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HCO+ and Radio Continuum Emission from the Star Forming Region G75.78+0.34

HCO+ and Radio Continuum Emission from the Star Forming Region G75.78+0.34 Hindawi Publishing Corporation Advances in Astronomy Volume 2014, Article ID 192513, 5 pages http://dx.doi.org/10.1155/2014/192513 Research Article HCO and Radio Continuum Emission from the Star Forming Region G75.78+0.34 Rogemar A. Riffel and Everton Lüdke Departamento de F´ısica, Centro de Ciencias ˆ Naturais e Exatas, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil Correspondence should be addressed to Rogemar A. Riffel; rogemar@ufsm.br Received 30 September 2013; Accepted 4 December 2013; Published 22 January 2014 Academic Editor: William Reach Copyright © 2014 R. A. Rieff l and E. L ud ¨ ke. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We present 1.3 and 3.6 cm radio continuum images and a HCO spectrum of the massive star forming region G75.78+0.34 obtained with the Very Large Array (VLA) and with the Berkley Illinois Maryland Association (BIMA) interferometer. Three structures were detected in the continuum emission: one associated with the well-known cometary H 00 region, plus two more compact structures located at 6 east and at 2 south of cometary H 00 region. Using the total flux and intensity peak we estimated an electron density 4 −3 7 −6 of≈1.5× 10 cm , an emission measure of≈6× 10 cm pc, a mass of ionized gas of≈3M , and a diameter of 0.05 pc for the cometary H 00 region, being typical values for an ultracompact H 00 region. eTh HCO emission probably originates from the molecular outflows previously observed in HCN and CO. 1. Introduction to observe high density gas. The high density cores ( n∼ 10 - 5 3 10 cm ) can be studied from mm and submm line emission H 00 regions are classified as ultracompact, compact, and clas- + of the HCN and HCO molecules [4]. sical according to their sizes, ionized gas masses, densities, In this work, we use 1.3 and 3.6 cm Very Large Array and emission measures (e.g., [1]). Classical H 00 regions such images to study the physical conditions of the gas in the ultra- as theOrion Nebulaehavesizes of ∼10 pc, densities of compact H 00 region G75.78+0.34, as well as Berkley Illi- 3 5 2 6 ∼100 cm , ∼10 M of ionized gas, and EM ∼ 10 pc cm . nois Maryland Association (BIMA) interferometric data at 3 3 Compact H 00 regions have densities≳5× 10 cm ,sizes of 3 mm. This object presents a well-known molecular outflow 7 −6 ≲0.5 pc, ionized gas mass of∼1M ,and EM≳ 10 pc cm observed in CO [5, 6]and HCN[7]islocated in thegiant while ultracompact H 00 regions have sizes of≲0.1 pc, and molecular cloud ON2 and was rst fi ly identified by Matthews 4 3 thermal electron densities of≳10 cm ,massofionized gasof et al. [8]withobservationsat5and10.7GHz.Sanc ´ hez- −2 7 −6 ∼10 M ,and EM≳ 10 pc cm [2, 3]. Monge et al. [9]usedthe VLAobservationstostudy thegas The study of the physical properties of H 00 regions and content of G75.78+0.34. eTh y found radio emission coming their classification is a fundamental key to understand how from three components: a cometary ultracompact H 00 region, excited by a B0 type star, and with no associated dust they evolute and how stars form. eTh radio continuum emis- sion at centimeter wavelengths traces the emission of the ion- emission, an almost unresolved ultracompact H 00 region, ized gas. On the other hand, the study of the molecular gas associated with a compact dust clump detected at milli- metric and midinfrared wavelengths, and a compact source next to H 00 regions provides significant information about these objects and issues of star formation theory applied to embedded in a dust condensation. eTh continuum emission theseobjects.Sofar,mostofthe studiesofthe moleculargas at 3.5 mm of G75.78+0.34 is dominated by free-free emission are based on CO emission, which has a low dipole moment, from the ionized gas surrounding the exciting star and dust implying that low rotational transitions do not trace dense emission may contribute only to a small fraction of its 3.5 mm gas, while molecules with higher dipole moments can be used continuum [7]. 