The Astrophysical Journal, 578:885–896, 2002 October 20 # 2002. The American Astronomical Society. All rights reserved. Printed in U.S.A. TESTING MODELS OF LOW-EXCITATION PHOTODISSOCIATION REGIONS WITH FAR-INFRARED OBSERVATIONS OF REFLECTION NEBULAE Rolaine C. Young Owl Department of Physics and Astronomy, University of California at Los Angeles, Mail Code 156205, Los Angeles, CA 90095-1562 Margaret M. Meixner1 and David Fong Department of Astronomy, University of Illinois, Urbana, IL 61801; [email protected], [email protected] Michael R. Haas NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035-1000; [email protected] Alexander L. Rudolph1 Department of Physics, Harvey Mudd College, 301 East 12th Street, Claremont, CA 91711; [email protected] and A. G. G. M. Tielens Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, Netherlands; [email protected] Received 2001 August 2; accepted 2002 June 24 ABSTRACT This paper presents Kuiper Airborne Observatory observations of the photodissociation regions (PDRs) in nine reflection nebulae. These observations include the far-infrared atomic fine-structure lines of [O i]63 and 145 lm, [C ii] 158 lm, and [Si ii]35lm and the adjacent far-infrared continuum to these lines. Our analysis of these far-infrared observations provides estimates of the physical conditions in each reflection nebula. In our sample of reflection nebulae, the stellar effective temperatures are 10,000–30,000 K, the gas densities are 4  102 2  104 cmÀ3, the gas temperatures are 200–690 K, and the incident far-ultraviolet intensities are 300–8100 times the ambient interstellar radiation field strength (1:2  10À4 ergs cmÀ2 sÀ1 srÀ1). Our observations are compared with current theory for low-excitation PDRs. The [C ii] 158 lmto[Oi]63lm line ratio decreases with increasing incident far-ultraviolet intensity. This trend is due in part to a positive cor- relation of gas density with incident far-ultraviolet intensity. We show that this correlation arises from a bal- ance of pressure between the H ii region and the surrounding PDR. The [O i] 145 to 63 lm line ratio is higher (greater than 0.1) than predicted and is insensitive to variations in incident far-ultraviolet intensity and gas density. The stellar temperature has little effect on the heating efficiency that primarily had the value 3  10À3, within a factor of 2. This result agrees with a model that modifies the photoelectric heating theory to account for color temperature effects and predicts that the heating efficiencies would vary by less than a factor of 3 with the color temperature of the illuminating field. In addition to the single-pointing observations, an [O i]63lm scan was done across the molecular ridge of one of our sample reflection nebulae, NGC 1977. The result appears to support previous suggestions that the ionization front of this well-studied PDR is not purely edge-on. Subject headings: dust, extinction — infrared: ISM — ISM: lines and bands — reflection nebulae 1. INTRODUCTION Far-infrared (FIR) spectroscopy is a standard tool for studying the energetics of PDRs. FIR emission from the Photodissociation regions (PDRs) are regions of the fine-structure transitions of singly ionized atomic carbon interstellar medium in which the astrochemistry, structure, ([C ii] 158 m) and neutral atomic oxygen ([O i] 63 and 145 and physical processes are determined by the interaction of l lm) dominate the cooling of the PDR gas (Tielens & far-ultraviolet (FUV; h ¼ 6 13:6 eV) radiation with inter- Hollenbach 1985a, hereafter TH85). Because dust grains stellar gas and dust. PDR chemistry is dominated by FUV absorb FUV photons and reemit the energy in the FIR, the photodissociation of molecular bonds. Many important FIR continuum dust emission is proportional to the inci- chemical reactions in PDRs are initiated by FUV-pumped dent FUV flux. Hence, PDRs in massive star-forming vibrationally excited molecular hydrogen. Most of the inci- regions were productive sites for early FIR analysis because dent FUV flux (e99%) is absorbed by dust grains and ree- mitted in the infrared continuum. A small percentage of their very bright FIR line and continuum emission. The regions proximal to massive stars contain very dense (e105 (d1%) of the incident FUV photons cause the photoejec- cmÀ3) gas being illuminated by very strong UV fields tion of electrons from small dust grains and large molecules; G e 5 G these electrons then collisionally heat the PDR gas. Energy ( 0 10 , where 0 is in units of the average interstellar radiation field [ISRF] intensity of 1:2 10À4 ergs cmÀ2 sÀ1 balance is maintained by radiative cooling, predominantly  srÀ1; Habing 1968). FIR observations of massive star-form- through line emission in the far-infrared ( 30 200 lm). ing regions in Orion (Melnick, Gull, & Harwit 1979; Russell et al. 1980; Genzel & Stutzki 1989 and references therein; Stacey et al. 1993; Herrmann et al. 1997; Luhman et al. 1 NSF CAREER Fellow. 