Top PDF Filament Fragmentation in High-Mass Star Formation

Filament Fragmentation in High-Mass Star Formation

Filament Fragmentation in High-Mass Star Formation

As outlined in Section 3.1, the separations between the main identified fragments along the filament exceed the Jeans length at the given densities and temperatures. Although our measured projected separations are upper limits because of potential un- resolved fragmentation and/or fragments below our sensitivity limit, the difference between Jeans length and projected frag- ment separation by more than a factor two appears significant. As a next step, we analyze the region in the framework of isothermal, gravitationally bound gaseous cylinders. Based on early work of Chandrasekhar & Fermi (1953); Nagasawa (1987) and Inutsuka & Miyama (1992) as well as more recent adaptions like Jackson et al. (2010), Beuther et al. (2011) or Kainulainen et al. (2013), we study the conditions of an infinite isothermal gas cylinder. Although IRDC 18223 is obviously not of infinite length, the filament is part of a much larger structure extending more than 50 pc projected on the sky. Hence, in this context, the approximation of IRDC 18223 being part of a much longer, al- most infinite structure seems justifiable. Furthermore, since star formation has already started at different locations in the fila- ment, it is not the perfect starless filament anymore. However, the infrared dark nature of IRDC 18223 clearly shows the youth of the whole structure. Hence, it is still an excellent target re- gion that represents conditions in the relatively early phase of filament fragmentation.
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The VLT-FLAMES Tarantula Survey IV. Candidates for isolated high-mass star formation in 30 Doradus

The VLT-FLAMES Tarantula Survey IV. Candidates for isolated high-mass star formation in 30 Doradus

The 30 Doradus region is a complex and dynamic region that contains multiple, although not necessarily spatially distinct, generations of stars (see Walborn & Blades 1997). The youngest population is dominated by the central cluster R136, with an age of 1-2 Myr (e.g. de Koter et al. 1998; Massey & Hunter 1998) and, most pertinently for the discussion here, there appears to be another young population to the north and west of R136, ex- emplified by the compact multiple systems in the dense nebular knots observed with the Hubble Space Telescope by Walborn et al. (1999, 2002). These comprise an apparently young, still embedded phase of star formation. Interestingly, Walborn et al. (2002) also presented imaging of two notable infrared (IR) sources, one of which was resolved into a small, embedded clus- ter, while the other was a point like source, seemingly single monolithic object (albeit at the distance of the LMC). This led the authors to note that the later object may have formed without an associated cluster or association.
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Simultaneous low  and high mass star formation in a massive protocluster : ALMA observations of G11 92 0 61

Simultaneous low and high mass star formation in a massive protocluster : ALMA observations of G11 92 0 61

regions of the IRDC G11.11-0.12 P6 by Wang et al. 2014). Estimates of the total mass in compact cores—e.g. includ- ing the previously-known sources MM1, MM2, and MM3– depend on the mass estimates for these sources, which are quite sensitive to the opacity correction term in equation 1 (in particular for MM2, see also Table 2 of Cyganowski et al. 2014). While a detailed analysis of the massive sources is beyond the scope of this paper (see also Ilee et al. 2016; Cyganowski et al. 2014), we conservatively estimate that MM1, MM2, and MM3 together account for ∼5-10% of the ATLASGAL clump mass. It is interesting that, at sub- arcsecond resolution, the fraction of the clump mass resolved into high-mass cores is of the same order of magnitude as that resolved into low-mass cores (indeed, within a factor of ∼2-3), and that all compact cores detected to date account for .15% of the total clump-scale mass reservoir. There may, of course, be additional low-mass cores below the sensitivity limit of our ALMA observations; the rms noise level varies significantly across the map (Section 2.1), corresponding to a 5σ mass sensitivity for T dust =20 K that ranges from ∼0.1
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What governs star formation in galaxies? A modern statistical approach

