Thushara Pillai

HOW MANY INFRARED DARK CLOUDS CAN FORM MASSIVE STARS AND CLUSTERS

We present a new assessment of the ability of Infrared Dark Clouds (IRDCs) to form massive stars and clusters. This is done by comparison with an empirical mass-size threshold for massive star formation (MSF). We establish m(r)>870 M_☉(r/pc)^(1.33) as a novel approximate MSF limit, based on clouds with and without MSF. Many IRDCs, if not most, fall short of this threshold. Without significant evolution, such clouds are unlikely MSF candidates. This provides a first quantitative assessment of the small number of IRDCs evolving toward MSF. IRDCs below this limit might still form stars and clusters of up to intermediate mass, though (like, e.g., the Ophiuchus and Perseus Molecular Clouds). Nevertheless, a major fraction of the mass contained in IRDCs might reside in few 10^2 clouds sustaining MSF.

Magnetic Fields in High-mass Infrared Dark Clouds

High-mass Stars are cosmic engines known to dominate the energetics in the Milky Way and other galaxies. However, their formation is still not well understood. Massive, cold, dense clouds, often appearing as Infrared Dark Clouds (IRDCs), are the nurseries of massive stars. No measurements of magnetic fields in IRDCs in a state prior to the onset of high-mass star formation (HMSF) have previously been available, and prevailing HMSF theories do not consider strong magnetic fields. Here, we report observations of magnetic fields in two of the most massive IRDCs in the Milky Way. We show that IRDCs G11.11-0.12 and G0.253+0.016 are strongly magnetized and that the strong magnetic field is as important as turbulence and gravity for HMSF. The main dense filament in G11.11-0.12 is perpendicular to the magnetic field, while the lower density filament merging onto the main filament is parallel to the magnetic field. The implied magnetic field is strong enough to suppress fragmentation sufficiently to allow HMSF. Other mechanisms reducing fragmentation, such as the entrapment of heating from young stars via high mass surface densities, are not required to facilitate HMSF.

Hierarchical fragmentation and differential star formation in the Galactic ‘Snake’: infrared dark cloud G11.11−0.12

We present Submillimeter Array (SMA) λ = 0.88 and 1.3 mm broad-band observations, and Very Large Array (VLA) observations in NH_3 (J, K) = (1,1) up to (5,5), H_2O and CH_3OH maser lines towards the two most massive molecular clumps in infrared dark cloud (IRDC) G11.11−0.12. Sensitive high-resolution images reveal hierarchical fragmentation in dense molecular gas from the ∼1 pc clump scale down to ∼0.01 pc condensation scale. At each 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 an important role in the early stages of clustered star formation. Masers, shock heated NH_3 gas, and outflows indicate intense ongoing star formation in some cores while no such signatures are found in others. Furthermore, chemical differentiation may reflect the difference in evolutionary stages among these star formation seeds. We find NH_3 ortho/para ratios of 1.1 ± 0.4, 2.0 ± 0.4, and 3.0 ± 0.7 associated with three outflows, and the ratio tends to increase along the outflows downstream. Our combined SMA and VLA observations of several IRDC clumps present the most in-depth view so far 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.

High mass star formation in the infrared dark cloud G11.11-0.12

We report detection of moderate to high-mass star formation in an infrared dark cloud (Gl 1.11-0.12) where we discovered class II methanol and water maser emission at 6.7 GHz and 22.2 GHz, respectively. We also observed the object in ammonia inversion transitions. Strong emission from the (3,3) line indicates a hot (60 K) compact component associated with the maser emission. The line width of the hot component (4 km s -1 ), as well as the methanol maser detection, are indicative of high mass star formation. To further constrain the physical parameters of the source, we derived the spectral energy distribution (SED) of the dust continuum by analysing data from the 2MASS survey, HIRAS, MSX, the Spitzer Space Telescope, and interferometric 3 mm observations. The SED was modelled in a radiative transfer program: a) the stellar luminosity equals ∼1200 L ○. corresponding to a ZAMS star of 8 M ○. ; b) the bulk of the envelope has a temperature of 19 K; c) the mass of the remnant protostellar cloud in an area 8 x 10 17 cm or 15 across amounts to 500 M ○. , if assuming standard dust of the diffuse medium, and to about 60 M ○. , should the grains be fluffy and have ice mantles; d) the corresponding visual extinction towards the star, Av, is a few hundred magnitudes. The near IR data can be explained by scattering from tenuous material above a hypothetical disk. The class II methanol maser lines are spread out in velocity over 11 km s -1 . To explain the kinematics of the masing spots, we propose that they are located in a Kepler disk at a distance of about 250 AU. The dust temperatures there are around 150 K, high enough to evaporate methanol-containing ice mantles.

