We have mapped the Balmer discontinuity temperature Te (BJ) along two long-slit positions in the Orion Nebula, using high-quality spectra obtained with the ESO 1.52 m telescope and with the 3.9 m AAT. One slit was oriented north-south and positioned 40” west of Ö1 C Ori. The second slit was oriented east-west, with its eastern end 10" west of Ö1 C Ori, identical to the slit position previously studied by Walter & Dufour (1994). For the NS slit, both the 1.52 m and the AAT data yield a constant temperature of about 9000 K, with variations of only a few hundred K over a total slit length of about 3#5. For the EW-slit, our data reveal two distinct regions of nearly constant temperatures of 8900 and 7200 K, respectively, possibly resulting from two separate H II regions. No evidence is found for the anomalously low temperatures reported by Walter and Dufour for the same region. © 1995, IOP Publishing Ltd

Barlow, MJ

Danziger, IJ

Liu, XW

Storey, PJ

English

UCL Discovery

BALMER DISCONTINUITY TEMPERATURES IN THE ORION NEBULAX.-W. LIU AND M. J. BARLOWDepartment of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UKI. J. DANZIGEREuropean Southern Observatory, Karl-Schwarzschild Str., D-85748 Garching bei München, GermanyANDP. J. STOREYDepartment of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UKReceived 1995 May 22; accepted 1995 July 6ABSTRACTWe have mapped the Balmer discontinuity temperature Te(BJ) along two long-slit positions in the OrionNebula, using high-quality spectra obtained with the ESO 1.52 m telescope and with the 3.9 m AAT. One slit wasoriented north-south and positioned 400 west of u1 C Ori. The second slit was oriented east-west, with its easternend 100 west of u1 C Ori, identical to the slit position previously studied by Walter & Dufour (1994). For the NSslit, both the 1.52 m and the AAT data yield a constant temperature of about 9000 K, with variations of only afew hundred K over a total slit length of about 3#5. For the EW-slit, our data reveal two distinct regions of nearlyconstant temperatures of 8900 and 7200 K, respectively, possibly resulting from two separate H II regions. Noevidence is found for the anomalously low temperatures reported by Walter and Dufour for the same region.Subject headings: H II regions— ISM: abundances— ISM: individual (Orion Nebula)1. INTRODUCTIONRecent abundance studies of a number of H II regions andplanetary nebulae (PNs) using optical recombination lines(Peimbert, Storey, & Torres-Peimbert 1993; Liu et al. 1995;Barlow et al., in preparation) suggest that the ‘‘standard’’method based on forbidden-line analyses (see e.g., Osterbrock1989) may underestimate the real abundances of heavy ele-ments by a factor of 2–3. Evidence of this effect was first givenby Peimbert (1967, 1971) who measured electron tempera-tures for three PNs and the Orion Nebula (M42) using theratio Ic(BJ)/I(Hb) of the nebular Balmer jump to Hb, andfound systematically lower temperatures than derived from the[O III] l5007/l4363 ratios for the same objects. He suggestedthat H II regions and PNs have significant temperature fluctu-ations, so that temperatures derived from forbidden line ratioswould overestimate the true mean temperature, while abun-dances derived from these forbidden lines would consequentlybe underestimated. Liu & Danziger (1993) measured Te(BJ)and Te([O III]) for 14 PNs and found that on average Te(BJ)tends to be lower than Te([O III]) by 11400 K. Recently,Walter & Dufour (1994, hereafter WD94) mapped Te(BJ)along a long slit positioned on M42 and found it to decreasefrom 8400 to 2800 K between 400 and 2200 west of u1 C Ori.This result, if confirmed, casts serious doubt on the reliabilityof Ic(BJ)/I(Hb) as a thermometer, since it is difficult tounderstand temperatures as low as 2800 K in a photoionizednebula such as M42. Given the potential importance of theirresult, we have mapped Te(BJ) for two regions in