󸀠󸀠 󸀠󸀠 2 Advances in Astronomy G75.78+0.34 22460.100 MHz This paper is organized as follows. In Section 2 we de- scribe the observations. Section 3 presents our results, which are discussed in Section 4. eTh conclusions of this work are 4 presented in Section 5. (A) 2. Observations 2.1. eTh BIMA Data. The observations of the emission of HCO (J =1-0)at89.1885GHzand HCN(J =1-0)at 1 88.63 GHz from G75.78+0.34 and G75.77+0.34 were per- (B) formed with the Berkley Illinois Maryland Association 0 (C) (BIMA) interferometer by Welch et al. [10] in June, 1999, using its shortest baseline configuration (D-array). −1 Mars and 3C273 observations were used as primary amplitude and bandpass calibrators and the data reduction −2 5 4 3 2 1 0 −1 −2 followed standard procedures with the software MIRIAD by (arc sec) Sault et al. [11]. The full width at half maximum (FWHM) of Center at RA 20 19 52.00000 DEC 37 16 60.0000 the synthesized map is about 18 arcsec and the resulting veloc- Peak flux = 8.3512E − 03 JY/BEAM −1 ity sampling is 𝛿 V ≈ 0.34 km s .TheHCN emission has already been presented and discussed in Riffel and L udk ¨ e [7], Figure 1: 1.3 cm radio continuum image of G75.78+0.34 obtained with VLA at “A” configuration. Flux levels are 0.5, 1.0, 1.5, 2.0, 2.5, where more details about the observations and data reduction 3.0, 3.5, 4.0, 4.5, and 5.0 mJy/beam. process can be found. Here, we present and discuss the spec- trum for the HCO . G75.78+0.34 8460.100 MHz 2.2. Radio Continuum Images. We used Very Large Array (VLA) radiocontinuum observations of G75.78+0.34 at 8.46 GHz (3.6 cm) and 22.46 GHz (1.3 cm) from the VLA data archive (program ID: AK440) obtained using the interfer- ometric array at congfi uration “A”. eTh se observations were processed following the standard procedure of VLA radio continuum imaging processing using the NRAO Astronomi- (A) cal Image Processing System (AIPS). eTh angular resolutions at these frequencies are 0 .24 and 0 .08 for 3.6 cm and 1.3 cm observations, respectively. eTh se resolutions correspond to a −3 −3 spatial resolution of 6.5 × 10 2009pc and 2.2 × 10 pc (B) 0 (C) assuming a distance of 5.6 kpc to G75.78+0.34 [12]from which we estimate that the sizes of the main components A, B, −1 and C are, respectively, 0.065, 0.013, and 0.027 pc. 6 5 4321 0 −1 −2 (arc sec) 3. Results Center at RA 20 19 52.00000 DEC 37 16 60.0000 Peak flux = 3.6009E − 03 JY/BEAM 3.1. Continuum Emission. In Figure 1 we show the 1.3 cm con- tinuum image of G75.78+34, showing thee components iden- Figure 2: 3.5 cm radio continuum image of G75.78+0.34 obtained with VLA at “A” configuration. Flux levels are 0.36, 0.72, 1.08, 1.44, tiefi dasA,B,and Cinthe gfi ure. Ourimage is in good agree- 1.80, 2.16, 2.52, 2.88, 3.24, and 3.6 mJy/beam. ment with the one presented by Sanc ´ hez-Monge et al. [9] using the same instrument configuration. eTh component A shows a complex structure, being more extended to the at 1.3 cm is not seen at 3.6 cm image, because of its lower north-south direction and presenting at least two knots of spatial resolution. Our 3.6 cm image is very similar to the one higher emission. It is associated with the cometary H 00 shown by Wood and Churchwell [12]. The linear sizes of the region observed at 6 cm by Wood and Churchwell [12]and components seen at 3.6 cm image are similar to those listed their image appears to be smoother than ours, probably above for the 1.3 cm image. Table 1 presents the measured total u fl x ( 𝑆 )and thepeak because of its lower spatial resolution. On the other hand, the intensity (𝐼 ) for each component identified in Figures 1 and components B and C are more compact and were not detected 2 at 1.3 cm and 3.6 cm, respectively. at 6 cm. The component C is cospatial with the water maser emission detected by Hofner and Churchwell [13]. The 3.6 cm image of G75.78+0.34 is shown in Figure 2, 3.2.𝑂 Line Emission. The signal-to-noise ratio of our showing the same three components observed at the 1.3 cm BIMA observations was not high enough to construct emis- image. The complex structure seen for the component A sion line u fl x maps and channel maps across the HCO profile (arc sec) (arc sec) 𝐻𝐶 󸀠󸀠 󸀠󸀠 Advances in Astronomy 3 Table 1: Total flux and peak intensity for each component of 3.5 G75.78+0.34 at 1.3 cm and 3.6 cm. 3.0 Component A B C 2.5 𝑆 (1.3 cm) (mJy) 42.4 8.6 8.2 2.0 𝑆 (3.6 cm) (mJy) 50.4 4.7 1.6 1.5 𝐼 (1.3 cm) (mJy/beam) 6.7 8.4 5.9 1.0 0.5 𝐼 (3.6 cm) (mJy/beam) 3.6 3.4 1.0 0.0 89190 89192 89194 89196 89198 89200 Table 2: Physical parameters obtained for the cometary H ii region Frequency (MHz) G75.78+0.34 from the 1.3 cm and 3.6 cm images. Figure 3: HCO spectrum for G75.78+0.34 and G75.77+0.34 from Parameter 1.3 cm 3.6 cm BIMA observations. eTh intensities are shown in arbitrary correlator 𝜃 (arc sec) 0.9 1.2 units. −3 4 4 𝑁 (cm ) 1.7×10 1.2×10 −6 6 6 EM (cm pc) 7.6×10 5.1×10 −2 −2 𝑀 (M ) 2.5×10 3.9×10 eTh mass of ionizedgas canbeobtainedby[ 14] 𝑇 (K) 2530 1100 0.25 0.5 2.5 𝜏 0.29 0.12 (𝑀/ M )=0.7934(𝑆 /Jy) (𝑇 /10 K) (𝐷/ kpc) ⊙ ] 𝑒 (4) Diameter (pc) 0.05 0.06 −0.5 1.5 −1 ×𝑏 (],𝑇 ) 𝜃 (1+𝑌 ) , + + for BIMA in the closest spacing configuration with hybrid where𝑌 is the abundance of He relative to H . self-calibration and closure mapping, as we were able to do for The brightness temperature for a compact radio source is the HCN in Rieff l and L udk ¨ e [7]discussed as ourfirstresults. given by (e.g., [12]) The HCO spectrum is shown in Figure 3 and it was obtained −29 2 by integrating over the whole eld fi of our BIMA observations 𝐼 10 𝑐 𝑇 = , (5) and includes both G75.78+0.34 and G75.77+0.34 H 00 regions. 