J \ 3 ] 2 and J \ 9 ] 8 lines of N O at 75 and 226 GHz, respectively, were also detected at Sgr B2 (N). Combined with the J \ 6 ] 5 2

Transcription

1 THE ASTROPHYSICAL JOURNAL, 561:44È5, 001 November 1 ( 001. The American Astronomical Society. All rights reserved. Printed in U.S.A. EVALUATING THE N/O CHEMICAL NETWORK: THE DISTRIBUTION OF N O AND NO IN THE SAGITTARIUS B COMPLEX D. T. HALFEN, A.J.APPONI, AND L. M. ZIURYS Department of Astronomy, Department of Chemistry, and Steward Observatory, University of Arizona, 9 North Cherry Avenue Tucson, AZ Received 001 April 0; accepted 001 June 18 ABSTRACT Mapping observations of the J \ 6 ] 5 transition of N O and the % `, J \ / ] 1/ line of NO in the mm band toward the core region of the Sagittarius B complex have 1@ been carried out using the Kitt Peak 1 m telescope. Emission from NO was found to be extended over a ] 5@ in size that includes the Sgr B (N), Sgr B (M), and Sgr B (OH) positions, very similar to the distribution found for HNO. In contrast, N O emission was conðned to a source approximately 1@ in extent, slightly elongated in the north-south direction and centered on the Sgr B (N) core. A virtually identical distribution was found for the J \ 14 ] 14 E transition of methanol, which lies 55 K above ground state and samples very hot gas. Kq Excitation 0 ~1 conditions are favorable for the J \ 6 ] 5 line of N O over the entire NO region; hence, the conðned nature of this species is a result of chemistry. The J \ ] and J \ 9 ] 8 lines of N O at 75 and 6 GHz, respectively, were also detected at Sgr B (N). Combined with the J \ 6 ] 5 data, these transitions indicate a column density for this molecule of N D 1.5 ] 1015 cm~ at this position and an abundance of f (N O/H ) D 1.5 ] 10~9. This fractional abundance tot is almost orders of magnitude higher than predicted by low-temperature chemical models. The N O observations suggest that this molecule is preferentially formed in high-temperature gas; a likely mechanism is the neutral-neutral reaction NO ] NH ] N O ] H, which has an appreciable rate only at T [ 15 K. The column density of NO found over the Sgr B cloud was N D (0.8È1.5) ] 1016 cm~, tot corresponding to a fractional abundance of f (NO/H ) D (0.8È1.5) ] 10~8, which is about 1 order of magnitude less than model predictions. The similar distributions of NO and HNO suggest a chemical connection. It is likely that the major route to HNO is from NO via the ion-molecule process NO ] HNO`]NO` ] HNO, which occurs readily at low temperatures. The NO molecule thus appears to be the main precursor species in the N/O chemical network. Subject headings: astrochemistry È Galaxy: center È ISM: abundances È ISM: molecules È molecular processes È radio lines: ISM 1. INTRODUCTION Predicting chemical abundances in molecular clouds is problematic. The presence of young stars, outñows, and small-scale clumping in such objects brings a high level of complexity to the problem such that simple, quiescent cloud models are no longer applicable. One such complicated source is the Sagittarius B cloud. The di use envelope of Sgr B extends over 40 pc in diameter, while the denser gas covers a region D5 ] 10 pc in size, elongated in the northsouth direction, and contains D106 M of material (see, e.g., Goldsmith et al. 1987; Lis et al. 199). _ Embedded in this denser gas are at least three compact sources, Sgr B (N), Sgr B (M), and Sgr B (S), which are thought to have densities of n(h ) D 107 cm~ (see, e.g., Vogel, Genzel, & Palmer 1987; Lis & Goldsmith 1991; Lis et al. 199). These cores contain ultracompact H II regions as well as H O and OH masers (see, e.g., Reid et al. 1988; Gaume & Claussen 1990). Hence, massive star formation is likely occurring in these sources. The chemistry of Sgr B is naturally quite complex and spatially dependent. Over 50 di erent molecules have been detected toward this cloud, as documented in spectral surveys of this region (Cummins, Linke, & Thaddeus 1986; Turner 1989; Sutton et al. 1991). Many of these surveys focused on the strong OH maser position Sgr B (OH), while others examined the compact cores. For example, a very recent band scan in the 1 mm region by Nummelin et al. (000), done with the Swedish-ESO Submillimeter Telescope (h surveyed three separate positions in the 44 cloud [Sgr B (N), Sgr B (M), and Sgr B (NW)] and found interesting chemical di erences between the northern and middle sources. Such di erentiation has also been investigated by Snyder and collaborators, who discovered that the distribution of the large organic molecules VyCN, EtCN, and HCOOCH was conðned to the Sgr B (N) position (Miao et al. 1995; Miao & Snyder 1997; Liu & Snyder 1999). No emission from these three species was found at Sgr B (M). Snyder thus called this region the Large Molecule Heimat ÏÏ or Sgr B (LMH). Chemical variation has also been found in nitrogencontaining molecules. Ammonia, for example, has an anomalously high abundance of f (NH /H ) Z 10~5 in the Sgr B (N) and Sgr B (M) cores (Hu ttemeister et al. 199). In fact, almost all the atomic nitrogen is contained in NH in these two regions. In contrast, HNCO emission appears to peak at an unusual position north of Sgr B (M), called Sgr B (N) (Wilson et al. 1996; Minh et al. 1998). There are no known embedded sources, UCH II regions, or gas density peaks at Sgr B (N). Wilson et al. proposed this emission peak to be the northern nitrogen core.ïï In addition, the Sgr B complex is the only source in which the three species containing an N-O bond have been detected, namely, NO, HNO, and N O (Ziurys et al. 1994a; Ziurys, Hollis, & Snyder 1994b). The complex structure of the Sgr B cloud is illustrated in Figure 1, which is a qualitative picture showing the positions of Sgr B (N), Sgr B (M), and Sgr B (OH), marked by stars, and that of Sgr B (N), indicated by an open