M42,including the area studied by WD94. The results are describedhere.We use the ratio of Ic(BJ) to the intensity of H11 at 3770 Å,instead of relative to the intensity of Hb, to determine Te(BJ).Because of the small wavelength difference between theBalmer discontinuity and H11, this approach minimizes pos-sible effects due to systematic errors (e.g., atmospheric differ-ential refraction, atmospheric extinction, and flux calibration).Furthermore it has the great advantage that the wavelengthdependence of the reddening correction toward M42, knownto be anomalous, becomes largely irrelevant. For example,using the extinction law derived for the Trapezium stars byCardelli, Clayton, & Mathis (1989), we find that Ic(BJ)/I(H11)changes by only 2.4% when c(Hb), the logarithmic extinctionat Hb, increases from zero to one. For the regions of M42 ofinterest here, the extinction falls well within this range (Bald-win et al. 1991, hereafter B91). Thus in our case, Ic(BJ)/I(H11) [ Fc(BJ)/F(H11), where we use I and F to denoteintrinsic and observed fluxes, respectively. The emissivities asfunctions of electron temperature for the nebular continuumand the H I Balmer lines are taken from Brown & Mathews(1970) and Storey & Hummer (1995), respectively. To takeaccount of the contribution from a small discontinuity at 3680Å produced by recombinations to the He I 2 1Po level, which isinseparable from the Balmer discontinuity, we follow Liu &Danziger (1993) and define the Balmer discontinuity asIc(BJ)[ Ic(l3643)2 Ic(l3681), where Ic(l3643) and Ic(l3681)are the nebular continuum fluxes at 3643 and 3681 Å, respec-tively. A power-law fit to the relationship between Ic(BJ)/I(H11) and Te is given by Te(BJ) 5 382.5 3 [Ic(BJ)/I(H11)]21.492 K, with a maximum error of 3.5% for Te between4000 and 20,000 K. A better fit can be achieved with a cubicspline function, which is used here. A diagram showing bothfits is available on request (contact xwl@star.ucl.ac.uk). Wehave assumed He1/H1 5 0.09 (B91); neglect of the He Idiscontinuity at 3680 Å would underestimate Te by 200 K.2. OBSERVATIONSOn 1994 December 20–24 M42 was observed using theB&C Spectrograph of the ESO 1.52 m telescope. The slit wasoriented north-south and positioned through the star JW 423(Jones & Walker 1988), 40"8 west and 75"6 north of u1 C Ori.THE ASTROPHYSICAL JOURNAL, 450 :L59–L62, 1995 September 10q 1995. The American Astronomical Society. All rights reserved. Printed in U.S.A.L59The detector was a 2048 3 2048 pixel Ford CCD and eachpixel along the slit projected to 0"815 on the sky. Severalgratings were used to provide coverage from 3410 to 8770 Å.Spectra of u1 A, B, C, and D Ori were obtained for two slitwidths: 20 and 60. Here we discuss only spectra coveringll3410–4410 from a single grating setup at a resolution of 1 ÅFWHM. The observations at the 3.9 m AAT were carried outon 1995 February 8–9 using the RGO Spectrograph, covering3510 to 3911 Å at 0.8 Å FWHM resolution. The detector wasa 1024 3 1024 pixel Tektronix CCD and each pixel along theslit projected to 0"772 on the sky. Two slit positions wereobserved, one identical to the ESO 1.52 m N-S position, andthe other oriented east-west with its eastern end starting 100west of u1 C Ori. This second slit position was identical to thatobserved by WD94. Again we observed the four Trapeziumstars using the same setup with 20 and 60 slit widths. All thenebular spectra obtained with the ESO 1.52 m telescope andwith the AAT were secured with a 20 slit width under typicalseeing conditions of 10 FWHM. All the spectra were flux-calibrated by observing the HST standard star HD 49798(spectral type O6, V 5 8.30; Walsh 1993). HD 49798 has onlyweak Balmer absorption lines and a very small Balmer jump.In addition it is calibrated every 6 Å in the near-UV and blueregions. All data were reduced using standard techniques forlong-slit spectra.3. RESULTSThe derived surface brightness