Showing papers on "Infrared dark cloud published in 2013"

Global collapse of molecular clouds as a formation mechanism for the most massive stars

Abstract: The relative importance of primordial molecular cloud fragmentation versus large-scale accretion still remains to be assessed in the context of massive core/star formation. Studying the kinematics of the dense gas surrounding massive-star progenitors can tell us the extent to which large-scale flow of material impacts the growth in mass of star-forming cores. Here we present a comprehensive dataset of the 5500(±800) M⊙ infrared dark cloud SDC335.579-0.272 (hereafter SDC335), which exhibits a network of cold, dense, parsec-long filaments. Atacama Large Millimeter Array (ALMA) Cycle 0 observations reveal two massive star-forming cores, MM1 and MM2, sitting at the centre of SDC335 where the filaments intersect. With a gas mass of 545(-385+770) M⊙ contained within a source diameter of 0.05 pc, MM1 is one of the most massive, compact protostellar cores ever observed in the Galaxy. As a whole, SDC335 could potentially form an OB cluster similar to the Trapezium cluster in Orion. ALMA and Mopra single-dish observations of the SDC335 dense gas furthermore reveal that the kinematics of this hub-filament system are consistent with a global collapse of the cloud. These molecular-line data point towards an infall velocity Vinf = 0.7( ± 0.2) km s-1, and a total mass infall rate Ṁinf ≃ 2.5(±1.0) × 10-3 M⊙ yr-1 towards the central pc-size region of SDC335. This infall rate brings 750(±300) M⊙ of gas to the centre of the cloud per free-fall time (tff = 3 × 105 yr). This is enough to double the mass already present in the central pc-size region in 3.5-1.0+2.2 × tff. These values suggest that the global collapse of SDC335 over the past million year resulted in the formation of an early O-type star progenitor at the centre of the cloud’s gravitational potential well.

Unveiling a network of parallel filaments in the infrared dark cloud g14.225–0.506

Abstract: We present the results of combined NH_3 (1,1) and (2,2) line emission observed with the Very Large Array and the Effelsberg 100 m telescope of the infrared dark cloud G14225–0506 The NH3 emission reveals a network of filaments constituting two hub-filament systems Hubs are associated with gas of rotational temperature T_(rot) ~ 15 K, non-thermal velocity dispersion σ_(NT) ~ 1 km s^(–1), and exhibit signs of star formation, while filaments appear to be more quiescent (T_(rot) ~ 11 K and σ_(NT) ~ 06 km s^(–1)) Filaments are parallel in projection and distributed mainly along two directions, at PA ~ 10° and 60°, and appear to be coherent in velocity The averaged projected separation between adjacent filaments is between 05 pc and 1 pc, and the mean width of filaments is 012 pc Cores within filaments are separated by ~033 ± 009 pc, which is consistent with the predicted fragmentation of an isothermal gas cylinder due to the "sausage"-type instability The network of parallel filaments observed in G14225–0506 is consistent with the gravitational instability of a thin gas layer threaded by magnetic fields Overall, our data suggest that magnetic fields might play an important role in the alignment of filaments, and polarization measurements in the entire cloud would lend further support to this scenario

Complex, quiescent kinematics in a highly filamentary infrared dark cloud

Abstract: Infrared Dark Clouds (IRDCs) host the initial conditions under which massive stars and stellar clusters form. We have obtained high sensitivity and high spectral resolution observations with the IRAM 30m antenna, which allowed us to perform detailed analysis of the kinematics within one IRDC, G035.39-00.33. We focus on the 1-0 and 3-2 transitions of N2H+, C18O (1-0), and make comparison with SiO (2-1) observations and extinction mapping. Three interacting filaments of gas are found. We report large-scale velocity coherence throughout the cloud, evidenced through small velocity gradients and relatively narrow line widths. This suggests that the merging of these filaments is somewhat "gentle", possibly regulated by magnetic fields. This merging of filaments may be responsible for the weak parsec-scale SiO emission detected by Jimenez-Serra et al. 2010, via grain mantle vaporization. A systematic velocity shift between the N2H+ (1-0) and C18O (1-0) gas throughout the cloud of 0.18 +/- 0.04 kms^{-1} is also found, consistent with a scenario of collisions between filaments which is still ongoing. The N2H+ (1-0) is extended throughout the IRDC and it does not only trace dense cores, as found in nearby low-mass star-forming regions. The average H2 number density across the IRDC is ~ 5 x 10^4 cm^{-3}, at least one order of magnitude larger than in nearby molecular clouds where low-mass stars are forming. A temperature gradient perpendicular to the filament is found. From our study, we conclude that G035.39-00.33 (clearly seen in the extinction map and in N2H+) has been formed via the collision between two relatively quiescent filaments with average densities of ~ 5 x 10^3 cm^{-3}, moving with relative velocities of ~ 5 kms^{-1}. The accumulation of material at the merging points started > 1 Myr ago and it is still ongoing.

