Hutsem´ekers : in Luminous Blue Variables : Massive Stars in Transi-

tion, A. Nota, H. Lamers (eds.), A.S.P. Conference Series 120, 316 (1997)

Dans cet article, certaines propri´et´es des n´ebuleuses associ´ees aux LBVs sont pass´ees en

revue et plus particuli`erement celles li´ees `a la pr´esence de poussi`ere.

Nous montrons tout d’abord que toutes les n´ebuleuses de type LBV contiennent de la

poussi`ere froide rayonnant dans l’infra-rouge lointain et, qu’`a cet ´egard, les n´ebuleuses de

type LBV du Grand Nuage de Magellan (LMC) ne diff`erent pas de celles de notre Galaxie.

Nous montrons que les LBVs du LMC suivent ´egalement la relation “masse n´ebulaire –

luminosit´e stellaire” ainsi que les n´ebuleuses associ´ees aux ´etoiles Wolf-Rayet de type

WN8, g´en´eralisant la relation ´etablie pr´ec´edemment pour les n´ebuleuses galactiques (Ar-

ticle 4). Ces r´esultats appuient l’hypoth`ese d’un lien ´evolutif entre les LBVs et les ´etoiles

de type WN8.

Enfin, nous montrons que dans quelques cas, on peut d´etecter la lumi`ere stellaire

diffus´ee par les poussi`eres et ceci jusqu’`a des distances exc´edant les dimensions de la

n´ebuleuse de gaz ionis´e.

De nouvelles n´ebuleuses de type LBV ayant ´et´e d´ecouvertes apr`es la publication de

cet article, nous pr´esentons une mise `a jour de la relation “masse n´ebulaire – luminosit´e

stellaire” qui se voit confirm´ee.

Luminous Blue Variables : Massive Stars in Transition ASP Conference Series, Vol. 120, 1997

A. Nota and H.J.G.L.M. Lamers, eds.

Dust in LBV-type Nebulae

Damien Hutsemekers 1

Institut d'Astrophysique, 5 av. de Cointe, B-4000Liege,Belgium

Abstra t. Somesystemati propertiesofdustinLBV-typenebulaeare reviewed, on thebasisof the far-infraredthermalemission measured by the IRASsatellite,andthe s atteringofvisiblestellarlight.

1. Introdu tion

Inthe framework of a systemati study of infrared properties of emission-line stars, M Gregor et al. (1988a) found that LBVs are surroundedby ool dust, while B[e℄ stars are predominantly asso iated with hot dust (although a few obje ts like  Car, or the B[e℄ star HD87643, both embedded in nebulosities, are simultaneously asso iated with hot and ool dust). They showed that, if in equilibrium, the ool dust must lie at large distan es from the star, most probably in the opti al nebula surrounding some of these obje ts. M Gregor et al. (1988b) subsequently resolved the far-infrared emission from AG Car, showingthatita tuallyarises withintheopti alnebula.

Table 1 summarizes the present situation for Gala ti and Large Magel- lani Cloud(LMC)LBVs,togetherwithfourGala ti WRstarsasso iatedwith eje ta-type nebulae(Esteban et al. 1992). For theGala ti obje ts, there is a nearly one-to-one orrelation between the presen e of ool dust and that of a nebula. Also,HD160529 andHD168607have beenimagedwithnodete tion of anyopti alnebulosity, anddonotshowanyinfraredex essindi atingthepres- en eofdust. FortheLMCobje ts, theobservationsaremu hmorein omplete: due to the larger distan e, only the biggest nebulae have been resolved (Nota et al.1995, 1996). The uxes measured by IRAS are low, or ontaminated by arelativelyhigh ba kground: only three obje tsare dete tedat 25and 60 m with a olor temperature signi antly dierent from that of the ba kground. Nevertheless, the presen e of ool dust in two nebulae, the dete tion in S119 ofan ex essat 25m, as well asthe fa t thatthe nebulaaround R71 is spe - tros opi ally dete ted (Stahl & Wolf 1986) and probably very small (see next se tion),suggestthatthenebulaearoundLMCLBVsarenotdierentfromthe Gala ti onesasfaras dustis on erned.