distributions of H11 alongthe NS and EW slits are shown in Figure 1. The mean Te(BJ)derived after integrating along each slit are given in Table 1.For comparison, results are listed both with and without thesubtraction of the stellar scattered light and sky backgroundcontributions (see below). Table 1 shows that the scatteredlight and sky background have only marginal effects on thederived Te(BJ). All three of our independent data sets give amean Te(BJ) of 9000 K within the estimated errors of less than300 K.The total observed continuum flux Fc(l) is assumed to begiven by Fc(l) 5 Fc(neb, l) 1 Fc(sca, l) 1 Fc(sky, l), wherethe contributions from the sky background Fc(sky, l) andstellar scattered light, Fc(sca, l) were determined as follows.During the ESO 1.52 m observing run, a total of eight spectraof M42 covering ll3410–4410, each of 20 minutes duration,were obtained. The first three exposures were secured beforethe moon rose and we assumed that the sky background wasnegligible for these spectra. The wavelength dependence ofthe sky spectrum was then derived from the spectra of twoplanetary nebulae observed later on the same night, and thiswas scaled to match the difference between the spectra withand without sky contributions. In order to derive the contri-bution of stellar scattered light, we made the following as-sumptions: (1) The scattered light is solely due to the fourTrapezium stars, with the contribution from each star beingproportional to its apparent brightness; (2) The dust scatteringefficiency varies as l21 (Costero & Peimbert 1970). Thus, thespectral distribution of the scattered light is given by Fc(sca,l) 5 c(l/l0)21Fc(Tr, l), where c is a constant, l0 is a referencewavelength, and Fc(Tr, l) is the mean continuum flux of theTrapezium stars, formed by averaging the spectra of u1 A, B,C, and D Ori with weights of 15:3:68:14, their brightness ratiosin the blue and near-UV wavelength regions. (Eq. [2] and therelation that leads to it in WD94 are incorrect. The positionsof the coefficients 3646 and 4580 should be reversed.) Theconstant c can be derived once the scattered light contributionat any particular wavelength is known. The magnitude of thenebular continuum, Fc(neb, l3835), can be inferred from theobserved intensity, F(H9), of the Balmer line at that wave-length, using the value of Te(BJ) derived before any scatteredlight or sky subtraction. Note that the value of Fc(neb, l3835)derived in this way is independent of the reddening correctiontoward M42. Using Fc(neb) and Fc(sky) at one wavelength,Fc(sca) can be deduced.The ESO 1.52 m spectrum of M42, integrated along the NSslit, is shown in Figure 2a. The derived scattered light and skybackground contributions are shown in Figure 2b, while Figure2c plots the spectrum after the scattered light and sky subtrac-tion. Also shown in Figures 2a and 2c are polynomial fits to theobserved continuum levels. The regions blueward and redwardof the Balmer discontinuity are fitted separately. On the longerFIG. 1.—F(H11), the observed surface brightness distributions of H11 (inunits of 10214 ergs cm22 s21 arcsec22), deduced from (a) the NS slit, ESO 1.52m data; and (b) the EW slit, AAT data. R(JW 423) and R(u1 C) are the angulardistances south of JW 423 and west of u1 C Ori, respectively. R(JW 423) andR(u1 C) are accurate to within 0"1 and 10, respectively. The NS- and theEW-slits intersect at R(JW 423) 5 75"6 and R(u1 C) 5 40"8.TABLE 1MEAN BALMER JUMP TEMPERATURES INTHE ORION NEBULAaPARAMETERNS SLITEW SLITAATESO 1.52 m AATBefore Scattered Light and Sky SubtractionF(H11)b . . . . . . . . . . 232 234 132Fc(BJ)c . . . . . . . . . . . 27.8 28.0 16.6Te(BJ) (K) . . . . . . . 8860 8810 8230After Scattered Light and Sky SubtractionF(H11)b . . . . . . . . . . 237 237 134Fc(BJ)c . . . . . . . . . . . 27.7 28.4 16.1Te(BJ) (K) . . . . . . . 