Distinct chemical regions in the “prestellar” infrared dark cloud g028.23–00.19

Abstract: We have observed the Infrared Dark Cloud (IRDC) G028.23–00.19 at 3.3 mm using the Combined Array for Research in Millimeter-wave Astronomy. In its center, the IRDC hosts one of the most massive (~1520 M_☉) quiescent, cold (12 K) clumps known (MM1). The low temperature, high NH2D abundance, narrow molecular line widths, and absence of embedded infrared sources (from 3.6 to 70 μm) indicate that the clump is likely prestellar. Strong SiO emission with broad line widths (6-9 km s^(–1)) and high abundances ((0.8-4) × 10^(–9)) is detected in the northern and southern regions of the IRDC, unassociated with MM1. We suggest that SiO is released to the gas phase from the dust grains through shocks produced by outflows from undetected intermediate-mass stars or clusters of low-mass stars deeply embedded in the IRDC. A weaker SiO component with narrow line widths (~2 km s^(–1)) and low abundances (4.3 × 10^(–11)) is detected in the center-west region, consistent with either a "subcloud-subcloud" collision or an unresolved population of a few low-mass stars. We report widespread CH_3OH emission throughout the whole IRDC and the first detection of extended narrow methanol emission (~2 km s^(–1)) in a cold, massive prestellar clump (MM1). We suggest that the most likely mechanism releasing methanol into the gas phase in such a cold region is the exothermicity of grain-surface reactions. HN^(13)C reveals that the IRDC is actually composed of two distinct substructures ("subclouds") separated in velocity space by ~1.4 km s^(–1). The narrow SiO component arises where the subclouds overlap. The spatial distribution of C2H resembles that of NH_2D, which suggests that C_2H also traces cold gas in this IRDC.

The Dynamical State of the Serpens South Filamentary Infrared Dark Cloud

Abstract: We present the results of N_2H^+ (J = 1-0) observations toward Serpens South, the nearest cluster-forming, infrared dark cloud. The physical quantities are derived by fitting the hyperfine structure of N_2H^+. The Herschel and 1.1 mm continuum maps show that a parsec-scale filament fragments into three clumps with radii of 0.1-0.2 pc and masses of 40-230 M_☉. We find that the clumps contain smaller-scale (~0.04 pc) structures, i.e., dense cores. We identify 70 cores by applying CLUMPFIND to the N_2H^+ data cube. In the central cluster-forming clump, the excitation temperature and line-width tend to be large, presumably due to protostellar outflow feedback and stellar radiation. However, for all the clumps, the virial ratios are evaluated to be 0.1-0.3, indicating that the internal motions play only a minor role in the clump support. The clumps exhibit no free fall but exhibit low-velocity infall, and thus the clumps should be supported by additional forces. The most promising force is the globally ordered magnetic field observed toward this region. We propose that the Serpens South filament was close to magnetically critical and ambipolar diffusion triggered the cluster formation. We find that the northern clump, which shows no active star formation, has a mass and radius comparable to the central cluster-forming clump and is therefore a likely candidate of a pre-protocluster clump. The initial condition for cluster formation is likely to be a magnetically supported clump of cold, quiescent gas. This appears to contradict the accretion-driven turbulence scenario, for which the turbulence in the clumps is maintained by the accretion flow.

Global collapse of molecular clouds as a formation mechanism for the most massive stars