The onlynebulawithoutdustisthataround PCygni: atadistan eof1.8 kp , and assuming a dust-to-gas ratio omparable to that estimated in other nebulae,far-infrared emission (otherthanwindfree-free emission) shouldhave beendete ted. Comparedtotheotherobje ts, thisisnottheonlydieren e of

1

Also,Cher heurQualieauFondsNationaldelaRe her heS ientique,FNRSBelgium

316

Table1. DustandnebulaearoundLBVandWRstars Obje t Type Opti alnebulosity? Dust?

Car LBV yes ool+hot

AGCar LBV yes ool

HRCar LBV yes ool

WRA751 LBV yes ool

PCygni LBV yes no

HD160529 LBV no no

HD168607 LBV no no

HD168625 LBV? yes ool

He3-519 Of/WN yes ool

R127 LBV yes ool R71 LBV  ool SDor LBV ? ? R110 LBV ? ? R143 LBV yes? ? BE294 LBV ? ?

S61 Of/WN yes ool

S119 Of/WN yes 

BE381 Of/WN yes ?

HD269927 Of/WN ? ? R99 Of/WN ? ? R84 Of/WN ? ? S9 Of/WN ? ? WR124 /M1-67 WN8 yes ool WR40 /RCW58 WN8 yes ool WR136 /NGC6888 WN6 yes ool WR6 /S308 WN5 yes ool

thePCygninebula: ithasamu hsmallerionizedgasmass,aswellasdierent morphology,ex itationme hanism,andabundan epattern(Johnsonetal.1992, Barlow etal.1994, Notaetal. 1995). Thispossiblysuggestsadierentorigin.

2. Dust properties derived from the far-infrared emission

For an opti ally thin loudof N dust grainsof uniformsize, omposition, and temperature,there eived uxdensityisequalto

F  =N a 2 d 2 Q  B  (T d ); (1)

where a is the grain radius, T d

the grain temperature, and d the distan e to the loud. In the far-infrared,the graineÆ ien y Q

 is/   with ' 1 2, andT d

maybederivedfromtwovaluesofF 

measuredatdierentwavelengths. Dusttemperatures are reported in Table2, for =1.5. Whatever thenature ofthe entral obje t, thedust temperature orrelates withthe opti alnebular radius (Fig.1a), hotter dust being in smaller nebulae, as expe ted at least in rstapproximation ifthedustismixedwiththeionizedgasorvery losetoit. Onthe basis ofthis orrelation, one an predi tthe size of thenebula around R71intheLMC:withT d = 127K, weexpe tR neb 0.1p ,or0: 00 3.

66

Table 2. DustpropertiesinLBVandWRnebulae Obje t d(kp ) log(L=L  ) R neb (p ) T d (K) log (M d =M  ) Car 2.5 6.70 0.08 133 -1.41 AGCar 6.0 6.22 0.50 88 -2.00 HRCar 5.2 5.58 0.43 100 -3.02 WRA751 7.1 6.06 0.40 103 -2.23 HD168625 2.2 5.34 0.08 124 -3.51 He3-519 7.6 6.03 1.0 73 -2.23 R127 51. 6.12 1.0 85 -2.04 R71 51. 5.90 ? 128 -2.29 S61 51. 6.03 0.3: 99 -2.50 WR124 /M1-67 4.5 5.97 0.92 69 -2.51 WR40 /RCW58 2.5 5.78 3.1 59 -2.77 WR136 /NGC6888 1.8 5.74 3.7 62 -2.22 WR6 /S308 1.8 5.50 9.2 55 -2.43

