Probing the role of polycyclic aromatic hydrocarbons in the photoelectric heating within photodissociation regions
Yoko Okada, Paolo Pilleri, Olivier Berné, Volker Ossenkopf, Asunción Fuente, Javier R. Goicoechea, Christine Joblin, Carsten Kramer, Markus Röllig, David Teyssier, Floris F. S. van der Tak
aa r X i v : . [ a s t r o - ph . GA ] M a r Astronomy&Astrophysicsmanuscript no. YOKADA c (cid:13)
ESO 2018January 30, 2018
Probing the role of polycyclic aromatic hydrocarbons in thephotoelectric heating within photodissociation regions ⋆ Yoko Okada , Paolo Pilleri , , , Olivier Bern´e , , Volker Ossenkopf , Asunci´on Fuente , Javier R. Goicoechea ,Christine Joblin , , Carsten Kramer , Markus R ¨ollig , David Teyssier , and Floris F. S. van der Tak , I. Physikalisches Institut der Universit¨at zu K¨oln, Z¨ulpicher Straße 77, 50937 K¨oln, Germany e-mail: [email protected] Centro de Astrobiolog´ıa, CSIC-INTA, 28850, Madrid, Spain Observatorio Astron´omico Nacional (OAN), Apdo. 112, 28803 Alcal´a de Henares (Madrid), Spain Los Alamos National Laboratory, Los Alamos, NM 87545, USA. Universit´e de Toulouse; UPS-OMP; IRAP; Toulouse, France CNRS; IRAP; 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4, France Instituto de Radioastronom´ıa Milim´etrica, Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain European Space Astronomy Centre, ESA, PO Box 78, 28691, Villanueva de la Ca˜nada, Madrid, Spain SRON Netherlands Institute for Space Research, P.O. Box 800, 9700 AV Groningen, The Netherlands Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The NetherlandsReceived; accepted
ABSTRACT
Aims.
We observationally investigate the relation between the photoelectric heating e ffi ciency in photodissociation regions (PDRs)and the charge of polycyclic aromatic hydrocarbons (PAHs), which are considered to play a key role in photoelectric heating. Methods.
Using PACS onboard
Herschel , we observed six PDRs spanning a wide range of far-ultraviolet radiation fields ( G = ). To measure the photoelectric heating e ffi ciency, we obtained the intensities of the main cooling lines in these PDRs, i.e., the [O i ]63 µ m, 145 µ m, and [C ii ] 158 µ m, as well as the far-infrared (FIR) continuum intensity. We used Spitzer / IRS spectroscopic mappingobservations to investigate the mid-infrared (MIR; 5.5–14 µ m) PAH features in the same regions. We decomposed the MIR PAHemission into that of neutral (PAH ) and positively ionized (PAH + ) species to derive the fraction of the positively charged PAHs ineach region, and compare it to the photoelectric heating e ffi ciency. Results.
The heating e ffi ciency traced by ([O i ] 63 µ m + [O i ] 145 µ m + [C ii ] 158 µ m) / TIR, where TIR is the total infrared flux,ranges between 0.1% and 0.9% in di ff erent sources, and the fraction of PAH + relative to (PAH + PAH + ) spans from 0 ( + ± + fraction show a low heating e ffi ciency, and all positions with a high heating e ffi ciency havea low PAH + fraction, supporting the scenario in which a positive grain charge results in a decreased heating e ffi ciency. Theoreticalestimates of the photoelectric heating e ffi ciency show a stronger dependence on the charging parameter γ = G T / / n e than theobserved e ffi ciency reported in this study, and the discrepancy is significant at low γ . The photoelectric heating e ffi ciency on PAHs,traced by ([O i ] 63 µ m + [O i ] 145 µ m + [C ii ] 158 µ m) / (PAH-band emission + [O i ] 63 µ m + [O i ] 145 µ m + [C ii ] 158 µ m), shows amuch better match between the observations and the theoretical estimates. Conclusions.
The good agreement of the photoelectric heating e ffi ciency on PAHs with a theoretical model indicates the dominantcontribution of PAHs to the photoelectric heating. This study demonstrates the fundamental role that PAHs have in photoelectricheating. More studies of their charging behavior are crucial to understand the thermal balance of the interstellar medium. Key words.
HII regions – ISM: lines and bands – photon-dominated region (PDR) – Infrared: ISM
1. Introduction
Photoelectric heating is a major heating process in photodissoci-ation regions (PDRs), and its e ffi ciency ( ǫ pe ) is one of the key pa-rameters to understanding the energy balance there. Theoreticalinvestigations suggest that small grains, in particular polycyclicaromatic hydrocarbons (PAHs), play a dominant role in pho-toelectric heating (Bakes & Tielens 1994). ǫ pe is defined as thefraction of energy absorbed by dust that is converted into ki-netic energy of the ejected electrons and therefore into gas heat-ing. Observationally it can be estimated by measuring the ra- ⋆ Herschel is an ESA space observatory with science instrumentsprovided by European-led Principal Investigator consortia and with im-portant participation from NASA. This work is based in part on obser-vations made with the Spitzer Space Telescope, which is operated bythe Jet Propulsion Laboratory, California Institute of Technology undera contract with NASA. tio of the energy emitted in the gas cooling lines ([O i ] 63 µ mand [C ii ] 158 µ m are the strongest ones from Av . ǫ pe in di ff erent sources, rangingfrom 10 − in W49N, which is illuminated by an intense UVfield (Vastel et al. 2001) to 1–2% in the relatively low-UV ex-cited PDR of the Horsehead Nebula (Goicoechea et al. 2009), al-though mechanical heating also plays a role in W49 (Nagy et al.2012). Mizutani et al. (2004) showed a variation of ǫ pe from 0.06to 1.2% across the 40 ′ × ′ area of the Carina Nebula, and ananti-correlation between ǫ pe and the intensity of the local UV ra-diation field. For extragalactic sources, Mookerjea et al. (2011)presented ǫ pe of 0.3–1.2% toward one H ii region in M 33 at aresolution of 50 pc. These variations have been attributed to dif-ferences in the mean charge state of the grains, i.e., a positive
1. Okada et al.: Probing the role of PAHs in the photoelectric heating within PDRs grain charge results in a decreased e ffi ciency, and the correlationwith the intensity of the UV radiation field supports this inter-pretation. However, considering two properties of di ff erent dustpopulations together, large grains that are the main carriers ofthe dust IR emission and small grains that are expected to be themain contributors of the photoelectric e ff ect, limits the interpre-tation. Habart et al. (2001) investigated the photoelectric heat-ing e ffi ciency on small grains and indicated that the observedspatial distribution of [O i ] 63 µ m and [C ii ] 158 µ m in L1721can be better explained by a model with a varying abundanceof small grains across the cloud. Joblin et al. (2010) concludedthat the evolution of PAHs and very small grains at the bor-der of PDRs should be considered to model the gas energetics.Before Herschel , estimating the gas-cooling energy in spatiallyresolved PDRs was only rarely possible because of the poorspatial resolution in the far-infrared (FIR) wavelength range.To investigate the charge state of PAHs, mid-infrared (MIR)spectroscopic observations are needed, which can be providedby the Infrared Spectrograph (IRS; Houck et al. 2004) onboardthe
Spitzer Space Telescope (Werner et al. 2004). The most evi-dent spectral di ff erence between neutral PAHs (PAH ) and ion-ized PAHs (PAH + ) is the strength of the 6–9 µ m complex rel-ative to the 11.3 µ m feature, which is supported by laboratoryexperiments (DeFrees et al. 1993; Pauzat et al. 1994; Langho ff ff erent environments, reflecting an evolution of the chargestate of PAHs in di ff erent physical conditions (Galliano et al.2008; Sakon et al. 2004; Peeters et al. 2002; Joblin et al. 1996).Recently, Joblin et al. (2008) and Pilleri et al. (2012b) proposedan alternative method of deriving the fraction of PAH + using aspectral decomposition approach based on a few template spec-tra (Bern´e et al. 2007; Rapacioli et al. 2005) that include PAH ,PAH + , and evaporating very small grains (eVSGs).The combination of the observations by Photodetector ArrayCamera and Spectrometer (PACS; Poglitsch et al. 2010) onboard Herschel (Pilbratt et al. 2010) and the IRS onboard
Spitzer en-ables us to investigate the relation between ǫ pe and the fraction ofionized PAHs in spatially resolved PDRs. In this paper, we reportthe results for six PDRs studied in the WADI (Warm And DenseInterstellar medium; Ossenkopf et al. 2011) guaranteed time keyprogram of Herschel . WADI is aimed to investigate the physicsand chemistry of PDRs and shocked regions with a wide rangeof physical properties. In this study, we investigate six PDRs lo-cated in five di ff erent regions that show clear detections of [C ii ]and [O i ] with PACS and were observed with the IRS.
