Multiwavelength study of low-luminosity 6.7-GHz methanol masers

Research in Astron. Astrophys. 2011 Vol. 11 No. 2. 137-155Research inhttp://www.raa-journal.org http://www.iop.org/jourmnals/raaAstronomy andAstrophysicsMultiwavelength study of low-luminosity 6.7-GHz methanolmasers*Yuan- Wei Wu, Ye Xu and Ji YangPurple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China;ywwu@ pmo.ac.cnReceived 2010 April 1I; accepted 2010 July 16Abstract We present results of 13CO(1- 0), Cl80(1-0), and HCO+(1- 0) map ob-servations and N2H+ (1-0) single point observations directed towards a sample ofnine low-luminosity 6.7-GHz masers. N2H+ line emission has been detected fromsix out of nine sources, Cl8O line emission has been detected from eight out of ninesources, and HCO+ and 13CO emission has been detected in all sources. In particular,a“blue profile" of the HCO+ spectrum, a signature of inflow, is found towards onesource. From integrated intensity emission maps, we identified 17 cores in the sample.Among them, nine cores are closely associated with low-luminosity methanol masers.For these cores, we derive the column densities, core sizes, masses and molecularabundances. Comparison of our results with similar molecular line surveys towardsthe southerm sky methanol masers indicates that linewidths of our sample, includingonly the low-luminosity masers, are smaller than the sample that includes both low-and high-luminosity masers. For the maser associated cores, their gas masses have thesame order of magnitude as their virial masses, indicating that these cores are gravi-tationally bound systems. In addition, we have found from our observations that thelow-luminosity methanol masers tend to coexist with H2O masers and outfows ratherthan with OH masers.Key words: masers 一ISM: abundances - - ISM: molecules - - stars: formation1 INTRODUCTIONInterstellar masers, such as the main-line hydroxy (OH) masers, 22-GHz water (H2O) masers andClass II methanol (CH3OH) masers, are one of the most readily observed signposts of star for-mation. These interstellar masers are powerful not only for signaling star formation regions, butalso for diagnosing physical conditions (Pavlakis et al. 1996a,b), probing the kinematics of theseregions (Torrelles et al. 2005; Motogi et al. 2008) and measuring trigonometric parallaxes (Xu etal. 2006a; Reid et al. 2009). Recently, with the advances of our understanding of these interstellarmasers, Ellingsen et al. (2007) even recommended using them to trace different evolutionary phasesof massive star formation, which may shed light on our understanding of early evolutionary stagesof massive stars.Among these species, Class II methanol masers have some advantages over H2O and OH maserssince they exclusively trace massive star-forming regions中国煤化工2008), while* Supported by the National Natural Science Foundation ofYHCNMHG138Y. W. Wu, Y. Xu& J. YangH2O and OH are known to also be associated with low mass star formation and evolved stars. To date,extensive surveys have yielded more than 800 6.7-GHz maser sites (Caswell et al. 1995; Pestalozziet al.2005; Pandian et al. 2007; Ellingsen 2007; Xu et al. 2008; Caswell 2009; Green et al. 2009).Recently, Purcell et al. (2009) found distinctions between radi-quiet and radio-loud subgroups of6.7-GHz methanol masers. Wu et al. (2010, hereafter Paper I) found remarkable physical and kine-matic distinctions between faint and bright subgroups of 6.7-GHz masers. Hence, there is the po-tential to use elaborate classifications of 6.7-GHz masers based on their associations, e.g., UCH IIregions, and inherent attributes. Such applications include using maser luminosities as a clock toindicate the evolutionary stage of ongoing massive star development.Observationally, molecular emission is a powerful tool for investigating the physical and chem-ical conditions in hot cores. Transitions requiring different temperatures and densities for excita-tion constitute an excellent probe of physical conditions. It is known that 12CO is the most usefulmolecule tracer. When the 12C0(1-0) lines are optically thick in most opaque regions of molecularclouds, rarer CO isotopes, i.e., 13CO and C18O, are usually used instead to trace the cloud's mass.In addition, HCO+ and N2H+ are important ionic molecules. Determining the abundance of HCO+can reflect the ionization rate of clouds. Saturated and self absorbed line profiles of HCO+ are alsoused to trace dynamics, like the outflow and inflow of young protostellar objects (Fuller et al. 2005;Klassen & Wilson 2007; Wu et al.2007; Sun & Gao 2009). In contrast, N2H+ is an excellent tracerof quiescent high density gas (Womack et al. 1992). Moreover, since the chemical