Circumstellar disks play an important role in many stages of the evolution of stars. However, it is only possible to directly image circumstellar disks for a few of the nearest stars. For massive stars, the situation is even more di±cult, as they are on average further away than the more numerous low mass stars. In order to shed light on such disks we are pursuing a programme of spectropolarimetry. This technique can reveal the presence of disks on a spatial scale of order stellar radii, while it has the potential for further modeling. Here we present our results on two groups of stars, on young, intermediate mass, premain sequence Herbig Ae/Be stars and on a sample of massive evolved objects, the Luminous Blue Variables. We also present initial results obtained at nearinfrared wavelengths

Davies, B.

Drew, J.E.

Oudmaijer, R.D.

Vink, J.S.

English

University of Hertfordshire Research Archive

Astronomical Polarimetry: Current Status and Future DirectionsASP Conference Series, Vol. 343, 2005Adamson, Aspin, Davis, and FujiyoshiSpectropolarimetry of Young and Evolved StarsRene´ D. Oudmaijer and Ben DaviesSchool of Physics and Astronomy, E.C. Stoner Building, Leeds LS29JT, UKJorick S. Vink and Janet E. DrewBlackett Laboratory, Imperial College of Science, Technology andMedicine, Prince Consort Road, London SW7 2AZ, UKAbstract. Circumstellar disks play an important role in many stages of theevolution of stars. However, it is only possible to directly image circumstellardisks for a few of the nearest stars. For massive stars, the situation is evenmore difficult, as they are on average further away than the more numerous lowmass stars. In order to shed light on such disks we are pursuing a programme ofspectropolarimetry. This technique can reveal the presence of disks on a spatialscale of order stellar radii, while it has the potential for further modeling. Herewe present our results on two groups of stars, on young, intermediate mass, pre-main sequence Herbig Ae/Be stars and on a sample of massive evolved objects,the Luminous Blue Variables. We also present initial results obtained at near-infrared wavelengths.1. Spectropolarimetry as a TechniqueThe principle of the method is rather simple: it exploits the fact that the freeelectrons in an extended, ionized disk scatter the continuum radiation from thecentral star more efficiently than the emission from hydrogen recombinationlines which are formed within the disk itself. Due to the electron scattering, theemerging light will be linearly polarized, but the photospheric light will be morepolarized than that of the emission line, as the latter travels a shorter distancethrough the disk.Therefore, a change in the polarization spectrum across an emission line suchas Hα indicates the presence of a flattened structure. In the case of a non-detection, the situation is less clear-cut. It could either be that the disk is face-on in the line of sight, or that the star is not surrounded by a disk at all, butby some spherically symmetric structure instead. In these cases all polarizationvectors cancel, giving zero polarization in both line and continuum.A low spectral resolution version of this technique was already explored inthe seventies by Poeckert & Marlborough (1976), who used narrow-band po-larimetry to prove that Be stars are surrounded by ionized disks. This resultcould be directly confirmed only many years later by high spatial resolution ra-dio and optical interferometry (Dougherty & Taylor 1992, and Quirrenbach etal 1997, respectively). As electron scattering occurs at scales of a few stellarradii (Cassinelli et al. 1987), the method provides a means to detect disks that227228 Oudmaijer et al.otherwise can not be found. With the advent of CCD detectors and the instal-lation of stable and efficient polarization optics, it has become possible to makeroutine spectropolarimetric observations at medium to high resolution.2. Pre-Main Sequence StarsWe have applied the technique to the intermediate mass pre-main sequenceHerbig Ae/Be stars with the primary goal of investigating whether these objectsaccrete material via a disk or not. The very presence of massive stars still posesa mystery; the radiation pressure from a newly born, yet still accreting star, mayblow away the infalling material, halting further growth (e.g. Yorke & Kruegel1977). However, a disk can withstand the radiation pressure much better (Yorke& Sonnhalter 2002), and may be able to supply the necessary material to allowthe star to reach higher masses. To within our sensitivity, more than half ofthe two dozen objects surveyed show a line effect. As the systems are orientedrandomly in the line of sight, it is not unexpected that some targets, even ifsurrounded by a disk, would not show a line effect in our data. In fact, thedetection rate does indicate the presence of a disk, and implies that all systemsare indeed surrounded by disks. Our survey therefore provides evidence that diskaccretion is in contention to explain the formation of massive stars (Oudmaijer& Drew 1999; Vink et al 2002).For a large fraction of the stars, more than just a simple depolarization signa-ture is seen, and instances of enhanced polarization are observed. This occursmore often in the cooler Herbig Ae stars than in the Herbig Be stars. It canbe explained by the presence of hot spots on the stellar surface - resulting inenhanced Hα line polarization compared to the stellar photosphere as a whole(e.g. Pontefract et al 2000). An added bonus is that we observed similar effectsin the lower mass T Tau stars (Vink et al 2003, see also Vink in these pro-ceedings). The similarity in the spectropolarimetric behaviour of the convectiveT Tauri stars and their hotter Herbig Ae counterparts strongly suggests thatmagnetically controlled accretion plays also a role in these more massive stars.3. Going Extra-Galactic: Luminous Blue VariablesWe are now pushing the technique to its limit by observing extra-galactic ob-jects. Specifically, we have observed a large sample of both Galactic and Magel-lanic Cloud Luminous Blue Variables (LBV). These stars are thought to be thelink between the main sequence O stars and the final products of massive-starevolution. It