The Magnetic Environments of Young Stellar Objects

THE MAGNETIC ENVIRONMENTS OF YOUNG STELLAR OBJECTS∗

ANTONIO CHRYSOSTOMOU, RACHEL CURRAN and DAVID AITKEN

Department of Physical Sciences, University of Hertfordshire, Hatfield, UK;E-mail: [email protected]

Abstract. We report on early results of submillimetre polarimetric measurements towards a sampleof young stellar objects. The results allow us to infer the magnetic field structure and show a varietyof configurations, providing evidence for axial, helical and pinched (i.e. ‘hourglass’) magnetic fieldconfigurations. We find that in some cases the field is curved over large scales, implying that it isinfluenced by the gas kinematics in the local environment, and that at these scales at least, the magneticfield plays a passive role in the star formation process.

Keywords: star formation, magnetic fields, submillimetre, polarimetry

1. Introduction

Star formation is a field of research striving to understand the processes which con-vert the mass in a molecular cloud into stars. There has been much work over thelast few decades to understand this phenomenon, from a theoretical standpoint sup-ported by observational evidence. We find that our understanding is best improvedupon when theory and observation advance together.

Recently, there have been a number of theoretical advances which attempt tounderstand the role that magnetic fields have in the formation of stars (witness, forinstance, the number of reviews and contributions in these proceedings). However,the observational evidence in support of these theories has been lacking, and inthis contribution we describe (in part) our research programme which attempts toredress the balance and help provide an observational basis to this issue.

1.1. THE IMPORTANCE OF MAGNETIC FIELDS

The standard theory of star formation (Shu et al., 1987) has the magnetic fieldplaying a fundamental role. It can regulate the collapse of material onto the protostarthrough the action of ambipolar diffusion (Mouschovias and Ciolek, 1999), and isinstrumental in driving and collimating molecular outflows.

More recently, magnetic fields have been regarded as important to the evo-lution of molecular clouds through MHD turbulence. There is a critical ratio of

∗The data presented here, apart from Figure 3, are all shown prior to full analysis and are currentlybeing prepared for publication.

Astrophysics and Space Science 292: 509–515, 2004.C© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

510 A. CHRYSOSTOMOU ET AL.

mass to magnetic flux above which the magnetic field can no longer support acloud from collapse and it is believed that low mass stars form in super-criticalcores while high mass stars form in sub-critical cores. However, the observa-tional evidence (see review by R. Crutcher in this volume) suggests that mostcores are either at or beyond criticality while Nakano (1998) showed that itis difficult for sub-critical cloud cores to maintain non-thermal velocity widths.Support through turbulence is believed to be important in explaining this appar-ent lack of sub-critical cloud cores and the sustenance of non-thermal velocitywidths.

1 .2 . MODELS OF MAGNETIC FIELDS IN MOLECULAR CLOUDS

There have been a number of recent studies which have looked at the effect of themagnetic field in molecular clouds with particular regard to turbulence (Padoanet al., 2001; Heitsch et al., 2001; Ostriker et al., 2001). Importantly for observers,these simulations all produce maps of the magnetic field structure which can be com-pared to observations. The results generally show that relatively strong magneticfields produce uniform field patterns, whereas weak fields produce more random,disorganised patterns. This behaviour is parameterised through the energy density(i.e. strength) of the field measured relative to the energy density in the gas, some-thing which Ostriker et al. (2001) express using the sound speed and the Alfvenvelocity, β = c2

s /v2A.

Other significant models include those of Fiege and Pudritz (2000) and Aitkenet al. (2002). Fiege and Pudritz (2000) looked at helical magnetic fields threadingthrough molecular clouds in order to explain the magnetic field structures seentowards the Orion Molecular Clouds (Matthews et al., 2001). As well as providinga working model for filamentary clouds, they found that helical fields providea simple, geometric explanation for the ‘polarization holes’ seen in single-dishpolarimetry data. Aitken et al. produced working models with the geometry of aprotostar (flared disk with a r−1.5 radial density profile). By treating the radiativetransfer properly they are able to determine the polarization vector pattern for anumber of magnetic field configurations at a range of wavelengths between themid-infrared and the submillimetre, providing the opportunity of constraining themodels using a multi-wavelength approach.

1.3. MEASURING MAGNETIC FIELD GEOMETRIES

The classical grain alignment theory has spinning and elongated dust grains whichbecome partially aligned to the local magnetic field direction through paramagneticrelaxation (Davis and Greenstein, 1951). The grains’ spin axes precess about thedirection of the magnetic field and the radiation emitted by a volume of suchgrains is polarized. Polarimetry, therefore, allows for a unique determination of

MAGNETIC ENVIRONMENTS OF YSOs 511

the direction and magnitude of the E-vector from which the B-vector may beinferred.

The observations presented here were all obtained using the polarimeter onthe Submillimetre Common User Bolometer Array (SCUBA) on the James ClerkMaxwell Telescope (JCMT) on the summit of Mauna Kea, Hawaii. The millime-tre/submillimetre wavelength regime is an ideal probe for star formation studiesprincipally because the radiation is optically thin and naturally traces the denseststructures where protostars form. Polarimetry data then provide a column-averaged(and intensity weighted) measurement of the magnetic field direction through thecloud and projected onto the plane of the sky.

