JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. A5, PAGES 10,353-10,361, MAY 1, 2000

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 15, NO. A5, PAGES 1,353-1,361, MAY 1, 2 Advanced Meteor Orbit Radar observations of interstellar meteoroids W. Jack Baggaley Department Physics and Astronomy, University of Canterbury, Christchurch, New Zealand Abstract. The Advanced Meteor Orbit Radar (AMOR) facility is providing an extensive data base of both the geophysical (atmospheric trajectory and velocity, height, ionization characteristics, etc.) parameters and astronomical parameters (heliocentric orbital elements, etc.) of Earth-impacting meteoroids of limiting particle radius 2/ m. This continuous operation multi-station complex provides an incisive probe of interplanetary dust orbital characteristics. Close calibration using meteoroid stream orbital elements delineated by other techniques (photographic, video, TV) permits robust dynamical information to be established. This unique technique allows the identification of the source geometry of the influx of extra-solar system particles: a general background influx from southern ecliptic latitudes exists with enhanced areas that appear to be discrete sources. The dominant compact directional inflow appears from the direction of the main-sequence debris-disk star /3 Pictoris. 1. Introduction 2. The AMOR facility The observational telescopic techniques that provide data on the interstellar dust population include stellar extinction, stellar spectral absorption features, scattering properties of star-light, thermal emission, and polarization measurements. These methods apply to sightlines that may extend for distances of many kpc. In contrast, the dust in the Hellosphere can be sampled by interplanetary probes carrying dust impact detectors and by ground-based radars sensing plasma signatures produced by interstellar dust (ISD) particles ablating in the Earth's atmosphere. The Ulysses and Galileo probes yield masses and approximate influx di- rections [Griin et al., 1994] for submicron dust (a size limit dictated by the statistical sample available from the physical size of the impact area). For such small particles, close coupling to the interstellar gas is ex- angles, atmospheric speed, speed measured by diffracpected (and confirmned by the impact characteristics). tion oscillations, atmospheric deceleration, and Doppler However, interpretation is complicated by the influence on the particle dynamics of solar radiation pressure and both interstellar and solar magnetic fields. In contrast, the Advanced Orbit radar (AMOR) facility possesses a relatively high directional accuracy for individual grains and can secure a large data rate and so can provide a valuable contribution to direct interstellar dust moni- toring. Copyright 2 by the American Geophysical Union. Paper number 1999JA //1999JA9383 $9. 1,353 The Advanced Meteor Orbit Radar (AMOR) [Baggaley et al., 1994a] is a 1-kW peak pulsed power phase coherent radar with a sampling rate 4s - designed specifically to fix the atmospheric trajectories and velocities of ablating meteoroids down to limiting grain diameter of 4/ m (mass 3 x 1-7 g) (the value is velocity dependent). The multistation geometry using 8 km baselines measures meteoroid velocity components, while radar echo position is fixed by use of a nar- row (-- 2 ø) fan beam located in the geographic meridian. A multiple spacing interferometer is able to determine echo elevation accurate to <.5 ø [Baggaley et al., 1996]. The archived topocentric parameters for each meteor are echo power time profile, elevation, range, height, radiant (influx direction) azimuth and zenith atmospheric motion. The celestial parameters are radiant coordinates, geocentric velocity components, heliocentric velocity components and derived heliocentric orbital elements semimajor axis a, perihelion distance q, eccentricity e, ecliptic inclination i, argument of perihe- lion w, longitude of ascending node f together with the ecliptic longitude and latitude of perihelion. The facility's location at temperate southern latitudes together with the radar antenna geometry limits the echo radi- ant coverage to declinations +2 ø to -9 ø. The radar complex, designed for continuous operation with.- 13 individual heliocentric orbits daily, is ideal for probing the population of Earth-impacting grains whose orbital parameters indicate an interstellar source [Baggaley et