has been observed that their large scale structures have bipolarmorphologies (Nota et al 1995). However, it remains unclear whether thesebipolar nebulae are due to spherically symmetric winds interacting with a pre-existing density contrast or whether the star is undergoing enhanced mass lossin the equatorial plane, perhaps due to rotation (Dwarkadas & Balick 1998;Dwarkadas & Owocki 2002). To probe the mass loss geometry of these objectsin more detail, and to constrain the role of rotation in massive star evolution,we embarked on a spectropolarimetric survey of these massive evolved stars.We observed all “confirmed” LBVs in the Galaxy (7) and Magellanic Clouds(6). The stars are listed as confirmed LBVs by Humphreys and Davidson (1994)Spectropolarimetry of Young and Evolved Stars 229Figure 1. The polarization spectrum across Hα of the Luminous Blue Vari-able RMC 110 in the Large Magellanic Cloud. The bottom panel shows theintensity spectrum, while the middle and upper panels show the polarizationand position angle respectively, rebinned to a corresponding accuracy in thepolarization of 0.13% (1σ). Note the de-polarization across the emission line.and van Genderen (2001), and constitute one of the largest samples of LBVs thathave been observed with the same instrumental set-up. We find several instancesof depolarization effects in the LBVs, indicating that these massive objects aresurrounded by flattened structures very close to the star. An example is shownin Figure 1.The analysis is in progress. The main result is that roughly half of the objectsshow a line effect and thus evidence for disks. In comparison Harries et al (1998)find that only 10% of Wolf-Rayet stars show this effect, while the majority ofevolved B[e] supergiants do (Schulte-Ladbeck et al 1993; Oudmaijer & Drew1999). Pending further analysis, the data may hint at different evolutionarysequences for WR stars and LBVs (see Lamers et al 2001, for a review on theevolutionary status of LBVs).4. Prospects at Longer WavelengthsThe previous sections discussed Hα data and the application of the method tooptically visible objects. As we aim to resolve the line-profiles, and the methodneeds millions of photons to achieve ∼ 0.1% accuracy in polarization, the objectsthat can be properly studied on 4m class telescopes are limited in brightness toabout V < 10. There are plans to equip the 8m telescopes’ spectrographs,operating at similar spectral resolution, with polarimetric capabilities, movingthe limit a few magnitudes fainter.However, a class of object that remains elusive even in the 8m era are thedeeply embedded massive Young Stellar Objects. These objects are the “HerbigAe/Be” equivalent of the most massive stars, and their formation is even more230 Oudmaijer et al.Figure 2. Paβ polarization data of MWC 349A. The left hand graph showsthe polarization data, as in Figure 1, now as a function of wavelength inµm. The bottom panel shows the (normalized) intensity spectrum, the dataare rebinned to a corresponding accuracy in the polarization of 0.25% Theright hand plot shows the Stokes QU vectors with the same binning applied.MWC349A shows an effect across the Paβ emission line, while the, muchweaker, He i λ 1.27842 µm line also displays a depolarization.enshrouded in mystery. This is mainly due to the fact that they are extremelyrare and that the Kelvin-Helmholtz contraction timescales are much shorter thanthe natal cloud’s freefall time. They are therefore optically invisible and havemost of their light output in the infrared.To be able to determine whether (accretion) disks around such objects arepresent we need to go to longer wavelengths where the objects become morevisible. Infrared hydrogen recombination lines are fairly strong (Bunn et al 1995)and spectropolarimetry could be a powerful tool to probe their inner regions.Little medium resolution spectropolarimetry has been performed in the near-infrared, and we present here some of the very first data across atomic lines ofa stellar object.We undertook proof-of-concept observations at UKIRT and observed amongothers the enigmatic object MWC 349A. The results around the Paβ line areshown in Figure 2; the data show a clear depolarization, and are consistent withthe optical data presented by Meyer et al (2002). This is a reassuring test, andobservations are planned for the embedded Young Stellar Objects.5. SummaryIn this contribution we have presented applications of spectropolarimetry in theoptical to pre-main sequence Herbig Ae/Be stars and evolved Luminous BlueVariables. In addition, we have shown our initial near-infrared data on embeddedobjects. The technique itself is powerful as it is capable of probing to scales ofSpectropolarimetry of Young and Evolved Stars 231order stellar radii, and shows promise for follow-up modeling as indicated byVink et al in these proceedings.The success in applying the method in the near-infrared is a motivation tomove on, and the natural next step will be to establish the presence or absence ofaccretion disks in the most massive pre-main sequence stars to provide insightson how such objects actually form.ReferencesBunn, J.C., Hoare, M. G., & Drew, J. E. 1995, MNRAS, 272, 346Cassinelli, J.P., Nordsieck, K. H., & Murison, M. 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Bjorkman: Electron scattering in a dense expanding wind produces redshiftedbroadening of the H alpha emission, which should be polarized for disklike sys-tems. Do you see any evidence for such an asymmetry in your H alpha polar-ization profiles?Oudmaijer: Indeed one could expect such an effect, particularly for the evolvedstars as they are known to have outflows. We are currently analyzing the data,but no obvious signs have been detected yet. This is most likely the result of thecombination of the “coarse” spectral resolution of our data (in velocity space it istypically similar to or larger than a typical outflow velocity) with the relativelylow signal-to-noise that one gets in the polarization data - of course, the totalspectra from which the polarization is derived have much higher SNR. With ourevolved star data we are pushing the instrumentation to its limit.

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