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Extinction and dust/gas ratio in LMC molecular clouds

Dobashi, K. and Bernard, J. P. and Hughes, A. and Paradis, D. and Reach, W. T. and Kawamura, A. (2008) Extinction and dust/gas ratio in LMC molecular clouds. Astronomy and Astrophysics, 484 (1). pp. 205-223. ISSN 0004-6361. https://resolver.caltech.edu/CaltechAUTHORS:DOBaanda08

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Abstract

Aims. The goal of this paper is to measure the dust content and distribution in the Large Magellanic Cloud (LMC) by comparing extinction maps produced in the near-infrared wavelengths and the spatial distribution of the neutral and molecular gas, as traced by Hi and CO observations. Methods. In order to derive an extinction map of the LMC, we have developed a new method to measure the color excess of dark clouds, using the 2MASS all-sky survey. Classical methods to measure the color excess (including the NICE method) tend to underestimate the true color excess if the clouds are significantly contaminated by unreddened foreground stars, as is the case in the LMC. We propose a new method that uses the color of the X percentile reddest stars and which is robust against such contamination. Using this method, it is possible to infer the positions of dark clouds with respect to the star distribution by comparing the observed color excess as a function of the percentile used and that predicted by a model. Results. On the basis of the resulting extinction map, we perform a correlation analysis for a set of dark molecular clouds. Assuming similar infrared absorption properties for the dust in the neutral and molecular phases, we derive the absorption-to-column density ratio AV/NH and the CO-to-H2 conversion factor X_(CO). We show that AV/NH increases from the outskirts of the LMC towards the 30 Dor star-forming region. This can be explained either by a systematic increase of the dust abundance, or by the presence of an additional gas component not traced by Hi or CO, but strongly correlated to the Hi distribution. If dust abundance is allowed to vary, the derived X_(CO) factors for the selected regions are several times lower than those derived from a virial analysis of the CO data. This could indicate that molecular clouds in the LMC are not gravitationally bound, or that they are bounded by substantial external pressure. However, the X_(CO) values derived from absorption can be reconciled with the virial results assuming a constant value for the dust abundance and the existence of an additional, unseen gas component. These results are in agreement with those derived for the LMC from diffuse far-infrared emission.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1051/0004-6361:20079151DOIArticle
http://www.aanda.org/index.php?option=article&access=doi&doi=10.1051/0004-6361:20079151PublisherArticle
ORCID:
AuthorORCID
Reach, W. T.0000-0001-8362-4094
Additional Information:© 2008. Received 27 November 2007 / Accepted 3 March 2008. This work was supported by a “Poste Rouge” from Observatoire Midi-Pyrénées, and also by Grants-in-Aid for Scientific Research from JSPS (17540214), MEXT (18026003), and Ito Science Society (H19). This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
Funders:
Funding AgencyGrant Number
Japan Society for the Promotion of Science17540214
Ministry of Education, Culture, Sports, Science and Technology18026003
Ito Science SocietyH19
NASAUNSPECIFIED
NSFUNSPECIFIED
Subject Keywords:ISM : dust; extinction galaxies : magellanic clouds; infrared : ISM.
Issue or Number:1
Record Number:CaltechAUTHORS:DOBaanda08
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:DOBaanda08
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:13745
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:25 Jun 2009 18:44
Last Modified:03 Oct 2019 00:43

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