2 2]𝑘Ω The HCO emission shows three intensity peaks, at 89.193, 89.195, and 89.197 GHz, suggesting the presence of HCO where ] is the frequency,𝐼 is the peak intensity in mJy/beam, clouds with distinct kinematics. 𝑘 is the Boltzmann constant, andΩ is thesolid angleofthe beam given by (e.g., [15]) 4. Discussion Ω =1.133Θ, (6) 4.1. Ionized Gas. The radio continuum images presented here where Θ is the angular resolution of the observations in canbeusedtoinvestigate thepropertiesofthe ionizedgas radians. associated with G75.78+0.34. We can use the total u fl xes Finally, the optical depth (𝜏 ) is estimated using and peak intensities from Table 1 to calculate some physical parameters for the component A associated with the com- −𝜏 𝑇 =𝑇 (1−𝑒 ). (7) 𝑏 𝑒 etary H 00 region. Following Panagia and Walmsley [14] under the assumption that the H 00 region has a roughly spherical In order to obtain the physical parameters above, we geometry, we can obtain the electron density (𝑁 )by assumed typical values for the electron temperature and abundance of𝑇 =10 Kand𝑌 = 0.07 ,respectively. Using 0.25 0.5 −3 2 4 𝑒 (𝑁 /cm )=3.113×10 (𝑆 /Jy) (𝑇 /10 K) 𝑒 ] 𝑒 the equations above and the angular resolutions of Section 2, (1) −13 we obtain𝑏( ],𝑇 ) = 0.7121 andΩ =1.7×10 sr for the −0.5 −0.5 −1.5 𝑒 𝑏 ×(𝐷/ kpc) 𝑏 (],𝑇 ) 𝜃 , −12 1.3 cm image and𝑏( ],𝑇 ) = 0.8025 andΩ = 1.5×10 sr 𝑒 𝑏 for the 3.6 cm image. The radius ( Θ )ofthe componentAcan where𝑆 is the total u fl x, 𝑇 is the electron temperature,𝐷 is ] 𝑒 be measured directly from Figures 1 and 2 and, thus, we can the distance to the object,𝜃 is the angular radius in arcmin- estimate relevant physical parameters for G75.78+0.34. utes, and In Table 2 we present the physical parameters estimated by the equations above, which can be compared with previous 𝑏 (],𝑇 )=1+0.3195 log(𝑇 /10 K)−0.2130 log(]/1 GHz). estimates from the literature. eTh values obtained for the (2) electron density are in reasonable agreement with those obtained by Wood and Churchwell [12]fromthe 6cmradio The emission measure is given by the following equation: 4 −3 emission (𝑁 =2.7×10 cm ) and with the value obtained −6 4 −3 from 7 mm observations−𝑁 ≈5×10 cm [16], as well as (EM/cm pc) (3) with the one found by Sanc ´ hez-Monge et al. [9]froma 4 4 −2 4 −3 =5.63810 (𝑆 /Jy)(𝑇 /10 K)𝑏 (],𝑇 )𝜃 . multifrequency study (𝑁 =3.7×10 cm ). The emission ] 𝑒 𝑅 𝑒 Intensity (a.u.) 4 Advances in Astronomy measure found here is between the values found by Matthews the one of the HCN (1-0) suggesting that the emission may 6 −6 occur within zones of similar physical and kinematical con- et al. [17]ofEM=1.1×10 cm pc, by Wood and Churchwell 6 −6 ditions. Our observations therefore suggest that SiO lines may [12]ofEM=2.3×10 cm pc, and by Sanc ´ hez-Monge et al. 7 −6 not be good observational tracers of shock-driven outflows in [9]ofEM = 2.5×10 cm pc. Wood and Churchwell [12] G75.78+0.34. found𝑇 = 2300Kand 𝜏 = 0.27 ,and thus,our values This conclusion is supported in a similar result obtained obtained from the 1.3 cm emission are in good agreement for the star forming core W3-SE using the Combined Array with theirs, while those estimated from the 3.6 cm are a bit for Research in Millimeter-wave Astronomy (CARMA) by smaller. The mass of G75.78+0.34 obtained here is about one Zhu et al. [19], in which the authors report the detection of order of magnitude smaller than the value found by Matthews the HCO associated with outflows, while the SiO (2-1) line et al. [17] and about one order of magnitude larger than the at 86.243 GHz was also not detected for W3-SE in indepen- valueobtainedbySanc ´ hez-Monge et al. [9], probably due to dent observations. This certainly brings the fact that there differences in the size of the region used to integrate the u fl x aredicffi ultiesinusing SiOtoobtaininformation on the by theseauthors andbyus, as well as duetothe assumptions astrophysical conditions of this compact object within the used in the calculations. The parameters presented in Table 2 production of SiO by destruction of