properties of hotcores vary with time, the relative molecular abundances can also be used as indicators of evolution(Bergin & Langer 1997; Langer et al. 2000). In this paper, we report our 3-mm spectral line observa-tions, including 13C0(1-0), C180(1-0), HCO+(1-0) and N2H+ (1-0) transitions, towards a sampleof nine faint 6.7-GHz methanol masers (Table 1). All nine sources have also been mapped in transi-tions of NH3(1,1), (2.2), (3,3) and 12C0(1-0) in Paper 1. In Section 2, we describe the sample andobservations. In Section 3, we present the results which include molecular line maps and individualdescriptions. Analysis is given in Section 4. Finally, conclusions are drawn in Section 5.Table 1 List of Faint 6.7-GHz MasersSourceRA(2000) Dec (J2000 Speak VisR DEpeakOtherRef.Name(hms)(°'")(kms~1) (kpc) .(Lo)(1)(2)(3(4)5)(6(7(8)106.80+5.31 22:19:18.3 +63:18:480.5-2.0 0.9 47.0E-10S140[13]11.25-0.77 23:16:09.7 +59:55:294.0-38.5 3.58 8.5E-08 IRAS 23139+5939 112121.24 -0.34 00:36:47.358 +63:29:02.18 10-22.80.85{15] 1.3E-08L 1287[9,14]133.72+1.22 02:25:41.9 +62:06:05-44.5 2.3 [54.6E-08W3 IRSS183.35-0.59 05:51:10.8 +25:46:1419. -4.5 2.1.161 1.5E-07 IRAS 05480+2545 [11188.80+1.03 06:09:07.8 +21:50:394.-5.5 2113.3E-08AFGL 5182.[12]189.03+0.78 06:08:40.671 +21:31:06.89 128.81.s[7 6.6E-08AFGL 6466[2,14]189.78+0.34 06:08:35.28 +20:39:06.75.8E-08 .S 252A[2.3]206.54-16.36 05:41:44.15 -01:54:44.9 1.480.4154.4E-10NGC 2024[10]Col. (1) is the source name which is named after Galactic coordinates, Cols. (2) and (3) are equatorial coordi-nates. Peak fux density, central velocity, distance and maser luminosity are listed in Cols. (4)-(7), respectively.Other names and references are listed in Cols. (8) and (9), respectively.References for sources and distances:[I] Carpenter et al. (1995); (2] Caswell et al. (1995); [3] Caswell (2009); [4] Crampton & Fisher (1974); [5]Georgelin & Georgelin (1976); [6] Hughes & Macleod (1993); {7] Humphreys (1978); 181 Larionov et al. (1999);[9] Macleod et al. (1998); [10] Minier et al. (2003); [11]Slysh el al. (999); [12]Szymczak et al. (2000); [13]Xu et al. (2008); [14] Xu et al. (2009); [15] Yang et al. (1991);* Heliocentric kinematic distance.中国煤化工MYHCNMHGMultiwavelength Study of Faint 6.7-GHz Methanol Masers1392 SAMPLE AND OBSERVATIONS .2.1 SampleThe low-luminosity 6.7-GHz methanol masers in this study were mostly selected (seven out of nine)from the catalog of Xu et al. (2003) according to their lowest luminosities (the luminosities arecalculated from the peak flux density assuming a typical linewidth of 0.25 km s-1 and isotropicemission). Two other sources with very low luminosities, 106.80+5.3 I and 206.54- 16.36, were se-lected from Xu et al. (2008) and Minier et al. (2003) respectively. The 6.7-GHz line luminositiesof these sources range from 4.4x10-10 to 1.5x10-7 Lo. Their properties, including galactic andequatorial coordinates, peak flux densities, central velocities, distances, luminosities, possible othernames and references, are listed in Table 1. All of the sources have already been mapped in transi-tions of NH3(1, 1), (2, 2), (3, 3) and 12CO(1 - 0) in Paper I.2.2 ObservationsThe observations were performed during 2008 January and 2010 March with the 13.7-m millime-ter wave telescope in Delingha, China. We mapped the nine sources in transitions of 13C0(1-0),C180(1-0) and HCO+(1-0). A transition of N2H+(l- -0) was observed with a single pointing, tar-geted at the maser sites. A cooled SIS receiver was employed (Zuo et al. 2004), and system tempera-tures were 200 ~ 300 K during the observations. The Acousto-Optical Spectrometer (AOS) was usedto measure the transitions of 13C0(1-0) and C80(1-0) and the Fast Fourier Transform Spectrometer(FFTS) was used to measure the transitions of HCO+(1-0) and N2H+(1-0). The HPBW was 60/'at 110 GHz. The observations were performed in a position switched mode. The grid spacing of themapping observations was 30", and the average integration time was 5 min per point. The pointingaccuracy was better than 10". Data were calibrated using the standard chopper wheel method. S140and NGC 2264 were used for flux calibration and observed every two hours during the observations.Absolute calibration is estimated to be accurate to about 15%. Basic information of the observationsis summarized in Table 2.Table 2 Observation ParametersTranslationHPBWBandwidthlσ rsma(GHz)(")(MH2)(kms-1)13C0(1-0)110.201353504:0.110.10C180(1-0)109.7821826040.12HC0+(1-0)89.188521742000.20N2H+(1-0)93.1718807110000.04Notes- a: typical value in the scale of brightness temperature for the reduced spectra.Data were processed using the CLASS and GREG packages of GILDASI software, includingbaseline subtraction, ftting