Using the Supernova / Acceleration Probe (SNAP) to Search for Microlensing Events Towards the LMC

9 Pages·0.14 MB·English

by  David S. Graff

Using the Supernova / Acceleration Probe (SNAP) to Search for
Microlensing Events Towards the LMC
David S. Graff
University of Michigan, Department of Astronomy, Ann Arbor, MI 48109
1 [email protected]
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0
2 Alex Kim1

n University of California, Department of Physics, Berkeley, CA 94720
a [email protected]
J

2
2

1
ABSTRACT
v
5 Microlensing experiments today have a tantalizing result; they have detected an excess of
9
microlensing events beyond what is expected from known stellar populations. These events
3
could be due to a possible form of Halo dark matter. However, study of these events is limited
1
0 by their small number and poor photometric precision. The SNAP satellite, which has been
1 proposedtosearchforsupernovae,wouldbeanidealinstrumenttosearchformicrolensingevents
0 towards the LMC. Due in part to its larger mirror, and largely to its superb space-based seeing
h/ and low background, with a modest program SNAP will be able to detect ∼ 250 microlensing
p events per year. In addition, SNAP will generate ∼ 16 high quality events per year with 1%
- photometric resolution. These data should break the microlensing degeneracy, and determine
o
r whether the events are indeed due to dark matter lenses in the Galactic Halo. Side benefits of
st such a program will include a never before equaled catalog of variable stars in the LMC. On
a of the most intriguing possibilities is a direct measurement of the distance to the LMC. Such a
:
v measurement will determine what is today the most uncertain rung on the distance ladder and
i thus lock down the Hubble constant.
X
r Subject headings: Dark matter — Gravitational lensing
a
1. Introduction events are difficult to interpret owing to the de-
generacies inherent in microlensing: the sole ob-
The principal motive for creating microlensing
servable in an event,its timescale,is a function of
searcheswastosearchfordarkmatterintheform
three parameters, the lens mass, its parallax, and
of discreet lumps, or MACHOs, by searching for
its proper motion.
microlensing towards the Magellanic Clouds. A
If the MACHO events are interpreted as be-
majorresultoftheseexperimentshasbeentoshow
ing due to lenses which are part of the halo, then
that the dark matter is not composed of objects
roughly 20% of the mass of the halo has to be
with mass in the range 10−7 −10−1M⊙ (Alcock made of MACHOs with mass about 0.5M⊙. This
et al. 1998, Lasserre et al. 2000). The MACHO
result would be quite significant, representing the
grouphasclaimedanintriguingpositivedetection
first detection of a component of the Milky Way
oflensing eventssignificantlyabovethatexpected
darkmatter. IftheMACHOswerebaryonic,their
from known stellar populations. However, these
masssuggeststhattheymightbewhitedwarfs,an
1LawrenceBerkeleyNationalLaboratory,Berkeley,CA interpretation which has been strengthened by a
94720 possible detection of halo white dwarfs by Ibata
1
et al. (1999) although their Hubble Deep Field tected a possibly non-baryonicform ofDark Mat-
propermotionsurveyisnotconsistentwithlarger ter, making up a good fraction of the mass of the
ground-basedproper motionsurveys(Flynn et al. Milky Way halo. However, this claim cannot be
2000). However, white dwarfs make poor dark verified until all possible background sources of
mattercandidates(Graffetal.1999;Fields,Freese events, such as previously unknown populations
& Graff 1998, 2000), so there is a real chance of variable stars and lensing due to ordinary stars
that the microlensing experiments have detected have been eliminated.
a new non-baryonic dark matter candidate such A variety of ways have been proposed to break
as primordial black holes (Carr & Hawking 1974; thisdegeneracy,whichweshallreviewlaterinthis
Jedamzik 1997). paper. These techniques either require very high
Microlensing’sability to detect Halo dark mat- signal to noise events, to look for subtle devia-
ter will always be cast in doubt as long as the tions from the standard microlensing light curve,
microlensing events could be due to a possible or they require many more events than have been
background. One possible background is a rare detected; either to do meaningful statistical tests
populationofvariablestars,hithertounknownbe- on the events, or to look for rare exotic events.