2 N/O CHEMICAL NETWORK: N O AND NO IN SGR B 45 FIG. 1.ÈSgr B complex showing the positions of Sgr B (N), Sgr B (M), and Sgr B (OH), indicated by stars, as well as the major H II regions ( Ðlled circles). The solid line indicates the outer extent of the 1 mm continuum emission (Goldsmith et al. 1987). Molecular emission shows distinctly di erent peaks, depending on the chemical species. The dashed contours represent the N \ ] transition of SO, which has a maximum at Sgr B (M) but also J extends 1 over Sgr B (N) (Goldsmith et al. 1987). The 10 ] 9 line of EtCN, on the other hand, is centered exclusively on Sgr 1,10 B (N) 1,9 (Liu & Snyder 1999), while the 4 ] transition of HNCO peaks at the N position (Minh et al. 1998), 04 the location 0 of the so-called northern nitrogen core (Wilson et al. 1996). The (0, 0) position on this map is Sgr B (OH): a \ 17h44m11s.0, d (B ). circle. (1@ corresponds to.6 pc at a distance of 8.5 kpc). Sgr B (S) is indicated by the single H II region directly south of Sgr B (OH). As the Ðgure shows, the 1 mm continuum encompasses the N, S, and M positions (from Goldsmith et al. 1987), where H II regions also exist, indicated by Ðlled circles. Varying molecular distributions are shown by emission contours of SO (N \ ] ), EtCN (J \ 10 ] 9 ), and HNCO J (J \ 1 4 ] ). SO, Ka,Kc for example, 1,10 has 1,9 a maximum at the Ka,Kc middle 04 position 0 but is strong toward Sgr B (N) as well (Lis & Goldsmith 1991). EtCN, on the other hand, exclusively exists at Sgr B (N) (Liu & Snyder 1999), while HNCO shows the unusual peak at Sgr B (N). The presence of several compounds contains an N-O bond, the large abundances of NH, and the uncharacteristic distribution of HNCO makes Sgr B a prime source for investigating nitrogen chemistry, in particular the N/O chemical network. Kuan & Snyder (1994) initiated the study of the N/O chemistry in this cloud by mapping the 1 ] 0 transition of HNO across Sgr B using the Kitt Peak 1 m telescope and the Berkeley-Illinois-Maryland Array. Although HNO was found to be extended over a several arcminute region, these authors found Ðve major concentrations of this molecule near the northern, middle, and southern cores. The emission of HNO seemed slightly o set from the core positions, however, suggesting chemical processing in these regions. Surveys of HNO and NO (Ziurys et al. 1991, 1994b) showed that these two species are present in a virtually identical set of sources. As a result, Ziurys et al. (1994b) concluded that their chemistry was likely to be related. In Sgr B, it probably involves N O as well. Therefore, to complement the work of Kuan & Snyder (1994), we have mapped the distribution of NO and N O in the Sgr B complex using the Kitt Peak 1 m telescope. The J \ 6 ] 5 transition of N O and the % `, J \ / ] 1/ hyperðne components of NO were measured, 1@ which both occur near 150 GHz. In addition, a high-energy transition of methanol was mapped. In this paper we present these results and compare them with previous HNO and HNCO studies. We also compare our observed abundances with those of recent chemical models and explain the varying spatial distributions in terms of the N/O chemical network.. OBSERVATIONS The data were taken during 1995 February and 001 January using the former NRAO1 1 m telescope at Kitt Peak, Arizona. The receivers used were dual-channel, cooled SIS mixers covering the 1,, and mm bands. Each mixer was operated in single-sideband mode with image rejection of Z0 db. The back ends used were 56 channel Ðlter banks of 500 khz and 1 MHz resolution operating in parallel mode ( ] 18). The temperature scale was determined by the chopper wheel method, corrected for forward spillover losses, and is given as T *. The radiation tem- R perature T, assuming the source Ðlls only the main beam, is R then T \ T */g. R R c Maps were made of the J \ 6 ] 5 line of N O (X 1&`) at 150,75.0 MHz and of the % `, J \ / ] 1/ transition of 1@ NO (X% ) at 150,176. MHz. These lines were mapped on r a5] 11 grid with 0A spacing in right ascension and declination, o set from the (0, 0) position at Sgr B (OH) (a \ 17h44m11s.0, d [B1950.0]). The data were taken in position-switching mode with the o position 0@ west in azimuth. The beam size of the 1 m telescope at 150 GHz was 4A such that the maps were oversampled and g \ In addition, the J \ ] and J \ 9 ] 8 tran- sitions c of N O at 75,69. and 6,094.0 MHz, respectively, were measured at a single position near Sgr B (N) (a \ 17h44m11s.0, d \[8 1@0@@ [B1950.0]; the N O peak). The J \ ] 1 line of NO` at 8,8. MHz (Bowman, Herbst, & DeLucia 198) and the main hyperðne component of the J \ 5 ] 6 transition of the NO radical at 70,589.7 Ka,Kc MHz (Baron, Godfrey, & Harris 1974) were also searched for toward one position close to the N O peak (a \ 17h44m11s.0, d \[8 1@0@@ [B1950.0]). Beam efficiencies and beam sizes at these other frequencies are given in Table 1.. RESULTS A summary of the observations for the N/O species at the Sgr B (N) and (M) positions is given in Table 1 as well as 1 NRAO is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation.

