C2H N=1-0 and N2H+ J=1-0 observations of Planck Galactic cold clumps
Xunchuan Liu, Yuefang Wu, Chao Zhang, Tie Liu, Jinghua Yuan, Sheng-Li Qin, Bing-gang Ju, Li-Xin Li
CC H N = − and N H + J = − observations of Planck Galactic coldclumps X.-C. Liu , , Y. Wu , , C. Zhang , , , T. Liu , J. Yuan , S.-L. Qin , B.-G. Ju , , L.-X. Li Abstract
A survey of C H N = − H + J = − H and N H + were chosen tostudy the chemical evolutionary states of PGCCs. Among 121 observed molecular cores associated withPGCCs, 71 and 58 are detected with C H N = − H + J = −
0, respectively. The detectedlines of most sources can be fitted with a single component with compatible V
LSR and line widths, whichconfirms that these PGCC cores are very cold (with gas temperatures 9–21 K) and quiescent while stilldominanted by turbulence. The ratio between the column densities of C H and N H + ( N (C H) / N (N H + ))is found to be a good tracer for the evolutionary states of PGCC cores. Gas-grain chemical model canreproduce the decreasing trend of N (C H) / N (N H + ) as a function of time. The cores with the lowestabundances of N H + ( X [N H + ] < − ) are the youngest, and have nearly constant abundances of C H.In evolved cores with X [N H + ] ∼ − , abundances of C H drop quickly as the exhaustion of carbonatoms. Although these PGCC cores are in di ff erent evolutionary states, they are all quite young ( < × yr) with N (C H) > N (N H + ). Mapping observations are carried out toward 20 PGCC cores. The PGCCcores in Cepheus have lower N (C H) / N (N H + ) and larger line widths compared with those in Taurus.This implies that PGCC cores in Taurus are less chemically evolved than those in Cepheus. Subject headings:
ISM: molecules – ISM: abundances – ISM: kinematics and dynamics – ISM: clouds – stars:formation
1. Introduction
The Planck satellite (Tauber et al. 2010; Planck Col-laboration et al. 2011a) carried out the first all skysurvey in the submillimeter to millimeter range withunprecedented sensitivity and provides a catalog ofcold clumps of interstellar matter in the Galaxy. TheCold Clump Catalog of Planck Objects (C3POs) re-leased by Planck Collaboration et al. (2011b) con-sists of 10 342 cold sources that stand out against awarmer environment. The C3PO clumps are cold withdust temperatures ranging from 7 to 19 K, peaking * [1501110219;ywu]@pku.edu.cn Department of Astronomy, Peking University, China KIAA, Peking University, 100871 Beijing, China Department of Astronomy, Yunnan University, China Korea Astronomy and Space Science Institute, Korea NAOC, Beijing 100101, China PMO, Qinghai Station, 817000, Delingha, China Key Laboratory for Radio Astronomy, CAS around 13 K. Among the C3PO clumps, 915 early coldcores (ECCs) were identified with most valid detectionand lowest dust temperatures ( < ff erent molecular spectral lines were conducted. Ob-servations with the J = − CO, CO, and C O toward 674 Planck cold clumps se-lected from the ECC catalog were performed by Wuet al. (2012) using the 13.7 m telescope of the PurpleMountain Observatory (PMO). Mapping observationsof the same transitions were followed up soon (Liuet al. 2012, 2013; Meng et al. 2013; Liu et al. 2015,1 a r X i v : . [ a s t r o - ph . S R ] J a n
00° 160° 120° 80° 40°Galactic Longitude-30°-15°0°15°30° G a l a c t i c L a t i t u d e R Fig. 1.— Spatial distribution in the Galactic plane of observed sources. The CO-selected cores with and withoutdetections of C H are denoted by the yellow and blue stars, respectively. The background image represents the H α emission (Finkbeiner 2003) in unit of R (10 / π photons cm − s − sr − ). The green contours represent CO (1-0)integrated emission detected by Planck HFI (Planck Collaboration et al. 2014). The red contours show the Planck 353 µ m continuum emission. The contour levels are (0.05, 0.1, 0.3, 0.5, 0.7, 0.9) × maximum value. Famous star formingregions such as Taurus-Perseus-California (Lombardi et al. 2010), Cepheus, Orion complex (Dame et al. 2001) aresketched with the black line.2016; Zhang et al. 2016; Liu et al. 2018a,b; Zhanget al. 2018; Tang et al. 2018). Meanwhile, single-point observations of HCO + J = − J = − H) (Beuther et al.2008) and the daughter molecule diazenylium (N H + )(Aikawa et al. 2003; Tatematsu et al. 2017) will behelpful.C H is the simplest hydrocarbon molecule with thecarbon-carbon triple bond (C ≡ C). Since being firstlydetected by Tucker et al. (1974), C H is found tobe widely distributed and detected in all evolutionarystages of star-forming regions (Sanhueza et al. 2013;Jiang et al. 2015). Beuther et al. (2008) suggest thatthis molecule could also be used to study the cold gasof forming stars to investigate their initial conditions.Meanwhile, N H + is also an excellent tracer of dense molecular cloud cores (Caselli et al. 2002). N H + isdurable in cold and dense regions owing to the de-pletions