Astrophysics in 1998

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Title
Astrophysics in 1998
Permalink
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Journal
Publications of the Astronomical Society of the Pacific, 111(758)
ISSN
0004-6280
Authors
Trimble, V
Aschwanden, M
Publication Date
1999
DOI
10.1086/316342
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PUBLICATIONSOFTHEASTRONOMICALSOCIETYOFTHEPACIFIC,111:385È437,1999April
(1999.TheAstronomicalSocietyofthePaciÐc.Allrightsreserved.PrintedinU.S.A.
Invited Review
Astrophysics in 1998
VIRGINIA TRIMBLE1 AND MARKUS ASCHWANDEN2
Received1998December10;accepted1998December11
ABSTRACT. From Alpha (Orionis and the parameter in mixing-length theory) to Omega (Centauri and
thedensityoftheuniverse),theGreekshadaletterforit.Inbetween,welookattheSunandplanets,some
verydistantgalaxiesandnearbystars,neutrinos,gammarays,andsomeoftheanomaliesthatariseinavery
largeuniversebeingstudiedbyroughlyoneastronomerper107Galacticstars.
1. INTRODUCTION tratedinafewhighlyregardedjournals,and,ofcourse,for
power to be concentrated in fewer and fewer editorial
Astrophysicsin1998welcomesanewco-author,Markus
hands.
Aschwanden, formerly of the University of Maryland
astronomy department, and as a direct result, gives some
attention to solar physics, which has been relatively
neglectedinrecentyears.Lucy-AnnMcFadden,meanwhile,
1.1. Up,Up,andAway
isuptoherverycapableshouldersintheNearEarthAster-
oid Rendezvous (NEAR) project, a Maryland honors A great many things got started during the year. Cassini
program,asciencemuseumforfamilies,andheavenknows was launched safely toward Saturn on 1997 October 15.
what else. Astrophysics in 1991 to 1997 appeared in PASP ThePlanetBmissiontoMarstooko†inJuly,1998andso
on page l of volumes 104È107, page 8 of volume 108, page wasrenamedNozomi(Hope),followinganexcellentcustom
78 of 109, and page 223 of 110, respectively. They are cited thathassavedtheJapanesespaceprogramfromhavinghad
belowasAp91,Ap92,etc. a named mission blow up on the launch pad or enter a
Thejournalsscannedweretheissuesthatreachedlibrary geostationaryorbitontheoceanÑoorforaverylongtime.
shelves between 1997 October 1 and 1998 September 30 of The Ðrst data came back from HALCA, the space-based
Nature,PhysicalReviewLetters,Science,TheAstrophysical radiointerferometer(Hirabayashietal.1998).Withabase-
Journal (plus Letters and Supplements), Monthly Notices of linetoearth-boundradiodishesexceeding25,000km,ithas
the Royal Astronomical Society, Astronomy and Astro- tightened the limits on the sizes of some components in
physics(plusSupplementsandReviews),SolarPhysics,Acta quasar jets and driven their apparent brightness tem-
Astronomica, Astrophysics and Space Science, Astronomy peratures above 1012 K (the limit for incoherent synchro-
Reports, Astronomy Letters, AstroÐzica, Astronomische tronemission).
Nachrichten, Journal of Astrophysics and Astronomy, Pub- Early data were also reported from the recommissioned
licationsoftheAstronomicalSocietyofJapan,Bulletinofthe Hooker 100A telescope on Mount Wilson (Hartkopf et al.
Astronomical Society of India, Baltic Astronomy, Astro- 1997); from the second OGLE survey for gravitational
physicalLettersandCommunications,NewAstronomy,IAU lensing events due to MACHOSÈand for variable starsÈ
Circulars, and (of course) Publications of the Astronomical (Udalski et al. 1997a; it uses a dedicated 1.3 m telescope at
Society of the PaciÐc. Several of these have become quite Las Campanas); from the Ðrst of the four 8 meter mirrors
difficult to locate, as more and more subscriptions are that will make up the Very Large Telescope (Kaper 1998);
dropped, and coverage of AstroÐzica, Astronomische Nach- fromtheSloanDigitalSkySurvey(Turner1998);from the
richten, and Astrophysical Letters and Communications is second Ñight of the ORFEUS extreme-ultraviolet telescope
not complete. This is part of a trend in astronomical pub- (Hurwitz et al. 1998a and about seven following papers; it
lication that can be traced back at least to the late 19th sawalotofOVI);andfromaverylargenumberofadaptive
century, for major papers to be more and more concen- optics systems, including one on the Russian 6 meter tele-
scope(Weigeltetal.1998),thoughtheyarenotthesolution
ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ to all the astronomical problems you ever thought of
1Department of Astronomy, University of Maryland, College Park, (Esslinger&Edmunds1998).
MD 20742; and Department of Physics and Astronomy, University of
Alpha Orionis (Betelgeuse) has been resolved at the 0A.2
California,Irvine,Irvine,CA92697.
2Lockheed-MartinATCSolarandAstrophysicsLaboratory,Depart- levelwithanullinginterferometer,whichgreatlysuppresses
mentH1-12,Building252,3251HanoverStreet,PaloAlto,CA94304. the Ñux from the central star, allowing one to see the sur-
385
386 TRIMBLE & ASCHWANDEN
rounding disk, and, someday, perhaps, planets (Hinz et al. Solar and Heliospheric Observatory (SOHO), which probed
1998). almost everything from the interior of the Sun (via
Rugate is a Ðlter to suppress the near-infrared OH lines helioseismology) out to 30 solar radii in the heliosphere,
produced by the earthÏs atmosphere (O†er & Bland- accompanied by an armada of other space-borne instru-
Hawthorn 1998). It is a Latin word, not a relative of the ments: Yohkoh, the Compton Gamma Ray Observatory
Rugrats. Oya et al. (1998) report data from a device with (CGRO), the Transition Region and Coronal Explorer
similar function (and a design with some resemblance to a (TRACE), Wind, Ulysses, and so forth. Short dives into
radial-velocityspectrometer),butwithoutaLatinname. spaceyieldinguniquesolarglimpseshavealsobeenaccom-
COME-ON-PLUS is a coronagraph to be used with plish by rocket Ñight such as NRL HRTS, NRL ETI, and
adaptiveoptics(Beuzitetal.1997),andwesuspectthatthe GSFCSERTS,andtheshuttlepayloadSPARTAN201.