3 46 HALFEN, APPONI, & ZIURYS Vol. 561 TABLE 1 OBSERVATIONS OF N/O SPECIES AND CH OH AT SELECTED POSITIONSa SGR B (N) SGR B (M) l h b T R * V LSR *V 1@ T R * V LSR *V 1@ MOLECULE TRANSITION (MHZ) (arcsec) g c (K) (km s~1) (km s~1) (K) (km s~1) (km s~1) NO... J \ /] 1/ F \ 5/] / ^ ^ ^ ^ ^ ^.9 F \ /] 1 / ^ ^ ^ ^ ^ ^ 6.0 F \ /] /b ^ ^..6 ^ ^ ^ ^ 6.0 F \ 1/] 1/b N O... J \ ] ^ ^ ^ J \ 6 ] ^ ^ ^ ^ ^ ^ 4.8 J \ 9 ] ^ ^ ^ NO`... J \ ] \ c 0c NO... N \ 5 ] Ka, Kc J \ 11/] 1/ F \ 1/ ] 15 / \ c 0c HNOd... J \ 1 ] ^ ^ ^ ^ 55.5 Ka, Kc CH OH... J \ 14 ] 14 E ^ ^ ^ ^ ^ ^ 5.4 Kq 0 ~1 a Source coordinates (epoch 1950) are Sgr B (N): 17h44m10s., [8 1@1A; Sgr B (M): 17h44m10s., All errors are p. b Blended components. c Assumed value. d From Kuan & Snyder the methanol line data. This table lists the line parameters measured for NO, N O, and CH OH (T *, *V, and V ) R 1@ LSR and the upper limits to the line intensities obtained for NO` and NO. NO has a % electronic ground state, and therefore its r pure rotational spectrum contains the e ects of spin-orbit coupling (i.e., ) \ 1 and / ladders) and j-doubling (positive and negative parity components). We observed the lower energy spin-orbit component ) \ 1 and the positive parity j-doublet, hence, the designation % `. This tran- 1@ sition, however, consists of four strong hyperðne components, indicated by quantum number F, which arise from the nitrogen spin of I \ 1. As Table 1 illustrates, the four hyperðne (HF) lines were detected at the N and M positions. (Two of the HF components, F \ / ] / and F \ 1/ ] 1/, are separated in frequency by 7 MHz [14 km s~1] and hence are blended together to form one single line.) The main hyperðne component, F \ 5/ ] /, was in fact observed at every position studied in the Sgr B cloud, including Sgr B (S) and Sgr B (OH). The intensity of this component was T * D 0.4 K at the N and M positions, and the weaker HF lines R were roughly a factor of weaker, as expected in the optically thin, LTE case. The line widths for NO are *V D 1È0 km s~1 for unblended HF lines, with velocities in 1@ the range D60È64 km s~1 at the M position and D64È67 km s~1 at the N position. These values are typical for Sgr B (Hu ttemeister et al. 199; Kuan & Snyder 1994). Observed in the same bandpass as the NO lines was the J \ 14 ] 14 E transition of CH OH. The energy above Kq ground 0 state ~1 for the upper level of this transition is 55 K. Hence, it traces warm gas. This line was detected at Sgr B (N), with a weaker feature toward Sgr B (M). While the LSR velocity of CH OH at these positions is comparable to that of NO, the line width appears to be somewhat narrower [*V D 1. vs km s~1 at Sgr B (N)]. A decrease in line 1@ width was found in HC N as a function of energy above ground state (de Vicente et al. 000). Groundstate lines had *V D 0 km s~1, while those originating 1@ from the v or v modes exhibited line widths as narrow as km s~1. The narrower line width in CH OH may be explained by a similar phenomenon. A spectrum illustrating the di erence in NO and CH OH line proðles is shown in Figure. These data were obtained at the northern source. The positions and relative intensities of the NO HF components are shown by lines underneath the spectra and clearly indicate low optical depth in this transition. The CH OH line is the stronger feature on the right, which appears noticeably narrower than the nearby NO transitions. There may be a second velocity component present in the main HF component (F \ 5/ ] /) of NO near V D 90 km s~1. A higher velocity component near LSR 85 km s~1 has been found at Sgr B (N) in metastable NH, FIG..ÈSpectrum of the % `, J \ / ] 1/ transition of NO near 150 GHz, observed with the Kitt Peak 1@ 1 m telescope toward Sgr B (N). This transition consists of four main hyperðne components, labeled by quantum number F. The lines under the spectrum show their positions and LTE relative intensities. The strong line to the right of the NO features is the 14 ] 14 E transition of CH OH, which lies 55 K above ground state. Spectral 0 resolution ~1 is 1 MHz ( km s~1).