2 1,354 BAGGALEY: AMOR RADAR MEASUREMENT al., 1994b]. Close astronomical calibration of the ex- open groups will contain a fraction that are mismeatensive orbital set is provided [Baggaley et al., 1992, sured members of the other population. Baggaley 1995] by data for those meteoroid streams for 4.1. Inclination which photographic and TV techniques have previously secured high-quality orbital elements (the r/ Aquarid, One indication of the presence of an interstellar mete- X Scorpidids, ct Scorpidids, Aquarids). ISD parti- oroid component can be provided simply by the geomecles on heliocentric hyperbolic orbits will generally pos- try with respect to the ecliptic plane of the distribution sess high atmospheric influx speeds: near-parabolic r/ of Earth-impacting meteoroids. Solar system dust is Aquarid meteors have an atmospheric mean speed of constrained to near the ecliptic plane with most mate- 65 km s -. The radar speed uncertainty for such high- rial in prograde motion: the small component which is speed meteors is < 3 kms -1 (lcr): calibrations also using retrograde is associated with streams originating from meteor speeds determined by the independent technique short-period comets (e.g., S/P Halley is the progenitor of echo diffraction characteristics confirm the robust orof the r/ Aquarid and Orionid streams). In order to bits measured by AMOR [Baggaley et al., 1993]. The provide a measure of the spatial dust distribution it is heliocentric orbits secured by AMOR have uncertainties necessary to weight the raw characteristics of the orin the angular elements i and w of - 3 ø, and in dimenbital elements for (1) the probability that a given class sions a -1 and q of - 5% and orbital speed 5%. With of meteorold with given q, e, i, Vn will impact the Earth; its sustained survey providing high-grade orbits, AMOR (2) the probability of radar detection, which depends on is a powerful contributor to securing observational inatmospheric speed (because of the dependence of ionizaformation on in[erstellar dust in the inner heliosphere. tion coe cient), and the radar antenna geometry. Figure. 1 shows the inclination distribution corrected for 3. Meteoric Evidence for Interstellar Dust The question of the reality of an interstellar component as probed by meteor techniques has been an outstanding topic over the last 6 years. The problem focuses on the validity of the small proportion of orbits that are measured as hyperbolic but which are possibly closed orbits because of the uncertainty in measured heliocentric speed at Earth orbit. However, with high-precision dynamical data becoming available, the Earth's atmosphere is a valuable detecting agency for ISD particles. 4. Orbital Data The transformation from observations, the recorded time of flight differences of echoes and radar echo position, to heliocentric orbit requires the input of several factors: atmospheric deceleration, Earth's rotation, Earth's gravity, and Earth's orbital elliptic motion [e.g., Baggaley et al., 1994a]. AMOR operates in near-continuous mode, and the archived orbit set for is analyzed here to interpret the hyperbolic contribution in the inner heliosphere. In analyzing the orbital characteristics to test for a real external dust Earth-impact probability and radar function for Earth longitudes 38 ø ø (November through February). It is clear that the two velocity groups are quite distinct. The closed orbit particles have the vast majority in prograde motion, with 4% retrograde similar to photographic data [e.g., McCrosky, 1968] as expected for the solar system dust cloud, whereas the open orbits have orbital planes distributed much more uniformly over the celestial sphere as would be expected for a source which lacks symmetry with respect to the ecliptic plane Fixing the Meteoroid Far-Sun Approach Directions Apse-line orientation is an important parameter in describing the geometry of heliocentric orbits: for example, for long-period comets (possessing near-parabolic orbits) the distribution of aphelion directions is an indicator of source locations. However, a more useful parameter for interstellar particles is the far-sun approach direction which is determined by both the aphelion direction and the hyperbolic asymptotic angle. Rotation from the aphelion direction in the orbital plane (the rotation direction depending on whether the orbit is prograde or retrograde) by the asymptotic angle fixes the particle influx direction. To describe the charac- teristics of the particle inflow directions, maps can be constructed of the ecliptic longitude and latitude / ) of the source direction for meteoroids in the sepacomponent it is useful to distinguish two groups depend- rate closed orbit (asymptotic angle zero) and open orbit ing on heliocentric speed Va' those (just dynamically groups. Figure. 2 shows influx coordinates for individclosed orbits) with 38 _< Vn _< 39 being a population ual meteoroids for the period Earth longitude 38 ø ø 1or less than the parabolic limit and those (just dynam- for the separate closed (Figure 2a) and open (Figure 2b) ically open hyperbolic orbits) with 45 _< Vn < 6. The groups and a grey-scale contour plot of the ratio of the chosen boundary values are +1or of the critical speed angular density of open orbits to that of closed (Figure which varies from 41.7 to 42.5 depending on the Earth's 2c). From the maps, and in conjunction with Figure.1 longitude. For this discriminant each of the closed and three features are clear (1) there is an overwhelming

3 BAGGALEY: AMOR RADAR MEASUREMENT 1,355 8 i i I i (a) IIIIIIIIIIIIIIIIIIIII INCLINATION (deg) Figure 1. Distribution of inclinations after correction for selection effects (see text) for (a) closed orbits heliocentric speed 38 _< Vh < 39 and (b) open orbits heliocentric speed 45 _<, < 6. population of orbits near the ecliptic plane as expected for the solar systems dust complex; (2) there is a poputernal to the solar system and to refine the mean source coordinates. lation of open orbits at all longitudes and south of the 5.1. Model of Orbital Element Seasonal ecliptic; and (3) there exist severalocalized inflow areas the most pronounced being a compact source of,. 3 ø Changes diameter centered at coordinates approximately ecliptic (3 ø, -5ø). All these features which are highly sta- Consider a unidirectional external source of particles specified by ecliptic coordinates ( s, s) having velocity tistically significant are present for all periods of solar longitude. These characteristics cannot be attributed to un- certainties introduced by the orbital element measurements of AMOR. Uncertainties arise from the limited radar pulsing rate so that for any atmospheric speeds in excess of the value corresponding to the parabolic limit for closely retrograde motion ( 73 kms -1) the uncertainties in position are rarely greater than,. 1 ø. For example, the elongation of the compact (3 ø, -5 ø ) source from the apex of the Earth's motion for all solar longitudes is large (; 5 ø) so that the atmospheric speed for these hyperbolic particles is only,. 5 kms -1 with corresponding influx direction uncertainty < 3 ø. AMOR data present strong evidence for the presence of a general inflow of extra-solar systems particles over a wide range of southern latitudes accompanied by more coherent streaming. 