Gaussian line profiles and hyperfine structure ftting (HFS) of N2H+(1一0) lines.3 RESULTSN2H+ emission is detected in six sources, Cl80 emission is detected in eight sources, and 13CO andHCO+ are detected in all sources. Detected spectral lines are at least 3_σ above the baseline and inmost cases have a signal to noise ratio greater than five. Th中国煤 化工dual sources1 CLASS and GREG are part of the Grenoble Image and Line Data.TYHC N M H GB group's soft.ware. ht:/://w iram.f/IRAMFR/GILDASN140Y.W.Wu,Y.Xu&J.YangTable 3 Detection Rates of Molecular LinesSource Name 13C0(1-0) C180(1-0) HC0+(1-0) N2H+(1-0)_106.80+5.31Y111.25-0.77N121.24 -0.34133.72+1.22183.35-0.59188.80+1.03189.03+0.78189.784+0.34206.54 -16.3Detection Rate9/8/9)/96/9Notes- “Y" indicates detection, “N" indicates no detection.are presented in Table 3. Recently, similar 3-mm spectral line surveys towards southem 6.7-GHzmethanol masers presented high detection rates (99%) of HCO+(1-0), N2H+(1- 0) and 13C0(1-0)(Purcell et al. 2009). Our detection rates of 13C0(1-0) and HC0+(1-0) are 100%, similar to the99% detection rates of Purcell et al. (2009), while our detection rate of N2H+(1-0) is 66%, lowerthan the 99% rate of Purcell et al. (2009). The lower detection rate of N2H+ (1-0) may be due to therelatively lower sensitivity of our observations.3.1 SpectraIn Figure 1, we present the 13CO(1-0), C180(1-0), HCO+(1-0) and N2H+(1-0) spectra at thepositions of the peak of the maser associated cores. 13C0(1-0), C18O(1-0), and HCO+(1-0) linesare ftted with Gaussian profiles. For HCO+(1-0) spectra with evidence of self absorbtion, thoughexhibiting a doubl-peak profile, are ftted with a single Gaussian function. Spectral parameters, i.e,bright temperatures, line width, velocity of the local standard of rest (VLsR), and integral intensitiesof 13C0(1-0), C180(1-0), and HCO+(1-0), are listed in Table 4. For N2H+(1-0), following Purcellet al. (2009), we fit the spectra with two methods: (1) the hyperfine structure ftting routine in theCLASS software, considering the seven-component structure; (2) three Gaussian ftting of the threeblended groups. The ftted parameters of both methods are presented in Table 5.3.2 MapsContours of 13C0(1-0), C80(1-0) and HC0+(1-0) integrated intensities are presented in Figure 2.We use infrared dust emission, from the Midcourse Space Experiment (MSX) E band (21 μm) im-ages, as the background for the contours. For 106.80+5.31, where no MSX data are available, theMultiband Imaging Photometer for Spitzer (MIPS) 24 um image is used. The central blank pixels inthe 24 μm image are due to saturation. The squares, triangles, pluses and ellipses in Figure 2 denotethe 6.7-GHz CH3OH masers, H2O masers, OH masers and error ellipses of the IRAS point sourcesin the fields, respectively. We use Cl80(1-0) (optical thin line) contours to define a concentratedstructure, i.e., a core, and determine the size and position of the core. Although for 133.72+1 .22where there is no detection of Ci8O, the cores are identified from the 13CO map and core 1 of189.78+0.34, where there is no C80 emission peak, is identified from the HCO+ map. Cores wereidentified using visual inspection. The methodology used中国煤化工the operationof the“CLUMPFIND" algorithm (Williams et al. 1994)rom the ninefields, with eight cores showing no maser associations.MHCNMHGMuliwavelength Study of Faint 6.7-GHz Methanol Masers141f068.80+5.31-- 13C0(1-0)fo06.80+5.31~7 C180(1-0)f06.845.317 HC0+(1-0) 315 t8区，区10!5520 v_mi10-20 vumf06.80+5.31 N2H+(1-0)]11.25- -0.77 13C0(1-0)‘” 11.25-.77 C180(1-0)1到20.smt -3a cm枳F21.24-0.3413C0(1-0)21.24-0.34 C180(1-0)511.2 -.77 HC0+(1-0) 31.5 t1.5s'-0.5o.s0.5 i--30-30 uon51:21.24- 0.34~ HC0+(1-0)F21.24-0.34 N2H+(1-0) 1[33.72+1.22 13C0(1-0)Zo.5吕5!-30-20-10-0-303202100-60_ -40.-1 -20Vm (xm :Fig.1 13C0(1-0). C180(1-0), HC0+(1-0) and N2H+(1-0) spectra at the positions of the peak ofthe maser associated cores. The velocities are the radial velocity with respect to the local standard ofrest. The y-axis is the main beam temperature (green lines are ftted profiles, color online).中国煤化工MHCNMHGY.W.Wu,Y.Xu&J.Yang33.2+1.22 HC0+(1-0)183.35-0.501300(1-0)~' 683.35-050 C180(1-0) 35}85s'o twwwww-60Vm (kml)-40-1 -20-30-20. a10°V (umnh)"0183.35-0.59 HC0+(1-0)83.35-0.59 N2H+(1-0) 18188.80+1.03 13C0(1-0) 1-30-2001910-4020-190102088.80+1.03 HC0+(1-0) ]10(89.03+0.78~ 130(1-0)?50.50 20-2020“f89.040.8 C1801-0) ]189.03+0.78~ HC0+(1-0) ]589.03+0.78 N2H+(1-0) 1.5 t&sr20.5oknbW^W* V M州.20 -1vm.IR-20-191R20-10hm.LR 20Fig.1 - Continued.中国煤化工MHCNMHGMultiwavelength Study of Faint 6.7-GHz Methanol Masers143189.78+0.3413C0(1-0)” f89.78+0.34 C180(1-0)689.78+034 HC0+(1-0)10-20 -10 019，20 30-20-10019，20 30).6189.78+0.34 ]N2H+(1-0) ] 20 206.54-1.38 T 13C0(1-0)3)* fo8.54-18.36 C180(1-0) 30.4.!