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Chemical Variation in Molecular Cloud Cores in the Orion A Cloud

We have observed molecular cloud cores in the Orion A giant molecular cloud (GMC) in CCS, HC3N, DNC, and HN13C to study their chemical characteristics. We have detected CCS in the Orion A GMC for the first time. CCS was detected in about a third of the observed cores. The cores detected in CCS are not localized but are widely distributed over the Orion A GMC. The CCS peak intensity of the core tends to be high in the southern region of the Orion A GMC. The HC3N peak intensity of the core also tends to be high in the southern region, while there are HC3N intense cores near Orion KL, which is not seen in CCS. The core associated with Orion KL shows broad HC3N line profile, and star formation activity near Orion KL seems to enhance the HC3N emission. The column density ratio of NH3 to CCS is lower near the middle of the filament, and is higher toward the northern and southern regions along the Orion A GMC filament. This ratio is known to trace the chemical evolution in nearby dark cloud cores, but seems to be affected by core gas temperature in the Orion A GMC: cores with low NH3 to CCS column density ratios tend to have warmer gas temperature. The value of the column density ratio of DNC to HN13C is generally similar to that in dark cloud cores, but becomes lower around Orion KL due to higher gas temperature.


Introduction
Most of stars in the Galaxy form in giant molecular clouds (GMCs). Molecular cloud cores are known to be the sites of star formation (e.g., Beichman et al. 1986). The evolution of molecular cloud cores in GMCs is less understood, compared with that of nearby dark cloud cores. In nearby dark clouds, molecules such as CCS, HC 3 N, NH 3 , and N 2 H + , and the neutral carbon atom C 0 are known to be good tracers of chemical evolution (e.g., Hirahara et al. 1992;Suzuki et al. 1992;Benson et al. 1998;Maezawa et al. 1999;Hirota et al. 2002;Hirota et al. 2009). The carbon-chain molecules, CCS and HC 3 N tend to trace the early chemical evolutionary stage, whereas N-bearing molecules, NH 3 and N 2 H + tend to trace the late stage. We wonder how different the chemical properties of molecular cloud cores in GMCs are, compared with those in nearby dark clouds.
The chemical variation has been suggested in limited regions in the Orion A GMC. Ungerechts et al. (1997) studied the region near Orion KL in many molecular lines, and have shown difference in molecular abundance between the Orion KL region, the Orion Bar, and the molecular ridge. Tatematsu et al. (1993) studied the region covering Orion KL and OMC-2 (see their Figure 4), and found that NH 3 tends to be stronger in the north (OMC-2 region) while CS tends to be stronger in the south. Tatematsu et al. (2008) showed that N 2 H + is widely distributed over the -shaped filament of the Orion A GMC, and pointed out that NH 3 and N 2 H + is relatively strong toward OMC-2, compared with the H 13 CO + and CS distribution. Johnstone et al. (2003) astrochemically compared submillimeter dust continuum sources in the Orion A GMC.
In this study, we investigate the global chemical characteristics of molecular cloud cores in the Orion A GMC through new molecular-line observations. The purposes of this study are (1) to search for the CCS emission, which may represent the young molecular cloud material, (2) to study the chemical properties of the cloud cores through the line intensity variation and the column density ratio variation ALONG the Orion A GMC filament, and (3) to compare the chemical properties of Orion cores with those of dark cloud cores.
The distance to the Orion A cloud is estimated to be 418 pc (Kim et al. 2008). At this distance, 1 ′ corresponds to 0.12 pc.