5. Orbital Elements of a Particle Stream For an approximately monodirectional stream source it is possible to verify that the meteoroids must be ex- at infinity Vc relative to the Sun and under solar gravity. For Earth impact at any solar longitude from this source, there exists just one heliocentric orbit possible having specified values of i, w, q, f, and impact parameter (L. Neslusan and W.J. Baggaley, unpublished 1998). Let L be the difference in the longitudes of Earth and the source L = ( e - 8). The longitude of ascending node f is fixed by the date of observation. Figures. 3a, 3b, and 3c show the model behavior for representative (for the present use) values of and three different eccentricities (and therefore influx speeds Vo,). For a given source direction, there is a clear cyclic pattern to be expected in i, w, and q. By matching the observed behavior of elements with the dynamical model the centroid coordinates of an external coherent source can be estimated. Meteoroids which provide a consistent presence throughout the year at the same ecliptic coordinates and whose orbital elements change in the ß modeled cyclic behavior cannot be due to sources inter- nal to the solar system Observations Figure. 4 shows the mean values of i, w, q together

4 ß ß 1,356 BAGGALEY: AMOR RADAR MEASUREMENT 9 I I I I I I I I I I I *. ß.... '.,,. '...;'. :.'. : :. ß, "*..ß. ß * ß.' ß:.ßß* * ß ß ß ß '' ß..'_ i'..:',";'...?.'"..:. >"::' '.". ß "',''. :;: ""....:,7...'ß' ' ß ß '. ß < ß.. o...:.. '..'.: -'... :;.'...'. ',..... ß... h : ".-'..:' : ':<.-";." :' :,.'½:'?.5 "':, ':. '. ' " " " 3- :.' : :: '..;. ;,'J.½...:,..'.'..'..-,:.,.o:...../ ,.--,:-..,.,r'---..,... l. w ;: :o! ',.-!.:.-: ß ' ß... ß ß ß. _1.. ' :::t":.'.' -:?..7-_..}' '..Z.: : ;:::,,",: ::-<,. ß..,., '. ß. '... :. '."-.". ß ",.',.,".',-"",.. ' '' ß,I ' ß i,.. ' ':.:.9" : $. :., ß.,.,,., ,..,,.,-..-,,,,.e..... '.. " ', :;.'.. a :..,,,.'z,.,.'..,,.'... :.._.'...&.': '. "ß... '... ;"-:'... '.? ':!' -. ' v... Z,.....,:- ' ß :. ":*:. ß :' '".' "' ' -.' ß ß ':....''½.';:..,;."L ß "'%"r ":.'":'.:.'' '-,,' %' a,< }. ß't..', '. ½..... :i;=' "' ':... :. ' ' '""'"': ' /: :" ;' ;' '" "'œ " ;- I': '.....'.-'... -:.'. v.:.-:. ß :.::.%; ß..:: ß....'.....: : ß.. ß... ß ß ß ß *.. ** * * ß ß ß ß * * ** **t* * * ß....:... ß :..... ß ß.. :..:..:.v'"...' ß. :..' ' : ß.' ß.' ii # *% ß. -*..***. ** ß. *... *.. ß ** ß ':':"".' ß.." ':.' ; ' ECLIPTIC LONGITUDE (deg) 9 i i i i I I i. i i i i ß.. i. *. ***. %.% ß....* *. ß.. ** o ß ß ß ß ** - 3.*... *. i.,o *½* t*_.*..,.... *** 1 ß., ** *... ** * ß.** ß ß.....,.....,: ß......: [ -...v.,. %,. -, :,,.....ß..:..' ß.. '-.-,'. $',... ß ß ß ß: ß. ß ß.. ß. ß ' I ß.,...:" ß '.:' -',, ' :..*-'- /t$ --,-,-',-,-' :-'.z ß j. ß... -z" -, -,, t:.'..:... :'"-- ß... :-..: ß ' '. ' I- [. ':.4-'-' '%..c,:,',,,:..%,,:?.,;, -.;' & ::2: '-5',".-:: ;'-' - ' : ' ß, ";.: oj':.;":.? '.'.'... '<<.:>/.';i;:,::..",r;,'-:. : i::½:½ ),: '..-' --. i : '".:' :.to -[i i: :': ":".,,--'-.-.,e,/-, c.. ' <... '.:.-:-_ ":-... '" :'. - -'. œ&e ' -,,-_ ::", :-- ', '.- '-'. ': '"... ß... t :'-.?. ß ':% ß" [. L%.,,,,-.;::,.,,.:.:,:v..-- na...-.:.,:'-...',- '. :?l. z'.' r ;:... : :.... ß. '..- '-'"': ',," a -,-., i'--:--.'-.. "': ". -.½,... 5'; &. l.?.:,',,,,.' _ o.,:..:,.,..:..:....::tac..,½:.ot...::.,.....,,,..,.::., : ;, 1...,.,,:..,:...:.,..,... "'.'...,.:. '::,..,: : a.i.., -,z,.,,...,.._... ß, X/..;;',.,. ; ½',--,,- :' ','--'", ß,... ' '".'t,-.,,,.,,.;.}i.,. a ' -..., -':',.,,.,.,..,. '." '.'-:" 'r. e.: `..? a. :.` :....`... ae.. ` a....`: ` :. `....: :..`.:`i! t ' '.h ' *J" - :t 7 _. g.' %ct " :::" '"' ' s =Z"ntl ';':' ' -" "" 1'"'ø *.-'W ":'ø'* - Ir.:e :;k,.. :: [.,... ß,.;. ', - z.-. ]... '.t- ' '. ""' ' '"..-'-.-"":'-/7.'.'.-,:"1 :.:.. -.:...: ':" ' ';" ½ ' ::-.''..':.,...½--.- ':'''' ''.z '":"::,-'""'l. o..:,.-,..,-n,'.',',... -,- ",... ', -- '-'e-,, ECLIPTIC LONGITUDE {deg) ECLIPTIC LONGITUDE ( g) Figure 2. (a) Approach directions of individual meteors for closed orbits 38< Vh < 39. (b) Approach directions of individual meteors for open orbits 45< Vh < 6. (c) Approach direction map showing the ratio of the angular density of open orbits to the angular density of closed orbits.