cause their frequency is so low (∼10−8). Some of We will show that the proposed SNAP (Su-
the detectedmicrolensingeventshavehighsignal- pernova / Acceleration Probe) satellite would be
to-noise and so precisely follow the microlensing an ideal tool to search for microlensing towards
light curve that their interpretation is unambigu- the LargeMagellanicCloud, andperhapstowards
ous. Other microlensing events are detected with the Small Magellanic Cloud. With a relatively
less signal-to-noise. During the course of the mi- small amount of observing time, it could detect
crolensingexperiments,awholeclassofsuchstars ordersof magnitude more events than the current
was discovered and eliminated from the samples, ground-based experiments. In addition, it would
thesocalled“bluebumpers”. However,theremay generate a reasonable ensemble of high signal-to-
be other sorts of variable stars lurking in the H- noiseevents,whichwouldeliminatealldoubtsthat
R diagram. For example, the EROS group has these events are indeed microlensing events, and
shown that one star in the LMC had a bump in allow them to be classifiedas Halo lenses or LMC
its light curve resembling sufficiently a microlens- lenses.
ing curve to be classified as a microlensing event.
Fiveyearslater,itslightcurvehadasimilarexcur- 2. SNAP
sion,suggestingthatatleastthisstarisavariable.
Both microlensing searches and supernovae
Anotherpossiblebackgroundisapreviouslyun-
searches require similar telescopes that are used
known population of ordinary stars acting as ei-
insimilarmannerssincebothexperiments involve
ther sourcesor lenses,sometimes called“selflens-
searchingfora point-likeobjectthatgetsbrighter
ing”. These stars would cause true microlensing,
and then dims over the course of tens of days. In
but would not indicate the presence of dark mat-
fact,theMACHOgrouphasfounddetectedsuper-
ter. Althoughsevereconstraintshavebeenputon
novae to be a substantial background in their ex-
this hypothesis (Gyuk, Dalal & Griest 2000), it is
periment(Alcocketal.2000a)andtheirtelescope
still viable. Weinberg (2000) has suggested that
has been used to search for supernovae (Reiss et
the LMC could be distorted out of virial equilib-
al. 1998), while the EROS group has a supernova
riumbythetidalforcingoftheMilkyWayinsuch
search program using their telescope (Hardin et
a manner as to have a large thickness (and thus a
al. 2000). In both types of experiment, a large
large microlensingoptical depth) while still main-
field is sampled frequently, with a frequency of
taining the low velocity dispersion measured by
hours to weeks. The “new” image is subtracted
several groups, and reviewed by Gyuk, Dalal &
from a reference field, and one looks for a point-
Griest(2000). Graffetal.(2000)reportapossible
like bright spot, which could be either due to a
detection of such a population along the line of
supernova, or the amplification of a microlensed
sighttotheLMC.Forarecentreviewoftheinter-
star. In both types of experiments, a “trigger”
pretationofmicrolensingevents,see Graff(2000).
allowsdetectionoftheeventinrealtime,allowing
Insum,microlensingexperimentsmayhavede-
2
for further, more intensive study. The primary observing resources.
difference between the two types of experiments
is that microlensing observations must be done 3. SNAP’s ability to detect microlensing
towards fields rich in source stars with low back- events
ground contamination, while supernova searches
In this section, we will estimate how many
arebestdone towardsdeepgalaxy-richfields with
events could be detected by a microlensing pro-
few foreground objects.