Probing the initial conditions of high-mass star formation - II. Fragmentation, stability, and chemistry towards high-mass star-forming regions G29.96−0.02 and G35.20−1.74

Most work on high-mass star formation has focused on observations of young massive stars in protoclusters. Very little is known about the preceding stage. Here, we present a new high-resolution study of pre-protocluster regions in tracers exclusively probing the coldest and dense gas (NH_2D). The two target regions G29.96−0.02 and G35.20−1.74 (W48) are drawn from the SCAMPS project, which searches for pre-protoclusters near known ultracompact Hii regions. We used our data to constrain the chemical, thermal, kinematic, and physical conditions (i.e., densities) in G29.96e and G35.20w. NH_3, NH_2D, HCO^+ , and continuum emission were mapped using the VLA, PdBI, and BIMA. In particular, NH_2D is a unique tracer of cold, precluster gas at high densities, while NH_3 traces both the cold and warm gas of modest-to-high densities. In G29.96e, Spitzer images reveal two massive filaments, one of them in extinction (infrared dark cloud). Dust and line observations reveal fragmentation into multiple massive cores strung along filamentary structures. Most of these are cold ( 10^5 cm^(-3)) and highly deuterated ([NH_2D/NH_3] > 6%). In particular, we observe very low line widths in NH_2D (FWHM ≲ 1 km s^(-1)). These are very narrow lines that are unexpected towards a region forming massive stars. Only one core in the center of each filament appears to be forming massive stars (identified by the presence of masers and massive outflows); however, it appears that only a few such stars are currently forming (i.e., just a single Spitzer source per region). These multi-wavelength, high-resolution observations of high-mass pre-protocluster regions show that the target regions are characterized by (i) turbulent Jeans fragmentation of massive clumps into cores (from a Jeans analysis); (ii) cores and clumps that are “over-bound/subvirial”, i.e. turbulence is too weak to support them against collapse, meaning that (iii) some models of monolithic cloud collapse are quantitatively inconsistent with data; (iv) accretion from the core onto a massive star, which can (for observed core sizes and velocities) be sustained by accretion of envelope material onto the core, suggesting that (similar to competitive accretion scenarios) the mass reservoir for star formation is not necessarily limited to the natal core; (v) high deuteration ratios ([NH_2D/NH_3] > 6%), which make the above discoveries possible; (vi) and the destruction of NH_2D toward embedded stars.

IS PROTOSTELLAR HEATING SUFFICIENT TO HALT FRAGMENTATION? A CASE STUDY OF THE MASSIVE PROTOCLUSTER G8.68–0.37

If star formation proceeds by thermal fragmentation and the subsequent gravitational collapse of the individual fragments, how is it possible to form fragments massive enough for O and B stars in a typical star-forming molecular cloud where the Jeans mass is about 1 M_⊙ at the typical densities (10^4 cm^(−3)) and temperatures (10 K)? We test the hypothesis that a first generation of low-mass stars may heat the gas enough that subsequent thermal fragmentation results in fragments ≥10 M_⊙, sufficient to form B stars. We combine ATCA and Submillimeter Array observations of the massive star-forming region G8.68−0.37 with radiative transfer modeling to derive the present-day conditions in the region and use this to infer the conditions in the past, at the time of core formation. Assuming that the current mass/separation of the observed cores equals the fragmentation Jeans mass/length and the region’s average density has not changed requires the gas temperature to have been 100 K at the time of fragmentation. The postulated first generation of low-mass stars would still be around today, but the number required to heat the cloud exceeds the limits imposed by the observations. Several lines of evidence suggest the observed cores in the region should eventually form O stars yet none have sufficient raw material. Even if feedback may have suppressed fragmentation, it was not sufficient to halt it to this extent. To develop into O stars, the cores must obtain additional mass from outside their observationally defined boundaries. The observations suggest that they are currently fed via infall from the very massive reservoir (~1500 M_⊙) of gas in the larger parsec scale cloud around the star-forming cores. This suggests that massive stars do not form in the collapse of individual massive fragments, but rather in smaller fragments that themselves continue to gain mass by accretion from larger scales.

The Mass-Size Relation From Clouds to Cores. I. A New Probe of Structure In Molecular Clouds

We use a new contour-based map analysis technique to measure the mass and size of molecular cloud fragments continuously over a wide range of spatial scales (0.05 ≤ r/pc ≤ 10), i.e., from the scale of dense cores to those of entire clouds. The present paper presents the method via a detailed exploration of the Perseus molecular cloud. Dust extinction and emission data are combined to yield reliable scale-dependent measurements of mass. This scale-independent analysis approach is useful for several reasons. First, it provides a more comprehensive characterization of a map (i.e., not biased toward a particular spatial scale). Such a lack of bias is extremely useful for the joint analysis of many data sets taken with different spatial resolution. This includes comparisons between different cloud complexes. Second, the multi-scale mass-size data constitute a unique resource to derive slopes of mass-size laws (via power-law fits). Such slopes provide singular constraints on large-scale density gradients in clouds.

