Polarization measurements of Vega-like stars

a r X i v :a s t r o -p h /0009483v 1 29 S e p 2000A&A manuscript no.(will be inserted by hand later)ASTRONOMYANDASTROPHY SICS1.IntroductionThe Infrared Astronomy Satellite (IRAS)detected (Aumann et al.1984)a large infrared excess at wave-lengths longward of 12µfrom the otherwise normal main-sequence A0V star Vega (αLyr).The infrared excess is thought to be due to emission from circumstellar dust,in thermal equilibrium with the stellar radiation from the central star,and distributed in a shell or ring,several tens to hundred AU in extent.Since then a large number of main-sequence stars have been found to have similar infrared excesses in the IRAS wavebands and are called Vega-like stars (see eg.Backman &Paresce 1993,Vidal-Madjar &Ferlet 1994,Lagrange-Henri 1995,Mannings &Barlow 1998).What is the spatial distribution of the circumstellar dust in Vega-like stars?Optical coronagraphic observa-tions of βPic by Smith &Terrile (1984)showed that the scattering dust in this object is distributed in a flattened disk being viewed nearly edge-on.Vega-like stars are now generally thought to have circumstellar dust distributed in disks.The disk structures in these2H.C.Bhatt and P.Manoj:Polarization measurements of Vega-like stars Table1.Polarization measurements of Vega-like starsHD Date of Spectral V B-V E(B-V)m-M Ref P(%)ǫP(%)θ(◦)ǫθ(◦) number Observation Type mag mag mag(1999)967217Jan A1V 5.60.060.04 3.94H0.170.05128 1720617Jan F5/F6V 4.40.480.030.73H0.100.03885 1744317Jan B9V8.70.300.387.67H 1.100.171395 3250916Jan A27.50.200.15 5.89H0.950.09513 3470017Jan G09.10.560.0010.32H0.110.123719 3738917Jan A08.3-0.060.007.040.460.12158 5330016Jan A08.00.310.31 5.79 1.000.13813 9333108Apr B9.5V7.20.020.07 5.90H0.240.061755 9880013Mar K58.8 1.150.10 3.34H0.540.13896 9921108Apr A9V 4.00.210.00 2.05H0.060.061755 10264707Apr A3V 2.10.090.010.22H0.150.05506 10908508May F2V 4.30.380.03 1.30H0.380.041475 11589208Apr A2V 2.70.060.18 1.27H0.180.061646 12184708May B8V 5.2-0.080.01 5.09H0.530.041722 12324708May B9V 6.40.000.07 5.02H0.210.06575 13188508Apr A0V 6.90.010.01 5.42H0.570.07133 13534411Mar A0V8.60.480.48 5.820.070.1110815 13961410Mar A7V8.20.230.03 5.760.650.10385 13966412Mar F5IV/V 4.60.410.00 1.22H0.760.071655 14209606may B3V 5.0-0.010.18 5.19H0.420.041732 14211408May B2.5V 4.5-0.070.15 5.62H0.450.05142 14216506May B5V 5.3-0.010.15 5.52H0.750.0632 14266611Mar A8V8.80.520.27 5.510.800.12724 14300608Apr G6/G810.20.730.03 4.820.690.0873 ********may B2V 4.5-0.170.06 5.78H0.500.051174 14991411Mar B9.5IV 6.70.240.29 6.09H 2.540.07661 23351710Mar K29.7 1.300.40 2.16 1.800.211774H.C.Bhatt and P.Manoj:Polarization measurements of Vega-like stars3 Table2.Polarization data for additional Vega-like stars from literatureHD P(%)ǫP(%)θ(◦)ǫθ(◦)E(B-V)V Spectral m-MNumber mag mag Type30030.0280.009127.79.10.00 5.2A2 2.31104760.0500.035178.119.30.00 5.2K1V-0.59106470.0400.03520.723.60.00 5.6G0I10.20107000.0050.007148.935.00.00 3.6G8V-2.47140550.0300.12018.063.40.00 4.0A0V 2.84161570.1310.047170.610.20.008.6K7V0.39189780.0060.00610.826.60.00 4.2A3V 2.07200100.0060.009115.336.90.00 4.0F8IV 1.15220490.0070.010147.635.50.00 3.7K2V-2.47383930.5700.03542.5 1.80.00 3.6F6V0.00439550.0900.03563.111.00.10 5.3B2V7.5949336 1.3000.035 2.00.80.10 6.2B3V7.66533760.0140.037175.554.00.019.3F3/F5V 6.10684560.0600.035166.416.30.00 4.8F7V 1.50698300.0800.035140.612.30.00 6.0G8V0.60711550.0200.12013.071.60.00 3.9A0V 2.95733900.1700.035119.5 5.90.10 5.3B3V 6.73954180.0000.12043.090.00.00 2.4A1V 1.271084830.1400.03559.57.10.00 3.9B2V 5.991149810.1400.03532.37.10.107.1B77.731281670.0900.12026.033.70.00 4.5F2 1.711390060.0600.12029.045.00.00 2.2A0V 1.271430180.3480.10737.28.70.10 2.9B1V 6.261496300.0800.12075.036.90.10 4.3B9V 3.161539680.9010.14133.2 4.50.089.3F0V7.701618680.0080.00133.3 3.60.10 3.8A0V 2.381629170.1000.03565.29.90.00 5.7F4V 1.991721670.0200.12075.071.60.000.0A0V-1.101766380.0300.03513.530.30.00 4.7A0V 3.991812960.0400.03599.923.60.00 5.0A0V 3.901818690.0100.03575.960.30.00 4.0B9II 4.402149530.0250.015115.316.70.00 6.3G0V 1.152169560.0050.005122.926.60.00 1.3A3V-0.772243920.0410.011128.57.60.00 5.2A2V 2.654H.C.Bhatt and P.Manoj:Polarization measurements of Vega-likestarsFig.1.Percentage