Abstract: The relative importance of primordial molecular cloud fragmentation versus large-scale accretion still remains to be assessed in the context of massive core/star formation. Studying the kinematics of the dense gas surrounding massive-star progenitors can tell us the extent to which large-scale flow of material impacts the growth in mass of star-forming cores. Here we present a comprehensive dataset of the 5500(+/-800) Msun infrared dark cloud SDC335.579-0.272 (hereafter SDC335) which exhibits a network of cold, dense, parsec-long filaments. Atacama Large Millimeter Array (ALMA) Cycle 0 observations reveal two massive star-forming cores, MM1 and MM2, sitting at the centre of SDC335 where the filaments intersect. With a gas mass of 545(+770,-385) Msun contained within a source diameter of 0.05pc, MM1 is one of the most massive, compact protostellar cores ever observed in the Galaxy. As a whole, SDC335 could potentially form an OB cluster similar to the Trapezium cluster in Orion. ALMA and Mopra single-dish observations of the SDC335 dense gas furthermore reveal that the kinematics of this hub-filament system are consistent with a global collapse of the cloud. These molecular-line data point towards an infall velocity V_{inf} =0.7(+/-0.2) km/s, and a total mass infall rate \dot{M}_{inf} = 2.5(+/-1.0) x 10^{-3} Msun/yr towards the central pc-size region of SDC335. This infall rate brings 750(+/-300) Msun of gas to the centre of the cloud per free-fall time (t_{ff}=3x10^5 yr). This is enough to double the mass already present in the central pc-size region in 3.5(+2.2,-1.0) x t_{ff}. These values suggest that the global collapse of SDC335 over the past million year resulted in the formation of an early O-type star progenitor at the centre of the cloud's gravitational potential well.

ALMA Observations of the IRDC Clump G34.43+00.24 MM3: Hot Core and Molecular Outflows

Abstract: We have observed a cluster forming clump (MM3) associated with the infrared dark cloud G34.43+00.24 in the 1.3 mm continuum and the CH3OH, CS, 13CS, SiO, CH3CH2CN, and HCOOCH3 lines with the Atacama Large Millimeter/submillimeter Array and in K-band with the Keck telescope. We have found a young outflow toward the center of this clump in the SiO, CS, and CH3OH lines. This outflow is likely driven by a protostar embedded in a hot core, which is traced by the CH3CH2CN, HCOOCH3, 13CS, and high excitation CH3OH lines. The size of the hot core is about 800 × 300 AU in spite of its low mass (<1.1 M ☉), suggesting a high accretion rate or the presence of multiple star system harboring a few hot corinos. The outflow is highly collimated, and the dynamical timescale is estimated to be less than 740 yr. In addition, we have also detected extended emission of SiO, CS, and CH3OH, which is not associated with the hot core and the outflow. This emission may be related to past star formation activity in the clump. Although G34.43+00.24 MM3 is surrounded by a dark feature in infrared, it has already experienced active formation of low-mass stars in an early stage of clump evolution.

ALMA Observations of the IRDC Clump G34.43+00.24 MM3: Hot Core and Molecular Outflows

Abstract: We have observed a cluster forming clump (MM3) associated with the infrared dark cloud G34.43+00.24 in the 1.3 mm continuum and the CH3OH, CS, 13CS, SiO, CH3CH2CN, and HCOOCH3 lines with the Atacama Large Millimeter/submillimeter Array and in K-band with the Keck telescope. We have found a young outflow toward the center of this clump in the SiO, CS, and CH3OH lines. This outflow is likely driven by a protostar embedded in a hot core, which is traced by the CH3CH2CN, HCOOCH3, 13CS, and high excitation CH3OH lines. The size of the hot core is about 800 x 300 AU in spite of its low mass (<1.1 M_sun), suggesting a high accretion rate or the presence of multiple star system harboring a few hot corinos. The outflow is highly collimated, and the dynamical timescale is estimated to be less than 740 yr. In addition, we have also detected extended emission of SiO, CS, and CH3OH, which is not associated with the hot core and the outflow. This emission may be related to past star formation activity in the clump. Although G34.43+00.24 MM3 is surrounded by a dark feature in infrared, it has already experienced active formation of low-mass stars in an early stage of clump evolution.

Star formation in the massive “starless” infrared dark cloud g0.253+0.016

Abstract: G0.253+0.016 is a remarkable massive infrared dark cloud located within ~100 pc of the galactic center. With a high mass of 1.3 × 105 M ☉, a compact average radius of ~2.8 pc, and a low dust temperature of 23 K, it has been believed to be a yet starless precursor to a massive Arches-like stellar cluster. We present sensitive JVLA 1.3 and 5.6 cm radio continuum observations that reveal the presence of three compact thermal radio sources projected against this cloud. These radio sources are interpreted as H II regions powered by ~B0.5 zero-age main sequence stars. We conclude that although G0.253+0.016 does not show evidence of O-type star formation, there are certainly early B-type stars embedded in it. We detect three more sources in the periphery of G0.253+0.016 with non-thermal spectral indices. We suggest that these sources may be related to the galactic center region and deserve further study.