FortheGala ti LBVs,dataarefromHutsemekers(1994,PaperI),withthedieren ethat opti al nebularradiimeasured inthe ontinuumare preferred wheneveravailable. Nebular radii are from Clampin et al. (1993) for R127, Stahl(1987) for S61, and from Esteban et al.(1993) for the WR nebulae. Stellar luminosities are from Stahl et al.(1983) for R127, Wolfetal.(1987)forS61,andLennonetal.(1994)forR71. FortheWRstars, weused the absolutemagnitudesfromHamannetal.(1995)togetherwiththebolometri orre tionsfrom L.F.Smithetal.(1994). Thedistan esarealsofromHamannetal.(1995),ex eptforWR124 (Crawford & Barlow 1991). Whenever ne essary, parameters are re-s aled to the adopted distan es. Dusttemperaturesandmassesare al ulatedasinPaperI,followingM Gregoret al.(1988a),i.e. usingthe25and60mIRAS uxdensities( olor- orre tedwhenne essary), =1.5,andK60=230 m

2 g

1

.FortheLMCLBVs,IRAS uxdensitiesarefromtheIRASPSC whenavailablewithgooda ura y,otherwisefromS hwering(1989). FortheWRnebulae,the IRAS uxdensitiesarefromtheIRASPSC(M1-67)andfromMathisetal.(1992). Estimates ofun ertaintiesaredis ussedinPaperI.

Thetotaldust massmay beevaluatedusingthe relation(Hildebrand1983)

M d = F  d 2 B  (T d )K  ; (2) where K  =3Q 

=4a, and isthe densityof the grain material. Adopting a suitablyweighted average of K



, Eq.2 maybe usedtoestimate the totaldust masswithoutdetailedknowledgeofthegrainsizedistribution. Dustmassesare reportedinTable2,usingK



=230 m 2

g 1

at60m. ConsideringonlyGala ti LBVs,wefoundinPaperIa orrelationbetweenthenebulardustmassandthe stellar luminosity. Fig. 1b illustrates this relation. The LMC LBVs seem to behavesimilarly,althoughthedustmassesaremoreun ertain(duetothelower IRAS uxes). ThetwoWN8obje ts areinex ellentagreementwiththeLBVs, supporting the relationship suggested by Smithet al. (1994) and Crowther et al. (1995). On the ontrary, S308 and NGC6888 (WN5-6) do not follow the generaltrend,possiblyindi atingthatthesenebulae-moremassive-arenot, or no longer, onstituted of similar material, or that they have a dierent origin and/orprogenitor(Mathisetal.1992,Smith1996).

It is important to note that, due to our poorknowledge of grain proper- ties,orders-of-magnitudedieren esexistinabsolutedustmassestimateswhen dierent grain models are onsidered. However, if we hange  and K

 for all

Figure 1. (A, left) The nebular dust temperature as a fun tion of the opti alnebularradius. (B,right)Thenebulardustmassasafun tionofthe stellarluminosity;thettedlineisbasedonGala ti LBVs(lledsquares, f. PaperI).OpensquaresrepresentLMCLBVs,losangesWN8, ir lesWN5-6.

obje ts, relative dust masses are essentially un hanged, and the slope of the \logM

d

logL"relation isunae ted. This an beseenin eliminatingT d

from Eq.2: withinagoodapproximation,M

d '1:710 4 (1:85)  K 1 60 d 2 F 0:71 25 F 1:71 60 , if F  isinJy, K  in m 2 g 1

, d inkp . The basi assumption isthat thegrain propertiesaresimilarinallLBV-typenebulae(i.e. LBV+WN8nebulae),anhy- pothesiswhi hinviewoftheobserved orrelationseemsaposteriorireasonable. Also,systemati dieren esinluminositieslikethosereportedbetweenempiri al estimatesandvaluesderivedfrommodelswillnotae t the orrelationaslong asthesedieren esremainsystemati .