2. Observations and data reduction
Table 1 shows the global properties of the investigated re-gions. They cover three orders of magnitude in far-ultraviolet(FUV; h ν = G in units of the Habing field(1 . × − W m − Habing 1968). Figure 1 shows the PACSfield-of-view and the extracted area for each PDR. Some ofthese sources have been studied and modeled in detail basedon the observations with the Herschel-Heterodyne Instrumentfor the Far-Infrared (HIFI; de Graauw et al. 2010) in WADI (seeJoblin et al. 2010 for NGC 7023 and Fuente et al. 2010 andPilleri et al. 2012a for Mon R2).
The Horsehead Nebula emerges from the western edge of L1630as a dark cloud at visible wavelengths, and its outer edge is de-lineated by a bright and narrow filament in the MIR emission(Abergel et al. 2003, Fig. 1a). The exciting source σ Ori illumi-nates the filament from the west, and a steep change in columndensity marks the western edge of the filament, while on theeastern side the MIR emission decreases because of the extinc-tion of the incident radiation by dense material (Abergel et al.2003). Using observations of the H to 10 cm − with a scale length of about 10 ′′ of thisfilament. Goicoechea et al. (2009) mapped the [O i ] 63 µ m emis-sion at a low spectral resolution and concluded that a nonlocaland non-LTE treatment can be important to model the [O i ] emis-sion. They derived a value of ǫ pe of 1–2%. Ced 201 is a reflection nebula, illuminated by the B9.5 star BD + ◦ ǫ pe to be < .
3% based on low spatial resolu-tion KAO observations of [C ii ] 158 µ m and the upper limit of[O i ] 63 µ m. NGC 7023 is a prototype PDR, which has been widely stud-ied at many wavelengths. The UV radiation from the excitingB star, HD200775, creates three main PDRs in this nebula; thebrightest NGC 7023 North-West (hereafter NGC 7023 NW)is located about 40 ′′ northwest of the star, another PDR liesabout 70 ′′ south (NGC 7023 S), and NGC 7023 East (hereafterNGC 7023 E) is located about 170 ′′ east of the star (Bern´e et al.2007). Pilleri et al. (2012b) showed a clear di ff erence in thespatial distribution of PAH and PAH + in these three PDRs.Joblin et al. (2010) presented the first results from HIFI observa-tions along a cut through NGC 7023 NW and S, suggesting thatboth the [C ii ] emission and the aromatic infrared-band (AIB)emissions in the MIR arise from the regions located in the tran-sition zone between atomic and molecular gas, providing newinsights into the importance of the PAH charge evolution in theenergetic studies of PDRs. The Carina Nebula is a massive star-forming region complexwith 65 O-type stars at a distance of 2.3 kpc (Smith 2006),which provides the nearest example of a very massive star-forming region and has been observed at many wavelengths(Smith & Brooks 2007). Trumpler 14 and 16 are the most promi-nent clusters. The PDR properties have been investigated us-ing FIR and submilimeter emission lines (Kramer et al. 2008;Mizutani et al. 2004; Brooks et al. 2003). The region excited byTrumpler 14 has a FUV flux of G = × and a high density of2 × cm − (Kramer et al. 2008). Preibisch et al. (2011) presenta large deep 870 µ m continuum map, showing that the total massis about 2 × M ⊙ . Gaczkowski et al. (2013) recently published
2. Okada et al.: Probing the role of PAHs in the photoelectric heating within PDRs D ec ( J ) (a) Horsehead IRAC 8μm [MJy sr −1 ] D ec ( J ) (b) Ced201 IRAC 8μm [MJy r −1 ] D ec ( J ) (c) NGC7023 E IRAC 8μ [MJy s" −1 ] D ec ( J ) (d) NGC7023 NW IRAC 8μm [MJy sr −1 ] D ec ( J ) (e) Carina N IRAC 8μm [MJy sr −1 ] D ec ( J ) PDR1
IRS1IRS2IRS3 IRS4 (f) Mon R2 IRS AIB band( [W m −2 (r −1 ] Fig. 1.
PACS footprints of the blue camera ([O i ] 63 µ m) and the red camera ([O i ] 145 µ m and [C ii ] 158 µ m) overlaid on IRAC 8 µ m,except for (f) Mon R2. (There IRAC 8 µ m is saturated, and the total intensity of AIB bands derived from fitting of IRS spectra (seetext) is shown.) Green circles or boxes show the areas from which our PACS and IRS spectra are extracted (subregions in Table 3).For Mon R2 (f), the positions of four IR sources (IRS1–4) and PDR 1 from Bern´e et al. (2009) are marked.
3. Okada et al.: Probing the role of PAHs in the photoelectric heating within PDRs
Table 1.
General properties of the observed targets. G only gives typical values within the individual regions. See Table 4 for localvariations. Object Distance Exciting source G a Reference[kpc] Name Spectral Type [Habing field]Horsehead 0.41 σ Ori O9.5V 100 1,2Ced 201 0.42 BD + ◦ × Menten et al. (2007), Habart et al. (2005), Young Owl et al. (2002), Pilleri et al. (2012b), Kramer et al. (2008), Downes et al.(1975), Rizzo et al. (2003)
PACS and SPIRE continuum maps of the Carina Nebula com-plex to study embedded young stellar objects. In this study, weanalyzed a PACS position in Carina North (hereafter Carina N),excited by Trumpler 14.
Mon R2 is a close-by ultracompact H ii (UCH ii ) region at a dis-tance of 850 pc, which comprises several PDRs that can be spa-tially resolved at both mm and IR wavelengths. The most intenseUV source, which coincides with the infrared source IRS1, is lo-cated at the center of the cometary shaped UCH ii . The brightestPDR of Mon R2 is illuminated by an extremely intense UV field( G ∼ × ; Rizzo et al. 2003), and its PAH emission peaksat about 20 ′′ northwest of IRS1 (Fig. 1). Several other PDRs liearound IRS1 and span di ff erent physical conditions such as tem-perature, density, column density, and UV field (e.g. Rizzo et al.2003; Bern´e et al. 2009; Pilleri et al. 2012a). All PDRs have verybright MIR spectra, consisting of the emission from AIBs, H rotational lines, and the continuum. The spatial extent of thesePDRs ( ∼ .