，0oWw Ym-20 vwm.-)200vm20 s08 06.54-16.36 HC0+(1-0) 356.54-16.36~ N2H+(1-0).5 t吕，!2ouwm叫wwww-10 010120 30-20-1001020 30Fig.1 - Continued.3.3 Comments on Individual Sources106.80+5.31- This source is located in the central part of the S 140 molecular complex. Boththe 13C0(1-0) and C180(1- 0) maps show a flamented structure extending southwest to north-east and peak at the central infrared source. The HCO+ map reveals a single core surroundingthe central infrared source. All the three maser species, i.e, CH3OH, H2O and OH masers, werefound in this region.111.25 -0.77一There is a single core peak towards the IRAS 23139+5939. The 13CO mapsreveal an extended emission elongating towards the northern direction. Both H2O and CH3OHmasers were found in this region.121.24- 0.34 一There is a single core which elongates northwest to southeast and peaks at IRAS00338+6312. Both H2O and CH3OH masers were fou中国煤化工w3 complex.133.72+1.22 - This is an intensively studied region13C0(1-0) and HC0+(1-0) spectra show two velodTYHCN M H Gity ranges of144Y.W.Wu,Y.Xu&J.Yang-48~- 42kms- 1 and -40~-36 km s-1, respectively. In the 13CO integrated intensity map .with a velocity range of . 48~- 42 km s-1, one can see a two-core structure, with the easterncore (Core 1) peaking towards the CH3OH maser and the western core (Core 2) associated withthe wester mid-infrared dust core. In the L3CO integrated intensity map with a velocity range of-40~- -36 km s-1 , there is a filament consisting of two cores, with the northwestern core (Core3) departing ~ 30" south of IRAS 02219+6152 and the southeasterm core (Core 4) departing ~2' southeast of the IRAS source. Emission of HCO+ shows a similar distribution. Three H2Oand one CH3OH masers are found in this field.- 183.35- -0.59一A single core is located southwest and with an offset of ~ 15" from IRAS05480+2545. Only the CH3OH maser is found in this region.188.80+1.03 - Both C180(1-0) and HCO+ maps reveal a filament consisting of two coreswhich extend southwest to northeast through the field. Core 1 (the southeast core) is offset by ~30" southeast towards IRAS source 06061+2151 and Core 2 (the northwest core) is offset by ~30"' northeast towards the IRAS source. H2O and CH3OH masers are found in this region.- 189.03+0.78 一The mid-infrared dust emission of this region shows two heat sources. Emissionof CO and HCO+ reveals a flament extending from the southwest infrared source towards thenortheast one. The filament in the C8O map consists of two cores. The southwestern core (Core1) is associated with IRAS 06056+2131. The northeastern core (Core 2) is located in the middleof the two mid-infrared cores departing ~ I' northeast towards IRAS 06056+2131. Both H2Oand CH3OH masers are found to be associated with Core 1.189.78+0.34一This source is part of the S252A molecular complex. 13CO and Cl80 mapsreveal a filamented structure consisting of two cores and extending from north to south, whilethe HCO+ map presents a filament extending northwest to southeast and peaking towards theCH3OH maser.206.54-16.36一It is located in the vicinity of the NGC 2024 HII region. A filament, called the“Molecular Ridge," is elongated in the north-south direction (Chandler & Carlstrom 1996). Thefilament is segregated into two segments on the C18O maps. Along the ridge, there are seven farinfrared sources (Mezger et al. 1988, 1992). The CH3OH maser is associated with FIR4 (Minieret al. 2003).Table 413C0(1-0), C180(1-0), and HC0+(1-0) spectral parameters, including bright tempera-tures, integrated intensities, central velocities and ftted line widths.13cocl8oHCO+Rogion Corea TR. ThdV VLsnov.TR S TAdV Visnov. TR I TRdV VLsR .(K) (Kkns-l) (kms-1) (kms-1) (K) (Kkms-) (kms-1) (kms-1) (K) (Kkms-1) (kms-1) (kms-106.80+5.3112.22 42.34 -7.46(01) 326(02) 1.855.73-752(04) 291(11) 17.82 53.71-6.74(01) 2.83(02)11.25-0.779.7827.43 . 44.09(01) 2.64(03) 1 .12.4. 44.28(04) 2.0511) 1.37 4.79. 44.02/05) 3.27(14)12124-0.34 19.0621.46 -17 39(01) 22(03) 1.57 3.1-17.15(0 1.87(08) 8.70 27.75-1753(02) 3.00(06)133.72+1.22 18.35 29.181.81 9.52- 4057(05) 4.91(14)133.72+1.22 25.67 29.7543.37(01) 4.93(03)-1.41 7.3941.89105) 4.91(14)133.72+1.22 33.17 46.05 -38 88(03) 5.4002)-1.407.00 . 36.1405) 4.71(14)133.72+1.22 49.06 28.15. -8.88(02) 292(02)-1.38 4.14 -3950 281015)183.35-0.59 17.30 18.44. 9.25(01) 237(02) 1.16 234 .9.24(04) 1.90(10) 4.25 12.17-9.76(05) 2.69(15)188.80+1.03 I6.9920.63 -0.58(01) 2.77(02) 1.00 28-0.3906) 265(15) 0.68 1.81-1.2506) 2.4(16)188.80+1.03 2 6.96 21.90 . 