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19 60MNRAS.120. .483D 0 a 0 .Shapley(2)foundthisbeltofstarstobeasub-systemthe the neutralhydrogendistribution andthelocalsystem(7). the Sunlieshavingpropertiessummarizedinfollowingtable(4,5) plane (3).Thesefactssuggestthatthereisagalacticsub-systemwithinwhich 200 parsecsoftheSunhavespatialdistributionshowninFig.1withasemi- northern andsouthernMilkyWayformedabeltinclinedatanangleof20tothe the southernpartsofsub-system.Apreliminary investigationofJodrell from theneutralhydrogendistributionaway thegalacticplane.Lilleyand it maybetheobservationaleffectofadust densitywhichfallsatincreasing distances oritmaybetheeffectofafortuitous irregulardistributionofstars. are plottedinFig.1,alsoindicateasysteminclinedatabout20tothegalactic Heeschen (6)havenotedacorrelationbetween neutralhydrogengasanddustin amplitude of12.Thesestarsaremainlyresponsibleforthenaked-eyeGould Belt. BrightBestarsofmagnitudelessthan6*5showthesamedistribution. Galaxy localizedwithinseveralhundredparsecsoftheSun.TheBstars Bank hydrogen-linerecordsshowedamuchmore extensivecorrelationbetween Some constituentsoftheinterstellarmedium,notablyreflectionnebulaewhich © Royal Astronomical Society • Provided by theNASA Data System The realityofthisproposedsub-systemhas beenquestioned(e.g.4,3); i. Introduction,—Gould(1)notedthatthebrighternaked-eyestarsof Strong newargumentsinfavourofthereality alocalsystemcanbederived 5 o0 position ofthelocalsystem(Gould’sBelt)early-typestarsanddust. The system(~10MO)appearstorepresentalaterformationinthelocal The majorfeatureofthedistributionisexcessneutralhydrogenin spiral arm.AnextendedregioninCepheusisprobablysimilar. of neutralhydrogencloudswhichcoincideinpositionwithdustcomplexes. been measuredbetweenDec.=+90and—32.Itshowsanumber A STUDYOFNEUTRALHYDROGENINTHESOLAR The distributionofneutralhydrogenawayfromthegalacticplanehas NEIGHBOURHOOD OFTHEMILKYWAY o 8 o0 N. Pole/'=170¿/=+74 Mass ~10MO? Radius ~500parsecs Centre V=240b'—3 (Communicated byA.C.B.Lovell) Properties oftheLocalSystem (Received 1959October1) R. D.Davies Summary Table I 35 484 R. D. Davies, A study of neutral hydrogen Vol. T 20

Reflection Nebulae

Fig. i.—The full-line represents in galactic coordinates the best fitting plane drawn through the bright B {after Shapley). The positions of large reflection nebulae {after Hubble) are also shown.

2. Programme of observations.—During the course of the hydrogen-line programme of the 30 ft paraboloid (beamwidth i°*5 x i°*7) in 1955 and 1956 a large number of drift curves at constant frequency were taken at various decli- nations. These employed bandwidths of either 18 Kc/s or 40 Kc/s and showed a clear separation from the galactic plane of a broad feature centred about 200 away from the plane. The preliminary analysis of this data and of Muller and Westerhout’s (8) spectra taken near the galactic plane pointed to the desirability of a complete spectral survey of the sky visible from Jodrell Bank. This complementary survey consisted of setting the 30 ft paraboloid at various declination settings 50 apart and allowing the rotation of the to sweep the beam through 24 hours in right ascension. During each drift the hydrogen comparison radiometer was constantly sweeping a frequency band i-2Mc/s in width so that spectra were obtained at least every 50 in the sky. The observing bandwidth was 100Kc/s; this was adequate to measure the line