of its destroyers such as CO and the delayedfreeze-out of its precursors such as N . We expect thatC H and N H + are enhanced in di ff erent evolutionarystates of PGCCs.In this paper, we report a survey of C H N = − H + J = − CO J = − + andHCN (Wu et al. 2012; Yuan et al. 2016) to reveal thecharacteristics of C H and N H + in PGCCs. We alsohave compared the abundances of C H and N H + withthose predicted by gas-grain chemical model to in-vestigate the evolutionary states of single PGCC andPGCCs in di ff erent regions. This paper is arranged asfollows. We present a description of the sample andobservations in Sect. 2. The results of the molecularline observations are presented in Sect. 3. We discussthe properties of these two species and the chemicalevolutionary states of detected sources in Sect. 4. Wesummarize the paper in Sect. 5.2able 1: Line parameters. Species Transition Freq (GHz) S ij µ (D ) E up (K)C H N = − J = / − / F = − N = − J = / − / F = − N = − J = / − / F = − N = − J = / − / F = − N = − J = / − / F = − N = − J = / − / F = − H + J = − F = − J = − F = − J = − F = −
2. Sample and observation2.1. Sample characteristics
A sample consisting of 121 CO-selected cores withstrongest emission of CO J = − H N = − H + J = −
0. Spectra of J = − CO andC O at the center of observed cores were extractedfrom previous mapping observations (Liu et al. 2012;Meng et al. 2013; Zhang et al. 2016). The preliminarywork of deriving line parameters from these CO datawas done as described in Yuan et al. (2016). Basicinformation about our detected sources including theirequatorial coordinates, distances, H column densitiesof host PGCCs derived from dust continuum ( N d (H ))(Planck Collaboration et al. 2016), and CO parametersare listed in Table 2.Distances of these sources are adopted from the lit-erature (Wu et al. 2012; Planck Collaboration et al.2016). For sources with no available distances in theliterature, distances are adopted as the values with thehighest probabilities given by a Bayesian distance cal-culator (Reid et al. 2016). The distances calculatedby the Bayesian distance calculator are on average 30percent higher than those adopted from the literature(Table 2). Figure 1 shows the spatial distribution ofthe observed sources. These sources are biased towardnearby star-forming regions while the Galactic planeis under-represented. These properties are inheritedfrom the whole sample of ECCs and CO-selected cores(Wu et al. 2012; Yuan et al. 2016). The red and greencontours represent Planck 353 µ m continuum emissionand Planck CO J = − µ m continuum is well cor-related with that of CO J = − J = − ex (CO)) for our CO-selected cores range from 9 Kto 21 K. The mean value of T ex (CO) is 14 K with astandard error of 0.3 K, and it is slightly larger thanthe value in Wu et al. (2012) and the average dust tem-perature (13 K) for C3POs (Planck Collaboration et al.2011b). Sources in our sample generally have higherH column densities than those of CO cores in otherPGCC samples. The H column densities of CO coresin nearby star-forming regions range from 1 × cm − to 10 × cm − with a mean value of 2.2 × cm − (Meng et al. 2013), and those in the Galacticsecond quadrant range from 0.6 × cm − to 36 × cm − with a mean value of 8 × cm − (Zhanget al. 2016). The H column densities ( N CO (H )) of ourCO-selected cores are derived from N ( CO) adoptingthe C / C isotope ratio and CO abundance ( X [CO])as the values in the solar neighbor with a Galactocen-tric distance ∼ × − , respectively. N CO (H )cover the range of (3-70) × cm − with a meanvalue of 2.2 × cm − .Futhermore, 20 cores with valid detection ofC H N = − H + J = − Single-point observations of C H N = − H + J = − H + (red).tal bandwidth of 1 GHz and 16 384 channels, corre-sponding to a velocity resolution of 0.21 km s − forC H N = − − for N H + J = − H N = − H + J = − ffi ciency at 90 GHz areabout 56 (cid:48)(cid:48) and 0.5, respectively. The pointing accuracyof the telescope was better than 4 (cid:48)(cid:48) . The typical systemtemperature (T sys ) is around 170 K and varies about tenpercent. Spectra of C H N = − H + J = − a ranged from 20 mKto 50 mK.Mapping observations were performed in June2015 using the PMO 13.7 m telescope. Same frontand back ends were employed as in single-point obser-vations. The on-the-fly (OTF) observation mode wasapplied. The antenna continuously scanned a region of18 (cid:48) × (cid:48) centered on CO-selected cores with a scanspeed of 20 (cid:48)(cid:48) s − . Only the central 10 (cid:48) × (cid:48) regionswere cut out for further analyses because the edges of the OTF maps are very noisy. Data were meshed witha grid spacing of 30 (cid:48)(cid:48) .The GILDAS package including CLASS andGREG (Guilloteau & Lucas 2000; Pety 2005) wasused to reduce the data. All figures were plotted usingthe open source Python package, Matplotlib.