What governs star formation in galaxies? A modern statistical approach

In order to monitor how the data behave, we created SOMs with two to fourteen neurons (Fig. 3.4). The 1 × 2 network (Fig. 3.4a) shows how the M31 data can be divided into two broad categories. The 1 × 14 network (Fig. 3.4c) is the first network in which all the regions in M31 are completely separated. In the higher network sizes, regions have more space to be separated based on their differences. In going from the 1 × 2 to the 1 × 14 network the distance between M31 regions increases, until they are completely separated. Fig. 3.4a shows that by forcing the regions in M31 to be divided into two groups, regions 1, 2, 9 and 10 (shown in Fig. 3.2a) occupy one neuron and the other regions occupy the other one. The medium grey colour between two neurons indicates that there are some similarities between two groups, but they are not very similar. By increasing the size of the neurons to three, in Fig. 3.4b, we can see that region 2 separates itself from the other regions and occupies the middle neuron. The white colour between the two left neurons suggests that the regions which occupy these neurons are very similar to each other, while the black colour between the two right neurons indicates otherwise. Hemachandra et al. (2015) showed that regions 1, 2, 9, and 10 have higher PAH fluxes compared to the other regions (fig. 5 in Hemachandra et al. 2015). These regions also have relatively high intensities in all the mid-infrared and far-infrared bands and have high dust luminosity and dust mass. The higher values for these quantities could be the reason that these four regions become separated from the others in the 1 × 2 network. Regions 1 and 9 are in the 10 kpc ring, region 2 is slightly out of the 10 kpc ring and region 10 is in the bulge of M31; however, regions 3 to 8 are located out of the inner ring or the 10 kpc ring (Fig. 3.2a). The differences in their positions account for their difference in input parameters. Since region 10 is located in the bulge of M31 and has high surface brightness for most of the input values, regions 1, 2, and 9 to gradually move towards other regions and away from region 10 in the higher grid SOMs.
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Observational signatures of massive star formation : an investigation of the environments in which they form, and the applicability of the paradigm of low mass star formation

Observational signatures of massive star formation : an investigation of the environments in which they form, and the applicability of the paradigm of low mass star formation

In addition, optical depth effects were included in the model. To find the optical depth of the path of the “photon” (luminosity/energy packet) from each point, the total emission measure E M (the integral of n 2 e along a line-of-sight from the generated point to the edge of the source) was found, and used to calculate the optical depth τ of the ionized gas via equation 2.3 in Chapter 2. This was added as a multiplicative weight e − τ to each point when summed into the image. Determination of different path lengths through the various components of the source, i.e. the cavity surface and the cavity grid in z, was carried out by finding the points of intersection of a ray directed towards the observer with these surfaces. Finally, the image was also convolved with the Gaussian synthesised beam of the 3.6 cm or 7 mm observations for comparison with the observed images. The initial estimate of the number of ionizing photons N phot was also varied until the model integrated 3.6 cm flux matched that observed (1.52 mJy, corresponding to 2 mJy without optical depth effects), giving N phot = 1.7 × 10 44 s − 1 , which produced an HII region of maximum radius 506 AU. The integrated model flux at 7 mm was 1.3 mJy (1.6 mJy without optical depth effects). The resulting spectral index between these two wavelengths was found to be approximately -0.1, consistent with optically thin emission. This is because the reduction of emission from the model HII region due to optical depth effects occurred in the high density cavity walls, which dominated the image morphology, however the emission from the cavity, which dominated the total flux, was unchanged, and therefore the spectral index of the model integrated flux corresponded to optically thin emission.
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Rapidly growing black holes and host galaxies in the distant universe from the Herschel Radio Galaxy Evolution Project

Rapidly growing black holes and host galaxies in the distant universe from the Herschel Radio Galaxy Evolution Project

