Owens Valley Radio Observatory, California Institute of Technology, MS 105-24, ... 1333/IRAS 4A made with the Owens Valley Millimeter Array with 5 resolution.
THE ASTROPHYSICAL JOURNAL, 456 : L45–L48, 1996 January 1 q 1996. The American Astronomical Society. All rights reserved. Printed in U.S.A.
MILLIMETER INTERFEROMETRIC POLARIZATION IMAGING OF THE YOUNG STELLAR OBJECT NGC 1333/IRAS 4A R. L. AKESON, J. E. CARLSTROM, J. A. PHILLIPS,
AND
D. P. WOODY
Owens Valley Radio Observatory, California Institute of Technology, MS 105-24, Pasadena, CA 91125 Received 1995 August 11; accepted 1995 October 23
ABSTRACT We present a 3.4 mm polarization image of the dust emission associated with the young stellar object NGC 1333/IRAS 4A made with the Owens Valley Millimeter Array with 50 resolution. The integrated linear polarization of the dust continuum is 4% of the total intensity. The polarization is produced by magnetically aligned dust grains and arises from very dense gas (n . 10 8 cm 23 ), which indicates that the dust alignment process remains viable in the dense protostellar envelope. The magnetic field directions inferred from our observations are aligned with features seen in the high-velocity outflow emanating from IRAS 4A. The magnetic field directions are not aligned with the field on much larger scales as measured by optical and infrared selective extinction, which suggests significant field structure in the cloud core. The peak of the polarized emission is offset from the total intensity peak, which perhaps indicates considerable unresolved structure in the magnetic field. Subject headings: magnetic fields — polarization — stars: formation — techniques: interferometric — techniques: polarimetric grains, grain growth in dense cores, or averaging of small-scale structure in the large beams. The molecular cloud complex L1450 in Perseus contains the active star formation region NGC 1333. Distance estimates for this cloud vary by a factor of 2, with a recent measurement of 220 pc (Cernis 1990); here we adopt a value of 350 pc (Herbig & Jones 1983) to be consistent with previous reports. This cloud contains several young stellar objects characterized by central infrared sources and associated bipolar outflows. The focus of the present investigation, IRAS 4A, is part of a wide (110,000 AU) double system, in which both components power a molecular outflow (Blake et al. 1995). NGC 1333/IRAS 4A and B are among the growing number of ‘‘class 0’’ objects. Other examples include VLA 1623 and IRAS 1629322422. These sources are believed to be extremely young; estimated ages are less than 10 5 yr 21 (Andre´ et al. 1993). The strong submillimeter fluxes observed from these sources suggest envelopes more massive than those surrounding older objects. Mass estimates of the envelope surrounding IRAS 4A range from 3 to 9 M J from submillimeter and millimeter continuum fluxes (Sandell et al. 1991; Blake et al. 1995). These massive structures may correspond to the magnetically supported envelopes predicted by collapse models and may make these sources ideal candidates for observing the magnetic field structure. Single-dish submillimeter polarization measurements have been made for both IRAS 4A and 4B at 800 mm (Minchin, Sandell, & Murray 1995). They found a continuum polarization for IRAS 4A of 3.2% in a 140 beam with a measured magnetic field direction of 428. In this Letter we report on millimeter polarization observations toward IRAS 4A with 50 resolution made with the Owens Valley Radio Observatory millimeter array.
1. INTRODUCTION
The energetics and dynamics of all aspects of star formation are influenced by magnetic fields. As the molecular cloud collapses into protostellar cores, the field lines will be dragged in with the ions. Ambipolar diffusion will begin to remove magnetic flux from the core at densities n . 10 4 cm 23 , but the magnetic field is still dynamically important during the collapse and accretion stages (Mouschovias 1991). Recent models of magnetized core collapse produce large (11000 AU), magnetically supported envelopes with hourglass morphologies of the field lines (Galli & Shu 1993; Fiedler & Mouschovias 1993). The magnetic field is also an important component in most viable models of the energetic outflows that emerge from protostellar sources (cf. Ko ¨nigl & Ruden 1993). Polarized dust emission offers an ideal probe of the magnetic field structure in protostellar disks and envelopes. The grains that cause the extreme extinction in the optical and near-IR emit in the far-IR to millimeter wavelengths. Elongated grains can become aligned perpendicular to the magnetic field when the component of rotation perpendicular to the magnetic field is removed by paramagnetic relaxation (Davis & Greenstein 1951). The emission from these grains will be linearly polarized perpendicular to the magnetic field direction. Observations of linear polarization thus measure directly the projected field direction. The field strength is not measured because the received polarization amplitude is dependent on the grain properties as well as the field strength and geometry. Polarized dust emission measurements have been made in the millimeter with single-dish telescopes, with angular resolution much larger than protostellar disks and envelopes. Millimeter linear polarization measurements have been made toward a few star formation regions, and the polarization is typically a few percent (Leach et al. 1991; Tamura et al. 1993). Observations of Orion show the polarization decreases toward BN-KL, a region of active star formation and massive outflows. Possible explanations for the lower polarization include collisional dealignment of the
2. OBSERVATIONS
To measure weak linear polarization with an interferometer, it is best to cross-correlate circular polarizations. The receivers of the Owens Valley Radio Observatory (OVRO) L45
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FIG. 1.—(a) Intensity of the linearly polarized emission (gra y scale) plotted with the total intensity (contours) toward NGC 1333/IRAS 4A. The contour levels are 25 mJy beam 21 . The linear polarization image has been clipped at 1 s. (b) Intensity of the linearly polarized emission (gray scale) plotted with the polarization vector shown at the position of the two peaks. The length of the vector is proportional to the linear polarization, and the direction is the position angle. Recall that the magnetic field direction is perpendicular to this position angle.