Probing the initial conditions of high mass star formation: I. Deuteration and depletion in high mass pre/protocluster clumps

Aims. UltraCompact Hit regions are signposts of high-mass star formation. Since high-mass star formation occurs in clusters, one expects to find even earlier phases of massive star formation in the vicinity of UltraCompact Hit regions. Here, we study the amount of deuteration and depletion toward pre/protocluster clumps found in a wide-field (10 x 10 arcmin) census of clouds in 32 massive star-forming regions that are known to harbour UCHII regions. Methods. We determine the column density of NH 3 , NH 2 D, CO, H 13 CN, and HC 15 N lines. We used the (J,K) =(1,1) and (2,2) inversion transitions of NH 3 to constrain the gas temperatures. Results. We find that 65% of the observed sources have strong NH 2 D emission and more than 50% of the sources exhibit a high degree of deuteration, (0.1 < NH 2 D/NH 3 < 0.7), 0.7 being the highest observed deuteration of NH 3 reported to date. Our search for NHD 2 in two sources did not result in a detection. The enhancement in deuteration coincides with moderate CO depletion onto dust grains. There is no evidence of a correlation between the two processes, though an underlying correlation cannot be ruled out as the depletion factor is very likely to be only a lower limit. Based on simultaneously observed H 13 CN and HC 15 N (J = 1-0) lines, we derive a high abundance ratio of H 13 CN to HC 15 N, which might indicate anomalous ratios of C and N isotopes relative to those derived toward the local ISM. Conclusions. We find CO depletion and high deuteration towards cold cores in massive star forming regions. Therefore, these are good candidates for sources at the early phases of massive star formation. While our sensitive upper limits on NHD 2 do not prove the predictions of the gas-phase and grain chemistry models wrong, an enhancement of ≃10 4 over the cosmic D/H ratio from NH 2 D warrants explanation.

NH3 OBSERVATIONS OF THE INFRARED DARK CLOUD G28.34+0.06

We present observations of the NH3 (J, K) = (1, 1) and (2, 2) inversion transitions toward the infrared dark cloud G28.34+0.06, using the Very Large Array. Strong NH3 emission is found to coincide well with the infrared absorption feature in this cloud. The northern region of G28.34+0.06 is dominated by a compact clump (P2) with a high rotation temperature (29 K) and large line width (4.3 km s−1), and is associated with a strong water maser (240 Jy) and a 24 μm point source with far-IR luminosity of 103 L☉. We infer that P2 has embedded massive protostars although it lies in the 8 μm absorption region. The southern region has filamentary structures. The rotation temperature in the southern region decreases with the increase of the integrated NH3 intensity, which indicates an absence of strong internal heating in these clumps. In addition, the compact core P1 in the south has small line width (1.2 km s−1) surrounded by extended emission with larger line width (1.8 km s−1), which suggests a dissipation of turbulence in the dense part of the cloud. Thus, we suggest that P1 is at a much earlier evolutionary stage than P2, possibly at a stage that begins to form a cluster with massive stars.

Masers as signposts of high-mass protostars A water maser survey of methanol maser sources

The 22 GHz H2O maser line was observed towards 79 candidate high-mass protostellar objects from a flux-limited sample of 6.7 GHz methanol sources. The emission was detected in 41 sources, towards 28 of these for the first time. The detection rate of 52% was similar to rates reported for other samples of high-mass protostars selected mainly with far-infrared (FIR) colour criteria. The median value of H2O maser luminosity of 10 −5.5 Lis equal to that of the CH3OH maser luminosity, whereas the median OH maser luminosity was found to be ∼1.5 orders of magnitude lower. Comparison of the velocity ranges showing maser emission implies that for the majority of sources the H2 Oa nd CH 3OH maser lines originate from different regions. The percentage of sources with emission in two or three of the maser species, their association with radio contin- uum and IR emission and the maser and IR luminosities are consistent with the view that evolutionary phases with H2 Oa nd CH3OH masers largely overlap and precede the OH maser phase, while at a later stage OH and CH3OH masers may coexist. Strong correlations of OH and CH3OH maser luminosities with IR luminosity and only a marginal correlation of H2 Oa nd IR luminosity confirm current pumping schemes of all three maser lines.