nario can be proposed. The galaxies can experience their growth through mergers. Several merging scenarios can be instigated, both major / minor and gas-poor / gas-rich mergers. Major merg- ers are rare events, but they are expected to be mostly gas rich at high-redshift as the gas fraction increases significantly (e.g. Tacconi et al. 2010). Notably, in the case of a major merger, gas can be e ffi ciently brought to the inner part of the galaxy (<1 kpc) and probably feeds the black hole and star formation simultane- ously, allowing a new episode of black hole growth. The trigger- ing event of the radio galaxy episode is still an open discussion, but recent studies suggest that major mergers can play an im- portant role (e.g. Ramos Almeida et al. 2013). In the case of minor mergers, gas-rich companions could form the stars and be accreted within the cosmic time. This scenario is supported by some observational evidence thanks to high-resolution imag- ing with HST (Miley et al. 2006; Seymour et al. 2012). This is also related to the size evolution of galaxies as well as the com- pactness of early-type galaxies at high-redshift (e.g. Daddi et al. 2005; van Dokkum et al. 2008; Delaye et al. 2014), the change in the mass function with cosmic time (Ilbert et al. 2013), the light profiles and elemental abundance ratios in the outer regions of massive ellipticals (Huang et al. 2013; Greene et al. 2013), and the fact that at constant co-moving density, the mass of mas- sive early-type galaxies grew by about a factor of 4 over ap- proximately the last 10 Gyr (e.g. van Dokkum et al. 2010; Ilbert et al. 2013). As HzRGs are found in dense environments, proba- bly in the centre of proto-clusters (e.g. Wylezalek et al. 2013a), they are likely to experience an important series of minor dry mergers, consistent with the size evolution scenario. Therefore,
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High mass star formation in the nearby region G352 630 1 067  I  parallax

High mass star formation in the nearby region G352 630 1 067 I parallax

mass star formation region ( HMSFR ) known, with only the Orion Nebula being closer. This may place this source, not within a Galactic spiral arm, but in the region between the Local and Sagittarius arms, indicating that molecular clouds in interarm regions may also generate high-mass stars. Kinematic association between this source and the Sagittarius Arm suggests that it may be located in a spur extending outward from this arm. Comparison with the known, nearby HMSFRs ( distances less than 1 kpc ) , reveal that G352.630-1.067 is in a more isolated environment than others, hence providing an excellent candidate for investigations of the processes that form individual high-mass stars. We fi nd a good spatial correlation between the 6.7 GHz methanol maser and high angular resolution images of the infrared out fl ow, suggesting that the class II methanol masers are closely associated with a jet / out fl ow in this source.
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Connecting low- and high-mass star formation: the intermediate-mass protostar IRAS 05373+2349 VLA 2

Connecting low- and high-mass star formation: the intermediate-mass protostar IRAS 05373+2349 VLA 2

To investigate how much millimetre emission might arise from the ionized material, we extrapolated the combined 6-cm flux of VLA 2 and VLA 3 to 2.7 mm, assuming a spectral index of α = 0.38 ± 0.14 and that the spectrum did not turn over. We found an expected flux density of ∼1.95 ± 0.73 mJy, which is ∼4 per cent of the measured millimetre continuum ( ∼ 54 mJy). Therefore, the contribution of ionized gas emission in this case is negligible. How- ever, if we use α = 1.20 ± 0.24, determined by excluding the 1.3 cm fluxes, the estimated contribution to the combined integrated flux of VLA 2 and 3 at 2.7 mm becomes ∼ 25 mJy, roughly half of the measured millimetre continuum. Scaling the 6 cm fluxes of VLA 2 and 3 by α = 1.2 and α = 0.45, respectively, where the latter is the measured spectral index for VLA 3, gives a total 2.7 mm flux of ∼20 mJy. We calculated the core mass for both cases below.
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Guo, Qi
  

(2009):


	Galaxy Formation and Evolution in a LCDM Universe.


Dissertation, LMU München: Fakultät für Physik

Guo, Qi (2009): Galaxy Formation and Evolution in a LCDM Universe. Dissertation, LMU München: Fakultät für Physik