gram involving SNAP. We will follow the opti-
The Supernova / Acceleration Probe (SNAP)
mal microlensing formalism developed by Gould
is a proposed satellite-borne telescope designed
(1999). This formalism assumes that the mi-
specifically for the detection and high-precision
crolensing experiment will use image subtraction
observation of cosmological supernovae. Since
to find microlensing events even when the source
searching for microlensing events is so similar
star is not isolated in the field, sometimes known
to searching for supernovae, it is unsurprising
as “Pixel Lensing”. SNAP will be well suited
that both the hardware and software capabilities
for pixel lensing since the image subtractiontech-
of the SNAP program are ideally suited to mi-
nique will already be implemented for its super-
crolensing. SNAP’s mission objectives require a
nova search program.
large field imager, high signal-to-noise observa-
The number of microlensing events detected
tions of point sources, frequent light-curve sam-
will depend on the strategy adopted by the mi-
pling,andsufferfromtelemetryandpointingcon-
crolensingexperiment. Forthepurposeofthispa-
straints. In the following, what we describe as
per,weshallassumethatSNAPtakesimageswith
“SNAP” is based on a feasibility-study design
exposuresof140seconds(whichcanaccommodate
(availableathttp://snap.lbl.gov). Ongoingdesign
its current feasibility-study draft telemetry rate).
andtradestudieswillformalize(andperhapsmod-
Following this strategy, SNAP could image 120
ify)SNAP’smissionparameters. Theyshouldnot,
deg2 of the LMC in 5.3 hours. The bulk of obser-
however,do so in a way that significantlychanges
vations should be made in one filter. Occasional
the characteristicsimportant for microlensing sci-
observations can be be made in other pass-bands
ence.
to provide color information on variable stars.
SNAP’s wide field of view is provided by a one
SNAP fields will be directed towards both
square-degree optical imager. It and the optical
Ecliptic poles and revisited with a frequency of
telescopeassembly’sphotongatheringcapabilities
∼ 4 days. We will assume that while SNAP is
are distilled concisely into Table 1, which gives
pointedtowardstheSouthEclipticpoleitwillob-
the signal-to-noise obtained from 140 second ex-
serve the entire LMC in addition to its supernova
posures of point sources of differing magnitude.
fields. This sampling frequency of four days is
Note that the noise is dominated by the source
short compared to the typical measured Einstein
itself and not background zodiacal light.
Radius crossing time of 40 days and the short-
SNAP’s current design maintains a fixed sin-
est measured Einstein Radius crossing time of 18
gle face directed towards the Sun; this constrains
days, so these events should be detected without
observing fields with full-time accessibility to be
much trouble. Additional target of opportunity
towards the ecliptic poles. Fortunately, however,
time could be allocated for deeper, better time-
theLMCisnearlyattheSouthEclipticpole,with
an ecliptic latitude of −85◦. The SMC would be sampled photometry and color measurements of
particularly interesting microlensing events.
a bit more difficult, with an ecliptic latitude of
−65◦. The Galactic center could not be observed Following the Gould (1999)estimate, the num-
ber of microlensing events that can be detected
by SNAP under its current design since it is near
with a particular χ2 significance level depends on
the ecliptic.
A modest search for t ∼ 20−100 day LMC thesignal-to-noiseratioofindividualobservations,
E and on the number of stars in the field, which in
microlensing events could be folded into or incor-
turn depends on the luminosity function of the
porated in the supernova search. Finer temporal
LMC and the surface brightness of the field in
resolution could be achieved with a more ambi-
question. The signal-to-noise of the current gen-
tious microlensing survey using more of SNAP’s
3
Table 1
Signal to Noise for exposure of 140 sec
Magnitude
Filter 20 21 22 23 24 25
V 135 85 53 32 19 10
R 157 99 61 37 22 12
I 128 80 49 29 16 8
eration of ground based experiments is severely terpret these events by using HST to count how
limited by the crowding of several stars into their many unresolved stars there are. SNAP too will
super-arcsecond seeing. With its larger mirror, measure microlensing in unresolved source stars,
andmostimportantly,withitssuperbspace-based but HST followup will also be unable to resolve
seeing, SNAP should easily achieve a signal-to- these stars. Interpretation of these SNAP events
noise far in excess of MACHO or EROS, or even will require either modeling the LMC luminos-
the proposed next generationexperiment (Stubbs ity function or measuring the luminosity function
1998). Furthermore, unencumbered by weather with a better resolution telescope such as NGST.