integral of neutral hydrogen at each point. Each declination run was repeated. The survey covered declinations — 320 to + 90°. In all, 2500 spectra were taken and analysed. The velocity characteristics of the neutral hydrogen in the local sub-system well away from the galactic plane were investigated by taking narrow-band spectra in sample regions. Bandwidths of 100, 25 and 3-5 Kc/s were used simultaneously ; the 25 Kc/s channel gave the spectra required while the 3*5 Kc/s channel was used as a check to show that the 25 Kc/s channel produced no broadening in the observed profiles. 3. Analysis and results (i) Drift curves at a single frequency.—In certain regions of the sky a drift curve across the galactic plane did not show a single broad peak but a broad peak centred on the radio galactic plane with a second fainter peak sometimes as much as 200 in galactic latitude from the first. Such constant declination drift curves

© Royal Astronomical Society • Provided by the NASA Astrophysics Data System 19 60MNRAS.120. .483D No. 5,i960inthesolarneighbourhoodofMilkyWay Fig. 3.—Driftcurvesacross thegalacticplaneandlocalsystematDec.=29°for various Dec. =40°,(b)2,6°,(c)—°.Galacticcoordinatesandthefrequencydifference Av fromthel.s.r.aregiven. © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Fig. 2.—Constantfrequencydriftcurvesacrossthegalacticplaneandlocalsystemat(a) +10 +5 « ^5AT?(Kt/s) frequencies relativetothel.s.r. 36 485 486 R. D. Davies, A study of neutral hydrogen Vol. 120

Fig. 4.—The distribution of N {the integrated number of neutral hydrogen atoms per cm2 in the line of sight) for the northern sky in units of 1020 cm~2. taken at frequencies near the local standard of rest (l.s.r.) are shown in Fig. 2. The ordinates are brightness temperature while the abscissae show, as well as the right ascension, the galactic coordinates (/', b') and the changing frequency Av (although constant relative to the Earth) relative to the local standard of rest in the solar neighbourhood of the Galaxy. Fig. 2 {a) is a drift curve across the Cygnus region and shows a strong asymmetry south of the plane lying between galactic latitudes — 50 and — 150. Fig. 2 {b) in the region gives a clear separation between the main galactic peak and the neutral hydrogen belt at latitudes — 150 to — 30o. A similar separation is visible above the plane in the Ophiuchus-Scorpio region shown in Fig. 2 (c). Hydrogen at different distances can be distinguished due to galactic rotation by taking drift curves at different frequencies. This criterion still applies in the anticentre region where galactic rotation is nearly perpendicular to the line of sight. Such a series of drift curves at different frequencies was taken at Dec. = 39 0 and the results are shown in Fig. 3. The frequency relative to the local standard of rest at any right ascension for a given curve is obtained by adding the frequency

© Royal Astronomical Society • Provided by the NASA Astrophysics Data System 19 60MNRAS.120. .483D 2 0 o No. 5,i960 number ofneutralhydrogen atomsina1cmcolumnthelineofsight ateach taken withabandwidth ofiooKc/swasdesignedtomeasureN,the integrated frequencies relativetothel.s.r.isvisibleover arangeofgalacticlongitudes.It point. which occursinnearbyhydrogenisobservedat thelowestfrequencies.Hydrogen broad peakonthegalacticplaneandasecondary maximum5toio°belowthe occurs abovetheplanearound/'=330°andbelow