3. Results
Among the 121 observed CO-selected molecularcores, 71 have detection of C H N = − H + J = −
0. Cores with or with-out detection of C H N = − H + J = − ff erent col-ors, and their projected spatial distributions have noobvious deviations. Typical spectra of several coreswith antenna temperature (T a ) of C H N = − J = / − / F = − H + F = −
4n Figure 2 as examples.Mapping observations of C H N = − H + J = − H + J = − All six hyperfine structure (HFS) componentsare well resolved for C H N = − H N = − J = / − / F = − H + J = −
0, as listed in Table 1,are well resolved for most of the sources. F = − H + J = − F = − F = − ∼ − .Using the HFS fitting program in GILDAS / CLASS,we performed hyperfine structure fitting toward spec-tra of C H N = − H + J = −
0. In theHFS fitting, the optical depths of di ff erent hyperfinelines are all assumed as Gaussian with the same width,and the excitation temperatures for di ff erent HFS com-ponents are the same (Feng et al. 2016). Hyperfinestructure fitting can give the parameters such as linewidth ( ∆ V) and velocity (V
LSR ) very precisely. Theresults of HFS fittings are listed in Table 3, includingT a , V LSR , ∆ V, and integrated intensities ( (cid:82) T a dV ) ofC H N = − J = / − / F = − H + J = − F = −
1. The optical depthsare not listed because most of the lines we detect areoptical thin ( τ < a of C H N = − J = / − / F = − a of N H + N = − J = − F = − H + J = − H N = −
0. Only 10 sourceshave T a of N H + J = − F = − H N = − J = / − / F = − σ (Table 3), and all of them have line widths ofC H N = − O larger than the average linewidth of C H ( ∼ − ) except G104.4 + LSR of C H N = − H + J = −
0. They agreewith each other very well, with δ (( V C H − V N H + ) /σ )smaller than three, where σ = σ ( V C H ) + σ ( V N H + ) .From Figure 3(b), one can see that line widths of C H N = − H + J = − δ (( ∆ C H - ∆ N H + ) /σ ) issmaller than 1.5, where σ = σ ( ∆ C H ) + σ ( ∆ N H + ) .The mean widths of CO J = − O J = − H N = − − , slightlylarger than the mean width of C H N = −
0. How-ever, from Figure 3(c) it can be clearly seen that thelarger mean width of C O mainly results from sev-eral sources (shown as red dots) with ∆ V C H smallerthan 1 km s − . The ∆ V of C H N = − O J = − H N = − H + J = −
0. Nonthermal ve-locity dispersions, σ NT , traced by C H N = − H + J = − X ( f X ) can be expressed withthree parameters (a, b, c) as f X ( x ) dx = √ π a exp (cid:32) − (ln( x − b ) − ln( c )) a (cid:33) d ln( x − b )(1)The mean ( µ ) and variance ( σ ) of lognormally dis-tributed random variable X can be expressed as µ = E [ X ] = exp( a / × c + b (2) σ = Var [ X ] = c (cid:16) exp(2 a ) − exp( a ) (cid:17) (3)The three parameters of the best lognormal fits are(0.88, 0.14, 0.20) and (0.87, 0.13, 0.16) (Figure 3(d)),corresponding to mean values and standard deviations( µ , σ ) of the fitted curves (0.43, 0.32) km s − and (0.36,0.25) km s − , respectively. The dispersions of thermal velocity ( σ therm ) and onedimensional nonthermal velocity ( σ NT ) can be calcu-lated as σ therm = (cid:34) kT therm m H µ H (cid:35) (4) σ NT = (cid:34) σ − σ therm m H µ H m X (cid:35) (5)where σ = ∆ V / √ T therm is the gas kinetic tem-perature which is adopted as excitation temperature5 V C H ( km/s ) V N H + ( k m / s ) y =1 . ± . x − . ± . , R =0 . δ ( V C H − V N H + σ ) = 2 . p ( δV ) = p π . exp ( − × . ) (a)0.4 0.0 0.4 V C H − V N H + N ∆ V C H ( km/s ) ∆ V N H + ( k m / s ) y =0 . ± . x − . ± . , R =0 . δ ( ∆ C H − ∆ N H + σ ) = 1 . (b)0.4 0.0 0.4 ∆ V C H − ∆ V N H + N ∆ V C H ( km/s ) ∆ V C O − ∆ V C H ( k m / s ) (c) NT ( km / s )010203040506070 C u m u l a t i v e N u m b e r (d) C H Data: (( , )=(0.43,0.22)Lognorm (0.88,0.14,0.20): P=0.30 N H + Data: (( , )=(0.36,0.18)Lognorm (0.87,0.13,0.16): P=0.95
Fig. 3.— Panel (a): Correlation between the centroid velocity of C H N = − H + J = −
0. Thegreen solid line represents the result of linear least-squares fitting. Panel (b): Correlation between the line width ofC H N = − H + J = −
0. Blue dashed line represents ∆ V C H = ∆ V N H + . Panel (c): Correlationbetween line width of C H N = − ff erence between the line width of C H N = − O. The two dashed lines denote the 1 σ values for the distribution of line width di ff erences. Panel (d): Fittings ofthe cumulative distribution functions of nonthermal velocities. Red and yellow solid lines show results of lognormalfittings. The three parameters (Equation 1) of the best lognormal fits are (0.88, 0.14, 0.20) and (0.87, 0.13, 0.16),respectively. Dashed pink lines show results of standard-normal fittings.of CO, k the Boltzmann’s constant, m X the molecu-lar mass, m H the mass of atomic hydrogen, and µ H = ρ / n(H ) the mean molecular weight of the gas (Kau ff -mann et al. 2008). µ H is adopted as 2.72 assum-ing n(He) / n(H) = σ NT derived from emission ofC H N = − H + J = − H and N H + can be cal- culated through (e.g. Mangum & Shirley 2015) N = k π ν Q (cid:80) S i j µ exp (cid:32) E up kT ex (cid:33) J ( T ex ) (cid:90) τ dV (6) T r ,ν = h ν k [ J ( T ex − J ( T bg ))] × [1 − exp ( − τ ν )] f (7)where J(T) = [ exp ( h ν/ kT ) − − , T bg (2.73 K) is thetemperature of the cosmic background radiation, h isthe Planck constant, and the beam-filling factor f is as-sumed as unit. The permanent dipole moment µ , linestrength S i j , partition function Q and upper level en-6rgy E u were adopted from the Cologne Database forMolecular Spectroscopy and partly listed in Table 1.Unfortunately, the excitation temperatures cannotbe given by HFS fittings because most lines we de-tected are optical thin and the exact beam filling fac-tors are unknown for single-point sources. Besides,the assumption that the excitation temperatures of dif-ferent hyperfine components stay the same is not al-ways valid. For example, the di ff erences among theexcitation temperatures of di ff erent hyperfine compo-nents of C H N = − ex ∼ ex = E u / k , was usuallymade to give the lower limits to the column densities(Miettinen 2014). It is also consistent with the typ-ical excitation temperatures of the N H + J = − J = − CO and CO. The column den-sities of C H and N H + as well as their ratios calcu-lated based on the two set of T ex assumptions are listedin the fourth-to-sixth and seventh-to-ninth columns ofTable 4, respectively. It is clear from Table 4 that col-umn densities calculated based on the two set of T ex assumptions do not deviate much from each other, andmost of them have deviations less than 15 percent. Anunderestimation of T ex (5 K) would introduce a higher τ through Equation 7, which compensates for the re-duced column densities introduced by it in Equation 6.The values of N (C H) / N (N H + ) change little within awide range of temperatures (Pan et al. 2017). The col-umn densities calculated with T ex = Among sources with detection of C H N = − H + J = −