Further,itisinterestingto onsiderthehighlyreddenedobje tsG79.9+0.46, G25.5+0.2, and M1-78, suspe ted to ontain LBV orWN entral stars (Higgs etal.1994,Subrahmanyanetal.1994,Gussie1995). ForG79.9+0.46, theIRAS ux densities from Waters et al. (1996) give logM

d

= 1:87 at d = 2 kp ; this perfe tly ts the orrelation in Fig. 1b with logL=L

= 6.30 from Higgs et al. (1994), supportingthe LBV nebula identi ation. On the ontrary, the dust masses evaluated for G25.5+0.2 (d ' 14 kp ) and M1-78 (d ' 8 kp ) using IRAS PSC ux densities (logM

d

= 1:22 and 1:13, respe tively) are denitelylargerthanforanyother obje t ( f. Table2), astingsomedoubtson theiridenti ationasLBV-type nebulae.

More detailed modelling has been arried out in a few ases, providing some information on the grains. After onsidering several models, M Gregor etal. (1988b)and Hyland& Robinson(1991) on ludedthat largegrains(a' 1 m) dominate in the AG Car nebula. The preponderan e of large grains wasalso suggested in the  Car homun ulus (Mit hell & Robinson1986), and possibly in He3-519 (Davidson et al. 1993), whileMathis et al. (1992) showed that omplex distributions of small grains may reprodu e the observations in threeWRnebulae.

Figure2. (A, left) HD168625: a 2.2m ontinuum image divided by a H+[Nii℄one,bothfromHutsemekersetal.(1994). (B,right)AGCar:ared ontinuum image divided by aH+[Nii℄ one, both obtained with the ESO 3.6mteles ope+EFOSC.Thedarkerregionsinthenebulaearethosewhere dusts atteringdominates. Note thatthe AGCarnebulaisalsodete tedat 2.2mbuttheimage,fainter,isofpoorquality.

3. LBV-type nebulae as re e tionnebulae

Inadditiontothewell-knownCarhomun ulus,dusts atteringhasbeen on- vin inglydete ted intwo LBV-typenebulae: AG Car (Viottietal. 1988), and HD168625 (Hutsemekers et al. 1994). Why not in other LBV-type nebulae? Assumingthe ontinuum surfa ebrightnessS

ont /S H =R neb , it appearsthat theotherLBV-typenebulaeareexpe tedtobe onsiderablyfainterinthe on- tinuum,andthereforeprobablyyet tobedis overedasre e tionnebulae.

There e tionnebulaaroundAGCarhasneutral olorsrelativetothestar, whilethataroundHD168625isabnormallyred. Inboth ases,thissuggeststhe presen eoflarge grains(Pares e & Nota1989, Hutsemekerset al.1994). Dust s attering seemsto extend to the near-IR (as in  Car, Allen1989), although this ontinuum ouldbeduetotheso- alledextended2.2memissionobserved in many re e tion nebulae (Sellgren et al. 1996). Imaging polarimetry ould distinguishbetween these two hypotheses. In fa t, detailed studies are om- pli ated by the fa t that LBVs are variable in magnitude and olor, the light rea hing the nebulawith atimedelay(typi allyone yearfor AG Car,whi his omparabletothetime-s aleof thestellar variations).

Thesizeandshapeofthere e tionnebulaeareinroughagreementwiththe H nebulae,although leardieren es exist ( f. Nota etal. 1995, and Fig. 2). ForHD168625, the re e tionnebulaextendsbeyondthe Hring,outliningthe southernrim;thisisalsothe ase insome partsof the AGCarnebula,andfor theHRCarnebula(Voorsetal. 1997).

4. Con lusions

Dust isa mainfeatureof LBV-type nebulae(= LBV+WN8 nebulae,ex ept PCygni).

Dust and ionized gas nebulae have roughly similarsize andshape, but their

detailedmorphology diers.

Thesmallernebulaehavethehotterdust,andthemostluminousstarshavethe mostmassivenebulae. Thelatterrelation(M

d /L

1:5

) onstrainstheinstability and/ornebulaformationme hanisms (Maeder1997,Stothers &Chin1996). Thereareseveral indi ationsforthepresen e oflargedustgrains(a'1m). The grain omposition is un lear: sili ate features are dete ted in  Car and R71(Ro heet al. 1993), whilePAH emissionisobservedin HD168625andAG Car(Cohenet al.1986, Tramsetal. 1997).

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