0.17(01) 296(02) 1.352.9-0.41(02) 208(05) 0.66 1.99-1.1507) 2.83(16)189.03+0.78 18.3126.702.79(01) 3.02(02)1.263.62.81(06) 2.74(13) 544 18.472.59(01) 3.19(04)189.03+0.78 2 8.56 24.272-32(01) 2.66(03) 1.63 3.82 213(03) 2.20(09) 3.06 10.19228(35) 3.12(09)189.78+0.34 1 12.26 41.28.6401) 3.16(02) 221 4.88.55(02) 2.24(05) 2.77 13.527.99(04) 4.58(09)189.78+0.34 2 1.9 49.238.1501) 3.89(01) 2.39 7.188.04(02) 3.03(05) 2.05 8.327.70(05) 3.83(13)189.78+0.34 310.92 45.76831(01) 3.94(01)2.685.097.67102)_ 1.96(07) 1.39.831.78(08) 3.94(19)206.54-16.3620.43 57.08 10.33(01) 268(01)3.817.4中国煤化工10.06103) 2.66(09)21.27 6.540.61(01) 294(02)__ 3.179.410.72(04) 3.70(07)Notes- - a is 1, 2, 3 and 4 denote the different components in the sYHCNMHGMultiwavelength Study of Faint 6.7-GHz Methanol Masers145+672306; 1680535 1300+8037236, 06065311 Cipo106 845.31 HCO"+632010+8J72030+5T18308631830terour zher4o 2z1r40r112507 130011260 1OO+574815+50*58155958158 -505出+59515g 1579515+5914152716190 21871 271002T1e182r1216030216-190广e 21650.03020 2+6372920+872020+632620+672820+872820+8372720030^8.0 0r34.0 0∞3x"58000307H0 0534800 003万'30 003050 03."4.0f 030573.0erer∞9-ro400Fig.2 13C0(1-0), C*80(1-0) and HC0+(1-0) maps of faint maser regions. The grey scale imagesare MSX 21 um images except for 106.80+5.31, where the grey scale image is the MIPS 24 μmimage. The central blank pixels in the 24 pum image are due to saturation. The squares, triangles,crosses and elipses denote the CH3OH masers, H20 masers, OH masers and error llipses of theIRAS point sources in the fields, respectively. The contours are chosen to highlight the most promi-nent features in each source, usually between 20% and 90% (steps of 10%) of the peak integratedintensity. The thicker lines denote the 50% levels of the peak integrated intensity which are used todetermine the core sizes.中国煤化工MYHCNMHG146Y. w. Wu, Y. Xu&J. Yang1201210--+28*4720+254720+8270100254820喜.8430.0270x00.4200-25420254202000 0250 025380 022528F s5i+180 051120 85150r051r180 0511200 051'T60PUL200001赫10+100 1800180.1.0.010a15450足+2"5010+28465042"4950-2490orowr17 080r000 orwrou.o1物0-.0 1sC010Q0-07BCUNO27513021209 217128+2700ororso orr8r erorsr oorsr" orowrse ororsgr corm" 0rornsr18 01-07 HCO.205950-212020220! 27320217120+20726020750650217020rors. orug orormo ororsr“ ororusr 001oror3ar" RAL200or1x2050“iworwo 0rn. orurPA.2000Fig.2 - Continued中国煤化工MYHCNMHGMultiwavelength Study of Faint 6.7-GHz Methanol Masers147206541636: C180015345. -01*55-015745-01*594505'41*56.0A1L2000Fig2一Continued.3.4 Physical Quantities3.4.1 Optical depth and excitation temperatureIn this section, we derive optical depths and excitation temperatures of CO and N2H+ transitions. For13CO and Cl80, the optical depths and excitation temperatures are estimated by assuming 12C0(1-0) lines are optically thick with formulae 1-3 of Wu et al. (2009). For N2H+ , due to the uncertaintyintroduced by the blended profles, optical depths of N2H+ cannot be directly ftted with the HFSroutine, but rather can be derived from the ratio of integrated intensities, j Tpdu, of the three blendedgroups with a method of Purcell et al. (2009), using equationfTB,1du_ 1-e-T1_ 1-e-n1(1)JTB,2du 1-e-2- 1-e-a7iwhere‘'a' is the expected ratio of T2/m. which should be 1: 5: 2 under optically thin conditions. It isnoted that the optical depth derived here is the“group optical depth" that contains contributions of(21-12), (23- 12) and (22- 11) components.Table 5 Fitted Profile Parameters for N2H+7-component HFS fts a3-Gaussian fts 7Group-IGroup-2Group-3SourceVisROV.fTRdVOV.STRdVoV、fTRdVoVkms-) (kms~1) (K kms-1) (kms~) (Kkmis-) (kms-l) (KkmS-1) (kms~ 1)106.80+5.31- 6.95(02) 1.71(05)1.202.15(19)4.822.48(20)2.87 2.41(39)121.24-0.34 -17.75(03) 1.56(07)0.431.84(24)1.992.39(19)1.02 1.96(24)183.35- -0.59 - -9.70(06) 1.60(11)0.261.41(30)1.242.39(20)0.70 2.1 1(34)189.03+0.78 2.58(06) 1.85(13)0.402.01(46)1.792. 50(30)1.27 3.14(17)189.78+0.347.44(06) 1.55(15)0.230.94(34)1.432.47(28)1.22 2.93(74)206.54-16.36 10.62(06) 1.30(12)| .072.09(25)0.63 1.82(23)Notes- -- a: fted with the CLASS HFS routine considering the 7~cc中国煤化工β3: VisR is rfrring to the Fr,F = 2,3→1,2 transition, i.e. theγr: Gaussian fits to the three groups.CNMHG148Y. W. Wu, Y. Xu &J. YangThen we derived (23- 12) excitation temperatures of N2H+(1-0) using equation=;1(1-e7),(2)where TR and T are brightness temperature and optical depth for the (23- 12) component, T。