5 BAGGALEY' AMOR RADAR MEASUREMENT 1,357 a (Ecc=1.5) b (Ecc=l.2) c (Ecc=l.4) O 3O A 1.2 ::3 '.8 t.6.4 I:1:.2 Q..8.. o.[7 ' EARTH-SOURCE LONGITUDE (deg) Figure 3. Model behavior of the observed orbital elements of particles from a point source at latitude/ s and different eccentricities (and therefore Voo) for changing longitude with respect to the Earth. (a) Eccentricity 1.5, (b) eccentricity 1.2, and (c) eccentricty 1.4. The Earth-source longitude is zero when the Earth and source have the same heliocentric longitude. For illustration, / s = 2 ø (solid line), 4 ø (dashed line), and 6 ø (dotted line). with their range of values at 3 ø intervals of Earth longitudes summed for the years 1995 to 1998 for all melongitude 38ø-158 ø summed for years ), and since interest lies in the high-velocity tail of the Vn disteoroids within the coordinate region of the selected tribution, then for convenience only Vn _> 3kms -1 is prominent feature 26 ø to 31 ø, -3 ø to -7 ø. A shown (corresponding to orbits with total energy denrange of values is expected because of the finite spread sity exceeding that of a test particle at the Earth's orof influx direction ( A, / ), the range of influx speeds bit). Figure. 5a shows those meteorolds having orbital Vc and measurement uncertainties. Detailed compar- inclinations i < 1 ø so that their far-sun approach diisons of Figure. 4 with the model behavior converge rections are within l ø of the ecliptic plane. Such partiestimates of the compact source centroid coordinates (As,/ s) to 28 ø, -56 ø with an uncertainty of--,5 ø and mean Vc km s -1 (corresponding to e 1.2). cles will have substantially closed orbits (see Figure la) so that the distribution for V > 42km s -1 represents largely uncertainties in orbital speed measurement' the fraction of those of speeds _> 45 kms -1 is 3.5 of the total, consistent with the cr - 3 km s-1 in measurement. The corresponding fraction for influx north of the eclip- 6. Particle Energy Distribution tic (latitudes +3 ø to 9 ø and 3 ø < i < 15 ø) is An additional and quite distinct method of demon- 4.8, whereas that for influx from southern latitudes is strating the extra solar system nature of any compact 17%, confirming the presence of a broad inflow south source is to examine the meteoroid orbital energy distribution. In all previous work an outstanding diffiof the ecliptic. For the discrete feature (within 15 ø of the centroid at 28 ø, -56 ø) the fraction is 33%. This culty in confirming the presence of interstellar particles discrete source exhibits a very clear hyperbolic compohas been the absence of a clear signature in the observed heliocentric velocity distribution: external particles would be evident as a distinct component in the Vn distribution in excess of the parabolic limit, additional to any velocities attributed to measurement uncertainties. Figure. 5 presents Vn distributions (data for Earth nent' there are two populations, one group comprising closed orbits which are in the tail of the error distribution and the other truly hyperbolic with a distinct peak at kms -1 and spread -- 3 kms -1. This feature of the compact source is present at all solar longitudes. The inflow speed to the solar system is found by

6 . 1,358 BAGGALEY' AMOR RADAR MEASUREMENT 15 i i i, i i i i i i i O I I I I I I I I I loo i i i i i i i i i i i i i i i i! i i i i i I--.9 o.., n-.3-13= ' O' I I I I I I I I I I I LONGITUDE Figure 4. The measured variation with season of the orbital elements inclination i, argument of perihelion w and perihelion distance, q, for meteoroids having upstream directions within the discrete feature (< 15 ø of ecliptic 28 ø - 56ø). Crosses are mean values, and lines show the range of values. subtracting the gravitational energy acquired at Earth lar motion direction (ecliptic 269 ø, +5 [Binney and orbit to yield 13kms - in accord with the geometrical Tremaine, 1987]) to yield ecliptic (49 ø, -72 ø) (galactic result (section 5.2). 259 ø, -28 ø) and LSR speed 26 kms Local Standard of Rest Influx Direction Two separate procedures refine the prominant, compact inflow of interstellar particles to upstream direction approximately ecliptic 28 ø, -56 ø and influx speed - 13 km s-. This heliocentric inflow direction and speed can be transformed to the local standard of rest (LSR) by noting the apparent elongation (16 ø ) from the so- 8. Source Candidates Sources that are known to seed the ISM are stars in the late stages of evolution: red giants, stars on the asymptotic giant branch (AGB) and post AGB evolutionary track stars. These are stars in their mass-losing phase characterized by strong stellar winds and which possess cool ( < 3K)extended atmospheres, conditions favorable for gas condensation and the formation

