _eiss et al., 1986).
In the head of comet Halley the dust to gas production rate ratio _ ---
_ / _. > 0.1 (5agdeev et al., 1986). The passage of such dusty comets
throug_ the Zodiacal dust cloud is accompanied, mainly, not by the meteor-
like, but by the explosion-type process, namely by production of expanding
plasma clots from high-velocity collisions of cometary and zodiacal dust
particles (Ibadov, 198761.
Using the continuity equation for the mass flow of the ionization
products of colliding dust particles we can find the relation for the
quasistationary concentration of metallic ions in the region of collision,
n ._(re, R); re represents the action radius of the explosion-type mecha-
niem, i.e. the cometocentric distance where the intense transformation of
dust particles onto plasma clots takes place.
According to GIOTTO measurements the concentration of ions in the
cometary coma varies as 1/r in the region of r ~< 10 4 knm (Balsiger et
al., 1986). Taking it into account, the radial distribution of the concen-
tration of ions, produced from collision of cometary and interplanetary
dust, may be presented in the form
ni(r,R) = [(k m Alp s V)/(m i Vi )](r e / r), re = N d Odd /(4=d Vd ks)"
Here k _ is the coefficient for producing plasma clots from cometary and
interplanetary dust; A _ is the mean abundance of the kind i element in
the colliding dust grains (in stony meteoroids A_ = 0.15, 0.2, 0.15 for
Fe, Si, Ng, respectively); 2s - v_: (R) is the spatial mass density of the
Zodiacal dust cloud; V -:V(R) is the relative velocity of the colliding
dust particles; m_ is the ion mass; V_ is the expansion velocity of the
coma ion Component; O_id= 2_a = ks the effective cross section for the
collisions of cometary and zodiacal dust particles, m d is the probable
mass of the coma dust particles, Vd is the mean velocity of their
outflowing from the nucleus.
Using the value of the concentration of Fe + ions measured by Veqa-2
spacecraft n_ (r = 2xI0 s cm, R = O.B AU) =I ion cm "a (Sringauz et_al.,
1987) and accepting k_ = 2, A L(Fe) = 0.2, V = BxlO a cm/s, m i= 10 "_ g
244 Asteroids, Comets, Meteors 1991
V_ - 10 5 cm/s, _I_ (R = 0.8 AU) 10T g/s, old 6xlO "J cm _
10 ¢_g, V_ = 5x104 scm/s, k = 0.3 (Sagdeev et al., 1986; Keller et
az., fi,d by above fo la = 0.8 = 6x10 g
This value corresponds to the expected mean mass density of the interpia- ::
netary dust (see, e.g., Stun et al., 1985).
Conclusion
Distribution of the emissions of atoms of volatile metals, such as Na, may
serve as indicators of physical conditions in the inner coma of comets,
particularly, -_n- the gas-dust jets.-- _c::_ _ =- _=_- : _ _=_
Ions of refractory elements, such as Fe + , detected in the coma of
comet Halley 1986 III by VEGAan_d GI_OTTO spacecraft, _y be generated due
to high-velocity impacts between cometary and zodiacal dust particles, and
may serve as indicators of interplanetary dust Clbu_= = _- _.
Spectral observations of comets with high sensitivity and spatial= _-
resolution are important for studying-both comets and interpianetary dust.
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Metallic atoms and ions in comets: Comet Halley 1986 3

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