0, 20 were performed with mappingobservations, and all have detection of C H N = − H + J = − σ ∼ − exceptG167.2-15A1. The nondetection of N H + J = − ∼ σ ∼ H N = − H + J = − shown as blue and red contours in Figure 4, respec-tively.From each map shown in Figure 4, one or severalsubstructures are resolved in one CO-select core. Thecontour with half maximum value is taken as the bor-der line of a substructure. Additional labels are used todistinguish di ff erent substructures if there are two ormore substructures resolved within a single map, forexample “NE” if one substructure was in the northeastrelative to its neighbor substructure. Since the emis-sion regions of C H N = − H + J = − H N = − H + J = − r ) of a substructure is defined as the radius ofa circle whose area is equal to the area enclosed bythe border line of that substructure. In total there are26 substructures resolved with r > (cid:48) . The averagevalue of the radius for C H substructures is 1.3 ± ± H + substructures, 0.9 ± ± LSR , ∆ V, and columndensities of peak points as well as the radii of thesesubstructures are also listed in Table 5.Because the abundances of C H are less variantthan those of N H + , the masses of substructures ( M sub )are calculated based on emission of C H N = − M sub = µ m H d X [ C H ] (cid:90) N ( C H ) dS (8)where the abundance of C H ( X [C H]) is assumed as10 − . It is reasonable because abundances of C H arenearly constant in di ff use molecular gas (4 ± × − )(Beuther et al. 2008), starless cores such as TMC-1 (3-5 × − ) (Liszt et al. 2018) and prestellar cores suchas L1498 (0.8 ± × − ) (Padovani et al. 2009) aswell as PGCCs (Sect. 4.2). The uncertainty of cal-culated M sub contributed by the assumption of fixed X [C H] can be as high as a factor of five. Virialmasses ( M vir ) of dense cores assumed as gravitation-ally bounded spheres with ρ ∝ R − can be calculatedthough (MacLaren et al. 1988; Williams et al. 1994) M vir = R σ D γ G (9)7ig. 4.— Contours of integrated intensities of C H N = − H + J = − CO emission. Yellow triangles and greenstars represent 2MASS sources and IRAS sources quoted from Simbad respectively.8here σ D = σ NT + σ therm ), G is the gravitationalconstant, γ = /
3. The M sub , M vir and virial parame-ters α = M vir / M sub are listed in the last three columnsof Table 5.The derived virial parameters range from 1.2 to21.8 with a median value of 4.8. Among the 26 sub-structures resolved, 20 have virial parameters smallerthan five. Considering the possible underestimationsof the masses of substructures for the overestimationsof X [C H], most of these substructures in PGCCs areapproximately virialized and slightly confined by ex-ternal pressures (Pattle et al. 2015).
4. Discussion4.1. Kinematics
All sources have only one velocity component withsingle peak, except for G120 + + δ V = (V thick -V thin ) /∆ V thin < -0.25 (Myers et al. 1996; Mardoneset al. 1997). A red profile would have δ V > + + J = − H + J = − O J = − H N = − CO J = − + H N = − H + J = − O J = −
0, but not with HCN J = − + J = − + J = − CO J = − ff erent species.The widths of di ff erent lines might be the resultof the di ff erent level of turbulence on di ff erent spatialscales. ∆ V of C H N = − H + J = − H N = − − andit is similar to that of HCN J = − O J = − CO J = − − ), Taurus (1.1 km s − ), and California(1.4 km s − ) (Liu et al. 2012; Meng et al. 2013). Formost of these PGCC sources, the line widths seem tobe uniform on di ff erent scales. It may indicate that tur-bulence has been dissipated on smaller scales. The en-tire PGCC region is nearly “transition-to-coherence”because of the low density, thus cuto ff wavelength be-low which Alfven waves cannot propagate and sup-port turbulence (Goodman et al. 1998) is large. Belowcoherence scale, constant residual line widths persistthroughout the volume (Tafalla 2005). The region out-side such a coherent core is more like filled with cloudcomponents with a radially power-law distributed ve-locity field. The Larson’s ∆ V-r relationship (Larson1981) can not be applied to PGCCs (Zhang et al. 2016)because the H column densities of PGCCs are low-est compared with other star formation samples suchas infrared dark clouds (IRDCs) (Wu et al. 2012) thus CO and C O traced the relatively dense componentsin PGCCs. It is also compatible with the concept thatthese PGCCs are quiescent and most of them seemto be in transitions from clouds to dense clumps (Wuet al. 2012). The cloud components may contribute tothe broad line widths of C O of several sources withnarrow C H lines. Sources with broad line widths ofC H and N H + may be more evolved since sourceswith emission of N H + stronger than that of C H allhave line widths broader than 1 km s − .The typical nonthermal velocity traced by C H N = − is ∼ − which corresponds toa σ NT ∼ − . The ratio between σ NT and σ therm ranges from 0.7 to 4.7 with a median value of 1.6.Among 71 sources, 12 have σ NT / σ therm <