In the standard scenario of galaxy formation, as originally proposed by White & Rees (1978), gas condenses at the center of hierarchically merging dark matter halos. The mass function of dark matter halos, which grow hierarchically by the merging of smaller systems which formed earlier is, however, a poor match in shape to galaxy luminosity functions (see also Chapter 1 and Chapter 4). Fewer stars have been observed in halos with higher and lower masses with respect to the Milky Way host halo than predicted, based on a constant mass-to-light ratio. One explanation of the low star formation efficiency in massive halos is that a supermassive black hole at the center releases vast amounts of energy when it absorbs mass from its surroundings, and this suppresses cooling and star formation in its host galaxy (AGN feedback) (Croton et al. 2006; Bower et al. 2006). The abundance of low mass halos could be reduced by replacing cold dark matter with warm dark matter (Bode et al. 2001), but the adoption of warm dark matter is challenged by the requirement to form relatively massive structures at high redshfit, within which galaxies could form to produce high energy photons to account for the reionization of the intergalactic medium (Spergel et al. 2003). White & Rees (1978) argued that in the hierarchical paradigm, feedback can help expel gas from small galaxies, making them less successful at forming stars, thus reducing the stellar masses of faint galaxies. Cosmic reionization, which could inhibit gas collapsing into shallow potential well and suppress gas cooling in low mass halos, could also affect galaxy formation in very small halos (see Chapter 1).
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Class I methanol masers in NGC253: alcohol at the end of the bar

Class I methanol masers in NGC253: alcohol at the end of the bar

temperatures (T ≥ 100 K) than the conditions under which the 44.1- GHz maser is the strongest class I methanol transition. Gorski et al. (2017) show that much of the 36.2-GHz methanol emission is lo- cated at the edge of supershells observed in CO and dense gas tracers (Sakamoto et al. 2011; Bolatto et al. 2013) and speculate that it could be produced by a large number of protostellar outflows or super- nova remnants in these regions. In Section 4.2, we demonstrated the significant differences between the 36.2- and 44.1-GHz methanol emission observed in NGC 253 and that observed in typical Galac- tic high-mass star formation regions (e.g. Voronkov et al. 2014), which would seem to rule out individual protostellar outflows as the mechanism. Supernovae in NGC 253 have also been studied in some detail (e.g. Lenc & Tingay 2006), but they are restricted to the inner nuclear region which shows no 36.2-GHz methanol emission.
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Hu, Chia-Yu
  

(2016):


	Star formation and molecular hydrogen in dwarf galaxies.


Dissertation, LMU München: Fakultät für Physik

Hu, Chia-Yu (2016): Star formation and molecular hydrogen in dwarf galaxies. Dissertation, LMU München: Fakultät für Physik

Fig. 3.18, panel (b), shows the mass loading η , dened as the ratio of the outow rate to the SFR. The outow rate is computed by summing the total mass of gas particles passing through a plane at z = ± 5 kpc, where the z-axis is the rotation axis of the disk. There is an initial burst of outow in all cases caused by the initial setup. Since all the gas particles are initially located in the disk, the environment outside of the disk is a vacuum which allows an unimpeded outow triggered by the SN explosions. These outowing particles will be pulled back down to the potential well of the disk due to gravity, leading to the onset of gas inow which can interact with the subsequent outow. Eventually the inow and outow reach a quasi-steady state and a hot gaseous halo is formed (after ∼ 150 Myrs). A small fraction of the gas can even escape the halo completely. The mass loading η shows a strong dependence on the adopted SPH schemes: both the PE formulation and the AC suppress the mass loading. One direct explanation is that the increased SFR transforms part of the gas into stars, thus reducing the amount of gas available to the outow. However, the mass loading drops almost an order of magnitude between the two most extreme cases while the dierence of SFR is only about a factor of 2. This suggests that the feedback eciency is directly aected by the SPH implementation. As it becomes weaker (for AC and PE models) the hot gas cannot be blown out of the disk as eciently. The driving force of the shock is dissipated as the shocked heated particles are continuously mixed with the ambient cold ISM. We also observe that the eect (reduced mass loading) seems stronger at higher resolution due to more ecient mixing. This is contrary to Hopkins et al. (2014) where they nd both the SFR and mass loading are insensitive to the adopted SPH scheme. The discrepancy might be due to the dierent feedback implementation: in Hopkins et al. (2014) the contribution of momentum input (the radiation pressure) is signicant, which is expected to be less aected by uid mixing.
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The formation of planets by disc fragmentation