dust grains by radiatively confirm that G75.78+0.34 is an ultracompact H 00 region [3], driven MHD shock front from young and massive stars. confirming previous results of S anc ´ hez-Monge et al. [9]. On the other hand, we must emphasize that there are For the components B and C, we estimate the physical several works for UCH 00 regions and more evolved objects parameters using only the 1.3 cm image, since it has the high- showing that the SiO is a good tracer of outflows (e.g., [ 20– est spatial resolution. For the component B, we obtain𝑁 ≈ 5 −3 7 −6 22]) andapossible explanation forthe absenceofexcited SiO 3.3×10 cm ,EM≈1.9×10 cm pc, and a mass of ionized −3 lines in G75.78+0.34 could be due to low sensitivity of the gas of𝑀≈1.6×10 M ,assuming aradiusof0.25arcsecfor SEST observations. New and more sensitivity observations theregion.Assumingaradiusof0.4arcsecforthecomponent 5 −3 6 −6 are henceforth needed to better constrain the SiO emission or C, we obtain𝑁 ≈1.6×10 cm ,EM≈7.4×10 cm pc, and −3 its absence in G75.78+0.34 and to unravel such apparently 𝑀≈3×10 M . es Th e values are similar to those expected contradictory statements. for ultracompact H 00 regions [2, 3]. In fact, Sanc ´ hez-Monge et al. [9] identified the component C as being an ultracompact H 00 region associated with dust emission in the millimetric 5. Conclusions and midinfrared wavelengths, while the component B seems We studied the radio continuum and molecular emission to be embedded in dust. from the star forming region G75.78+0.34 using VLA and BIMA observations. Our main conclusions are as follows. 4.2. Molecular Gas. Although the signal-to-noise ratio of the HCO data was not high enough to construct u fl x and (i) eTh 3.6 and 1.3 cm radio continuum images for the channel maps, as it has been done for the HCN in Riffel and H 00 region G75.78+0.34 present three components, Ludk ¨ e [7]using thesamedata, itsdetection cantellussome one associated with the cometary H 00 region, other information about the star forming region G75.78+0.34. located at 6 east of the cometary H 00 region, and eTh complexlineprofileshown in Figure 3 suggests that + another structure located at 2 south of it. the HCO emission originate from gas with a disturbed (ii) The cometary H 00 region presents an electron density kinematics and since it usually originate from high density + 4 −3 gas, the HCO emission may be tracing also the bipolar of ≈1.5 × 10 cm ,anemissionmeasure of ≈6 × 7 −6 molecular outflows observed in HCN for G75.78+0.34 in our 10 cm pc, ionized gas of≈3M , and a diameter of previous paper. A similar conclusion has been presented for 0.05 pc, consistent with the values expected for an the Serpens star forming region using BIMA observations by ultracompact H 00 region. Hogerheijde [18], for which it is found that the HCN and + (iii) The HCO (J = 1-0) seems to originate from the gas HCO present enhanced emission near outflows, while the which follows the same astrophysical conditions of N H reflects the distribution of the cloud. outflows previously observed in main HCN transi- tions. Nevertheless, new interferometric observations 4.2.1. SEST Observations and eTh Nondetection of SiO Emis- with enhanced sensitivity and resolution are needed sion. Besides the BIMA data, we have also employed the to better constraint the HCO origin in G75.78+0.34. Swedish-ESO Submillimeter Telescope (SEST) in 20th April (iv) The SiO higher transitions up to 200 GHz were not 2003 in order to detect higher excitation lines like SiO (2- detected using single dish observations with SEST of 1at86.646GHz), SiO(3-2at130.268GHz),SiO (5-4 at G75.78+0.34, suggesting that either it is not a good 217.104 GHz), and SiO (8-7, 347.330 GHz) to make a spec- tracer of outo fl ws in dense cores or that more sensi- tral line survey of maser emission from G75.78+0.34 and tivity is needed at those relevant frequencies. G75.77+0.34 at higher excitations. Even with longer integra- tion times of about one hour per transition, we were unable to detect these lines at a confidence better than 5𝜎 above the Conflict of Interests instrumental noise level. On the other hand, the H CN at 86.338 GHz was present eTh authors declare that there is no conflict of interests in our SEST observations, presenting a profile similar to regarding the publication of this paper. 󸀠󸀠 󸀠󸀠 Advances in Astronomy 5 Acknowledgments [15] K. Rohlfs and T. L. Wilson, Tools of Radio Astronomy,Springer, New York, NY, USA, 2000. eTh authors thank the referee for valuable suggestions that [16] P. Carral,S.E.Kurtz,L.F.Rodr´ıguez, C. de Pree, and P. Hofner, helped to improve the present paper. Everton Ludk ¨ e partic- “Detection of 7 millimeter sources near cometary H II regions,” ularly would like to thank the BIMA Staff and the SEST Ob- The Astrophysical Journal Letters ,vol.486,no. 2, pp.L103–L106, serving Team at ESO La Silla for comments which improved the observations. The SEST Radiotelescope was operated by [17] N. Matthews,M.Andersson,and G. H. 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HCO+ and Radio Continuum Emission from the Star Forming Region G75.78+0.34