1. It is con-sistent with the idea that supersonic isothermal turbu-9 -10 -9 -8 N ( N H + ) /N CO ( H ) N ( C H ) /N CO ( H ) N d ( H )( c m − ) (a) 1.8 1.4 1.0 0.6 0.2 log( N ( N H + ) /N ( C H )) N T M C L C B G MM I MM G . S (b) Fig. 5.— Panel (a): Relation between the abundances of C H and N H + of CO-selected cores and the H columndensities of their host PGCCs ( N d (H )). The abundances of C H and N H + of our sources locate in two di ff erent re-gions separated by the green line. The dashed red line shows the result of linear least-square fitting on data representedby red circles. Panel (b): Blue line shows number density distribution of log( N (N H + ) / N (C H)) for our detectedPGCC sources. The value of this parameter for typical sources are also shown, including well known starless coresTMC-1 (Hirota et al. 2004; Liszt et al. 2018), L1498 and CB246 (Padovani et al. 2009), massive clumps associatedwith infrared dark clouds I18151-1208 MM3 (abbreviated as I18151MM3) and G019.27 + H and N H + traces the innercoherent regions of PGCCs, where radial density dis-tributions of pressure-confined Bonnor-Ebert spheres(Pattle et al. 2015) may be established.All these characteristics indicate that most of ourselected cores in PGCCs are very cold (with an aver-age gas temperature 14 K), quiescent and with singlecomponent, while still turbulence dominant. However,there are still some obviously more envolved sources.Our sample is made up of di ff erent components includ-ing clouds, relatively isolated cold clumps and evolvedgas cores. Emission of C H and N H + originates fromthe inner dense regions but may have di ff erent states ofchemical evolutions (Sect. 4.2). H and N H + These early cores in our sample with low tempera-tures but high enough column densities to shield in-terstellar radiation field (Tatematsu et al. 2017) aregood sites to test the evolutions of those two kind of molecules. C H is generally the most abundanthydrocarbon (Liszt et al. 2018) in di ff use moleculargas and dark cloud gas. It has been known to be atracer of photo-dissociation regions (PDRs) (Fuenteet al. 1993). Recent evidences suggest that it couldalso trace the cold and dense gas associated with theearly stage of star formation. In dark clouds, C H hasan extended distribution (Beuther et al. 2008). In theearly stage of dark clouds, C H is thought to residein the inner regions instead of in the external photo-dissociated layers of clumps (Sanhueza et al. 2013).In the latter stages C H can still has a high abun-dance in the outer region when it is oxidized to formother species such as CO, OH and H O in the densecenter regions (Beuther et al. 2008; Miettinen 2014;Feng et al. 2016). However, N H + usually shows cen-trally peaked emission for its durability in dense re-gions. N H + forms through proton transfer reaction N + H + → N H + + H (Aikawa et al. 2001). N H + isimpeded if CO is present in the gas phase for the com-petition of CO to react with its precursor H + through CO + H + → HCO + + H . Furthermore, CO also playsthe role of the direct destroyer of N H + through reac-tion N H + + CO → HCO + + N (Bergin et al. 2002).Anticorrelation between N H + and gas-phase CO waspresented in envelopes around the pre-stellars and pro-10 A b un d a n c e (a) C HN H + C
11 10 9 log( N [ N H + ] /N [ H ]) l og ( N [ C H ] / N [ N H + ] ) ∆ V = 1 km/s ∆ V = 2 km/s(b) 912151821
Fig. 6.— Panel (a): Time evolution of species according to the result of gas-grain chemical model. Panel (b): Rela-tion between abundances of N H + and N [C H] / N [N H + ]. The color of each dot represents according CO excitationtemperature, and dot-size represents column density of H induced from CO ( N CO (H )). The green line representsthe center line of the green band. The blue line shows the result of the gas-grain chemical model.tostars, such as L1544 (Caselli et al. 1999) and IC 5146(Bergin et al. 2001). In star-forming cores, heating andradiations of protostars will lead to the generation ofC H and the destruction of N H + , which make it hardto predict the evolution trends of the abundances ofthese two species.Figure 5(a) shows the relation between the abun-dances of C H and N H + of CO-selected cores and theH column densities of their host PGCCs ( N d (H )).The H column densities of CO-selected cores de-rived from emission of CO J = − N CO (H ))are adopted to calculate the abundances of C H andN H + . N d (H ) are much lower than N CO (H ) for rela-tively large beams ( ∼ (cid:48) at 350 µ m) (Planck Collabo-ration et al. 2016) and should be treated as the densitiesof the environments of CO-selected cores. N H + abun-dances of CO-selected cores are positively correlatedwith N d (H ), but with a large dispersion (Figure 5(a)).It suggests that N H + abundances are positively corre-lated with the evolutionary ages if the cores in PGCCswith larger N d (H ) tend to be more evolved. On theother hand, the abundances of C H are weakly corre-lated with N d (H ). From Figure 5(a), it is clear to seethat the abundances of C H and N H + of our sourceslocate in two di ff erent regions separated by a greenline. Our sources all have N (C H) > N (N H + ) thusin pretty young states ( < × yr). The abundancesof C H and N H + as well as their ratios can serve asintrinsic parameters to trace the evolutionary states ofPGCC gas cores. Figure 5(b) shows the number den- sity distribution of the ratio between the column den-sity of N H + and that of C H ( N (N H + ) / N (C H)). Thevalue of N (N H + ) / N (C H) for typical starless coresand IRDCs are also shown in Figure 5(b). Our PGCCcores generally have N (N H + ) / N (C H) higher thanthose of starless cores such as TMC-1 and L1498, butlower than those of IRDCs such as G028.34S, whichis consistent with the result of (Tatematsu et al. 2017).We built a very simple gas-grain chemical model tounveil the evolution of C H and N H + in cold gas. Inthis simulation, a single-point (zero-dimension) chem-ical code is run under an ordinary di ff erential equationsolver DVODE (Brown et al. 1989) with most physi-cal parameters fixed and dynamical processes are notcoupled. The temperature is adopted as 10 K, the vis-ible extinction A v =