The Dynamical State of The Serpens South Filamentary Infrared Dark Cloud

Abstract: We present the results of N$_2$H$^+$ ($J=1-0$) observations toward Serpens South, the nearest cluster-forming, infrared dark cloud. The physical quantities are derived by fitting the hyperfine structure of N$_2$H$^+$. The Herschel and 1.1-mm continuum maps show that a pc-scale filament fragments into three clumps with radii of $0.1-0.2$ pc and masses of $40-230M_\odot$. We find that the clumps contain smaller-scale ($\sim 0.04$ pc) structures, i.e., dense cores. We identify 70 cores by applying CLUMPFIND to the N$_2$H$^+$ data cube. In the central cluster-forming clump, the excitation temperature and line-width tend to be large, presumably due to protostellar outflow feedback and stellar radiation. However, for all the clumps, the virial ratios are evaluated to be $0.1-0.3$, indicating that the internal motions play only a minor role in the clump support. The clumps exhibit no free-fall, but low-velocity infall, and thus the clumps should be supported by additional forces. The most promising force is the globally-ordered magnetic field observed toward this region. We propose that the Serpens South filament was close to magnetically-critical and ambipolar diffusion triggered the cluster formation. We find that the northern clump, which shows no active star formation, has a mass and radius comparable to the central cluster-forming clump, and therefore, it is a likely candidate of a {\it pre-protocluster clump}. The initial condition for cluster formation is likely to be a magnetically-supported clump of cold, quiescent gas. This appears to contradict the accretion-driven turbulence scenario, for which the turbulence in the clumps is maintained by the accretion flow.

Star formation in the massive "starless" infrared dark cloud G0.253$+$0.016

Abstract: G0.253+0.016 is a remarkable massive infrared dark cloud located within $\sim$100 pc of the galactic center. With a high mass of $1.3 \times 10^5 M_\odot$, a compact average radius of $\sim$2.8 pc and a low dust temperature of 23 K, it has been believed to be a yet starless precursor to a massive Arches-like stellar cluster. We present sensitive JVLA 1.3 and 5.6 cm radio continuum observations that reveal the presence on three compact thermal radio sources projected against this cloud. These radio sources are interpreted as HII regions powered by $\sim$B0.5 ZAMS stars. We conclude that although G0.253+0.016 does not show evidence of O-type star formation, there are certainly early B-type stars embedded in it. We detect three more sources in the periphery of G0.253+0.016 with non-thermal spectral indices. We suggest that these sources may be related to the galactic center region and deserve further study.

G048.66?0.29: Physical State of an Isolated Site of Massive Star Formation

Abstract: We present continuum observations of the infrared dark cloud (IRDC) G48.66–0.22 (G48) obtained with Herschel, Spitzer, and APEX, in addition to several molecular line observations. The Herschel maps are used to derive temperature and column density maps of G48 using a model based on a modified blackbody. We find that G48 has a relatively simple structure and is relatively isolated; thus, this IRDC provides an excellent target to study the collapse and fragmentation of a filamentary structure in the absence of complicating factors such as strong external feedback. The derived temperature structure of G48 is clearly non-isothermal from cloud to core scale. The column density peaks are spatially coincident with the lowest temperatures (~17.5 K) in G48. A total cloud mass of ~390 M ☉ is derived from the column density maps. By comparing the luminosity-to-mass ratio of 13 point sources detected in the Herschel/PACS bands to evolutionary models, we find that two cores are likely to evolve into high-mass stars (M ≥ 8 M ☉). The derived mean projected separation of point sources is smaller than in other IRDCs but in good agreement with theoretical predications for cylindrical collapse. We detect several molecular species such as CO, HCO+, HCN, HNC, and N2H+. CO is depleted by a factor of ~3.5 compared to the expected interstellar abundance, from which we conclude that CO freezes out in the central region. Furthermore, the molecular clumps, associated with the submillimeter peaks in G48, appear to be gravitationally unbound or just pressure confined. The analysis of critical line masses in G48 shows that the entire filament is collapsing, overcoming any internal support.

