Warm C2H2 toward NGC 7538 IRS9: Grain Surface Origin


We consider models for the observed ro-vibrational absorption lines of acetylene toward NGC 7538 IRS9. The data are fit with multiple screens, each having separate column densities, rotational and vibrational excitation temperatures, and filling factors. The best fit was determined using a chi-squared minimization scheme. We find that only one screen is necessary—multiple screens gave rise to either making one of the screens transparent, or very occasionally making the two screens the same. As a result, we can place constraints on Trot, Tvib, NC2H2, and the filling factor, f. In particular we find 0.03 < f < 0.3 with a best fit of f ~ 0.1. We also find Tvib < 200 K, with a best fit of Tvib < 20 K. We find NC2H2 = 2.4 +/- 0.6 × 1016 cm-2, or that N × f ~ 2 × 1015 cm-2. Lastly, we find 80 < Trot < 140 K, with a best fit of Trot ~ 100 K. Physically, this can be interpreted as: (1) no vibrational excitation, (2) the warm region only fills a small fraction of the beam, (3) the C2H2 arises very near a region of 100 K. Chemically, this is in consistent with a model where the C2H2 is formed in the gas phase. It is however consistent with a scenario where the C2H2 is evaporated at 100 K from the grain surface, suggesting either a grain-surface origin or earlier origin followed by condensation. Finally, the C2H2 column density is consistent with a disk geometry.

Share and Cite:

Doty, S. , Doty, S. , Cochran, J. , Lacy, J. , Barentine, J. and Field, R. (2014) Warm C2H2 toward NGC 7538 IRS9: Grain Surface Origin. International Journal of Astronomy and Astrophysics, 4, 479-490. doi: 10.4236/ijaa.2014.43044.

Conflicts of Interest

The authors declare no conflicts of interest.