5, the grain radius σ g = µ m,and the rate of ionization by cosmic-ray γ is set as1 . × − s − (Lee et al. 2004). At such a low temper-ature that is lower than the thermal evaporation tem-perature of CO 22–25 K (Bergin et al. 1995; Rippleet al. 2013), there is nearly no feedback of gas par-ticles except H from grain surfaces, although grainsurface reactions are very active. The gas phase re-actions were downloaded from UMIST Database forAstrochemistry 2012 (McElroy et al. 2013) with 6 173reactions for 467 kind of species. The metal abun-dances are adopted as the low-metal abundance caseof Graedel et al. (1982). Initially, the elements areall ionized except the hydrogen atoms. The volumedensity n(H ) is fixed as 10 cm − , and the results of11he simulation are shown in Figure 6(a). Adopting alower or higher n(H ) has little influence on the evo-lution trends of the abundances except the timescale,especially for the early stages when the chemical pro-cesses are mainly driven by the atoms and ions gener-ated from photo-dissociation and photo-ionization dur-ing prior more di ff use phase instead of externally in-duced ionizations. It is natural that the abundances ofspecies will evolve slower or faster under a lower orhigher n(H ) (Pan et al. 2017). The values along the x-axis of Figure 6(a) have limited meanings and shouldnot be interpreted as the exact chemical ages consid-ering the variances of volume densities of PGCCs. In-stead, we find that X [C H] / X [N H + ] is an intrinsic pa-rameter to trace the evolutionary state of a PGCC.In early stage, the abundance of N H + increaseswith time while that of C H stays nearly constant.The abundance of C H drops down quickly after thecarbon atoms are depleted, while that of N H + keepsgrowing. As shown in Figure 6(b), the ratio betweenthe abundance of N H + and that of C H can tracethe evolution states of PGCCs quite well. The coreswith the lowest abundances of N H + ( < − ) are inthe youngest evolutionary states compared with othersources, and most of them are located in the green bandshown in Figure 6(b) whose center line has a power-law index of 0.75. The power law index is slightlylower than one maybe because of the depletions ofCO (Liu et al. 2012) and thus the H column densi-ties in evolved regions are underestimated. For moreevolved cores, X [N H + ] versus X [C H] / X [N H + ] de-viates from the green band for dropping down of theabundances of C H. The cores with high abundancesof N H + ( ∼ − ) but still located in the green bandmay have the harshest depletions of CO. Another pos-sibility is that the C H emission regions in these coresare dominated by the outer regions where the abun-dances of C H have not yet dropped down. Observa-tions with higher resolutions will be helpful to inves-tigate the depletions in the most inner dense regionsof PGCCs, and exam the validity and general applica-bility of this molecule pair as a tracer of evolutionarystate.
Most of our mapped sources are located in nearbystar forming regions such as Taurus and Cepheus (Wuet al. 2012, and the references therein). Cepheus re-gion consists of two velocity components called fea-ture A (300–500 pc) and feature C (800 pc), and they are associated with the Gould belt and the local armor Orion arm, respectively (Olano et al. 2006). Thedistance of taurus ranges from 130–160 pc (Loinardet al. 2011). Among these 20 mapped CO-selectedcores in PGCCs, six are associated with IRAS andeight with 2MASS objects, both within CO emis-sion regions. There are no cores associated with bothIRAS and 2MASS objects. This is unlike the case ofPGCCs in the second quadrant (with 98 ◦ < l < ◦ and-4 ◦ < b < ◦ as defined by Dame et al. (1987)) in whichmost of the associated IR objects are IRAS pointsources and the rest are 2MASS objects (Zhang et al.2016). However, the ratio of the count of cores associ-ated with IR objects to the size of the sample (referredas core-associated-ratio below) of our sample ( ∼ + H and N H + substructures are much smaller thantheir host clumps and CO emission regions. The av-erage value of r (CO) / r (C H) and r (CO) / r (N H + ) is2.3 and 2.9, respectively. Objects locating within theborder line of a substructure are considered as its asso-ciated objects. In total there are ten substructures as-sociated with IRAS or 2MASS objects. Among them,six are in Cepheus and only one in Taurus. The PGCCsin Cepheus region may be generally more evolved thanthose in the Taurus Complex.The average value of line widths of substructuresin Cepheus, 1.13 km s − , is larger than that in Taurusregion, 0.88 km s − (Table 5). The average value of r (C H) / r (N H + ) for the cores in Taurus, 1.33, is largerthan that in Cepheus, 1.15. Similarly the average valueof the N (C H) / N (N H + ) in Taurus, 23, is larger thanthat in Cepheus, 13. These characteristics again con-firm that PGCCs in Taurus are less evolved than thosein Cepheus. For young sources such as PGCCs in Tau-rus, distribution of C H is much more extended thanthat of N H + . For more evolved sources like PGCCsin Cepheus, N H + is generated in the dense region(Tatematsu et al. 2017) and the area of emission re-gion of N H + continuously expands till close to that ofC H.Our finding is compatible with the statistics of My-ers (1998). Among four complexes, Taurus, Perseus,12rion, and Cepheus, the percentage of supercriticaland cluster associated cores is lowest in Taurus, whilehighest in Cepheus. The cores in Cepheus tend to bedynamically and chemically more evolved than thosein Taurus, because the Taurus complex is younger thanCepheus complex as a whole. Another possibility isthat the materials in Cepheus are significantly a ff ectedby the large void between Cassiopeia and Cepheus(Grenier et al. 1989; Tachihara et al. 2005).Among the four mapped cores not located inTaurus and Cepheus, C H emission regions of twocores (G192.3-11A2, G172.8 + H with annular distribution is detected in G070.4-01A2 (Figure 4). Similar emission distribution hadbeen detected by Tatematsu et al. (2017) in PGCCwith C S surrounding centrally peaked N H + . Abun-dance of C H is positively correlated with that ofC S in dark cloud cores beacuse of the reactions C H + H −→ C H + S −→ C S (Suzuki et al. 1992). Emis-sion of C H shows annular distribution attributed tothe depletion of C H in the central regions of prestellarcores such as L1498 (Padovani et al. 2009). SimilarC H distributions are also observed in various mas-sive star formation regions (Li et al. 2012) such asNGC 6334I (Walsh et al. 2010) and PDRs aroundH ii regions (Pilleri et al. 2013). Although the de-pletion factors of CO are usually not high ( <
2) inearly PGCC cores (Liu et al. 2013), the depletion fac-tors in C H N = − H (0.77 D; Wilson & Green 1977) is about seventimes higher than that of CO. Depletions in the densestplaces of PGCCs may produce the annular distribu-tions of C H emission regions.