The positions for the observations were chosen from intensity peak positions of 125 CS cores in Tatematsu et al. (1993). The core name used here is 'TUKH' followed by three digits of the core number of Tatematsu et al. (1993). Note that the degrees of the declination were printed incorrectly for cores TUKH068 and TUKH070 in Table 1 of Tatematsu et al. (1993): "−5 • " should read "−6 • ." We selected 81 peak positions toward which both CS J = 1−0 and CS J = 2−1 were clearly and consistently detected without velocity-component confusion, on the basis of Tatematsu et al. (1998). In the CCS and HC 3 N observations, we lost some data due to a mistake in the receiver set-up, and we do not use data with high noise levels . The final number of the cores successfully observed in CCS and HC 3 N is 62 (out of 81). The observed data were reduced by using the software package "NewStar" of Nobeyama Radio Observatory. Table 1 summarizes the observed line parameters: the peak intensity T * A , the LSR velocity ∆v LSR , and the linewidth of CCS, HC 3 N. HN 13 C, and DNC. The HC 3 N peak intensity in the table represents T * A of the observed spectrum rather than the hyperfine-component intensity. 'Three dots' in T * A means either 'not observed' or 'omitted because noise level is too high (the rms noise level is larger than 0.14 K).' Figures 1 and 2 show the CCS spectra obtained toward cores TUKH083 and TUKH122, respectively. To our knowledge, the detection of CCS has not been reported in Orion GMC cores previously, and our observations serve as the first CCS detection in this cloud. Because it is suggested that CCS traces young molecular gas (< 10 6 yr; e.g. Suzuki et al. 1992) in nearby dark clouds, it is very interesting that the Orion A GMC, whose age is at least of order 10 7 yr (e.g. Bally et al. 1987), harbors an early-type molecule CCS. Furthermore, the detection rate is high. We detected CCS in 20 core peaks out of 62, implying the detection rate is 32%. For HC 3 N, the detection rate is 53% (33 out of 62). The detection rates for DNC and HN 13 C are 32% (26 out of 81) and 37% (30 out of 81), respectively. Tatematsu et al. (2008) did not detected CCS anywhere in the OMC-2/3 region, but better rms noise levels in the present study allowed us to detect one core TUKH003 in CCS out of six observed cores in the OMC-2/3 region, resulting in a detection rate of 17% in this region. We compare our detection rates with previous studies. Hirota et al. (2009) observed CCS and HC 3 N toward nearby dark cloud cores and their detection rates were 42% and 42%, and Sakai et al. (2008) observed the same molecules toward infrared dark clouds and their detection rates were 0% and 78%, respectively. These studies including the present study used the same telescope. The rms noise level was ∼0.1 K (CCS) and ∼0.15 K (HC 3 N) in the present study, ∼0.05 K (CCS) and ∼0.1 K (HC 3 N) in Hirota et al. 2009, and∼0.2 K in Sakai et al. (2008). The non-detection of CCS in infrared dark clouds is in contrast with the detection rate of CCS in Orion A GMC cores and in nearby dark cloud cores. This can be partly due to somewhat higher rms noise level for observations toward infrared dark clouds. Infrared dark clouds are farther than Orion A GMC and nearby dark clouds, and smaller beam filling factor of CCS may explain difference. However, the HC 3 N detection rate is higher in infrared dark clouds, and it is impossible to explain difference between CCS and HC 3 N detection rates only in terms of sensitivity and beam dilution. It is most likely that different detection rates reflect difference in chemical properties between infrared dark clouds and the others. The detection rates of CCS and HC 3 N in Orion are not very different from those in nearby dark clouds. It seems that Orion GMC cores are more similar to nearby dark cloud cores than infrared dark clouds.

Line Intensities and Their Variations
The observed lines are thought to be optically thin. We investigate the peak intensity variation along the Orion A GMC filament. Figure 3 plots the CCS peak intensity T * A against the declination. Note that the Orion A GMC is roughly elongated in the NS direction (e.g., Bally et al. 1987). CCS-detected cores are not localized, but are distributed over the Orion A GMC. It seems that the CCS peak intensity is stronger in the south (Dec. < −6.5 deg). We wonder whether there is any relationship between the CS and CCS intensities, because these molecules are thought to be chemically related (Suzuki et al. 1992). Figure 4 compares the CCS peak intensity with the CS peak intensity of Tatematsu et al. (1993). There is no positive correlation between these intensities. Most intense CCS cores are weak in CS. From the CS intensity, it is hard to predict the CCS detection. Figure  with the NH 3 rotation temperature (Wilson et al. 1999). CCS was not detected toward Orion KL, which is warm (T rot > 50 K and kinetic temperature T kin = 70−150 K; Genzel & Stutzki 1989;Wilson et al. 1999). See Danby et al. (1988) for the T rot − T kin relation. Next, we show an observational result in HC 3 N. Figure 6 plots the HC 3 N peak intensity against the declination. Again, HC 3 N-detected cores are not localized, but distributed over the Orion A GMC filament. There are two characteristics in the HC 3 N peak intensity distribution. First, the HC 3 N peak intensity tends to be stronger toward the south, as the CCS peak intensity distribution does. Because CCS and HC 3 N are known to be early-type molecules in nearby dark clouds, this similarity can be understood as they trace similar chemical evolutionary stages. Second, the HC 3 N intensity is very strong around Orion KL (Dec. = −5.375 deg), which is not seen in the CCS intensity distribution. The HC 3 N linewidth toward Orion KL is broad (∼ 6 km s −1 ), and it seems that the HC 3 N emission is also affected by star formation activities such as molecular outflows.