03 pc) yields an angular size of ∼ ′′ , as shown byPAH and H emissions in the MIR (Bern´e et al. 2009). The shapeof the MIR spectra varies at di ff erent PDRs, reflecting the photo-processing of the AIB carriers. The FIR spectroscopic observations were performed with PACSas part of the WADI program. The observational parameters arelisted in Table 2. All measurements are single pointed observa-tions with the PACS spectrometer consisting of a 5 × . ′′ each (see Fig. 1). Wepipelined the data from level 0 to 2 using the Herschel data pro-cessing system (HIPE; Ott 2010) version 8. We converted theunit of the pipelined spectra, Jy / spaxel, to W m − µ m − sr − us-ing the field of view of a spaxel (9 . ′′ × . ′′ ). For the Chop / Nodobservations, we checked the contamination at the OFF positionsfor [O i ] 63 µ m, 145 µ m, and [C ii ] 158 µ m using the script pro-vided in HIPE (ChopNodSplitOnO ff .py), which provides sep-arate spectra for the ON and OFF positions. Only the [C ii ]158 µ m emission in the Horsehead shows significant OFF con-tamination. Since the northern side of the OFF positions showsstronger [C ii ] 158 µ m, and the emission from the southern sidecan be attributed to a di ff use component that is not associatedwith the region, we used only the OFF measurement of thesouthern side to obtain the flux from the source. For the un-chopped grating-scan observations, some emission is detected at the OFF positions in [C ii ] 158 µ m (Carina N and Mon R2)and [O i ] 63 µ m (Carina N). The line intensities at the OFF po-sitions are 4–7%, 2–5%, and 1–4% of that at the ON positions.We subtracted the OFF emission in Mon R2 but not in Carina N,because the detection of [O i ] 63 µ m in the Carina N OFF po-sition indicates a real contamination from local dense clouds,while the [C ii ] 158 µ m emission traces more di ff use region andcan be attributed to the large-scale di ff use Galactic [C ii ] emis-sion. To determine the continuum levels, we subtracted the OFFmeasurements in all observations to remove the telescope back-ground.The uncertainty of the absolute flux calibration is 11–12%for the line and continuum emission . A larger uncertainty isexpected in weak sources for unchopped grating scans. Thereproductivity in total absolute flux (telescope + source) ofthe unchopped grating scan is 4% (peak-to-peak). The rela-tive uncertainty against the source flux can be expressed as0 . × F (telescope + source) / F (source), where F is the flux. ForMon R2, this means an uncertainty of <
13% and <
5% for thecontinuum at 105–180 µ m and at the peak of [C ii ] 158 µ m. ForCarina N, it is <
10% and <
5% at the peak of the [O i ] 63 µ mand [C ii ] 158 µ m emission lines and 20–50% for the underlyingcontinuum level.We defined the region to be used in our study depending onthe morphology of each source so that the widest range of phys-ical conditions is included. Since our PACS observations are notfully sampled, we cannot exploit the entire spatial information.Instead, we selected a few typical areas in each source and ex-tracted the [O i ] and [C ii ] line intensities and the continuum fluxfrom PACS observations and combined them with the IRS re-sults. These areas are listed in Table 3, shown as green boxes orcircles in Fig. 1, and are explained in detail in the following. Weextracted spectra based on the geometrical area and did not applya beam-size correction. The FWHM of the PACS spectrometerbeam at 63 µ m and 158 µ m is ∼ ′′ and ∼ . ′′ , respectively,and the uncertainty from the di ff erence of the beam size is esti-mated to be ∼
15% (see Appendix A).For Horsehead, Carina N, and Mon R2, all observations weremade with almost the same position angle, i.e., the PACS spax-els observed almost the same area in the sky for di ff erent wave-lengths (Fig. 1). In this case, we extracted the areas to be ex-amined on the basis of the PACS spaxels. In the Horsehead, thespaxels are aligned to the ridge. We defined three areas; the firstone lies at the western side of the ridge in the ionized gas (an av-erage over four spaxels except for the north-end spaxel), the sec-ond area is along the ridge (over five spaxels), and the third area PACS spectroscopy performance and calibration document v2.44. Okada et al.: Probing the role of PAHs in the photoelectric heating within PDRs
Table 2.
PACS observation summary.
Object RA (J2000) DEC (J2000) Obs. ID Obs. mode LinesHorsehead 5 h m s . − ◦ ′ ′′ . / Nod [O i ] 63 µ m1342228245 LineSpec, Chop / Nod [C ii ] 158 µ m1342228246 LineSpec, Chop / Nod [O i ] 145 µ mCed 201 22 h m s . + ◦ ′ ′′ . / Nod [O i ] 63 µ m1342216208 LineSpec, Chop / Nod [O i ] 145 µ m, [C ii ] 158 µ mNGC 7023 E 21 h m s . + ◦ ′ ′′ . / Nod [O i ] 63 µ m1342222231 LineSpec, Chop / Nod [O i ] 145 µ m, [C ii ] 158 µ mNW 21 h m s . + ◦ ′ ′′ . / Nod [O i ] 63 µ m1342222230 RangeSpec, Chop / Nod [O i ] 145 µ m, [C ii ] 158 µ mNW 21 h m s . + ◦ ′ ′′ . / Nod [O i ] 63 µ m1342197033 RangeSpec, Chop / Nod [O i ] 145 µ m, [C ii ] 158 µ mCarina N 10 h m s . − ◦ ′ ′′ . i ] 63 µ m, [C ii ] 158 µ mMon R2 6 h m s . − ◦ ′ ′′ . / Nod [O i ] 63 µ m1342228454 / i ] 145 µ m, [C ii ] 158 µ m Table 3.
Line intensities observed by PACS. Subregions are shown by green circles or boxes in Fig. 1, and are described in detail inSect. 2.1.
Object Subregion Line intensities [W m − sr − ][O i ] 63 µ m [O i ] 145 µ m [C ii ] 158 µ mHorsehead 1 H ii region side (1 . ± . × − (1 . ± . × − (9 . ± . × − . ± . × − (5 . ± . × − (2 . ± . × − . ± . × − (2 . ± . × − (1 . ± . × − Ced 201 1 9.4 ′′ diameter (6 . ± . × − (5 . ± . × − (7 . ± . × − ′′ diameter (4 . ± . × − (4 . ± . × − (6 . ± . × − NGC 7023 E 1 9.4 ′′ diameter (1 . ± . × − (4 . ± . × − (3 . ± . × − ′′ diameter (1 . ± . × − (3 . ± . × − (3 . ± . × − NGC 7023 NW 1 cavity (6 . ± . × − (6 . ± . × − (5 . ± . × − . ± . × − (3 . ± . × − (9 . ± . × − . ± . × − (3 . ± . × − (6 . ± . × − Carina N 1 8 µ m clump (4 . ± . × − – (4 . ± . × − . ± . × − – (4 . ± . × − × . ± . × − – (3 . ± . × − Mon R2 1 around PDR1 (7 . ± . × − (1 . ± . × − (1 . ± . × − . ± . × − (3 . ± . × − (3 . ± . × − . ± . × − (2 . ± . × − (2 . ± . × − is in molecular gas, at the eastern side of the ridge (five spaxels).In Carina N, we selected three regions; a single spaxel toward aclump seen in the IRAC 8 µ m map, a ridge-like east-west struc-ture in the 8 µ m map including this clump (five spaxels), and thesouthern 5 × + fraction is suggested to be low, and toward the southwestand northeast inner edges of the PAH + distributions shown inBern´e et al. (2009). They correspond to the three green boxes inFig. 1f from southwest to northeast.For Ced 201, NGC 7023 E, and NGC 7023 NW, the [O i ]63 µ m line was observed in di ff erent seasons of the year than the[O i ] 145 µ m and [C ii ] 158 µ m observations, and the di ff erenceof the position angle is significant. Therefore, we defined cir-cles as areas to be studied, and took a weighted mean of PACSspectra based on the geometrical overlap between the definedcircles and the PACS spaxels. For Ced 201 and NGC 7023 E,two circles were defined with a common center for both PACSfootprints and diameters of 9 . ′′ and 28 . ′′ . For NGC 7023 NW,we selected three typical regions that trace di ff erent ionizationfractions of PAHs based on the analysis in Pilleri et al. (2012b).The areas were defined by circles with 12 ′′ diameter, at the in- terface with strong PAH emission, toward the molecular cloud,and in the cavity toward the exciting star (Fig. 1).After extracting the spectra in each region as describedabove, we obtained the [O i ] 63 µ m and [C ii ] 158 µ m intensi-ties, as well as [O i ] 145 µ m when available, by a Gaussian fitafter linear baseline subtraction (Fig. 2, Table 3). In general,the ratio of [O i ] 63 µ m / [C ii ] 158 µ m traces the density of thePDRs (R¨ollig et al. 2006). Among our PDRs, NGC 7023 E andCed 201 show a low ratio ( < & i ] 145 µ m / µ m is > .
09 for all tar-gets and 0 . i ] 63 µ m emission, a suprathermal population of[O i ] by collision with H , and foreground absorption in [O i ]63 µ m (Liseau et al. 2006). Although the optical depth of the[O i ] 63 µ m line is taken into account by the PDR models forsimple geometries, the overlapped several PDR clumps along theline-of-sight or the edge-on geometry can cause more significantself-absorption of [O i ] 63 µ m (Habart et al. 2003; Okada et al.2003), and the non-local calculation suggests a higher [O i ]145 µ m / µ m ratio (Elitzur & Asensio Ramos 2006). The fore-
5. Okada et al.: Probing the role of PAHs in the photoelectric heating within PDRs I n t en s i t y [ - W m - µ m - s r - ] NGC7023 E - 1 0246810 NGC7023 NW - 263.0 63.1 63.2 63.3 63.4 63.5Wavelength [ µ m]0510152025 Carina N - 1 63.0 63.1 63.2 63.3 63.4 63.5Wavelength [ µ m]0102030405060 Mon R2 - 2 0.00.10.20.30.40.50.60.7 Horsehead - 2 0.000.020.040.060.080.100.12 Ced201 - 1 0.00.20.40.6 I n t en s i t y [ - W m - µ m - s r - ] NGC7023 E - 1 012345 NGC7023 NW - 2145.0 145.2 145.4 145.6 145.8 146.0Wavelength [ µ m]010203040 Mon R2 - 2 0.000.050.100.150.20 Horsehead - 2 0.000.020.040.06 Ced201 - 1 0.00.10.20.3 I n t en s i t y [ - W m - µ m - s r - ] NGC7023 E - 1 0.00.20.40.60.81.0 NGC7023 NW - 2157.0 157.5 158.0 158.5Wavelength [ µ m]01234 Carina N - 1 157.0 157.5 158.0 158.5Wavelength [ µ m]01234 Mon R2 - 2 Fig. 2.