Advances in Astronomy , Volume 2014 – Jan 22, 2014

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Abstract

Hindawi Publishing Corporation Advances in Astronomy Volume 2014, Article ID 192513, 5 pages http://dx.doi.org/10.1155/2014/192513 Research Article HCO and Radio Continuum Emission from the Star Forming Region G75.78+0.34 Rogemar A. Riffel and Everton Lüdke Departamento de F´ısica, Centro de Ciencias ˆ Naturais e Exatas, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil Correspondence should be addressed to Rogemar A. Riffel; rogemar@ufsm.br Received 30 September 2013; Accepted 4 December 2013; Published 22 January 2014 Academic Editor: William Reach Copyright © 2014 R. A. Rieff l and E. L ud ¨ ke. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We present 1.3 and 3.6 cm radio continuum images and a HCO spectrum of the massive star forming region G75.78+0.34 obtained with the Very Large Array (VLA) and with the Berkley Illinois Maryland Association (BIMA) interferometer. Three structures were detected in the continuum emission: one associated with the well-known cometary H 00 region, plus two more compact structures located at 6 east and at 2 south of cometary H 00 region. Using the total flux and intensity peak we estimated an electron density 4 −3 7 −6 of≈1.5× 10 cm , an emission measure of≈6× 10 cm pc, a mass of ionized gas of≈3M , and a diameter of 0.05 pc for the cometary H 00 region, being typical values for an ultracompact H 00 region. eTh HCO emission probably originates from the molecular outflows previously observed in HCN and CO. 1. Introduction to observe high density gas. The high density cores ( n∼ 10 - 5 3 10 cm ) can be studied from mm and submm line emission H 00 regions are classified as ultracompact, compact, and clas- + of the HCN and HCO molecules [4]. sical according to their sizes, ionized gas masses, densities, In this work, we use 1.3 and 3.6 cm Very Large Array and emission measures (e.g., [1]). Classical H 00 regions such images to study the physical conditions of the gas in the ultra- as theOrion Nebulaehavesizes of ∼10 pc, densities of compact H 00 region G75.78+0.34, as well as Berkley Illi- 3 5 2 6 ∼100 cm , ∼10 M of ionized gas, and EM ∼ 10 pc cm . nois Maryland Association (BIMA) interferometric data at 3 3 Compact H 00 regions have densities≳5× 10 cm ,sizes of 3 mm. This object presents a well-known molecular outflow 7 −6 ≲0.5 pc, ionized gas mass of∼1M ,and EM≳ 10 pc cm observed in CO [5, 6]and HCN[7]islocated in thegiant while ultracompact H 00 regions have sizes of≲0.1 pc, and molecular cloud ON2 and was rst fi ly identified by Matthews 4 3 thermal electron densities of≳10 cm ,massofionized gasof et al. [8]withobservationsat5and10.7GHz.Sanc ´ hez- −2 7 −6 ∼10 M ,and EM≳ 10 pc cm [2, 3]. Monge et al. [9]usedthe VLAobservationstostudy thegas The study of the physical properties of H 00 regions and content of G75.78+0.34. eTh y found radio emission coming their classification is a fundamental key to understand how from three components: a cometary ultracompact H 00 region, excited by a B0 type star, and with no associated dust they evolute and how stars form. eTh radio continuum emis- sion at centimeter wavelengths traces the emission of the ion- emission, an almost unresolved ultracompact H 00 region, ized gas. On the other hand, the study of the molecular gas associated with a compact dust clump detected at milli- metric and midinfrared wavelengths, and a compact source next to H 00 regions provides significant information about these objects and issues of star formation theory applied to embedded in a dust condensation. eTh continuum emission theseobjects.Sofar,mostofthe studiesofthe moleculargas at 3.5 mm of G75.78+0.34 is dominated by free-free emission are based on CO emission, which has a low dipole moment, from the ionized gas surrounding the exciting star and dust implying that low rotational transitions do not trace dense emission may contribute only to a small fraction of its 3.5 mm gas, while molecules with higher dipole moments can be used continuum [7]. 