4 No. 1, 001 N/O CHEMICAL NETWORK: N O AND NO IN SGR B 47 which is thought to trace warm, low-density material. Another candidate for the identiðcation of this feature is dimethyl ether. The J \ 6 ] 5 transition of N O was detected at Sgr B (N) but not at Sgr B (S), and only a weak feature was present at the middle core (see Table 1), similar to the 14 ] 14 E line of CH OH. In addition, the J \ ] and 0 9 ] 8 ~1 lines of this molecule were detected at Sgr B (N). The J \, 6, and 9 levels lie 7., 5., and 54. K above ground state, respectively. Consequently, for the J \ 9 ] 8 transition of N O to be observed, this molecule must arise from moderately warm gas. The LSR velocities of N O are similar to those of NO and CH OH at the same positions, except for the J \ ] transition, whose velocity is a little higher than the others (V D 7 vs. 64È67 km s~1). However, this transition was LSR measured with a considerably larger beam size (85A vs. [4A), so it may be sampling a slight velocity gradient. Also, the J \ 9 ] 8 transition of N Ohas a narrower line width than the other NO and N O lines, more comparable to that of CH OH. Again, this e ect may result from excitation. The three transitions of N O are plotted in Figure. As the Ðgure shows, the line shapes are very similar for each transition, indicating that they indeed arise from the same molecule. Three unidentiðed lines are present in the FIG..ÈSpectra of the J \ ], 6 ] 5, and 9 ] 8 transitions of N O near 75, 150, and 6 GHz, respectively, observed with the 1 m telescope toward Sgr B (N), the N O peak. Spectral resolutions are 500 khz ( km s~1) for the ] line and 1 MHz for the 6 ] 5(kms~1) and 9 ] 8 (1. km s~1) transitions. The N O lines may consist of two velocity components, similar to what has been observed for EtCN and VyCN at this position (Miao et al. 1995). Various unidentiðed features appear in the J \ 6 ] 5 spectrum. J \ 6 ] 5 spectrum; these are not likely to arise from other velocity components of N O because they do not appear in the other transitions. The N O lines themselves may be comprised of two velocity components. Two Gaussian curves can be Ðtted to the N O J \ 6 ] 5 line proðle, with V D 6.5 and 71.9 km s~1 and *V D 8.4 and 4.8 km s~1. LSR Similar velocity components are 1@ found in VyCN and EtCN by Miao et al. (1995). However, the supposed velocity components ÏÏ in N O are barely above the noise, and longer integrations need to be carried out for these spectra before they can be established with any certainty. Additionally listed in Table 1 are the upper limits for the searches for NO` and NO (T * [ 0.0 K) and line parameter data for HNO from Kuan R & Snyder (1994). HNO, like NO, was readily observed at both cloud cores. The LSR velocities of HNO, NO, and N O agree within the quoted errors at these positions. The line widths of HNO, on the other hand, are unusually broad at the middle and northern positions relative to the other molecules (*V D 1È46 km s~1 for HNO as opposed 1È19 km s~1 for 1@ NO and N O). The p errors on the HNO line widths, which were measured with an interferometer, are larger than the numbers themselves. The single-dish data of HNO from Kuan & Snyder do not appear to exhibit these broad line widths but look very similar to the NO spectra. Based on line proðles, it can therefore be concluded that HNO, NO, and N O emission arises from similar material. The mapping data for the J \ 6 ] 5 line of N O, the J \ / ] 1/ (% ` ), F \ 5/ ] / transition of NO, and 1@ the J \ 14 ] 14 E line of CH OH is easily sum- Kq 0 ~1 marized. Of the 55 separate positions observed, N O was detected at only seven of these clustered around Sgr B (N). The methanol distribution mimics that of N O, although it was observed in the NO spectral bandpass. In contrast, NO was detected at every single position; however, the emission becomes weaker at the northern and southern edges of the map. These e ects are apparent in Figures 4 and 5. Figure 4 presents the NO/CH OH spectra as a function of position. The line in the middle of each spectrum is the strongest HF component of NO, the weaker lines to the left are the other HF lines of this molecule, and the other strong feature to the right is the CH OH line (see Fig. ). The (0, 0) position on this map is that of Sgr B (OH) (a \ 17h44m11s.0, d [B1950.0]), while the star marks the position of Sgr B (N). The NO lines are clearly visible in every panel, and there appears to be slight northsouth and east-west velocity gradients. To the south and east, velocities are lower, typically V D 55 km s~1, but increase to the north and west, with LSR V D 65 km s~1. These gradients were also observed in other LSR molecules such as HC N (de Vicente et al. 000) and HNO (Kuan & Snyder 1994). The line widths are relatively constant in value across the map, with *V D 15È0 km s~1. In contrast to NO, the CH OH emission 1@ is sharply conðned to the Sgr B (N) position. There is some extended emission 0A south and west of Sgr B (N), but the methanol line disappears 1@ away from this region. In Figure 5, spectra of the J \ 6 ] 5 transition of N O are presented as a function of spatial position (see middle panel of Fig. ). Again, the strongest line intensity is at Sgr B (N), the panel marked by the asterisk, with weaker emis- Gaussian Ðts to the line proðles or intensity upper limits for all positions can be obtained from the authors.

5 48 HALFEN, APPONI, & ZIURYS Vol. 561 FIG. 4.ÈPosition-spectrum map of the J \ / ] 1/ transition of NO (% ` ) and the J \ 14 ] 14 E transition of CH OH in the Sgr B complex (see Fig. ). The (0, 0) o set is at the Sgr B (OH) position, and the star marks 1@ the position of Kq Sgr B 0 (N). The ~1 spectra are plotted on the same scale as Fig. : LSR velocity for the x-axis and T * (K) for the y-axis. It is evident from the spectra that NO is extended over the entire region, while the CH OH transition sharply peaks near Sgr B (N). The R spectra are spaced in position by 0A. sion 0A west and south of this core; the line is not visible at any other position. In Figures 6 and 7, contour maps of the peak line intensities, in T * (K), of NO, N O, and CH OH are shown. R Figure 6 shows a contour map of the strongest HF component of NO (F \ 5/ ] /). Positions of Sgr B (N), (M), and (OH) are indicated, as well as the (N) peak where HNCO shows a maximum (Wilson et al. 1996; Minh et al. 1998). Overlaid on the NO contours are those of HNO, indicated by dashed lines, from the interferometer maps of Kuan & Snyder (1994). Although extended over the entire region, NO appears to show local maximum near Sgr B (M) and Sgr B (N). The peak near the middle position is coincident with an HNO maximum, but there are no other small-scale correlations between these two molecules. The left- and right-hand panels of Figure 7 present the contour maps for N O (J \ 6 ] 5) and CH OH (J \ Kq 14 ] 14 E). The source distributions of these two lines 0 ~1 are extremely similar. Both transitions show a distinct maximum at Sgr B (N), with weaker emission near Sgr B FIG. 5.ÈPosition-spectrum map of the J \ 6 ] 5 transition of N O in the Sgr B region (see middle panel of Fig. ). The (0, 0) o set is at Sgr B (OH), and the star marks the position of Sgr B (N). The axes are identical to those in Fig.. The N O line is only observed near the Sgr B (N) position. The spectra are spaced in position by 0A.