Far-infrared extinction mapping of infrared dark clouds

Abstract: Progress in understanding star formation requires detailed observational constraints on the initial conditions, i.e., dense clumps and cores in giant molecular clouds that are on the verge of gravitational instability. Such structures have been studied by their extinction of near-infrared and, more recently, mid-infrared (MIR) background light. It has been somewhat more of a surprise to find that there are regions that appear as dark shadows at far-infrared (FIR) wavelengths as long as ~100 ?m! Here we develop analysis methods of FIR images from Spitzer-MIPS and Herschel-PACS that allow quantitative measurements of cloud mass surface density, ?. The method builds on that developed for MIR extinction mapping by Butler & Tan, in particular involving a search for independently saturated, i.e., very opaque, regions that allow measurement of the foreground intensity. We focus on three massive starless core/clumps in the Infrared Dark Cloud (IRDC) G028.37+00.07, deriving mass surface density maps from 3.5 to 70 ?m. A by-product of this analysis is the measurement of the spectral energy distribution of the diffuse foreground emission. The lower opacity at 70 ?m allows us to probe to higher ? values, up to ~1 g cm?2 in the densest parts of the core/clumps. Comparison of the ? maps at different wavelengths constrains the shape of the MIR-FIR dust opacity law in IRDCs. We find that it is most consistent with the thick ice mantle models of Ossenkopf & Henning. There is tentative evidence for grain ice mantle growth as one goes from lower to higher ??regions.

Triggered massive and clustered stars formation by together H II regions G38.91-0.44 and G39.30-1.04

Abstract: We present the radio continuum, infrared, and CO molecular observations of infrared dark cloud (IRDC) G3895-047 and its adjacent H II regions G3891-044 (N74), G3893-039 (N75), and G3930-104 The Purple Mountain Observation (PMO) 137 m radio telescope was used to detect12CO J=1-0,13CO J=1-0 and C18O J=1-0 lines The carbon monoxide (CO) molecular observations can ensure the real association between the ionized gas and the neutral material observed nearby To select young stellar objects (YSOs) associated this region, we used the GLIMPSE I catalog The13CO J=1-0 emission presents two large cloud clumps The clump consistent with IRDC G3895-047 shows a triangle- like shape, and has a steep integrated-intensity gradient toward H II regions G3891-044 and G3930-104, suggesting that the two H II regions have expanded into the IRDC Four submillmeter continuum sources have been detected in the IRDC G3895-047 Only the G03895-0047-M1 source with a mass of 117 Msun has outflow and infall motions, indicating a newly forming massive star We detected a new collimated outflow in the clump compressed by G3893-039 The derived ages of the three H II regions are 61*10^5yr, 25*10^5yr, and 90*10^5yr, respectively In the IRDC G3895-047, the significant enhancement of several Class I YSOs indicates the presence of some recently formed stars Comparing the ages of these H II regions with YSOs (Class I sources and massive G03895-0047-M1 source), we suggest that YSOs may be triggered by G3891-044 and G3930-104 together, which supports the radiatively driven implosion model It may be the first time that the triggered star formation has occurred in the IRDC compressed by two H II regions The new detected outflow may be driven by a star cluster

Triggered massive and clustered star formation by combined H II regions G38.91-0.44 and G39.30-1.04

Abstract: Aims. We investigate the triggered star formation occurring in the Infrared dark clouds (IRDC) G38.95-0.47 between H II regions G38.91-0.44 and G39.30-1.04, and study the detailed morphology, distribution, and physical parameters of the molecular gas and dust in this region. Methods. We present the radio continuum, infrared, and CO molecular observations of infrared dark cloud (IRDC) G38.95-0.47 and its adjacent H II regions G38.91-0.44 (N74), G38.93-0.39 (N75), and G39.30-1.04. The Purple Mountain Observation (PMO) 13.7 m radio telescope was used to detect 12 CO J=1-0, 13 CO J=1-0 and C 18 O J=1-0 lines. The carbon monoxide (CO) molecular observations can ensure the real association between the ionized gas and the neutral material observed nearby. To select young stellar objects (YSOs) associated this region, we used the GLIMPSE I catalog. Results. The 13 CO J=1-0 emission presents two large cloud clumps. The clump consistent with IRDC G38.95-0.47 shows a trianglelike shape, and has a steep integrated-intensity gradient t oward H II regions G38.91-0.44 and G39.30-1.04, suggesting that the two H II regions have expanded into the IRDC. Four submillmeter continuum sources have been detected in the IRDC G38.95-0.47. Only the G038.95-00.47-M1 source with a mass of 117 M⊙ has outflow and infall motions, indicating a newly forming ma ssive star. We detected a new collimated outflow in the clump compressed by G 38.93-0.39. The derived ages of the three H II regions are 6.1×10 5 yr, 2.5×10 5 yr, and 9.0×10 5 yr, respectively. In the IRDC G38.95-0.47, the significant e nhancement of several Class I YSOs indicates the presence of some recently formed stars. Comparing the ages of these H II regions with YSOs (Class I sources and massive G038.9500.47-M1 source), we suggest that YSOs may be triggered by G38.91-0.44 and G39.30-1.04 together, which supports the radiatively driven implosion model. It may be the first time that the trigg ered star formation has occurred in the IRDC compressed by two H II regions. The new detected outflow may be driven by a star clust er.

