Star clusters form in dense, hierarchically collapsing gas clouds. Bulk kinetic energy is transformed to turbulence with stars forming from cores fed by filaments. In the most compact regions, ...stellar feedback is least effective in removing the gas and stars may form very efficiently. These are also the regions where, in high-mass clusters, ejecta from some kind of high-mass stars are effectively captured during the formation phase of some of the low mass stars and channeled into the latter to form multiple populations. Star formation epochs in star clusters are generally set by gas flows that determine the abundance of gas in the cluster. We argue that there is likely only one star formation epoch after which clusters remain essentially clear of gas by cluster winds. Collisional dynamics is important in this phase leading to core collapse, expansion and eventual dispersion of every cluster. We review recent developments in the field with a focus on theoretical work.
The observational consequences of the merger scenario for massive star formation are explored and contrasted with the gradual accumulation of mass by accretion. In high-density protostar clusters, ...envelopes and disks provide a viscous medium that can dissipate the kinetic energy of passing stars, greatly enhancing the probability of capture. Protostellar mergers may produce high-luminosity infrared flares lasting years to centuries followed by a luminosity decline on the Kelvin-Helmholtz timescale of the merger product. Mergers may be surrounded by thick tori of expanding debris, impulsive wide-angle outflows, and shock-induced maser and radio continuum emission. Collision products are expected to have fast stellar rotation and a large multiplicity fraction. Close encounters or mergers will produce circumstellar debris disks with an orientation that differs from that of a preexisting disk. Thus, massive stars growing by a series of mergers may produce eruptive outflows with random orientations; the walls of the resulting outflow cavities may be observable as filaments of dense gas and dust pointing away from the massive star. The extremely rare merger of two stars close to the upper-mass end of the initial mass function may be a possible pathway to hypernova-generated gamma-ray bursts. In contrast with the violence of merging, the gradual growth of massive stars by accretion is likely to produce less infrared variability, relatively thin circumstellar accretion disks that maintain their orientation, and collimated bipolar outflows that are scaled-up versions of those produced by low-mass young stellar objects. While such accretional growth can lead to the formation of massive stars in isolation or in loose clusters, mergers can only occur in high-density cluster environments. It is proposed that the outflow emerging from the OMC-1 core in the Orion molecular cloud was produced by a protostellar merger that released between 1048 and 1049 ergs less than a thousand years ago.
In this chapter, we review some historical understanding and recent advances on the Initial Mass Function (IMF) and the Core Mass Function (CMF), both in terms of observations and theories. We focus ...mostly on star formation in clustered environment since this is suggested by observations to be the dominant mode of star formation. The statistical properties and the fragmentation behaviour of turbulent gas is discussed, and we also discuss the formation of binaries and small multiple systems.
The CO-dark molecular gas mass in 30 Doradus Chevance, Mélanie; Madden, Suzanne C; Fischer, Christian ...
Monthly Notices of the Royal Astronomical Society,
06/2020, Volume:
494, Issue:
4
Journal Article
Peer reviewed
Open access
ABSTRACT
Determining the efficiency with which gas is converted into stars in galaxies requires an accurate determination of the total reservoir of molecular gas mass. However, despite being the most ...abundant molecule in the Universe, H2 is challenging to detect through direct observations and indirect methods have to be used to estimate the total molecular gas reservoir. These are often based on scaling relations from tracers such as CO or dust, and are generally calibrated in the Milky Way. Yet, evidence that these scaling relations are environmentally dependent is growing. In particular, the commonly used CO-to-H2 conversion factor (XCO) is expected to be higher in metal-poor and/or strongly UV-irradiated environments. We use new SOFIA/FIFI-LS observations of far-infrared fine-structure lines from the ionized and neutral gas and the Meudon photodissociation region model to constrain the physical properties and the structure of the gas in the massive star-forming region of 30 Doradus in the Large Magellanic Cloud, and determine the spatially resolved distribution of the total reservoir of molecular gas in the proximity of the young massive cluster R136. We compare this value with the molecular gas mass inferred from ground-based CO observations and dust-based estimates to quantify the impact of this extreme environment on commonly used tracers of the molecular gas. We find that the strong radiation field combined with the half-solar metallicity of the surrounding gas is responsible for a large reservoir of ‘CO-dark’ molecular gas, leaving a large fraction of the total H2 gas (≳75 per cent) undetected when adopting a standard XCO factor in this massive star-forming region.
Abstract
The Small Magellanic Cloud (SMC) provides the only laboratory to study the structure of molecular gas at high resolution and low metallicity. We present results from the
Herschel
...Spectroscopic Survey of the SMC (HS
3
), which mapped the key far-IR cooling lines C
ii
, O
i
, N
ii
, and O
iii
in five star-forming regions, and new ALMA 7 m array maps of
and
with coverage overlapping four of the five HS
3
regions. We detect C
ii
and O
i
throughout all of the regions mapped. The data allow us to compare the structure of the molecular clouds and surrounding photodissociation regions using
,
, C
ii
, and O
i
emission at
(
pc) scales. We estimate
using far-IR thermal continuum emission from dust and find that the CO/C
ii
ratios reach the Milky Way value at high
in the centers of the clouds and fall to
the Milky Way value in the outskirts, indicating the presence of translucent molecular gas not traced by bright
emission. We estimate the amount of molecular gas traced by bright C
ii
emission at low
and bright
emission at high
. We find that most of the molecular gas is at low
and traced by bright C
ii
emission, but that faint
emission appears to extend to where we estimate that the
-to-H
i
transition occurs. By converting our
gas estimates to a CO-to-
conversion factor (
X
CO
), we show that
X
CO
is primarily a function of
, consistent with simulations and models of low-metallicity molecular clouds.