󸀠󸀠 󸀠󸀠 2 Advances in Astronomy G75.78+0.34 22460.100 MHz This paper is organized as follows. In Section 2 we de- scribe the observations. Section 3 presents our results, which are discussed in Section 4. eTh conclusions of this work are 4 presented in Section 5. (A) 2. Observations 2.1. eTh BIMA Data. The observations of the emission of HCO (J =1-0)at89.1885GHzand HCN(J =1-0)at 1 88.63 GHz from G75.78+0.34 and G75.77+0.34 were per- (B) formed with the Berkley Illinois Maryland Association 0 (C) (BIMA) interferometer by Welch et al. [10] in June, 1999, using its shortest baseline configuration (D-array). −1 Mars and 3C273 observations were used as primary amplitude and bandpass calibrators and the data reduction −2 5 4 3 2 1 0 −1 −2 followed standard procedures with the software MIRIAD by (arc sec) Sault et al. [11]. The full width at half maximum (FWHM) of Center at RA 20 19 52.00000 DEC 37 16 60.0000 the synthesized map is about 18 arcsec and the resulting veloc- Peak flux = 8.3512E − 03 JY/BEAM −1 ity sampling is 𝛿 V ≈ 0.34 km s .TheHCN emission has already been presented and discussed in Riffel and L udk ¨ e [7], Figure 1: 1.3 cm radio continuum image of G75.78+0.34 obtained with VLA at “A” configuration. Flux levels are 0.5, 1.0, 1.5, 2.0, 2.5, where more details about the observations and data reduction 3.0, 3.5, 4.0, 4.5, and 5.0 mJy/beam. process can be found. Here, we present and discuss the spec- trum for the HCO . G75.78+0.34 8460.100 MHz 2.2. Radio Continuum Images. We used Very Large Array (VLA) radiocontinuum observations of G75.78+0.34 at 8.46 GHz (3.6 cm) and 22.46 GHz (1.3 cm) from the VLA data archive (program ID: AK440) obtained using the interfer- ometric array at congfi uration “A”. eTh se observations were processed following the standard procedure of VLA radio continuum imaging processing using the NRAO Astronomi- (A) cal Image Processing System (AIPS). eTh angular resolutions at these frequencies are 0 .24 and 0 .08 for 3.6 cm and 1.3 cm observations, respectively. eTh se resolutions correspond to a −3 −3 spatial resolution of 6.5 × 10 2009pc and 2.2 × 10 pc (B) 0 (C) assuming a distance of 5.6 kpc to G75.78+0.34 [12]from which we estimate that the sizes of the main components A, B, −1 and C are, respectively, 0.065, 0.013, and 0.027 pc. 6 5 4321 0 −1 −2 (arc sec) 3. Results Center at RA 20 19 52.00000 DEC 37 16 60.0000 Peak flux = 3.6009E − 03 JY/BEAM 3.1. Continuum Emission. In Figure 1 we show the 1.3 cm con- tinuum image of G75.78+34, showing thee components iden- Figure 2: 3.5 cm radio continuum image of G75.78+0.34 obtained with VLA at “A” configuration. Flux levels are 0.36, 0.72, 1.08, 1.44, tiefi dasA,B,and Cinthe gfi ure. Ourimage is in good agree- 1.80, 2.16, 2.52, 2.88, 3.24, and 3.6 mJy/beam. ment with the one presented by Sanc ´ hez-Monge et al. [9] using the same instrument configuration. eTh component A shows a complex structure, being more extended to the at 1.3 cm is not seen at 3.6 cm image, because of its lower north-south direction and presenting at least two knots of spatial resolution. Our 3.6 cm image is very similar to the one higher emission. It is associated with the cometary H 00 shown by Wood and Churchwell [12]. The linear sizes of the region observed at 6 cm by Wood and Churchwell [12]and components seen at 3.6 cm image are similar to those listed their image appears to be smoother than ours, probably above for the 1.3 cm image. Table 1 presents the measured total u fl x ( 𝑆 )and thepeak because of its lower spatial resolution. On the other hand, the intensity (𝐼 ) for each component identified in Figures 1 and components B and C are more compact and were not detected 2 at 1.3 cm and 3.6 cm, respectively. at 6 cm. The component C is cospatial with the water maser emission detected by Hofner and Churchwell [13]. The 3.6 cm image of G75.78+0.34 is shown in Figure 2, 3.2.𝑂 Line Emission. The signal-to-noise ratio of our showing the same three components observed at the 1.3 cm BIMA observations was not high enough to construct emis- image. The complex structure seen for the component A sion line u fl x maps and channel maps across the HCO profile (arc sec) (arc sec) 𝐻𝐶 󸀠󸀠 󸀠󸀠 Advances in Astronomy 3 Table 1: Total flux and peak intensity for each component of 3.5 G75.78+0.34 at 1.3 cm and 3.6 cm. 3.0 Component A B C 2.5 𝑆 (1.3 cm) (mJy) 42.4 8.6 8.2 2.0 𝑆 (3.6 cm) (mJy) 50.4 4.7 1.6 1.5 𝐼 (1.3 cm) (mJy/beam) 6.7 8.4 5.9 1.0 0.5 𝐼 (3.6 cm) (mJy/beam) 3.6 3.4 1.0 0.0 89190 89192 89194 89196 89198 89200 Table 2: Physical parameters obtained for the cometary H ii region Frequency (MHz) G75.78+0.34 from the 