[1] McKee, C.F. and Tan, J.C. (2002) Massive Star Formation in 100,000 Years from Turbulent and Pressurized Molecular Clouds. Nature, 416, 59-61. http://dx.doi.org/10.1038/416059a
[2] McKee, C.F. and Tan, J.C. (2003) The Formation of Massive Stars from Turbulent Cores. The Astrophysical Journal, 585, 850. http://dx.doi.org/10.1086/346149
[3] Krumholz, M.R., Klein, R.I. and McKee, C.F. (2007) Molecular Line Emission from Massive Protostellar Disks: Predictions for ALMA and EVLA. The Astrophysical Journal, 665, 478.
[4] Bonnell, I.A., Bate, M.R., Clarke, C.J. and Pringle, J.E. (2001) Competitive Accretion in Embedded Stellar Clusters. MNRAS, 323, 785-794. http://dx.doi.org/10.1046/j.1365-8711.2001.04270.x
[5] Bonnell, I.A. and Bate, M.R. (2002) Accrection in Stellar Clusters and the Collisional Formation of Massive Stars. MNRAS, 336, 659-669. http://dx.doi.org/10.1046/j.1365-8711.2002.05794.x
[6] Bonnell, I.A. and Bate, M.R. (2006) Star Formation through Gravitational Collapse and Competitive Accretion. MNRAS, 370, 488.
[7] Bonnell, I.A., Bate, M.R. and Vine, S.G. (2003) The Hierarchical Formation of a Stellar Cluster. MNRAS, 343, 413. http://dx.doi.org/10.1046/j.1365-8711.2003.06687.x
[8] Moscadelli, L., Reid, M.J., Menten, K.M., et al. (2009) Trigonometric Parallaxes of Massive Star Forming Regions. II. Cep A and NGC 7538. The Astrophysical Journal, 693, 406.
[9] Gibb, E.L., Whittet, D.C.B., Boogert, A.C.A. and Tielens, A.G.G.M. (2004) Interstellar Ice: The Infrared Space Observatory Legacy. The Astrophysical Journal, 151, 35. http://dx.doi.org/10.1086/381182
[10] van der Tak, F.F.S., van Dishoeck, E.F. and Caselli, P. (2000) Abundance Profiles of CH3OH and H2CO toward Massive Young Stars as Tests of Gas-Grain Chemical Models. A&A, 361, 327.
[11] Lacy, J.H., Richter, M.J., Greathouse, T.K., Jaffe, D.T. and Zhu, Q. (2002) TEXES: A Sensitive High-Resolution Grating Spectrograph for the Mid-Infrared. Publications of the Astronomical Society of the Pacific, 114, 153. http://dx.doi.org/10.1086/338730
[12] Barentine, J.C. and Lacy, J.H. (2012) A Comparative Astrochemical Study of the High-Mass Protostellar Objects NGC 7538 IRS 9 and IRS 1. Astrophysical Journal, 757, 111.
[13] Ulrich, B.L. and Haas, R.W. (1976) Absolute Calibration of Millimeter-Wavelength Spectral Lines. Astrophysical Journal Supplement Series, 30, 247-258. http://dx.doi.org/10.1086/190361
[14] Doty, S.D., van Dishoeck, E.F., ven der Tak, F.F.S. and Boonman, A.M.S. (2002) Chemistry as a Probe of the Structures and Evolution of Massive Star-Forming Regions. Astronomy & Astrophysics, 389, 446-463. http://dx.doi.org/10.1051/0004-6361:20020597
[15] Doty, S.D. and Neufeld, D.A. (1997) Models for Dense Molecular Cloud Cores. Astrophysical Journal, 489, 122.
[16] Gonzalez-Alfonso, E. and Cernicharo, J. (1997) Explanation of 29SiO, 30SiO, and High-v 28SiO Maser Emission. Astronomy & Astrophysics, 322, 938.
[17] Visser, R., Kristensen, L.E., Bruderer, S., van Dishoeck, E.F., Herczeg, G.J., Brinch, C., et al. (2012) Modeling Herschel Observations of Hot Molecular Gas Emission from Embedded Low-Mass Protostars. Astronomy & Astrophysics, 537, Article No. A55. http://dx.doi.org/10.1051/0004-6361/201117109
[18] Boogert, A.C.A., Helmich, F.P., van Dishoeck, E.F., Schutte, W.A., Tielens, A.G.G.M. and Whittet, D.C.B. (1998) The Gas/Solid Methane Abundance Ratio toward Deeply Embedded Protostars. Astronomy & Astrophysics, 336, 352.
[19] Jacquinet-Husson, N., Scotta, N.A., Chédina, A., Garcerana, K., Armantea, R., Chursinb, A.A., et al. (2005) The 2003 Edition of the GEISA/IASA Spectroscopic Database. Journal of Quantitative Spectroscopy and Radiative Transfer, 95, 429-467. http://dx.doi.org/10.1016/j.jqsrt.2004.12.004
[20] Moré, J.J., Garbow, B.S. and Hillstrom, K.E. (1980) User Guide for MINPACK-1. Argonne National Laboratory Report ANL-80-74, Argonne, Ill.
[21] Doty, S.D. and Palotti, M.L. (2002) A Study of Some Current Methods of Analysing Observations of Star-Forming Regions. Monthly Notices of the Royal Astronomical Society, 335, 993-1004.
[22] Bruderer, S., Doty, S.D. and Benz, A.O. (2009) Chemical Modeling of Young Stellar Objects, I. Method and Benchmarks. Astrophysical Journal Supplement Series, 183, 179-196.
[23] Mitchell, G.F., Curry, C., Maillard, J.P. and Allen, M. (1989) The Gas Environment of the Young Stellar Object GL 2591 Studied by Infrared Spectroscopy. Astrophysical Journal, 341, 1020-1034.