theplanearound/'=150. plane. Thislocalhydrogensomedistancefrom theplaneatzeroornegative at negativefrequencies(recession)relativeto the localstandardofresthasone and consequentlyreferstomoredistantneutral hydrogen.Thesamepheno- half temperaturepoints.Thedriftcurveforhigherfrequenciesisnarrow menon isobservedintheabsorptionspectrum ofTaurus-A(9)whereabsorption correction Avatthatrightascensiontothefrequencyassignedcurve.The Hi spectruminthisregionoftheskyisasinglelinei6oKc/swidthat © Royal Astronomical Society • Provided by theNASA Astrophysics Data System (ii) Thesurveyofneutralhydrogenspectrainthe NorthernSky.—Thissurvey, in thesolarneighbourhoodofMilkyWay +80 N= 15 J Ndl=2-3x10 H Fig. 4.—Continued. 3IO 290270 Longitude (/) 36* 487 19 60MNRAS.120. .483D 0202 0 o 0 0 3 the planeinFig.4;regionsGouldBeltareawereincluded.Thecentral with thesamepropertiesofprofilesobtainednearbyongalacticplanewhich column inthelineofsight. than 5indiameter.TheunitsofNare10neutralhydrogenatomspercm taken withthe25Kc/sbandwidth.Peakbrightnesstemperaturesofupto of 25and3*5Kc/sweretakenatthepositionsenhancedNfoundawayfrom less detailedmap,allowingforthedifferenceingridspacing,isgood. temperature pointsof7km/sec.Ifthekineticistakenas125°K eight regionsawayfromtheplanearereproducedinFig.5.Theprofiles were checkedagainsttheWesterhoutandMullerprofiles. frequency, peaktemperatureandhalf-widthoftheseprofileswerecompared a io°gridforlatitudesmorethan20fromtheplane.Theagreementwiththis 488 R.D.Davies,AstudyofneutralhydrogenVol.120 may becomparedwithvaluesof12to10km/secforthegalacticplane the opticaldepthspectraindicateavalueofhalfwidth(77)5*8km/sec.This obtained withthe3*5Kc/schannelwerenotsignificantlydifferentfromthose temperature oftheneutralhydrogenwastakentobei25°K. profiles atdistancesof4to8-2kiloparsecsfromthegalacticcentre(11). 55 °Kweremeasured20offtheplanealongbothaxesofGouldBeltsystem. profiles. Allbrightnesstemperaturespectrawerethenconvertedintooptical the surveywithsimilarspectrainMullerandWesterhout’s(8)catalogueof Figs. 4(a),(¿),(c)and(d).Noattempthasbeenmadetorepresentdetailless depth spectraandthevalueofNwasderivedforeach.The meter foreachspectrum.T(v)istheobservedtemperatureonanarbitraryscale. following way.Theintegratedquantity¡T{v)dvwasmeasuredwithaplani- referring spectratakennearthegalacticplane tospectrainthecatalogueof plotted inFig.5wasdeterminedtoanaccuracy of±5Kc/s(±1km/sec)by structure. Thoseat/'=330°,¿'=+20°and /'=330°,¿'=+40°showeda The latterprofilesprobablyincludemanyneutral hydrogencloudsoftheGould contours weredrawnthroughthesepointsandtheresultantmapisshownin declination range—32to+90°wereplottedingalacticcoordinates.Smooth These quantitiesarestandardizedbycomparingspectratakenneartheplanein of neutralhydrogenwithavelocityvcm/sec.Forthisevaluationthekinetic away fromtheplanehavevaluesofrjaslarge as 12km/sec. broad basewithanarrowbrightpeak.Some oftheseregionsmorethan20 Belt typeinrelativemotion. where Nisthenumberofatomspercm,ldepth,t(v)opticaldepth centre frequencyat—15 Kc/s(+3km/sec)whichrepresentssmallbut significant of mostspectrafromthe zeroofthel.s.r.Thespectrarepresentative ofthe Gould Beltregion,i.e. near/'=330°,b'=+20°and150°,b'= 20° showa Muller andWesterhout. Thereisasignificantdepartureofthecentre frequency recession ofthisneutral hydrogen. H © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Averages ofthreeprofilestakenwiththe25KC/Sbandwithchannelineach This mapmaybecomparedwithapreparedbyEricksonetal.