=hv/k, and Tbg = 2.7K.The optical depths and excitation temperatures of 13CO, Cl80 and N2H+ are tabulated inTable 6. The typical optical depths are 0.7, 0.08 and 0.3 for 13CO, Cl80 and N2H+ , respectively.CO excitation temperatures range from 15 K to 48 K, with a mean value of 28 K. Excitation tem-peratures for N2H+ range from 5 K to 12 K, with a mean value of 7 K. Since the calculations arebased on a uniform beam flling factor, the excitation temperatures (especially for N2H+) derivedhere should be the lower limits of the true values.Table 6 Optical Depths, Column Densities and AbundancesOptical Depth T_ Exciation TemperatureColumn Density NAbundanceRegionCore 13COCI8ON2H+ a Tex (CO)Tex (N2H+)月13CO C180 N2H+ x(C180) X(N2H+)K)(cm-2) (cm-2) 1012cm-2 10-7(1(2) (3) (4)(5)6)7)9)(10)(11)(12)106.80+5.3110.62 0.0.30 32.8(4.9) 12.1(2.9) 9.50E+16 1.01E+16 8.962.12 1.89E-1011.25- -0.77.85 0.0023.8(3.6)5.09E+16 3.26E+151.28121.24 -0.340.80 0.10 0.2722.0(3.3)7.4(2.7) 3.63E+163.85E+15 2.482.12 1.37E -10133.72+1.2235.0(5.7)6.01E+160.2438.3(5.7)6.46E+160.3332.3(4.8)8.60E+160.4927. 7(4.5)5. 14E+16183.35-0.59 1.02 0.13 0.32 21.2(3.2)5.1(3.1) 3.32E+162.85E+15 1.231.727.42E-11188.80+1.030.69 0.0720.7(3.1)3.15E+16 3.27E+152.08188.80+1.03 2 1.23 0.1714.6(2.2)3.12E+16 2.68E+151.72189.03+0.780.72 0.08 0.2523.1(3.5)7.3(6.3) 4.56E+164.69E+15 2.192.06 9.62E-110.89 0.1417.2(2.6)3.45E+16 3.89E+152.26189.78+0.340.72 0.09 0.3733.1(4.9)5.0(2.3) 9.76E+168.58E+15 1.441.76 2.94E-11189.78+0.34 20.82 0.1329.1(4.4)1.08E+17 1, 16E+162.15189.78+0.34 3 0.76 0.1627.1(4.1)9.17E+16 7.82E+151.71206.54- -16.360.940.09 .33 36.0(5.4)5.0(4.3) 1.60E+17 1.44E+16 1.051.80 1.31E-11206.54-16.36 2 1.54 0.1336.7(5.5)2.42E+17 1.90E+161.57mean'0.74 0.09 0.3127.526.976.78E+166.37E+15 2.891.878.98E-11median0.72 0.09 0.3123.806.205.09E+16 4.27E+151.83 .1.93 8.52E-11Notes- a: T(N2H+ ) is the“group optical depth" that contains contributions of (21- 12), (23- 12) and (22- 11) com-ponents; β: Tex(N2H+) is (23- 12) excitation temperature; r: the abundances were derived with the assumption ofX(13CO)~2 x 10-6; 8: mean and median values are only for cores associated with masers, ie, Core 1. .3.4.2 Column density and chemical abundanceWith optical depths and excitation temperatures, total column densities of 18CO, C18O and N2H+can be obtained from (eq. (A1), Scoville et al. 1986)_3k_ exp[hBJ(Ji + 1)/kTex]Tex + hB/3k8π3Bμ2 .(J1+1)1- expl-hy/kT2x[rdu,(3)中国煤化工，where B is the rotational constant of the molecule andloment of themolecule. The values of B and μ are taken to be 55.101:MHC N M H G3CO, 54.891Multiwavelength Study of Faint 6.7-GHz Methanol Masers149MHz and 0. 1098 Debye for Cl8O (Lovas & Krupenie 1974), and 46.587 MHz and 3.37 Debyefor N2H+ (Botschwina 1984). J is the rotational quantum number of the lower state in the ob-served transitions. Column densities of 13CO, C18O and N2H+ are presented in Table 6. Typicalcolumn densities of 13CO, Cl8O and N2H+ are 5x1016, 4x1015 and 1x1012 cm-2, respectively.For HCO+, whose excitation temperature and optical depth are lacking, we are unable to calculatereliable column densities. It is noted that the column densities, especially for N2H+, estimated hereshould be the lower limits, because of the underestimate of excitation temperatures. The total N2H+column density calculated with an excitation temperature of 20 K should be three times larger thanthe value estimated with an excitation temperature of5 K.The relative abundance X between two species may be found directly from the ratio of theirvolume densities. Assuming both molecules occupy the same volume of space, the ratio of twospecies X=n1/n2≈N1/N2. Recently, the COMPLETE molecular survey gave an estimated valueof [H2/3CO] in the range of 2.8 to 4.9 x 105 (Pineda et al. 2008). The chemical evolution modelsuggested a more stable CO abundance relative to HCO+ and N2H+ (Bergin & Langer 1997). Thus,assuming a moderate 13CO abundance of 3x 10-6, we derived the molecular abundances of C18Oand N2H+ for the targeted low-luminosity 6.7-GHz maser regions. The results of column densitiesand chemical abundances are presented in Table 6.The abundances of Cl80 range from 1.3 to 2.3x 10-7, with a median value of 1.9x10-7, butthe abundances of N2H+ range from 1.3x 10-11 to 1.9x 10-10, with a median value of 8.5x 10-11.In contrast, the abundance fluctuation of N2H+ is more dramatic than that of Cl8O.3.4.3 Core size and massThe nominal core sizes, l, were determined from contours of the Cl8O integral intensities by de-convolving the telescope beam, using Equation (4),1/2l= D(号/2- 0明B)(4)where D is the distance, θ1/2 is the half-power angular size of the core, and 0MB is the half-powerbeam width of the telescope.Core masses were computed by assuming a Gaussian column density distribution with a full-width at half-maximum (FWHM) of l. If N(H2) is the molecular hydrogen column density of thepeak position, mH2 is the mass of a hydrogen molecule and μ is the ratio of total gas mass tohydrogen mass (assumed to be 1.36 based on Hildebrand 1983), the core mass is given byMga≈umH2 I 2rrN (H2)e" 41n26(7)dr.