7 BAGGALEY: AMOR RADAR MEASUREMENT 1, I I I I I I I (a) 1 5OO I I I I i! i 6 7 I I I I 1 IJ.I 5OO o- i i i I I i i L ' ' ' ' ' ' ' ' 1 5,,, mm..., % m m m m m m m m 6O 4O - (d) i 8o 2O i i i i i i i HELIOCENTRIC VELOCITY (km/s) Figure 5. Distributions of orbital heliocentric speeds at Earth impact: (a) orbits for < i < 1 ø and -1 < < 1 ø, (b) orbits for < i < 1 ø and 3 < < 9 ø, (c) orbits for 3 < i < 15 ø and -3 < < -9 ø, and (d) orbits for the discrete feature. of dust grains. Other objects known to possess dust environments are pre main-sequence young stellar objects (YSO) with protoplanetary disks and several main sequence stars known to be surrounded by debris disks was given a major impetus by the International Astronomical Satellite (IRAS) observations [Auraann et al., 1984] of many main sequence stars that radiate a flux of IR radiation in excess of that expected from photoconsidered to contain evolved material. Cool red stars spheric emission alone (Vega-type stars). Recent cata- (distances 2 pc) are rare in the solar neighbor- logues [e.g., Mannings and Barlow, 1998] contain some hood and in the travel time of any ejected dust, those stars evolve into other objects making identification hazardous. YSO are very distant (kpc). However, there 2 Vega-like sources in the solar neighborhood ( < 5 pc) with major examples being fi Pic (distance 19.2 pc), a PsA (7.7pc), a Lyr (Vega) (7.8pc), and e Eri (3.2pc). are several debris-disk stars in the solar neighborhood The / Pic prominant dust disk has been successfully ( < 5 pcs) and being long-lived (> 17 years) could be observed (by imaging the scattered starlight by the ciridentifiable sources of detectable solar system influxing cumstellar grains). dust. Although dust ejection models from such objects have been little considered, the debris disks are known to contain large numbers of planetesimals undergoing 9. collisional destruction, evaporation, and gravitational Stellar Kinematics perturbations embedded in a dust population having Aside from stellar evolution in which a dust losing a spectrum of sizes from 1/ m up to- 3/ m [Zucker- star may have significantly evolved (e.g., Mira-type variman and Becklin, 1993]. The study of circumstellar dust ables into planetary nebulae and white dwarfs), identifi-

8 1,36 BAGGALEY: AMOR RADAR MEASUREMENT cation of a possible stellar dust source is complicated by proper motion. In the dust travel time ( 16 yr for a 25 pc distant star and dust velocity VLSR of 27kms -1) the apparent galactic coordinates can change significantly. As an illustration we examine the four major Vega-like sources above and, considering the time-scales involved ( < 16 yr), will assume linear motion rather than a full dynamical model employing a full description of the gravitational potential of the galaxy. The choice of a suitable candidate is constrained not only by the galactic coordinate change during the dust travel time but also by the condition that the emitted dust must celestial position and observed dust speed. However, in the case of Pic, there is close agreement with (258, -28.5) j only 2 ø away from the observed true influx direction and Vej = 29 km s - Note that Pic has a low LSR speed 3.26kms -1 at an angle of 19 ø so that its relative motion is small. 