name was selected by someone who was not a native WhatÏs the scientiÐc harvest? Despite the overwhelming
speakerofAmericanEnglish. multicolor vision in invisible wavelengths, the complex
AreplicaofthetelescopewithwhichKarlJanskydidhis intricacies of solar plasma physics could not yet be suffi-
pioneeringworkinradioastronomyhasbeenerectedclose ciently disentangled to yield solutions to the problems
totheoriginalsiteinNewJersey(Tyson&Wilson1998).It posedbycoronalheating,accelerationofthesolarwind,or
isnotintendedasaworkingmodel. particle acceleration in solar Ñares. However, encouraging
Finally,asdriversofrelativelysedatevehicles(““myother newÐndingswerepresented.
car is a VolvoÏÏ), we wish to record with awe a disk wind Coronal heating by dissipative Alfve(cid:2)n waves in multiple
thatgoesfrom0to3000kms~1in6È8minutes.Itbelongs resonancelayersthatdriftthroughoutcoronalloopsispre-
to the cataclysmic variable BZ Cam (Ringwald & Naylor dicted by recent theories (Ofman et al. 1998). Obser-
1998). vationally,quasi-periodicwavetrainshavebeendetectedin
solar plumes; they carry an energy Ñux that is almost
enough to heat coronal holes (DeForest & Gurman 1998).
Falconeretal.(1998)localizethesourcesofcoronalheating
1.2. LÏEnvoi
concentrations in the magnetic network. Krucker & Benz
Some things also came to an end. After a long (in both (1998) inquire into the faintest extreme-ultraviolet (EUV)
timeandwavelength)and,wehope,happylife,theInfrared brightness Ñuctuations in the quiet Sun and Ðnd that a
Space Observatory (ISO) was turned o† (Bonnet 1998). surprisinglyhighpercentage,atleast85%ofallpixelsinthe
ROSAT made the mistake of an inexperienced eclipse EUV images they investigated, vary signiÐcantly all the
observer and looked too closely at the Sun (with the time.SuchsmallEUVbrighteningshavebeeninterpretedin
declaredterminationoftheguestobserverprogramcoming terms of ““miniÑaresÏÏ or ““network ÑaresÏÏ that heat the
justafterouryearended).SOHO,incontrast,sortofforgot plasma of tiny, low-lying loops to EUV ([1 MK) tem-
to look at the Sun until its batteries had run down badly, peratures but do not show up in (Z2 MK) soft X-rays
butseemstohavemadearemarkablerecovery. (Berghmans et al. 1998). The energy for these tiny EUV
It was not quite so devastating a year as 1997 for the brightenings is believed to come from magnetic reconnec-
Americanastronomicalcommunity,butdeathsreportedto tion events of low-lying quiet-Sun loops, which reconnect
the AAS and/or ASP during the reference year included preferentially near the chromospheric network and require
RichardHerr,BarryRappaport,VictorSzebehely,Andrew a magnetic Ñux replacement within 48 hr on average
Michalitsianos, Walter McAfee, Charles Worley, Anthony (Schrijver et al. 1998). The Ðnest structures of the magnetic
Jenzano, Georgeanne Caughlan, David Schramm, John Ðeld detected with SOHO/MDI down to a resolution of 1A
Baer, Frank Bradshaw Wood, Gabriel Kojoian, Emil arenowdescribedas““magneticcarpetÏÏtextures.
Herzog, Robert L. Wildey, John Wang, Charles Franklin Exciting progress was also reported from helio-
Prosser, Jr., Karl Kamper, Alexander Rodgers, Robert seismology.Globalp-modeoscillationsofthesolarinterior
Light, James B. Willett, Guenter Brueckner, William Titte- are now so accurately measured that even slight dis-
man,andOlinJ.Eggen.Manyofthemwereinthemidstof turbancesfromactiveregionscanberegisteredanddubbed
activeastronomicalcareers,andtheirpapersarecitedelse- ““seismic holography of solar activityÏÏ (Braun et al. 1998;
whereinthesepages. Chenetal.1998b).EvenmoredramaticistheÐrstdetection
of a ““Sun quakeÏÏ caused by a solar Ñare (Kosovichev &
Zharkova 1998). It was compared with the ““energy that
would be released if the earthÏs continents were covered
2. SOLAR PHYSICS
with a yard of dynamite and detonated all at once!ÏÏ Said
The Sun has never been watched so closely in all wave- SunquakewasalsoratedontheRichterscaleinoneofthe
lengths and with such high spatial resolution as this year. spaceweather-prone web-page advertisements. It rated an
The solar literature received a substantial boost from the 11.3!
1999 PASP, 111:385È437
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ASTROPHYSICS IN 1998 387
Talking about the most energetic energy releases in the 3. XI, KAPPA, ALPHA, AND OTHER DEVILISH
daily life of our Sun, the solar Ñares, the understanding of DETAILS OF STELLAR STRUCTURE
the underlying plasma physics did not become easier.
Stellarstructureandevolutionaregenerallyputforward
MasudaÏs discovery of ““above-the-loop-topÏÏ hard X-ray
as the best example of solved problems in astrophysics.
sources (Masuda et al. 1994) was celebrated as the
What is meant is that we can write down four di†erential
revelationoftheprimaryparticleaccelerationsite(Tsuneta
equations(fordM/dr,dP/dr,dL/dr,anddT/dr),whichmake
&Naito1998).ThiswaslocalizedintheplasmaoutÑowin
useofthreeauxiliaryquantities(i,theopacitytoradiation;
the cusp of X-type magnetic reconnection processes in the
v, the rate of energy generation; and an equation of state,
recent literature and glamorously enhanced in hard X-ray
each as a function of temperature, density, and
images reconstructed with pixon methods (Alexander &
composition).Nextwesolvetheequationsnumericallyand
Metcalf 1997). But the big picture became clouded again
evolvethemforwardthroughsmalltimesteps.Theresultis
withthedemonstrationofsecondaryÑareloops,seeninsoft
setsofstellarmodelsandevolutionarytracksorisochrones
X-raysandradio,thatinteractwiththeprimaryÑareloop,
thatreallydolookalotlikerealstars.Itseemsunlikelythat
seen in hard X-rays (Nishio et al. 1997), because the mag-
any fundamental problems remain, except in the realm of
netictopologyina““three-leggedÑareloopconÐgurationÏÏ
star formation, for which, notoriously, we have no theory.
is not easily understood. Models with three-dimensional
Thissectionis,therefore,mostlyaboutdetailsÈwhyisthere
magneticreconnectionseemtobeinescapable(Longcope&
structureontheredgiantandhorizontalbranchesofglobu-
Silva 1997; Somov et al. 1998). X-type reconnection pro-
larclusters,whathappenswhenstarsbecomefullyconvec-
cesses were observationally traced by ejected plasma blobs
tive,istherestillmissingopacity,andsoforth?