1997) and M17 SW (Storey, Watson, & Townes 1979; 885 886 YOUNG OWL ET AL. Vol. 578 Russell et al. 1981; Harris et al. 1987; Stutzki et al. 1988; illuminated by stars of spectral type ranging from B0 to Meixner et al. 1992) have been well modeled by the theory B9.5. The results are compared with predictions from the low- of dense PDRs (TH85; Sternberg & Dalgarno 1995; Hollen- density PDR theory of Hollenbach, Takahashi, & Tielens bach & Tielens 1997 and references therein). Because the (1991, hereafter HTT91) and from Spaans et al. (1994). 2 À2 5 sensitivity of FIR detectors has continued to increase stead- HTT91 modeled PDRs with density 10 cm < n0 < 10 À2 3 ily over the past 25 yr, it has become possible to extend the cm and incident FUV intensity 1 G0 10 . Spaans et FIR study of PDR energetics to the lower energy and lower al. (1994) modeled the effects of color temperature on the density regimes more commonly found in the interstellar energetics of PDRs around cool stars. medium (ISM). An initial focus was on the lower excitation PDRs found in reflection nebulae illuminated by early B stars (e.g., Makinen et al. 1985; Chokshi et al. 1988; Howe 2. OBSERVATIONS AND RESULTS et al. 1991; Steiman-Cameron, Haas, & Tielens 1997). Table 1 lists our target sample consisting of nine reflection Visual reflection nebulae are expected to be excellent can- nebulae illuminated by stars of spectral type ranging from didates for studying low-excitation PDRs. The stars illumi- B0 to B9.5 and effective temperatures 10,000–30,000 K. nating them are usually type B or later stars, compared to These sources were chosen to study the neutral work func- the O-type stars that illuminate massive star-forming tion and photoelectric yield of interstellar dust grains under regions. Because of the lower UV flux emitted from later- a range of low-excitation conditions. The sample consists of type stars, the ionization of atomic hydrogen is expected to six quiescent reflection nebulae, two reflection nebulae near balance recombination at a smaller Stro¨mgren radius than active star-forming regions (NGC 1333 and NGC 1977), for O-type stars, resulting in a smaller H ii region. More- and a reflection nebula illuminated by a post–asymptotic over, confusion caused by shock-excited [O i] is less likely, giant branch (AGB) star (Red Rectangle). The observations because the gas is more quiescent (i.e., the ionization shock were carried out in 1989 March and in 1992 January, June, fronts are weaker). Another advantage to observing reflec- and July using the 91 cm telescope of the Kuiper Airborne tion nebulae is that even though the IR surface brightness Observatory (KAO) with the facility cryogenic grating spec- may be lower than in very luminous high-density PDRs, trometer (CGS; Erickson et al. 1984). Each reflection nebula reflection nebulae PDRs are likely to be more spatially was observed at the peak in its FIR continuum emission. extended because of more gradual FUV attenuation, allow- Guiding was done using offsets D (arcsec), D (arcsec) from ing the structure to be spatially resolved. A reflection nebula the illuminating star. The exception was NGC 1333, where PDR is expected to be less dense by a factor of 10–104 than BD +30549 was the guide star instead of SVS 3, which the PDR in a massive star-forming region such as Orion, illuminates the FIR peak. 5 6 À3 which has n0 10 10 cm . Standard chopping techniques were employed through- In this paper, FIR analysis following the method in out; the chopper amplitude was 50–90, and the chopper Chokshi et al. (1988) is used to estimate the physical condi- rotation angle was individually selected for each source to tions in reflection nebula PDRs and to test how spectral type avoid background contamination (see Table 2). The detec- affects PDR energetics. FIR line and continuum observa- tor arrays were flat-fielded by dividing each spectrum by tions were conducted for a group of reflection nebulae that of a known calibration source (Table 2). In 1989 March TABLE 1 Target Reflection Nebulae for FIR Spectroscopy Offsets R.A. Decl. L* (arcsec) Distance a Name Illuminating Source (B1950.0) (B1950.0) Stellar (L ) (D , D) (pc) Reference NGC 1977........................................ 42 Ori 05 32 55.1 À04 52 11 B1 V 29,000 À176, À284.b 450 Æ 40 0.70 1, 2 À190, À283.c 450 Æ 40 0.70 1, 2 À249, À211.d 450 Æ 40 0.70 1, 2 NGC 2068........................................ HD 38563N 05 44 10.9 00 04 17 B2 IIÀIII 3,000 0, 0 500 Æ 100 0.65 3, 4, 5 IC 446 .............................................