Non-detection of magnetic fields in the central stars of the planetary nebulae NGC 1360 and LSS 1362
F. Leone, M. J. Martinez Gonzalez, R. L. M. Corradi, G. Privitera, R. Manso Sainz
aa r X i v : . [ a s t r o - ph . S R ] A p r Non detection of magnetic fields in the central stars of theplanetary nebulae NGC1360 and LSS1362
Francesco Leone , Mar´ıa J. Mart´ınez Gonz´alez , , Romano L.M. Corradi , , GiovanniPrivitera , Rafael Manso Sainz , Universit`a di Catania, Dipartimento di Fisica e Astronomia, Sezione Astrofisica, Via S.Sofia 78, I–95123 Catania, Italy Instituto de Astrof´ısica de Canarias, E-38200 La Laguna, Tenerife, Spain Departamento de Astrof´ısica, Universidad de La Laguna, E-38205 La Laguna, Tenerife
ABSTRACT
The presence of magnetic fields is an attractive hypothesis for shaping PNe.We report on observations of the central star of the two Planetary NebulaeNGC1360 and LSS1326. We performed spectroscopy on circularly polarized lightwith the
FOcal Reducer and low dispersion Spectrograph at the Very Large Tele-scope of the European Southern Observatory. Contrary to previous reports (Jor-dan et al. 2005, A& A, 432, 273), we find that the effective magnetic field,that is the average over the visible stellar disk of longitudinal components of themagnetic fields, is null within errors for both stars. We conclude that a directevidence of magnetic fields on the central stars of PNe is still missing — eitherthe magnetic field is much weaker ( <
600 G) than previously reported, or morecomplex (thus leading to cancellations), or both. Certainly, indirect evidences(e.g., MASER emission) fully justify further efforts to point out the strength andmorphology of such magnetic fields.
Subject headings:
Stars: magnetic field — (ISM:) planetary nebulae: individual(NGC1360, LSS13632) — Polarization
1. Introduction
A large fraction of planetary nebulae (about 80%) are bipolar or elliptical rather thanspherically symmetric. Many of them also harbor complex structures on small scales, such asknots, filaments, jets and jet-like features, etc. (see e.g. Corradi (2006)). But the reason forthe departure during the PN phase from the spherical symmetry that has characterized mostof the evolution of the progenitor stars, is still a matter of debate. Modern theories invoke 2 –magnetic fields, among other causes, to explain the rich variety of aspherical componentsobserved in PNe (see the review by Balick & Frank (2002)). The presence of magnetic fieldswould indeed help to explain some features of the complicated shapes of planetary nebulae,as the ejected matter is trapped along magnetic field lines. There are several ways magneticfields can be created in the vicinity of planetary nebulae. Magnetic fields can be producedby a stellar dynamo during the phase when the nebula is ejected. It is also possible thatthe magnetic fields are fossil relics of previous stages of stellar evolution (Blackman et al.2001). Under most circumstances, the matter in stars is so highly electrically conductivethat magnetic fields can survive for millions or billions of years. In both cases, the magneticfield combined with other physical processes including stellar rotation, winds interaction,interaction wit the interstellar medium, and the dynamical action of evolving photoionizationfronts, would produce the complex morphologies observed in PNe.Until recently, the idea that magnetic fields are an important ingredient in the shaping ofplanetary nebulae was mostly a theoretical claim, since no such magnetic field was measuredin the nebulae themselves. To obtain direct evidence for the presence of magnetic fields inplanetary nebulae one can focus on their central stars, where the magnetic fields should havesurvived.Jordan et al. (2005) report the detection of magnetic fields in the central star of two non-spherical planetary nebulae, namely NGC1360 and LSS 1362. The claim is based on circularlight spectropolarimetry carried out with the