1. Conclusions The AM OR radar facility is a powerful probe of the local ISM: a near continuous survey provides high-grade heliocentric orbits of grains down to the a limiting size diameter 4/ m. This ground-based radar can provide detections which complement those in situ measurements of interplanetary probes carried out by Galileo and Ulysses. AMOR has identified the existence of a widespread south-ecliptic latitude dust influx together with a discrete stream feature. Arguments based on geometry and energy distribution have confirmed the discrete feature as a dust source external to the solar system. In addition, the upstream direction has been fixed. Solar system hyperbolic meteorolds have been observed previously [e.g., Baggaley et al., 1994b, Taylor et al., 1996], but this is the first report of directional properties for individual grains and the first report of a coherent stream and a source identification that matches well with the dust-debris disk star/ Pic. The dust mapping reported here is unlikely to have an origin in the Kuiper belt (since the observed dust has an inflow well away from the ecliptic) or in the have a space speed VctLSR 26kms -1 (as measured, Oort cloud (where the very low orbital speeds of bodies see section 7 above) and assuming a representative stel- would make collision-induced solar-directed speeds of lar dust ejection speed < 4 km s -1. Additionally, those those measured, 13 km s -1, impossible). There exists stars where the observed radial velocity (i.e., relative a population of dust of a quite different mass regime to the Sun) is in excess of approximately + 4 kms - than is accessible to space probes. The present work (recession) may be discounted. has focused on the dynamics of this large dust compo- From the Hil:imrchos catalogue [European Space Agency, nent and mapping directional characteristics. Absolute 1992, 1997] a star's galactic coordinates X, Y, Z, and fluxes of the particles require Earth-impact probabilvelocity components U, V, W (heliocentric) enable the ities and radar detection sensitivity: such fluxes will LSR velocity components to be computed and also the enable the contribution of the radar-detected infiowing angle between the star's sightline and the star's space grains to be matched to the other dust components in motion. The angle ½ between the star's present Galac- the heliosphere. tic position (9, l) and its position (9,1)ej at dust ejection AMOR mapping is able to reliably distinguish inter- is then stellar particles from interplanetary dust. This is signif- V, LSR sin sin ½ = icant because at i AU the contribution of ISD is small V'dL SR in contrast to the regions of space sampled by space where V, LSR is the star's space speed and for simplicity we assume V als -- V, LS COS(18 - ) + Vej with Vej the speed with which dust is ejected from the circumstellar disk of the star (for calculations a free paprobes where the contribution may be 3%. Earth ground-based radar surveys are an important adjunct to spacecraft impact detections of ISD in the solar system: in situ probes on the Pioneer series, Helios, Galileo, rameter -4 km s- ). The angular shift is in the plane Ulysses and the current Stardust and Cassini missions. containing the velocity vector and the line-of-site vec- AMOR-observed grain sizes are very large compared tor. The resulting coordinate (g, 1) j can be compared to those responsible for space probe detections (size with the direction of dust influx into the solar system < 1/ m) where small collecting area and statistics mitobserved galactic (259 ø, -28 ø) from section 7 above. Of the four candidates considered here, three, c PsA, c Lyr and ½ Eri are well removed from the constraints of both igate against sampling of large particles. Astronomical techniques are not sensitive to the detection of large grains. A further possible method of solar system detection of ISD particles is the mapping of IR radiation from grains resulting from solar heating. In contrast to the large material mapped by AM OR, where dynamics is gravity-dominated, dust impact probe grains are strongly influenced by solar radiation pressure and solar and interstellar magnetic fields. Additionally, large particles are much less susceptible to grain-grain collisions produced by interstellar supernova shockwaves [Jones et al., 1997]. The existence of large interstellar particles (size 4/ m) in the heliosphere, if representative of the local diffuse interstellar medium, has profound consequences: the mass loading of the ISM is a critical parameter for models of stellar evolutionary processes and stellar mass loss. Acknowledgments. Janet G. Luhmann thanks Stephen Knowles and another referee for their assistance in evaluating this paper.

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