We investigate the variation also in the velocity-integrated intensity T * A dv. linewidths are larger near the Orion Nebula region, and become smaller toward the south (not shown) as observed in other moleculs (Tatematsu et al. 1993). The integrated intensity does not tend to be high toward the south in CCS, and tends to be high only weakly toward the south in HC 3 N. It seems that the variation in the peak and integrated intensities are similar in CCS and HC 3 N except for the strong HC 3 N emission near Orion KL.

Column Densities
We investigate the chemical properties of the cloud cores by using the column density ratio of the molecules. The column density is calculated by assuming local thermodynamic equilibrium (LTE) with two excitation temperatures: T ex = T rot and T ex = T rot /2. T rot is taken from NH 3 observations of Wilson et al. (1999). The NH 3 rotation temperature is very close to the gas kinetic temperature T k for T k < ∼ 30 K (Danby et al. 1988). We use T ex = T rot as (almost) thermalized cases and T ex = T rot /2 as subthermal cases. When Wilson et al. (1999) lists an upper or lower limit to the rotation temperature, we employ the limit value as T rot . When Wilson et al. (1999) does not list a rotation temperature due to no detection, we assume T rot = 20 K, i.e., T ex = 20 and 10 K. The dipole moments of CCS, HC 3 N, DNC, and HN 13 C are 2.81 (Murakami 1990), and 3.724, 3.050, and 2.699 Debye (JPL Catalog 2 ), respectively. The upper limit to the column density is defined as 3σ for 1 km s −1 velocity width. To see the chemical properties, we compare the column density of the late-type molecule NH 3 to the early-type molecule CCS. Figures 9 and 10 plot the N(NH 3 )/N(CCS) column density ratio against the declination, with CCS excitation temperatures of T rot and T rot /2, respectively. The ratio is not constant, is lowest in the middle, and becomes higher toward the north (Dec. > ∼ −5.5 deg) and south (Dec. < −7 deg). This suggests that the Orion A GMC filament has global chemical variation along the filament. We found that the CCS and HC 3 N peak intensities are stronger in the south region. However, this tendency is not seen (CCS) or not very prominent (HC 3 N) in the integrated intensity. Furthermore, the NH 3 column density is also higher in the south. As a result, southern cores are found to have large N(NH 3 )/N(CCS). The logarithmic column density ratio of NH 3 to CCS, log N(NH 3 )/N(CCS) ranges from 0.9 to 2.3. If CCS and NH 3 represent early and late evolutionary stages, respectively, also in GMCs, the Orion A GMC is chemically younger in the middle and older in the north and south regions.
We plot starless cores and star-forming cores separately in Figures 9 and 10 (and hereafter). Here, the cores associated with molecular outflows and/or IRAS point sources are classifies as star-forming cores (Table 1 of Tatematsu et al. 1993). Furthermore, we visually investigated the Spitzer 24 µm MIPS images. We found that observed cores TUKH008, TUKH056, TUKH079, TUKH087, TUKH092, TUKH097, TUKH111, and TUKH125 are associated with point sources. Cores TUKH092 and TUKH097 are associated with several and two point sources, respectively, while the other cores are associated with one point source each. These are also classified as star-forming cores. On the other hand, core TUKH89 associated with Morgan 18 does not accompany a Spitzer source. Morgan et al. (1991) identified a molecular outflow but its distribution is not well defined. We exclude this core as a star-forming A toward Orion cores is plotted against NH 3 rotation temperature T rot (Wilson et al. 1999). Orion KL is shown as the upper limit to the CCS intensity and the lower limit to the NH 3 rotation temperature. The most intense CCS core in Figure 3 is not plotted here because the T rot is not listed. core and classified as a starless core. Note that identification of the Spitzer MIPS images is difficult near the Orion Nebula due to intense diffuse infrared emission. We investigate the relationship between N(NH 3 )/N(CCS) and the association with young stellar objects. High  Hirota et al. (2009). N(NH 3 )/N(CCS) ranges from ∼3 to ∼100 in Orion A GMC, from ∼10 to ∼1000 in Perseus, and from ∼20 to ∼1000 in Ophiuchus. On the other hand, it ranges from 2 to 400 in Taurus. Taurus have cores with very low N(NH 3 )/N(CCS) (∼ 2−7), which corresponds to "Carbon-Chain-Producing Regions (CCPRs)" (Hirota et al. 2009). Cores in Perseus and Ophiuchus do not show very low N(NH 3 )/N(CCS). region is more compact than a CCS emitting region in L1544 in Taurus (Aikawa et al. 2001 and references therein). A possibility is that NH 3 , which may have emitting reqions smaller than CCS also in Orion, will be beam diluted in Orion, which is three times more distant than