G048.66-0.29: Physical State of an Isolated Site of Massive Star Formation

Abstract: We present continuum observations of the infrared dark cloud (IRDC) G48.66-0.22 (G48) obtained with Herschel, Spitzer, and APEX, in addition to several molecular line observations. The Herschel maps are used to derive temperature and column density maps of G48 using a model based on a modified blackbody. We find that G48 has a relatively simple structure and is relatively isolated; thus this IRDC provides an excellent target to study the collapse and fragmentation of a filamentary structure in the absence of complicating factors such as strong external feedback. The derived temperature structure of G48 is clearly non-isothermal from cloud to core scale. The column density peaks are spatially coincident with the lowest temperatures (~ 17.5 K) in G48. A total cloud mass of ~390Msun is derived from the column density maps. By comparing the luminosity-to-mass ratio of 13 point sources detected in the Herschel/PACS bands to evolutionary models, we find that two cores are likely to evolve into high-mass stars (M>8 Msun). The derived mean projected separation of point sources is smaller than in other IRDCs but in good agreement with theoretical predications for cylindrical collapse. We detect several molecular species such as CO, HCO+, HCN, HNC and N2H+. CO is depleted by a factor of ~3.5 compared to the expected interstellar abundance, from which we conclude that CO freezes out in the central region. Furthermore, the molecular clumps, associated with the sub-millimeter peaks in G48, appear to be gravitationally unbound or just pressure confined. The analysis of critical line masses in G48 show that the entire filament is collapsing, overcoming any internal support.

A multi-transition molecular line study of infrared dark cloud G331.71+00.59

Abstract: Using archive data from the Millimeter Astronomy Legacy Team Survey at 90 GHz (MALT90), carried out using the Mopra 22-m telescope, we made the first multi-transition molecular line study of infrared dark cloud (IRDC) MSXDC G331.71+00.59. Two molecular cores were found embedded in this IRDC. Each of these cores is associated with a known extended green object (EGO), indicating places of massive star formation. The HCO + (1–0) and HNC(1–0) transitions show prominent blue or red asymmetric structures, suggesting outflow and inflow activities of young stellar objects (YSOs). Other detected molecular lines include H 13 CO + (1–0), C 2 H(1–0), HC 3 N(10–9), HNCO(4 0,4 –3 0,3 ) and SiO (2–1), which are typical of hot cores and outflows. We regard the two EGOs as evolving from the IRDC to hot cores. Using public GLIMPS data, we investigate the spectral energy distribution of EGO G331.71+0.60. Our results support this EGO being a massive YSO driving the outflow. G331.71+0.58 may be at an earlier evolutionary stage.

The Sequential Growth of Star Formation Seeds in the Galactic Snake: Infrared Dark Cloud G11.11-0.12

Abstract: We present Submillimeter Array (SMA) 1.3 and 0.88 mm broad band observations, and Very Large Array (VLA) observations in NH3 (J,K) = (1,1) up to (5,5), as well as H2O and CH3OH maser lines toward the two most massive molecular clumps in Infrared Dark Cloud (IRDC) G11.11-0.12, also known as the Snake nebula. The sensitive high-resolution images reveal hierarchical fragmentation from the ~1pc clump scale down to ~0.01pc condensation scale. At each fragmenting scale, the mass of the fragments is orders of magnitude larger than the Jeans mass. This is common to all four IRDC clumps we studied, suggesting that turbulence plays a dominant role in the initial stages of clustered star formation. Masers, shock heated NH3 gas, and outflows indicate intense ongoing star formation in some cores while none of such signatures are found in others. Furthermore, chemical differentiation between cores reflects a sequential growth of these star formation seeds. The same applies to condensations and clumps. The mass function of the resolved condensations is consistent with a power law with an index of alpha = 2.0+/-0.2 and a turnover at 2.7 solar mass. Our combined SMA+VLA observations of several IRDC clumps have presented so far the deepest view of the early stages prior to the hot core phase, revealing snapshots of physical and chemical properties at various stages along an apparent evolutionary sequence, subject to further tests by detailed modeling.