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Register a new userMolecular Cloud Evolution VI. Measuring cloud ages
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Manuel Zamora-Aviles Dr.
Publication date
2018
fields
Physics
and research's language is
English
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In previous contributions, we have presented an analytical model describing the evolution of molecular clouds (MCs) undergoing hierarchical gravitational contraction. The clouds evolution is characterized by an initial increase in its mass, density, and star formation rate (SFR) and efficiency (SFE) as it contracts, followed by a decrease of these quantities as newly formed massive stars begin to disrupt the cloud. The main parameter of the model is the maximum mass reached by the cloud during its evolution. Thus, specifying the instantaneous mass and some other variable completely determines the clouds evolutionary stage. We apply the model to interpret the observed scatter in SFEs of the cloud sample compiled by Lada et al. as an evolutionary effect so that, although clouds such as California and Orion A have similar masses, they are in very different evolutionary stages, causing their very different observed SFRs and SFEs. The model predicts that the California cloud will eventually reach a significantly larger total mass than the Orion A cloud. Next, we apply the model to derive estimated ages of the clouds since the time when approximately 25% of their mass had become molecular. We find ages from $sim 1.5$ to 27 Myr, with the most inactive clouds being the youngest. Further predictions of the model are that clouds with very low SFEs should have massive atomic envelopes constituting the majority of their gravitational mass, and that low-mass clouds ($M sim 10^3$-$10^4 , M_odot$) end their lives with a mini-burst of star formation, reaching SFRs $sim 300$-$500, M_odot$ Myr$^{-1}$. By this time, they have contracted to become compact ($sim 1$ pc) massive star-forming clumps, in general embedded within larger GMCs.
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I describe the scenario of molecular cloud (MC) evolution that has emerged over the past decade or so. MCs can start out as cold atomic clouds formed by compressive motions in the warm neutral medium (WNM) of galaxies. Such motions can be driven by large-scale instabilities, or by local turbulence. The compressions induce a phase transition to the cold neutral medium (CNM) to form growing cold atomic clouds, which in their early stages may constitute thin CNM sheets. Several dynamical instabilities soon destabilize a cloud, rendering it turbulent. For solar neighborhood conditions, a cloud is coincidentally expected to become molecular, magnetically supercritical, and gravitationally dominated at roughly the same column density, $N sim 1.5 times 10^21 psc approx 10 Msun$ pc$^{-2}$. At this point, the cloud begins to contract gravitationally. However, before its global collapse is completed ($sim 10^7$ yr later), the nonlinear density fluctuations within the cloud, which have shorter local free-fall times, collapse first and begin forming stars, a few Myr after the global contraction started. Large-scale fluctuations of lower mean densities collapse later, so the formation of massive star-forming regions is expected to occur late in the evolution of a large cloud complex, while scattered low-mass regions are expected to form earlier. Eventually, the local star formation episodes are terminated by stellar feedback, which disperses the local dense gas, although more work is necessary to clarify the details and characteristic scales of this process.
We present a numerical study of the evolution of molecular clouds, from their formation by converging flows in the warm ISM, to their destruction by the ionizing feedback of the massive stars they form. We improve with respect to our previous simulations by including a different stellar-particle formation algorithm, which allows them to have masses corresponding to single stars rather than to small clusters, and with a mass distribution following a near-Salpeter stellar IMF. We also employ a simplified radiative-transfer algorithm that allows the stellar particles to feed back on the medium at a rate that depends on their mass and the local density. Our results are as follows: a) Contrary to the results from our previous study, where all stellar particles injected energy at a rate corresponding to a star of ~ 10 Msun, the dense gas is now completely evacuated from 10-pc regions around the stars within 10-20 Myr, suggesting that this feat is accomplished essentially by the most massive stars. b) At the scale of the whole numerical simulations, the dense gas mass is reduced by up to an order of magnitude, although star formation (SF) never shuts off completely, indicating that the feedback terminates SF locally, but new SF events continue to occur elesewhere in the clouds. c) The SF efficiency (SFE) is maintained globally at the ~ 10% level, although locally, the cloud with largest degree of focusing of its accretion flow reaches SFE ~ 30%. d) The virial parameter of the clouds approaches unity before the stellar feedback begins to dominate the dynamics, becoming much larger once feedback dominates, suggesting that clouds become unbound as a consequence of the stellar feedback. e) The erosion of the filaments that feed the star-forming clumps produces chains of isolated dense blobs reminiscent of those observed in the vicinity of the dark globule B68.