5. Summary
We have made C H N = − H + J = − H J = − H + J = −
0. Lineparameters were derived through HFS fittings. Ourcolumn densities calculated assuming T ex equal to 5K and excitation temperatures of CO J = − H and N H + with peaks slightly dislocated. Our main find-ings are as follows:1. Most spectra of detected sources are singlepeaked. Sources that show red and blue profiles inHCO + are identified with multicomponents under jointanalysis of spectra of C H, N H + , CO as well as HCNand HCO + . Centroid velocities and line widths ofC H N = − H + J = − H and N H + is a good tracer of evolution forPGCCs. Gas grain chemical model based on UMISTnetwork is applied to fit N (C H) / N (N H + ) versus N (N H + ). At the most early stage ( N (C H) / N (N H + ) > H is nearly invariable while thatof N H + increases continuously. Later on ( N (C H) / N (N H + ) < H + keeps growing while that ofC H drops rapidly as the exhaustion of carbon atoms.These PGCCs in our sample are in quite early stagesand chemistry driven by residual atoms and ions gen-erated from photo-dissociation and photo-ionizationduring prior more di ff use phase still plays a importantrole.3. The PGCC cores mapped are approximately viri-alized ( α <
5) and slightly confined by external pres-sures. Sources in Cepheus have lower ratios between N (C H) and N (N H + ) and larger line widths com-pared with those in Taurus. The probability of findingan associated IR source within PGCC substructuresin Cepheus, 55%, is larger than that in Taurus, 10%.These indicate that PGCCs in Taurus are less chemi-cally evolved than those in Cepheus. The C H emis-sion region of G074.4 + ff of PMO Qinghai Sta-tion. We also thank Ken’ichi Tatematsu and JunzhiWang for the helpful discussions. This project wassupported by the grants of the National Key R&DProgram of China No. 2017YFA0402600, NSFCNos. 11433008, 11373009, 11373026, 11503035,11573036 and U1631237, and the China Ministry ofScience and Technology under State Key DevelopmentProgram for Basic Research (No.2012CB821800),and the Top Talents Program of Yunnan Province(2015HA030). J. Y. is supported by the Young Re-13earcher Grant of National Astronomical Observato-ries, Chinese Academy of Sciences. REFERENCES
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( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . - A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . - A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . - A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . + A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . - A : : . + : : . . ( . ) . . ( . ) . ( . ) . ( . ) . ( . ) . ( . ) G . + A : : . + : : . . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . - A : : . - : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . - A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . - B : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . + A : : . + : : . ( ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . + A : : . + : : . . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) O n l y t h e d e t ec t e d s ou r ce s a r e li s t e d . D i s t a n cea dop t e d fr o m lit e r a t u r e s ( W u e t a l . ; P l a n c k C o ll a bo r a ti on e t a l . ) . D i s t a n ce g i v e nby B a y e s i a n D i s t a n ce C a l c u l a t o r( R e i d e t a l . ) . H c o l u m nd e n s iti e s o f ho s t P G CC s d e r i v e d fr o m du s t c on ti nuu m . H c o l u m nd e n s iti e s o f C O - s e l ec t e d c o r e s d e r i v e d fr o m N ( C O ) . a b l e : C on ti nu e d . D e s i gn a ti on R A ( J ) D E C ( J ) d i s t r d i s t N d ( H ) ∆ V ( C O ) ∆ V ( C O ) τ ( C O ) T ex ( C O ) N C O ( H ) kp c kp c c m − k m / s k m / s K c m − G . - A : : . - : : . . ( . ) . . ( . ) . ( . ) . ( . ) ( ) ( ) G . + A : : . - : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . + A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . - A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . + A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . + A : : . + : : . . ( . ) . . ( . ) . ( . ) . ( . ) . ( . ) . ( . ) G . + A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . - A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) ( ) G . - A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . - A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . + A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . + A : : . + : : . . ( . ) . . ( . ) . ( . ) . ( . ) . ( . ) . ( . ) G . + A : : . + : : . . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . + A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . + A : : . + : : . . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . - A : : . + : : . . ( . ) . . ( . ) . ( . ) . ( . ) ( ) ( ) G . - A : : . + : : . . ( . ) . . ( . ) . ( . ) . ( . ) ( ) ( ) G . + A : : . + : : . . ( . ) . . ( . ) . ( . ) . ( . ) . ( . ) . ( . ) G . + A : : . + : : . . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . + A : : . + : : . . ( . ) . . ( . ) . ( . ) . ( . ) . ( . ) . ( . ) G . - B : : . + : : . . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . + A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . - A : : . + : : . . ( . ) ( ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . - A : : . + : : . . ( . ) ( ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . + A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) . ( . ) . ( . ) G . + A : : . + : : . . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . + A : : . + : : . . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . + A : : . + : : . . ( . ) . . ( . ) . ( . ) . ( . ) . ( . ) . ( . ) G . - A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . - A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) ( ) . ( . ) G . - A : : . + : : . . ( . ) . ( . ) . . ( . ) . ( . ) . ( . ) . ( . ) . ( . ) Designation C H N H + T a V LSR ∆ V (cid:82) T a dV T a V LSR ∆ V (cid:82) T a dV K K km s − km s − km s − K K km s − km s − km s − G120.1 + + + + + + + + + + + + + Designation C H N H + T a V LSR ∆ V (cid:82) T a dV T a V LSR ∆ V (cid:82) T a dV K K km s − km s − km s − K K km s − km s − km s − G224.2-00A1 0.15(0.03) 14.04(0.05) 1.4(0.1) 0.32(0.02) 0.17(0.05) 13.83(0.10) 1.4(0.3) 0.39(0.07)G026.4 + + + + + + + + + + + + + + + + + + Designation σ NT ( C H ) σ NT ( N H + ) T ex = T ex (CO) T ex = N (C H) N (N H + ) ratio N (C H) N (N H + ) ratiokm s − km s − cm − cm − cm − cm − G120.1 + + + + + + + + + + + + + Designation σ NT ( C H ) σ NT ( N H + ) T ex = T ex (CO) T ex = N (C H) N (N H + ) ratio N (C H) N (N H + ) ratiokm s − km s − cm − cm − cm − cm − G224.2-00A1 0.61(0.04) 0.6(0.1) 34(2) 19(3) 17(3) 36(2) 21(3) 17(3)G026.4 + + + + + + + + + + + + + + + + + + a b l e : M a pp i ngp a r a m e t e r s . C HN H + D e s i gn a ti on s ub r ( C O ) ce n t e r V L S R ∆ V r N ce n t e r V L S R ∆ V r N M s ub M v i r α (cid:48) ( (cid:48) , (cid:48) ) k m s − k m s − (cid:48) c m − ( (cid:48) , (cid:48) ) k m s − k m s − (cid:48) c m − M (cid:12) M (cid:12) G . + A ∆ –2 . (- . , - . )- . . . ( )(- . , - . )- . . . . ( . ) . ( . ) ( ) . G . + A ∆ –2 . ( . , . )- . . . ( )(- . , - . )- . . . ( ) ( ) ( ) . G . - A (cid:63) –3 . (- . , . ) . . . ( )(- . , . ) . . . ( ) . ( . ) ( ) . G . - A (cid:63) –2 . (- . , . ) . . . ( )(- . , . ) . . . . ( . ) . ( . ) . ( . ) . G . - A (cid:63) –– ( . , . ) . . . ( )( . , . ) . . . . ( . ) . ( . ) . ( . ) . G . - A (cid:63) –– (- . , . ) . . . ( )(- . , . ) . . . . ( . ) . ( . ) . ( . ) . G . - A (cid:63) –– (- . , - . )- . . . ( )(- . , - . )- . . . . ( . ) . ( . ) ( ) . G . - A (cid:63) W . ( . , . ) . . . ( )( . , . ) . . . . ( . ) . ( . ) . ( . ) . G . - A (cid:63) E . ( . , . ) . . . ( )( . , . ) . . . . ( . ) . ( . ) . ( . ) . G . - A (cid:63) –3 . ( . , . ) . . . ( )(- . , . ) . . . . ( . ) . ( . ) . ( . ) . G . - A (cid:63) S . ( . , . ) . . . ( )( . , - . ) . . . . ( . ) . ( . ) . ( . ) . G . - A (cid:63) N . (- . , . ) . . . ( )(- . , . ) . . . . ( . ) . ( . ) . ( . ) . G . - A . ( . , - . ) . . . ( )( . , - . ) . . . . ( . ) . ( . ) . ( . ) . G . + A . (- . , . )- . . . ( )(- . , . )- . . . . ( . ) ( ) ( ) . G . - A . ( . , - . ) . . . ( )( . , . ) . . . . ( . ) ( ) ( ) . G . + A ∆ S W . ( . , - . ) . . . ( )(- . , - . ) . . . . ( . ) . ( . ) . ( . ) . G . + A ∆ N E . ( . , . ) . . . ( )( . , . ) . . . . ( . ) . ( . ) . ( . ) . G . - A N W . (- . , - . ) . . . ( )(- . , - . ) . . . ( ) ( ) ( ) . G . - A S E . ( . , - . ) . . . ( )( . , - . ) . . . ( ) ( ) ( ) . G . + A ∆ –3 . (- . , . )- . . . ( )(- . , . )- . . . . ( . ) . ( . ) ( ) . G . + A ∆ –2 . ( . , . ) . . . ( )( . , . ) . . . . ( . ) . ( . ) ( ) . G . + A ∆ –2 . (- . , - . )- . . . ( )(- . , - . )- . . . . ( . ) . ( . ) ( ) . G . + A ∆ W . (- . , - . )- . . . ( )(- . , - . )- . . . . ( . ) . ( . ) ( ) . G . + A ∆ E . ( . , . )- . . . ( )( . , . )- . . . . ( . ) . ( . ) . ( . ) . G . + A ∆ W . ( . , - . )- . . . ( )(- . , - . )- . . . . ( . ) . ( . ) ( ) . G . + A ∆ E . ( . , . )- . . . ( )( . , - . )- . . . . ( . ) . ( . ) ( ) . (cid:63) a nd ∆ r e p r e s e n tt h a tt h i s c o r e i s i n T a u r u s a nd C e ph e u s r e g i on r e s p ec ti v e l y . T h e un ce r t a i n t y c on t r i bu t e dby a ss u m i ng a fi x e d a bund a n ce o f X [ C H ] i s no ti n c l ud e d ..