Examples of [O i ] 63 µ m, 145 µ m, and [C ii ] 158 µ m spec-tra in each PDR. The numbers labeled in figures after the nameof the object show subregions (see Table 3).ground absorption cannot be quantified in our PDRs because ofthe lack of velocity-resolved [O i ] 63 µ m observations. However,the HIFI observations of the velocity-resolved [C ii ] 158 µ m im-ply only minor e ff ects toward our PDRs. We analyzed the MIR spectra observed with the Short-Low(SL1 and SL2) module of IRS onboard
Spitzer except forNGC 7023 E, for which we used the ISOCAM highly-processeddata product (Boulanger et al. 2005). We used the same datacubes as in Pilleri et al. (2012b) for the Horsehead, Ced 201, and NGC 7023, and the data reduction is described in that paper aswell. For Mon R2, the IRS data and their reduction are shownin Bern´e et al. (2009). For Carina N, we collected the IRS obser-vations from the
Spitzer data archive and analyzed them usingCUBISM (Smith et al. 2007). To estimate the continuum, Short-High (SH) and Long-Low (LL) spectra were also reduced andextracted when available. Their intensities are scaled to matchthe SL spectra, then the whole spectra are scaled to match thephotometric data; IRAC 8 µ m and MIPS 24 µ m for Horsehead,Ced 201, and NGC7023 E, and IRAC 8 µ m for NGC7023 NWand Carina N. For Mon R2, the absolute flux scaling was notapplied because IRAC 8 µ m is saturated, and the available spec-tral range does not cover the MIPS 24 µ m band. The correctionshould be less significant in Mon R2 because it is the brightestsource in our samples. Applying this correction does not a ff ectany trend in the following results and changes none of our con-clusions. We extracted a spectrum from the same area as for thePACS spectra described in the previous subsection.
3. Analysis
Observationally, ǫ pe is measured as ([O i ] + [C ii ]) / TIR,where TIR is the total infrared flux (3–1100 µ m; Dale & Helou2002), representing the fraction of the input energy that is con-verted into the cooling lines. The total FIR flux, 42.5–122.5 µ m(Helou et al. 1988) or the integrated flux of the thermal emis-sion by large dust grains, is often used as a tracer of the inputenergy. Here we used the TIR instead, which is the emissionfrom all dust grains, including PAHs and VSGs, to representthe total input energy. As dominant cooling lines, we summedthe intensities of [O i ] 63 µ m, 145 µ m, and [C ii ] 158 µ m, exceptfor Carina N, where the [O i ] 145 µ m was not observed, and weused ([O i ] 63 µ m + [C ii ] 158 µ m). A second PACS observation1.3 ′ away in Carina N, which is not used in this study becauseit does not overlap with the IRS observations, shows an [O i ]145 µ m / µ m intensity ratio of 0 . .
15. If our position hasthe same ratio, neglecting the [O i ] 145 µ m emission underesti-mates ǫ pe by 8% at most.To estimate TIR as the energy input tracer we need to inte-grate the grain emission of 3–1100 µ m. The contribution from3–5.5 µ m is negligible because of the low intensity at 5.5 µ m inall regions. To obtain the energy in the range of 5.5–14 µ m, wefit the IRS spectra using the procedure described in Pilleri et al.(2012b) and integrated the resulting fit (see Sect. 3.2).For λ > µ m, the available data and the quality are notuniform, and we fine-tuned approaches from region to regionto estimate the integrated flux (see Fig. 3), although the basicapproach was the same; we fit the FIR spectra from PACS ob-servations with the thermal dust model, extrapolated the fit inthe MIR to longer wavelengths, and connected the two fits at theintersection point.In Horsehead, Ced 201, and NGC 7023 E, MIR spectra areavailable up to 35 µ m. We estimated the integrated flux for 14–28 µ m by direct integration excluding the strong emission linesfor Ced 201 and NGC 7023 E, and by a linear fit to the spectra inthe Horsehead because of the lower signal-to-noise ratio (S / N).The 28–32 µ m range is fitted by a linear function, and the resultsare extrapolated to longer wavelengths. In NGC 7023 NW andCarina N, IRS / SH data are available for 14–19 µ m, which aredirectly integrated, and we assumed a flat spectrum from 19 µ mup to the wavelength where the big-grain emission, peaking in
6. Okada et al.: Probing the role of PAHs in the photoelectric heating within PDRs -7 -6 Horsehead - 2 -7 -6 -5 Ced201 - 1 -7 -6 -5 I n t en s i t y [ W m - µ m - s r - ] NGC7023 E - 1 -6 -5 -4 NGC7023 NW - 2
10 100Wavelength [ µ m]10 -6 -5 -4 Carina N - 1
10 100Wavelength [ µ m]10 -5 -4 -3 Mon R2 - 2
Fig. 3.
Continuum fit at the same positions as in Fig. 2. The black lines are the observed spectra, while the red and blue lines arethe models used to estimate the TIR. The blue asterisks are the data points that are used for fitting the FIR thermal dust emission(see text). The blue curve in the FIR is the fit with the thermal dust model, that in the MIR indicates either the direct integration,the linear fit, or the assumed constant value. The red curve below 14 µ m shows the fit described in Sect. 3.2. The black asterisk inNGC7023 NW is the MIPS 24 µ m flux.the FIR, exceeds this level. For Mon R2, we assumed that theflux at λ > µ m is constant (Fig. 3).For the PACS Chop / Nod LineSpec observations (Table 2),the continuum levels are available around the emission lines.They are indicated as blue asterisks in Fig. 3. In NGC 7023 NWand Mon R2, the full spectral data from the PACS RangeSpecobservations at > µ m, after excluding strong emission lines,were used to fit the FIR continuum. In Carina N, the un-certainty of the absolute continuum flux is too large (seeSect. 2.2). Therefore we obtained the photometry data atblue (70 µ m) and red (160 µ m) bands from the Herschel dataarchive. We fit the FIR continuum using the dust emissivity ofOssenkopf & Henning (1994), for a gas density of 10 cm − andgrains with thin ice mantles, considering the temperature and thecolumn density as free parameters. Since we discuss only the in-tegrated IR flux, the choice of the dust emissivity is not critical.Where this fit was exceeded by the extrapolation from the MIRspectrum, we connected these two at the intersection point. Thefinal fits are shown as blue lines in Fig. 3.Since the PACS observations are not fully sampled and theIRS spectra were extracted just using the geometrical area, thedi ff erence of the PSF could a ff ect the resulting fit of the spectralenergy distributions (SEDs). However, the uncertainty decreasesfor larger regions (average over several spaxels) and it does notchange the general trend of ǫ pe (Sect. 4; Fig. 5). In NGC 7023 NW, the MIPS 24 µ m flux is available but wasnot used to scale the IRS spectra because the spectral range of theIRS does not cover the MIPS 24 µ m band. Figure 3 shows thatthe MIPS 24 µ m flux matches the assumed constant continuumflux.We translated the obtained TIR into the impinging FUV flux.By assuming that the entire FUV energy is absorbed by dustgrains and reradiated as infrared flux, we estimate G (TIR) as4 π × TIR / (1 . × − ) (Table 4). The systematic uncertaintyof TIR is ∼ ff erence of the beamsizes at di ff erent wavelengths (Sect. 2.2). In this formula we as-sumed a spherical geometry. When we resolve an edge-on PDRwith a shorter thickness compared to the length of the line-of-sight, G (TIR) is overestimated (Meixner et al. 1992). Anotheruncertainty of G (TIR) is the contribution of photons with anenergy outside of 6 eV < h ν < < G (TIR).The SED of a B9 star by Castelli & Kurucz (2004) indicates thatonly 1 / / < h ν < G (TIR) as an upper limit. Detailed compar-isons of G (TIR) with previous studies and other diagnostics aredescribed in Appendix B, and the final G estimates, which areused in the following analysis, are also listed in Table 4.