(10)on (iii) Narrowbandspectra.—Spectraobtainedwithhalf-powerbandwidths The groupofspectraobtainedatagivendeclinationwasanalysedinthe The centrefrequencyreferredtothelocalstandard ofresttheprofiles Other profilesinthepresentsurveywerebroader andshowedmorecomplex A numberofthebrightnesstemperaturespectrashowhalfwidthsto 19 60MNRAS.120. .483D No. 5>i960inthesolarneighbourhoodofMilkyWay Dec. — 12 — 26 “ 3 + 26 + 54 + 54 + 30 + 10 - 3 +10 o © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Fig. 5.—Narrowband(25Kcjs)profilestakeninregionsawayfromthegalacticplane. Properties ofspectratakeninsomeregionslargeN observedawayfrom 16 i? 15 (1958) h m R.A. 4 20 2 50 3 14 40 43 43 28 30 30 10 334 330 330 174 150 130 130 120 140 152 V Frequency (Kc/s)relativetothel.s.r. + 20 + 40 b' 25 35 20 25 18 38 o o the galacticplane temp Peak 106 °K 53 Table II 93 42 37 48 27 55 10 18 T (°k) ~60 LO 20 -20 40 iocnr units ofspectrum, 20 95 80 N WidthofT 28 19 16 16 9 Kc/s 127 105 170 100 125 115 70 70 67 80 Km/ 11*5 lo-i IO-5 ii-i 12*1 sec 6-0 7*4 7-6 9-0 5*8 centre freq. l.s.r., Kc/s relative to -15 - 5 + 3 -13 — 20 -13 —10 +10 + 13 “ 3 489 19 60MNRAS.120. .483D 2 0 202 0 o as follows: sight, aretobefoundplottedinFig.4(a),(b),(c)and(d)maydescribed of N,theintegratednumberneutralhydrogenatomspercminline of thespectratakeninpresent25Kc/ssurveyaresummarizedTableII. hydrogen neartheSunathighgalacticlatitudesasdistinctfromanoutward is inclinedati*5totheLundplane(¿'=0).ThemaximumvalueofiVon movement withintheplaneofGouldBeltsystem.Thepropertieseach present results.Thusthereappearstobeaninwardmovementofneutral 490 R.D.Davies,Astudyofneutralhydrogen plane isabout150x10atomspercmandtheminimum70 l.s.r. ofspectraforwhich|ô'|>20.Theseshowavelocitycontraction 5 km/secfor|¿'|>40andattheirlowlatitudelimitof¿'=±20theycheckthe northern hemisphereisafull lineabove,andthatforthesouthernabrokenbelow a fiducial level foreachlatitude. © Royal Astronomical Society • Provided by theNASA Astrophysics Data System 4. FeaturesofthespatialcontoursN.—Themainfeaturesdistribution Erickson etaZ.haveplottedthedepartureofcentrefrequencyfrom (i) ThegreatestvaluesofNliealongthe‘‘radio”galacticplane(b"=o)which Fig. 6.—ThedistributionofN inlongitudebetweenlatitudesio°and6o°.Theplotfor the Vol. 120 19 60MNRAS.120. .483D 202o 0 0o o latitude distributionofN.Accordinglythe average valueofNobservedat at highgalacticlatitudes. in magnitude;theyprobably representthebroaddistributionofneutral hydrogen the tenlowestvaluesof N(foundinanyio°longitudeinterval)foreach latitude. These areofsimilarshape totheformerplotsbutarereducedbyabout 30percent regions asecondsetof plots foreachhemispherewasderivedfromthe averageof in Fig.7.Toavoidanyeffectofthebiasing theseplotsbythesumoflocalized each latitudewasplottedagainstforthe northernandsouthernhemisphere the galacticcentrearesummarizedinFig.6whichshowsdistributionofATwith northern hemisphereareplottedabove,andthoseforthesouthern galactic longitudeplottedatio°intervalsoflatitude.ThevaluesNforthe 4 x10atomspercmarefoundat\b'\>70. of neutralhydrogenathighgalacticlatitudes can begainedfromastudyofthe the galacticplaneinanumberofregions.Fourteenwhichwereseparatedfrom assuming akinetictemperatureof125°K.Therearesignificantamounts No. 5,i960inthesolarneighbourhoodofMilkyWay491 are plottedbelowafiducialbaseline.Themajorasymmetries of neutralhydrogeninbothpolarregionswherevaluesNtherange2to to 