Aims: We derive the probability density functions (PDFs) of column density for a complete sample of prominent molecular cloud complexes closer than 200 pc. Methods: We derive near-infrared dust extinction maps for 23 molecular cloud complexes, using the nicest colour excess mapping technique and data from the 2MASS archive. The extinction maps are then used to examine the column density PDFs in the clouds. Results: The column density PDFs of most molecular clouds are well-fitted by log-normal functions at low column densities (0.5 mag < A_v < 3-5 mag). However, at higher column densities prominent, power-law-like wings are common. In particular, we identify a trend among the PDFs: active star-forming clouds always have prominent non-log-normal wings. In contrast, clouds without active star formation resemble log-normals over the whole observed column density range, or show only low excess of higher column densities. This trend is also reflected in the cumulative PDFs, showing that the fraction of high column density material is significantly larger in star-forming clouds. These observations are in agreement with an evolutionary trend where turbulent motions are the main cloud-shaping mechanism for quiescent clouds, but the density enhancements induced by them quickly become dominated by gravity (and other mechanisms) which is strongly reflected by the shape of the column density PDFs. The dominant role of the turbulence is restricted to the very early stages of molecular cloud evolution, comparable to the onset of active star formation in the clouds.
We investigate the formation and evolution of giant molecular clouds (GMCs) by the collision of convergent warm neutral medium (WNM) streams in the interstellar medium, in the presence of magnetic fields and ambipolar diffusion (AD), focusing on the evolution of the star formation rate (SFR) and efficiency (SFE), as well as of the mass-to-magnetic-flux ratio (M2FR) in the forming clouds. We find that: 1) Clouds formed by supercritical inflow streams proceed directly to collapse, while clouds formed by subcritical streams first contract and then re-expand, oscillating on the scale of tens of Myr. 2) Our suite of simulations with initial magnetic field strength of 2, 3, and 4 $muG$ show that only supercritical or marginal critical streams lead to reasonable star forming rates. 3) The GMCs M2FR is a generally increasing function of time, whose growth rate depends on the details of how mass is added to the GMC from the WNM. 4) The M2FR is a highly fluctuating function of position in the clouds. 5) In our simulations, the SFE approaches stationarity, because mass is added to the GMC at a similar rate at which it converts mass to stars. In such an approximately stationary regime, the SFE provides a proxy of the supercritical mass fraction in the cloud. 6) We observe the occurrence of buoyancy of the low-M2FR regions within the gravitationally-contracting GMCs, so that the latter naturally segregate into a high-density, high-M2FR core and a low-density, low-M2FR envelope, without the intervention of AD. (Abridged)
Using wide-field $^{13}$CO ($J=1-0$) data taken with the Nobeyama 45-m telescope, we investigate cloud structures of the infrared dark cloud complex in M17 with SCIMES. In total, we identified 118 clouds that contain 11 large clouds with radii larger than 1 pc. The clouds are mainly distributed in the two representative velocity ranges of 10 $-$ 20 km s$^{-1}$ and 30 $-$ 40 km s$^{-1}$. By comparing with the ATLASGAL catalog, we found that the majority of the $^{13}$CO clouds with 10 $-$ 20 km s$^{-1}$ and 30 $-$ 40 km s$^{-1}$ are likely located at distances of 2 kpc (Sagittarius arm) and 3 kpc (Scutum arm), respectively. Analyzing the spatial configuration of the identified clouds and their velocity structures, we attempt to reveal the origin of the cloud structure in this region. Here we discuss three possibilities: (1) overlapping with different velocities, (2) cloud oscillation, and (3) cloud-cloud collision. From the position-velocity diagrams, we found spatially-extended faint emission between $sim$ 20 km s$^{-1}$ and $sim$ 35 km s$^{-1}$, which is mainly distributed in the spatially-overlapped areas of the clouds. We also found that in some areas where clouds with different velocities overlapped, the magnetic field orientation changes abruptly. The distribution of the diffuse emission in the position-position-velocity space and the bending magnetic fields appear to favor the cloud-cloud collision scenario compared to other scenarios. In the cloud-cloud collision scenario, we propose that two $sim$35 km s$^{-1}$ foreground clouds are colliding with clouds at $sim$20 km s$^{-1}$ with a relative velocity of 15 km s$^{-1}$. These clouds may be substructures of two larger clouds having velocities of $sim$ 35 km s$^{-1}$ ($gtrsim 10^3 $ M$_{odot}$) and $sim$ 20 km s$^{-1}$ ($gtrsim 10^4 $ M$_{odot}$), respectively.
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