9180 8840 8800a For the NS slit, the measurements were made on thespectrum integrated from 40 to 1250 south of JW 423.For the EW-slit, the spectrum was integrated from 100 to1900 west of u1 C Ori. The power-law fit given in § 1yields Te(BJ) 2.5%–3.1% higher than those listed here,derived from the more accurate cubic spline fit.b In units of 10216 ergs cm22 s21 arcsec22.c In units of 10216 ergs cm22 s21 arcsec22 Å21.L60 LIU ET AL. Vol. 450wavelength side of the jump, a straight line is sufficient to fitthe continuum level over the whole wavelength range ofconcern here. However, on the short-wavelength side of thejump, sometimes a second order polynomial is required,especially before the scattered light subtraction. Immediatelyredwards of the Balmer jump, the spectrum is crowded by highorder Balmer lines and, even more seriously, the extendedwings of the extremely strong [O II] doublet at 3727 Å distortthe nearby continuum. Thus only data points longwards of3750 Å were used to fit the continuum level longwards of theBalmer jump.The analyses carried out for the spectra integrated along theslits were applied to each individual spectral scan line alongboth the NS and EW slits. Examples of spectra at someselected points are shown in Figure 3. The upper three panelsshow spectra near the peak nebular surface brightness,whereas the lower three are for low surface brightness points(see Fig. 1). The derived surface brightness distributions ofscattered light at 3810 Å along the slits are shown in Figure 4.The deduced distributions of Te(BJ) along the NS and EW slitsare given in Figure 5. The results deduced both before andafter scattered light and sky subtraction are shown. Note thatboth Table 1 and Fig. 5 show that after correction for theFIG. 2.—ESO 1.52 m spectra of M42, integrated along the NS-slit from 40 to1250 south of JW 423. (a) The spectrum before the scattered light and skybackground corrections. (b) Upper curve: the estimated stellar scattered light;lower curve: the sky background. (c) Spectrum (a) after subtraction of thescattered light and sky contributions shown in (b). Note that the scatteredstellar absorption line wings seen in (a) disappear in the corrected spectrum (c).Also shown in (a) and (c) are polynomial fits to the continuum bluewards andredwards of the Balmer jump at 3646 Å.FIG. 3.—Spectra of M42 near the Balmer jump region at some selected points along the NS and the EW slits. Panels (a) and (b) are from the ESO 1.52 m datawhile panels (c–f) are from the AAT data. (a, c) 620 south of JW 423; (b, d) 50 south of JW 423; (e) 370 west of u1 C Ori; ( f) 2200 west of u1 C Ori. Each spectrumrepresents one pixel along the slit on the detector, sampling 0"815 3 20 and 0"772 3 20, respectively, for the ESO 1.52 m data and the AAT data. Also shown arepolynomial fits to the nebular continuum bluewards and redwards of the Balmer jump. The stellar scattered light and sky background contributions have beensubtracted.FIG. 4.—F(sca), the surface brightness distribution of stellar scattered lightat 3810 Å (in units of 10216 ergs cm22 s21 arcsec22 Å21), along (a) the NS; and(b) the EW slit. The small peak at R(JW 423)5 0 in (a) is from JW 423. Seealso the caption to Fig. 1.No. 2, 1995 BALMER DISCONTINUITY TEMPERATURES L61scattered light contribution, the derived temperatures arehigher than those without the correction, contrary to previousexpectations (Peimbert 1971). For the NS-slit, the resultsderived from the ESO 1.52 m data and from the AAT datashow excellent agreement. Across the whole 3#5 slit length,Te(BJ) shows only very small variations, of a few hundred K.The average temperature is about 9000 K, in excellent agree-ment with the value deduced from the integrated spectra(Table 1). The small variations might be real, as they can beseen in both data sets. Along the AAT EW-slit, two regions ofslightly different mean temperature can be seen, one of 8900 Kfrom 100 to 900 west of u1 C Ori and another of 7200 K from900 