(Ohyama & Shibata 1998), by bidirectional plasma jets in
EUV (Innes et al. 1997), and by microwave signatures in
soft X-ray jets (Kundu et al. 1997). Some Ñares were
3.1. Xi,Kappa,andSigma
observedingammaraystoproducenuclearlinesforaslong
as2hr(Murphyetal.1997).Takingabreakfromexploring Microturbulence, m, is the parameter in which we hide
theviolentÑareactionperse,patientresearchersstartedto our remaining ignorance about the mechanisms that
study the ““waiting timeÏÏ distribution between Ñares broadenabsorptionlinesinstellaratmospheres(anditmay
(Wheatlandetal.1998).Othersstartedtoplaywithcellular ormaynotmeananythingthatyoualmostneverÐndyour-
automata models (MacKinnon et al. 1996; MacKinnon & self wanting to use a negative one to Ðt your curve of
MacPherson 1997). However, serious work was resumed growth). It is supposed to represent gas motions on scales
by transforming these numerical toys into discretized smallerthanthemeanfreepathofphotons.Butthemodels
magnetohydrodynamic (MHD) equations (Isliker et al. of stellar atmospheres donÏt actually give the values of m
1998). thatyouuseincurvesofgrowthforthesamestars(Gilletet
Acceleration of the solar wind is another hot topic that al.1998).
was stimulated by recent SOHO, Ulysses, and Wind data. The atomic parameters needed for detailed analysis of
SOHO/UVCSmeasurementsdiscoveredtemperatureaniso- stellarspectracontinuetobeassembledfromcombinations
tropiesoforderT /T B100, which were interpreted oflaboratorydataandcalculations.Theyincludecrosssec-
perp parallel
as dissipation of high-frequency, ion-cyclotron, resonance tions for collisional excitation and deexcitation, recombi-
Alfve(cid:2)nwaves(Kohletal.1998).Habbaletal.(1997)present- nation and ionization, atomic energy levels and even
ed the Ðrst evidence that the slow solar wind originates in wavelengths (Biemont et al. 1998), and (standing for them
streamer stalks, while the fast wind is ubiquitous in the allhere)f-valuesandopacitiesthatmeasurethedifficultya
inner corona. The geoe†ective impact of the Sun was photon in a line or a continuum experiences in getting out
demonstratedwithcoronalmassejectionslaunchedduring of the star and on his way to our telescopes. (Bosons are
Ñares (Dere et al. 1997; Chen et al. 1997a), dramatically masculine, fermions are feminine, as you will know from
recorded with wide-angle cinematic views from the SOHO/ largecommittees,spectatorsports,andfraternallodges.)
LASCOcoronagraph,leadingtoalertsinpressreleases. That we still need better opacity information, especially
Miscellaneous: Also perhaps of astronomical interest is for cool stars, is not in doubt (Tripicchio et al. 1997). And
the degree of roundness of the Sun. The latest meridian ““betterÏÏ nearly always means ““more,ÏÏ partly because it is
transit measurements, combined with the solar limb dark- easiertohaveforgottensomethingthantohaveinventedit
ening, yield a radius ofR \695,508^0.026 Mm (Brown out of whole cloth, and partly because there is still opacity
_
& Christensen-Dalsgaard 1998). Careful measurements of missinginvariouswavelengthregimes,assuming,ofcourse,
the EUV limb with SOHO/EIT revealed that the polar that you want your model atmospheres to look like the
radiuswaslargerthantheequatorialradius,anodditythat stars they are modeling (Golimowski et al. 1998). The
was attributed to spicules giving the chromosphere a problem becomes acute close to the dividing line between
bumpysurface(Zhangetal.1998a). Mdwarfsthatfusehydrogenandbrowndwarfsthatdonot,
1999 PASP, 111:385È437
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388 TRIMBLE & ASCHWANDEN
especiallywhenyouneedtoconverttoobserversÏunitsand being able to use their period change to study white dwarf
decidewhererealstarsfallrelativetotheline(Luhmanetal. cooling(OÏBrienetal.1998).
1997;Brocatoetal.1998). The obvious way to make very low mass (helium-core)
Inthebrowndwarfregime,dustmay(Schultzetal.1998) white dwarfs is in close binary systems, with mass transfer
ormaynot(Oppenheimeretal.1998)beconfusingtheissue. settinginafterthecoreformsbutbeforeitgetshotenough
It is certainly confusing the astronomers, since the two to start helium burning (Je†ery & Pollacco 1998). But a
groupsarelookingatthesamestar,G1229B.Artifactsyou planet or brown dwarf companion that spirals in and
can achieve with sufficiently odd absorption conditions merges with the core can also do the job (Nelemans &
includeapparentmetallicities10timesthecorrectvalue(in Tauris1998),andthisisagoodthing,becauseatleastafew
BAL [broad absorption line] quasars; Hamann 1998) and single,slowlyrotatingwhitedwarfsoflowmassseemtobe
mainsequencesdepressed2magatagivencolor(Harriset out there (Maxted & Marsh 1998). Even the hottest white
al. 1998b on dwarf carbon stars). Just as improving his dwarfsdonotquitecoolfastenoughtoseeinrealtime,but
handwritingforcedtheapocryphalstudenttolearntospell, thePG1159starPG0122]200isgettingclose:afewmore
better opacities will sometimes force you to do other parts years should either establish changes in its oscillation
of the physics better as well (StarrÐeld et al. 1998 on nova periodsorsettightlimits.
explosions).