millimeter array are equipped with single linear feeds. To produce circular polarization, we installed adjustable reflecting polarizers in the optical path of each telescope. Each polarizer consists of a wire grid placed in front of a flat mirror. The ‘‘grids’’ are composed of parallel 25 mm gold-plated tungsten wires on 125 mm centers. Incident radiation polarized parallel to the wires is reflected. The wire size and spacing were chosen to balance the reflection and transmission losses of the grid. Circular polarization is produced when the polarizer is set so the component reflected from the mirror travels one-fourth wave farther than the component reflected from the grid and the two components have equal magnitudes. Adjustment of the angle of the wire grid and the grid-mirror spacing allows operation at all frequencies accessible to the receiver. To measure all four Stokes parameters, we crosscorrelate the four combinations of right and left circular on each baseline. Because there is only one receiver on each telescope, we time-multiplex between right and left circular polarizations to obtain all the correlations. If the instrumental response is not purely circular, errors will be introduced in the measured Stokes parameters. Possible sources of instrumental polarization include imperfect wire grids, errors in the grid angle or grid-mirror spacing, misalignment of the optical path, and polarization cross-coupling in the mirrors and feed horn. Each baseline has a different instrumental term for each polarization state. The OVRO telescopes have altitude-azimuth mounts, so the angle of the feed with respect to the sky—the parallactic angle— changes with hour angle. This allows us to measure the instrumental polarization, as it has no hour angle dependence. The instrumental polarization can also be measured with an unpolarized source or a source with known linear polarization. Extensive tests were made to characterize and reduce the instrumental
polarization. The remaining instrumental terms are measured for each baseline and removed from the u-v data before the Stokes parameters are calculated. Individual maps are made for each Stokes parameter and then combined to form an image of the linear polarization. The IRAS 4A data were taken with the six-element OVRO millimeter array in 1994 October. The 1 GHz continuum correlator was centered at 86.2 GHz. The single-sideband system temperatures ranged from 250 to 400 K. We measured the instrumental polarization immediately before obtaining the IRAS 4A data by observing Neptune, which we assumed to be unpolarized. The total instrumental polarization as measured from the Neptune image with no corrections is 2.5% at the center of the field of view. After the removal of constant instrumental terms from the u-v data, the residual instrumental polarization was 0.4% as measured in the corrected Neptune image, which was 4 times the noise level. This residual is a combination of errors in measuring the instrumental polarization and any time-varying terms that might be present. The errors in the polarization flux and angles for IRAS 4A are calculated by adding this residual in quadrature with the measured map rms uncertainties. The FWHM beam for these observations is 5"1 3 4"3 at a position angle of 211$3. 3. RESULTS AND DISCUSSION
In Figure 1 we present the linear polarization emission detected toward IRAS 4A plotted with the total intensity and with the polarization vectors. The peak linear polarization is 8.2 H 1.4 mJy beam 21 and is offset to the northeast of the total intensity peak. The angle of the linear polarization at the peak is 1038 H 68, which corresponds to a magnetic field direction of 138 H 68 (Fig. 1b). The peak in the linear polarization to the
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west of the total intensity peak has a polarization direction of 1358 H 78, corresponding to a field direction of 458 H 78. The average polarization is 4%, and the polarization within the 50% contour of the total intensity varies from 0% to 8%. The signal-to-noise ratio outside this contour is not sufficient to measure reliably the polarization percentage. Assuming the dust continuum is optically thin, we derive a mass for the envelope surrounding IRAS 4A of 12 M J . Our integrated 3.4 mm flux of 230 mJy agrees with the fluxes measured by Blake et al. (1995) and Sandell et al. (1991) at 2.8 and 2.0 mm, respectively, using a dust opacity proportional to n 1 . If we estimate a source size of 1750 AU (50) from the 50% contour in the continuum map, the average H 2 density is 10 8 cm 23 , and the column density is 10 24 cm 22 . For dust grain alignment by paramagnetic relaxation in a uniform magnetic field, the polarization increases with higher column densities or with higher field strengths (Davis & Greenstein 1951). However, several effects may act to counter grain alignment at higher densities. Collisions, which will dealign the grain, increase as n. In dense cores, dust grains may grow more rapidly, and if this growth produces spherical rather than elongated grains, the polarization percentage will decrease. In fact, previous low angular resolution studies observed a decrease in the polarization percentage toward denser regions. In far-infrared polarization observations toward several star formation regions, the maximum polarization percentage was found to decrease with increasing optical depth (Hildebrand et al. 1995). Furthermore, millimeter polarization measurements of Orion show lower polarization percentage in the most active regions (Leach et al. 1991). Based on our polarization observations toward IRAS 4A, we