7. Okada et al.: Probing the role of PAHs in the photoelectric heating within PDRs
Table 4.
TIR and G (TIR) and the parameters used to estimate the charging parameter γ = G T / / n e . The uncertainty of TIR and G (TIR) is ∼
15% (see text).
Object Sub TIR G (TIR) Adopted parameters for γ region [W m − sr − ] [Habing field] G n H [cm − ] T [K]Horsehead 1 2 . × −
210 100–210 (0 . × . × −
550 100–550 (0 . × . × −
470 100–470 (0 . × . × −
940 200–940 4 × –1 . × . × −
600 200–600 4 × –1 . × . × −
970 120–970 (0 . × . × −
900 120–900 (0 . × . × − . × (0 . × ±
100 430–4502 9 . × − . × (2 . . × (0 . × . × − . × (0 . . × (0 . × . × − . × (0 . . × (0 . × . × − . × (0 . . × (0 . × . × − . × (0 . . × (0 . × . × − . × (5 . . × (0 . × . × − . × (0 . . × (0 . × . × − . × (1 . . × (0 . × Horsehead - 2
Ced201 - 1 I n t en s i t y [ M Jy s r - ] NGC7023 E - 1
NGC7023 NW - 2 µ m]01000200030004000 Carina N - 1 µ m]01000200030004000 Mon R2 - 2
Fig. 4.
Fit of the MIR spectra using the PAHTAT procedure (Pilleri et al. 2012b) at the same positions as in Fig. 2, assuming themixed extinction model and R V = .
1. The black line shows the observed spectrum, the red line is the fitted spectrum, the blue linerepresents the continuum, green, orange, light-green and light-blue show the PAH , PAH + , PAH x , and eVSG components. To derive the contribution of PAH + in each region, we appliedthe PAHTAT (PAH Toulouse Astronomical Templates) proce-dure described in Pilleri et al. (2012b). This procedure fits MIRspectra using a minimal set of template spectra. The PAH-relatedtemplates comprise PAH , PAH + , larger ionized PAHs (named PAH x , see Joblin et al. 2008), and evaporating very small grains(eVSGs). In the MIR the PAH and PAH + templates are char-acterized by a strong di ff erence in the relative strength of their7.7 µ m and 11.3 µ m band features; PAH + has a stronger 7.7 µ mband. The PAH x population consists of large ionized PAHs,which was introduced to provide a better fit to planetary neb-
8. Okada et al.: Probing the role of PAHs in the photoelectric heating within PDRs
Table 5.
The fraction of each component relative to the total band emission in the PAH fit. The variation in each column describesthe range covered by the di ff erent assumptions in the fit. Object sub Fraction of each componentregion PAH PAH + PAH x eVSGHorsehead 1 1 .
00 0 .
00 0 .
00 0 .
002 0 .
39 0 .
00 0 .
00 0 .
613 0 .
42 0 .
00 0 .
00 0 . . .
25 0 . .
44 0 . .
14 0 . .
342 0 . .
22 0 . .
25 0 . .
10 0 . . . .
30 0 . .
15 0 . .
07 0 . .
572 0 . .
30 0 . .
15 0 . .
08 0 . . .
13 0 .
86 0 .
00 0 .
002 0 . .
61 0 . .
27 0 . .
05 0 . .
153 0 . .
60 0 .
03 0 . .
01 0 . . . .
94 0 .
00 0 . .
13 0 . .
112 0 . .
00 0 .
00 0 . .
12 0 . .
033 0 . .
00 0 .
00 0 . .
12 0 . . . .
54 0 . .
35 0 . .
18 0 . .
252 0 . .
25 0 . .
67 0 . .
20 0 . .
203 0 . .
24 0 . .
67 0 . .
22 0 . . ula spectra: its MIR spectrum is similar to PAH + , but the7.7 µ m band is shifted to longer wavelengths (Joblin et al. 2008;Pilleri et al. 2012b). eVSGs are an intermediate population be-tween PAHs and classical VSGs. They present both a broad bandand continuum emission in the MIR range. The PAHTAT proce-dure also allows simultaneous fitting of the gaseous emissionlines, underlying continuum, and the extinction by dust grainsalong the line-of-sight. More details of the fitting tool can befound in Pilleri et al. (2012b). We assumed a linear continuumexcept for Mon R2, where a continuum of two slopes connectedat 10 µ m was adopted since big grains contribute significantly tothe MIR continuum because of the strong UV field. To explorethe parameter space of the fitting, we used all combinations ofthe following di ff erent assumptions: the dust extinction curveswith R V = . .
5, geometries in which the absorbing ma-terials are placed in the foreground or are fully mixed with theemitting materials, and since the presence of PAH x in PDRs withan intermediate UV radiation field strength is doubtful, we alsotested the results including or excluding the PAH x componentfrom the fit. The di ff erence between these di ff erent assumptionsis included into the uncertainty of the results, although they aretypically smaller than the errors estimated by the discrepancy be-tween the observed spectra and the model (see below). The fittedspectra are shown in Fig. 4, and the derived fraction of each PAHand eVSG contribution to the total band flux is shown in Table 5.In the following we define the fraction of positivelyionized PAHs by the integrated intensity ratio as f ( + ) = R I PAH + / ( R I PAH + R I PAH + ), where the integration is made overthe wavelength from 5 . µ m to 14 µ m. The uncertainty of thisfraction can be attributed to two di ff erent e ff ects: (1) fitting errorsand (2) systematic errors in the assumptions within PAHTAT,i.e., due to intrinsic uncertainties in the templates. To estimatethe first contribution, we integrated the absolute discrepancy ofthe observed spectra from the model. We assumed that this resid-ual consists of the uncertainty of the band emissions of smallgrains (PAH , PAH + , PAH x , and eVSGs) and the continuumemissions; R (residual) = ∆ (band) + ∆ (continuum), and the ra-tio of the uncertainty ∆ (band) / ∆ (continuum) is the same as theratio of the integrated flux R I band / R I continuum . Thus, the uncer-tainty of the integrated band emission is defined as ∆ (band) = R (residual) × R I band / ( R I band + R I continuum ). Then we attribute itto the uncertainty of the PAH + flux, i.e., the final uncertainty of f ( + ) is expressed as ∆ (band) / ( R I PAH + R I PAH + ). This is conser-vative, because it assumes that the entire uncertainty in the bandemission comes from that of PAH + .The second uncertainty in the obtained f ( + ) comes from thetemplates for PAH and PAH + . The templates are constructedbased on observational data and a mathematical blind signal sep-aration (BSS) method (Pilleri et al. 2012b). The spectral proper-ties of the BSS-extracted spectra, templates, and their assign-ment to PAH + and PAH have been discussed in Rapacioli et al.(2005), Bern´e et al. (2007), Joblin et al. (2008), and Bern´e et al.(2009). Recently, Rosenberg et al. (2011) compared the BSS ex-traction to the theoretical spectra with the density functional the-ory and showed good agreement, although this is for a spectralrange of 10–19.5 µ m. A precise determination of the uncertaintyis di ffi cult, however, given the nature of the extraction methodand our limited knowledge of the PAH populations that exist inspace. Using a Monte Carlo approach, Rosenberg et al. (2011)showed in their BSS extraction a 1 σ uncertainty of typically10% and up to 30% in some parts of the spectrum (see theirFig. 3). However, this uncertainty in the templates systematicallypropagates to the uncertainty of f ( + ) and does not change thetrend and our conclusion discussed below. Therefore, we presentour errorbars in the following figures based on the fitting errordescribed above.