But the winner of the oddity of the year award in the
opacity class is the contribution ofH at low temperatures 3.3. PlanetaryNebulae
2
and high densities. The H molecule acquires a modest
2 Planetaries, like mayÑies, have to do their thing quickly,
pressure-induced dipole (canÏt you just see it clutching a
andsomewhereintheMilkyWaythereissurelyayear-old
dressinggownarounditssuddenlyseparatedchargeclouds)
toddler,thoughwemightnotrecognizeit,sinceturn-onisa
and thereby adds a blue tinge, in infrared colors, to the
rather complex balance between stellar winds and photo-
coolestwhitedwarfs(Hansen1998)andtothecoolestmain-
ionization (Chevalier 1997). He 3-1357, whose emission
sequencestars(Chabrier&Bara†e1997).Bothhaveconse-
lines turned on only about 5 yr ago, comes very close. A
quencesfordesigningsearchesfortheseelusivestarsandfor
HubbleSpaceTelescope(HST)imageshowscollimatedÑow
interpretingthenumbersyouÐnd.
feeding it (Bobrowski et al. 1998). The basic shape of the
Crosssectionsfornuclearreactionsareoneofthe(many)
outÑow may already be set before you can even recognize
thingssymbolizedbyp.1998istheÐrstyearinmanynotto
thenebula(Hani†&Buscher1998).
have to cope with a paper (or more often a preprint) pur-
Marching logarithmically away in time, we hit IC 4997,
portingtorevolutionizestellartheorybychangingtherate
whoseradiomorphologyhaschangedsince1995(Miranda
of some key nuclear reaction by an enormous factor. But
&Torrelles1998);Vy2-2,whoseexpansionageis213^26
Brown(1998)notesthatberylliumisdestroyedinlow-mass
yr (Christianto & Seaquist 1998); and two nebulae with
starsbyasubthresholdreaction,whosepoorlyknownspin
mixtures of carbon-rich and oxygen-rich characteristics,
andparityleaveastrayfactorof10inthereactionratenear
indicating a thermal pulse in the progenitor star in the last
4]106K.
1000yr(Watersetal.1998).
3.2. WhiteDwarfAnomalies 3.4. ThermalPulses,LastHeliumFlashes,and
GallopingGiants
Normal discussions of stellar evolution begin with the
mainsequenceorstarformationandlumberonthroughto Multi-shelled circumstellar material (Schroeder et al.
end points. We will lumber backward (and note that there 1998; Ste†en et al. 1998), chemical anomalies (Frost et al.
aresomeotheraspectsofstellarastronomyin°8). 1998b),andveryrapidchangesinstellarcolorsandvisible-
The temperature scale for DA (hydrogen atmosphere) light output are all signs that the end is at hand. The
whitedwarfssu†eredamajorrecalibrationinthedirection examplethatcaughteveryoneÏseyewasFGSge,whichhas
of lower temperatures at a given color over the range displayedallofthesetraitsoverthepastcentury,butisnow
50,000È70,000 K (Barstow et al. 1998). The revision comes settling down as a sort of R CrB variable and looks rather
from including the e†ects of previously neglected atmo- dull(Gonzalezetal.1998;Tatarnikovetal.1998).
spheric metals. Other temperature ranges are likely to be V605AqlapparentlyexperiencedalastheliumÑashearly
a†ectedaswell;theyjusthavenÏtdonetheworkyet.Heavy inthiscentury.Thestarhasbeenrecovered(Clayton&De
elementsareapparentlyalsothereasonthathalfthestarsin Marco1997).Itisthenucleusofapreviouslyknownplanet-
the GW Vir or PG 1159 instability strip donÏt pulse. The arynebula,Abell58,andisnowquitefaintbecauseofdust
onesthatdohavenitrogen-richsurfaces,ratherthancarbon obscuration. But it is probably at least as bright as M \
B
oroxygen(Dreizler&Heber1998),andwearenotfarfrom [4.7 and as hot as 50,000 K. The 1921 spectrogram,
1999 PASP, 111:385È437
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ASTROPHYSICS IN 1998 389
exposedbyKnutLundmarkontheLickCrossleytelescope, 3.6. TheAsymptoticGiantBranch
has also been recovered, in Sweden, with a little help from
Theeye-catcherherewasthereportofanOBassociation
Bidelman (1974). Taking oneÏs plates home with one is an
in the Large Magellanic Cloud, where most of the bright
old astronomical custom that observatory directors strug-
stars are asymptotic giant branch (AGB) members, includ-
gled for decades to discourage. You might think that the
ing the long-period variables (Kodaira et al. 1998).
advent of digital electronic everything would have solved
Somehow,wethoughtonlystarsofsomewhatlowermasses
the problem; but the modern equivalent is not archiving
didthis.Therestofthemembersareverybrightbluestars.
your data, and it is also very popular. Incidentally, V605
Apparently, to make details on the HR diagram come out
Aql near maximum light looked like an undimmed R CrB,
right, it is essential to include the e†ects of radiation pres-
withane†ectivetemperaturenear5000K.
sure (Ste†en et al. 1997), a range of initial metallicities
MostrecentisSakuraiÏsobject,whichduringtheyearhas
(Frogel&Whitelock1998;vanLoonetal.1998),andmass
acquired a planetary nebula (seen as a radio source; Eyres
loss (Frost et al. 1998a). As a side beneÐt, you also get
etal.1998),avariablestardesignation,V4334Sgr,andan
carbon stars as numerous and almost as bright as we see
expanding literature (Duerbeck et al. 1997, who note that
them, depending on whether M \[6.8 (Frost et al.
CKVulin1671wasprobablyasimilarsortofbeast,though bol
observations) and M \[6.5 (Marigo et al. 1998
nospectrogramshavebeenrecovered).V4334Sgrhasbeen bol
calculations)aredi†erentnumbers.
coolingevenfasterthanFGSgeinitsheyday(Arkhipovaet
A last helium Ñash automatically gives you a second
al.1998).StillotherobjectsthatmayberelatedareAbell30
AGB phase (as if one werenÏt enough). Koesterke et al.
and78andtheplanetarynebulainM22(Jacobyetal.1998).
(1998)suggestthatthismayberelevanttotheoriginofPG
1159 stars with large current rates of mass loss. In this
context,10~7M yr~1is““large.ÏÏ
_
3.5. TheTwilightofaMassiveStar
Massivestarsdoeverythingfaster,themostconspicuous
3.7. TheHorizontalBranch
recent example having been the postÈred-supergiant IRC
]10 420, which has gone from F8If in the 1970s to A5 in TheHRdiagramfortheglobularclusterinyourelemen-
the 1990s and is probably en route to landing among the tary astronomy textbook almost certainly shows a gap
Wolf-Rayetstars(Klochkovaetal.1997;Oudmaijer1998). somewhere in the middle of the horizontal branch. That is
Theprocessisarguablypartofthetotalphenomenoncalled where the RR Lyrae stars live, and this is not what Kravt-
““blue loops,ÏÏ meaning that massive stars sashay back and sovetal.(1997)meanwhentheyaskwhethertherearegaps
forth across the HR diagram when burning zones cross in the distribution of HB stars in M79. The study with the
compositional discontinuities (de Jager 1998; Dohm- biggest sample (Catelan et al. 1998) concludes that many
Palmeretal.1997). purported gaps simply reÑect statistics of small numbers,
JustwhichstarsbecomeWolf-Rayetsisstillbeingsorted butthattherearesomerealones,includingNGC6229,and
out, but a couple of papers agree that it is easier if you are that they should provide clues to the nature of the second
metal rich (Massey 1998; Smith & Maeder 1998). We are parameter. Ferraro et al. (1998) and Grundahl et al. (1998)
notsurprised,havingconcludedthatmostthingsareeasier agree about the reality and suggest di†erent amounts of
ifyouarerichÈnot,unfortunately,onthebasisofpersonal masslossormixingontheredgiantbranchasthecause.
