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Register a new userCharacterizing filaments in regions of high-mass star formation: High-resolution submilimeter imaging of the massive star-forming complex NGC 6334 with ArTeMiS
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Herschel observations of nearby molecular clouds suggest that interstellar filaments and prestellar cores represent two fundamental steps in the star formation process. The observations support a picture of low-mass star formation according to which ~ 0.1 pc-wide filaments form first in the cold interstellar medium, probably as a result of large-scale compression of interstellar matter by supersonic turbulent flows, and then prestellar cores arise from gravitational fragmentation of the densest filaments. Whether this scenario also applies to regions of high-mass star formation is an open question, in part because Herschel data cannot resolve the inner width of filaments in the nearest regions of massive star formation. We used the bolometer camera ArTeMiS on the APEX telescope to map the central part of the NGC6334 complex at a factor of > 3 higher resolution than Herschel at 350 microns. Combining ArTeMiS data with Herschel data allowed us to study the structure of the main filament of the complex with a resolution of 8 or < 0.07 pc at d ~ 1.7 kpc. Our study confirms that this filament is a very dense, massive linear structure with a line mass ranging from ~ 500 Msun/pc to ~ 2000 Msun/pc over nearly 10 pc. It also demonstrates that its inner width remains as narrow as W ~ 0.15 +- 0.05 pc all along the filament length, within a factor of < 2 of the characteristic 0.1 pc value found with Herschel for lower-mass filaments in the Gould Belt. While it is not completely clear whether the NGC 6334 filament will form massive stars or not in the future, it is two to three orders of magnitude denser than the majority of filaments observed in Gould Belt clouds, and yet has a very similar inner width. This points to a common physical mechanism for setting the filament width and suggests that some important structural properties of nearby clouds also hold in high-mass star forming regions.
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Massive stars and their associated ionized (HII) regions could play a key role in the formation and evolution of filaments that host star formation. However, the properties of filaments that interact with H regions are still poorly known. To investigate the impact of HII regions on the formation of filaments, we imaged the Galactic HII region RCW 120 and its surroundings where active star formation takes place and where the role of ionization feedback on the star formation process has already been studied. We used the ArTeMiS camera on the APEX telescope and combined the ArTeMiS data at 350 and 450 microns with Herschel-SPIRE/HOBYS. We studied the dense gas distribution around RCW 120 with a resolution of 8 arcsec (0.05 pc at a distance of 1.34 kpc). Our study allows us to trace the median radial intensity profile of the dense shell of RCW 120. This profile is asymmetric, indicating a clear compression from the HII region on the inner part of the shell. The profile is observed to be similarly asymmetric on both lateral sides of the shell, indicating a homogeneous compression over the surface. On the contrary, the profile analysis of a radial filament associated with the shell, but located outside of it, reveals a symmetric profile, suggesting that the compression from the ionized region is limited to the dense shell. The mean intensity profile of the internal part of the shell is well fitted by a Plummer like profile with a deconvolved Gaussian FWHM of 0.09 pc, as observed for filaments in low-mass star-forming regions. This study suggests that compression exerted by HII regions may play a key role in the formation of filaments and may further act on their hosted star formation. ArTeMiS data also suggest that RCW 120 might be a 3D ring, rather than a spherical structure
The full stellar population of NGC 6334, one of the most spectacular regions of massive star formation in the nearby Galaxy, have not been well-sampled in past studies. We analyze here a mosaic of two Chandra X-ray Observatory images of the region using sensitive data analysis methods, giving a list of 1607 faint X-ray sources with arcsecond positions and approximate line-of-sight absorption. About 95 percent of these are expected to be cluster members, most lower mass pre-main sequence stars. Extrapolating to low X-ray levels, the total stellar population is estimated to be 20-30,000 pre-main sequence stars. The X-ray sources show a complicated spatial pattern with about 10 distinct star clusters. The heavily-obscured clusters are mostly associated with previously known far-infrared sources and radio HII regions. The lightly-obscured clusters are mostly newly identified in the X-ray images. Dozens of likely OB stars are found, both in clusters and dispersed throughout the region, suggesting that star formation in the complex has proceeded over millions of years. A number of extraordinarily heavily absorbed X-ray sources are associated with the active regions of star formation.
