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Limits to the Magnetic Field in the Planetary Nebula Ngc 246 From Revista Mexicana de Astronom´ıa y Astrof´ısica, 53, 45–52 (2017) LIMITS TO THE MAGNETIC FIELD IN THE PLANETARY NEBULA NGC 246 FROM FARADAY ROTATION L. F. Rodr´ıguez,1 C. Carrasco-Gonz´alez,1 J. Cant´o,2 A. Pasetto,3 A. C. Raga,4 and D. Tafoya1 Received August 4 2016; accepted September 29 2016 RESUMEN Presentamos observaciones de radiocontinuo de la fuente extragal´actica li- nealmente polarizada J0047−1150, cuya l´ınea de visi´on atraviesa a la nebulosa pla- netaria gal´actica NGC 246. Determinamos el ´angulo de posici´on del vector el´ectrico para siete frecuencias entre 1.3 y 1.8 GHz, sin encontrar evidencia de rotaci´on de Faraday y obtenemos un l´ımite superior a 4-σ de 9.6 rad m−2 para la medida de rotaci´on, lo cual implica un l´ımite superior de 1.3 µG para el valor promedio del campo magn´etico en la l´ınea de visi´on. Sin embargo, demostramos que la medida de rotaci´on a lo largo de una fuente con campo magn´etico de morfolog´ıa dipolar pr´acticamente se cancela. Entonces, si el campo mag´etico tiene esta morfolog´ıa, los valores locales del campo magn´etico en NGC 246 pueden ser mucho mayores y no se har´ıan evidentes en un experimento de rotaci´on de Faraday. ABSTRACT We present radio continuum observations of the linearly polarized extragalac- tic source J0047−1150, whose line of sight traverses the galactic planetary nebula NGC 246. We determine the position angle of the electric vector at seven frequen- cies between 1.3 and 1.8 GHz, finding no evidence of Faraday rotation and setting a 4-σ upper limit to the rotation measure of 9.6 rad m−2, which implies an upper limit to the average line-of-sight component of the magnetic field in NGC 246 of 1.3 µG. However, we show that the rotation measure across a source with a dipolar magnetic field morphology practically cancels out. Therefore, if the magnetic field has this morphology, the local values of the magnetic field in NGC 246 could be much larger and will not be evident in a Faraday rotation experiment. Key Words: ISM: magnetic fields — planetary nebulae: individual (NGC 246) — plasmas — polarization 1. INTRODUCTION etary nebulae, there are only a handful of measure- Magnetic fields are a key component in the under- ments. For HII regions, the technique of Faraday standing of the origin and evolution of astrophysical rotation has provided values in the range of about 1 objects. From theoretical and observational studies to 40 µG for a few sources (Heiles & Chu 1980; Heiles © Copyright 2017: Instituto de Astronomía, Universidad Nacional Autónoma México we know that in the Universe there are magnetic et al. 1981; Harvey-Smith et al. 2011; Rodr´ıguez et fields going from values in the range of 10−17 to al. 2012; Purcell et al. 2015; Costa et al. 2015). 10−14 G in the intergalactic medium (Essey et al. The search for the Zeeman effect in radio recombi- 2011) to values as high as 1015 G in some magnetars nation lines from the ionized gas in HII regions has (Tiengo et al. 2013). set upper limits of a few (Troland & Heiles 1977) to A variety of techniques has been used to estimate a few tens of mGauss (mG; Roberts et al. 1991). these magnetic fields. In the case of diffuse galactic To our knowledge, the only detection of the Zee- regions of ionized gas, namely HII regions and plan- man effect in hydrogen radio recombination lines is that of the H30α emission from the photoevaporat- 1 Instituto de Radioastronom´ıa y Astrof´ısica, UNAM, ing disk of the Be star MWC 349 (Thum & Morris M´exico. 2Instituto de Astronom´ıa, UNAM, M´exico. 1999). An average strength of 22 mG was derived 3Max-Planck Institut f¨ur Radioastronomie, Germany. from these observations. This discussion indicates 4Instituto de Ciencias Nucleares, UNAM, M´exico. 45 46 RODR´IGUEZ ET AL. that the Faraday rotation technique is much more magnetic field has a value of ≈ 5 µG (Cox 2005) and sensitive than the Zeeman technique. However, it using the B ∝ n1/2 scaling proposed by Fleck (1983) requires the fortuitous circumstance of the studied we obtain an estimate of ≈ 50 → 500 µG for the source being located in front of a relatively bright, ionized shell of a classical PNe. linearly polarized source, typically a quasar. The In this paper we present radio observations to- Faraday rotation is the rotation of the electric vec- ward the PN NGC 246. This PN has an angular tor position angle (EVPA) of the linearly polarized extent of ≈ 4′, and was found by us to have an ex- emission that occurs when an electromagnetic wave tragalactic source behind it. In § 2 we describe the passes through a magnetized medium. It is described nebula in more detail as well as the observations. In by the value of the rotation measure (RM), that con- § 3 we report our results and discuss the implications tains information about two important elements of for magnetic fields in PNe. Finally, we present our the intervening medium: the electron density and conclusions in § 4. the strength of the component of the magnetic field parallel to the line of sight. 