experience. The ““extended horizontal branchÏÏ is extended toward
All in all, a massive star is very busy in its last 104 yr, the blue and is a source of excess ultraviolet emission in
establishing asymmetries that may later appear in the globularclustersandotheroldpopulations(e.g.,OÏConnell
supernova explosion (Kastner & Weintraub 1998), storing et al. 1997). Eggen (1998a) reported that NGC 6791 is
itsangularmomentum(Heger&Langer1998),sheddingits unique among the open clusters of the Galactic disk in
outer layers and then turning o† its wind (Benetti et al. having such stars. ““Super horizontal branch starsÏÏ are
1998,onSN1994aj),andgenerallyreadjustingitsstructure fairly common, at least in NGC 6522 (Shara et al. 1998).
to make a better supernova (Heger et al. 1997; Bazan & Their evolutionary status is uncertain, but supposing that
Arnett1998,a3Dcalculationwithplumesandribbons). theyaredescendedfrombluestragglersisattractivebecause
We all agree, of course, that the last thing a pre- there are such a lot of ways of making those (Ouellette &
supernovadoestogetreadyistobuildupaniron(etc.)core Pritchet 1998; Preston & Landolt 1998, on mergers and
tothepointofcollapse.Justwhereisthatpoint?Well,two masstransfer,respectively).
modern codes, applied to a 25 M star of solar composi- A few blue stragglers are also Am stars, which must
_
tion, Ðnd 1.51 and 1.78 M (Chieffi et al. 1998). There are surelyhavesomethingtodowiththepriceofbeans,butwe
_
also di†erences in the extent of the convective zones and arenotquitesurewhat,andneitherwasAndrievsky(1998).
otherdetails. Bidelman(1998),ontheotherhand,hasprovidedauniÐed
1999 PASP, 111:385È437
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390 TRIMBLE & ASCHWANDEN
hypothesis for the Am, Ap, and blue straggler stars, using 3.9. RedGiants
processesinclosebinariesforallofthem.Healsonotesthat
Why did the star cross the Herzsprung gas? has at least
blue stragglers Ðrst show up in an HR diagram from
as many answers as Why did the chicken cross the road?
Ebbighausen(1940).
Answers to the latter can be found on web sites associated
withthenameofPaulErdos(no,weÏrenotgoingtoexplain,
3.8. PulsatingStars butseeBar-Ilan1998forthedetails).Itisnotclearthatthe
right answer to the former question is entirely in hand yet,
Cepheid pulsating variables still present a discrepancy
but Eggleton et al. (1998) point out that having a helium
between the surface gravities (masses) determined from
corethatgrowstothelimitingChandrasekhar-Schoenberg
spectroscopic or pulsational properties and those deter-
mass(about10%ofthestar)isnotenoughunlessyoualso
minedfromevolutionarytracks(Lucketal.1998).Thereis
haveaverypeculiarequationofstate.Thediscontinuityin
alsoconsiderablescatterintheradiideterminedfromdi†er-
meanmolecularweightattheedgeofthecoreisalsopartof
ent methods, including the Baade-Wesselink (Bono et al.
thecrossingmotivation.
1998).Theobservedrelationshipsamongmass,period,and
Theluminosityfunctionforredgiantsinglobularclusters
luminosity are not very well Ðtted by models for some
has a bump, which Cassisi et al. (1997) attributed to di†u-
populations(Bara†eetal.1998),butatleastthereisnogap
sion. Di†usion is, we suppose, mixing in a modest mood,
in the N(P) distribution (Buchler et al. 1997a). One would
andsoareasonablelead-intotheancienttopicofcomposi-
expect HIPPARCOS parallaxes and proper motions for
tion anomalies among red giants (and other evolved stars)
Cepheids to have had something to say about these issues,
in globular clusters. Many of the patterns are associated
buttheprimaryfocushasbeenongettingthebestpossible
with products of nuclear reactions that might reasonably
zero-pointtotheP-L relation(ratherthanontidyingupits
occur in elderly, low-mass stars, especially for the isotopes
internalstructure),soastomoveontotheLargeMagella-
ofCNO,butalsosometimesNa,Ne,Al,andproductsofthe
nic Cloud and the Hubble constant (Feast et al. 1998;
s-process. Here, extra mixing does seem to be most of the
Madore&Freedman1998;Bergeatetal.1998).
answer (Charbonnel & do Nascimento 1998; Denissenkov
The existence of non-pulsating stars within the Cepheid
etal.1998;Cavalloetal.1998;Kingetal.1998b).Thelast
stripremainspuzzling.Agoodmanydisplaysmallvelocity
group notes that reactions in a somewhat more massive
wiggles (0.05È3.0 km s~1) with periods of 60È80 days, as if
companion,whichdumpsitswretchedrefuseandthenhides
embarrassmentattheirnon-compliancehadcausedabitof
itsdegeneratehead,isalsoapossiblescenario.
nonradial oscillation (Butler 1998). And some good news
But nothing you can do in a low-mass star will account
about Polaris: its fading away as a pulsational variable
forthecorrelationsinthegiantsofM92,whereeuropium(a
seemstohavestoppedinabout1996(Cox1998;Kamper&
classic r-process product) is involved. Langer et al. (1998)
Fernie1998).
hence conclude that the stars must have begun with di†er-
RR Lyrae stars resemble Cepheids in several ways, of
ent heavy element abundances, contributed by supernovae
which the least inspiring is that the pulsation calculations
beforetheyformed.Donot,however,trytoholdyourstock
are an only moderately good match to observed light and
in the metal factory for too longÈthe di†usion boys may
radial-velocity curves (Kovacs & Kanbur 1998; Jurcsik
trytosellitshortwhentheygettiredofApstars.