and the moon and covering every one of the 120 In addition to the shear increase in quantity,
fields during eachobservingrun,its efficiency, the SNAP will also provide a large sample of high
fractionofmicrolensingeventsoccurringwhichare quality events. An examination of Table 1 shows
actuallydetected,shouldbecloseto100%ascom- that under the campaign we propose, SNAP will
pared with less than 50% for the MACHO exper- have 1% photometric resolution on all events
iment (Alcock et al. 2000a)and less than 30% for brighter than V < 21. Nearly all LMC events
the EROS experiment (Lasserre et al. 2000). discovered to date by the ground-based experi-
Weestimatetheexpectednumberofmicrolens- ments are at least this bright, thus, all of these
ingeventstobefoundbyourproposedexperiment eventswouldbefollowedwith1%photometricres-
by scaling from the ground-based “next genera- olution. SinceSNAPwouldcoveralargerfraction
tion”microlensingexperimentproposedbyStubbs of the LMC than the ground based experiments
(1998) and whose detection rates were given in under our test observation strategy, and since its
Figure 2 of Gould (1999). The majority of SNAP efficiency would be higher, it would find roughly
events will come close to the detection threshold 4 times as many bright events as the MACHO
(e.g. R=24.5foraconservativeS/N =15)where collaboration finds (one factor of 2 coming from
a 140 second SNAP exposure is equivalent to a the higher efficiency, and one factor coming from
220minute groundexposureatnew moon2. From the increasedareaofourproposedSNAPsurvey),
Gould (1999) this observing depth will produce or perhaps 10 bright events per year.
∼ 245 events per year of current ground-based We can make a quantitative estimate of this
quality across the entire face of the LMC, factor- number as follows: The detection rate of mi-
ing in the difference between theoretical and real crolensing events is
MACHO events.
2 τǫu
0
The ground based experiments frequently see Γ= N∗ (1)
π t
microlensing in unresolved source stars. They in- E
where N∗ is the number of observed stars, τ is
2At these magnitudes, SNAP is source noise dominated the optical depth, ǫ is the efficiency of detection,
whereas the ground experiment is sky dominated; SNAP u ≈ 1 is the minimum impact parameter for an
is thus faster by the ratio of sky (168 e−/sec) and source ev0ent to be classified a “detection”, and t ≈ 40
fluxes (3 e−/sec). Mirrorapertures and quantum efficien- E
days is the typical Einstein radius crossing time
ciesgivesimilarcountsforthesamesource.
of the event. This method is applicable in the
4
case where the source stars are not crowded, true notbe donewithSNAP withitsenormousfieldof
for the brighter sample of stars under considera- view: with proper coordination, HST could serve
tionherewiththeexpectedSNAPresolution. Ac- instead.
cording to the MACHO collaboration (Alcock et The microlensing parallax signal scales as the
al. 2000a), the optical depth towards the LMC is squareoftheeventtimescale,sowewouldonlybe
roughly τ ≈ 10−7. Using the luminosity func- able to measure the parallax of the longer events.
tion of Alcock et al. (2000b), we see that there Thus, the parallax technique would not be sensi-
are roughly N∗ ≈ 2.5 × 107 stars brighter than tive to the possibility that, for some reason, the
V <21 in the LMC. Assuming that the efficiency longer events are due to LMC self lensing while
ofdetecting suchbrightevents is ǫ≈1, we expect the shorter events are due to, e.g., brown dwarfs
an event rate of 16 bright events per year. in the halo. Therefore, we note other methods
SNAP will be able to use to break the microlens-
4. How will SNAP data resolve the Mi- ing degeneracy which will make use of the shear
crolensing Degeneracy? quantityofmicrolensingeventsdetectedbySNAP.