1.3 cm and 3.6 cm images. Figure 3: HCO spectrum for G75.78+0.34 and G75.77+0.34 from Parameter 1.3 cm 3.6 cm BIMA observations. eTh intensities are shown in arbitrary correlator 𝜃 (arc sec) 0.9 1.2 units. −3 4 4 𝑁 (cm ) 1.7×10 1.2×10 −6 6 6 EM (cm pc) 7.6×10 5.1×10 −2 −2 𝑀 (M ) 2.5×10 3.9×10 eTh mass of ionizedgas canbeobtainedby[ 14] 𝑇 (K) 2530 1100 0.25 0.5 2.5 𝜏 0.29 0.12 (𝑀/ M )=0.7934(𝑆 /Jy) (𝑇 /10 K) (𝐷/ kpc) ⊙ ] 𝑒 (4) Diameter (pc) 0.05 0.06 −0.5 1.5 −1 ×𝑏 (],𝑇 ) 𝜃 (1+𝑌 ) , + + for BIMA in the closest spacing configuration with hybrid where𝑌 is the abundance of He relative to H . self-calibration and closure mapping, as we were able to do for The brightness temperature for a compact radio source is the HCN in Rieff l and L udk ¨ e [7]discussed as ourfirstresults. given by (e.g., [12]) The HCO spectrum is shown in Figure 3 and it was obtained −29 2 by integrating over the whole eld fi of our BIMA observations 𝐼 10 𝑐 𝑇 = , (5) and includes both G75.78+0.34 and G75.77+0.34 H 00 regions. 2 2]𝑘Ω The HCO emission shows three intensity peaks, at 89.193, 89.195, and 89.197 GHz, suggesting the presence of HCO where ] is the frequency,𝐼 is the peak intensity in mJy/beam, clouds with distinct kinematics. 𝑘 is the Boltzmann constant, andΩ is thesolid angleofthe beam given by (e.g., [15]) 4. Discussion Ω =1.133Θ, (6) 4.1. Ionized Gas. The radio continuum images presented here where Θ is the angular resolution of the observations in canbeusedtoinvestigate thepropertiesofthe ionizedgas radians. associated with G75.78+0.34. We can use the total u fl xes Finally, the optical depth (𝜏 ) is estimated using and peak intensities from Table 1 to calculate some physical parameters for the component A associated with the com- −𝜏 𝑇 =𝑇 (1−𝑒 ). (7) 𝑏 𝑒 etary H 00 region. Following Panagia and Walmsley [14] under the assumption that the H 00 region has a roughly spherical In order to obtain the physical parameters above, we geometry, we can obtain the electron density (𝑁 )by assumed typical values for the electron temperature and abundance of𝑇 =10 Kand𝑌 = 0.07 ,respectively. Using 0.25 0.5 −3 2 4 𝑒 (𝑁 /cm )=3.113×10 (𝑆 /Jy) (𝑇 /10 K) 𝑒 ] 𝑒 the equations above and the angular resolutions of Section 2, (1) −13 we obtain𝑏( ],𝑇 ) = 0.7121 andΩ =1.7×10 sr for the −0.5 −0.5 −1.5 𝑒 𝑏 ×(𝐷/ kpc) 𝑏 (],𝑇 ) 𝜃 , −12 1.3 cm image and𝑏( ],𝑇 ) = 0.8025 andΩ = 1.5×10 sr 𝑒 𝑏 for the 3.6 cm image. The radius ( Θ )ofthe componentAcan where𝑆 is the total u fl x, 𝑇 is the electron temperature,𝐷 is ] 𝑒 be measured directly from Figures 1 and 2 and, thus, we can the distance to the object,𝜃 is the angular radius in arcmin- estimate relevant physical parameters for G75.78+0.34. utes, and In Table 2 we present the physical parameters estimated by the equations above, which can be compared with previous 𝑏 (],𝑇 )=1+0.3195 log(𝑇 /10 K)−0.2130 log(]/1 GHz). estimates from the literature. eTh values obtained for the (2) electron density are in reasonable agreement with those obtained by Wood and Churchwell [12]fromthe 6cmradio The emission measure is given by the following equation: 4 −3 emission (𝑁 =2.7×10 cm ) and with the value obtained −6 4 −3 from 7 mm observations−𝑁 ≈5×10 cm [16], as well as (EM/cm pc) (3) with the one found by Sanc ´ hez-Monge et al. [9]froma 4 4 −2 4 −3 =5.63810 (𝑆 /Jy)(𝑇 /10 K)𝑏 (],𝑇 )𝜃 . multifrequency study (𝑁 =3.7×10 cm ). The emission ] 𝑒 𝑅 𝑒 Intensity (a.u.) 4 Advances in Astronomy measure found here is between the values found by Matthews the one of the HCN (1-0) suggesting that the emission may 6 −6 occur within zones of similar physical and kinematical con- et al. [17]ofEM=1.1×10 cm pc, by Wood and Churchwell 6 −6 ditions. Our observations therefore suggest that SiO lines may [12]ofEM=2.3×10 cm pc, and by Sanc ´ hez-Monge et al. 7 −6 not be good observational tracers of shock-driven outflows in [9]ofEM = 2.5×10 cm pc. Wood and Churchwell [12] G75.78+0.34. found𝑇 = 2300Kand 𝜏 = 0.27 ,and thus,our values This conclusion is supported in a similar result obtained obtained from the 1.3 cm emission are in good agreement for the star forming core W3-SE using the Combined Array with theirs, while those estimated from the 3.6 cm are a bit for Research in Millimeter-wave Astronomy (CARMA) by smaller. The mass of G75.78+0.34 obtained here is about one Zhu et al. [19], in which the authors report the detection of order of magnitude smaller than the value found by Matthews the HCO associated with outflows, while the SiO (2-1) line et al. [17] and about one order of magnitude larger than the at 86.243 GHz was also not detected for W3-SE in indepen- valueobtainedbySanc ´ hez-Monge et al. [9], probably due to dent observations. This certainly brings the fact that there differences in the size of the region used to integrate the u fl x aredicffi ultiesinusing SiOtoobtaininformation on the by theseauthors andbyus, as well as duetothe assumptions astrophysical conditions of this compact object within the used in the calculations. The parameters presented in Table 2 production of SiO by destruction of dust grains by radiatively confirm that G75.78+0.34 is an ultracompact H 00 region [3], driven MHD shock front from young and massive stars. confirming previous results of S anc ´ hez-Monge et al. [9]. On the other hand, we must emphasize that there are For the components B and C, we estimate the physical several works for UCH 00 regions and more evolved objects parameters using only the 1.3 cm image, since it has the high- showing that the SiO is a good tracer of outflows (e.g., [ 20– est spatial resolution. For the component B, we obtain𝑁 ≈ 5 −3 7 −6 22]) andapossible explanation forthe absenceofexcited SiO 3.3×10 cm ,EM≈1.9×10 cm pc, and a mass of ionized −3 lines in G75.78+0.34 could be due to low sensitivity of the gas of𝑀≈1.6×10 M ,assuming aradiusof0.25arcsecfor SEST observations. New and more sensitivity observations theregion.Assumingaradiusof0.4arcsecforthecomponent 5 −3 6 −6 are henceforth needed to better constrain the SiO emission or C, we obtain𝑁 ≈1.6×10 cm ,EM≈7.4×10 cm pc, and −3 its absence in G75.78+0.34 and to unravel such apparently 𝑀≈3×10 M . es Th e values are similar to those expected contradictory statements. for ultracompact H 00 regions [2, 3]. In fact, Sanc ´ hez-Monge et al. [9] identified the component C as being an ultracompact H 00 region associated with dust emission in the millimetric 5. Conclusions and midinfrared wavelengths, while the component B seems We studied the radio continuum and molecular emission to be embedded in dust. from the star forming region G75.78+0.34 using VLA and BIMA observations. Our main conclusions are as follows. 4.2. Molecular Gas. Although the signal-to-noise ratio of the HCO data was not high enough to construct u fl x and (i) eTh 3.6 and 1.3 cm radio continuum images for the channel maps, as it has been done for the HCN in Riffel and H 00 region G75.78+0.34 present three components, Ludk ¨ e [7]using thesamedata, itsdetection cantellussome one associated with the cometary H 00 region, other information about the star forming region G75.78+0.34. located at 6 east of the cometary H 00 region, and eTh complexlineprofileshown in Figure 3 suggests that + another structure located at 2 south of it. the HCO emission originate from gas with a disturbed (ii) The cometary H 00 region presents an electron density kinematics and since it usually originate from high density + 4 −3 gas, the HCO emission may be tracing also the bipolar of ≈1.5 × 10 cm ,anemissionmeasure of ≈6 × 7 −6 molecular outflows observed in HCN for G75.78+0.34 in our 10 cm pc, ionized gas of≈3M , and a diameter of previous paper. A similar conclusion has been presented for 0.05 pc, consistent with the values expected for an the Serpens star forming region using BIMA observations by ultracompact H 00 region. Hogerheijde [18], for which it is found that the HCN and + (iii) The HCO (J = 1-0) seems to originate from the gas HCO present enhanced emission near outflows, while the which follows the same astrophysical conditions of N H reflects the distribution of the cloud. outflows previously observed in main HCN transi- tions. Nevertheless, new interferometric observations 4.2.1. SEST Observations and eTh Nondetection of SiO Emis- with enhanced sensitivity and resolution are needed sion. Besides the BIMA data, we have also employed the to better constraint the HCO origin in G75.78+0.34. Swedish-ESO Submillimeter Telescope (SEST) in 20th April (iv) The SiO higher transitions up to 200 GHz were not 2003 in order to detect higher excitation lines like SiO (2- detected using single dish observations with SEST of 1at86.646GHz), SiO(3-2at130.268GHz),SiO (5-4 at G75.78+0.34, suggesting that either it is not a good 217.104 GHz), and SiO (8-7, 347.330 GHz) to make a spec- tracer of outo fl ws in dense cores or that more sensi- tral line survey of maser emission from G75.78+0.34 and tivity is needed at those relevant frequencies. G75.77+0.34 at higher excitations. Even with longer integra- tion times of about one hour per transition, we were unable to detect these lines at a confidence better than 5𝜎 above the Conflict of Interests instrumental noise level. 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