6 No. 1, 001 N/O CHEMICAL NETWORK: N O AND NO IN SGR B 49 and 40 K from the extended component of NH over this region. Moreover, Lis & Goldsmith (1991) found n D 105 cm~ over the same 10 pc (4@) region, while Hu ttemeister et al. (199) derived n D 104 cm~. Hence, there appears to be sufficient density and gas hot enough to readily excite the J \ 6 ] 5 transition of N O over at least 4@ of the Sgr B cloud. Further evidence for sufficient excitation conditions comes from observations of the J \ 1 ] 11 transition of HC N (Lis & Goldsmith 1991). This line is extended over 4@ in the Sgr B cloud complex, encompassing the major cores. The energy above ground state of the J \ 1 level is 8 KÈcomparable to the J \ 6 level of N O. However, HC N has a dipole moment of.6 DÈover an order of magnitude larger than that of N O. If this transition is excited over an extended region, then the J \ 6 ] 5 line of N O should be also, provided that the molecule is present. The conðned nature of N O emission must therefore be a chemical e ect. FIG. 6.ÈContour map of the peak line intensity (T *) of the % `, J \ / ] 1/; F \ 5/ ] / transition of NO (1 MHz resolution). R The 1@ lowest contour level is at 0.10 K and increases in increments of 0.0 K. The (0, 0) position is Sgr B (OH). The three stars mark the positions of Sgr B (N), Sgr B (M), and Sgr B (OH), and the open circle indicates the N source, the so-called northern nitrogen core. The beam size for the observations is shown in the bottom left-hand corner. NO appears to be extended throughout the complex, with local maxima near the M and N cores. The dashed contours show the four major HNO peaks (from Kuan & Snyder 1994). (M) and no evidence of emission at Sgr B (OH) or Sgr B (N). The maximum in these maps is very close to the HNO (N) source (Kuan & Snyder 1994). The HNO (S) and HNO (NW) peaks have no counterparts in either NO or N O. 4. ANALYSIS 4.1. A ConÐned N O Source: Excitation or Chemistry? The conðned distribution of N O emission could be a result of either excitation or preferential chemistry. Excitation does not explain this result. First of all, N O has a rather small dipole moment of 0.16 D. The Einstein A- coefficient for the J \ 6 ] 5 transition is therefore 4.7 ] 10~7 s~1. Consequently, the critical density needed to equate the A-coefficient downward with the collisional rate upward is n D 4 ] 104 cm~, assuming a collisional cross section of p c D 10~15 cm and a gas kinetic temperature of 0 K. Such a kinetic temperature is likely to be a lower limit for the Sgr B complex. Lis & Goldsmith (1991) for example, found T D 0È40 K over 10 pc (or 4@) ofthe cloud, which includes K Sgr B (M), (N), and (OH). Hu ttemeister et al. (199) derived temperatures between Column Densities and Abundances of N/OSpecies The column density of N O at its peak near Sgr B (N) was determined from a rotational temperature diagram (see Turner 1991), as shown in Figure 8. This diagram is a plot of upper-state column density per detected transition versus energy of the upper state. The three observed transitions of N O plotted here follow a reasonably straight line and yield T D 40 K and N D 1.5 ] 1015 cm~. No correction was rot tot made for beam size in this plot. (With about a 1@ source, the beam should be nearly Ðlled for every transition.) For the other molecules observed, only one rotational transition was measured, and hence the column densities had to be calculated assuming a rotational temperature from a single formula, which is N \ k105t R *V 1@ f rot. (1) tot 8nlk S e~*egd@trotr 0 ij HF In the expression, l is the frequency of the transition, k is 0 the permanent dipole moment, f is the rotational parti- rot tion function, S is the line strength, *E is the energy of ij gd the Jth level above ground state, and T and *V are the R 1@ radiation temperature and line widths (in km s~1), respectively, of the transition J ] 1 ] J. R is the relative HF HF intensity, if present in the transition (NO, NO ); otherwise, R \ 1. For the diatomic or linear species involved (NO, NO`, HF N O), an exact value was calculated for the partition function for J ¹ 100. In the case of NO, an asymmetric top with C symmetry, the partition function was estimated using the v equation (see Turner 1991) f \ 1[(kT /h)n/abc]1@. () rot Here A, B, and C are the rotational constants of the molecule. Rotational temperatures were chosen based on the N O result for Sgr B (N) and on kinetic and excitation temperatures derived from measurements of other molecules for the other positions (Lis & Goldsmith 1991; Hu ttemeister et al. 199). These values are given in Table. It should be noted that the dipole moments of NO and NO are comparable to that of N O(k B 0.È0. D; see Table ). The dipole moment of NO` has 0 not been measured; it was estimated to be D1 D, based on dipole moments of other molecular ions. Resulting column densities for NO and N O at the core positions in Sgr B are tabulated in Table, along with the