IfthemicrolensingeventsaretrulyduetoHalo
Thereareseveralproposedtechniquestodeter-
lensing,thenwewouldexpectonlyasmallchange
minewhethertheobservedlensingisindeeddueto
in the optical depth and time scales of microlens-
lenses in the Halo. These techniques have hereto-
ing events across the face of the LMC. The MA-
fore been limited by the low quantity and quality
CHO group (Alcock et al. 2000) has already used
ofmicrolensingevents. However,thetwoordersof
this technique to rule out one particular model of
magnitude increase in quantity of SNAP events,
LMCselflensing,buthasnotruledoutallrealistic
and order of magnitude increase in photometric
models of self lensing, being hampered by the rel-
precision will allow these techniques to come into
atively low number of events in their sample, and
their own.
by the limited spatial extent of their fields. The
As noted above, SNAP should detect some 16
SNAPprogram,withitsabilitytodetecthundreds
events with 1% photometric resolution and occa-
ofeventsacrossthefaceoftheLMC,wouldbeable
sional color measurements. If these events con-
to clearly rule out models of LMC self lensing.
form to microlensing lightcurves, there should be
In addition to being uniform across the face
nofurtherdoubtthattheyareduetomicrolensing
of the LMC, the optical depth must also be uni-
and not to some exotic unclassified variable star.
form across the H-R diagram. Zhao, Graff &
Gould (1998) suggested that if a typical LMC
Guhathakurta (2000) have suggested that under
microlensingeventismeasuredeverydaywith1%
LMC self lensing models, the source stars will
photometric resolution, then one could measure
tend to be towards the back of the LMC. They
the leading term of its microlensing parallax (the
should thus should be somewhat more reddened
mean acceleration of the earth) (Gould 1992), a
and somewhat dimmer than the LMC field stars.
small deviation from the standard microlensing
The MACHO group has looked for this effect in
light curve due to the motion of the earth around
their sample, but has been unable to draw firm
the sun. In that case,one canstill not completely
conclusions owing to the relatively small number
break the degeneracy of the microlensing event,
of events in their sample (Alcock et al. 2000c).
but it should still be possible to separate halo
Also, by verifying that the much larger sample of
lensing (which should have detectable parallax)
events are evenly drawn from the HR diagram,
from LMC lensing (which should not). This tech-
SNAP will confirm that the events are true mi-
niquecouldonlybeappliedtoeventsbrighterthan
crolensing events and not due to some heretofore
V <21 and as discussed above, SNAP should see
unknown type of variable star.
∼16 of these events per year.
By generating a large number of events, SNAP
Tomeasuretheparallaxofthese16events,they
will generate a decent sample of rare events, such
should be followed up daily. Since only 4 of these
as finite source size events (Alcock et al. 1997),
events will be visible at any one time on aver-
amplification of a binary source (Han & Gould
age, this daily follow-up requires an additional 18
1997)or events due to a binary lens (Schneider &
minutes per day on average. This follow-up need
Weiss 1986). All of these variations on the stan-
5
dard microlensing curve allow a partial breaking 5.1. Variable stars
ofthe microlensingdegeneracy,byuncoveringone
Microlensingsurveyshavealreadybegunarev-
of the two hidden parameters, the parallax of the
olution in the study of variable stars (e.g. Bauer
lens and its proper motion (relative to the source
etal.1999,Alcocketal.1999,Udalskietal.1999).
in both cases). Still, this partial breaking will be
Thetwoorderofmagnitudeincreaseindepthand
enough to simply distinguish between the Halo-
precisionmadepossiblebySNAPcannothelpbut
lensinghypothesisandtheself-lensinghypothesis,
increase the pace of this revolution.
though it won’t be able to do finer measurements
The ground based microlensing experiments
of, for example, the mass function of the lenses.
have been limited in their studies to relatively
As shown by Honma (1999), high amplifica-
bright, high amplitude stars, such as Cepheids.