polarization plotted against distance modulus for Vega-like stars.Vega-like stars are represented by the asterix symbol while crosses are used for normal starsTherefore,to supplement our observations,we have madea search for polarization measurements of additional Vega-like stars in the SIMBAD data base at CDS,Strasbourg.We found polarization measurements for34such stars inthe catalogue of stellar polarization by Heiles(2000).Ina manner similar to Table1,we list data for these starsfrom Heiles(2000)in Table2.Altogether,in Tables1and2,we now have61Vega-like stars with polarization mea-surements.In the following we compare the polarimetricbehaviour of the Vega-like stars with normalfield stars.In order to assess the strength of the intrinsic compo-nent in the polarization of the Vega-like stars,one needsto have an estimate of the interstellar polarization.Formore distant Vega-like stars the interstellar componentcould be relatively large,since the interstellar polarizationin general increases with distance,and should be sub-tracted from the observed polarization.This is,however,not possible for the programme stars individually,asthe interstellar polarization in the direction of thesestars,as a function of distance,is generally not known.We therefore make a statistical comparison between thepolarization observed for the Vega-like stars and normalfield stars with similar magnitudes and distances.Forthe comparison,polarization measurements for normalfield stars have been taken from Heiles(2000).Normalstars within∼1◦of the Vega-like stars were chosen forcomparison.Stars with any known peculiarities(likethe presence of emission lines in the spectra,infraredexcesses,association with nebulosities,variability etc.)were excluded.In Fig.1we plot the observed degreeof polarization against the distance modulus for theVega-like stars as well as the normalfield stars.It canbe seen from Fig.1that,on an average,the Vega-likestars have polarization values generally larger than thenormalfield stars at comparable distances.The averagevalue of polarization for the Vega-like stars is0.34%with a large dispersion of0.48%,while the normalfieldstars have an average polarization of0.2%and a smallerdispersion of0.22%.Fig.2shows frequency distribution(as a fraction of the total number of stars in the twosamples seperately)of stars with observed polarizationlarger than a given value.Now the difference betweenH.C.Bhatt and P.Manoj:Polarization measurements of Vega-like stars5 Fig.2.Frequency distribution of observed polarization values for Vega-like and normalfield stars.f is the fractionalnumber of stars with observed polarization larger than a given value.Solid line represents Vega-like stars and dashedline normalfield starsthe polarimetric behaviour of the two samples of starsis more clear.About30%of the Vega-like stars showpolarization values larger than0.5%,whereas only13%of the normalfield stars show polarization values largerthan0.5%.There are nofield stars with polarizationvalues larger than1%,while about10%of the Vega-likestars show polarization values larger than1%.For thestars plotted in Fig.1a two-sided two-dimensionalKolmogorov-Smirnofftest shows that the two samples(Vega-like and normalfield stars)are different to99.7%.For the Vega-like stars that show relatively largedegree of polarization,the observed polarization can notbe accounted for by normal interstellar polarization andmust be circumstellar in origin.Circumstellar dust aroundthese stars,to which the observed infrared excesses areascribed,can cause polarization by the the process ofscattering of the light from the central star.In order tobe able to produce a net polarization in the integratedlight the dust must be distributed in a non-sphericalgeometry.These non-spherical distributions could beflattened disks around the stars similar to those inβPic.The disk planes should have relatively small inclinationswith the observer’s line of sight.Given this constraint onthe inclination of the disks,the difference between theVega-like stars and the normal stars noticed in Fig.1becomes even more significant,because only a fraction ofthe stars with disks will have have favourable inclinations.The large values of polarization observed for