[24] Mitchell, G.F., Maillard, J.P., Allen, M., Beer, R. and Belcourt, K. (1990) Hot and Cold Gas toward Young Stellar Objects. Astrophysical Journal, 363, 554-573. http://dx.doi.org/10.1086/169365
[25] Fraser, H.J., Collings, M.P., McCoustra, M.R.S. and Williams, D.A. (2001) Thermal Desorption of Water Ice in the Interstellar Medium. Monthly Notices of the Royal Astronomical Society, 327, 1165-1172. http://dx.doi.org/10.1046/j.1365-8711.2001.04835.x
[26] Collings, M.P., Anderson, M.A., Chen, R., Dever, J.W., Viti, S., Williams, D.A. and McCoustra, M.R.S. (2004) A Laboratory Survey of the Thermal Desorption of Astrophysically Relevant Molecules. Monthly Notices of the Royal Astronomical Society, 354, 1133-1140. http://dx.doi.org/10.1111/j.1365-2966.2004.08272.x
[27] Collings, M.P., Dever, J.W., Fraser, H.J., McCoustra, M.R.S. and Williams, D.A. (2003) Carbon Monoxide Entrapment in Interstellar Ice Analogs. Astrophysical Journal, 583, 1058-1062.
[28] Hasegawa, T.I., Herbst, E. and Leung, C.M. (1992) Models of Gas-Grain Chemistry in Dense Interstellar Clouds with Complex Organic Molecules. Astrophysical Journal Supplement Series, 82, 167-195. http://dx.doi.org/10.1086/191713
[29] Brooke, T.Y., Tokunaga, A.T., Weaver, H.A., Crovisier, J., Bockelée-Morvan, D. and Crisp, D. (1996) Detection of Acetylene in the Infrared Spectrum of Comet Hyakutake. Nature, 383, 606-608. http://dx.doi.org/10.1038/383606a0
[30] Moore, M.H. and Hudson, R.L. (2005) Astrochemistry: Recent Successes and Current Challenges. Lis, D., Blake, G. and Herbst, E., Eds., Cambridge University Press, Cambridge, 247-260.
[31] Tielens, A.G.G.M. and Charnley, S.B. (1997) Circumstellar and Interstellar Synthesis of Organic Molecules. In: Whittet, D.C.B., Ed., Planetary and Interstellar Processes Relevant to the Origins of Life, Kluwer Academic Publishers, Dordrecht, 23.
[32] Knez, C., Lacy, J.H., Evans, N.J., Richter, M.J., Boonman, A.M.S. and van Dishoeck, E.F. (2003) The Study of Interstellar Chemistry through Mid-Infrared Spectroscopy. RevMexAA (Serie de Conferencias), 18, 45-47.
[33] Mueller, K.E., Shirley, Y.L., Evans II, N.J. and Jacobson, H.R. (2002) The Physical Conditions for Massive Star Formation: Dust Continuum Maps and Modeling. Astrophysical Journal Supplement Series, 143, 469-498.
[34] Bruderer, S., Benz, A.O., Bourke, T.L. and Doty, S.D. (2009) Evidence of Warm and Dense Material along the Outflow of a High-Mass YSO. Astronomy & Astrophysics, 503, L13-L16.
[35] Bruderer, S., Benz, A.O., Doty, S.D., van Dishoeck, E.F. and Bourke, T.L. (2009) Multidimensional Chemical Modeling of Young Stellar Objects. II. Irradiated Outflow Walls in a High-Mass Star-Forming Region. Astrophysical Journal, 700, 872.
[36] Bruderer, S., Benz, A.O., St?uber, P. and Doty, S.D. (2010) Multidimensional Chemical Modeling of Young Stellar Objects. III. The Influence of Geometry on the Abundance and Excitation of Diatomic Hydrides. Astrophysical Journal, 720, 1432.
[37] Sandell, G., Goss, W.M. and Wright, M. (2005) Protostars and Outflows in the NGC 7538 IRS 9 Cloud Core. Astrophysical Journal, 621, 839-852.
[38] Peretto, N., Fuller, G.A., Duarte-Cabral, A., Avison, A., Hennebelle, P., Pineda, J.E., et al. (2013) Global Collapse of Molecular Clouds as a Formation Mechanism for the Most Massive Stars. Astronomy & Astrophysics, 555, Article No. A112. http://dx.doi.org/10.1051/0004-6361/201321318
[39] Sánchez-Monge, A., Beltrán, M.T., Cesaroni, R., Etoka, S., Galli, D., Kuma, M.S.N., et al. (2014) A Necklace of Dense Cores in the High-Mass Star Forming Region G35.20-0.74N: ALMA Observations. http://www.arxiv.org/pdf/1406.4081v1.pdf
[40] Krumholz, M.R., Klein, R.I., McKee, C.F., Offner, S.S.R. and Cunningham, A.J. (2009) The Formation of Massive Star Systems by Accretion. Science, 323, 754-757.
[41] Fallscheer, C., Beuther, H., Sauter, J., Wolf, S. and Zhang, Q. (2011) A High-Mass Dusty Disk Candidate: The Case of IRAS 18151-1208. Astrophysical Journal, 729, 66.
[42] Doty, S.D., van Dishoeck, E.F. and Tan, J.C. (2006) Astrochemical Confirmation of the Rapid Evolution of Massive YSOs and Explanation for the Inferred Ages of Hot Cores. Astronomy & Astrophysics, 454, L5-L8. http://dx.doi.org/10.1051/0004-6361:20065320
[43] Woitke, P., Kamp, I. and Thi, W.F. (2009) Radiation Thermo-Chemical Models of Protoplanetary Disks. I. Hydrostatic Disk Structure and Inner Rim. Astronomy & Astrophysics, 501, 383-406.

Copyright © 2023 by authors and Scientific Research Publishing Inc.

Creative Commons License

This work and the related PDF file are licensed under a Creative Commons Attribution 4.0 International License.