2. Magnetic Fields and Outflows

2.1. HH 211 AND NGC 1333 IRAS2

In Figure 1 is shown our SCUBA polarimetry of the Class 0 low-mass sources,HH 211 and NGC 1333 IRAS2. The polarization vectors have been rotated by 90◦

to depict the B-vector projected onto the plane of the sky.The outflow direction for HH 211 lies at a position angle of ∼135◦ (Gueth and

Guilloteau, 1999) and the data show that the inferred field pattern is predominantlyuniform and axial, i.e. the magnetic field lies along the direction of the outflow axis.However, there is significant departure from a truly axial field with a fair degree ofcurvature in the projected field pattern through the source. We feel that this maybe indicative of a slightly helical field running through the envelope of the youngstellar object (YSO).

HH 211 forms part of a campaign at the JCMT to measure the magnetic fieldgeometries towards a number of Class 0 YSOs. The data we have begun to collectseem to be indicating that such a field pattern may be quite common. However, ex-ceptions remain. NGC 1333 IRAS2, for instance, exhibits quite a different scenario.The field pattern appears randomized and the degree of polarization decreases atthe source.

Chandler and Richer (2000) studied the radial density profiles of these twosources at 450 and 850 µm. Although not altogether conclusive, they found thatthey could describe NGC 1333 IRAS2 as an object undergoing free-fall collapsewhile HH 211 was still enjoying some form of extra hydrostatic support. If this istrue, then our results lend evidence to the suggestion that the extra support comesfrom a relatively strong magnetic field.

2.2. CEPHEUS A

In Figure 2 we show our polarimetry towards the high-mass YSO, Cepheus A. Thepolarization vectors have been rotated by 90◦ to depict the B-vector projected onto


Figure 1. Magnetic field geometry projected onto the plane of the sky for HH 211 and NGC 1333IRAS2. The greyscale gives the intensity distribution of the 850 µm emission, while the short linesgive the degree and direction of the B-vector (scale shown on the side). Coordinates are given in J2000.


Figure 2. Same as Figure 1 but for Cepheus A.

the plane of the sky. The outflow direction lies at a position angle of ∼40◦. Thereare some important features to note here:

– in general, the field runs parallel to the outflow axis,– the degree of polarization decreases close to the source,– as they approach the source, the vectors bend in towards it.

This field pattern is strongly reminiscent of pinched, or ‘hourglass’, field modelspresented in Aitken et al. (2002)—their model III in particular. Such field patternshave for some time been predicted to be an inevitable consequence of the gravita-tional collapse of a core into a protostar.

3. Curvature in Magnetic Fields

One of the striking features in our dataset has been the discovery, towards a few ofour sources, of significant degrees of curvature in magnetic field patterns. Matthewset al. (2001) find some evidence of this in OMC-3 but it is not clear whether anycurvature is due to the dense filamentary structure of the cloud or to the field itself.In Figure 3, we present SCUBA polarimetry of the W 51 region with contours of the6 cm emission overlaid. Curvature of the field towards this source is seen across theregion. A crude comparison to MHD turbulence simulations shows that the vectorpattern is consistent with a relatively weak field which has β ∼ 1 (Chrysostomouet al., 2002).

There is a quite distinct correspondence between the field pattern and theposition and extent of the HII-region, W 51 IRS1. The 6 cm emission showsthat the gas from the HII-region is expanding and enveloping itself around the


Figure 3. Same as Figure 1 but for W 51. The contours show the 6 cm emission. The core to thenorth-west is associated with W 51 IRS2. The 6 cm emission in the centre of the field is associatedwith W 51 IRS1.

dense submillimetre core to the east. The field follows this structure/pattern quiteclosely1

A similar relationship between gas kinematics and the magnetic field structure isseen for NGC 7538 (Momose et al., 2001). Here the correlation is between the fieldand the molecular outflow, as traced by CO emission. Our observations of S 140show similar field curvature, although we have as yet not been able to attribute adirect correlation with gas kinematics.

These results imply that, at the scales sampled by the JCMT beam (∼15′′), themagnetic field plays a passive role, allowing itself to be manipulated by the energydensity and pressure gradients in the gas. At closer scales, such as those sampledby interferometers such as the BIMA, this may not be the case. As pointed out inChrysostomou et al. (2002), interferometric data of the core associated with W 51IRS1 (Lai et al., 2001) shows polarization vectors consistent with the single-dishdata in Figure 3 but show no evidence of a weak field. Instead, we see a uniformfield pattern indicative of a relatively strong field (β � 1) and suggesting that themagnetic field may only start to play an active role in the star formation process atsmall scales after cloud collapse and accretion have commenced.

1Our measurements have been confirmed through comparison with data taken of W51 at 350 µm usingthe Hertz polarimeter on the Caltech Submillimetre Observatory. The authors thank and acknowledgeJohn Vaillancourt for allowing this comparison to be made.


4. Conclusions

We have presented preliminary results of a campaign to measure the projectedmagnetic field structures towards a sample of high- and low-mass young stellarobjects. The data are still being analyzed but preliminary results have allowed usto come to the following conclusions.

– We have found evidence to suggest the presence of a variety of magnetic fieldconfigurations. These include axial (with the general field orientation parallel tothe outflow axis) and helical fields, and even what may be the first direct evidenceof an ‘hourglass’ magnetic field. At the same time, one source in our sample oflow-mass objects shows the absence of a definitive magnetic field structure.

– It is important to understand how the magnetic field interacts with the dynamicsof the gas in its environment. If correlations with gas dynamics are found, thenthis opens up the exciting possibility of probing and mapping the magnetic fieldenergy density in these systems by simply assuming equipartition between thegas and the field.

– Combining data using single-dish telescopes and interferometers provides uswith an understanding of how the field behaves as a function of scale.

References

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The Magnetic Environments of Young Stellar Objects