west of u1 C Ori to the western end of the slit. In parallelto this sudden drop in Te, the electron density derived from the[O II] doublet ratio l3729/l3726 decreases abruptly from 2600to 1100 cm23. A similar change in the [S II] densities wasobserved previously (B91). Although collisional suppression ofthe [O III] fine-structure cooling lines can lead to slightlyhigher Te in the inner, denser regions, photoionization mod-eling calculations show such an effect is only of the order of afew hundred K. Thus the sudden decrease in Te of 2000 K isdifficult to understand in a picture of M42 as a single H IIregion ionized by a single central source, i.e., u1 C Ori. Figure15 of B91 shows that the thickness of the H1 zone along theline of sight increases abruptly by a factor of 2 near thisposition and then increases rapidly beyond. In addition, theirFigures 10 and 11 show evidence of irregularities in theionization structure around this region. One possible explana-tion is that there is another low surface brightness H II region,ionized by stars cooler than u1 C Ori and thus of lower Te,surrounding the ‘‘Trapezium’’ H II region, and its contributionto the observed fluxes becomes more important beyond R(u1C) ? 900. The lowest derived temperatures, of about 5500 K,seen in Figure 5f near R(u1 C) 1 2000, correspond to very lowsurface brightness emission (see Fig. 1b), so that we do notconsider them to be significant.In conflict with results presented here, WD94 found Te(BJ)to drop from 8400 to 2800 K between 400 and 2200 west of u1C Ori. As described earlier, by adopting the ratio of theBalmer jump to the intensity of H11, we minimize the effectsof many possible systematic errors in determining the temper-ature, including uncertainties arising from the reddening cor-rections for M42. As we have observed the four Trapeziumstars using the same instrumental setup, we have been able tosubtract the scattered light in a consistent way. In contrast, theresults of WD94 were more sensitive to reddening corrections.In addition, inspection of their Figure 1 spectrum shows anirregular continuum shape. We conclude that the dramaticvariations in Te(BJ) across M42 that they reported are not real.REFERENCESBaldwin, J. A., et al. 1991, ApJ, 374, 580 (B91)Brown, R. L., & Mathews, W. G. 1970, ApJ, 160, 939Cardelli, J. A., Clayton, G. C., & Mathis, J. S. 1989, ApJ, 345, 245Costero, R., & Peimbert, M. 1970, Bol. Obs. Tonantzintla Tacubaya, 5, 229Jones, B. F., & Walker, M. F. 1988, AJ, 995, 1755Liu, X. W., & Danziger, I. J. 1993, MNRAS, 263, 256Liu, X. W., Storey, P. J., Barlow, M. J., & Clegg, R. E. S. 1995, MNRAS, 1995,272, 369Osterbrock, D. E. 1989, Astrophysics of Gaseous Nebulae and Active GalacticNuclei (Mill Valley: University Science Books)Peimbert, M. 1967, ApJ, 150, 825———. 1971, Bol. Obs. Tonantzintla Tacubaya, 6, 29Peimbert, M., Storey, P. J., & Torres-Peimbert, S. 1993, ApJ, 414, 626Storey, P. J., & Hummer, D. G. 1995, MNRAS, 272, 41Walsh, J. R. 1993, ST-ECF Newsletter, 19, 6Walter, D. K., & Dufour, R. J. 1994, ApJ, 434, L29 (WD94)FIG. 5.—Variations of the nebular Balmer discontinuity temperature Te(BJ) in M42 along (a–d) the NS-slit and (e–f) the EW-slit. (a) and (b) were derived fromthe ESO 1.52 m data, whereas (c–f) are from the AAT data. The upper and lower panels are, respectively, results before and after corrections for stellar scatteredlight and sky background. In each panel, we give the mean temperature and the standard deviation for all points equally weighted, with the mean indicated by a line.For the EW-slit, two mean values are given, one for R(u1 C) 5 100–900 and another for R(u1 C) 5 900–2300. See also the caption to Fig. 1.L62 LIU ET AL.

Balmer Discontinuity Temperatures in the Orion Nebula

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Balmer Discontinuity Temperatures in the Orion Nebula

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