7 50 HALFEN, APPONI, & ZIURYS Vol. 561 FIG. 7.ÈContour maps of the peak line intensity T * (K) of the J \ 6 ] 5 transition of N O (left) and the J \ 14 ] 14 E line of CH OH (right). The lowest contour of the N Omap is K and increases R in increments of K. For CH OH, the lowest contour Kq is K ~1 with increments of 0.04 K. Beam sizes are shown in the lower left-hand corners, and the three stars mark the positions of Sgr B (N), (M), and (OH). The northern nitrogen core, Sgr B (N), is marked by an open circle. The (0, 0) position is that of Sgr B (OH). The emission of both molecules shows a distinct peak at Sgr B (N), with some north-south elongation. upper limits for NO and NO`. Additionally listed are column density values for HNO derived by Kuan & Snyder (1994). Fractional abundances, relative to H, are also listed, which assume N(H ) D 104 cm~ (see Nummelin et al. 000). As the table shows, the most abundant N/O molecule is NO itself, which has a fairly constant column density of N D (0.8È1.5) ] 1016 cm~ across several arc- tot minutes with f (NO/H ) D (0.8È1.5) ] 10~8. N O has N D 1015 cm~ at its peak near Sgr B (N), which corre- tot sponds to f D 10~9. HNO, on the other hand, has a column density and abundance almost orders of magnitude smaller than that of NO across the north-south ridge TABLE COLUMN DENSITIES AND ABUNDANCES OF N/O MOLECULES IN SGR B f (X / H ) k 0 N tot Early-Times Model Steady-State Model MOLECULE (D) T rot (K) POSITION (cm~) Observeda (. ] 105 yr)b (1 ] 108 yr)b N O N 1.50 ] ] 10~9.96 ] 10~ ] 10~11 NO N 1. ] ] 10~8.189 ] 10~7.67 ] 10~6 7 N 1.54 ] ] 10~ M 9. ] ] 10~ S 7.7 ] ] 10~ NO`... D1.0 7 N \4.5 ] 101 \4.5 ] 10~1.808 ] 10~ ] 10~10 NO N \. ] 1015 \. ] 10~9.07 ] 10~ ] 10~10 HNOc N.5 ] ] 10~ ] 10~ ] 10~9 9 N.6 ] ] 10~ N 1.1 ] ] 10~ a Assumes cm~; Nummelin et al N(H ) \ 1 ] 104 b Millar et al c Kuan & Snyder 1994.

8 No. 1, 001 N/O CHEMICAL NETWORK: N O AND NO IN SGR B 51 temperature-sensitive gas-phase reactions. A search through the most recent versions of the UMIST reaction rate catalog (see, e.g., Le Tue, Millar, & Markwick 000) suggests that the only viable processes leading to N O are neutral-neutral reactions. No ion-molecule processes were found in the data set that produce N O. Of the neutralneutral reactions, the only ones with a rate-constant k Z 10~1 cm s~1 were k1 NO ] N ÈÈÈÕ k NO ] NH ÈÈÈÕ N O ] O, () N O ] OH, (4) FIG. 8.ÈRotational diagram for N O, based on the J \ ], 6 ] 5, and 9 ] 8 transitions observed. The three data points lie approximately along a straight line and suggest N D 1015 cm~ and T D 40 K. rot [N D 1È4 ] 1014 cm~ and f (HNO/H ) D (1È4) tot ] 10~10]. The upper limit for NO` places its abundance over a factor of 1000 less than that of NO, while that for NO indicates N [ 1015 cm~,orf [ 10~9. tot 5. DISCUSSION 5.1. High-Temperature Gas-Phase Production of N O The most striking result of this study is the limited spatial extent of N O in the Sgr B complex as compared to NO and HNO. This molecule appears to exist solely in the vicinity of Sgr B (N), with a deconvolved source size of h [ 45@@. It is not present at the so-called northern nitrogen core s of Wilson et al. (1996); in fact, the emission drops sharply to the north, avoiding this region altogether. Moreover, the line widths of the N O transitions toward their peak are *V D 11È19 km s~1, as opposed to *V D 0 km s~1 found 1@ in HNCO (Wilson et al. 1996). 1@ Some chemical e ect is limiting the production of N O across the Sgr B complex. Miao et al. (1995) also found such molecules as EtCN, VyCN, and HCOOCH conðned to a small region centered on Sgr B (N). These authors attributed this e ect to grain surface chemistry in the dusty core of this source followed by subsequent evaporation caused by the high temperatures of this material. Indeed, both NH observations (Hu ttemeister et al. 199) and HC N measurements (de Vicente et al. 000) indicate gas temperatures near 00È50 K at Sgr B (N). However, this chemical scenario probably does not apply to N O, whose structure involves two unsaturated bonds, i.e., NxNxO. If N O were formed on grain surfaces, these bonds would likely be saturated. Another alternative for the synthesis of this molecule is k NO ] NH ÈÈÈÕ N O ] H. (5) The Ðrst two processes may not be important because the abundance of NO is not high, given our failure to detect this species. Furthermore, the rate of the Ðrst process is k D ] 10~1 cm s~1. The second rate is not well known. 1 It has an inverse temperature dependence between 00 and 00 K, but at lower temperatures the rate is highly uncertain. At 00 K, k D 4 ] 10~11 cm s~1. The third reaction may be more probable because both reactants are known interstellar molecules. One reactant, NO, is abundant throughout the Sgr B complex, as this work has demonstrated. The other reaction partner, NH, is a simple hydride that has been detected optically (Meyer & Roth 1991; the pure rotational transitions of NH lie in the far-infrared and thus pose difficulties for ground-based detections). NH is likely to be widespread in molecular clouds similar to OH and CH. The third reaction additionally obeys a rate law that increases rapidly with temperature (Mallard et al. 1998): k \ 1.16 ] 10~10(T /00)~1.0 exp ([40/T ). (6) In Figure 9, this rate is plotted against temperature. As this diagram shows, the rate is negligible near 50 K but increases to k D (1È) ] 10~11 cm s~1 between 15 and 00 K. Hence, this