190;itcanbetracedoutlatitudesof30and40abovebelowtheplane. (a) abulgeaboutthegalacticplaneinlongituderange60to120 discussed below. position, extentandavalueofAiV,theincrementNabovesurroundings. the backgrounddistributionarelistedinTableIII.Foreachisassigneda extending fromè'=4-15°to¿'=+5°and(b)thelocalsub-systemofGould’s Belt seenabovetheplanefromV=220to350andbelow/'1io° In additionthelastthreecolumnsoftableshowopticaldatawhichwillbe designation H iregion © Royal Astronomical Society • Provided by theNASA Astrophysics Data System 5. ThedistributionofNinlatitude.—Arepresentative pictureoftheamount (ii) ThemaindeparturesfromsymmetryofiVaboutthegalacticplaneandabout (iii) Concentrationsofneutralhydrogenlimitedextentwereseenawayfrom N A J H B M L E K F D C I G to 190 to 120 to 350 The propertiesof14regionsenhancedneutralhydrogenemission 340 32° 220 I40 120 ITS IIO I55 70 65 60 70 60 V o to —40 to +30 to +15 -40 + 50 -40 -30 -28 + 35 + 45 + 5 — 20 -55 -50 + 5 — 20 b' o 130x25 30 XIO 60 X10 80 X40 30 X20 35 x10 15x15 10 x15 15 X8 10 x5 10 x25 10 X 15 x5 10 X7 Extent o Ixb 20 JV unitsObscuring - of 10clouds cm Table III 15 18 15 10 10 5 6 6 8 5 Present Present Present Present Present ? Present Present Present C (Pleiades)2 C +E()Avoidance C (Scorpioetc)Avoidance C Avoidance nebulae absorption Bright Extragalactic o ? o Avoidance o 1-2 1-2 1-2 1-2 1-2 1-2 (mags) 19 60MNRAS.120. .483D 20-2 an 20_2 202 20 2 20-2 492 R.D.Davies,AstudyofneutralhydrogenVol.120 below theSunofiV=3*5xiocmd4’3xiorespectively.The respectively. Theaveragedistributionswereequivalenttolayersaboveand layers aboveandbelowtheSunof^=2*3xiocm3-im“ hydrogen usingasecant-lawanalysis.Theminimaldistributionswerefittedby effect ofthenon-uniformneutralhydrogendistributionnearSunalready difference betweentheminimalandaveragedistributionsmaybeascribedto hydrogen galacticplanewithsome20percentmorebelowthe than aboveit.Ifthedistributionofneutralhydrogeninsolarneighbour- discussed. BothdistributionssuggestthattheSunliesaboveneutral the neutralhydrogenplane.Analysisofstellar distributionsindicatesthatthe hood istakenasuniformwithathicknessof220 parsecs(12)thentheabove20 for theminimaldistributions. per centdifferencewouldbeaccountedforifthe Sunwereabout22parsecsabove of theaverageinline ofsightinthedirectionCygnus.Inview ofthis expected athighlatitudes withadepthofabout100parsecsisaccordingly about plane andtheknowndimensionsofgalaxy asmappedinneutralhydrogen. compared withthatexpectedfromtheobserved valueofNwithinthegalactic 2 xiocm“ifweassume theH1densityiñsolarneighbourhood tobethat For example,thevalueofNwithingalactic planeintheCygnusdirectionis assumption, theagreement isgoodwiththevalueof2—3xiocm~ observed is13parsecsabovethestellargalacticplane (13). 150 xiocmwhichisintegratedoveradepth ofabout8kpc.Thevalue southern. values arealsoplotted.Thefulllineisforthenorthernhemisphere,broken © Royal Astronomical Society • Provided by theNASA Astrophysics Data System The plotswerefittedtoauniformthicknessmodelofthegalacticneutral The observedamountofneutralhydrogenat highgalacticlatitudescanbe Fig. 7.