1998). The instability strip for horizontal-branch stars
narrows as metallicity increases, but the stars cross slower
(Bono et al. 1997). They are the BL Her stars, in case it
3.10. MixingItUpontheMainSequence
might have slipped your mind. HIPPARCOS data may or
may not have clariÐed any of these issues. Again, analysis The more mixed-up author caught more than a dozen
has focused on the calibration of extra-galactic distance papers during the year in which a major thrust was that
scales, and the focus has not resulted in a ““best-betÏÏ dis- main-sequence stars show evidence of more mixing and
tancetoanything(McNamara1997;Fernleyetal.1998). earlier mixing than standard models produce. Some things
The single most important advance in 9 Aurigae vari- arereducedatthesurface,especiallylithium,beryllium,and
ableswastherecognitionthattheyshouldreallybecalledc boron (Boesgaard et al. 1998), and the process must start
Dorstars.Thismeansthatwewillnolongerhavetostrug- even before hydrogen fusion is fully established (Je†ries
gletopersuadecopyeditorsthat9mustnotbespelledout 1997). Other things are built up, especially helium
asnine.Inaddition,evidencecontinuestomountthatnon- (Lyubimkovetal.1997),andtheSunwasnotexemptfrom
radial pulsation is the best Ðt to their multiple, day-long theprocess,whateveritis(Basu1997).
modes (Zerbi et al. 1997a, 1997b; Ushomirsky & Bildsten Themaincompetitiontoextramixingasanexplanation
1998).Thenumberscontinuetoincrease(Porettietal.1997; for oddities in stellar surface composition is mass loss
Kaye & Strassmeier 1998; Hatzes 1998a), and the Ðrst one (Morel et al. 1997), and it was outvoted, at least on the
inaclusterhasbeenfound(Zerbietal.1998). lower main sequence during 1998. Very massive stars and
1999 PASP, 111:385È437
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ASTROPHYSICS IN 1998 391
helium stars are, of course, a di†erent story (Langer 1998; Among more massive preÈmain-sequence stars, many
Leushinetal.1998) Herbig Ae/Be stars have disks, as pre-MS stars should
Some other unexpected aspects of main-sequence struc- (Mannings&Sargent1997),butitisnotentirelyclearthat
ture concern convection. Our Sun apparently began its life theyhavetherightsurfacegravitiesfortheirexpectedposi-
withaconvectivecore(Venturietal.1998).Thecoreformed tion on an HR diagram (Kovalchuk & Pugach 1997). For
because abundant carbon was initially converted to nitro- once, the HIPPARCOS data help. Van den Ancker et al.
gen in a beginning of a CNO cycle, with the usual strong (1998)reportabsolutemagnitudesfor44examplesandÐnd
temperature dependence of the reaction rate producing a thatnotonlyarethestarsperchedproperlyabovetheMS,
gradientofenergygenerationsteepenoughtodriveconvec- but also the most variable ones are farthest from the zero-
tion.Assoonasallthecarbongetshungupas14N,thecore agemainsequence.
disappears.
Stars with masses of less than about 0.25È0.30 M are
_
convective throughout, but nothing much happens to 3.12. StarFormation
stellar activity (that is, dynamo-generated magnetic Ðelds,
When you describe star formation to a class of under-
angular momentum loss, etc.) there, at least in the Pleiades
graduate science majors, you always begin by deriving an
(Krishnamurthi et al. 1998). There is, however, a glitch in
expressionfortheJeansmass.Itwas,therefore,asurpriseto
themass-radiusrelationforstarsinthesolarneighborhood
readthattheclumpsinsevengiantmolecularcloudshavea
thatoccursataboutthismass,whichtheauthors(Clemens
power-law mass distribution from 10~4 to 10`4 M , with
etal.1998a)donotattributetothiscause. _
no sign of any glitch or discontinuity at the Jeans mass
Nothing much is also what happens at the color,
(Krameretal.1998b).TheimportanceofmagneticÐeldsin
B[V \0.2È0.3, where convective envelopes begin to
the substructure of star-forming clouds was, however,
appear (Newberg & Yanny 1998). A sizeable number of
affirmed during the year (Schleuning 1998). A mild rumble
other papers during the index year dealt with convection,
of dispute continues about the extent to which ambipolar
especially overshoot and alternatives to mixing-length
di†usionisresponsibleforremovingexcessÐeldfromcores
theory (Deupree 1998; Canuto & Dubovikov 1998), who
that are trying to contract. Important (Li 1998; Ciolek &
need no fewer than Ðve serious equations to replace one
Mouschovias 1998); unimportant (Nakano 1998), said the
alpha, the parameter of MLT. Bernkopf (1998) actually
king, trying to Ðgure out which sounded better. Zweibel
requires two di†erent values of alpha at di†erent places in
(1998)meanwhilesaidthattheprocessisimportantbecause
his stars. But we must hurry on to earlier evolutionary
it drives a new instability that in turn leads to turbulence
phases,orwewillnevercomebackthepreviousnight.
andacceleratedfragmentationinmolecularclouds.
Other words that go with star formation (usually as
3.11. BeforetheBeginning adjectives), and for which observations provide some evi-
denceofimportanceinclude:
Youngstellarobjectsarenotasbrightastheyusedtobe.
The maximum achieved by a 0.5 M star any time from I turbulence(Goodmanetal.1998;Lisetal.1998)
_
class 0 to the main sequence is 0.4L , down from 3L in I hierarchical(Efremov&Elmegreen1998)
_ _
earlier calculations (Myers et al. 1998). Among T Tauri I dynamical (and non-hierarchical) fragmentation
stars, the di†erence between the classical ones and the (Whitworthetal.1998;Klessenetal.1998,whopredictthat
weak-lined ones is not (or not always) an evolutionary fragmentationwillmakeapower-lawdistributionofstellar
sequence. In the 1È3 Myr old cluster IC 348, Herbig (1998) masses, while processes dominated by the Jeans mass will
Ðndsthatthetwoclassesare,onaverage,thesameage.The makealog-normalone)
WWTTs are, however, more concentrated to the cluster I triggered;towhichonecanonlysay,someofthemare
core,whichmightsuggestalossofoutdoorclothingintidal (de Boer et al. 1998) and some of them arenÏt (Braun et al.
encounters. 1997);both,asithappens,intheLMC
TTauristarsareoftenspotted,sothatonecansaysome- I episodic;undoubtedly, but it shows only in dwarf gal-
thing about their rotation. It is di†erential, with what axies(Hurley-Kelleretal.1998;Gallagheretal.1998)
Johns-Krull & Hatzes (1997) call ““anti-solar di†erential I bar driven, no (Perea et al. 1997) or yes (Boeker et al.
