The Second Survey of the Molecular Clouds in the Large Magellanic Cloud by Nanten. Ii. Star Formation

Abstract

We studied star formation activities in the molecular clouds in the Large Magellanic Cloud. We have utilized the second catalog of 272 molecular clouds obtained by NANTEN to compare the cloud distribution with signatures of massive star formation including stellar clusters, and optical and radio HII regions. We find that the molecular clouds are classified into three types according to the activities of massive star formation; Type I shows no signature of massive star formation, Type II is associated with relatively small HII region(s) and Type III with both HII region(s) and young stellar cluster(s). The radio continuum sources were used to confirm that Type I GMCs do not host optically hidden HII regions. These signatures of massive star formation show a good spatial correlation with the molecular clouds in a sense they are located within ∼100 pc of the molecular clouds. Among possible ideas to explain the GMC Types, we favor that the Types indicate an evolutionary sequence; i.e., the youngest phase is Type I, followed by Type II and the last phase is Type III, where the most active star formation takes place leading to cloud dispersal. The number of the three types of GMCs should be proportional to the time scale of each evolutionary stage if a steady state of massive star and cluster formation is a good approximation. By adopting the time scale of the youngest stellar clusters, 10 Myrs, we roughly estimate the timescales of Types I, II and III to be 6 Myrs, 13 Myrs and 7 Myrs, respectively, corresponding to a lifetime of 20-30 Myrs for the GMCs with a mass above the completeness limit, 5 × 10 4 M ⊙ .

The formation and destruction of molecular clouds and galactic star formation

“…Since the mean density of the initial multiphase HI medium is an order of magnitude higher than the typical warm neutral medium (WNM) density, this formation timescale is shorter than that of molecular cloud formation solely through the accumulation of diffuse WNM (see, e.g., Koyama & Inutsuka 2002;Hennebelle et al 2008;Heitsch & Hartmann 2008;Banerjee et al 2009;Vázquez-Semadeni et al 2011, for the cases of WNM flows). The resulting timescale of molecular cloud formation of 10 Myr is consistent with the evolutionary timescale of molecular clouds in the Large Magellanic Cloud (LMC; Kawamura et al 2009). …”

Multi-frequency observations of SNR J0453–6829 in the LMC

“…We also note that prominent Hα emission is visible in the southwest, which might suggest that higher density ISM material is causing the brightening in the radio shell emission at the outer rim of the SNR in this direction. While no molecular clouds are reported in this region (Fukui et al 2008;Kawamura et al 2009), we do note that SNR J0453-6829 lies in a region of low H i column density with the rim density highest in the south west (Kim et al 1998(Kim et al , 1999. The radio spectral index (Fig.…”

Alma Observations of the Antennae Galaxies. I. A New Window on a Prototypical Merger

“…Other relevant papers are Kawamura et al (2009) and Miura et al (2012). Kawamura et al (2009;see Yamaguchi et al 2001 for an earlier version of related work) used NANTEN observations of molecular gas observations in the Large Magellanic Clouds and developed a model with four evolutionary stages (no star formation, H ii regions, H ii regions and <10 Myr young stellar groups, and H ii regions and 10-30 Myr young stellar groups).…”

“…These include selection and completeness effects, the possibility that many of the clusters may be destroyed with time (e.g., see Whitmore et al 2007), the fact that the star formation rate is not constant, and low-number statistics. Nonetheless, if we cautiously forge ahead we can at least make order-of-magnitude estimates, as has been done by Kawamura et al (2009) and Miura et al (2012).…”