—ThedistributionofNinlatitudeaveragedoverallvalueslongitude.Minimal 19 60MNRAS.120. .483D 0 0o o o 0 the radiogalacticplane(b"=o).Theseclouds ofdustandgasextendfor the quadrantscentredonaxisoflocal system(/'=150°,l'=33°)in gives thedistanceaswellsizeofclearlydistinguishable galacticnebulosities. system andalsowiththeCepheusbulge. zone ofavoidanceextragalacticnebulaecoincideswiththeshapelocal has acounterpartinLundmark’smap;thisregionisdustcomplexabout No. 5,i960inthesolarneighbourhoodofMilkyWay493 fluctuations instardensity.Allthosedustcloudsgreaterthani-odegree their spectralcharacter isgiven(EforgaseousemissionandC continuum below theplaneandtheirdiametershavebeen plotted.Thelocalsystemis been convertedintomagnitudesofabsorptionincolumn8TableIII.The counts ofextragalacticnebulae.Hubble’sobservednumberdeficiencieshave measures andLundmark’sobscuringcloudstherewasacleardeficiencyinthe above theplanebetweenlongitudes60and120inCepheus(regionD)also above thegalacticcentre.Thebulgeseeninneutralhydrogendistribution below thegalacticanticentreandregionsofOphiuchus,ScorpioScutum with galacticlatitudeintegrated overeachquadrantisgivenasapolar plotbythe Those nebulaewithdistanceslessthan1000parsecs havebeendividedamong obscuring matter.TheagreementwiththegeneralfeaturesofLundmark’s counts ofextragalacticnebulaegiveanotherindicationthedistributionlocal are indicatedincolumn6ofTableIII. clouds hasalreadybeenestablishedforanumberofregionswithinthegalaxy full-line inFig.8{a)and (è).ThecorrelationbetweenNandthe existence of plotted onLundmark’s mapandareatthesamedistance.Thevariation ofN reflection fromdust). These regionsareinterwovenwiththeobscuring matter 200 parsecsinthedirectionof/'=330and300 parsecs inthedirectionofV=150. Fig. 8(a)andtheperpendicularaxisin 8(b).Theirpositionsaboveor catalogue isgoodandinallcaseswheretherewasacorrelationbetweenthe21cm obscuring matterinthesmallregionsA,H,I,KandM.Thesecorrelations 500 parsecsdistant(4).Therealsoappearstobegoodevidenceforanexcessof matter andthedistributionofneutralhydrogenawayfromgalacticplane. diameter havebeenplottedinthemapsofNFig.4(0),(ô),(c)and{d), correlation betweenthedensityofneutralhydrogenandexistencedust The moreextensivebrightcomplexesarenoted incolumn7ofTableIIIwhere clearly seenseparatedfromthegalacticdistribution. Itliesinclinedat20to dust regionsinwhichthestardensityislessthan5or6timessurroundings. spiral armsofthegalaxy. interstellar dust isagainmarked. G andL)inthewell-knowndustregionsofTaurus,OrionAurigawhichlie There isanexcessofobscuringmatterintheplanelocalsystem(regions In consequencethehigherlatituderegionsmaybeinerror,beingonlystatistical Population Iobjectswhichareknowntoco-existwithneutralhydrogeninthe (14, 15,16).Thepresentstudyhasincludedanexaminationoftypes © Royal Astronomical Society • Provided by theNASA Astrophysics Data System A mappreparedbyLundmark(17)showsthepositionsandsizesofthose A correlationisvisiblebetweenmanyfeaturesofthedistributionobscuring 6. Acomparisonoftheneutralhydrogendistributionwithopticalfeatures,—The Cederblad’s (19)catalogueofcontinuousreflectionandemissionnebulae Hubble’s (18)measurementsoftheeffectgalacticobscuringmatteron (ii) Brightnebulae (i) Obscuringclouds 19 60MNRAS.120. .483D 6 6 0 494 system. Thismaybecomparedwith100-200x10yearsforthebulkof of spectraltypeBtowhichindicates(20)anage20x10yearsforthelocal system inclinedatabout20°tothegalacticplane.Alargenumberoftheseare the GalaxyandBlythe(22)hasshownthatsome38Mc/scontinuumdetailsseen hydrogen andthebrightnessdistributionofcontinuumradiationfrom discussed intheintroduction.Thenearbyearly-typestarsliewithinlocal indicated thattheremaybesomecorrelationbetweenthedistributionofneutral stars inthesolarspiralarm. with beamwidthsoflessthanaboutio°i.e.comparable withtheresolutionof neutral hydrogensurveyandfromcontinuum surveyswhichhavebeenmade in theTaurusregionarealsovisibleneutral hydrogencontours.More 38 (22),200(23),and400(24)Mc/s.The valuesaretheaveragebrightness present survey.TheresultsareplottedinFig. 