tion events andbinary lens events should be com-
SNAP will be able to generate similar catalogs of
pletelysolvableifintenselymonitoredfromasatel-
dimmer variable stars, such as δ Scuti stars, and
litesuchasSNAPbyusingthemotionofthesatel-
of stars with low amplitudes, such as CP stars
lite as its orbit moves the satellite in and out of
(Hensberge 1993). SNAP will also possibly dis-
the caustic. The non-linear motion of the satel-
cover new types of variable stars which had been
lite as the satellite orbits the earth causes a small
missedin the past because they are rareandhave
(1%)deviationintheshapeofthelightcurvedur-
small amplitudes.
ing the caustic crossing. This technique will only
There are too many types of variable stars to
work if the caustic crossing time scale is about as
detailindividually inthis paper. However,wecan
long, or longer than the satellite orbit time, 7–14
layoutthegeneralbenefitsofsuchaprogramthat
days. Thus,theHonmatechnique canonlybe ap-
will apply to all types of variable stars.
plied to events which are known to be self lensing
events, which have much longer caustic crossing AllthevariablestarsfoundbySNAPwillbeat
time scales than halo lensing events. Still, this the same distance (to the extent that the LMC is
technique, by directly solving the event, will re- thin), and with relatively low reddening, facilitat-
solve the mass function of the binary population ing the construction of color magnitude diagrams
oftheLMC.Thatis,wewillbesureofthemassof ofvariables,perhapsyielding their mass,age,and
the componentsofthe lens,whereasnow,wehave metallicity. Theefficiencyoffindingvariablestars
to estimate the mass of the lens statistically, and will be constant across the face of the LMC and
based on assumptions of the parallax and proper well modeled; thus we will be able to characterize
motions of the lenses. the frequency of variable stars. The high reso-
lution of SNAP will find many variable stars in
Insummary,SNAPwillbeabletobreakthemi-
LMC clusters, allowing a measurement of their
crolensing degeneracy using several independent
age. Perhaps most importantly, the LMC has a
techniques, and thus unambiguously determine
lower metallicity than the Milky Way, though the
whether or not the lenses lie in the Halo or in
ageandperhapsenvironmentsaresimilar,sostud-
the LMC. SNAP thus has the potential to iden-
ies of LMC variable stars will be able to examine
tify a large component of the dark matter in the
the effect of metallicity on variable stars.
Galactic Halo.
For all variables which are driven through in-
5. Non-microlensing programs ternal pulsations, even those with low amplitude,
SNAPwillgeneratepreciselightcurves. Studiesof
Inadditiontothemicrolensingresultsfromthis the detailed shapes of light curves of stars driven
LMCprogram,therewillbeotherscientificresults by internal pulsations will yield new measures of
whichmaybe just asinterestingas the microlens- the internal structure of these stars.
ing results themselves. It is beyond the scope of
this work to detail them all, and we challenge the 5.2. Distance to the LMC
astronomicalcommunity,especiallythe expertsin
The distance to the LMC is at present the
variable stars, to propose further benefits of these
mostuncertainrunginthecosmicdistanceladder.
observations.
Cepheids in the LMC are well studied (and will
be even better studied by SNAP as discussed in
6
theprevioussection)andareusedtocalibratethe where the seeing is of order the pixel size (the
distance to other galaxies, which may in turn be V band is undersampled for SNAP) will depend
usedtocalibrateotherdistanceindicators. Witha on the details of the point spread function and
major campaign involving the Hubble Telescope, the detailed shape of the individual pixels. Given
much of the traditional uncertainty surrounding the possibility of dithering and the stability and
the Hubble Constant has dissipated, to the ex- planned study of the PSF, we can assume that
tent that the reported values of the Hubble con- the astrometric precision is roughly the seeing
stant tend to vary within a narrower band than times the photometric uncertainty. Thus, for a
the factor of two common ten years ago. For ex- single pointing towards a 20th mag star, SNAP
ample, Gibson et al. (2000) report a value of should have an statistical astrometric precision of
H = 68±2±4kms−1Mpc−1 while Saha et al. ∼1.5mas.