rotation.ÏÏThismeansthatwhetherthepolesortheequator 1997),and
is faster is backwards from the Sun. You have half a sen- I cooling Ñows, yes; Lemonon et al. (1998) present a
tencetorememberwhichwayitgoesfortheSun,beforewe reasonably clean case for Abell 2390, based on infrared
hop on to Hatzes (1998b) and Ruediger et al. (1998), who Ñuxes seen by ISOCAM; some indirect evidence in color
explain, or at least describe, which sort occurs at which gradients and line strengths, suggesting that only the rela-
rotationperiods,andothercorrelationswithstellarproper- tively small fraction of the gas that gets into emission-line
ties. regions(coolanddense)formsstars,isshownbyCardielet
1999 PASP, 111:385È437
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392 TRIMBLE & ASCHWANDEN
al.(1998).AndwestilldonÏtknowwhatbecomesoftherest a VLA radio image (Lim et al. 1998). Alpha Cen has dis-
(Ap94,° 10). tinctly nonsolar composition, with [CNO/Fe]\]0.24
(Neuforge-Verheecke&Magain1997).
Sober truth is that star formation remains a heavily
BetaCetiisoneofasmallhandfulofsingleX-raystarsin
empiricalsubject,thoughtheobservationalempire(no,the
its class (M III), according to Maggio et al. (1998) and
wordsarenotcognate)cansometimesberemarkablyinfor-
Huensch et al. (1998). Beta Lyrae has bipolar outÑow
mative.Considerhowmanytheoristshavelaboredhardto
(Ho†manetal.1998).Well,toquotethechapwhothrewup
produce relaxation (odd, that) in star clusters, so that the
after breakfast every morning, doesnÏt everybody? Beta
biggeststarswouldÐndtheirwaytothecenter.Apparently,
OrionisisexperiencingbothinfallandoutÑow(Israelianet
the massive stars simply started out there in many cases
al. 1997). Beta Cephei is the prototype of the Beta Cepheid
(Hillenbrand&Hartmann1998andBonnell&Davies1998
variables,andthereareatleastafewinCygOB2(Pigulski
onOrion;Fischeretal.1998onNGC2157; Su et al. 1998
&Kolaczkowski1998).BetaCrucisisaBetaCepheid, and
on M 11; Chen et al. 1998a on NGC 4815; Raboud &
a fairly complicated, binary one at that (Aerts et al. 1998).
Mermilliod 1998 on NGC 6231). Bonnell et al. (1998) have
Delta Ceti is also a Beta Cepheid, and if the pulsation is
put forward a mechanism for the formation of stars above
driven by opacity, its mass is 10.6 M (Cugier & Nowak
10 M from collisions and mergers of intermediate ones. _
_ 1997).
Themodelaccountsbothforcentralconcentrationsandfor
Beta Pictoris, as you may have heard, has a dust disk. If
the most massive stars being formed only in big, dense
you hadnÏt heard, at least 14 papers during the year were
clouds.
anxious to tell you about it. We note only a model for
TheBonnelletal.(1998)picturealsopredictslotsofclose
warpingthedisklateinlife,whichmightpermittheforma-
(tidal capture) binaries. Fragmentation, on the other hand,
tion of non-coplanar planets (Armitage & Pringle 1997).
makes lots of wide binaries (Nelson 1998; Sigalotti 1998),
Notinourbackyardsolarsystem,ifyoudonÏtmind.
with the distribution of mass ratios that is seen, at least, in
Gamma Cas is the secondary of a bright X-ray source,
the Hyades for binaries with a\5È50 AU (Patience et al.
andwhethertheprimaryisawhitedwarforaneutronstar
1998). And we recorded a dozen other papers under a
isstillbeingdisputed(Kuboetal.1998;Smithetal.1998b).
heading ““one, two, many,ÏÏ addressing the formation of
Gamma Cygni experienced a major outburst in the 1600s
binaries, singletons, and small groups, selecting out here,
andcurrentlyhasaspectrummorecomplexthanitwas60
perversely, a couple that deal speciÐcally with triple
yrago.Inotherwords,itactsagooddeallikeEtaCarinae
systems. These indicate that a good deal of post-formation
(Markova&deGroot1997;Najarroetal.1997).
dynamicalevolutiongoesintodeterminingthepopulations
This has the happy e†ect of allowing us to skip Delta
weactuallysee(Valtonen1998;Tokovinin1997).
Scuti stars (oh, all right; they show a period-luminosity
Is this the place to put our anthropologist and native
relation in HIPPARCOS data; Petersen & HÔg 1998;
stories?““One,two,manyÏÏis,ofcourse,thewayveryprimi-
Antonello & Pasinetti Fracassini 1998) and a few other
tivepeoplesaresupposedtohavecounted;hencetheNean-
intermediatelettersandgostraightontoEtaCarinae,orat
derthal chemist who studied the series ““methane, ethane,
leastasubsetofthedozenorsopapersthatdiscussedit.It
paraffin.ÏÏ Two other favorites end ““ask him the word for
hadasecondoutburstin1890,almostaspowerfulinkinetic
index ÐngerÏÏ (on the use of native informants in language
energyasthe1843one(Smithetal.1998c; Smith & Gehrz
learning) and ““did you know they are serving free beer in
1998). Since then, it has been quite active in the bands we
the village?ÏÏ (on how to deal with the tribes of liars and
callradio(Duncanetal.1997),X-ray(Corcoranetal.1997),
truthtellers).
andtheory(Dwarkadas&Balick1998a;Pittardetal.1998,
ontheX-raysascollidingwinds).Thestarmayactuallybea
3.13. SomeReallyBrightStars
binary (Davidson 1997) or even a triple (Livio & Pringle
AlphaCrBistheonly““alphaÏÏwithtotaleclipses.Italso 1998), though it is not clear that having company made
displays apsidal motion (Schmitt 1998). Alpha Ari and muchcontributiontothemaineventsof1843(incidentally,
Alpha Cas have revealed their limb darkening directly to a ittakesalargehistorybooktoÐndanythingthathappened
three-element optical interferometer with closure phase in1843moreexcitingthantheeruptionofEtaCarinae).