The formation of planets by disc fragmentation

material from the disc [55]. The objects that form first and migrate inwards gain enough mass to become stars, whereas the ones that stay in the outer disc region increase in mass but not as much, becoming brown dwarfs. If one of the brown dwarfs from the outer disc region drifts inwards, then it is quickly ejected again into the outer disc region due to dynamical interactions with the higher-mass objects of the inner region. The inner disc region will also be populated by planets that form by core accretion at a later stage (after ∼ 1 Myr). Most of the brown dwarfs are either ejected from the system becoming field brown dwarfs, or stay bound to the central star at relatively wide orbits (∼ 200 − 10 4 AU). Therefore, it
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The coordinated radio and infrared survey for high-mass star formation III. A catalogue of northern ultra-compact H II regions

The coordinated radio and infrared survey for high-mass star formation III. A catalogue of northern ultra-compact H II regions

tween the measured and the theoretical unattenuated flux is be- low 10%. The bright UCHII G049.4905 − 00.3688 has the high- est computed di ff erence ( ∼ 56.4%). The distribution of the flux difference due to attenuation tapers off above differences greater than ∼ 20%, indicating that there is most likely no significant fraction of sources that have been missed altogether. The same should be true even if the electron temperature varies from re- gion to region (within the expected physical bounds). It should also be noted that the computed Lyman continuum flux in this case is not significantly underestimated due to optically thick free-free emission, but could still be affected by loss of ionising photons (e.g. via dust absorption), or for radio flux that was not recovered.
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Parallaxes of 6 7 GHz methanol masers towards the G 305 2 high mass star formation region

Parallaxes of 6 7 GHz methanol masers towards the G 305 2 high mass star formation region

The uncertainties in μx , μy for both sources correspond to internal motions of 10 km s−1 in the maser emission and are consistent with proper motion estimates of 6.7-GHz methanol maser[r]

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Sokolov, Vlas
  

(2018):


	Early stages of massive star formation.


Dissertation, LMU München: Fakultät für Physik

Sokolov, Vlas (2018): Early stages of massive star formation. Dissertation, LMU München: Fakultät für Physik

This work presented analyses of observations towards IRDC G035.39–00.33, previously found to be an excellent target to probe the initial conditions for high-mass star formation. Below the summary of preceding chapters is given, and interesting future prospects for IRDC and high- mass star formation are outlined. With the large-scale GBT observations of ammonia inversion transitions and the complementary archival Herschel photometric data, a quantitative compar- ison between the two temperature estimating methods is made. Without careful modelling of the background and foreground emission, the conventional method of deriving dust temperature is shown not only to systematically overestimate the IRDC temperatures by 2 − 3 K, but also conceals signs of protostellar heating of the core envelopes. What does this entail for the future studies of IRDCs? Because the spectral line surveys that can reliably measure the gas temper- ature are time-consuming, far-infrared surveys are commonly used in their place for large-scale temperature measurements of cold clouds. Given the richness of the available Herschel Galactic plane surveys, any systematic search for starless core candidates through far-infrared emission would benefit from tighter constraints of the spatial temperature distribution, which should take the line-of-sight contamination into account. Additionally, any chemical modelling of the in- frared dark clouds has to take the systematics above into account, because the chemical reaction rates are highly dependant on the temperature, making the 2 − 3 K bias found significant enough to account for.
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Diagnostics of a nuclear starburst: water and methanol masers

Diagnostics of a nuclear starburst: water and methanol masers

temperatures > 300 K to mase if it is collisionally pumped. The line can also be radiatively pumped with background infrared radi- ation with temperatures of ∼1000 K (Gray et al. 2016). 22 GHz water masers are classified into three groups: the stellar class (L < 0.1 L ), kilomasers (0.1 L < L < 1 L ), and megamasers (L > 20 L ) (see Hagiwara et al. 2001 for a description of this nomenclature). The stellar and kilomaser classes are mostly asso- ciated with star formation (e.g. Hagiwara et al. 2001; Walsh et al. 2011; Tarchi 2012). Walsh et al. (2011) estimate that 90 per cent of these stellar class water masers are associated with high-mass star formation (Young Stellar Objects, YSOs) and the other 10 per cent associated with either low-mass star formation or evolved stars. Their survey is sensitivity limited with 50 per cent completeness at a flux limit of 5.5 Jy. We will use the 22 GHz H 2 O maser as a sign
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Star Formation in Magnetized, Turbulent and Rotating Molecular Cloud: The Critical Mass