We have studied four complex organic molecules (COMs), methyl formate ($CH_3OCHO$), dimethyl ether ($CH_3OCH_3$), formamide ($NH_2CHO$), and ethyl cyanide ($C_2H_5CN$), towards a large sample of 39 high-mass star-forming regions representing different evolutionary stages, from early to evolved phases. We aim to identify potential correlations between the molecules and to trace their evolutionary sequence through the star formation process. We analysed spectra obtained at 3, 2, and 0.9 mm with the IRAM-30m telescope. We derived the main physical parameters for each species by fitting the molecular lines. We compared them and evaluated their evolution, also taking several other interstellar environments into account. We report detections in 20 sources, revealing a clear dust absorption effect on column densities. Derived abundances are ~$10^{-10}-10^{-7}$ for $CH_3OCHO$ and $CH_3OCH_3$, ~$10^{-12}-10^{-10}$ for $NH_2CHO$, and ~$10^{-11}-10^{-9}$ for $C_2H_5CN$. The abundances of $CH_3OCHO$, $CH_3OCH_3$, and $C_2H_5CN$ are very strongly correlated (r>0.92) across ~4 orders of magnitude. $CH_3OCHO$ and $CH_3OCH_3$ show the strongest correlations in most parameters, and a nearly constant ratio (~1) over a remarkable ~9 orders of magnitude in luminosity for a wide variety of sources: pre-stellar to evolved cores, low- to high-mass objects, shocks, Galactic clouds, and comets. This indicates that COMs chemistry is likely early developed and then preserved through evolved phases. Moreover, the molecular abundances clearly increase with evolution. We consider $CH_3OCHO$ and $CH_3OCH_3$ to be most likely chemically linked: they could e.g. share a common precursor, or be formed one from the other. We propose a general scenario for all COMs, involving a formation in the cold, earliest phases of star formation and a following increasing desorption with the progressive heating of the evolving core.
To constrain models of high-mass star formation, the Herschel/HOBYS KP aims at discovering massive dense cores (MDCs) able to host the high-mass analogs of low-mass prestellar cores, which have been searched for over the past decade. We here focus on NGC6334, one of the best-studied HOBYS molecular cloud complexes. We used Herschel PACS and SPIRE 70-500mu images of the NGC6334 complex complemented with (sub)millimeter and mid-infrared data. We built a complete procedure to extract ~0.1 pc dense cores with the getsources software, which simultaneously measures their far-infrared to millimeter fluxes. We carefully estimated the temperatures and masses of these dense cores from their SEDs. A cross-correlation with high-mass star formation signposts suggests a mass threshold of 75Msun for MDCs in NGC6334. MDCs have temperatures of 9.5-40K, masses of 75-1000Msun, and densities of 10^5-10^8cm-3. Their mid-IR emission is used to separate 6 IR-bright and 10 IR-quiet protostellar MDCs while their 70mu emission strength, with respect to fitted SEDs, helps identify 16 starless MDC candidates. The ability of the latter to host high-mass prestellar cores is investigated here and remains questionable. An increase in mass and density from the starless to the IR-quiet and IR-bright phases suggests that the protostars and MDCs simultaneously grow in mass. The statistical lifetimes of the high-mass prestellar and protostellar core phases, estimated to be 1-7x10^4yr and at most 3x10^5yr respectively, suggest a dynamical scenario of high-mass star formation. The present study provides good mass estimates for a statistically significant sample, covering the earliest phases of high-mass star formation. High-mass prestellar cores may not exist in NGC6334, favoring a scenario presented here, which simultaneously forms clouds and high-mass protostars.
We present a study of molecular outflows using six molecular lines (including 12CO/13CO/C18O/HCO+(J = 1-0) and SiO/CS(J = 2-1)) toward nine nearby high-mass star-forming regions with accurate known distances. This work is based on the high-sensitivity observations obtained with the 14-m millimeter telescope of Purple Mountain Observatory Delingha (PMODLH) observatory. The detection rate of outflows (including 12CO, 13CO, HCO+, and CS) is 100%. However, the emission of SiO was not detected for all sources. The full line widths ($Delta V$) at 3$sigma$ above the baseline of these molecular lines have the relationship $Delta V_{rm ^{12}CO} > Delta V_{rm HCO^{+}} > Delta V_{rm CS} approx Delta V_{rm ^{13}CO} > Delta V_{rm ^{18}CO}$. 12CO and HCO+ can be used to trace relatively high-velocity outflows, while 13CO and CS can be employed to trace relatively low-velocity outflows. The dynamical timescales of the 13CO and CS outflows are longer than those of the 12CO and HCO+ outflows. The mechanical luminosities, masses, mass-loss rates and forces of all outflows (including 12CO, 13CO, HCO+, and CS) are correlated with the bolometric luminosities of their central IRAS sources.
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