(Hajianetal.1998,andwehopekeepingitclosedprevented
some of the embarrassment the stars might otherwise have
3.14. BinaryEvolution
felt). Alpha Cyg variables may all be periodic, though the
dataSterkenetal.(1997)presentareforamereZeta(Sco); The common-envelope binary phase, responsible for
and all evolved massive stars are Alpha Cyg variables (van casting out both mass and angular momentum to make
Genderenetal.1998;andthereforeSocratesismortal?). short-period evolved systems, is itself a remarkably short
AlphaOrionishasconvectivehotspotsinaninterferome- period, only a few years to lose 40% of the eventual total
terimage(Wilsonetal.1997a)andlargeconvectivecellsin mass excretion (Sandquist et al. 1998). Two X-ray binaries
1999 PASP, 111:385È437
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ASTROPHYSICS IN 1998 393
haveprobablybeencaughtclosetothetimewhentheywill trinospecies(ormore!).Allthreeareofthesortthatrequire
reappear as binary pulsars: Aquila X-1 has begun its pro- us to express the Ðndings in the form of two slightly odd
peller phrase to shut o† accretion and turn on rotation parameters: (1) the square of the di†erence in masses
powering (Campana et al. 1998), while SAX J1808.4[3658 between two species, and (2) sin2 2h, where h is generally
(XTE J1808[369) is the Ðrst low-mass X-ray binary to called the Weinberg angle. The Ðrst new result concerns
reveal a well-deÐned, unique rotation period. It is only 2.3 muonneutrinosmadeatLosAlamosNationalLaboratory
ms,sotheendofmassaccretionwillsignaltheappearance and requires that they be able to oscillate (change) into
of a binary millisecond pulsar (Wijnands & van der Klis electron neutrinos at a rate that could be explained by a
1998;Chakrabarty&Morgan1998;White1998). fairly wide range of mass di†erences and Weinberg angles,
Every year we intend to write a comprehensive section though*m2\0.1eV2looksgoodand10~3eV2isreallyo†
entitled““allperiodschangefasterthanyouexpectthemto.ÏÏ thescale(Athanassoulaetal.1998).
And every year we fail. But an explanation in terms of Second, and accompanied by the pressiest releases, was
magnetic cycles may even apply to the eclipsing binary an announcement from the owners and operators of the
millisecondpulsarJ2051[0827(Stappersetal.1998). Superkamiokande detector in Japan that they had con-
Ðrmed the disappearance of muon neutrinos made in the
upperatmospherewhentheymusttravelalongwaytoget
4. NU IS FOR NEUTRINO
tothedetector(whichmeansgoingthroughtheearth,butit
AtleastinYiddish,““Nu?ÏÏisaquestion.Perhapsalsoin is transparent for this purpose). The missing ones are not
physics. In recent years, the dominant question has been, becomingelectronneutrinos,whichSuperkamiokandealso
What are the rest masses of the three known species of sees, and must be rotating into q neutrinos or some pre-
neutrino? Closely related questions concern the possible viously unknown species separated by *m2 in the range
role of stable or decaying neutrinos as dark matter and as 0.5È6]10~3eV2andalargishWeinbergangle(Fukudaet
promoters of the formation of large-scale structure in the al. 1998a, a Physical Review Letter that stretched to six
universe, neutrino contributions to the ejection of the pagesbecausethenamesofthe120authorsoccupiedmore
envelopes of massive stars in Type II supernovae, and the thanone!).
long-standing shortage of observed neutrinos coming from Thisbringsus,thirdly,backtotheSun.Datacontinueto
theSuncomparedtopredictionsusingthestandardmodel. accumulate,with26yrofHomestakemineresults(the37Cl
Ap95, ° 5, touched on neutrino contributions to super- detector,orDavisexperiment)summarizedbyClevelandet
novae (for which their mass is not a major factor). At that al. (1998), and the Ðrst year of Superkamiokande results,
moment, it seemed as if the worrisome problem of how to pertaining only to the B8 (highest energy) neutrinos sum-
eject the stellar envelope had been solved by three- marized by Fukuda et al. (1998b). There is no doubt that
dimensional, neutrino-driven convection. This may still be eachdetectedÑuxislessthanthestandardmodelprediction
so,butthelatestwordisnegative.Mezzacappaetal.(1998) (e.g.,Bahcalletal.1995),byfactorsofatleast3andatleast
report that their neutrino-driven convection is vigorous, 2,respectively.Thereisnoevidence,sofar,fortimevariabil-
but dynamically unimportant, even for Newtonian gravity ity in what Superkamiokande sees, and that in the Home-
(afavorableprevarication)anda15M star.ItjustdoesnÏt stakedataisinsomedispute(Walther1997;Sturrocketal.
_
blowup. 1997). No new numbers for average detected Ñux or varia-
NeutrinosasdarkmatterappearverybrieÑyin° 11.4in bilityinthegalliumexperimentsappearedduringtheindex
among the other candidates, but we note here that using year.Thesearetheonlydetectorsthatcanseetheneutrinos
them to make supermassive objects at the centers of gal- from the commonest form of hydrogen fusion in the Sun,
axies,inlieuofblackholes,wouldrequirerestmassesofat the ppI chain. Gallium, unfortunately, is a good deal more
least 12È14 keV, which is not what the laboratory experi- expensive than cleaning Ñuid and presents temptations to
mentshavebeensayingthisyear(Tsiklauri&Viollier1998). nonscientiÐcuses.
A decaying neutrino with a rest mass near 25 eV (cf. Ap91, The solar data can, in any case, also be explained by a
° 11.6)alsodoesnotexactlyjumpoutatyoufromthe1998 range of combinations of *m2 and sin2 2h. The only
literature, but the emission line it will leave at 915^7 A(cid:2) problem is that, putting together the Los Alamos, Super-
might when the EURD instrument Ðnishes its mission on kamiokande, and solar neutrino data sets, we have more
Minisat-01(Sciama1998).EURDisclearly““extremeultra- equations than unknowns if only three sorts of neutrino
violet something or other,ÏÏ and Bharadwaj & Sethi (1998) participate,andnoconsistentsetofnumberscanbefound.
wouldsaythatthissortofdecayingneutrinocanalreadybe Thus, either the physics of neutrino oscillation must be
excluded as a contributor either to intergalactic ionization rather di†erent from what is generally assumed, or there
ortotheformationoflarge-scalestructure. mustbe(atleast)afourthsortofneutrinointowhichoneor
Three separate lines of evidence nevertheless strongly moreoftheknownonescandisappear,butwhichdoesnot
suggest non-zero rest masses, probably for all three neu- show up either as a contributor to energy density in the
1999 PASP, 111:385È437
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