2. SEARCH FOR SOURCES AND A planetary nebulae (PN) is an ionized shell of OBSERVATIONS gas around an evolved low mass star that is on its We searched for planetary nebulae with a back- way to becoming a white dwarf. Planetary nebu- ground extragalactic source that could be used to lae are classic objects of astrophysical study, and measure the Faraday rotation by analyzing images parameters such as the electron density and tem- from the NRAO Very Large Array (VLA) Sky Sur- perature, as well as the chemical composition are vey (NVSS; Condon et al. 1998). Finding back- well known. However, similarly to many astronom- ground objects in the solid angle of the relatively ical objects, there are no reliable measurements of compact planetary nebulae is difficult, given the their magnetic fields. Magnetic fields have been de- modest angular resolution of the NVSS (45′′). How- termined for the central stars of a handful of PNe, ever, in the case of extended (> 1′) planetary nebu- with values in the range of hundreds to thousands of lae we were successful in finding one planetary neb- Gauss (Jordan et al. 2005; Steffen et al. 2014). Also, ula, NGC 246, that appeared to have a background from observations of the Zeeman effect in OH and source. To verify the presence of the compact source H2O masers, magnetic fields have been estimated to we obtained images of NGC 246 (using VLA archive be in the order of mG to tens of mG in the dense, data from project AP110, taken at 20 cm in the D molecular structures found surrounding protoplan- configuration in 1986 January 5), both with all the etary nebulae and young planetary nebulae (e. g. (u,v) data and also removing the short spacings (be- Hu et al. 1993; Trigilio et al. 1998; Miranda et low 1 kλ), so as to suppress the extended emission al. 2001; Vlemmings & van Langevelde 2008; Leal- with angular dimensions above ≈ 3′. In Figure 1 Ferreira et al. 2013; Qiao et al. 2016). More re- we show the radio images of NGC 246, where it is cently, the linearly polarized dust emission observed apparent that when the extended radio emission is in the envelopes of protoplanetary nebulae has been suppressed, we clearly see the compact background proposed as a tool to estimate the magnetic field in source. We checked that this compact emission was these environments (Sabin et al. 2014). not due to an ionized knot in the nebula by a com- However, there are no measurements for the ion- parison with the red image of the Digital Sky Survey ized envelope that characterizes classical PNe (with (DSS) that does not show increased emission at the 2 4 3 © Copyright 2017: Instituto de Astronomía, Universidad Nacional Autónoma México electron densities in the range of 10 − 10 cm− ), position of the radio source (see Figure 1). These although Ransom et al. (2010a, 2010b; 2015) have data did not have the sensitivity to search for Fara- recently reported magnetic field estimates for a few day rotation and we requested time in the JVLA for old, diffuse, PNe that turn out to be similar to an ad hoc observation. those of the interstellar medium. From the previ- The new observations were made with the Karl ous range of magnetic fields and taking into account G. Jansky Very Large Array (JVLA) on 2014 Jan- that the average magnetic field in the diffuse inter- uary 20 at L band. On that date, the JVLA was stellar medium is ≈5 µG (Cox 2005), we can crudely in its B configuration providing an angular resolu- “interpolate” that a magnetic field of the order of tion of 4′′.3. We observed continuum emission in the ≈10-100 µG is expected for the ionized shell that frequency range from 1 to 2 GHz. The total band- forms the classical PNe. Another estimate can be width was divided in 16 spectral windows with 64 made considering the fact that in the average Galac- channels of 1 MHz each. Nine out of the 16 spectral tic interstellar medium (of density n ≈ 1 cm−3) the windows were not used due to strong RFI. Flux and LIMITS TO MAGNETIC FIELD IN THE PN NGC 246 47 -11 49 50 51 52 DECLINATION (J2000) 53 54 55 -11 49 50 51 J0047-1150 52 DECLINATION (J2000) 53 54 > 1 klambda 55 -11 49 50 51 52 DECLINATION (J2000) 53 54 © Copyright 2017: Instituto de Astronomía, Universidad Nacional Autónoma México 55 Red DSS 00 47 15 10 05 00 46 55 50 RIGHT ASCENSION (J2000) Fig.
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