FOcal Reducer and low dispersion Spectrograph (FORS) at the Very Large Telescope (VLT) of the European Southern Observatory (ESO).The impact of Jordan et al. (2005) paper is testified by the 50 citations counted at the endof 2010, and increasing efforts to include magnetic fields in theory of planetary nebulae. Asan example, Tsui (2008) performed MHD calculations to model an equatorial plasma torusin around the central stars of PNe.Because of the achieved noise level and adopted set-up, Jordan et al. (2005) could notobtain a direct measurement of these magnetic fields (see their figures 2 and 3). Controversialvalues were also obtained from different Balmer lines. These authors found necessary toperform a large number of simulations to associate a statistical significance to their results.In this paper, we present the results of new spectropolarimetric measurements of NGC1360and LSS1362 obtained on Dec. 22, 2010 with FORS2 at the VLT, at higher signal to noiseratio, reciprocal dispersion and spectral resolution than previously done by Jordan and co-workers, with the aim to finally obtain a direct evidence of magnetic fields on the surface ofthe central star of these planetary nebulae. 3 –Fig. 1.— Observed Stokes F I / F c , F V / F I and F N / F I (see text) of the central stars of thePNe NGC1360 and LSS1362. The Zeeman signature in the Stokes V /I spectra of Balmerlines is absent in the case of PNe and well visible in the magnetic star HD94660.Fig. 2.— The slope of Stokes F V / F I vs. − Cg eff λ
20 1 F I d F I ( λ )d λ measures the effective magneticfield (B e ). No slope is observed in the case of PNe. A B e value consistent with the literaturedata is obtained for the magnetic star HD94660. The zero slope of F N / F I spectra shows nospurious polarization effects. 4 –
2. Observations and data reduction
Jordan et al. (2005) observed NGC1360 and LSS1362 with the FORS1 spectrograph atthe VLT using the R600B+22 grating, a 0.8 arcsec slit, a MIT 24 µ m CCD and adopting atotal exposure time of 624 s. Dispersion was 1.18˚A pix − , and spectral resolution R ∼ λ/ α = +45 and − o .To improve the precision of the measurements, we observed NGC1360 and LSS1362 for3072 s each with the R1200B+97 grating, a 0.5 arcsec slit and the E2V blue-optimized 15 µ m CCD. This setup results in a linear dispersion equal to 0.35˚A pix − and R ∼ α between +45 and − o . On the coadded spectra of NGC1360 and LSS1362, we measuredS/N ∼ o -rdinary and e -xtraordinary beams emerging from the polar-izer to measure the circular polarization degree is critical. There is a time independent(instrumental) sensitivity G , for example due to a pixel-to-pixel efficiency, together with atime dependent sensitivity F of spectra obtained at different α angles for example due tovariation of sky transparency and slit illumination. Photon noise dominated Stokes I and V can been obtained from the recorded spectra at α =+45 o and − o : S +45 ◦ ,o = 0 . I + V ) G o F +45 ◦ S +45 ◦ ,e = 0 . I − V ) G e F +45 ◦ S − ◦ ,o = 0 . I − V ) G o F − ◦ S − ◦ ,e = 0 . I + V ) G e F − ◦ Table 1: B eff measurements from single Balmer lines, g eff = 1. Average values are from asimultaneous fit of all available Balmer lines. NGC 1360 LSS 1362 HD 94660HJD/B/Seeing 2455553.503/10.99/1” 2455553.832/12.27/2” 2454181.658/6.02/0.6”B eff ( V/I ) [G] B eff ( N/I ) [G] B eff ( V/I ) [G] B eff ( N/I ) [G] B eff ( V/I ) [G] B eff ( N/I ) [G]All +154 ±
113 +121 ± − ±
286 +34 ± − ± − ± θ – – – – − ± − ± η – – – – − ±
371 +293 ± ζ − ± − ± ± ± − ±
378 +109 ± ǫ +220 ±
867 +200 ±
787 +372 ± − ± − ± − ± δ − ± − ± − ± ± − ± − ± γ +104 ±
379 +35 ± − ±
882 +389 ± − ±
362 +370 ± β +450 ±
318 +325 ± − ± − ± − ± − ± VI = R V − R V + 1 with R V = S +45 o ,o /S +45 o ,e S − o ,o /S − o ,e We have reduced the data following the previous relations as in Leone (2007). In additionto
V /I we have also computed the
Noise spectrum: NI = R N − R N + 1 with R N = S +45 o ,o /S − o ,e S − o ,o /S +45 o ,e In an ideal polarimeter, signal extraction and wavelength calibration of ordinary and ex-traordinary spectra,
N/I is null and its absolute error equal to (N total ) − / , where N total isthe total number of photons. Any anomalous behavior of N/I would be present, at the samelevel, in Stokes
V /I by definition.To test our capability to recover the circular polarized signal from FORS spectra andmeasure stellar magnetic fields, we have applied the previous procedures to the spectropo-larimetric data, obtained from ESO archive, of the magnetic star HD94660, whose fieldis at the intensity level of NGC1360 and LSS1362 as claimed by Jordan et al. (2005).Landstreet & Mathys (2000) have shown that the magnetic field of HD94660 is variablewith a 2700 day period between − − v e sin i <
20 km s − (Garc´ıa-D´ıaz et al. 2008) and HD94660 <
30 km s − (Levato et al. 1996). We did not find any estimate in the literature for the pro-jected rotation velocity of LSS1362, whose spectral lines appears in our spectra as broad asNGC1360 lines.Fig. 1 shows the observed spectra of NGC1360, LSS1362 and HD94660.