(5)With linewidths and core sizes, we also estimated the virial masses following the approach byMacLaren et al. (1988)Mvir = 126rOu2 ,(6)where r is the radius of the core in pc, Ov is the FWHM linewidth of Cl80 in kms-1 and Mvir isthe virial mass in Mo.The estimated nominal core sizes, gas masses and virial masses are tabulated in Table 7.4 DISCUSSION4.1 Line Profiles中国煤化工Line profiles of 13C0 and Cl80 are rlatively simple andEgle Gaussian.In Figure 3 we present example spectra for the source 121YHC N M H G1-0)contour.150Y.W.Wu,Y.Xu&JYangTable 7 Physical Quantities of the CoresRegionCoreR.A.Dec.Angular SizeN(H2)Mgas MvirName(“",")(pe) (x1022cm-2) (Mo) (Mo)(1)(2)(3)(4)(5)(6)(7)(8)9)106.80+5.31 .22:19:20.0 +63:19:10 (120, 90) 0.334.75134823:16:10.0，559125-0.77+59:55:30 (70,60) 0.372.5549121.24 -0.340:36:47.8+63:28:57 (110, 70) 0.231.8225133.72+1.2202:25:41.8 +62:06:05 (30, 30) 0.293.016622002:25:32.0 +62:06:22 (120, 90) 0.843.2359263802:25:41.8 +62:05:40 (150,90) 0.984.3010738502:25:53.5 +62:04:35 (90, 60) 0.422.5711805183.35-0.5905:51:10.5+25:46:05 (120, 90) 0.761.662436188.80+1.0306:09:06.5+21:50:20 (90, 70) 0.451.588310006:09:09.4+21:51:06 (75, 70)0.351.56 .548189.03+0.7806:08:41.0+21:31:08 (70, 70)2.28315406:08:44.5 +21:31:40 (90, 60) 0.271.733341189.78+0.3406:08:34.5 +20:39:00 (100, 80) 0.434.882355806:08:40.5 +20:38:00 (90,60) 0.275.40107806:08:40.5 +20:36:30 (90, 60) 0.274.5987206.54 -16.3605:41:43.5- 01:54:05 (90, 80) 0.118.00213206.54- -16.36205:41:45.0-01:56:05 (120, 100) 0.1612.1040Mean0.363.39Median0.33Cols. (3) and (4) are equatorial coordinates, Col. (5) is angular extensions of the major and minor axes of thecore assuming a spherical geometry. Col. (6) is the nominal core size. Col. (7) is molecular hydrogen columndensity. Gas masses and virial masses are listed in Cols. (8)- (9), respectively. Mean and median values of thesequantities are listed in the last two rows.The spectrum of N2H+ is ftted with the HFS routine in the CLASS software to simultaneously ftthe N2H+ profiles with seven Gaussian distributions. In case of a cold quiescent environment, e.g.,a dark cloud, the spectrum of N2H+(1-0) can exhibit seven hyperfine components (Womack et al.1992). In our case, however, the seven components of N2H+ are blended into three groups due tolarge linewidths.In Figure 3, the spectrum of HCO+ exhibits a double-peak profile. By contrast, the spectrumof Cl8O, an optically thin line, shows a single peak located at the dip of HCO+ , which indicatesthat the double-peak profile of HCO+(1-0) is a self-absorption feature. Therefore, we fit the HCO+spectrum with a single Gaussian by blanking the absorption dip. This "blue profile" is considered asa signature of inflow. Wu et al. (2007) conducted an HCO+ survey towards high mass star formingregions and obtained a high rate of incidence for this kind of“blue profile" (29%). However, in oursample, this inflow signature is only found towards 121.24 _0.34, corresponding to a rate of 11%.In addition, the self-absorption feature is also found in the HCO+ spectrum of 206.54 -16.36, butin contrast to 121.24-0.34, the HCO+ spectrum of 206.54-16.36 shows an excess of the red profilewhich may be evidence of expansion. Emprechtinger et al. (2009) established a sophisticated modelbased on the scenario of a PDR and the“Blister model" to interpret the complex line shapes of themultiple CO transition from 206.54 -16.36.Figure 4 shows the linewidth of the four molecules as an function of maser luminosity. The meanlinewidths towards these low-luminosity maser regions are as follows: HCO+ (3.3 kms-l), 13CO(2.8km s- }), Cl80 (2.3 kms-') and N2H+ (1.6 kms~ l). In general, linewidths of HCO+ are largerthan 13CO, Cl8O and N2H+. Purcell et al. (2006, 2009) surveyed 83 southem methanol masers andobtained larger mean linewidths: HCO+ (5.1 km s- 1), I[ 中国煤化工3.0 km s-').It is obvious that linewidths in our sample, including onl: smaller thanPurcell et al. (2006, 2009)'s sample, which are composed:YHC N M H Gosity masers.Multiwavelength Study of Faint 6.7-GHz Methanol Masers151N2H+(1-0)HC0+(1-0)124-034 13C0C180(1-0)+63*2920+63'2820'+63"2720"00^36"58.0"00^36^-48.000'36"38.0*AlJ2000Fig.3 N2H+, HCO+ and Cl80 characteristic spectra for 121.24 0.34 alongside the 13C0(1-0)contours overlaid on the 21 μm MSX image. On the image, the square marks the 6.7-GHz CH3OHmaser, the triangle marks the H2O maser and the ellipse denotes the error elipse of the IRAS pointsource. Green lines are hyperfine structure/Gaussian fttings. The seven hyperfine components ofN2H+ have blended into three groups due to large linewidths. The spectrum of HCO+ reveals aself- absorbed line profile and is ftted with a Gaussian by masking the self-absorption dip.3.5牛*2.5;1.5!1.010log[Luminosity(Lo)]中国煤化工Fig.4 