Vega-likestars thus support the existence of dusty disks aroundthese stars.The polarization is produced by scatteringof light from the central star.In contrast,the interstellarpolarization is caused by selective extinction due to non-spherical dust grains aligned by the intertellar magneticfield.The interstellar polarization shows a correlation withreddening given by the average relation:P V/E(B−V)=3%mag−1.The circumstellar dust in Vega-like stars isbelieved to consist of relatively large grains(a few tensof microns(Backman&Parsce1993))that are not ex-6H.C.Bhatt and P.Manoj:Polarization measurements of Vega-likestarsFig.3.Percentage polarization against reddenning for Vega-like stars.The average interstellar relation P(%)/E(B−V)=3is shown as the solid linepected to produce any additional reddening at opticalwavelengths.Thus large values of polarization may resulteven with small values of reddening at small distances.Fig.3shows a plot of the observed polarization against redden-ing E(B−V)for the Vega-like stars,together with the linerepresenting the average relation followed by interstellarpolarization and reddening.It can be seen that there aremore stars above this line than below it.Also,there isa lack of correlation between polarization and reddeningin Vega-like stars for smaller values of reddening.Onlyat relatively large distances when the interstellar compo-nent becomes large does the observed polarization beginto generally increase with reddening.Circumstellar dust in Vega-like stars absorbs light fromthe central star and radiates thermally in the infrared.Dust also scatters starlight and produces polarization.One may therefore expect some correlation between theobserved polarization in the optical and the excess infraredemission from these objects.In Fig.4we plot the observedpercentage polarization against the excess infrared lumi-nosity(L IR)as a fraction of the total bolometric luminos-ity(L∗)of the star(the ratio L IR/L∗)for the Vega-likestars.The infrared excess L IR has been computed fromthe observedflux densities in the IRAS bands using theequation(Cox2000)F IR(7−135µm)=1.0×10−14(20.653f12+7.538f25+4.578f60+1.762f100)W m−2for the total infraredflux and the appropriate distancefactor4πd2where d is the distance to the star.The stel-lar bolometric luminosity L∗corresponds to the bolomet-ric luminosity of a normal star of the same spectral typeas that of the Vega-like star under consideration.TheL IR/L∗values are listed in Table3.A positive correla-tion is apparent from Fig.4.Vega-like stars with relativelylarger infrared excesses tend to have larger values of po-larization in the optical.The ratio L IR/L∗can be takento be a rough measure of the dust optical depthτin theoptical where the absorption of starlight takes place.Theobserved values of the ratio L IR/L∗implyτin the range∼10−4-∼10−1.For large dust grains(grain size a>>λ,the wavelength of light),that are generally thought toH.C.Bhatt and P.Manoj:Polarization measurements of Vega-like stars7 Table3.Estimated fractional infrared excesses(L IR/L∗)for Vega-like starsHD(IRAS Flux Densities)L IR/L∗Number F12(Jy)F25(Jy)F60(Jy)F100(Jy)96723.31×10−014.06×10−012.00×10+001.91×10+001.64×10−3172061.88×10+006.54×10−011.63×10+005.01×10+001.67×10−3174434.19×10−012.39×10+001.45×10+012.28×10+016.32×10−2325094.33×10−016.87×10−012.48×10+006.59×10+001.22×10−2347006.05×10−014.42×10+001.41×10+019.38×10+003.48×10−1373893.84×10−011.40×10+001.33×10+014.21×10+011.65×10−1533007.48×10−013.15×10−014.14×10−012.80×10+006.75×10−3933312.50×10−012.54×10−016.71×10−011.32×10+003.38×10−3988001.98×10+009.28×10+007.28×10+004.46×10+001.63×10−1992111.57×10+004.27×10−014.00×10−011.00×10+007.34×10−41026476.97×10+002.11×10+001.18×10+001.00×10+004.56×10−41090852.18×10+007.73×10−015.44×10−011.00×10+001.18×10−31158923.56×10+009.72×10−014.00×10−011.00×10+002.44×10−41218472.39×10−013.62×10−011.08×10+002.16×10+007.06×10−41232471.33×10−011.56×10−015.13×10−014.40×10+001.80×10−31318851.06×10−012.14×10−013.52×10−011.07×10+001.84×10−31353441.59×10+006.71×10+002.56×10+012.57×10+018.38×10−21396144.11×10+001.81×10+011.93×10+011.39×10+013.41×10−11396641.42×10+006.93×10−016.61×10−012.37×10+001.44×10−31420965.55×10−013.89×10−016.46×10−011.30×10+001.55×10−41421146.64×10−011.73×10+