0
report a value of H0 =60±2kms−1Mpc−1. The more crowded LMC fields may have some
Both of these measurements assume that the 106 stars as bright as magnitude 20, and we are
distancetotheLMCis50kpc,butthepresentday interested in the average parallax of these stars.
uncertainty of the distance to the LMC is larger Hence, the astrometric precision may be limited
than the reported uncertainties (which do not in- by the number of distant astrometric references
clude the LMC distance), or than the difference rather than by the number of foreground LMC
betweenthesetwovalues. Incontrastto the Hub- stars. There are about 30 quasars brighter than
ble Constant, there continues to be disagreement mag 20 in each 1 degree square SNAP field, not
overthedistancetotheLMC.Thisdistanceisusu- enough to achieve the required statistical preci-
ally measured with standard candles, lately nor- sion. Ibata et al. (1999) used compact bright
malizedagainstlocalcandles calibratedwith Hip- galaxies as their reference. They found 50 in the
parcos. For example, Feast and Catchpole (1997) 0.0015 deg2 Hubble field of view, so there should
find a distance to the LMC of 55±2.5 kpc using be ∼3×105 inthe SNAP field (roughly the same
trigonometric parallaxes of Cepheids while Udal- as the number of brighter stars).
ski (2000), using a variety of distance indicators, With an exposure every 4 days for a period of
finds a value of 44 kpc. 4 years, there will be 365 total exposures. Thus,
By co-adding 100 images of the LMC, SNAP the totalstatisticalparallaxprecisiontowardsthe
shouldineffectgenerateatotalexposureof14,000 LMC is roughly
seconds covering the entire LMC. This will gener-
ate an HR diagram containing 100 million stars σ ≈0.14µas 365 3×105 1/2 . (2)
π
measured with 1% photometric resolution, down (cid:18)Nexp Nobj (cid:19)
to V < 25, or down to absolute magnitude of
This estimate of the parallax precision is about
M ∼6.5. This data set will allow a largergroup
V 0.6%oftheparallaxoftheLMC,20µas,compared
of standard candles to be used to calibrate the
to the present day uncertainty of about 20%.
LMC distance. The extra precisionwill alsoallow
Thisestimateonlydealswiththe purelystatis-
a study of the orientation and three dimensional
tical uncertainties involved with such a program.
structure of the LMC.
This project will obviously be limited by system-
The most satisfying solution would be to di-
atic uncertainties, which cannot be estimated at
rectly measure the distance to the LMC. With
this stageinthe designofthe satellite. Thesesys-
such a detailed study of the LMC, and with such
tematic errors may prove too large to be useful.
highresolution,itmaybepossibletodirectlymea-
Nonetheless, with such high statistical precision,
sure the distance via trigonometric parallax. The
it is worth pursuing such a program in the hopes
ability of SNAP to measure parallax will be lim-
that the systematic uncertainties can be beaten
ited by its relative astrometric precision. For the
down to an interesting level.
moment, in this paper, we will discuss the purely
statistical limits (due to photon noise, and a lim-
6. Conclusion
ited number of astrometric sources) to the astro-
metric precision. We have shown that the SNAP satellite will
Theastrometricprecision,especiallyinthecase haveordersofmagnitudegreatersensitivitytomi-
7
crolensing events than ground-based searches. A Fields, B., Freese, K., & Graff, D.S. 1998, New
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Flynn, C., Sommer-Larsen, J., Fuchs, B., Graff,
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D.S. & Salim, S. 2000, to be published in MN-
photometric resolution. Armed with these data,
RAS, astro-ph/9912264
it will be possible to identify the location of the
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Part of this work has been funded by NSF
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gan. We thank Andy Gould, Saul Perlmutter,
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