Abstract
Multi-epoch narrowband Hubble Space Telescope images of the bipolar H
ii
region Sh2-106 reveal highly supersonic nebular proper motions that increase with projected distance from the massive ...young stellar object S106 IR, reaching over ∼30 mas yr
−1
(∼150 km s
−1
at D = 1.09 kpc) at a projected separation of ∼1.′4 (0.44 pc) from S106 IR. We propose that S106 IR experienced a ∼10
47
erg explosion ∼3500 yr ago. The explosion may be the result of a major accretion burst or a recent encounter with another star, or a consequence of the interaction of a companion with the bloated photosphere of S106 IR as it grew from ∼10 through ∼15
M
⊙
at a high accretion rate. Near-IR images reveal fingers of H
2
emission pointing away from S106 IR and an asymmetric photon-dominated region surrounding the ionized nebula. Radio continuum and Br
γ
emission reveal a C-shaped bend in the plasma, indicating either the motion of S106 IR toward the east, or the deflection of plasma toward the west by the surrounding cloud. The H
ii
region bends around a ∼1′ diameter dark bay west of S106 IR that may be shielded from direct illumination by a dense molecular clump. Herbig–Haro and Molecular Hydrogen Objects tracing outflows powered by stars in the Sh2-106 protocluster such as the Class 0 source S106 FIR are discussed.
Stratospheric Observatory for Infrared Astronomy C ii 157 m, APEX 860 m J = 3−2 CO, and archival James Clerk Maxwell Telescope J = 2−1 CO and 13CO observations of the Horsehead Nebula are presented. ...The photon-dominated region (PDR) between the Orion B molecular cloud and the adjacent IC 434 H ii region is used to study the radial velocity structure of the region and the feedback impacts of UV radiation. Multiple west-facing cloud edges are superimposed along the line of sight with radial velocities that differ by a few kilometers per second. The Horsehead lies in the foreground blueshifted portion of the Orion B molecular cloud and is predominantly illuminated from the rear. The mean H2 density of the Horsehead, , results in a spatially thin PDR where the photoablation flow has compressed the western cloud edge to an H2 density of . The associated C ii 157 m layer has a width L < 0.05 pc. The background parts of the Orion B cloud in the imaged field consist of a clumpy medium surrounded by molecular gas with H2 densities lower by one to two orders of magnitude. Along the straight part of the IC 434 ionization front, the PDR layer probed by C ii 157 m emission is much thicker with L ∼ 0.5 pc. A possible model for the formation and evolution of this edge-on ionization front and PDR is presented. The C ii data were independently analyzed and published by Pabst et al.
Based on deep Very Large Telescope Infrared Spectrometer and Array Camera JHK photometry, we have derived the present-day mass function (MF) of the central starburst cluster NGC 3603 YC (Young ...Cluster) in the giant H II region NGC 3603. The effects of field contamination, individual reddening, and a possible binary contribution are investigated. The MF slopes resulting from the different methods are compared and lead to a surprisingly consistent cluster MF with a slope of = -0.9 ± 0.15. Analyzing different radial annuli around the cluster core, no significant change in the slope of the MF is observed. However, mass segregation in the cluster is evidenced by the increasing depletion of the high-mass tail of the stellar mass distribution with increasing radius. We discuss the indications of mass segregation with respect to the changes observed in the binned and cumulative stellar MFs and argue that the cumulative function, as well as the fraction of high- to low-mass stars, provides better indicators for mass segregation than the MF slope alone. Finally, the observed MF and starburst morphology of NGC 3603 YC are discussed in the context of massive local star-forming regions such as the Galactic center Arches cluster, R136/30 Dor in the LMC, and the Orion Trapezium cluster, all providing resolved templates for extragalactic star formation. Despite the similarity in the observed MF slopes, dynamical considerations suggest that the starburst clusters do not form gravitationally bound systems over a Hubble time. Both the environment (gravitational potential of the Milky Way) and the concentration of stars in the cluster core determine the dynamical stability of a dense star cluster, such that the long-term evolution of a starburst is not exclusively determined by the stellar evolution of its members, as frequently assumed for globular cluster systems.
We present new sensitive CO(2-1) observations of the 30 Doradus region in the Large Magellanic Cloud. We identify a chain of three newly discovered molecular clouds that we name KN1, KN2, and KN3 ...lying within 2-14 pc in projection from the young massive cluster R136 in 30 Doradus. Excited H2 2.12 m emission is spatially coincident with the molecular clouds, but ionized Brγ emission is not. We interpret these observations as the tails of pillar-like structures whose ionized heads are pointing toward R136. Based on infrared photometry, we identify a new generation of stars forming within this structure.
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