3. Measuring magnetic fields
High resolution circular spectropolarimetry of metal lines gives the possibility to distin-guish photospheric regions with positive and negative magnetic fields, as for instance doneon HD24712 by Leone & Catanzaro (2004, R =115 000). It is also proved useful at moderateresolution (Leone & Catanzaro 2001, R = 15 000), but is still prohibitive to detect magneticfields of faint stars. For faint white dwarfs, Angel & Landstreet (1970) introduced a methodbased on narrowband ( ∼
30 ˚A ) circular photopolarimetry on the wings of the H γ Balmerline. In the weak field approximation for stellar atmospheres (Landstreet 1982; Mathys 1989),the disk integrated Stokes-V parameter (the difference between the opposite circular polar- 6 –ized intensities) F V , is proportional to the derivative of the intensity flux F I : F V F I = − Cg eff λ F I d F I ( λ )d λ B eff (1)where C = − . × − G − ˚A − , g eff is the effective Land´e factor of the transition, λ the wavelength in ˚A, and B eff = 32 π Z π d φ Z B k µ d µ (2)is the longitudinal component of the magnetic field ( B k ) integrated over the stellar disk.The slope of the linear regression of F V / F I versus − Cg eff λ
20 1 F I d F I ( λ )d λ (forced to passthrough the origin), gives the effective magnetic field. In other words, we minimize the χ merit function χ = X ij σ (cid:20) ( F V / F I ) ij + C g i eff ( λ i ) F I ) ij d( F I ) ij d λ B eff (cid:21) (3)where the standard deviation of the noise σ is independent of the spectral line i andwavelength j . If ( F ′ I ) ij = ( λ i ) g i eff 1( F I ) ij d( F I ) ij d λ , after some algebra: B eff = − P ij ( F V / F I ) ij ( F ′ I ) ij C P ij [( F ′ I ) ij ] (4)while the error is obtained from the covariance matrix: δB eff = ± p ∆ χ σC qP ij [( F ′ I ) ij ] (5)where ∆ χ are isocontours of the χ function that contain a certain confidence level. Thevalues of ∆ χ are tabulated and depend on the number of degrees of freedom. In our case,with only one degree of freedom, ∆ χ = 1 , , ±
113 and 337 ±
286 G, 7 –for NGC1360 and LSS 1362, respectively. In other words, the magnetic field is essentiallyundetermined within errors, with the most probable value compatible with the observationslying well below the kG. It is important to stress that we measured the magnetic field byminimizing the sum of the merit function ( χ ) for all spectral lines simultaneously, and notas the weighted average of the magnetic fields obtained from individual spectral lines, whichis not correct.Magnetic fields ∼ kG should be apparent in the circular polarization spectrum, as it is inthe case of the magnetic star HD94660. Fig. 1 shows that a clear Zeeman signature appearsin all individual Balmer lines present in the spectrum of HD94660. This is evident alsofrom the clear linear relationship between F V / F I and − Cg eff λ
20 1 F I d F I ( λ )d λ , in contrast to thebehavior shown by the central stars of the PNe (Fig. 2). Table 1 summarizes the magneticfield inferred from individual lines. They are all consistent within errors B eff = − ±