4. Results and discussion ǫ pe and f ( + )Figure 5 shows the relation between the ǫ pe = ([O i ] 63 µ m + [O i ] 145 µ m + [C ii ] 158 µ m) / TIR and the fraction of positivelyionized PAHs f ( + ). Both ǫ pe and f ( + ) strongly vary among theregions; ǫ pe varies from 0.1% to 0,9%, and f ( + ) from 0 ( + ± f ( + ) show a low ǫ pe ,and all positions with a high ǫ pe show a low f ( + ). This trendsupports the theoretical expectation in which ǫ pe decreases whengrains are positively ionized, because the energy required to ejectelectrons from positively charged grains is higher than that fromneutral grains. Here we take the ionization of PAHs as a grossindicator of the positive charging of grains in general, which isgenerally quantified by the charging parameter ( γ ) as shown inSect. 4.2. Figure 5 directly compares of the observationally de-rived ǫ pe and f ( + ), independent of the use of PDR models to
9. Okada et al.: Probing the role of PAHs in the photoelectric heating within PDRs + / (PAH +PAH + )0.0000.0020.0040.0060.0080.010 ( [ O I] µ m + [ O I] µ m + [ C II] µ m ) / T I R HorseheadCed201NGC7023 ENGC7023 NWCarina NMon R2
Fig. 5. ǫ pe = ([O i ] 63 µ m + [O i ] 145 µ m + [C ii ] 158 µ m) / TIRversus the fraction of ionized PAHs ( f ( + )).quantify the physical conditions, over a wide range of the phys-ical properties of PDRs.Using ([O i ] 63 µ m + [O i ] 145 µ m + [C ii ] 158 µ m) / TIR asa tracer of ǫ pe contains several assumptions. We neglect the heat-ing by the collisional de-excitation of vibrationally excited H .The H de-excitation heating becomes a dominant heating pro-cess in a dense PDR with a low UV field (R¨ollig et al. 2006).Among our PDRs, only G and the upper value of the densityof the Horsehead are close to the regime where the contributionfrom the photoelectric heating and H heating is comparable.Those in other PDRs indicate that the photoelectric heating isdominant. We also neglect the cooling by H emission. This ispartly justified because the H emissions as a consequence ofthe excitation by the UV-pumping and the H formation do notcontribute to the estimate of the photoelectric heating e ffi ciency.Habart et al. (2011) showed intense H lines in the Horsehead(0–0 S(0) to S(3) and 1–0 S(1)), which are, as a sum, com-parable with the ([O i ] + [C ii ]) intensity. In NGC 7023 E andCed 201, the sum of the H pure rotational emissions S(0) toS(3) (Habart et al. 2011 for NGC 7023 E, and from the IRS spec-tra in the Spitzer archive for Ced 201) is ∼
80% and ∼
60% ofthe ([O i ] + [C ii ]) intensity, respectively. Neglecting these H emissions may underestimate ǫ pe by a factor of .
2. In otherregions, the H pure rotational emissions from the IRS spectrain the Spitzer archive show the intensity of .
20% of ([O i ] + [C ii ]).As mentioned in Sect. 2.2, the [O i ] 63 µ m emission is indi-cated to be optically thick in most targets. Therefore, the optical-depth-corrected [O i ] 63 µ m intensity is higher than the observedvalues. However, we observe the emission that leaves the cloud,and this contributes to the net cooling of the cloud. This justifiesthe direct use of the observed ([O i ] + [C ii ]) to express the gascooling, except if the foreground absorption is significant, whichis unlikely for our PDRs, as mentioned above.As discussed for G (TIR) (Sect. 3.1), photons with an energyof < ǫ pe with the observed TIR provides alower limit. For the exciting source(s) of spectral types earlierthan early B, the e ff ect is not significant in terms of the abso-lute factor and the variation between di ff erent spectral types. ForCed 201, which has an exciting source of B9.5, we may under-estimate ǫ pe by a factor of a few. This uncertainty, however, doesnot change the overall trend in Fig. 5 and our conclusions. γ ( [ O I] µ m + [ O I] µ m + [ C II] µ m ) / T I R HorseheadCed201NGC7023 ENGC7023 NWCarina NMon R2
Fig. 6. ǫ pe = ([O i ] 63 µ m + [O i ] 145 µ m + [C ii ] 158 µ m) / TIRversus the charging parameter ( γ = G T / / n e ). Solid line isthe theoretical calculation from Bakes & Tielens (1994), dashedlines are that from Weingartner & Draine (2001b) for R V = . b C = . . R V = . b C = . . γ ε PA H = I li ne / ( I PA H + I li ne ) HorseheadCed201NGC7023 ENGC7023 NWCarina NMon R2
Fig. 7. ǫ PAH = ([O i ] 63 µ m + [O i ] 145 µ m + [C ii ] 158 µ m) / (PAH + [O i ] 63 µ m + [O i ] 145 µ m + [C ii ] 158 µ m) versus thecharging parameter ( γ ). The black curve depicts our calculationsusing 25 di ff erent size distributions from Weingartner & Draine(2001a), which looks degenerated (see text).Our definition of ǫ pe assumes a spherical geometry, whereboth line and continuum emissions radiate isotropically. If oneassumes an edge-on geometry, where the continuum emissioncan escape through the molecular side while the line emissions,especially [O i ] 63 µ m, becomes completely optically thick andcan be emitted only from the front side, ǫ pe should be divided by2 (Tielens 2005). ǫ pe with theory Theoretically, ǫ pe can be expressed as a function of the charg-ing parameter, defined as γ = G T / / n e , where T is thegas temperature and n e is the electron density. γ is propor-tional to the ratio of the ionization and recombination rate
10. Okada et al.: Probing the role of PAHs in the photoelectric heating within PDRs γ PA H + / ( PA H + PA H + ) HorseheadCed201NGC7023 ENGC7023 NWCarina NMon R2
Fig. 8.
Fraction of PAH + ( f ( + )) versus the charging parameter( γ ).(Bakes & Tielens 1994). We calculated γ from G , n H , and T listed in Table 4. They were carefully estimated in each subre-gion (Appendix B) to be independent of the existing PDR mod-els. We estimated n e by assuming that most electrons are pro-vided by carbon ionization and all carbon atoms are singly ion-ized, i.e., n e = x (C) n H , where x (C) = . × − (Sofia et al.2004) is the elemental carbon abundance. In Fig. 6 we showthe relation between ǫ pe and γ . There is a trend that ǫ pe de-creases when γ increases. Bakes & Tielens (1994) theoreticallyestimated ǫ pe based on the physics of the photoelectric e ff ecton dust grains. Weingartner & Draine (2001b) modeled ǫ pe withsome improved physical parameters and various size distribu-tions of dust grains in Weingartner & Draine (2001a). Dashedand solid lines in Fig. 6 show some of their estimates, represent-ing the strongest variation of ǫ pe depending on the dust size dis-tribution. Dense regions characterized by R v = . ffi ciency. The parameter b C is the totalC abundance in the log-normal populations of the size distribu-tion, and the model with a larger b C shows a stronger enhance-ment at the size of <
10Å (Weingartner & Draine 2001a). CaseA is the case without the constraint on the total grain volume(Weingartner & Draine 2001a). Although the observed trend that ǫ pe decreases when γ increases is reproduced by models, theobservation indicates a weaker dependence of ǫ pe on γ , and allmodels overestimate ǫ pe at low γ . In the following, we focus onthe photoelectric heating only on PAHs.When we consider ǫ pe on PAHs, ([O i ] 63 µ m + [O i ] 145 µ m + [C ii ] 158 µ m) / TIR is not an appropriate definition, becausethe TIR expresses the total energy emitted by all dust grains.The total energy that PAHs absorb is converted into either MIRAIB emission or cooling line emission through the photoelec-tric e ff ect on PAHs. A fraction of the energy (generally taken tobe 0 .
5) remains behind as electronic excitation energy after thephotoelectric e ff ect (Tielens 2005), which also results in MIRAIB emission. Therefore ǫ PAH is defined as ([O i ] 63 µ m + [O i ]145 µ m + [C ii ] 158 µ m) / (PAH band emission + [O i ] 63 µ m + [O i ] 145 µ m + [C ii ] 158 µ m). The trend of ǫ PAH against γ (Fig. 7)is very similar to that reported in Fig. 6. Following Tielens (2005), we computed a simple theoreti-cal estimate of ǫ PAH . For a given PAH containing the number ofcarbon atoms of N c , ǫ PAH , N c = f ( Z = h ν − IPh ν ! , (1)where f ( Z =
0) is the neutral fraction and IP is the ionizationpotential. With a typical photon energy of 10 eV and an ioniza-tion potential of 7 eV, this equation becomes ǫ PAH , N c = . f ( Z = . (2)The neutral fraction is given by f ( Z = = (1 + γ ) − , (3)where γ is the ratio of the ionization rate over the recombinationrate, which for small PAHs is given by γ = . × − N / c γ. (4)We substitute Eq.(4) into Eq.(3) and then into Eq.(2), and in-tegrate over the size distribution from Weingartner & Draine(2001a) up to N c = ǫ pe , in which a di ff erentsize distribution gives a large di ff erence (Fig. 6), ǫ PAH is insensi-tive to the adopted size distribution. In Fig. 7, the results with 25di ff erent size distributions in Weingartner & Draine (2001a) areplotted, but they look degenerated. Although the match betweenthe theoretical estimates and the observed values is not excel-lent, it is much better than in Fig. 6. This result suggests that thephotoelectric heating is dominated by PAHs.In Fig. 8, the relation between the fraction of PAH + ( f ( + ))and γ is shown. PAHs are almost neutral at γ < and almostfully ionized at γ > , and there is a transition in-between.Since γ is defined by the environment properties and it is pro-portional to the ratio of the grain ionization and recombinationrate, Fig. 8 confirms that the fraction of PAH + is a good indi-cator for the positive charging of grains in general. This trendis also consistent with the correlation between the intensity ratioof PAH(6.2 µ m) / PAH(11.3 µ m) and γ presented in Galliano et al.(2008).