suggest that large-scale beams are averaging over small-scale field structure. The linear polarization image of IRAS 4A shows structure at 50 resolution. Additionally, the polarization percentages we observed are consistent with levels from single-dish observations and indicate the grain alignment mechanism is still effective at densities of 110 8 cm 23 . While our observations of polarized emission do not directly measure the magnetic field strength, we can estimate the field strength from empirical relationships and from the virial theorem. For isotropic contraction of a spherical cloud core in which a uniform magnetic field is frozen to the gas, the field strength will scale as n 2/3 , while for an oblate cloud B F n 1/ 2 (Mouschovias 1976). Determinations of magnetic field strengths from Zeeman splitting in star formation regions show that B F n 1/ 2 for the range of densities spanning 10 4 –10 10 cm 23 (Heiles et al. 1993; Mouschovias 1995). Using the empirical relation between field strength and density derived from Zeeman measurements toward Orion (Heiles et al. 1993), a field strength of 10 mG is expected for a density of 10 8 cm 23 . The field strength in the envelope of IRAS 4A can also be roughly estimated using equipartition of energy. Assuming the cloud is spherical, equipartition of gravitational and magnetic energy gives a field strength of 8 mG, while including turbulent support with a line width of 1 km s 21 gives a field strength of 5 mG. Observations of the magnetic field structure on all size scales can help reveal the relative importance of the magnetic field in the collapse of the molecular core. If clouds collapse along uniform field lines, we would expect to see outflows parallel and disks perpendicular to the large-scale field. For both Orion IRc2 and IRAS 1629322422 the field
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direction as measured by single-dish millimeter polarization observations agrees with the large-scale field direction, and the outflows are directed roughly along the field lines (Leach et al. 1991; Tamura et al. 1993). In IRAS 4A, our observed small-scale field direction spans a range of 138– 458, which agrees with the single-dish submillimeter polarization measurement of Minchin et al. (1995). The well-collimated outflow is directed almost north-south close to the source but bends to a position angle of 1458 over larger scales (Blake et al. 1995). Recent submillimeter observations by Lay et al. (1995) with the CSO-JCMT interferometer found that IRAS 4A contains a 1"8 binary at a position angle of 1308, although they were unable to determine the absolute position. These directions are consistent with the picture of core collapse along the field lines, as the field is parallel to directions seen in the molecular outflow and perpendicular to the binary separation angle. The large-scale magnetic field in Perseus has been studied with selective extinction observations by Turnshek, Turnshek, & Craine (1988) and Goodman et al. (1990). Both groups covered an area of several square degrees in the Perseus cloud and found magnetic field components with peaks near 708 and 1458. Observations of two velocity components toward this cloud (Loren 1976) led both Turnshek et al. (1988) and Goodman et al. (1990) to infer the presence of two clouds along the line of sight to Perseus. Infrared extinction measurements toward the 11 pc core of NGC 1333 indicated a local field direction of 1258 (Tamura et al. 1988). None of the large-scale field directions match the directions we measured for the field in the vicinity of IRAS 4A. This nonalignment suggests substantial structure of the magnetic field in the cloud core, as the optical and infrared observations are probing the magnetic field structure in the outer, less dense layers of the cloud, while our millimeter observations are sensitive only to the fields in the dense gas immediately surrounding the protostellar source.
4. SUMMARY
We have made 3.4 mm polarization measurements of the young stellar source NGC 1333/IRAS 4A on 1750 AU (50) size scales. The linearly polarized emission is 4% of the total intensity, although the peak of the polarized emission is offset from the continuum peak. The measured field directions are aligned with directions seen in the well-collimated molecular outflow. The polarized emission arises from the dense envelope and shows that dust grains are still aligned by the magnetic field. The offset of the peak linear polarization from the center of the continuum emission may indicate structure in the source that is still unresolved. Our detection of polarization structure in IRAS 4A clearly shows the usefulness of interferometric polarimetry at millimeter wavelengths in probing the magnetic field structure in protostellar cores and envelopes. We plan to make higher resolution observations at 3 mm and are installing polarizers for the 1 mm system at OVRO to allow imaging of the polarized emission with 10 resolution. We would like to thank Cosmo Papa and the staff at the CfA Sub-millimeter Array Receiver lab for invaluable assistance in constructing the wire grids and Marty Gould for assistance with the polarizer assembly. We would also like to thank the
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staff at OVRO, particularly Steve Scott. Research at OVRO is supported by NSF grant AST-9314079, and polarization studies at OVRO are supported by a NASA Origins grant NAGW-
4193. J. E. C. gratefully acknowledges support from the David and Lucile Packard Foundation and a NSF Young Investigator award.
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