other regions. The telescope beam size is ∼ 40 ′′ in both of the NH 3 and CCS observations. We need to check this possibility with higher angular resolution observations. Regarding the Here, for Taurus, we used the column density derived from the spatiallysmoothed spectrum, which compensates for the difference in linear resolution due to a factor of 3 difference in distance. The difference in the average column density is a factor of 4, and we should take into account this "vertical shift" to compare the N(NH 3 )/N(CCS)-N(CCS) diagram. Next, we investigate the origin of CCS. Ikeda et al. (1999)  of the neutral carbon atom and CO over the Orion A GMC, and suggested that the origin of the neutral carbon atom might be the photodissociation region due to the cloud clumpiness under UV irradiation. We wonder whether CCS in the Orion GMC is related with photodissociation. CCS was not detected toward cores TUKH034 and TUKH036 located along the Orion Bar, which is an archetypal photodissociation region (PDR). Then, it is not likely that photodissociation in a clumpy cloud explains CCS in the Orion A GMC. It is suggested that CCS exists also in evolved molecular gas as secondary late-stage peak due to CO depletion (Li et al. 2002 and references therein). On the other hand, CO depletion will be less effective in warm molecular gas in Orion. The gas kinetic temperature of Orion cores ranges from ∼ 10 K to > 50 K (Wilson et al. 1999 We found two very interesting Orion cores. TUKH122, whose CCS spectrum is shown in Figure 2, is the most intense core in CCS, which is strong also in NH 3 . This core has a substantial amount of both CCS and N-bearing molecule. TUKH083, whose CCS spectrum is shown in Figure 1, is the core with the lowest N(NH 3 )/N(CCS) column density ratio. This core has a substantial amount of th early-type molecule CCS. These two cores look starless through the visual inspection of Spitzer 24 micron images. The core mass is 50-60 solar masses from the CS observations by Tatematsu et al. (1993), and these cores will be very good candidates for future cluster formation.

Deuterium Fractionation
We show the column density ratio of DNC to HN 13 C to see the deuterium fractionation.  et al. 1999). The ratio is lower for warmer cores. This indicates that the deuterium enhancement is less efficient in warm cores. Similar results have been shown previously by, e.g., Snell & Wootten (1979), Wootten (1987, and Schilke et al. (1992). We conclude that the tendency seen in N(DNC)/N(HN 13 C) is due to variation in the gas temperature: D/H decreases with increasing gas kinetic temperature. There are two possibilities for this: dependence of exchange reactions between HD and molecular ion on temperature (Snell & Wootten 1979) or dependence of CO depletion on temperature. At low temperatures, the depletion of molecules becomes efficient, the H 2 D + /H + 3 abundance ratio increases, and then molecular D/H ratios increase (Roberts & Miller 2000). It is found that the deuterium fractionation is a tracer of the chemical age of dark cloud cores (Hirota & Yamamoto 2006). However, the variation observed in the present study is due to variation of gas temperature rather than core evolution, because Orion cores do not have CCPRs. Figures 18 plots N seems that high-density cores tend to show low N(DNC)/N(HN 13 C). Depletion will be more serious in high-density cores at cold temperatures, and therefore the above result cannot be explained in terms of different depletion degrees due to different density. We conclude that different N(DNC)/N(HN 13 C) is due to difference in gas temperature, not due to difference in density.

Summary
We have detected CCS in the Orion A GMC for the first time. CCS-detected cores are not localized in the Orion cloud and widely distributed. The CCS peak intensity tends to be high in southern cores. The HC 3 N peak intensity of cores tends to be high toward the south, while there are also HC 3 N intense cores near Orion KL. The core associated with Orion KL show intense wing emission, and star formation activity near Orion KL seems to enhance the HC 3 N intensity. The NH 3 /CCS column density ratio is higher in the north and south regions  Figure 9 but for the CCS excitation temperature equal to T rot /2. and lower in the middle region, suggesting the chemical variation along the Orion A GMC filament. This ratio is a good tracer of chemical evolution in nearby dark cloud cores, but seems to be affected by core gas temperature in the Orion A GMC. This ratio tends to be low for warmer cores. The DNC to HN 13 C column density ratio is generally similar to that observed in dark cloud cores, but becomes lower around Orion KL. Variation in the DNC to HN 13 C column density ratio seems to reflect the temperature variation among cores.