Line widh as a function of maser luminosity. OpeTYHCNMHG3Co andci8o; pluses and flled triangles indicate HCO+ and NzH152Y. w. Wu, Y. Xu &J. YangPrevious observations of ammonia also indicate that NH3(1,1) (2,2) linewidths of low-luminositymaser regions are smaller than those of high-luminosity maser regions (Paper I).4.2 Gas Mass vs Virial MassIn Figure 5 we plot the diagram of gas mass versus virial mass For the maser associated cores, Mgasranges from 25 to 250 Mg, with a mean value of 104 Mo and Mvir ranges from 13 to 220 Mo, witha mean value of 78 Mo. We can see that the value of Mvir and Mgas are consistent with having thesame magnitude. As a general rule, clouds with Mas/Mvir > 1 are considered to be gravitationallybound systems. Generally, we propose that cores that contain low-luminosity 6.7-GHz methanolmasers should be gravitationally bound. .3.5 r3.0一0▲重22.0冶:1.51.0-1.02.53.03.5Fig.5 Gas masses versus virial masses. Filled triangles and open squares denote cores with andwithout maser associations, respectively. The solid line is the line where Mgas equals Mvir.4.3 AssociationsApart from Class II methanol masers, outlows, and bright IRAS sources, H2O and OH masers arealso signatures of star formation. Since our targeted sources are all methanol masers, it is meaning-ful to investigate the associations of methanol masers with other star-forming phenomena, such asoutflows, bright IRAS sources and two other kinds of masers. In Table 8, we list these associations.The association rates for the IRAS source, outflow and H2O are all 7/9, but the association rate forthe OH maser is only 2/9. Positions of OH, H2O and IRAS sources are also marked in Figure 2 toshow their spatial coexistence. We can see that these low-luminosity methanol masers are highly co-incident with bright IRAS sources, outflows and H2O masers. In contrast, associations of 0H masersare relatively weak. Though the number of studied cases in our sample is limited, our sample clearlyindicates the fact that the Class II methanol maser phase is” 中国煤化工outlw andthe H2O maser phrase than to be overlaid on the OH mas_07; Breen etal. 2010).TYHCNMHGMultiwavelength Study of Faint 6.7-GHz Methanol Masers153Table 8 Associations of These Low-luminosity 6.7-GHz MasersSource NameIRAS NameOutlowH2O106.80+5.3122176+6303Y[1Y [5]Y [7]111.25. -0.7723139+5939YWY[S]N[8121.24 -0.34 .00338+6312Yi1j133.72+1.2202219+6152([1jN[7]183.35-. -0.5905480+2545N[2N[5]188.80+1.0306061+2151 .N[3]NT189.03+0.7806056+2131Y[1]YI5I189.78+0.34Y[4]Y |61]206.54- -16.3NY山N [5][9]Association Rate/9792/9Notes-“Y" indicates association,“N" indicates no association,“.." indicatesthat no information is available;References- (1] Wu et al. (200); [2] Snell et al. (1990); [3] Kim & Kutz(2006); [4] Xu et al. (2006b); [S] Vldetaro e al. (2001); [6] Lada e al. (1981);[7] Baudry et al. (1997); [8] Szymczak & Kus (2000); [9] Knowles et al. (1976).5 CONCLUSIONSWe have performed multi-line observations, including transitions of 13C0(1 -0), Cl80(1-0),HCO+(1-0) and N2H+(1-0), towards nine low-luminosity 6.7-GHz methanol masers. From inte-grated intensity emission maps, we identified 17 cores, among which nine cores are closely asso-ciated with low-luminosity masers and eight cores lack maser associations. Physical quantities ofthese cores were derived, including column densities, core sizes, masses and molecular abundances.Our major findings are as follows:(1) Linewidths of 13CO, HCO+ and N2H+ of these low-luminosity maser regions are smaller thanPurcell et al. (2006, 2009)'s sample.(2) A "blue profile" of the HCO+ (1-0) spectrum, signature of inflow, is found towards one source,the detection rate of which is three times less than Wu et al. (2007).(3) N2H+ abundances of these regions show larger fuctuations than those of CO.(4) Virial masses and gas masses of the maser associated cores are consistent with having the samemagnitude, indicating that these cores that contains low-luminosity 6.7-GHz masers are gravi-tationally bound systems.(5) These low-luminosity masers are more inclined to coexist with H2O masers, outfows and brightIRAS sources rather than to coexist with OH masers.We however caution that the number of sources in our study is limited. Consequently, the find-ings of this work have to be confirmed by a much larger sample.Acknowledgements We wish to thank all the staff at Qinghai Station of Purple MountainObservatory for their assistance with our observations. This work was supported by National NaturalScience Foundation of China (Grant Nos. 11073054, 10733030, 10703010 and 10621303) and theNational Basic Research Program of China (2007CB8 15403). 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