9(0)and(b)forfrequenciesof Galaxy. Mills(21)hasfound81Mc/sradiationfromspiralstructurewithin the continuumemissionatanyfrequencyand theneutralhydrogendistribution evidence relatingtothiscorrelationwasderived fromastudyofthepresent temperatures within±5ofthelatitudesindicated. is notunexpectedsince thecontinuumradiationcomesfromanextended region both lineandcontinuum radiationistheOrioncomplexofgasand dust. This showing theHaemission centredontheOrionNebula.Moreover continuum at galacticlatitudesnear±20°.Theonlyfeature whichproducesaneffectin 59 spur at/'=o°reaching to highpositivelatitudeshasnocounterpartin theneutral hydrogen distributionwith theresolutionofpresentsurvey. line polarplot. averaged foreachquadrantoflongitude.ThecorrespondingdistributionNisgivenbythefull © Royal Astronomical Society • Provided by theNASA Astrophysics Data System 7. Acomparisonwithradiocontinuumsurveys.—Recentpublicationshave The spatialdistributionofstarsinthesolarneighbourhoodhasalreadybeen (iii) Thedistributionofstars An examinationoftheseplotsshowsthatthere isnoclosecorrelationbetween Fig. 8.—Thedistributioninheight(z)abovethegalacticplaneofbrightnebulae{afterCederblad) R. D.Davies,Astudyofneutralhydrogen Vol. 120 19 60MNRAS.120. .483D 0 3 0 o o 5 5 No. 5,i960inthesolarneighbourhoodofMilkyWay within thelocalspiralarm. will beassumedherethatthehydrogenisdistributed indepththesamewayas hydrogen inthelocalsystem(Gould’sBelt)canonlybeestimatedif the widthtohalf-intensityinlatitudeof~15 . Thecorrespondingbreadthin the dust.Thushydrogenwillbetakento haveauniformdensitytowards is socloselycorrelatedinpositionwiththedust complexesofthelocalsystem,it representation ofthelocal systemis2-0atomspercminexcessofthe gasdensity the directionZ'=6o°toV240isestimatedat 150parsecs(width~70)although /'= 150outto300parsecs(theTaurus,Orion clouds).Theequivalentdepth Z' =330°outto200parsecs(theOphiuchus and Scorpioclouds)towards dimensions ofthesystemcanbederived.Since theneutralhydrogendistribution and inthelatter2*6x ioMO-Thehydrogengasdensityinthis idealized it isdifficulttodeterminebecausethelocalsystem herecrossesthegalacticplane of thehydrogenperpendiculartogalacticplane is30parsecsasderivedfrom and mightjustaswell have beentakenas500parsecsi.e.makingthe regiona circular disk.Theneutral hydrogenintheformercaseis10solarmasses (MQ) 0 Fig. 9.—ThedistributioningalacticlongitudeofNandcontinuumradiationfor{a)ö'=+15 © Royal Astronomical Society • Provided by theNASA Astrophysics Data System 8. Themassofgas,dustandstarsinthelocalsystem.—Theneutral and (b)b'=-20°. 495 19 60MNRAS.120. .483D 3 8 52 0 45 496 R.D.Davies,AstudyofneutralhydrogenVol.120 and takingtheobscurationalongmajoraxistobe2magnitudesasreadfrom in magnitudes.Thedustmassesofthetwomodelsusedneutralhydrogen calculations were2-5xioMQand6respectively.Theneutral Hubble’s countsofextragalacticnebulae.Thelinesightdustmass,m',was hydrogen gasmassis40timesthatofthedust. the system.AnapproachtoproblemcanbemadeusingfactthatA gives anapproximateestimateof10MQforasystem500parsecsinradius. derived usingtheexpression(i6)m'=3x10~mgrcmwheremisabsorption type starswithapparentmagnitudeslessthan6lieinaplaneinclinedtothe galactic