reaction becomes important for T [ 15 K. K Temperatures are known to exceed this value toward Sgr B (N), as demonstrated by other molecular measurements (Hu ttemeister et al. 199; de Vicente et al. 000) and as indicated by the detection of the 14 ] 14 E transition of CH OH. The distribution and existence 0 of ~1 N O at Sgr B (N) can therefore be explained by the reaction of NO ] NH, which proceeds only at elevated temperatures. The importance of high-temperature chemistry in the production of N O is additionally supported by chemical models. Millar, Farquhar, & Willacy (1997) have calculated the abundance of this species in their most recent model, at both early times ÏÏ (D. ] 105 yr) and steady state (108 yr). Their calculations are for a 10 K cloud. The calculated abundances are f D ] 10~11 at both epochsèalmost orders of magnitude less than the observed value of f D ] 10~9 (see Table ). Therefore, 10 K gas is not likely to contain a signiðcant amount of N O. 5.. Relationship of NO and HNO NO appears to be an important precursor molecule to N O. Is it also linked to HNO? It is clear from this work and the HNO study of Kuan & Snyder (1994) that both species are present in the same bulk gas throughout Sgr B. Examining the UMIST rate tables, the fastest reaction

9 leading to HNO is via NO: k4 NO ] HNO` ÈÈÈÕ HNO ] NO`. (7) The rate here is k D 7 ] 10~10 cm s~1, i.e., close to the 4 Langevin value. All other reactions leading to HNO are neutral-neutral processes and are at least an order of magnitude slower. Furthermore, HNO` is rapidly produced via the ion-molecule reaction (Le Tue et al. 000) NO ] H ] H. (8) `]HNO` Hence, provided there is sufficient NO and H HNO can be synthesized. `, The observed abundance of HNO in the Sgr B complex is f D (1È) ] 10~10 (see Table ), while it is f D (0.6È 1.5) ] 10~8 for NO. Therefore, NO is at least a factor of 10 more abundant than HNO and, consequently, could be the main precursor for HNO via the above reaction scheme. Curiously, the early-time calculations of the model of Millar et al. (1997) reproduce the observed HNO abundance very well (see Table ). For NO, on the other hand, the calculated value is about a factor of 15 too high, even at early times. 5.. TheN/O Chemical Network Two other molecules searched for in this study are NO` and NO. The latter species is very similar to N O in that there are no ion-molecule reactions leading to its synthesis (Le Tue et al. 000). It seems to be produced only by neutral-neutral processes. In fact, many of these reactions have extremely large activation energy barriers and there- 5 HALFEN, APPONI, & ZIURYS Vol. 561 fore are not feasible even at 00 K. The only possible route to NO seems to be the reaction FIG. 9.ÈPlot of the rate of the reaction NO ] NH ] N O ] H vs. temperature. The rate increases rapidly for T Z 50 K and has a value k Z 10~11 cm s~1 for T Z 15 K. k5 O ] HNO ÈÈÈÕ NO ] H. (9) The rate here is relatively slow, with k D 1 ] 10~1 cm s~1. All other rates considered for NO and 5 N O are at least an order of magnitude faster. Therefore, the failure to detect NO does not contradict chemical rate predictions. The Millar et al. (1997) model in fact calculates an NO abundance of f D ] 10~11 at early times (Table ). The upper limit found here is f \ 10~9. In contrast, NO` is rapidly produced from NO through charge exchange reactions with many di erent ions (H`, H C`, CH`, CH NH etc.). Most of these processes proceed `, near the Langevin `, `, rate (D10~9 cm s~1; Le Tue et al. 000). However, the reverse processes lead rapidly back to NO, such that the NO` abundance, as predicted by Millar et al. (1997), is only f D 4 ] 10~11. (HNO and N O can also produce NO`, but there are many more pathways from NO with comparable or faster rates.) The upper limit obtained for NO` from our observations is f [ 10~1. However, this is based on an estimated dipole moment of 1 D. (The dipole moment of NO`, to our knowledge, has never been measured.) If this value is smaller by a factor of.5, for example, then the observed upper limit would exactly match the model predictions. A schematic summarizing the chemical network linking NO, N O, HNO, NO, and NO` is shown in Figure 10. The key molecule in this scheme appears to be NO, which is the main precursor to HNO (via HNO`) as well as N O (by the temperature-dependent process NO ] NH). It also is the key reactant leading to NO`. The major route to NO is through HNO, and NO can subsequently form N O through the processes already discussed. Finally, NO can form NO via a reaction with O (NO ] O ] O ] NO), but this process is slow. Because a fair amount of nitrogen is contained in ammonia, especially toward the Sgr B (M) and Sgr B (N) cores (see, e.g., Hu ttemeister et al. 199), it is important to FIG. 10.ÈDiagram illustrating the N/O chemical network. The key species in this scheme is NO, which can be converted to N O via the temperature-dependent process NO ] NH ] N O ] H and also to HNO by the fast ion-molecule reaction HNO` ] NO ] HNO ] NO`. NO` arises from NO and HNO via charge exchange processes; NO is produced slowly from HNO ] O ] NO ] H. The network is highly dependent on neutral-neutral reactions.