71 G,as expected from Landstreet & Mathys (2000). Moreover, the null spectra F N / F I shows nospurious polarization effects (Table 1, Fig. 2).In order to push forward the detection limit, we decrease the noise level by addingBalmer lines in the velocity frame. The line addition technique introduced by Semel &Li (1996) is a widespread technique that has been successfully applied to detect Zeemansignatures in a large variety of stars (e.g., Donati & Landstreet 2009). The mean spectrallines thus obtained, of the two central stars of NGC 1360 and LSS 1362, and the magneticstar HD94660 are represented in Fig. 3. In the selected velocity interval, the r.m.s. of F V / F I and F N / F I spectra are similar, σ ( F V / F I ) ∼ σ ( F N / F I ) ∼ . × − , for NGC 1360 and, σ ( F V / F I ) ∼ σ ( F N / F I ) ∼ . × − , for LSS 1362. The large difference in the case ofHD94660, σ ( F V / F I ) ∼ σ ( F N / F I ) ∼ × − , is a further unquestionable evidence of astrong magnetic field in this star. Fig. 2 shows how large indeed is the circular polarizationin Balmer lines of a star harboring a ∼ ∼
300 G ( ∼
600 G) for the central stars of NGC1360(LSS1362) with a probability of 68.2%, and below ∼
400 G ( ∼
900 G) with a probability of95.4%. These values correspond to the magnetic field obtained with all the spectral linesplus the values of the error at 68.2% and 95.4% confidence levels, see equation (4). 8 –
4. Conclusions
Contrary to Jordan et al. (2005), we find no evidence for the existence of kG magneticfields in the central stars of the PNe NGC1360 and LSS1326. Our conclusion is basedon spectropolarimetric observations deeper and at higher spectral resolution than those ofJordan et al. (2005), as well as on a rigorous analysis of the polarization signal in severalBalmer lines, considered individually or added in the velocity space. The upper limits thatwe found for the longitudinal magnetic field integrated over all stellar disc is ∼
300 and ∼
600 G for NGC1360 and LSS1362, respectively. An application of our method to theJordan et al. (2005) data, obtained from ESO archive, gives an upper limit of ∼
400 G(NGC1360) and ∼
600 G (LSS1362).With this conclusion, no evidence is left for magnetic fields on PN central stars . Onthe other hand, positive indication of magnetic fields was obtained for the nebulae in ahandful of objects: mG fields were found in the young PN OH 0.9+1.3 by OH circularpolarization (Zijlstra et al. 1989), and in the bipolar PNe NGC 7027, NGC 6537, and NGC6302 by polarimetry of magnetically aligned dust grains (Greaves 2002; Sabin et al. 2007).Therefore the negative result for the two PNe studied in this paper should not stop furtherefforts to detect magnetic fields in other PNe central stars. The method described in thispaper, sensitive to ∼ kG fields, may be attempted on other PNe which display morphologicalfeatures expected for magnetically active PN central stars, such as elongated bipolar lobes,jets, and ansae (cf. e.g. Garc´ıa-Segura et al. (1999)). It should be remarked, however,that NGC 1360 was exactly one of these promising targets, as it possesses polar jets withincreasing speed with distance from the central star, expected for a magnetically collimatedoutflow (Garc´ıa-D´ıaz et al. 2008). Other morphologies should be tested.Based on observations made with ESO Telescopes at the Paranal Observatories underprogramme 386.D-0325(A) and ESO Science Archive Facility. The Spanish contribution hasbeen funded by the Spanish Ministry of Science and Innovation under the projects AYA2010-18029 and AYA2007-66804. REFERENCES
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10 –Fig. 3.— Mean Stokes F I / F c , F V / F I and F N / F I Balmer line profiles of the central stars ofPNe NGC1360 and LSS1362. For comparison we report the same profiles of the magneticstar HD94660, whose effective magnetic field is −−