planeat4comparedwith16fortheBstarswithin200parsecs(2). The majoruncertaintyliesindeterminingthenumberoflowluminositystars from theknowndensityoffieldstars(25).Thestellarmasslocalsystem This indicatesthatAstarswithinthelocalsystemamounttoonlyabout30per mass ofthelocalsystemconsistsmainlyAandlatertypestarswithaspace cent oftheAstardensityinfieldstarslocalspiralarm.Assuming appear tobelessthanthemassofhydrogeninsystem.Thisresultmay calculated inthiswayliestherangeof10tosolarmasses,andwould compared withthefractionofneutralhydrogeninspiralarmnearSun density 30percentofthatinthelocalspiralarm,ausefulestimatecanbemade which amountsto10percent(26). stars inthelocalsystemproposedbyShapley.Neitherdistributionissmooth away fromthegalacticplaneshowsaclosesimilaritytodistributionofyoung although themajorfeaturesofoneappearonother.Therealitythis of ayoungPopulationIobject—youngstarsandhighpercentagegasdust. system isfurtherstrengthenedbecauseitconsistentlyexhibitsthecharacteristics the spectralsurveydescribedinSection3(ii). the dustandneutralhydrogendistributionsindirectionofCepheusmaybe this datacanbeinterpretedinanevolutionarysense.The“bulge”seen played amajorpartinthedevelopmentandoperationofequipmentused a similartypeofsystem.Thiscanonlybesubstantiatedifdataontheand It thenwouldappeartobea“recent”phenomenoninthelocalspiralarmif distance ofthestarsembeddedwithinitbecomeavailable. (3) Hubble,E.,Ap.J.,56, 162,1922. (2) Shapley,H.,Ap.J.,49,311,1919. (1) Gould,B.A.,UranometriaArgentina,335,1879. (4) Bok,B.J.,TheDistribution ofStarsinSpace,Chicago,1937. (6) Lilley,A.E.,andHeeschen, D.S.,Pub.Nat.Acad.Sei.40,1095,1954. (5) Schmidt,H.,Bonn.Univ. Veroff,No.35,1949. (7) Davies,R.D.,ParisSymposium onRadioAstronomy,Ed.R.N.Bracewell, Stanford © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Jodrell BankExperimentalStation, The massofthedustinsystemwascalculatedusingsamedimensions The massofstarsinthelocalsystemisnotreadilycalculable.Schmidt(5) 1959 September. 9. Conclusion.—Thepresentstudyofthedistributionneutralhydrogen Acknowledgments.—The authorwishestothankDrR.C.Jennison,who Lower Withington, Univ. Press,Stanford,1959. Cheshire : References No. 5, i960 in the solar neighbourhood of the 497 (8) Muller, C. A., and Westerhout, G., 13, 151, 1957. (9) Muller, C. A., Ap.J., 125, 830, 1957. (10) Erickson, W. C., Heifer, H. L., and Tatel, H. E., Paris Symposium on Radio , Ed. R. N. Bracewell, Stanford Univ. Press, Stanford, 1959. (11) Kwee, K. K., Muller, C. A. and Westerhout, G., B.A.N., 12, 211, 1954. (12) Schmidt, M., B.A.N.y 13, 247, 1957. (13) van Tulder, J. J. M., B.A.N., 9, 315, 1942. (14) Lilley, A. E., Ap. J.y 121, 559, 1955- (15) Heeschen, D. S., Ap.J., 121, 569, 1955. (16) Davies, R. D., M.N.y 116, 443, 1956. (17) Lundmark, K., Med. fr. Astron. Obs. Upsala, No. 30, 1927. (18) Hubble, E., Ap. J.y 79, 8, 1934. (19) Cederblad, S., Med. fr. Astron. Obs. Lund, Ser. II, 119, 1946. (20) Sandage, A., Stellar Populationsy Vatican Observatory, p. 41, 1958. (21) Mills, B. Y., Paris Symposium on Radio Astronomy, Ed. R. N. Bracewell, Stanford Univ. Press, Stanford, 1959. (22) Blythe, J. H., M.N.y 117, 652, 1957. (23) Droge, F., and Priester, W., Z. f. Ap., 40, 236,1956. (24) Seeger, C. L., Westerhout, G., and Conway, R. G. (in preparation). (25) Allen, C. W., Astrophysical Quantities, London, 1955. (26) Kerr, F. J., and Hindman, J. V., P.A.S.P., 69, 558, 1957.

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