5. Summary
We analyzed
Herschel / PACS and
Spitzer / IRS spectroscopic ob-servations in six PDRs and showed that the photoelectric heatinge ffi ciency ( ǫ pe ) is lower in regions with a large fraction of posi-tively ionized PAHs ( f ( + )). Based on examining the photoelec-tric heating e ffi ciency on PAHs, we found a dominant contribu-tion of PAHs to the photoelectric heating.Our PDR sample covers a wide range of physical conditions(100 . G . ) and provides a good test case for investigat-ing the relation between ǫ pe and the charge state of PAHs. Weestimated ǫ pe as ([O i ] 63 µ m + [O i ] 145 µ m + [C ii ] 158 µ m) / TIR. ǫ pe varies in our PDRs between 0.1 and 0.9%. f ( + ) wasobtained from a fit of the MIR spectra with a set of templatespectra representing PAH-related species and varied from 0%( + ± f ( + ) show alow ǫ pe , and all positions with a high ǫ pe show a low f ( + ). Thistrend supports a scenario in which a positive grain charge re-sults in a decreased heating e ffi ciency. The theoretical estimateof ǫ pe shows a stronger dependence on the charging parameter( γ ) than the observed ǫ pe reported in this study, and overesti-mates ǫ pe at low γ . The photoelectric heating e ffi ciency on PAHs, ǫ PAH = ([O i ] 63 µ m + [O i ] 145 µ m + [C ii ] 158 µ m) / (PAH band
11. Okada et al.: Probing the role of PAHs in the photoelectric heating within PDRs emission + [O i ] 63 µ m + [O i ] 145 µ m + [C ii ] 158 µ m), showsa much better match between the observations and the theoreti-cal estimates, indicating a dominant contribution of PAHs on thephotoelectric heating. PDR models that fully account for the rel-ative contribution of di ff erent PAH and eVSGs populations areneeded. Velocity-resolved observations of [O i ] 63 µ m in the fu-ture with for instance SOFIA / upGREAT (Heyminck et al. 2012)will enable us to investigate the foreground absorption and theoptical depth e ff ect in detail (c.f. Boreiko & Betz 1996). Acknowledgements.
PACS has been developed by a consortium of insti-tutes led by MPE (Germany) and including UVIE (Austria); KU Leuven,CSL, IMEC (Belgium); CEA, LAM (France); MPIA (Germany); INAF-IFSI / OAA / OAP / OAT, LENS, SISSA (Italy); IAC (Spain). This developmenthas been supported by the funding agencies BMVIT (Austria), ESA-PRODEX(Belgium), CEA / CNES (France), DLR (Germany), ASI / INAF (Italy), andCICYT / MCYT (Spain). HIFI has been designed and built by a consor-tium of institutes and university departments from across Europe, Canadaand the United States under the leadership of SRON Netherlands Institutefor Space Research, Groningen, The Netherlands and with major contribu-tions from Germany, France and the US. Consortium members are: Canada:CSA, U.Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA,MPIfR, MPS; Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF, OsservatorioAstrofisico di Arcetri-INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK;Spain: Observatorio Astron´omico Nacional (IGN), Centro de Astrobiolog´ıa(CSIC-INTA). Sweden: Chalmers University of Technology - MC2, RSS &GARD; Onsala Space Observatory; Swedish National Space Board, StockholmUniversity - Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA:Caltech, JPL, NHSC.We thank the
Herschel helpdesk and the
Spitzer helpdesk for their supportin analyzing data. We thank the referee for useful suggestions that greatly im-proved the paper. Part of this work was supported by the German
Deutsche For-schungsgemeinschaft, DFG , project number SFB956 C1, by the Spanish pro-gram CONSOLIDER INGENIO 2010, under grant CSD2009-00038 MolecularAstrophysics: The Herschel and ALMA Era (ASTROMOL), by CNES, by aRam´on y Cajal research contract, and by the Spanish MICINN through grantsAYA2009-07304 and CSD2009-00038.
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12. Okada et al.: Probing the role of PAHs in the photoelectric heating within PDRs beam size = 9.0" r = . * ( F ( s p . ) / F ( s p . ) + F ( s p . ) / F ( s p . )) (a) beam size = 11.5" F ( " bea m ) / F ( . " bea m ) @ s pa x e l (c) Fig. A.1.
Black curves show the simulated r , defined asEq. (A.1), as a function of the position of the point source for thebeam size of (a) 9 ′′ and (b) 11.5 ′′ . The relative contribution ofthe extended source compared to the height of the point source at63 µ m is 0 .
0, 0 .
02, 0 .
1, 0 .
3, and 0 . ± σ (blue lines in Fig. A.2). (c) The flux ratio of 9 ′′ beam and 11.5 ′′ beam at spaxel 2 as a function of the position of the point sourcewith the same model. Shaded areas show ranges when r in (a) or(b) matches the observed value. µ m0.0 0.5 1.0 1.5 2.0 2.5 3.0Averaged flux ratio of adjacent spaxels (r)0102030405060 [CII] 158 µ m Fig. A.2.
Histograms of the observed r for [O i ] 63 µ m and [C ii ]158 µ m. Red lines show the fit with a Gaussian profile, and bluelines show the center and ± σ of the fitted Gaussian. Appendix A: Relative flux uncertainty between63 µ m and 158 µ m caused by the difference ofthe beam size We estimate the uncertainty of the relative flux in one spaxel atdi ff erent wavelengths that arises from the di ff erent PSF and theunknown spatial structure of the sources by considering the com-bination of two extremes, pure point sources and flat extendedemission, and by comparing the contrast between neighboringspaxels for these combinations with the contrast actually mea-sured in the observations.We consider three spaxels 1–3 along a line whose centersare located at (0 . . . . ′′ ). We put a point source represented by a Gaussianprofile with the FWHM of 9 ′′ and 11.5 ′′ for 63 µ m and 158 µ m,respectively, centered at various positions between (0 . . r = F (spaxel 2) F (spaxel 1) + F (spaxel 2) F (spaxel 3) ! , (A.1)where F is the flux falling in one spaxel. r is lowest when thecenter of the point source is located at (1 . r is 7 . . µ mand 158 µ m, respectively (upper lines in Figs. A.1a,b). On theother hand, we derive the observed r of the [O i ] 63 µ m and [C ii ]158 µ m line intensities from the original 5 × r is well repre-sented by a Gaussian with a small number of outliers at high r values (Fig. A.2), and the fitted Gaussian has a center of 1 . σ of 0 . . µ m and 158 µ m. Even theupper 3 σ (1 . .