10 No. 1, 001 N/O CHEMICAL NETWORK: N O AND NO IN SGR B 5 consider how the N/O chemical network relates to NH. Although NH is involved in a few charge exchange reac- tions with NO and NO`, it in general does not participate in the N/O network (Le Tue et al. 000). In fact, it is not part of any pathways in the formation or destruction of N O. The chemistry of N/O compounds is therefore almost totally distinct from that of NH. Because NO is the cornerstone of the N/O scheme, it is of interest to consider what the major routes to its formation are. There are various ion-molecule reactions of HNO` that lead to NO, but the reverse processes are also fast, making this pathway circular. (Rest frequencies for HNO` are unavailable, so its abundance and distribution are unknown.) Examination of other pathways suggests that the most promising reaction leading to the synthesis of NO is k6 NH ] O ÈÈÈÕ NO ] H. (10) The rate here is k D 1. ] 10~10 cm s~1, with no activa- tion energy, while 6 the reverse process has E D 0,000 K (Le Tue et al. 000) and hence will not occur. act This neutralneutral reaction is therefore critical to the N/O network. It is quite interesting to note that many of the important reaction routes in this network are not ion-molecule processes but rather neutral-neutral reactions. 6. CONCLUSION Mapping observations of the NO and N O molecules across the Sgr B complex has lead to a new interpretation of the chemical processes producing these species. The widespread distribution of NO across 5@ of the cloud suggests that it is formed by a low-temperature process, which is likely to be the reaction NH ] O ] NO ] H. N O, in contrast, is conðned to a D1@ source centered at Sgr B (N)Èa nearly identical distribution as found for the highenergy (E B 55 K) 14 ] 14 E transition of CH OH. This e ect u cannot arise 0 from ~1 excitation. Therefore, N O must be exclusively formed in hot gas near the northern core. Gas-phase reactions likely account for this hightemperature synthesis, in particular the process NH ] NO ] N O ] H, which becomes efficient only at temperatures T Z 15 K. Indeed, the so-called Large Molecule Heimat at Sgr K B (N) may actually be the Hot Molecule Heimat,ÏÏ or Sgr B (HMH). HNO, on the other hand, is probably produced by the ion-molecule reaction HNO` ] NO ] HNO ] NO`. Its distribution is qualitatively similar to NO across the Sgr B cloud. Also, emission from N O was not present at the so-called northern nitrogen core, although NO may have a weak maximum at this position. Finally, these observations demonstrate that neutral-neutral reactions, even with activation energy barriers, may play an important role in the synthesis of simple molecules in complex cloud cores like those found in Sgr B. This research was supported by NSF Grant AST The authors thank John P. Schaefer and the Research Corporation for providing the funding to keep the Kitt Peak 1 m telescope operational so that this project could be completed. Baron, P. A., Godfrey, P. D., & Harris, P. O. 1974, J. Chem. Phys., 60, 7 Bowman, W. C., Herbst, E., & De Lucia, F. C. 198, J. Chem. Phys., 77, 461 Cummings, S. E., Linke, R. A., & Thaddeus, P. 1986, ApJS, 60, 819 de Vicente, P., Mart n-pintado, J., Neri, R., & Colom, P. 000, A&A, 61, 1058 Gaume, R. A., & Claussen, M. T. 1990, ApJ, 51, 58 Goldsmith, P. F., Snell, R. L., Hasegawa, T., & Ukita, N. 1987, ApJ, 14, 55 Hu ttemeister, S., Wilson, T. L., Henkel, C., & Mauersberger, R. 199, A&A, 76, 445 Kuan, Y.-J., & Snyder, L. E. 1994, ApJS, 94, 651 Le Tue, Y. H., Millar, T. J., & Markwick, A. J. 000, A&AS, 146, 157 Lis, D. C., & Goldsmith, P. F. 1991, ApJ, 69, 157 Lis, D. C., Goldsmith, P. F., Carlstrom, J. E., & Scoville, N. Z. 199, ApJ, 40, 8 Liu, S.-Y., & Snyder, L. E. 1999, ApJ, 5, 68 Mallard, W. G., Westley, F., Herron, J. T., Hampson, R. F., & Frizzell, D. H. 1998, NIST Chemical Data Base, Ver. Q98 (Gaitnersburg: NIST) Meyer, D. M., & Roth, K. C. 1991, ApJ, 76, L49 REFERENCES Miao, Y., Mehringer, D., Kuan, Y.-J., & Snyder, L. E. 1995, ApJ, 445, L59 Miao, Y., & Snyder, L. E. 1997, ApJ, 480, L67 Millar, T. J., Farquhar, P. R. A., & Willacy, K. 1997, A&AS, 11, 19 Minh, Y. C., Haikala, L., Hjalmarson, A., & Irvine, W. M. 1998, ApJ, 498, 61 Nummelin, A., Bergman, P., Hjalmarson, A., Friberg, P., Irvine, W. M., Millar, T. J., Ohishi, M., & Saito, S. 000, ApJS, 18, 1 Reid, M. J., Schneps, M. H., Moran, J. M., Gwinn, C. R., Genzel, R., Downes, D., & Ro nna ng, B. 1988, ApJ, 0, 809 Sutton, E. C., Jaminet, P. A., Danchi, W. C., & Blake, G. A. 1991, ApJS, 77, 55 Turner, B. E. 1989, ApJS, 70, 59 ÈÈÈ. 1991, ApJS, 76, 617 Vogel, S. N., Genzel, R., & Palmer, P. 1987, ApJ, 16, 4 Wilson, T. L., Snyder, L. E., Comoretto, P. R., Jewell, P. R., & Henkel, C. 1996, A&A, 14, 909 Ziurys, L. M., Apponi, A. J., Hollis, J. M., & Snyder, L. E. 1994a, ApJ, 46, L181 Ziurys, L. M., Hollis, J. M., & Snyder, L. E. 1994b, ApJ, 40, 706 Ziurys, L. M., McGonagle, D., Minh, Y., & Irvine, W. M. 1991, ApJ, 7, 55