Star Formation in Magnetized, Turbulent and Rotating Molecular Cloud: The Critical Mass

The standard theory simply related to the magnetic fields of the progenitor main-sequence stars frozen during collapse or flux conservation [4]. This model does not deliver the mechanism how magnetic field supports and opposes gravity in star formation. Understanding the origin and evolution as well as physical behaviour of the complex process of star formation (SF) is still an ongoing research area in modern astrophysics. Solving those problems will help to develop a theory regarding origin of stars. Some of unanswered questions regarding the role of magnetic fields in star formation are: What is the effect of magnetic fields on MC core collapse? It has been suggested that magnetic fields suppress fragmentation [5]. The other way suggested is that the presence of magnetic fields may enhance fragmentation [6]. To this end, we perform theoretical formulation how magnetic field counteracts gravity and how mag- netic tension supports gravity in a gravitating, magnetized turbulent rotating MCc.
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VIS3COS I:Survey overview and the role of environment and stellar mass on star formation

VIS3COS I:Survey overview and the role of environment and stellar mass on star formation

Fig. 8. Top: SFR (from SED fitting) distribution as a function of stel- lar mass. Each small circle represents a single galaxy. Large squares show the median value for the population in stellar mass bins. Error bars show the error on the median of each bin. Higher density regions are coloured in blue while low density galaxies are shown in green colours. The empty symbols represent the bins considering star-forming galax- ies only, with log 10 (sSF R) > −11. The symbols are horizontally shifted for visualization purposes. The vertical dotted line shows the complete- ness limit of our survey. Globally, we find that galaxies in higher density regions have lower SFRs, but only when considering the entire popula- tion. When selecting star-forming galaxies, we find no difference be- tween the median SFRs in low and high density environments. Bot- tom: Dust corrected [O II ] luminosity distribution as a function of stellar mass. We show as small arrows the upper limits on [O II ] luminosity for the galaxies which we have not a measure with sufficient S/N. We show as horizontal lines three values of SFR = 1, 10, 50 M yr −1 as derived
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High-mass star formation at sub-50 au scales

High-mass star formation at sub-50 au scales

or 6000/13200 au at 1.0/2.2 kpc distance. Projected separations between sub-sources range from more than 1000 au to several 100 au but can be even smaller below 100 au. Since we filter out the large-scale emission, very extended structures are not visible in the data, but the largest features we can identify are between approximately 200 and 600 au in size, similar to the size-scales predicted by theoretical simulations for disks around embed- ded high-mass protostars. Brightness temperatures toward the main continuum peak positions are in excess of 1000 K, indicat- ing high optical depth toward these positions. This high optical depth as well as the missing flux only allows us to derive lower limits for the masses and column densities of the sub-sources. In the hierarchical picture of fragmentation from large-scale clouds to small-scale cores, we find that the fragmentation properties of this high-mass core with a size of several 1000 au is consistent with thermal Jeans fragmentation.
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Resolving the fragmentation of high line-mass filaments with ALMA: the integral shaped filament in Orion A

Resolving the fragmentation of high line-mass filaments with ALMA: the integral shaped filament in Orion A

to ∼1000 AU scales. However, it is not obvious how to arrive to this conclusion, because both the free-fall time and the fragmen- tation time depend on density with the same power ( ∝ρ −0.5 ). One possible route is provided by magnetic fields that may provide support against a global collapse while not strongly a ff ecting the fragmentation time-scale (e.g., Fiege & Pudritz 2000a,b, see also Seifried & Walch 2015, for a numerical study showing an analo- gous e ff ect in filaments that are thermally near-critical). Another possibility could be large-enough, pre-existing density fluctua- tions that can grow and collapse faster than the global, longitudi- nal collapse ensues (Larson 1985; Pon et al. 2011). Supporting this possibility, filaments with low star formation activity have been observed to contain significant density fluctuations (e.g., Roy et al. 2015; Kainulainen et al. 2016). Developing this car- toon framework into a coherent theory is beyond this paper, but it would be a crucial topic for future works that aim at understand the fragmentation of highly super-critical filaments.
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