4) is smaller compared to the above modelwith a point source, indicating that the contribution of extendedemission is significant in our regions. Therefore, we modify themodel by adding a constant extended component to all spaxels,and estimate r , which is shown as black lines in Figs. A.1a andb with varying constant values. Then we estimate F(spaxel 2,63 µ m) / F(spaxel 2, 158 µ m), which expresses the uncertainty ofthe relative flux at 63 µ m and 158 µ m, for each model (blacklines in Fig. A.1c). The observed r with 3 σ errors are shown asshaded areas in Figs. A.1a and b, and the corresponding rangesare shaded in Fig. A.1c, which is numerically calculated by sur-veying the constant emission value and determining the positionrange of the point source, where r is in the observed range foreach constant emission value. It reaches to within 15% of unity.Therefore, we take 15% as the uncertainty of the relative fluxbetween 63 µ m and 158 µ m caused by the beam size di ff erence. Appendix B: Estimate of the charging parameter
Here we describe the estimate of G , the gas density n H , and thegas temperature T to be used for deriving the charging parameter γ in individual subregions (Table 4). The rotational temperaturederived from the low- J H pure rotational emissions up to S(4)is adopted as T in all regions. G and n H a ff ect γ linearly andwe estimate them independent of an existing PDR model. Theseproperties are representative of the warm PDR surface wheremost of the H rotational emission and PAH emission comesfrom. B.1. Horsehead
Habart et al. (2005) estimated G (star) ∼
100 at the PDR inter-face based on the e ff ective temperature of the exciting star andassuming geometrical dilution at the projected distance. Underthis assumption, our three subregions are close enough to eachother to make no di ff erence for the G (star) between them. G derived from TIR, G (TIR), is listed in Table 4. We adopt both G (star) and G (TIR) for the possible range in individual sub-regions. For the gas temperature T , we use the range of the es-timate from the intensity ratio of H S(2) / S(0), S(3) / S(1), andS(4) / S(2) in Habart et al. (2011).Abergel et al. (2003) showed that the lower limit of the den-sity behind the filament is ∼ × cm − from the infraredbrightness profile. Habart et al. (2005) modeled the spatial dis-tribution of H , PAH, CO and 1 mm continuum emissions us-ing a PDR model and suggested the density gradient from n H = cm − in the H emitting region to n H = × cm − in theinner cold molecular layers. Since the region we are interested
13. Okada et al.: Probing the role of PAHs in the photoelectric heating within PDRs in is the H -emitting gas, we still include the n H value in theH -emitting region even for subregion 3, i.e. we adopt n H = (0 . × cm − for subregions 2 and 3. n H ∼ cm − at the inter-face is also confirmed by the pressure equilibrium with the ion-ized gas. In the ionized gas, n e is estimated to be 100–350 cm − with T e = n e T e = n H T , gives n H = (0 . × cm − .Therefore, we adopt n H = (0 . × cm − for subregion 1. B.2. Ced 201
Kemper et al. (1999) estimated G =
200 by comparing the ob-servations with a PDR model in units of the average interstel-lar radiation between 2 eV and 13.6 eV. On the other hand,Young Owl et al. (2002) derived G =
300 by the infrared con-tinuum emission, with a geometry correction factor of 2. Weadopt G =
200 as a lower limit and G (FIR) as an upperlimit for each subregion. The fraction of eVSGs determined byPAHTAT is correlated to G (Pilleri et al. 2012b) and the adopted G for each subregion is also consistent with it.For n H , Young Owl et al. (2002) derived n H = × cm − from the lower limit of [C ii ] 158 µ m / [O i ] 63 µ m using a PDRmodel. Kemper et al. (1999) estimated n H of (5 ± × cm − bysimple excitation models of CO and CO emissions. They alsomodeled the emission of CO, C, C + , CS, and HCO + using a PDRmodel, which suggests n H of 1 . × cm − . We use all theseranges for the two subregions. T is estimated as ∼
330 K fromthe ratio of H S(1) and S(3) emissions (Kemper et al. 1999).
B.3. NGC 7023 E
Pilleri et al. (2012b) modeled the spatial distribution of the MIRAIB emission, assuming a spherical shell geometry and the en-ergy balance taking into account, and derived the spatial varia-tion of n H and G along a cut in NGC 7023 E. At the PDR front, G is calculated to be 250 based on the spectral type of the star,geometrical dilution and assuming an attenuation of A v = . . × cm − . Around the MIR AIB emissionpeak, which corresponds to the regions in this study, G is esti-mated to be 120–170 and n H = (0 . × cm − . On the otherhand, G (TIR) is 970 and 900 for two subregions. We adopt 120as a lower limit of G , and the corresponding G (TIR) as an up-per limit for each subregion, and n H = (0 . × cm − for bothsubregions. For the gas temperature T , we use 258–370 K fromthe intensity ratio of H S(2) / S(0) and S(3) / S(1) (Habart et al.2011).
B.4. NGC 7023 NW
The same modeling as for NGC 7023 E was made by Pilleri et al.(2012b). Our three subregions are not exactly on their cut be-cause we chose the subregions to maximize the variation ofthe PAH + fraction. Subregion 2 is located at the PDR inter-face. Pilleri et al. (2012b) estimate G (star) = G (TIR) = G = G (star) by theprojected distance gives G = , higher than G (TIR), pos-sibly because either the real distance to the star is larger thanthe projected distance, and / or the assumption that all UV radi-ation is absorbed and converted into the IR emission underesti-mates G in such a cavity because many UV photons pass the region. Nevertheless, we conservatively cover both values, i.e., G = (0 . × , for subregion 1. The adopted G in sub-regions 1 and 2 is also consistent with the correlation betweenthe fraction of eVSGs and G from Pilleri et al. (2012b). Forsubregion 3, we use this correlation to derive G (see Eq.(5) inPilleri et al. 2012b), which results in G = G (TIR) = G = . × cm − to 2 × cm − within ∼ ′′ .Fuente et al. (1996) showed high-density filaments of a few10 cm − based on HCO + observations. Therefore, we adopt10 –2 × cm − for subregion 2, and (0 . × cm − forsubregion 3. Bern´e & Tielens (2012) examined n H in the cavityin detail from several diagnostics and suggested 150 ±
100 cm − .We take this as n H for our subregion 1. For the gas temperature T , we use 430–450 K from the intensity ratio of H S(3) / S(1)and S(4) / S(2) (Fuente et al. 2000).
B.5. Carina N
We estimate G (star) by computing the contribution from all OBstars of Trumpler 14 listed in Smith (2006), which gives (7–8) × for the three subregions. On the other hand, G (TIR)is (1 . . × . The point-source catalog of the Wide FieldInfrared Survey Explorer (WISE; Wright et al. 2010) containsseveral sources with a flux stronger at 4.6 µ m than at 3.4 µ mclose to our region, which indicates a contribution from embed-ded protostars. We consider both G (star) and G (TIR) as a pos-sible range in individual subregions.Kramer et al. (2008) modeled CO and [C i ] emissions inthe Carina N region and suggested n H of 2 × cm − . Thisis consistent with a pressure equilibrium with the H ii region.Using the emission lines ratio from ions with similar ioniza-tion potential observed with IRS, we can estimate n e assum-ing the elemental solar abundance and isothermal thin emission.[Ne ii ] 12.8 µ m / [S iii ] 18.7 µ m, [P iii ] 17.9 µ m / [Ne ii ] 12.8 µ m,and [Ar iii ] 8.99 µ m / [S iii ] 18.7 µ m give n e of 10 , 2 × , and9 × –10 cm − , respectively, when T e = K. Since the gastemperature T in PDRs is derived as T = emission lines, n H can be estimated as 3 × –10 cm − . We use2 × –10 cm − for all subregions. B.6. Mon R2
We calculate G (star) from the contribution of IRS1-4 (seeFig. 1) with the luminosity listed in Henning et al. (1992), as(5 . . × , 10 , and (1 . . × in the three subregions.While G = × is used to characterize the radiation fieldat the ionization front (Rizzo et al. 2003), our subregions do notcover the nearest regions of infrared sources to avoid unresolvedcomplex spatial distributions. G (star) matches G (TIR) well.We consider both G (star) and G (TIR) as a possible range inindividual subregions.Rizzo et al. (2005) and Pilleri et al. (2013) performed anLVG analysis with C H and C H at several positions in Mon R2and derived n H > cm − . Ginard et al. (2012) showed similarresults from di ff erent molecules; the molecular hydrogen den-sity of a few 10 cm − to 10 cm − . These molecules tracesthe cold molecular gas with a temperature of T ∼
45 K(Giannakopoulou et al. 1997). On the other hand, Bern´e et al.(2009) derived n H of (0 . × from PDR model calculationsusing H rotational emission lines, and the temperature derivedfrom the H rotational emission lines is T =
14. Okada et al.: Probing the role of PAHs in the photoelectric heating within PDRs n H and T are consistent with a pressure equilibrium with thecold molecular gas. In the H ii region, the emission measure of1 . × pc cm − and the geometrical mean diameter of 23.7 ′′ (Takahashi et al. 2000; Wood & Churchwell 1989) give an esti-mate of n e = . × cm − . With T e = T = n H = (4–7) × cm − , which is also compatible with the estimatefrom H rotational emissions, although the pressure equilibriumis an inadequate assumption in an UCH ii like Mon R2. We adoptthe estimate range from H emissions, i.e., (0 . × for allsubregions.for allsubregions.