004.20×10+001.16×10+014.02×10−41421652.50×10−013.40×10−012.77×10+001.16×10+019.01×10−41426668.57×10+001.12×10+017.23×10+005.46×10+002.32×10−11430068.57×10−013.16×10+006.57×10+004.82×10+004.80×10−11454826.58×10−011.04×10+003.13×10+001.11×10+013.78×10−41499143.90×10−015.25×10−013.53×10+001.23×10+014.77×10−32335175.02×10−013.60×10+007.60×10+005.10×10+009.62×10−230034.78×10−013.21×10−014.00×10−011.00×10+008.26×10−4104762.00×10+006.64×10−014.00×10−011.25×10+002.40×10−3106478.22×10−013.42×10−018.51×10−011.08×10+002.10×10−3107009.56×10+002.16×10+005.12×10−011.36×10+002.37×10−3140551.07×10+004.70×10−018.58×10−018.48×10−014.71×10−4161575.43×10−012.01×10−014.00×10−011.07×10+001.35×10−2189781.38×10+003.53×10−014.00×10−011.00×10+006.75×10−4200104.11×10+009.70×10−012.87×10−011.00×10+001.65×10−3220499.52×10+002.65×10+001.66×10+001.89×10+002.46×10−3383934.60×10+001.08×10+004.00×10−011.00×10+001.28×10−3439551.47×10−011.45×10−014.42×10−011.46×10+001.00×10−4493362.39×10−011.74×10−011.72×10−011.09×10+002.67×10−4533766.86×10−012.79×10−014.00×10−014.46×10+006.85×10−2684561.46×10+002.72×10−014.00×10−011.85×10+001.42×10−3698309.66×10−013.75×10−014.00×10−011.00×10+002.80×10−3711551.15×10+004.49×10−014.00×10−011.00×10+004.13×10−4733905.11×10−012.50×10−015.32×10−011.71×10+002.29×10−4954184.80×10+001.39×10+006.27×10−011.00×10+003.85×10−41084835.61×10−012.64×10−014.00×10−015.20×10+009.83×10−51149811.56×10−011.40×10−012.16×10−017.38×10−011.00×10−31281671.60×10+003.76×10−014.00×10−011.00×10+001.12×10−31390065.92×10+001.73×10+007.51×10−011.00×10+003.76×10−41430183.05×10+005.47×10+009.25×10+008.41×10+001.25×10−41496309.64×10−012.47×10−012.64×10−011.00×10+003.23×10−41539683.68×10−013.77×10−014.00×10−011.71×10+003.32×10−21618681.41×10+005.10×10−011.29×10+002.35×10+004.14×10−41629175.44×10−012.50×10−013.80×10−011.26×10+001.60×10−31721674.16×10+011.10×10+019.51×10+007.76×10+003.55×10−41766384.74×10−013.95×10−014.00×10−011.28×10+005.17×10−41812965.39×10−014.77×10−015.33×10−011.00×10+007.63×10−41818698.21×10−013.48×10−014.00×10−011.00×10+003.16×10−42149536.26×10−014.43×10−014.00×10−011.00×10+003.16×10−32169561.82×10+014.81×10+009.02×10+001.12×10+016.43×10−42243925.21×10−011.93×10−014.00×10−011.00×10+008.01×10−48H.C.Bhatt and P.Manoj:Polarization measurements of Vega-likestarsFig.4.Plot of percentage polarization against the fractional infrared excess for Vega-like stars(Shawl1975).Larger values of polarization can result ifthe direct light from the central star is extincted by ob-scuring dust in front of the star.Also,the observed polar-ization depends on theflatness and orientation of the dustdisk.It is to be noted from Fig.4that several stars withL IR/L∗∼10−3or less also show polarization∼0.5%,much larger than the expected values<∼0.1%.It is pos-sible that these stars have an additional dust componentwith smaller grains with high albedo.This population ofgrains would cause relatively large polarization by scat-tering without producing large infrared excesses.4.ConclusionsThe results of the present study of the polarimetric be-haviour of Vega-like stars can be summarized as follows.–Vega-like stars generally show optical linear polariza-tion that is much larger than that which can be as-cribed to interstellar polarization along the line of sightto these relatively nearby objects.–The anomalous polarization in Vega-like stars is causedby scattering of stellar light due to circumstellar dustdistributed in non-spherically symmetric envelopes,and is fully consistent with a distribution inflatteneddisks.–The absence of any excess reddening in these stars isconsistent with the dust grains in their circumstellardisks being relatively large in size.–In some Vega-like stars that show relatively small in-frared excesses but large values of polarization,an ad-ditional component of dust consisting of smaller grainswith high albedo may also be present.Multiwavelength polarization measurements of Vega-likestars would be able to shed more light on the nature anddistribution of the dust in their circumstellar environment.Acknowledgements.We would like to thank the referee Prof.P.F.Bastien for several very useful comments.ReferencesAumann H.H.,et al.,1984,ApJ,278,L23H.C.Bhatt and 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